Hydrothermal reduction and phase transformation of Fe(III) minerals induced by rice straw to improve the heterogeneous Fenton degradation of metolachlor

Abstract

Heterogeneous Fenton technology is effective in degrading residual pesticides in soil, but the reduction of Fe(III) in the mineral structure presents a bottleneck. This study combined rice straw with Schwertmannite (Sch), ferrihydrite (Fh), and magnetite (Mag) via a hydrothermal process to obtain iron oxides-hydrothermal carbon composites (Sch@HTC, Fh@HTC, and Mag@HTC). Poor-crystallized Sch and Fh, which were more capable of accepting electrons compared to well-crystallized Mag, exhibited obvious phase transformation to highly active Fe(II)-mineral (humboldtine) via the combination of oxalic acid, an intermediate product, with reduced Fe(II), while Mag was hard to achieve. After hydrothermal treatment, all composites showed enhanced catalytic activity, which increased with the degree of phase transformation. Especially, Sch@HTC demonstrated the highest catalytic activity, degrading 85 % of metolachlor in soil within 24 hours, 2–10 times faster than the others. Surprisingly, the solid-phase Fe(II) in soil increased slightly after the Fenton reaction. Moreover, the in-situ fluorescence intensity of HO^• in soil was continuously enhanced, and the effective utilization of H₂O₂ to HO^• was improved. These results confirmed that HTC could provide electrons to Fe(III) during the hydrothermal process, facilitating the Fe(III)/Fe(II) redox cycle and sustaining reactive Fe(II), thus overcoming key challenges in heterogeneous Fenton catalysis.