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Alkali-activated materials for radionuclide immobilisation and the effect
of precursor composition on Cs/Sr retention
Peer-reviewed author version
VANDEVENNE, Niels; Ion Iacobescu, Remus; CARLEER, Robert; SAMYN, Pieter;
D'HAEN, Jan; Pontikes, Yiannis; SCHREURS, Sonja & SCHROEYERS, Wouter
(2018) Alkali-activated materials for radionuclide immobilisation and the effect of
precursor composition on Cs/Sr retention. In: JOURNAL OF NUCLEAR
MATERIALS, 510, p. 575-584.
DOI: 10.1016/j.jnucmat.2018.08.045
Handle: http://hdl.handle.net/1942/26732
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Alkali-activated materials for radionuclide immobilisation and the
2
effect of precursor composition on Cs/Sr retention
3
Niels Vandevenne a, Remus Ion Iacobescu b, Robert Carleer c, Pieter Samyn c, Jan D’Haend,e, Yiannis Pontikes b,
4
Sonja Schreurs a, Wouter Schroeyers a,*
5
a
6
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Hasselt University, CMK, Nuclear Technological Centre, Agoralaan, Gebouw H, 3590 Diepenbeek, Belgium
b
c
KU Leuven, Department of Materials Engineering, Kasteelpark Arenberg 44, 3001 Leuven, Belgium
Hasselt University, CMK, Research Group of Applied and Analytical Chemistry, Agoralaan, Gebouw D, 3590
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9
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Diepenbeek, Belgium
d
Hasselt University, Institute for Materials Research (IMO), Wetenschapspark 1, 3590 Diepenbeek, Belgium
e
IMEC, Division IMOMEC, Wetenschapspark 1, 3590 Diepenbeek, Belgium
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12
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* Corresponding author: Prof. Dr. Wouter Schroeyers, e-mail address:
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wouter.schroeyers@uhasselt.be
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1
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Abstract
19
One of the major challenges for the nuclear industry is the safe and sustainable immobilisation of
20
radioactive wastes (RAW). Currently, the most commonly used immobilisation matrices for low and
21
intermediate level wastes are based on ordinary Portland cement. For the more difficult to
22
immobilise nuclides, such as caesium (Cs+) and strontium (Sr2+), researchers have been studying
23
alternative immobilisation matrices, of which alkali-activated materials (AAM) are a very promising
24
option. However, the differences in precursor compositions and the use of different types of
25
activating solutions make it difficult to fully understand the effects of precursor composition on the
26
immobilisation of introduced nuclides. Therefore, six different compositions of laboratory-
27
synthesized Ca-Si-Al slags were developed to serve as precursors for low-alkaline AAMs to study their
28
immobilisation behaviour. Immobilisation capacities up to 97.6 % Cs+ and 99.9 % Sr2+ were achieved
29
with 1 wt% waste loading when leaching for 7 days at 20 °C in Milli Q water. Cs+ immobilisation is
30
higher at lower Si/Al and Ca/(Si+Al) ratios. Immobilisation of Sr2+ is higher at a lower Ca/(Si+Al) ratio
31
and independent of Si/Al ratio. The results of this study offer a deeper understanding of the
32
immobilisation behaviour of AAMs and can encourage further research and application of AAMs for
33
RAW immobilisation.
34
Keywords
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Alkali-activated material; immobilisation; radioactive waste; caesium; strontium
36
1
37
In the pursuit of a sustainable society, one of the main challenges for researchers and industries is
38
the safe management and disposal of industrial wastes, with radioactive waste (RAW) being
39
particularly important. RAW is produced by many different sources; the main ones being the energy
40
sector (nuclear fuel cycle), the dismantling of nuclear installations, and applications in medicine,
Introduction
2
41
agriculture and industry. Because of the ever-increasing amounts of RAW, continuous innovation in
42
immobilisation is becoming more and more important. As an alternative to the currently widely used
43
cementitious immobilisation matrices, alkali-activated materials (AAM) have been increasingly
44
studied. Depending on the type of precursor and the composition of the hydration products, AAMs
45
(or subclasses thereof) are also known as geopolymers, inorganic polymers, soil cements,
46
geocements, alkaline cements, zeoceramics, alkali-activated slag cement and a variety of other
47
names [1,2]. AAMs have demonstrated promising results in immobilising radionuclides such as
48
caesium and strontium [3-22]. As an example, the superior Cs+ retention of AAMs was reported by
49
Shi and Fernández-Jiménez (2006) [9], who tested leaching of Cs+ and Sr2+ from AAMs containing
50
zeolites and/or metakaolin (MK) as additives. They concluded that wastes show much less
51
interference with the hydration of AAMs than that of ordinary Portland cement. Blackford et al.
52
(2007) [11] developed a geopolymer matrix derived from MK, in which Cs+ was introduced. They
53
concluded that Cs+ was fully incorporated into the amorphous geopolymer phase, proving the
54
potential of AAMs for RAW immobilisation. In addition to the immobilisation of Cs+ and Sr2+, other
55
elements such as Cd2+ and Pb2+ [23], and radionuclides such as 152Eu, 60Co, 59Fe and isotopes of Am
56
and Pu have also been successfully immobilised in AAMs [24–26].
