Preamble

NUCLEAR TECHNIQUES, Vol. XX, No. X, XXX 20XX

Reversibility of Bentonite Colloid Aggregation Induced by Calcium and Sodium Ions: Insights from Scattering Techniques

DU Jin¹,², ZHU Yi², LI Jiebiao³, TIAN Qiang¹, LIU Chunli²

¹State Key Laboratory of Environment-Friendly Energy Materials, School of Materials and Chemistry, Southwest University of Science and Technology, Mianyang 621010, China

²Beijing National Laboratory for Molecular Sciences, Fundamental Science on Radiochemistry and Radiation Chemistry Laboratory, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

³Beijing Research Institute of Uranium Geology, Beijing 100029, China

Abstract

[Background]: Stable and mobile bentonite colloids can adsorb various radionuclides and facilitate their migration. Therefore, a thorough understanding of the stability and aggregation behavior of bentonite colloids in groundwater environments is of great significance for accurately assessing the long-term safety performance of high-level radioactive waste disposal repositories.

[Purpose]: This study aims to investigate the reversibility and mechanism of bentonite colloid aggregation induced by typical cations Ca²⁺ and Na⁺.

[Methods]: This study combined synchrotron radiation small-angle X-ray scattering and dynamic light scattering techniques to analyze the aggregation structure of single-layer bentonite colloids influenced by Ca²⁺ and Na⁺ at the nanoscale to microscale, and compared the structural changes before and after dialysis.

[Results]: The results demonstrated that Ca²⁺ promoted the assembly of face-to-face aligned lamellar structures with a characteristic interlayer spacing of 1.86 nm, mediated by a distinctive ionic bridging mechanism. Following dialysis, only microscale secondary aggregates exhibited partial disaggregation, while the cyclic lamellar structures maintained their structural integrity. In contrast, Na⁺ promoted the formation of loosely packed edge-to-face aggregates that underwent complete dissociation into individual monolayers upon dialysis. In mixed Ca²⁺-Na⁺ electrolyte systems, the colloidal aggregation behavior of bentonite was found to be dominantly controlled by Ca²⁺.

[Conclusions]: Considering the characteristics of the groundwater at the Beishan site, it can be inferred that bentonite colloids near the disposal site are likely to remain in an aggregated and sedimented state for long time, which to some extent inhibits the migration of radionuclides adsorbed onto the bentonite colloids.

Keywords: Bentonite colloids, Aggregation reversibility, SAXS, DLS

Classification Codes: O434.19; TL942

Introduction

The safe disposal of high-level radioactive waste has become a critical constraint on the sustainable development of nuclear energy. By 2050, China is projected to generate 6,600 tonnes of spent fuel annually, with a cumulative total reaching 110,000 tonnes. The reprocessing of this spent fuel will produce substantial quantities of high-level radioactive waste. Deep geological disposal based on a multi-barrier design concept is considered the only viable solution for the safe management of high-level radioactive waste, and China has already entered the underground laboratory construction phase for this purpose.

Bentonite is a crucial component of the engineered barrier system in deep geological repositories, serving as a buffer/backfill material between waste canisters and the surrounding rock formation to ensure long-term safety and stability. Numerous studies have demonstrated that bentonite, primarily composed of montmorillonite, is an excellent buffer/backfill material. China has designated the GMZ (Gaomiaozi) sodium-based bentonite from Inner Mongolia as the preferred buffer/backfill material. When compacted bentonite at the interface with fractured host rock is eroded by groundwater, colloids ranging from 1 to 1000 nm in size can form. Research has shown that stable, mobile bentonite colloids can adsorb various radionuclides and facilitate their migration along fracture water. Therefore, investigating the occurrence forms of bentonite colloids in groundwater is essential for safety assessment of disposal repositories.

Groundwater ionic strength, ion type, pH, and temperature significantly influence bentonite colloid stability. Bentonite colloids are unstable under high ionic strength and acidic conditions, with divalent cations causing more pronounced aggregation than monovalent cations. Xian et al. investigated pH effects on GMZ bentonite colloid stability and proposed aggregation mechanisms based on edge charge distribution. Xu et al. found that alkaline conditions promote stability while high temperature and salinity have opposite effects, discussing Ca²⁺ and Na⁺ influences on sedimentation behavior through DLVO theory. Notably, long-term geological activities may reduce groundwater ionic strength through recharge from rivers, lakes, and rainfall. Consequently, studying aggregation reversibility—whether aggregated colloidal clusters can revert to stable, mobile states under favorable ionic conditions—is critically important for repository safety assessment. However, research on bentonite colloid aggregation reversibility remains limited. Mayordomo et al. confirmed that reduced ionic strength facilitates disaggregation of aggregated FEBEX bentonite colloids, though particle size cannot fully recover. Xu et al. observed that in GMZ bentonite colloid systems containing Na⁺, hydrodynamic diameter returned to initial levels after Na⁺ concentration reduction. Due to experimental design and technique limitations, current understanding of bentonite colloid aggregation structures and reversibility remains inadequate.

