Element mobilization and immobilization from carbonate rocks between CO2 storage reservoirs and the overlying aquifers during a potential CO2 leakage

Accepted Manuscript Element mobilization and immobilization from carbonate rocks between CO2 storage reservoirs and the overlying aquifers during a potential CO2 leakage Amanda R. Lawter, Nikolla P. Qafoku, R. Matthew Asmussen, Ravi K. Kukkadapu, Odeta Qafoku, Diana H. Bacon, Christopher F. Brown PII:

S0045-6535(17)32177-X

DOI:

10.1016/j.chemosphere.2017.12.199

Reference:

CHEM 20580

To appear in:

ECSN

Received Date: 30 August 2017 Revised Date:

21 December 2017

Accepted Date: 31 December 2017

Please cite this article as: Lawter, A.R., Qafoku, N.P., Asmussen, R.M., Kukkadapu, R.K., Qafoku, O., Bacon, D.H., Brown, C.F., Element mobilization and immobilization from carbonate rocks between CO2 storage reservoirs and the overlying aquifers during a potential CO2 leakage, Chemosphere (2018), doi: 10.1016/j.chemosphere.2017.12.199. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Element mobilization and immobilization from carbonate rocks

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between CO2 storage reservoirs and the overlying aquifers during a

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potential CO2 leakage †

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Amanda R. Lawter*a, Nikolla P. Qafokua, R. Matthew Asmussena, Ravi K. Kukkadapua, Odeta

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Qafokua, Diana H. Bacona and Christopher F. Browna a

Pacific Northwest National Laboratory (PNNL), 902 Battelle Boulevard, Richland, WA

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99352

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Submitted to Chemosphere: August 2017

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*Author to whom correspondence should be addressed.

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Address: Pacific Northwest National Laboratory (PNNL), P.O. Box 999, MSIN P7-54,

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Richland, WA 99352

E-mail: [email protected]

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Phone: (509) 375-2709 Fax: (509) 371-7249

† Electronic Supplementary Information (ESI) available.

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ABSTRACT

Despite the numerous studies on changes within the reservoir following CO2 injection and the effects of CO2 release into overlying aquifers, little or no literature is available on the effect

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of CO2 release on rock between the storage reservoirs and subsurface. This is important, because

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the interactions that occur in this zone between the CO2 storage reservoir and the subsurface may

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have a significant impact on risk analysis for CO2 storage projects. To address this knowledge

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gap, relevant rock materials, temperatures and pressures were used to study mineralogical and

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elemental changes in this intermediate zone. After rocks reacted with CO2-acidified 0.01M NaCl,

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liquid analysis showed an increase of major elements (e.g., Ca and Mg) and variable

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concentrations of potential contaminants (e.g., Sr and Ba); lower aqueous concentrations of these

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elements were observed in N2 control experiments, likely due to differences in pH between the

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CO2 and N2 experiments. In experiments with As/Cd and/or organic spikes, representing

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potential contaminants in the CO2 plume originating in the storage reservoir, most or all of these

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contaminants were removed from the aqueous phase. SEM and Mössbauer spectroscopy results

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showed the formation of new minerals and Fe oxides in some CO2-reacted samples, indicating

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potential for contaminant removal through mineral incorporation or adsorption onto Fe oxides.

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These experiments show the interactions between the CO2-laden plume and the rock between

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storage reservoirs and overlying aquifers have the potential to affect the level of risk to overlying

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groundwater, and should be considered during site selection and risk evaluation.

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Keywords: CO2 sequestration, carbonate, CO2 transport, storage reservoir

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1. INTRODUCTION

CO2 sequestration is a viable option for reduction of anthropogenic CO2 released to the atmosphere. Carbon capture and storage (CCS) involves long term storage of CO2 in deep, saline

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reservoirs or depleted oil and gas reservoirs. A major concern with implementation of CCS is the

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effect of inadvertent releases of CO2 on overlying groundwater aquifers. It is well known that

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CO2 will cause a decrease in pH when dissolved in water due to the formation of carbonic acid

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(Harvey et al., 2013; Qafoku et al., 2015). This lowered pH can increase mineral dissolution,

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which may result in an increased concentration of undesirable trace elements in the groundwater

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aquifer.

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Studies related to the overlying groundwater aquifers have focused on the potential effect on groundwater quality following an unintentional release of CO2 or CO2-rich brine into the aquifer.

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These studies have covered interactions with sandstone (Keating et al., 2010; Vong et al., 2011),

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carbonate (Bacon et al., 2016; Wang et al., 2016; Yang et al., 2014) and unconsolidated sand and

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gravel aquifers (Humez et al., 2013; Lawter et al., 2016; Little and Jackson, 2010; Shao et al.,

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2015; Xiao et al., 2016). Results of this research vary greatly, ranging from increases in

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concentrations of mobilized contaminants causing degradation of groundwater quality, to the

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removal of contaminants from the groundwater through adsorption or other mechanics.

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Studies have also been focused on the interaction between reservoir rock, CO2 plumes, and

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CO2-laden brines. These interactions have been studied at temperatures ranging from 25 °C to

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200 °C and pressures ranging from 1 bar to 337.8 bar (Garcia-Rios et al., 2014; Horner et al.,

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2015; Icenhower et al., 2015; Jung et al., 2013; Karamalidis et al., 2013; Lu et al., 2010; Shao et

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al., 2014). These studies cover interactions with sandstone (Horner et al., 2015; Icenhower et al.,

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2015), siltstone (Shao et al., 2014) and carbonate rocks (Garcia-Rios et al., 2014; Lu et al.,

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2016). The focus has been to study chemical, and mineralogical, and physical changes within the

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reservoir and especially the caprock following contact with CO2-enriched reservoir brines.

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Previous studies have also considered that contaminants present in the reservoir brine may travel upwards with the CO2 plume (Carroll et al., 2014; Karamalidis et al., 2013; Xiao et al.,

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2017). Recent studies have been conducted to determine the fate of these contaminants after

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entering an overlying groundwater aquifer; these studies concluded that aquifer sediments may

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have sufficient adsorption capacities to remove most or all of these contaminants from the liquid

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phase (Lawter et al., 2015; Shao et al., 2015). Organic material can also be present in the

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reservoir, and can migrate with the CO2 plume (Scherf et al., 2011; Zhong et al., 2014b). Studies

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on organics in CO2 sequestration environments have shown n-aliphatic and aromatics are the

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most likely to be extracted from reservoir rock and mobilized (Jarboe et al., 2015; Kolak and

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Burruss, 2006, 2014; Kolak et al., 2015; Zhong et al., 2014a; Zhong et al., 2014b). A modeling

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study conducted by Cantrell and Brown (2014) indicated that the heavier of the extracted

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organics would condense out of the plume prior to reaching an overlying groundwater aquifer,

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but significant concentrations of lighter organics, such as benzene and toluene, could reach the

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aquifer and negatively impact the groundwater quality.

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While there is currently a moderate amount of literature available covering changes within

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the reservoir following CO2 injection as well as effects of CO2 release into overlying

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groundwater aquifers, there is no literature available, to the best of our knowledge, on the effect

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of inadvertent releases of CO2 on the rock between the storage reservoir and the shallow

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subsurface. There is potential for the CO2 plume to interact with the rock, increasing the release

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of hazardous elements that may then continue upward movement into an overlying aquifer, or 4

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the interactions may instead lead to the formation of carbonate or other secondary minerals that

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may incorporate or sorb contaminants from the CO2 plume, resulting in a reduced risk to the

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overlying aquifer. For contaminants originating in the reservoir brine, interactions with rock have

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the potential to remove the contaminants before they reach an overlying groundwater aquifer,

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reducing the concern for degradation of the groundwater. In addition, precipitation of minerals

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would likely change the permeability and porosity of the rock, which will also have an effect on

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the risk related to CO2 leakage.

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To fill this current knowledge gap, the objective of this study was to use relevant rock

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materials from different depths, temperatures and pressures to study solid phase mineralogical

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changes and measure major, minor and trace elements removed from or released into the

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aqueous phase through adsorption, mineral precipitation or mineral dissolution. Carbonate rocks

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were chosen for this study due to the prevalence of carbonates in potential CO2 sequestration

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locations (Dai et al., 2014; Qafoku et al., 2017); however, it is important to note that the rock

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type found between a CO2 reservoir and overlying aquifer is highly site specific, and these tests

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are meant as a first step to understanding the role of these intermediary rocks during CO2

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leakage. Some experiments included an As and Cd spike, with and without organics, added to the

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solution to represent possible contaminants from the reservoir brine. The pH of the system was

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determined by geochemical modeling. Acid extraction, microscopic inspections, spectroscopic

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investigations and spectrometric measurements were used to determine solid phase elemental

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composition and experimentally induced mineralogical and liquid phase composition changes at

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different experimental times.

