Artificial weathering as a function of CO2 injection in Pahang Sandstone Malaysia: investigation of dissolution rate in surficial condition.

Formation of carbonate minerals by CO2 sequestration is a potential means to reduce atmospheric CO2 emissions. Vast amount of alkaline and alkali earth metals exist in silicate minerals that may be carbonated. Laboratory experiments carried out to study the dissolution rate in Pahang Sandstone, Malaysia, by CO2 injection at different flow rate in surficial condition. X-ray Powder Diffraction (XRD), Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectroscopy (EDX), Atomic Absorption Spectroscopy (AAS) and weight losses measurement were performed to analyze the solid and liquid phase before and after the reaction process. The weight changes and mineral dissolution caused by CO2 injection for two hours CO2 bubbling and one week' aging were 0.28% and 18.74%, respectively. The average variation of concentrations of alkaline earth metals in solution varied from 22.62% for Ca(2+) to 17.42% for Mg(2+), with in between 16.18% observed for the alkali earth metal, potassium. Analysis of variance (ANOVA) test is performed to determine significant differences of the element concentration, including Ca, Mg, and K, before and after the reaction experiment. Such changes show that the deposition of alkali and alkaline earth metals and the dissolution of required elements in sandstone samples are enhanced by CO2 injection.

Carbon dioxide (CO2) emissions from fossil fuels are estimated at 6 GtC/year1. Fifty percent of the increased concentration of CO2 has occurred during the last 40 years and is mainly due to human activities2. Means to reduce the atmospheric carbon emissions from the energy and/or process industries has become increasingly emphasized as a primary environmental concern, because it increases the global warming and rise in sea level. In nature, the formation of carbonate minerals from atmospheric CO2 is one of the major processes in the long-term global carbon cycle3. Gaillardet et al.4 stated that about 0.1 Gt of carbon per year is bound by silicate-mineral weathering throughout the world; at this rate, the consumption of global atmospheric CO2 inventory would be after 8000 years. To accelerate this process, transformation of atmospheric CO2 into carbonate minerals can be done via two effective ways; ex-situ and in-situ approaches. The former is performed via the industrial processes5 and the later by injection into subsurface geological formations where the required elements are available for creation of carbonate minerals6. CO2 mineralization or mineral carbonation, as an artificial rock weathering, mimics the natural rock weathering, was first proposed by Seifritz in 19907. It provides a permanent and leakage-free CO2 disposal method such that the carbonate production is environmentally benign and stable8. However, mechanisms controlling mineral stabilities in contact with injected supercritical fluids containing water are relatively unknown9. Chemical reaction of CO2 with minerals to form stable, solid compounds like carbonates considered as the main target of CO2 mineralization in surface and subsurface rocks.

