Sludge-based activated biochar for adsorption treatment of real oil sands process water: Selectivity of naphthenic acids, reusability of spent biochar, leaching potential, and acute toxicity removal

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 (qt) for total NAs (classical and oxidized NAs) increased from 2.3 to 26.6 mg/g. The increase in qt 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 7C-7C interactions are the adsorption mechanisms. Thermal regeneration of spent activated biochar was studied, and in the fifth cycle of regeneration, the regenerated biochar had qt 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.

Automated summary

Large volumes of oil sands process water containing naphthenic acids (NAs) and other contaminants are effectively treated using activated biochar from sludge, reducing environmental impact and promoting circular economy. The activated biochar exhibited higher adsorption capacity for NAs due to increased surface area, presence of mesopores, and oxygen-containing surface groups. Adsorption mechanisms include pore-filling, hydrogen bonding, and 7C-7C interactions. Thermal regeneration of spent activated biochar further increased its adsorption capacity. OSPW treated with activated biochar also showed reduced levels of metals, organic matter, acute toxicity, and bioavailability of organics.