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2. MATERIALS AND METHODS Six core plugs from the Austin Core Research Center (Austin, TX) were used to represent different depths (approximate depths of 1000’ (2 plugs), 1500’, 2000’, 3000’, and 10,000’). One

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of these core plugs (“Prinz”) was chosen due to signs of high organic matter content (due to an

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oily look, feel, and smell). All of the core plugs came from the San Antonio region in Texas; the

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county where each core plug originated from is listed in Table 1. These carbonate rocks were

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chosen due to the prevalence of carbonates in potential CO2 sequestration locations (Dai et al.,

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2014; Qafoku et al., 2017). Total organic carbon (TOC) results confirm the relatively higher

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organic matter content of the Prinz core plug compared to the other core plugs (section 3.1.1.).

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Water was used to cut the plugs from the original cores. The core plugs ranged in length from 3

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to 6 cm and were all ~2.5 cm in diameter (Fig. S1).

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Experiments were conducted in 0.01M NaCl solutions. NaCl was used to increase salinity to represent brine present in the deep storage reservoir that is likely to be displaced along with

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CO2. An increase in salinity has been shown to increase dissolution through ion exchange

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reactions (Shao et al., 2011) while diminishing the corrosiveness of de-ionized water (Lu et al.,

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2010; Smyth et al., 2009); the low concentration of NaCl for these experiments does not well

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represent brine, but was chosen to avoid Na interference during subsequent analysis (i.e., Na can

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plug system components and potentially cool the plasma of the inductively coupled plasma-mass

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spectrometry (ICP-MS) or ICP- optical emission spectrometry (ICP-OES) instruments, which

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can depress detection of other analytes) while still including the potential for ion exchange

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reactions.

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2.1 Pre- and post-treatment solid phase characterization

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Characterization of these samples included quantitative X-ray diffraction (QXRD), 8M nitric acid extractions, and scanning electron microscopy (SEM) combined with energy-

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dispersive X-ray spectroscopy (EDS). Coupons (3.33 cm2) and ground (<212 µm) samples were

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inspected with SEM/EDS and the ground samples were analyzed by QXRD before and after the

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experiments. The analyses were conducted using a Field Emission Focused Ion Beam-SEM,

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Quanta 3D, after the powdered samples and coupons, secured into a double sticky C-tape

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attached to an aluminum holder, were coated with a conductive carbon layer with a varying

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thickness of 15 to 20 nm to inhibit charging effects. An energy-dispersive X-ray detector was

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used for qualitative analysis using an acceleration voltage of 20 keV, and a current of 1.4 nA

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with an acquisition time of 45-60 seconds. In addition to these methods, the Prinz and Otto 1060

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samples were analyzed for total organic carbon (TOC) content.

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2.2.1 8M nitric acid extractions

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Acid extractions were conducted on all of the samples in duplicate. The 8M nitric acid

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extractions are useful, due to their harsh nature, in determining the maximum, potentially

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mobilized concentrations of contaminants within the sample. Details on the procedure used for

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the acid extraction can be found in section 3 of the ESI .

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2.1.2. 57Mössbauer spectroscopy

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Despite the Fe detected in the Otto 1060 acid extraction results (Table 2), SEM detection

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of pyrite in the sample, and Fe-oxide coatings on pyrite in the CO2-treated sample (Fig. 3), native

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Fe-containing phases or secondary minerals were not evident in QXRD (Table 3). This result

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suggests that Fe in the sample was either amorphous, existed in multiple Fe phases with

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individual quantities below QXRD detection, and/or was incorporated in carbonate- and

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phyllosilicate-minerals. To characterize the Fe mineralogy in detail, 57Fe-specific Mössbauer 7

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spectroscopy measurements were carried out on unreacted and reacted Otto 1060 ground

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samples. Mössbauer spectroscopy data of the samples was collected using a WissEl Elektronik

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(Germany) instrument that included a closed-cycle cryostat SHI-850 obtained from Janis

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Research Co., Inc. (Wilmington, MA), a Sumitomo CKW-21 He compressor unit, and an Ar-Kr

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proportional counter detector (LND, Inc. NY). A 57Co/Rh source (50-mCi to 75-mCi, initial

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strength) was used as the gamma energy source. The transmitted counts were stored in a

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multichannel scalar (MCS) as a function of energy (transducer velocity) using a 1024-channel

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analyzer. The raw data were folded to 512 channels to provide a flat background and a zero-

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velocity position corresponding to the center shift (CS) of a metal Fe foil at room temperature

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(RT). Calibration spectra were obtained with a 25-µm-thick Fe foil (Amersham, England) placed

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in the same position as the samples to minimize any geometry errors. The Mössbauer

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spectroscopy data was modeled with Recoil software (University of Ottawa, Canada) using a

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Voigt-based structural fitting routine (Rancourt and Ping, 1991). The sample preparation (type of

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the sample holder, etc.) was identical to the procedures reported in Peretyazhko et al. (2012).

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2.2 Batch experiments

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The core plugs were cut into ~0.3 cm thick slices (Fig. S1b). Each circular piece was then

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quartered. Two quarters (“coupons”) from the discs were used for each experiment and each

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duplicate, as well as a control experiment with the same pressure and temperature, but using N2

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instead of CO2. The N2 control was used to determine the effect of the pressure and temperature

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on elemental release for comparison with the CO2 experiments, where the presence of CO2 can

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also have an effect on changes in the aqueous concentration of elements. Prior to starting the

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Otto 1060, Otto 2010, and Devine experiments, two coupons (one slated for the CO2 experiment 8

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and one for the N2 control experiment) were inspected by SEM/EDS, and the same ones were re-

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inspected after the experiment. Humble coupons were inspected post-experiment. Prinz and Otto

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1084 experiments did not include coupons, only ground material, and were inspected by

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SEM/EDS post-experiment. Some Otto 1060, Otto 1084 and Prinz experiments included an

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As/Cd spiked experiment, and one Otto 1084 experiment included the As/Cd spike as well as an

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addition of organics (with an N2 control included as well). The As/Cd and organic spikes were

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included in these tests because of the potential for these additional contaminants to travel from

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the deep storage reservoir along with the CO2-laden brine upwards towards overlying

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groundwater aquifers (Cantrell and Brown, 2014; Carroll et al., 2014).

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Experiments was conducted in 25 mL Parr vessels for 7 days. For each experiment, two coupons were placed into the Parr vessel and 15 mL of 0.01 M NaCl was added to the vessel.

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The coupons were each approximately 0.8-0.9 g, resulting in a solid to solution ratio of

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approximately 1.7 g : 15 mL. The coupons were placed flat in the bottom of the reactor with one

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lying flat and the other slightly overlapping the first (Fig. S1c). The coupons and brine were

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allowed to equilibrate overnight and then a 2 mL “time 0” aqueous sample was taken from the

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reactor. During equilibration, the experiments do not contain gas; therefore, the CO2 experiments

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and the N2 controls should have similar aqueous chemistries in the “time 0” samples; differences

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in these samples indicate variability in the core plug material. The reactor was then heated using

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a rigid mantle heater (Parr Instrument Company, IL) with an Omega heat controller receiving

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feedback from a J-type thermocouple (Omega Engineering, Inc., CT). After heating, CO2 (or N2)

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was injected into each Parr vessel to reach the predetermined pressure. The pressure and

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temperature for each core plug varied, using USGS pressure gradient and temperature data as a

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guideline (Burke et al., 2011), based on the depth of the individual core plug (see Table 1 and

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Table S2). Due to equipment limitations, Humble experiments were conducted at 137.9 bar and

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61°C, which is lower than the calculated pressure and temperature shown in Table 1. One sample

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blank was also conducted for each set of experiments, using the same pressure and temperature,

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pressurized with CO2, but with no solid sample in the reactor, to reveal any potential reactor,

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synthetic groundwater, or gas related contamination in the system. After 7 days of reaction time,

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the liquid in the Parr vessel was collected and filtered using a 0.2 µm syringe filter. A subsample

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of this collected liquid was acidified with concentrated nitric acid to reach a final HNO3

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concentration of 1% in the subsample for ICP-OES and ICP-MS analysis and the remaining

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sample was used for ion chromatography (IC, Dionex DX-600) analysis.

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In addition to the core plug coupon experiments, ground sample experiments were conducted to test the effect of surface area. The remaining quartered sections of the core plugs

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were ground with an agate mortar and pestle until the sample passed through a No. 70 (ASTM

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type) sieve (<212 µm). The ground material was also used for 8M nitric acid extractions, XRD,

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and SEM/EDS.

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The ground experiments were conducted the same way as the coupon experiments, using 1.7 g of ground sample for each experiment. For the Devine experiments, 1.5 g was used for

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each experiment to better compare with the coupon experiments, which were lighter than the

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coupons from the other samples. The ground sample was weighed onto a weight boat, and then

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the 15 mL of brine was used to rinse the weigh boat into the pressure vessel. Following the 7 d

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reaction time, the solid material from the reactor was collected and the liquid was filtered (0.2

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µm). Double deionized (DDI) water was used to rinse the solids; the rinse water was also

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collected and filtered (0.2 µm). Liquid samples were analyzed by ICP-OES, ICP-MS, and IC.

