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[Display omitted] • ZnCl 2 -activated biochar from sludge successfully adsorbed total NAs in raw OSPW. • Pore filling, hydrogen bonding, and π − π interactions were the adsorption mechanisms. • Biochar had selectivity for NAs with hydrophobic and higher cyclicity characteristics. • Microtox® assay indicated 66% reduction in acute toxicity after biochar treatment. • 91% reduction of bioavailability of organics was observed in OSPW treated with sludge biochar. Large volumes of oil sands process water (OSPW) containing naphthenic acids (NAs) and other contaminants are generated by oil sands operations. For the first time, pristine and zinc chloride-activated biochars from sludge from a municipal wastewater treatment plant are studied in the adsorption of NAs in real OSPW. By using activated biochar instead of pristine biochar, the adsorption capacity (q t) for total NAs (classical and oxidized NAs) increased from 2.3 to 26.6 mg/g. The increase in q t for activated biochar was related to the improvement of surface area from 7.9 to 513 m2/g, the abundance of mesopores (89% of pore volume), and the oxygen-containing surface groups. Equilibrium was reached at 2 h of contact time. Considering the thousands of structures of NAs in the OSPW, the activated biochar presented higher affinity for NAs with higher carbon and double bond equivalency numbers, therefore, NAs with more hydrophobic and higher cyclicity characteristics. Pore-filling, hydrogen bonding, and π-π interactions are the adsorption mechanisms. Thermal regeneration of spent activated biochar was studied, and in the fifth cycle of regeneration, the regenerated biochar had q t of 7.1 mg/g, which was 5 times higher than the adsorption capacity of fresh pristine biochar. Adsorption treatment of OSPW using activated biochar decreased the following parameters: metals Mg (7%), Ca (21%), Sr (27%), and Ba (60%), organic matter by means of DOC (63%) and COD (66%), acute toxicity towards V. fischeri (66%) and bioavailability of organics (91%). These findings promote circular economy as it presents an efficient adsorbent for NAs in OSPW developed from sludge, reducing the environmental impact associated with sludge disposal and allowing the efficient reclamation and reuse of OSPW. [ABSTRACT FROM AUTHOR]