Title : From waste to resource: Exploring the double-edged benefits of contaminated biomass in biochar production
Abstract:
In recent years, the significance of biochar as a fertilizer and soil improver in sustainable agriculture has increased. It is claimed to improve the physicochemical properties of the soil, particularly by providing and retaining nutrients, increasing water holding capacity and immobilizing contaminants. However, biochar can also contain harmful elements and chemicals, inherent to biomass used, but also those produced through pyrolysis. The issue concerns primarily heavy metals as well as organic compounds. Once applied to the soil, it poses threat, due to the possibility of secondary contamination. Improper application can lead to negative effects on soil flora and fauna. Therefore, it has become necessary to assess the environmental impact of biochar using ecotoxicity tests. In this study two types of biomass and their biochars, distinguished by heavy metals content, were incorporated into reference OECD soil in concentration of 0.01, 0.1 and 1%. The primary objective was to determine the environmental risk of soil storing the materials based on toxicity-based approach. The ecotoxicity tests included: (a) Terrestrial Plants Seedling Emergence and Seedling Growth (OECD 208), utilizing garden cress Lepidium sativum and cucumber Cucumis sativus as dicotyledonous plants, as well as common wheat Triticum aestivum as monocotyledon plant, and (b) Earthworm Acute Toxicity (OECD 207) and (c) Earthworm Avoidance (ISO 17512-1), utilizing redworm Eisenia fetida. Endpoints in the following tests included: stem growth and plant biomass increase, animals’ mortality, and worms avoidance of soil containing the materials.In the Seedling Emergence and Seedling Growth Test, garden cress proved to be the most sensitive plant. At all concentrations of biomass amendment there was a statistically significant inhibition of plant stem growth, and at the highest concentration (1%), also the inhibition of plant biomass growth. For the biochar, there was a reduction in toxicity in comparison to biomass. The addition of either biochar at any concentration did not cause any toxic effect. In the case of cucumber, no inhibition of stem growth was observed for any of the samples, while there was less than 30% inhibition of plant biomass growth for contaminated biomass samples at dosage of 0.1 and 1%. Wheat proved to be the least sensitive to biomass and biochar samples – for both stem growth and biomass growth, inhibition did not occur with any of the tested groups. In the Earthworm Acute Toxicity Test, no statistically significant mortality was detected. In the Avoidance Test, in all tested groups, the organisms preferred the soil with the addition of the materials, but there were no statistically significant differences between the samples. Pyrolysis reduced the toxicity of the biomass, so that the biochar amendment in soil at concentrations ≤ 1% did not inhibit plant growth or induce mortality in earthworms, and did not cause earthworms to avoid soils with these materials. This is also consistent with the literature. However, it should be noted that the use of biochar at higher concentrations can lead to negative effects in model organisms.
