Ecotoxicology of biochar-bound PAHs in runoff from amended soils

Abstract

While the appeal of biochar application to soils continues growing, so does the concern about the possibility for surface and groundwater contamination, due to biochar-bound contaminants, such as polycyclic aromatic hydrocarbons (PAHs). Up to now, insufficient information exists on to what extent that PAH fraction can become bioavailable in the soil solution over time and which are the associated potential ecotoxicological implications, as a result of processes such as runoff. This pilot study aimed to evaluate the ecotoxicological effects of biochar-bound PAHs in runoff from soils amended with biochar, having been identified as a gap in current knowledge. Combining soil wetting-drying cycles with PAH water-extraction, a good first approach was obtained for evaluating their potential occurrence in soil solution, while taking into consideration natural soil processes and soil-biochar interactions. LUFA 2.2 soil alone and containing biochar (at usual field rates, 4% ww-1) was subjected to 0, 6 or 12 (sampling times ST-0, ST-1 or ST-2 respectively) consecutive wetting-drying cycles, after which the corresponding test elutriates were extracted. Alongside PAH quantification, a battery of standard aquatic bioassays were used with representative test organisms (Vibrio fischeri, Pseudokirchneriella subcapitata and Daphnia magna), for a robust ecotoxicological evaluation of the biochar-soil (BS) aqueous extracts, while LUFA soil elutriates were used as control (SS). Compared to the control (SS) and to elutriates of biochar alone (B), BS extracts showed the highest total PAH contents, suggesting that a relevant PAHs fraction in biochar-amended soil may be easily water-extractable, perhaps due to interactions between biochar and soil components. Yet, the number of soil wet-dry cycles on aqueous total PAH concentrations was often not significant, suggesting that natural soil wetting-drying events might have little influence on increased PAH bioavailability in pore water, on the short term. BS extracts induced toxicity in all tested species, although its extent was species-specific and varied with the number of wet-dry cycles. For example, the highest sensitivity was observed in the acute assay with D. magna exposed to BS extract for ST-0, while P. subcapitata and V. fischeri were most sensitive when exposed to BS, ST-1. Nevertheless, sub-lethal effects were also observed for P. subcapitata and V. fischeri, when exposed to the control (SS) extracts. Although the levels of individual PAHs in all samples (BS, SS and B) were below the acutely toxic concentrations reported in the literature, it cannot be excluded the combined effects of the multiple PAHs in the test elutriates when explaining these results. Furthermore, although individual PAH concentrations were below that to produce acute effects, chronic effects can occur, and therefore, long-term exposure to these elutriates and using additional non-target species, various biochars and soil properties are necessary for a full evaluation of the bioavailability and ecotoxicity of biochar-bound PAH contaminants in runoff from treated soils.