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Biochar: contaminant source or sink?

Biochar is a promising organic recycling material to be used as a soil conditioner and feed supplement. However, it may contain pollutants originating from feedstock, or formed during pyrolysis. The latter origin is eminent in the case of polycyclic aromatic hydrocarbons (PAHs). Inherent inorganic contaminants in biochars are unlikely and studies on this topic are rare. Concentrations of metals are probably similar to those in other recycling fertilizers, depending on the input material. Conversely, pyrolysis of organic matter produces condensed carbonaceous structures which strongly adsorb organic pollutants such as PAHs, pesticides, polychlorinated biphenyls, etc. Biochars are also used to immobilize heavy metal pollutions in soils. Carboxylized biochars are employed to enhance ionic interactions with the pollutants to avoid leaching into (ground) waters. The raise of pore water pH in biochar amended soil in contrast mobilizes anionic contaminants such as antimony or arsenic which exhibit mostly negative speciation in such alkaline environments. All in all, biochar is probably both, a source and sink of contaminants.