Effect of biochar chemical modification (acid, base and hydrogen peroxide) on contaminants content depending on feedstock and pyrolysis conditions

The effect of biochars (BCs) chemical modifications on the content of various contaminants is poorly studied and understood, despite the fact that such results would be important in terms of estimating the safety of BCs application. The aim of this study was to evaluate the content of different types of contaminants - heavy metals (HMs), polycyclic aromatic hydrocarbons (solvent extractable - Ctot and freely dissolved - Cfree PAHs) and environmentally persistent free radicals (EPFRs) in post-pyrolytic chemically modified (HNO3, KOH and H2O2) BCs produced under diverse conditions (temperature and carrier gas) and from various feedstocks (willow, rice husk, sewage sludge, oil seed rape straw, wheat straw). The physio-chemical properties of BCs were determined (such as morphology, elemental composition and chemical state, and crystalline structure) and correlated with contaminants content. Regardless of the BCs type, acid modification reduced HMs and Ctot PAHs the most effectively but simultaneously increased Cfree PAHs and EPFRs content (compared to pristine BCs). Changes in contaminant content after H2O2 and alkali treatment were more varied and strictly related to the physiochemical properties of BCs (such as porosity, DOC, and polarity). H2O2 treatment reduced Ctot and Cfree PAHs content but was less effective than acid modification. In contrast, KOH treatment significantly increased the Ctot in most BCs, which depends mainly on porosity and DOC changes. Also, the reduction in HMs content and increase in EPFR after alkali and H2O2 treatment were less substantial and more differentiated by BC origin than after acid modification.