Retardation of dissolved radiocarbon through a carbonated matrix

doi:10.1016/0016-7037(85)90163-2

Geochimica et Cosmochimica Acta

Volume 49, Issue 3, March 1985, Pages 683-693

https://doi.org/10.1016/0016-7037(85)90163-2 Get rights and content

Abstract

Spiked solutions (¹⁴C and ³H) were passed through two columns, one filled with silica and one with a mixture of silica and carbonate sand. Significant delay of ¹⁴C vs. ³H appeared in the second material and leads to a reduction of the relative flow rate which reaches 40%. Three types of effects causing modifications of the ¹⁴C transfer are analysed: (i) flow rate, (ii) sediment aging (iii) solution chemical composition and load. The reside with respect to water is governed by a linear process (assuming a constant number of reacting sites on the matrix) and kinetic process (where mass transfer effects are significant). It is also shown that much less than one mono-molecular layer of solid carbonate is involved and a distribution coefficient was estimated for these dynamic runs. Under field conditions, the interaction between ¹⁴C species (in the mobile and in the stationary phases) make ages obtained by ¹⁴C overestimated.

References (23)

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Cited by (23)

Rates of carbon and oxygen isotope exchange between calcite and fluid at chemical equilibrium
2022, Geochimica et Cosmochimica Acta
Citation Excerpt :
Trace metal, major element, and isotopic compositions of carbonate minerals can be used to infer conditions occurring at the time of mineral formation (Bernard et al., 2017; Coggon et al., 2010; Dekens et al., 2002; Friedrich et al., 2012; Hoefs, 1997; Ravelo and Hillaire-Marcel, 2007; Urey et al., 1951). Carbon, oxygen, and calcium isotopes in carbonates can be used to provide insights into geochemical processes ranging from marine to groundwater environments including acting as proxies for temperature, carbon cycle dynamics, and pH (Avrahamov et al., 2013; Bernard et al., 2017; Fantle and Tipper, 2014; Friedrich et al., 2012; Füger et al., 2019; Garnier, 1985; Gussone et al., 2016; Hoefs, 1997; Katz et al., 2010; Marriott et al., 2004; Mavromatis et al., 2013, 2015, 2019; Mozeto et al., 1984; Ravelo and Hillaire-Marcel, 2007; Riechelmann et al., 2018; Spero et al., 1997; Urey et al., 1951). To use any of these proxies effectively requires first that isotopic and trace element compositions are preserved over timescales up to millions of years, and second that the mechanisms of isotope fractionation are known.
The isotopic composition of carbonate minerals provides a record of historical geochemical and environmental conditions, but the ability to interpret these compositions as paleo-proxies hinges on their preservation over thousand to million year timescales. At chemical equilibrium, alteration of initial isotopic compositions of calcite can occur in the presence of a fluid without visible changes in morphology at the submicron scale, complicating the interpretation of stable isotope compositions of carbonates. However, the rates and mechanisms of isotope exchange at chemical equilibrium are poorly understood. To evaluate the rates and processes by which C and O isotopes are exchanged between calcite and fluid, batch reactor experiments were conducted at chemical equilibrium between calcite and a fluid enriched in ¹³C and ¹⁸O relative to the solid at 25 °C. Both natural and synthetic calcite of different grain sizes were investigated to evaluate the impact of mineral surface area and size on C and O isotope exchange rates. Our experimental results indicate that rapid exchange of both C and O isotopes occurs within 72 h for all calcite grain sizes studied, likely indicative of exchange of surface species in combination with a backward reaction during dissolution and Ostwald ripening of high energy surface sites. After 72 h, C and O isotope exchange rates were slower but near constant for timescales of thousands of hours. Surface-area normalized C and O isotope exchange rates were similar for all calcite grain sizes studied, and O and C were exchanged in a ∼3:1 ratio consistent with exchange of CO₃^2–. The rates of C and O exchange were ∼4 orders of magnitude lower than far-from-equilibrium calcite dissolution rates, suggesting exchange was controlled either by dissolution-precipitation of pre-existing reactive sites alone, or a combination of dissolution-precipitation and solid state/aqueous mediated diffusion. Overall, the results of this study suggest alteration of O and C isotope compositions of calcite at ambient temperatures can proceed readily over short time scales, though the extent to which this process continues to operate over geologic time scales is difficult to predict at present. The results of this study further highlight the importance of generating a mechanistic understanding of the process of isotope exchange at chemical equilibrium, and represent a step towards this understanding.
Characterization of a carbonate karstic aquifer flow system using multiple radioactive noble gases (3H-3He, 85Kr, 39Ar) and 14C as environmental tracers
2018, Geochimica et Cosmochimica Acta
Citation Excerpt :
In fact, secondary calcite can be created. The clear correlation between 14C values and δ13C of the DIC in the EMA (Fig. 5), as well as the increase in Sr/Ca molar ratio downstream (Fig. 9), indicates advancing stages of water-rock interaction for samples with low 14C (see similar trends in Münnich et al., 1967; Wendt et al., 1967; Thilo and Münnich, 1970; Wendt, 1971; Mozeto et al., 1984; Garnier, 1985; Gonfiantini and Zuppi, 2003; Avrahamov et al., 2013). This ratio increase downstream is due to a relativity low partition coefficient of Sr2+ in secondary calcite (5.7 · 10−2; Katz et al., 1972).
