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Organic Ligand and pH Effects on Isotopically Exchangeable Cadmium in Polluted Soils

^a Dep. of Soil and Water, Adelaide University, PMB No. 1, Glen Osmond, South Australia 5064, Australia
^b CSIRO Land and Water, PMB No. 2, Glen Osmond, South Australia 5064, Australia
^c Laboratoire Sols et Environnement, ENSAIA-INRA/INPL, 2 avenue de la Forêt de Haye, BP 172, F-54505 Vandoeuvre-les-Nancy, France

View larger version (10K): [in a new window]   Fig. 1. The distribution of Cd between the solid and solution phases (represented as Kd) for the soils as affected by changes of pH. Values and error bars represent the mean and standard deviation of triplicate measurements. The five-parameter nonlinear regression fitted for the acidic soil was Kd = 48.4 + 2499/(1 + exp[-(pH-5.77)/0.18]0.86: R2 = 0.99, P < 0.0001 and for the calcareous soil was Kd = 112.4 + 3421/(1 + exp[-(pH-7.49)/0.0034]0.0112: R2 = 1.00, P < 0.0001. See text for explanation of equations.

View larger version (32K): [in a new window]   Fig. 2. The Kd of Cd in the acidic soil as affected by organic ligands compared with estimated values at the same equilibrium pH in the absence of ligand. Values and error bars of Kd in the presence of ligands represent the mean and standard deviation of triplicate measurements. The Kd values in the absence of ligands were calculated using the five-parameter nonlinear regression derived from the data of Fig. 1 and the mean pH values measured from triplicate samples containing the ligands.

View larger version (38K): [in a new window]   Fig. 3. The Kd of Cd in the calcareous soil as affected by organic ligands compared with estimated values at the same equilibrium pH in the absence of ligand. Values and error bars of Kd in the presence of ligands represent the mean and standard deviation of triplicate measurements. The Kd values in the absence of ligands were calculated using the five-parameter nonlinear regression derived from the data of Fig. 1 and the mean pH values measured from triplicate samples containing the ligands.

View larger version (8K): [in a new window]   Fig. 4. The E value of Cd in the acidic soil as affected by pH and the presence of organic ligands. The fitted line is a linear regression of the pH data where E = -0.8683pH + 18.223: R2 = 0.91, P > 0.001. The symbol () and error bars represent the mean value and standard deviation of triplicate measurements of pH. The other symbols represent: (0.25–5 mM) citric acid (x), (0.25–5 mM) sodium citrate (o), (0.25–1 mM) histidine (), and (0.25–1 mM) tartrate ().

View larger version (11K): [in a new window]   Fig. 5. The E value of Cd in the calcareous soil as affected by pH and the presence of citric acid. The solid line represents the fitted five-parameter non-linear regression of E = 3.44 + 7.83/(1 + exp[-(pH-8.23)/-0.41]21: R2 = 0.99, P < 0.0001. Hashed lines represent 95% confidence limits. The symbol () represents the mean E value of Cd, in the absence of ligands, measured with triplicate samples. The symbol (x) and error bars represent the mean value and standard deviation of triplicate measurements of (0.25–5 mM) concentrations of citric acid.

View larger version (8K): [in a new window]   Fig. 6. The effect of organic ligands on the E value of Cd in the calcareous soil. Values represent the ratio of the E value measured in the presence of the ligand against that measured in the absence of the ligand at pH 7.5. Symbols and error bars represent the mean and standard deviation of triplicate measurements. The symbols represent: (0.25–5 mM) sodium citrate (), (0.25–1 mM) histidine (), (0.5–1 mM) tartrate (), and (0.25–0.5 mM) DMA (+).