NONREVERSIBLE ADSORPTION OF DIVALENT METAL-IONS (MN-II, CO-II NI-II CU-II AND PB-II) ONTO GOETHITE - EFFECTS OF ACIDIFICATION, FE-II ADDITION, AND PICOLINIC-ACID ADDITION

Differences in the coordination chemistry of divalent metal ions affect their adsorption onto goethite (alpha-FeOOH) surfaces and their subsequent release by acidification, Fe-II addition, and picolinic acid addition. Equilibrium constants for adsorption onto FeOOH (goethite) decrease in the order: Cu-II > Pb-II > Ni-II approximate to Co-II > Mn-II. Equilibrium constants for picolinic acid complexation follow the Irving-Williams series; complex formation with Pb-II and Mn-II is nearly negligible. Divalent metal ion release 24 h following acidification or picolinic acid addition is typically less than the amount predicted using an equilibrium speciation model, indicating adsorption/desorption hysteresis. Experiments performed under strict exclusion of O-2 reveal a slight increase in Co-II, Ni-II, and Cu-II adsorption upon the addition of 1000 mu M Fe-II. Fe-II interferes with the ability of picolinic acid to cause divalent metal ion release.