57
Concerning the use of AAMs for RAW immobilisation, most literature covers AAMs based on
58
commercial recipes, MK, fly ash (FA), ground granulated blast furnace slag (GGBFS) or combinations
59
thereof. Despite the large body of research and the promising immobilisation results, a systematic
60
study of the influence of the precursor composition on the immobilisation capacities for Cs+ and Sr2+
61
is lacking. According to Aly et al. (2008) [12], MK-based AAMs show optimal leach resistance at Si/Al
62
ratios higher than 2. They reported a sharp decrease in the Cs+ release when the Si/Al ratio increased
63
from 1.5 to 2 followed by a gradual increase at Si/Al ratios higher than 3, reaching the lowest value at
64
Si/Al ratio of 2. For Sr2+, the lowest release was obtained at a Si/Al ratio of 1.5, increasing gradually
65
with increasing Si/Al ratio [12].
3
66
Almost all concerning literature describes AAMs made from industrial residues, making it difficult to
67
exclude effects of trace elements in the precursor on the immobilisation capacities. Also, the variety
68
in precursor origin and composition in most studies make it difficult to generalize the drawn
69
conclusions, since the immobilisation performance is very dependent on the design parameters. In
70
addition, there exists a wide variety of alkali-activators used, most often consisting of highly-alkaline
71
activating solutions and often containing sodium silicates.
72
According to the IAEA [27], the lack of standards for precursors, experience of process optimization,
73
and demonstration of long-term stability limit their use in RAW immobilisation, despite the reported
74
favourable experiences using AAMs. They stated that novel materials need a better benchmarking,
75
and emphasised that it is also important to realise that existing test methods do not always give
76
comparable results with different classes of materials [27]. Therefore, the effect of precursor
77
composition on the immobilisation of Cs+ and Sr2+ in AAMs is studied in this work, by developing
78
synthetic Ca-Si-Al slag precursors with different compositions from laboratory reagents, yielding Si/Al
79
and Ca/(Si+Al) molar ratios of 0.95 – 5.1 and 0.42 – 1.0 respectively. In this way, immobilisation is
80
studied excluding possible effects of trace elements in the mixture. The present results can be
81
further used as a guideline for choosing industrial residues with a proper composition or for using
82
proper mixing ratios.
83
2
84
2.1
85
Ca-Si-Al slags were synthesised from analytical grade laboratory reagents Al2O3, SiO2 and CaCO3 (all
86
Sigma-Aldrich, > 99 % pure). The studied compositions (Table 1) were chosen to broadly resemble
87
GGBFS and to be fully liquid at 1550 °C (see Figure 1). The CaCO3 was first calcined in a muffle
88
furnace at 1050 °C overnight to expel CO2. For each composition, the exact mass of CaCO3 necessary
89
for achieving the stoichiometric amount of CaO was weighed and inserted into the muffle furnace.
Materials and methods
Ca-Si-Al slag synthesis
4
90
Immediately after cooling, the mass of the obtained CaO was determined to verify the complete
91
decarbonation. The resulting CaO was then mixed with weighed amounts of Al2O3 and SiO2 for two
92
hours in a Turbula T2C mixer for homogenization. The mixtures were placed in a platinum crucible
93
and inserted into a bottom loading furnace (AGNI ELT 160-02) at 1630 °C. A higher temperature was
94
applied to account for possible differences in temperature between the location of the crucible and
95
the thermocouple of the furnace, to ensure that the sample would melt completely. After an
96
isothermal period of 2.5 h, the melt was quenched by pouring in water at room temperature. The
97
cooled slag was then dried at 110 °C to constant weight before being crushed and milled in a Retsch
98
disk mill RS200 at 1000 rpm for 60 s.
99
Homogeneity of the finely milled slag was confirmed by X-ray diffraction spectroscopy (XRD). The
100
measurements were performed with a Bruker D8 diffractometer. This theta-theta diffractometer is
101
equipped with a Göbel mirror (line focus, Cu kɑ radiation). The X-rays are detected with a 1D lynxeye
102
detector.
103
The specific surface area of the resulting powders was measured in threefold by use of Blaine
104
method. The procedures described in the standard EN 196-6 [28] were followed as closely as possible
105
and the measurements were performed against a reference cement sample. It is, however,
106
important to note that the Blaine method is designed for cements and that results may deviate when
107
using other types of materials.
108
Table 1: Designed compositions of synthetic slags (wt fraction) and the initial Si/Al and Ca/(Si+Al)
109
molar ratios. The labelling of the samples is based on the initial Si/Al ratio.
SiO2
Al2O3
CaO
Si/Al
Ca/(Si+Al)
wt fraction
wt fraction
wt fraction
mol/mol
mol/mol
S_1
0.37
0.33
0.30
0.95
0.42
S_1.1
0.40
0.30
0.30
1.1
0.43
S_2
0.49
0.21
0.30
2.0
0.44
S_2.4
0.47
0.17
0.36
2.4
0.59
5
S_3.4
0.40
0.10
0.50
3.4
1.0
S_3.4b*
0.40
0.10
0.50
3.4
1.0
S_5.1
0.60
0.10
0.30
5.1
0.45
*This composition, randomly chosen, has been made as a replicate to check the reproducibility of the experimental design
110
111
112
Figure 1: Phase diagram at 1550 °C. The designed compositions are all within the liquid (white) area.