Formation of carbonate minerals such as calcite (CaCO3), dolomite (Ca0.5Mg0.5CO3) and magnesite (MgCO3) is the most stable, long-term storage tool for atmospheric carbon dioxide3. The fixation of CO2 into divalent cations, such as Ca2+, Mg2+, Fe2+ is known as mineral carbonation10. Magnesium and calcium are proven to be the most suitable alkaline earth metals due to their stability11, abundance12 (they comprise ~2.0 and 2.1 mol% of the earth's crust, respectively), and solubility13. Alkaline earth metals, such as calcium and magnesium oxides, readily react with carbon dioxide to produce stable carbonate minerals, and this reaction in sandstone will cause precipitation and dissolution of minerals14. CO2 is dissolved in water to form carbonic acid (H2CO3) as depicted in Eq. 1, which dissociates to H+ and HCO3. Dissolved CO2 is expected to acidify the system and to deliver a source for carbon for the precipitation of carbonate minerals15. As shown in Eq. 2, the hydrogen ion hydrolyzes the mineral, liberating Mg2+ cations and forming silicic acid or free silica and water (Eq.2). The free Mg2+ cations react with the bicarbonate ions to form the solid carbonate as shown in Eqs. 3 and 4. Carbonate phases and dissolution of the minerals are energetically favored to form from interaction of CO2 with silicate phases as Forsterite and Anorthite in accordance with the theorized reaction equations shown in Eqs. 3 and 5, respectively161718. Indeed, one of the major challenges in the mineral sequestration of CO2 is to obtain the appropriate cations such as calcium and Magnesium. The most abundant cation source is silicate minerals, and sandstone rocks are dominant in Earth surface and subsurface geological formations. It is well documented that the reaction of CO2 with sandstone triggering precipitation and dissolution of minerals36. A number of studies have been published to assess the degree to which in situ CO2 mineralization is possible within sandstones rocks141521. Nonetheless, only a few reports have focused on the dissolution of silicate minerals and precipitation of carbonate through numerical modeling2223, and experimental works2425. Other researchers were conducted mineral carbonation in other rocks such as basalt or ultramafic rocks1026, fly ash27, Olivine and Serpentine13, Wollastonite28. However, carbonates precipitation is depending on the composition of the host rock. Actually, enormous volumes of sandstone rocks with different compositions are present on the Earth's surface, which are rich of alkaline earth metals. So, these widespread volumes of feedstock may have a correspondingly large CO2-sequestration capacity providing vast means of ex-situ carbon-mineralization sites throughout the world. Moreover, despite of the enormous CO2-EOR projects that are injecting millions of tons CO2 per year in sandstone rocks29, there are limited studies of injection of CO2 for the sake of disposal in the literature and consequently little data about the effect of CO2 injection on dissolution rate are currently available38. Moreover, due to the slow extraction rate of alkaline earth metals (e.g., Ca) in direct reaction with CO220, one might speed up the reaction to make the weathering process feasible for CO2 sequestration. Lackner et al.17, Kakizawa et al.19 and Ba3dyga et al.40 used hydrochloric acid, acetic acid, and succinic acid, respectively as acceleration medium. However, the major disadvantage of those acids is corrosiveness. In order to accelerate the reaction, injection of different CO2 flow rates into the acid-free carbonated water filled with sieved sandstone rocks, would be a new exploration option in this field. This mineral carbonation scenario should have contributes to large potential capacity of CO2 sequestration, reduction of CO2 emission, and minimal environmental impact.

This experimental analysis has been developed to assess the suitability of Pahang Sandstone for mineral carbonation. Accordingly, this paper concentrated on understanding of the impacts in weathering process and/or mineral carbonation by injection of CO2 at different flow rates (0.5 L/min, 1 L/min, and 1.5 L/min) at 80°C and 1 bar in a carbonated water filled with 4.0 mm size Pahang Sandstone Malaysia, which constitutes the optimum temperature and injection time as recommended by Bob et al.30, to reach the steady state pH, or the steady state CO2 concentration. In this paper, after XRD analysis and determination of the major cations of Pahang Sandstone, a complete study with the purpose of analyzing the solid and liquid phase before and after the reaction process, has been carried out. The approximate changes in weight and composition before and after the experiment and the effect of CO2 bubbling with different flow rates on the dissolution rate of Pahang Sandstone are presented. Investigation of dissolution effects of bubbling of different CO2 flow rates in rich carbonated waters on sandstone rocks is of great importance to increase the potential capacity of CO2 carbonation in such abandoned rocks.

Results

XRD analysis of the Pahang Sandstone showed quartz is the major mineral, with minor amounts of K-feldspar. Figure 1 shows XRD pattern of the main mineral components [silica (SiO2) and K-feldspar (CaAl2Si2O8)] before and after the reaction experiment. XRD analyses before and after experiments did detect contamination by clay minerals or the presence of secondary phases1536. Illitization of smectite minerals has been widely documented by XRD studies1531. The X-ray diffraction spectra of the Pahang Sandstone showed more or less amorphous nature with very little crystalline areas between 2θ 20–22°, 26–28°, and 42–44°. Such peaks were less visible in 20–22° and 26–28° after the reaction experiment indicating the dissolution of quartz and K-feldspar minerals. The tracer presence of smectite, kaolinite, illite and calcite is verified by XRD patterns.

Figure 1

XRD for mineralogical analysis of Pahang Sandstone before (a) and after (b) the reaction experiment with water/CO2.

It showed less noise in (b) indicating reduced amorphous content.

