Conversion of Waste Corn Straw to Value-Added Fuel via Hydrothermal Carbonization after Acid Washing

Unlocking the potential of biochar in the remediation of soils contaminated with heavy metals for sustainable agriculture

Agricultural soils contaminated with heavy metals (HMs) impose a threat to the environmental and to human health. Amendment with biochar could be an eco-friendly and cost-effective option to decrease HMs in contaminated soil. This paper reviews the application of biochar as a soil amendment to immobilise HMs in contaminated soil. We discuss the technologies of its preparation, their specific properties, and effect on the bioavailability of HMs. Biochar stabilises HMs in contaminated soil, enhance the overall quality of the contaminated soil, and significantly reduce HM uptake by plants, making it an option in soil remediation for HM contamination. Biochar enhances the physical (e.g. bulk density, soil structure, water holding capacity), chemical (e.g. cation exchange capacity, pH, nutrient availability, ion exchange, complexes), and biological properties (e.g. microbial abundance, enzymatic activities) of contaminated soil. Biochar also enhances soil fertility, improves plant growth, and reduces the plant availability of HMs. Various field studies have shown that biochar application reduces the bioavailability of HMs from contaminated soil while increasing crop yield. The review highlights the positive effects of biochar by reducing HM bioavailability in contaminated soils. Future work is recommended to ensure that biochars offer a safe and sustainable solution to remediate soils contaminated with HMs.

Hydrothermal Treatment of Residual Forest Wood (Softwood) and Digestate from Anaerobic Digestion—Influence of Temperature and Holding Time on the Characteristics of the Solid and Liquid Products

Hydrothermal treatment (HTT) offers the potential to upgrade low-value biomass such as digestate (DG) or forest residue (FR) by producing solids and liquids for material use or energetic utilization. In this study, microwave-assisted HTT experiments with DG and FR as feedstocks were executed at different temperatures (130, 150, 170 °C) and with different holding times (30, 60, 90 min) to determine the influences on product properties (ash and elemental concentrations, calorific values and chemical compounds). In general, DG and FR reacted differently to HTT. For the DG solids, for instance, the ash concentration was reduced to 8.68%DM at 130 °C (initially 27.67%DM), and the higher heating value increased from 16.55 MJ/kgDM to 20.82 MJ/kgDM at 170 °C, while the FR solids were affected only marginally. Elements with importance for emissions in combustion were leached out in both HTT solids. The DG and FR liquids contained different chemical compounds, and the temperature or holding time affected their formation. Depending on the designated application of HTT, less severe conditions can deliver better results. It was demonstrated that different low-temperature HTT conditions already induce strong changes in the product qualities of DG and FR. Optimized interactions between process parameters (temperature, holding time and feedstock) might lead to better cost–benefit effects in HTT.

Evaluation of Physical and Chemical Properties of Residue from Gasification of Biomass Wastes

Thermochemical conversion of biomass waste is a high potential option for increasing usage of renewable energy sources and transferring wastes into the circular economy. This work focuses on the evaluation of the energetic and adsorption properties of solid residue (char) of the gasification process. Gasification experiments of biomass wastes (wheat straw, hay and pine sawdust) were carried out in a vertical fixed bed reactor, under a CO2 atmosphere and at various temperatures (800, 900 and 1000 °C). The analysis of the energy properties of the obtained chars included elemental and thermogravimetric (TGA) analysis. TGA results indicated that the chars have properties similar to those of coal; subjected data were used to calculate key combustion parameters. As part of the analysis of adsorption properties, BET, SEM, FTIR and dynamic methanol vapor sorption tests were conducted. The specific surface area has risen from 0.42–1.91 m2/g (biomass) to 419–891 m2/g (char). FTIR spectroscopy confirmed the influence of gasification on the decomposition of characteristic chemical compounds for biomass. Methanol sorption has revealed for the 900 °C chars of pine sawdust the highest sorption capacity and its mass change was 24.15% at P/P0 = 90%. Selected chars might be an appropriate material for volatile organic compounds sorption.