Effect of chelators on the production and nature of the reactive intermediates formed in Fe(II) activated peroxydisulfate and hydrogen peroxide processes

Iron chelators are often used to improve the performance of Fe(II) activated peroxides (e.g., peroxydisulfate (PDS) and hydrogen peroxide (H2O2)) for oxidative water treatment over a wide pH range due to the enhanced solubility of iron in the presence of chelators at high pH. In this study, we compared the effect of various chelators on the production and nature of the reactive intermediate formed in Fe(II)/PDS and Fe(II)/H2O2 systems by using methyl phenyl sulfoxide (PMSO) as a probe, which could distinguish ferryl ion (Fe(IV)) from free radicals (•OH and SO4•-) due to their marked difference in product formation. Six representative chelators (oxalate acid (OA), citric acid (CA), nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), pyrophosphate (PPP), and tetrapolyphosphate (TPP)) which covered the commonly used polycarboxylates, aminocarboxylates, and polyphosphates ligands were selected. In chelator assisted Fe(II)/PDS systems, the highest PMSO transformation efficiency at pH 3-9 was obtained in cases with polycarboxylates, due to their higher reactivity to PDS activation, lower steric hindrance, and stronger ability in promoting Fe(II)/Fe(III) cycle. Comparatively, in chelator assisted Fe(II)/H2O2 systems, TPP addition achieved the best performance in PMSO transformation at pH > 5. Moreover, the yield of Fe(IV) indicative product (methyl phenyl sulfone, PMSO2) decreased with increasing chelator/Fe(II) molar ratio, but was independent on pH in cases of PDS, indicating that chelator altered reactive intermediate nature from Fe(IV) to SO4•- and Fe(IV) yield was not sensitive to pH. In cases of H2O2, chelator decreased PMSO2 production while promoting PMSO loss at near-neutral pH, suggesting that Fe(II)-chelator complexes also tended to catalyze H2O2 to generate •OH rather than Fe(IV).