14This study assessed the properties and toxicity (water cress germination trials) of 38 15 waste-derived, novel biochar-mineral composites (BMCs) produced via slow pyrolysis 16 and hydrothermal carbonization (hydrochars). The biochars were produced from sewage 17 sludge and compost-like output (CLO) by varying the type of mineral additive (zeolite, 18 wood ash and lignite fly ash), the mineral-to-feedstock ratio and the carbonization 19 process. While pure hydrochars completely inhibited germination of water cress, this 20 effect was ameliorated by mineral additives. Seedlings grew best in pyrolysis chars and 21 while wood ash addition decreased plant growth in many cases, 1:10 addition to CLO 22 doubled germination rate. The factors responsible for the phytotoxicity can be attributed 23 to pH, salinity and organic contaminants. Importantly, while pure minerals inhibited 24 germination, conversion of minerals into BMCs reduced their inhibitory effects due to 25 buffered release of minerals. Overall, mineral wastes (e.g., combustion ashes) and waste 26 biomass can be used safely as sources of nutrients and stable organic carbon (for soil 27 carbon sequestration) when converted into specific biochar-mineral composites, 28 exploiting synergies between the constituents to deliver superior performance. 29
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