Review on the treatment of electroplating industry wastewater by electrochemical methods

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Review on the treatment of electroplating industry wastewater by electrochemical methods

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Review on the treatment of electroplating industry wastewater by electrochemical methods

Sonal Rajoria, Manish Vashishtha, Vikas K. Sangal

^⇑

Department of Chemical Engineering, Malaviya National Institute of Technology, Jaipur 302017, India

a r t i c l e i n f o

Article history:

Received 10 December 2020 Received in revised form 6 April 2021 Accepted 12 April 2021

Available online 4 May 2021

Keywords:

AOP

Electrochemical methods Electroplating industry effluents Heavy metals

Wastewater treatment

a b s t r a c t

Fate and health exposure linked with pollutants in water are one of the major environmental protests.

Electroplating wastewater involves toxic pollutants, which are harmful to living organisms. During the last two decades, advanced oxidation processes (AOP’s) for effectively decontaminate water have drawn great attention. Among various AOP’s, electrochemical technologies appear to be the most promising methods for removing organic pollutants from industrial wastewater. The main objective of this review is to study the feasibility of electrochemical methods for the treatment of electroplating effluents and also found that the rate of toxic pollutants removal was significantly influenced by different anode materials.

Selection and peer-review under responsibility of the scientific committee of the Sustainable Technolo- gies in Water Treatment and Desalination; ‘‘Materials Science’’.

1. Introduction

In the upcoming years, global trouble may begin for water, food, or/and energy; however, all three are linked and water is needed for all social and commercial progress among the society. Today, drinking water scarcity is arising due to the discharge of polluted or untreated industrial wastewater into the environment [1]. A huge amount of pollutant wastewater with heavy metals and other persistent toxic substances is discharged at the time of electroplating operation, resulting in the electroplating industry is one of the most risky among the chemical-intensive industries worldwide [2,3]. An electroplating process can be applied through the involve- ment of thin metal over another metal through electro-deposition.

The use of different chemicals and metal salt generates pollution problems. About 2–20% of the chemicals used along with the valu- able metals and cyanide are lost in wastewater. Most of the hazardous metal ions such as copper (Cu), nickel (Ni), Chromium (Cr), and lead (Pb) are coming from electroplating industry wastewater, which can be easily consumed by living organisms.

Once these hazardous substances enter our human body, they may cause severe health disorders. Other toxic effluents from the electroplating industry and the Maximum Contaminant Level (MCL) standards, for those heavy metal ions, approved by USEPA,

WHO, ISI are listed in (Table 1). To prevent the harmful effects generated by the existence of these chemicals, many conventional methods directly being applied for electroplating wastewater treatment such as chemical precipitation, ion-exchange[4], membrane filtration [5], adsorption [6]. But due to high capital and operational cost, sludge production/disposal problems, these conventional methods are ineffective to use. After the treatment of wastewater, the recovery of most of the chemicals is also problematic that important to be addressed conventional treatment techniques are noted to be ineffective, as some pollutants found in water are refractory to some degree.

Since industries are searching for a better treatment method that can handle the electroplating effluent efficiently without producing any secondary effluents. The electrochemical methods are more active and efficient than other conventional technologies.

This process does not require additional consumption of chemicals and only electrons are combined to the processes to stimulate reactions. The comparison of conventional methods and electrochemical methods is shown in (Table 2). Many researchers have initiated working towards thorough degradation of different pollutants from the environment by growing and accepting the new techniques for the destruction of toxic pollutants. Among the various techniques proposed for the problematic organic pollutants’

deterioration, electrochemical processes are more attractive for the thorough degradation of a major variety of pollutants.

Selection and peer-review under responsibility of the scientific committee of the Sustainable Technologies in Water Treatment and Desalination; ‘‘Materials Science’’.

⇑Corresponding author.

E-mail address:[email protected](V.K. Sangal).

Contents lists available atScienceDirect

Materials Today: Proceedings

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t p r

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In this review, an overview of the different AOPs, based on the electrochemical process (i.e electrochemical oxidation, electro- Fenton process, and photo electrocatalysis process) for pollutant degradation is described. Furthermore, the comparison of conventional and electrochemical methods are listed in (Table 2). This review also provides major areas for future development and directions of research.

