# Contribution of Ternary Reaction to Pd Sorption on MX-80 in Na-Ca-Cl Solution at High Ionic Strength

Authors: Shinya Nagasaki; Justin Riddoch; Joshua Racette
Journal: Science and Technology of Nuclear Installations
 (2019)

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## Abstract

In this study, we first examined the sorption of Pd on MX-80 in Na-Ca-ClO4 solution as a function of pHc (3–9) and ionic strength (0.1 M–4 M) and confirmed that the experimentally derived Kd values could be fitted by a 2-site protolysis nonelectrostatic surface complexation and cation exchange (2SPNE SC/CE) model using three binary surface complexation constants previously estimated. Then, we investigated the sorption of Pd on MX-80 in Na-Ca-Cl-ClO4 solution as a function of pHc (3–9) and Cl-/ClO4- molar concentration ratio (0–∞) at the ionic strength = 4 M. We found that the sorption of Pd on MX-80 in Na-Ca-Cl-ClO4 solution could be simulated only by the three binary and one ternary surface complexations (S-OH+Pd2++4Cl-↔S-OPdCl43-+H+). This suggests that the contribution of other ternary surface complexations such as ≡S-OH +Pd2++xCl-↔ ≡S-OPdClxx-1-+H+ (x = 1, 2 and 3) to Pd sorption in Na-Ca-Cl-ClO4 solution with ionic strength = 4 M was negligibly small.


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## Body

## 1. Introduction

Canadian sedimentary rocks are being considered as potential host rocks for a deep geologic repository (DGR) for used nuclear fuel. Some of these rocks in Canada (for example, Southern Ontario, Michigan Basin [1]) contain Na-Ca-Cl type brine solutions with total dissolved solids (TDS) concentration up to 350 g/L or with the ionic strength (I) as high as 7.2 M. Sorption of radionuclides onto host rocks surrounding a DGR and onto materials comprising the engineered barrier system is considered an important mechanism for retarding their subsurface transport from the repository to the biosphere [2–6]. Therefore, it is important to elucidate the sorption behaviour of radionuclides not only on host rocks, but also on engineered barrier materials such as bentonite.The Nuclear Waste Management Organization of Canada (NWMO) has been maintaining a database of sorption distribution coefficients,Kd, for Canadian sedimentary rocks and bentonite [2]. Palladium is selected as an element of interest by the NWMO, but the only existing relevant sorption data for Pd on bentonite had been measured in 0.01 and 0.1 M NaCl solutions [7]. Hence, sorption of Pd onto Queenston shale, illite (the major mineral constituent of Queenston shale), and MX-80 bentonite (the candidate material for engineered barrier in Canada) in Na-Ca-Cl solutions with pHc ranging from 3 to 9 andI ranging from 0.01 M to 6 M was studied in our previous paper [8]. The 2-site protolysis nonelectrostatic surface complexation and cation exchange (2SPNE SC/CE) model [9, 10] with SIT parameters was applied to the pHc dependence of Kd values for illite and MX-80 (I ≤ 4 M), and the optimized surface complexation and cation exchange constants were estimated [8]. Since Pitzer parameters for Pd in Na-Ca-Cl solution are not completely compiled, we did not apply the 2SPNE SC/CE model to the Kd values atI = 6 M. We found that Pd sorption could largely be accounted for with the formation of the ≡S-OPdOH, ≡S-OPdOH2-, ≡S-OPdCl43- surface species, and the ≡S-O2Pd exchange species with 2Na+ for both sorbents, with the additional ≡S-OPd+ surface species for MX-80. We included four possible ternary surface complexations (≡S-OH + Pd2+ +xCl- ↔  ≡S-OPdClx(x-1)- + H+;x = 1 – 4) in the fitting by 2SPNE SC/CE model, but only one reaction (≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+) was found to contribute to the sorption in Na-Ca-Cl solutions with highI.In this work, we first measured the sorption of Pd on MX-80 in Na-Ca-ClO4 solutions as a function of pHc andI and studied whether the experimentally derived Kd values in the absence of Cl- can be simulated by using our previously optimized constants. Then, we measured the sorption of Pd on MX-80 in Na-Ca-Cl-ClO4 solutions as a function of pHc and Cl-/ClO4- molar concentration ratio (0–∞) atI = 4 M and investigated the contribution of ternary surface complexation to the Pd sorption on MX-80 atI = 4 M.


