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

摘要

Differences in the coordination chemistry of divalent metal ions affect their adsorption onto goethite (α-FeOOH) surfaces and their subsequent release by acidification, Fe" addition, and picolinic acid addition. Equilibrium constants for adsorption onto FeOOH (goethite) decrease in the order: Cu~Ⅱ > Pb~Ⅱ > Ni~Ⅱ ≈ 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 μM Fe~Ⅱ. Fe~Ⅱ interferes with the ability of picolinic acid to cause divalent metal ion release.