2. Advanced oxidation processes (AOP’s): Basic principles and classifications

Advanced oxidation processes are appearing as alternative processes for effective degradation of metal-contaminated wastewater with high chemical stability. AOP’s produce actively oxidizing agents such as hydroxyl radical (OH) for complete decaying of organic pollutants into non-harmful products like water and CO2

or other inorganic salts. AOP’s can be sub-categorized into three

different categories which depend upon the different criteria as shown in (Table 3).

3. Electrochemical processes

The electrochemical process is the most effective technique for the degradation of different variety of pollutants. This technique offers several advantages over conventional techniques such as total mineralization, require few or no chemical reagents, prevent- ing the formation of new toxic species, energy costs have as low as possible. And the disadvantages are initial investment is high and sometimes electrode fouling may occur. Furthermore, a summary of the advantages and disadvantages of the electrochemical processes for degradation of the electroplating industry effluents are also listed in (Table 4).

The condition of the electrochemical processes is moderate and the effectiveness depends on the electrolytes in solution, pH, the applied potential, and electrodes[13,14]and this process can be used with other technologies. electrochemical methods are one of the type of AOP’s. Among electrochemical oxidation, electro- Fenton and photo electrocatalysis process (i.e. photochemical process) are classified under the category of electro-chemical AOPs, which have drawn more and more attention.

3.1. Electrochemical oxidation process

Electrochemical oxidation is considered to be most the remark- able appliance for the degradation of toxic pollutants for industrial effluents. The mechanism involves the electron transfer reaction that made electro-oxidation phenomenon of the organic effluent Table 1

Effluents from the electroplating industry (milligrams per liter, except for pH) and MCL standards for the toxic heavy metals and their toxicities[7,8].

Parameter Toxicities Maximum Value Maximum effluent discharge standards (mg/

L)

USEPA^a WHO^b ISI^c

TSS Fever, sunburn-like rash 25 —— —— ——

Oil and grease Increased cancer risk, reproductive damage 10 —— —— ——

Arsenic (As) Vascular disease, visceral cancers 0.1 0.050 0.05 0.05

Cadmium (Cd) Kidney damage, renal disorder 0.1 0.01 0.005 0.01

Chromium (VI) Carcinogenic, diarrhea 0.1 0.05 —— 0.05

Copper (Cu) Liver damage, insomnia 0.5 0.25 1.0 0.05

Lead (Pb) Kidney’s diseases, damage nervous system 0.2 0.006 0.05 0.10

Mercury (Hg) Rheumatoid arthritis, damage nervous system 0.01 0.00003 —— ——

Nickel (Ni) Nausea, Coughing 0.5 0.20 —— ——

Zinc (Zn) Increased thirst, depression 2 0.80 —— ——

Total metals Kidney damage, visceral cancers 10 —— —— ——

Cyanides (CN) Fast heart rate, shortness of breath 0.2 —— —— ——

Trichloroethane Headache, hypotension 0.05 —— —— ——

Trichloroethylene Dizziness, headaches, 0.05 —— —— ——

Phosphorus Hypotension, death 5 —— —— ——

aUnited States Environmental Protection Agency (USEPA).

bWorld Health Organization (WHO).

c Indian Standards Institution (ISI).

Table 2

Comparison of conventional methods and electrochemical methods[9].

Conventional methods Electrochemical methods Producing secondary

effluent

Low chemical usage, produce less sludge, high separation

Slow process and High maintenance

Total Mineralization of toxic pollutants High sludge production

and disposal problems

High efficiency and rapid organic matter separation than in traditional coagulation Requires adjunction of

non-reusable chemicals

Clean techniques due to its less harmful and inexpensive chemical reagent utilization

Table 3

Three categories of AOPs technologies among different criteria[10].

CATEGORY I:Based on the tested methodology for the formation of the oxidizing radicals (HO2,OH,O)

CATEGORY II:Based on whether oxidation processes develop with UV and Vis light irradiation or not

CATEGORY III:Based on whether oxidation processes develop in a single phase

Chemical AOPs:Peroxonation (O3/H2O2)Fenton’s reagent (Fe^2+/H2O2) Photo-chemical AOPs:Photo-Fenton process (UV/ Fe²⁺/H2O2)Photolysis of H2O2(UV/H2O2)Photocatalysis (UV/TiO2)

Photochemical AOPs:UV/H2O2Photo-Fenton process (UV/Fe²⁺/H2O2)

Homogenous AOPs:Persulfate- based technologies

Electro-chemical AOPs:Electro-FentonElectrochemical Oxidation Non-photo-chemical AOPs:(AEOP) Advanced electro-oxidation processes

Heterogenous AOPs:

Heterogeneous Fenton likeCatalytic ozonation

S. Rajoria, M. Vashishtha and V.K. Sangal Materials Today: Proceedings 47 (2021) 1472–1479

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complex with a dissociate chemisorptions step is observed in (Fig. 1). Two types of oxidation phenomenon occur at the electrode surface i.e; (i) direct oxidation and (ii) indirect oxidation[15].

