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Microbubbles loaded with nickel nanoparticles: A perspective for carbon sequestration Seokju Seo, Minh Nguyen, Mohammad Mastiani, Gabriel Navarrete, and Myeongsub (Mike) Kim Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02205 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 26, 2017
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Analytical Chemistry
Microbubbles loaded with nickel nanoparticles: A perspective for carbon sequestration Seokju Seo,† Minh Nguyen,† Mohammad Mastiani,† Gabriel Navarrete,† and Myeongsub "Mike" Kim*† †
Department of Ocean and Mechanical Engineering, Florida Atlantic University, 777 Glades Road, Boca Raton, FL33431, USA. *
Tel:+1 5612973442. E-mail:
[email protected] Abstract This work reports a microfluidic study investigating the feasibility of accelerating gaseous carbon dioxide (CO2) dissolution into a continuous aqueous phase with the use of metallic nickel (Ni) nanoparticles (NPs) under conditions specific to carbon sequestration in saline aquifers. The dissolution of CO2 bubbles at different pH levels and salinities was studied to understand the effects that the intrinsic characteristics of brine in real reservoir conditions would have on CO2 solubility. Results showed that an increased shrinkage of CO2 bubbles occurred with higher basicity, while an increased expansion of CO2 bubbles was observed with proportionally increasing salinity. To achieve acceleration of CO2 dissolution in acidic brine containing high salinity content, the catalytic effect of Ni NPs was investigated by monitoring change in CO2 bubble size at various Ni NPs concentration. The optimal concentration for Ni NPs suspension was determined to be 30 mg L-1; increasing the concentration up to 30 mg L-1 showed significant increase in the dissolution of CO2 bubbles, but increasing from 30 to 50 mg L-1 displayed a decrease in catalytic potential, due to the decreased translational diffusion coefficient that occurs at higher concentrations. The optimal additive concentration of Ni NPs was tested with variations of solution at acidic and basic conditions and different levels of salinity to reveal how effectively the Ni NPs behave under real reservoir conditions. At the acidic level, Ni NPs proved to be more effective in catalyzing CO2 dissolution and can sufficiently alleviate the negative impact of salinity in brine. Keywords: Carbon Sequestration, CO2 Dissolution, Deep Saline Aquifers, Nickel (Ni) Nanoparticles (NPs)
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1. Introduction Carbon capture and sequestration technology for mitigating anthropogenic greenhouse gas emissions has attracted much attention in response to international concerns about global warming and ocean acidification.1-3 Among the types of reservoirs that could be employed in geologic carbon sequestration, deep saline aquifers are considered to be one of the most appropriate options for large–scale commercial carbon dioxide (CO2) storage due to their large storage capacities. The potential capacity of saline reservoirs was estimated to be approximately fivefold higher than that of depleted oil–natural gas reservoirs.4 A critical characteristic of saline aquifers for CO2 storage is the amount of metal cations found in the brine, because metals such as magnesium (Mg), calcium (Ca), and iron (Fe) could react with the introduced CO2 to promote carbon mineralization, thereby increasing permanent storage of CO2.5 Another contribution to safe storage potential that brine possesses is the solubility trapping mechanism; mobile free– phase CO2 can migrate upwards and risk leakage, but CO2 that dissolves into brine and reaches an equilibrium state increases the density of the brine which then carries the dissolved CO2 downwards and reduces risk of leakage.6 However, the feasibility of carbon sequestration via the solubility trapping and mineralization processes in saline aquifers is controversial mainly due to the slow reaction rate of carbonic acid (H2CO3) formation from CO2 injected into brine.7 This reaction rate can further be negatively impacted by site–specific conditions of saline aquifers. The preferable depths for CO2 storage are usually at 800 to 2000 meters from surface level, and these depths can contain salinity concentrations of 5% to more than 25% weight per volume, respectively.8 The pH of brine is also site–specific and varies depending on depth. For example, some known pH values of saline aquifers are 2.3~2.4 in 2800 m deep wells in Indiana County, Pennsylvania, 4.2 in an 1158 m well in Guernsey, Ohio,9 5.4 and 6.5 in Paradox Valley, Colorado,10 and 7.1 in Utsira, North Sea.11 Because of these large variations in salinity and pH values of brine found in saline aquifers, there still exist substantial uncertainties about how the parameters may affect the reaction rate of H2CO3 formation. Therefore, quantification of these particular effects may adequately resolve global uncertainties, help identify suitable storage sites based on reservoir conditions, and promote carbon sequestration in saline aquifers. There has been a paucity of research for overcoming the major challenge of slow reaction rates, so contributing to this endeavor is another critical step in enhancing the feasibility of carbon sequestration in saline aquifers. There is significant potential for nickel (Ni) nanoparticles (NPs) to meet this demand, as two previous studies have reported evidence of Ni NPs accelerating the formation of H2CO3 in CO2 added to brine.7, 12 In these studies, the catalytic activity of Ni NPs was studied through a titration method (to measure the concentration of all species of CO2 (CO2 (aq), H2CO3 and H+ and HCO3-) in water)7 and pH measurements.12 However, such methods utilizing bulk experimentation are insufficient in explaining the mechanisms by which Ni NPs can enhance CO2 solubility in brine at different pH levels with different salinities. Other techniques conventionally used in evaluating CO2 gas dissolution, such as reaction towers of bubble generators13 and bubble columns,14 are expensive to manufacture. They also require substantial laboratory space and a longer–time span to adequately provide results, especially when investigating a broad range of conditions. Field studies, which mainly focus on the long– term fate of CO2 by measuring final and initial stage, are expensive to conduct and, more
