Hybrid tungstate-functionalized nitrogen-doped rice husk biochar for selective dual adsorption of methylene blue and methyl orange.

The development of multifunctional and sustainable sorbents for simultaneous removal of oppositely charged dyes remains a significant challenge in wastewater treatment. In this study, a tungstate-modified rice husk biochar composite (WRHB) was engineered through the synergistic integration of K2CO3 activation, nitrogen heteroatom doping, and sodium tungstate functionalization. The resulting hybrid material exhibits a mesoporous structure (SBET = 145.4 m2 g-1; pore diameter ≈ 3.8 nm), an alkaline surface charge (pHpzc ≈ 8.6), and uniformly dispersed crystalline Na2WO4 domains within an amorphous carbon matrix. Adsorption behavior toward methylene blue (MB) and methyl orange (MO) was systematically investigated under identical conditions. Under equilibrium isotherm conditions of pH 6.01, an initial dye concentration range of 20-200 mg L-1, a WRHB dosage of 3.0 g L-1, a contact time of 240 min, and a temperature of 25 ± 1 °C, maximum Langmuir monolayer capacities reached 24.2 mg g-1 for MB and 31.5 mg g-1 for MO, with Sips modeling confirming quasi-monolayer adsorption on energetically comparable sites. Kinetic data followed the pseudo-second-order model, indicating surface-site-controlled uptake with multi-step diffusion contributions. Thermodynamic analysis revealed spontaneous and exothermic adsorption (ΔG° < 0; ΔH° = -23.4 and -46.3 kJ mol-1 for MB and MO, respectively), with stronger affinity toward MO. Mechanistically, adsorption proceeds through cooperative electrostatic attraction, π-π stacking, hydrogen bonding, and ion-dipole interactions involving tungstate-derived polar domains. The sorbent demonstrated high regenerability using dilute mineral acids and maintained strong decolorization efficiency (≈85%) in real textile wastewater. The combined structural tunability, dual-dye functionality, and operational stability position WRHB as a scalable and application-oriented platform for dye-contaminated wastewater remediation.