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Competitive Sorption of Cobalt, Copper, and Nickel Ions by a Calcium-Saturated Soil

Dep. of Natural Resources, Univ. of New Hampshire, Durham, NH 03824

* Corresponding author.

ABSTRACT

Solution ionic composition strongly affects ion sorption onto solid surfaces. This study evaluated the effect of competing ions on heavymetal sorption by soil. Investigations were conducted on Ca-saturated soil in 0.5 mmol L^–1 CaCl₂ solution. Sorption of Ni²⁺, Co²⁺, and Cu²⁺ in binary metal-Ca and ternary Ni-Co-Ca or Ni-Cu-Ca systems were evaluated. Sorption occurred in the order Cu > Ni Co, with the soil exhibiting selectivity to Cu²⁺, but not to Ni²⁺ or Co²⁺. Nickel sorption was equivalent to Ca²⁺ release, but neither Co²⁺ nor Cu²⁺ sorption was accompanied by an equivalent Ca²⁺ release. In the ternary systems, neither Co²⁺ nor 0.016 mmol L^–1 Cu²⁺ caused a significant decrease in Ni²⁺ sorption. At 0.079 mmol L^–1, Cu²⁺ did cause an observable decrease in Ni²⁺ sorption. Copper sorption was not affected by Ni²⁺, but Co²⁺ sorption decreased in direct proportion to Ni²⁺ sorption. Apparently, Ni²⁺ was retained by an exchange mechanism, but Co²⁺ and Cu²⁺ sorption was more complex. Nickel competed with Co²⁺ for sorption sites to a greater extent than did Co²⁺ for Ni²⁺ sites. Copper was selectively sorbed from the Ca²⁺ solution, while Co²⁺ and Ni²⁺ were able to compete with Ca²⁺ only in very dilute solutions. Maintaining a near-neutral soil pH may not always be the best strategy for minimizing heavy-metal mobility. Some ions appear unable to compete with Ca²⁺ for sorption sites at ionic strengths representative of near-neutral pH. Thus, while absolute solubility may be controlled, mobility in solution may be enhanced by increased ionic strength as the soil pH approaches neutrality.

Contribution no. 1720 from the New Hampshire Agric. Exp. Stn. This study was part of Northeast Regional Research Project NE-96/NE-159.

Received for publication March 27, 1991.

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