As the important earth metals; K, Mg and Ca are utilized for carbon mineralization. Therefore, it is necessary to determine the dissolution concentration of them in the host rock after reaction experiment. The chemical composition of the solution after the interaction of sandstone/water/CO2 in 300 mL with different CO2 flow rates was analyzed and depicted in Figure 2. Pretreatment of the sandstone samples by size reduction will result in major improvements of the reaction rate. As shown in Figure 2, dissolved Ca and Mg show decreasing trend in concentration to 6.84 mg/L and 0.435 mg/L, respectively after increasing the flow rates to 1.5 L/min and reached to their equilibrium state after two weeks to 7.59 mg/L and 0.457 mg/L, respectively. In contrast, dissolved K was rapidly increased from 5.05 mg/L in 0.5 L/min CO2 flow rate to 5.31 mg/L and 5.98 mg/L, for 1 L/min and 1.5 L/min, respectively and reached its equilibrium state to 6.2 mg/L after two weeks. As illustrated in Figure 2, dissolved K concentrations were lower than dissolved Ca by 1–2 orders of magnitude.

Figure 2

Element concentration in residual solution as a function of CO2 flow rate for Ca, K, and Mg.

To further study the precipitation of secondary minerals, SEM-EDS investigation was conducted on the surface textures of the Pahang Sandstone sample. SEM microphotographs and EDS charts are shown in Figures 3a, 3b and 3b, 3c, respectively. Figures 3a and 3b show SEM microphotographs of sandstone surface textures after the reaction experiment with obvious surface abnormality and clear softening of the middle of the marked rectangular, which indicates the fluid-rock interaction2537. Figures 3c and 3d show EDS charts that detect chemicals surrounding the section being scanned.

Figure 3

(a) and (b) ESM of the sandstone surface; 3 (c) and (d) EDS that detects chemicals surrounding the section being scanned.

Figure 4 shows the result for pH variation of the reactant solution as a function of different CO2 flow rates. The pH of the solution for each injection flow rate was measured after two weeks' aging. At zero flow rate of CO2 the figure indicates the solution will be slightly alkaline. As CO2 flow rate injection is increased, the solution becomes more and more acidic. As CO2 flow rate increases to 1.5 L/min, pH drops, and much of the carbonate ion is converted to bicarbonate ion, which results in higher solubility of Ca2+, Mg, and K. The effect of the latter is especially evident in AAS results. Increasing the flow rate injection from 0 to 0.5 L/min, 0.5 to 1 L/min, and 1 to 1.5 L/min increase the pH by the factor of 2.29, 5.41, and 16.69, respectively.

Figure 4

pH as a function of CO2 flow rate.

Figure 4 depicts the pH of element concentration in residual solution (Ca, K, and Mg) measured at surficial condition as a function of CO2 flow rate. The initial pH was about 7.0. At a CO2 flowrate of 0.5 L/min, a pH of 6.84 was reached in 2 hours. The measured pH for 1 L/min and 1.5 L/min were reached the values of 6.47 and 5.39 respectively. Figure 4 shows the solution to be more acidic by increasing the injection flow rate of CO2. Increasing CO2 flowrate and addition of alkali and alkaline earth metals affect the dissolution rates of Pahang Sandstone (Figure 2).

Figure 5 shows formation of secondary minerals on quartz surface through SEM microphotographs. As shown in Figure 5, the identification of some clay minerals was confirmed by SEM microphotographs. Figures 5d, 5e and 5f illustrate enlargement of one of the etch spots of quartz outlined by the white box in 5d, showing dissolution features and presence of kaolinite platelets. Enlarged view of the kaolinite coating is shown in Figure 5f that do not form a continuous cover. The dissolution of feldspar and precipitation of kaolinite (Figures 5d, 5e, 5f), is a weathering reaction that accelerated by CO2 bubbling. The secondary minerals are likely to be illite (ribbons or fibrous) intergrown with smectite, which suggests the progressive illitization of smectite (Figures 5g, 5h, 5i). As indicated by Lu et al.15, removal of cations such as Na+, K+ and silica by fluid flow is the requirement of illitization process. Those cations, such as Mg2+, were detected by AAS analysis after the experiment and their concentration in residual solution as a function of CO2 flow rate is shown in Figure 2.

Figure 5

SEM microphotograph of Pahang Sandstone dissolution by CO2 bubbling.

