In-situ resource recovery of Fenton sludge via sulfuric acid leaching and zero-valent iron reduction

This study aims to develop an economical, facile, and efficient approach for the recovery and reuse of Fenton sludge (FS) by proposing an in-situ reutilisation strategy that integrates acid leaching with micron-sized zero-valent iron (mZVI) reduction. Given the high iron content of FS (50.68%), single-factor experiments were performed to investigate the influence of critical parameters in acid leaching (liquid-to-solid ratio, sulfuric acid concentration, and acid leaching time) and mZVI reduction (mZVI/Fe³⁺ (mol/mol), reaction time, and stirring speed). Furthermore, response surface methodology was applied to optimise the process parameters. The optimisation results showed that the optimal acid leaching conditions were a liquid-to-solid ratio of 2.1, a sulfuric acid concentration of 6.2 mol/L, and an acid leaching time of 32 min, achieving a total iron leaching efficiency of 94.24%. For mZVI reduction, the optimal conditions were an mZVI/Fe³⁺ of 2.15, a reaction time of 134.4 min, and a stirring speed of 156.7 r/min, resulting in an Fe³⁺ reduction efficiency of 99.32%. Comparative experiments confirmed that the TOC removal efficiency of the FS-derived reduction solution was comparable to that of commercial FeSO₄ in treating aniline-containing wastewater, achieving 44.11% and 44.36%, respectively. Economic analysis showed that the treatment cost using FS decreased from 0.63 $/kg-FS to 0.32 $/kg-FS. This study provides a feasible pathway for energy conservation and emission reduction in the Fenton process by minimising FS generation and enabling its in-situ resource recovery.