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

Abstract

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.