Potential leakages of CO2 from storage sites to shallow aquifers could have adverse impacts on the quality of potable groundwater. The mineralogy of well-sorted silica sand is modified by the pH-controlled precipitation of eight metals (Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd). Continuous flow tests are performed in two fixed-bed columns packed with the modified sand by coinjecting gas CO2/distilled water (2-phase column) and distilled water (1-phase column/control test) at constant influx rates for a period of two months. The concentration of dissolved metals is measured in the effluents of columns with atomic absorption spectroscopy (AAS). Mineralogical analysis of the surface of sand grains is done before and after the flow tests with scanning electron microscopy-X-ray energy dispersive spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS), whereas the precise quantitative measurement of the metal content in the sand is done with AAS. A dynamic numerical model that couples the flow and mass-transfer processes in porous media with the equilibrium and kinetically driven metal desorption processes is developed. Inverse modeling of the continuous flow test enables us to quantify and rank the selectivity of metal mobility in terms of equilibrium and kinetic desorption parameters. The continuous CO2 dissolution and water acidification causes significant mobilization and dissolution of several metals (Mn, Ni, Cu, Zn, Co), moderate mobilization of Cr, acceleration of Cd dissolution, whereas Fe remains strongly bonded on the sand grains as goethite. The parameters estimated from lab-scale column tests might be helpful for interpreting field-scale CO2 leakage scenarios and installing relevant early warning monitoring systems.
Since materials science is of great importance, the application of composite materials based on carbon nanotubes (CNTs) is widely investigated. Current work is aimed at preparing nanocomposites of TiO 2 /MWCNT using an impregnation method combined with slow hydrolysis. Different titanium alkoxide compounds as Ti(OEt) 4 , Ti(OiPr) 4 , and Ti(OBu) 4 , respectively, were used as precursor materials to cover the surface of CNTs in ethanolic medium. In our samples the mass ratio was 10:1 (TiO 2 :MWCNT).The produced composite materials were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), Raman microscopy, transmission (TEM) and scanning electron microscopy (SEM) techniques; moreover, thermal analysis and the determination of the specific surface area (BET) were also carried out. These as-prepared TiO 2 /MWCNT composites were also tested as photocatalysts in degradation reaction of the model compound salicylic acid.
The mineralogy of well-sorted silica sand is modified by mixing it with an aqueous solution of metal (Fe, Mn, Cu, Co, Ni, Cr, Zn, Cd) salts and increasing very slowly the pH so that insoluble metal oxides/hydroxides coprecipitate on sand grains. The modified sand is packed in two fixed-bed columns saturated with water which is injected at a constant and low influx rate. In the one column, gas CO 2 is co-injected at a flow rate that is one order of magnitude higher than that of water, while the flow test in the other column is used as control experiment. The pressures drop across the aqueous and gas phases are recorded continuously to track any potential changes of the (column averaged) relative permeabilities. Water effluent samples are collected to measure physicochemical parameters (e.g. pH, conductivity, etc) and metal cation concentration by atomic absorption spectroscopy. The mineralogy of sand is identified with scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and quantified with atomic absorption spectroscopy. The kinetics of CO 2 dissolution and aqueous phase acidification along the porous medium is simulated under conditions of constant and uniform distribution of water saturation. The highest rates of metal cation release, due to geochemical solid/aqueous phase reactions, are observed during the initial and transient phase of the experiment as the CO 2 dissolution and water acidification have not yet been completed, whereas the fluid saturation and relative permeability change respectably. The CO 2 dissolution rate is enhanced as the water saturation decsreases due to the higher specific interfacial area.
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