Groundwater age in a carbonate karstic aquifer was assessed using a multiple tracer method that enables identification of modern groundwater (recharged after 1955; using ³H-³He, ⁸⁵Kr CFCs, SF₆,), older components (³⁹Ar, ¹⁴C) and quantification of the mixing between them. Twelve wells were sampled in the Eastern Mountain Aquifer (EMA) of Israel along two trajectories, from the recharge area in the mountains, to the natural outlets in the Dead Sea area. The concentration of the dissolved ³⁹Ar in the groundwater decreased from 96 to 12% along the trajectories, indicating recent recharge upstream, and groundwater aged more than 800y downstream. Other tracers present a similar general trend of decreasing concentrations with distance from the recharge area at two distinct rates, suggesting two different groundwater flow velocities in the two different groundwater flow paths. In most of the wells, pronounced mixing was observed according to the presence of young (after 1955) and older water components. The fraction of the young water was quantified by tritium (³H) and by the combination of ³H and ⁸⁵Kr measurements and found to be between 1 and 67%. The wide age distribution is likely caused by the karstic nature of the aquifer with pronounced dispersion and exchange between highly permeable flow channels and stagnant water stored in the rock matrix. Another mixing mechanism is vertical leakage from the upper to the lower sub-aquifer and vice versa according to the groundwater head differences between the two sub-aquifers. Mixing, diffusive exchange and water rock interaction lead to a reduction of ¹⁴C in DIC, resulting in an apparent half-life of ∼900 y instead of 5730y for radioactive decay only. This is concluded from the comparison of ¹⁴C and ³⁹Ar ages.
Mechanisms of inorganic carbon-14 attenuation in contaminated groundwater: Effect of solution pH on isotopic exchange and carbonate precipitation reactions
2017, Applied Geochemistry
Citation Excerpt :
Pre-existing carbonates will be equilibrated with natural 14C-DIC concentrations, but where 14C is present as a contaminant, 14C-DIC will be present at concentrations far above natural abundance levels, potentially driving rapid exchange kinetics. Indeed, isotope exchange of H14CO3− with natural carbonate sand at circumneutral pH was reported to occur on a time scale of only a few days (Garnier, 1985). Isotopic disequilibrium is also the main mechanism in aqueous-gaseous isotopic exchange where disequilibria created by the addition of aqueous 14C species leads to exchange reactions occurring with atmospheric CO2 to restore isotopic equilibrium among all species of carbonate across the aqueous-gaseous pools (Krauskopf and Bird, 1995; Gonfiantini and Zuppi, 2003; White, 2013).
Radioactive ¹⁴C is a significant contaminant associated with nuclear fuels and wastes that is potentially highly mobile in the environment as dissolved inorganic carbonate species. This study investigated the mechanisms by which dissolved inorganic ¹⁴C is retained in surface and groundwater environments via precipitation and isotopic exchange reactions. Precipitation of calcite in the presence and absence of nucleation sites is considered along with isotopic exchange with both atmospheric CO₂ and solid carbonates. Precipitation occurs at calcite supersaturation values of SI_CAL > 1.5 in the absence of nucleation sites and SI_CAL > 0–0.5 in the presence of nucleation sites, suggesting that precipitation of ¹⁴C-bearing carbonates is much more likely in subsurface environments where nucleation sites are abundant. The maximum ¹⁴C removal in solid isotopic exchange experiments occurred after approximately 2 weeks equilibration. In these experiments the amount of ¹⁴C removed from solution was proportional to the amount of calcite surface area present, and removal from solution was equivalent to rapid equalisation of the isotope ratio in an 8–10 Å active surface layer. Although the reactivity of natural carbonates may be lower than the calcite samples used in this study, these results suggest isotopic exchange with solids will be an important ¹⁴C retardation mechanism in subsurface environments containing only modest TIC concentrations. These results suggest that if inorganic ¹⁴C is released into sub-surface environments, both precipitation and solid phase isotopic exchange can result in non-conservative ¹⁴C-DIC transport and ¹⁴C contamination may persist in groundwater for decades following accidental releases. In contrast, in experiments open to atmosphere with pH values below 9.3, complete loss of dissolved inorganic ¹⁴C was very rapid and occurred with timescales of 10's of hours. ¹⁴C loss was due to a rapid exchange of dissolved ¹⁴C species with ¹²CO₂ (g) and the kinetics of ¹⁴C removal increased as pH values were lowered (i.e. atmospheric isotopic exchange was first order with respect to the concentration of carbonic acid present). Thus these results suggest that release of inorganic ¹⁴C to surface waters with pH values <9.3 would result in rapid exchange with ¹²CO₂ (g) and ¹⁴C would not persist in the aqueous environment, whereas ¹⁴C-DIC released to saturated subsurface environments may persist close to the release site for decades due to precipitation and solid phase exchange reactions preventing/retarding transport with the groundwater.
The curved 14C vs. δ13C relationship in dissolved inorganic carbon: A useful tool for groundwater age- and geochemical interpretations
2014, Chemical Geology
