INTRODUCTION Cd Usually it combined with other elements such: oxygen, chlorine, or sulfur. Cd 2+ activities in soils is about M. levels of Cd in most waters is one µg/L or less. Typical topsoil levels of Cd about 260 µg/kg. Intake of Cd from the diet is about 0. 4 µg/kg/day

Sources of Cadmium : Most Cd is obtained as a by-product from the smelting of Zn, lead, or copper. Cd used in metal plating, pigments, batteries, and plastics. Igneous rocks ( mg/Kg) Metamorphic ~.3 & Sedimentary rocks ( g/kg) ZnS minerals, ZnCO3 Up to 5% Fertilizers in cultivated soils

Several processes tend to keep Cd concentrations low in groundwater Adsorption by organic matter and clay minerals. Complexes by oxides. Precipitation of Cd as CdS when Sulfide present Precipitation of Cd as carbonate at relatively high pH.

Adsorption: can be described as the process in which cadmium entering the soil becomes physically or chemically bonded to the colloidal surfaces, therefore causing a net decrease in its concentration in the solution phase.

When the nature of cadmium adsorption and the influence of soil characteristics on the process are better understood, cadmium contamination of ground water and plant availability may be assessed more accurately. Important of the subject :

OBJECTIVES To measure the solid activity coefficients (SAC) of adsorbed Cd. To model Cd Adsorption on Hydrous ferric oxide (HFO)& humic acid (HA) surfaces

Ion activity product (IAP) of Cd(OH) 2 is usually less than the solubility products (Ksp)of pure mineral phase Reasons Isomorphous substitution Adsorption. Nonequilibrium condition Non Ideal Solid Mixtures IAP & Ksp

Chemistry of Cadmium The oxidation states of cadmium in a soil is limited to Cd 2+. In most environment the solubility of cadmium is governed by hydroxide or carbonate.

METHODOLOGY Adsorption Isotherms: A 25 ml of M NaNO 3 containing 0.8, 1.6, 3.2, 6.4, and 12.8 ppm Cd 2+ were added to 0.5 g of soil in a 50 mL polyethylene centrifuge tubes and shaken at room temp. for 24 hr. For pH Cd adsorption study : pH (5, 6, 7, 8) and 12.8 ppm Cd addition were used

CADAMIUM ANALYSIS Total Cd by an atomic absorption spectrophotometer Cd 2+ activity by Cd selective ion lectrode

SAC of Cadmium adsorbed Approximate SAC can be calculated from IAP in the aqueous soil solution. From ISE & pH measurements IAP = (Cd(OH) 2 ) = (Cd 2+ )(OH - ) 2 For nonideal solid mixtures: Bohn & Bohn 1987 IAP i = SAC i *X i *K spi X= Mole Fraction. The Ksp = Lindsay, 1979.

Speciation and Modeling Adsorbed Cd The amount of Cd adsorbed onto each of the humic and hydrous ferric oxide surfaces was modulated using HYDRAQL program Assumption for Humic acid Two unique diprotic complexation sites, H 2 A and H 2 B, associated with the humic materials. Bolton et al.,1996

The total [H 2 A] T and [H 2 B] T calculated by : [ H 2 L] T = (Msoil*OC*THA*0.50*1.724) V M soil = mass of soil (g), OC = organic carbon (g/kg), THA = total humic acidity (mol/g) of H 2 L, V = volume of solution (ml). The 0.5 and included to account for the diprotic nature humic acid and to convert from carbon to humic acid

Cd adsorption on hydrous ferric oxide (HFO) surfaces USING: The diffuse layer model Dzombak and Morel 1990 The total concentration of adsorption sites associated with [FeOOH] T calculated as: [FeOOH] T = (Feo-Fep)*Msoil*Ns (V*MW Fe ) Feo-Fep is the inorganic poorly crystalline Fe fraction, N s is the site density (mol mol Fe -1 ) of the strong or weak sites, and MW Fe is the molecular weight of Fe.

RESULTS AND DISCUSSION General Description of Soils The soil pH Y 4.9 to 8.8 The CEC Y 4. to 99.0 cmol/kg The O.C Y 0.6 to g/kg. Iron Oxides: The Feo Y 1.7 to 31.0 g/kg The Fep Y 0.1 to 8.0 g/kg The Fed Y 2.7 to 51.0 g/kg.

Cadmium Activities in Soils At ( ppm) Cd Load & Unadjusted pH: The Cd 2+ in the soils studied varied M It increased with increasing total Cd added. At (12.8 ppm ) & different levels of pH (5-8) The Cd 2+ activities ranged from M The Cd 2+ were highly correlated with soil pH The Cd 2+ activities were inversely related to soil pH.

Cd2+ activities as a function of pH for soils (1, 2, 4, 6, and 10).

Solubility diagram for the Cd-soil equilibrium for soils (1, 2, 4, 6, 10).

Log Rcd versus pH for two different amounts of HFO in a soil/water system.

Modeling of Cadmium Adsorption on Soils The complexation model prediction values for the % of adsorbed Cd are within 5% of the batch data at the pH 8 Soils have high contents of O.M & hydrous ferric oxides showed relatively good agreements with the batch data. The Humic materials are important in Cd retention at pH>3 & HFO surfaces at pH>7.

Measured and predicted Cd adsorption onto soil (2) using the surface complexation model.

Measured and predicted Cd adsorption onto soil (18) using the surface complexation model.

Measured and predicted Cd adsorption (Aridic Soils)

Measured and predicted Cd adsorption (Andisols Soils )

Cadmium speciation for the non electrostatic and the diffuse layer models,where [H 2 A] T = 1.66x10 -3 M, [H 2 B] T =7.44 x10 -4 & [FeOOH] T =3.76x M.

SAC of Cadmium Adsorbed SAC of Cd Adsorbed (Unadjusted pH and varied Cd loads). IAP of increased with Cd addition. & IAP < Ksp The IAP Y 2.3x M to 5.6x M. The SAC of Cd(OH) 2 Y 8.2x10 -5 to 470 The single ion SAC Cd2+ Y to 7.8 The large SAC values of Cd(OH) 2 imply that the substituting ion fits poorly into the mineral structure

SAC of Cadmium Adsorbed SAC of Cd Adsorbed (pH 5-8 &12.8 ppm Cd ) IAP of is strongly pH dependent. The SAC of Cd(OH) 2 Y 3.9x10 -6 to 4.6 SAC increased with pH The SAC results indicate that Soil Cd Sorption Capacity would increase with increasing iron (Feo-Fep, Feo, and Fed), CEC, and O.M in soils.

IAP of Cd(OH)2 as a function of soil pH

CONCLUSIONS 1) The relationship between Cd activity and soil pH was highly significant 2) The Cd adsorption prediction using the non electrostatic and diffuse layer models would work well in soils with high contents of O.M & HFO 3) A small change in pH can cause a dramatic change in the adsorption capacity of the soil specially at pH below 6.

4) The humic materials are an important factor in Cd retention at pH > 3 and HFO surfaces are an importantat pH greater than 7. 5). The correlation between SAC Cd2+ and soil properties suggests that soil adsorption capacity for Cd would increase with increasing Iron Fraction and O.M in 6) The Rcd values increased with increasing amounts of the iron oxide fractions. 7) The Rcd values increased with increasing amounts of the iron oxide fractions.

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