Olivine, a magnesium- and iron-rich silicate mineral abundant in ultramafic rocks, has long been celebrated as a candidate for CO2 mineralization. When CO2 dissolved in water encounters olivine, a chain of chemical reactions converts the carbon into carbonate minerals such as magnesite, effectively turning a greenhouse gas into stone. This process, known as mineral carbonation, is attractive because the captured carbon is stored permanently, with no risk of leakage back into the atmosphere over human timescales. Yet the practical challenge has always been speed. Natural weathering of olivine at Earth’s surface takes thousands to millions of years, and even under engineered conditions the reaction rates can vary dramatically depending on the composition of the mineral and the chemistry of the surrounding fluids.

One puzzling observation has stood out in the literature: calcium-bearing olivines, such as the mineral kirschsteinite, appear to react with carbon dioxide far more readily than their magnesium-bearing counterparts, such as the common mineral forsterite. The origin of this striking difference in reactivity has remained poorly understood, and the new study set out to resolve it. Rather than treating the cation itself as the sole actor, the researchers built a multiscale theoretical framework that connects the electronic structure of the mineral surface to the macroscopic dissolution rates that can be measured in the laboratory.

At the heart of the framework is a seemingly humble chemical species: the nonbridging oxygen. In silicate minerals, oxygen atoms either bridge two silicon tetrahedra or dangle from a single silicon atom, the latter being known as nonbridging oxygens. These dangling oxygens are chemically more reactive, and the new work identifies them as the critical oxygen source for the surface reactions that initiate CO2 mineralization. When a CO2 molecule approaches the olivine surface, it is these nonbridging oxygens that participate directly in forming carbonate species, setting the stage for the dissolution of the metal cations and the eventual precipitation of solid carbonate.

The calculations, grounded in density functional theory, revealed that the type of cation modulates the reactivity of these surface oxygens in profound ways. In calcium-bearing olivine, the presence of Ca2+ alters the local electronic environment such that the energy barrier for calcium dissolution is reduced by 60 percent compared with the magnesium analogue. That seemingly modest energetic change has enormous kinetic consequences: the rate of ion release accelerates by six orders of magnitude, a millionfold increase. In chemical kinetics, a millionfold rate difference is the difference between a reaction that finishes in an afternoon and one that grinds on for centuries.

To test whether these atomic-scale predictions hold up at the laboratory scale, the team compared their calculated apparent dissolution rate constants with experimental observations. The agreement was remarkable. The computed rates reproduced the experimentally observed complete conversion of calcium olivine within hours under a carbon dioxide pressure of just one atmosphere, conditions that are mild by the standards of industrial carbonation processes. This cross-scale consistency, from quantum mechanical calculations of bond breaking to bulk conversion measurements in a reactor, provides strong evidence that surface carbonation reactions, rather than bulk transport or precipitation, are the governing bottleneck in olivine carbonation.

The implications of this mechanistic insight extend well beyond explaining a laboratory curiosity. For engineers designing CO2 mineralization systems, the framework offers a predictive basis for selecting and tailoring materials. If the reactivity of a silicate surface can be tuned by controlling the density and environment of nonbridging oxygens, then synthetic materials could be engineered to maximize the availability of these reactive sites, dramatically accelerating carbonation rates without resorting to extreme temperatures or pressures. The same logic applies to natural reservoirs: basaltic and ultramafic formations, which host vast quantities of reactive divalent cations, could be evaluated and ranked according to their surface chemistry rather than merely their bulk composition.

The study also carries a conceptual lesson for geochemistry more broadly. Dissolution rates of silicate minerals have traditionally been parameterized by empirical rate laws that depend on pH, temperature, and solution composition, but the atomic origins of those dependencies have often remained obscure. By demonstrating that cation identity acts on dissolution kinetics through its modulation of oxygen reactivity, the work provides a bridge between electronic structure theory and the macroscopic rate laws used in reactive transport models. Such bridges are essential if models of subsurface carbon storage are to move from descriptive to predictive, allowing operators to forecast how quickly injected CO2 will mineralize in a given formation and to design injection strategies accordingly.

