Journal of Environmental Chemical Engineering xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering
journal homepage: www.elsevier.com/locate/jece
Technological trends in heavy metals removal from industrial wastewater:
A review
Rakesh Shrestha a, 1, Sagar Ban a, 1, Sijan Devkota a, 1, Sudip Sharma a, 1, Rajendra Joshi a,
Arjun Prasad Tiwari b, *, Hak Yong Kim c, *, Mahesh Kumar Joshi a, d, **
a
Department of Chemical Science and Engineering, School of Engineering, Kathmandu University, Dhulikhel, Kavre, Nepal
Carbon Nano Convergence Technology Center for Next Generation Engineers (CNN), Jeonbuk National University, Republic of Korea
Department of Nano Convergence Engineering, Jeonbuk National University, Jeonju 561-756, Republic of Korea
d
Department of Chemistry, Tri-Chandra Multiple Campus, Tribhuvan University, Kathmandu, Nepal
b
c
A R T I C L E I N F O
A B S T R A C T
Editor: Teik Thye Lim
Rapid industrialization, with economic prosperity set as the prime goal, has always created some secondary
intolerable problems such as heavy metal contamination, wastewater that need remediation. Industrial wastewater is the major contributors to contamination of aquatic and terrestrial ecosystems with toxic heavy metals
like arsenic, copper, chromium, cadmium, nickel, zinc, lead, and mercury whose hazardous bio-accumulative
nature in biotic systems is attributed to their high solubility in the aquatic environments. There has, therefore, always been a need for the removal and/or recovery of these toxic, non-biodegradable, and persistent heavy
metals from the industrial wastewater. For several decades, extensive investigations have been performed for
easy, efficient, and economic removal of heavy metals with a varying degree of success. Chemical precipitation,
adsorption, ion floatation, ion-exchange, coagulation/flocculation and electrochemical methods have been the
most readily available conventional methods for the removal of these heavy metals. These methods however
have posed some serious shortcomings such as high sludge production needing further treatment, low removal
efficiency and high energy requirements. In the present years, newer more efficient, more economic and innovative technologies are being investigated. Recently photocatalysis, electrodialysis, hydrogels, membrane separation technique and introducing newer adsorbents have been developed for better adsorption. Hence in this
paper, we have reviewed efforts and technological advances achieved so far in the pursuit of more efficient
removal and recovery of heavy metals from industrial wastewaters and have evaluated their efficiency dependence on various parameters such as pH, temperature & initial dosing.
Keywords:
Heavy metals
Wastewater treatment
Chemical precipitation
Electrodialysis
Adsorption
Photocatalysis
Membrane separation
1. Introduction
Chemical industries with their rapid growth have increasingly
created nuisances for living beings and the environment. One of these
nuisances is the generation of large volumes of wastewater contaminated with heavy metals whose high solubility in the aquatic environment poses serious threats to all living organisms. Several industries
have contributed to this problem, with the most prevalent contaminants
being Cadmium(Cd), Zinc(Zn), Lead(Pb), Chromium(Cr), Nickel(Ni),
Copper(Cu), Vanadium(V), Platinum(Pt), Silver(Ag), Tin(Sn) and Titanium(Ti) [1–3]. These heavy metals, which are generally categorized
with a density exceeding 5 g/cm3, are well-known to be toxic and can act
as carcinogenic agents and are usually generated by electroplating,
electrolytic depositing, conversion-coating, anodizing-cleaning, milling,
and etching industries [4]. Other industries contributing notable
amounts of Sn, Pb, Ni, As include printed circuit boards (PCBs)
manufacturing industries and wood processing industries. Similarly,
petroleum refinery generates Ni, V and Cr containing effluents from
conversion catalysts, also films with high concentrations of Ag and
ferrocyanide are produced from photographic operations [5]. The most
common sources of heavy metals have been listed in Table 1.
World health organization (WHO) has set guidelines for the
* Corresponding authors.
** Correspondence to: Department of Chemical Science and Engineering, School of Engineering, Kathmandu University, PO BOX 6250, Dhulikhel, Kavre, Nepal.
E-mail addresses: tiwariarjuna@gmail.com (A.P. Tiwari), dragon4875@gmail.com (H.Y. Kim), joshimj2003@yahoo.com (M.K. Joshi).
1
These authors contributed equally.
https://doi.org/10.1016/j.jece.2021.105688
Received 7 January 2021; Received in revised form 8 May 2021; Accepted 12 May 2021
Available online 17 May 2021
2213-3437/© 2021 Elsevier Ltd. All rights reserved.
Please cite this article as: Rakesh Shrestha, Journal of Environmental Chemical Engineering, https://doi.org/10.1016/j.jece.2021.105688
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
maximum permissible limits on these toxic heavy metals and their
consequences to human health when those limits are surpassed in
drinking water and industrial waste water, as listed in Table 2 [10,11].
Contamination of agricultural soils and crops by heavy metals produced
from industries and municipal waste has been considered serious environmental problem due to their non-biodegradable nature and long
biological half-life. Consumption of heavy metal content vegetables can
seriously deplete some essential nutrients in body that are further
responsible of decreasing immunological defenses, intrauterine growth
retardation, impaired psychological faculties, disabilities related to
malnutrition, and high prevalence of upper gastrointestinal cancer rates
[12,13]. Many reports showed plants uptake preferentially the divalent,
trivalent, or multivalent metal present in nutrient solution in the ionic or
chelating form [14,15]. When human injested the plant source containing the heavy metals like Cd, it affects the adsorption of more
essential metals such as Fe and Zn in gastrointestinal tract. In such a
case, Fe content can be diminished while increasing Cd content, a toxic
metal due to the common Fe transporter [16]. This results in the
intoxication of the heavy metal and showed side effects associated with
it finally. The level of toxicity of the selected metals for humans follows
the order Zn<Fe<Cu<Mn< Ni<Cr <Pb< <Cd<Hg [10,17]. To achieve
heavy metal concentrations in wastewater well below the limits set, it
has to be subjected to a treatment process before discharging them into
the environment or reusing them in the industry. Besides affecting
humans and animals, heavy metals concentration should not exceed a
permissible limit in plants as well and when exceeded, there can be
serious consequences as seed germination & lipid content get declined
by Cd, photosynthesis gets inhibited by Cu and Hg, chlorophyll production and plant growth get hindered by Pb [18]. Therefore, it is
extremely necessary to treat these toxic metals contaminated wastewater before their discharge to the environment.
Over several decades, heavy metals have been removed from these
industrial effluents by conventional treatment methods such as chemical
precipitation [21,22], ion flotation [23,24], ion exchange [25,26],
coagulation/flocculation [27,28], adsorption [29], and electrochemical
removal [30]. These low-cost advantage-oriented methods, however,
have significant disadvantages and inadequacies, some of them being
incomplete removal of heavy metals, high-energy requirements and
toxic sludge production [31,32]. Several studies have been made on the
development of a cheaper and more efficient technology. Substantial
achievements have already been made on different types of membrane
separation processes such as ultrafiltration (UF), nanofiltration (NF) and
reverse osmosis (RO) [33,34]. Newer adsorbents have been introduced
for better adsorption. In the other hand, innovative technologies such as
photocatalysis and electrodialysis techniques have been proven efficient
and promising to not only organic waste management but also removal
of heavy metal [35,36]. Fig. 1 provides a summary of different technologies that have been developed for heavy metals removal under
adsorption, membrane separation, electrochemical, photocatalytic and
gravity settling processes. In this review, we have reviewed some of
these better changes that have been made on technologies over recent
years along with their advantages and disadvantages on efficient
removal of heavy metals from industrial wastewater. Before that, we
have discussed briefly the techniques used conventionally.
2. Conventional technologies for heavy metals removal from
industrial wastewater
The physical and chemical conventional techniques that have been
employed for decent amounts of removal of heavy metals include
chemical precipitation, adsorption, ion floatation, ion-exchange, coagulation/flocculation and electrochemical processes.
2.1. Chemical precipitation
Chemical precipitation is used to remove the ionic constituents in
wastewater by the addition of precipitating agents resulting in a
chemical reaction that converts the soluble compound into an insoluble
form. It is always followed by some other separation techniques
including coagulation or filtration to remove the precipitates. Most
metals are precipitated as hydroxides, however other methods such as
sulfide and carbonate precipitation are also in common use [37]. The
mechanism of precipitation can be generalized as.
M2+ + 2(OH)- = M(OH)2↓
Where, M2+, OH-, and M(OH)2 are the metal ions, the precipitating
agent and finally the insoluble metal hydroxide formed, respectively.
AbiD et al. compared the effectiveness of using magnesia (MgO) over
Table 1
Some of the most common sources of heavy metals.
Industries
Metals
Paper mills
Organic chemistry
Allies, Chlorine
Fertilizers
Petroleum refinery
Steel works
Aircraft
Glass/Cement
Textile mills
Tanning
Power plants
Pharmaceutical
Engineering
Fine chemicals
Dyes
Pesticides
Fungicides
Metal smelters
Welding
Electroplating
Nuclear fission
Mining
Ferromanganese production
Batteries
Brass manufacture
Al
As
Cd
Cr
Cu
Hg
Pb
Ni
Zn
✓
✓
✓
✓
✓
✓
✓
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✓
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Co
Fe
Mn
References
✓
[6]
✓
✓
✓
✓
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✓
[7]
[8]
✓
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[9]
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R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
hydroxide as a precipitating agent and the final precipitate was absorbed
by the biofilm absorbent [40]. They have studied the effect of pH, reaction time, adsorbent dosage and the initial concentration of metal ions
on the removal efficiency and concluded that the removal efficiency was
optimum at pH 5. The removal rate increased with an increase in the
amount of adsorbent. From the above-depicted studies, it can be suggested that while the chemical precipitating technique is among the
least expensive technology requiring simple and easily operable equipment, it is not quite effective for treating wastewater having high acid
content besides also producing a large amount of toxic sludge that needs
treatment with chemical stabilization followed by proper disposal. Some
of the metal salts are not water-soluble, in this case, correct anion should
be added to get precipitated. This can lead to the production of a high
water content sludge and disposal process became expensive. On other
hand, precipitation with lime and bisulphide lacks specificity. This
process can be ineffective in the removal of metal ions of low
concentration.
