10:45 AM - 12:15 PM
[BCG06-P02] Geochemical modelling of cation-exchange reaction and whole-rock compositional change in Neogene marine sediments
Keywords:Cation exchange reaction, Meteoric water infiltration, PHREEQC, Whole-rock chemistry
1. Introduction
In order to validate assessment methodology of radioactive waste disposal, JAEA has been studying geochemical data of boring cores and groundwater in Horonobe. The relationship between petrological and hydrochemical characteristics of Neogene marine sediments in Horonobe, target sample of this study, has been investigated by Ishii et al. (2007) and leaching of Na in meteoric water infiltration zone has been revealed. However, interpretation of the infiltration depth remains inconsistent between based on δD and δ18O compositions of groundwater (Mochizuki&Ishii, 2022) and based on whole-rock chemistry (Na2O/Al2O3 decrease, Ishii et al., 2007). In this study, calculated compositional changes in exchangeable cation and whole-rock chemistry of the sediments are compared with measured geochemical data, and then validity and applicability of the geochemical model are discussed including interpretation of the infiltration depth. In this paper, results of static cation-exchange reaction in a batch system were reported.
2. Cation-exchange reaction model
In this study, reactive group relating to cation-exchange was simply assumed to one site as first approximation, and the reaction system of Na-K-Ca-Mg-H-X (X is the reactive group) was considered. A thermodynamic constant of an exchange reaction between Na+ and Men+ (Me = K, Ca, Mg; n = charge of Me), Men+ + nNaX = MeXn + nNa+, can be written,
KNa/Me = {(γNa+･mNa+)n/(γMen+･mMen+)}･{(γMeXn･mMeXn)/( γNaX･mNaX)n}
where γi is the activity coefficient of chemical species i, mi is molar concentration of i.
The activity coefficient of adsorbed ions is considered to be same with that of aqueous ions (c.f., Neal&Cooper, 1983; Appelo, 1994). When this assumption is applied, KNa/Me can be denoted as follows.
KNa/Me = {EMeXn/(ENaX)n}･{(ENa+)n/EMen+}･(Ctot/CEC)
where Ei means the mole equivalent fraction of i, Ctot is total concentration of aqueous ions and CEC is exchangeable cation capacity.
The parameter KNa/Me was set based on Bradbury&Baeyens (2002). The KNa/H were given the values for sandy aquifers used in Appelo (1994). The moles of exchange site were 1.59–1.72 mol/kgw.
Exchangeable cationic composition during meteoric water infiltration was calculated by using PHREEQC code (Parkhurst&Appelo, 2013) at the temperature of 25degC. A thermodynamic database of phreeqc.dat was used.
Whole-rock base cation concentrations after the infiltration were calculated from the sum of initial amount and net change in exchangeable cation. The calculated results are shown as ratio in weight concentration with Al2O3.
3. Results and Discussion
Compositional change of exchangeable cation was simulated assuming that hydrochemical variation during the infiltration process can be simply estimated by mixing between fossil seawater and meteoric water (Fig. 1). Dominant exchangeable cation changed from NaX to CaX2, and Na2O/Al2O3 ratio decreased by about 0.05 and CaO/Al2O3 increased by about 0.02. In the infiltration zone of measured depth profiles, these compositional changes should be observed.
Figure 2 shows measured whole-rock chemistry of borehole HDB-5. According to Mochizuki&Ishii (2022), isotopic signature of meteoric water can be confirmed up to the depth of about 400 m. At the depth below 250 m, Na2O/Al2O3 decrease can be observed whereas the decrease cannot be observed at deeper zone from 250 m. Moreover, the infiltration zone below 250 m can be subdivided into two zones: CaO/Al2O3 increase is observed at the depth below 150 m (Zone-1); CaO/Al2O3 is not changed at the depth from 171 to 225 m (Zone-2). Whole-rock feature of Zone-1 is consistent with static cation-exchange calculation, on the other hand, that of Zone-2 and -3 is inconsistent. This implies that whole-rock feature of the infiltration zones would be affected by geochemical buffering effect due to additional chemical reactions and/or chromatographic effect induced by reactive-transport.