## 2. Materials and Methods

### 2.1. Chemicals, Solids, and Solutions

All chemicals used were reagent grade and were supplied from Fisher Scientific. Deionized water was prepared using a Milli-Q Direct 8 (18.2 MΩ·cm). The MX-80 bentonite sample was supplied by the American Colloid Company and was used as received. Pd used in experiments was derived from a 1000 ± 1 μg/mL standard solution with a natural isotopic abundance supplied by Agilent Technology. A Precise Controlled Atmosphere Glove Box (GB) supplied by Labconco was filled with N2 gas (>99.999 %). The concentration of O2 in the GB was smaller than 2 ppm.The reference brine solution in Southern Ontario, SR-270-PW, is proposed by the NWMO [2]. It has a Na/Ca molar concentration ratio of 2.7. In this study, Na-Ca-ClO4 solutions (Na/Ca molar concentration ratio = 2.7) withI = 0.1, 1, 2, 4 M were prepared by NaClO4·H2O and Ca(ClO4)2·4H2O, and Na-Ca-Cl-ClO4 solutions (Na/Ca ratio = 2.7; Cl-/ClO4- ratio = 0.01, 0.1, 1, 10, 100, ∞) withI = 4 M were prepared by NaCl, CaCl2·2H2O, and NaClO4·H2O. Na-Ca-ClO4 solution withI = 4 M was considered a case of Cl-/ClO4- ratio = 0.The pH values indicated on the pH meter (pHmeasure) should be considered as operational values at high ionic strength (I ≥ 0.1 M) [11]. The relationships between the operational pHmeasure and the molar H+ concentrations (pHc=-log⁡cH+) or the molal H+ concentration (pHm=-log⁡mH+) were discussed in detail by Altmaier et al. [12, 13]. In this study, the relationships between pHmeasure (Fisher Scientific Accumet AB 150/micro accupHast pH combination electrode) and pHc in solutions were determined by titration (Metrohm Ti-Touch 916), and the pHmeasure values were converted to the pHc values.


### 2.2. Sorption Experiments

All sorption experiments were carried out in triplicate at 25°C. In preliminary tests, the sorption of Pd on the wall of a polycarbonate reaction vessel was found to be negligible. Initial concentration of Pd for all sorption tests was 1.0 × 10−7 M and was confirmed to be smaller than solubility of Pd [8]. The liquid/solid ratio was set to be 0.5 m3/kg.We followed the sorption experimental procedure we conducted in Np(V) and Np(IV) sorption previously [14–16]. In the GB, MX-80 was added into a polycarbonate reaction vessel and Na-Ca-ClO4 or Na-Ca-Cl-ClO4 solution was added into the reaction vessel. The suspensions were kept in the GB for 3 to 4 days for preequilibration (the pHc of solutions in contact with MX-80 was confirmed to be stabilized within 1 day in the preliminary tests). The reaction vessels were removed from the GB and the liquid and solid were separated by centrifugation for 120 min at 18,000 rpm (Beckman Coulter, Allegra X-30R). The reactions vessels were then transferred back into the GB and the liquid was removed by pipette. Na-Ca-ClO4 or Na-Ca-Cl-ClO4 solution was then added into the reaction vessel again, and a portion of the Pd standard solution was spiked into the reaction vessel. The pHc of the solution was adjusted to the predecided values at 3 ≤ pHc ≤ 9 by addition of 0.01 M HClO4, 0.01 M HCl, or 0.01 M NaOH solution in the GB, and the reaction vessel was tightly sealed. Change inI and Cl-/ClO4- ratio by addition of HClO4, HCl, or NaOH could be neglected.The reaction vessels were transferred from the GB to an incubator at 25°C (Infors HT Ectron) and were shaken for 14 days. Fourteen days were enough to reach to the sorption equilibrium [8]. The pHc of the solution was measured once a day in the GB and readjusted to the original pHc value if the pHc changed by more than ± 0.3 from the original value. After 14 days, the liquid was separated from the solid by centrifugation for 120 minutes at 18,000 rpm at 25°C. We previously confirmed that the liquid phase was sufficiently separated from the solid phase by this method [14]. The pHc of the liquid was measured in the GB, and an aliquot was sampled from the liquid phase of each reaction vessel. The concentration of Pd in the aliquot was measured by ICP-MS (Agilent ICP-MS 8800).TheKd value (m3/kg) is calculated according to Equation (1), whereCi is the initial concentration of Pd in solution (1.0 × 10−7 M),Ceq the concentration at sorption equilibrium (M),L the volume of liquid used in the sample (m3), andM the mass of solid used (kg). In this paper, the sorption of Pd on MX-80 is described by Kd values.(1)Kd=Ci-CeqCeq×LM