Table 2
Permissible limits on toxic heavy metals and their toxic effects on human health
[10,17].
Contaminants
Recommeneded
safe limits by
WHO in drinking
water
Recommeneded
safe limits by
WHO in waste
water
Toxic effects
Copper
< 2 mg/L
1 mg/L
Zinc
< 3 mg/L
2–5 mg/L
Manganese
< 0.12 mg/L [11,
17]
< 0.2 mg/L
Arsenic
< 0.01 mg/L
n/a
Gastrointestinal
effects, carriers of the
gene for Wilson
disease, metabolic
disorder of copper
homeostasis
Skin irritations,
anemia, nausea and
vomiting
Neurological effects
following inhalation
exposure,
psychological
symptoms (irritability,
emotional lability).
Chronic arsenicism
including dermal
lesions such as hyperand
hypopigmentation,
peripheral
neuropathy, skin
cancer, bladder and
lung cancers and
peripheral vascular
disease
Carcinogenic by the
inhalation route,
kidney problem due to
accumulation in the
kidneys
Chromium(VI) is
carcinogenic to
humans, lung cancer
Accumulates in the
skeleton and creates
bone problems,
adverse effects on
central and peripheral
nervous systems
Long-term exposure is
manifested in nails,
hair and liver, effects
on synthesis of a liver
protein
Oral ingestion creates
problems in
gastrointestine; kidney
damage, increase the
incidence of some
benign tumors.
Irritability, nausea,
vomiting, difficulty
sleeping
Cadmium
0.003–0.005 mg/
L
0.003 mg/L
Chromium
< 0.05 mg/L
0.05 mg/L
Lead
< 0.01 mg/L
0.01 mg/L [20]
Selenium
< 0.01 mg/L
n/a
Mercury
< 0.006 mg/L
0.05 mg/L [20]
Nickel
0.02–0.07 mg/L
0.02 mg/L
2.2. Adsorption
Adsorption is a surface phenomenon and is defined as an attachment
of a particular compound at the surface of a solid object by physical
forces or by chemical bonds [41]. Compound (pollutant) that attaches to
the solid surface is called an adsorbate while the solid surface is known
as an adsorbent. There are mainly three sequential steps involved in
heavy metals adsorption: the transport of heavy metals from the bulk
solution to the absorbent surface followed by adsorption on particle
surface, and finally the transport within the adsorbent particle. Fig. 2
illustrates different types of adsorption. Factors affecting the adsorption
are temperature, nature of the adsorbate and adsorbent, presence of
other pollutants, and atmospheric & experimental conditions (pH, the
concentration of pollutants, and contact time and particle size of the
adsorbent). In addition, the presence of suspended particles, oils and
greases reduces the efficiency of the process and, therefore, pre-filtration
may sometimes be required [42]. Graphene, activated carbon, carbon
nanotubes, rice husk, surfactant modified waste, modified sugarcane
bagasse, modified wheat bran, modified coconut waste, modified orange
peel waste, modified saw dust, modified eggshell, mesoporous silica,
chitosan, zeolite, red mud, coffee residue, powdered olive stones, apple
pomace, magnetic adsorbents, alumina, clay, fungal biomass, yeast,
algal biomass and microbial (bacteria) are among the most studied adsorbents [43–46].
Since the introduction of adsorption in the 1940s, activated carbon
has been the important and prime choice for the treatment and recycling
of community and industrial wastewater to potable quality water
because of its good adsorption capacity due to small particle size, active
free valences and high surface area. However, large-scale utilization of
activated carbon for water treatment is not feasible because of its high
cost of production and regeneration [47,48]. Kadirvelu et al. demonstrated pH dependence of adsorption with maximum removal of 73% for
Cu(II), 100% for Hg(II), Pb(II) and Cd(II) and 92% for Ni(II) by coir pith
carbon at pH of 5.0 for Cu(II), 4.0 for Pb(II), 3.5 for Ni(II) & Hg(II), and
4.0 for Cd(II) [49]. The maximum adsorption was detected at a pH range
of 4.0–5.0. Santhy et al. found that at pH value below 3, the adsorption
of metal ions was very low and was effective only when the pH value was
above 6 [50]. In the batch mode operation, the efficiency of adsorption
of metal ions (Cu(II), Cd(II), and Zn(II)) was studied on porous carbon,
increased with an increase in contact time, pH, carbon dose, and
decrease in initial metal–ion concentrations. The presence of common
anions such as chloride and sulfate did not affect the removal of metal
ions up to 3000 mg/L. Nickel showed the highest removal of 90% by
activated carbon at every concentrations but the removal percentage
decreased with the increase in the concentrations of the heavy metal
[51]. Table 3 summarizes some of the adsorbents, their specific heavy
metal removal efficiencies and further weighing of merits with demerits.
lime for the removal of Fe(III), Cr(III), Cu(II), Pb(II), Ni(II) & Cd(II) and
reported that the removal efficiency was more than 97% at an optimum
MgO dose [38]. With the use of MgO as a precipitating agent, the sludge
was grainy, dense and easily settleable & dewatered, while lime usage
resulted in a low settling rate and dewatering difficulty. Meunier et al.
studied the electrocoagulation and chemical precipitation for the
removal of heavy metals from acidic soil saline leachate and found that
the chemical precipitation using Ca(OH)2 was effective in reducing Cr,
Cu, Ni and Zn but not Cd and Pb [39]. While both the techniques needed
to be operated at higher pH values; electrocoagulation was found to be
effective at pH between 7 and 8. The electrochemical treatment is
suitable for the removal of Pb and can be applicable of producing an
effluent having a pH value close to the neutral. Wu et al. used calcium
3
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
Fig. 1. Commonly used technologies used for removal of heavy metals from industrial wastewater so far.
kinetics.
Recently, low-level removal of arsenic, mercury, lead, chromium and
cadmium ions from aqueous solution have been achieved using cysteine
reacted with octanoyl, decanoyl and dodecanoyl chloride [72]. The
highest removal efficiency of 99.9% by re-crystallized octanoyl-cysteine
(octanoyl-cys) surfactant for mercury ion using pure nitrogen gas and the
removal of other ions was also around 99.1–99.7% efficient using either
pure nitrogen or air. Twice recrystallized S-octanoyl-cys surfactant
showed high efficiency in removing low levels of arsenic ions from
aqueous solutions. Hoseinian et al. also investigated removal of Ni(II) and
Zn(II) ions from wastewater using Ethyl-hexadecyl-dimethyl-ammonium
bromide (EHDABR) and sodium dodecyl sulfate (SDS) as collector and
Dowfroth250 and methyl isobutyl carbonyl (MIBC) as frothers [73]. In
their study, the optimal conditions for removal of Ni(II) and Zn(II) at an
initial concentration of 10 ppm were’ pH 3, SDS = 300 ppm Downforth
250 = 90 ppm and airflow rate of 108 mL/h. They reported an optimal
recovery of Zn(II) and Ni(II) to be 92% and 88%, respectively further
adding the optimal recovery of Zn(II) being achieved with a shorter
period than Ni(II). The advantages include its simplicity, flexibility, low
energy consumption, small volume sludge production, and acting selectively in industrial applications. However, metal ions are difficult to be
separated from the collectors due to they remain still in the form of
complexion. This limits large-scale application for the remediation of
environmental pollution [74].
Fig. 2. Schematic diagram showing the different types of adsorption.
2.3. Ion flotation
Ion flotation is one of the most accepted techniques in metal removal
from industrial wastewater. Various lab scale and industrial scale
removal of metals from wastewater have been reported with various
efficiencies. Removal of achieved 90% Fe(III) and Mn(II) in flotation
column has been achieved with biodegradable surfactant as collector at
a lowest airflow rate of 1098 mL/min after 15 min of operation [69].
The synthetic surfactant like sodium oleate as a collector also showed a
similar collection of heavy metals. The use of bio-surfactant is a more
promising alternative to a synthetic surfactant with dissolved air flotation system compared to column flotation [69]. Bodagh et al. studied the
potentiality of removal of cadmium from aqueous solution by foam
flotation with rhamnolipid bio-surfactant [70]. In their study, cadmium
separation from zinc and copper was investigated with different operating conditions like rhamnolipid and cadmium concentration, solution
pH, aeration rate, frother type and concentration. Superior selectivity
coefficient of cadmium over copper and zinc was found to be 57%, 36%
and 48% respectively following first-order kinetics with rate of
0.0071 min-1 with maximum separation of cadmium from zinc and
copper contaminated solution. Using sodium dodecyl sulfate (SDS) and
hexadecyl trimethyl ammonium bromide (HTAB) as anionic and
cationic surfactant resulted removal of 98% and 76% of copper at
optimal operation condition, respectively [71]. The presence of NaCl in
the solution reduced recoveries while addition of 1% ethanol increased
removal efficiencies with removal kinetics following the first order
2.4. Ion exchange
Another classical method for heavy metal removal from industrial
effluent with a solid capable of exchanging cations/anions is known ion
exchanger. Over the last two decades, performance investigations of
several ion exchange resins have been done. Most advantages of using
this method are that it can remove the parts per billion (PPB) levels
while managing the relatively large volume. The other important
advantage is that this technique can be effective to clean up either cations or anions. Rengaraj et al. studied the performance of cation exchange resins IRN77 and SKN1 in removing Cr from synthetic coolant
water with study of the influence of parameters such as initial resin
dosage, agitation time and pH, and reported that the resins were able to
4
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
Table 3
Adsorbents with their efficiencies compared alongside their shortcomings.
