Tailoring the surface chemistry of rice husk biochar through N,S functionalization for enhanced methyl orange adsorption

Abstract

Rice husk, an abundant agricultural by-product in Vietnam, holds great potential as a sustainable precursor for carbon materials but remains largely underexploited. Conventional rice husk biochars often exhibit low surface area and poor adsorption performance. To address this limitation, a nitrogen and sulfur co-doped biochar (N,S–RBC) was synthesized through a simple and low-cost route, achieving efficient removal of the anionic dye methyl orange (MO). The N,S–RBC exhibited a high surface area (1483.34 m2 g−1) and a well-developed micro–mesoporous network, leading to a maximum adsorption capability of 454.54 mg g−1 according to the Langmuir model. Kinetic analysis revealed that the adsorption followed a pseudo-second-order model (R2 = 0.999), suggesting a chemisorption-dominated process involving electron-rich surface groups such as –NH2,–C=S,–SOx, and –COOH. Thermodynamic studies confirmed the spontaneous (ΔG° < 0) and endothermic (ΔH° = 38.69 kJ mol−1) nature of the uptake, accompanied by an increase in system randomness (ΔS° = 172.37 J mol·K−1), implying a mixed physisorption–chemisorption mechanism with monolayer adsorption predominance. The N,S–RBC achieved a uptake efficiency of 96.17% within 120 min and retained over 70% efficiency after six regeneration cycles, demonstrating acceptable stability and reusability. Furthermore, the presence of coexisting ions (Na+, K+, Ca2+, SO42−) had negligible effects on performance, indicating suitability for high-salinity wastewater. With its superior adsorption capacity, rapid kinetics, multi-interaction mechanism, and sustainable recyclability, N,S–RBC shows potential as a bio-adsorbent for practical applications in the treatment of dye-laden and persistent organic pollutants in textile wastewater.