113
2.2
114
AAM pastes were prepared by mixing the powdered slag precursors with a 2 M NaOH solution
115
(prepared from NaOH pellets (Fischer Scientific, 98.44 % pure) and type II distilled water) using a
116
laboratory mixer and maintaining a liquid to solid ratio (L/S) of 0.30. These parameters were chosen
117
based on our earlier study [29] and adjusted for optimal workability and setting time. At this molarity
118
and L/S ratio, the amount of Na+ added is 1 wt% for each sample. Cs+ and Sr2+ were added as nitrates
119
(CsNO3, Alfa Aesar 99.8%; Sr(NO3)2, Emsure 99.0%) to account for 1 wt% and 0.1 wt% respectively of
120
the final AAM-mass (solid precursor + activating solution). A lower wt% of Sr2+ was chosen to avoid
121
significant interference with the polymerisation kinetics, as reported in our earlier studies [29,30].
122
The resulting mixtures were poured into plastic (polymethylmethacrylate, PMMA) 25 x 25 x 20 mm³
123
moulds. The moulds were manually tapped during 60 seconds to remove air bubbles before being
AAM elaboration
6
124
sealed with a PMMA cap. These pastes were then allowed to cure for 28 days at 23 ± 1 °C. Each
125
composition was made in threefold for the leaching experiments.
126
2.3
127
The release of introduced caesium and strontium, and of the structural elements silicon, aluminium,
128
calcium and sodium was measured using a semi-dynamic diffusion test described in our earlier study
129
[29] and based on the standards ASTM C1220-98 [31] and CEN/TS 15863:2015 [32]. The samples
130
were demoulded, cleaned with a dry brush and measured for dimensions and weight before being
131
submerged in 400 ml of pre-heated Milli-Q water (90 °C, polypropylene bottle and sample holder).
132
Before and after the eluate was refreshed, the mass of the closed container was measured to
133
determine water loss through evaporation. In all cases, the loss of mass through evaporation was
134
lower than 2 %, which is in line with the standard ASTM C1220-98 [31]. At each sampling time (1 h,
135
24 h, 7 d, 28 d), 10.0 ml of the eluate was filtered over a 0.2 µm syringe filter and acidified
136
immediately after sampling to a concentration of 2 % HNO3 (MERCK Suprapur 65 %). The
137
concentrations of water-soluble Sr2+, Si4+, Al3+, Ca2+, and Na+ were measured by ICP-OES (Perkin Elmer
138
type Optima 8300) in axial mode. The concentration of water-soluble Cs+ was measured by ICP-MS
139
(Perkin Elmer NexION 350S). At each sampling time, the eluate was also measured for pH (calibrated
140
electrode HI1043B, Hanna Instruments) and conductivity (Schott Geräte CG 858, calibrated with 0.1
141
M KCl).
142
The release 𝑟𝑟 (mg/m²) of element 𝑖𝑖 at leaching interval 𝑛𝑛 is calculated as:
143
With
144
𝐶𝐶𝑖𝑖,𝑛𝑛
145
𝐵𝐵𝑖𝑖,𝑛𝑛
Leaching of introduced and structural elements
𝑟𝑟𝑖𝑖,𝑛𝑛 =
�𝐶𝐶𝑖𝑖,𝑛𝑛 − 𝐵𝐵𝑖𝑖,𝑛𝑛 � ∙ 𝑉𝑉
𝐴𝐴𝑠𝑠
(1)
= concentration of element 𝑖𝑖 in the filtered aliquot of leaching interval 𝑛𝑛 (mg/ml)
= concentration of element 𝑖𝑖 in the filtered blanc aliquot of leaching interval 𝑛𝑛 (mg/ml)
7
146
147
148
𝑉𝑉
𝐴𝐴𝑠𝑠
= initial volume of eluate in the bottle containing the sample AAM (ml)
= geometric surface area of the sample AAM (m²)
The cumulative release 𝑅𝑅 (mg/m2) of each constituent is calculated as:
𝑛𝑛
149
150
151
152
𝑅𝑅𝑖𝑖 = � 𝑟𝑟𝑖𝑖,𝑛𝑛
0
(2)
The normalized leach rate 𝐿𝐿𝐿𝐿 (mg/(m².s)) of element 𝑖𝑖 at leaching interval 𝑛𝑛 is calculated as:
𝐿𝐿𝐿𝐿𝑖𝑖,𝑛𝑛 =
𝑟𝑟𝑖𝑖,𝑛𝑛
𝛥𝛥𝛥𝛥𝑛𝑛 ∙ 𝑓𝑓𝑖𝑖
(3)
with 𝛥𝛥𝛥𝛥𝑛𝑛 the time of leaching interval 𝑛𝑛 (in seconds) and 𝑓𝑓𝑖𝑖 the fraction of element 𝑖𝑖 in the sample, as
to account for possible differences in waste loadings. The total release of element 𝑖𝑖 (% 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑖𝑖 ) and
the percentage of wash-off in the first leaching interval (%𝑊𝑊𝑊𝑊𝑖𝑖 ) are defined as:
% 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑖𝑖 =
%𝑊𝑊𝑊𝑊𝑖𝑖 =
𝑅𝑅𝑖𝑖,28𝑑𝑑 ∙ 𝐴𝐴𝑠𝑠
∙ 100 %
𝑚𝑚𝑠𝑠 ∙ 𝑓𝑓𝑖𝑖
𝑅𝑅𝑖𝑖,1ℎ
∙ 100 %
𝑅𝑅𝑖𝑖,28𝑑𝑑
(4)
(5)
153
with 𝑚𝑚𝑠𝑠 the mass of the sample (mg) after demoulding.