For the global climate effort, the stakes are considerable. Ultramafic rocks are a global resource with vast potential for permanent CO2 mineralization, and basaltic reservoirs such as those already being used for pilot carbon storage projects in Iceland and the Columbia River Basalt Group in the United States have demonstrated that injected CO2 can mineralize within years rather than millennia. Understanding at the atomic level why certain minerals convert so much faster than others gives scientists a lever to pull: the possibility of accelerating the process further, whether through fluid chemistry manipulation, thermal treatment, or the design of engineered reactive materials that mimic the fast-reacting calcium olivines identified in this study.

The research, funded by the Japan Society for the Promotion of Science through KAKENHI grants and by ENEOS Xplora Inc., represents a convergence of quantum chemistry, geochemistry, and reaction engineering aimed squarely at one of the hardest problems in decarbonization: what to do with the CO2 once it is captured. By showing that the answer to rapid mineralization lies in the fine-grained choreography of oxygen atoms at a mineral surface, the Tokyo team has turned a long-standing puzzle into a design principle. As carbon capture and storage infrastructure scales up worldwide, the ability to predict and engineer mineral reactivity from first principles may prove to be one of the quiet but decisive advances that turns permanent geological carbon storage from a promising idea into a dependable industrial reality.

Subject of Research: Atomic-scale mechanisms of olivine surface carbonation and CO2 mineralization kinetics

Article Title: Surface carbonation governs olivine reactivity for rapid CO2 mineralization

Article References: Shao, Z., Liang, Y., Jo, G., Nakamura, K., & Tsuji, T. (2026). Surface carbonation governs olivine reactivity for rapid CO2 mineralization. Nature Communications. https://doi.org/10.1038/s41467-026-78548-z

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78548-z

Keywords: CO2 mineralization, olivine, carbon capture and storage, geochemistry, density functional theory, nonbridging oxygens, dissolution kinetics, ultramafic rocks, carbonate minerals, basalt reservoirs, reaction mechanisms, climate mitigation

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: 10.1016/j.cscee.2026.101499

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

The research team, led by Liuhua Yang and Haikun Liu, worked with cemented paste backfill, a material that mines routinely pump underground as a mixture of tailings and Portland cement. Their tailings, sourced from the Nanchang Copper Mine in Jiangxi Province, China, were rich in reactive calcium — containing 24.05 percent calcium oxide alongside abundant silica, iron and aluminum oxides. Rather than testing a handful of discrete conditions, the researchers designed a continuous water-content gradient spanning five groups, from completely dry samples held at 55 percent relative humidity, through liquid-to-solid ratios of 0.4 and 1, all the way to fully suspended slurries at ratios of 3 and 20. Every sample was carbonated under identical conditions: pure carbon dioxide at 200 milliliters per minute, ambient temperature and atmospheric pressure, sampled at 6, 12 and 24 hours.

What emerged from this systematic sweep is a story of two fundamentally different crystallization regimes. In the water-starved samples, derivative thermogravimetry revealed bimodal decomposition profiles and broadened shoulder peaks — thermal fingerprints of amorphous calcium carbonate and poorly crystalline vaterite, the metastable precursors that normally transform into stable calcite but here remained kinetically frozen. A persistent portlandite peak at around 460 degrees Celsius showed that without free water, calcium transport channels were severed, confining the reaction to the outermost particle layer. The nanometric water films that remained simply could not carry ions fast enough: diffusion coefficients under such conditions fall two to three orders of magnitude below those in bulk solution.

As water availability increased, the thermal signatures transformed. The low-temperature shoulder vanished, the main decomposition peak narrowed and sharpened, and by the highest liquid-to-solid ratio the entire carbonate signal coalesced into a single intense peak near 800 degrees Celsius — the hallmark of highly crystalline calcite. Intriguingly, the total amount of carbonate barely changed across the entire gradient: after 24 hours, apparent calcium carbonate content varied only from 30.4 to 32.4 percent by weight, a relative difference of roughly 6.6 percent. Water, it turns out, is not primarily a lever on how much carbon gets captured, but on what form that carbon takes and how cleanly it forms.

