Precursor-dependent reconstruction of iron-carbon interface for high-efficiency removal of Cr(VI)

Using kitchen waste digestate as the feedstock, we assessed the impacts of precursor carbonization degree on modified chars' characterization, compared their Cr(VI) removal behavior, and clarified the high-efficiency Cr(VI) removal mechanisms. K₂FeO₄ promoted pore development, surface oxygen-containing group enrichment, and in-situ anchoring of iron species in torrefied chars and hydrochars, whereas it mainly introduced iron-based active sites in pyrolysis chars. K₂FeO₄ modification effectively improved the Cr(VI) removal performance of the chars. The equilibrium adsorption capacity increased by 83.8-100.8% from 39.46 to 54.29 mg·g^-1 to 79.24-100.20 mg·g^-1. Isotherm results suggested monolayer adsorption for modified torrified char and heterogeneous multilayer adsorption for modified pyrolysis char and hydrochar. The maximum adsorption capacity of modified torrified char increased from 90.91 to 96.91 mg·g^-1, whereas the Freundlich adsorption coefficient of modified pyrolysis char and hydrochar increased markedly by 467% and 39%, respectively. The thermodynamic driving force, heat‑absorbing property, and randomness at solid‑liquid interface of Cr(VI) removal were evidently promoted. Cr(VI) was removed by K₂FeO₄-modified chars via several mechanisms, including surface electrostatic accumulation, inner-layer complexation capture, reduction of Cr(VI) to Cr(III), and Fe/Cr co-precipitation. These findings lay a solid scientific foundation for optimizing the preparation and usage of the K₂FeO₄ modified chars for Cr(VI) removal.