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After drying, the solid was collected in glass vials and stored for further analysis.

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For the Otto 1060 contaminant spiked tests, coupon and ground experiments were conducted the same way, but the starting 0.01M NaCl solution contained 114 µg/L As and 40

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µg/L Cd (these experiments are referred to as Otto 1060B). These concentrations and

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contaminants were chosen because previous studies have shown the potential for these

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concentrations of As and Cd to reach groundwater aquifers in CO2 leakage scenarios (Carroll et

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al., 2014), when As and Cd are present in the CO2 storage reservoir brine. One CO2 and one N2

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experiment were conducted with the coupons and ground material. Prinz ground material was

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also used for experiments with and without the As/Cd spike (experiments with the spike are

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referred to as PrinzAsCd).

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The Otto 1084 material was used to conduct longer term tests; these experiments were conducted in 300 mL Parr pressure vessels equipped for in-situ sampling to allow for long term

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testing without disturbing the pressure system. Due to solid material limitations as well as the

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need for a higher volume of solution for sampling, the solid to solution ratio of these experiments

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was decreased to approximately 0.07g : 1 mL (compared to 0.13g : 1 mL in the 7 day tests). In

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addition to the CO2 and N2 long term tests, the experiments were repeated with an addition of

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As/ Cd or As/Cd with organics (31 µl of toluene was added to the 106.5 mL of solution,

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representing 80% of the solubility of toluene at the experimental temperature and pressure,

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immediately before sealing the reactor). Toluene was chosen based on the results of a modeling

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study conducted by Cantrell and Brown (2014) that showed lighter organics, such as toluene, in

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CO2 storage reservoirs can potentially reach groundwater aquifers when present in leaking CO2-

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laden brine plumes. These tests were conducted for 38 to 64 days, with 9 to 13 aqueous samples.

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A two-valve system was utilized to remove approximately 2 mL per sample without opening the

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reactor. No substantial decrease in pressure was observed as a result of sampling. The sample

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was then filtered (0.2 µm) and analyzed by ICP-OES and ICP-MS. After the initial 21 days of

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the As/Cd and As/Cd/organics experiments, the pressure in the reactors was reduced at regular

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intervals for the remaining 43 days to represent the decreasing pressure that would be

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experienced as the plume travelled from a deep storage reservoir towards the subsurface. The

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reactors started at 38.3 bar, and then were reduced to 20.7 bar, then 10.3 bar, remaining at each

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pressure for two weeks while being sampled weekly. Following the second 10.3 bar sampling

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period, the remaining pressure was released, and the reactors were sampled after one week at

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approximately 1.0 bar (atmospheric conditions). These experiments were included to represent

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the decreasing pressure that a plume would experience as it migrates upward.

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2.3 pH modeling

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Because pH could not be measured during the pressurized and temperature controlled batch experiments, The Geochemists Workbench was used to simulate the experiments and

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model the pH (Table 4) for several of the shorter term experiments (Otto 1060, Otto 2010,

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Devine, and Humble). First, the mass of gas in the pressure vessels was estimated based on the

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solution volume and gas density as a function of pressure (Table S2). Next, the moles of each

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mineral in the pressure vessel (Table S5) was estimated based on the total mass of the sample,

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the volume fraction of minerals in each sample (Table S3, based on QXRD results from Table 3),

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mineral densities and molecular weights from the THERMODDEM database (Table S4), and

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kinetic rate parameters from Palandri and Kharaka (2004). Finally, the surface area of the

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coupons was calculated based on the dimensions of the coupons (Table S6) and the surface area

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of the ground samples was estimated based on particle size (Table S7).

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3. RESULTS

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3.1 Pre-treatment solid phase characterization

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3.1.1 8M nitric acid extractions, and total organic carbon

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Acid extraction results showed that the Otto 1060 sample was clearly different from the other samples that were used in these studies, which was also evident in the QXRD and SEM/EDS

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analyses (Sections 3.1.2 and 3.3.1). Acid extraction results showed that potential contaminants

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such as As, Cr, and Pb were present in at least one sample (Table 2).The Prinz results showed

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similar concentrations to the Devine and Humble core plugs, with no obvious difference despite

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the unique appearance (Fig. S1) of this material. These analyses confirmed that there were many

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potential contaminants in the solid phase chemistry of the rocks tested in these experiments that

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could potentially be released during interactions with the CO2-brine plume..

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TOC analysis was conducted on the Prinz sample due to the obvious signs of organic matter on the sample. Results showed the Prinz material contained 39±1.4 mg/g organic carbon,

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compared to 7.2±0.8 mg/g in the Otto 1060 sample (used to represent the samples that did not

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have noticeable organic content).

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3.1.2 Quantitative XRD (QXRD)

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QXRD determined that six of the seven core plug samples were dominated by calcite (Table

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3). Devine, Humble, Otto 2010, and Prinz were 96-99% calcite, with small amounts of dolomite

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in Devine, Humble, and Prinz, and small amounts of quartz in Humble and Otto 2010. Otto 1084

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was dissimilar to the other samples, although still dominated by calcite; this sample also

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contained 12% ankerite, 8% quartz, and 3% kaolinite. Otto 1060 varied greatly from the other

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core plug samples, containing 46% quartz, 27% dolomite, 14% feldspar, and 12% kaolinite.

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Post-experiment XRD results fell within the expected variability range for all samples, indicating

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no discernable XRD-detected differences between the reacted and unreacted materials for all of

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the core plug samples.

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3.2 Batch experiments

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For each solid tested, experiments were conducted with solid and ground material, reacted with CO2 at field-relevant pressures and temperatures (see section 2.2). Each of these

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experiments was repeated using N2 to reach the target pressure instead of CO2. The N2 control

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experiments were included to determine the effects of the pressure and temperature to compare

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with the CO2 experiments.

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3.2.1. Changes in aqueous chemistry

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General changes in aqueous chemistry were similar to previously reported results from carbonate rock-CO2 experiments [see Qafoku et al. (2017) and references therein] and are briefly

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summarized here, with full data shown in Tables S9 and S10. As expected, the aqueous

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concentrations of several elements increased in all experiments, in both the CO2 experiments and

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in the N2 control experiments, with higher concentrations in the CO2 experiments compared to

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their N2 control counterparts. Some elements had slightly higher concentrations in the CO2

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experiments compared to their N2 control counterparts for most or all of the solids tested, such as

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Ba, S, and K. Other elements had more pronounced differences when comparing the CO2

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experiments with the N2 control, including Ca and Mg, with higher concentrations in the CO2

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experiments.

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Several of the aqueous concentrations did not follow an increasing pattern; in all of the

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experiments, Na remained relatively stable while the Zn concentrations decreased following the

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equilibration period (likely indicating contamination from the metal pressure vessel), and Mo 14

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decreased in the Otto 1060, Devine, and Humble CO2 experiments (and remained stable in the

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Otto 2010 CO2 experiment) and increased in the corresponding N2 control experiments (Fig. 1).

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The decreasing (or stable) Mo in the CO2 experiments and increasing Mo in the N2 controls was

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likely pH controlled. In the CO2 experiments, Mo was likely adsorbing to Fe oxides and/or

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sulfides due to the lowered pH of the solution; Mo adsorption decreases with increasing pH,

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explaining the increase of Mo in the N2 experiments (Goldberg et al., 1996; Smedley et al.,

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2014). The modeled pH of the CO2 experiments was 2.6-3.5 whereas the N2 experiments had a

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modeled pH of 7.0-9.4 (Table 4).

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3.2.2. Size fraction effect

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Humble experiments were unique in that the size fraction (as opposed to the gas present)

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played a larger role in the concentration of many of the elements. This was evident by the greater

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concentration of elements, including S, Mg, K, Si, and Mo in the CO2 and N2 ground

322

experiments than the CO2 coupon experiment (Fig. 1, Table S10, S11). The effect of the size

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fraction could be due to the relatively higher pressure in the Humble experiments. Solubility of

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calcite increases with increasing pressure at pressures below 6 kbar (Fein and Walther, 1987),

325

pointing to pressure being the driving factor for increased release in the ground N2 experiments.

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However, Wunsch et al. (2014) found that calcite dissolution rates increased with increased

327

pCO2; while the increase in pressure may have increased the dissolution in both the CO2 and N2

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ground experiments, the CO2 ground experiments still released greater concentrations of

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potential contaminants than the N2 ground experiments due to a higher pCO2, in agreement with

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the conclusions from Wunsch et al. (2014).

