Date of Award

1989

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Michael C. Amacher

Abstract

A nonlinear multireaction model was used to describe kinetic retention data sets for chromate, phosphate, arsenate, borate, fluoride, molybdate, selenite, and silicate by goethite, an iron oxide mineral, and selected soils. These data sets were obtained from laboratory batch experiments with time up to 24 h. Either a three-parameter version of the model consisting of one nonlinear reversible and one first order irreversible reaction, or a five-parameter version consisting of two nonlinear reversible and one first order irreversible reactions was capable of predicting anion sorption by goethite. A mechanistic model consistent with the nonlinear multireaction approach was also proposed in order to account for the adsorption of inorganic oxyanions onto oxide surfaces. The retention of chromate by goethite as a function of pH showed equilibrium constants to decrease with increasing pH. This was supported by the rate equations derived from the mechanistic model which predicted the pseudo rate coefficients for ligand exchange to be pH dependent. Two equilibrium type models, namely the Freundlich and two-site Langmuir were found to adequately describe the sorption (after 24 h of reaction) of chromate, phosphate, arsenate, selenite, and silicate by goethite. Both the Freundlich model and the multireaction model were found to be consistent in establishing an affinity sequence for anion adsorption by goethite. The data suggested the following affinity sequence: arsenate $>$ selenite $>$ phosphate $>$ molybdate $>$ chromate $>$ fluoride $>$ borate $>$ silicate. The retention of chromate by six soils having different soil chemical properties were well described by either the three- or five-parameter versions of the multireaction model. Soils with high iron oxide contents and low pH were capable of retaining chromate to a greater degree than soils low in iron oxides and high pH. It was also found that significant amounts of chromate were incapable of being adsorbed by amorphous aluminum oxide and humic acid. The data sets generated for chromate and phosphate retention by goethite and various soils, as well as the models used for the prediction of their behavior, are a prerequisite for the quantification of their amounts remaining in the soil solution and thus susceptible to transport to the groundwater.

Pages

172

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