How to avoid the danger of pollutants in Swiss biochar production and use?

The study of pollutants found in biochar and its effects on agricultural soil was conducted in this paper. The aim of the research was to analyse organic and inorganic pollutants found in biochar which either originate from the biomass itself or are formed in the process of pyrolysis. Pollutants that were analysed are polycyclic aromatic hydrocarbons (PAHs), heavy metals, dioxins, metal cyanide, persistent free radicals and volatile organic compounds (VOCs). Biomass categories and pyrolysis conditions are examined and linked to pollutant concentrations found in biochar. Secondary aim of the study was to connect biochar types with the overall positive or negative effects that are observed when biochar is applied on one of the four Swiss soil types (silt loam, loam, sandy loam and silty clay) at a specific application rate. After assessing the previously stated, instructions for safe biochar production and use on agricultural soils will be provided. Heavy metals are identified as pollutants with the highest risk for the ecosystem due to their toxicity. Sewage sludge, animal manure and phytoremediation residues are non-woody biomass which are observed to be rich in heavy metals. Heavy metals can concentrate during the pyrolysis process in biochar so they end up having higher heavy metal concentrations than the biomass itself. Acid washing prior to the pyrolysis or co-pyrolysis with a heavy metals-poor biomass are mitigation strategies for heavy metal-rich biomass. Miscanthus and paper mill sludge are identified as non-woody biomass with high potential of forming PAHs during the pyrolysis process. Sawdust is a woody biomass identified to have high potential of forming PAHs and dioxins during the pyrolysis. Pyrolysis at high temperatures or with a long residence time is a potential solution. Correlation between heavy metal polluted biomass and its corresponding negative effects on the ecosystem is proposed due to observed results. Non-woody biomass is identified with larger overall negative effects on the ecosystem, when applied to the soil, than woody biomass. Biochar is identified to have overall more positive effects on the soil ecosystem than negative effects. Soil bulk density, soil water content, soil aggregate stability and water retention capacity are positively affected, while invertebrates and microorganisms are negatively affected. Soil erosion, soil salinity, soil pH, soil conductivity and plant biomass are categories which are influenced both positively and negatively, thus further studies are necessary in order to conclude on the specific overall effect of biochar when applied to the soil. Effects are highly contradictory as different studies observed different effects. Considering this, due to the inconsistent results, it is concluded that no specific recommendations can be given on safe use of biochar on the agricultural soils. One use of biochar that is significant and has a big potential is carbon sequestration which is used as a climate change mitigation tool. Long term effects, biochar stability in the soil, pollutants bioavailability and application rates of biochar are proposed topics for further research which is necessary to conduct a more thorough analysis.