ELECTROCHEMICAL TREATMENT OF SYNTHETIC PURIFIED TEREPHTHALIC ACID WASTEWATER

Abstract

Large amount of wastewater is generated during production process of terephthalic acid (TPA) in purified terephthalic acid (PTA) plant. It is estimated that about 3-10 m3 of wastewater is generated during production of 1 ton of TPA, having 5-20 g/L of COD. Aromatic compounds are the major pollutants of purified terephthalic acid (PTA) wastewater. TPA, BA and p-TA are the three major pollutants which contribute about 75% of the COD of PTA wastewater (Macarie and Guypt, 1992; Kleerebezem et al., 1997; Karthik et al., 2008). Since last few decades, various physiochemical and biological processes have been used to treat PTA wastewater. However, nobody has worked on electrocoagulation (EC) process for treatment of such wastewater. Therefore, in present study major pollutants (TPA, BA, p-TA) of PTA wastewater has been treated in single and combined form by using EC process using Al and Fe electrodes. Initial concentrations of TPA (400 mg/L), BA (400 mg/L) and p-TA (500 mg/L) were chosen based on the previous studies (Noyala et al., 1990; Macarie and Guyot, 1992; Chidambararaj et al., 1997; Young et al., 2000; Verma et al., 2010, 2011; Kim et al., 2012; Anand et al., 2013; Marashi et al., 2013). Response Surface Methodology (RSM) in Design Expert Software (8.0.7.1, 2010, Stat-Ease Inc. Minnepolis) was used for optimization of all operating parameters. In first study various operating parameters viz. pH (7-11), current density (A/m2): (38.4-105), electrolysis time (min): (15-60) and supporting electrolyte concentration (NaCl) (g/L): (0.5-1.25) was used for treatment of synthetic terephthalic acid (TPA) wastewater. Optimum percentage removal of TPA: 89.75, 73.86; COD: 83.88, 67.63 and minimum consumption of electrical energy (kWh/kgCODremoved): 57.25, 70.84 were achieved using Al and Fe electrodes respectively at optimum operating conditions - pH:7; CD (A/m2):84.33, 91.47; time (min): 37.57, 40.0 and NaCl concentration (g/L):1.10, 0.94 for Al and Fe electrodes respectively. In second study various operating parameters viz. pH: (4-12), CD (A/m2): (76.00- 230.0), electrolysis time (min): (15-90) and supporting electrolyte concentration (NaCl) (g/L): (0.5-1.5) were used for treatment of benzoic acid (BA) wastewater. Maximum percentage removal of BA: 70.52, 64.42; COD: 65.65, 60.70 and energy consumption: 102.89, 133.52 were found by using Al and Fe electrodes respectively at optimum operating conditions- pH: 8.48, 7.99; CD (A/m2): 150.26, 155.30; time (min): 53.92, 57.18 and NaCl concentration: 0.97, 1.0 for Al and Fe electrodes respectively. In third study effects of various operating parameters viz. pH: (4-12), CD (A/m2): (76.0-215.0), electrolysis time (min): (15-90) and electrolyte concentration (NaCl) (g/L): (0.5-1.5) were studied on treatment of synthetic para-toulic acid wastewater. Maximum percentage removal of p-TA: 60.49, 50.76; COD: 53.80, 46.75 and energy consumption (kWh/kgCODremoved): 90.25, 135.50 are achieved using Al and Fe electrodes respectively at optimum operating conditions viz. pH: 8.75, 8.81; CD (A/m2): 158.23, 154.62; time iii (min): 58.93, 60.23 and NaCl concentration (g/L): 0.99, 1.10 for Al and Fe electrodes respectively. In final study various operating parameters viz. pH: (5-13), CD (A/m2): (49.5- 255.50), electrolysis time (min): (15-95) and supporting electrolyte dosages (NaCl) (g/L): (0.25-2.25) were used to treat multicomponent wastewater of TPA, BA and p-TA. Maximum percentage removal of TPA: 56.21, 54.10; BA: 59.52, 53.84; p-TA: 45.71, 39.91; COD: 49.91, 42.95 and energy consumption (kWh/kgCODremoved): 80.15, 110.85 were achieved using Al and Fe electrodes at optimum operating conditions viz. pH: 8.18, 8.0; CD(A/m2): 172.97, 180.33; Time (min): 63.47, 65.55 and NaCl concentration (g/L): 1.69, 1.74 for Al and Fe electrodes respectively. Sludge and scum obtained at optimum operating conditions were analysed for isoelectric point (pHIEP), settling characteristics, X-ray diffraction spectroscopy (XRD), Scan electron microscopy (SEM)/Energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), Differential thermal analysis (DTA)/Thermo gravimetric analysis (TGA). Sludge samples of TPA, BA, p-TA, and multicomponent wastewater show zero net charge (PZC) at pH (7.6, 9.4, 9.2, 9.0) and (7.4, 8.2, 8.4, 8.4) using Al and Fe electrodes respectively. Fe electrodes generated sludge shows better settling properties in comparison of Al electrodes generated sludge. XRD spectra of sludge generated by Al electrodes show crystalline nature while sludge and scum generated by Fe electrodes show porous structure. SEM micrographs of anode surfaces of Al and Fe electrodes show dents and pits at surfaces and Fe anodes show large number of cracks in comparison of Al electrodes. EDX analysis of sludge and scum samples of Al and Fe electrodes shows high weight percentage of Al and Fe in comparison to other components like carbon, chlorine, oxygen, sodium, etc. FTIR spectra of sludge and scum generated by Al and Fe electrodes show presence of carbonyl group, C=C stretching, halogen compounds and hydrogen ring or hydrogen atom. The thermal analysis of sludge showed highest weight loss (≈ 96%) in case of treatment of BA wastewater by Fe electrodes and lowest weight loss (≈ 12%) in case of treatment of p-TA wastewater by Fe electrodes. Al electrodes generated sludge shows highest weight loss (≈ 38%) in case of treatment of multicomponent wastewater and lowest weight loss (≈ 32%) in case of treatment of p-TA wastewater. Operating costs ($/kgCODremoved) of the process were (Al:7.26, Fe:8.15), (Al:12.40, Fe:14.12), (Al:10.77, Fe:13.95) and (Al:13.58, Fe:14.40) in case of treatment of TPA, BA, p-TA and multicomponent wastewater respectively based on model results. Conclusion of the present research work is that the removal efficiency of individual components i.e., TPA, BA and p-TA at optimum operating conditions in combined form was lower than the single component wastewater treatment using Al and Fe electrodes. Process shows higher operating costs in case of Fe electrodes in comparison to Al electrodes for all wastewater used in present study.