Integrated valorization of rice straw through stigmasterol extraction and KOH-containing residue-derived biochar for methyl tert-butyl ether adsorption and cytotoxicity assessment

Rice straw is an abundant agricultural residue that is often underutilized, despite its potential for circular economy applications. This study develops an integrated bio-refinement method for rice straw valorization by extracting high-value stigmasterol and converting extraction residues into KOH-modified biochar to remove methyl tert-butyl ether (MTBE) from water. Ultrasound-assisted extraction and alcoholic KOH saponification are used to recover stigmasterol. The optimal extraction conditions are pre-ultrasound-assisted extraction saponification, a solid-to-liquid ratio of 1:5 g/mL, an extraction time of 30 min and an extraction power of 600 W, to produce a stigmasterol yield of 307 mg/kg dry rice straw. The extraction residues are pyrolyzed at 600 and 700 ° C to produce rice straw biochar (RSBC). KOH-RSBC-700 features the greatest BET surface area (511 m 2 /g) and pore volume (0.323 cm 3 /g). MTBE adsorption is best described by the pseudo-second-order kinetic model and the Langmuir isotherm model and indicates monolayer adsorption on relatively homogeneous adsorption sites. The maximum adsorption capacity is 6.784 mg/g, with approximately 85% MTBE removal achieved within 12 h at 25 ° C. Thermodynamic analysis shows that the adsorption process is spontaneous and exothermic. Adsorption is primarily governed by pore filling and weak hydrogen-bonding interactions. KOH-RSBC-700 also demonstrates good regeneration performance over 5 adsorption–desorption cycles and remains a high MTBE removal efficiency (80%) in a real wastewater matrix. Cytotoxicity is observed at elevated biochar concentrations but practical groundwater remediation applications minimize direct biological exposure through immobilized treatment configurations. This study demonstrates a sustainable circular economy strategy that maximizes resource utilization and minimizes waste.