Figure 6 shows the rock sample weight loss before and after the interaction procedure. The initial weight before the experiment was 32.70 gram and started to decrease from 32.50 gram in 0.5 L/min to 32.47 gram and 32.43 gram in 1 L/min and 1.5 L/min, respectively. The decrease for 0.5 L/min flow rate was 0.612% and 0.094% for 1 L/min, and this trend increased to 0.123% for the flow rate of 1.5 L/min.

Figure 6

Rock sample weight loss as a function of CO2 flow rate.

Discussion

Rock weathering reaction plays an important role in global carbon cycle, and has been previously used by a few scenarios2021, which is principally the reaction of calcium silicates or magnesium silicates, as shown in in Eqs. 4 and 5, with the atmospheric CO2 to form carbonates. Calcium and Magnesium are capable of being carbonated because carbonic acid is a stronger acid than silicic acid19, and alkaline earth metals can be found in sandstone rocks that are dominant in Earth surface. It is well documented that the reaction of CO2 with sandstone triggering precipitation and dissolution of such minerals36. Consequently, in this paper, the focus of the artificial weathering effort by injection of CO2 at different flow rates (0.5 L/min, 1 L/min, and 1.5 L/min) at 80°C and 1 bar in a carbonated water filled with 4.0 mm size Pahang Sandstone Malaysia has thus far been mainly on the sandstone dissolution rates, addressing mineral carbonation of alkali as well as alkaline earth metals.

Our observation on K-feldspar precipitation show that carbonation process occurred on Pahang Sandstone sample. The percentage change in SiO2 and CaAl2Si2O8 showed a decrease in those minerals over the flooding period from 96.16% to 13.49% and 14.43% to 2.57%, respectively. These minerals are predicted by many researchers (e.g., Lu et al.15) to play active role in geochemical interaction of sandstone with aqueous CO2. As indicated by Lu et al.15, usually some clay minerals, such as smectite and kaolinite persists on K-feldspar surfaces, which can cause pore throat clogging and reduction of permeability and result in significant decline in productivity32. In Pahang Sandstone, a noticeable decrease in the K-feldspar content is commonly observed suggesting that it has been dissolved in the process of illitization of kaolinite and possibly also smectite36. As confirmed by AAS analysis, K shows increasing trend from 5.05 mg/L in 0.5 L/min CO2 flow rate to 5.31 mg/L and 5.98 mg/L, for 1 L/min and 1.5 L/min, respectively and reached its equilibrium state to 6.2 mg/L after two weeks. SEM observations confirmed the illitization of smectite. Dissolution of K-feldspars and conversion of smectite to illite are likely to be the two reactions that contribute to the release of SiO2.

Consistent with the XRD results, SEM-EDS analyses indicates that Si and C are the major components of sandstone, while Al, and K, (~6 atoms %) are minor components (Tables 1 and 2).

Table 1

Quantitative results of ESM-EDS of spectrum 1

Element	App Conc.	Intensity Corrn.	Weight%	Weight% (Sigma)	Atomic%
C K	0.10	0.6308	18.79	1.60	27.86
O K	0.56	1.3863	44.02	1.14	48.99
Al K	0.09	1.1202	8.92	0.39	5.89
Si K	0.23	1.0460	24.65	0.74	15.62
K K	0.03	1.0179	3.61	0.44	1.65
Totals			100.00

Table 2

Quantitative results of ESM-EDS of spectrum 2

Element	App Conc.	Intensity Corrn.	Weight%	Weight% (Sigma)	Atomic%
C K	0.05	0.5056	12.66	1.93	19.92
O K	0.47	1.4422	42.39	1.23	50.05
Al K	0.05	1.1450	6.03	0.37	4.22
Si K	0.30	1.0758	37.02	1.07	24.90
K K	0.01	1.0188	1.90	0.45	0.92
Totals			100.00

