## 3 Results
### Sorption of Individual Heavy Metals
To understand the distribution of solutes in equilibrium between the different phases, Freundlich and Langmuir models were tested for the adsorption of heavy metals individually spiked in the soils. Details of the Freundlich and Langmuir are provided in the Supplementary Materials (Table S1) and it was observed that the models did not fit to the results obtained from present study. Sorption coefficients of Cd with 1:100 dilution ratio with concentration of 100 mg/L for untreated red soil was 3.3564 mg/L and 4.711 mg/L for untreated black soils with pH values 5.69 and 5.84 respectively whereas the sorption increased to 6.583 mg/L and 6.6487 mg/L after treating red soil and black soil by E2 and solution pH being 6.29 and 7.32 respectively. Similarly, Ni adsorbed to the maximum extent when compared to Cd and Ni. Table. 1 gives the comparison of sorption coefficients of Cd, Ni and Pb before and after treatment with E2. Comparison with results obtained after E2 treatment is done since E2 is found to provide maximum sorption on the soil. Figure 1: Sorption coefficients at different initial concentrations, (**a**) sorption of Cd, (**b**) sorption of Ni. Impact of pH on the adsorption mechanism is vital, CaCO\({}_{3}\) surface is charged positively when pH value happens to be more than 8, even though negative charges exist but are ineffective, thereby attracting the anionic heavy metal content in the solution leading to adsorption [62]. Sorption of Pb on the CaCO\({}_{3}\) surfaces is reported to be very high by [PERSON] et al. [63] and they also reported that the mechanism of Pb sorption on the calcite surface also showed the formation of PbCO\({}_{3}\) (cerussite), formation of cerussite is helpful in reducing hazardous effects of Pb. Mechanism of sorption of heavy metals on the CaCO\({}_{3}\) precipitates is proposed to be ion exchange, precipitation of heavy metal carbonates and hydroxycarbonate forms, adsorption and complexation [64]. Literature available on the use of CaCO\({}_{3}\) as a sorbent portrays the use of eggshells [65], precipitates extracted from earthworms [66], naturally available limestone [7], Calcite precipitated by microbes [59], lime [67] it is reported by [PERSON] [65] that sorption behavior of heavy metal does not depend on the source of CaCO\({}_{3}\) but the initial concentration of the heavy metal in the solution. Hence the mechanism taking place in sorption process by the CaCO\({}_{3}\) precipitates by the Enzyme solutions in the present work can be proposed to be similar to that taking place by CaCO\({}_{3}\) precipitated by other sources. Ni adsorption was comparatively more than Cd and Pb, the reason for the same may be the presence of Ni in urease used in the work, about 17% of Ni is present in jack bean extracted urease [41], Ni ions already present in CaCO\({}_{3}\) precipitated due to the urease enzyme probably club with the Ni ions present in the contaminant solution. It can be observed that the Table 2 shows the statistical analysis of the sorption coefficients for both the soils for different metals and it can be observed that the sorption range for soils treated with E2 is better than that of untreated one.