Ian Burke
University of Leeds, Earth and Environment, Faculty Member
The tailings dam breach at the Ajka alumina plant, western Hungary in 2010 introduced 1 million m3 of red mud suspension into the surrounding area. Red mud (fine fraction bauxite residue) has a characteristically alkaline pH and contains... more
The tailings dam breach at the Ajka alumina plant, western Hungary in 2010 introduced 1 million m3 of
red mud suspension into the surrounding area. Red mud (fine fraction bauxite residue) has a characteristically
alkaline pH and contains several potentially toxic elements, including arsenic. Aerobic and anaerobic
batch experiments were prepared using soils from near Ajka in order to investigate the effects of red
mud addition on soil biogeochemistry and arsenic mobility in soil–water experiments representative of
land affected by the red mud spill. XAS analysis showed that As was present in the red mud as As(V) in
the form of arsenate. The remobilisation of red mud associated arsenate was highly pH dependent and
the addition of phosphate to red mud suspensions greatly enhanced As release to solution. In aerobic
batch experiments, where red mud was mixed with soils, As release to solution was highly dependent
on pH. Carbonation of these alkaline solutions by dissolution of atmospheric CO2 reduced pH, which
resulted in a decrease of aqueous As concentrations over time. However, this did not result in complete
removal of aqueous As in any of the experiments. Carbonation did not occur in anaerobic experiments
and pH remained high. Aqueous As concentrations initially increased in all the anaerobic red mud
amended experiments, and then remained relatively constant as the systems became more reducing,
both XANES and HPLC–ICP-MS showed that no As reduction processes occurred and that only As(V)
species were present. These experiments show that there is the potential for increased As mobility in
soil–water systems affected by red mud addition under both aerobic and anaerobic conditions.
red mud suspension into the surrounding area. Red mud (fine fraction bauxite residue) has a characteristically
alkaline pH and contains several potentially toxic elements, including arsenic. Aerobic and anaerobic
batch experiments were prepared using soils from near Ajka in order to investigate the effects of red
mud addition on soil biogeochemistry and arsenic mobility in soil–water experiments representative of
land affected by the red mud spill. XAS analysis showed that As was present in the red mud as As(V) in
the form of arsenate. The remobilisation of red mud associated arsenate was highly pH dependent and
the addition of phosphate to red mud suspensions greatly enhanced As release to solution. In aerobic
batch experiments, where red mud was mixed with soils, As release to solution was highly dependent
on pH. Carbonation of these alkaline solutions by dissolution of atmospheric CO2 reduced pH, which
resulted in a decrease of aqueous As concentrations over time. However, this did not result in complete
removal of aqueous As in any of the experiments. Carbonation did not occur in anaerobic experiments
and pH remained high. Aqueous As concentrations initially increased in all the anaerobic red mud
amended experiments, and then remained relatively constant as the systems became more reducing,
both XANES and HPLC–ICP-MS showed that no As reduction processes occurred and that only As(V)
species were present. These experiments show that there is the potential for increased As mobility in
soil–water systems affected by red mud addition under both aerobic and anaerobic conditions.
Research Interests:
The failure of the Ajka red mud depository in October 2010 led to the largest single release of red mud into the surface water environment. This study provides a comparative assessment of stream sediment quality in the... more
The failure of the Ajka red mud depository in October 2010 led to the largest single release of red mud into
the surface water environment. This study provides a comparative assessment of stream sediment quality in
the Torna–Marcal–R´aba catchment between post-disaster surveys (2010) and follow up surveys at an
identical suite of 21 locations in 2013. The signature of red mud apparent in initial surveys with high Al,
As, Cr, Na, V was only apparent at a small number of sample stations in recent surveys. These constitute
<1 km of stream, compared to the >20 km reach of affected sediments in the immediate aftermath of
the spill. Concentrations of red mud-derived contaminants are predominately associated with fine
fractions of the red mud (<8 mm). This enhances transport out of the system of red mud-derived
contaminants and, along with extensive remedial efforts, has substantially limited the within-channel
inventory of potentially ecotoxic metals and metalloids.
the surface water environment. This study provides a comparative assessment of stream sediment quality in
the Torna–Marcal–R´aba catchment between post-disaster surveys (2010) and follow up surveys at an
identical suite of 21 locations in 2013. The signature of red mud apparent in initial surveys with high Al,
As, Cr, Na, V was only apparent at a small number of sample stations in recent surveys. These constitute
<1 km of stream, compared to the >20 km reach of affected sediments in the immediate aftermath of
the spill. Concentrations of red mud-derived contaminants are predominately associated with fine
fractions of the red mud (<8 mm). This enhances transport out of the system of red mud-derived
contaminants and, along with extensive remedial efforts, has substantially limited the within-channel
inventory of potentially ecotoxic metals and metalloids.
Research Interests:
Red mud (RM) is a byproduct of aluminum production, worldwide between 70 and 120 million tons are produced annually. We analyzed RM which was released in the course of the Kolonatar disaster in Hungary into the environment in acute and... more
Red mud (RM) is a byproduct of aluminum production, worldwide between 70 and 120 million tons are produced annually. We analyzed RM which was released in the course of the Kolonatar disaster in Hungary into the environment in acute and genotoxicity experiments with plants which are widely used for environmental monitoring. We detected induction of micronuclei which reflect
chromosomal damage in tetrads of Tradescantia and in root cells of Allium as well as retardation of root growth with contaminated soils and leachates. Chemical analyses showed that RM contains metals, in particular high concentrations of vandadium. Follow up experiments indicated that vanadate causes the effects in the plants. This compound causes also in humans DNA damage and positive results were obtained in carcinogenicity studies. Since it was found also in RM from other production sites our findings indicate that its release in the environment is a global problem which should be studied in more detail.
chromosomal damage in tetrads of Tradescantia and in root cells of Allium as well as retardation of root growth with contaminated soils and leachates. Chemical analyses showed that RM contains metals, in particular high concentrations of vandadium. Follow up experiments indicated that vanadate causes the effects in the plants. This compound causes also in humans DNA damage and positive results were obtained in carcinogenicity studies. Since it was found also in RM from other production sites our findings indicate that its release in the environment is a global problem which should be studied in more detail.
Research Interests:
137Cs (t1/2 = 30 years) is a common contaminant at nuclear legacy sites. Often the mobility of 137Cs in the environment is governed by its sorption to charged sites within the sediment. To this end it is important to understand the... more
137Cs (t1/2 = 30 years) is a common contaminant at nuclear legacy sites. Often the mobility of 137Cs in the environment is governed by its sorption to charged sites within the sediment. To this end it is important to understand the sorption behaviour of Cs across a wide range of environmental conditions. This work investigates the effect of varying solution composition (pH and competing ions) on the sorption of Cs to micaceous aquifer sediment across a large concentration range (1.0x10-11 – 1.0x10-1 mol L-1 Cs). Experimental results show that Cs exhibits three distinct sorption behaviours at three different concentration ranges. At very low concentrations <1.0x10-6 mol L-1 Cs sorption was unaffected by competition with Na+ or H+ but significantly reduced in high ionic strength K+ solution. Secondly between 1x10-6 and 1.0x10-3 mol L-1 Cs is strongly sorbed in a neutral pH, low ionic strength background but sorption is significantly reduced in solutions with either a high concentration of Na+ or K+ ions or low pH. At high concentrations > 1.0x10-3 mol L-1 Cs sorption is reduced in all systems due to saturation of the sediment’s sorption capacity. A multi-site cation exchange model was used to interpret the sorption behaviour. From this it was determined that at low concentrations Cs sorbs to the illite frayed edge sites only in competition with K+ ions. However once the frayed edge sites are saturated the Cs sorbs to the Type II and Planar sites in competition with K+, Na+ and H+ ions. Therefore sorption of Cs at concentrations > 1.0 x 10-6 mol L-1 is significantly reduced in both high ionic strength and low pH solutions. This is a significant result with regard to predicting the migration of 137Cs in acidic or high ionic strength groundwaters.
Research Interests:
The biochemical and molecular mechanisms used by alkaliphilic bacterial communities to reduce metals in the environment are currently unknown. We demonstrate that an alkaliphilic (pH > 9) consortium dominated by Tissierella, Clostridium... more
The biochemical and molecular mechanisms used by alkaliphilic bacterial communities to reduce metals in the environment are currently unknown. We demonstrate that an alkaliphilic (pH > 9) consortium dominated by Tissierella, Clostridium and Alkaliphilus sp. are capable of using iron (Fe3+) as a final electron acceptor under anaerobic conditions. Iron reduction is associated with the production of a freely diffusible species that upon rudimentary purification and subsequent spectroscopic, HPLC and electrochemical analysis has been identified as a flavin species displaying properties indistinguishable from riboflavin. Due to the link between iron reduction and the onset of flavin production, it is likely that riboflavin has an import role in extracellular metal reduction by this alkaliphilic community.