### 2.3. Sorption Modelling

ThepHc,I, and Cl-/ClO4- ratio dependence of Kd values for MX-80 was fitted using the 2SPNE SC/CE model with the program PHREEQC [17]. The specific-ion interaction theory (SIT) was used for the computational method, as it accounts for the electrostatic interactions of all other ions in solution with the one ion of interest. This is necessary for accuracy when computing activity coefficients in solutions of highI. The Pitzer computational method can be more accurate in higherI solutions; however, Pitzer parameters for Pd were not completely known at the present. SIT is known to be inaccurate atI > 4.0 M and, therefore, sorption was modelled atI ≤ 4.0 M in this study [18].The Japan Atomic Energy Agency (JAEA) thermodynamic database (TDB) was chosen as the reference database since it contains the necessary thermodynamic data for Pd [19]. However, no thermodynamic data for PdOH+ and PdOH42- are available in JAEA TDB, so the values contained in the SIT database (v9a) included in the PHREEQC [20] were added to the JAEA TDB. The hydrolysis and complexation constants used in this work are summarized in Table 1.Table 1
Thermodynamic data of Pd(II) species used in this work [19, 20].
Hydrolysis and complexation reaction log ⁡ K Reference Pd2+ + H2O - H+ ↔ PdOH+ 1.86 [18] Pd2+ + 2H2O - 2H+ ↔ Pd(OH)2 -3.49 [17] Pd2+ + 3H2O - 3H+ ↔ PdOH3- -15.48 [17] Pd2+ + 4H2O - 4H+ ↔ PdOH42- -29.36 [18] Pd2+ + Cl- ↔ PdCl+ 5.00 [17] Pd2+ + 2Cl- ↔ PdCl2 8.42 [17] Pd2+ + 3Cl- ↔ PdCl3- 10.93 [17] Pd2+ + 4Cl- ↔ PdCl42- 13.05 [17] Pd2+ + 3Cl- + H2O - H+ ↔ PdCl3OH2- 3.77 [17]The specific surface area and the cation exchange capacity of MX-80 were 26.2 m2/g [21] and 102.1 meq/100g [22], respectively. However, the sorption site capacity was not measured. Hence, we assumed that the sorption site capacity is proportional to the specific surface area [14]. As shown in Supplementary Materials, the contribution of cation exchange reaction to the sorption of Pd on MX-80 was negligibly small. Therefore, the slight difference in the value of the cation exchange capacity did not affect the calculation results. Since the concentration of Pd in the solution was much smaller than the total sorption site number in the MX-80 used, the slight difference in the value of the specific surface area also did not influence the calculation results.In the 2SPNE SC/CE model, Bradbury and Baeyens considered both strong and weak surface sites [9, 10]. In our previous work [8], we could simulate the Pd sorption on MX-80 in Na-Ca-Cl solution by considering the strong site only and estimate the optimized surface complexation and cation exchange constants atI = 0 M, as shown in Table 2.Table 2
Optimized surface complexation and cation exchange constants [8].
Sorption reaction log ⁡ K Binary surface complexation: ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+ 8.2 ± 0.2 ≡S-OH + Pd2+ + H2O ↔  ≡S-OPdOH + 2H+ 2.3 ± 0.2 ≡S-OH + Pd2+ + 2H2O ↔  ≡S-OPdOH2- + 3H+ -5.5 ± 0.2  Ternary surface complexation: ≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+ 13.05 ± 0.2  Cation exchange: 2≡S-ONa + Pd2+ ↔ (≡S-O)2Pd + 2Na+ 5.90 ± 0.2For the Pd sorption in Na-Ca-ClO4 solutions, the 2SPNE SC/CE model calculation was conducted using the binary surface complexation and cation exchange constants illustrated in Table 2.For the Pd sorption in Na-Ca-Cl-ClO4 solutions, we included the additional three ternary surface complexations (≡S-OH + Pd2+ +xCl- ↔  ≡S-OPdClx(x-1)- + H+;x = 1 – 3) in the modelling. Bradbury and Baeyens proposed the linear free energy relationships (LFER) between the surface complexation constants for montmorillonite and the hydrolysis constants [9]. In the fitting of the 2SPNE SC/CE model to the Kd values experimentally derived, we set the initial values for additional ternary surface complexation constants, by assuming that the proposed LFER equation for montmorillonite can be used for the surface complexation constants and the PdClx(x-2)- complexation constants (x = 1 – 3). The initial value of ternary surface complexation constant for ≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+ was set as 13.05. In the fitting, the values of four ternary sorption reaction constants were used as the fitting parameters to obtain the best optimized fitting, and other values were fixed.


