Preparation of reed straw hydrochar adsorbent and its environmental impact assessment

Hydrothermal carbonization technology has attracted increasingly the amount of attention due to its efficient conversion of biomass resources. Such technology benefits both the economy and the environment. In this study, hydrothermal carbonization of reed was conducted in a hydrothermal reactor under laboratory conditions. The factors that control the hydrochar yield and the efficiency of methylene blue adsorption, such as temperature, residence time, and solid-liquid ratio were investigated. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the structure of the reed hydrochar. The experimental results indicated that the order of the factors affecting hydrochar yield was as follows: residence time > maximum temperature > solid-liquid ratio. The order of factors that determine the extent of methylene blue adsorption was: solid-liquid ratio > maximum temperature > residence time. The optimum condition for hydrochar production was with a reaction temperature at 260 °C, a residence time of 2 h, and a solid-liquid ratio of 1:10. The optimum condition for the highest amount of methylene blue adsorption was with a reaction temperature at 260 °C, a residence time of 1 h, and a solid-liquid ratio of 1:10. The SEM imaging revealed a pore structure at the surface of the hydrochar comprising carbon microspheres and multilayer structure. Whereas the FTIR analysis indicated that the hydrochar had a high number of oxygen-containing surface functional groups. The environmental impact of the reed hydrochar preparation process was evaluated using the life cycle assessment approach by the SimaPro 9.2.0.2 software. The results revealed that the hydrothermal carbonization process majorly impacted the toxicity to the environment, the land use and the global warming. Future optimization of the hydrothermal carbonization process should focus on: (1) reducing the energy consumption during this process; (2) evaluating the environmental impact of the hydrochar-producing system, including the resource recycling of the by-products during this process.