Reactive Transport of Heavy Metals in Lake Sediments: Impacts of Multi-Component Diffusion, Diffuse Double Layer, Dissolution of Manganese Oxides, and Water Transport in Nanotubes

摘要

Reactive transport modeling is a particularly important tool for understanding and managing the complex interrelationships between the dynamic microbial community and redox-stratified aquatic and sedimentary environments. Inorganic processes including aqueous speciation, surface complexation, mineral precipitation, and dissolution were first quantified by Sengor et al. (2007) in a biogeochemical model of heavy metal cycling in Lake Coeur d'Alene (LCdA) sediments by which data were quantified and coupled to a biotic reaction network that included multiple microbial community groups using different terminal electron acceptors. The goals of this study are to use the LCdA as an example site to apply reactive transport modeling in order to elucidate (i) the impact of MCD and EDL on reactive-diffusive transport of heavy metals; and (ii) the impact of reductive dissolution of manganese oxides and surface complexation of heavy metals onto the manganese oxides on to the overall biogeochemical cycling.;Chapter 2 assesses the impact of MCD through Nernst-Planck representation of diffusion compared to classical Fickian diffusion, on the overall dynamics of heavy metals and biogeochemical processes in environments purely governed by diffusion. The results demonstrate that the use of single uniform diffusion coefficient for modeling all species in purely diffusion dominated sediments may underestimate the mobility of heavy metals undergoing complex, multi-process reactions. This outcome is further signified when explicit treatment of EDL effects is considered in addition to MCD. The simulation results also illustrate the importance of aqueous metal (bi)sulfide complexes, especially when MCD and EDL effects are implemented in reactive transport simulations, impacting the solubility and dynamics of heavy metals in diffusion dominated systems. The competitive effects of FRB and SRB activities on pH and overall biogeochemical processes show that the system is more sensitive to the changes in Fe 3+ reduction compared to sulfate reduction. The impact of EDL implementation (in addition to MCD) using a wide range of microporosity spectrum is also assessed on the overall ion transport dynamics in the system, demonstrating the significance of accurate determination of EDL layer, based on the ionic strength of the solution.;Also building on the previous model developed by Sengor et al. (2007), Chapter 3 presents the implementation of microbial reductive dissolution of MnO2. Surface complexation of heavy metals (Pb, Zn, Cu) onto manganese oxides (HMO sites) is also considered. The model effectively simulates the mobilization of initially sorbed heavy metals (Pb, Zn, Cu) onto both manganese oxides and hydrous ferric oxides through the reductive dissolution of MnO 2 and ferrihydrite, respectively. The results suggest that the relative rate of Mn reduction (in addition to Fe3+ and sulfate reduction) plays an important role in controlling pH and mobility of heavy metals sorbed onto HMO and HFO phases. Comparative simulations of biotic and abiotic dissolution of MnO2 showed that the reductive dissolution of MnO2 in the LCdA sediments was possibly microbially mediated, rather than abiotically controlled. The biotic and abiotic reaction networks were both extended to investigate the impact of Fe and Mn adsorption onto Fe(III) (hydr)oxides (HFO sites) and MnO2 (HMO sites) on the overall dynamics. A visual comparison of the model predictions with the available field data showed that the incorporation of especially Fe sorption onto HFO sites resulted in differences between simulated and measured porewater chemistry for LCdA sediments. This could be due to the uncertainties in the concentration of surface sites available for adsorption, the presence of impurities substituted within the mineral structure affecting the reactivity, the effects of competition among dissolved species for sorption, potential pitfalls on the available sorption constant datasets, limitations on the direct applicability of sorption constants derived in the laboratory environment to the field, or reactions that are not included in the model.;Chapter 4 investigates transport in nanoporous media. For this purpose, pressure-driven transport is studied through a nano-scale media through bench-scale flow experiments. Advective transport driven by Darcy flow is the predominant mechanism of fluid transport, which is based on the continuum representation of the porous media. However, in nano-scale porous formations, such as those observed in shale reservoirs, transport of fluids exhibit substantially different physics than what is observed in larger-scale systems. In nano-scale confinements, atomic interactions between liquid and wall molecules may lead to velocity slip, no-slip or liquid adsorption on the walls (Karniadakis, et al., 2005). In this study, flow rate (Q) over a range of constant pressure conditions is measured and compared to the flow rate predictions based on the Hagen--Poiseuille equation, which corresponds to the flow rate determination without the influence of the boundary velocity slip or liquid adsorption layer effects (QNS ). (Abstract shortened by ProQuest.).