ZVI and ball milling co-modified biochar for chromium (Cr(Ⅵ)) removal from groundwater: Performance optimization and mechanisms

Abstract

Ball milling (BM) and Fe(0) (ZVI) co-modified walnut shell biochar (BC) was used for Cr(VI) removal from groundwater. In this study, the effects of pyrolysis temperature, ball milling conditions, and key factors (solution pH, dosage, and co-existing ions) on Cr(VI) removal, as well as the changes in Cr(Ⅲ) concentrations during the removal process, were investigated. Results revealed that the Cr(VI) removal efficiency of BM-ZVI-BC reached 90.92 %, which was significantly higher than that of BC prepared at 700 ℃ (38.48 %). Notably, BM-ZVI-BC performed high removal efficiency (90.92 % - 99.79 %) and adsorption capacity (9.09 mg·g⁻¹ - 9.98 mg·g⁻¹) across a wide pH range (2.00–7.00) and at a low dosage (0.50 g·L⁻¹). NO₃^- co-existing ions enhanced Cr(VI) removal, while HCO₃^- significantly inhibited it. Adsorption data were fully consistent with the Freundlich and pseudo-second-order models, indicating that multilayer adsorption and chemisorption were the primary mechanisms in the removal process. XRD and XPS analyses showed that Fe(Ⅱ) reduced Cr(VI) to Cr(Ⅲ), which was identified as the dominant removal mechanism. Furthermore, BM-ZVI-BC maintained a high Cr(VI) removal efficiency of over 80 % after five cycles of adsorption-desorption. Therefore, BM and ZVI co-modification of BC provided a promising adsorbent, with potential applications in the development of green, sustainable, and efficient remediation technologies.