Fate of metals in presence of minerals and mineral-organic assemblages

Metals can enter aquatic systems from natural and anthropogenic processes associated with weathering, sediment re-suspension, industrial activities, and atmospheric deposition. Metals pose health and environmental risks at high concentrations due to their potential toxicity and bioaccumulation, but many trace metals also serve as essential micronutrients for biogeochemical processes in natural aquatic systems. Biogeochemical processes such as methanogenesis, denitrification, and mercury methylation require transition metals such as nickel (Ni), cobalt (Co), copper (Cu), and molybdenum (Mo) for completion. These biogeochemical processes can be substantial contributors of greenhouse gases, such as methane (CH4) and nitrous oxide (N2O), into the atmosphere. The behavior, mobility, and bioavailability of metals in water systems are controlled by their interactions with mineral phases and mineral-organic assemblages. Understanding the reactivity of metals with minerals and organic moieties will not only help in developing effective removal techniques but will also aid in developing robust fate and transport models to predict metal mobility in environmental systems. The knowledge of how metals reactivity affects their bioavailability in environmental systems can be important in improving the accuracy of ecosystem models to estimate greenhouse gas emissions from natural landscapes. The dissertation pursued four main objectives to understand dominant reaction mechanisms controlling metals mobility in natural and engineered systems: (i) to optimize the reaction conditions for removing uranium from water systems by using synthesized biosurfactant-coated iron oxide nanoparticles, (ii) to elucidate the factors governing the fate of trace metals added in dissolved form to soils and sediments collected from three different natural aquatic systems, (iii) to determine the role of available Cu in a biogeochemical process in natural aquatic systems, and (iv) to understand the mobility behavior of trace metals from wetland soils and stream sediments upon fluctuating redox conditions.Based on tunable properties, engineered nanoparticles hold significant promise for water treatment technologies. Motivated by concerns regarding toxicity and non-biodegradability of some nanoparticles, we explored engineered magnetite (Fe3O4) nanoparticles with a biocompatible coating. These were prepared with a coating of rhamnolipid, a biosurfactant primarily obtained from Pseudomonas aeruginosa. By optimizing synthesis and phase transfer conditions, particles were observed to be monodispersed and stable in water under environmentally relevant pH and ionic strength values. The rhamnolipid-coated iron oxide nanoparticles (IONPs) showed high sorption capacities for U(VI) removal under different pH and dissolved inorganic carbon concentrations. Equilibrium sorption behavior was interpreted using surface complexation modeling (SCM). Two models (diffuse double layer and non-electrostatic) were evaluated for their ability to account for U(VI) binding to the carboxyl groups of the rhamnolipid coating as a function of the pH, total U(VI) loading, and dissolved inorganic carbon concentration. The diffuse double layer model provided the best simulation of the adsorption data and was sensitive to U(VI) loadings as it accounted for the change in the surface charge associated with U(VI) adsorption. Natural aquatic systems can act as a sink for trace metals through adsorption, precipitation, and complexation. We conducted batch experiments under anoxic conditions on soils and sediments collected from three different natural aquatic systems to understand their response to influxes of dissolved Cu, Ni and Zn. X-ray absorption spectroscopy indicated that the speciation of the freshly added metals taken up by the solids differs substantially from the speciation of the metals originally present in unamended samples. Cu speciation was dominated by sulfides at low loadings (1 µmol/g), whereas complexation to thiol groups and formation of metallic Cu governed speciation at high loadings (10 µmol/g). For Ni and Zn, adsorption to mineral surfaces and organic matter governed their speciation in materials from most sites. Our findings imply that geochemical processes controlling trace metal speciation may vary considerably with metal loading in different natural systems. Laboratory studies of pure cultures have highlighted that the availability of Cu, required for the multicopper enzyme nitrous oxide reductase, can limit nitrous oxide (N2O) reduction during denitrification. However, in natural aquatic systems, the role of Cu in controlling denitrification had not been well understood. Our study indicated that natural systems with background Cu concentrations below or around geological levels (40 - 280 nmol g-1) may lack sufficient bioavailable Cu to carry out the conversion of N2O to nitrogen (N2). By providing Cu at dissolved concentrations of 10-300 nM, the conversion of N2O to N2 can be enhanced substantially. Our results indicated that including Cu bioavailability in ecosystem models could improve the accuracy of estimates of N2O emissions from natural landscapes. Wetland soils and hyporheic zones of stream beds undergo fluctuating redox conditions due to microbial activity, varying water saturation levels, and nutrient dynamics. With fluctuating redox conditions, trace metals can be mobilized or sequestered in response to changes in iron and sulfur speciation and the concentrations and lability of organic carbon. We conducted systematic studies to examine the effect of redox fluctuations on samples collected from a riparian wetland and a stream. Water-saturated soils and sediments were incubated under three cycles of anoxic-oxic conditions (τanoxic:τoxic = 3) spanning 24 days to observe the change in dissolved and bioavailable metal concentrations. We observed that the trace metal dynamics in microcosms with materials from natural environments under events of redox fluctuations is strongly coupled to solid-phase speciation of the trace metals and the redox status of the recent past. This study illustrated that different trace metals display distinct bioavailability patterns during redox fluctuations in soils and sediments. The information gained from the research projects improved our understanding of metal interactions with engineered nanoparticles and soils and sediments from natural aquatic subsurface systems. We believe that with effective optimization methods, biodegradable engineered materials can be successfully implemented in water treatment systems for the removal of potent contaminants using environmentally benign materials. The insights from the studies broadened our knowledge of the factors controlling trace metal speciation and bioavailability in natural systems. High association of trace metals to iron oxides, sulfide minerals, and dissolved organic matter decreased the bioavailability of trace metals in wetland soils and hyporheic zones of streams for biogeochemical processes