{"title":"Dissolution behavior of mixed calcium‑cobalt carbonates at 25 °C in contact with different gas phases","authors":"","doi":"10.1016/j.gexplo.2024.107558","DOIUrl":null,"url":null,"abstract":"<div><p>The potential immobilization of cobalt in various environments can be achieved through the incorporation of Co into carbonate minerals, forming solid solutions of (Ca<sub>1-x</sub>Co<sub>x</sub>)CO<sub>3</sub>. However, the thermodynamic properties of these minerals are not well-understood due to conflicting data from natural observations and experiments. In this work, a series of mixed calcium‑cobalt carbonates were prepared and their interaction with aqueous solution was investigated. Depending on the Co/(Ca + Co) mol ratio (X<sub>Co</sub>) of the mixed solution, ranging from 0.00 to 1.00, pure calcite, Co-bearing calcite, Co-bearing aragonite, Ca-bearing spherocobaltite and pure spherocobaltite were successively synthesized using a precipitation method. Upon dissolution of the Co-bearing solids (X<sub>Co</sub> = 0.10–1.00) in N<sub>2</sub>-degassed water (NW) and air-saturated water (AW), the Co concentration of the aqueous solutions increased gradually to a stable state of 0.017–0.191 and 0.018–0.186 mmol/L after 240–360 d dissolution, respectively. When the dissolution occurred in CO<sub>2</sub>-saturated water (CW), the Co concentration initially spiked to 0.372–2.258 mmol/L within 6 h ∼ 15 d and then decreased to a stable range of 0.030–0.559 mmol/L after 240–360 d. The Co/(Ca + Co) mol ratio in the aqueous solution (X<sub>Co2+,AS</sub>) was significantly lower than the Co/(Ca + Co) atomic ratio in the solids (X<sub>Co,SS</sub>), particularly when dissolved in NW and AW. During these dissolution processes in NW, AW and CW at 25 °C, the average log IAP values at the final stable state were determined as follows: for calcite (CaCO<sub>3</sub>), the values were −8.25 ± 0.03 in NW, −8.34 ± 0.11 in AW, and −8.10 ± 0.08 in CW; for spherocobaltite (CoCO<sub>3</sub>), they were −9.24 ± 0.26 in NW, −9.39 ± 0.23 in AW, and −9.38 ± 0.09 in CW. Furthermore, the log IAP values increased from those typical for calcite to −7.89 ± 0.01 ∼ −7.84 ± 0.10 for the solid with X<sub>Co,SS</sub> = 0.187 as X<sub>Co,SS</sub> increased, eventually aligning with those typical of spherocobaltite. Lippmann diagrams, constructed using the Guggenheim parameters <em>a</em><sub>0</sub> = 2.30 and <em>a</em><sub>1</sub> = 0.265 for the “subregular” calcite-spherocobaltite solid solutions [(Ca<sub>1-x</sub>Co<sub>x</sub>)CO<sub>3</sub>] with a miscibility gap ranging from X<sub>Co,SS</sub> = 0.251 to 0.858, highlighted the “peritectic” point at X<sub>Co</sub><sub>2+</sub><sub>,AS</sub> = 0.0538 on the <em>solutus</em>. This analysis revealed that the solids dissolved non-stoichiometrically in water. Consequently, the Co-poor aqueous solution would reach equilibrium with the Co-rich calcite-structure phase at the solid surface.</p></div>","PeriodicalId":16336,"journal":{"name":"Journal of Geochemical Exploration","volume":null,"pages":null},"PeriodicalIF":3.4000,"publicationDate":"2024-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Geochemical Exploration","FirstCategoryId":"89","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0375674224001742","RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
引用次数: 0
Abstract
The potential immobilization of cobalt in various environments can be achieved through the incorporation of Co into carbonate minerals, forming solid solutions of (Ca1-xCox)CO3. However, the thermodynamic properties of these minerals are not well-understood due to conflicting data from natural observations and experiments. In this work, a series of mixed calcium‑cobalt carbonates were prepared and their interaction with aqueous solution was investigated. Depending on the Co/(Ca + Co) mol ratio (XCo) of the mixed solution, ranging from 0.00 to 1.00, pure calcite, Co-bearing calcite, Co-bearing aragonite, Ca-bearing spherocobaltite and pure spherocobaltite were successively synthesized using a precipitation method. Upon dissolution of the Co-bearing solids (XCo = 0.10–1.00) in N2-degassed water (NW) and air-saturated water (AW), the Co concentration of the aqueous solutions increased gradually to a stable state of 0.017–0.191 and 0.018–0.186 mmol/L after 240–360 d dissolution, respectively. When the dissolution occurred in CO2-saturated water (CW), the Co concentration initially spiked to 0.372–2.258 mmol/L within 6 h ∼ 15 d and then decreased to a stable range of 0.030–0.559 mmol/L after 240–360 d. The Co/(Ca + Co) mol ratio in the aqueous solution (XCo2+,AS) was significantly lower than the Co/(Ca + Co) atomic ratio in the solids (XCo,SS), particularly when dissolved in NW and AW. During these dissolution processes in NW, AW and CW at 25 °C, the average log IAP values at the final stable state were determined as follows: for calcite (CaCO3), the values were −8.25 ± 0.03 in NW, −8.34 ± 0.11 in AW, and −8.10 ± 0.08 in CW; for spherocobaltite (CoCO3), they were −9.24 ± 0.26 in NW, −9.39 ± 0.23 in AW, and −9.38 ± 0.09 in CW. Furthermore, the log IAP values increased from those typical for calcite to −7.89 ± 0.01 ∼ −7.84 ± 0.10 for the solid with XCo,SS = 0.187 as XCo,SS increased, eventually aligning with those typical of spherocobaltite. Lippmann diagrams, constructed using the Guggenheim parameters a0 = 2.30 and a1 = 0.265 for the “subregular” calcite-spherocobaltite solid solutions [(Ca1-xCox)CO3] with a miscibility gap ranging from XCo,SS = 0.251 to 0.858, highlighted the “peritectic” point at XCo2+,AS = 0.0538 on the solutus. This analysis revealed that the solids dissolved non-stoichiometrically in water. Consequently, the Co-poor aqueous solution would reach equilibrium with the Co-rich calcite-structure phase at the solid surface.
期刊介绍:
Journal of Geochemical Exploration is mostly dedicated to publication of original studies in exploration and environmental geochemistry and related topics.
Contributions considered of prevalent interest for the journal include researches based on the application of innovative methods to:
define the genesis and the evolution of mineral deposits including transfer of elements in large-scale mineralized areas.
analyze complex systems at the boundaries between bio-geochemistry, metal transport and mineral accumulation.
evaluate effects of historical mining activities on the surface environment.
trace pollutant sources and define their fate and transport models in the near-surface and surface environments involving solid, fluid and aerial matrices.
assess and quantify natural and technogenic radioactivity in the environment.
determine geochemical anomalies and set baseline reference values using compositional data analysis, multivariate statistics and geo-spatial analysis.
assess the impacts of anthropogenic contamination on ecosystems and human health at local and regional scale to prioritize and classify risks through deterministic and stochastic approaches.
Papers dedicated to the presentation of newly developed methods in analytical geochemistry to be applied in the field or in laboratory are also within the topics of interest for the journal.