Biochar-enhanced removal of naphthenic acids from oil sands process water: Influence of feedstock and chemical activation

Bitumen extraction generates massive volumes of oil-sands-process-water (OSPW), which are stored in tailings ponds in Alberta, Canada, requiring treatment to reduce freshwater use and ensure safe discharge. Naphthenic acids-NAs, which are toxic and persistent organic contaminants, pose major challenges. While biochars show promise for adsorption, their effectiveness for real-OSPW remains largely unexplored, as most studies focus on model compounds. This study investigated the adsorption of NAs from real-OSPW via biochars-derived from municipal sludge and peat—a novel feedstock in this context—with and without chemical activation, including a dual FeCl 3 +ZnCl 2 strategy not previously evaluated for OSPW treatment. The results demonstrate that feedstock type and activation methods significantly influence biochar properties and adsorption performance. Chemical activation increased mesopore-volume and surface functional-groups (-OH and C C), improving adsorption capacity. Pristine-biochars removed ∼23 ​% of total-NAs, whereas FeCl 3 +ZnCl 2 -activated biochars achieved >90 ​% removal of both classical-&-oxidized-NAs. The FeCl 3 -activated biochars exhibited higher selectivity for NAs with larger carbon numbers, whereas FeCl 3 +ZnCl 2 -activated biochars efficiently removed a broader range of NAs. The study also revealed that biochars performed better in real-OSPW than in model compound solutions, highlighting the necessity of testing adsorbents in actual-wastewater matrices. Among biochar properties, mesopore-volume identified as the most critical factor for maximizing NA removal, emphasizing the importance of pore-structure over total surface area. These findings suggest that FeCl 3 +ZnCl 2 -activated biochars are promising materials for OSPW treatment. Future research should focus on integrating biochar into passive treatment systems e.g., wetlands and exploring its potential for land reclamation and carbon sequestration in oil and sand remediation. • Activated biochars removed >90 ​% of NAs in OSPW with adsorption capacity of 30 ​mg/g. • Biochars presented higher selectivity for NAs with higher carbon numbers in the OSPW. • Higher mesopore fraction improved adsorption capacities of NAs from OSPW. • Sludge and peat biochars have potential for application in passive treatment of OSPW.