Evaluation of Soil Potential of Urmia Landfill as Cadmium (Cd) Absorbent Liner in Construction Engineering-health Landfill

 
Introduction
One concern in the disposal of municipal solid waste (MSW) by landfill is the production of leachate. Acidic water is capable of dissolving elements from the waste; and, as a result, the leachate can become quite contaminated. One of the heavy metals present in leachate, cadmium poses a great concern in terms of environmental contamination since it is toxic to nearly all living organisms and it is not used in any biological functions. Clay liners are used to contain contaminant such as heavy metals present in leachate from landfill sites containing MSW. In this study, the chemical and physical characteristics of soils of Urmia city landfill site in Nazloo region were examined to evaluate their cadmium sorption potential.
Materials & Methods
Surface soil samples (0-30 cm) were collected from around Urmia landfill soils. Soil physical and chemical properties were determined using standard methods. The pH was determined using a 1:2.5 soil to 0.01 M CaCl₂ suspension and a glass electrode. Electrical conductivity was determined in saturated extracts of the soils. Particle size distribution was measured by the hydrometer method. Total soil carbonates expressed as calcium carbonate equivalent (CCE) were determined by a rapid titration method. Organic carbon was determined by wet digestion. Cation exchange capacity (CEC) of the soils was determined by the 1 M NaOAc (pH 8.2) methods. Sorption isotherms were obtained using the batch equilibrium method. Soils of 1 g were separately equilibrated in tubes containing 20 ml of different concentrations of Cd: 2.5-40 mgl^-1 by dissolving Cd(NO₃)₂ in 0.01M CaCl₂ as background electrolyte solution. Samples was equilibrated on an end-over-end shaker for 24 h, followed by centrifugation; filtration and Cd concentrations was determined using Shimadzu 6300 atomic absorption spectrophotometer. The Langmuir and Freundlich one-surface sorption equations were applied to describe the reaction of Cd with soil. The desorption experiment was carried out by adding additional 0.01M CaCl₂ solution as a background solution to the soil remaining in the centrifuge tubes and maintaining the total amount of the solution exactly at 20 ml. Then the desorbed solutions were centrifuged, decanted and analyzed. The Langmuir and Freundlich models, applicable to heavy metals sorption processes, were used to determine the sorption capacity of different soils. The equations used were:



 

(1




Where C₀= the initial concentration of Cd⁺²(mg/l), V_sol= the volume of the solution (L) and M_s= the soil mass (g). The Freundlich and Langmuir isotherm equations are adopted as expressed by Eq (2 & 3).



 

(2



 

(3




In which K_f (Sorption capacity or distribution coefficient) and n (Intensity Sorption) are the Freundlich sorption parameters and K(Bonding Energy) and b (Maximum Sorption) are Langmuir parameters adjusted to fit Eq (2 & 3) to the experimental data.
 
Discussion of Results
Chemical and mechanical characteristics of soil samples
Soil properties are given in table 1. There is a direct relationship between content of clay and CEC in soils. Also CaCO₃ observed in soil number 1 have a noticeable difference in comparison whit other soils (2 & 3). Calcium carbonate is one of the important factors in sorption of heavy metals and directly influence sorption this metals, so increase in amount of CaCO₃ lead to enhancement sorption of Cd.
Table 1: Some of chemical properties of soil samples





Soil texture


CEC


EC


pH


CCE


OM


Clay


Silt


Sand


Sample




 


cmolckg^-1


dSm^-1


0.01M CaCl₂


%


 




Silty Clay


29


5.6


7.17


16.7


0.26


43.9


55


1.12


1




Sandy loam


18


0.67


7.53


3.64


0.68


19.2


11.3


69.2


2




Sandy Clay Loam


19


1.11


6.56


4.68


1.17


29.2


20


50.8


3





 
  The materials most commonly used for compacted clay liner construction are natural cohesive soils. The soil may be used in fine liner, total pore volume and porosity turns more and more surface area to adsorb provided. The term compacted clay liner (CCL) is used for all mineral liners which predominantly consist of fine grained soils like clays, silty clays and clayey silts. Soils with more than 20 percent clay are used for construction of engineering-sanitary landfill liner (Table 2).
Table 2: Mechanical properties of soil samples





