K2S-mediated hierarchical pore engineering in lignin-derived sulfur-enriched activated carbons for enhanced elemental mercury capture

Sulfur-functionalized porous carbons have garnered significant attention for multifunctional applications ranging from energy storage to environmental remediation. This study introduces an innovative single-step K₂S activation strategy for converting renewable lignin into high-surface-area sulfur-enriched porous carbons with hierarchical porosity. The results showed that the activation parameters of K₂S/lignin mass ratios (0.5:1 to 3:1) and thermal activation temperatures (600–850 °C) critically govern the resultant pore structure, sulfur content (5.0–12.5 wt%), and sulfur speciation distribution. The characterization reveals three dominant sulfur configurations: elemental (S⁰), thiophenic (C–S–C), and sulfonic (C–S=O) moieties, with their relative distribution being thermally modulable. Post-synthesis treatments through methanol extraction and thermal annealing (800 °C/N₂) were found to effectively eliminate pore-blocking S⁰ species while enhancing textural properties, achieving exceptional Brunauer–Emmett–Teller surface areas (2500 m² g⁻¹) and total pore volumes (2.4 cm³ g⁻¹) at optimal conditions. This pore evolution suggests a dual mechanism where elemental sulfur acts as both a porogen and reactive intermediate during K₂S activation. The optimized sulfur-containing porous carbons exhibited superior elemental mercury adsorption capacity (44 mg g⁻¹), outperforming conventional sulfur-impregnated carbons. This work establishes K₂S activation as a sustainable paradigm for fabricating hierarchically sulfur-containing porous carbons, combining renewable feedstocks with tailorable surface chemistry for advanced environmental applications.

Graphical abstract