## 2.1. Chemicals, Solids, and Solutions

All chemicals used were reagent grade and were supplied from Fisher Scientific. Deionized water was prepared using a Milli-Q Direct 8 (18.2 MΩ·cm). The MX-80 bentonite sample was supplied by the American Colloid Company and was used as received. Pd used in experiments was derived from a 1000 ± 1 μg/mL standard solution with a natural isotopic abundance supplied by Agilent Technology. A Precise Controlled Atmosphere Glove Box (GB) supplied by Labconco was filled with N2 gas (>99.999 %). The concentration of O2 in the GB was smaller than 2 ppm.The reference brine solution in Southern Ontario, SR-270-PW, is proposed by the NWMO [2]. It has a Na/Ca molar concentration ratio of 2.7. In this study, Na-Ca-ClO4 solutions (Na/Ca molar concentration ratio = 2.7) withI = 0.1, 1, 2, 4 M were prepared by NaClO4·H2O and Ca(ClO4)2·4H2O, and Na-Ca-Cl-ClO4 solutions (Na/Ca ratio = 2.7; Cl-/ClO4- ratio = 0.01, 0.1, 1, 10, 100, ∞) withI = 4 M were prepared by NaCl, CaCl2·2H2O, and NaClO4·H2O. Na-Ca-ClO4 solution withI = 4 M was considered a case of Cl-/ClO4- ratio = 0.The pH values indicated on the pH meter (pHmeasure) should be considered as operational values at high ionic strength (I ≥ 0.1 M) [11]. The relationships between the operational pHmeasure and the molar H+ concentrations (pHc=-log⁡cH+) or the molal H+ concentration (pHm=-log⁡mH+) were discussed in detail by Altmaier et al. [12, 13]. In this study, the relationships between pHmeasure (Fisher Scientific Accumet AB 150/micro accupHast pH combination electrode) and pHc in solutions were determined by titration (Metrohm Ti-Touch 916), and the pHmeasure values were converted to the pHc values.


## 2.2. Sorption Experiments

All sorption experiments were carried out in triplicate at 25°C. In preliminary tests, the sorption of Pd on the wall of a polycarbonate reaction vessel was found to be negligible. Initial concentration of Pd for all sorption tests was 1.0 × 10−7 M and was confirmed to be smaller than solubility of Pd [8]. The liquid/solid ratio was set to be 0.5 m3/kg.We followed the sorption experimental procedure we conducted in Np(V) and Np(IV) sorption previously [14–16]. In the GB, MX-80 was added into a polycarbonate reaction vessel and Na-Ca-ClO4 or Na-Ca-Cl-ClO4 solution was added into the reaction vessel. The suspensions were kept in the GB for 3 to 4 days for preequilibration (the pHc of solutions in contact with MX-80 was confirmed to be stabilized within 1 day in the preliminary tests). The reaction vessels were removed from the GB and the liquid and solid were separated by centrifugation for 120 min at 18,000 rpm (Beckman Coulter, Allegra X-30R). The reactions vessels were then transferred back into the GB and the liquid was removed by pipette. Na-Ca-ClO4 or Na-Ca-Cl-ClO4 solution was then added into the reaction vessel again, and a portion of the Pd standard solution was spiked into the reaction vessel. The pHc of the solution was adjusted to the predecided values at 3 ≤ pHc ≤ 9 by addition of 0.01 M HClO4, 0.01 M HCl, or 0.01 M NaOH solution in the GB, and the reaction vessel was tightly sealed. Change inI and Cl-/ClO4- ratio by addition of HClO4, HCl, or NaOH could be neglected.The reaction vessels were transferred from the GB to an incubator at 25°C (Infors HT Ectron) and were shaken for 14 days. Fourteen days were enough to reach to the sorption equilibrium [8]. The pHc of the solution was measured once a day in the GB and readjusted to the original pHc value if the pHc changed by more than ± 0.3 from the original value. After 14 days, the liquid was separated from the solid by centrifugation for 120 minutes at 18,000 rpm at 25°C. We previously confirmed that the liquid phase was sufficiently separated from the solid phase by this method [14]. The pHc of the liquid was measured in the GB, and an aliquot was sampled from the liquid phase of each reaction vessel. The concentration of Pd in the aliquot was measured by ICP-MS (Agilent ICP-MS 8800).TheKd value (m3/kg) is calculated according to Equation (1), whereCi is the initial concentration of Pd in solution (1.0 × 10−7 M),Ceq the concentration at sorption equilibrium (M),L the volume of liquid used in the sample (m3), andM the mass of solid used (kg). In this paper, the sorption of Pd on MX-80 is described by Kd values.(1)Kd=Ci-CeqCeq×LM


