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Our research is directed at elucidating the fundamental physical and chemical mechanisms that determine the fate of chemical compounds in natural and engineered systems. This information is necessary for accurate risk estimation, cost-effective selection of remedial options for contaminated sites, and efficient treatment of water and wastewater. The systems examined are inherently heterogeneous, typically composed of a mixture of several mineral, organic, biological, water and gaseous phases. Accordingly, much of our work emphasizes reactions occurring at phase boundaries, or interfaces. Our work is interdisciplinary in nature and combines careful experimentation, mathematical analysis and fundamental theory.
Current research topics include:
The Influence of Natural Organic Matter
Natural organic matter (NOM) governs the behavior of chemical contaminants in natural and engineered systems and I have devoted significant effort at evaluating a range of NOM-regulated processes. NOM can be roughly divided into two pools comprised of heterogeneous humic substances that resist characterization and molecules of known structure that share functionality with humic substances but owing to their known structure they are more readily evaluated. My work with NOM focusses on improving our understanding of their influence on particle behavior and contaminant fate. To this end, we have evaluated how NOM alters particle interactions because as they interact with these surfaces they change their interfacial properties. For example, our recent work with silver nanoparticles demonstrates that humic substances interact with the particle surface and that this interaction enhances particle stability and minimizes dissolution. With organic acids of known structure, we demonstrated that even small molecules (e.g., maleate) induce steric effects and that small differences in molecular orientation at interfaces results in different particle behavior. Our work evaluating the influence of NOM on contaminant binding demonstrated the underlying surface structure of the oxide dominates interactions with lead as the adsorbed NOM influence is modest and does not appear to reflect differences in NOM structure.
Colloid and Nanoparticle Behavior
The fate of nano-sized mineral, biological or organic colloids in the environment is the subject of intense scrutiny as these particles can themselves be hazardous or act as vectors for the transport of adsorbed organic and inorganic contaminants. Our recent efforts focused on evaluating the environmental fate of anthropogenic nanomaterials, namely silver nanoparticles due to their widespread use as non-specific biocides and we determined that the aggregation of these particles is mediated by their simultaneous dissolution. In recognition of the fact that particle properties change upon their introduction to the environment, we evaluated the fate of “aged” particles and demonstrated interactions with native particles, such as clay colloids, and NOM are important controls on nanoparticle fate.
Sustainable Engineering Applications
Another area of research focusses on seeking sustainable outcomes associated with the management and disposal of large-volume wastes, such as membrane concentrate, lime softening sludge, arsenic-laden adsorbent materials and coal combustion byproducts. Our recent work with arsenic-laden adsorbents demonstrates the potential for arsenic release into municipal waste landfills exceeds that predicted through regulatory leaching tests, reflecting the influence of biological and chemical processes not accounted for in these tests. We are investigating the recovery of Rare Earth Elements from acid mine drainage (AMD) using stabilized flue gas desulfurization materials and although in its preliminary stages this work demonstrates potential as an approach to mitigate the harmful environmental effects of AMD while recovering marketable products.
Algal Toxin Fate in Natural and Engineered Systems
The recent increase in the occurrence and severity of harmful algal blooms (HABs) spurred research to mitigate the formation of HABs as well as research to mitigate their impacts. Our research evaluates the role of interfacial processes on the fate of toxins produced by HABs in both natural and engineered systems. In natural systems, we demonstrated that clays, which comprise a significant portion of the mineral phase of sediments, exhibit only a modest affinity for the algal toxin microcystin-LR. As expected, this interaction is dependent upon solution conditions, with NOM exhibiting a controlling effect. In water treatment, one approach to removal algal toxins utilizes activated carbon and we have initiated research designed to prepare guidance for water treatment utilities that use granular activated carbon (GAC) or powdered activated carbon (PAC). Our work with GAC is still preliminary, but it demonstrates a controlling influence on the GAC material. With PAC, we see similar dependence on the source of the carbon, but this can be somewhat alleviated based upon the contact time and presence of natural organic matter.