Date of Award

Fall 1980

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

Abstract

Land treatment of wastewater involves the use of plants and the soil to remove unwanted constituents. Removal of phosphorus is particularly important to avoid excessive biological activity in water systems receiving the treated wastewater. This research examines the phosphate adsorption - desorption behavior of two soils: Charlton silt loam, a typical acid soil from New England which is being used in experimental wastewater treatment, and Tujunga coarse sandy loam from a wastewater land treatment facility located at Manteca, California, which has failed to remove phosphate from wastewater efficiently.

The effects of changing the pH and phosphate content of municipal wastewater on adsorption and desorption were determined. Sorption isotherms were determined for each soil using wastewater as the equilibration solution. Phosphate was added to the wastewater to obtain the concentration range necessary for the isotherms. After 96 hour equilibration periods, radioactive phosphorus was added to the suspensions. Measurements of the redistribution of the P-32 were used to determine the exchangeability of the sorbed phosphate.

Radioactive phosphorus-32 was used to follow the adsorption and desorption rates in suspensions of soil and wastewater. Parallel experiments were conducted in which phosphate concentrations were determined by conventional spectrophotometric analysis. Effluent from a conventional secondary treatment facility and the whole soil (all particles < 2 mm) were used in these studies to model the normal situation in land treatment facilities. Concentration changes of solution phosphate or P-32 were measured over 46 hour equilibration periods. After this period, soil samples were separated from the suspension, treated to remove interstitial solution, and dried at room temperature.

The dried soil samples were resuspended with either a fresh portion of effluent or 0.005 M NaCl solution. Resuspension with effluent mimics a repeat application of wastewater to a treatment field. Suspension in sodium chloride solution allows determination of phosphate desorption in the absence of exchange reactions.

The complementary use of P-32 and non-radioactive phosphate permitted the evaluation of adsorption and desorption of freshly sorbed and native phosphate independently as well as the amount of exchange occurring.

Charlton soil sorbed large amounts of phosphate with a capacity of over 1000 mg P/g soil. At concentrations typical of wastewater, complete removal of phosphate from solution was rapid. When small amounts (< 100 mg/g) were sorbed, phosphate was bound in very stable forms which were largely unavailable to exchange. Sorption was slightly less complete in suspensions at pH 8.0 as compared to pH 5.0 and 6.5. At higher loadings of phosphate, an increasing amount of exchangeable phosphate was sorbed on the Charlton soil. However, at a single pH, the proportion of exchangeable phosphate to sorbed phosphate was constant at all amounts of sorbed P studied.

Tujunga soil capacity for phosphate was much lower than Charlton soil. Sorbed phosphate was susceptible to a large amount to exchange as well as net desorption. Efficiency of sorption decreased with a decrease in pH--opposite to the trend observed with the Charlton soil. A three step mechanism of P sorption was used to explain the results of these studies.

Determination of relative concentrations of Fe (II) and Fe (III) by Mossbauer spectroscopy was examined. Mossbauer spectra were obtained which demonstrate the successful application of the technique to the study of reduced iron in soils.

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