The compounds become adsorbed on the anode surface during direct anodic oxidation. Direct anodic oxidation of contaminants is due to the production of physically adsorbed ‘‘active oxygen”, or chemical absorbed ‘‘active oxygen” (Oxygen in the oxide lattice, MOx+1), by direct anodic oxidation. Indirect EO means that the metal ions are oxidized on an anode and converted from a stable state into a reactive species. Strong oxidants, such as hypochlo- rite/chlorine, ozone and hydrogen peroxide are produced electro- chemically in an indirect oxidation method. By the oxidation of the generated oxidant, contaminants are then eliminated in the bulk solution.

Kazeminezhad and Mosivand used an electro-oxidation process for the elimination of nickel (Ni) and copper (Cu) from synthetic wastewater. For 60 min operation, the concentration of Ni or Cu was decreased at pH value~4.5 by applying~28 V[16]. Guan et al., also investigated an integrated approach for the removal of nickel-ammonia complexes by the combination of an electrooxidation process and electrodeposition processes (ED) by applying RuO₂/Ti and stainless steel (SS) as an electrode. About 99% of Ni removal was achieved at pH 9.0 [17]. The different results obtained from some research work using the electrochemical process for degradation of heavy metals ions from industrial wastewater are shown in (Table 5).

3.2. Electro-Fenton process

Fenton process is an efficient and/or powerful process that can reduce various amounts of toxic pollutants. Homogeneous and heterogeneous processes are the two types of Fenton process.

3.2.1. Homogeneous Fenton process

In this process, hydrogen peroxide (H2O2) and iron (Fe) reacts with each other and form strong reactive OH radicals, which in turn interact with dissolved germs, organics, bacteria, and colloids up to complete degradation into water and CO2. For the deterioration of heavy metal ions, the mechanism of the Fenton process is shown in (Fig. 2). Wang et al. examined that by applying homogeneous Fenton oxidation, the degradation efficiency of Ni²⁺and Ni- EDTA was 99.8% and 93.4% respectively[23]. Basically, the advantages of the electro-Fenton process for the treatment of wastewater such as a straightforward and flexible operation, simple-to-use and comparably cheap chemicals, no requirement for energy input.

This process offers few disadvantages, e.g., strict requirements for pH[24].

3.2.2. Heterogeneous Fenton process

The heterogeneous Fenton process also called the Fenton-like oxidation process which applies solid catalysts to facilitate reduce the soluble metal ions with an extended pH scale. Liu et al. devel- oped a polymer-supported, nanosized, and hydrated Fe (III) oxide (HFOD) applied as a catalyst and found that a Fenton-like catalyst was effectively applied to eliminate heavy metal complexes[25].

The heterogeneous Fenton process has several benefits such; the rate of sludge formation is controllable due to the slight leaching of metals ion from catalysts; by rising in the reaction time.

3.3. Photo Electro-Catalysis process

The photo electrocatalysis process is effective in use and offers several advantages such as easy-to-use, clean, relatively economical, high stability, and high removal efficiency[26]. The photocatalytic process is one of the photo electrocatalysis process for the efficient degradation of environmental pollutants. TiO2, CdS, ZnS, etc. are examples of various semiconductors which widely used in this process. As commonly examined, the super photocatalytic achievement with maximal quantum yields is constantly attained with TiO2.

The photocatalytic oxidation principle includes excitation of photogenerated radicals at the surface of semiconductors by sup- plying enough energy for photocatalyst illumination and metal ion deterioration (Eq. (1)) [27,28]. In the reaction valance band holes of photocatalyst get shifted to the surface, which in turn directly oxidize absorbed heavy metal ion or hydroxide ion. Subse- quently, which is used to destroy the ligands (Eqs.(2)–(4)). Finally, the discharged metal ions are eliminated onto the photocatalyst by adsorption (Eq.(5))[29].