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In the Otto 2010 experiments, the coupon experiment resulted in higher concentrations of several elements compared to the ground experiment. The coupon experiments had lower

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concentrations of Ba, Sr, and Mg compared to the ground experiments, but higher concentrations

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of Ca, S, K, Cu, and Cr (Fig. 1, Table S10, S11). The opposite was expected, with higher

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dissolution expected in the ground sample due to the greater surface area (surface area was

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estimated at 4.35 cm2/g for the coupon experiments compared to 600 cm2/g in the ground

337

experiments; Table 4); however, the sorption capacity of the material is also increased with

338

increased surface area, and potentially resulted in increased aqueous elemental removal for this

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solid sample. In the SEM micrographs of these two samples, there appears to be a coating over

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much of the ground sample, whereas the reacted coupon sample micrographs show less frequent

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precipitated formations (see section 3.3.2 and Fig. S3). Further investigation is needed to

342

determine the source of these elements in the Otto 2010 experiments. This was unique to the

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Otto 2010 solid sample; the other solids all demonstrated greater concentrations of aqueous

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elements in the smaller size fraction experiments compared to their coupon counterparts.

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In the other core plug experiments, the concentration of most elements in the CO2

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experiments, regardless of the size fraction, were greater than either of the N2 experiments. This

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greater role of the size fraction could be a sample specific characteristic or due to the increased

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pressure used in the Humble experiments compared to the other four core plug experiments, and

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sorption capacity differences may explain the differences seen in the Otto 2010 experiments.

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3.2.3. Solid phase chemistry

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Despite having a significantly lower carbonate content (27% carbonates versus 77-100% in the other core plug samples), the modeled pH of the Otto 1060 experiments was similar to the

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modeled pH of the other core plugs (pH values ranged from 2.6 to 3.5, Table 4) indicating that

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the pH-induced extent of mineral dissolution was similar in the samples from different depths.

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However, compared to the other samples, Otto 1060 released higher concentrations of K, Ba,

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Mn, Cs, B and S. Otto 1060 also released higher concentrations of Mg, likely from the

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dissolution of dolomite in the sample, and released the lowest amount of Ca (Fig. 1). The

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chemical analysis of the solid phase of the Otto 1060 sample showed that this sample contained

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greater amounts of these elements (i.e., K, Ba, Mn, Cs, B, S, and Mg), as determined by 8M

360

nitric acid extractions, suggesting the solid phase chemistry of the rock was a dominant factor in

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elemental release for this core plug. In addition to the solid phase chemistry of this core plug, the

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mineral phases and microcracks within the solid could have contributed to these results.

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In the Otto 1060B experiments, an As and Cd spike (114 and 40 µg/L, respectively) was

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added to the 0.01M NaCl solution. Following the overnight equilibration period, approximately

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14 µg/L Cd remained in solution in the coupon CO2 and N2 experiments. Cd was decreased

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overnight to just slightly over the detection limit in the ground CO2 experiment and not detected

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in the ground N2 experiment (detection limit = 0.7 µg/L; units are in µg/L instead of µg/L for As

370

and Cd to compare to the original spike concentrations but conversion to µg/g can be found in

371

Table 5). After 7 days, 6-7 µg/L Cd was detected in the liquid phase of both CO2 experiments, 4

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µg/L remained in the N2 coupon experiment, and Cd was not detected in the ground N2

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experiment. After the equilibration period, approximately 80 µg/L As remained in the liquid

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phase in the coupon experiments and 35 µg/L remained in the ground experiments. After 7 days,

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As was below detection limits in all four tests (detection limit = 4.2 µg/L) (Table 5). Results for

376

other elements are comparable to the Otto 1060 experiments conducted without the As and Cd

377

spike (Table S11). The Otto 1060B As/Cd spiked experiments have important implications for

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future modeling efforts of contaminants from reservoir brine that may travel upward with CO2;

379

the results indicated that much of the As and Cd would not reach the aquifer due to the

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adsorption capacity of the rock material contacted by the CO2-laden brine plume during upward

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migration.

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In the PrinzAsCd experiments, the As remained relatively high after the overnight

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equilibration period, decreasing from 114 µg/L in the starting solution to an average of 108 µg/L

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before the start of the experiment. After 7 days, the As concentration remained at 83 µg/L in the

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CO2 experiment and higher at 94.8 µg/L in the N2 control experiment. The Cd concentrations

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decreased from a 40 µg/L starting concentration to approximately 3 µg/L after the equilibration

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period in the CO2 experiment (and below detection limits for the N2 equilibration period sample).

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After 7 days, the Cd was still not detectable in the N2 liquid sample (detection limit = 0.7 µg/L),

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and had decreased to 0.78 µg/L in one duplicate in the CO2 experiment (and was below detection

390

limits in the other duplicate) (Table 5).

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Compared to the Otto 1060B experiments conducted with the As/Cd spike, the remaining As

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in the liquid phase of the Prinz experiments was substantially higher, whereas the Cd in the final

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PrinzAsCd experiments samples were lower than those in the Otto 1060B samples. The Prinz

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material clearly has a lower adsorption capacity for As and a slightly higher capacity for Cd. The

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higher TOC content of the Prinz material compared to the Otto 1060 material (see section 2.1)

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implies the Prinz material has a higher natural organic matter (NOM) content, which is a likely

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candidate for the reduced As adsorption as NOM decreases As adsorption due to competition for

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adsorption sites on metal oxides between the As and NOM (Bauer and Blodau, 2006; Redman et

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al., 2002). While the anionic As and NOM compete for sites, this does not affect the adsorption

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of the positively charged Cd.

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3.2.5. Long term experiment results

When experiments were conducted for longer periods of time (i.e, up to 64 days), less

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pronounced differences between the CO2 and N2 experiments were noticeable by the end of the

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experiments (Fig. 2, Table S12); see Table 1 for experiment variables.

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Across the experiments, several elements had a continuous increase/decrease pattern, including Ca, Ba, Sr, and Si. The increase and decrease of these elements suggests a

412

precipitation/dissolution cycle. By the end of the 21 to 38 days for experiments without added

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organics, the N2 experiment concentrations were the same (e.g., Ni, Mn, Mo, Ba) or higher (Cs,

414

Si, Mg) than those in the CO2 experiments. Several elements were approximately the same

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concentration in the N2 and CO2 experiments for the duration of the tests, including Cr, Na, and

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K (Fig. 2, Table S12).

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Comparing the tests with and without organics, S, Sr, K, Si, Ba, Ca, and Mo were lower in the CO2 and N2 organic tests than in the non-organic tests. The Mg was also low in the CO2

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organic test compared to others. The CO2 AsCd+ O test had higher levels of Mn, Ni, Cr, and Fe

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and significantly lower concentrations of S, Na, and Mg compared to the CO2 AsCd and N2

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AsCd+O tests. In the tests that included the As/Cd spike, the Cd was quickly fell below or

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remained near the detection limit (0.7-1.4 µg/L) in all of the tests. The As concentrations in all

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three tests decreased below detection limits (4.2-8.5 µg/L) within the first 9 days of the

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experiment, but the tests with organics had As levels below detection slightly faster (within 6

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days).

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After 28 days, three of the tests (CO2 AsCd, CO2 AsCd+O, and N2 AsCd+O) continued on

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with gradually decreasing pressure. The tests were reduced from 39.2 bar to 20.7 bar for 14 days,

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then 10.3 bar for 14 days, and then 1 bar (atmospheric pressure) for 7 days. With the decrease in

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pressure, the solubility of the CO2 and N2 in water was also expected to decrease (Dodds et al.,

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1956; Krichevsky and Kasarnovsky, 1935). The decrease in CO2 and N2 made the tests

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increasingly similar in dissolved gas composition; this was reflected in the way the tests had

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increasingly similar aqueous element concentrations as the pressure was decreased for elements

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including Ba, Cd, Cr, Fe, and Mn (Fig. 2, Table S12). After remaining at atmospheric pressure

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for 7 days, many of the elements were below detection limits in all three tests (e.g., As, Ca, Cd,

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Cr, Cu, Fe, Mg, Mn, Mo, Ni, K, Si, Na, Sr, S, and Zn). This removal of elements from the liquid

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phase after depressurization is in agreement with findings from a study by Wunsch et al. (2013)

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in which elemental concentrations decreased immediately after depressurization at the end of

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batch experiments involving CO2 exposed dolomite samples. The reduction of pressure in three

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of the long term experiments is especially important to consider; as a CO2 plume migrates

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upward, it will experience a decrease in pressure. The results from these tests indicate that CO2

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will have a decreasing effect on elemental release as the pressure is decreased.

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3.3 Post-treatment solid phase characterization

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3.3.1 SEM/EDS

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Otto Samples (1060, 2010, 1084): SEM and EDS of the unreacted Otto 1060 coupon found

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mainly SiO2 in the sample, but also K, Ca, Fe, and S. The Fe and S were found together, in a 1:2

446

atomic weight % ratio, indicative of pyrite (Figs. 3, S2).

