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Anion Exchange Chemistry of Middle Atlantic Soils: Charge Properties and Nitrate Retention Kinetics

Delaware Agric. Exp. Stn., Dep. of Plant Science, College of Agric. Sci., Univ. of Delaware, Newark, DE 19717-1303

T. H. Carski

E.I. DuPont de Nemours Corp., Stine-Haskell Laboratories, Newark, DE

*Corresponding author.

ABSTRACT

The negative impact of nitrate (NO₃) on groundwater supplies has sparked a great deal of interest and concern in recent years, particularly in areas where coarse-textured soils abound. In light of this concern, the anion exchange chemistry of eight soils from the Middle Atlantic region was studied with particular emphasis on NO₃ retention and kinetics. The soils were chosen to encompass a range of physicochemical and mineralogical properties and were extensively characterized. Anion exchange capacity (AEC) was determined on Cl-saturated samples by desorption of Cl with SO₄. Anion exchange capacity ranged from 0 to 1.35 cmol_c kg^–1 for the eight soils and was found to parallel increases in clay and Fe oxide contents in the soil profiles. Point of zero salt effect (PZSE) values were determined by potentiometric titration with 0.001, 0.01, and 0.1 M NaCl as the indifferent electrolyte. These were of little value in predicting the development of AEC for the soils. The kinetics of NO₃ adsorption and desorption were studied using a stirred-flow reaction chamber and a first-order reaction best described the data. Nitrate adsorption was found to be completely reversible, indicating a simple electrostatic retention mechanism. The effect of pH and NO₃ concentration on cumulative NO₃ adsorbed (CNA) and on NO₃ adsorption kinetics was also studied. The CNA was found to increase with a decrease in pH below 5.5 and to increase with increasing NO₃ concentration. The latter indicated an increase in competitiveness by NO₃ for positively charged sites.

Published as Miscellaneous Paper no. 1275 of the Delaware Agric. Exp. Stn. Contribution no. 247 of the Dep. of Plant Science, Univ. of Delaware.

Received for publication August 12, 1988.

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