Photocatalystþh

v

ðUV=solar lightÞ !Photocatalyst ð1Þ

h^þ þ OH ! OH ð2Þ

OHþCA !Simple organics ð3Þ

MetalCAþ OH=h^þ!M^xþ Oxidation productsþ CO2 ð4Þ

M^x^þ e !PhotocatalystþMetal ð5Þ

Table 4

Advantages and disadvantages of electrochemical techniques.

Process Advantages Disadvantages References

Electro-chemical process Effective for the elimination of heavy metal ions, low chemical usage, produce less sludge, high separation and it regarded as rapid and provides good reduction yields

Could be easily connected with other conventional technologies

Initial investment is high

Sometimes electrodes fouling occurs

[11,12]

Fig. 1.Mechanisms of electrochemical oxidation of organics with simultaneous oxygen evolution on (i) active anodes (reactions a, c, d and f) and on (ii) non-active anodes (reactions a, b and e). (a) Generation of hydroxyl radicals, OH*; (b) and (d) Evolution of oxygen; (c) Formation of higher metal oxide, (MO); (e) Combustion of R (organic molecules); (f) electrochemical conversion of R to RO.

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Various attempts were made by Zhao et al., to degrade Cr(VI) to less harmful and immobile Cr(III) by photocatalytic degradation [30]. For this purpose (TiO2) is used as a photocatalyst along with neodymium (Nd), in presence of UV radiation. As a result, the Cr (VI) gets adsorbed on the TiO2surface and reduced to Cr(III)[31].

Rhoads and Davis used TiO2 as a photo-catalysis to degrade Cu- EDTA and found that about 80–100% of Cu-EDTA degradation and 50–80% of Cu recovery was obtained within a 60-min operation [32]. Vohra and Davis also established photo-catalytic oxidation with TiO₂photocatalysis and observed 60% of Pb-EDTA was eliminated in a 60-min operation[33]. The different results obtained from some research work using the photo electrocatalysis process for degradation of the heavy metal complex from wastewater are shown in (Table 6).

4. Electrode materials

The performance of these electrochemical methods depends on the electrode material. With good chemical resistance and high efficiency in the treatment of wastewater, different anodes can be used such as Ti-supported metal and metal oxide like Pt, RuO2-TiO2and TaO2-IrO2, graphite, etc. The performance of different electrodes for the degradation of organic pollutants is examined by the response of electrodes towards oxygen evolution reaction (OER) in an acidic medium.

It is clearly seen in (Fig. 3.), Ti/SnO₂–Sb₂O₅anodes together with BDD anodes have the higher oxygen evolution reaction (OER) overpotential, and they are highly active for the complete oxidation of organic contaminants, followed by Ti/Ta₂O₅–SnO₂ and Ti/PbO₂ anodes[36]. As specified in (Fig. 3.), the higher oxygen evolution reaction (OER) overpotential the weaker bonding of (OH) radicals on the anode material and little energy is absorbed for the side reaction of water oxidation. Quantitative information about the OER over-potential of electrodes is given in (Table 7). Pt, RuO2– TiO2, and IrO2–Ta2O5anodes with a lower oxygen evolution reaction (OER) in acidic medium and, as a result, lower response towards pollutants oxidation of pollutants[37]. The major benefits and limitations of distinct electrodes in electrochemical technique are summarized in (Table 8).

5. Current and future developments

Treatment of electroplating effluents is a critical environmental problem because traditional methods, which are often insufficient to achieve the degree of purification required to minimize the neg- ative environmental effects. Electrochemical techniques have attracted much interest as a result of their high treatment efficien- cies, which lead to almost total mineralization of organic pollutants or the production of intermediate species that can increase the biodegradability of the final effluent. But for complex effluents, combined processes have higher treatment efficiency. Electro- chemical technologies should not be the only treatment proposed for a given industrial waste but combined with other technologies looking for an efficient solution to the environmental problem. The combination of electrochemical and other chemical or biological processes for the complete degradation of toxic chemicals can lead to a more cost-effective process. (Table 9) shows the summary for treatment of electroplating wastewater by the combination of other convention methods with electrochemical methods.

