Removal of copper from sulfate solutions using biochar derived from crab processing by-product

Increasing metal demand is accelerating the mining and processing of minerals, however to ensure sustainable growth innovative approaches are required to better manage associated effluents. Biochar from the fast pyrolysis of residues from fishery and forestry operations has been studied as a low-cost, environmentally and economically friendly method for treating mine tailings and processing effluents. However, the bulk of the studies focus on terrestrial biomass (e.g. wood) and do not include potential inhibition/enhancement of adsorption due to pH controlling compounds. In this work biochar generated from snow crab (Chionoecetes Opilio) processing was studied as an adsorbent for copper solutions containing sulfate (a key compound in sulfide ore mining waters) with the objective of assessing adsorption capacity and the impact of sulfate on copper adsorption. The biochar, a porous structure comprised of calcite (CaCO₃), was alkaline and has a negative zeta potential under neutral and basic conditions. The crab biochar removed over 99% of Cu²⁺ from a 100 mg/L solution (sourced as CuSO₄) at a dosage of 5 g/L, which was higher than lignocellulosic biochar at the same biochar dosage. While metal adsorption can often be impacted at acidic conditions, Cu²⁺ adsorption was not impacted by initial acidic pH due to the biochar's buffering capacity. The Pseudo-Second Order (PSO) model fit the adsorption rate with maximum adsorption achieved in approximately 2 h. The maximum adsorption isotherm capacity was 184.8 ± 10.2 mg/g for Cu²⁺, much higher than existing commercial activated carbons and previously studied lignocellulosic biochars and followed the Freundlich isotherm. The adsorption mechanism responsible for removal of Cu²⁺ was found to be precipitation, in the form of the mineral posnjakite (Cu₄[(OH)₆SO₄]·H₂O). These results indicate for the first time that crab-based biochars are capable of adsorbing large quantities of Cu²⁺ from sulfate-rich solution, while also buffering solution pH, demonstrating promise as an acid mine drainage treatment for removal of harmful metals and reduction of acidity.