Degradation of three typical hydroxamic acids collectors via UVA-B activated H2O2 and persulfate: Kinetics, transformation pathway, DFT calculation and toxicity evaluation

Abstract

This work systematically studied the kinetics and mechanism of degradation of salicylhydroxamic acid (SHA), benzhydroxamic acid (BHA) and N-hydroxyphthalimide (NOP) by UVA-B/H2O2 and UVA-B/peroxodisulfate (PDS). UVA-B irradiation could induce a direct photolysis of SHA and dominated SHA destruction in both systems. BHA and NOP were effectively degraded via HO•- and SO4•−-mediated oxidation. UVA-B/PDS displayed a better degradation performance for HAAs investigated than UVA-B/H2O2. An acidic pH was more suitable for three HAAs removal in the UVA-B/H2O2 system. However, basic pH was more efficient for HAAs degradation in the UVA-B/PDS system. The degradation of BHA and NOP was predominantly driven by SO4•− at all pH levels used (5.0–9.0). The second-order rate constants for SHA, BHA and NOP reactions with HO• and SO4•− were calculated to be (4.16–5.22) × 109 M−1·s−1 and (1.19–7.22) × 109 M−1·s−1, respectively. Presence of various water constituents had different influence on HAA removal, with a enhancement in the presence of HCO3–, Fe2+ and Cu2+. When real waters were used as a background, dissolved organic carbon and Cl− were the main factors that consumed radicals and affected the degradation performance of HAAs. Analysis of the transformation products and density functional theory revealed that all of the investigated HAAs first generated amidated products but the formation mechanisms might have been different. HAAs degradation pathways mainly included hydrolysis, hydroxylation, decarboxylation and ring opening processes. Toxicity evaluation showed that the UV/AOP degradation of HAAs generated some transformation products with higher acute toxicity than the parent compounds.