Further understanding the role of hydroxylamine in transformation of reactive species in Fe(II)/peroxydisulfate system

Hydroxylamine is commonly known to enhance the performance of Fe(II)-based advanced oxidation processes (AOPs) by promoting Fe(II) regeneration from Fe(III). Recently, ferryl ion (Fe(IV)) was found as an important reactive specie in Fe(II)/peroxydisulfate (PDS) system, while the interaction between hydroxylamine and Fe(IV) in Fe(II)/hydroxylamine/PDS system was still ambiguous. Therefore, the role of hydroxylamine, especially in interaction with reactive species, was systematically studied therein. With methyl phenyl sulfoxide (PMSO) as a probe compound, an original function of hydroxylamine to transform the major reactive species from Fe(IV) to radicals (SO4 center dot- and center dot OH) for potential pollutants oxidation was clarified. Experimental results combined with Density Functional Theory (DFT) calculations showed that SO4 center dot-, center dot OH and Fe(IV) all contributed to PMSO oxidation. Hydroxylamine showed the greatest promotional effect on the production of SO4 center dot-, while the production of Fe(IV) was inhibited with concentrations of hydroxylamine increasing from 0.2 to 1.4 mM. Compared with PMSO, hydroxylamine showed stronger scavenging ability towards Fe(IV) than SO4 center dot- and center dot OH. Thus, more than 50% of radicals was consumed by PMSO and Fe(IV) mainly reacted with hydroxylamine to regenerate Fe (II). In a word, hydroxylamine played a dual role during potential pollutants oxidation in Fe(II)/hydroxylamine/ PDS system, namely (1) transforming the major reactive species from Fe(IV) to radials (SO4 center dot- and center dot OH) and (2) regenerating Fe(II) from both Fe(IV) and Fe(III). This study provides an insight into the role of hydroxylamine and its interactions with Fe(IV), which advances our understanding on similar Fe(II)/reductants/PDS systems.