Industry type
Metals
Efficiencies
Merits
Demerits
References
Waste sludge from industries
Cd(II)
37.8 mg/g
No recovery
[52]
Melon peel
Cd
81.97 mg/g
Metal-laden adsorbent immobilization in
cement for ultimate disposal.
Effective use of agricultural waste
[53]
Pyrophillite waste
Silty clay
Cd, Ni
Cd(II), Ni(II)
Biochars
Cr(III), Cr
(VI),
As(V)
Cr(VI)
Cd = 75%, Pb = 98%
Cd(II) = 5.48 mg/g
Ni(II) = 3.60 mg/g
Cr(III) = 95%, Cr(VI) =
89%,
As(V) = 53%
22.96 mg/g
Higher mixing rates needs
higher energy
Low Cd removal rate
n/a
Lower removal rates
[56]
Absorbent was derived from the green
source
Suitable substitute for commercial
activated carbon
Performance evaluation in
short time
High limiting capacity for
adsorption
[57]
Effective environment friendly adsorbent
Lacking of mechanism of
adsorbtion
Cr(VI) to Cr(III) slower
reduction
Difficult to harvest specific
fungi
Metal based-derivative to
remove metal ions
[59]
Needs high temperature to
prepare char
Low scale production of
adsorbent
Low scale production of
adsorbent
Not suitable for highly
concentrated samples
Nature of adsorption not
spontaneous
n/a
[63]
Orthophosphoric acid-treated Green
coconut shell
Coconut coir and bituminious coal
based activated carbon
Cr(VI)
Low cost adsorbent
Metal ions removal by SC were
spontaneous, and feasible
Biochars could retain metal ions compared
to soil
Pea waste
Cr
For coconut
coir = 27.8 mg/g
For bituminious
coal = 38.5 mg/g
3.56 mg/g
Sulfur reducing bacteria and zerovalent iron
Ectomycorrhizal fungi
Cr(VI)
Complete removal
Complete removal for low Cr concentration
Cr(VI)
99%
Chromium doped nickel nano metal
oxide
Cd(II), Pb
(II), Cu(II)
Surface modified biochar
Cr(VI)
Cd(II) = 98%, Pb(II) =
99.5%,
Cu(II) = 97.5%
45,366 µg/g
pH reduced by Hydrogen ion present in the
fungi
OH- formation on Cr doped NiO surface for
aqueous solutes
Fe2O3 nanoparticles
Cr(VI)
Fe2O3/A1SBA-15
Fe3O4 nanoparticles
Cr(VI)
As(V)
As(V)
Cr(VI) = 85%,
As(V) = 94%
99.2%
Tea waste
As(V)
72%
Treated magnetite waste
As
> 90%
Eco-friendly and economic adsorbent
Cost-effective adsorbent with high removal
efficiency
Superior adsorbent for the industrial
effluent
Utilization of waste from the Aluminum
production process
Low-cost adsorbent over a wide
concentrated range
Cost-effective and potential candidate to
captor other metals
remove up to 98% of chromium from an aqueous solution of 100 mg/L
with the influence of parameters being optimal pH of 3.5, decrease in
equilibrium concentration with increasing resin doses and finally SKN1
resin requiring less agitation time than IRN77 [75]. Another study
demonstrated nearly 99.9% Cd, Cu and Hg removal technique from
waste sludge using commercially available magnetic ion exchange resin
having a capacity of 4.5 meq/g and particle size of 100–300 µm [76].
Cavaco et al. evaluated the temperature dependence of a chelating
[54]
[55]
[58]
[60]
[61]
[62]
[64]
[65]
[66]
[67]
[68]
exchange resin Diaion CR11 and Amberlite IRC86 having capacity of
1.21 and 2.77 meq Na+/g resin, respectively on removing Cr (III) from
industrial effluents [77]. Three basic anion exchange resins D301, D314
and D354 have been used to investigate the Chromium (VI) adsorption
capacities in an aqueous solution and found their dependence on varying
experimental conditions [78]. The result indicated more than 99.4%
removal of Cr6+ ions in 1–5 pH range. At room temperature, D301 and
D314 resins showed higher adsorption capacities than D314 but when
Table 4
Ion exchange resins with their efficiencies compared with their shortcomings.
Resin type
Metals
Efficiency
Merits
Demerits
References
Amorphous hydrous
Manganese oxide
(HMO)
B. longum 46, L.
fermentum ME3 and
B. lactis Bb12
Golden Apple Snail
(GAS)
Cd(II), Ni
(II), Zn(II)
Increase in the amount of Ca(II)
shows higher removal efficiency
HMO can be regenerated by HCL, advantageous than
other polymeric exchangers
High pressure drop in
column operation
[82]
Cd(II), Pb
(II)
Cd(II) = 54.7 mg/g and Pb
(II) = 175.7 mg/g of dry
biomass
81.301 mg/g
Fast removal and metabolism-independent surface
process
Needs further water
purification
[83]
Economic and effective biomaterial
[84]
Nitrated Styrofoam and
sulfonated Styrofoam
Cd(II), Pb
(II), Hg(II
Utilization of waste styrofoam leading to decrease in
environmental pollution, potential use in the
floatation system due to its low density
Ferrous chloride
Cr(VI)
For Nitrated Styrofoam,
Cd = 600 µmol/g,
Pb = 1100 µmol/g
Hg = 2450 µmol/g
For sulfonated Styrofoam
Cd = 500 µmol/g,
Pb = 1000 µmol/g
Hg = 2100 µmol/g
> 77%
Recovery of Harmful
invasive gastropod, yet to
be studied
No Cr removal was
measured in the
bicarbonate field brine
[85]
Cd(II)
Adsorption to the precipitated solids
5
[52]
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
the temperature was 60 ◦ C. Alyüz et al. [79] reported more than 98% of
Ni2+ and Zn+ ions removal from an aqueous solution using Dowex HCR
S/S cation exchange resin. The agitation time for reaching equilibrium
was 90 min for nickel and 120 min for zinc. The operation was 99%
efficient when the initial resin dosage was 0.2–0.3 g/100 mL for both
zinc and nickel. The effects of pH on the removal of Cd2+ by
ion-exchange resin Amberjet 1200H from a metal solution revealed that
metal removal was optimum within the pH range 4–7 [80]. Table 4
further lists some of the ion exchange resins with their specific heavy
metal removal efficiencies and their merits and demerits. The drawback
of this method is that it cannot be effective in the case of using
concentrated metal solution due to exchange matrix gets easily fouled by
organic and other solids in the waste [81].
2.6. Electrochemical process
Electrochemical treatment of wastewater deals with an electron
transfer reaction that may be either electroreduction or electrooxidation
[6]. Electrolytic metal recovery has been investigated for a long time
now. In this method as illustrated in Fig. 4, a direct current is passed
through an aqueous solution containing metal ions between the cathode
plates and insoluble anodes. The positively charged metal ions adhere to
the negatively charged cathode [87]. It can be summarized basically as a
cathodic deposition given by the equation:
Mn+ (aq) + neᅳ = M (s)
In the past, the electrochemical reactors for metal recovery, electrocoagulation, electrofloation and electrooxidation were popular [88].
The removal efficiencies of the metals have been reported to be influenced by the type of cell and the electrode used. The hexavalent chromium can be reduced to trivalent chromium and can be
electrochemically precipitated in an alkaline medium [89]. The complete reduction after 5 h at pH of 1 and with 0.25 A current was achieved
and after 8 h of electrolysis at pH 5.5, 98.6% removal of chromium can
be achieved. Cheballah et al. studied the effect of pH, current density,
number, and the types of electrodes used for the reduction of hexavalent
to trivalent chromium by bipolar electrocoagulation, and found that
100% Cr(VI) was reduced to Cr(III) at pH 3, current density of 200 A/m2
and conductivity of 2.6 mS/cm [90]. The energy requirement is much
lower with the use of iron electrode than with Al electrode [90]. Khattab
et al. found that the removal efficiencies were directly proportional to
the current density [91]. Maarof et al. concluded that electrosorption
technology could be an emerging method that offers a cost-effective
solution for wastewater treatment [92]. The 3D electrodes could also
be brought into practice compared to 2D electrodes. Overall, electrochemical processes provide following advantages: (1) metal selectivity,
(2) no additional requirements of chemicals, (3) rapid and
well-controlled operation with high removal efficiency and (4) lower
level of produced sludge. While limitation comprises: (1) pH-sensitive
process, (2) need for replacement of sacrificial electrode and (3)
requirement for high-cost electrodes and its subsequent electrical energy
requirement make this technology of relatively higher cost [93].
