Hydroxylamine promoted Fe(III) reduction in H₂O₂/soil systems for phenol degradation

Production of hydroxyl radicals (•OH) upon the oxidation of solid Fe(II) by O₂ or H₂O₂ in soils and sediments has been confirmed, which benefits in situ remediation of contaminants. However, Fe(III) reduction by H₂O₂ is rate-limiting. Accelerating the Fe(III)/Fe(II) cycle could improve the efficiency of remediation. This study intended to use hydroxylamine to promote Fe(III)/Fe(II) cycle during 100 g/L soil oxidation by H₂O₂ for phenol degradation. The removal of phenol was 76% in 3 h during soil oxidation with 1 mM H₂O₂ in the presence of 1 mM hydroxylamine but was negligible in the absence of hydroxylamine. Fe(III) in the soil was reduced to 0.21 mM Fe(II) by 1 mM hydroxylamine in 30 min. The accelerated cycle of Fe(III)/Fe(II) in the soil by hydroxylamine could effectively decompose H₂O₂ to produced •OH, which was responsible for the effective enhancement of phenol degradation during soil oxidation. Under the conditions of 1 mM H₂O₂ and 100 g/L soil, the pseudo-first-order kinetic constant of phenol degradation increased proportionally from 0.0453 to 0.0844 min^-1 with the increase of hydroxylamine concentrations from 0.5 to 1 mM. The kinetic constant also increased from 0.0041 to 0.0111 min^-1 with H₂O₂ concentration increased from 0.5 to 2 mM, while it decreased from 0.0100 to 0.0051 min^-1 with soil dosage increased from 20 to 200 g/L. In addition, column experiments showed that phenol (10 mg/L) degradation ratio kept at about 48.7% with feeding 2 mM hydroxylamine and 2 mM H₂O₂ at 0.025 PV/min. Column experiments suggested an optional application of hydroxylamine and H₂O₂ for in situ remediation. The output of this study provides guidance and optional strategies to enhance contaminant degradation during soil oxidation.