Arabian Journal of Chemistry (2011) 4, 361–377
King Saud University
Arabian Journal of Chemistry
www.ksu.edu.sa
www.sciencedirect.com
REVIEW ARTICLE
New trends in removing heavy metals from
industrial wastewater
M.A. Barakat
*
Department of Environmental Sciences, Faculty of Meteorology and Environment, King Abdulaziz University (KAU),
P.O. Box 80202, Jeddah 21589, Saudi Arabia
Received 1 February 2010; accepted 17 July 2010
Available online 21 July 2010
KEYWORDS
Heavy metals;
Wastewater treatment;
Removal;
Advanced techniques
Abstract Innovative processes for treating industrial wastewater containing heavy metals often
involve technologies for reduction of toxicity in order to meet technology-based treatment standards. This article reviews the recent developments and technical applicability of various treatments
for the removal of heavy metals from industrial wastewater. A particular focus is given to innovative physico-chemical removal processes such as; adsorption on new adsorbents, membrane filtration, electrodialysis, and photocatalysis. Their advantages and limitations in application are
evaluated. The main operating conditions such as pH and treatment performance are presented.
Published studies of 94 cited references (1999–2008) are reviewed.
It is evident from survey that new adsorbents and membrane filtration are the most frequently studied and widely applied for the treatment of metal-contaminated wastewater. However, in the near
future, the most promising methods to treat such complex systems will be the photocatalytic ones
which consume cheap photons from the UV-near visible region. They induce both degradation of
organic pollutants and recovery of metals in one-pot systems. On the other hand, from the conventional processes, lime precipitation has been found as one of the most effective means to treat inorganic effluent with a metal concentration of >1000 mg/L. It is important to note that the overall
treatment cost of metal-contaminated water varies, depending on the process employed and the local
conditions. In general, the technical applicability, plant simplicity and cost-effectiveness are the key
factors in selecting the most suitable treatment for inorganic effluent
ª 2010 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.
* Permanent Address: Central Metallurgical Research and Development Institute, P.O. Box 87, Helwan 11421, Egypt.
E-mail address: mabarakat@gmail.com
1878-5352 ª 2010 King Saud University. Production and hosting by
Elsevier B.V. All rights reserved.
Peer-review under responsibility of King Saud University.
doi:10.1016/j.arabjc.2010.07.019
Production and hosting by Elsevier
362
M.A. Barakat
Contents
1.
2.
3.
4.
5.
6.
7.
8.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Heavy metals in industrial wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1. Definition and toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2. Industrial wastewater sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3. Conventional processes for removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Adsorption on new adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Adsorption on modified natural materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Adsorption on industrial by-products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Adsorption on modified agriculture and biological wastes (bio-sorption) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. Adsorption on modified biopolymers and hydrogels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Membrane filtration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrodialysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Evaluation of heavy metals removal processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Due to the discharge of large amounts of metal-contaminated
wastewater, industries bearing heavy metals, such as Cd, Cr,
Cu, Ni, As, Pb, and Zn, are the most hazardous among the
chemical-intensive industries. Because of their high solubility
in the aquatic environments, heavy metals can be absorbed
by living organisms. Once they enter the food chain, large concentrations of heavy metals may accumulate in the human
body. If the metals are ingested beyond the permitted concentration, they can cause serious health disorders (Babel and
Kurniawan, 2004). Therefore, it is necessary to treat metalcontaminated wastewater prior to its discharge to the environment. Heavy metal removal from inorganic effluent can be
achieved by conventional treatment processes such as chemical
precipitation, ion exchange, and electrochemical removal.
These processes have significant disadvantages, which are,
for instance, incomplete removal, high-energy requirements,
and production of toxic sludge (Eccles, 1999).
Recently, numerous approaches have been studied for the
development of cheaper and more effective technologies, both
to decrease the amount of wastewater produced and to
improve the quality of the treated effluent. Adsorption has
become one of the alternative treatments, in recent years, the
search for low-cost adsorbents that have metal-binding capacities has intensified (Leung et al., 2000). The adsorbents may be
of mineral, organic or biological origin, zeolites, industrial byproducts, agricultural wastes, biomass, and polymeric materials (Kurniawan et al., 2005). Membrane separation has been
increasingly used recently for the treatment of inorganic effluent due to its convenient operation. There are different types of
membrane filtration such as ultrafiltration (UF), nanofiltration
(NF) and reverse osmosis (RO) Kurniawan et al., 2006. Electrotreatments such as electrodialysis (Pedersen, 2003) has also
contributed to environmental protection. Photocatalytic process is an innovative and promising technique for efficient
destruction of pollutants in water (Skubal et al., 2002).
Although many techniques can be employed for the treatment
of inorganic effluent, the ideal treatment should be not only
suitable, appropriate and applicable to the local conditions,
362
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362
362
363
363
363
364
365
366
367
370
371
373
374
374
but also able to meet the maximum contaminant level
(MCL) standards established. This article presents an overview
of various innovative physico-chemical treatments for removal
of heavy metals from industrial wastewater. Their advantages
and limitations in application are evaluated. To highlight their
removal performance, the main operating conditions such as
pH and treatment efficiency are presented as well.
2. Heavy metals in industrial wastewater
2.1. Definition and toxicity
Heavy metals are generally considered to be those whose density exceeds 5 g per cubic centimeter. A large number of elements fall into this category, but the ones listed in Table 1
are those of relevance in the environmental context. Arsenic
is usually regarded as a hazardous heavy metal even though
it is actually a semi-metal. Heavy metals cause serious health
effects, including reduced growth and development, cancer,
organ damage, nervous system damage, and in extreme cases,
death. Exposure to some metals, such as mercury and lead,
may also cause development of autoimmunity, in which a
person’s immune system attacks its own cells. This can lead
to joint diseases such as rheumatoid arthritis, and diseases of
the kidneys, circulatory system, nervous system, and damaging
of the fetal brain. At higher doses, heavy metals can cause
irreversible brain damage. Children may receive higher doses
of metals from food than adults, since they consume more
food for their body weight than adults. Wastewater regulations
were established to minimize human and environmental exposure to hazardous chemicals. This includes limits on the types
and concentration of heavy metals that may be present in the
discharged wastewater. The MCL standards, for those heavy
metals, established by USEPA (Babel and Kurniawan, 2003)
are summarized in Table 1.
2.2. Industrial wastewater sources
Industrial wastewater streams containing heavy metals are produced from different industries. Electroplating and metal sur-
New trends in removing heavy metals from industrial wastewater
Table 1
363
The MCL standards for the most hazardous heavy metals (Babel and Kurniawan, 2003).
