Sulfur-Functionalized Rice Straw Biochar for Enhanced Cadmium Sorption: Spectroscopic, Kinetic and Computational Insights

Cadmium (Cd) contamination necessitates effective remediation approaches. This study valorizes rice straw biochar by enhancing its Cd sorption capacity through sulfur functionalization. Biochar was functionalized with sulfur compounds (H₂SO₄, Na₂S, and Na₂S₂O₃) to introduce Lewis soft base characteristics for improved affinity towards Cd(II), a Lewis soft acid. Characterization confirmed successful grafting of various sulfur groups, increasing the sulfur content from 0.33% to 13.27% and boosting oxygen-containing functional groups. The sulfur-functionalized biochar demonstrated significantly enhanced Cd(II) sorption compared to pristine biochar, with a maximum capacity of 34.40 mg g^-1 at pH 5.0 – a 17.3-fold increase – and reduced equilibrium time from 8 hours to 5-10 minutes. Density functional theory calculations suggested that sulfur doping facilitated Cd-S bond formation and modulated the electronic properties of the biochar-Cd systems, contributing to the superior sorption performance. Sulfur functionalization offers a cost-effective, eco-friendly method for enhanced Cd(II) removal, validated by experiments and theory.