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A widely applicable model for predicting the cadmium concentration in soil solution

We developed a method for predicting the total cadmium (Cd) concentration [CdT] in soil solution. Disregarding minor ion pairs, [CdT] was approximated by . We assumed that Cd2+ activity (Cd2+) is dominated by the adsorption–desorption equilibrium, expressed stoichiometrically as . From this, we derived the balance equation We calculated (CdCl+) as the product of (Cd2+), chloride activity (Cl–), and a production constant. Our proposed prediction model was We conducted an experiment using six types of soil and manipulated the Cd content, pH, and chloride concentration [Cl−] to determine the effect on Cd concentration in the soil solution. The soil solution measurements produced rectilinear regression relationships between log(Cd2+) and log Q Cd – 0.6pH with slopes of 1.0 in all soil types, which validated the proposed Cd adsorption–desorption stoichiometry; the y-intercept of the regression lines represented log(K Cd/C Cd) and depended on the soil type. Furthermore, we were able to predict log(K Cd/C Cd) as a function of cation exchange capacity (CEC), and the activity coefficient (f Cd) as a function of electrical conductivity (EC) alone. Substituting these functions into the proposed prediction model, we obtained [CdT] = 0.288Q Cd10−0.6pH CEC −2.37{1 + 2.23EC 0.490 + 95.5[Cl−]}. The model provided a high goodness-of-fit (r 2 = 0.956) between the predicted and measured values over a concentration range of more than five-orders of magnitude. This prediction equation therefore appears to have wide applicability, suggesting that Cd concentration in soil solution can be reliably predicted from measurements of Cd content, pH, CEC, EC, and [Cl−].