Pharmaceutical Wastewater Treatment by Electrooxidation Method

Abstract

Advancement in medical science over years has led to bulk production and consumption of pharmaceutical compounds all over the world. Antibiotics, particularly have become an indispensable part of human and animal health systems, which are intended to treat bacterial infections. As per a report published in 2018, over a period of fifteen years (2000 – 2015), antibiotic consumption in India increased by 103%. Moreover, India was the largest consumer of antibiotics in the world in 2010, with cephalosporins, broad-spectrum penicillins and fluoroquinolones among the top three categories of antibiotics being consumed by Indians Such large production and intake of antibiotics has led to their occurrence and accumulation in aquatic environment. Scientific communities worldwide have conducted comprehensive studies and reported occurrence of antibiotics in effluents from waste water treatment plants, sewage treatment plants, sea water, river water, lakes and ground water, which proved the incapability of conventional treatment technologies to remove antibiotics. This alarming issue requires immediate attention due to growing antibiotic resistance in native bacterial population. Over the years, this problem has worsened in India, leading to highest number of deaths caused by antibiotic resistance. Electro-oxidation (EO) has fetched much attention of researchers in past two decades. EO is a type of electrochemical AOP, which utilizes strong oxidant species (•OH, H2O2, O3) generated on anode for pollutant degradation in a much simpler equipment and without involvement of any hazardous chemicals. Moreover, it is an environmentally benign technology as no sludge generation takes place. In present study, two commonly prescribed antibiotics: ofloxacin (OFLX) and amoxicillin trihydrate (AMT), belonging to fluroquinolone and penicillin category, respectively, were chosen as model pollutants for testing the degradation/mineralization efficiency of EO cell comprising of Ti/RuO2 electrodes in batch as well continuous mode of treatment. In case of batch EO treatment of antibiotics (OFLX and AMT), treatment efficiency was evaluated in terms of antibiotic removal efficiency (%ARE), TOC removal efficiency (%TRE), specific energy consumption (SEC, kW h (g TOC removed)-1) and mineralization current efficiency (%MCE). The effects of operating parameters such as applied current (I, 0.25–1.0 A), initial pH of wastewater (2–9) and supporting electrolyte (NaCl) concentration (S0, 0.5–2 g L-1), on ARE, TRE, SEC and MCE were systematically studied. Effect of initial antibiotic concentration (C0, 10–50 mg L-1) and applied current (I) on kinetics of antibiotic degradation was analysed. Analysis of transformation products of antibiotics formed during EO was done by ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-Q-TOF-MS). In case of batch EO treatment of OFLX, optimum pH and current were found to be 6.8 ± 0.1 (natural pH of OFLX wastewater) and 1 A, at which 80% ARE and 46.3% TRE were achieved in 30 min and 240 min of electrolysis, respectively. MCE decreased from 7.8 to 4.9% with increase in I value from 0.25 to 1 A. De-fluorination, breaking of piperazinyl ring and dealkylation or replacement by •OH and Cl- led to the formation of seven major OFLX transformation products. Whereas, when batch EO of AMT was performed, 60% of antibiotic and 48% of TOC removal took place in 60 and 240 min, respectively, at optimum conditions of 1 A of applied current and neutral pH 7.0. MCE decreased from 11.77 to 7.67% with increase in applied current from 0.25 to 1.0 A. This was attributed to increase in undesirable but inevitable reactions occurring simultaneously during EO, which consumed a considerable part of energy. Three major transformation products of AMT were identified which were generated after hydroxylation of AMT molecule and attack of active chlorine on aromatic moiety. Degradation of both the drugs followed pseudo-first order kinetics. Increase in applied current resulted in rapid degradation, which was proved by increase in kf value. SEC decreased with the increase in NaCl concentration. The operating cost of EO process for removal of 1 g of TOC was found to be INR 35.38 ($ 0.54) and INR 28.88 ($ 0.44) for OFLX and AMT, respectively. The reactor set-up for continuous EO treatment was similar to batch except that an inlet and outlet points were incorporated in the reactor, and a continuous supply of wastewater from reservoir to reactor at required flow-rate (corresponding to retention time) was facilitated by a peristaltic pump. The continuous EO of antibiotics was performed after designing the experiments by response surface methodology (RSM) based on five level central composite design (CCD) . Set of 30 experiments with 6 replications were performed for each antibiotic. Individual and interactive effects of operating parameters (independent variables): pH (2–10), I (0.25–1.25 A), retention time (RT, 15 – 195 min) and elapsed time (t, 20 -180 min), on the responses %TOC removal (Z1), % antibiotic removal (Z2) and SEC (Z3) were investigated. Synthetic wastewater comprising 50 mg L-1 of drug and 2 g L-1 of NaCl was freshly prepared before each experiment. As suggested by ANOVA, the quadratic model was found to be significant for both the antibiotics. The optimum condition for continuous EO treatment of OFLX as found by RSM was: pH = 6.0, I = 0.64 A, RT = 157.68 min and t = 170 min. The experimental values of the responses (Z1 = 23.85%, Z2 = 76.78%, Z3 = 0.706 kW h (g TOC removed)-1) were in good agreement with the predicted values (Z1 = 23.94%, Z2 = 78.16%, Z3 = 0.658 kW h (g TOC removed)-1). Seven major reaction intermediates of OFLX were identified and a plausible degradation scheme was proposed. In case of AMT, optimum condition achieved by RSM was: pH = 7.53, I = 0.7 A, RT = 175.6 min and t = 128.89 min. The experimental values of the responses (Z1 = 38.36%, Z2 = 52.86%, Z3 = 0.385 kW h (g TOC removed)-1) were in good agreement with the predicted values (Z1 = 37.82%, Z2 = 51.64%, Z3 = 0.408 kW h (g TOC removed)-1). MCE at optimum run came out to be 5.07 and 9.81% for OFLX and AMT, respectively. Decay kinetics of both the antibiotics as well as mineralization at optimum run of continuous EO followed pseudo-first order kinetic model. However, degradation was found to be almost five times faster than mineralization of antibiotics by Ti/RuO2 electrodes. A plausible degradation mechanism was proposed on the basis of eight major reaction intermediates of AMT. Apart from direct degradation at anode surface, the AMT molecule was indirectly oxidized by active chlorine leading to opening of its beta-lactam ring. The chlorinated transformation products underwent reduction at the Ti/RuO2 cathode, leading to their de-chlorination. The operating cost of continuous EO treatment of antibiotics came out to be INR 102.69 (US $ 1.57) (OFLX) and INR 56.5 (US $ 0.87) (AMT) for removal of 1 g of TOC from the wastewater. The characterization of fresh and used Ti/RuO2 electrodes was done by SEM–EDX and XRD analysis. SEM images revealed that the morphology of electrodes was same, except diminutive loss of RuO2 coating after performing EO treatment reactions. EDX study of used electrode indicated increase in amount of carbon content as compared to fresh electrode, which could be attributed to •OH mediated EO. XRD pattern of fresh and used electrode exhibited Ti, TiO2 and RuO2 rich broad and symmetric peaks. The peak positions in used electrode remained intact, signifying unaltered micro structure, however the intensity of peaks was slightly diminished.