Contaminants in biochar and suggested mitigation measures – a review

Biochars produced by pyrolysis at high temperatures under oxygen limited conditions can contain both well-known contaminants (polycyclic aromatic hydrocarbons (PAHs), potentially toxic elements (PTEs), dioxins, volatile organic compounds (VOCs)) and emerging contaminants (e.g., persistent free radicals, metal cyanide). Their potential to induce phytotoxicity, cytotoxicity, and neurotoxicity highlight the need to establish effective strategies to control and eliminate contaminants for sustainable biochar use. Although some articles have reviewed the ecotoxic potential of biochar in relation to some of these contaminants, strategies to mitigate the whole suite of contaminants potentially present in biochar have not been systematically reviewed so far. This review discusses (i) the formation mechanism of such contaminants and (ii) evaluates their potential risk to ecosystems, a prerequisite (iii) to understand and explore effective control strategies for producing biochars with minimum contamination. A pyrolysis unit design where pyrolysis liquids and solids are fully separated following the pyrolysis zone and optimization of pyrolysis parameters ensures organic contaminants in the pyrolysis liquids do not condense onto the biochar. Post-production treatments, such as thermal treatment or natural and artificial ageing help organic contaminant removal and breakdown. Co-pyrolysis of metal-rich feedstock with metal-free biomass is the only way of reducing total PTE levels, though available PTE levels are low in biochars and decrease further with pyrolysis temperature. More specific strategies for reducing the concentration of individual contaminants are discussed in the review. With our proposed recommendations, biochars with little risk for the environment can be produced.