Research Interests:
Zero-valent iron (ZVI) and green rusts can be used as reductants to convert chromium from soluble, highly toxic Cr(VI) to insoluble Cr(III). This study compared the reduction rates of Cr(VI) by ZVI and two carbonate green rust phases in... more
Zero-valent iron (ZVI) and green rusts can be used as reductants to convert chromium from soluble, highly toxic Cr(VI) to insoluble Cr(III). This study compared the reduction rates of Cr(VI) by ZVI and two carbonate green rust phases in alkaline/hyperalkaline solutions. Batch experiments were carried out with synthetic chromate solutions at pH 7.7–12.3 and a chromite ore processing residue (COPR) leachate (pH ≈ 12.2). Green rust removes chromate from high pH solutions (pH 10–12.5) very rapidly (<400 s). Chromate reduction rates for both green rust phases were consistently higher than for ZVI throughout the pH range studied; the surface area normalized rate constants were two orders of magnitude higher in the COPR leachate solution at pH 12.2. The performances of both green rusts were unaffected by changes in pH. In contrast, ZVI exhibited a marked decline in reduction rate with increasing pH to become almost ineffective above pH12.
Research Interests:
Red mud leachate (pH 13) collected from Ajka, Hungary is neutralised to < pH 10 by HCl, gypsum or seawater addition. During acid neutralisation >99 % Al is removed from solution during the formation of an amorphous boehmite-like... more
Red mud leachate (pH 13) collected from Ajka, Hungary is neutralised to < pH 10 by HCl, gypsum or seawater addition. During acid neutralisation >99 % Al is removed from solution during the formation of an amorphous boehmite-like precipitate and dawsonite. Minor amounts of As (24 %) are also removed from solution via surface adsorption of As onto the Al oxyhydroxides. Gypsum addition to red mud leachate results in the precipitation of calcite, both in experiments and in field samples recovered from rivers treated with gypsum after the October 2010 red mud spill. Calcite precipitation results in 86 % Al and 81 % As removal from solution, and both are non-exchangeable with 0.1 mol L-1 phosphate solution. Contrary to As associated with neoformed Al oxyhydroxides, EXAFS analysis of the calcite precipitates revealed only isolated arsenate tetrahedra with no evidence for surface adsorption or incorporation into the calcite structure, possibly as a result of very rapid As scavenging by the calcite precipitate. Seawater neutralisation also resulted in carbonate precipitation, with >99 % Al and 74 % As removed from solution during the formation of a poorly ordered hydrotalcite phase and via surface adsorption to the neoformed precipitates, respectively. Half the bound As could be remobilised by phosphate addition, indicating that As was weakly bound, possibly in the hydrotalcite interlayer. Only 5-16 % V was removed from solution during neutralisation, demonstrating a lack of interaction with any of the neoformed precipitates. High V concentrations are therefore likely to be an intractable problem during the treatment of red mud leachates.
Research Interests:
Results are presented from one year batch experiments where K-rich hyperalkaline pH 13.5 young cement water (YCW) was reacted with sediments to investigate the effect of high pH, mineral alteration and secondary mineral precipitation on... more
Results are presented from one year batch experiments where K-rich hyperalkaline pH 13.5 young cement water (YCW) was reacted with sediments to investigate the effect of high pH, mineral alteration and secondary mineral precipitation on 90Sr sorption. After reaction with YCW, Sr sorption was found to be >75 % in all samples up to 365 days, and 98 % in a sample reacted for 365 days at 70 °C. SEM analysis of sediment samples reacted at room temperature showed surface alteration and precipitation of a secondary phase, likely a K-rich aluminosilicate gel. The presence of Sr-Si(Al) bond distances in Sr K-edge EXAFS analysis suggested that the Sr was present as an inner-sphere adsorption complex. Sequential extractions, however, found the majority of this Sr was still exchangeable with Mg2+ at pH 7. For the sample reacted for one year at 70 °C, EXAFS analysis revealed clear evidence for ~6 Sr-Si(Al) backscatters at 3.45 Å, consistent with Sr incorporation into the neoformed K-chabazite phase that was detected by XRD and electron microscopy. Once incorporated into chabazite, 90Sr was not exchangeable with Mg2+ and chemical leaching with pH 1.5 HNO3 was required to remobilise 60 % of the 90Sr. These results indicate that in high pH cementitious leachate there is significantly enhanced Sr retention in sediments due to changes in the adsorption mechanism and incorporation into secondary silicate minerals. This suggests that Sr retention may be enhanced in this high pH zone and that the incorporation process may lead to irreversible exchange of the contaminant over extended time periods.
Research Interests:
"It is not currently known if the widely used reaction of zero valent iron (ZVI) and Cr(VI) can be used in a permeable reactive barrier (PRB) to immobilise Cr leaching from hyper alkaline chromite ore processing residue (COPR). This study... more
"It is not currently known if the widely used reaction of zero valent iron (ZVI) and Cr(VI) can be used in a permeable reactive barrier (PRB) to immobilise Cr leaching from hyper alkaline chromite ore processing residue (COPR). This study compares Cr(VI) removal from COPR leachate and chromate solution by ZVI at high pH. Cr(VI) removal occurs more rapidly from the chromate solution than from COPR leachate. The reaction is first order with respect to both [Cr(VI)] and the iron surface area, but iron surface reactivity is lost to the reaction. Buffering pH downwards produces little change in the removal rate or the specific capacity of iron until acidic conditions are reached. SEM and XPS analysis confirm that reaction products accumulate on the iron surface in both liquors, but that other surface precipitates
also form in COPR leachate. Leachate from highly alkaline COPR contains Ca, Si and Al that precipitate on the iron surface and significantly reduce the specific capacity of iron to reduce Cr(VI). This study suggests that although Cr (VI) reduction by ZVI will occur at hyper alkaline pH, other solutes present in COPR leachate will limit the design life of a PRB. "
also form in COPR leachate. Leachate from highly alkaline COPR contains Ca, Si and Al that precipitate on the iron surface and significantly reduce the specific capacity of iron to reduce Cr(VI). This study suggests that although Cr (VI) reduction by ZVI will occur at hyper alkaline pH, other solutes present in COPR leachate will limit the design life of a PRB. "
Research Interests:
Red mud is highly alkaline (pH 13), saline and can contain elevated concentrations of several potentially toxic elements (e.g. Al, As, Mo and V). Release of up to 1 million m3 of bauxite residue (red mud) suspension from the Ajka... more
Red mud is highly alkaline (pH 13), saline and can contain elevated concentrations of several potentially toxic elements (e.g. Al, As, Mo and V). Release of up to 1 million m3 of bauxite residue (red mud) suspension from the Ajka repository, western Hungary, caused large scale contamination of downstream rivers and floodplains. There is now concern about the potential leaching of toxic metal(loid)s from the red mud as some have enhanced solubility at high pH. This study investigated the impact of red mud addition to three different Hungarian soils with respect to trace element solubility and soil geochemistry. The effectiveness of gypsum amendment for the rehabilitation of red mud-contaminated soils was also examined. Red mud addition to soils caused a pH increase, proportional to red mud addition, of up to 4 pH units (e.g. pH 7 11). Increasing red mud addition also led to significant increases in salinity, dissolved organic carbon (DOC) and aqueous trace element concentrations. However, the response was highly soil specific and one of the soils tested buffered pH to around pH 8.5 even with the highest red mud loading tested (33% w/w); experiments using this soil also had much lower aqueous Al, As, and V concentrations. Gypsum addition to soil / red mud mixtures, even at relatively low concentrations (1% w/w) was sufficient to buffer experimental pH to 7.5-8.5. This effect was attributed to the reaction of Ca2+ supplied by the gypsum with OH- and carbonate from the red mud to precipitate calcite. The lowered pH enhanced trace element sorption and largely inhibited the release of Al, As and V. Mo concentrations, however, were largely unaffected by gypsum induced pH buffering due to the greater solubility of Mo (as molybdate) at circumneutral pH. Gypsum addition also leads to significantly higher porewater salinities and column experiments demonstrated that this increase in total dissolved solids persisted even after 25 pore volume replacements. Gypsum addition could therefore provide a cheaper alternative to recovery (dig and dump) for treatment of red mud affected soils. The observed inhibition of trace metal release within red mud affected soils was relatively insensitive to either the percentage of red mud or gypsum present, making the treatment easy to apply. However, there is risk that over-application of gypsum could lead to detrimental long term increases in soil salinity.
