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

Production of hydroxyl radicals (•OH) upon the oxidation of solid Fe(II) by O or HO in soils and sediments has been confirmed, which benefits in situ remediation of contaminants. However, Fe(III) reduction by HO 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 HO for phenol degradation. The removal of phenol was 76% in 3 h during soil oxidation with 1 mM HO 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 HO to produced •OH, which was responsible for the effective enhancement of phenol degradation during soil oxidation. Under the conditions of 1 mM HO and 100 g/L soil, the pseudo-first-order kinetic constant of phenol degradation increased proportionally from 0.0453 to 0.0844 min with the increase of hydroxylamine concentrations from 0.5 to 1 mM. The kinetic constant also increased from 0.0041 to 0.0111 min with HO concentration increased from 0.5 to 2 mM, while it decreased from 0.0100 to 0.0051 min 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 HO at 0.025 PV/min. Column experiments suggested an optional application of hydroxylamine and HO for in situ remediation. The output of this study provides guidance and optional strategies to enhance contaminant degradation during soil oxidation.