Hop-derived biochar for efficient heavy metal removal: Unraveling and quantifying dominant mechanisms

This study introduces a novel application of Humulus lupulus (hop) waste for producing a biochar with a high adsorption capacity for Cd, Cu, Pb, and Zn, rivaling those of chemically modified counterparts. Batch experiments revealed maximum adsorption capacities following the order Cu (1.98 mmol g −1 ) > Pb (1.09) > Zn (0.79) > Cd (0.77). Uptake kinetics were best described by a pseudo-second-order model. Advanced spectroscopic and thermodynamic characterization (DRIFT, XRD, SEM-EDS, TGA), combined with demineralization experiments, unequivocally identified mineral-driven mechanisms, specifically precipitation and cation exchange, as the dominant sequestration pathways. These mechanisms account for 66.2%, 85.5%, 65.8%, 71.8% of the maximum adsorption capacity for Cd, Cu, Pb, and Zn, respectively. The inherent carbonate content in the hop-derived BC originates from the pyrolysis of calcium oxalates, which are naturally abundant in the feedstock. Our findings reveal that the strategic selection of oxalate-rich feedstocks presents a novel and sustainable criterion for designing inherently effective, unmodified BCs for heavy metal remediation. This approach offers a direct waste-to-resource strategy and establishes a superior baseline for potential further performance enhancement. • Hop biochar removes Cu, Pb, Zn, and Cd effectively, outperforming other biochars. • Calcite enables 66–86% metal removal via precipitation and ion exchange. • Brewery waste transforms into effective adsorbent, supporting sustainability. • Feedstock composition with calcium oxalate may enhance biochar performance.