Research Interests:
Soil collected from beneath a chromite ore processing residue (COPR) disposal site contained a diverse population of anaerobic alkaliphiles, despite receiving a continuous influx of a Cr(VI) contaminated, hyperalkaline leachate (pH... more
Soil collected from beneath a chromite ore processing residue (COPR) disposal site contained a diverse population of anaerobic alkaliphiles, despite receiving a continuous influx of a Cr(VI) contaminated, hyperalkaline leachate (pH 12.2). Chromium was found to have accumulated in this soil as a result of an abiotic reaction of Cr(VI) with Fe(II) present in the soil. This sediment associated Fe(II) was therefore acting as a natural reactive zone beneath the COPR and thereby preventing the spread of Cr(VI). In anaerobic microcosm experiments soil microorganisms were able to reduce nitrate at pH 11.2 coupled to the oxidation of electron donors derived from the original soil organic matter, but progressive anoxia did not develop to the point of iron reduction over a period of 9 months. It is not clear, therefore, if Fe(II) can be actively replenished by microbial processes occurring within the soil at in situ conditions. Sodium bicarbonate was added to this soil to investigate whether bioreduction of iron in hyperalkaline chromium contaminated soils could be enhanced by reducing the pH to a value optimal for many alkaliphilic bacteria. The addition of sodium bicarbonate produced a well buffered system with a pH of ~9.3 and iron reducing conditions developed within 1 month once complete denitrification had occurred. Iron(III) reduction was associated with an increase in the proportion of genetic clone libraries that were from the phylum Firmicutes, suggesting that these species are responsible for the Fe(III) reduction observed. Amendment of the pH using bicarbonate may provide a suitable strategy for stimulating the bioreduction of Fe(III) in COPR leachate contaminated soils or other environments where microbial reduction is inhibited by elevated pH.
Research Interests:
Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of 90Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments.... more
Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of 90Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments. The extent of sorption is controlled by the geochemistry of the groundwater and sediment mineralogy. Here, batch sorption experiments were used to examine the sorption behaviour of 90Sr in sediment-water systems representative of the UK Sellafield nuclear site based on groundwater and contaminant fluid compositions. In experiments with low ionic strength groundwaters (< 0.01 mol L-1), pH variation is the main control on sorption. The sorption edge for 90Sr was observed between pH 4 – 6 with maximum sorption occurring (Kd ~ 103 L kg-1) at pH 6 - 8. At ionic strengths above 10 mmol L-1, and at pH values between 6 – 8, cation exchange processes reduced 90Sr uptake to the sediment. This exchange process explains the lower 90Sr sorption (Kd ~ 40 L kg-1) in the presence of artificial Magnox tank liquor (IS = 29 mmol L-1). Strontium K-edge EXAFS spectra collected from sediments incubated with Sr2+ in either HCO3-buffered groundwater or artificial Magnox tank liquor, revealed a coordination environment of ~ 9 O atoms at 2.58 - 2.61 Å after 10 days. This is equivalent to the Sr2+ hydration sphere for the aqueous ion and indicates that Sr occurs primarily in outer sphere sorption complexes. No change was observed in the Sr sorption environment with EXAFS analysis after 365 days incubation. Sequential extractions performed on sediments after 365 days also found that ~ 80 % of solid associated 90Sr was exchangeable with 1 M MgCl2 in all experiments. These results suggest that over long periods, 90Sr in contaminated sediments will remain primarily in weakly bound surface complexes. Therefore, if groundwater ionic strength increases (e.g. by saline intrusion related to sea level rise or by design during site remediation) then substantial remobilisation of 90Sr is to be expected.
Research Interests:
Results are presented from X-ray absorption spectroscopy based analysis of As, Cr and V speciation within samples of bauxite ore processing residue (red mud) collected from the spill site at Ajka, Western Hungary. Cr K-edge XANES analysis... more
Results are presented from X-ray absorption spectroscopy based analysis of As, Cr and V speciation within samples of bauxite ore processing residue (red mud) collected from the spill site at Ajka, Western Hungary. Cr K-edge XANES analysis found that Cr is present as Cr3+ substituted into hematite, consistent with TEM analysis. V K-edge XANES spectra have E½ position and pre-edge features consistent with the presence of V5+ species, possibly associated with Ca-aluminosilicate phases. As K-edge XANES spectra identified As present as As5+. EXAFS analysis reveals arsenate phases in red mud samples. When alkaline leachate from the spill site is neutralised with HCl, 94 % As and 71 % V is removed from solution during the formation of amorphous Al-oxyhydroxide. EXAFS analysis of As in this precipitate reveals the presence of arsenate Al-oxyhydroxide surface complexes. These results suggest that in the circumneutral pH, oxic conditions found in the Torna and Upper Marcal catchments, incorporation and sorption respectively will restrict the environmental mobility of Cr and As. V is inefficiently removed from solution by neutralisation, therefore, the red mud may act as a source of mobile V5+ where the red mud deposits are not removed from affected land.
Research Interests:
Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-rich wastes. The high pH produced in these areas has the potential to lead to highly specialised microbial communities and unusual... more
Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-rich wastes. The high pH produced in these areas has the potential to lead to highly specialised microbial communities and unusual biogeochemical processes. This paper reports an investigation into the geochemical processes that are occurring in a buried, saturated, organic–rich soil layer at pH 12.3. The soil has been trapped beneath calcite precipitate (tufa) that is accumulating where highly alkaline leachate from a lime kiln waste tip is emerging to atmosphere. A population of anaerobic alkaliphilic bacteria dominated a single, unidentified specie within the Comamonadaceae family of β-proteobacteria has established itself near the top of the soil layer. This bacterial population appears to be capable of nitrate reduction using electron donors derived from the soil organic matter. Below the zone of nitrate reduction a significant proportion of the 0.5N HCl extractable iron (a proxy for microbial available iron) is in the Fe(II) oxidation state indicating there is increasing anoxia with depth and suggesting that microbial iron reduction is occurring.
Research Interests:
A number of emergency pollution management measures were enacted after the accidental release of caustic bauxite processing residue that occurred in Ajka, western Hungary in October, 2010. These centred on acid and gypsum dosing to reduce... more
A number of emergency pollution management measures were enacted after the accidental release of caustic bauxite processing residue that occurred in Ajka, western Hungary in October, 2010. These centred on acid and gypsum dosing to reduce pH and minimise mobility of oxyanion contaminants mobile at high pH. This study assesses the effectiveness of gypsum dosing on contaminant mobility and carbon sequestration through assessment of red mud and gypsum-affected fluvial sediments via elemental analysis, sequential extraction and stable isotope analysis. There is a modest uptake of contaminants (notably As, Cr, and Mn) on secondary carbonate-dominated deposits in reaches subjected to gypsum dosing. C and O stable isotope ratios of carbonate precipitates formed as a result of gypsum dosing are used to quantify the importance of the neutralisation process in sequestering atmospheric carbon dioxide. This process is particularly pronounced at sites most affected by gypsum addition, where up to 36% of carbonate-C appears to be derived from atmospheric in-gassing of CO2. The site is discussed as a large scale analogue for potential remedial approaches and carbon sequestration technologies that could be applied to red mud slurries and other hyperalkaline wastes. The results of this work have substantial implications for the aluminium production industry in which 3-4% of the direct CO2 emissions may be offset by carbonate precipitation. Furthermore, carbonation by gypsum addition may be important for contaminant remediation, also providing a physical stabilisation strategy for the numerous historic stockpiles of red mud.
Research Interests:
This paper identifies the spatial extent of red mud-derived contaminants and modes of transport within the Marcal and Rába river systems after the dyke failure at Ajka, western Hungary. The geochemical signature of the red mud is apparent... more
This paper identifies the spatial extent of red mud-derived contaminants and modes of transport within the Marcal and Rába river systems after the dyke failure at Ajka, western Hungary. The geochemical signature of the red mud is apparent throughout the 3076km2 Marcal system principally with elevated Al, V, As and Mo. Elevated concentrations of Cr, Ga and Ni are also observed within 2km of source areas in aqueous and particulate phases where hyperalkalinity (pH <13.1) is apparent. While the concentrations of some trace elements exceed aquatic life standards in waters (e.g. V, As) and fluvial sediments (As, Cr, Ni, V), the spatial extent of these is limited to the Torna Creek and part of the upper Marcal. Source samples show a bi-modal particle size distribution (peaks at 0.7 and 1.3µm) which lends the material to ready fluvial transport. Where elevated concentrations are found in fluvial sediments, sequential extraction suggests the bulk of the As, Cr, Ni and V are associated with residual (aqua-regia/HF digest) phases and unlikely to be mobile in the environment. However, at some depositional hotspots , association of As, Cr and V with weak acid-extractable phases is observed.
Research Interests:
Between 600,000—700,000 m³ of caustic (pH> 13) red mud suspension were released from the Ajkai Timfoldgyar Zrt alumina plant on the 4th October 2010. This study highlights the dispersal of key red mudderived contaminants in downstream... more
Between 600,000—700,000 m³ of caustic (pH> 13) red mud suspension were released from the Ajkai Timfoldgyar Zrt alumina plant on the 4th October 2010. This study highlights the dispersal of key red mudderived contaminants in downstream fluvial sediments from surveys undertaken within two months of the spill. Source samples contain abundant V (> 1000mg/kg), Cr (> 800mg/kg), Ni (> 250mg/kg) and As (> 80mg/kg).