154
2.4
155
To confirm the observed relation between immobilisation capacity and the AAM design ratios, the
156
immobilisation capacity of selected compositions has been tested under varying conditions. Samples
157
of compositions AAM_1, AAM_1.1, AAM_2 and AAM_5.1 were leached at a lower temperature of 20
158
± 1 °C for 7 days. The specific surface area of these samples was measured. The Brunauer Emmett
159
Teller (BET) surface area, was measured for these samples by continuous flow method using 0.3
160
ml/min of nitrogen gas (Tristar 3000).
Effect of leaching temperature and specific surface area on immobilisation performance
8
161
2.5
162
The effect of slag fineness on the immobilisation performance is studied by further milling slag
163
precursors S_1 and S_2 for an additional 30 s. From these finer slags, additional samples of
164
composition AAM_1 and AAM_2 were prepared for leaching during 7 d at 20 ± 1 °C.
Effect of slag fineness on immobilisation performance
165
166
3
Results and discussion
167
3.1
168
All mixtures were completely molten during the isothermal period in the bottom loading furnace.
169
Water-quenching of the melt gave rise to a clear transparent glass for all mixtures. XRD patterns of
170
selected slag samples S_1.1, S_2.4, S_3.4 and S_5.1 are presented in Figure 2. No crystalline phases
171
were detected in any of the measured slags, indicating homogeneity without crystalline inclusions.
Homogeneity of the slag
172
173
Figure 2: XRD patterns of the finely milled slags S_1.1, S_2.4, S_3.4 and S_5.1.
9
174
3.2
175
Figure 3 shows the cumulative release for Cs+, Sr2+ and Na+ in function of the leaching time. The
176
177
% 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 is summarized in Table 2. The highest Na+ leaching is measured in AAM_5.1, which is also
178
7.8 ± 0.3 % of the introduced Cs+, which is almost twice as good as the second best (AAM_1). In our
179
earlier study [29], GGBFS-based AAMs activated with 6 M NaOH were leached for 7 days under the
180
same conditions. The composition of the GGBFS closely resembles that of S_3.4 (excluding the
181
contribution of MgO, SO3 and other trace elements). With a 1 wt% Cs+ loading, the GGBFS-AAM
182
leached 66 % of the introduced Cs+ [29]. This is much higher than the Cs+ release observed in
183
AAM_3.4 and AAM_3.4b which could be due to e.g. the differences in activating solution molarity or
184
the presence of magnesium and other elements. Among the compositions tested in this study,
185
AAM_3.4 performs worst regarding both Cs+ and Sr2+ immobilisation indicating that the composition
186
of GGBFS requires optimisation regarding immobilisation purposes.
187
The Cs+ and Sr2+ normalized leach rates are given in Table 3. The effect of initial wash-off (%𝑊𝑊𝑊𝑊𝐶𝐶𝐶𝐶 )
188
Immobilisation of Cs+, Sr2+ and Na+
the sample with the highest Si/Al ratio. AAM_1.1 shows the best Cs+ immobilisation, releasing only
seems to be the least pronounced for AAM_1.1 (see Table 3) with a percentage released in the first
189
hour of leaching of 7 % of the total amount leached. Also for Sr2+, AAM_1.1 clearly shows the best
190
immobilisation, releasing only 0.50 ± 0.04 % of the introduced Sr2+, which is about 50 % better than
191
the second best (AAM_2). The overall percentage of wash-off (%𝑊𝑊𝑊𝑊𝑆𝑆𝑆𝑆 , see Table 3) is lower for Sr2+
192
than for Cs+ indicating a slower release from the sample surface.
10
193
194
Figure 3: Cumulative release of Cs+, Na+ and Sr2+ during a 28-d leaching test at 90 °C.
195
Table 2: Percentage of introduced Cs+, Sr2+ and Na+ that has been released by leaching for 28 d at 90
196
°C. Uncertainties are calculated as standard deviations from 3 samples per composition.
Cs+
Sr2+
% 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑖𝑖
AAM_1
14 ±
2
AAM_1.1
7.8 ±
20.1 ±
AAM_2
Na+
1.11 ±
0.07
58 ±
2
0.3
0.5 ±
0.04
49 ±
2
0.5
0.74 ±
0.3
46 ±
1
AAM_2.4
40 ±
2
1.4 ±
0.2
65 ±
2
AAM_3.4
46 ±
2
4.9 ±
0.5
59 ±
2
AAM_3.4b
40.9 ±
0.7
5 ±
0.4
53 ±
1
AAM_5.1
27.8 ±
0.9
0.9 ±
0.1
71 ±
2
197
11
198
Table 3: Average leach rates (mg/(m².s)) of Cs+ and Sr2+ during a 28-d leaching period at 90 °C. The
199
mean value calculated per sample is weighted for the time of the respective leaching interval. %𝑊𝑊𝑊𝑊𝑖𝑖
200
(wash-off) is the percentage of the total released amount leached in the first hour of the test.