## 2.3. Sorption Modelling

ThepHc,I, and Cl-/ClO4- ratio dependence of Kd values for MX-80 was fitted using the 2SPNE SC/CE model with the program PHREEQC [17]. The specific-ion interaction theory (SIT) was used for the computational method, as it accounts for the electrostatic interactions of all other ions in solution with the one ion of interest. This is necessary for accuracy when computing activity coefficients in solutions of highI. The Pitzer computational method can be more accurate in higherI solutions; however, Pitzer parameters for Pd were not completely known at the present. SIT is known to be inaccurate atI > 4.0 M and, therefore, sorption was modelled atI ≤ 4.0 M in this study [18].The Japan Atomic Energy Agency (JAEA) thermodynamic database (TDB) was chosen as the reference database since it contains the necessary thermodynamic data for Pd [19]. However, no thermodynamic data for PdOH+ and PdOH42- are available in JAEA TDB, so the values contained in the SIT database (v9a) included in the PHREEQC [20] were added to the JAEA TDB. The hydrolysis and complexation constants used in this work are summarized in Table 1.Table 1
Thermodynamic data of Pd(II) species used in this work [19, 20].
Hydrolysis and complexation reaction log ⁡ K Reference Pd2+ + H2O - H+ ↔ PdOH+ 1.86 [18] Pd2+ + 2H2O - 2H+ ↔ Pd(OH)2 -3.49 [17] Pd2+ + 3H2O - 3H+ ↔ PdOH3- -15.48 [17] Pd2+ + 4H2O - 4H+ ↔ PdOH42- -29.36 [18] Pd2+ + Cl- ↔ PdCl+ 5.00 [17] Pd2+ + 2Cl- ↔ PdCl2 8.42 [17] Pd2+ + 3Cl- ↔ PdCl3- 10.93 [17] Pd2+ + 4Cl- ↔ PdCl42- 13.05 [17] Pd2+ + 3Cl- + H2O - H+ ↔ PdCl3OH2- 3.77 [17]The specific surface area and the cation exchange capacity of MX-80 were 26.2 m2/g [21] and 102.1 meq/100g [22], respectively. However, the sorption site capacity was not measured. Hence, we assumed that the sorption site capacity is proportional to the specific surface area [14]. As shown in Supplementary Materials, the contribution of cation exchange reaction to the sorption of Pd on MX-80 was negligibly small. Therefore, the slight difference in the value of the cation exchange capacity did not affect the calculation results. Since the concentration of Pd in the solution was much smaller than the total sorption site number in the MX-80 used, the slight difference in the value of the specific surface area also did not influence the calculation results.In the 2SPNE SC/CE model, Bradbury and Baeyens considered both strong and weak surface sites [9, 10]. In our previous work [8], we could simulate the Pd sorption on MX-80 in Na-Ca-Cl solution by considering the strong site only and estimate the optimized surface complexation and cation exchange constants atI = 0 M, as shown in Table 2.Table 2
Optimized surface complexation and cation exchange constants [8].
Sorption reaction log ⁡ K Binary surface complexation: ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+ 8.2 ± 0.2 ≡S-OH + Pd2+ + H2O ↔  ≡S-OPdOH + 2H+ 2.3 ± 0.2 ≡S-OH + Pd2+ + 2H2O ↔  ≡S-OPdOH2- + 3H+ -5.5 ± 0.2  Ternary surface complexation: ≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+ 13.05 ± 0.2  Cation exchange: 2≡S-ONa + Pd2+ ↔ (≡S-O)2Pd + 2Na+ 5.90 ± 0.2For the Pd sorption in Na-Ca-ClO4 solutions, the 2SPNE SC/CE model calculation was conducted using the binary surface complexation and cation exchange constants illustrated in Table 2.For the Pd sorption in Na-Ca-Cl-ClO4 solutions, we included the additional three ternary surface complexations (≡S-OH + Pd2+ +xCl- ↔  ≡S-OPdClx(x-1)- + H+;x = 1 – 3) in the modelling. Bradbury and Baeyens proposed the linear free energy relationships (LFER) between the surface complexation constants for montmorillonite and the hydrolysis constants [9]. In the fitting of the 2SPNE SC/CE model to the Kd values experimentally derived, we set the initial values for additional ternary surface complexation constants, by assuming that the proposed LFER equation for montmorillonite can be used for the surface complexation constants and the PdClx(x-2)- complexation constants (x = 1 – 3). The initial value of ternary surface complexation constant for ≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+ was set as 13.05. In the fitting, the values of four ternary sorption reaction constants were used as the fitting parameters to obtain the best optimized fitting, and other values were fixed.


