Oilfield Brine Meets CO2 in Ammonia Twist on the Solvay Process

Every barrel of oil that reaches the surface brings water with it, and as oilfields mature, that water becomes an ever-growing burden. In arid producers such as Oman, treatment of this produced water leaves behind concentrated, sodium-chloride-rich brines that are difficult to discharge or reuse, and that typically end their lives in evaporation ponds or deep wells. A new laboratory study suggests these waste brines could instead serve double duty: soaking up carbon dioxide while yielding a recoverable, bicarbonate-bearing salt product. The work, published in Case Studies in Chemical and Environmental Engineering, applies an ammonia-assisted, Solvay-type carbonation route to real oilfield brine, a feedstock that has largely been overlooked in the brine-carbonation literature.

The research team, led by Mansour Al-Haddabi of Sultan Qaboos University, collected saline produced-water brine from evaporation ponds at an Omani oil field and characterized it in detail before any reaction took place. The brine was alkaline, with a pH of 9.1, and dominated by sodium at 2,708.5 milligrams per liter and chloride at 4,227.8 milligrams per liter. It also carried measurable alkalinity, modest residual oil of 0.5 milligrams per liter, and a suite of minor constituents including calcium, magnesium, sulfate, boron, and organic carbon. That composition matters enormously, because most prior carbonation studies have targeted seawater, desalination reject brine, or calcium- and magnesium-rich solutions where carbonate precipitation is thermodynamically straightforward. Sodium-dominated brines are a different chemical beast altogether.

The difficulty lies in two intertwined effects. High ionic strength suppresses the solubility of carbon dioxide through salting-out, meaning less of the gas dissolves in the first place. Meanwhile, the scarcity of divalent calcium and magnesium ions closes off the conventional pathway to calcium or magnesium carbonate precipitation. What sodium-rich brines do offer, in abundance, is sodium itself, and sodium can in principle be converted to sodium bicarbonate if enough dissolved inorganic carbon and alkalinity are supplied. The classic route to exactly that transformation is the Solvay process, the nineteenth-century industrial chemistry that turns salt, ammonia, and carbon dioxide into soda ash. The Omani team borrowed that logic, using ammonia to enhance carbon dioxide absorption and push the dissolved carbon toward bicarbonate chemistry.

The experimental setup was deliberately simple: a custom acrylic bubble-column reactor, 60 centimeters tall, through which high-purity carbon dioxide was sparged at a fixed flow rate of 2 liters per minute into ammoniated brine, all held at a controlled 23 degrees Celsius. Before touching the real brine, the researchers ran controlled screening experiments on synthetic sodium chloride solutions to isolate the chemistry. Liquid samples were withdrawn at intervals and analyzed for pH, total carbon, and dissolved sodium, with total carbon measured on a Shimadzu TOC-L analyzer serving as a proxy for carbon derived from absorbed carbon dioxide and its aqueous reaction products.

The synthetic experiments delivered a clear mechanistic picture. In ammoniated solutions, liquid-phase total carbon climbed rapidly during the early minutes of bubbling and then leveled off, and raising the ammonia concentration from 1.5 to 4.5 molar increased the amount of carbon retained. The chemistry runs through carbamate formation: carbon dioxide reacts with two ammonia molecules to form a carbamate ion and ammonium, and the carbamate then hydrolyzes to yield bicarbonate and regenerate ammonia. A non-ammoniated sodium chloride control accumulated almost no carbon, which the authors describe as the clearest evidence that ammonia materially enhances retention of carbon-dioxide-derived carbon and that simple physical dissolution cannot explain the results.

Salinity, however, worked against the process. As sodium chloride concentration rose from 1 to 3 molar, the final measured total carbon fell from roughly 6,805 to 3,081 milligrams of carbon per liter, a decline of about 54 percent. This is the salting-out effect in action: increasing ionic strength reduces carbon dioxide solubility and suppresses reactive carbon retention. Ammoniation can partially compensate by converting whatever gas does dissolve into ionic species, but it cannot erase the fundamental constraint imposed by the brine’s salt load. The pH trajectory told a consistent story, dropping rapidly from strongly alkaline conditions to around 9 within the first 30 minutes and then stabilizing slowly, a pattern compatible with bicarbonate-rich salt formation.

