In situ voltammetric determinations of metal ions in goethite suspensions: Single metal ion systems

In the present paper voltammetric techniques have been used to study the sorption of Pb(II), Cd(II), Zn(II), and Cu(II) at low concentrations (approximate to 10(-6) M) in goethite (alpha-FeOOH) suspensions with varying solid concentrations as a function of pH. Metal ion adsorption on the cell walls was minimized by using a Teflon cell. Measurements in the absence of goethite showed the applicability of the voltammetric techniques for trace analysis in a wide pH range. The analysis seems to be limited mainly by the onset of metal ion hydrolysis at high pH values, where neutral and negatively charged species are formed. The results obtained in the presence of goethite were compared with previously determined surface complexation models, evaluated from relatively high metal ion (approximate to 10(-3) M) and solid concentrations (approximate to 10 g/L). Application of these models to more dilute conditions could thus be questioned. The results obtained indicated that no "high-affinity" sites are present on the surface of alpha-FeOOH. The voltammetric measurements showed good agreement for the Zn(II) and Cu(II) goethite systems. In the case of Pb(II), deviations between calculated and experimentally determined metal ion concentrations were detected at pH >7 and a molar ratio of Pb(II) to the surface concentration of goethite of 0.17. These deviations may be explained by assuming either formation of polymeric species or surface precipitates at high total concentrations (instead of the previously suggested surface complex =FeOPbOH) or slow adsorption kinetics at low concentrations. Also in the Cd(II) goethite system, a corresponding weaker adsorption than predicted was found. These observations strongly emphasize the importance of evaluating surface complexation models from data within concentration ranges as wide as possible. This holds for all components including the solid concentration. (C) 1997 Academic Press.