X-ray diffraction and infrared spectroscopy told the same story from different angles. In dry samples, the principal calcite reflection at 29.4 degrees remained weak and broad even after a full day of reaction, while vaterite peaks lingered — evidence of a crystallization pathway stuck in first gear. In the wettest samples, the calcite peak became the dominant feature and vaterite disappeared entirely. Infrared spectra traced the carbonate vibrational modes sharpening and intensifying with water content, while the silicate framework signals shifted in parallel, revealing depolymerization and repolymerization of the aluminosilicate matrix as water enabled dissolved species to migrate and reorganize.

Perhaps the most visually striking evidence came from electron microscopy. Under water-deficient conditions, tailings particle surfaces became blanketed in a dense, gel-like encapsulation — flocculent, edgeless material formed when explosive local supersaturation triggers a storm of tiny nuclei that can neither grow nor ripen. This coating acts as a physical barrier, blocking carbon dioxide from penetrating and calcium from escaping, which explains why carbonation efficiency in the driest group stagnated almost immediately. At intermediate moisture, the surfaces sprouted spheroidal, cauliflower-like aggregates with rounded edges — crystals beginning to emerge but still crowded and intergrown. Only in the water-rich slurries did the researchers observe the prize: abundant rhombohedral calcite crystals with smooth faces, sharp edges and well-defined cleavage planes, grown to micron scale through solution-mediated dissolution and reprecipitation.

The elemental maps added a crucial twist about impurities. In dry samples, calcium signals overlapped intimately with silicon, aluminum, magnesium and iron, indicating that carbonate precipitated entangled with the unreacted matrix while magnesium ions became trapped at lattice growth sites — a form of crystal poisoning that distorts calcite morphology. In water-rich samples, the maps showed dramatic elemental segregation: discrete calcium-rich zones with impurity signals at background levels. Real-time solution chemistry explained why. Magnesium concentrations in the slurry rose steadily as the reaction proceeded, showing that magnesium was eluted into the liquid phase rather than incorporated into the solid. The researchers attribute this to hydration energetics — magnesium’s hydration free energy of roughly minus 1,921 kilojoules per mole substantially exceeds calcium’s minus 1,577, making dehydration and lattice incorporation less favorable. Alkali metal behavior added a further nuance: at the moderate liquid-to-solid ratio of 3, higher ionic strength compressed the electrical double layer around growing nuclei, encouraging impurity adsorption, whereas the dilute conditions at ratio 20 fostered a cleaner crystallization environment.

Solid-state nuclear magnetic resonance revealed that the silicate skeleton itself undergoes a water-driven reconstruction. Dry samples retained low-polymerization silicate dominated by Q1 and Q2 units — dimers and short chains left stranded after calcium leaching. As water content rose, the spectra shifted upfield, and the wettest samples showed a broad resonance spanning minus 95 to minus 115 parts per million, deconvolving into Q3 and Q4 sites characteristic of layered and three-dimensional framework structures. In other words, abundant water allowed detached silicate fragments to repolymerize into a porous, highly connected amorphous silica network — one that, unlike the dense passivation films of dry carbonation, appears to serve as an ion-transport conduit rather than a barrier, consistent with the complete consumption of portlandite observed in the wettest group.

The pH trajectory of the aqueous experiments delineated three kinetic stages: an initial alkaline plateau near 12.5 to 13 buffered by portlandite and calcium silicate hydrates, a rapid decline as those phases were consumed, and a final stabilization near 6.0 to 6.5 during which carbonate precipitation and calcite growth continued. Calcium concentrations traced a textbook dissolution-precipitation cycle — surging as minerals dissolved, plummeting during massive carbonate precipitation, and settling at equilibrium with calcite’s solubility product. Together these measurements sketch a self-purifying mechanism in which water physically separates the desired product from the contaminants that would otherwise sabotage it.