AAS analysis also shows that K and Ca concentrations are present in the solution of sandstone/water/CO2 reaction system, whereas the concentration of Mg is less than 0.522 mg/l after the first injection flow rate of CO2 as illustrated in Figure 2. Dissolved Ca and Mg show increasing trend by increasing the CO2 flow rate and reach equilibrium state after two weeks. The K concentrations measured at 0.5 L/min, 1 L/min, and 1.5 L/min were 5.04 mg/L, 5.31 mg/L, and 5.98 mg/L, respectively. By increasing the CO2 flow rate, dissolved K concentration increased gradually to above 5.97 mg/L. Approximately 12% and 17% lower concentrations of dissolved Ca and Mg were observed in the experiments, respectively. Whereas dissolved concentrations of K was increase 16% by in the slurry created by the interaction of sandstone/water/CO2. Experimental results showed that CO2 flow rate has a significant influence on the interactions between sandstone and synthetic water/CO2 system. The lower amount of carbonate mineral precipitation is due to the solubility trapping in aqueous phase that is the dominant CO2 sink152333.

ANOVA test is performed to determine significant differences of the element concentration (Ca, Mg, K) before and after the reaction experiment and the results were tabulated in Table 3. The F critical value (F crit) is the number that the test statistic (F) must exceed to reject the test. As shown in Table 3, Fcrit at α = 0.05 for Ca, Mg, and K equal to 5.98, 5.99, and 5.97, respectively. Since F for Ca, Mg, and K equal to 61.80, 652.37, and 343.62, respectively, and they are higher than their F crit, the results are significant at the 5% significance level. One would reject the null hypothesis, concluding that there is strong evidence that the expected values in the two groups, before and after the reaction experiment for the elements, considerably differ. The p-value for these tests are tabulated in Table 3 that are 0.0002, 2.37E-07, and 1.59E-06 for Ca, Mg, and K, respectively, which are smaller than α and considered an extra evidence to reject that the null hypothesis. Therefore, high confidence is present about the difference between the mean of the first group (before experiment) and the mean of the other group (after experiment). In Table 3, the divisor is called the degrees of freedom (df), the summation is called the sum of squares (SS), the result is called the mean square (MS), and they show the variations between groups. Sum, average, and variance show the three statistics of the second groups (the results after reaction experiment) of the studied elements. Bowker et al.41 confirmed the significance of the alkali and alkaline earth metals concentration before and post CO2 breakthrough and the rate of increase of Ca, Mg, and K after CO2 injection reported 19%, 29%, and 10%, respectively.

Table 3

ANOVA test result comparing element concentrations of the solution before and after the reaction experiment

Element	Sum	Average	Variance	SS	df	MS	F	P-value	F crit
Ca	29.67	7.42	0.17	58.89	1	58.89	61.80	0.0002	5.98
Mg	1.89	0.47	0.001	0.45	1	0.445	652.37	2.37E-07	5.99
K	22.54	5.63	0.98	52.72	1	52.72	343.62	1.59E-06	5.97

It's clearly shows that quartz dissolution is the major process liable for the weight loss in the sample and increasing the concentration of Si and K in the residual solution. Also, as clearly illustrated in Figure 4, the pH of the solution has an increased trend toward acidic from 0.5 L/min to 1.5 L/min CO2 injection, indicating that there has been precipitation of secondary minerals.

The results of the weight loss (e.g., mineral-fluid interaction) show that higher flow rate (1.5 L/min) is greater than 1 L/min flow rates in 80°C, but lower than initial flow rate. The higher weight loss in 0.5 L/min flow rate may associate with loosening of the rock samples and dissolution of some clay minerals. As showed in Figure 1, the differences in silica and K-feldspar before and after the reaction/experiment procedure are noticeable and they counts were reduced after the experiment. This unusual phenomenon arises from the increase in the increase in CO2 flowrate and removal of cations, including silica, by illitization process that well documented in the literature153639. The dissolution behavior of silica/feldspar minerals reveal more acidic fluid conditions in the sandstone/water/CO2 system, greater porosity and fluid mobility and as inferred by Liu et al.34, the sample weight change can be accommodate by deposition of secondary minerals and by the corresponding fixation of CO2 in the rock sample. Also, the presence of oxygen in the solution35 may be contributed to enhanced dissolution of calcite and attributed to acid generation resulting from oxidative dissolution of Pahang Sandstone.