𝑳𝑳𝑳𝑳𝑪𝑪𝑪𝑪
Mean
%𝑾𝑾𝑾𝑾𝑪𝑪𝑪𝑪
𝑳𝑳𝑳𝑳𝑺𝑺𝑺𝑺
201
Mean
%𝑾𝑾𝑾𝑾𝑺𝑺𝑺𝑺
Leachin
g time
(h)
1
24
168/
192*
672
1
24
168/
192*
672
AAM_1
AAM_1.1
*
AAM_2
AAM_2.
4
AAM_3.
4
AAM_3.4b
*
AAM_5.
1
49
4.7
12
2.0
120
7.8
150
9.3
150
23
120
21
120
12
0.47
0.30
0.55
0.94
1.4
1.1
0.80
0.13
0.43
16
0.68
0.14
0.09
0.23
7
0.53
0.070
0.09
0.64
28
0.39
0.24
0.13
0.83
25
1.5
0.36
0.18
1.5
15
10
0.94
0.14
1.3
14
12
1.1
0.12
0.83
20
0.77
0.20
0.047
0.017
0.036
0.035
0.27
0.22
0.033
0.022
0.032
3
0.0089
0.014
5
0.0079
0.023
2
0.0088
0.029
8
0.054
0.15
10
0.049
0.15
11
0.012
0.024
4
202
The overall leaching behaviour as seen in Figure 3 is comparable for all samples and comprises a
203
decreasing leaching rate with increasing leaching time. E.g. 𝐿𝐿𝐿𝐿𝐶𝐶𝐶𝐶 for AAM_1 is 49 mg/(m².s) in the
204
first hour of leaching, decreases with a factor 10 for each following step (24 h, 7 d) and levels off in
205
the final interval (see Table 3). For both Cs+ and Sr2+, the leach rate for all samples is highest in the
206
first hour of leaching. This behaviour is typical for materials with an initial surface wash-off, which is a
207
process that occurs often in this type of tank test for monolithic materials and comprises the fast
208
dissolution of soluble salts from the surface of the monolith [33]. In addition to surface wash-off,
209
diffusion of the element of interest through the sample and eventual depletion of the element from
210
the sample are important processes governing the leaching behaviour.
211
Diffusion-controlled release from a monolithic waste form is related to the surface area of the waste
212
form and the time of exposure [33]. In semi-dynamic tank leaching tests with a monolithic sample, a
213
one-dimensional semi-infinite diffusion model, based on Fick’s second law, is often assumed [34,35].
12
214
In this model, mass transfer is assumed to take place in response to concentration gradients in the
215
pore water solution of the structure [35]. When Fickian diffusion is considered as the dominant
216
release mechanism, the mass release should be proportional to the square root of the release time
217
(𝑅𝑅 ~ 𝑡𝑡 0.5 ); this results in a straight line with a slope of 0.5 when the logarithm of the cumulative
218
release is plotted against the logarithm of the release time [35]. Initial surface wash-off causes a
219
higher release in the first stage of leaching, while depletion leads to a levelling-off of the cumulative
220
release curve [33].
221
Figure 4 shows the cumulative caesium and strontium release of a selected subset of samples (the
222
remaining samples show similar behaviour) plotted against the release time on a double logarithmic
223
scale. None of the samples closely follow the 0.5 slope. Since an initial wash-off and depletion of Cs+
224
is clear from Table 2 and Table 3, the results from the leaching test can be divided into the intervals
225
[1 h ; 24 h] and [24 h ; 28 d] as shown in Figure 4a for Cs+ and Figure 4b for Sr2+.
226
For caesium, it is clear from Figure 4a that the sample with limited wash-off (AAM_1.1, as seen in
227
Table 3) has a slope close to 0.5 in the interval [1 h ; 24 h] indicating possible diffusion-controlled
228
release. AAM_2 shows a slope of 0.28 in this first interval, indicating a significant wash-off (high
229
concentration in the first hour of leaching).
230
In the interval [24 h ; 28 d], the regression lines flatten for all samples (slope < 0.5) indicating signs of
231
depletion. Since depletion is only assumed to occur when the total release is higher than 20 % [35],
232
not all samples have actually been significantly depleted. Table 2 shows that the release of Cs+ from
233
the AAMs is between 46 and 7 %. The fact that depletion-like behaviour occurs even in samples with
234
a low release indicates a possible distribution of the present Cs+ into an easily-leachable fraction and
235
a strongly-bound fraction. The occurrence of the wash-off and the depletion effects can then be
236
contributed to the easily-leachable fraction. The levelling-off of the regression curve could thus be
237
caused by the depletion of the readily available Cs+, while the remaining fraction is more strongly
238
bound in the AAM structure. The leaching behaviour of Cs+ can thus be summarized as a combination
13
239
of initial wash-off, diffusion, and depletion of an easily-released fraction. The lower the Si/Al and
240
Ca/(Si+Al) ratio, the smaller this easily-leachable fraction (as discussed further).
