EFFECT OF CROPPING SYSTEMS ON ADSORPTION OF METALS BY SOILS .3. COMPETITIVE ADSORPTION

Because Cd, Cu, Ni, Pb, and Zn coexist in soil solution, competitive adsorption isotherms of these metals were generated for two soils, each selected under continuous corn from different long-term cropping systems. Competitive adsorption of the metals as a function of pH was evaluated by equilibrating soil with 200, 1,000, or 2,000-mu-M each of Cd, Cu, Ni, Pb, and Zn perchlorate in 10 mM Ca(ClO4)2 after adjusting the pH with NaOH or HClO4 solution to range from pH 4 to 8. In general, metal affinity by both soils was: Pb > Cu > Ni greater-than-or-equal-to Cd = Zn. Competition among metals for exchange sites was enhanced as the initial metal concentration was increased. The metal affinity sequence of the soils was compared with the predicted affinity sequences obtained from metal properties. Poor agreement was found between observed metal affinity sequence and metal affinity sequence predicted by ionic potential, indicating that bonding of metals by these soils was not predominantly electrostatic. The observed affinity of Pb and Cu for the soils was consistent with that based on metal hydrolysis, but the affinity sequence of Cd, Ni, and Zn could not be explained by metal hydrolysis. With the exception of Cd, affinity sequence based on the hard and soft acids and bases theory of Pearson agreed with the metal affinity sequence for these soils. Comparison of results in this work with previous findings suggested that metal adsorption by these soils could be described by: (i) Adsorption of Cu as CuOH+ and Pb as PbOH+ and/or (ii) replacement of indigenous exchangeable cations (Ca, Mg) by transition- and heavy-metal cations.