Enhancement of hexavalent chromium adsorption by hydroxyapatite-modified biochar derived from <i>Mimosa pigra</i> invasive plant

Chromium (VI) contamination in water presents significant environmental and public health challenges, demanding efficient and sustainable remediation strategies. This study explores the use of biochar (BC) derived from the invasive plant Mimosa pigra, modified with hydroxyapatite (HAp), to form a composite (BC@HAp) for effective Cr VI removal. HAp is known for its high adsorption capacity and biocompatibility, and its incorporation into BC enhances heavy metal removal through synergistic effects. BC and BC@HAp were synthesized via the sol–gel method and tested under various conditions including pH, adsorbent dosage, Cr VI concentration, contact time, and temperature. Structural and morphological analyses confirmed improved surface characteristics of BC@HAp. The composite exhibited a significantly higher Cr VI adsorption capacity (67.68 mg/g) compared to unmodified BC (39.95 mg/g), attributed to increased surface area and new functional moieties. The adsorption mechanism was facilitated by electrostatic attraction between Cr VI anions (HCrO 4 − , Cr 2 O 7 2− ) and positively charged Ca 2+ sites, ion exchange with surface carbonate and phosphate groups, and surface complexation involving hydroxyl (–OH) and carboxyl (–COOH) groups. Adsorption followed the Freundlich and Temkin isotherms, indicating heterogeneous surface interactions, and was best described by the Elovich kinetic model. Thermodynamic parameters revealed the process to be spontaneous and endothermic, favoring higher temperatures. The enhanced performance of BC@HAp demonstrates its potential as a cost-effective and eco-friendly solution for both Cr VI remediation and invasive species utilization.