The practical implications are considerable but carefully caveated. The authors identify three water regimes: an unfavorable deficient domain up to a liquid-to-solid ratio of 0.4, a transitional domain at ratio 1, and a favorable water-sufficient domain at ratios of 3 and above — while stressing that these thresholds are composition-specific rather than universal. More water is not automatically better: carbon dioxide diffuses through liquid roughly four orders of magnitude more slowly than through gas, so excessive slurry depth could throttle the gas supply to reactive surfaces, and high water volumes carry real costs in reactor size, agitation energy, dewatering and wastewater treatment — particularly since the self-purification mechanism transfers magnesium and other dissolved ions into the process liquor. The engineering optimum, the authors suggest, lies near the minimum water content that can trigger dissolution-reprecipitation and impurity exclusion. Within that window, however, the message is transformative: a variable as simple as moisture can be tuned to control the size, purity and morphology of carbonate products, turning one of mining’s largest waste streams into a precision instrument for permanent carbon removal.

Subject of Research: Water-content-dependent mineral carbonation of cemented mine tailings for CO2 sequestration

Article Title: Water-content-dependent carbonate formation in cemented paste backfill: Phase assemblage, morphology, and ion partitioning

Article References: Yang, L., Liu, H., Li, T., Jiao, H., Niu, H., & Chen, S. (2026). Water-content-dependent carbonate formation in cemented paste backfill: Phase assemblage, morphology, and ion partitioning. Results in Engineering, 32, Article 113319. https://doi.org/10.1016/j.rineng.2026.113319

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113319

Keywords: mineral carbonation, CO2 sequestration, mine tailings, calcite, cemented paste backfill, water content, crystallization, passivation layer, magnesium inhibition, silicate polymerization, carbon capture, Ostwald ripening

Phosphogypsum is the bulky, problematic by-product of phosphoric acid production for fertilizer manufacturing. Global stockpiles have now exceeded six billion tonnes, with another 200 to 250 million tonnes added every year, while utilization rates remain stubbornly low. Chemically, the material is dominated by gypsum, calcium sulfate dihydrate, with calcium oxide content commonly around 30 percent by weight. That makes it an tantalizingly abundant calcium reservoir for mineral carbonation, the process in which CO2 reacts with calcium- or magnesium-bearing phases to form stable carbonate minerals such as calcite. Unlike steel slag or carbide slag, however, phosphogypsum holds its calcium in sparingly soluble gypsum rather than in readily reactive lime or calcium silicates, so its carbonation depends entirely on whether that bound calcium can first be coaxed into solution.

Researchers led by Ying Shi and colleagues at Central South University in China tackled this problem with a comparative experiment of unusual rigor. They tested four activators, ammonium chloride, sodium acetate, ammonium acetate, and sodium gluconate, against plain deionized water, to dissolve calcium out of washed phosphogypsum from Guizhou Province. The resulting calcium-rich leachates were then carbonated under an identical protocol: pH was raised above 12 with sodium hydroxide, and each leachate was exposed to a CO2 atmosphere in sealed gas-tight bags for fourteen days, with the gas renewed daily. By measuring dissolved calcium at every stage and characterizing every solid that emerged, the team could link three successive steps that most studies treat in isolation: calcium release, calcium consumption, and carbonate product formation.

The leaching results were dramatic. Water alone extracted only modest amounts of calcium, leaving dissolved concentrations between roughly 726 and 833 milligrams per liter. Ammonium chloride nearly tripled that, sodium acetate and ammonium acetate pushed higher still, and sodium gluconate was in a class of its own, sustaining dissolved calcium concentrations around 15,300 milligrams per liter, roughly twenty times the blank system. Under optimized conditions, sodium gluconate released up to 98.5 percent of the available calcium from the solid, dissolving so much gypsum that only 5.4 grams of residue remained from an initial 240 grams of phosphogypsum. Electron microscopy showed the characteristic plate-like gypsum particles almost completely dismantled, while X-ray diffraction confirmed the near-total removal of gypsum and a relative enrichment of insoluble quartz. The gluconate anion, the study notes, forms stable soluble complexes with calcium ions, shifting the gypsum dissolution equilibrium and driving continued release.

