Comparative Study of Transport and Diffusion Coefficient of Copper and Cadmium in Saturated Clay and Fined Grained Soil

Abstract

Heavy metal pollution is a major problem for soil and water because of their toxicity and hazardous nature. The main sources of heavy metals are activities such as mining, smelting, use of chemical fertilizers, discharging of untreated heavy metal industrial waste or effluent etc. To find out, how much their potential hazard, it is very important to understand their mobility in the soil. The objective of this study is to quantify heavy metals (Cu and Cd) transport and diffusion in two different simulated soils (Soil X and Soil Y) having different properties. Two separate tests, column test and diffusion test, have been conducted to meet this objective. Column study tries to investigate the BTC’s of Cu and Cd, and relationship between depth and time in fined grained soil (Soil Y). While
diffusion test carried out to find the effective diffusion coefficient (D*) of Cu and Cd in clay soil (Soil X). In this study, batch equilibrium sorption experiments are carried out for different range of concentrations of Cu and Cd. The results shows the sorption of Cu and Cd in simulated fine grained soil and clay is nonlinear and follows Freundlich sorption isotherm. Cd exhibits higher Kf value and Cu exhibits lower N value for both types of soils. Moreover both metals have higher affinity in clayey soil, which has also higher cation exchange capacity than fine grained soil. In column test, vertical transport of Cu and Cd in saturated fined grained soil was also evaluated. Known concentration of simulated leachate of CuSO4 and CdI2 were introduced in the soil column. Samples of leachate flowing down the column were taken from different sampling ports at different height for Cu and Cd concentration. BTC’s of Cu and Cd at different depth of soil column were drawn. Concentration ratio (C/C0) versus depth curves were also drawn. Then t1/2 (C/C0 = 0.5) and t1 (C/C0 = 1) for different depths were calculated from the BTC’s and investigated for their linear and polynomial relationship with depth. Results indicated that BTC’s of Cu and Cd are “S” patterned. Concentration ratio (C/C0) decreases sharply for initial number of days for smaller depths. At same depths, t1 for Cd is significantly less than Cu. Cd was the more mobile with the higher recovery in the effluent solution. Time t1 follows second order polynomial relationship with depth for both Cu and Cd, while t1/2 and depth gives approximate same values of R2 for linear and second order polynomial equations. In diffusion study, effective diffusion coefficient (D*) of Cu and Cd in saturated clay soil were calculated using solution of transient diffusion equation. Different unknown parameters of equation were measured from laboratory experiments. Cu and Cd ions were contained in simulated leachate. The calculated D* range for Cu and Cd is 2.02×10-10 to 2.25 ×10-10 and 2.109×10-10 to 2.28×10-10 respectively. The effective diffusion coefficient ranges are in good agreement with previous studies. In this study a very small range of D* is given for Cu and Cd instead of a particular value. Since D* value measurement is very sensitive and requires computer programs to solve equation, it is suitable to give a range of D* to compensate the requirement of any numerical method