Arsenic Dissolution from Sedimentary Formations under Geologic Carbon Dioxide Storage Conditions
Creators: name: Hariprasad  Parthasarathy
Publisher: name: Figshare
Year: 2018
Subjects: subject: 90799 Environmental Engineering not elsewhere classified
subject: FOS: Environmental engineering
subject: 90599 Civil Engineering not elsewhere classified
subject: FOS: Civil engineering

description: The overall goal of this Ph.D. study was to investigate the mobilization of arsenic (As) from sedimentary formations under conditions representative of geologic carbon dioxide storage (GCS) i.e., high pressure, temperature, and salinity. GCS is a promising technology for the mitigation of increasing CO2 emissions in the atmosphere. It primarily involves the capture of CO2 from point sources, followed by transport and injection into deep subsurface formations for long-term storage. Of the potential subsurface formations under consideration in the United States, saline formations, characterized by the presence of high salinity brines, are estimated to have the largest storage capacity. Potential for leakage of injected CO2, native brines, and CO2- saturated brines from these reservoirs exists and may lead to an increase in mineral dissolution from reservoir formations, and leakage pathways. Of particular interest in the risk assessment of GCS is the dissolution and mobilization of toxic metals such as arsenic (As) and lead. The primary mineral source of As in high and low permeability sedimentary formations is arsenopyrite (FeAsS (s)). While the oxidative dissolution of FeAsS (s) has been reported in the literature, the dissolution of FeAsS (s) under anoxic, high salinity conditions of GCS remains unexplored. To conduct dissolution experiments at high pressure, temperature, and salinity, a small-scale plug-flow system capable of measuring dissolution rates without mass transfer limitations was designed and constructed. The capacity of the system in measuring dissolution rates under GCS conditions was validated. The plug-flow system is capable of accurate and rapid measurement of dissolution rates for minerals with slow and moderate dissolution rates, with a maximum rate limitation of 5 x10-5 mol/m2s at a flow rate of 10 ml/min. To enable accurate determination of reaction rates, a method for preparation of uniformly sized arsenopyrite particles free of surface oxides was developed. The method involves sonication of crushed minerals with ethanol, washing with 12N HCl, and 50% ethanol, followed by drying in N2. Analysis of the arsenopyrite surface with X-ray photoelectron spectroscopy revelealed that the method was successful in removing all the oxides of As and S on the surface, while only 12% of Fe was left oxidized. Subsequently, the dissolution of arsenopyrite, galena, and pyrite in low-concentration alkali and alkaline metal chloride solutions under anoxic conditions was investigated. Further, the effect of Na-Ca-Cl brines on the release of arsenic was determined under ambient as well as GCS conditions. The result of these experiments revealed that electrolytes traditionally considered inert, such as NaCl, CaCl2, and MgCl2 are capable of effecting sulfide mineral dissolution. In particular, the dissolution of As increased with increasing cation activity, and the dissolution of sulfur decreased with an increase in chloride ion activity in solution. Dissolution experiments with 1.5M Na-Ca-Cl brines resulted in arsenic dissolution rates in the range of 10-10 to 10-11 mol/m2 s under anoxic conditions. The rate of As release was found to be dependent on the CaCl2 content of these Na-Ca-Cl brines. Upon the introduction of CO2 into the system, the dissolution rate of As decreased and was determined to be in the range of 10-11 to 10-12 mol/m2s. For comparison, the rate of As release from arsenopyrite under oxic conditions is in the range of X to Y mol/m2 s. Finally, dissolution experiments aimed at understanding the release of As from naturally occurring seal rocks of a GCS formation were conducted. A primary seal rock and two secondary seal rocks were obtained from the Cranfield oil field CO2- EOR site in Mississippi. The rock samples were characterized by micro Xray adsorption near edge structure analysis, which revealed that multiple sources of As exist in the reservoir seal rocks studied. Dissolution experiments with seal rocks and anoxic brines of 105g/L NaCl resulted in the dissolution of arsenic in concentrations of 70 to 80 ppb at steady state. Dissolution of CO2 in the brine had no discernible effect on the steady state release concentration of As.