Potential for colloid removal from petrochemical secondary effluent by coagulation-flocculation coupled with persulfate process

The selective removal and characterization of colloids from petrochemical secondary effluent by treatment with coagulation-flocculation (CF) coupled with the persulfate (PS) process was investigated. The results showed that the optimal dosages for CF-PS were as follows: 25.0 mg L-1 poly aluminium chloride (PAC), 6.0 mmol L-1 Fe2+, and a Na2S2O8 : 12COD ratio of 1.0; the removal ratio of dissolved organic carbon (DOC) reached 55.0%. The importance of the factors affecting the efficiency followed the order Fe2+ > S2O82- > pH, and the DOC removal followed the pseudo-first-order kinetic model. Ultrafiltration technology was used to analyze the molecular weight (MW) distribution of the colloids and to investigate the selective removal of the colloids. CF-PS can effectively remove high-MW (>50 K) colloids, with a DOC removal of 41.3%. PS-only treatment can effectively remove medium-WM (3-50 K) and low-MW (<3 K) colloids, with DOC removals of 62.7% and 40.3%, respectively. Excitation emission matrix fluorescence spectroscopy combined with fluorescence regional integration analysis indicated that the medium-MW and low-MW fractions were more removed for the fulvic-acid-like, soluble microbial metabolite, and humic-acid-like substances by the PS process. Moreover, CF-PS, with the addition of the CF pretreatment, enhanced the removal of high-MW and medium-MW colloids in the tyrosine-like and tryptophan-like substances. The relationship between the fluorescence and the DOC further reveals that humic-acid-like/soluble microbial metabolite substances are the dominant contributors to colloidal organic carbon in the CF-PS. This study provides a new approach to evaluate the colloid selectivity during PS-related processes and advanced oxidation processes.