## 3. Results and Discussion

### 3.1. Sorption in Na-Ca-ClO4 Solution

ThepHc andI dependence of Kd values for MX-80 is plotted in Figure 1. It was found that the Kd values decreased with pHc (3–7) and then became constant against pHc (7–10). Little dependence of Kd values onI was also observed. These tendencies are consistent with the previous study by Tachi et al. [7].Figure 1
p H c andI dependence of Kd in Na-Ca-ClO4 solutions. Plots are the experimentally measured Kd values. The solid lines are the 2SPNE SC/CE model calculation results. The error bars represent the standard deviation of triplicate measurements.The 2SPNE SC/CE model with the binary surface complexation and cation exchange constants previously estimated in the Na-Ca-Cl solutions [8] was applied to the pHc andI dependence of Kd values in the Na-Ca-ClO4 solutions. The model calculation results are also illustrated in Figure 1. We could find the consistency between the experimental and calculation results.In the pH region of 3 – 10, Pd(OH)2 is a dominant species in the solution (Pd speciation in Na-Ca-ClO4 solution atI = 4 M is shown in Figure S-1. Pd speciation atI < 4 M is not shown here). As shown in Figure S-8, the binary surface complexations ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+, ≡S-OH + Pd2+ + H2O ↔  ≡S-OPdOH + 2H+, and ≡S-OH + Pd2+ + 2H2O ↔  ≡S-OPdOH2- + 3H+ dominate the sorption at pH = 3–6, 6–8, and 8–10, respectively. This indicates that Pd sorbs on MX-80 markedly in the acidic pH region through the ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+ surface complexation in the absence of Cl-. From Figure S-8, the contribution of cation exchange reaction was also found to be negligibly small. These demonstrate that the binary surface complexation constants we previously estimated [8] can be used to simulate the sorption of Pd on MX-80 in the Na-Ca-ClO4 solutions quantitatively.


### 3.2. Sorption in Na-Ca-Cl-ClO4 Solutions

Figure2 illustrates the pHc and Cl-/ClO4- ratio dependence of Kd values for MX-80. We found that the Kd values at pHc < 7 decreased as the Cl-/ClO4- ratio increased. Particularly, the Kd values at pHc < 6 for MX-80 were drastically suppressed in the presence of Cl- even at Cl-/ClO4- ratio = 0.01, compared to the Kd values in the absence of Cl- (Figure 1). On the other hand, the Kd values at pHc = 7 – 9 slightly increased with Cl-/ClO4- ratio up to 1 and did not change any more at Cl-/ClO4- ratio ≥ 10.Figure 2
p H c and Cl-/ClO4- molar concentration ratio dependence of Kd in Na-Ca-Cl-ClO4 solutions atI = 4 M. Plots are the experimentally measured Kd values. The solid lines are the 2SPNE SC/CE model calculation results. The error bars represent the standard deviation of triplicate measurements.The Pd speciation in Na-Ca-Cl-ClO4 solutions atI = 4 M at Cl-/ClO4- ratio = 0.01, 0.1, 1, 10, 100 and infinity are shown in Figures S-2–S-7, respectively. It was found that the pH region of PdCl42- dominance expanded with Cl-/ClO4- ratio, while that of Pd(OH)2 dominance shrank. Figure 2 suggests that the formation of PdCl42- complex strongly suppressed the Pd sorption, especially the surface complexation ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+, on MX-80 at pHc < 7.The fitting results of the 2SPNE SC/CE model to the experimentally measuredKd values using three binary and four ternary surface complexations and cation exchange reaction are also drawn in Figure 2. It was shown that the pHc dependence of Kd values in the wide range of Cl-/ClO4- ratio could be also quantitatively fitted by the 2SPNE SC/CE model. It was found that the fitting results could be obtained only with three binary and one ternary surface complexations as we studied in Na-Ca-Cl solutions [8]. Namely, only the ternary surface complexation, ≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+, among the four complexations considered in the modelling contributed to the Pd sorption on MX-80 in Na-Ca-Cl-ClO4 solutions. The contribution of other three ternary surface complexations was negligible. The surface complexation constant for this ternary sorption reaction was estimated as 13.3 ± 0.3. This value is consistent with 13.05 ± 0.2 which was previously estimated [8].The pH dependence of surface complexation and cation exchange reaction at Cl-/ClO4- ratio = 0, 0.01, 0.1, 1, 10, 100 and infinity is shown in Figures S-8–S-14, respectively. Three binary surface complexations dominated the sorption at Cl-/ClO4- ratio = 0. In the presence of Cl-, the surface complexation, ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+, was drastically suppressed even at Cl-/ClO4- ratio = 0.01. As the Cl-/ClO4- ratio increased, the contribution of surface complexations, ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+ and ≡S-OH + Pd2+ + H2O ↔  ≡S-OPdOH + 2H+, became small and was finally zero. This led to the decrease in the measured Kd values at pHc < 7. On the other hand, the contribution of ternary surface complexation, ≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+, became large at Cl-/ClO4- ratio = 0.1 and was significant and dominated the sorption at Cl-/ClO4- ratio ≥ 1. The increase in the measured Kd values at pHc 7–9 was attributed to this ternary surface complexation.The complexation constant for Pd2+ + 3Cl- + H2O - H+ ↔ PdCl3OH2- is compiled in the JAEA TDB (Table 1). Although PdCl3OH2- did not appear in Na-Ca-Cl-ClO4 solutions atI = 4 M (Figures S-1–S-7), the 2SPNE SC/CE model including the ternary surface complexation ≡S-OH + Pd2+ + 3Cl- + H2O ↔  ≡S-OPdCl3OH3- + 2H+ was also applied to the measured Kd values to make sure of the contribution of ternary surface complexation in the Pd sorption on MX-80. However, it was found that this ternary surface complexation could be negligible.


