Predicting thermodynamic and transport parameters for metal contaminant sorption to hydrated metal oxides in aquatic systems
Department of Civil and Environmental Engineering
Doctor of Philosophy
Hsieh, Hsin Neng
Meegoda, Jay N.
Sirkar, Kamalesh K.
Due Of Metal Contaminants
Hydrated oxides such as HAO,'HFO, goethite, and BA40 are prevalent in soils and sediments as discrete particles or as coatings. These microporous oxides have large surface areas and high affmity for metal ions, and hence they act as both a sink and a source for anthropogenically released metal contaminants. To better understand risks posed by metals in the environment and to develop effective waste management programs, mechanistic models are needed to accurately predict their fate in soils and sediments.
To achieve this objective, sorption of metal ions Sr, Cd, Zn, Ni, and Ca to these oxides were studied with macroscopic as well as spectroscopic experiments, as a function of pH, ionic strength, concentration, temperature, and reaction time. Macroscopic studies in combination with the XAS investigations suggest that the sorption of divalent metal ions to amorphous oxides is a two-step process: rapid adsorption to the external surface followed by slow intraparticle diffusion along the micropore walls. Adsorption is an endothermic physical reaction that can be represented by one average mechanism or site independent of pH and adsorbate concentration. Accordingly, the sorbed ions retain their primary hydration shell and form an outer sphere complex. Hence, adsorption enthalpies (ΔH°) can be predicted from their primary hydration number (N) and the hydrated radius (RH). The site capacities of these oxides are a function of pH and can be estimated from their surface charge densities.
On the other hand, metal ions form mononuclear inner sphere complexes with goethite. Although goethite may show a higher affiiiity for metal ions than HFO, its site capacity is much smaller than that of BFO. Macroscopic analyses disclosed two sets of adsorption sites on the goethite surface: a small set of high affinity sites available to transition metal ions and a large set of low affmity sites to which only alkaline earth metals bind. This limited availability of high affinity sites induces competitive adsorption between Ni and Zn, which can be described with the single-site Langmuir model.
XAS investigations of intraparticle diffusion studies revealed that the local structure of metal ions sorbed to amorphous oxides do not change with time suggesting that the internal sites are similar to the external ones. Modeling resulted in diffusivities ranging from 10-16 to 10-10 cm2 s-1. Therefore, surface diffusion of metal ions along the micropore walls may take from a few days to few years to reach equilibrium. Based on site activation theory, surface diffusivities can be estimated knowing the activation energy and site capacity. From Polanyi relation, EA is linearly proportional to ΔH°, and is comparable for adsorption of a specific metal to HAO, BFO, and HMO. Furthermore, because metals of the same group in the Periodic Table form similar sorption complexes, the Polanyi constant (α) was equivalent. Strong similarities in the local structure of Ni and Zn ions sorbed to HMO corroborate this hypothesis. Interestingly, surface diffusion was not important with goethite.
Overall this research renders an insight into the mechanisms by which hydrated metal oxides control the partitioning and bioavailability of metal contaminants. This study provides methods that can accurately predict important transport and thermodynamic parameters for describing the fate of these pollutants.
njit-etd2001-087 (339 pages ~ 15,225 KB pdf)
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Created June 26, 2002