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Effects of Pyrophosphate, EDTA, and DTPA on Zinc Sorption by Montmorillonite¹

ABSTRACT

The effects of pyrophosphate (PP), pH, ionic strength, EDTA, and DTPA on zinc (Zn) sorption by montmorillonite were studied in order to evaluate the role of pyrophosphate in Zn solubilization. In such a system PP dissolution of organic matter and its subsequent complexation of Zn was limited and would not affect the interpretation of the results. The effectiveness of PP in complexing Zn was dependent on the levels of Ca, initial PP concentration, and pH of the montmorillonite suspension. Decreased sorption of PP with increasing pH, as well as markedly greater sorption of PP by Ca-montmorillonite than by K-montmorillonite, suggests the existence of a sorption process which could include surface adsorption and precipitation.

At initial PP concentrations of 1.92 mM, Zn solubilization decreases monotonically with increasing pH, while at 9.6 mM initial PP, a minimum in Zn solubilization is observed at about pH 6.0. This trend of solubilization could be attributed to decreased Zn sorption and increased Zn complexing with PP. At pH greater than about 7.5, PP is comparable to EDTA and DTPA in Zn solubilization, but only at PP molar concentrations some 40 times greater than the organic ligands. Equilibration time (2 to 192 hours) had little effect on PP concentration or on Zn concentration. Regardless of which of the ligands was used, the sorption of Zn reached completion in most cases within 2 hours. For EDTA and DTPA, provided that their concentration was greater than Zn concentration, sorption of Zn was found to be a relatively slow process. Approximately 40% (EDTA) and 20% (DTPA) of the initial Zn was sorbed after 192 hours.

¹ Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 119-E, 1981 series.

² Scientists, Division of Soil Chemistry and Plant Nutrition, Institute of Soils and Water, ARO, The Volcani Center, Bet Dagan, Israel.

Received for publication March 4, 1981. Accepted for publication September 29, 1981.