3.3. Effect of the addition of cavitated charcoal on the content of selected heavy metals in soils
Regardless of the soil (LS or C), the applied CHAR-C rates did not significantly change the total contents of the studied heavy metals (Table 4). Significant differences in the studied element contents were dictated by the type of soil used.
The effect of the application of CHAR-C on the bioavailability of heavy metals was tested after extracting the most mobile forms of Cu, Cd, Pb, and Zn with a 0.01 mol∙dm-3 solution of CaCl2 (Table 4). Regardless of the element and soil used, the application of CHAR-C reduced the content of CaCl2-extracted forms of heavy metals. Following the application of CHAR-C, the LS soil presented a relatively low content of the most mobile forms of the studied elements. It should also be noted that, regardless of the soil, mobile forms decreased with an increased CHAR-C rate.
Recent studies indicate that thermally transformed organic materials, including charcoal and biochar, exhibit sorption capacity for heavy metal ions (Miura and Shiratani, 2018; Mierzwa-Hersztek et al., 2019). These materials have a large specific surface area, i.e., a surface on which there are functional groups that are active in the sorbing of heavy metal ions. The sorption capacity of charcoal and biochar is determined, among others, by the concentration of heavy metals in the soil solution. In unpolluted soils, the natural consequence of using these materials is the reduction in the content of the most available element forms (Miura and Shiratani, 2018; Mierzwa-Hersztek et al., 2019). In our study, the increase in soil pH after the use of CHAR-C was one of the important factors reducing the content of the most available heavy metals (Gondek and Mierzwa-Hersztek, 2016).