Dynamic -camera imaging of the radiotracer 99mTc(VII) was used to assess the impact of biostimulation of metal-reducing bacteria on the mobility of the radioactive contaminant technetium in sediments. Additions of electron donor... more
Dynamic -camera imaging of the radiotracer 99mTc(VII) was used to assess the impact of biostimulation of metal-reducing bacteria on the mobility of the radioactive contaminant technetium in sediments. Additions of electron donor (acetate) were used to construct a redox gradient, from oxic to Fe(III)-reducing conditions with increasing depth, in sediment columns. When 99mTc was pumped through the columns, real-time -camera imaging in combination with geochemical analyses showed that the 99mTc was irreversibly bound to regions of elevated Fe(II) formed via microbial reduction of Fe(III). Electron microscopy with EDX mapping confirmed that the Tc was associated with iron phases, while XAS confirmed reduction of Tc(VII) to insoluble Tc(IV). Molecular analyses of the microbial communities in the columns further emphasized a direct link between the accumulation of biogenic Fe(II) and Tc(VII) reduction and precipitation, with Fe(III)-reducing bacteria more abundant in zones of technetium immobilization. This study offers a novel non-invasive approach to monitor radionuclide mobility at trace concentrations in real-time biogeochemical experiments and confirms the effectiveness of biostimulation of Fe(III)-reducing bacteria in immobilising technetium.
Research Interests:
Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and understanding these reactions is essential for the safe management of radioactive wastes. Neptunium, an alpha-emitting transuranic element, is... more
Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and understanding these reactions is essential for the safe management of radioactive wastes. Neptunium, an alpha-emitting transuranic element, is of particular importance because of its long half-life, high radiotoxicity, and relatively high solubility as Np(V)O2+ under oxic conditions. Here, we describe experiments to explore the biogeochemistry of Np where Np(V) was added to oxic sediment microcosms with indigenous microorganisms and anaerobically incubated. Enhanced Np removal to sediments occurred during microbially mediated metal reduction, and X-ray absorption spectroscopy showed this was due to reduction to poorly soluble Np(IV) on solids. In subsequent reoxidation experiments, sediment-associated Np(IV) was somewhat resistant to oxidative remobilization. These results demonstrate the influence of microbial processes on Np solubility and highlight the critical importance of radionuclide biogeochemistry in nuclear legacy management.
Research Interests:
Here we examine the bioreduction of technetium-99 in sediment microcosm experiments with varying nitrate and carbonate concentrations added to synthetic groundwater to assess the influence ofpHand nitrate on bioreduction processes. The... more
Here we examine the bioreduction of technetium-99 in sediment microcosm experiments with varying nitrate and carbonate concentrations added to synthetic groundwater to assess the influence ofpHand nitrate on bioreduction processes. The systems studied include unamended-, carbonate buffered-, low nitrate-, and high nitrate-groundwaters. During anaerobic incubation, terminal electron accepting processes (TEAPs) in the circumneutral pH, carbonate buffered system progressed to sulfate reduction, and Tc(VII) was removed from solution during Fe(III) reduction. In the high-nitrate system, pH increased during denitrification (pH 5.5 to 7.2), then TEAPs progressed to sulfate reduction. Again, Tc(VII) removal was associated with Fe(III) reduction. In both systems, XAS confirmed reduction to hydrous Tc(IV)O2 like phases on Tc removal from solution. In the unamended and low-nitrate systems, the pH remained low, Fe(III) reduction was inhibited, and Tc(VII) remained in solution. Thus, nitrate can have complex influences on the development of the metal reducing conditions required for radionuclide treatment. High nitrate concentrations stimulated denitrification and caused pH neutralization facilitating Fe(III) reduction and Tc(VII) removal; acidic, low nitrate systemsshowed no Fe(III)-reduction. These results have implications for Tc cycling in contaminated environments where nitrate has been considered undesirable, but where it may enhance Fe(III)-reduction via a novel pH “conditioning” step.
Research Interests:
Groundwaters at nuclear sites are often characterised by low pH and high nitrate concentrations (10-100 mM). These conditions are challenging for bioremediation, often inhibiting microbial Fe(III)-reduction that can limit radionuclide... more
Groundwaters at nuclear sites are often characterised by low pH and high nitrate concentrations (10-100 mM). These conditions are challenging for bioremediation, often inhibiting microbial Fe(III)-reduction that can limit radionuclide migration. Here, sediment microcosms representative of the UK Sellafield site were used to study the influence of variable pH and nitrate concentrations on microbially-mediated TEAP (terminal electron accepting processes) progression. The rate of bioreduction at low pH (~ 5.5) was slower than that in bicarbonate-amended systems (pH ~ 7.0), but in the low pH systems, denitrification and associated pH buffering resulted in conditioning of the sediments for subsequent Fe(III) and sulfate reduction. Under very high nitrate conditions (100 mM), bicarbonate amendment (pH ~ 7.0) was necessary for TEAP progression beyond denitrification and the reduction of 100 mM nitrate created alkaline conditions (pH 9.5). 16S rRNA gene analysis showed that close relatives of known nitrate reducers Bacillus niacini and Ochrobactrum grignonense dominated the microbial communities. In the 100 mM nitrate system, close relatives of the Fe(III)-reducing species Alkaliphilus crotonatoxidans and Serratia liquifaciens were observed. These results highlight that denitrification can support bioreduction via pH conditioning for optimal metal reduction and immobilization.
Research Interests: Environmental Science, Soil Science, Climate Change, Environmental Geochemistry (Environmental Studies), Geomicrobiology, and 9 moreWater Pollution, Environmental Pollution, Salinity, Nitrate Contamination, Biological Nitrogen Fixation, Biofertilizers, Groundwater Quality, Soil Microbiology, and Mycorrhizae
The behaviour of strontium (Sr2+) during microbial reduction in nitrate impacted sediments was investigated in sediment microcosm experiments relevant to nuclear sites. Although Sr2+ is not expected to be influenced directly by redox... more
The behaviour of strontium (Sr2+) during microbial reduction in nitrate impacted sediments was investigated in sediment microcosm experiments relevant to nuclear sites. Although Sr2+ is not expected to be influenced directly by redox state, bioreduction of nitrate caused reduced Sr2+ solubility due to an increase in pH during bioreduction and denitrification. Sr2+ removal was greatest in systems with the highest initial nitrate loading and consequently more alkaline conditions at the end of denitrification. After denitrification, a limited re-release of Sr2+ back into solution occurred coincident with the onset of metal (Mn(IV) and Fe(III)) reduction which caused minor pH changes in all microcosms with the exception of the bicarbonate buffered system with initial nitrate of 100 mM and final pH > 9. In this system ~ 95 % of Sr2+ remained associated with the sediment throughout the progression of bioreduction. Analysis of this pH 9 system using X-ray absorption spectroscopy (XAS) and electron microscopy coupled to thermodynamic modelling showed that Sr2+ became partially incorporated within carbonate phases which were formed at higher pH. This is in contrast to all other systems where final pH was < 9, here XAS analysis showed that outer sphere Sr2+ sorption predominated. These results provide novel insight into the likely environmental fate of the significant radioactive contaminant, 90Sr, during changes in sediment biogeochemistry
induced by bioreduction in nitrate impacted nuclear contaminated environments.
induced by bioreduction in nitrate impacted nuclear contaminated environments.
Research Interests:
The speciation and behaviour of chromium leached from highly alkaline chromite ore processing residue (COPR) within the soils beneath the waste is investigated. Water in contact with the COPR had a pH of 12.2 and contained 990 µmols L-1... more
The speciation and behaviour of chromium leached from highly alkaline chromite ore processing residue (COPR) within the soils beneath the waste is investigated. Water in contact with the COPR had a pH of 12.2 and contained 990 µmols L-1 Cr(VI). This water has been entering the soils beneath the waste for over 100 years. The soil immediately beneath the waste has a pH value between 11 - and 12.5. It contains between 0.3 - 0.5 % w/w chromium, and 45 - 75 % of the 0.5 N HCl extractable Fe is present as Fe(II). Both soil pH and Cr concentration decrease with distance from the waste suggesting that the Cr content of the soil is due to interaction of a COPR leachate. XANES analysis of soil samples indicated that Cr is predominantly present as Cr(III) and EXAFS analysis of high concentration samples suggest Cr(III)OOH association with Fe(III)OOH in both incorporated and surface precipitate coordination environments. In microcosms containing soil from beneath the COPR and water from the waste pile Cr(VI) is removed from solution in all experiments, including sterile controls, within 40 days. Cr(VI) removal is most likely via abiotic reaction with microbially produced Fe(II) present in these soils. Indeed, a viable consortium of microbial iron reducers, dominated by members of the phylum Firmicutes, has been isolated from the same COPR affected soil. In separate oxidation experiments, where Cr-rich soil from beneath the COPR was shaken in DIW exposed to atmosphere, less than 2 % of the total Cr was remobilised despite the fact that significant reoxidation of Fe(II) was observed. XAS analysis of the reoxidised soil showed no change in Cr-speciation occurred during oxidation indicating the Cr(III)-Fe(III)-oxy hydroxide phase is a stable long term host for Cr. This work suggests that sequestration of Cr(VI) via reductive precipitation is an effective method of contaminant immobilisation in soils where microbially produced Fe(II) is present.