Then came the twist. When the leachates were carbonated, the leachate with the most dissolved calcium performed the worst at consuming it. The ammonium chloride system, starting from a moderate 2,376 milligrams per liter of dissolved calcium, plummeted below 50 milligrams per liter within a single day. Sodium acetate and ammonium acetate followed similar trajectories, and after fourteen days all three had achieved calcium consumption efficiencies of 98.7 to 99.0 percent. The sodium gluconate leachate, despite beginning with 15,315 milligrams per liter, crawled down slowly, still holding 2,177 milligrams per liter after two weeks, for a final consumption efficiency of just 85.8 percent. Its pH also stayed persistently high, remaining near 11.8 at the end of the experiment while the other systems settled around neutral.

The authors attribute this behavior to lingering calcium-gluconate complexation. Even though the total dissolved calcium was enormous, much of it was locked in soluble complexes rather than existing as free calcium ions, reducing the ion activity product needed to trigger calcium carbonate precipitation and delaying supersaturation and nucleation. In other words, a high calcium concentration on paper does not guarantee that the calcium is chemically available to react with carbonate. The study is careful to note that carbon dioxide was supplied from a static gas phase without stirring or sparging, so gas-liquid mass-transfer resistance may also have shaped the apparent kinetics, but all leachates faced the same conditions, making the comparison fair.

The solid products told the same story from a different angle. Every leachate produced calcite, the crystalline form of calcium carbonate, confirmed by X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis. But the quality of the products diverged sharply. The water, ammonium chloride, sodium acetate, and ammonium acetate systems yielded solids that were overwhelmingly carbonate, with apparent calcium carbonate equivalents of 93.56 to 96.90 percent by weight. The sodium gluconate product was a different beast entirely: quantitative X-ray diffraction showed crystalline calcite made up only about 20 percent of the recovered solid, with the remainder predominantly X-ray amorphous, accompanied by an unusual silicon-oxygen signature and a distinct thermal profile featuring substantial low-temperature mass loss.

Translated into climate terms, the differences are meaningful. Normalized to the initial phosphogypsum mass, the water, ammonium chloride, sodium acetate, and ammonium acetate routes mineralized an estimated 3.96, 14.36, 15.87, and 20.00 kilograms of CO2 per tonne of phosphogypsum, respectively. The sodium gluconate route, judged only by its quantified crystalline calcite, incorporated 72.51 kilograms of CO2 per tonne into calcite, a far higher figure that reflects its extraordinary calcium supply. Yet the authors caution that this headline number masks the complexity of the product and the substantial calcium left stranded in solution, which matters enormously if the goal is clean, well-defined carbonate for construction fillers or cementitious applications.

The broader lesson is a methodological one that could ripple through the field. Most phosphogypsum carbonation studies optimize a single leaching-carbonation system and report calcium recovery or carbonate yield, without asking whether more calcium release actually translates into better carbonation. By systematically decoupling the two stages, this study shows they can even be inversely related. Ammonium chloride emerges as the balanced performer, offering moderate calcium release, near-complete consumption, and clean calcite-dominated products, while sodium gluconate demonstrates that powerful complexing agents can extract calcium superbly yet sabotage its subsequent use. For engineers designing carbonation-based materials from industrial waste, the criterion for a good recycled calcium source must therefore expand from how much calcium a leachate contains to how much of that calcium can actually be consumed and converted into well-defined carbonate solids.

