3.2.2. Heavy Metal Immobilization

Biochar can also act as a barrier to prevent heavy metals from percolating into groundwater aquifers and surface water resources [15,24,25]. Biochar has a high ability of filtering of heavy metals in contaminated soil and faecal sludge due to the potential of adsorbing metals on its surface [16]. Sequestration of Pb, Cd, Cu, and Ni has been reported from cottonseed hull biochar because of functional groups on the biochar surfaces [24]. As pH, volatile matter, O:C, and N:C ratios in the biochar increase, biochar's capacity to adsorb heavy metals also increases [16,24]. In a study using poultry litter biochar and green waste biochar, it was found that Cd and Pb elements in soil water decreased [94]. Conversely, Cu increased in the soil water because of more mobility through increased dissolved organic C [94]. Other studies observed no effect on Cu when the soil was mixed with a hardwood biochar produced at 750 ◦C [20]. Decreases in Cd, Cr, Cu, Pb, Ni, and Zn in excess faecal effluent leachates from a soil column amended with a wood-based biochar were also found [25]. Cu, Cd, and Pb were removed from aqueous solution after amending the soil with bamboo, sugarcane, hickory, and peanut hull biochars, with the bamboo biochar being most effective [62]. The high adsorption of heavy metals in the study by Zhou et al. [62] was associated with pH increases in the solution. An increase in Cu and Zn removal as pH increased has also been seen using hardwood and corn straw biochar produced at temperatures of 450 ◦C and 600 ◦C [110]. These findings are similar to Krueger et al. [111], showing the immobilization occurring when faecal sludge is treated with biochar (Figure 3). The long-term impacts of biochar on heavy metal elements sorption are limited and require more research due to the recalcitrance of biochar [16].

**Figure 3.** Mobility of heavy metals in faecal sludge (FS) and their derived biochars (BC); sludge sourced from Narsapur (N-FS) and Warangal (W-FS) Faecal Sludge Treatment Plants [111]. Reprinted from the reference with the permission, © 2019 Krueger et al., CC-BY-4.0.