Determination of the ¹⁴C content of dissolved inorganic carbon (DIC) is useful for dating of groundwater. However, in addition to radioactive decay, the ¹⁴C content in DIC (¹⁴C_DIC) can be affected by many geochemical and physical processes and numerous models have been proposed to refine radiocarbon ages of DIC in groundwater systems. Changes in the δ¹³C content of DIC (δ¹³C_DIC) often can be used to deduce the processes that affect the carbon isotopic composition of DIC and the ¹⁴C value during the chemical evolution of groundwater. This paper shows that a curved relationship of ¹⁴C_DIC vs. δ¹³C_DIC will be observed for groundwater systems if (1) the change in δ¹³C value in DIC is caused by a first-order or pseudo-first-order process, e.g. isotopic exchange between DIC and solid carbonate, (2) the reaction/process progresses with the ageing of the groundwater, i.e. with decay of ¹⁴C in DIC, and (3) the magnitude of the rate of change in δ¹³C of DIC is comparable with that of ¹⁴C decay. In this paper, we use a lumped parameter method to derive a model based on the curved relationship between ¹⁴C_DIC and δ¹³C_DIC. The derived model, if used for isotopic exchange between DIC and solid carbonate, is identical to that derived by Gonfiantini and Zuppi (2003). The curved relationship of ¹⁴C_DIC vs. δ¹³C_DIC can be applied to interpret the age of the DIC in groundwater. Results of age calculations using the method discussed in this paper are compared with those obtained by using other methods that calculate the age of DIC based on adjusted initial radiocarbon values for individual samples. This paper shows that in addition to groundwater age interpretation, the lumped parameter method presented here also provides a useful tool for geochemical interpretations, e.g. estimation of apparent rates of geochemical reactions and revealing the complexity of the geochemical environment.
Carbon isotope exchange rate of DIC in karst groundwater
2003, Chemical Geology
Citation Excerpt :
Deviations from the predicted isotopic trend will also be observed when DIC is derived in part from organic matter disproportion reactions, which produce CO2 and methane (Barker and Fritz, 1981; Eichinger, 1987). The carbon isotope exchange of DIC with CaCO3 was also investigated by means of laboratory experiments (Münnich et al., 1967; Thilo and Münnich, 1970; Wendt et al., 1967; Wendt, 1971; Mozeto et al., 1984a,b; Garnier, 1985). The experiments showed that the exchange does take place, but the kinetics is much faster than under natural conditions.
The kinetics of isotopic exchange between dissolved inorganic carbon (DIC) of groundwater and calcite of the matrix of karst aquifers of Cyrenaica, Libya, can be deduced from ¹³C and ¹⁴C data. The aquifers are mostly confined, and the majority of the wells do not show any occurrence of modern recharge: in 1976–1980, in fact, the tritium content was below 1 tritium unit (TU) in most sites.
Assuming that the isotopic exchange takes place through a first order reaction such as ¹⁴C radioactive decay, it can be shown that a linear correlation occurs between lnA and ln(δ_M−δ−ε_P), where A is the ¹⁴C activity, δ_M and δ are the ¹³C contents of matrix calcite and DIC, respectively, and ε_P is the ¹³C enrichment in CaCO₃ precipitation. The slope of the correlation provides the half-life of the isotopic exchange process. For Cyrenaica karst groundwater, a half-life of about 11,000 years is obtained, i.e. about double that of ¹⁴C radioactive decay.
The isotopic exchange kinetics also depends on the ratio between groundwater volume and the calcite surface exposed to the exchange process. Thus, other aquifers will show different exchange half-life values. The Cretaceous chalk aquifers of the Paris Basin, France and Lägerdorf, Germany give a half-life of about 4000 years, much shorter than that of Cyrenaica, which may be due to the high porosity, i.e. to the large surface available for the isotope exchange process. The Berkshire Chalk aquifer, UK, gives a half-life of about 10,000 years. Much higher half-lives, above 20,000 years, are obtained for two sandy aquifers in Flevoland, The Netherlands, and Texas, USA, which could be explained by the low CaCO₃ content of the aquifer matrix. The highest half-life value, about 40,000 years, is obtained in an artesian limestone aquifer in Florida, USA.
Sorption of inorganic 14C on to calcite, montmorillonite and soil
1998, Applied Geochemistry
Although ¹⁴C occurs naturally, it is also a waste product of the nuclear industry, and can be important because of its long half-life, high mobility as an anion, and ready incorporation into biota. Some aqueous inorganic species are anionic with migration minimally retarded by most geological and soil materials. Substantial retardation is expected when calcite is present, but there are few data to quantify this effect. The present study measured partition coefficient values, R_d (concentration on solids divided by concentration in liquids), of 8–85 l kg⁻¹ for a series of calcite materials and for a carbonated soil. In contrast, R_d was zero for montmorillonite. The series of calcite materials varied in particle size. In order to investigate the effects of particle size, dissolution and degassing of ¹⁴C and ¹²C were monitored as pH was slowly decreased. The change in pH with addition of acid was strongly affected by particle size, as expected, but there was no systematic effect of particle size on the relative dissolution rates of ¹⁴C vs ¹²C, or on R_d. Apparently, surface area was not a limiting factor in the interaction of ¹⁴C with these materials. The ¹⁴C in soil behaved most like the very fine calcite, indicating that the specific surface of the soil carbonate was similar to that of the very fine calcite.

