XRD and FT-IR

The mineral compositions of the samples were determined by XRD and FT-IR analyses.

Figure 2a shows the XRD patterns of the samples. The predominant mineral components of granite are quartz, albite, potassium feldspar, and biotite, with albite and K-feldspar together constituting approximately 70% of the mass percentage of the granite minerals. Bentonite contains montmorillonite as the main mineral component as well as secondary minerals, such as quartz, illite, and albite (Fig. 2b). The interplanar spacing of the bentonite colloid at the (001) plane differs slightly from that measured for the soil sample (1.52 nm). The diffraction curve of the bentonite colloid contains only a montmorillonite peak, indicating that secondary minerals were removed by centrifugation and dialysis [24]. The FT-IR analysis of the surface functional groups revealed that granite mainly consists of Si- and Al-based functional groups, including O-H, Si-O, Al-O, and Si-O-Si, which are similar to those present in bentonite [25, 26]. The bentonite colloids and bentonite have similar primary functional groups with minimal differences in the corresponding peak shapes and positions in the FT-IR spectra. This suggests that colloid formation has a negligible effect on the surface functional groups and chemical bonds of bentonite.

Fig. 2Full-screen View

(Color online) XRD (a) and FT-IR (b) images of the sample

Zeta and particle size distribution

The Zeta potential of the geological materials and bentonite colloids at different pHs is shown in Fig. 3a. Granite reaches the zero-point charge site at a pH of approximately 5.30, whereas bentonite and bentonite colloids have negative zeta potentials across the pH range of 3–11, with the bentonite colloids having a slightly lower potential than bentonite. The negative charge of bentonite arises from both the permanent charge of the structure and variable edge charge [27]. The colloid preparation process can affect the deprotonation of -SiOH and -AlOH, resulting in a bentonite colloid with a slightly lower potential than bentonite. However, the charges of bentonite and bentonite colloids were similar, indicating that the contribution from the edge charge was small. Figure 3b shows the average size and size distribution of the colloidal particles determined by dynamic light scattering. The colloidal particle sizes ranged from 100.9 nm to 433.5 nm with an average of 209.1 nm, suggesting relatively good colloidal stability. The polydispersity index (PDI) is a crucial parameter for characterizing particle size uniformity in a system. The bentonite colloid sample had a PDI of 0.217 in ultrapure water, indicating excellent dispersion.

Fig. 3Full-screen View

(Color online) (a) Zeta potential of the sample; (b) Particle size distribution of bentonite colloid

SEM-EDS before and after adsorption

The TEM images of the samples are shown in Fig. 4. Granite exhibits a massive shape with a dense internal texture. Both bentonite and bentonite colloids display similar sheet morphologies with irregular edges, and the layered structure of the colloid is more pronounced, confirming the XRD results. In addition, the TEM diagram illustrates that the prepared bentonite colloid has good dispersion, and the particle size of the colloid is approximately 200 nm, which is consistent with the particle size distribution test results of the colloid.

Fig. 4Full-screen View

TEM images of (a) granite, (b) bentonite, and (c) bentonite colloid

The surface morphologies of the samples before and after adsorption were characterized by SEM-EDS. Figure 5 shows that the granite samples have a fragmented distribution with unevenly sized layered fragments, and the sample surface is smooth with angular features. There is no significant difference in the morphology of the adsorbed granite before and after adsorption although adsorption makes the surface rougher and more fragmented, possibly because of immersion and agitation. The EDS results for the samples after adsorption indicate an increase in the quantity of adsorbed Sr²⁺ on the granite surface; however, there is no discernible distribution pattern for Sr²⁺ or a direct correlation between the distributions of Sr²⁺ and the other ions. The nonuniform particle size of bentonite indicates a distinct flaky structure with numerous irregular pores and cavities that facilitate the migration and diffusion of nuclides [28]. After adsorption, the bentonite surface curls and becomes rough, presumably because adsorption is followed by drying [29].

