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Activities of Cu²⁺ and Cd²⁺ in Soil Solutions as Affected by pH¹

ABSTRACT

The concentrations of free copper and cadmium ions in soil suspensions were measured by an ion-selective electrode as a function of pH after equilibrating metal salt solutions with the soils. Similar experiments were done with water-extractable organic matter. Free (uncomplexed) Cu²⁺ concentrations were strongly pH-dependent in the soil systems, but well below the level where precipitation could have occurred. Free Cd²⁺ concentrations were much less affected by pH adjustment, and the nonacid soil systems approached saturation or oversaturation with respect to CdCO₃ precipitation at high pH. Similiar dependence of the free metal ion concentration on pH was found in water extracted organic matter solutions. Precipitation of CdCO₃ in oversaturated solutions containing the extracted organic matter was slow, while none of the copper-organic matter solutions attained oversaturation with respect to Cu(OH)₂ or Cu₂(OH)₂CO₃.

¹ Contribution of the Dep. of Agronomy, Cornell University, Ithaca, NY 14853. Agronomy Paper no. 1329. Funding was provided by the Regional Research Project NE-96.

² Graduate Research Assistant and Assistant Professor of Soil Science, respectively.

Received for publication December 14, 1979. Accepted for publication March 6, 1980.