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 heavy-metal 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 almost-equal-to 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.