Heavy metal
Toxicities
MCL (mg/L)
Arsenic
Cadmium
Chromium
Copper
Nickel
Zinc
Lead
Mercury
Skin manifestations, visceral cancers, vascular disease
Kidney damage, renal disorder, human carcinogen
Headache, diarrhea, nausea, vomiting, carcinogenic
Liver damage, Wilson disease, insomnia
Dermatitis, nausea, chronic asthma, coughing, human carcinogen
Depression, lethargy, neurological signs and increased thirst
Damage the fetal brain, diseases of the kidneys, circulatory system, and nervous system
Rheumatoid arthritis, and diseases of the kidneys, circulatory system, and nervous system
0.050
0.01
0.05
0.25
0.20
0.80
0.006
0.00003
face treatment processes generate significant quantities of
wastewaters containing heavy metals (such as cadmium, zinc,
lead, chromium, nickel, copper, vanadium, platinum, silver,
and titanium) from a variety of applications. These include
electroplating, electroless depositions, conversion-coating,
anodizing-cleaning, milling, and etching. Another significant
source of heavy metals wastes result from printed circuit board
(PCB) manufacturing. Tin, lead, and nickel solder plates are the
most widely used resistant overplates. Other sources for the
metal wastes include; the wood processing industry where a
chromated copper-arsenate wood treatment produces arseniccontaining wastes; inorganic pigment manufacturing producing pigments that contain chromium compounds and cadmium
sulfide; petroleum refining which generates conversion catalysts
contaminated with nickel, vanadium, and chromium; and photographic operations producing film with high concentrations
of silver and ferrocyanide. All of these generators produce a
large quantity of wastewaters, residues, and sludges that can
be categorized as hazardous wastes requiring extensive waste
treatment (Sorme and Lagerkvist, 2002).
and the long-term environmental impacts of sludge disposal
(Aziz et al., 2008).
Ion exchange is another method used successfully in the
industry for the removal of heavy metals from effluent. An
ion exchanger is a solid capable of exchanging either cations
or anions from the surrounding materials. Commonly used
matrices for ion exchange are synthetic organic ion exchange
resins. The disadvantage of this method is that it cannot handle concentrated metal solution as the matrix gets easily fouled
by organics and other solids in the wastewater. Moreover ion
exchange is nonselective and is highly sensitive to the pH of the
solution. Electrolytic recovery or electro-winning is one of the
many technologies used to remove metals from process water
streams. This process uses electricity to pass a current through
an aqueous metal-bearing solution containing a cathode plate
and an insoluble anode. Positively charged metallic ions cling
to the negatively charged cathodes leaving behind a metal deposit that is strippable and recoverable. A noticeable disadvantage was that corrosion could become a significant limiting
factor, where electrodes would frequently have to be replaced
(Kurniawan et al., 2006).
2.3. Conventional processes for removal
3. Adsorption on new adsorbents
The conventional processes for removing heavy metals from
wastewater include many processes such as chemical precipitation, flotation, adsorption, ion exchange, and electrochemical
deposition. Chemical precipitation is the most widely used
for heavy metal removal from inorganic effluent. The conceptual mechanism of heavy metal removal by chemical precipitation is presented in Eq. (1) Wang et al., 2004:
M2þ þ 2ðOHÞ $ MðOHÞ2 #
ð1Þ
where M2+ and OH represent the dissolved metal ions and
the precipitant, respectively, while M(OH)2 is the insoluble metal hydroxide. Adjustment of pH to the basic conditions (pH
9–11) is the major parameter that significantly improves heavy
metal removal by chemical precipitation (Fig. 1). Lime and
limestone are the most commonly employed precipitant agents
due to their availability and low-cost in most countries (Mirbagherp and Hosseini, 2004; Aziz et al., 2008). Lime precipitation
can be employed to effectively treat inorganic effluent with a
metal concentration of higher than 1000 mg/L. Other advantages of using lime precipitation include the simplicity of the process, inexpensive equipment requirement, and convenient and
safe operations. However, chemical precipitation requires a
large amount of chemicals to reduce metals to an acceptable
level for discharge. Other drawbacks are its excessive sludge
production that requires further treatment, slow metal precipitation, poor settling, the aggregation of metal precipitates,
Sorption is transfer of ions from water to the soil i.e. from
solution phase to the solid phase. Sorption actually describes
a group of processes, which includes adsorption and precipitation reactions. Recently, adsorption has become one of the
alternative treatment techniques for wastewater laden with
heavy metals. Basically, adsorption is a mass transfer process
by which a substance is transferred from the liquid phase to
the surface of a solid, and becomes bound by physical and/
or chemical interactions (Kurniawan and Babel, 2003). Various low-cost adsorbents, derived from agricultural waste,
industrial by-product, natural material, or modified biopolymers, have been recently developed and applied for the removal of heavy metals from metal-contaminated wastewater.
In general, there are three main steps involved in pollutant
sorption onto solid sorbent: (i) the transport of the pollutant
from the bulk solution to the sorbent surface; (ii) adsorption
on the particle surface; and (iii) transport within the sorbent
particle. Technical applicability and cost-effectiveness are the
key factors that play major roles in the selection of the most
suitable adsorbent to treat inorganic effluent.
3.1. Adsorption on modified natural materials
Natural zeolites gained a significant interest, mainly due to
their valuable properties as ion exchange capability. Among
the most frequently studied natural zeolites, clinoptilolite was
364
M.A. Barakat
Figure 1
Processes of a conventional metals precipitation treatment plant (Wang et al., 2004).
shown to have high selectivity for certain heavy metal ions such
as Pb(II), Cd(II), Zn(II), and Cu(II). It was demonstrated that
the cation-exchange capability of clinoptilolite depends on the
pre-treatment method and that conditioning improves its ion
exchange ability and removal efficiency (Babel and Kurniawan,
2003; Bose et al., 2002). The ability of different types of synthetic zeolite for heavy metals removal was recently investigated. The role of pH is very important for the selective
adsorption of different heavy metal ions (Basaldella et al.,
2007; Ŕıos et al., 2008; Barakat, 2008a). Basaldella et al.
(2007) used NaA zeolite for removal of Cr(III) at neutral pH,
while Barakat (2008a) used 4A zeolite which was synthesized
by dehydroxylation of low grade kaolin. Barakat reported that
Cu(II) and Zn(II) were adsorbed at neutral and alkaline pH,
Cr(VI) was adsorbed at acidic pH while the adsorption of
Mn(IV) was achieved at high alkaline pH values. Nah et al.
