Partitioning Behavior of Organic Contaminants in Carbon Storage Environments: A Critical Review

Carbon capture and storage is a promising strategy for mitigating the CO₂ contribution to global climate change. The large scale implementation of the technology mandates better understanding of the risks associated with CO₂ injection into geologic formations and the subsequent interactions with groundwater resources. The injected supercritical CO₂ (sc-CO₂) is a nonpolar solvent that can potentially mobilize organic compounds that exist at residual saturation in the formation. Here, we review the partitioning behavior of selected organic compounds typically found in depleted oil reservoirs in the residual oil–brine–sc-CO₂ system under carbon storage conditions. The solubility of pure phase organic compounds in sc-CO₂ and partitioning of organic compounds between water and sc-CO₂ follow trends predicted based on thermodynamics. Compounds with high volatility and low aqueous solubility have the highest potential to partition to sc-CO₂. The partitioning of low volatility compounds to sc-CO₂ can be enhanced by cosolvency due to the presence of higher volatility compounds in the sc-CO₂. The effect of temperature, pressure, salinity, pH, and dissolution of water molecules into sc-CO₂ on the partitioning behavior of organic compounds in the residual oil–brine–sc-CO₂ system is discussed. Data gaps and research needs for models to predict the partitioning of organic compounds in brines and from complex mixtures of oils are presented. Models need to be able to better incorporate the effect of salinity and cosolvency, which will require more experimental data from key classes of organic compounds

Measurement and Modeling of Setschenow Constants for Selected Hydrophilic Compounds in NaCl and CaCl₂ Simulated Carbon Storage Brines

ConspectusCarbon capture, utilization, and storage (CCUS), a climate change mitigation strategy, along with unconventional oil and gas extraction, generates enormous volumes of produced water containing high salt concentrations and a litany of organic compounds. Understanding the aqueous solubility of organic compounds related to these operations is important for water treatment and reuse alternatives, as well as risk assessment purposes. The well-established Setschenow equation can be used to determine the effect of salts on aqueous solubility. However, there is a lack of reported Setschenow constants, especially for polar organic compounds.In this study, the Setschenow constants for selected hydrophilic organic compounds were experimentally determined, and linear free energy models for predicting the Setschenow constant of organic chemicals in concentrated brines were developed. Solid phase microextraction was employed to measure the salting-out behavior of six selected hydrophilic compounds up to 5 M NaCl and 2 M CaCl₂ and in Na–Ca–Cl brines. All compounds, which include phenol, <i>p</i>-cresol, hydroquinone, pyrrole, hexanoic acid, and 9-hydroxyfluorene, exhibited log–linear behavior up to these concentrations, meaning Setschenow constants previously measured at low salt concentrations can be extrapolated up to high salt concentrations for hydrophilic compounds. Setschenow constants measured in NaCl and CaCl₂ brines are additive for the compounds measured here; meaning Setschenow constants measured in single salt solutions can be used in multiple salt solutions.The hydrophilic compounds in this study were selected to elucidate differences in salting-out behavior based on their chemical structure. Using data from this study, as well as literature data, linear free energy relationships (LFERs) for prediction of NaCl, CaCl₂, LiCl, and NaBr Setschenow constants were developed and validated. Two LFERs were improved. One LFER uses the Abraham solvation parameters, which include the index of refraction of the organic compound, organic compound’s polarizability, hydrogen bonding acidity and basicity of the organic compound, and the molar volume of the compound. The other uses an octanol–water partitioning coefficient to predict NaCl Setschenow constants. Improved models from this study now include organic compounds that are structurally and chemically more diverse than the previous models. The CaCl₂, LiCl, and NaBr single parameter LFERs use concepts from the Hofmeister series to predict new, respective Setschenow constants from NaCl Setschenow constants. The Setschenow constants determined here, as well as the LFERs developed, can be incorporated into CCUS reactive transport models to predict aqueous solubility and partitioning coefficients of organic compounds. This work also has implications for beneficial reuse of water from CCUS; this can aide in determining treatment technologies for produced waters

