## Materials and Methods
### Sorption Tests  
Soil samples were initially mixed with distilled water and shaken for 24 h and desired dosage of heavy metal solution was spiked and shaken for 24 h. The solution was filtered using Whatman 42 ashless filter paper and the amount of heavy metal sorbed is calculated using atomic absorption spectrophotometer (AAS) (PerkinElmer Model A-Analyst 400). To ascertain accuracy and reproducibility, each test was performed on three samples and the average value was taken. The amount of heavy metal sorbed on the soil surface was obtained by calculating the difference between the initial and final mass of heavy metal concentration in the filtrate solution. To achieve the desired concentration of heavy metal, i.e., 10, 20, 50, and 100 mg/L; 50 mL of deionized double distilled water is spiked with 0.5, 1, 2.5 and 5 mL of stock solution (1000 mg/L) respectively. All contaminant solutions were maintained to have a solid to liquid (s/l) ratio of 1:20, addition of 1000 mg/L stock solution varies the s/l ratio to 1:20.5, 1:21, 1:22.5 and 1:25 respectively and this variation in s/l ratio is justified by ASTM D3987 [54]. pH of the test samples was maintained to 5 by using 1M HCL (unit molarity). Soil particles get dispersed and hydrated when shaken for a duration of 24 h in deionized double distilled water, and attain steady state. Contaminants are spiked at this stage to enable sorption onto every possible soil grain. To ensure a constant dilution ratio, stock solution of very high concentration was added in a comparatively lesser volume of the aqueous solution to maintain the desired contaminant concentration, i.e., 100 mg/L. It can be observed that the ratio of s/l ratio increases slightly with a difference of 0.1, this is an acceptable method of maintaining the desired contaminant concentration [55]. Different dilution ratios were maintained to carry out the sorption studies. Dilution ratio is the ratio of the amount of liquid in mL taken for a specific mass of soil in grams to be mixed together to achieve the desired ratio. The ratios maintained in the study were 1:5, 1:10, 1:20, 1:50 and 1:100 by mixing 5 g of dried soil in deionized distilled water of volumes 25, 50, 100, 250 and 500 mL respectively. Further contaminant concentration was maintained at 100 mg/L, by adding 2.5, 5, 10, 25, and 50 mL of 1000 mg/L stock solution. The contaminant stock solutions were added to the samples using the heavy metal stock solution using a micropipette. The solution was shaken for 24 h at 30 rpm before and after adding contaminants. pH value of each aqueous solution was maintained at 5 before adding contaminants to avoid precipitation of the heavy metals and the final pH values of the solutions were determined soon before testing in AAS. Further the same procedure was followed to spike more than one contaminant simultaneously in the following combinations, i.e., Cd + Ni, Ni + Pb, Pb + Cd and Ni + Cd + Pb. Amount of contaminant sorbed is expressed as sorption coefficient (\(q_{e}\)) (mg/g) using the expression:  
\[q_{e}=\frac{(C_{0}-C_{e})V}{m} \tag{1}\]  
where:  
\(C_{0}=\) initial concentration of heavy metal (mg/L),  
\(C_{e}=\) sorbed heavy metal on the soil (mg/L),  
\(m=\) mass (mg) of dry soil, and  
\(V=\) volume (L) of the solution. Results obtained from the sorption tests were modeled using the Langmuir and Freundlich Isotherms for the soil surfaces. In Langmuir model, the sorption surface offers a single layer coating for heavy metal ion on the specific surface, whereas, the Freundlich model gives insight for sorption on heterogeneous surface(s). The models can be expressed as follows  
Langmuir Isotherm  
\[q_{e}=\frac{V_{m}KC_{e}}{1+KC_{e}} \tag{2}\]  
where:  
\(q_{e}=\) sorption coefficient (mg/g),  
\(V_{m}=\) capacity of monolayer,  
\(K=\) equilibrium constant,  
\(C_{e}=\) sorbed heavy metal on the soil (mg/L).