(2006) prepared synthetic zeolite magnetically modified with
iron oxide (MMZ). MMZ showed high adsorption capacities
for the Pb(II) ion and a good chemical resistance in a wide
pH range 5–11. The natural clay minerals can be modified with
a polymeric material in a manner that this significantly improves their capability to remove heavy metals from aqueous
solutions. These kinds of adsorbents are called clay–polymer
composites (Vengris et al., 2001; Sölenera et al., 2008;
Abu-Eishah, 2008). Different phosphates such as; calcined
phosphate at 900 °C, activated phosphate (with nitric acid),
and zirconium phosphate have been employed as new adsorbents for removal of heavy metals from aqueous solution (Aklil
et al., 2004; Moufliha et al., 2005; Pan et al., 2007). Fig. 2 shows
the adsorption isotherm of Pb(II), Cu(II), and Zn(II) onto
calcined phosphate at pH 5 (Aklil et al., 2004). Table 2 presents
the highest reported metal adsorption capacities of low-cost
adsorbents from various modified natural materials.
3.2. Adsorption on industrial by-products
Industrial by-products such as fly ash, waste iron, iron slags,
hydrous titanium oxide, can be chemically modified to enhance
its removal performance for metal removal from wastewater.
Figure 2 Adsorption isotherm of Pb(II), Cu(II), and Zn(II) onto
calcined phosphate (Aklil et al., 2004).
New trends in removing heavy metals from industrial wastewater
Table 2
365
Adsorption capacities of modified natural materials for heavy metals.
Adsorbent
Adsorption capacity (mg/g)
Zeolite, clinoptilolite
Modified zeolite, MMZ
HCl-treated clay
Clay/poly(methoxyethyl)acrylamide
Calcined phosphate
Activated phosphate
Zirconium phosphate
Cd2+
Zn2+
Cu2+
1.6
123
2.4
0.5
1.64
63.2
20.6
83.3
29.8
81.02
85.6
155.0
4
398
Several studies have been conducted; Lee et al. (2004) studied
green sands, another by-product from the iron foundry industry, for Zn(II) removal. Feng et al. (2004) investigated Cu(II)
and Pb(II) removal using iron slag. A pH range from 3.5 to
8.5 [for Cu(II)] and from 5.2 to 8.5 [for Pb(II)] was optimized.
Fly ashes were also investigated as adsorbents for removal of
toxic metals. Gupta et al. (2003) explored bagasse fly ash, a solid waste from sugar industry, for Cd(II) and Ni(II) removal
from synthetic solution at pH ranging from 6.0 to 6.5. Alinnor
(2007) used fly ash from coal-burning for removal of Cu(II)
and Pb(II) ions. Sawdust treated with 1,5-disodium hydrogen
phosphate was used for adsorption of Cr(VI) at pH 2 Uysal
and Ar, 2007. Iron based sorbents such as ferrosorp plus
(Genç-Fuhrman et al., 2008) and synthetic nanocrystalline
akaganeite (Deliyanni et al., 2007) were recently used for
Ti
O
Ti
O
Ti
O
Ti
O
Cu
O
Cu
+
O
O
OH
OH
Cu OH
OH
Cu OH
OH
Ti
O
Ti
O
Ti
O
Ti
O Cu
O Cu
OH
OH
O Cu OH
OH
O Cu OH
OH
Figure 3 The adsorption mechanism of Cu(II) on hydrous TiO2
(Barakat, 2005).
Figure 4
References
Pb2+
Cr6+
Ni2+
0.4
8
80.9
Babel and Kurniawan (2003)
Nah et al. (2006)
Vengris et al. (2001)
Sölenera et al. (2008)
Aklil et al. (2004)
Moufliha et al. (2005)
Pan et al. (2007)
simultaneous removal of heavy metals. Ghosh et al. (2003)
and Barakat (2005) studied hydrous titanium oxide for adsorption of Cr(VI) and Cu(II), respectively. Barakat reported that,
the adsorbed Cu(II) aqueous species can undergo surface
hydrolysis reaction as pH rises. This yields a series of surface
Cu(II) complexes such as TiO–CuOH+, TiO–Cu(OH)2, and
TiO–Cu(OH)3 species. The formation of surface metal complexes can also be depicted conceptually by the following
scheme (Fig. 3).
Zeta potential of TiO2 and its adsorption behavior to
Cu(II) in aqueous solution are shown in Fig. 4(a and b) Barakat, 2005. TiO2 particles are negatively charged at pH P6, and
so complete Cu(II) adsorption was achieved at such pH range.
3.3. Adsorption on modified agriculture and biological wastes
(bio-sorption)
Recently, a great deal of interest in the research for the
removal of heavy metals from industrial effluent has been
focused on the use of agricultural by-products as adsorbents.
The use of agricultural by-products in bioremediation of heavy
metal ions, is known as bio-sorption. This utilizes inactive
(non-living) microbial biomass to bind and concentrate
heavy metals from waste streams by purely physico-chemical
pathways (mainly chelation and adsorption) of uptake (Igwe
et al., 2005). New resources such as hazelnut shell, rice husk,
pecan shells, jackfruit, maize cob or husk can be used as an
adsorbent for heavy metal uptake after chemical modification
or conversion by heating into activated carbon. Ajmal et al.
(a) Zeta potential of TiO2 in aqueous solution. (b) Adsorption of Cu(II) on TiO2.
366
Table 3
M.A. Barakat
Adsorption capacities of some agricultural and biological wastes for heavy metals.
Adsorbent
Maize cope and husk
Orange peel
Coconut shell charcoal
Pecan shells activated carbon
Rice husk
Modified rice hull
Spirogyra (green alga)
Ecklonia maxima – marine alga
Ulva lactuca
Oedogonium species
Nostoc species
Bacillus – bacterial biomass
Adsorption capacity (mg/g)
Pb2+
Cd2+
Zn2+
456
493.7
495.9
References
Cu2+
Cr6+
Ni2+
Igwe et al. (2005)
Ajmal et al. (2000)
Babel and Kurniawan (2004)
Bansode et al. (2003)
Bishnoi et al. (2003)
Tang et al. (2003)
Gupta et al. (2006)
Fenga and Aldrich (2004)
El-Sikaily et al. (2007)
Gupta and Rastogi (2008)
Gupta and Rastogi (2008)
Ahluwalia and Goyal (2006)
158
3.65
13.9
31.7
2.0
0.79
23.4
133
90
235
112.3
145
93.5
467
85.3
418
(2000) employed orange peel for Ni(II) removal from
simulated wastewater. They found that the maximum metal
removal occurred at pH 6.0. The applicability of coconut shell
charcoal (CSC) modified with oxidizing agents and/or chitosan
for Cr(VI) removal was investigated by Babel and Kurniawan
(2004). Cu(II) and Zn(II) removal from real wastewater were
studied using pecan shells-activated carbon (Bansode et al.,
2003) and potato peels charcoal (Amana et al., 2008). Bishnoi
et al. (2003) conducted a study on Cr(VI) removal by rice
husk-activated carbon from an aqueous solution. They found
that the maximum metal removal by rice husk took place at
pH 2.0. Rice hull, containing cellulose, lignin, carbohydrate
and silica, was investigated for Cr(VI) removal from simulated
solution (Tang et al., 2003). To enhance its metal removal, the
adsorbent was modified with ethylenediamine. The maximum
Cr(VI) adsorption of 23.4 mg/g was reported to take place at
pH 2. Other type of biosorbents, such as the biomass of marine
dried green alga (biological materials) (Gupta et al., 2006;
Fenga and Aldrich, 2004; El-Sikaily et al., 2007; Gupta and
Rastogi, 2008; Ahmady-Asbchin et al., 2008), were investigated for up-taking of some heavy metals from aqueous solution. Some of the used alga wastes were; Spirogyra species
(Gupta et al., 2006), Ecklonia maxima (Fenga and Aldrich,
2004), Ulva lactuca (El-Sikaily et al., 2007), Oedogonium sp.