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After the Otto 1060 coupon was reacted with CO2 gas, SEM/EDS inspections revealed a

448

carbonate coating on the top of the coupon, and many of the dips and holes seen prior to the

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experiment were no longer visible (Fig. S2). Additionally, many dissolution pits were visible

450

after being reacted with CO2; in contrast, the N2 reacted coupon had fewer dissolution pits (Fig.

451

S2). On one of the Otto 1060 coupons, an Fe –rich phase with a morphology similar to hematite-

452

like growths (Qafoku et al., 2012) was found on top of a pyrite grain (Fig. 3). To further

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investigate the changes in the iron mineralogy of this sample, Mössbauer spectroscopy data was

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collected (section 3.3.2).

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For Otto 2010, changes in the coupon samples before and after CO2 exposure are hard to

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discern. Otto 2010 SEM micrographs depicted small precipitates coating areas on the ground

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CO2-reacted sample whereas precipitates were not widespread on the reacted coupon experiment

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samples (Fig. S3), which may indicate that the powdered experiments released enough elements

459

to supersaturate the liquid, resulting in a precipitation of solids. The higher concentrations of

460

elements in the coupon experiments compared to the ground experiments (Section 3.2.2) could

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be explained by the removal of elements by precipitation in the ground experiments.

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462

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S7. Devine: In general, the Devine SEM micrographs show minimal changes before and after the

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SEM results for Otto 1084 are similar to the other two Otto samples and can be seen in Fig.

465

experiments (Fig. S4). There may be a slight increase in dissolution features, but other changes

466

could not be discerned.

Humble: Humble coupons exhibited a noticeable change in color and features on the CO2

468

reacted coupon, but not the N2 reacted coupon (Fig. 4, Fig. S5). The SEM combined with EDS

469

examination of these coupons revealed various minerals on the surface of the CO2 reacted

470

coupon (i.e., calcite and silicate) that were not visible on the N2 reacted coupon (Fig. 4).

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Prinz: In the Prinz SEM micrographs, most of the particles appear to have a smooth coating covering them (Fig. S6). An iron rich phase was found in the N2 reacted solids, but was not

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prevalent in this or the other solids. QXRD did not identify any iron-rich minerals in this sample,

474

indicating that this mineral composes of <1% (QXRD detection limit) of the solid.

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3.3.2 Mössbauer spectroscopy

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Mössbauer spectroscopy measurements were carried out to determine the Fe minerology of the Otto 1060 core plug, before and after being in contact with CO2-acidified NaCl solution.

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The Otto 1060 solid material was chosen for this analysis because it contained the highest

479

amount of Fe, determined by 8M nitric acid extractions (16.3 mg/g, Table 2). Untreated sample:

480

The Fe-mineral suite in the untreated sample was comprised of ankerite

481

[CaFe(II)0.6Mg0.2Mn0.1(CO3)2 – high spin Fe], pyrite [Fe(II)S2 – low-spin Fe], kaolinite Fe(III),

482

and a ferrihydrite-like mineral (i.e., the results most likely indicate ferrihydrite, but were not

483

definitive) (Figure 5, Figure S6, and Table S13). Identification of ankerite was based on derived

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RT Mössbauer parameters of the most intense doublet in the spectrum, indicated by the red trace

485

in Figure 5a (Milodowski et al., 1989) and similarity of its 12 K spectral features to a 10 K

486

natural ankerite sample spectrum, Figure S6 (De Grave, 1986). Furthermore, absence of an Fe(II)

487

magnetic octet feature in the 12 K spectrum indicated that the sample was free of siderite, FeCO3

488

(Peretyazhko et al., 2012); resolution of siderite and ankerite peaks from each other above a 35 K

489

spectrum is rather difficult. Similar to ankerite, the presence of pyrite is unambiguous and its

490

identification was based on the RT Mössbauer spectral parameters of the second most intense

491

doublet (blue trace in Figure 5a) (Shao et al., 2014). The nature of the other doublet in the

492

spectrum is not obvious, but its RT parameters are somewhat similar to octahedral Fe(III) in

493

kaolinite (Komusinski et al., 1981). Finally, assignment of the rest of the Fe (minor doublet;

494

brown trace in Fig. 5) in the sample to a ferrihydrite-like mineral (with some coatings) was in

495

accordance with transition of the RT doublet to a sextet feature below 77 K (Fig. S6). This oxide

496

was definitely not small-particle goethite, magnetite, nor hematite; these oxides display well-

497

defined sextets at 77 K.

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N2-treated sample: The spectra of the N2-treated sample are virtually similar to the untreated sample, based on qualitative comparison of RT and 12 K spectra (Figures 5b and 6.

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This similarity implied that the reaction conditions are not severe enough to induce

501

dissolution/oxidation of Fe phases in the sample, despite presence of oxidant (O2).

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CO2-treated sample: In addition to the native Fe minerals (i.e., ankerite, pyrite,

503

ferrihydrite-like oxide, and Fe(III)-kaolinite), hematite was present in the CO2-treated sample.

504

Assignment of hematite was based on RT derived Mössbauer spectral parameters of the well-

505

defined sextet (Figure 5c, Table S13). Importantly, despite one-tenth of the total Fe as hematite,

506

the Fe mineral composition of the untreated and CO2-treated sample is similar (Table S13). This 23

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discrepancy suggests that the Fe mineral composition of the untreated and CO2-treated samples

508

was not uniform. From Mössbauer data it was not possible to identify the source of hematite Fe

509

(i.e., pyrite, ankerite or both). SEM detection of Fe-oxide coatings on pyrite, the presence of

510

sulfate in the solution (likely due to oxidation of pyrite sulfide), and formation of Fe(III)-oxides

511

in dolostone rock samples (Lu et al., 2016), however, suggest that both pyrite and ankerite were

512

sources of Fe. Non-precipitation of hematite in the N2-treated sample (Fig. S6b) despite similar

513

amounts O2 in the head-space under similar reaction conditions, however, implies that the source

514

of Fe for hematite is most probably Fe(II) from dissolution of ankerite; the CO2/O2 solution was

515

likely more acidic than N2/O2 reactant (Shao et al., 2014). Finally, the samples were free of

516

siderite, implying that CO3- (from dissolution of ankerite and amended CO2), based on below 12

517

K spectra (Fig. S6), was not participating in secondary mineral formations. Finally, it appears

518

from comparison of 12 K spectra (Fig. S6b) that the ferrihydrite-like mineral was stable under

519

the experimental conditions.

520

5. DISCUSSION

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Several factors were likely controlling elemental release into the liquid phase of these

522

experiments. For example, the relatively higher concentration of Mg, K, Ba, and S in the Otto

523

1060 experiments (compared to the other core plug experiments) was likely controlled by the

524

chemistry of the solid sample, as determined by 8M nitric acid extractions. Particle size and

525

increased pressure played a larger role in the Humble experiments, and the physical properties,

526

such as surface area, in Otto 2010 coupon experiments may have been the controlling factors for

527

increased release of several elements compared to the ground experiments from the same

528

material. Despite the underlying factors, each of these (i.e., chemical, physical, and

529

mineralogical) properties, as well as the depth-dependent calculated pressure and temperature for

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each experiment are, in some extent, related to the depth of the samples. The inclusion of toluene

531

in one Otto 1084 set of experiments demonstrated the importance of including possible reservoir

532

contaminants when evaluating risk; the organic tests showed lower release of elements such as Sr

533

and Ba and a slightly faster removal of As and Cd. The decrease of As and Cd from the liquid

534

phase in the Otto 1060B experiments also showed rapid removal of these contaminants from the

535

liquid phase, but removal in the Prinz experiments was significantly lower. This could be due to

536

the presence of kaolinite in the Otto 1060B sample and the adsorption properties of the clay

537

mineral, or due to the decrease in adsorption of As due to competition with NOM for adsorption

538

sites (Redman et al., 2002) in the Prinz sample, or a combination of the two. This is another

539

indication of the importance of considering the interactions occurring along the CO2 migration

540

pathway with variable rock types when conducting risk assessments. In the final long term

541

experiments, when pressure was reduced to atmospheric pressure, most elemental concentrations

542

decreased to below detection limits.

543

6. CONCLUSION

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The results from this study give strong evidence for the need to incorporate the changing

545

pressure and temperature of the CO2 plume into future laboratory and modeling studies. Through

546

these experiments, it is evident that the interactions between the CO2-rich brine and the rock

547

between the deep subsurface storage reservoir and the overlying groundwater aquifer have the

548

potential to affect the level of risk to groundwater resources overlying the CO2 sequestration site,

549

and should be further investigated for consideration during CCS site selection and risk

550

evaluation.