There remain open questions. The authors acknowledge that a complete calcium-sulfur-carbon material balance was not established, and they call for future work with phase-resolved elemental accounting to track where every element ends up. Impurities in phosphogypsum, including soluble phosphorus, fluoride, organic matter, and heavy metals, may also interact with calcium phases and carbonate nucleation in ways not yet fully resolved, and the sample from a single Guizhou source may not represent phosphogypsum from other phosphate ores. Still, the core message stands: in the race to turn a six-billion-tonne waste mountain into a carbon sink, chemistry that looks strongest at the extraction stage may quietly undermine the storage stage, and only a full-chain view, from dissolution to precipitation, can identify the leachates truly fit for locking CO2 into stone.

Subject of Research: Activator-assisted calcium release from phosphogypsum and its aqueous carbonation for CO2 mineralization

Article Title: Comparative study of phosphogypsum-derived leachates as recycled calcium sources for carbonation: Linking Ca release, consumption and carbonate product formation

Article References: Shi, Y., Guo, Q., Li, S., Zhou, X., Yan, K., & Liu, P. (2026). Comparative study of phosphogypsum-derived leachates as recycled calcium sources for carbonation: Linking Ca release, consumption and carbonate product formation. Case Studies in Construction Materials, 25, Article e06557. https://doi.org/10.1016/j.cscm.2026.e06557

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06557

Keywords: phosphogypsum, mineral carbonation, CO2 sequestration, calcium leaching, calcite, sodium gluconate, ammonium chloride, industrial waste valorization, gypsum dissolution, carbonate precipitation, low-carbon construction materials, recycled calcium sources

Serpentinite forms through serpentinization, a coupled fluid–rock reaction in which primary olivine and pyroxenes are progressively replaced by serpentine-group minerals—lizardite, chrysotile, and antigorite—often accompanied by brucite and magnetite. The reaction is geochemically consequential in its own right: oxidation of ferrous iron during secondary mineral growth releases molecular hydrogen, generating hyperalkaline, strongly reducing fluids capable of driving abiotic methane formation. This mechanism, the authors note, places serpentinite at the center of the emerging geologic hydrogen conversation, with field observations from ophiolite belts—including the northern Semail Ophiolite in the United Arab Emirates—documenting natural hydrogen anomalies associated with fault-controlled migration pathways and alkaline springs in actively carbonating settings.

The mineralogical makeup of serpentinite is far from uniform, and this variability is the linchpin of the review’s argument. Lizardite typically dominates low-temperature serpentinites, forming the familiar mesh and bastite replacement textures; antigorite, the most ordered and thermally stable polymorph, characterizes higher-temperature, tectonized rocks; and chrysotile, the fibrous asbestos-form habit, records deformation-assisted fluid flow but simultaneously imposes serious occupational and environmental constraints. Magnesium-recovery experiments demonstrate that these polymorphs behave differently during activation and leaching, meaning that a serpentinite’s dominant serpentine phase is a process-relevant variable rather than a mere crystallographic curiosity. Accessory phases such as brucite, magnetite, and relict Cr-spinel further modulate reactivity, density, and impurity burdens.

In construction, serpentinite already has a long commercial track record as dimension stone, aggregate, and refractory feedstock, marketed widely as “green marble” for its polishability and veining. The review highlights more ambitious upgrades: waste serpentine cutting has been converted into forsterite-rich refractories, cordieritic ceramics have been engineered from serpentinite blends with tailored porosity and thermal expansion, and magnesia–silicate ceramics benefit from controlled forsterite–enstatite phase evolution during firing. Perhaps most striking is serpentinite’s role in radiation-shielding concrete, where chemically bound hydroxyl water enhances neutron attenuation. Yet durability studies caution that serpentinite aggregates can produce wider, more porous interfacial transition zones with cement paste, so shielding benefits must be balanced against careful mix design and transport-property management.