Multimodel Predictive System for Carbon Dioxide Solubility in Saline Formation Waters

The prediction of carbon dioxide solubility in brine at conditions relevant to carbon sequestration (i.e., high temperature, pressure, and salt concentration (T-P-X)) is crucial when this technology is applied. Eleven mathematical models for predicting CO₂ solubility in brine are compared and considered for inclusion in a multimodel predictive system. Model goodness of fit is evaluated over the temperature range 304–433 K, pressure range 74–500 bar, and salt concentration range 0–7 <i>m</i> (NaCl equivalent), using 173 published CO₂ solubility measurements, particularly selected for those conditions. The performance of each model is assessed using various statistical methods, including the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Different models emerge as best fits for different subranges of the input conditions. A classification tree is generated using machine learning methods to predict the best-performing model under different T-P-X subranges, allowing development of a multimodel predictive system (MMoPS) that selects and applies the model expected to yield the most accurate CO₂ solubility prediction. Statistical analysis of the MMoPS predictions, including a stratified 5-fold cross validation, shows that MMoPS outperforms each individual model and increases the overall accuracy of CO₂ solubility prediction across the range of T-P-X conditions likely to be encountered in carbon sequestration applications

Determination of Rare Earth Elements in Hypersaline Solutions Using Low-Volume, Liquid–Liquid Extraction

Complex, hypersaline brines–including those coproduced with oil and gas, rejected from desalination technologies, or used as working fluids for geothermal electricity generation–could contain critical materials such as the rare earth elements (REE) in valuable concentrations. Accurate quantitation of these analytes in complex, aqueous matrices is necessary for evaluation and implementation of systems aimed at recovering those critical materials. However, most analytical methods for measuring trace metals have not been validated for highly saline and/or chemically complex brines. Here we modified and optimized previously published liquid–liquid extraction (LLE) techniques using bis­(2-ethylhexyl) phosphate as the extractant in a heptane diluent, and studied its efficacy for REE recovery as a function of three primary variables: background salinity (as NaCl), concentration of a competing species (here Fe), and concentration of dissolved organic carbon (DOC). Results showed that the modified LLE was robust to a range of salinity, Fe, and DOC concentrations studied as well as constant, elevated Ba concentrations. With proper characterization of the natural samples of interest, this method could be deployed for accurate analysis of REE in small volumes of hyper-saline and chemically complex brines

Rare Earth Element Distributions and Trends in Natural Waters with a Focus on Groundwater

Systematically varying properties and reactivities have led to focused research of the environmental forensic capabilities of rare earth elements (REE). Increasing anthropogenic inputs to natural systems may permanently alter the natural signatures of REE, motivating characterization of natural REE variability. We compiled and analyzed reported dissolved REE concentration data over a wide range of natural water types (ground-, ocean, river, and lake water) and groundwater chemistries (e.g., fresh, brine, and acidic) with the goal of quantifying the extent of natural REE variability, especially for groundwater systems. Quantitative challenges presented by censored data were addressed with nonparametric distributions and regressions. Reported measurements of REE in natural waters range over nearly 10 orders of magnitude, though the majority of measurements are within 2–4 orders of magnitude, and are highly correlated with one another. Few global correlations exist among dissolved abundance and bulk solution properties in groundwater, indicating the complex nature of source-sink terms and the need for care when comparing results between studies. This collection, homogenization, and analysis of a disparate literature facilitates interstudy comparison and provides insight into the wide range of variables that influence REE geochemistry

New Linear Partitioning Models Based on Experimental Water: Supercritical CO₂ Partitioning Data of Selected Organic Compounds

