# 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
These values are higher than those obtained under the same parameters but in soil containing individual metals. The differences range from 8% for AW and chromium ions to 59% for AC and lead ions. A similar trend was observed in the analysis of the degree of metal removal (Figure 6B). Here, the reduction ranged from 51% to 82% for AW--Cr, while for cadmium removal by AW, the reduction varied between 35.4% and 6.4%. The sorption capacity of the materials with respect to Pb, Cd, and Cr also showed the same trend (Figure 6C). The differences between the results for soil containing individual metals and soil containing all three metals can be attributed to competitive sorption that occurs when removing multiple metals simultaneously. In such cases, the available sorption sites on the tested materials are occupied by various metal ions, reducing the efficiency of removal and sorption capacity for each metal. When removing individual metals, the sorbents were able to sorb specific metal ions more effectively, leading to higher removal efficiency and sorption capacity [51; 52]. These results highlight the significant impact of competitive adsorption on the cleaning processes and emphasize the importance of selecting appropriate process conditions for practical heavy metal sorption applications. Many researchers have studied the use of biochar made from various raw materials to remove heavy metals from soil. [PERSON] et al. obtained biochar from sludge by pyrolysis carried out at different temperatures to remove chromium, lead, cadmium, arsenic, copper and zinc. The biochar produced at 900 \({}^{\circ}\)C reduced chromium from 73.51% to 9.57%, lead from 55.91% to 4.87%, cadmium from 78.20% to 12.50%, arsenic from 97.91% to 52.11%, copper from 91.65% to 9.44% and zinc from 98.82% to 63.34% [53]. [PERSON] et al. used rice husk biochar to remove copper and zinc, among others, which allowed them to reduce Cu from 53.9 mg/kg to 51.57 mg/kg and Zn from 210.82 mg/kg to 194.59 mg/kg [54]. [PERSON] et al. investigated the possibility of using rice straw biochar and a mixture of biochar and compost to remove cadmium from soil. After 30 days, they observed a 65.8% reduction in cadmium using biochar alone and an 87.1% reduction in cadmium using a mixture of biochar and compost [55]. [PERSON] et al. used magnetic macroporous biochar beads to remove cadmium and arsenic from soil in their study, achieving reductions in cadmium from 2.81 mg/kg to 1.39 mg/kg and arsenic from 60.23 mg/kg to 27.34 mg/kg [56]. The results presented on the removal efficiency of the studied metal ions by the analyzed materials show that lead ions are removed with higher efficiency compared to cadmium and chromium ions. The main reasons for this difference are due to the chemical and physical properties of Pb(II), which has a larger ionic radius and lower hydration energy compared to Cr and Cd ions, which facilitates its sorption on the sorbent surface (lower hydration energy results in easier adsorption) [57; 58]. Lead ions easily form strong complexes with functional groups on the surface of the adsorbent, which  
Figure 5: The influence of the material and its concentration on the removal of heavy metals from the soil: (**A**)—Metal concentration [mg/kg], (**B**)—removal efficiency [%], (**C**)—sorption capacity [mg/g] (Cs = 2%,5%,10%, (**A**–**C**)—Series I, (**A**\({}^{\prime}\)**-**C**\({}^{\prime}\))—Series II). increase its removal efficiency [59]. Lead, due to its properties, better fills available spaces on the adsorbent surface compared to smaller and more complex forms of metals, such as chromium ions. Chromium in the form of Cr(VI) (e.g., as chromate) is less susceptible to adsorption compared to Pb(II) because Cr(VI) is more soluble in water and has less tendency to form permanent bonds with functional groups on the adsorbent surface. Although the obtained biochar shows lower effectiveness in removing metals compared to activated carbon, it remains a valuable sorbent material.