Competition of copper, lead, and cadmium adsorption to goethite

Competition of copper, lead, and cadmium adsorption on goethite depends on metal ion and oxide surface characteristics. Goethite was characterized via X-ray diffraction, potentiometric titrations, site density determination, particle size analysis, and optical microscopy. In this thesis, a hypothesis was tested that electronegativity is the most important factor in metal affinities and adsorption competition to goethite. Metal affinity and site density for the goethite surface were assessed by performing adsorption edges and isotherms. Based on the results of these experiments, competition studies were designed to observe adsorption and displacement of the competing metals by limiting the number of available sites on the oxide surface. Adsorption competition was studied by conducting competition isotherms in two-metal and three-metal systems, where the temperature and pH were maintained constant. Competing metals of equivalent, below saturation, concentrations were introduced in solution in alternative orders as well as simultaneously. In adsorption edges, ionic strength was varied and no changes were observed in the amount of metal adsorbed, suggesting that copper, lead, and cadmium were specifically adsorbed on goethite. Metal affinity and capacity for the goethite surface were found to increase with metal electronegativity:Cu\u3ePb\u3eCd. Neutrally charged hydrolysis products of metal ions were found to slightly decrease metal affinities for goethite. Goethite posses distinct types of sites with varying affinities for adsorbates. In addition to the sites used by heavy metals (Cu, Pb, and Cd), strontium, apparently binds to another group of, possibly lower energy sites. Modeling of copper, lead, and cadmium adsorption and competition was best described with a two-site Langmuir isotherm. From the equilibrium constants obtained from the two-site Langmuir model, more electronegative species were preferably adsorbed over less electronegative species on the high affinity sites of goethite. However, lead showed higher affinity than copper for the low energy sites. In Cu and Cd competition studies, cadmium was completely displaced. In the Pb and Cd studies, lead displaced cadmium from the sites it required. Because sites were not limited in this experiment, unoccupied ones were then filled with cadmium. During Cu and Pb as well as Pb, Cu, and Cd competition studies, copper preferably adsorbed over lead to the high energy sites, whereas lead showed greater affinity for the lower energy sites. During Pb, Cu and Cd competition studies, cadmium was completely desorbed. Furthermore, during the same competition study, the hydrolysis product Cu(OH)2(aq) decreased copper adsorption on goethite where only 30% of this species was adsorbed. The two-site Langmuir model worked well with Cu and Cd competition and Pb and Cd competition, but it was not able to predict the Cu and Pb and the Cu, Pb, and Cd systems\u27 results. The findings from this thesis suggest that electronegativity is an important factor in adsorption competition of pollutants on high affinity sites on the goethite surface. However, lead showed higher affinity than copper for the low energy sites. This study confirms that adsorption competition plays a crucial role in contaminant mobility in the environment