Estimating Divalent Cadmium Activities Measured in Arid-Zone Soils using Competitive Chelation
1990
Workman, S. M. | Lindsay, W. L.
Competitive chelation was explored as a method of measuring divalent Cd activity in neutral to alkaline soils. The most successful combination of chelating agents and competing metals comprised dtpa (diethylenetriaminepentaacetic acid) and Pb²⁺ in equilibrium with added PbCO₃(c). A series of 10⁻⁵ M dtpa solutions containing varying initial mole fractions of Cd and Pb were reacted with soils for 140 h. Some solutions lost Cd and gained Pb, while others gained Cd and lost Pb. From these results it was possible to estimate the composition of a unique solution that does not change upon reaction with a given soil. The composition of this unique solution reflects equilibrium conditions and was used to calculate Cd²⁺ activity. Eight soils ranging in pH from 7.15 to 8.25 showed Cd²⁺ activities ranging from 10⁻⁶.⁶ to 10⁻⁹.⁴ M. The Cd²⁺ activities measured in these soils could be predicted using the following expression: (Cd²⁺) = 10⁷.⁶¹ (H⁺)² This solubility of Cd²⁺ in soils is approximately two orders of magnitude lower than the activity of Cd²⁺ in equilibrium with CdCO₃(c) and atmospheric CO₂. No known Cd mineral has yet been identified whose solubility reflects this low activity of Cd²⁺. The competitive chelation method used here avoids many of the pitfalls encountered in measuring extremely low levels of Cd in neutral and alkaline soils where Cd-organic complexes may be present. Contribution from Colorado Agric. Exp. Stn., with financial support from Binational Agric. Res. and Development Fund no. US-1145-86.
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