Oxidation of 1,3-diphenylguanidine (DPG) by goethite activated persulfate: Mechanisms, products identification and reaction sites prediction

As emerging pollutants continue to be discovered, studies on the degradation behavior of emerging pollutants have proliferated, but few studies have focused on the reactivity of the new pollutants themselves. The work investigated the oxidation of a representative roadway runoff-derived organic contaminant, 1,3-diphenylguani-dine (DPG) by goethite activated persulfate (PS). DPG exhibited the highest degradation rate (k(d) = 0.42 h(-1)) with present of PS and goethite at pH 5.0, then started to decrease with increasing pH. Chloride ion inhibited DPG degradation by scavenging HO center dot. Both HO center dot and SO4-center dot were generated in goethite activated PS system. Competitive kinetic experiments and flash photolysis experiments were conducted to investigate free radical reaction rate. The second-order reaction rate constants for DPG reacting with HO center dot and SO4-center dot were quantified (kDPG + HO center dot, kDPG + SO4-center dot), which both reached above 10(9) M-1 s(-1). Chemical structures of five products were identified, four of them were previously detected in DPG photodegradation, bromination and chlorination processes. By density functional theory (DFT) calculations, ortho-and para-C were more easily attacked by both HO center dot and SO4-center dot. Abstraction of H on N by HO center dot and SO4-center dot were the favorable pathways, and the product TP-210 might be generated by cyclization of DPG radical from abstraction of H on N (3). The results of this study help us to better understand the reactivity of DPG with SO4-center dot and HO center dot.