Constraining bioavailable polyaromatic hydrocarbons effectively during the production and application of biochar

Biochar has substantial potential as a globally significant negative emissions technology (NET) provided that it can be deployed economically at a sufficiently large scale without introducing unintended negative impacts. The generation of polyaromatic hydrocarbons (PAHs) during the production of biochars has been identified as a potential risk. A commonly applied risk management and certification mechanism is to set an upper limit on the total PAH loading of the produced biochar. However, this simple metric does not take into account the strong affinity for biochar to bind PAHs, which can significantly reduce their bioavailablity and associated potential risks to health and the environment. Nor does reflect the fact that not all PAHs are the same, and that the distribution of more and less toxic species within the total is influenced by a number of feedstock and production factors. The capacity for biochar to adsorb potential toxins, not only PAHs, but also heavy metals and other organic pollutants from contaminated soils and aqueous media is an important and affordable mechanism to help reduce the uptake of such pollutants and their associated risks to health. Elevated concentrations of PAHs are often to be found in soils located in the vicinity of roads, heavy industries and crop-burning activities. The addition of suitably prepared biochar in such situations can result in a net reduction in bioavailable PAHs. Whilst the imposition of tight constraints is justifiable for certain applications, such as when used as a ruminant feed supplement, adherence to excessively tight constraints in general may unintentionally inhibit the generation of biochar by more affordable processes that would otherwise be sufficient to deliver net environmentally positive benefits, local and global. By managing overall risk on a more class-by-class basis, the potential for large-scale deployment of biochar as a NET may be realised effectively and safely. Crown Copyright (C) 2021 Published by Elsevier B.V. All rights reserved.

Automated summary

Biochar has significant potential as a negative emissions technology, but the generation of PAHs during its production poses risks. Setting limits on total PAH loading without considering biochar's ability to bind PAHs and the different toxicity levels within the total may hinder its affordable and effective large-scale deployment.