(Amino)carboxylate coordination reactions with ferric (hydr)oxides: Adsorption and ligand-assisted dissolution

This study examines 27 closely related (amino)carboxylate compounds, including EDTA, HEDTA, NTA, IDA and benzoic acid, to determine structure/activity relationships for adsorption and ligand-assisted dissolution within the pH range 3–10. The two ferric (hydr)oxide surfaces examined are goethite and ferrihydrite. Capillary electrophoresis enables a direct measurement of chelating agent speciation that was previously unavailable. In addition, the low ionic strength present in the samples increases the significance of long-range electrostatic interactions that have previously been overlooked, but are important under environmental conditions.; The protonation of Lewis Base groups was found to be critical to the adsorption of the compounds investigated. Carboxylate group protonation diminished the long-range electrostatic contributions to adsorption and the ability of the carboxylate groups to coordinate surface sites. With benzoic acid and its analogues, these effects created an adsorption maximum near the pK _a. The protonation of amine groups also decreased the long-range electrostatic contributions to adsorption, and prevented the deprotonated carboxylate groups from interacting with surface sites due to intramolecular interactions between the amine and carboxylate groups. Compounds containing amine groups also adsorbed to a lesser extent than compounds without amine groups.; As pH was varied, both the extent of adsorption and the rate of ligand-assisted goethite dissolution changed. At slightly alkaline pH values the extent of adsorption was near the detection limit for all compounds investigated. In contrast, the rate of ligand-assisted dissolution reached a maximum value at slightly alkaline pH values for strong chelating agents, such as EDTA. For weak chelating agents, the formation of metal complexes was not thermodynamically favorable at neutral and alkaline pH values, and ligand-assisted dissolution was not observed. Several types of surface complexes may form, each with a characteristic ability to remove iron from the (hydr)oxide surface. For the strong chelating agents, the surface complexes formed at low pH values were less efficient at removing iron from the surface than the surface complexes formed at slightly alkaline pH values. Despite extensive modeling efforts, the identity of the adsorbed surface complexes could not be determined based upon macroscopic measurements.