Overlooked enhancement of chloride ion on the transformation of reactive species in peroxymonosulfate/Fe(II)/NH2OH system

Abstract

Though hydroxylamine (NH2OH) is effective for accelerating pollutants degradation in Fenton and Fenton-like systems, the effect of anions simultaneously introduced by the hydroxylamine salts have always been ignored. Herein, effect of two commonly used hydroxylamine salts, hydroxylamine hydrochloride (NH2OH·HCl) and hydroxylamine sulfate [(NH2OH)2·H2SO4], for the degradation of dimethyl phthalate (DMP) in peroxymonosulfate (PMS)/Fe(II) system was comparatively investigated. Degradation efficiency of DMP with NH2OH·HCl was 1.6 times of that with same dosages of (NH2OH)2·H2SO4. SO4·−, Fe(IV) and ·OH formed in the PMS/Fe(II)/NH2OH system, but ·OH was the major species for DMP degradation. Addition of Cl− significantly improved the production of ·OH and Cl·, and the exposure dose of ·OH (CT·OH) was more than 10 times that of CTCl· as the concentration of Cl− increased to 1 mM. Calculations based on branching ratios of Cl· and ·OH indicated that the reactions of Cl− with SO4·− and Cl· with H2O were not the only production sources of ·OH in the system. Further experiments with methyl phenyl sulfoxide (PMSO) as the probe indicated that Cl− would facilitate the shift of reactive species from Fe(IV) to radicals (SO4·− or ·OH) in the system. Both hydroxylation and nitration intermediate products were detected in the oxidation of DMP. Cl− promoted the formation of hydroxylation intermediates and reduced the formation of nitration intermediates. This study revealed for the first time that Cl− could shift reactive species from Fe(IV) to radicals in PMS/Fe(II) system, raising attention to the influence of the coexisting anions (especially Cl−) for pollutants oxidation in iron-related oxidation processes.