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.