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552 553

ACKNOWLEDGEMENTS Funding for this research was provided by the National Risk Assessment Partnership (NRAP) in the U.S. DOE Office of Fossil Energy under DOE contract number DE AC05

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76RL01830. XRD, SEM/EDS, and Mössbauer spectroscopy were performed in the

556

Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility

557

sponsored by the Department of Energy’s Office of Biological and Environmental Research and

558

located at PNNL. PNNL is operated by Battelle for the U.S. DOE under Contract DE-AC06-

559

76RLO 1830. The authors would like to thank those who analyzed samples, reviewed data, and

560

helped with experimental equipment, including Keith Geiszler, Steven Baum, Ian Leavy, Megan

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Nims, Michelle Valenta Snyder, Ben Williams, and Guohui Wang. Core plugs were received

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from the Austin Core Research Center.

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Bacon, D. H., Qafoku, N. P., Dai, Z., Keating, E. H., and Brown, C. F. (2016). Modeling the impact of carbon dioxide leakage into an unconfined, oxidizing carbonate aquifer. International Journal of Greenhouse Gas Control 44, 290-299. Bauer, M., and Blodau, C. (2006). Mobilization of arsenic by dissolved organic matter from iron oxides, soils and sediments. Science of the Total Environment 354, 179-190. Burke, L. A., Kinney, S. A., and Kola-Kehinde, T. B. (2011). "Digital archive of drilling mud weight pressures and wellbore temperatures from 49 regional cross sections of 967 well logs in Louisiana and Texas, onshore Gulf of Mexico basin," Rep. No. 2331-1258. US Geological Survey. Cantrell, K. J., and Brown, C. F. (2014). Source term modeling for evaluating the potential impacts to groundwater of fluids escaping from a depleted oil reservoir used for carbon sequestration. International Journal of Greenhouse Gas Control 27, 139-145. Carroll, S. A., Keating, E., Mansoor, K., Dai, Z., Sun, Y., Trainor-Guitton, W., Brown, C., and Bacon, D. (2014). Key factors for determining groundwater impacts due to leakage from geologic carbon sequestration reservoirs. International Journal of Greenhouse Gas Control 29, 153-168. Dai, Z., Keating, E., Bacon, D., Viswanathan, H., Stauffer, P., Jordan, A., and Pawar, R. (2014). Probabilistic evaluation of shallow groundwater resources at a hypothetical carbon sequestration site. Scientific reports 4. De Grave, E. (1986). 57 Fe Mössbauer effect in ankerite: Study of the electronic relaxation. Solid state communications 60, 541-544. Dodds, W., Stutzman, L., and Sollami, B. (1956). Carbon dioxide solubility in water. Industrial & Engineering Chemistry Chemical & Engineering Data Series 1, 92-95. Fein, J. B., and Walther, J. V. (1987). Calcite solubility in supercritical CO2 H2O fluids. Geochimica et Cosmochimica Acta 51, 1665-1673. Garcia-Rios, M., Cama, J., Luquot, L., and Soler, J. M. (2014). Interaction between CO2-rich sulfate solutions and carbonate reservoir rocks from atmospheric to supercritical CO2 conditions: Experiments and modeling. Chemical Geology 383, 107-122. Goldberg, S., Forster, H. S., and Godfrey, C. L. (1996). Molybdenum adsorption on oxides, clay minerals, and soils. Soil Science Society of America Journal 60, 425-432. Harvey, O. R., Qafoku, N. P., Cantrell, K. J., Lee, G., Amonette, G. E., and Brown, C. F. (2013). Geochemical Implications of Gas Leakage associated with Geologic CO2 Storage-A Qualitative Review. Environ. Sci. Technol. 47, 23-26. Horner, K. N., Schacht, U., and Haese, R. R. (2015). Characterizing long-term CO2-water-rock reaction pathways to identify tracers of CO2 migration during geological storage in a low-salinity, siliciclastic reservoir system. Chemical Geology 399, 123-133. Humez, P., Lagneau, V., Lions, J., and Negrel, P. (2013). Assessing the potential consequences of CO2 leakage to freshwater resources: A batch-reaction experiment towards an isotopic tracing tool. Applied Geochemistry 30, 178-190. Icenhower, J., Saldi, G., Daval, D., and Knauss, K. (2015). Experimental determination of the reactivity of the Frio Sandstone, Texas, and the fate of heavy metals resulting from carbon dioxide sequestration. Environmental Earth Sciences 74, 5501-5516. Jarboe, P. J., Candela, P. A., Zhu, W. L., and Kaufman, A. J. (2015). Extraction of Hydrocarbons from HighMaturity Marcellus Shale Using Supercritical Carbon Dioxide. Energy & Fuels 29, 7897-7909. Jung, H. B., Um, W. Y., and Cantrell, K. J. (2013). Effect of oxygen co-injected with carbon dioxide on Gothic shale caprock-CO2-brine interaction during geologic carbon sequestration. Chemical Geology 354, 1-14.

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Karamalidis, A. K., Torres, S. G., Hakala, J. A., Shao, H., Cantrell, K. J., and Carroll, S. (2013). Trace Metal Source Terms in Carbon Sequestration Environments. Environmental Science & Technology 47, 322-329. Keating, H. E., Fessenden, J., Kanjorski, N., Koning, D. J., and Pawar, R. (2010). The impact of CO2 on shallow groundwater chemistry: observations at a natural analog site and implications for carbon sequestration. Environmental Earth Science 60, 521-536. Kolak, J. J., and Burruss, R. C. (2006). Geochemical investigation of the potential for mobilizing nonmethane hydrocarbons during carbon dioxide storage in deep coal beds. Energy & Fuels 20, 566574. Kolak, J. J., and Burruss, R. C. (2014). The use of solvent extractions and solubility theory to discern hydrocarbon associations in coal, with application to the coal-supercritical CO2 system. Organic Geochemistry 73, 56-69. Kolak, J. J., Hackley, P. C., Ruppert, L. F., Warwick, P. D., and Burruss, R. C. (2015). Using Ground and Intact Coal Samples To Evaluate Hydrocarbon Fate during Supercritical CO2 Injection into Coal Beds: Effects of Particle Size and Coal Moisture. Energy & Fuels 29, 5187-5203. Krichevsky, I., and Kasarnovsky, J. (1935). Thermodynamical calculations of solubilities of nitrogen and hydrogen in water at high pressures. Journal of the American Chemical Society 57, 2168-2171. Lawter, A., Qafoku, N., Shao, H., Bacon, D., and Brown, C. (2015). Evaluating Impacts of CO2 and CH4 Gas Intrusion into an Unconsolidated Aquifer: Fate of As and Cd. Frontiers in Environmental Science 3. Lawter, A., Qafoku, N. P., Wang, G., Shao, H., and Brown, C. F. (2016). Evaluating impacts of CO 2 intrusion into an unconsolidated aquifer: I. Experimental data. International Journal of Greenhouse Gas Control 44, 323-333. Little, M. G., and Jackson, R. B. (2010). Potential Impacts of Leakage from Deep CO2 Geosequestration on Overlying Freshwater Aquifers. Environ. Sci. Technol. 44, 9225-9232. Lu, J., Mickler, P. J., Nicot, J.-P., Yang, C., and Darvari, R. (2016). Geochemical impact of O2 impurity in CO2 stream on carbonate carbon-storage reservoirs. International Journal of Greenhouse Gas Control 47, 159-175. Lu, J., Partin, J. W., Hovorka, S. D., and Wong, C. (2010). Potential risks to freshwater resources as a result of leakage from CO2 geological storage: a batch-reaction experiment. Environ. Earth Sci. 60, 335-348. Palandri, J. L., and Kharaka, Y. K. (2004). "A compilation of rate parameters of water-mineral interaction kinetics for application to geochemical modeling." GEOLOGICAL SURVEY MENLO PARK CA. Peretyazhko, T., Zachara, J. M., Kukkadapu, R. K., Heald, S. M., Kutnyakov, I. V., Resch, C. T., Arey, B. W., Wang, C. M., Kovarik, L., and Phillips, J. L. (2012). Pertechnetate (TcO 4−) reducKon by reacKve ferrous iron forms in naturally anoxic, redox transition zone sediments from the Hanford Site, USA. Geochimica et Cosmochimica Acta 92, 48-66. Qafoku, N., Zheng, L., Bacon, D., Lawter, A., and Brown, C. (2015). "A Critical Review of the Impacts of Leaking CO2 Gas and Brine on Groundwater Quality. PNNL-24897." PNNL. Qafoku, N. P., Lawter, A. R., Bacon, D. H., Zheng, L., Kyle, J., and Brown, C. F. (2017). Review of the impacts of leaking CO 2 gas and brine on groundwater quality. Earth-Science Reviews. Qafoku, O., Kovarik, L., Kukkadapu, R. K., Ilton, E. S., Arey, B. W., Tucek, J., and Felmy, A. R. (2012). Fayalite dissolution and siderite formation in water-saturated supercritical CO2. Chemical Geology 332–333, 124-135. Rancourt, D., and Ping, J. (1991). Voigt-based methods for arbitrary-shape static hyperfine parameter distributions in Mössbauer spectroscopy. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 58, 85-97.

