Uranium and lead are two of the most important contaminants of concern in natural waters in Earth’s critical zone. In order to create detailed experimental and theoretical models of molecular interactions between these cations and sediment-water interfaces, it is crucial to understand important modalities and key characteristics of complex natural solid-solution interfaces. Historically, model systems have employed well-defined iron or aluminum oxide minerals, in the presence or absence of simplified organic molecules or humic or fulvic acids. Yet, reactive solid surfaces in aquifers and reservoirs may be decorated with other types of organic matter such as microbial biomass, or may be non-mineral particulate organic matter, may exhibit nano-scale heterogeneity with inorganic and organic domains, or may be modified through reaction with water and solutes. It is helpful to consider studies of natural systems to gain a better understanding of aqueous-metal ion interactions at these complex natural surfaces and to better represent this complexity them in model systems. In this talk, we will address the question: <i>What types and compositions of solid substrates (inorganic vs organic and functional groups present) control U(IV) and Pb(II) behavior in natural sediments and groundwater? </i>Recent studies of redox-variable riparian soils have demonstrated a greater and more complex role for organics in governing U(IV) and Pb(II) sorption at solid surfaces than previously appreciated. For example, in spite of the low solubility of U(IV) mineral phases, U(IV) surface complexes on particulate organic matter and clay-organic assemblages dominate uranium inventories in contaminated anoxic sediments and influence groundwater quality regionally across the upper Colorado River Basin. Similarly, Pb(II) is widely considered to partition strongly to iron/manganese oxides, sulfides, clays, and dissolved organic matter. Yet, recent studies show that solid-associated organic matter plays a major role in controlling Pb(II) behavior in riparian sediments under variable redox conditions. These studies broadly emphasize the need to better understand molecular processes occurring at complex organic-dominated solid-water interfaces.

Importance of organic matter as a control over U(IV)- and Pb(II)-surface interactions in natural sediments

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.