241
For strontium, the leaching behaviour in most samples does not seem to be dependent on the
242
leaching interval as seen in Figure 4b. The overall slopes of the regression curves are smaller than
243
0.5, indicating depletion. Since the total amount of leached Sr2+ is small for all samples (< 5 %, see
244
Table 2), it seems reasonable that Sr2+ leaches only from the sample surface, slowly depleting while
245
the bulk of the Sr2+ remains encapsulated in the sample, not available for leaching.
246
The amount of sodium leached does not correspond with the amount of Cs+ leached (see Table 2), as
247
would be expected since Cs+ is known to act as a charge-balancing ion, replacing Na+ in the
248
framework [13,15]. This indicates that Cs+ and Na+ immobilisation is not completely similar, and
249
shows that Cs+ is retained better than Na+ (since Na+ release is higher and both were introduced as 1
250
wt%). This is consistent with the findings of Kuenzel et al. (2015) [15], who reported that the reaction
251
between Cs+ and Al(OH)4- is favoured over that of Na+ because of the lower charge density of Cs+. In
252
addition, the difference in immobilisation between Cs+ and Na+ is dependent on the AAM
253
composition, since from Table 2 it can be seen that e.g. AAM_2 releases the least Na+ (46 %) while
254
releasing 20 % Cs+; in contrast, AAM_1 leaches less Cs+ (14 %) and more Na+ (58 %). This could
255
indicate a difference in hydrate phases formed depending on the AAM composition. The findings of
256
Kuenzel et al. (2015) [15] were based on MK-AAMs, resulting in an amorphous sodium
257
aluminosilicate hydrate (N-A-S-H) as the main binder phase.
258
A difference in the amount of N-A-S-H formed (as compared to calcium sodium aluminosilicate
259
hydrate (C-(N)-A-S-H)) could be the cause of the difference in leaching behaviour of Na+ as compared
260
to Cs+. Based on the initial composition of the slags and the use of a NaOH activator, the AAMs are
261
expected to consist mainly of C-(N-)A-S-H gel, N-A-S-H gel (due to high Al content), strätlingite, and
262
some zeolitic phases [36–38]. For the low Si/Al samples AAM_1 and AAM_1.1, the ratio of leached
263
Na+/Cs+ is much higher than for the high Si/Al samples (see Table 2).
14
(a)
(b)
264
265
266
Figure 4: (a) 𝑅𝑅𝐶𝐶𝐶𝐶 , dividing the leaching time into two intervals [1 h ; 24 h] and [24 h ; 28 d] and (b)
𝑅𝑅𝑆𝑆𝑆𝑆 , dividing the leaching time into two intervals. R²-values are all > 0.97 for (a) and (b). The dashed
267
line represents the slope of 0.5 from a diffusion-based release. The slopes are indicated on the graph
268
next to the regression lines.
269
When linking the immobilisation capacity for Cs+ and Sr2+ with the precursor composition, an initial
270
Si/Al ratio of about 1.1 (Table 2) seems to be best. Figure 5a shows the cumulative 28-d release of
271
Cs+ in function of the Si/Al and Ca/(Si+Al) ratios of the precursor slags. For the studied compositions,
272
Cs+ is immobilised better in AAMs with lower Si/Al ratios and lower Ca/(Si+Al) ratios.
15
273
Figure 5b shows the cumulative 28-d release of Sr2+ in function of the Si/Al and Ca/(Si+Al) ratios of
274
the precursor slags. For the studied compositions, the immobilisation of Sr2+ is dependent on the
275
Ca/(Si+Al) ratio, but not on the Si/Al ratio. This indicates a competition between Ca2+ and Sr2+ for
276
incorporation into the AAM structure. The independence of strontium leaching to the Si/Al ratio is in
277
contrast with the results of the study of Aly, et al. (2008) [12], where Sr2+ release increased in
278
samples with an increasing Si/Al ratio from 1.5 to 4. This could be due to the fact that the samples
279
studied by Aly, et al. (2008) [12] were low in calcium, indicating the possible differences in leaching
280
behaviour in function of the type of AAMs.
(b)
(a)
281
Figure 5: The cumulative 28-d release of Cs+ (a) and Sr2+ (b) (10³ mg/m²) in function of the Si/Al and
282
Ca/(Si+Al) ratios of the precursor slags. The regression planes are a result of multiple linear
283
regressions with R²-values > 0.93.
284
3.3
285
The Ca2+ release of AAM_1, AAM_1.1, AAM_3.4 and AAM_3.4b are similar, while S_1 and S_1.1 have
286
a CaO content of only 30 % and S_3.4 and S_3.4b have an initial CaO content of 50 %. Also, the Ca2+
287
release decreases with increasing Si/Al ratio reaching a value of 7300 ± 200 mg/m² for AAM_2 and
288
5000 ± 200 mg/m² for AAM_5.1. This higher Ca2+ release at lower Si/Al ratios explains why AAM_1,
289
AAM_1.1, AAM_3.4 and AAM_3.4b show a similar Ca2+ release while having a lower calcium content.
Release of structural elements
16
290
This behaviour of calcium leaching could be an indication of less C-(N)-A-S-H formation at lower Si/Al
291
ratios since the formation of this phase would better immobilise Ca2+ into the structure. An increased
292
Al content (and thus lower Si/Al ratio) promotes the formation of N-A-S-H in addition to C-(N)-A-S-H,
293
while an increased Ca content impedes the formation of N-A-S-H [38]. Higher Si/Al ratios will thus
294
give rise to more C-(N)-A-S-H formation.