Partitioning coefficients of organic compounds between water and supercritical CO₂ (sc-CO₂) are necessary to assess the risk of migration of these chemicals from subsurface CO₂ storage sites. Despite the large number of potential organic contaminants, the current data set of published water–sc-CO₂ partitioning coefficients is very limited. Here, the partitioning coefficients of thiophene, pyrrole, and anisole were measured in situ over a range of temperatures and pressures using a novel pressurized batch-reactor system with dual spectroscopic detectors: a near-infrared spectrometer for measuring the organic analyte in the CO₂ phase and a UV detector for quantifying the analyte in the aqueous phase. Our measured partitioning coefficients followed expected trends based on volatility and aqueous solubility. The partitioning coefficients and literature data were then used to update a published poly parameter linear free-energy relationship and to develop five new linear free-energy relationships for predicting water–sc-CO₂ partitioning coefficients. A total of four of the models targeted a single class of organic compounds. Unlike models that utilize Abraham solvation parameters, the new relationships use vapor pressure and aqueous solubility of the organic compound at 25 °C and CO₂ density to predict partitioning coefficients over a range of temperature and pressure conditions. The compound class models provide better estimates of partitioning behavior for compounds in that class than does the model built for the entire data set

Effects of Ligand Chemistry and Geometry on Rare Earth Element Partitioning from Saline Solutions to Functionalized Adsorbents

Rare earth elements (REE) are elements that drive the development of new technologies in many sectors, including green energy. However, the supply chain of REE is subject to a complex set of technical, environmental, and geopolitical constraints. Some of these challenges may be circumvented if REE are recovered from naturally abundant alternative sources, such as saline waters and brines. Here, we synthesized and tested aminated silica gels, functionalized with REE-reactive ligands: diethylenetriaminepentaacetic acid (DTPA), diethylenetriaminepentaacetic dianhydride (DTPADA), phosphonoacetic acid (PAA), and N,N-bisphosphono­(methyl)­glycine (BPG). A suite of characterization techniques and batch adsorption experiments were used to evaluate the properties of the functionalized silica adsorbents and test the REE-uptake chemistry of the adsorbents under environmentally relevant conditions. Results showed that BPG and DTPADA yielded the most REE-reactive adsorbents of those tested. Moreover, the DTPADA adsorbents demonstrated chemical and physical robustness as well as ease of regeneration. However, as in previous studies, amino-poly­(carboxylic acid) adsorbents showed limited uptake at midrange pH and low-sorbate concentrations. This work highlighted the complexity of intermolecular interactions between even moderately sized reactive sites when developing high-capacity, high-selectivity adsorbents. Additional development is required to implement an REE recovery scheme using these materials; however, it is clear that BPG- and DTPADA-based adsorbents offer a highly reactive adsorbent warranting further study

Effects of Ligand Chemistry and Geometry on Rare Earth Element Partitioning from Saline Solutions to Functionalized Adsorbents

Rare earth elements (REE) are elements that drive the development of new technologies in many sectors, including green energy. However, the supply chain of REE is subject to a complex set of technical, environmental, and geopolitical constraints. Some of these challenges may be circumvented if REE are recovered from naturally abundant alternative sources, such as saline waters and brines. Here, we synthesized and tested aminated silica gels, functionalized with REE-reactive ligands: diethylenetriaminepentaacetic acid (DTPA), diethylenetriaminepentaacetic dianhydride (DTPADA), phosphonoacetic acid (PAA), and N,N-bisphosphono­(methyl)­glycine (BPG). A suite of characterization techniques and batch adsorption experiments were used to evaluate the properties of the functionalized silica adsorbents and test the REE-uptake chemistry of the adsorbents under environmentally relevant conditions. Results showed that BPG and DTPADA yielded the most REE-reactive adsorbents of those tested. Moreover, the DTPADA adsorbents demonstrated chemical and physical robustness as well as ease of regeneration. However, as in previous studies, amino-poly­(carboxylic acid) adsorbents showed limited uptake at midrange pH and low-sorbate concentrations. This work highlighted the complexity of intermolecular interactions between even moderately sized reactive sites when developing high-capacity, high-selectivity adsorbents. Additional development is required to implement an REE recovery scheme using these materials; however, it is clear that BPG- and DTPADA-based adsorbents offer a highly reactive adsorbent warranting further study