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

Though hydroxylamine (NHOH) 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 (NHOH·HCl) and hydroxylamine sulfate [(NHOH)·HSO], for the degradation of dimethyl phthalate (DMP) in peroxymonosulfate (PMS)/Fe(II) system was comparatively investigated. Degradation efficiency of DMP with NHOH·HCl was 1.6 times of that with same dosages of (NHOH)·HSO. SO, Fe(IV) and ·OH formed in the PMS/Fe(II)/NHOH 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) was more than 10 times that of CT 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 SO and Cl· with HO 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 (SO 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.