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611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657

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Redman, A. D., Macalady, D. L., and Ahmann, D. (2002). Natural organic matter affects arsenic speciation and sorption onto hematite. Environmental Science & Technology 36, 2889-2896. Scherf, A. K., Zetzl, C., Smirnova, I., Zettlitzer, M., Vieth-Hillebrand, A., and Grp, C. s. (2011). Mobilisation of organic compounds from reservoir rocks through the injection of CO2 - Comparison of baseline characterization and laboratory experiments. Energy Procedia 4, 4524-4531. Shao, H., Kukkadapu, R. K., Krogstad, E. J., Newburn, M. K., and Cantrell, K. J. (2014). Mobilization of metals from Eau Claire siltstone and the impact of oxygen under geological carbon dioxide sequestration conditions. Geochimica Et Cosmochimica Acta 141, 62-82. Shao, H., Qafoku, N. P., Lawter, A. R., Bowden, M. E., and Brown, C. F. (2015). Coupled Geochemical Impacts of Leaking CO2 and Contaminants from Subsurface Storage Reservoirs on Groundwater Quality. Environmental science & technology 49, 8202-8209. Smedley, P. L., Cooper, D. M., Ander, E. L., Milne, C. J., and Lapworth, D. J. (2014). Occurrence of molybdenum in British surface water and groundwater: Distributions, controls and implications for water supply. Applied Geochemistry 40, 144-154. Vong, C. Q., Jacquement, N., Picot-Colbeaux, G., Lions, J., Rohmer, J., and Bouc, O. (2011). Reactive transport modeling for impact assessment of a CO2 intrusion on trace elements mobility within fresh groundwater and its natural attenuation for potential remediation. Energy Procedia 4, 3171-3178. Wang, G., Qafoku, N. P., Lawter, A. R., Bowden, M., Harvey, O., Sullivan, C., and Brown, C. F. (2016). Geochemical impacts of leaking CO2 from subsurface storage reservoirs to an unconfined oxidizing carbonate aquifer. International Journal of Greenhouse Gas Control 44, 310-322. Wunsch, A., Navarre-Sitchler, A. K., Moore, J., and McCray, J. E. (2014). Metal release from limestones at high partial-pressures of CO2. Chemical Geology 363, 40-55. Wunsch, A., Navarre-Sitchler, A. K., Moore, J., Ricko, A., and McCray, J. E. (2013). Metal release from dolomites at high partial-pressures of CO 2. Applied Geochemistry 38, 33-47. Xiao, T., Dai, Z., Viswanathan, H., Hakala, A., Cather, M., Jia, W., Zhang, Y., and McPherson, B. (2017). Arsenic mobilization in shallow aquifers due to CO2 and brine intrusion from storage reservoirs. Scientific Reports 7. Xiao, T., McPherson, B., Pan, F., Esser, R., Jia, W., Bordelon, A., and Bacon, D. (2016). Potential chemical impacts of CO 2 leakage on underground source of drinking water assessed by quantitative risk analysis. International Journal of Greenhouse Gas Control 50, 305-316. Yang, C., Dai, Z., Romanak, K. D., Hovorka, S. D., and Trevino, R. H. (2014). Inverse Modeling of WaterRock-CO2 Batch Experiments: Potential Impacts on Groundwater Resources at Carbon Sequestration Sites. Environ Sci Technol 48, 2798-2806. Zhong, L., Cantrell, K. J., Bacon, D. H., and Shewell, J. (2014a). Transport of organic contaminants mobilized from coal through sandstone overlying a geological carbon sequestration reservoir. International Journal of Greenhouse Gas Control 21, 158-164. Zhong, L. R., Cantrell, K., Mitroshkov, A., and Shewell, J. (2014b). Mobilization and transport of organic compounds from reservoir rock and caprock in geological carbon sequestration sites. Environmental Earth Sciences 71, 4261-4272.

698

FIGURE CAPTIONS

699 700

Fig. 1: ICP analysis results for Ca (a), Mg (b), Sr (c), and Mo (d) in µg/g or mg/g from the Otto 1060, Otto 2010, Devine, and Humble coupon (C) and ground (G) experiments.

AC C

EP

TE D

M AN U

SC

RI PT

658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697

29

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Fig. 2: Changes in Sr (a) and Ba (b) concentrations over time in the longer term Otto 1084 experiments, reacted with CO2 or N2, with and without As/Cd or As/Cd and organic spikes; changes in Ca (c) and Mg (d) concentrations over time in the Otto 1084 longer term experiments with As/Cd or As/Cd and organic spikes, with pressure reduction time indicated with vertical lines.

706 707 708

Fig. 3: SEM micrographs and EDS analysis of Otto 1060 coupon samples. Left: Unreacted Otto 1060 coupon with pyrite (Fe and S in a 1:2 atomic weight ratio; circled in red); Center (red circle) and right: Possible hematite growth on pyrite on the Otto 1060 CO2-reacted coupon.

709

Fig. 4: SEM micrographs for Humble coupons. Left and center: CO2-reacted, right: N2-reacted.

710 711 712 713 714 715

Fig. 5: Modeled RT Mössbauer spectra of untreated (a), N2-treated (b) and CO2-treated (c) Otto 1060 samples showing ankerite, pyrite, Fe(III)-kaolinite, and ferrihydrite-like phases, at two different velocity ranges – a,i), a,ii), b, c,i), and c,ii) in -5 to + 5 mm/sec range (to clearly show various doublets), and the rest in in -12 to +12 mm/sec range (to clearly show hematite sextet peaks). Relative % of individual phases and derived RT Mössbauer spectral parameters are shown in Table S13 in the ESI.

M AN U

SC

RI PT

701 702 703 704 705

716

EP

720

AC C

719

TE D

717 718

30

ACCEPTED MANUSCRIPT

TABLES

723 724 725

Table 1: Details for each core plug used in these experiments. Using the sample depth, the pressure and temperature was found in Burke et al. (2011); the pressure was then converted from ppg to psi and bar. . All of the core plugs came from Texas; the county is listed in the table. An explanation of how these pressures and temperatures were calculated can be found in the ESI.

SC

RI PT

721 722

M AN U

726

727

Core plug

Location (County)

Depth (ft)

Pressure (ppg)

(P=0.052*ppg*depth)

[bar] = [psi] / 14.50

Temp (°C)

Otto 1060

Medina

1060

10.1

556.7

38.4

50.6

Ground (G) and/or Coupon (C) G, C

Otto 1084

Medina

1084

10.1

569.3

39.3

50.6

G

Prinz Otto 2010 Devine Humble

Bexar Medina Medina Atascosa

1497 2010 3102 9754

10.1 9.8 9.9 12.4*

786.2 1024.3 1596.9 6289.4*

54.2 70.6 108.2 433.6*

50.6 47.8 60.6 102.2*

G G, C G, C G, C

Pressure (bar)

TE D

Pressure (psi)

733 734

AC C

732

EP

*Due to equipment limitations, the Humble experiments were conducted at 137.9 bar and 60°C

735 736 31

Spikes Added 728 As/Cd 729 As/Cd, Organics As/Cd 730 None None 731 None

ACCEPTED MANUSCRIPT

Otto 2010

Prinz

Devine

Humble

St Dev

Ave.

St Dev

Ave.

St Dev

Ave.

St Dev

Ave.

St Dev

672.5 3.2 32.7 32.8 0.2 0.3 14.5 14.6 7.3 2.6 7.2 221.5 0.4 6.6 449.0 345.5 795.5 541.5 5.1 14.2 5.2

40.3 0.1 0.1 1.2 0.0 0.1 0.1 0.4 0.1 0.4 7.8 0.0 0.4 21.2 16.3 24.7 10.6 0.5 0.6 0.3

38.4 ND 0.4 41.3 ND ND 0.3 3.1 0.01 0.1 1.0 9.9 0.1 1.7 70.8 ND 64.3 76.6 1.7 ND ND

9.1 0.0 6.2 0.1 0.3 0.0 0.0 0.1 1.3 0.0 0.3 7.4 41.6 10.3 0.3 -

65.6 ND 0.8 37.9 ND ND 0.5 13.6 0.04 0.5 3.8 54.6 0.4 2.3 65.4 ND 55.2 87.2 1.8 1.8 ND

2.5 0.1 1.8 0.0 2.3 0.0 0.1 0.4 3.4 0.1 0.3 3.7 13.4 23.8 0.1 0.6 -

55.0 ND 0.7 44.6 ND ND 1.2 7.7 0.04 1.0 4.3 7.1 0.3 2.5 76.8 ND 102.0 302.0 2.1 ND ND

1.3 0.0 1.4 0.0 0.1 0.0 0.1 0.1 0.1 0.0 0.0 1.9 1.4 2.8 0.0 -

170.0 ND 3.0 43.9 ND ND 0.9 3.1 0.7 2.0 4.5 76.7 0.6 2.5 91.8 106.0 223.5 378.5 2.7 ND ND

7.1 0.1 0.4 0.0 0.9 0.0 0.9 0.0 0.4 0.0 0.1 1.9 5.7 7.8 13.4 0.0 -

M AN U

SC

Ave.