When the team turned to the real produced-water brine, dosing it with nominal ammonia concentrations of 4.5, 5.5, and 6.5 molar, the highest apparent conversion of dissolved sodium reached 47 percent at 6.5 molar ammonia, equivalent to removing about 1.27 grams of sodium per liter. The authors are careful with the word apparent. Because no measurements were made of ammonia remaining in solution, bound to the solid, or escaping in the off-gas, a fully ammonia-normalized conversion could not be calculated. Still, the 47 percent figure compares favorably with prior Solvay-based brine studies, which have reported sodium removals of roughly 33 to 46 percent using ammonia, calcium oxide, or potassium hydroxide variants on desalination reject brines.

The recovered solid told a more complicated story than the liquid-phase numbers alone. X-ray diffraction identified ammonium bicarbonate as the dominant crystalline phase, not the phase-pure sodium bicarbonate that the idealized Solvay equation promises. Several diffraction peaks overlapped with sodium bicarbonate and related carbonate phases, but the overlap prevented reliable quantification of minor sodium-bearing contributions. Infrared spectroscopy confirmed carbonate, bicarbonate, and ammonium-related functional groups, while scanning electron microscopy revealed irregular, agglomerated crystals with layered, plate-like surfaces, the signature of simultaneous nucleation and co-precipitation from a concentrated multicomponent brine. Energy-dispersive spectroscopy detected carbon, oxygen, and nitrogen as major elements, with variable sodium and chlorine, underscoring the heterogeneous character of the product.

This heterogeneity is not a footnote; it is the central engineering challenge. The sequential Solvay chemistry explains the outcome: ammonia, water, and carbon dioxide first form ammonium bicarbonate, which then reacts with sodium chloride to precipitate sodium bicarbonate and leave ammonium chloride in solution. When the second step is incomplete or displaced by the brine’s complex crystallization conditions, ammonium bicarbonate dominates the recovered solid. Any application demanding a defined sodium bicarbonate or sodium carbonate specification would therefore require upgrading steps such as controlled washing, selective recrystallization, or thermal treatment, each carrying trade-offs in water use, energy demand, and product recovery.

Ammonia management looms equally large on the path to scale. The gas enhances absorption but represents a significant chemical inventory and a potential environmental liability, and the study performed no direct measurement of ammonia in the reactor off-gas. The authors sketch the necessary next steps with unusual candor for a laboratory paper: closed mass balances on sodium, carbon, nitrogen, chlorine, and water; quantitative phase analysis of the solid; ammonia recovery and recycle testing; and only then process simulation, techno-economic assessment, and targeted pilot validation. On an upper-bound basis, if all the removed sodium were recovered as sodium bicarbonate, the process would yield roughly 4.6 kilograms of bicarbonate-equivalent per cubic meter of brine, several tonnes per day at a hypothetical throughput of 1,000 cubic meters daily, though not necessarily saleable product. The study’s real contribution is the demonstration that a field-derived, sodium-rich produced-water brine, despite its punishing salinity and chemical complexity, can participate meaningfully in carbon dioxide utilization, provided engineers treat sodium conversion, product purity, and ammonia recovery as inseparable parts of a single problem.

Subject of Research: Ammonia-assisted Solvay-type carbonation of sodium-chloride-rich oilfield produced-water brine for CO2 uptake and recoverable salt formation

Article Title: Ammonia-assisted carbonation of NaCl-rich produced-water brine: CO 2 uptake, sodium conversion, and recoverable salt formation

Article References: Al-Haddabi, M., Al-Salmi, M., Ahmed, M., & Al-Quraini, H. (2026). Ammonia-assisted carbonation of NaCl-rich produced-water brine: CO2 uptake, sodium conversion, and recoverable salt formation. Case Studies in Chemical and Environmental Engineering, 14, Article 101499. https://doi.org/10.1016/j.cscee.2026.101499

Image Credits: AI Generated

DOI: Not provided

Keywords: produced water, carbon dioxide utilization, Solvay process, ammonia carbonation, sodium bicarbonate, brine management, mineral carbonation, bubble column reactor, salting-out effect, ammonium bicarbonate, XRD characterization, oilfield brine