Advanced treatment technologies, especially electrochemical techniques are one of the most popular and active processes to remove harmful contaminants from industrial effluents. The directions of research is specified in (Fig. 4.), that indicated the electrochemical techniques with the efficient electrodes applied for degradation of electroplating industrial effluents. This technique Table5 Summaryfortreatmentofelectroplatingwastewaterbyelectro-oxidationorelectrochemicalprocesses. MetalMethodWaste-waterElectrodesInitialconc.TimeCurrent-densityTemp (⁰C)pHRemoval(%)References Cu,NiEOSyntheticWWIronsheet—1hr————4.5100[16] NiEO-EDIndustrialWWRuO2/Ti2156±50mg/L180min32mA/cm260999[17] Cr(VI)ElectrochemicalSynthetic/industrial WWFe/FeandFe/Cu180mg/L15min0.1–0.2(A)—2and 4—[18] CrElectrochemicalElectroplatingWWScrapiron11,136mg/L—1.2–3.5(mA/cm2252.9100[19] CuandNiElectrochemicalSyntheticWWTi/PtCu2+Ni2+0.06(M)20hrs20–25(A/m2)256.897[20] Cu,Cr,andNiElectrochemicalIndustrialWWTi/RuO2Cu14.67Cr18.54Ni 7.97mol/m3——Cu-10Cr,Ni-90(A/ m2)25–271Cu->99Cr,Ni->99.9[21] Cu(I)andCu (CN32–EOSyntheticWWActivatedcarbonfiber andSScoupledwith DSA

Cu(CN)32–0.2,0.4,0.8mM75min50A/m2—9Cu(I)-95.0±3.0CN- 91.0±4.9[22]

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Table 6

Removal of the heavy metal complex using photo electro catalysis process.

Catalysts Metal complex Rxtime (min) pH Removal (%) Recovery (%) References

TiO2 Cu-EDTA, Cd-EDTA 40 4–6 100 – [34]

TiO2 Pb-EDTA 60 6.0 >99 70–80 [33]

TiO2,Zno,CdS Cr (VI) 180 2.0 96.8 – [35]

TiO2 Fe-EDTA, Cu-EDTA 60 6.0 80–100 50–80 [32]

Fig. 3.The activity of various anode materials towards oxygen evolution reaction (OER) in acidic medium.

Fig. 2.Mechanisms of Electro-Fenton process for decomposition of metal ion.

Table 7

Quantitative information about OER over-potential of electrodes[37].

Electrodes RuO2 IrO2 Pt Graphite SnO2 PbO2 BDD

V/SHE 1.47 1.52 1.6 1.7 1.9 1.9 2.3

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Table 8

Comparison of electrode materials in electrochemical treatment[37-39].

Electrode Advantages Disadvantages Comparison with the other electrodes

Ti Highly stable Costly ——

Pt Inert in corrosion, superb repeatability properties Costly, low efficiency tomineralize organics Low performance in anodic oxidation

PbO2 Low-cost, simple to assemblerelatively high overpotentialtowards OER, relatively high mineralization efficiency

Corrosive Pb²⁺ions could be released, insufficient application in industrial wastewater treatment

——

SnO2 Mostly chemically, electro chemically inert, good conductivity properties Costly Degradation rate is low as compared to BDD

BDD Favorable current efficiency, high overpotential towards OER, high electrochemical stability Very costly, performance decreased in diluted solutions Higher activity

Table 9

Summary for treatment of electroplating wastewater by combination of other convention methods with electrochemical methods.

Method Pollutant Anode Cathode Initial conc. Time Current-

density Temp (⁰C)

Voltage pH Removal (%) References

(EO-ED) Electrooxidation- Electrodeposition

Ni-NH3 RuO2/Ti Stainless

steel

Ni- 2156NH39680 (mg/L) 180 min 32 mA/

cm²

60 — 9 Ni99,NH370 [17]

(EC-EO)electrocoagulation–

electrooxidation

Industrial Wastewater

Boron-doped diamond (BDD)

Iron rectangular plates

—— 2 hrs 200–

800 A/

m²

— 3 V 8 COD -<99 BOD- <99 [40]

Electrocatalytic-membrane process (ECMP) integrated with Chemical or electrocoagulation

Cu²⁺, Ni²⁺and chemical oxygen demand (COD)

Dimensional stable anode (DSA)

Stainless- steel

COD- 130Heavy metal 1.0 (mg/L)

