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Fluoro-Aluminum Complexes on Model and Soil Exchangers

Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

P. M. Bertsch^*

Div. of Biogeochemistry, Savannah River Ecology Lab., Univ. of Georgia, Drawer E, Aiken, SC 29801

* Corresponding author.

ABSTRACT

A number of investigators have suggested that complexes may exist as discrete species on the exchange phase and, thus, be important to a complete description of exchange reactions. Little evidence establishing the presence of complexes on the exchange phase has hitherto been provided, however. Adsorption experiments in which µM F solutions were equilibrated with K-, Ca-, and Al-saturated cation-exchange resins demonstrated substantial F removal to the Al-saturated resin, with the adsorbed F quantitatively recovered with a single wash with 1.0 M NH₄Cl, whereas no F removal was observed when either K or Ca occupied the exchange sites. Removal of both Al and F from Al-F solutions equilibrated with K- and Ca-saturated resins provided additional evidence for adsorption as fluoro-Al complexes. Modeling of the competitive complexation-exchange reactions with MINTEQA2 yielded results that under-predicted net Al sorption in the presence of F, and thus also inaccurately predicted solution composition, by neglecting potential adsorption of fluoro-Al complexes. Kaolinite (KGa-2) samples also sorbed appreciable F, though both surface OH displacement and sorption to exchange-phase Al were apparently responsible for the observed decrease in F solution concentrations. In the absence of exchange-phase Al, however, all sorbed F was fixed against displacement by 1.0 M NH₄Cl. Samples of a Fuquay soil (loamy, siliceous, thermic Arenic Plinthic Paleudult) profile from South Carolina were also extracted with water, 0.01 M CaCl₂, and 1.0 M NH₄Cl; no detectable F was found in the water extracts, while considerable F was solubilized in the CaCl₂ and NH₄Cl extracts, with NH₄Cl-extractable F being highly correlated with exchangeable Al (r² = 0.98). These results suggest that part of the Al on the soil exchange phase may in fact exist as fluoro-Al complexes, and also imply a role of exchange-phase Al in regulating apparent F solubility within acid soils.

Contribution of the Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State Univ., and the Savannah River Ecology Lab., Univ. of Georgia. Research was partially supported by contract DE-AC09-76SR00819 between the Univ. of Georgia and the U.S. Dep. of Energy.

Received for publication December 10, 1989.