Sorption of copper (II) and sulphate to different biochars before and after composting with farmyard manure

Biochar application has been suggested for reducing toxic levels of metals in contaminated soils and enhancing nutrient retention in agro-ecosystems. We studied sorption of copper (Cu(II)) and sulphate-sulphur (SO4-S) to charcoal, gasification coke and flash-pyrolysis biochar in order to relate sorption to char properties. Furthermore, we investigated the effect of composting of charcoal and gasification coke on sorptive properties. Langmuir sorption affinity coefficients for Cu(II) for non-composted biochars increased in the order flash-pyrolysis char < charcoal < gasification coke. The sorption capacity for Cu(II) of the chars decreased in the order gasification coke (629 mg kg-1) > flash-pyrolysis char (196 mg kg-1) > charcoal (56 mg kg-1). Composting significantly increased the sorption affinity coefficient approximately by a factor of 5 for charcoal (up to 1.1 l mg-1) and by a factor of 34 for gasification coke (up to 3.2 l mg-1). Whereas Cu(II) sorption to gasification coke (composted or not) was largely irreversible, sorption to flash-pyrolysis char and charcoal showed higher reversibility. Relationships between Cu(II) sorption and biochar properties such as cation exchange capacity, specific surface area or aromaticity suggest that sorption was largely determined by complexation with organic matter. Sorption of SO4-S was negligible by non-composted and composted biochars. Composted gasification coke might be suited to reducing toxic Cu(II) concentrations in contaminated soils. Composted charcoal can potentially improve Cu(II) retention in a plant available form in acidic, sandy soils with small organic matter contents. Transient effects of biochars on soil pH can over-ride the influence of sorption to biochars on concentrations of trace elements in soil solution and their availability to plants.