and Nostoc sp. (Gupta and Rastogi, 2008), and brown alga Fucus serratus (Ahmady-Asbchin et al., 2008). On the whole, an
acidic pH ranging 2–6 is effective for metal removal by adsorbents from biological wastes. The mechanism of up-taking
heavy metal ions can take place by metabolism-independent
metal-binding to the cell walls and external surfaces (Deliyanni
et al., 2007). This involves adsorption processes such as ionic,
chemical and physical adsorption. A variety of ligands located
on the fungal walls are known to be involved in metal chelation. These include carboxyl, amine, hydroxyl, phosphate
and sulfhydryl groups. Metal ions could be adsorbed by
complexing with negatively charged reaction sites on the cell
surface. Table 3 shows the adsorption capacities of different
biosorbents.
3.4. Adsorption on modified biopolymers and hydrogels
Biopolymers are industrially attractive because they are, capable of lowering transition metal ion concentrations to sub-part
381
39.9
per billion concentrations, widely available, and environmentally safe. Another attractive feature of biopolymers is that
they posses a number of different functional groups, such as
hydroxyls and amines, which increase the efficiency of metal
ion uptake and the maximum chemical loading possibility.
New polysaccharide-based-materials were described as modified biopolymer adsorbents (derived from chitin, chitosan,
and starch) for the removal of heavy metals from the wastewater (Table 4). There are two main ways for preparation of sorbents containing polysaccharides: (a) crosslinking reactions, a
reaction between the hydroxyl or amino groups of the chains
with a coupling agent to form water-insoluble crosslinked networks (gels); (b) immobilization of polysaccharides on insoluble supports by coupling or grafting reactions in order to give
hybrid or composite materials (Crini, 2005). Chitin is a naturally abundant mucopolysaccharide extracted from crustacean
shells, which are waste products of seafood processing industries. Chitosan, which can be formed by deacetylation of chitin, is the most important derivative of chitin. Chitosan in
partially converted crab shell waste is a powerful chelating
agent and interacts very efficiently with transition metal ions
(Pradhan, 2005). Recently other modified chitosan beads were
proposed for diffusion of metal ions through crosslinked chitosan membranes (Lee et al., 2001). The excellent saturation
sorption capacity for Cu(II) with the crosslinked chitosan
beads was achieved at pH 5. Liu et al. (2003) prepared new hybrid materials that adsorb transition metal ions by immobilizing chitosan on the surface of non-porous glass beads. Column
chromatography on the resulting glass beads revealed that they
have strong affinities to Cu(II), Fe(III) and Cd(II). Yi et al.
(2003) proposed the use of chitosan derivatives containing
crown ether. The materials had high adsorption capacity for
Pb(II), Cr(III), Cd(II) and Hg(II). The materials can be
Table 4 Adsorption capacities of modified biopolymers for
heavy metals (Crini, 2005).
Adsorbent
Adsorption capacity (mg/g)
Pb2+ Cd2+ Zn2+ Cu2+ Cr6+ As5+
Crosslinked chitosan
Crosslinked starch gel
433
Alumina/chitosan composite
150
164
135
200
230
New trends in removing heavy metals from industrial wastewater
CH
H2C
H2C
367
CH
O
O
HN
HN
Redox
Polymerization
HN
O
N
Cl
H2C
Monomer
Figure 5
CH
Crosslinking agent
Three-dimensional network formation of cationic hydrogel (Barakat and Sahiner, 2008).
regenerated and their selectivity properties were better than
crosslinked chitosan without crown ether. The sorption mechanism of polysaccharide-based-materials is different from
those of other conventional adsorbents. These mechanisms
are complicated because they implicate the presence of different interactions. Metal complexation by chitosan may thus involve two different mechanisms (chelation versus ion
exchange) depending on the pH since this parameter may affect the protonation of the macromolecule (Crini, 2005).
Chitosan is characterized by its high percentage of nitrogen,
present in the form of amine groups that are responsible for
metal ion binding through chelation mechanisms. Amine sites
are the main reactive groups for metal ions though hydroxyl
groups, especially in the C-3 position, and they may contribute
to adsorption. However, chitosan is also a cationic polymer
and its pKa ranges from 6.2 to 7. Thereby, in acidic solutions
it is protonated and possesses electrostatic properties. Thus, it
is also possible to sorb metal ions through anion exchange
mechanisms. Sorbent materials containing immobilized thiacrown ethers were prepared by immobilizing the ligands into
sol–gel matrix (Saad et al., 2006). The competitive sorption
characteristics of a mixture of Zn(II), Cd(II), Co(II), Mn(II),
Cu(II), Ni(II), and Ag(I) were studied. The results revealed
that the thiacrown ethers exhibit highest selectivity toward
Ag(I).
0.14
q, g adsorbate/g adsorbent
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
5
Three-dimensional network
10
15
20
25
c, residual As(V) ppm
Figure 6 Adsorption isotherm of As(V) onto the hydrogel
(Barakat and Sahiner, 2008).
Hydrogels, which are crosslinked hydrophilic polymers, are
capable of expanding their volumes due to their high swelling
in water. Accordingly they are widely used in the purification
of wastewater. Various hydrogels were synthesized and their
adsorption behavior for heavy metals was investigated. Kesenci et al. (2002) prepared poly(ethyleneglycol dimethacrylate-coacrylamide) hydrogel beads with the following metals in the
order Pb(II) > Cd(II) > Hg(II); Essawy and Ibrahim (2004)
prepared poly(vinylpyrrolidone-co-methylacrylate) hydrogel
with Cu(II) > Ni(II) > Cd(II); while Barakat and Sahiner
(2008) prepared poly(3-acrylamidopropyl)trimethyl ammonium chloride hydrogels for As(V) removal. The removal is
basically governed by the water diffusion into the hydrogel,
carrying the heavy metals inside especially in the absence of
strongly binding sites. Maximum binding capacity increases
with pH increase to >6. Fig. 5 shows the schematic representation of polymerization/crosslinking reaction that results in
three-dimensional network formation of cationic hydrogel,
while the adsorption isotherm of As(V) onto the hydrogel is
shown in Fig. 6.
