Role of inorganic ions and dissolved natural organic matters on persulfate oxidation of acid orange 7 with zero-valent iron

Abstract

The impacts of common anions and organic matter, initial pH and PS dosage on the oxidation of acid orange 7 (AO7) by persulfate (PS) activated with zero-valent iron (ZVI) were investigated. The present findings revealed that maximum AO7 decolorization occurred at pH 3.0, and increasing the system pH resulted in a greater decrease in AO7 decolorization rates. AO7 decolorization efficiency was 100% at 120 min when the molar ratio of PS : AO7 was 5 : 1. Interestingly, ClO₄⁻, CH₃COO⁻ and humic acid (HA) were found to accelerate AO7 decolorization rates while other anions retarded AO7 decolorization in the following sequence: NO₂⁻ > H₂PO₄⁻ > HPO₄²⁻ > EDTA > SO₄²⁻ > CO₃²⁻ > HCO₃⁻ > NO₃⁻ > Cl⁻. ClO₄⁻, CH₃COO⁻ and HA with 50 mM, 10 mM and 1.0 mg L⁻¹, respectively were found to be the optimal concentrations for AO7 decolorization. The removal efficiencies of AO7 were decreased by 90.3%, 51.5%, 58.7% and 38.2%, respectively over 120 min in addition of NO₂⁻ (50 mM), H₂PO₄⁻ (50 mM), HPO₄²⁻ (50 mM) and EDTA (50 mM). The other anions including SO₄²⁻, CO₃²⁻, HCO₃⁻, NO₃⁻ and Cl⁻ led to a decrease change of less than 20%. The mechanisms for the influence were complexation reactions with Fe²⁺ generated from ZVI, consuming of sulfate radicals (SO₄⁻˙) by scavenging reactions, and oxidation reactions involving inorganic ions. The reason for the acceleration by CH₃COO⁻ and HA was probably through acting as an electron ‘shuttle’ and facilitating electron transfer from the ZVI surface to PS and resulted in more Fe²⁺ and SO₄⁻˙. However, the acceleration caused by ClO₄⁻ was presumably ascribed to the oxidizing of ZVI directly by ClO₄⁻ to produce more Fe²⁺.