Fig. 5Full-screen View

SEM-EDS images of granite, bentonite, and bentonite colloid before and after adsorption

The immersion and dialysis of bentonite generates bentonite colloids with a distinct layered morphology [30] with clearer boundaries and a more regular distribution than that of the soil samples. Adsorption does not change the morphology of the colloids considerably but makes the layered structure denser, more compact, and agglomerated, with less distinct interlayers. The EDS results presented in Fig. 6 show that O, Si, Al, and Mg are still the main elements in the colloidal samples after simulated nuclide adsorption. The Sr²⁺ ions are adsorbed unevenly on the colloids, mainly accumulating in the layered stacking regions, indicating the presence of different adsorption sites on the colloids, with the stacking edge sites having possibly a stronger affinity for Sr²⁺ [31]. The distribution of Sr on the colloids appears to be correlated with that of Ca, suggesting that Ca in the colloids might participate in the adsorption of Sr via an ion exchange reaction between Ca and Sr. Bentonite colloids primarily consist of montmorillonite. Missana et al. [32] observed similar phenomena for Sr adsorption onto Na-montmorillonite.

Fig. 6Full-screen View

(Color online) Distribution of various elements in adsorbed bentonite colloid

Adsorption kinetics

The results of the kinetic experiments are presented in Fig. 7. The adsorption capacities of the three geological materials for Sr²⁺ are observed to decrease in the order of bentonite colloid (BC) > bentonite (Bt) > granite (Gt). The adsorption equilibrium time for Sr²⁺ at five days is notably shorter for Gt than for BC and Bt, possibly because of the differences in the number and specificity of surface adsorption sites among the materials. The pseudo-second-order kinetic equation fits the adsorption data for Gt with a high correlation coefficient (R²=0.998), suggesting that Sr²⁺ adsorption by Gt is primarily chemical [33]. The fitting curve for the Gt data based on the W-M model has three distinct regions corresponding to surface diffusion, internal diffusion, and equilibrium, indicating that intraparticle diffusion is not the sole rate-controlling step for Gt adsorption. In contrast, BC rapidly reaches adsorption equilibrium within 1 h, with q_e increasing from 2.136 mg/g to 13.989 mg/g, indicating a considerably greater adsorption capacity than that of Bt. This result is attributed to the larger specific surface area and lower surface charge of BC compared with that of Bt, leading to an increased number of adsorption sites and an enhanced ability to capture Sr²⁺ ions. Both BC and Bt data were most accurately fitted by pseudo-second-order kinetics, suggesting a chemical adsorption process involving surface complexation between functional groups and ion exchange [34]. Fitting the adsorption data of BC and Bt using the W-M equation revealed a two-stage process, unlike the adsorption on Gt. The fitted BC and Bt curves do not go through the origin, implying that the adsorption of Sr²⁺ is primarily determined by the adsorption sites and that the activation energy for adsorption and internal diffusion plays a minor role in the process [35]. Overall, the adsorption capacity of bentonite surpassed that of granite, suggesting a potential synergy between the two materials in blocking nuclide diffusion. However, the strong adsorption capabilities of bentonite colloids introduce considerable uncertainty and risk factors for the long-term disposal at repositories.

Fig. 7Full-screen View

(Color online) Adsorption kinetics of (a) granite, (b) bentonite and bentonite colloids. The illustration shows the linear fitting of quasi-second-order kinetics. Adsorption of (c) granite, (d) bentonite and bentonite colloids fit using the W-M model. (T=298 K, pH=7, S/L_Gt=20 g/L, S/L_Bt=5 g/L, S/L_BC=1 g/L, C₀=25 mg/L)