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Geochimica et Cosmochimica Acta

Abstract

References (23)

Dispersion in porous media

Adv. Hydroscience

Experimental observations on carbon isotope exchange in carbonate-water systems

Geochim. Cosmochim. Acta

Adsorption, partition, ion exchange and chemical reaction in batch reactors or in columns. A review

J. Hydrol.

Analyses des processus chromatographiques linéaires à l'aide de modèles phénoménologiques

Chem. Eng. Sci.

Relation entre la forme des pics chromatographiques et les paramètres physiques et opératoires de la colonne

J. Chromatogr. Sci.

Modèle représentatif de la distribution des temps de séjour dans un réacteur semi-infini à dispersion axiale avec zones stagnantes

Chem. Eng. Sci.

Sorption studies of H¹⁴CO₃⁻ on some geologic media and concrete

Dissolution kinetics of calcium carbonate in sea water. IV: Theory of calcite dissolution

Amer. J. Sci.

Effects of solution and exchange on the radiocarbon dating of sediments and natural waters

Dead-end pore volume and dispersion in porous media

J. Soc. Petrol. Eng.

Carbon isotope fractionation during the precipitation of calcium carbonate

Earth Planet. Sci. Lett.

Cited by (23)

Rates of carbon and oxygen isotope exchange between calcite and fluid at chemical equilibrium

Characterization of a carbonate karstic aquifer flow system using multiple radioactive noble gases (<sup>3</sup>H-<sup>3</sup>He, <sup>85</sup>Kr, <sup>39</sup>Ar) and <sup>14</sup>C as environmental tracers

Mechanisms of inorganic carbon-14 attenuation in contaminated groundwater: Effect of solution pH on isotopic exchange and carbonate precipitation reactions

The curved <sup>14</sup>C vs. δ<sup>13</sup>C relationship in dissolved inorganic carbon: A useful tool for groundwater age- and geochemical interpretations

Carbon isotope exchange rate of DIC in karst groundwater

Sorption of inorganic <sup>14</sup>C on to calcite, montmorillonite and soil

Retardation of dissolved radiocarbon through a carbonated matrix

Abstract

Adv. Hydroscience

Geochim. Cosmochim. Acta

J. Hydrol.

Chem. Eng. Sci.

J. Chromatogr. Sci.

Chem. Eng. Sci.

Sorption studies of H14CO3− on some geologic media and concrete

Dissolution kinetics of calcium carbonate in sea water. IV: Theory of calcite dissolution

Amer. J. Sci.

Effects of solution and exchange on the radiocarbon dating of sediments and natural waters

Dead-end pore volume and dispersion in porous media

J. Soc. Petrol. Eng.

Carbon isotope fractionation during the precipitation of calcium carbonate

Earth Planet. Sci. Lett.

Sorption studies of H¹⁴CO₃⁻ on some geologic media and concrete