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

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 CaCl 2 solution. Sorption of Ni 2+ , Co 2+ , and Cu 2+ 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 Cu 2+ , but not to Ni 2+ or Co 2+ . Nickel sorption was equivalent to Ca 2+ release, but neither Co 2+ nor Cu 2+ sorption was accompanied by an equivalent Ca 2+ release. In the ternary systems, neither Co 2+ nor 0.016 mmol L −1 Cu 2+ caused a significant decrease in Ni 2+ sorption. At 0.079 mmol L −1 , Cu 2+ did cause an observable decrease in Ni 2+ sorption. Copper sorption was not affected by Ni 2+ , but Co 2+ sorption decreased in direct proportion to Ni 2+ sorption. Apparently, Ni 2+ was retained by an exchange mechanism, but Co 2+ and Cu 2+ sorption was more complex. Nickel competed with Co 2+ for sorption sites to a greater extent than did Co 2+ for Ni 2+ sites. Copper was selectively sorbed from the Ca 2+ solution, while Co 2+ and Ni 2+ were able to compete with Ca 2+ 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 Ca 2+ 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.