Making hydrochar suitable for agricultural soil: A thermal treatment to remove organic phytotoxic compounds

Unlike biochars, hydrochars often have initial phytotoxic effects that prevent their being directly applied to soil, an undesirable property thought to be due to the volatile organic compounds they contain. Thermal treatment in the 200-600 °C temperature range was proposed for mitigation purposes and it was shown that at a temperature of 275 °C phytotoxicity is eliminated from the carbonaceous solids. The phytotoxic property was "recovered" or maintained in the separated liquid, which can be used to generate heat in the HTC process, thereby achieving the safe disposal of harmful compounds at no additional cost. The post-treatment unlocks the potential of transforming the organic fraction of municipal solid waste into a soil amendment product in a zero-waste process. The absence of phytotoxicity was confirmed by the Zucconi method on seed germination. A gas-chromatographic method was also developed, based on the quantification of the total amount of substances detected by desorption, for a rough but quick estimation of hydrochar phytotoxicity. It was shown that the absence of these volatile compounds is in line with the absence of phytotoxicity as evaluated by the Zucconi method.