Scaling up biochar based remediation: Continuous column adsorption of toxic elements from oil sands wastewater and sequential valorization of spent biochar in cement

This study examined simultaneous removal of potentially toxic elements from spiked oil sands process affected water, using canola straw biochar in fixed bed columns. Elements were arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), selenium (Se), and zinc (Zn). Adsorption hierarchy (Cd>Co>Ni>Zn) was consistent with alkalinity governing selectivity. Under optimal conditions (2 mL min⁻¹, 12 cm bed depth, 240 h), maximum equilibrium adsorption capacities 0.279, 0.210, 0.126, 0.104 mg g⁻¹ were for Cd, Co, Ni, Zn, respectively. As, Cr, and Se uptake was negligible, consistent with electrostatic repulsion; Cu adsorption was limited by complexation with dissolved organic matter. A 17.2-29.4 min empty bed contact time maximized Cd and Co service time. Thomas and Yoon-Nelson models captured breakthrough behaviour for Cd, Co, and Ni, not oxyanions. A techno-economic analysis showed per litre treatment costs declined 84% (CAD 17.80-2.65). Cement valorization avoided CAD 0.375 kg⁻¹ of spent biochar, yielding 168.75 savings over 150 runs across 3 columns at industrial scale. Incorporating spent biochar into cement at 1-2% (w/w) suppressed leaching of most potentially toxic elements; Cr leaching increased in cement treatments, consistent with release from cement, supporting a circular end-of-life pathway for spent biochar.