The effect of organic solvent washing on the structure of hydrochar-based dissolved organic matters and its potential environmental toxicity

With the increased interest in the practical use of hydrochar, concerns about the possible environmental biotoxicity of hydrochar and its released dissolved organic matters (DOM) have grown. As a common method for removing bio-oil on the surface of hydrochar, the effect of organic solvent washing on the properties of hydrochar released DOM remains unclear. In this study, we made a comprehensive comparison of hydrochar properties and molecule structure as well as biotoxicity of DOM released from HC (raw hydrochar) and THC (hydrochar washed by tetrahydrofuran). The results indicated that the mass loss of hydrochar was obvious after tetrahydrofuran (THF) washing, and a decline of H/C atomic ratio and increase of N/C and O/C atomic ratios was observed based on Van Krevelen (VK) diagram. This result was further confirmed by FTIR, 13 C NMR, and XPS results. Meanwhile, the molecule structure of DOM was shifted to lower molecule weight with higher O-contain compounds after THF extraction due to the demethanation process. However, the biotoxicity experiments indicated that both extracted DOM had no significant impact on germination rate of wheat, and HC-treated sample even exhibited growth superiority. Nevertheless, potential toxicity was observed with the increase of the activity of antioxidant enzymes, and THF washing aggravated the potential oxidative damage through increasing the aromaticity of DOM. Such understanding highlights the importance of evaluating hydrochar and its released DOM before applications, so as to reduce the potential environment biotoxicity.