Results show that the Pahang Sandstone will react with carbonated water through injection of CO2 in 80°C and 1 atm. Also, pH decreased by 2.30%, 5.41%, and 16.70% after CO2 flow rates of 0.5 L/min, 1 L/min, and 1.5 L/min, respectively. Lower the pH cause more severe reactions in the sandstone/water/CO2 system. Therefore, dissolved CO2 acidify the solution and carbon source will be provided by carbonate minerals. Based on the XRD, AAS, and ESM-EDS results, Ca and Mg were released as calcite and dolomite were dissolved by carbonic acid, while K and Si were released by cation exchange of K with Ca and dissolution35. The presence of Ca, Mg, and K in considerable amounts in the solution of reaction experiment with sandstone samples is an indication that the Pahang Sandstone is the right candidate for CO2 mineralization and utilization of CO2 flow rate as acceleration medium shows good results, thus accelerating the mineral carbonation for CO2 sequestration.

Clearly, increasing CO2 flowrate accelerates the reactions between the carbonated water and Pahang Sandstone and consequently the rate of dissolution increased. As verified by XRD, SEM and AAS, mineral dissolution is the explanation of K-feldspar's loss. Also, the C signal on one EDS points on Figures 3c and 3d are the direct evidence on carbonate chartered crystal morphology. This confirmation is supported by XRD analysis and AAS results after carbonation process. Therefore, in this study, the existence of carbonate minerals is validated. However, special care must be taken because the C signal can result from organic contamination, trace original carbonates in sandstone or C coating one the sample. Fourier transform infrared (FTIR) spectroscopy is sensitive to carbonate C-O bond, compare the FTIR signal before and after the reaction can provide useful information on carbonate existence.

For mineral carbonation the use of alkali and alkaline earth metals, K, Mg, Ca, is favoured because they are worldwide available in huge amounts. Mineral carbonation in sandstone rocks contain alkaline earth metals is potentially significant for carbon sequestration. As a technique for CO2 storage in solid form, an aqueous process for the direct carbonation of alkaline and alkali minerals in Pahang Sandstone has been developed in this study. The abundant in the Earth's surface, make sandstone rocks as a liable candidates for the purpose. This study focuses on understanding the potential impacts of injection of different CO2 flow rates on the sandstone/water/CO2 system. Three CO2 injection flow rates with the same pressure and temperature conditions in laboratory scale have been examined to assess the impact of CO2 flow rate to sandstone/water/CO2 systems. Si and C appear as the prevailing components in all of the samples, while Al and K are minor constituents in Pahang Sandstone. The weight changes and mineral dissolution caused by CO2 injection for two hours were 0.28% and 18.74%, respectively. The average variation of concentrations of alkaline earth metals in solution varied from 22.62% for Ca2+ to 17.42% for Mg2+, with in between 16.18% observed for the alkali earth metal, potassium. Such changes show that the deposition of alkali and alkaline earth metals and the dissolution of required elements in sandstone samples are enhanced by CO2 injection. Finally, injection of higher rates CO2 showed a particularly strong effect. To the best of our knowledge, the major sequestration product appears to be calcium and potassium carbonates; however, more detailed material characterization, such as FTIR, may useful on pre- and post-carbonated samples to better interpret carbonation products in sandstone rocks. The undertaken assessment is generally representative for Pahang Sandstone, Malaysia.

Methods

Materials

In this research, high purity CO2, carbonate water, sandstone rock samples were used. Sandstone samples were collected from an outcrop of a local sandstone formation from Bukit Bangkong, Kuala Rompin, Pahang, which were grind into 4.0 mm in size. In order to have CO2 equilibrated aqueous solution, CO2 gas has been injected into deionize water and stirred accordingly to from carbonated water. The slurry and rock samples collected after each injection CO2 flow rates (0.5 L/min, 1.0 L/min, 1.5 L/min) preserved for two weeks and then characterized by performing XRD, SEM with EDX, AAS, which were used to further analyze the mineralogy and major element concentrations of the residual solution (Ca, K, and Mg) of several selected samples. Pre-treatment of the minerals performed by size reduction of the sandstone rock samples to 4.0 mm to increase reactivity.