On carbon management, the review positions serpentinite as a promising feedstock for permanent CO2 sequestration through mineral carbonation, in which magnesium from the silicate lattice reacts with CO2 to form thermodynamically stable carbonates such as magnesite. The catch is kinetics: magnesium is tightly bound in layered crystals, and silica-rich reaction products form passivating layers that choke further dissolution. Overcoming these barriers requires process intensification—fine grinding, thermal activation in the roughly 600–750 degree Celsius window, acid or ammonium-salt leaching, and exfoliation. Pilot-scale aqueous carbonation of serpentinite tailings exposed to cement-plant flue gas has demonstrated feasibility, and techno-economic analyses indicate that residue-based routes co-located with CO2 point sources and waste-heat supplies offer the most credible path to commercial deployment.

Serpentinite also underpins an industrial chemistry platform. Thermal activation followed by acid leaching liberates magnesium for production of high-purity Mg(OH)2, MgO, and soluble Mg salts, while generating silica-rich residues that may themselves become saleable co-products if purity and leachability are controlled. Exfoliation with potassium salts has produced mesoporous, hydroxyl-rich nanosheets that efficiently bind divalent heavy metals such as cadmium and lead, opening doors to engineered water-treatment sorbents. Meanwhile, serpentinite-derived phases have served as precursors for solid-base biodiesel catalysts and magnesium potassium phosphate matrices for radioactive waste immobilization, and serpentinite-hosted deep-sea hydrothermal precipitates—brucite-rich lamellae with aligned nanocrystals—have even demonstrated osmotic energy conversion, offering geomimetic inspiration for nanofluidic membrane design.

Environmental and occupational risks thread through every potential pathway. Chrysotile-bearing lithotypes, particularly foliated and cataclastic serpentinites with abundant fibrous veins, show heightened fiber-release potential during quarrying, crushing, and handling, and documented soil contamination around serpentinite mining operations reinforces the need for integrated petrographic screening and dust control. Elevated nickel and chromium inventories—geologically expected in depleted mantle-derived rocks—raise concerns about trace-metal mobility during weathering, leaching, enhanced rock weathering deployment, and residue disposal. The review treats these hazards not as an afterthought but as cross-cutting criteria that determine which serpentinite lithotypes are suitable for which end uses, arguing for fit-for-purpose classification in every deployment scenario.

On the energy frontier, the review frames geologic hydrogen not as another application category but as a resource-system problem. Serpentinization generates hydrogen wherever Fe(II)-bearing phases oxidize and magnetite forms with sustained fluid access, but viable accumulations require generation rates that exceed leakage and microbial consumption, effective migration corridors, and credible trapping conditions. Regional assessments of Arabian ophiolites identify serpentinites as prospective hydrogen targets, and magnetotelluric phase-tensor mapping has been proposed to delineate serpentinization-related electrical anisotropy beneath cover in the UAE. The authors stress that geophysical signatures and surface anomalies are targeting tools, not proof of commercial resources; direct gas monitoring, generation-rate quantification, and subsurface validation remain essential next steps.

Looking ahead, the review identifies structural gaps that must close before serpentinite can realize its promise. The field lacks polymorph- and texture-resolved performance databases that link mineralogical attributes to engineering outcomes, leaving a persistent disconnect between rock names and measurable properties. Laboratory demonstrations of carbonation, adsorption, and binder performance have yet to be translated systematically through pilot-scale validation under realistic feedstock variability, water constraints, and continuous-operation conditions. Integrated techno-economic and life-cycle assessment is similarly overdue, particularly for arid-region economies where water demand for aqueous carbonation and leaching may dominate feasibility. The authors’ prescription is a shift from treating serpentinite as an abundant commodity to managing it as a qualified, risk-governed resource—a reframing that could turn one of Earth’s most underappreciated rocks into a cornerstone of sustainable construction, carbon management, and the clean-energy transition.

