Efficient Cr(VI) Removal from Acidic Wastewater by Tannic-Acid/Fe3O4-Modified Corn Straw Biochar: Performance and Mechanism

The problem of chromium contamination, especially Cr(VI), in acidic wastewater has drawn significant attention, requiring effective and sustainable remediation measures. In this study, tannic-acid/Fe3O4-modified corn straw biochar (Fe-TA-CSB) is prepared by a grinding-calcination method to remove Cr(VI). The factors influencing the removal effect of Fe-TA-CSB are investigated through static adsorption experiments. The removal mechanism is explored by combining adsorption kinetics, isothermal adsorption, and thermodynamics, as well as characterization methods. The results show that the removal efficiency of Cr(VI) increases with the increase in pH, contact time (t), and solid–liquid ratio (m/v), but decreases with the increase in initial concentration (C0). Under optimal conditions of TA/Fe3O4 mass ratio = 12.5%, pH = 3.0, m/v = 1.0 g/L, and C0 = 10 mg/L, the removal efficiency value is 94.02%, which is approximately 81.44% after four adsorption–desorption cycles. The adsorption behavior is fitted well by the Sips isotherm model and Elovich kinetics model, suggesting the adsorption process of heterogeneous monolayer chemisorption. The removal mechanism of Cr(VI) by Fe-TA-CSB involves electrostatic interaction with Cr(VI), reduction in Cr(VI) to Cr(III) through C–O and Fe(II), and complexation of reduced Cr(III) with the introduced Fe–O and phenolic hydroxyl groups. Fe-TA-CSB is an environmentally friendly and renewable adsorbent with good potential for the treatment of acidic wastewater.