Sulfur-functionalized sawdust biochar for enhanced cadmium adsorption and environmental remediation: A multidisciplinary approach and density functional theory insights

Pristine biochar typically exhibits limited capacity for heavy metal adsorption due to its inadequate pore development and insufficient surface functionality. This study introduces an innovative chemical strategy to enhance the surface of sawdust biochar with sulfur-based functional groups (C=S, C-S, S-S, S^2-, S-H, -SO₃^2-, -SO₄^2-) to significantly improve cadmium (Cd) adsorption. Sulfur-doping using H₂SO₄, Na₂S, and Na₂S₂O₃ markedly increased the sulfur content from 0.11% (pristine) to 2.81% (H₂SO₄), 0.57% (Na₂S), and 13.27% (Na₂S₂O₃). Characterization techniques such as SEM-EDS, FTIR, and XPS confirmed the successful incorporation of sulfur moieties and additional oxygen-containing groups, improving surface functionality. The Cd adsorption capacity of S-modified biochar increased by 4.8-9.0 times compared to pristine biochar, with peak values of 39.38, 20.84, and 34.14 mg g^-1 for H₂SO₄, Na₂S, and Na₂S₂O₃-modified biochar, respectively. The equilibrium time was significantly reduced from 4 h (pristine) to 5-10 min (S-modified). The enhanced Cd adsorption was attributed to the synergistic interplay of electrostatic attraction, cadmium-π electron interactions, complexation, and ion exchange mechanisms, facilitated by the presence of oxygen and sulfur functional groups. Density Functional Theory (DFT) calculations showed that sulfur doping modulated the electronic properties of the biochar-Cd systems, narrowing the band gap and enhancing the Cd-O bonds, thereby improving the Cd adsorption performance. Additionally, the binding energies of the S-modified biochar-Cd complex were found to be more stable compared to those before Cd adsorption. This study demonstrates that both oxygen and sulfur-functionalized sawdust biochar is an effective and eco-friendly adsorbent for Cd removal, highlighting the significance of tailored surface modifications to augment biochar's reactivity and affinity towards specific contaminants. The developed material offers a sustainable and scalable solution for Cd removal from aqueous environments, contributing to advanced water treatment technologies and environmental remediation strategies.