Pre-reduction of Goethite by hydroxylamine hydrochloride enhances sulfamethoxazole degradation via accelerated Fe(III)/Fe(II) cycling in Fenton-like systems

The heterogeneous Fenton-like process using ferric oxide is a promising method for environmental remediation, but the slow regeneration of Fe(II) limits the production of reactive oxygen species. This study developed a pre-reducing Fenton system in which hydroxylamine hydrochloride (HA) pre-reduced goethite (α-FeOOH) before H 2 O 2 addition to degrade sulfamethoxazole (SMX). The degradation rate constant in this system was found to be 2.62 times higher than that of the synchronous Fenton system, where H 2 O 2 and HA were added simultaneously. The faster degradation rate can be attributed to efficient Fe(III)/Fe(II) cycling, leading to increased production of ·OH and ·O₂ − radicals. Degradation pathways for SMX were proposed based on eight identified intermediate products, and both acute and chronic toxicity of SMX and its intermediate products were significantly reduced. Anions such as HCO₃ − and NO₃ − hindered SMX removal, while SO₄ 2− and Cl − had negligible effects. Cyclic tests showed that iron oxide remained stable after four reaction cycles. The pre-reducing Fenton system was also effective in degrading intracellular antibiotic resistance genes, promoting their release into the extracellular environment. To facilitate the practical application of pre-reducing Fenton, HA and α-FeOOH were introduced into the aqueous phase of the Electro-Fenton (EF) system to activate the in situ generated H 2 O 2 , resulting in the degradation of SMX at a faster rate compared to the shaking flask experiment. Moreover, the use of α-FeOOH-modified carbon cloth (CC) (α-FeOOH/CC) as the cathode in the EF system demonstrated efficient removal of SMX without the need for exogenous addition of H 2 O 2 and goethite. Overall, this study highlights the effectiveness of the pre-reducing Fenton system in enhancing SMX removal and its potential for in situ application in the EF system. • Pre-reducing Fenton outperformed synchronous Fenton due to higher radical generation. • OH, O₂· − , and Fe IV O 2+ were identified as the primary reactive species. • Pre-reducing Fenton treatment significantly reduced the toxicity of SMX products. • Pre-reducing strategies can be implemented in EF system using in-situ generated H 2 O 2 .