Adsorption of acid-extractable organics from oil sands process-affected water onto biomass-based biochar: Metal content matters

- Biochar from six local biomass sources were prepared and characterized.
- Aluminum and iron content of biochar correlated positively with adsorption capacity.
- Adding iron in biochar by impregnation method increased adsorption capacity.
- Metal sites acting as Lewis acids can form coordination bonds with the naphthenates.
- Effects of combined acidification with biochar adsorption was purely additive.

The impact of biochar properties on acid-extractable organics (AEO) adsorption from oil sands process-affected water (OSPW) was studied. Biochar from wheat straw with the highest ash content (14%) had the highest adsorption capacity (0.59 mg/g) followed by biochar from pulp mill sludge, switchgrass, mountain pine, hemp shives, and aspen wood. The adsorption capacity had no obvious trend with surface area, total pore volume, bulk polarity and aromaticity. The large impact of metal content was consistent with the carboxylates (i.e., naphthenate species) in the OSPW binding to the metals (mainly Al and Fe) on the carbon substrate. Although the capacity of biochar is still approximately two orders of magnitude lower than that of a commercial activated carbon, confirming the property (i.e., metal content) that most influenced AEO adsorption, may allow biochar to become competitive with activated carbon after normalizing for cost, especially if this cost includes environmental impacts.