## 3 Results  
### Sorption of Individual Heavy Metals  
Sorption coefficients (q\({}_{e}\)) in mg/g were found for the tests conducted on the treated soils, an understanding of the behavior of heavy metal sorption was obtained with the plots shown in Figure 1. Amount of heavy metal getting sorbed on the untreated soil was comparatively lesser than that of the treated soil. Values of sorption coefficients for 100 mg/L initial concentration of Ni were 1.513 mg/g (pH 6.32) and 7.0838 mg/g (pH 7.63) for raw red soil and red soil treated with E2 respectively. For black soil the sorption coefficients for Ni were maximum for treatment with E2, for raw black soil Ni sorbed to 1.8754 mg/g (pH 6.35) and that after treatment with E2 showed an improvement of 7.6873 mg/g while the solution pH was 7.88. Further the sorption was comparatively less effective for Cd and Pb than Ni, and E2 being the most effective in holding the heavy metals. Sorption of Cd increased to 5.4235 mg/g (pH 6.44) and 5.8519 mg/g (pH 7.09) for red soil and black soil treated with E2 from 1.352 mg/g (pH 6.44) and 2.4523 mg/g (pH 6.11) for untreated red soil and black soil respectively. Pb sorption was 5.6925 mg/g (pH 7.01) and 6.6741 mg/g (pH 8.24) for red soil and black soil respectively treated with E2 and that for untreated red soil and black soil was 1.5476 mg/g and 1.8139 mg/g with pH values 6.87 and 7.01 respectively. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Curing (Days)**} & \multicolumn{3}{c}{**RS (Permeability Samples)**} & \multicolumn{3}{c}{**BS (Swell Test Samples)**} \\ \cline{2-7}  & **E1** & **E2** & **E3** & **E1** & **E2** & **E3** \\ \hline
7 & 1.784 & 1.687 & 1.583 & 1.693 & 1.983 & 1.365 \\ \hline
14 & 1.965 & 2.813 & 1.691 & 1.835 & 2.387 & 1.578 \\ \hline
21 & 2.681 & 2.967 & 1.833 & 2.465 & 2.592 & 1. Sorption on soils treated with E1 and E2 were also better than that of untreated soils, Figure 1a,b show the plots for sorption coefficients against different initial concentrations for both soils spiked with Cd and Ni respectively, similar plot for Pb is given in the Supplementary Materials (Figure S1), it can be observed that sorption for untreated soils was less than that for the treated soils, Thereby indicating the effectiveness of enzyme treated soil in providing suitable environment for the heavy metal to sorb. The mechanism of increase in sorption can be attributed to the precipitates of CaCO\({}_{3}\) on the soil grains which provide sites for heavy metal ions to adsorb [59]. CaCO\({}_{3}\) plays a vital role in the improvement of sorption of heavy metals [60]. CaCO\({}_{3}\) is known for its involvement in biomineralization of heavy metals like Cu, Ni, Co, Zn, Pb, Ca and Cd and converting them in to precipitates [61], In the present study, precipitated CaCO\({}_{3}\) in the soil is the main source to initiate the sorption of heavy metal ions. Adsorption of the heavy metals depends on the parameters like, solubility, adsorbent affinity, size of cations etc. [PERSON] et al. [51] reported that the heavy metal adsorption takes place due to exchange of protons and precipitation of heavy metals at micro level. Adsorption trend of the heavy metals was Ni \(>\) Pb \(>\) Cd in both soils. Further the results obtained for sorption tests conducted with different dilution ratios portrayed that, decrease in dilution ratio decreased the sorption coefficient, maximum sorption was observed at a dilution ratio of 1:100 because greater amount of heavy metal will be available to get adsorbed on the soil grain surface and even greater availability of solid surface sites on the soil grains support greater sorption [55]. Figure 2 represents the variation of sorption coefficients for different dilution ratios for Cd and Ni. Sorption coefficients of Pb for different dilution ratios are provided in Supplementary Materials (Figure S2).