Dissertation Defenses

Doctoral Dissertation Announcement

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Candidate: Soumya Das

Degree of: Doctor of Philosophy

Department: Geosciences

Title: Adsorption of Lead on Single and Mixed Mineral Assemblages

Committee:
Dr. Carla M. Koretsky, Chair
Dr. Johnson R. Haas
Dr. Alan E. Kehew
Dr. John B. Miller

Date: Monday, August 27, 2007 1:00 p.m. – 5:00 p.m.
1122 Rood Hall

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, pure silica and pure kaolinite and on binary and ternary assemblages of HFO, silica and koalinite have been measured as a function of pH (~2-9), total metal concentration (10-4 to 10-6 M), ionic strength of the electrolyte (0.1 to 0.001M NaNO3) and mineral-to-mineral ratio. Results of the single systems showed that HFO is by far the strongest adsorbent for Pb comparing to the other two at any given pH. Pb forms strong inner-sphere complexes on HFO surfaces comparing weak outer-sphere complexes with both silica and kaolinite. Adsorption decreases with increasing ionic strength for those pure mineral phases.
Binary mixtures containing HFO and silica are in excellent agreement with experimental data sing component additivity model. Fits are less impressive whenkaolinite is present in the system. Results of the ternary systems showed that the component additivity approach could also predict very well the natural Pb adsorption. So, it can be concluded that if the single system has been studied well enough and the stability constants have been derived adequately, adsorption of Pb can be modeled and estimated for a single as well as mixed mineralogy. Though the estimation is somewhat off when kaolinite is present in the system, when studying the binary systems and ternary systems, it is clear that this study using the SCMs approach is the first step to get the hints of adsorption behavior of Pb in the natural systems. Most of the natural systems are too complex to model as it might contain numerous amounts of solid phases. So using pure well-controlled laboratory experiments may be a too simplistic approach for that purpose, but this study would be an excellent foundation for understanding the fate and transport of metal pollutants in the environment.