(288l) Cadmium Sorption by Some Alkaline Soils of North-West India

Abstract: Cadmium (Cd) contamination of the environment has been attributed to industrial activities like smelting and electroplating of metals, combustion of lubricating oils, vulcanization of rubber for motor vehicle tires and use of cadmium compounds as fungicide sprays. The divalent Cd is slightly soluble in an aqueous solution when associated with carbonate anions and in alkaline soils and therefore their leaching must be limited. Thus higher Cd status of surface soils entails its accumulation in plants and eventually in food stuffs. Therefore, the present investigation was undertaken to study cadmium adsorption by alkaline soils of north-west India. Surface samples of alkaline soils representing alluvial plains in north -west India were collected from different locations in Punjab. For sorption study, 1 g of soil in duplicate was equilibrated for 24 hours in polyethylene bottles containing 50 ml of 0.01 M NaNO3 solution of different Cd (added as Cd(NO3)2 ) concentrations ranging from 0.25 to 1.35 ìmol Cd cm-3. Adsorption isotherms were determined by equilibrating the soil solution suspension at 293 0K and 313 0K. A clear solution was separated from soil-solution suspension with Whatman no.42 filter paper. The pH and EC was determined of the filtrate solutions at 298 0K by pH meter and Conductivity Bridge, respectively. The Cd in the equilibrating solution was determined on ICAP-AES. The Langmuir equation was used to interpret the equilibrium adsorption data. The linear form of the Langmuir equation is: C/x/m = 1/Kb + C/b, where, C is the equilibrium Cd concentration (ìmol Cd cm-3); x/m is the amount of Cd adsorbed (ì mol Cd g-1 soil); b is the adsorption maxima (ìmol Cd g-1 soil) and k is the constant related to the bonding energy of the soil for Cd (Cm-3 ìmol Cd). The differential isosteric heat of Cd adsorption, ÄH ads((adsorption), was obtained by collecting adsorption data at 293 and 3130K and applying the Clausius-Clapeyron equation to the system. For a given surface coverage è, log(C2/C1)è = ÄH ads/2.303R (1/T1 -1/T2 ),Where C1 and C2 are the equilibrium concentration (ìmol Cd cm-3) at temperature T1 (2930K) and T2 (3130K), respectively and R is the molar gas constant (Kcal mol-1). The differential isosteric heat of adsorption (ÄH ads) is related to heat adsorbed or liberated during adsorption reaction at constant surface coverage but at different temperatures (Hayward and Trapnel, 1964). By convention, it is equal but opposite in sign to that portion of the change in enthalpy of the system attributable to adsorption. ÄH = -ÄH ads. For solubility isotherms, pCd values, the negative logarithm of the cadmium activities were calculated for soils equilibrated with 0.01M NaNO3 solution containing no initial Cd and with the equilibrated initial Cd concentrations of 40, 70, 90 and 140 ìmol Cd cm-3. For study of solubility of cadmium minerals, equilibrium reaction of octavite (CdCO3) and cadmium hydroxide (Cd(OH)2 phase was considered at 0.003 atmospheric pressure of CO2Corrections for the following ion-pairs were made: CdHCO3+, CdCO30, CaNO3+, Cd(NO3)02 and CdHPO04. Cadmium adsorption on five alkaline soils of north-west India followed Langmuir equation. The analogues shape of Linear Langmuir plot in each soil at 298 and 3130K elucidate that temperature does not change the nature of reaction but influence the bonding energy constant and adsorption maxima for Cd ion. The magnitude of adsorption maxima enhanced with increase in CaCO3 and organic content in soils. The positive and constant value of differential isosteric heat of adsorption (ÄH) on each soil between 298 and 313oK elucidate that adsorption of Cd is an endothermic process which leads to precipitation of Cd rather than physical adsorption. However, the variation of ÄH within different soils might be due to variation in their physico-chemical characteristics. Solubility isotherm diagram (Fig.1) revealed that CdCO3 reaction product formed in equilibrated soil-Cd system used for adsorption study. It was concluded from Linear Langmuir plots, differential isosteric heat of adsorption ÄH (ads) and solubility isotherm studies that adsorption of Cd in alkaline soils occurred due to precipitation of CdCO3 mineral. Keywords: Cadmium adsorption isotherm, solubility isotherm, differential isosteric heat of adsorption ÄH (ads).