Pb and Cd chemisorption by acid mineral soils with variable Mn and organic matter contents

Abstract

Adsorption of Pb2+ and Cd2+ on acidic A and B horizon mineral soils of New York (MP) and Vermont (BH) were determined in 0.2 M Na acetate buffered at pH 5.5 to determine strength and capacity of the soils for chemisorption of these trace metal ions. Adsorption isotherms over a very wide concentration range were better modeled using the linearized Freundlich compared to the linearized Langmuir equation. Adsorption of Pb was much stronger than that of Cd for all soils, and A horizon soils adsorbed Pb and Cd more strongly than their B horizon counterparts. The logarithms of the Freundlich binding coefficients (log KF) were greater by about 1.0 unit for Pb than for Cd for all but the B-horizon of the MP soil, which was the least strongly adsorptive of all soils tested. Pb binding constants were significantly correlated to hydroxylamine- and NH4 oxalate-extractable Fe and Mn in addition to soil CEC, but the Cd binding constants were correlated significantly to extractable Fe only. Partition coefficients (KP) were calculated for Pb and Cd from the adsorption experiments, and for the lowest additions of Pb to the A-horizon soils, ranged from more than 4000 L kg−1 in the BH soils to about 300 in the more acidic MP soils. Cd partition coefficients were much lower than those of Pb in all soils, and the MP soils retained Cd particularly weakly, with a KP in the range of 3–20 for the A-horizon and even lower for the B-horizon. Pb and Cd were adsorbed more strongly on A-horizon than B-horizon soils, indicating the importance of organic matter in both Pb and Cd retention by chemisorption at low pH. For both Pb and Cd, partition coefficients diminished markedly with increasing metal addition levels.Adsorption of Pb and Cd ions caused release of alkaline earth metals and Mn into solution during the adsorption process on the high-Mn BH soils, whereas the more strongly acidic MP soils released Al. Decreasing solubility of P and S corresponding to increasing Pb adsorption on the soils suggested that, in addition to chemisorption, precipitation of Pb phosphate and Pb sulfate may have been involved in Pb removal from solution.