ELECTROCHEMICAL TREATMENT OF PETROCHEMICAL WASTEWATER

Abstract

Purified terephthalic acid (PTA) is xylene-based petrochemical product manufactured by air oxidation of para-xylene in presence of cobalt or manganese salt catalyst with bromine promoter. Approximately 3-4 m3 of wastewater with very high chemical oxygen demand (5-20 g/L) is generated per ton of PTA production. Aromatic compounds such as benzoic acid (BA), terephthalic acid (TPA), para-toluic acid (p-TA) and phthalic acid (PA) are the major pollutants of PTA wastewater which contribute about 75% of the COD of PTA wastewater (Macarie and Guypt, 1992; Kleerebezem et al., 1997). Since last few decades, various physiochemical and biological processes have been used to treat PTA wastewater. Very few studies have been reported on pretreatment of PTA wastewater using electrocoagulation (EC) process. However, no study has been reported for the treatment of PTA wastewater by electrooxidation (EO) and electro-Fenton (EF) processes. Therefore, in this study, EC, EO and EF processes have been employed for the treatment of major pollutants of PTA wastewater. Prior to electrochemical treatment, acid precipitation (AP) treatment of wastewater was performed. Single, binary and multicomponent solution of BA, p- TA, TPA and PA were synthetically prepared with distilled water at laboratory. Actual PTA wastewater was collected from a nearby petrochemical plant and treated. Optimization of the process was done using Central Composite Design (CCD) in Response Surface Methodology (RSM) tool of Design Expert Software. In first study, pre-treatment of BA (400 mg/L) containing synthetic solution was carried out by AP in pH range of 1-3 and temperature 15-60 oC and maximum removal of BA and COD, 61% and 56% respectively was obtained at pH: 1 and temperature: 15 oC. The filtered supernatant (BA- 156 mg/L and COD-330 mg/L) was further treated by EC and EF processes separately. Optimization of operating parameters namely initial pH: (3-11), current density (CD) (A/m2): (15.24-76.21), electrolyte concentration (Na2SO4) (mol/L): (0.03-0.07), H2O2 concentration (mg/L): (100-500) and electrolysis time (min): (15-95) has been performed using CCD during EC and EF treatments. Maximum removal efficiencies of BA- 76.83%, 88.50%; chemical oxygen demand (COD)- 69.23%, 82.21% and energy consumption (kWh/kgCODremoved) - 30.86, 21.15 were achieved at optimum conditions, pH-7.34, 2.99, CD iii (A/m2) - 50.97, 44.87, electrolyte concentration (mol/L)- 0.05 , H2O2 concentration (mg/L)- 307, time (min)- 58.86, 55.10 during EC and EF treatments respectively. In second study, pretreatment of aqueous solution of p-TA (500 mg/L) was carried out by AP at different pH (3-6) and temperature (15-60 oC) and resulted in 43.1% of p-TA and 38.2% COD removals respectively at optimum pH: 3 and temperature: 15 oC. Optimization of operating parameters viz. initial pH: (1-9), CD: (30.48-152.44 A/m2), electrolyte concentration: (0.03-0.07 mol/L), H2O2 concentration: (300-700 mg/L) and electrolysis time: (10-90 min) was performed. Maximum removal of p-TA- 64.83%, 74.50% COD- 61.27%, 68.21% with minimum energy consumption (kWh/kgCODremoved)- 69.71, 41.60 at optimum conditions, pH-8.1, 3.1, CD (A/m2)- 103.39, 88.90, electrolyte concentration- 0.05 mol/L, H2O2 concentration- 522 mg/L and time (min)- 50.42, 58.86 were obtained by EC and EF processes respectively. In third study, the TPA (400 mg/L) containing solution was initially treated by AP at different pH (2-5) and temperature (15-60 oC). Approximately 87.1% of TPA and 68.85% of COD were removed by AP treatment at optimum pH: 3 and temperature: 15 oC. Operating parameters viz. pH- (4-12), CD (A/m2)- (15.24-45.72), electrolyte concentration (mol/L)- (0.02- 0.04), H2O2 concentration (mg/L)- (50-250) and time (min)- (10-70) were optimized during EC and EF treatments. Maximum removal of TPA- 82.76%, 91.87% and COD- 79.56%, 89.68% with energy consumption (kWh/kgCODremoved)- 22.65, 18.11 at pH- 7.44, 3.20, CD (A/m2)- 32.71, 27.89, electrolyte concentration- 0.04 mol/L, H2O2 concentration -142.7 mg/L time (min)- 43.49, 38.55 were achieved during EC and EF treatments respectively. In fourth part of study, AP treatment of PA (400 mg/L) solution was studied initially at various pH (1-3) and temperature (10-55 oC) and, reduction in concentrations was found 32% and 29.1% for PA and COD respectively at optimum pH: 1 and temperature: 10 oC. Operating parameters viz. pH- (3-11), CD (A/m2)- (30.48-152.44), electrolyte concentration (mol/L)- (0.03- 0.07), H2O2 concentration (mg/L)- (100-500) and time (min)- (15-95) were optimized. Maximum removal of PA- 68.21 %, 82.25% and COD- 64.79%, 75.21% with minimum energy consumption (kWh/kgCODremoved)- 120.95, 65.68 at optimum pH- 8.1, 3, CD (A/m2)- 110.96, 86.54, electrolyte concentration (mol/L) - 0.05, H2O2 concentration -328.3 mg/L, time (min)- 58.44, during EC treatment and PA- 82.25% and COD- 75.21% with energy consumption (kWh/kgCODremoved)- 65.68 at