Crop straw biochar for adsorption of potentially toxic elements from oil sands process water: a multi-element performance assessment

Canola straw-derived biochar (CSB) was assessed for adsorption efficacy of arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), selenium (Se), and zinc (Zn) from an alkaline, geochemically complex oil sands process affected water (OSPW). The CSB exhibited strong, selective adsorption, with removal efficiencies exceeding 90 % for Cd, Co, Cu, Ni, and Zn. Selectivity order Cd(II) > Zn(II) > Cu(II) > Co(II) > Ni(II) > As(III) > Se(IV) > Cr(VI) was influenced by potential toxic element (PTE) speciation in OSPW. Kinetic and isotherm analyses revealed a dual adsorption process: initial rapid chemisorption, followed by intraparticle diffusion. Spectroscopic and textural analyses confirmed surface complexation and mineral precipitation were dominant removal mechanisms; ion exchange was secondary. Negligible As, Cr, and Se (≤0.01 mg g −1 ) uptake was attributed to electrostatic repulsion between anionic species and the negatively charged biochar surface. Regeneration tests showed 0.5 M HCl desorbed PTEs from CSB; with adsorption efficiency declining upon reuse. Removal efficiencies decreased 20-30 % after the first cycle for several metals, with progressive losses for Cd, Co, Ni, and Zn over four cycles; Cu removal declined 9 % (98-89 %), indicating high regeneration stability. Thus, waste-derived biochar can serve as a selective, regenerable sorbent for removal of divalent metals from industrial wastewaters. • Eight potentially toxic element adsorption assessed in oil sands process water. • Surface complexation and precipitation identified as dominant removal mechanisms. • Selectivity sequence: cadmium > zinc > copper > cobalt > nickel > arsenic > selenium > chromium. • Multi-model kinetic and isotherm analysis revealed dual-stage adsorption process. • Copper removal remained high at 89 % after four regeneration cycles with acid.