2.5. Coagulation and flocculation
Coagulation and flocculation are essential treatment processes of
wastewater disposal and subsequently achieving drinking water. Coagulation is obtained when a chemical reaction occurs after the use of a
coagulant or a chemical in wastewater. In an aqueous solution, colloidal
materials join together to form flocs or small aggregates. Suspended
materials like metals are attracted to these small aggregates or flocs.
Slow mixing of water could help form small flocs which could increase in
size such that they settle down inside the solution. This process is known
as flocculation. Both of these phenomena are illustrated in Fig. 3. Mostly
organic contaminants are removed with these procedures.
Hossain et al. investigated the performance of ferrous sulfate heptahydrate FeSO4⋅7H2O removing biochemical oxygen demand (BOD),
chemical oxygen demand (COD) and total suspended solids (TSS) from
palm oil mill effluent of waste from Titanium Oxide industry [86].
Freundlich isotherm model described the coagulation adsorption
mechanism showing that coagulation adsorption occurred in a multilayer formation with non-uniform distribution of adsorbed particles.
FeSO4⋅7H2O waste could remove about 70% COD, over 80% BOD, and
over 85% TSS in a single-stage coagulation treatment. Although having
the advantage of being a simple and non-metal selective heavy metal
separation process, the method produces huge amount of sludge and
further creates a separation problem by transferring the toxic compounds to solid phase. This method is suitable for the effective removal
of dissolved metals, dyes and suspended solids.
3. Modern technologies for heavy metals removal from
industrial wastewater
Several inadequacies are attributed to each of the conventional
methods such as chemical precipitation and coagulation/flocculation
while being among the simplest and least expensive technology
requiring easily operable equipment, are not quite effective for treating
wastewater with high acid content. Besides conventional techniques
produce large amount of toxic sludge (in solid-phase for coagulation/
flocculation) that needs treatment with chemical stabilization followed
by proper disposal. Although previously studied adsorbents have shown
good retention and almost complete removal of heavy metals, they have
Fig. 3. Schematic diagram showing the coagulation & flocculation methods for
heavy metal removal.
Fig. 4. Electrolytic method for heavy metals recovery.
6
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
however also shown higher mixing rates needing higher energy, lower
removal rates, high limiting capacity and slower reduction. IonExchange, despite being economic and effective, has several disadvantages such as the ion exchange media being fouled by the metals and also
with the oil, grease, clay, organic materials and microbes [94]. This
fouling should be removed with proper cleaning program that increases
the maintenance cost. The recovery of metals can be difficult after the
cleaning. The efficiency of this treatment is also reduced by the presence
of free acids. There is high dependence of Ion floatation technology on
pH and temperature. With an increase in ionic strength, a decrease in the
removal efficiency has been observed [76–78,95]. The surfactant concentration is also affected by the temperature which affects the removal
efficiency. Electrochemical methods with metal selectivity, rapid and
well-controlled operation and high removal efficiency need replacement
of high-cost sacrificial electrode. Hence, to fulfill the need for better
technology to overcome these inadequacies several new methods, adsorbents and technologies have been developed which are described
briefly.
high performance and lower cost. Graphene [96,97], activated carbon
[98], carbon nanotubes [98,99], and Zeolite [100] have been the most
common and commercially studied adsorbents. Modification on shape,
size, physical and chemical nature of these adsorbents have influenced
the removal efficiency. Some of these newly developed adsorbents with
their removal efficiency for the most prevalent heavy metals have been
listed in Table 5.
3.2. Hydrogels
A hydrogel is a three-dimensional (3D) network of hydrophilic
polymers that maintaining the structure by the physical and chemical
linking between the individual chain. The important property of the
hydrogel is that it can swell in water and hold a large amount of water
while maintaining the structure. For a material to be considered a
hydrogel, at least 10% of total weight (or volume) must constitute water.
In most of the cases, hydrogel can return to its initial state as soon as the
stimulus
is
removed
[160].
pH-sensitive
hydrogels,
temperature-sensitive hydrogels, electro-sensitive hydrogels, light sensitive hydrogels are some common types of hydrogels. Along with heavy
metals removal, hydrogels have been used for drug delivery, as scaffolding in tissue engineering, in contact-lenses, pH sensors, biosensors,
biomedical engineering [161].
3.1. Nano-adsorbents
Large numbers of studies have been focussed to develop high efficient nano-adsorbents to remove heavy metals from wastewater with
Table 5
List of adsorbents and their removal parameters for each heavy metal.
Adsorbents
Graphene
Activated carbon
Carbon nanotubes
Rice husk
Zeolite
Heavy metals
Adsorption capacity, mg/g
Conc., mg/L
Optimum pH/ Temp, K
Efficiency (%)
Contact time, min.
References
Pb(II)
Cd(II)
Cr
Cu++
As+++
Co(II)
Ni(II)
Hg++
Zn++
256
136.98
92.65
207.27
42.75
21.28
66.01
280.8
208.33
3.225–0.01
3.225–0.01
52
50
5
2.0–25
10–100
100
10
5/313
6/313
5.5
5/298–313
5/298–313
5.5/298
4–9/318
5/298
7/293
99
98.46
92.65
86.77
80.39
93.8
78.31
98
100
120
120
12
300
900
30
120
120
1440
[101,102]
[101,103,104]
[105,106]
[104,106,107]
[104,107]
[104,108]
[104,109]
[110,111]
[112,113]
Pb(II)
Cd(II)
Cr
Cu++
As(V)
Co(II)
Ni(II)
Hg++
Zn++
Mn++
Fe++
134.22
38.03
21.57
38
14
22.57
67.56
154
31.11
51.23
58.76
40
200
50–100
37
40
100
20
40
200
20
20
6.5/298
6.8/298
5/293
5/300
6.5/303
6.0/303
5.8/293
5/
4.5/298
5.8/293
5.8/293
97.95
95
98.2
97
99
90.3
94
100
95
88
90
50
120
60
75
150
100
120
40
120
120
120
[114,115]
[115,116]
[117–119]
[115,120,121]
[122]
[121,123]
[121,124]
[125]
[115]
[124]
[124,126]
Pb(II)
Cd(II)
Cr
Cu++
Co(II)
Ni(II)
Hg++
102.04
14.09
264.5
8.84
69.63
47.86
35.89
540
26.5
100
0.01
100–1200
10–80
0.55
5/298
6/298
2/298
4/298
7/298
7/NA
6/298
96.03
97.25
95
96
58
93.4
26.5
80
600
240
30
NA
900
600
[127,128]
[129]
[130,131]
[132,133]
[134]
[135,136]
[129,137]
Pb(II)
Cd(II)
Cr
Fe++
Cu++
Ni(II)
4.0
125.94
63.69
0.22
0.10
0.029
20
20–400
50–300
11.78–0.088
5.43–0.099
1.74–0.053
11/298
6.5/298
2/NA
2–10/298–300
2–10/298–300
2–10/298–300
99.6
99.1
71
99.253
98.177
96.954
40
120
120
120–150
120–150
120–150
[138–140]
[141,142]
[143]
[140,144]
[144,145]
[144,146]
Pb(II)
Cd(II)
Cr
Cu++
Co(II)
Ni(II)
Hg++
Zn++
16.81
47
3.27
2.1
5.5–16.8
24.65
16.354
28.6
1–200
2500
2.89
100–400
50–100
20
13.20–575
20
5/293
4/298
3.5/300
5/NA
3–4/298
7.5/298
5–6/NA
7.5/298
92
97.5
96
99
66.10
99
90
99
25
30
240
60
60
60
420
30
[147,148]
[149,150]
[151]
[152–154]
[155,156]
[155,157]
[158,159]
[154,155]
7
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
Cross-linked acrylamide (AM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) homopolymers and copolymers have been prepared by free radical solution polymerization and studied to separate Cd
(II), Cu(II) and Fe(III) from aqueous solutions [162]. The pH of the solution affects significantly on the capability of hydrogel to adsorb metal
ions. Increasing pH also increased the adsorption capacity of AM/ AMPS
hydrogels. The affinity order of metal ions adsorbed by hydrogel is Cd
(II) > Cu(II) > Fe(III), which is suggested to be mainly dependent on
polarity, electronic configuration, ionic radius, etc. and most importantly on nature of interaction with the functional groups of hydrogel
[163,164]. The binding capacities of all metal ions decreased under
competitive conditions; 730–800 mg/g for Cd(II), 650–720 mg/g for Cu
(II), and 610–700 mg/g for Fe(III). The binding capacities for other
metal ions Zn(II), Mn(II), Co(II), Pb(II), and Na(I), are 580–620,
610–680, 500–581, 300–350, and 200–280 mg/g, respectively.
Desorption of the absorbed metal ions from saturated AM/AMPS
hydrogels can be achieved by decreasing the pH of the medium. The
recoveries of Cd(II), Cu(II) and Fe(III) using AM (50 mol%)/ AMPS
(50 mol%) cross linked with 10% of N, N’ -methylenebisacrylamide
(MBA) were reported to be 98.5%, 93% and 94%, respectively [162].
Self assembled TEMPO-oxidized cellulose nanofibers (TOCN)/
cationic guar gum (CGG) hydrogels was developed to remedy wastewater containing oil, heavy metal ions or organic dyes [165]. The
monolithic TOCN/CGG hydrogel could also effectively remove copper
ions (Cu II) and dyes (i.e. thioflavin T and methyl orange), based on an
adsorption mechanism. CMC/PAM composite hydrogel has been found
efficient for the removal of Cu(II), Pb(II) and Cd(II) ions from the
wastewater [166]. The adsorption followed the Langmuir model and
exhibited pseudo-second-order kinetics. Further, in situ reductions of
adsorbed Cu(II) ions lead to a Cu NPs-loaded composite hydrogel,
serving as a heterogeneous catalyst for the reduction of 4-NP. The
cost-effectiveness, excellent adsorption and catalytic properties proved
the hydrogel to be a promising material for the cascaded treatment/reuse of the heavy metal ions [166].