Research Interests:
Acetate was added to two closed soil-water systems that are representative of the subsurface environment close to chromium ore processing residue disposal sites; one had a pH of 7.7, the other 9.3. Cr(VI) reduction occurred in both... more
Acetate was added to two closed soil-water systems that are representative of the subsurface environment close to chromium ore processing residue disposal sites; one had a pH of 7.7, the other 9.3. Cr(VI) reduction occurred in both systems as part of a cascade of microbially mediated terminal electron accepting processes, occurring between nitrate and iron reduction. Cr(VI) and subsequently iron reduction took longer to start and were slower in the more alkaline system. At the point when Cr(VI) reduction was essentially complete, the microbial populations in both systems showed a significant increase in species closely related to -proteobacteria that are capable of nitrate reduction.
Research Interests:
Bitumen is used to embed low and intermediate radioactive waste because of its high impermeability and its great resistance to chemicals. This study focused on the bituminous wastes used for stabilisation of soluble and insoluble salts,... more
Bitumen is used to embed low and intermediate radioactive waste because of its high impermeability and its great resistance to chemicals. This study focused on the bituminous wastes used for stabilisation of soluble and insoluble salts, mainly of nitrate and sulphate.
This study describes the biogeochemical behaviour of the radionuclide technetium (99Tc) in background area sediments from the US Department of Energy Field Research Center (FRC) in Oak Ridge, TN, USA. Microcosm experiments with trace... more
This study describes the biogeochemical behaviour of the radionuclide technetium (99Tc) in background area sediments from the US Department of Energy Field Research Center (FRC) in Oak Ridge, TN, USA. Microcosm experiments with trace levels of 99Tc(VII) were used to examine Tc reduction and reoxidation. Efficient removal of 0.5 μM Tc(VII) from solution was seen under Fe(III)-reducing conditions, and was attributed to a lower valence insoluble form of the radionuclide. Molecular and cultivation-dependent analysis confirmed the presence of known Fe(III)-reducing bacteria (Geothrix and Geobacter species) in these sediments. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic analysis of analogous microcosm experiments, challenged with higher (550 μM) concentrations of Tc(VII), confirmed the presence of reduced insoluble Tc(IV) as hydrous TcO2 in the Fe(II)-bearing sediments. Reoxidation experiments of pre-reduced microcosms challenged with 0.5 μM99Tc showed very limited (<3 %) remobilization of the reduced 99Tc with 100 mM nitrate but significant (ca 80%) remobilization of 99Tc under air reoxidation conditions. Fe(II) oxidation was, however, significant in all oxidation treatments. EXAFS analyses of Fe(II)-bearing sediments challenged with higher (550 μM) concentrations of Tc(VII) and then reoxidized with 100mMnitrate contained both Tc(IV) and Tc(VII) immobile phases. These results suggest that under anaerobic oxidation
conditions, Tc(IV) will not remobilize rapidly, even in the presence of high concentrations of nitrate. This has implications for the biogeochemical cycling of technetium in contaminated environments, including those where bioreduction has been stimulated to minimize transport of the radionuclide.
conditions, Tc(IV) will not remobilize rapidly, even in the presence of high concentrations of nitrate. This has implications for the biogeochemical cycling of technetium in contaminated environments, including those where bioreduction has been stimulated to minimize transport of the radionuclide.
Research Interests:
Technetium is a long lived (2.13 x 105 y), beta emitting radionuclide which is a groundwater contaminant at a number of nuclear facilities throughout the world. Its environmental behaviour is primarily governed by its redox state: Under... more
Technetium is a long lived (2.13 x 105 y), beta emitting radionuclide which is a groundwater contaminant at a number of nuclear facilities throughout the world. Its environmental behaviour is primarily governed by its redox state: Under oxic conditions it forms the highly soluble pertechnetate (TcO4-) ion; under reducing conditions it forms the poorly soluble, reduced forms of Tc, particularly the Tc(IV) ion which, above its solubility limit (10-9 mol l-1 at ~ pH 7), is expected to precipitate as hydrous TcO2. Thus the redox cycling behaviour of technetium is predicted to be key to its environmental behaviour in the natural and engineered environment. Here we present the results of a series of X-ray absorption spectroscopy (XAS) experiments which examine the oxidation state and coordination environment of technetium in a range of estuarine and freshwater sediment suspensions, and in an environmentally relevant Fe(II)- mineral suspension under both reduced and reoxidised biogeochemical conditions. In both reduced sediments and Fe(II) containing minerals prior to reoxidation, XAS results show that Tc was retained as a hydrous TcO2 like phase which was remarkably common across all samples. Under air reoxidation, experiments showed significant (up to 80 %) remobilisation of Tc to solution as TcO4-, and XAS indicated that in pre-reduced freshwater sediments and Fe(II) minerals oxidised with air, Tc remained associated with solid phases as a hydrous TcO2 like phase. By contrast, in air reoxidised estuarine sediment XAS analysis suggested that both hydrous TcO2 and TcO4- were retained within the sediment phase. Finally, when microbially mediated nitrate reoxidation occurred in estuarine and freshwater sediment slurries, experiments showed low (< 8%) remobilisation of Tc from solids over similar timescales to air reoxidation experiments, whilst XAS again showed that both hydrous TcO2 and TcO4- were retained within the sediment phase. These results are discussed in the context of the redox cycling behaviour of Tc in the natural and engineered environment.
Research Interests:
Technetium is a redox active radionuclide which is present as a contaminant at a number of sites where nuclear fuel cycle operations have been carried out. Recent studies suggest that Tc(VII), which is soluble under oxic conditions, will... more
Technetium is a redox active radionuclide which is present as a contaminant at a number of sites where nuclear fuel cycle operations have been carried out. Recent studies suggest that Tc(VII), which is soluble under oxic conditions, will be retained in sediments as Fe(III)-reducing conditions develop, due to reductive scavenging as hydrous TcO2. However, the behaviour of technetium during subsequent reoxidation of sediments remains poorly characterized. Here we describe a microcosm based approach to investigate the reoxidation behaviour of reduced, technetium contaminated sediments. In reoxidation experiments, the behaviour of Tc was strongly dependent on the nature of the oxidant. With air, reoxidation of Fe(II) and, in sulfate-reducing sediments, sulfide occurred accompanied by ~50% remobilisation of Tc to solution as TcO4-. With nitrate, reoxidation of Fe(II), and in sulfate reducing sediments, sulfide only occurred in microbially active experiments where Fe(II) and sulfide oxidation coupled to nitrate reduction was occurring. Here, Tc was recalcitrant to remobilisation with < 10 % Tc remobilized to solution even when extensive Fe(II) and sulfide reoxidation had occurred. X-ray absorption spectroscopy on reoxidised sediments suggested that 15 – 50 % of Tc bound to sediments was present as Tc(VII). Overall, these results suggest that Tc reoxidation behaviour is not directly coupled to Fe or S oxidation and that the extent of Tc remobilisation is dependent on the nature of the oxidant.
Research Interests:
Technetium is a fission product that is highly mobile in its oxic form (as Tc(VII)O4-), but is scavenged to sediments in its reduced forms (predominantly as Tc(IV)). In progressive microcosms, a cascade of stable element terminal electron... more
Technetium is a fission product that is highly mobile in its oxic form (as Tc(VII)O4-), but is scavenged to sediments in its reduced forms (predominantly as Tc(IV)). In progressive microcosms, a cascade of stable element terminal electron accepting processes developed as a result of indigenous microbial activity. TcO4- removal from solution occurred during microbial Fe(III) reduction, and was essentially complete (>99%) by the onset of SO42- reduction. Microbial community analysis revealed a similar and complex microbial population at all three sample sites. At the intermediate salinity site, Paull, a broad range of NO3--, Mn(IV)-, Fe(III)- and SO42-- reducers were present in sediments including microbes with the potential to reduce Fe(III) to Fe(II). When sterilised sediments were incubated with pure cultures of NO3--, Fe(III)- and SO42--reducing bacteria, TcO4- removal occurred only during active Fe(III) reduction. X-ray absorption spectroscopy confirmed that TcO4- removal in these sediments was due to reduction to hydrous Tc(IV)O2 in both Fe(III)- and SO42--reducing sediments.
