Mechanisms through which reductants influence the catalytic performance of a pyrophosphate-modified Fenton-like process under circumneutral pH conditions

Abstract

The application of reductants to accelerate the transformation from Fe(III) to Fe(II) has been proposed as a strategy to overcome the rate limitations of Fenton and Fenton-like processes. However, this strategy has been given little consideration to date in ligand-modified homogeneous Fenton-like (LMF) processes. Therefore, this study investigated the roles of ordinary reductants, namely gallic acid (GA), ascorbic acid (AA), acetylacetone (AAc), and three kinds of sulfhydryl compounds (SHs), in a pyrophosphate-modified Fenton-like (PMF) process for degrading polychlorinated biphenyls (PCBs) under circumneutral pH conditions. Interestingly, the inhibitory effects of SHs/AAc, the promoting effect of AA, and the inhibitory following with promoting effects of GA were observed on PCB degradation in the PMF process. The decrease of PP-Fe complex reactivity with decrease of pH and the scavenging effect of SHs/AAc caused their inhibitory effects, while the high reducibility of AA resulted in its promoting effect. The inhibitory pathways of GA in the initial stage were similar to those of SHs/AAc, whereas its promoting effect in the later stage was ascribed to the ligand exchange between GA and PP and the reducibility of GA towards Fe3+ ions at pH 3.0. Results obtained in aqueous and soil systems under optimal reaction conditions further reveal that the application of suitable reductants, particularly AA, readily facilitates the development of the PMF process for the removal of refractory organic pollutants in natural environments.