4. Membrane filtration
Membrane filtration has received considerable attention for
the treatment of inorganic effluent, since it is capable of
removing not only suspended solid and organic compounds,
but also inorganic contaminants such as heavy metals.
Depending on the size of the particle that can be retained, various types of membrane filtration such as ultrafiltration, nanofiltration and reverse osmosis can be employed for heavy metal
removal from wastewater.
Ultrafiltration (UF) utilizes permeable membrane to separate heavy metals, macromolecules and suspended solids from
inorganic solution on the basis of the pore size (5–20 nm) and
molecular weight of the separating compounds (1000–
100,000 Da). These unique specialties enable UF to allow the
passage of water and low-molecular weight solutes, while
retaining the macromolecules, which have a size larger than
the pore size of the membrane (Vigneswaran et al., 2004).
Some significant findings were reported by Juang and Shiau
(2000), who studied the removal of Cu(II) and Zn(II) ions
from synthetic wastewater using chitosan-enhanced membrane
filtration. The amicon-generated cellulose YM10 was used as
368
M.A. Barakat
the ultrafilter. About 100% and 95% rejection were achieved
at pH ranging from 8.5 to 9.5 for Cu(II) and Zn(II) ions,
respectively. The results indicated that chitosan significantly
enhanced metals removal by 6–10 times compared to using
membrane alone. This could be attributed to the major role
of the amino groups of chitosan chain, which served as coordination site for metal-binding. In acidic conditions, the amino
groups of chitosan are protonated after reacting with H+ ions
as follows:
RNH2 þ Hþ $ RNHþ
3;
logKp ¼ 6:3
ð2Þ
Having the unshared electron pair of the nitrogen atom as
the sole electron donor, the non-protonated chitosan binds
with the unsaturated transition metal cation through the formation of coordination bond. For most of the chelating adsorbent, the functional groups with the donor atoms are normally
attached to the metal ions, thus leading to a donor–acceptor
interaction between chitosan and the metal ions (Fei et al.,
2005), as indicated by the Eq. (3):
M2þ þ nRNH2 $ M–ðRNH2 Þ2þ
n
ð3Þ
where M and RNH2 represent metal and the amino group of
chitosan, respectively, while n is the number of the unprotonated chitosan bound to the metal. Combination of Eqs. (2) and
(3) gives the overall reaction as follows:
2þ
þ
M2þ þ nRNHþ
3 $ M–ðRNH2 Þn þ nH
ð4Þ
Eq. (4) suggests that an increase in pH would enhance the
formation of metal–chitosan complexes. To explore its potential to remove heavy metals, Saffaj et al. (2004) employed
low-cost ZnAl2O4–TiO2 UF membranes to remove Cd(II)
and Cr(III) ions from synthetic solution. They reported that
93% Cd(II) rejection and 86% Cr(III) rejection were achieved.
Such high rejection rates might be attributed to the strong interactions between the divalent cations and the positive charge of
the membranes. These results indicate that the charge capacity
of the UF membrane, the charge valencies of the ions and the
ion concentration in the effluent, played major roles in determining the ion rejection rates by the UF membranes. Depending on the membrane characteristics, UF can achieve more than
90% of removal efficiency with a metal concentration ranging
from 10 to 112 mg/L at pH ranging from 5 to 9.5 and at 2–
5 bar of pressure. UF presents some advantages such as lower
driving force and a smaller space requirement due to its high
packing density. However, the decrease in UF performance
due to membrane fouling has hindered it from a wider applica-
Figure 7
tion in wastewater treatment. Fouling has many adverse effects
on the membrane system such as flux decline, an increase in
transmembrane pressure (TMP) and the biodegradation of
the membrane materials (Kurniawan et al., 2006). These effects
result in high operational costs for the membrane system.
The application of both reverse osmosis (RO) and nanofiltration (NF) technologies for the treatment of wastewater
containing copper and cadmium ions was investigated (Abu
Qdaisa and Moussab, 2004). The results showed that high
removal efficiency of the heavy metals could be achieved by
RO process (98% and 99% for copper and cadmium, respectively). NF, however, was capable of removing more than
90% of the copper ions existing in the feed water (Fig. 7).
Lv et al. (2008) investigated amphoteric polybenzimidazole
nanofiltration hollow fiber membrane for both cations and anions removal NF membranes perform separation in between
those of UF and RO ones. The molecular weight of the solute
that is 90% rejected by NF membrane range from 200 to
1000 Da with pore diameters varying from 0.5 to 2 nm (Lv
et al., 2008; Khedr, 2008). A multiple membrane process was
developed for selective separation to reduce cost and mitigated
the increasing heavy metal pollution. The process was divided
into three stages: firstly, microfiltration (MF) and UF were
used to separate the possible organic and suspended matters,
secondly, electrodialysis (ED) was carried out for effective
desalination, and thirdly, the concentrate from ED was treated
by NF and RO separately to increase the recovery rate of
water. Results showed that filtration characteristics of UF
membrane here was not so good as is usually, even if compared
with MF membrane. And RO performed better than NF in
wastewater separation, especially in anti-compacting ability
of membrane (Zuoa et al., 2008).
Polymer-supported ultrafiltration (PSU) technique has
been shown recently to be a promising alternative for the removal of heavy metal ions from industrial effluent (Rether
and Schuster, 2003). This method employs proprietary watersoluble polymeric ligands to bind metal ions of interest, and
the ultrafiltration technique to concentrate the formed macromolecular complexes and produce an effluent, essentially free
of the targeted metal ions (Fig. 8). Advantages of the PSU
technology over ion exchange and solvent extraction are the
low-energy requirements involved in ultrafiltration, the very
fast reaction kinetics, all aqueous based processing and the
high selectivity of separation if selective bonding agents are
applied. Polyamidoamine dendrimers (PAMAM) have been
surface modified, using a two-step process with benzoylthiou-
Concentration of (a) Cu(II) and (b) Cd(II) ions in the permeate from RO and NF (Abu Qdaisa and Moussab, 2004).