Super adsorbents with ultrahigh adsorption capabilities are highly
beneficial for effective wastewater remediation. A super-adsorbent
hydrogel spheres (SAHSs) are based on self-sacrificing micro-reactors
obtained from the interior cross-linking polymerization of the hydrogel
network and subsequent self-disintegration within only 150 s [167]. The
as-prepared poly(2-acrylamido-2-methyl-propanesulfonic acid-co-acrylic
acid) hydrogel spheres exhibited excellent adsorption capabilities towards organic dyes (4625 ± 231 mg g−1 for methylene blue) and heavy
metallic ions (4312 ± 185 mg g−1 for Pb(II)). In addition, the hydrogel
also showed good recyclability, selective adsorption and smart separation–regeneration. The chitosan/gelatin hydrogel particles prepared by
inverse emulsion from the aqueous solutions of chitosan, gelatin, and
glutaraldehyde showed a maximum removal efficiency of 98% for Hg (II)
ions in a solution [168]. The results were affected by the composition of
hydrogels rather than the pore size or degree of swelling. The removal
efficiencies for Pb (II), Cd (II), Hg (II) and Cr (III) ions reached about
73−94% in a multiple metal ion solution. The result indicated that the CG
hydrogel could be used to remove co-existing heavy metal ions from
wastewater, providing a versatile method to remove multiple metal ions
from natural or industrial wastes [168].
Cr, Cu, Cd, etc. [171]. Debnath et al. produced and tested a synthetic
oxide as hydrated and agglomerated nanocrystallite (11–13 nm) titanium oxide for Ni(II) adsorption from aqueous solution [172]. Film
diffusion seemed to control the reaction rate following a
pseudo-first-order reaction. Maghemite (γ-Fe2O3) nanoparticles were
investigated for selective removal of toxic heavy metals such as Cr(VI),
Cu(II) and Ni(II) from electroplating wastewater [173]. Electrostatic
attraction and ion exchange caused adsorption of Cr and Cu whereas Ni
was adsorbed due to electrostatic attraction of maghemite. Wang et al.
[174] separated lead and chromium from wastewater using magnetite
nanoparticles (Fe3O4 NPs). Fe3O4 able to adsorb 90% Pb ion within
2 min from the solution containing 10 ppm lead (Pb) ion.
3.4. Membrane separation
Membrane separation process is a technique in which feed water is
forced through a semipermeable membrane at high pressure to separate
specific materials from the solution. According to the pore size, this
process can be categorized as microfiltration (MF), ultrafiltration (UF),
nanofiltration (NF), and reverse osmosis (RO). Schematic representation
of membrane separation process is given in Fig. 5. The molecules or ions
are passed through the membrane by diffusion and the rate of diffusion
depends upon the pressure, temperature, permeability of membrane and
the concentration of molecules or ions present in the solution. Membrane separation process mainly be governed by three basic principles,
namely adsorption, sieving and electrostatic phenomenon. Adsorption
mechanism is based on the hydrophobic interactions of the membrane
and the solute. The separation of materials through the membrane depends on membrane pore size and solute size. Based on these principles,
various membrane processes with different separation mechanisms have
been developed [175].
The uses of polymeric membranes are increasing due to their
attractive properties such as flexibility, high mechanical integrity, and
easy fabrication. Water-soluble polymers are commonly involved in
polymer enhanced ultrafiltration processes (PEUF), and allowed rapid
sorption of metallic cations in water. The interactions between metallic
cations and functional acidic groups depend on pH of the solution. The
sorption capacities decrease in the following order: SO3H > P (O)(OH)2
> COOH, whereas selectivity increases in the opposite way: COOH > P
(O)(OH)2 > SO3H [176]. The cost-effective analysis and selectivity of
materials are the most important parameters that need to be analyzed
for use of these materials.
The other efficient solution for the treatment of wastewater is the use
of super adsorbent polymers. Recently, graft copolymers of polyacrylonitrile (PAN) onto Arabic gum (AG) are prepared in aqueous solution using (KMnO4/HNO3) as a redox initiator [177]. Chemical
modification of the graft copolymer has been carried out by reaction
with hydrazine hydrochloride and subsequent hydrolysis in the basic
medium [178]. This polymer could be used to remove Pb2+, Cu2+ and
Cd2+ from the wastewater. The maximum super adsorbent capacities
were found to be 1017, 413 and 396 mg/g for Pb2+, Cu2+ and Cd2+
respectively. Further, treatment with 0.2 M HNO3 resulted in the
3.3. Multifunctional nanomaterials
One of the most rapidly growing sectors in wastewater treatment
technologies is the use of multifunctional nanomaterials that can solve
many problems more efficiently. Magnetic nanoparticles can be easily
and more efficiently recovered after adsorption compared to other
typical adsorbents [169,170]. Zinc Oxide (ZnO), Titanium Dioxide
(TiO2), Cerium Oxide (CeO2) are major metal oxides used as nano adsorbents since they have high surface area, and their affinity can be
improved by using numerous functionalized groups. Zeolites are also
used as an ion-exchange medium for heavy metals removal like Zn, Ni,
Fig. 5.. Membrane separation process for heavy metal removal.
8
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
recovery of 96%, 99% and 99% for Pb2+, Cu2+ and Cd2+ respectively
[178]. A large variety of materials have been observed recently for
fabricating the membranes and the adsorbents to find the best removal
efficiency. The filtration membranes and adsorbents containing aromatic conjugated polymers (ACPs) such as polyaniline and polypyrrole
exhibited superior metal removal efficiency as they were found to
possess high surface areas and charge and site density within their
structures to attract and attach metal ions. The uses of the polymer
membranes on the removal of metal ions from aqueous solution mostly
dependents on the porosity of the membranes [179].
The performance of PVP modified membranes in removal of copper
and iron ions from liquid phase revealed that the membranes with the
lowest content of PVP were found to be most effective while the membranes with the highest content of PVP showed the lowest resistance
[179]. The nanoparticles incorporated nanofibrous membranes allowed
high adsorption and high filtration capacity due to nanofiber’s large
surface area and porosity. The adsorption isotherm and kinetics used in
membrane adsorption for the characterization of parameters are similar
to that used in the adsorption techniques. Wang et al. showed polysulfonebenzylthiourea hollow fiber membrane could be operated at a
high feed flow rate and large-scale removal of Cd(II) and Zn(II) were
realized [180]. Novel chelating membrane, the PVA/poly(ethyleneimine) (PEI) has been prepared and studied for the removal of heavy
metal ions from aqueous solutions. The maximum adsorption capacities
of the membrane are 0.729 mmolg-1 for Pb(II), 0.692 mmolg-1 for Cu
(II) and 0.525 mmolg-1 for Cd(II). Gao et al. examined the adsorption
property of grafted polymer of polysulfone and sodium p-styrenesulfonate membrane for three heavy metal ions, Pb(II), Zn(II), and Hg(II) ions
[182]. Results showed that adsorption capacities of all metal ions are
dependent on the pH values. The adsorption capacity first increases and
then decreases with the increase of pH, and there is a maximum at pH 5
for Pb2+ and at pH 5.5 for Zn2+ while adsorption is maximum at pH 2 for
Hg2+ (Fig. 6).
Removal of heavy metals using Polymer Inclusion Membranes (PIMs)
has gained significant interest. The selectivity and the permeability of
the PIMs can be enhanced by choosing the right components in PIMs
which are base polymer, extractant and plasticizer [184]. PVC and CTA
are the popular base polymers, and Aliquat 336 & D2EHPA are the
popular extractant. The dual system of PIM could successfully transport
two metal ions into their stripping compartment by leaving the third
metal ion in the feed phase [184].
Nanofiltration (NF) is an advanced membrane separation technique
that has the size of the pores smaller than microfiltration and ultrafiltration but larger than reverse osmosis. This method is effective for the
removal of higher metal concentrations up to 12,000 ppm. NF membranes are capable of treating wastewater containing more than one
heavy metal with a removal efficiency of more than 80%. The membrane
filtration with the combination of floatation showed a high rejection rate
and it can be used for any type of water for the treatment of inorganic
effluent in the water with a metal concentration of more than
10,000 ppm [185]. Wei et al. prepared the NF hollow fiber membranes
for the treatment of wastewater under different operating conditions. At
0.4 MPa and pH 2.31, the membrane showed good stability and the
removal rate for Cr, Cu and Ni were 95.76%, 95.33% and 94.99%
respectively [186]. Basaran et al. studied the ability of two commercially
available membranes for the removal of Ni(II) and Cr(VI) from metal
plating wastewater in the aviation industry [187]. The effects of the
operating pressure (10–30 bar) and the feed pH (3.5–10) on the performance of the membranes revealed that the Ni(II) rejection increased
more than the Cr(VI) rejection, as the pH value changed from 3.5 to 10.
The membrane rejection values for both metals were over 95% using a
cross-flow filtration system. The polyethersulfone (PES) nanofiltration
membrane with 0.5 wt% MMGO nanoparticles had the highest copper
removal of about 92% [188].
