Potential Impact of Biochar Water-Extractable Substances on Environmental Sustainability

Abstract

Application of biochar as a soil amendment could be a significant approach for carbon sequestration to possibly control climate change for energy and environmental sustainability. However, more studies are needed in a number of research areas, including the development of clean biochar materials free of any harmful substances, before this approach could be implemented at a global scale. In this study, biochar water-extractable substances were tested for their potential harmful effects on the growth of aquatic photosynthetic microorganisms including both blue–green alga (cyanobacteria Synechococcus) and eukaryotic green alga (Desmodesmus) that represent the primary photosynthetic producers of the aquatic environment. The water extracts from three different biomass-derived biochar materials varied widely in their dissolved organic and inorganic contents, as well as in their characteristics including their pH values. Bioassays with pinewood-derived biochar water extract showed a significant inhibitory effect on aquatic photosynthetic microorganism growth in a dose-dependent manner, while chicken litter and peanut shell-derived biochar water extracts showed no signs of growth inhibition. The pinewood-derived biochar water-extracted substances were further separated into three fractions based on their molecular sizes and electric charges through an electrodialysis separation process using a cellulose–acetate membrane with a 500-delta cutoff pore size. Our analysis showed that the active component of pinewood-derived biochar water-extracted substances that are toxic to both blue–green alga (cyanobacteria Synechococcus) and eukaryotic green alga (Desmodesmus) is likely a 500-delta (or smaller) organic chemical species that carries at least one carboxyl group. This finding is important to engineering a high-tech biochar that can be free of any undesirable substances for its soil applications toward agricultural and environmental sustainability.