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Concentration and pH Dependence of Calcium and Zinc Adsorption by Iron Hydrous Oxide Gel¹

ABSTRACT

The specific adsorption of calcium (Ca) (pH 8.0) and zinc (Zn) (pH 5.5 and 6.5) by iron (Fe) gel in 1M NaNO₃ could be adequately described over three to five decades of concentration (c. 10^-2M to 10^-7M) by splitting the adsorption isotherm into one, two, or three regions and fitting the Freundlich isotherm to each region separately. Over the range of concentrations, 10^-4M < c < 10^-2M, Zn adsorption at pH 6.5 was about 10 times greater than at pH 5.5. Zinc adsorption at pH 5.5 was about the same as Ca adsorption at pH 8.0. No adsorption maxima were found. At trace concentrations (c < 10^-7M) where the adsorption isotherm is linear, adsorption (x/m) may be described by the equation: log (x/m) = log k₁ + n* pH + log c, where k₁ and n* are empirical constants derived from percent adsorption against pH data. Estimates of log k₁ and n* for trace adsorption of Ca, Sr, Ba, Co, Cu, Zn, Cd, Ag, Y, and Ce were derived from various sources. Naturally occurring iron oxides (and other hydrous oxides) could exert a significant control on Zn concentrations in soil solutions above about pH 6.0.

Key Words: specific adsorption • cation exchange

¹ Supported in part by the College of Agric. and Life Sci., Univ. of Wisconsin, Madison; in part by the Ecological Sciences Branch, Div. of Biomedical and Environmental Research, U.S. Energy Res. and Develop. Admin. Contract DE-AS02-76-S-02-1515-Jackson (Paper COO-1515-81); and in part by the National Science Foundation, EAR-76-19783; through an Intl. Consortium for Interinstitutional Cooperation in the Advancement of Learning (ICICAL).

² Hydrogeology Unit, Inst. of Geological Sciences, Wallingford, Oxon, England (formerly Research Assistant, UW-Madison), and the Franklin Hiram King Professor of Soil Science, Univ. of Wisconsin-Madison, respectively.

Received for publication June 23, 1980. Accepted for publication August 17, 1981.