Chung et al. developed a novel dual-layer polybenzimidazole/polyethersulfone NF hollow fiber membrane to remove the heavy metal ions
[189]. The simultaneous co-extrusion of PBI and PES polymers not only
kept the high rejection property of the PBI material but also significantly
reduced the transport resistance across the membrane as well as the
material cost for the membrane manufacture. As-developed membrane
showed superior rejection efficiency to metal ions. For example, the
membrane had a rejection efficiency of 95% and 93% for Cd(II) and Pb
(II), respectively at pH 12 While the rejection to Cr2O27 can reach 98%.
Otero et al. made a thin film composite (TFC) on a porous polysulfonate
substrate for the removal of chromium in an industrial pilot plant [190].
For a feed concentration from 1 to 120 ppm, the result achieved was
above 95% for chromium. Composite NF membrane comprising a
molecularly designed pentablock copolymer selective layer is a useful
platform for the development of new NF for the removal of heavy metal
ions [191]. The membrane showed an effective removal efficiency
greater than 98% for Pb(II), Cd(II), Zn(II) and Cu(II). The high membrane rejection of these cations is ascribed to the small mean effective
pore diameter of the composite membrane (0.5 nm), which is significantly smaller than the hydrated diameters of these cations. Liu et al.
prepared a positively charged nanofiltration membrane by phase
inversion method for the removal of heavy metals from pollutant water.
The obtained membrane fabricated via ion complexation induced phase
inversion process had high water permeability and possessed high
Fig. 6.. Adsorption isotherms of Pb2+ on PSF-g-PSSS membrane at different pH values (A) and effect of pH value on adsorption capacities of three metal ions on PSFg-PSSS membrane Temperature: 25 ◦ C (B) [183].
9
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
R. Shrestha et al.
rejection towards divalent cations such as Mg2+, Ca2+ and Pb2+ that can
be used for the removal of heavy metals from wastewater. At the optimal
condition, the pure water reflux from the membrane was 24.3 Lm-2h-1
with MgCl2 rejection of 92.2% [192].
The building block of tunable quaternary ammonium groups was
used to prepare quaternized polyelectrolyte complex membranes
(QPECMs) for the nanofiltration process via surface coating and
glutaraldehyde crosslinking method [193]. As prepared membrane had
efficient and anti-scaling properties with a low decline ratio of 11.5%
and a high recovery ratio of 96.5%. The design strategy of this membrane presents facile adaptability to other polyelectrolyte complex
membranes for specific water treatments and environmental remediation. A positively charged nanofiltration membrane had been developed
using 2-chloro-1-methyliodopyridine as an active agent onto the
membrane surface via covalent bonding with surface carboxylic groups
[194]. The prepared membrane had a highly selective separation for
divalent and monovalent salts and also exhibited high removal efficiency for toxic metal ions and dyes. PES/B-Cur membranes were
fabricated by incorporating boehmite nanoparticles functionalized with
curcumin (B-Cur) into PES membrane via phase inversion method
[195]. The PES/B-Cur exhibited higher pure water reflux of
120–140 kg/m2h than PES membrane due to an increase in pore size,
and porosity.
Surface functionalized ZnO nanoparticles were incorporated to prepare polyether-imide-based nanofiltration membranes using phase
inversion method [196]. The prepared PEI/PZ membranes showed superior separation performance and anti-fouling properties compared
with neat PEI and PEI/ZnO membranes. Zhu et al. prepared thin-film
Table 6
Summary of removal of heavy metals from wastewater by membrane technologies.
Membranes
Metals
Sources
Removal efficiencies
References
Spiral bound RO membrane
Ni(II), Pb(II), Cu(II)
Industrial wastewater
[198]
Functional polymers
Metallic cations
Wastewater
Super adsorbent polymer
Pb(II), Cu(II), Cd(II)
Industrial wastewater
Polyethersulfone membrane with pore generating agent (PVP)
Polysolfonebenzylthiourea reactive hollow fiber ultrafiltration
membrane
PVA/poly(ethyleneimine) (PEI) membrane
Cu, Fe
Cd(II), Zn(II)
Aqueous solutions
Aqueous solutions
Ni(II) = 98.5%
Pb(II) = 97.5%
Cu(II) = 96%
Sorption capacities: SO3H > P
(O)(OH)2 > COOH
Pb(II) = 1017 mg/g
Cu(II) = 413 mg/g
Cd(II) = 396 mg/g
High removal efficiency
High rejection rate
Hg(II), Pb(II), Cd
(II), Cu(II)
Aqueous solutions
Grafted polymer of polysulfone and sodium p-styrenesulfonate
membrane
Hollow fiber membrane with polyamide film on the inner side
Pb(II), Zn(II), Hg(II)
Aqueous solutions
Cr, Cu, Ni
Electroplating wastewater
DL and DK membrane
Cu, Zn, Cd
Industrial waste water from
Tunisian wiring industry
NF90 and NF270 membranes
Ni(II), Cr(VI)
Aviation industry metal plating
wastewater
Magnetic grapheme based composite embedded polyethersulfone
(PES) polymer
Dual layer nanofiltration hollow fiber membrane
Cu
Aqueous solutions from Cu
(NO3)2
Model wastewater
Aromatic polyamide NF membrane
Cr(VI)
Pentablock copolymer
Pb(II), Cd(II), Zn(II),
Cu(II)
Divalent cations
Pb(II) = 0.729 mmolg-1
Cd(II) = 0.525 mmolg-1
Cu(II) = 0.692 mmolg-1
95%
[199]
[178]
[179]
[180]
[182]
Cr = 95.76%
Cu = 95.33%
Ni = 94.99%
For DL membrane;
Cu = > 84%
Zn = < 93%
Cd = 69–86%
For DK membrane;
Cu = 94–97%
Zn = 80–92%
Cd = 81–85%
For NF90 membrane;Ni(II) =
99.2%
Cr(VI) = 96.5%
For NF270 membrane;Ni(II) =
98.7%
Cr(VI) = 95.7%
92%
[200]
Mg(II) = 98%
Cd(II) = 95%
Pb(II) = 93%
> 95%
[189]
> 98%
[203]
Pollutant water
MgCl2 rejection of 92.2%
[192]
Metal cations
Cu, Ni, Cr
Wastewater
Aqueous solution containing
metal ions and dyes
[193]
[204]
PES/B-Cur membranes
Fe2+ Cu2+, Pb2+,
Mn2+, Ni3+, Zn2+
Aqueous solution containing
metal ions
Polyether-imide based nanofiltration membranes
Pb2+, Cu2+
Aqueous solution containing
metal ions
Rejection ratio of 96.5%
Cu = 96%
Ni = 95.8%
Cr = 98%
Fe2+ = 99.88%
Cu2+ = 98.72%
Pb2+ = 99.61%
Mn2+ = 99.31%
Ni3+ = 99.11%
Zn2+ = 99.51%
Pb(NO3)2 = 59%
Cu(NO3)2 = 67%
Positively charged nanofiltration membrane prepare from the mixture
of polyacids and polybases by phase inversion method
Quaternized polyelectrolyte complex membranes
Nanofiltration membrane
Mg(II), Cd(II), Pb(II)
10
Samples of varying
concentration were prepared
Aqueous solutions
[201]
[202]
[188]
[190]
[195]
[196]
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
composite (TFC) hollow fiber nanofiltration (NF) membranes by grafting poly (amidoamine) dendrimer (PAMAM) on the interfacially polymerized layer of polyethersulfone (PES) membranes [197]. The
membrane showed a rejection efficiency of over 99% for Pb(II), Cu(II),
Zn(II), Ni(II) and Cd(II). On changing the pH of a solution, the rejection
efficiency for As(V) reached about 97%. The water permeability gets
increased with the presence of –NH2 groups on PAMAM that makes the
surface of the TFC membrane more positively charged in the acidic
medium (Tables 6 and 7).
that can be applied for the removal of aqueous ions through electrically
driven ion-exchange membranes [216]. The method further has the
advantage of producing effluent water that can be reused besides
removing even the lower concentrations of metal ions from the effluents.
A laboratory-scale ED system is used in the treatment of synthetic
effluent based on industrial nickel electroplating process for concentrating and extracting Nickel (Ni) and its salts and also can be used for
the removal of organic compounds from the additives [217]. Simultaneous recovery of heavy metals and phosphorus has been achieved using
3-compartmental electrodialysis method at low pH with the electrodes
separated by ion-exchange membranes from the sample [218]. The
heavy metals extraction pattern, at 3.7 pH, followed Zn(85%) > Ni
(56%) > Cd(31%) > Cu(22%) > Cr(6%) > Pb(1%) in the order of most
to least released or extracted. Due to rise in pH caused by the
half-reactions at cathode, followed by the precipitation, 2-compartmental ED cell setup was less effective in extraction [218]. Semerci
et al. conducted a two-part experiment with the first part containing
bioleaching process for solubilization of phosphorus at different conditions and the second part of electrodialysis separating heavy metal ions
from phosphorus ions. The pH of the Sewage Sludge Ash (SSA) was 11.6
and conductivity of 2.04 mS cm-1 and electrodialysis experiments with
copper electrodes coated with gold resulted in 24.6% phosphorus passage to the anode chamber and showed Zn, Cu and Ni separation efficiency of 64.2%, 100% and 68.6% respectively [219]. Kirkelund et al.
subjected leachable heavy metals and salts which pose most problems in
Municipal Solid Waste Incineration (MSWI) to electrodialytic remediation and reported a reduction in Cd, Cu, Pb and Zn leaching with a pH
below 8 [220]. Santos et al. used ED process as a sequential treatment for
removal of Cr(VI) from an effluent that was already subjected to an
anaerobic biological treatment in a hybrid anaerobic bioreactor and was
able to effectively remove more than 99% of Cr(VI) in 75 min operation
time [221]. They studied the influence of electrical current as an
adjustable operational factor on the performance of ED systems and the
quality of treated wastewater.
