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Zinc Solubility Under Alkaline Conditions in a Zinc-Bentonite System¹

ABSTRACT

The possibility of zincate formation in alkaline soils has been investigated. Utah bentonite suspensions were titrated to , 1, and 2% of their base-exchange capacities with Zn⁶⁵ in zinc chloride. The pH of the suspensions was varied by treatment with sodium, potassium, or calcium hydroxide. After a period of shaking, the suspensions were centrifuged and the supernatant liquid analyzed for Zn⁶⁵.

In both the sodium and potassium systems, zinc solubility reached a minimum in the pH range of 5.5 to 6.7. As the alkalinity of the systems was increased, the solubility of zinc also increased. This suggests the formation of soluble alkali zincates. In the calcium system, zinc solubility reached a minimum at a pH of 7.6, and no increase in soluble zinc was noted as the pH of the system was increased. The formation of insoluble calcium zincate can be postulated. Since analyses did not reveal the ionic species of zinc in solution, this study can only be regarded as circumstantial evidence in support of zincate formation in alkaline soils.

The movement of various ionic forms of zinc was observed in a column of calcareous silty calay soil. Zn⁶⁵ was applied to the surface as the zinc ion, the zinc-ammonia complex, and the zincate anion at a rate equivalent to 50 pounds zinc per acre 6 inches. The columns were then leached with 18.6 inches of water. The zincate complex moved the furthest, being found at a depth of 3 cm.; the zinc ion and the zinc-ammonia complex both moved to a depth of 2 cm. The difference in movement was attributed to the difference in sign of charge between the zinc treatments.

Contribution from the Department of Agronomy, Utah State Agricultural College. This work was supported in part by the U. S. Smelting, Mining, and Refining Co. of Salt Lake City, Utah, and in part by the U. S. Atomic Energy Commission under Contract At(11-1)80.

² Graduate Assistant and Professor of Agronomy, respectively.

Received for publication November 20, 1954.