Degradation of p-arsanilic acid and simultaneous in-situ removal of arsenic species with ferrate(VI): Kinetics, intermediate and degradation pathway

In this study, potassium ferrate(VI) was employed for the degradation of p-arsanilic acid (p-ASA) and simultaneous removal of the generated arsenic species though in-situ coagulation. The effects of several factors, including ferrate(VI) dosage, initial pH, coexisting anions, and humic acid, on the p-ASA degradation and arsenic species removal were investigated. Under various water quality conditions, an excellent removal of p-ASA could always be achieved with the 20:1 mol radio dosage of ferrate(VI) and p-ASA within 30 min. But the removal of arsenic species is significantly affected by the water quality. It could be inhibited by alkalinity and presence of phosphate and humic acid. The generated ferric oxide was mainly 2-line ferrihydrite according to the XRD and XPS analysis. Based on the HPLC-HG-AFS and GC-MS analysis, the major degradation products were detected and the mechanism of the p-ASA degradation by ferrate(VI) and the released arsenic removal by 2-line ferrihydrite generated from ferrate(VI) was given. Two simultaneous degradation pathways of p-ASA were proposed. The p-ASA in solution attacked by ferrate(VI) in solution would lead cleavage of the As-C bond firstly to generate As(III) and p-aminophenol. On the other hand, small amounts of p-ASA initially absorbed on 2-line ferrihydrite would be firstly converted to 4-nitrophenylarsonic acid through the oxidation of -NH2 to -NO2 by ferrate(VI), subsequently transformed to As(III) and p-nitrophenol. Combined with the strong oxidizing and the high removal capacity of released arsenic by its generated product, ferrate(VI) have well potential applications for controlling the risk of p-ASA in wastewater.