One-step microwave dual-alkali activation of porous biochar for efficient Cr(VI) removal

Hexavalent chromium (Cr(VI)) is a highly toxic and mobile heavy metal pollutant. Developing efficient, low-cost, and sustainable adsorbents for Cr(VI) removal from wastewater is urgently needed. In this study, wet okara (soybean-processing residue) was used as a biomass precursor to fabricate porous biochar via one-step microwave-assisted KOH/K 2 CO 3 activation. The synergistic activation effect of dual alkalis, together with the rapid volumetric heating provided by microwave treatment, enabled the simultaneous construction of abundant micropores and mesopores. The optimized biochar, denoted as BC-KOH + K 2 CO 3 , exhibited a high specific surface area of 941 m 2 ·g −1 , a total pore volume of 0.44 cm 3 ·g −1 , and a micropore volume of 0.31 cm 3 ·g −1 , indicating the successful formation of a micropore-dominated hierarchical porous structure. Batch adsorption experiments demonstrated that BC-KOH + K 2 CO 3 displayed excellent Cr(VI) removal performance from a 100 mg/L solution under acidic conditions, achieving a maximum adsorption capacity of 250.74 mg·g −1 at pH = 1. The adsorption capacity reached 230.8 mg·g −1 at pH = 2 in the first adsorption cycle and remained at 107.7 mg·g −1 in the fourth adsorption cycle, and showed strong resistance to interference from common coexisting ions. The dual-alkali activation strategy also improved the adsorption performance of biochars derived from other biomass precursors (rice husk and bran), demonstrating preliminary cross-feedstock transferability at the adsorption-performance level. This work provides a rapid, energy-efficient, and generalizable approach for converting agricultural and food-processing wastes into high-performance biochar adsorbents for heavy metal-contaminated wastewater remediation.