# Characteristics of Removal of Lead, Cadmium and Chromium from Soil Using Biosorbent and Biochar
## 3 Result and Discussion
### Removal of Heavy Metals from the Soil
Figure 5A,A\({}^{\prime}\) indicate that the concentrations of the tested Pb, Zn, and Cr ions in the soil decrease in both Series I and Series II after adding the tested sorbents. The highest metal contents in the soil were observed for 2% Cs after the first series: AW: Pb--879.1 mg/kg, Cd--951.8 mg/kg, Cr--972.7 mg/kg; BC: Pb--852.8 mg/kg, Cd--928.5 mg/kg, Cr--944.0 mg/kg; AC: Pb--816.4 mg/kg, Cd--912.1 mg/kg, Cr--914.2 mg/kg. The lowest metal contents were observed for 10% Cs after the second series: AW: Pb--521.9 mg/kg, Cd--629.5 mg/kg, Cr--842.2 mg/kg; BC: Pb--483.2 mg/kg, Cd--557.0 mg/kg, Cr--722.6 mg/kg; AC: Pb--281.2 mg/kg, Cd--457.5 mg/kg, Cr--  
Figure 4: The influence of the material on the removal of heavy metals from the soil: **(A)**—Metal concentration [mg/kg], **(B)**—removal efficiency [%], **(C)**—sorption capacity [mg/g] (Cs = 10%, I—Series I, II—Series II). 611.8 mg/kg. These data show that lead was the metal most effectively removed from the soil, with removal ranging from 9% (AW, Cs = 2%, I) to 71% (AC, Cs = 10%, II). In contrast, chromium was the most difficult to remove, with removal ranging from 2% (AW, Cs = 2%, I) to 38% (AC, Cs = 10%, II). The percentage of heavy metal removal increases rapidly with the increasing sorbent dosage due to greater availability of replacement sites or a larger surface area. Additionally, the adsorption percentage of metal ions on the adsorbent depends on the adsorption capacity of the adsorbent for different metal ions [47]. The obtained results are consistent with studies by other researchers, where increasing the sorbent dose led to an increase in the degree of contamination removal [6; 48; 49]. However, the amount of heavy metal ions adsorbed per unit mass of sorbent decreased as the amount of sorbent increased (Figure 5C). The highest differences were observed for the process carried out for 36 days. The difference in sorption capacity between Cs = 2% and Cs = 10% was: AW: Pb--78.0%, Cd--31.9%, Cr--60.3%; BC: Pb--78.3%, Cd--47.0%, Cr--53.3%; AC: Pb--70.4%, Cd--55.6%, Cr--55.2%. Increasing the sorbent dose increases the number of active sites available for adsorption, which facilitates the adsorption of heavy metal ions and explains the increase in removal percentage. However, the reduction in sorption capacity with higher sorbent doses (resulting in a higher ratio of sorbent mass to heavy metal ions) can be attributed to several factors, the most important of which is the saturation of sorption sites during the sorption process. Increasing the sorbent dose leads to a less proportional increase in total sorption due to lower utilization of the sorbent's sorption capacity. As the sorbent dose increases, the number of available sorption sites increases, resulting in a larger amount of heavy metal ions absorbed. However, the reduction in sorption capacity at higher sorbent doses can mainly be attributed to the saturation of sorption sites during the process [47; 50]. The observed higher sorption capacity of activated carbon compared to that of aspen wood and biochar is mainly due to its highly developed specific surface area and larger pore volume, which allow better access for metal ions. Activated carbon is also characterized by a more diverse distribution of pores and the presence of numerous functional groups that favor the creation of strong bonds with heavy metals. In turn, sawdust and biochar have a less developed porous structure and a limited number of active adsorption places, which translates into their lower adsorption capacity. Figure 6 shows the results of removing heavy metal ions from soil containing all three metals simultaneously by the tested materials. Figure 6A illustrates that all three tested metals were partially removed by the sorbents used. The metal concentrations obtained were: AW: Pb--790.5 mg/kg, Cd--912.4 mg/kg, Cr--918.9 mg/kg; BC: Pb--740.3 mg/kg, Cd--868.7 mg/kg, Cr--896.1 mg/kg; AC: Pb--684.3 mg/kg, Cd--865.2 mg/kg, Cr--854.4 mg/kg.