Electroplating wastewater treatment and resource recovery via a hybrid process of stepwise alkalization, Fenton, and chlorination

Electroplating wastewater rich in metal ions and acids has dual attributes of real environmental pollutant and potential resource carrier. Targeting resource recovery and up-to-standard discharge from electroplating wastewater treatment in this study, a hybrid process of stepwise alkalization, Fenton, and chlorination was proposed to treat real electroplating wastewater from Guangzhou, China, where pH was 1.1 and Ag, Fe, Ni, Cu, Zn, Cr, P, F, chemical oxygen demand (COD), NH3-N was 624.9, 11266.9, 914.7, 1169.6, 13.8, 1971, 8750.8, 7433.1, 382.3, 451.3 mg/L, respectively. Pollutants removal from liquid phase and precipitates composition were systematically investigated during stepwise alkalization via pH control, showing that first adding NaOH to pH of 3 produced high-purity (98.4 %) Fe(PO3)3 precipitates suitable for high value-added resource recovery, second adding Ca(OH)2 to pH of 9 achieved high pollutants removal (especially F and P) and cost savings, and centrifugation with 2000 g and 10 min was appropriate for precipitates separation. Aiming at COD and chelated/ complexed Ni removal from treated electroplating wastewater by stepwise alkalization, the optimal parameters for Fenton process in this study were determined as the initial pH of 3, mass ratio of H2O2 to COD of 2.0, molar ratio of Fe2+ to H2O2 of 1.11, and reaction time of 2 h, under which all pollutants except NH3-N met the discharge standards in Guangdong Province, China (DB 44/1597-2015), where the maximum concentration of Ag, Fe, Ni, Cu, Zn, Cr, P, F, COD, NH3-N was set as 0.1, 2, 0.5, 0.5, 1, 0.5, 1, 10, 80, 15 mg/L, respectively. Further chlorination treatment via adding NaClO for NH3-N removal showed that the optimal molar ratio of NaClO to NH3-N was 2.89 and the optimal reaction time was 90 min in this study, under which NH3-N removal followed first-order reaction kinetics with rate constant of 0.01831 min-1, NH3-N decreased to 12.6 mg/L satisfying the discharge standard, and COD was simultaneously reduced to 26.7 mg/L. These findings demonstrate the proposed hybrid process of stepwise alkalization, Fenton, and chlorination is promising for electroplating wastewater treatment and resource recovery.