A bipolar membrane electrodialysis (BMED) system has been
developed for the treatment of raffinate generated during copper ore
hydrometallurgical processing for their reuse as leaching influent and
reported removal rates of heavy metals as 99.3% (iron), 99.1% (zinc),
99.0% (copper), 84.9% (nickel), 70.6% (chromium), 95.8% (cadmium),
and 94.8% (arsenic) during an optimal 40 h operating time. The study
3.5. Biosorption
In recent years, biosorption is considered to be an efficient and ecofriendly alternative technology for the removal of heavy metals from
wastewater effluent generated from different industries. Biosorption is a
physiochemical process that involves binding of metal ions to the surface of a biosorbent. The potential metal sorbents could be algae, fungi,
bacteria, yeasts, agricultural and industrial wastes and biopolymers
[205,206]. Biosorption is considered to be a significant technology not
just only for the removal of even lower concentrations of heavy metals
but also their recovery. Sarrafzadeh et al. [46] developed a tool to
measure the efficiency of nutrient removal, carbon captor and metabolite generation using the different ratios of Chlorella vulgaris and
nitrifier-enriched-activated-sludge (NAS). The influent was based on the
municipal wastewater and heavy metals adsorption were not considered. Kanamarlapudi et al. have inferred simple operation, no additional
nutrients requirement, lower amount of sludge generation, lower operational cost, higher efficiency and regeneration of biosorbents as prime
advantages of this process [207]. The percentage of biosorption (RE)
also termed as biosorption efficiency for the metal can be evaluated as
RE =
Ci − Ce
× 100%
Ci
where, Ci is the initial concentration of metal ions in the solution (mg/L)
and Ce is the equilibrium concentration of metal ions in the solution
(mg/L).
3.6. Electrodialysis
Electrodialysis (ED) is among the most economic recent technology
Table 7
Some of the biosorbents with their performance parameters and inadequacies.
Psidium guvajava L. leaf
powder
Rice bran and pine sawdust
Bellamya bengalensis (snail
made up of CaCO3)
Chlorella
vulgaris and Scenedesmus
acutus
Metals
Efficiencies
Cd(II)
93.2%
Cd(II), Pb
(II)
For rice bran, Cd(II) =
98.25%, Pb(II) =
99.25%For pine
sawdust,
Cd(II) = 90.21%, Pb
(II) = 95.25%
30.33 mg/g
Cd
Cr
For C. vulgaris 88.2%
and for S. acutus 87.1%
Lactobacillus acidophilus
Dicerocaryum eriocarpum
plant modified with sodium
and potassium chlorides
As(III)
Zn(II), Cd
(II), Ni(II),
Cr(III), Fe
(II)
Klebsiella sp. 3S1
Floating (Azolla filiculoides)
and submerged (Hydrilla
vertcillata) macrophytes
PEI-modified sericin beads
Zn(II)
Cu(II), Cr
(VI), As(III),
Pb(II)
Cr(Vi)
60%
Zn(II) = 99.94%
Ni(II) = 99.87%
Fe(II) = 95.54%
Cr(III) = 96.21%
Cd(II) = 93.7%
48.4 mg/g
Pb(II)>Cu(II)>As(III)>
Cr(VI)
Merits
Demerits
References
[208]
Effective biosorbent for the
electroplating industry and also
potential removal for Zn, Ca and
Mg
Comparatively effective for Pb
[209]
Efficient, low cost, environment
friendly, sustainable adsorbent
Mostly influenced by pH than biomass dose and
time
Reduction in efficiency can be seen due to the
interaction between functional groups of the
microalgae and other compounds present in water
that are competitor of chromium
[210]
Highly efficient in the removal of
metal ions than unmodified
mucilage
Potential use of aquatic
macrophytes for biosorption
capability
11
Pb(II) and Cu(II) strongly inhibited Cu(II) removal
and Cr(VI) inhibits Pb(II) removal
Reduction of Cr(Vi) to Cr(III)
[211]
[212]
[213]
[214]
[215]
[206]
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
degradation and best removal efficiency at pH 6 for both Ni2+ and Fe3+
metal ions.
Recently, clay-TiO2 composite has been developed from Degussa P25
(80% anatase and 20% rutile) and clay, for advanced treatment of
wastewaters by single-step combination of photocatalysis and adsorption [233]. The adsorption capacities and photocatalytic properties of
as-synthesized composites on pollutant matrix containing Methylene
Blue (MB) as dye and cadmium ion (Cd2+) as a heavy metal resulted that
the composites could be efficiently used for simultaneous removal of MB
and Cd2+ more simply and cost-effectively for industrial wastewater
treatment processes [233]. TiO2/C3N4 heterojunctions on carbon-fiber
cloth are developed by in situ growth of TiO2 particles via
dip-coating/hydrothermal method on carbon-fiber (CF) cloth substrate
followed by incorporation of C3N4 nanosheets on their surface by thermal polymerization. The prepared CF/TiO2/C3N4 cloth exhibited
improved photocurrent and photocatalytic activity for various organic
pollutants degradation and on reducing Cr(VI) heavy metal ions [234].
Multifunctional silver nanoparticles incorporated chitosan thin films
has been studied photodegradation, heavy metals removal and its
antibacterial activity [235]. The photodegradation rate of organic pollutants under sunlight irradiation with and without oxygen was found to
be increased with raising catalyst concentrations using incorporated
polymer thin films. As-synthesized composites also showed antimicrobial activity against gram-negative (Escherichia coli) and gram-positive
(G. bacillus). CuCo2S4 modified Z-scheme MoSe2/BiVO4 hybrid composites have been synthesized via a facile hydrothermal method for the
removal of heavy metals efficiently under visible light [236]. Incorporation of CuCo2S4 inhibited the electrons/holes pair recombination
during photocatalysis. The highest photocatalytic removal was attained
at pH of 9, 0.5 mg catalyst dosage and with 210 min of irradiation time
for almost all metals. They attributed the excellent removal efficiency of
photocatalyst to the wide adsorption of visible light region, high surface
area and suitable content of CuCo2S4 [236].
Carbon nitride (CN) nanorods are synthesized by solvothermal route
followed by growing Ag/AgCl nanoparticles on CN surface by a facile coprecipitation method [237]. The Ag/AgCl-decorated CN heterojunctions
with diverse molar ratios (0.3,0.5,0.7) of Ag/CN precursor exhibited
wide adsorption from UV to visible-light region. With 120 min of
visible-light illumination, Ag/AgCl-CN nanorods degraded 98.5%
rhodamine B (RhB), 75.4% tetracycline (TC) and adsorbed 39.5% Cr(VI)
ions better than Ag/AgCl, CN and Ag-CN [237]. Wang et al. synthesized
mesoporous zeolite Beta (mBeta) via one-step hydrothermal crystalline
process investigating the removal efficiency of Cd2+ and Zn2+ from the
wastewater. The mBeta adsorbed heavy metal ions and sulfurizing to
obtain metal sulfide loaded mBeta [238]. In the coexisting solution of
100 ppm Cd2+ and 100 ppm Zn2+, the adsorption capacities reached 30
and 16 mg/g respectively, and the hydrogen evolution rate of
5.52 mmol g-1 was achieved on the sample CdS/ZnS-mBeta in 4 h under
visible light irradiation [238].
An “adsorbent-to-photocatalyst” strategy has been proposed by Chen
et al. connecting heavy metals removal from wastewater to photocatalytic reduction of CO2. In this strategy, heavy metals were collected
by adsorbents are converted to photocatalysts without any secondary
treatment [239]. They prepared calcium silicate hydrate (CSH) nanosheets by rate-controlled precipitation at ambient temperature and
studied the Cu2+, Zn2+, Ni2+ and Pb2+ ions. Additionally, they have
evaluated the photocatalytic activity of CO2 reduction of CSH-M that
were recycled and reported that CSH-Pb was inert to CO2 reduction and
very less gas generated for CSH-Cu and CSH-Zn whereas CSH-Ni showed
superior photocatalytic activity with CO2 adsorbed being 31.2 cm3 g-1
than 29.1 cm3 g-1 and 28.5 cm3 g-1, for CSH-Zn and CSH-Cu, respectively
[239]. Fenton process is another recommended advanced oxidation
method that involves the oxidation of organic substrates by iron and
hydrogen peroxide. The significance of this effective process is it produces non-toxic compounds, can be used in various scales, reduces the
amount of sludge, increases the biodegradability of biological sludge,
goal-focused on the removal of heavy metals and also H2SO4 [222].
3.7. Photocatalysis
One of the most innovative solutions for rapid and efficient
destruction of environmental pollutants is with photocatalysis, a method
in which a semiconductor-electrolyte interface is illuminated with light
energy that exceeds the semiconductor bandgap resulting in a formation
of electron-hole (e-/h+) pairs in both conduction and valence band of the
semiconductors [223–227]. These migrating charge carriers can reduce
or oxidize species with suitable redox potential in solution. TiO2, ZnO,
CeO2, SnO2, CdS, ZnS are some of the commonly studied
semiconductors.