## 3.1. Sorption in Na-Ca-ClO4 Solution

ThepHc andI dependence of Kd values for MX-80 is plotted in Figure 1. It was found that the Kd values decreased with pHc (3–7) and then became constant against pHc (7–10). Little dependence of Kd values onI was also observed. These tendencies are consistent with the previous study by Tachi et al. [7].Figure 1
p H c andI dependence of Kd in Na-Ca-ClO4 solutions. Plots are the experimentally measured Kd values. The solid lines are the 2SPNE SC/CE model calculation results. The error bars represent the standard deviation of triplicate measurements.The 2SPNE SC/CE model with the binary surface complexation and cation exchange constants previously estimated in the Na-Ca-Cl solutions [8] was applied to the pHc andI dependence of Kd values in the Na-Ca-ClO4 solutions. The model calculation results are also illustrated in Figure 1. We could find the consistency between the experimental and calculation results.In the pH region of 3 – 10, Pd(OH)2 is a dominant species in the solution (Pd speciation in Na-Ca-ClO4 solution atI = 4 M is shown in Figure S-1. Pd speciation atI < 4 M is not shown here). As shown in Figure S-8, the binary surface complexations ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+, ≡S-OH + Pd2+ + H2O ↔  ≡S-OPdOH + 2H+, and ≡S-OH + Pd2+ + 2H2O ↔  ≡S-OPdOH2- + 3H+ dominate the sorption at pH = 3–6, 6–8, and 8–10, respectively. This indicates that Pd sorbs on MX-80 markedly in the acidic pH region through the ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+ surface complexation in the absence of Cl-. From Figure S-8, the contribution of cation exchange reaction was also found to be negligibly small. These demonstrate that the binary surface complexation constants we previously estimated [8] can be used to simulate the sorption of Pd on MX-80 in the Na-Ca-ClO4 solutions quantitatively.


