Recent Advance on Torrefaction Valorization and Application of Biochar from Agricultural Waste for Soil Remediation

With the increase of global proportion of soil pollution and the number of areas at risk, researchers have sought to develop various pathways to repair or relieve the pollutants in soil. Among them, biochar represents one multidimensional soil amendment which has got great deal of attention on its physicochemical properties towards the removal or mitigation of contaminants in soil. A variety of agricultural wastes like straw and manure prepared from different torrefaction process have been employed as feedstock for the production of biochar, which can be applied to the contaminated soil to facilitate the growing environment for crops, and to improve soil fertility and microbial environment. In addition, the utilization of biochar for soil remediation is also considered as a process of carbon sequestration. The purpose of this review is to summarize the latest research progress in torrefaction processes and mechanism of agricultural waste, the effects of different torrefaction methods on the formation and properties of biochar were explained, coupled with the effects of process parameters. Especially, the conversion and mechanisms of biochar prepared from agricultural wastes composed mainly with lignocellulosic material were discussed, and the characteristics of biochar prepared for improving soil physical and chemical characteristics, microbial community characteristics, nutrients, and the stability and relief of soil pollutants, especially heavy metals, are compared. Finally, this work discussed the application and future technical challenges of soil remediation based on agricultural waste derived biochar.