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importantly, may not be appropriate for studying the catalytic activity of Ni NPs since the intermediate processes associated with interactions between Ni NPs and CO2 gas have not yet been explained. To effectively study reaction rates involving CO2 gas for the purpose of sequestration, ideal experiments should be able to accurately quantify CO2 dissolution across the gas/liquid interface over a short timescale. In response to the need for better experimental techniques, microfluidic technology has been adapted specifically for this purpose by virtue of its ability to reveal key information about CO2 dissolution and the associated reactions. A microfluidic approach provides controllability of reaction times through fine modulation of geometries and can achieve results in a cost–effective and time–efficient manner.15-22 These factors are instrumental in understanding the intermediate processes that drive CO2 dissolution, thus pairing the microfluidic solution with an investigation of the Ni NPs solution can yield critical data for the study of carbon sequestration in saline aquifers. Here, we present a microfluidic research that addresses all the main strategies outlined above to develop a cohesive report that could prove conducive to facilitating feasibility of carbon dissolution in saline aquifers. Individual microfluidic examinations of salinity and pH parameters would offer concrete evidence as a preliminary step that could first establish the site–specific effects of saline aquifers on CO2–brine reactions. To this end, a quantitative measurement of salinity– and pH–mediated dissolution of CO2 bubbles was carried out under experimental parameters chosen based on real reservoir conditions. Then, building on the knowledge of the potential of Ni NPs for enhancing CO2 dissolution rates, the interaction between Ni NPs and brine was studied with microfluidics to explore the driving mechanisms of Ni NPs assisted CO2 dissolution. Furthermore, a broad range of Ni NPs concentration was tested in this manner so as to find the optimal concentration for catalyzing CO2 dissolution, thereby determining the optimized additive level could then be considered under varying conditions of pH and ionic strength. This final optimization step is for the ultimate purpose of investigating which real– world environment of saline aquifers would be most ideal for implementation of the Ni NPs– assisted carbon sequestration process. In every segment of the study, microfluidics methodology allowed data to be successfully quantified by monitoring the change in micrometer–diameter of CO2 bubbles.
2. Experimental section 2.1. Microfluidic Chips Polymethylmethacrylate (PMMA) was used for a chip material due to its excellent chemical resistance and mechanical stability. PMMA microfluidic chips were conveniently fabricated through cost–effective laser ablation using a laser cutter (VersaLaser VLS2.30, Universal Laser Systems, Inc.). Figure 1a illustrates the overall design of the microfluidic device that was used for all experiments. By adjusting the power and speed of the laser, 100 µm wide channels for gas, an orifice channel, and a 200 µm wide channel for the liquid were fabricated based on the optimal geometry for large surface–to–volume ratio of spherical CO2 bubbles derived from numerical simulation (Fig. 1c). Numerical simulations were conducted for the comparison of shapes of CO2 bubbles depending on geometries of T–junction using CFD software ANSYS Fluent 17.0. (Fig. S1). Introducing an orifice at the T–junction can decrease the size of droplets,23
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and this mechanism for droplet generation in liquid–liquid systems should apply as efficiently to the generation of bubbles in gas–liquid systems.24 As can be seen from Fig. 1b, the average diameter with the standard deviation of 72 CO2 bubbles generated at the initial position—right after the orifice—was 57.89 ± 0.08 µm.
Figure 1 (a) Schematic diagram of a T–junction microfluidic device for CO2 experiments, (b) Histogram of measured diameters of CO2 bubbles after the orifice, determined from optical microscopy images fitted using the Lorentzian function, and (c) A simulation result of spherical CO2 bubble formation in the T–junction with an orifice. 2.2. Materials and Experimental Conditions PMMA sheets were purchased from Good Fellow (Berwyn, PA) for manufacturing microfluidic devices. Sodium chloride (NaCl, ≥99.5% purity), sodium hydroxide (NaOH, ≥98% purity), hydrochloric acid (HCl, 37%), phosphate buffer powder, and Ni NPs powder (≥99% purity, 0.05) in the effect of pH were confirmed between pH 3 and 5 or between pH 5 and 7, respectively. The significant changes in volumes of CO2 bubbles at pH 9 and 11 can be explained by applying the effective Henry’s constant.26 The total quantity of dissolved CO2 comprises the amount of aqueous CO2, along with the content of CO2 present in the intermediate HCO3- form and the equilibrium CO32- form that occurs when dissolved CO2
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reacts with water. Physical absorption of gaseous CO2 is governed by Henry’s law, where the amount of mass fraction of the gas in the liquid at the interface [CO2]aq is given as a function of Henry’s constant (kH = 3.45 × 103 kPa L mol-1) and applied inlet CO2 gas partial pressure (PCO2 = 13.79 kPa).
() + ↔ ( ) , =
(1)
CO2 (aq) reacts with water to form H2CO3, which then dissociates into H+ and HCO3- ions at the intermediate stage, and lastly H+ and CO32- ions form. These chemical reactions with the equilibrium constants (K1 and K2) are described as following: ( ) + ↔
(2)
↔ + , !" = ↔ + , ! =
# ∙ %&
= 4.45 × 10 - .
(3)
= 4.69 × 10 "" .
(4)
( )
# %& %&
Simultaneously solving for K1 and K2 with Henry’s constant can yield an effective Henry’s constant, kH*, that accounts for the total dissolved CO2 present. (121 3) = ( ) + + =
∗ =
(9:9;)
=
45
6
6 6
1 + 7# + 7# .
45
