Dioxin-Like Polychlorinated Biphenyls (dl-PCB) in Hydrochars and Biochars: Review of Recent Evidence, Pollution Levels, Critical Gaps, Formation Mechanisms and Regulations

Contamination of chars with dioxin-like polychlorinated biphenyls (dl-PCB) significantly limits their use and hinders their deployment in the circular bioeconomy, specifically in applications that may lead to dietary exposure. Here, for the first time, we review the levels of contamination of chars produced from pyrolysis and hydrothermal carbonisation (HTC) with dl-PCB congeners. We conduct a detailed and critical examination of the role played by the processing parameters, such as temperature and residence time, and the reaction mechanisms, to detoxify the biomass under an oxygen-free atmosphere during its valorisation. The PCB-based toxicity of biomass depends mostly on the abundance of dl-PCB in the raw material, and on the dechlorination and other transformation processes that operate during the treatment. The key dechlorination steps make the toxicity of hydrochars pass through a maximum with increasing treatment time, whereas the toxicity of biochars in pyrolysis decreases monotonically. Pyrolysis displays more complex mechanistic pathways of volatilisation, dechlorination, degradation of PCB rings, minor de novo formation of dl-PCB in case of air leaks, and concentrating persistent organic pollutants (POP) in char matrices. In contrast, the mechanisms responsible for the evolution of toxicity in HTC processes comprise the dechlorination, possible chlorine position shift, and biomass densification. The kinetic model developed in this review affords insight into the evolution of the hydrochar toxicity that depends on process temperature and treatment time. The dl-PCB concentrations in treated biomass generally range from 1.06 ng WHO₂₀₀₅-TEQ (kg DM)^-1 to 11.7 ng WHO₂₀₀₅-TEQ (kg DM)^-1, whereas for biochars produced from contaminated sediments the toxicity varies from 0.00662 ng WHO₁₉₉₈-TEQ (kg DM)^-1 to 1.42 ng WHO₁₉₉₈-TEQ (kg DM)^-1. DM stands for dry matter, TEQ for toxic equivalency, and WHO₁₉₉₈ means the application of the toxic equivalency factors (TEF) set by the World Health Organization (WHO) in 1998 to calculate the TEQ. Finally, we identify the crucial gaps in the literature, review the regulations governing the use of biomass in feed and in the environment, and provide suggestions for future research. The findings in this article provide both the technical understanding of how to minimise the formation of dl-PCB in the production of chars and suggest modifications to the current guidelines. The latter will increase the consumer’s trust in valorised biomass, leading to its wider acceptance in the circular bioeconomy as feed supplements and soil additives.