Chapter 14 - Biochar as an (Im)mobilizing Agent for the Potentially Toxic Elements in Contaminated Soils

Abstract

Biochar has exhibited a greater potential for treatment of potentially toxic elements (PTEs) contaminated soils than other sorbents. However, biochar can cause contradictory effects on the PTEs mobilization and phytoavailability. Therefore, the aim of this chapter is to present the contrasting effects of different types of biochar on the mobilization and phytoavailability of different PTEs in different soils. Here, we critically reviewed and discussed the (im)mobilization efficiency of biochar for PTEs in soils with respect to its physicochemical characteristics and type of soil and element. Application of biochar to PTEs contaminated soils may affect the bioavailability of soil PTEs either by immobilizing or mobilizing these elements, which may depend on the element type, biomass feedstock, application rate, and soil type. Biochar may immobilize many PTEs as a result of increasing soil pH and increasing soil sorption capacity. On the other side, biochar may increase the mobilization of some other PTEs like Cu and As as a result of the increase of dissolved organic carbon. Also, increasing mobility and bioavailability of As in the biochar-treated soils might be explained by the increase of soil pH and promoting the reduction of As(V) to As(III). The properties of biochar (e.g., surface functional groups, mineral content, ionic content, and π-electrons) directly influenced PTEs (im)mobilization mechanisms, which in turn determined the capacity of biochar for remediation of PTEs contaminated soils. Hence, the combined effects of those properties on the (im)mobilization of PTEs by biochar are complex and highly PTE-specific. This chapter demonstrates the overarching scientific opportunities for a comprehensive understanding of using biochars as promising low-cost and effective materials for the remediation of PTEs contaminated soils.