Heavy metal adsorption on iron oxide and iron oxide-coated silica : macroscopic, spectroscopic, and modeling studies
Department of Civil and Environmental Engineering
Doctor of Philosophy
Hsieh, Hsin Neng
Dyer, James A.
X-ray adsorption spectroscopy
To accurately model metal mobility and bioavailability in soils and sediments, systematic sorption studies are needed using representative and well-characterized minerals. Two important surfaces are iron oxide and silica, which are ubiquitous and associated with one another in the environment playing important roles in metal distribution. The objectives of this research are to develop models for predicting speciation and mobility of toxic trace metal ions in groundwater, soils, and sediments when competing ions are present. A model system for soils and sediments, iron oxide-coated silica, was synthesized; the degree of coatings was highly sensitive to the particle size of silica and ranged between 0.59 and 21.36 mg Fe g-1 solid. The iron oxide coatings increased surface area and introduced small pores. Surface charge distribution suggested that both silica and goethite surfaces are important for adsorption. The sub-micrometer-sized coatings exhibited a larger capacity for metals ions than discrete ones and, although in low concentration, they greatly affected metal affinity to the coated media. X-ray absorption spectroscopy (XAS) analysis revealed that both Ni(II) and Pb(II) ions form mononuclear bidentate edge-sharing surface complexes on FeO6 octahedra. This mechanism appeared to be invariant of pH, ionic strength, metal loading, and reaction time. Constrained with spectroscopic information, the 2-pK triple layer model successfully predicted Ni or Zn adsorption in single adsorbate systems. The curvature in adsorption isotherms was accurately described using two types of sites - high affinity and low affinity ones. A unique set of parameters was found for each metal ion that can successfully describe adsorption over a large range of experimental conditions, covering six or seven orders of magnitude in concentration, ionic strength from 10-3 to 10-2, and an environmentally relevant pH range. Competition was also predicted quantitatively with parameters calibrated using single adsorbate data. Results of this work will assist in better understanding and predicting the mobility and bioavailability of trace metals in soils, sediments, and groundwater.
njit-etd2006-042 (228 pages ~ 11,739 KB pdf)
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Created July 17, 2007