Valorization of Rice Husk into MgO/Al₂O₃-Modified Biochar for Remediating Aqueous Emerging Contaminants

Emerging contaminants, such as pharmaceutical residues, in aqueous environments provide serious threats to public health, aquatic life, and deteriorate water quality, demanding long-term and economical remediation techniques. These drugs are frequently used all around the world, and high residual concentrations are reported in wastewater across continents, including Asia. Biochar has drawn more attention as an adsorbent due to its high stability, surface functional groups, and potential for surface modification. In this study, biochar derived from rice husk was modified using the co-precipitation method to enhance acetaminophen and trimethoprim adsorption. Engineered biochar (surface area = 419 m²/g) easily adsorbed aqueous acetaminophen and trimethoprim (∼8 h equilibrium time) with adsorption capacities of 69.7–137.4 mg/g and 54.2–269 mg/g, respectively, vs. pristine biochar (surface area = 182 m²/g). The Elovich kinetic model (R² = 0.90-0.99) showed the best correlation for both acetaminophen and trimethoprim. All isotherm models gave R² > 0.95, suggesting simultaneous sorption processes (monolayer/multilayer and homogeneous/heterogeneous) are taking place. Mg or Al leaching from the adsorbent is well within the drinking water limit and not a concern. Spent adsorbent was regenerated using EDTA, HCl, H₂SO₄, ethanol, and methanol. Potential sorption interactions were hydrogen bonding, pore diffusion, π-π interaction, and electrostatic interactions. These findings demonstrate the potential of engineered biochar as a versatile and sustainable water treatment. The study contributes to advancing green materials for environmental remediation and provides insights for scaling biochar technologies within circular-economy frameworks.