Applying biochar to treat and detoxify whisky distillery effluent

Spent lees, the acidic, copper-rich liquid industrial effluent from the second distillation step of whisky production, present a significant environmental management challenge. This study explores the use of biochar-based materials as a low-cost, sustainable treatment to reduce dissolved copper (dissCu) and neutralise acidity. Three biochar types were assessed: raw biochar (RB), metal-modified biochar (MMB), and a biochar–mineral composite (BMC). RB was produced from softwood pyrolysed at ∼500 °C, while MMB was prepared by incorporating Fe, Mn, and Mg salts into the biochar matrix. BMC was generated by co-carbonising orchard residues with manure and digestate, resulting in a mineral-rich structure. Both MMB and BMC significantly elevated pH and removed dissCu, with performance improving at 4–8 g/L doses. Ecotoxicity testing using Lepidium sativum (germination index) and Escherichia coli (growth inhibition) showed marked reductions in toxicity in treated effluents, particularly at higher doses of MMB and BMC. Surface and structural analyses (SEM, XRD, FTIR, XPS) confirmed that metal oxides and mineral phases enhanced pH buffering and metal adsorption capacity, supported by point of zero charge (pHpzc) measurements. This is one of very few studies to apply engineered biochars for the treatment of Scotch whisky effluent. By integrating chemical removal data with rapid ecotoxicity screening, the study offers a novel and practical framework for evaluating treatment performance. These findings demonstrate the potential of modified biochars as scalable, effective solutions for distillery wastewater and other agro-industrial waste streams. • Oxide-enriched biochars neutralised acidity and immobilised copper in wastewater. • These biochars also mitigated plant stress caused by toxic whisky effluents. • Surface analyses revealed strong Cu binding on the engineered biochar surfaces. • Biochar-based treatments offer a promising solution for whisky wastewater effluent management.