ECMP for COD- 10 minAnd ECG for Cu²⁺and Ni²⁺- 35 min

—— —— 10 V 7 COD77 [41]

(EO-PS)Persulfate enhanced electrochemical oxidation

CN containing organic WW

Boron-doped diamond (BDD)

Stainless steel

COD-11,290,TOC4456And CN-1280.15(mg/L)

24 hrs 10 mA/

cm²

40 – 5.6 COD95.8 TOC87.8 and

CN98.4 [42]

In-situ ion exchange electrocatalysis biological coupling (i-IEEBC)

COD, TOC, Cr and Cu ions,

Multi-hole Stainless steel

Multi-hole Titanium plate

COD274.89–319.42TOC 89.55–96.40Cu²⁺0.056–

0.137Cr⁶⁺0.442–1.111 (mg/

L)

— 0.40 mA/

(cm)²

20–23 – 7.1–

7.8

COD-87.23TOC-80.42Cr 91.25Cu95.97

[43]

Reduction/precipitation, Chemical oxidationand Biological aerated filter

Cu, Cr, Ni, CN, COD

———— ——— Cu108Cr62Ni85CN-

136COD- 450 (mg/L)

—— – 25 —— 9–

11

Cu–9.94Cr-99.5Ni- 99.0CN- 99.7COD-84.2

[44]

Combined electrochemical and Ozonation methods

Cr, Fe, Ni, Cu, Zn, Pb, TOC, and COD

Iron plate —— Cr- 6.3Fe440.4Ni0.532Cu

3.5Zn262.5Pb0.847TOC 370COD1430 (mg/L)

Electrochemical- 15 minAnd Ozonation – 30 min

—— —— 12 V 6.9 Cr- 99.94Fe-100.00 Ni-

95.86Cu- 98.66Zn-99.97 Pb- 96.81 TOC-93.2and COD- 93.43

[45]

EO with coagulation–

flocculation (CF)

COD andNH4+

- N

Aluminum plate

Graphite plate

COD –233 and NH4+

-N185

(mg/L)

90–120 min 60 mA/

cm²

20 — 8.9 100 [46]

S.Rajoria,M.VashishthaandV.K.SangalMaterialsToday:Proceedings47(2021)1472–1479

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easily combined with other conventional techniques (such as physical, chemical and, biological) for the complete mineralization of organic pollutants or the production of intermediate species that can enhance the biodegradability of the final effluent.

Recently the basic aspects for upgrowing development in research direction are characterized:

i) The electrolytic reactor have to be designed to achieve excel- lent mass transfer and significantly enhance the efficiency of the treatment;

ii) In addition to the geometrical structure of electrode study, research work have to be focused on material sciences for better and more economical electrodes to make the electrolytic systems more competitive than other processes (physical, chemical and biological process); and

iii) The efficiency of the proposed method is significantly improved by collaborating electrochemical techniques with the other conventional techniques to further minimize the energy consumption as well as time and cost.

The future of electrochemical techniques is immensely promising not only at industrial level but also in the decentralized water treatment of urban and domestic wastewater. More studies on electrocatalytic anodic materials are needed to focus for minimiz- ing the initial investments on the acquisition of electrodes. The future developments of electrochemical will be directly related to the scale-up process and the electrochemical reactors design to establish the efficient and cost-effective exploitation of this technology.

6. Conclusion

Advanced oxidation process (AOP) is achieving importance in the treatment of industrial wastewater. In this critical review, we have shown that the electrochemical process, one of the AOP’s is a very adequate technique for the degradation of wastewater con- sists of hazardous contaminants. The performance of these electrochemical methods depends on the electrode materials. The future progress of the electrochemical process will require the advance- ment of anode materials with unique aspects that can make the process competitive with other conventional technologies. Most of the studies reported in the literature have been carried out at the laboratory scale. Efforts should be made to operate electrochemical experiments at pilot plant scale using real industrial effluent to analyze the probability of using electrochemical methods for proper treatment of real industrial effluents. A Continuous treatment system has more scope for industrial applications than the batch system. Most of the study has focused mainly on the batch system; hence there is a need to work on the development of a continuous system. Subsequently, one should need to highlight the significance of electrochemical methods as a technological appliance for environmental management.

Declaration of Competing Interest

The authors declare that they have no known competing finan- cial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 4.Electrochemical treatment methods for electroplating wastewater and its future direction[15,47].

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