Research Interests:
Technetium-99 is a high yield product of nuclear fission and has been found as a contaminant at sites where nuclear wastes have been processed or stored. In addition, its long half-life (2.1 × 105 years) makes Tc one of the radionuclides... more
Technetium-99 is a high yield product of nuclear fission and has been found as a contaminant at sites where nuclear wastes have been processed or stored. In addition, its long half-life (2.1 × 105 years) makes Tc one of the radionuclides of interest when considering long-term disposal options for nuclear waste. Under oxic conditions, Tc occurs as the highly mobile Tc(VII)O4-, under reducing conditions, the lower valency, poorly soluble Tc(IV) tends to predominate. Thus, under anoxic conditions, Tc(IV) commonly precipitates as hydrous TcO2 or is associated with mineral surfaces. Here we use microcosm experiments containing soil samples representative of the UKAEA site at Dounreay to examine the effect of varying solution chemistry on the fate of technetium during microbial reduction. Analysis of a suite of stable element redox indicators demonstrated that indigenous microbial activity occurred in a range of microcosm experiments including un-amended Dounreay sediments, carbonate buffered sediments and microcosms amended with EDTA, a complexing ligand used in nuclear fuel cycle operations. During the development of microbial anoxia, TcO4- was removed from solution in experiments. In all systems, TcO4- removal from solution was greatest during active microbial Fe(III) reduction. The addition of TcO4- to sediments which had been microbially reduced and then sterilized also resulted in the removal of Tc from solution, indicating that removal was likely to be caused by abitoic reaction with Fe(II) in the reduced sediments. The potential stability of reduced Tc(IV) in the soil was examined as reduced Tc-labelled sediments were contacted with de-oxygenated EDTA. No evidence of remobilisation of Tc(IV) was observed. The results indicate that technetium bound to Dounreay soils is resistant to remobilisation by strong chelating agents such as EDTA.
Research Interests:
The release of radionuclides from nuclear sites and their subsequent mobility in the environment is a subject of intense public concern. Natural sources of radioactivity include U (present in Earth's crust at concentration of 1.8 ppm),... more
The release of radionuclides from nuclear sites and their subsequent mobility in the environment is a subject of intense public concern. Natural sources of radioactivity include U (present in Earth's crust at concentration of 1.8 ppm), Th, Ra isotopes, and radon, while significant quantities of natural and artificial/manmade radionuclides were also released as a consequence of nuclear weapons testing in the 1950s and 1960s, and via accidental release eg from Chernobyl in 1986.
ABSTRACT This project concerns the ongoing remediation of UK nuclear legacy sites with particular focus on in situ remediation strategies. Microcosm based studies have been used to investigate the effectiveness of bioremediation... more
ABSTRACT This project concerns the ongoing remediation of UK nuclear legacy sites with particular focus on in situ remediation strategies. Microcosm based studies have been used to investigate the effectiveness of bioremediation approaches such as bio-stimulation whereby an electron donor is added to the subsurface to stimulate bioreduction. Here microbial action changes the geochemistry of sediments leading to a favorable change in radionuclide speciation.
Technetium is a radioactive contaminant found in groundwaters at sites where nuclear wastes have been processed or stored. The redox chemistry of technetium is a major control on its environmental mobility. Under oxic conditions,... more
Technetium is a radioactive contaminant found in groundwaters at sites where nuclear wastes have been processed or stored. The redox chemistry of technetium is a major control on its environmental mobility. Under oxic conditions, technetium exists as the pertechnetate ion, Tc(VII)O4-, which is poorly sorbed by minerals across a wide range of environmentally relevant pH values. Under reducing conditions pertechnetate is converted to lower valency species, of which Tc(IV) tends to predominate. Typically, the Tc(IV) oxidation state readily precipitates as insoluble hydrous Tc(IV) oxides or, at low concentrations, is removed from solution by association with mineral surfaces. Here, we present novel X-ray absorption spectroscopy (XAS) data examining Tc associations with reduced Dounreay soils. In reduced unamended microcosms and in microcosms where we added the co-contaminants ethylenediaminetetraacetic acid (EDTA) or bicarbonate to investigate their effect on Tc biogeochemistry, Tc was removed from solution on exposure to the reduced sediments and was present on solids as hydrous Tc(IV)O2-like phases. Furthermore, to investigate the long term stability and remobilization of solid phase associated Tc in reduced soils, we reoxidised reduced, Tc(IV)-labeled soils, in the presence of air and nitrate. The extent of remobilisation of Tc to solution was dependent on the oxidant used. After reoxidation with air for 60 days, 42 ± 6% of the initial soil bound Tc was resolubilised. In the presence of 25 or 100 mmol l-1 nitrate as an oxidant, negligible microcosm reoxidation or remobilization of Tc to solution occurred. XAS analysis of soils treated with the two oxidants showed that in both systems, the remaining soil associated Tc was present as hydrous TcO2-like phases. The recalcitrance of Tc remobilisation under reoxidising conditions has implications for the fate of Tc in contaminated environments.
Research Interests:
Technetium -99 is an important fission product in radioactive wastes. As Tc(VII)O4-, Tc is highly mobile in oxic environments but, under reducing conditions, Tc becomes strongly associated with sediments as hydrous Tc(IV)O2 like phases.... more
Technetium -99 is an important fission product in radioactive wastes. As Tc(VII)O4-, Tc is highly mobile in oxic environments but, under reducing conditions, Tc becomes strongly associated with sediments as hydrous Tc(IV)O2 like phases. In order to further examine the behaviour of Tc over a range of concentrations in estuarine sediments, anoxic incubation experiments were combined with a range of direct (transmission electron microscopy and gamma camera imaging) and indirect (incubation experiments and chemical extractions) experimental techniques. When TcO4- was incubated in sediment microcosms at micro-molar (10-6 mol L-1) concentrations, > 99 % TcO4- was removed from solution over the course of 36 days in systems undergoing active microbial Fe(III)-reduction. By contrast, when spiked into pre-reduced estuarine sediments that were predominantly Fe(III)-reducing (incubated for 60 days) or sulfate-reducing (incubated for 270 days), > 99 % TcO4- was removed from solution in under 10 minutes in both microbially active and heat sterilised systems. Chemical extraction techniques showed that 70 ± 3 % of Tc bound to sediments was remobilised when sediments were exposed to the first strong oxidant (H2O2) in the extraction scheme. At higher Tc concentrations (~ 0.05 mol kg-1 of sediment) scanning transmission electron microscopy, combined with energy dispersive X-ray mapping, was used to examine the associations of Tc in sediments. At these concentrations, Tc was localised and co-associated with nano-metre size Fe(II)-rich particles, consistent with the hypothesis that removal of Tc may be controlled by reduction of Tc(VII) to Tc(IV) by biogenic Fe(II) in sediments. In addition, gamma camera imaging with the gamma emitting 99mTcO4- (half-life 6 hours) at pico-molar (10-12 mol L-1) concentrations, was used to visualise the interaction of Tc in sediments at very low concentrations. Here, over the course of 24 hours the scavenging of Tc to sulfate-reducing sediments was observed. As the Tc concentrations used in the 99mTc experiments were below the solubility limits for hydrous Tc(IV)O2 (ca 10-9 mol L-1 at pH 7 - 9), sorption of Tc(IV) species is likely to be a significant control on Tc behaviour in these sediments even at very low concentrations. Overall, the results of this study show that multiple approaches are essential to understanding Tc speciation in complex heterogeneous sediments over the wide range of concentrations relevant to contaminated natural and engineered environments.
Research Interests: Biogeochemistry, Climate Change, MARINE POLLUTION, Groundwater Pollution, Water Pollution, and 8 moreEnvironmental Pollution, Environmental (soil and water) pollution prevention and monitoring., Salinity, Biological Nitrogen Fixation, Biofertilizers, Groundwater Quality, Soil Microbiology, and Mycorrhizae
The deep basins of the Baltic Sea are commonly anoxic, and finely laminated diatomaceous sediments have been deposited at intervals throughout the last 8000 yr. The origin and composition of individual laminae in Gotland Deep sediments... more
The deep basins of the Baltic Sea are commonly anoxic, and finely laminated diatomaceous sediments have been deposited at intervals throughout the last 8000 yr. The origin and composition of individual laminae in Gotland Deep sediments have often proved difficult to characterise using traditional micropalaeontological and sedimentological studies. Here, we present a scanning electron microscope study in which lamina down to 0.03 mm in thickness with distinct mineralogical or micropalaeontological composition have been identified and described. Depositional laminae sequences in the form of couplets, triplets and quadruplets of diatomaceous and lithogenic laminae are observed with an average thickness of approximately 0.7 mm. Diagenetic Ca-rhodochrosite laminae also occur within these depositional sequences. Examination of the diatom assemblages suggests that these bundles of laminae represent annual deposits, or varves. Varves are relatively uncommon, and typically occur in small intervals of two to five varves, which are interrupted by more diffusely laminated and homogenous sediments. The origin of these more massive sediments probably relates to periodic re-oxygenation of the basin on inter-annual time scales and destruction of varves by bioturbation.
