RP Rietra | PubMed Health

PubMed2000

Electrolyte Anion Affinity and Its Effect on Oxyanion Adsorption on Goethite.

The influence of various types of background electrolytes (NaCl, NaNO(3), and NaClO(4)) on the proton adsorption and on the adsorption of sulfate and phosphate on goethite have been studied. Below the PZC the proton adsorption on goethite decreases in the order Cl>NO(3)>ClO(4). The decreasing proton adsorption affects the adsorption of oxyanions on goethite. Anion adsorption of strongly binding polyvalent anions is lower in the studied electrolytes in the order Cl<NO(3)<ClO(4). The ion pair formation constants of monovalent electrolyte anions on the goethite surface are derived from proton adsorption data. It is shown that the derived ion pair formation constants enable the prediction of the effect of different electrolyte anions on adsorption of polyvalent anions. Especially at low oxyanion and high electrolyte concentrations the differences between the electrolytes influence the anion adsorption considerably. The effect is in principle not different for anions with a higher affinity for goethite, such as phosphate in comparison with sulfate, but the effect is only there if the particles are positively charged, which in the case of phosphate is only the case at relatively low P concentration and sufficiently low pH values. Copyright 2000 Academic Press.

Journal Article↗

PubMed1999

Sulfate Adsorption on Goethite.

Recent spectroscopic work has suggested that only one surface species of sulfate is dominant on hematite. Sulfate is therefore a very suitable anion to test and develop adsorption models for variable charge minerals. We have studied sulfate adsorption on goethite covering a large range of sulfate concentrations, surface coverages, pH values, and electrolyte concentrations. Four different techniques were used to cover the entire range of conditions. For characterization at low sulfate concentrations, below the detection limit of sulfate with ICP-AES, we used proton-sulfate titrations at constant pH. Adsorption isotherms were studied for the intermediate sulfate concentration range. Acid-base titrations in sodium sulfate and electromobility were used for high sulfate concentrations. All the data can be modeled with one adsorbed species if it is assumed that the charge of adsorbed sulfate is spatially distributed in the interface. The charge distribution of sulfate follows directly from modeling the proton-sulfate adsorption stoichiometry since this stoichiometry is independent of the intrinsic affinity constant of sulfate. The charge distribution can be related to the structure of the surface complex by use of the Pauling bond valence concept and is in accordance with the microscopic structure found by spectroscopy. The intrinsic affinity constant follows from the other measurements. Modeling of the proton-ion stoichiometry with the commonly used 2-pK models, where adsorbed ions are treated as point charges, is possible only if at least two surface species for sulfate are used. Copyright 1999 Academic Press.

Journal Article↗