## 3 Results
### Simultaneous Sorption of Heavy Metals
with Enzyme solutions, further E2 happened to be most effective in simultaneous sorption of Cd + Ni. Cd sorption was 7.1624 mg/g and for Ni was 10.6642 mg/g when black soil was treated with E2 and the corresponding pH value was 7.08 and while the sorption of Cd was 9.8116 mg/g and that for Ni was 12.6730 mg/g for black soil treated with E2 and the pH of the contaminant solution was 6.97 at dilution ratio of 1:100. pH is one of the main controlling factors in sorption and solubility of contaminants in the soil [68; 69]. Further the sorption of Ni + Pb with different dilution ratios are plotted in Figure 4b. It was observed that E2 treatment gave the best results in sorption of heavy metals whereas treatment with E3 portrayed comparatively lesser sorption than E2 and E1, although sorption after E3 treatment was better than the sorption observed in untreated soil. Precipitated CaCO\({}_{3}\) in the soil has high capability to adsorb metal ions encircling its surface [70; 71; 72]. CaCO\({}_{3}\) is considered to have large specific surface, hollow structure and steady chemical characteristics, because of which it has found vast application in adsorption of metal ions [73; 74] same reasons hold good in this work. Greater adsorption on soils treated with E2 than those treated with E1 and E2 was observed because of higher precipitation of CaCO\({}_{3}\) due to the use of non-fat milk powder in E2 [21]. Further electronegativity of metal ions played a vital role in the adsorption process, i.e., metal ions with greater electronegativity get sorbed more than those with lesser electronegativity [75; 76]. The electronegativities of the heavy metals used in this study have the order Pb > Ni > Cd that is 2.33, 1.91 and 1.69 respectively. The effect of electronegativity on the sorption process was found justified in present study for different dilution ratios (refer Figure 4a,b) but this reason was not justified in the sorption tests conducted for different initial concentrations (refer Figure 3b). Ionic radii of the heavy metals influence the adsorption process by competing for sites on the adsorbent and metal ions with higher ionic radius induce faster saturation of sites on the adsorbent, therefore the heavy metals move into calcite's crystal lattice and get adsorbed [77], since ionic radius of Pb (202 pm) is greater than that of the other two metals ions (Cd-158 pm and Ni-163 pm) sorption of Pb is comparatively more. [PERSON] et al. [78] studied applicability of bioremediation on Pb contaminated soils by biologically precipitating calcite. The results revealed that Pb changed from soluble/exchangeable (toxic) form to carbonate bound Pb (PbCO\({}_{3}\)), which is insoluble and nontoxic. Sorption of Cd on the calcite surface is found to be appreciable due to the similarity of its ionic radii with that of Ca ions in the precipitates [79; 80]. [PERSON] et al. [81] used CaCl\({}_{2}\) for washing Cd contaminated soils and observed 55% removal of Cd from soil. It is attributed to the potential of Cd to form complexes with anions like CI, CO\({}_{3}\), PO\({}_{4}\) and SO\({}_{4}\) and assisted in the precipitation of CaCO\({}_{3}\) to immobilize it even in calcareous soils [30; 82]. Results obtained from sorption  
Figure 4: Sorption coefficients of heavy metals spiked simultaneously for different dilution ratios: (**a**) combination of Cd + Ni (**b**); combination of Ni + Pb. studies portrayed that sorption of heavy metals is lesser when compared to that with individual sorption studies probably because of unavailability of sites on the sorbent for the metal ions. This phenomenon is generally found in the sorption processes [83]. A schematic representation of adsorption process has been shown in the Figure 5 accumulation of metal ions encircling the calcite precipitated in the soil pores can be observed there.