Research Interests:
The manganese carbonate deposits of the anoxic Littorina sediments of the Gotland Deep have been commonly related to the periodic renewal of deep water by inflowing saline water from the North Sea. The use of scanning electron microscopy–... more
The manganese carbonate deposits of the anoxic Littorina sediments of the Gotland Deep have been commonly related to the periodic renewal of deep water by inflowing saline water from the North Sea. The use of scanning electron microscopy– based techniques allows identification of small-scale sedimentary and geochemical features associated with Mn-carbonate laminae, which has significant implications for models of Mn-carbonate formation. Varves occurring in the Littorina sequence contain up to four laminae that may be placed in a seasonal cycle, and kutnahorite laminae occur within varves only as a winter– early spring deposit. This kutnahorite laminae seasonality is in agreement with the seasonal distribution of major Baltic inflow events recorded in historical records, and a direct causal link between inflows and kutnahorite deposition is implied. Benthic foraminifera tests are found to be heavily encrusted in kutnahorite, implying that benthic recolonization during oxidation events occurs concurrently with kutnahorite formation. The relatively common occurrence of small (50 to 100 m) hexagonal -Mn-sulfide pseudomorphs, associated with 13% of kutnahorite laminae studied, is reported in Gotland Deep sediments for the first time. Although Mn-sulfide crystals are not usually preserved in the sediment, the discovery of Mn-sulfide pseudomorphs
suggests that initial formation of Mn-sulfide in the Gotland Deep may occur much more commonly during the process of kutnahorite formation than previous reports of Mn-sulfide occurrence have implied.
suggests that initial formation of Mn-sulfide in the Gotland Deep may occur much more commonly during the process of kutnahorite formation than previous reports of Mn-sulfide occurrence have implied.
Research Interests:
Abstract Iron minerals influence the environmental redox behaviour and mobility of metals including the long-lived radionuclide technetium. Technetium is highly mobile in its oxidized form pertechnetate (Tc (VII) Formula), however, when... more
Abstract Iron minerals influence the environmental redox behaviour and mobility of metals including the long-lived radionuclide technetium. Technetium is highly mobile in its oxidized form pertechnetate (Tc (VII) Formula), however, when it is reduced to Tc (IV) it immobilizes readily via precipitation or sorption.
Iron minerals are known to influence the redox behaviour and mobility of metals in the environment, including the long-lived radioactive element technetium. In this study, we have investigated several environmentally relevant, abiotically... more
Iron minerals are known to influence the redox behaviour and mobility of metals in the environment, including the long-lived radioactive element technetium. In this study, we have investigated several environmentally relevant, abiotically and biogenically precipitated Fe(II)-bearing minerals with the potential to control the environmental biogeochemistry of technetium and using geochemical and X-ray absorption spectroscopy (XAS) approaches. Technetium is highly mobile in its oxidized form pertechnetate (Tc(VII)O4-), however, when it is reduced to Tc(IV) it immobilises readily via precipitation or sorption. In low concentration tracer experiments, and in higher concentration XAS experiments, pertechnetate was added to samples of both biogenic and abiotically synthesized Fe(II)-bearing minerals (bio-magnetite, bio-vivianite, bio-siderite and an abiotically precipitated Fe(II) gel). In tracerexperiments, each mineral scavenged different quantities of Tc(VII) from solution with essentially complete removal in Fe(II)-gel and bio-magnetite systems and with 84 ± 4 % removal onto bio-siderite and 68 ± 5 % removal onto bio-vivianite. In higher concentration XAS experiments, the mechanism of removal was confirmed as reductive precipitation with the Fe(II)-bearing minerals and Tc was speciated as hydrous Tc(IV)O2-like phases. Low concentration reoxidation experiments with air-, and in the case of the Fe(II) gel, nitrate-oxidation of the Tc(IV)-labelled samples resulted in only partial resolubilisation of Tc. Upon exposure to air, Tc bound to the Fe-minerals was resistant to oxidative remobilisation with a maximum of ~25 % Tc remobilised in the bio-vivianite system over 2 - 3 months of vigorous air reoxidation. Nitrate mediated oxidation of Fe(II)-gel inoculated with a stable consortium of nitrate-reducing, Fe(II)-oxidizing bacteria showed only 3.8 ± 0.4 % remobilisation of reduced Tc(IV), again highlighting the recalcitrance of Tc(IV) to remobilisation in partially reoxidised, Fe-bearing systems. Selected reoxidation experiments with the higher concentrations of Tc required for XAS-analysis of Tc speciation identified only hydrous TcO2 like phases in reoxidised samples suggesting Tc(IV) was associated with Fe(III)-bearing reoxidation products. This study highlights the role that Fe(II)-bearing biogenic mineral phases have in controlling reductive scavenging of Tc(VII) to hydrous TcO2 like phases onto a range of Fe(II)-bearing minerals. In addition, it suggests that on reoxidation of these phases, Fe-bound Tc(IV) is likely to be recalcitrant to reoxidation potentially over long timescales. This has implications when considering bioremediation approaches and in predictions of the long term fate of Tc.
Research Interests:
This paper reports an investigation of microbially mediated Cr(VI) reduction in a hyperalkaline, chromium-contaminated soilwater system representative of the conditions at a chromite ore processing residue (COPR) site. Soil from the... more
This paper reports an investigation of microbially mediated Cr(VI) reduction in a hyperalkaline, chromium-contaminated soilwater system representative of the conditions at a chromite ore processing residue (COPR) site. Soil from the former surface layer that has been buried beneath a COPR tip for over 100 years was shown to have an active microbial population despite a pH value of
10.5. This microbial population was able to reduce nitrate using an electron donor(s) that was probably derived from the soil organic matter.With the addition of acetate, nitrate reduction was followed in turn by removal of aqueous Cr(VI) from solution, and then iron reduction. Removal of ∼300 μM aqueous Cr(VI) from solution was microbially mediated, probably by reductive precipitation, and occured over a few months. Thus, in soil that has had time to acclimatize to the prevailing pH value and Cr(VI) concentration, microbially mediated Cr(VI) reduction can be stimulated at a pH of 10.5 on a time scale compatible with engineering intervention at COPR-contaminated sites.
10.5. This microbial population was able to reduce nitrate using an electron donor(s) that was probably derived from the soil organic matter.With the addition of acetate, nitrate reduction was followed in turn by removal of aqueous Cr(VI) from solution, and then iron reduction. Removal of ∼300 μM aqueous Cr(VI) from solution was microbially mediated, probably by reductive precipitation, and occured over a few months. Thus, in soil that has had time to acclimatize to the prevailing pH value and Cr(VI) concentration, microbially mediated Cr(VI) reduction can be stimulated at a pH of 10.5 on a time scale compatible with engineering intervention at COPR-contaminated sites.
Research Interests:
Anoxic Tc(IV)-containing sediments representative of the UK Sellafield reprocessing facility were exposed to either air or NO3− to investigate redox cycling of technetium and iron. With air, oxidation of Fe(II) in the reduced sediments... more
Anoxic Tc(IV)-containing sediments representative of the UK Sellafield reprocessing facility were exposed to either air or NO3− to investigate redox cycling of technetium and iron. With air, oxidation of Fe(II) in the reduced sediments was accompanied by ∼75% mobilization of Tc to solution, as soluble Tc(VII). Nitrate additions resulted in the bio-oxidation of Fe(II), coupled to microbially mediated NO−3 reduction but was accompanied by only very limited (<5%) mobilization of the reduced, sediment-bound Tc, which remained as Tc(IV). PCR-based 16S rRNA and narG gene analyzes were used to investigate changes in the microbial community during sediment oxidation by air and nitrate. Contrasting microbial communities developed in the different treatments and were dominated by Betaproteobacteria (including Herbaspirillum and Janthinobacterium spp.) in the presence of high NO−
3 concentrations. This suggests that the Betaproteobacteria are involved in the redox cycling of Fe and N in these systems, but are unable to mediate NO3
−-dependent Tc(IV) oxidation. These microorganisms may play a previously unrecognized yet pivotal role in nfluencingcontaminant fate and transport in these environments which can have implications to the long-term stewardship of radionuclidecontaminated sediments.