The photocatalysis mechanism over semiconductor particles follows
a generation of electron-hole pairs with a subsequent need of a trap for
these generated electron-hole pairs to avoid their recombination and the
process is schematically represented in Fig. 7. The Hydroxyl ions (OH-)
act as a trap for holes which leads to strong oxidizing capable hydroxyl
radical’s formation while the adsorbed oxygen species act as a trap for
electrons leading to the formation of unstable and reactive superoxides
which may finally evolve through several ways [228].
+
TiO2 + hv = TiO2 + e−
CB + hVB
+
⋅
TiO2(substrate) − OH−
s + h = TiO2(substrate) − OHads
−(ads)
O2(ads) +e− = O2
Solar irradiated photocatalytic reductions of Cr(VI), Ni(II), Zn(II)
and Cu(II) at different TiO2 concentrations [229]. Minimal reduction of
Cr(VI) was reported in the absence of citric acid but the acidic pH
favored rapid and higher reduction of Cr(VI). However, Ni(II), Zn(II) and
Cu(II) showed their reduction rates independent of citric acid concentrations supporting thermodynamic feasibility and water acted as an
equally efficient hole scavenger for these metals. All of Cr(VI) was
reduced at a pH level of 2 in a 1.5 h minimum period and the period
increased on increasing pH values. Adsorption and removal of Ni(II), Zn
(II) and Cu(II) however were significant only in the alkaline pH range
with maximum Ni(II) removal of 60.8% at 7 pH and Zn(II) removal of
70.6% at 8 pH and Cu(II) removal of 31%. On varying TiO2 concentrations for Cr(VI) reduction at its optimum removal pH of 2, all of the Cr
(VI) was reduced with effect being increased reduction time with
decreasing TiO2 concentrations [229].
Wahaab et al. compared three commercial TiO2 specimens namely
TiO2 Degussa P25 (80% anatase, 20% rutile), TiO2 (100% anatase) and
TiO2 (100% rutile) suspended in a constant amount of 0.25 g/l of cyanide solution with a medium pressure Hg lamp as UV-radiation source
[230]. The results indicated TiO2 Degussa P25 gave the best photocatalytic activity in the removal of cyanides with 90% removal at
60 min. The removal was further enhanced by the addition of hydrogen
peroxide along with a decrease in the complete removal time [230]. In
another study Rahimi et al. showed the maximum removal of 99.8% and
99.2% for Cd2+ and Pb2+ respectively, when the TiO2 dosage was 0.9 g/l
and at a pH of 11 [231]. Zhao et al. synthesized nanoparticles assembled
SnO2 nanosheets and studied the organic dyes degradation ability and
heavy metals such as Fe3+ and Ni2+ removal ability [232]. The SnO2
nanosheets showed an excellent catalytic property for organic dye
Fig. 7. Photocatalytic method for heavy metal removal.
12
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
R. Shrestha et al.
and can lead to a decrease in volatile solids. Moreover uses of this
process synergistically with other remediation processes such as photocatalysis can enhance the adsorption for the heavy metal ions and
degradation of the organic pollutants [240].
researchers. Due to the excessive pressure drop, the nanoparticles
cannot be used in a packed column. A technique should be developed to
incorporate nanoparticles in a nanofibrous membrane and also for the
regeneration of heavy metals and these nanoparticles.
Different techniques are being employed for the better and efficient
performance of hydrogels for intensive use in industrial wastewater
treatment. The shift in the preparation techniques from pure synthetic
chemical polymers to bio-based and ultra-high adsorption polymers
have seen significant improvement and utilization. Smart hydrogels
have the capacity of being regenerated to their initial state as soon as the
external stimulus is removed. Hydrogels were previously prepared from
(AM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) homopolymers and co-polymers by free radical solution polymerization
using N, N’-methylenebisacrylamide (MBP) as the cross-linker and many
other similar synthetic chemicals. Nowadays, hydrogels are being prepared by inverse emulsion from the aqueous solutions of chitosan,
gelatin, and glutaraldehyde, making them more environmentally
friendly. A super-adsorbent hydrogel sphere (SAHSs) based on selfsacrificing micro-reactors, which achieve the interior cross-linking
polymerization of the hydrogel network and subsequent selfdisintegration within only 150 s was developed for ultra-high adsorption capabilities. A new bio-based bilateral hydrogel containing carboxymethyl cellulose (CMC) and polyacrylamide (PAM) can be
considered for wastewater remediation in the future.
Recent researches have been focused to develop cost-effective strategies. However, the overall cost can be varied upon the technologies
based on the sources of wastewater and scale of remediation. Hence, it is
a difficult task to compare the specific cost of remediation for different
techniques. This suggests the future works should be considered the cost
of operation while remediation that would make more useful for realworld applications.
4. Advantages/disadvantages of modern technologies
In recent years, physiochemical treatments offer various advantages
such as low cost, ease of operation, economic feasibility and flexibility in
modification of a chemical plant wherever necessary. These benefits
exceed the number of drawbacks such as high operational cost, high
sludge production, high energy consumption and also the metal selectivity encountered in the conventional methods. However, if the cost for
the disposal of these toxic sludge is reduced by any means, the physiochemical treatment becomes the most suitable treatment method for
the wastewater with inorganic effluents. Studies on biological methods
have also sought the interest of many researchers and companies.
Various biological methods such as biosorption and activated sludge
process have been used at present. Simple operation, low amount of
sludge production, high metal removal efficiencies and regeneration of
the biosorbents are the advantages of using biological methods. However, the advantages are followed by some drawbacks such as high pH
and temperature dependence, high energy requirement and maintenance requirement. The technology targets to have high metal removal
efficiency as well as the recovery of metals. The most promising and
innovative technologies to treat the effluents at present have been the
photocatalysis and electrodialysis. In photocatalysis, the photons serve
as an electron rays from organic substrate to the metal ions whereas in
electrodialysis, the ions are transported through a semi-permeable
membrane under the influence of an electric potential. Table 8 summarizes the main advantages and disadvantages of the recent technologies for the removal of heavy metals from industrial effluents presented
in this study.
6. Conclusions
5. Future perspective of the technologies
Over the past few decades, with more stringent environmental regulations, there has been a need for an improved quality of treated industrial effluent. Technologies like adsorption, chemical precipitation,
ion-flotation, ion-exchange, electrodialysis, membrane filtration, biosorption, coagulation/flocculation and photocatalysis have been used to
treat the effluent containing heavy metals. Ion-exchange is recognized to
be the most frequently used technique owing to its higher efficiency at
even higher metal concentrations. Meanwhile, adsorption by low-cost
adsorbents and biosorption is identified to be efficient and costeffective even applicable for lower concentrations of heavy metals.
Furthermore, few technologies are also emerging as promising techniques for industrial heavy metals containing effluent treatment.
Several polymeric materials and variety of nanomaterials have been
developed to enhance the removal efficiency of the heavy metals. Factors including pH, temperature and pressure affect the cost and the
removal efficiencies of the membrane technology. The parameters
should be adjusted to their appropriate values so the cost could be
lowered and the efficiency could be enhanced. Different heavy metal
ions can be removed from wastewater by the use of polymer-based adsorbents. The selectivity and the cost analysis can be compared for
different functional polymers for their role in the removal of heavy
metals. The use of nanofiltration has grabbed the attention of many
Table 8
Evaluation of recently developed technologies for heavy metals removal.
Heavy metals removal
technologies
Advantages
Disadvantages
References
Adsorption with recent
adsorbents
Hydrogels
Low cost, high metal binding capacities, wide pH range, easy operability,
flexibility and design simplicity
Easy operability, low cost, more effective, biodegradable, reusable and
recyclable
Suitable for sulfate salts and hardness ions such as Cu(II) and Cd(II), lowpressure requirement
Suitable for removal of organic and inorganic wastes, small space
requirement, high separation selectivity, low pressure
Requirement of adsorbent regeneration, low selectivity,
excess waste production
Highly dependent on pH, temperature, Metal conc. and
nature of the material used
Costly, prone to membrane fouling
[41,241]
High operational cost, high energy consumption due to
membrane fouling, high concentration of sludge
production
Highly dependent on pH, temperature, type of bioadsorbent used, reactive site and agitation speed
[186,188,
192,222]
Membrane fouling and high energy consumption,
formation of large particles, high sludge production
Significant amount of O2 requirement, long duration
time, limited application
[222,246]
Nanoparticles and
nanotechnology
Membrane separation
Biosorption
Electrodialysis
Photocatalysis
Simple operation, no additional nutrients requirement, low quantity
sludge, low operational cost, high efficiency, bio-sorbent regeneration and
low COD
High separation selectivity, rapid process and effectiveness for certain
metal ions, economic
No sludge production, effective at lab scale, simultaneous removal of
metals as well as organic pollutants, less harmful by-product
13
[166,167,
242]
[243–245]
[41,206,207]
[247–249]
R. Shrestha et al.
Journal of Environmental Chemical Engineering xxx (xxxx) xxx
Membrane filtration is widely studied and is recognized to have a high
efficiency but there is a cost factor affecting its wider use. Photocatalysis
has recently emerged as a promising innovative technique for clean and
efficient treatment. Biosorption has also been given high importance
keeping the environment-friendly nature of the biosorbents in mind.
Although many techniques have been developed and used, most of
the techniques depend on the critical factors like pH, initial concentration of heavy metals in the wastewater, temperature of operation and
others. Environmental impact, overall treatment performance,
compared to other technologies, as well as economic parameters such as
capital investment and operational cost should be considered before its
application. This review serves as a resource for further studies on the
treatment of industrial wastewater for heavy metals removal.
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgment
This work did not receive any specific grant from any funding
agencies in the public, commercial or non-profit sectors.
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