New trends in removing heavy metals from industrial wastewater
Figure 8
369
Principles of polymer-supported ultrafiltration (PSU) technique (Rether and Schuster, 2003).
rea groups to provide a new excellent water-soluble chelating
ion exchange material with a distinct selectivity for toxic heavy
metal ions. Studies on the complexation of Co(II), Cu(II),
Ni(II), Pb(II) and Zn(II) by the dendrimer ligand were performed using the PSU method. The results show that all metal
ions can be retained almost quantitatively at pH 9. Cu(II) form
the most stable complexes with the benzoylthiourea modified
PAMAM derivatives (can be completely retained at pH >4),
and can be separated selectively from the other heavy metal
ions investigated (Fig. 9).
Another similar technique, complexation–ultrafiltration,
proves to be a promising alternative to technologies based
on precipitation and ion exchange. The use of water-soluble
metal-binding polymers in combination with ultrafiltration
(UF) is a hybrid approach to concentrate selectively and to
recover valuable elements as heavy metals. In the complexation – UF process cationic forms of heavy metals are first
complexed by a macroligand in order to increase their molecular weight with a size larger than the pores of the selected
membrane that can be retained whereas permeate water is then
purified from the heavy metals (Petrov and Nenov, 2004;
Barakat, 2008b; Trivunac and Stevanovic, 2006). The advantages of complexation–filtration process are the high separation
selectivity due to the use of a selective binding and low-energy
requirements involved in these processes. Water-soluble polymeric ligands have shown to be powerful substances to remove
trace metals from aqueous solutions and industrial wastewater
through membrane processes. Carboxyl methyl cellulose
(CMC) Petrov and Nenov, 2004; Barakat, 2008b, diethylaminoethyl cellulose (Trivunac and Stevanovic, 2006), and polyethyleneimine (PEI) Aroua et al., 2007 were used as efficient
water-soluble metal-binding polymers in combination with
ultrafiltration (UF) for selective removal of heavy metals from
water. Barakat (2008b) investigated the removal of Cu(II),
Ni(II), and Cr(III) ions from synthetic wastewater solutions
by using CMC and polyethersulfon ultrafiltration membrane.
The efficiency of the metals rejection is shown in Table 5.
Ferella et al. (2007) examined the performance of surfactants-enhanced ultrafiltration process for removal of lead
and arsenic by using cationic (dodecylamine) and anionic
(dodecylbenzenesulfonic acid) surfactants. The removal of lead
ions was >99%, while with arsenate ions it was 19%, in both
the systems. Modified UF blend membranes based on cellulose
acetate (CA) with polyether ketone (Arthanareeswaran et al.,
2007), sulfonated polyetherimide (SPEI) Nagendran et al.,
2008, and polycarbonate (Vijayalakshmi et al., 2008) were recently tested for heavy metals removal from water. It was
found that CA/blend membranes displayed higher permeate
flux and lower rejection compared to pure CA membranes.
A new integrated process combining adsorption, membrane
separation and flotation was developed for the selective separation of heavy metals from wastewater (Mavrov et al.,
2003). The process was divided into the following three stages:
firstly, heavy metal bonding (adsorption) by a bonding agent,
secondly, wastewater filtration to separate the loaded bonding
agent by two variants: crossflow microfiltration for low-contaminated wastewater (Fig. 10), or a hybrid process combining
flotation and submerged microfiltration for highly contaminated wastewater (Fig. 11), and thirdly, bonding agent regeneration. Synthetic zeolite R selected as a bonding agent, was
Table 5 Metal rejection in both individuals and simultaneous
solutions (Barakat, 2008a) (pH 7, CMC = 1 g/L, metal ion
concentration = 25 mg/L, p = 1 bar).
Figure 9
2003).
Selectivity of PSU polymer (Rether and Schuster,
Metal ion
Ni(II)
(%)
Cu(II)
(%)
Cr(III)
(%)
Metal rejection (independently) (wt.%)
Metal rejection (simultaneously) (wt.%)
95.1
94.4
98.6
98
99.1
98.3
370
M.A. Barakat
Bonding agents
Wastewater
containing heavy
metals
Crossflow pressuredriven microfiltration
Selective
bonding
of metal ions
Purified water
for reuse or
discharge
Crossflow
Bleed /
BA concentrate
or regeneration or discharge
Figure 10 The integrated processes combining metal bonding and separation by cross flow membrane filtration (for low-contaminated
wastewater) (Mavrov et al., 2003).
Hybrid process:
combining flotation and
submerged membranes
Purified water
for reuse or
discharge
Bonding agents
Wastewater
containing heavy
metals
MF
Selective
bonding
of metal ions
Flotation
Froth /
BA concentrate
for regeneration
or discharge
Figure 11 The integrated processes combining metal bonding and separation by a new hybrid process (for highly contaminated
wastewater) (Mavrov et al., 2003).
characterized and used for the separation of the zeolite loaded
with metal (Mavrov et al., 2003). Bloocher et al. (2003) and
Nenov et al. (2008) developed a new hybrid process of flotation
and membrane separation by integrating specially designed
submerged microfiltration modules directly into a flotation
reactor. This made it possible to combine the advantages of
both flotation and membrane separation. The feasibility of this
hybrid process was proven using powdered synthetic zeolites as
bonding agents. The toxic metals, copper, nickel and zinc, were
reduced from initial concentrations of 474, 3.3 and 167 mg/L,
respectively, to below 0.05 mg/L, consistently meeting the discharge limits.
Another hybrid process, membrane contactor, is not only
combined with an extraction or absorption process but both
processes are fully integrated and incorporated into one piece
of equipment in order to exploit the benefits of both technologies fully (Klaassen et al., 2008). It offers a flexible modular energy efficient device with a high specific surface area. It is
important to note that the selection of the appropriate membrane depends on a number of factors such as the characteristics of the wastewater, the concentration of the heavy metals,
pH and temperature. In addition, the membranes should be
compatible with the feeding solution and cleaning agents to
minimize surface fouling. It is observed that membranes with
polyamide as their skin materials have a higher removal of
heavy metals and can workin a wide range of temperature
(5–45 °C). This may be attributed to the fact that polyamide
membranes have a higher porosity and hydrophilicity than
other materials such as cellulose acetate (Madaeni and Mansourpanah, 2003).
5. Electrodialysis
Electrodialysis (ED) is a membrane separation in which ionized species in the solution are passed through an ion exchange
membrane by applying an electric potential. The membranes
are thin sheets of plastic materials with either anionic or cationic characteristics. When a solution containing ionic species
passes through the cell compartments, the anions migrate toward the anode and the cations toward the cathode, crossing
the anion exchange and cation-exchange membranes (Chen,
2004), Fig. 12 shows the principles of electrodialysis.
Some interesting results were reported by Tzanetakis et al.