3 concentrations. This suggests that the Betaproteobacteria are involved in the redox cycling of Fe and N in these systems, but are unable to mediate NO3
−-dependent Tc(IV) oxidation. These microorganisms may play a previously unrecognized yet pivotal role in nfluencingcontaminant fate and transport in these environments which can have implications to the long-term stewardship of radionuclidecontaminated sediments.
Research Interests:
Under anaerobic conditions, uranium solubility is significantly controlled by the microbially-mediated reduction of relatively soluble U(VI) to poorly soluble U(IV). However, the reaction mechanism(s) for bioreduction are complex with... more
Under anaerobic conditions, uranium solubility is significantly controlled by the microbially-mediated reduction of relatively soluble U(VI) to poorly soluble U(IV). However, the reaction mechanism(s) for bioreduction are complex with prior sorption of U(VI) to sediments significant in many systems, and both enzymatic and abiotic U(VI) reduction pathways potentially possible. Here, we describe results from sediment microcosm and Fe(II)-bearing biomineral experiments designed to assess the relative importance of enzymatic vs. abiotic U(VI) reduction mechanisms and the long-term fate of U(IV). In oxic sediments, U(VI) was rapidly and significantly sorbed to surfaces and during microbially-mediated bioreduction, XAS analysis showed that sorbed U(VI) was reduced to U(IV) commensurate with Fe(III)-reduction. Additional control experiments with Fe(III)-reducing sediments that were sterilised after bioreduction and then exposed to U(VI), indicated that U(VI) reduction was inhibited, implying that enzymatic as opposed to abiotic mechanisms dominated in these systems. Further experiments with Fe(II)-bearing biomineral phases (magnetite and vivianite) showed that significant U(VI) reduction occurred in co-precipitation systems, where U(VI) was spiked into the biomineral precursor phases prior to inoculation with Geobacter sulfurreducens. In contrast, when U(VI) was exposed to pre-formed, washed biominerals, XAS analysis indicated that U(VI) was recalcitrant to reduction. Reoxidation experiments examined the long-term fate of U(IV). In sediments, air exposure resulted in Fe(II) oxidation and significant U(IV) oxidative remobilisation. By contrast, only partial reoxidation of U(IV) and no remobilisation to solution occurred with nitrate mediated bio-oxidation of sediments. Magnetite was resistant to biooxidation with nitrate. On exposure to air, magnetite changed from black to brown in colour, yet there was no mobilisation of uranium to solution and XAS confirmed that U(IV) remained dominant in the oxidised mineral phase. Overall these results highlight the complexity of uranium biogeochemistry and highlight the importance of mechanistic insights into these reactions if optimal management of the global nuclear legacy is to occur.
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It is well known that microbially mediated reduction can result in the removal of U(VI)(aq) from solution by forming poorly soluble U(IV) oxides; however, the fate of U(VI) already associated with mineral surfaces is less clear. Here we... more
It is well known that microbially mediated reduction can result in the removal of U(VI)(aq) from solution by forming poorly soluble U(IV) oxides; however, the fate of U(VI) already associated with mineral surfaces is less clear. Here we describe results from both oxic adsorption and anaerobic microcosm experiments to examine the fate of sorbed U(VI) during microbially mediated bioreduction. The microcosm experiments contained sediment representative of the nuclear facility at Dounreay, UK. In oxic adsorption experiments, uptake of U(VI) was rapid and complete from artificial groundwater and where groundwater was amended with 0.2 mmol l-1 ethylenediaminetetraacetic acid (EDTA) a complexing ligand used in nuclear fuel cycle operations. By contrast, uptake of U(VI) was incomplete in groundwaters amended with 10 mmol l-1 bicarbonate. Analysis of sediments using X-ray adsorption spectroscopy showed that in these oxic samples, U was present as U(VI). After anaerobic incubation of U(VI) labelled sediments for 120 days, microbially mediated Fe(III)- and SO42-- reducing conditions had developed and XAS data showed uranium was reduced to U(IV). Further investigation of the unamended groundwater systems where oxic systems were dominated by U(VI) sorption showed that reduction of sorbed U(VI) required an active microbial population and occurred after robust iron- and sulfate- reducing conditions had developed. Microbial community analysis of the bioreduced sediment showed a community shift compared to the oxic sediment with close relatives of Geobacter and Clostridium species, which are known to facilitate U(VI) reduction, dominating. Overall, efficient U(VI) removal from solution by adsorption under oxic conditions dominated in unamended and EDTA amended systems. In all systems bioreduction resulted in the formation of U(IV) in solids.
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The formation of Mn-enrichments in Gotland Deep sediments has been linked to the occurrence of periodic inflows of saline water from the North Sea. In turn these saline inflow events are very strongly linked to variation in North... more
The formation of Mn-enrichments in Gotland Deep sediments has been linked to the occurrence of periodic inflows of saline water from the North Sea. In turn these saline inflow events are very strongly linked to variation in North Atlantic atmospheric conditions. Here we measure sedimentary Mn-concentrations in mid Holocene sediments with a 0.5mm sampling resolution using scanning electron microscope techniques. As the sedimentation rate in core 20001-5 was estimated to be approximately 0.5-1mm per year, examining the variation in Mn-enrichments may potentially provide an annual record of variation in saline inflow, and by extension, North Atlantic climate on interannual timescales. Processes that can affect Mn-cycling in the Gotland Deep were considered together producing a complex web of factors. These could potentially act to remove or significantly weaken the transmission of the primary saline inflow signal to the measured geochemical record, producing an effectively random Mn-record. Analysing the Mn-record as a time series of discrete events revealed that the Mn-record was not consistent with a random distribution of events, and contains some long term order. Spectral analysis of the Mn-record then indicates a significant periodicity in the Mn-record between 33-35.5mm. This represents a discrete decadal periodicity in Mn-enrichment at between 25-55 years that is consistent with the timing of previously reported Mn-enrichments in Gotland Deep sediments.
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CHAPTER ELEVEN The microbial ecology of land and water contaminated with radioactive waste: towards the development of bioremediation options for the nuclear industry ANDREA GEISSLER, SONJA SELENSKA-POBELL, KATHERINE MORRIS, IAN T. BURKE,... more
CHAPTER ELEVEN The microbial ecology of land and water contaminated with radioactive waste: towards the development of bioremediation options for the nuclear industry ANDREA GEISSLER, SONJA SELENSKA-POBELL, KATHERINE MORRIS, IAN T. BURKE, FRANCIS R. LIVENS AND JONATHAN R.
Chromium ore processing residue (COPR) has been deposited at a site in the North of England, probably at the end of the nineteenth century. The site covers an area of approximately 2.2 ha, and is situated between a canal and a river that... more
Chromium ore processing residue (COPR) has been deposited at a site in the North of England, probably at the end of the nineteenth century. The site covers an area of approximately 2.2 ha, and is situated between a canal and a river that are about 150m apart. It is in a glacial valley underlain by millstone grit and in-filled with alluvial deposits (silt, clay and sand). The original surface deposit is a thin layer of sandy clay that was probably deposited during over-bank flow of the river.
Abstract The formation of Mn-enrichments in Gotland Deep sediments has been linked to the occurrence of periodic inflows of saline water from the North Sea. In turn these saline inflow events are very strongly linked to variation in North... more
Abstract The formation of Mn-enrichments in Gotland Deep sediments has been linked to the occurrence of periodic inflows of saline water from the North Sea. In turn these saline inflow events are very strongly linked to variation in North Atlantic atmospheric conditions. Here sedimentary Mn-concentrations in mid-Holocene sediments are measured with a 0.5 mm sampling resolution using scanning electron microscope techniques.
This paper reports an investigation into the possible fate of Cr(VI) that is migrating downwards from a chromium ore processing residue (COPR) tip into the underlying soils. Thiswastewas deposited at a site in the north of England more... more
This paper reports an investigation into the possible fate of Cr(VI) that is migrating downwards from a chromium ore processing residue (COPR) tip into the underlying soils. Thiswastewas deposited at a site in the north of England more than 100 years ago and is currently a cause for environmental concern because groundwater emerging from thewaste is alkaline, visibly yellowand has an elevated Cr(VI) concentration. Sandy clay from immediately beneath the waste (assumed to be the topsoil layer prior to waste tipping) contains between about 600 and 3000mgkg−1 of Cr, and around 60% of 0.5N HCl extractable iron was present as reduced Fe(II). DNA fragments fromsoil bacteria were extracted from this soil, and microcosm experiments where the pH was adjusted to more neutral values showed that it contains a viable bacterial population capable of iron-reduction. This sandy clay layer, despite a pH value of 10.5, appears to be acting as a natural reactive zone beneath the waste as it is accumulating chromium. It is thought that the mechanism of Cr(VI) reduction is most likely to be an abiotic reaction with the Fe(II) present in the soil, and that Fe(II) in the soil is being replenished by microbial iron-reduction (although the rate of replenishment is unknown).