The global distribution of serpentinite adds a geographic dimension to its strategic profile. Ophiolite belts—fragments of oceanic lithosphere thrust onto continental margins—occur along the Tethyan suture from the Alps through Anatolia, Oman, and the Himalaya, as well as in the Circum-Pacific and Appalachian orogens. Because these belts frequently lie within or near rapidly industrializing economies in the Middle East, South Asia, and Southeast Asia, serpentinite resources are often located closer to cement plants, steelworks, and other concentrated CO₂ point sources than many alternative mafic feedstocks such as basalt. That co-location advantage matters for mineral carbonation economics, since transporting low-value rock over long distances erodes the already thin margins of carbonation processes, and the review’s emphasis on residue-based routes near industrial emitters reflects this logistical reality.

The thermodynamic underpinning of serpentinite’s carbonation appeal is worth underscoring. Magnesium silicates react with CO₂ to form magnesite and quartz with a substantial negative free-energy change, meaning the reaction is strongly favored at ambient conditions and the resulting carbonates are stable over geological timescales without monitoring requirements comparable to those of injected supercritical CO₂. The challenge is therefore not thermodynamic feasibility but reaction speed: the same strong Mg–O bonds that make serpentinite carbonates permanent also make the precursor silicates slow to dissolve. This asymmetry between thermodynamics and kinetics is what drives the entire pretreatment literature, from attrition grinding that exposes fresh surfaces to dehydroxylation that destabilizes the layered structure and dramatically accelerates Mg liberation.

Water chemistry imposes further constraints that are easy to underestimate. Aqueous carbonation routes consume water both as a reaction medium and for solids handling, and they generate alkaline process streams that require management. In arid ophiolite terrains—precisely where some of the most reactive, Mg-rich serpentinites occur—water allocation may become the binding constraint on deployment, which is why the review flags integrated water accounting alongside techno-economic assessment. Recyclable ammonium-salt chemistry offers one response, since it decouples Mg extraction from high water throughput and allows reagent recovery between cycles, but each such intensification step adds capital cost and process complexity that must be justified against the value of the sequestered carbon and any co-produced magnesium chemicals.

Enhanced rock weathering represents a lower-intensity alternative that trades reaction rate for simplicity. Spreading finely ground serpentinite on agricultural soils or coastal environments lets ambient CO₂ and organic acids dissolve the silicate gradually, with co-benefits including pH buffering and magnesium supply to crops. However, the same Ni and Cr enrichments that characterize mantle-derived ultramafics raise questions about trace-metal accumulation in amended soils, and the slow dissolution kinetics make verification of actual carbon removal difficult without robust measurement and monitoring protocols. The review’s insistence on lithotype-specific screening before such deployment reflects a broader principle: the geological variability that makes serpentinite interesting also makes blanket claims about its performance scientifically indefensible.

Finally, the occupational legacy of serpentine mining provides important historical context. Regulatory attention to naturally occurring asbestos since the late twentieth century has shaped quarrying practice, product standards, and public perception of serpentinite worldwide, and any expansion of its use must reckon with that institutional history. The review’s risk-governed framework effectively extends this legacy into new domains, arguing that fiber screening, dust suppression, and trace-metal characterization should be embedded in resource qualification from the outset rather than retrofitted after problems emerge.

Subject of Research: Serpentinite as a strategic geomaterial linking mineralogy, carbon mineralization, magnesium recovery, and geologic hydrogen

Article Title: Serpentinite as a strategic Earth material: geological controls, mineral reactivity, carbon mineralization, geologic hydrogen, and sustainable resource pathways

Article References: Abukhadra, M. R., Hamed, S. M., Allam, A. A., Hamdan, M. A., & Saima, M. A. (2026). Serpentinite as a strategic Earth material: geological controls, mineral reactivity, carbon mineralization, geologic hydrogen, and sustainable resource pathways. Environmental Earth Sciences, 85(15), Article 402. https://doi.org/10.1007/s12665-026-13127-5

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13127-5

Keywords: serpentinite, carbon mineralization, CO2 sequestration, geologic hydrogen, serpentinization, magnesium recovery, low-carbon binders, radiation shielding concrete, enhanced rock weathering, Semail Ophiolite, chrysotile risk, mineral carbonation