1 Korean J. Chem. Eng., 35(3), 1-13 (2018) DOI: 10.1007/s11814-017-0336-2 INVITED REVIEW PAPER pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: vksandhwar@gmail.com Copyright by The Korean Institute of Chemical Engineers. Comparison of electrocoagulation, peroxi-electrocoagulation and peroxi-coagulation processes for treatment of simulated purified terephthalic acid wastewater: Optimization, sludge and kinetic analysis Vishal Kumar Sandhwar † and Basheshwar Prasad Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India (Received 9 October 2017 • accepted 1 December 2017) Abstract-This study mainly focuses on a comparative study of electrocoagulation (EC), peroxi-electrocoagulation (PEC) and peroxi-coagulation (PC) processes for the treatment of aqueous solution containing major toxic compo- nents of purified terephthalic acid wastewater: benzoic acid (BA), terephthalic acid (TPA), para-toluic acid (p-TA) and phthalic acid (PA). The solution was initially treated by acid treatment method at various pH (2-4) and temperature (15-60 o C). The supernatant was further remediated by EC, PEC and PC methods independently. Process variables such as pH (4-12) and pH (1-5), current density (45.72-228.60 A/m 2 ), electrolyte concentration (0.04-0.08 mol/L), elec- trode gap (1-3 cm), H 2 O 2 concentration (600-1,000 mg/L) and reaction time (20-100 min) during EC, PEC and PC treatment were effectively optimized through central composite design under Design Expert software. Maximum COD removal of 60.76%, 73.91%, 66.68% with energy consumption (kWh/kg COD removed) of 95.81, 49.58, 69.26 was obtained by EC, PEC and PC treatments, respectively, at optimum conditions. Electrochemical methods were com- pared by removal capacities, consumption of energy, operating cost, degradation kinetics and sludge characteristics. PEC treatment was found most effective among EC, PEC and PC processes due to its highest removal capacity and lowest energy consumption features. Keywords: Purified Terephthalic Acid, Electrochemical Treatment, Response Surface Methodology, Optimization, Kinetic Study INTRODUCTION Many organic chemical industries use the intermediates and raw materials generated from processing of crude oil and natural gas. Para-xylene, which is an essential raw material for these indus- tries, is also used for the production of PTA. Various waste steams (liquid and gaseous) turn out during PTA production. Industrial PTA wastewater contains several hazardous aromatic compounds like phthalic acid, benzoic acid, terephthalic acid, and para-toluic acid as its major components [1-3]. These aromatic compounds are responsible for liver, bladder and kidney damage as well as also show- ing carcinogenic effects [4-6]. Their phthalate forms are more haz- ardous, particularly due to the adverse impact on reproduction capacity of humans [7]. US EPA has mentioned phthalate, its esters and its degradation products as the priority pollutants based on their toxicity. Due to strict environmental regulations, highly effi- cient and eco-friendly techniques are required to treat these toxic compounds from wastewater before its use or surface discharge. In recent years several bioremediation and physico-chemical processes have been utilized for PTA wastewater treatment. Some of these techniques have operational issues like partial degradation of pol- lutants, toxic intermediates formation, high sludge formation and generation of secondary phases which affect the economy of the process. Among these wastewater remediation techniques, electro- chemical methods have aroused a great deal of interest due to eco- friendly, versatility, automation, high efficiency and cost effective- ness features [8-14]. Recently, more emphasis has been given to the advancement of efficient electrochemical processes for the treat- ment of aromatic compounds present in petrochemical wastewater. Electrocoagulation (EC) is one of the most famous electrochemi- cal techniques for wastewater treatment. During this treatment, coag- ulant species form through the dissolution of a sacrificial anode in electrochemical cell. These generated coagulants interact with pol- lutants present in the solution resulting, solubility reduction or entrap- ping into growing flocs of precipitates, eventually promoting removal of pollutants by sedimentation and flotation/filtration stage [15,16]. Chemical reactions take place during EC treatment with Fe anode as shown below [17]. At anode: Fe→Fe 2+ +2e - (1) In acidic conditions 4Fe 2+ +O 2 +H 2 O→4Fe 3+ +4OH - (2) In alkaline conditions Fe 2+ +2OH - →Fe(OH) 2 (3) At cathode: 2H 2 O+2e - →H 2 +2OH - (4) Electro-Fenton (EF) method is the combination of electrochemical as well as Fenton methods. This method was basically developed for the improvement in oxidation of organic pollutants. Oxidation