Effect of nuclide concentrations

The concentrations of nuclides carried in groundwater fluctuate with natural precipitation and water flow, which can deteriorate the blocking performance of barrier materials. Figure 8 shows that BC has the highest adsorption potential for Sr²⁺, whereas the adsorption capacities of Gt, Bt, and BC for Sr²⁺ all increase with the Sr²⁺ concentration, gradually tending toward equilibrium. This phenomenon may be attributed to the low probability of Sr²⁺ binding to sites with low Sr²⁺ concentrations, with the probability increasing with Sr²⁺ concentration, which facilitates the capture of Sr²⁺ by adsorption sites. Consequently, the saturation of the binding sites decreases, resulting in a slower increase in the adsorption capacity, leading to gradual equilibrium [36]. To further explore the adsorption mechanism, we used the Langmuir, Freundlich, Temkin, and D-R isotherm models to fit the adsorption data at different Sr²⁺ concentrations (Table 3). Compared with the Freundlich and Temkin models, the fit of the Langmuir adsorption model to the Gt, Bt, and BC data had higher correlation coefficients of 0.996, 0.991, and 0.989, respectively. This indicates that Sr²⁺ is mainly adsorbed as a surface monolayer [37]. Within the experimental concentration range, the maximum adsorption capacities of Gt, Bt, and BC for Sr²⁺ were 1.229 mg/g, 4.407 mg/g, and 30.303 mg/g, respectively, and were positively correlated with the specific surface areas of the materials. The fit of the Gt, Bt, and BC data to the Temkin model also showed high linearity, suggesting strong intermolecular interactions between the adsorbent and adsorbate [38]. As the Freundlich constant, 1/n, is less than 1, adsorption occurs easily. In the D-R equation, E denotes the energy required for 1 mol of nuclide to react with the adsorbent surface. Table 4 shows that Gt, Bt, and BC have E values above 16 kJ/mol; therefore, the adsorption mechanism of Gt, Bt, and BC is a chemical process [39], which is consistent with the kinetic fitting results.

Fig. 8Full-screen View

(Color online) Adsorption isotherms of (a) granite, (b) bentonite, and (c) bentonite colloid. (d) Adsorption data fitted to the D-R isothermal model. (T = 298 K, pH = 7, S/L_Gt=20 g/L, S/L_Bt=5 g/L, S/L_BC=1 g/L)

Effect of pH

The pH of the environment strongly influences the mineral surface potential and species distribution of ions, which affects the adsorption of ions on minerals [40, 41]. Figure 9a shows the impact of the pH on Sr²⁺ adsorption. The adsorption of Sr²⁺ by Gt, Bt, and BC is impeded by both acidic and alkaline environments. The adsorption capacities of BC, Bt, and Gt peak at pH6 displaying amplitudes of 13.303 mg/g, 2.014 mg/g, and 0.733 mg/g, respectively. This phenomenon may be attributed to the combined effects of the distribution of the strontium species, zeta potential, and colloidal stability. Figure 9b shows that Sr²⁺ is the primary strontium species under acidic conditions, where the competition between excess H⁺ and Sr²⁺ for adsorption sites hinders adsorption [42]. Consequently, adsorption is generally inhibited in acidic environments. As the pH increases, complexes are formed between OH^- and Sr²⁺, such as Sr(OH)⁺ and Sr(OH)⁺, which hinder adsorption. Therefore, the adsorption of Gt, Bt, and BC is impeded in alkaline environments. The poor stability of bentonite colloids in acidic environments results in a greater inhibitory effect on BC than on Gt or Bt [43, 44]. Zeta potential affects the affinity of the adsorbent for nuclides. The pH_ZPC value of Gt is approximately 5.30. At pHs below 5.30, the Gt surface is positively charged, which impedes the adsorption of Sr²⁺ via electrostatic repulsion. Conversely, at pHs > 5.30, the Gt surface becomes negatively charged, facilitating adsorption. Given that the permanent structural charge of Bt and BC is negative, the zeta potentials of these materials remain negative within the experimental pH range and decrease with increasing pH, indicating that the affinity of Bt and BC for Sr²⁺ increases with pH. However, this behavior does not alter the inhibition of adsorption by Bt and BC under alkaline conditions. This result can be explained in terms of the minor role that charge attraction plays in adsorption by Bt and BC. The pH of groundwater near disposal facilities is typically approximately 6.70. Around this pH, BC demonstrated high Sr²⁺ adsorption activity. Therefore, considering the potential effect of colloids on Sr²⁺ migration during disposal is important.