295
The highest silicon release is observed in AAM_5.1 (≈ 24 000 mg/m²), while AAM_3.4, AAM_3.4b,
296
and AAM_1.1 show the lowest silicon release (≈ 5000 mg/m²). AAM_2.4, AAM_2, and AAM_1 all
297
show a 28-d cumulative silicon release around 10 000 mg/m². The lower silicon leaching of AAM_3.4
298
and AAM_3.4b as compared to AAM_2.4, AAM_2, and AAM_1 (while having similar initial SiO2
299
contents) indicates that a higher calcium content increases the immobilisation of silicon, which again
300
indicates the formation of C-(N)-A-S-H.
301
Regarding aluminium, the release follows the Si/Al ratio, with AAM_5.1 showing the lowest
302
aluminium release and increasing with decreasing Si/Al. For AAM_1, the amount of silicon and
303
aluminium released is almost equal (AAM_1: 𝑅𝑅𝑆𝑆𝑆𝑆 = 10000 ± 600 mg/m², 𝑅𝑅𝐴𝐴𝐴𝐴 = 9200 ± 500 mg/m²),
304
indicating congruent dissolution.
17
305
306
Figure 6: Cumulative release of silicon, aluminium and calcium during a 28-d leaching test at 90 °C.
307
3.4
308
The measured pH of the eluates was similar for all samples. After 1 h of leaching the average pH over
309
all samples was 10.7 ± 0.3. After 24 h, the pH of the eluates increased to 11.2 ± 0.3. After 28 d, the
310
pH-values were 11.3 ± 0.6. The conductivity of selected eluates is given in Figure 7.
311
AAM_5.1, AAM_3.4, AAM_3.4b, AAM_2.4 and AAM_2 show a high peak in conductivity at 24 h (only
312
AAM_2 and AAM_3.4 shown in Figure 7), while decreasing at later sampling times. This high early
313
peak in ionic conductivity followed by a gradual decrease indicates a more profound early wash-off
314
and subsequent depletion, which is consistent with the results seen for Cs+ in Figure 4a. This effect is
315
not so pronounced for AAM_1.1, which shows a slighter decrease in conductivity at the 7 d sampling
316
time. Regarding the validity of the experimental design, the duplicates AAM_3.4 and AAM_3.4b show
pH and conductivity
18
317
very similar results in all measured leaching aspects, confirming that the slag development, the AAM
318
development, and the leaching tests are very well reproducible.
319
320
321
Figure 7: Conductivity values (µS/cm) of AAM_1.1, AAM_2 and AAM_3.4 at each sampling time.
3.5
Effect of leaching temperature on immobilisation capacity
322
323
Table 4 and Table 5 show that the leaching temperature has a profound effect on the release of all
324
constituents. However, this effect is not the same for all samples. AAM_1 exhibits significantly less
325
release at 20 °C than at 90 °C for Cs+, Sr2+ and Na+, while AAM_5.1 exhibits less release for Sr2+ and
326
Na+ but not for Cs+. In contrast, AAM_1.1 has a significantly higher release at 20 °C as compared to 90
327
°C for all constituents, while AAM_2 has a higher release for Cs+ and Na+ at 20 °C and a similar release
328
for Sr2+. Although all samples were completely set after 28 days of curing, it appears that the high
329
temperature (90 °C) of the leaching environment influenced the curing of AAM_1.1 and AAM_2
330
which significantly improved their immobilisation potential. This does not seem to be the case for
331
AAM_1 and AAM_5.1.
332
Table 4: Slag Blaine values, % 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 and BET values for samples leached for 7 d at 20 °C. The
333
334
% 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 values are given as individual measurements since only one or two samples were tested
per composition.
19
Slag Blaine (m²/kg)
Sample
% 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 7 d, 20 °C
Cs+
Sr2+
Na+
BET (m²/g)
AAM_1
110
±
20
2.97;
3.06
0.13;
0.22
31.52;
31.77
10.7
±
0.1
AAM_1.1
200
±
20
11.22
4.02
60.21
14.8
±
0.1
AAM_2
140
±
10
26.56;
27.94
0.27;
0.49
41.29;
42.76
3.8
±
0.1
AAM_5.1
310
±
10
22.45;
22.71
0.20;
0.22
53.88;
55.21
37.5
±
0.1
335
336
Table 5: Percentages of Cs+, Sr2+ and Na+ released after 7 d leaching at 90 °C.
% 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 7 d, 90 °C
Cs+
Sample
Na+
Sr2+
AAM_1
10
±
1
0.48
±
0.05
46
±
2
AAM_1.1
5.1
±
0.2
0.23
±
0.03
41
±
1
AAM_2
17.5
±
0.3
0.51
±
0.2
36
±
1
AAM_5.1
24.2
±
0.7
0.49
±
0.06
62
±
2
337
338
339
3.6
340
The BET specific surface area (before leaching) of the samples used for leaching at 20 °C is given in
341
Table 4. The BET specific surface area is lowest for AAM_2, while AAM_5.1 exhibits the highest BET
342
area. The specific surface areas of AAM_1 and AAM_1.1 are comparable. AAM_2 releases more Cs+
343
and Sr2+ than AAM_5.1 at 20 °C, although having a much lower BET value. From the tested samples,
344
no clear correlation could be established between the BET values and the immobilisation capacities.
