Enhanced diclofenac elimination in Fe(II)/peracetic acid process by promoting Fe(III)/Fe(II) cycle with ABTS as electron shuttle

Abstract

Fe(II)/peracetic acid (Fe(II)/PAA) process exhibits a great potential in abatement of micropollutants because of the fast reaction between Fe(II) and PAA. Similar to Fenton system, the slow reduction from Fe(III) to Fe(II) leads to the high dosage of Fe(II) in Fe(II)/PAA process. This study introduced 2,2′-azino-bis(3-ethylbenz- -othiazoline-6-sulfonate) (ABTS) as electron shuttle to improve the oxidation capacity of Fe(II)/PAA process by accelerating the redox cycle of Fe(III)/Fe(II). ABTS/Fe(II)/PAA process possessed high elimination efficiency of diclofenac in acidic conditions. Alcohols quenching experiments confirmed that ABTS•+ was the primary reactive species responsible for diclofenac elimination, although FeIVO2+ and carbon-centered radicals (R-O•) also existed in ABTS/Fe(II)/PAA process. ABTS•+ was generated via various pathways, including the reactions of ABTS with Fe(III), FeIVO2+ and R-O•. ABTS was not an ideal electron shuttle because R-O• could decompose partial ABTS•+ to sulfoxides and sulfones. Two possible degradation pathways of diclofenac were proposed based on the detected transformation products. ABTS/Fe(II)/PAA process was highly-efficient for diclofenac elimination in underground water and reservoir water, while high concentration of humic acid had inhibition effect on diclofenac degradation. The present study provides a promising way to alleviate the intrinsic drawbacks of Fe(II)/PAA process and might advance its potential application in water treatment.