(2003), who evaluated the performance of the ion exchange
membranes for the electrodialysis of Ni(II) and Co(II) ions
from a synthetic solution. Two cation-exchange membranes,
perfluorosulfonic Nafion 117 and sulfonated polyvinyldifluoride membrane (SPVDF), were compared under similar oper-
New trends in removing heavy metals from industrial wastewater
371
Figure 12 Electrodialysis principles (Chen, 2004). CM – cation-exchange membrane, D – diluate chamber, e1 and e2 – electrode
chambers, AM – anion exchange membrane, and K – concentrate chamber.
ating conditions. By using perfluorosulfonic Nafion 117, the
removal efficiency of Co(II) and Ni(II) were 90% and 69%,
with initial metal concentrations of 0.84 and 11.72 mg/L,
respectively. Effects of flow rate, temperature and voltage at
different concentrations using two types of commercial membranes, using a laboratory ED cell, on lead removal were studied (Mohammadi et al., 2004). Results show that increasing
voltage and temperature improved cell performance; however,
the separation percentage decreased with an increasing flow
rate. At concentrations of more than 500 ppm, dependence
of separation percentage on concentration diminished. Using
membranes with higher ion exchange capacity resulted in better cell performance. Electrodialytic removal of Cd(II) from
wastewater sludge, was studied (Jakobsen et al., 2004). During
the remediation a stirred suspension of wastewater sludge was
exposed to an electric dc field. The liquid/solid (mL/g fresh
sludge) ratio was between 1.4 and 2. Three experiments were
performed where the sludge was suspended in distilled water,
citric acid or HNO3 (Fig. 13). The Cd(II) removal in the three
experiments was 69%, 70% and 67%, respectively.
ED process was modeled based on basic electrochemistry
rules and copper ion separation experimental data (Mohammadi et al., 2005). The experiments were performed for zinc,
lead and chromium ions. It was found that performance of
Figure 13 Electrodialytic remediation of cadmium from wastewater sludge (Jakobsen et al., 2004) (AN: anion exchange
membrane, CAT: cation-exchange membrane, (a) stirrer).
an ED cell is almost independent on the type of ions and only
depends on the operating conditions and the cell structure. In
spite of its limitation, ED offers advantages for the treatment
of wastewater laden with heavy metals such as the ability to
produce a highly concentrated stream for recovery and the
rejection of undesirable impurities from water. Moreover,
valuable metals such as Cr and Cu can be recovered. Since
ED is a membrane process, it requires clean feed, careful operation, periodic maintenance to prevent any stack damages.
6. Photocatalysis
In the recent years, photocatalytic process in aqueous suspension of semiconductor has received considerable attention in
view of solar energy conversion. This photocatalytic process
was achieved for rapid and efficient destruction of environmental pollutants. Upon illumination of semiconductor–
electrolyte interface with light energy greater than the
semiconductor band gap, electron–hole pairs (e/h+) are
formed in the conduction and the valence band of the semiconductor, respectively (Herrmann, 1999). These charge carriers,
Figure 14 The conceptual reaction path of photocatalysis over
TiO2 (Herrmann, 1999).
372
Figure 15
UV-light.
M.A. Barakat
Cu(II) removal by UV illuminated TiO2 at various Cu(II)/CN ratios Barakat et al., 2004. (a) Without UV-light and (b) with
which migrate to the semiconductor surface, are capable of
reducing or oxidizing species in solution having suitable redox
potential. Various semiconductors have been used: TiO2, ZnO,
CeO2, CdS, ZnS, etc. As generally observed, the best photocatalytic performances with maximum quantum yields are always
obtained with titanium dioxide. Fig. 14 shows the conceptual
reaction path of photocatalysis over titanium dioxide particle.
The mechanism of photocatalysis over titanium dioxide
particle was reported (Zhang and Itoh, 2006). The generated
electron–hole pairs must be trapped in order to avoid recombination. The hydroxyl ions (OH) are the likely traps for holes,
leading to the formation of hydroxyl radicals which are strong
oxidant agents, while the traps for electrons are adsorbed oxygen species, leading to the formation of superoxide species
(O2) which are unstable, reactive and may evolve in several
ways.
þ
TiO2 þ hm ¼ TiO2 þ e
CB þ hVB
ð5Þ
þ
TiO2ðsubstrateÞ –OH
s þ h ¼ TiO2ðsubstrateÞ –OHðadsÞ
ð6Þ
ðadsÞ
O2ðadsÞ þ e ¼ O2
ð7Þ
Barakat et al. (2004) studied the photocatalytic degradation
using UV-irradiated TiO2 suspension for destroying complex
cyanide with a con-current removal of copper. Results revealed
Figure 16 Comparison of photocatalytic reduction of Cr(VI)
using thin film TiO2 and Degussa P-25 (Kajitvichyanukula et al.,
2005).
that free copper (102 M) was completely removed in 3 h. The
co-existence of Cu(II) and CN enhanced the removal efficiency of both CN and copper; the removal (%) increased
with increase of Cu:CN molar ratio reaching a complete removal for both copper and cyanide at a ratio of 10:1 (Fig. 15).
Several studies were reported for the photocatalytic reduction of Cr(VI), which is mobile and highly toxic, compared to
Cr(III), which is immobile and less harmful. Various unmodified and modified semiconductors were synthesized and characterized as photocatalysts. TiO2 thin films immobilized on
glass plates and prepared by sol–gel technique wereinvestigated (Kajitvichyanukula et al., 2005). Cr(VI) was successfully removed, the photoactivity of the prepared TiO2 thin films
exhibited a comparable efficiency with TiO2 powder, Degussa
P-25 (Fig. 16). TiO2 modified with sulfate (Mohapatra et al.,
2005) and TiO2 loading on zirconium phosphate (ZrP) and
titanium phosphate (Dasa et al., 2006) were prepared and
tested. Samples prepared at lower pH exhibit more surface
area and higher reactivity than those prepared at higher pH
(Fig. 17a and b). Polyoxometalates (POM) PW12O403 or
SiW12O404 as photocatalyst and an organic substrate (salicylic acid or propan-2-ol) as electron donor were also investigated (Gkika et al., 2006). Increase of POM or salicylic acid
(SA) concentration accelerated, till a saturation value, with
both the reduction of metal and the oxidation of the organic
compound. The method is suitable for a range of chromium
concentration from 5 to 100 ppm achieving complete reduction
of Cr(VI) to Cr(III).
