Urea/acrylamide-assisted nitrogen-functionalized rice husk biochar for Cd(ii) removal: adsorption performance and...

Cadmium-bearing wastewater remains a major environmental concern, requiring effective sorbents that can operate under complex aqueous conditions. Herein, urea/acrylamide-assisted nitrogen-functionalized activated rice husk biochar, denoted UA-NARB, was synthesized through preliminary carbonization followed by K₂CO₃-assisted activation and nitrogen functionalization using urea and acrylamide as nitrogen-containing precursors. XRD, FTIR, SEM, N₂ adsorption-desorption, DLS, and zeta potential analyses revealed a heterogeneous carbon-mineral sorbent with disordered carbon domains, O-/N-containing functionalities, accessible mesoporosity, and a moderately negative surface charge. Cd(ii) adsorption was strongly pH-dependent, reaching 94.5% removal at pH 6, which was selected as the working condition. A sorbent dose of 2.0 g L^-1 provided an effective balance between Cd(ii) removal and material utilization. Kinetic analysis showed rapid initial uptake followed by gradual equilibration, with the pseudo-second-order model providing the best empirical description. Equilibrium adsorption exhibited saturation-type behavior and was well described by the Langmuir model, with a maximum adsorption capacity of 14.6 mg g^-1. Thermodynamic parameters indicated spontaneous and endothermic Cd(ii) uptake. From an application perspective, UA-NARB showed efficient regenerability, with 1.0 M H₂SO₄ desorbing 92.7% of adsorbed Cd(ii), and maintained 80.7% Cd(ii) removal from synthetic multimetal wastewater. The adsorption process is proposed to proceed through cooperative contributions from pH-regulated electrostatic attraction, interactions with polar surface functionalities and mineral-associated sites, and diffusion through accessible mesoporous domains. These results position UA-NARB as a rice-husk-derived sorbent with practical potential for Cd(ii)-contaminated wastewater treatment.