Surface-activated biochars not only represent a useful carbon sink, but can also act as useful filtering materials to extract plant nutrients (e.g. NH₄⁺) from wastes (e.g. animal or municipal waste streams) and added thereafter to soils. Biochars produced by low-temperature pyrolysis of fibrous debarking waste from pine (PI) and eucalyptus (EU) were pre-treated with either diluted (L) or undiluted (S) alkaline tannery waste (L-PI, S-PI, L-EU, S-EU). Biochars produced from untreated feedstock were used as controls. Samples were characterised by FT-IR, solid-state CP MAS ¹³C NMR, XPS, SEM microphotographs, and BET specific surface area. Elemental composition, carbon recovery, yield, surface charge, and NH₄⁺ sorption/desorption properties were also studied.

Carbon recovery was lower in biochars prepared from L-EU and S-EU (43 and 42%, respectively) than in control EU (45%) but these biochars showed greater changes in their chemical characteristics than those made from L-PI and S-PI, which showed minimal decrease in recovered carbon. The specific surface area of the biochars decreased with treatments, although acidic surface groups increased. In subsequent sorption experiments, treated biochars retained more NH₄⁺ from a 40 mg N/L waste stream (e.g. 61% retention in control EU and 83% in S-EU). Desorption was low, especially in treated biochars relative to untreated biochars (0.1–2% v. 14–27%). The results suggest that surface activated biochars can be obtained with negligible impairment to the carbon recovered.