Fig. 9Full-screen View

(Color online) (a) Effect of pH on adsorption; (b) Distribution of Sr species in pure water (T=298 K, S/L_Gt=20 g/L, S/L_Bt=5 g/L, S/L_BC=1 g/L)

Effect of ions

The complex disposal environment for radioactive waste can result in a groundwater solution containing numerous ions that can affect the adsorption of nuclides by colloids and barrier materials. Preliminary analysis of the groundwater solution showed that the primary anions and cations in the groundwater near the disposal site were ${\text{CO}}_3^{2-}$, ${\text{SO}}_4^{2-}$, Fe³⁺, Ca²⁺, Mg²⁺, and K⁺. Given the potential of Sr²⁺ to form insoluble precipitates with anions such as ${\text{CO}}_3^{2-}$ and ${\text{SO}}_4^{2-}$ in groundwater, various electrolytes (KCl, FeCl₃, CaCl₂, and MgCl₂) have been used to investigate the adsorption behavior of Sr²⁺ in the presence of cations. Figure 10 shows that coexisting cations inhibit adsorption of Sr²⁺ by silicate minerals. These inhibitory effects were consistent for BC, Bt, and Gt, with the effectiveness decreasing in the order Fe³⁺ > Ca²⁺ > Mg²⁺ > K⁺. BC was inhibited the least because of its high unit adsorption capacity, whereas Gt displayed the most sensitivity to coexisting cations. Typically, high-valence cations compete more effectively for adsorption sites than low-valence cations [17], which accounts for Fe³⁺ having a more pronounced inhibitory effect than K⁺. The difference between the inhibitory effects of Ca²⁺ and Mg²⁺ arises from the difference in the radii of the corresponding hydrated ions. The smaller the radius of a hydrated ion, the more strongly the ion competes with Sr²⁺ for adsorption. Given the same positive charge, the hydrated ion radius of Ca²⁺ (0.200 nm) is smaller than that of Mg²⁺ (0.310 nm), leading to a greater inhibitory effect for Ca²⁺ than Mg²⁺.

Fig. 10Full-screen View

(Color online) Effect of ions on adsorption at T=298 K, pH = 7, S/L_Gt=20 g/L, S/L_Bt=5 g/L, S/L_BC=1 g/L, C_Sr=25 mg/L, C_ion=0.01 mol/L

Effect of bentonite colloid on migration

Batch adsorption experiments confirmed the robust adsorption capacity of BC for Sr²⁺, motivating us to perform dynamic shower experiments to further investigate the influence of BC on Sr²⁺ migration in the granite system. Figure 11a shows a penetration curve in which neither the peak values of BC-Sr comigration nor Sr migration alone reach 1, indicating retention of Sr²⁺ in the granite column. The longitudinal dispersion length (D_L) measured during Sr-BC comigration was 4.27×10^-4. Penetration of the granite column commenced at 18 h, which was considerably shorter than 70 h for Sr migration alone, indicating that the presence of colloids strongly promotes Sr²⁺ transport. This effect can be attributed to colloidal adsorption and inherent migration. Both bentonite colloids and granite exhibit negative charges across a broad pH range, resulting in repulsion between the colloids and granite during nuclide transport. Static adsorption experiments revealed that the adsorption of Sr²⁺ by BC is robust, advancing BC as a carrier for accelerating Sr²⁺ transport. Many studies [30, 45-47] have corroborated that bentonite colloids strongly promote the migration of Em(III), Pu(IV), U(VI), and other nuclides.