Photocatalytic reduction of Cr(VI) over TiO2 catalysts was
investigated in both the absence and presence of organic compounds (Papadama et al., 2007; Wang et al., 2008). A marked
synergistic effect between the photocatalytic reduction of
Cr(VI) and organic compounds was observed over the photocatalyst with the largest specific surface area. These results
demonstrated that the photocatalytic reduction of Cr(VI)
alone was dependent on both the specific surface area and crystalline structure of the photocatalyst in the absence of any organic compounds, but was dominated by the specific surface
area of the photocatalyst in the presence of organic compounds because of the synergistic effect between the photocatalytic reduction of Cr(IV) and the photocatalytic oxidation of
organic compounds.
New trends in removing heavy metals from industrial wastewater
373
Figure 17 Effect of pH of the solution on the photocatalytic reduction of Cr(VI) over (a) TiO2 modified with sulphate (Lv et al., 2008),
(b) TiO2 loading on zirconium phosphate (ZrP) and titanium phosphate (Dasa et al., 2006).
A novel photocatalyst, titanium dioxide (TiO2) doped with
neodymium (Nd), was prepared by the sol–gel method and
used for the photocatalytic reduction of Cr(VI) under UV illumination (Rengaraj et al., 2007). The results indicated that the
presence of Nd(III) in TiO2 catalysts substantially enhances
the photocatalytic reaction of Cr(VI) reduction. The neodymium ions deposited on the TiO2 surface behave as sites at which
electrons accumulate. The improved separation of electrons
and holes on the modified TiO2 surface allows more efficient
channeling of the charge carriers into useful reduction and oxidation reactions rather than recombination reactions. The
presence of sacrificial electron donors such as formic acid
enhances the photocatalytic reduction. The Cr(VI) adsorbed
on the surface of the TiO2 particles was observed to be almost
completely photoreduced to Cr(III). To overcome the limitation of powder TiO2, a novel technique of immobilization
based on anodization was applied and investigated (Yoona
et al., 2009). Immobilized TiO2 electrode was applied to
reduce toxic Cr(VI) to non-toxic Cr(III) in aqueous solution
under UV irradiation. The anodization was performed with
0.5% hydrofluoric acid, and then the anodized samples were
annealed under oxygen stream in the range 450–850 °C. The
photocatalytic Cr(VI) reduction was favorable in acidic conditions, with 98% of the Cr(VI) being reduced within 2 h at pH
3.
Heterogeneous photocatalytic oxidation of arsenite to
arsenate in aqueous TiO2 suspensions has also been proved
recently to be an effective and environmentally acceptable
technique for the remediation of arsenite contaminated water.
The process was performed using an adsorbent developed by
loading iron oxide and TiO2 on municipal solid waste melted
slag (Zhang and Itoh, 2006). A concentration of 100 mg/L
arsenite could be entirely oxidized to arsenate within 3 h in
the presence of the adsorbent and under UV-light irradiation
(Fig. 18).
7. Evaluation of heavy metals removal processes
Figure 18 Effect of illumination time on the oxidation of
arsenite to arsenate (Zhang and Itoh, 2006).
In general, physico-chemical treatments offer various advantages such as their rapid process, ease of operation and control,
flexibility to change of temperature. Unlike in biological system, physico-chemical treatment can accommodate variable
input loads and flow such as seasonal flows and complex
discharge. Whenever it is required, chemical plants can be
modified. In addition, the treatment system requires a lower
space and installation cost. Their benefits, however, are
outweighed by a number of drawbacks such as their high operational costs due to the chemicals used, high-energy consumption and handling costs for sludge disposal. However, with
reduced chemical costs (such as utilizing of low-cost adsor-
374
M.A. Barakat
Table 6 The main advantages and disadvantages of the various physico-chemical methods for treatment of heavy metal in
wastewater.
#
Treatment method
Advantages
Disadvantages
References
1
Chemical precipitation
Low capital cost, simple operation
Kurniawan et al. (2006)
2
Adsorption with new
adsorbents
3
Membrane filtration
Electrodialysis
5
Photocatalysis
High operational cost due to
membrane fouling
High operational cost due to
membrane fouling and energy
consumption
Long duration time, limited
applications
Kurniawan et al. (2006)
4
Low-cost, easy operating conditions,
having wide pH range, high metalbinding capacities
Small space requirement, low
pressure, high separation selectivity
High separation selectivity
Sludge generation, extra operational
cost for sludge disposal
Low selectivity, production of waste
products
Removal of metals and organic
pollutant simultaneously, less
harmful by-products
bents) and a feasible sludge disposal, physico-chemical treatments have been found as one of the most suitable treatments
for inorganic effluent (Kurniawan et al., 2006).
In wastewater systems containing heavy metals with other
organic pollutants, the presence of one species usually impedes
the removal of the other. For instance, hydrometallurgy, a
classical process to recover metals, is inhibited by the presence
of organic compounds and a pre-treatment step, to remove or
destroy organics, is generally required, pyrometallurgy which
is able to decontaminate systems from organic pollutants
and recover metals suffers from lack of controllability,
demanding extremely high temperatures. The most promising
methods to treat such complex systems are the photocatalytic
ones which consume cheap photons from the UV-near visible
region. These photo catalysts serve as electron relays, from the
organic substrates to metal ions. Thus, they induce both
degradation of organic pollutants and recovery of metals in
one-pot systems, operable at traces of the target compounds
(less than ppm). Table 6 summarizes the main advantages
and disadvantages of the various physico-chemical treatments
presented in this study.
8. Conclusion
Over the past two decades, environmental regulations have become more stringent, requiring an improved quality of treated
effluent. In recent years, a wide range of treatment technologies such as chemical precipitation, adsorption, membrane filtration, electrodialysis, and photocatalysis, have been
developed for heavy metal removal from contaminated wastewater. It is evident from the literature survey of 94 articles
(1999–2008) that: lime precipitation has been found as one
of the most effective conventional means to treat inorganic
effluent with a metal concentration higher than 1000 mg/L;
new adsorbents and membrane filtration are the most frequently studied and widely applied for the treatment of the
heavy metal-contaminated wastewater; photocatalysis is a
promising innovative technique for a clean and efficient
treatment.
Although many techniques can be employed for the treatment of wastewater laden with heavy metals, it is important
Babel and Kurniawan (2003);
Aklil et al. (2004)
Mohammadi et al. (2005)
Barakat et al. (2004);
Kajitvichyanukula et al. (2005)
to note that the selection of the most suitable treatment for metal-contaminated wastewater depends on some basic parameters such as pH, initial metal concentration, the overall
treatment performance compared to other technologies, environmental impact as well as economics parameter such as
the capital investment and operational costs. Finally, technical
applicability, plant simplicity and cost-effectiveness are the key
factors that play major roles in the selection of the most suitable treatment system for inorganic effluent. All the factors
mentioned above should be taken into consideration in selecting the most effective and inexpensive treatment in order to
protect the environment.
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