## 3.2. Sorption in Na-Ca-Cl-ClO4 Solutions

Figure2 illustrates the pHc and Cl-/ClO4- ratio dependence of Kd values for MX-80. We found that the Kd values at pHc < 7 decreased as the Cl-/ClO4- ratio increased. Particularly, the Kd values at pHc < 6 for MX-80 were drastically suppressed in the presence of Cl- even at Cl-/ClO4- ratio = 0.01, compared to the Kd values in the absence of Cl- (Figure 1). On the other hand, the Kd values at pHc = 7 – 9 slightly increased with Cl-/ClO4- ratio up to 1 and did not change any more at Cl-/ClO4- ratio ≥ 10.Figure 2
p H c and Cl-/ClO4- molar concentration ratio dependence of Kd in Na-Ca-Cl-ClO4 solutions atI = 4 M. Plots are the experimentally measured Kd values. The solid lines are the 2SPNE SC/CE model calculation results. The error bars represent the standard deviation of triplicate measurements.The Pd speciation in Na-Ca-Cl-ClO4 solutions atI = 4 M at Cl-/ClO4- ratio = 0.01, 0.1, 1, 10, 100 and infinity are shown in Figures S-2–S-7, respectively. It was found that the pH region of PdCl42- dominance expanded with Cl-/ClO4- ratio, while that of Pd(OH)2 dominance shrank. Figure 2 suggests that the formation of PdCl42- complex strongly suppressed the Pd sorption, especially the surface complexation ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+, on MX-80 at pHc < 7.The fitting results of the 2SPNE SC/CE model to the experimentally measuredKd values using three binary and four ternary surface complexations and cation exchange reaction are also drawn in Figure 2. It was shown that the pHc dependence of Kd values in the wide range of Cl-/ClO4- ratio could be also quantitatively fitted by the 2SPNE SC/CE model. It was found that the fitting results could be obtained only with three binary and one ternary surface complexations as we studied in Na-Ca-Cl solutions [8]. Namely, only the ternary surface complexation, ≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+, among the four complexations considered in the modelling contributed to the Pd sorption on MX-80 in Na-Ca-Cl-ClO4 solutions. The contribution of other three ternary surface complexations was negligible. The surface complexation constant for this ternary sorption reaction was estimated as 13.3 ± 0.3. This value is consistent with 13.05 ± 0.2 which was previously estimated [8].The pH dependence of surface complexation and cation exchange reaction at Cl-/ClO4- ratio = 0, 0.01, 0.1, 1, 10, 100 and infinity is shown in Figures S-8–S-14, respectively. Three binary surface complexations dominated the sorption at Cl-/ClO4- ratio = 0. In the presence of Cl-, the surface complexation, ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+, was drastically suppressed even at Cl-/ClO4- ratio = 0.01. As the Cl-/ClO4- ratio increased, the contribution of surface complexations, ≡S-OH + Pd2+ ↔  ≡S-OPd+ + H+ and ≡S-OH + Pd2+ + H2O ↔  ≡S-OPdOH + 2H+, became small and was finally zero. This led to the decrease in the measured Kd values at pHc < 7. On the other hand, the contribution of ternary surface complexation, ≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+, became large at Cl-/ClO4- ratio = 0.1 and was significant and dominated the sorption at Cl-/ClO4- ratio ≥ 1. The increase in the measured Kd values at pHc 7–9 was attributed to this ternary surface complexation.The complexation constant for Pd2+ + 3Cl- + H2O - H+ ↔ PdCl3OH2- is compiled in the JAEA TDB (Table 1). Although PdCl3OH2- did not appear in Na-Ca-Cl-ClO4 solutions atI = 4 M (Figures S-1–S-7), the 2SPNE SC/CE model including the ternary surface complexation ≡S-OH + Pd2+ + 3Cl- + H2O ↔  ≡S-OPdCl3OH3- + 2H+ was also applied to the measured Kd values to make sure of the contribution of ternary surface complexation in the Pd sorption on MX-80. However, it was found that this ternary surface complexation could be negligible.


## 4. Conclusions

TheKd values of Pd for MX-80 were systematically measured as a function of pHc,I, and Cl-/ClO4- molar concentration ratio in Na-Ca-ClO4 (3 ≤pHc ≤ 9 and 0.1 M ≤I ≤ 4M) and Na-Ca-Cl-ClO4 (3 ≤pHc ≤ 9 and 0 ≤ Cl-/ClO4- ratio ≤ ∞;I = 4 M) solutions.It was found that thepHc,I, and Cl-/ClO4- ratio dependence of Kd values was quantitatively fitted by the 2SPNE SC/CE model. The model calculation indicated that, in the presence of Cl-, the contributions of binary surface complexations were suppressed while that of the ternary surface complexation dominated the sorption as the Cl-/ClO4- ratio increased.It was also found that thepHc dependence of Kd values for MX-80 in the Na-Ca-Cl-ClO4 solutions atI = 4 M could be simulated by considering only one ternary surface complexation reaction (≡S-OH + Pd2+ + 4Cl- ↔  ≡S-OPdCl43- + H+) besides the binary surface complexation reactions in all range of Cl-/ClO4- ratio (0–∞).



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