345
3.7
346
The remainder of slag precursors S_1 and S_2 were further milled for an additional 30 s. The higher
347
Blaine values (HB) of these slags are given in Table 6. From these finer slags, additional samples of
Effect of specific surface area on immobilisation capacity
Effect of slag fineness on immobilisation capacity
20
348
composition AAM_1 and AAM_2 were prepared for leaching during 7 d at 20 ± 1 °C. The percentages
349
of Cs+, Sr2+ and Na+ leached under these new leaching conditions are given in Table 6.
350
Increasing the slag fineness of S_2 to a comparable value of that of S_5.1 (see Table 4) reduces the
351
Cs+ and Sr2+ release of AAM_2 by a factor 2, increasing its immobilisation capacity beyond that of
352
AAM_5.1. AAM_1 shows the greatest immobilisation potential for all constituents at 20 °C, despite
353
having the smallest Blaine specific surface area of the slag. Increasing the slag fineness of S_1 and
354
S_2 demonstrates its significant effect on the immobilisation potential. The increased Blaine value of
355
220 m²/kg for S_1 (which is now comparable to that of S_1.1) further increases the immobilisation
356
potential for all constituents. The same effect is observed for AAM_2. Increasing the slag fineness
357
leads to a larger fraction of small slag particles that can be readily dissolved to form hydrate phases.
358
This, in turn, raises the need for more charge-balancing ions, thus increasing the incorporation
359
potential.
360
The results of the effect of leaching temperature, and the differences in specific surface area and slag
361
fineness support the earlier established relationship between the immobilisation potential and
362
design parameters Si/Al and Ca/(Si+Al). Leaching at 20 °C strengthens the conclusion that low Si/Al
363
and Ca/(Si+Al) ratios provide a higher immobilisation potential since AAM_1 now exhibits the best
364
immobilisation potential (see Table 5 and Table 6). Under the experimental conditions, a Si/Al ratio
365
of 1 thus seems optimal for immobilising Cs+ and Sr2+.
366
367
368
Table 6: Higher Blaine values (HB) after additional milling and % 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 after 7 d leaching at 20 °C
of AAM_1 and AAM_2 prepared with HB-slag.
Sample
AAM_1
AAM_2
Slag Blaine HB
(m²/kg)
220
380
±
±
10
10
Cs+
2.38
12.79
% 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 7 d, 20 °C
Sr2+
0.05
0.18
Na+
22.03
31.48
369
21
370
371
4
Conclusion
372
The effect of AAM composition regarding Si/Al and Ca/(Si+Al) ratios on the immobilisation capacity of
373
introduced Cs+ and Sr2+ is discussed. Stoichiometrically controlled slags were designed from analytical
374
grade chemicals to serve as precursors for monolithic AAM samples for immobilisation purposes.
375
Under the given experimental conditions, the following conclusions are made:
376
a) Very effective immobilisation of Cs+ and Sr2+ was achieved by use of low-alkaline AAMs. An
377
immobilisation potential of up to 97.6 % and 99.9 % of introduced Cs+ and Sr2+ respectively was
378
achieved for composition AAM_1, after 7 days of leaching at 20 °C.
379
b) The leaching behaviour of Cs+ from the AAMs consists of a combination of initial wash-off,
380
diffusion and depletion of an easily-leachable fraction. Sr2+ leaching appears to be limited to a
381
small fraction present on or near the surface, showing less wash-off and exhibiting slow
382
depletion.
383
c) Cs+ immobilisation is dependent on the Si/Al and Ca/(Si+Al) ratios of the precursor while Sr2+
384
immobilisation is only dependent on the Ca/(Si+Al) ratio. Better immobilisation is achieved at
385
lower ratios, independent of the observed differences in precursor fineness or AAM specific
386
surface area.
387
d) Lowering the leaching temperature from 90 °C to 20 °C has a varying effect on the immobilisation
388
capacity of different compositions. This highlights the importance of being aware that leaching
389
conditions affect obtained results, especially when comparing compositions. Leaching at higher
390
temperature affects the curing of the AAMs, which can lead to misinterpretations of leaching
391
results when comparing immobilisation capacities of different compositions.
392
In general, the developed AAMs show very effective Cs+ and Sr2+ immobilisation which is very
393
promising for the use of AAMs for waste immobilisation purposes. This study offers a deeper
22
394
understanding of the immobilisation mechanism of AAMs, which could encourage further research in
395
finding better alternatives for RAW immobilisation, and, in turn, encourage its large-scale
396
application.
397
Acknowledgements
398
The authors thank Joris Van Dyck (KU Leuven, Department of Materials Engineering) for his
399
assistance in developing the slag precursors, Elsy Thijssen and Martine Vanhamel (Hasselt University,
400
CMK, Research Group of Applied and Analytical Chemistry) for their help regarding ICP-OES and ICP-
401
MS measurements, and Bart Ruttens (IMEC, Division IMOMEC) for the XRD measurements.
402
Data availability
403
The raw/processed data required to reproduce these findings cannot be shared at this time as the
404
data also forms part of an ongoing study.
405
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406
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