Persistent free radicals in biochar enhance superoxide-mediated Fe(III)/Fe(II) cycling and the efficacy of CaO2 Fenton-like treatment

Abstract

Superoxide radicals (O2•-) produced by the reaction of Fe(III) with H2O2 can regenerate Fe(II) in Fenton-like reactions, and conditions that facilitate this function enhance Fenton treatment. Here, we developed an efficient Fenton-like system by using calcium peroxide/biochar (CaO2/BC) composites as oxidants and tartaric acid-chelated Fe(III) as catalysts, and tested it for enhanced O2•--based Fe(II) regeneration and faster sulfamethoxazole (SMX) degradation. SMX degradation rates and peroxide utilization efficiencies were significantly higher with CaO2/BC than those with CaO2 or H2O2 lacking BC. The CaO2/BC system showed superior activity to reduce Fe(III), while kinetic analyses using chloroform as a O2•- probe inferred that the O2•- generation rate by CaO2/BC was one-half of that by CaO2. Apparently, O2•- is utilized more efficiently in this system to regenerate Fe(II) and enhance SMX degradation. Additionally, a positive correlation between SMX degradation rate constants and EPR signal intensities of biochar-derived persistent free radicals (PFRs) in CaO2/BC was obtained. We postulate that PFRs enhanced Fe(III) reduction by shuttling electrons donated by O2•-. This represents a new strategy to augment the ability of superoxide to accelerate Fe(III)/Fe(II) cycling for increased hydroxyl radical production and organic pollutant removal in Fenton-like reactions.