Sample Code


1


2


3




Liquid limit(W_L)


30.9


22.5


25.3




Plastic limit(W_p)


15.8


14


14.5




Plastic Index(PI)


15.1


8.5


10.8




Specific density(Gs)


2.73


3.01


2.89




Maximum dry density(γd_max) (Kg/m³)


1882


1841


17.91




Optimum moisture content (ω_opt) (%)


14.2


17


16





 
Based on the results of the test samples 1 and 3 have about Aterbrg plasticity index is over 10. Environment Agency standards, plasticity index of the clay liner should be between 10 to 65 units. Standard Proctor test results on soil samples showed that soil 1 than in soil 2 and 3 of the maximum dry density and moisture content had significant difference and the difference is evident in the density plots. Assessed in terms of compaction to soil moisture content, soil 1, has the highest density at optimum moisture content to less than soils 2 and 3. This means that in soil 1, to achieve the maximum density of water is less and in this respect the use of soil 1, is to build more affordable liner.
Cadmium absorption characteristics
Adsorption isotherm diagram is shown in Figure 1. Results showed that the absorption curve of soil 1, due to the high adsorption capacity, appears to be linear and for other soils, the curve appeared. This suggests that adsorption sites of soil 1, with a maximum concentration of cadmium (40 mg/L of soil solution using a ratio of 1:20) in the adsorption isotherm tests are as half full and not completely occurring and increase in the cadmium concentration, adsorption in sites also will be blank. While adsorption sites of 2 and 3 soils immediately filled, and precipitation of cadmium also were absorbed on surface particles. To evaluate the intensity and amount of absorbed cadmium concentration in soil absorption data of 2 and 3, Sigma Plot v12 software using the Langmuir and Freundlich nonlinear equations were fitted. Freundlich and Langmuir adsorption equation adsorption parameters are shown in Table 3.



 
 
 



Figure 1: Adsorption isotherms of soils
 
 
 
 
Table 3: Freundlich and Langmuir adsorption parameters





Samples


 


 


 


 


 


Linear equation




1


 


 


 


 


 


a


b


SE


R²




 


 


 


 


 


 


85.9


7331


12.5


0.994




 


Langmuir adsorption parameters


 


Freundlich adsorption parameters




 


K


b


SE


R²


 


K_f


n


SE


R²




2


1.90


1121


25.76


0.992


 


764.2


1.73


25.15


0.993




3


2.30


1623


31.5


0.988


 


1501.4


1.43


51.40


0.969





 
The regression coefficient equations, Freundlich model and Langmuir adsorption data showed good fit disruption, But Freundlich than Langmuir model was more uniform. Slope steep and linear soil 1, indicating a high degree of absorption and the absorption maximum, the maximum amount of soil 2 and 3, the concentrations of these results. Desorption of cadmium levels in soils 2 and 3 is almost 3 times the amount of desorption in soil 1.This process of absorbing and filtering pollutants by soil particles, it is considered as a strong point. Therefore, soil 1, has been increase amount of sorption, and on the other hand, reduces the amount of cadmium desorption. Consequently, due to the high potential for soil 1, the absorption and desorption of cadmium decreases as the soil was suitable for use in the construction of clay liners.
Conclusions
The main objective of this study was to evaluate and select a suitable soil for the construction of clay liners that to this end, the parameters of cadmium absorption on sorption surface of soils in Urmia landfill, and their relationship with the engineering properties of soils were studied. Maximum absorption of cadmium in soil 1, due to high clay content, pH, CEC, CaCO₃, and great places absorption surface than other soils were higher. Furthermore, desorption of cadmium in soil 1, compared to the other soils, the minimum amount that is an important factor in evaluating the facility. Also, based on the mechanical properties of the soil, being fine aggregate, dry the high density, low moisture optimize, and plasticity index above 10 percent, the soil 1, the better option is to use a liner, is evaluated.