Experimental procedure

Three sets of experiments were conducted with analogous temperature and pressure experimental conditions, to investigate the effect of CO2 flow rate on the dissolution of primary minerals and deposition of secondary minerals in sandstone/water/CO2 system. The reaction experiments performed under atmospheric pressure and controlled temperature (80°C) conditions, as well as all solution preparations, were carried out using a glass reactor with a volume of 300 ml. The experimental setup is shown in Figure 7, which contained a glass reactor in which solutions were stirred by a magnetic stirrer.

Figure 7

Experimental apparatus for CO2 mineralization.

The glass reactor was surrounded by an open water bath, which was heated using a separate, temperature-controlled water bath connected to the open water bath. To avoid evaporation losses, the reactor was equipped with a condenser cooled with tap water. The pH and temperature of the solution were measured allowing data to be monitored and logged in real time. CO2 gas bottle was connected through a pressure regulator and a flow meter, allowing CO2 to be injected through the solutions by different flow rates (0.5 L/min, 1 L/min, and 1.5 L/min). Grinded rock sieved by 4.0 mm and then weighted. 32.70 gram of sandstone rock sample was poured into glass reactor, which contained 300 ml of carbonate water. Then slurry bubbled by 0.5 l/min CO2 flow rate. The carbon dioxide flow was switched off after two hours and the solution was immediately filtered through a Whatman 40 μm filter paper, and the solution was sealed for two weeks. Same procedure repeated for the second and third tests, which 32.50 and 32.47 grams of rocks samples were bubbled by 1.0 L/min and 1.5 L/min CO2, respectively. The filtered debris and rocks were washed using deionize water, dried at 120°C overnight then weighted and prepared for the second and third experiments. At the end of each experiment, the reactor glass was washed and the reacted sandstone was collected for solid phase characterization. The slurry of each experiment was analyzed for alkali and alkaline earth metals using AAS. The rock samples from the third experiment (1.5 l/min) were analyzed using XRD and SEM-EDS.

Analytical methods

Rock and fluid samples were taken from the reactor at consistent intervals, in which they remained in the solution for two weeks after two hours of CO2 injection. Then they were analyzed for phase identification by XRD, phase relationships and surface morphology by SEM-EDX, and chemical analysis of products by AAS. XRD were used to identify crystalline mineral phases of the sandstone samples before and after the mineral carbonation experiment. The crystalline minerals of the rock samples were characterized using XRD with Cu Kα radiation (λ = 1.5418 Å) in the range of 2θ = 4°–60° at scanning speed of 0.05° per second and dwell time of 1.5 second to generate the XRD pattern of the material. Magnesium, potassium, and calcium concentrations in solution were assessed by Aanalyst 400 Perkin Elmer Atomic Absorption Spectrophotometer. Samples were filtered using Whatman 40 μm filter paper and the filtrates after suitable dilutions, were analyzed. AAS is performed to evaluate the slurry after each step of CO2 injection. 32.70 grams of the rock sample were reacted with 300 ml of carbonated water in three injection flow rates of CO2 at 80°C and 1 atm. The rate of CO2 injection was kept constant at each corresponding flow rate (two hours) for the whole experiment. The dissolved concentrations of Ca, Mg, and K were measured in each CO2 flow rate. SEM is used to visualize very small topographic details on the surface or entire or fractioned objects and provide semi-quantitative elemental analysis of the sandstone rock sample after the carbonation experiment. In this study, JSM-6701F was used to obtain the scanning images of sandstone rock sample after the experiment. The JSM-6701F is an ultra-high resolution SEM suitable for observation of fine structures such as multi-layered film and nanoparticles fabricated by the nano-technology. High resolution at 1 nm (15 kV) and 2.2 nm (1 kV) with the maximum 2 nA probe current and without changing the objective lens aperture size. The specimen's size will be up to 200 mm diameter. In addition, an energy dispersive X-ray spectroscopy (EDS) is attached to this system that used for the elemental analysis or chemical characterization of the sandstone samples after reaction with CO2/water.

Author Contributions

M.Z. designed and directed the study. M.J. and F.A. performed the experiments as well as most of the coordination with XRD, SEM, and AAS laboratories. R.J. and R.M. took part in the supplementary materials and discussion of chemical processes, Figures 3–5 and statistical analysis. M.Z. prepared the manuscript and all authors reviewed the manuscript.