Removal Performance and Mechanism of Cr(VI) by Zero-Valent Iron Doped Chlorapatite

Chromium has been considered as one of the most hazardous heavy metals because of its strong and persistent toxicity to the ecosystem and human beings. In this study, a chlorapatite (CLAP) loaded zero-valent iron (ZVI) composites (ZVI-CLAP) were constructed using Photinia serrulata as green reducing reagent, and applied to the removal of Cr(VI) from aqueous solution. In order to determine the optimum adsorption conditions, a series of single factor experiments such as iron-phosphorus ratio, pH and dosage were carried out. Results indicated that CLAP and ZVI-CLAP exhibited optimal removal efficiency which was 86.59% for Cr(VI) at a mass ratio of ZVI to CLAP of 3:1 and a pH of 3. The adsorption process for both materials predominantly involved monolayer adsorption driven by chemical adsorption, with increasing temperature favoring the adsorption reaction. Finally, the Cr(VI) removal mechanisms of CLAP, both pre- and post-modification, were investigated using BET, SEM, EDS, FT-IR, and XRD analyses. The Cr(VI) adsorption mechanism of ZVI-CLAP involved the reduction of Cr(VI) to Cr(III) by ZVI, followed by the adsorption of Cr(III) onto the CLAP surface through electrostatic adsorption, ion exchange, and complexation. The co-precipitation of phosphate ions with Cr(III) further enhanced the pollutant removal efficiency.