The introduction of pyrochar boosts the generation of singlet oxygen in a ferrihydrite-based Fenton-like system to facilitate the Cr(III)-citrate decomplexation under Cr(VI) inhibition

Abstract

Cr(III)-organic complexes are frequently detected in tanning effluent, but they are challenging to elimination. Ferrihydrite (Fh)/carbon materials (CMs) composites showed superior performance in Fenton-like oxidation process, but their effectiveness and mechanism in Cr(III)-organic complex treatment need to be deep explored. In this study, pyrochar (PC)/Fh compounds were successfully synthesized and the experimental results found that the 5%-PC/Fh/H2O2 system exhibited efficient elimination of Cr(III)-citrate complexes with 89% reduction in TOC and almost 100% elimination of total Cr. The cooperation of PC with Fh promoted the decomposition of H2O2 by accelerating the electron transfer between Fe(III)/Fe(II) pairs. In contrast to the traditional Fenton-like catalysts, 5%-PC/Fh equipped with Fe–O–C bond could boost the production of 1O2, resulting in decomplexation of Cr(III)-citrate rather than direct attack of the Cr(III) site in the Cr(III)-citrate complex. Consequently, Cr(VI) could not be produced in the 5%-PC/Fh/H2O2 system. Moreover, the developed 5%-PC/Fh/H2O2 system exhibited enhanced reusability and excellent stability when applied to natural water and real wastewater. Finally, the oxidation intermediates could even promote the growth of C. vulgaris, indicating the structure of Cr(III)-citrate was destroyed and their toxicity was decreased. All these phenomena suggested the great potential of the 5%-PC/Fh/H2O2 system for practical wastewater treatment. This study has provided theoretical guidance and technical support for the efficient treatment of Cr(III)-organic complexes in real wastewater.