TE D

Analyte Units Ave. St Dev Aluminum µg/g 903.0 36.8 Arsenic µg/g 2.0 0.0 Barium 26.3 0.8 µg/g Bismuth 6.5 0.3 µg/g Cadmium µg/g ND Cesium µg/g 0.9 0.0 Chromium µg/g 13.7 0.3 Copper µg/g 11.7 1.1 Iron mg/g 16.3 0.1 Lead µg/g 9.6 0.4 Magnesium mg/g 8.6 0.1 Manganese µg/g 164.5 0.7 Molybdenum µg/g 0.3 0.0 Nickel µg/g 7.7 0.2 Phosphorus µg/g 201.0 0.0 Potassium µg/g 528.5 10.6 Sodium µg/g 942.5 0.7 Strontium µg/g 273.0 0.0 Sulfur mg/g 4.5 0.1 Zinc µg/g 28.5 2.1 Zirconium µg/g 8.7 0.0

Otto 1084

EP

Otto 1060

RI PT

Table 2: 8M Nitric Acid Extraction Results (in mg/g or µg/g). Results with no standard deviation were either not detectable (ND) in both duplicates, or were detectable in only one duplicate. Results for additional elements can be found in the ESI, Table S8.

AC C

737 738

32

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739 Table 3: QXRD results

Quartz 46% 51% 51% 0.3% 0.1% 0.3% 0.4% 1.2% 8.2% 10% -

Feldspar 14% 15% 11% -

753

CO2 2.70 2.78 3.50 3.22

AC C

EP

CO2 N2 Otto 1060 2.70 7.00 Otto 2010 2.61 8.18 Devine 3.04 8.01 Humble 2.86 8.06 Coupon: surface area = 4.35 cm2/g Ground: surface area = 600 cm2/g

752

741

Ankerite -742 -743 744 -745 -746 12% 747 10% -748 -

749

Table 4: Calculated pH for batch experiments, based on data shown in tables S4-S9 in the ESI. Coupon

751

Kaolinite 12% 11% 13% 2.9% 3.0% -

RI PT

Dolomite 27% 23% 25% 0.8% 2.3% 1.4% 0.9% 3.9% 4.1%

TE D

750

Calcite 99% 100% 100% 99% 98% 98% 98% 77% 77% 96% 96%

SC

Sample Otto 1060 Otto 1060 (CO2) Otto 1060 (N2) Otto 2010 Otto 2010 (CO2) Otto 2010 (N2) Devine Devine (CO2) Humble Humble (CO2) Otto 1084 Otto 1084 (CO2) Prinz Prinz (CO2)

M AN U

740

754 755 756

33

Ground N2 7.00 9.35 9.15 9.15

ACCEPTED MANUSCRIPT

757 758 759

Table 5: As and Cd results for the Otto 1060B and PrinzAsCd experiments (conducted with As and Cd spiked NaCl solution).

Sample

CO2 Coupon

CO2 ground

N2 Coupon

0.01M NaCl + As and Cd Starting concentrations

RI PT

Otto 1060B N2 ground

Units

Eq

7d

Eq

7d

Eq

7d

Eq

7d

µg/g

µg/L

Arsenic

µg/g

0.63

ND

0.34

ND

0.64

ND

0.31

ND

0.94

113

Cadmium

µg/g

0.12

0.05

0.01

0.05

0.09

0.03

ND

ND

Prinz AsCd Arsenic

µg/g

0.97

0.63

Cadmium

µg/g

0.03

0.01

TE D EP AC C 34

0.33 28.8 0.01M NaCl + As and Cd

0.93

0.70

1.01

113

ND

ND

0.35

28.8

M AN U

760

SC

Analyte

ACCEPTED MANUSCRIPT

Element mobilization and immobilization from carbonate rocks between CO2 storage reservoirs and the overlying aquifers during a potential CO2 †

Amanda R. Lawter*a, Nikolla P. Qafokua, R. Matthew Asmussena, Ravi K. Kukkadapua, Odeta Qafokua, Diana H. Bacona and Christopher F. Browna Pacific Northwest National Laboratory (PNNL), Richland, WA

AC C

EP

TE D

M AN U

SC

Figures

RI PT

a

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

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Figure 1: ICP analysis results for Ca (a), Mg (b), Sr (c), and Mo (d) in µg/g or mg/g from the Otto 1060, Otto 2010, Devine, and Humble coupon (C) and ground (G) experiments.

ACCEPTED MANUSCRIPT Figure 2: Changes in Sr (a) and Ba (b) concentrations over time in the longer term Otto 1084 experiments, reacted with CO2 or N2, with and without As/Cd or As/Cd and organic spikes; changes in Ca (c) and Mg (d) concentrations over time in the Otto 1084 longer term experiments with As/Cd or As/Cd and organic spikes, with pressure reduction time indicated with vertical lines. Strontium

200

2

100

50

0

0 -10

0

10

20

30

Time (days)

Calcium

LT CO2 AsCd + O

10.3 bar

20.7 bar

1 bar

10

0 EQL = 0.005 mg/g 30

EP

5

20

10

20

30

Time (days)

Magnesium

d

600

TE D

LT N2 AsCd + O

40

Time (days)

AC C

10

0

LT CO2 AsCd LT CO2 AsCd + O LT N2 AsCd + O

LT CO2 AsCd

0

-10

800

c

15

M AN U

1

LT CO2 LT CO2 + AsCd LT CO2 + AsCd + O LT N2 LT N2 + AsCd + O

SC

Ba (µg/L)

150

50

60

Magnesium (µg/g)

Sr (mg/L)

3

Calcium (mg/g)

blank CO2 or N2 injection

b

LT CO2 LT CO2 + AsCd LT CO2 + AsCd + O LT N2 LT N2 + AsCd + O

4

20

Barium

250

CO2 or N2 injection

blank

a

RI PT

5

400

20.7 bar

1 bar

200

0 70

10.3 bar

EQL = 0.4 µg/g 0

10

20

30

40

Time (days)

50

60

70

ACCEPTED MANUSCRIPT

M AN U

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Figure 3: SEM micrographs and EDS analysis of Otto 1060 coupon samples. Left: Unreacted Otto 1060 coupon with pyrite (Fe and S in a 1:2 atomic weight ratio; circled in red); Center (circled in red) and right: Possible hematite growth on pyrite on the Otto 1060 CO2-reacted coupon.

AC C

EP

TE D

Figure 4: SEM micrographs for Humble coupons. Left and center: CO2-reacted, right: N2-reacted.

ACCEPTED MANUSCRIPT

a, i) -6

-4

-2

0

2

4

6

8

10

12

SC

-12 -10 -8

RI PT

Figure 5: Modeled RT Mössbauer spectra of untreated (a), N2-treated (b) and CO2-treated (c) Otto 1060 samples showing ankerite, pyrite, Fe(III)-kaolinite, and ferrihydrite-like phases, at two different velocity ranges – a,i), a,ii), b, c,i), and c,ii) in -5 to + 5 mm/sec range (to clearly show various doublets), and the rest in in -12 to +12 mm/sec range (to clearly show hematite sextet peaks). Relative % of individual phases and derived RT Mössbauer spectral parameters are shown in Table S13 in the ESI.

-12 -10 -8 -6 -4 -2

0

2

4

6

8 10 12

M AN U

a) Untreated

a, ii) -4

-3

-2

-1

0

1

2

3

4

5

TE D

Ankerite Pyrite Ferrihydrite-like Fe(III)-kaolilnieclay Hematite

b) + N2 (red) untreated (blue) -3

-2

-1

0

1

2

3

4

5

AC C

EP

-4

c, i)

-12 -10 -8

-6

-4

-2

0

2

4

6

8

10

12

c, i)

c) + CO2

-12 -10 -8 -6 -4 -2

0

2

4

6

8 10 12

Velocity (mm/sec)

-4

-3

-2

-1

0

1

2

3

4

5

ACCEPTED MANUSCRIPT

Element mobilization and immobilization from carbonate rocks

potential CO2 leakage † Highlights:

RI PT

between CO2 storage reservoirs and the overlying aquifers during a

Interaction of CO2 with rock prior to reaching aquifers can influence potential risk.

•

Contaminants can be mobilized/immobilized by interactions with intermediary rocks.

•

These interactions should be included in CO2 sequestration risk assessments.

AC C

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TE D

M AN U

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•