## 3 Results
### Simultaneous Desorption of Heavy Metals  
Desorption studies give a better understanding of the mechanism of encapsulation since heavy metal solution is introduced in the soil and left for around 40 days to cure.  
Figure 6 gives the details variation of removal efficiencies of heavy metals with concentrations 50 and 100 mg/kg for EDTA and citric acid extractants with different molarities. Removal efficiencies of heavy metals in the solution had reduced after treatment with E1, E2 and E3. Figure 6a gives the variation of removal efficiencies for Cd and Ni spiked together with 50 mg/kg load ratio in the soils and extracted by EDTA solution. It was found that removal efficiency of Cd decreased to 11.2% for red soil treated with E2 from 21.28% which was for untreated red soil and removal efficiency of Ni for red soil treated with E2 decreased to 11.64% from 15.24% of untreated red soil. Further for black soil the removal efficiency for Cd and Ni decreased to 11.18% and 10.84% respectively after treatment with E2 from 15.68% and 18.33% for Cd and Ni respectively for untreated black soil. For load ratio of 100 mg/kg of Cd and Ni spiked in the soils and extracted by EDTA the results obtained were again better for E2 treated soils, removal efficiency for Cd and Ni reduced to 10.98% and 6.08% from 12.66% and 10.84% respectively (refer Figure 6c). Similarly, removal efficiencies of Cd and Ni for both load ratios of 50 mg/kg and 100 mg/kg extracted by Citric acid are shown in Figure 6b,d. Similar figures for desorption of other combinations (Ni + Pb, Pb + Cd and Ni + Cd + Pb) are given in Supplementary Materials (Figures S5-S7). CaCO\({}_{3}\) precipitated in the soil grains plays vital role in encapsulating the heavy metal heavy metals and method of precipitating CaCO\({}_{3}\) by enzymes is specifically effective for Cd [84].  
Figure 5: Schematic representation of precipitation of calcite in the soil grains and formation of calcite biofilms on the soil grains and adsorption of heavy metal ions on calcite surface.  
Desorption Studies conducted on all combinations showed that simultaneous immobilization of heavy metals in the soil is better after its treatment, E2 was the most effective solution in decreasing removal efficiencies. It is being noted by [PERSON] et al [85] that removal efficiency can be up an average of 80% and removal can be up to 99% with increase in heavy metal concentration thereby posing severe effect on the environment. Continuous accumulation of heavy metal ions from the contamination source leads to serious effects. Applying immobilization techniques on the contaminated sites can reduce the effects by either crystalizing or encapsulating the contaminants. Precipitates of CaCO\({}_{3}\) in the soils have different behavior to different heavy metals [77]. The formation of carbonates (CdCO\({}_{3}\), NiCO\({}_{3}\) and PbCO\({}_{3}\)) in the form of bio-minerals in contaminated soils was observed by [PERSON] et al. [86]. These bio-minerals have different morphologies and include spherical shape, needle shape or rhombohedral shape and minimum dimension of 10 \(\upmu\)m to a maximum of 50 \(\upmu\)m. [PERSON] et al. [87] carried out lead bio-mineralization in mine tailings using Bacillus sp. KK1 bacteria and found 26% reduction in exchangeable Pb due to the bio-mineralization process triggered by the precipitated calcite.  
Figure 6: Removal Efficiencies of Cd + Ni spiked in both soils with: (**a**) 50 mg/L ethylene diamine tetra-acetic acid (EDTA) extractant; (**b**) 50 mg/L citric acid extractant (**c**); 100 mg/L EDTA extractant, (**d**) 100 mg/L citric acid extractant.