In order to validate assessment methodology of radioactive waste disposal, JAEA has been studying geochemical data of boring cores and groundwater in Horonobe. The relationship between petrological and hydrochemical characteristics of Neogene marine sediments in Horonobe, target sample of this study, has been investigated by Ishii et al. (2007) and leaching of Na in meteoric water infiltration zone has been revealed. However, interpretation of the infiltration depth remains inconsistent between based on δD and δ18O compositions of groundwater (Mochizuki&Ishii, 2022) and based on whole-rock chemistry (Na2O/Al2O3 decrease, Ishii et al., 2007). In this study, calculated compositional changes in exchangeable cation and whole-rock chemistry of the sediments are compared with measured geochemical data, and then validity and applicability of the geochemical model are discussed including interpretation of the infiltration depth. In this paper, results of static cation-exchange reaction in a batch system were reported.
2. Cation-exchange reaction model
In this study, reactive group relating to cation-exchange was simply assumed to one site as first approximation, and the reaction system of Na-K-Ca-Mg-H-X (X is the reactive group) was considered. A thermodynamic constant of an exchange reaction between Na+ and Men+ (Me = K, Ca, Mg; n = charge of Me), Men+ + nNaX = MeXn + nNa+, can be written,
KNa/Me = {(γNa+･mNa+)n/(γMen+･mMen+)}･{(γMeXn･mMeXn)/( γNaX･mNaX)n}
where γi is the activity coefficient of chemical species i, mi is molar concentration of i.
The activity coefficient of adsorbed ions is considered to be same with that of aqueous ions (c.f., Neal&Cooper, 1983; Appelo, 1994). When this assumption is applied, KNa/Me can be denoted as follows.
KNa/Me = {EMeXn/(ENaX)n}･{(ENa+)n/EMen+}･(Ctot/CEC)
where Ei means the mole equivalent fraction of i, Ctot is total concentration of aqueous ions and CEC is exchangeable cation capacity.
The parameter KNa/Me was set based on Bradbury&Baeyens (2002). The KNa/H were given the values for sandy aquifers used in Appelo (1994). The moles of exchange site were 1.59–1.72 mol/kgw.
Exchangeable cationic composition during meteoric water infiltration was calculated by using PHREEQC code (Parkhurst&Appelo, 2013) at the temperature of 25degC. A thermodynamic database of phreeqc.dat was used.
Whole-rock base cation concentrations after the infiltration were calculated from the sum of initial amount and net change in exchangeable cation. The calculated results are shown as ratio in weight concentration with Al2O3.
3. Results and Discussion
Compositional change of exchangeable cation was simulated assuming that hydrochemical variation during the infiltration process can be simply estimated by mixing between fossil seawater and meteoric water (Fig. 1). Dominant exchangeable cation changed from NaX to CaX2, and Na2O/Al2O3 ratio decreased by about 0.05 and CaO/Al2O3 increased by about 0.02. In the infiltration zone of measured depth profiles, these compositional changes should be observed.
Figure 2 shows measured whole-rock chemistry of borehole HDB-5. According to Mochizuki&Ishii (2022), isotopic signature of meteoric water can be confirmed up to the depth of about 400 m. At the depth below 250 m, Na2O/Al2O3 decrease can be observed whereas the decrease cannot be observed at deeper zone from 250 m. Moreover, the infiltration zone below 250 m can be subdivided into two zones: CaO/Al2O3 increase is observed at the depth below 150 m (Zone-1); CaO/Al2O3 is not changed at the depth from 171 to 225 m (Zone-2). Whole-rock feature of Zone-1 is consistent with static cation-exchange calculation, on the other hand, that of Zone-2 and -3 is inconsistent. This implies that whole-rock feature of the infiltration zones would be affected by geochemical buffering effect due to additional chemical reactions and/or chromatographic effect induced by reactive-transport.