Fig. 11Full-screen View

(Color online) (a) Comigration curves of bentonite colloids and Sr²⁺; (b) Adsorption/desorption of Sr²⁺ by bentonite and granite (comigration: T = 298 K, pH=7, C_BC=250 mg/L, C_Sr=100 mg/L, adsorption/desorption: C_Sr = 50 mg/L, S/L_Gt=20 g/L, S/L_BC =0.2 g/L)

Retention of Sr²⁺ in the granite column was observed in the migration experiments. To investigate the influence of colloids on Sr retention in granite, experiments were conducted on the adsorption/desorption of Sr²⁺ on BC and Gt. As shown in Fig. 11b, the adsorption results indicate that the BC-Gt system has strong adsorption capacity for Sr²⁺. Under alkaline conditions, the adsorption capacity of the BC-Gt system is considerably greater than that of the Gt system because the stability of BC is enhanced in an alkaline environment [48]. The desorption experiments revealed that Gt has a lower adsorption capacity for Sr²⁺, with most sites being reversible [49], which resulted in a low retention rate of (15.1% to 35.6%) for Sr²⁺ in Gt after desorption, which is consistent with the observations in the migration experiments. The retention of Sr²⁺ on Gt was slightly higher for the BC-Gt system, which is attributed to the small number of colloids attached to the surface of the granite. Notably, acidic conditions considerably increased Sr²⁺ retention, which can be attributed to the reduced stability of BC under acidic conditions [50], leading to sedimentation and adhesion with Gt, which weakens desorption. Overall, fractures in granite significantly increase the risk of Sr²⁺ leakage into the biosphere, and a small amount of colloid adhering to the surface of granite in the repository may slightly enhance the retention of Sr²⁺ in granite. However, colloid promotion was not observed to have a significant impact on the retention of Sr²⁺ in granite during dynamic migration experiments. This indicates that the influence of colloids on Sr²⁺ retention in granite should be considered alongside multiple factors such as flow rate, concentration, and pH, which are crucial for the safety assessment of nuclear waste disposal facilities.

Comigration of Bentonite Colloids and Polynuclides

As shown in Fig. 12b, the adsorption capacity of bentonite colloids for nuclides in multicomponent systems increased in the order of Co²⁺ < Ni²⁺ < Sr²⁺ < Cs⁺, indicating that BC has the strongest affinity for Cs⁺ in a multicomponent system. Analysis of the breakthrough curve revealed that the detection time for Sr²⁺ in the multinuclide comigration system surpassed that of the BC-Sr system. This delay may arise from the interaction between the numerous positively charged ions and bentonite colloids, mitigating electrostatic repulsion and reducing colloidal fluidity to promote bentonite agglomeration. These agglomerates were retained in the column because of size exclusion effects. Similar outcomes have been reported in studies on coexisting ions, in which fluctuations in ion content were attributed to external interference. In particular, the coexistence of ions with the same valence state causes steric hindrance, and filtering exerts more pronounced inhibitory effects on colloidal transport [51]. The effluent recovery rate increased in the order Co²⁺ < Ni²⁺ < Sr²⁺ < Cs⁺ because of the nuclide adsorption capacity of BC. Nuclides adsorbed on BC can be retained in granite columns, diminishing the nuclide recovery rates in the effluent. Differences in affinities and binding capabilities of the colloids and different simulated nuclides resulted in Sr²⁺ and Cs⁺ eluting faster than Co²⁺, which had a more pronounced elution curve. Nuclides with a stronger affinity for BC were affected more during migration. The penetration times of the nuclides in the multielement system increased in the order Sr²⁺ < Cs⁺ < Ni²⁺ < Co²⁺. Hence, considering the role of bentonite colloids in predicting Sr²⁺ migration is important in the safety assessment of repositories.

Fig. 12Full-screen View

(Color online) (a) Co-migration curves of bentonite colloids and polynuclides; (b) Adsorption behavior of bentonite colloids on nuclides in polynuclides systems (T=298 K, pH=7, C_BC=250 mg/L, C_nuclide=100 mg/L)