pH- 3, CD- 86.54 A/m2, H2O2 concentration -328.3 mg/L, time- 49.12, 49.12 were obtained during EC and EF treatments respectively. iv In fifth study, acid treatment of binary aqueous solution of BA (400 mg/L) and TPA (mg/L) was studied initially at various pH (2-5) and temperature (15-60 oC) and subsequently, the supernatant was subjected to EO and EF processes separately. After acid treatment, reduction in concentrations of BA, TPA and COD in solution was approximately 48.7%, 83.4% and 57% respectively at temperature: 15 oC and pH: 2. Optimization of variables such as initial pH: (1-9), CD (A/m2): (30.48-91.45), NaCl concentration (g/L): (0.5-1.5), Fe2+ concentration (mmol/L): (0.5-1.5) and electrolysis time (min): (10-90) were studied for EO and EF processes. Maximum percentage removal of BA-70.76, 80.45, TPA- 68.52, 76.83, COD- 67.27, 73.70 with energy consumption (kWh/kgCODremoved)- 31.01, 19.39 at optimum conditions i.e. pH- 4.6, 3.1, CD (A/m2)- 65.15, 54.39, NaCl concentration- 1.0 g/L, Fe2+ concentration- 1.0 mmol/L, time (min)- 58.02, 50.11 were found during EO and EF treatments respectively. In sixth study, AP treatment of binary component solution of PA (400 mg/L) and p-TA (mg/L) was studied initially at various pH (2-4) and temperature (15-60 oC) followed by EO and EF processes separately. After acid treatment, approximately 22.7% PA, 47.8% of p-TA and 48.7% of COD reduction was observed at optimum pH: 1 and temperature: 15 oC. Prameters such as initial pH: (1-9), CD: (30.48-91.45 A/m2), NaCl concentration: (0.5-1.5 g/L), Fe2+ concentration (mmol/L): (0.5-1.5) and electrolysis time (min): (15-95) for EO and EF processes. Maximum removal PA- 64.55%, 75.21%, p-TA- 60.24%, 65.19%, COD- 62.77%, 68.15% with energy consumption (kWh/kgCODremoved)- 28.50, 20.11were obtained at optimum pH- 5.4, 3.2, CD (A/m2)- 67.13, 61.30, NaCl concentration- 1.0 g/L, Fe2+ concentration- 1.1 mmol/L, time (min)- 60.12, 53.31 during EO and EF treatments respectively. In seventh study, multicomponent solution of 400 mg/L each of BA, p-TA, TPA and PA was initially treated by AP treatment. Influence of pH (2-4) on COD removal at various temperatures (15-60 oC) was observed in acid treatment and obtained 60.78% of COD removal at optimum conditions (pH -2 and temperature- 15 oC). At this optimum condition, removal of BA, p-TA, TPA and PA was found 41.6%, 44.8%, 80.9% and 19.8% respectively. Operating parameters viz. pH- (4-12), CD (A/m2) - (45.72-228.60), Na2SO4 concentration (mol/L)- (0.04- 0.08), H2O2 concentration (mg/L)- (600-1000) and time (min)- (20-100) were optimized during EC and EF treatments. Maximum removal of BA- 66.16%, 75.24%, p-TA- 59.15%, 68.11 %, TPA- 64.07 %, 73.49 %, PA- 65.14%, 74.25%, and COD- 60.76 %, 73.91% with energy consumption (kWh/kgCODremoved)- 95.81, 49.58 at optimum conditions at pH- 8.3, CDv 140.11 A/m2, Na2SO4 concentration- 0.06 mol/L, time- 66.51 min and pH- 3, CD- 131.54 A/m2, H2O2 concentration -826.62 mg/L, time- 56.12 min during EC and EF processes respectively. In the final study, treatment of actual PTA wastewater was performed by AP followed by EC and EF processes. Effect of pH (2-4) on COD removal at various temperatures (15-60 oC) was observed in AP treatment and obtained 55.2% of COD removal at the optimum pH: 2 and temperature: 15 oC. Optimization of parameters viz. pH- (4-12), CD (A/m2)- (60.97-243.91), Na2SO4 concentration (mol/L)- (0.04-0.08), H2O2 concentration (mg/L)- (1000-1800), and time (min)- (25-125) were studied. Maximum removal of COD- 64.23%, 73.45% with energy consumption (kWh/kgCODremoved)- 77.58, 51.11 was found at pH- 8.7, 3.1, CD (A/m2)- 164.3, 148.5, Na2SO4 concentration- 0.06 mol/L, H2O2 concentration -1473.4 mg/L, time (min)- 86.26, 78.02 during EC and EF processes respectively. Degradation kinetics during EC and EF treatments at optimum conditions was studied for multicomponent wastewater and actual PTA wastewater and found that 2nd order kinetic model provides best correlation for COD removal rate with high R2 and low error values during EC and EF treatments. Sludge generated after treatment of BA, p-TA, TPA, PA, multicomponent and actual PTA wastewater at optimum conditions during EC and EF treatments were analyzed by settling characteristics, PZC, XRD, FESEM, FTIR, DTA/ TGA/ DTG techniques. Operating costs (US $) of the EC and EF processes for the treatment of BA, p-TA, TPA and PA in single component wastewater were (EC-5.21, EF- 5.18), (EC-9.52, EF-8.16), (EC- 4.57, EF-4.16) and (EC-13.48, EF-9.02) respectively. Operating cost ($) for multicomponent and actual PTA wastewater was found 13.23 and 13.24 during EC treatment and 9.19 and 10.84 during EF treatment respectively. Therefore, on the basis of obtained results, it can be concluded that EC, EO and EF processes provide remarkable removal efficiencies during treatment of synthetic and actual wastewaters. It can also be concluded that EF process is more efficient based on removal efficiencies, energy consumption and operating cost.