Date of Award

12-2007

Degree Name

Doctor of Philosophy

Department

Geosciences

First Advisor

Dr. Carla M. Koretsky

Abstract

Metal oxy-hydroxides and phyllosilicate minerals play a significant role in the fate and transport of heavy metals in the environment (Bertsch and Seaman, 1999). Chemical speciation of metals affects their bioavailability and chemical reactivity (Stumm and Morgan, 1996). Surface complexation models (SCMs) based on equilibrium thermodynamic principles have been successfully used to quantify adsorption of heavy metals on pure solid minerals, including phyllosilicates and oxy-hydroxides. For natural sediments with mixed mineralogy, Davis et al. (1998) suggested two different SCM approaches, namely, the component additivity and generalized composite models. In this study, adsorption of lead on pure HFO, silica and kaolinite and on binary and ternary assemblages of HFO, silica and kaolinite have been measured as a function of pH (∼2-9), total metal concentration (10-4 to 10-6 M Pb), ionic strength of the electrolyte (0.1 to 0.001 M NaNO3) and solid-to-solid ratio. Results of the single solid systems show that HFO is by far the strongest adsorbent for Pb of the three solids at any given pH. Adsorption of Pb is not affected by ionic strength for HFO, but adsorption decreases strongly with increasing ionic strength for silica and kaolinite systems. This suggests that Pb forms strong inner-sphere complexes on HFO surfaces in contrast to weaker outer-sphere complexes with both silica and kaolinite. Data for the single solid systems are used to derive best fit DLM thermodynamic stability constants for Pb adsorption and are used to predict adsorption edges for binary and ternary solid mixtures at varying sorbent ratios. Predictions made using the thermodynamic data with a simple component additivity approach are in excellent agreement with experimental data for binary mixtures containing HFO and silica. Fits are poorer when kaolinite is a component in the system. Results for two ternary systems show that the component additivity approach can predict Pb adsorption very. Therefore, this study suggests that if a single solid system has been studied well enough to adequately derive stability constants for a robust surface complexation model, then predictions based on the single systems can provide reasonable estimates of adsorption behavior for systems with mixed mineralogy.

Access Setting

Dissertation-Open Access

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