This work combined dynamic light scattering (DLS) and synchrotron radiation small-angle X-ray scattering (SAXS) techniques to investigate the aggregation behavior of single-layer bentonite colloids across nanometer to micrometer scales, using both hydrodynamic diameter and local lamellar stacking structure as parameters. We systematically compared the effects of Ca²⁺ and Na⁺ on bentonite colloid aggregation structures before and after dialysis, analyzed the influence of ion valence on aggregation reversibility mechanisms, and deepened understanding of bentonite colloid environmental behavior to provide critical scientific basis for high-level radioactive waste repository safety assessment.

Experimental Section

Materials and Instruments

Sodium-based bentonite (99% purity) was purchased from Shanlin Shiyu Mineral Products Co., Ltd. Analytical grade CaCl₂, NaCl, KCl, and MgCl₂ reagents were obtained from China Chemical Reagent Company. A DLS instrument (NanoBrook Omni, Brookhaven, USA) measured sample hydrodynamic diameters using a 640 nm laser at 90° scattering angle, with scattering signals collected at 25°C. Each sample was measured three times consecutively, with 60 s acquisition time per measurement. SAXS data were acquired at beamline BL19U2 of the National Facility for Protein Science in Shanghai, using 0.103 nm X-ray wavelength and 2678.6 mm sample-to-detector distance, covering a scattering vector range of q = 0.06–4.26 nm⁻¹ (where q = 4πsinθ/λ, with θ being half the scattering angle). BioXTAS RAW software processed experimental data, including one-dimensional scattering curve conversion, direct beam normalization, and solvent scattering background subtraction. X-ray diffraction (XRD) patterns were measured using a MiniFlex 600 diffractometer (Rigaku, Japan). Transmission electron microscopy (TEM) images were obtained with a Libra 200FE microscope (Zeiss, Germany). Turbidity was measured using a TB200 turbidimeter (Shanghai Bante Instruments Co., Ltd.).

Experimental Methods

Ten grams of sodium-based bentonite powder was dispersed in 1000 mL ultrapure water (resistivity ≥ 18.2 MΩ·cm), magnetically stirred at 200 rpm for one day, and sonicated for 30 minutes. The resulting bentonite suspension was centrifuged at 6000 rpm for 30 minutes, and the supernatant was transferred using a pipette into dialysis bags with molecular weight cutoff of 8000–14000 kDa for three days of dialysis in ultrapure water to remove impurity ions. Fifty milliliters of the prepared bentonite colloid solution was placed in three beakers and dried at 70°C for four days. The average mass concentration was determined gravimetrically. The stock solution was precisely diluted to obtain a series of known concentrations, and turbidity values were measured to establish a standard calibration curve. The concentration of this batch of bentonite colloids was determined to be 1.8 mg/mL, which was diluted to 1.0 mg/mL for subsequent experiments.

In the Beishan groundwater of Gansu Province, typical concentration ranges are 10–50 mmol/L for Na⁺ and 1–6 mmol/L for Ca²⁺. Therefore, high-concentration CaCl₂ and NaCl solutions were prepared and mixed with bentonite colloid solution at a 1:20 volume ratio. Final concentrations were approximately 1, 2, and 6 mmol/L for Ca²⁺, and 10, 20, and 50 mmol/L for Na⁺. Samples were left to stand for 72 hours before DLS and SAXS testing.

Aggregated colloid solutions were centrifuged to obtain precipitates. Ten milliliters of ultrapure water was added, and the mixture was transferred into dialysis bags (8000–14000 kDa cutoff) for 72 hours of dialysis in ultrapure water (dialysate changed every 12 hours, final conductivity <10 μS/cm). To compare aggregation reversibility induced by Ca²⁺ and Na⁺, dialyzed samples were re-analyzed by DLS and SAXS.

Testing Principles and Data Analysis

DLS is a common method for measuring hydrodynamic diameters of colloidal particles in solution, obtaining size distribution information by detecting scattering intensity fluctuations caused by Brownian motion. For monodisperse particle systems, the intensity correlation function C(τ) is given by the Siegert relation:

$$
C(\tau) = 1 + \beta|g(\tau)|^2 = 1 + \beta e^{-2Dq^2\tau}
$$

where g(τ) is the electric field correlation function, τ is the delay time, q is the light scattering vector magnitude (q = 4nπsinθ/λ, with n being the medium refractive index and θ half the scattering angle), D is the diffusion coefficient, and β is the instrument efficiency factor. According to the Stokes–Einstein relation, the hydrodynamic diameter is expressed as:

$$
D_h = \frac{k_BT}{3\pi\eta D}
$$

where k_B is the Boltzmann constant, T is absolute temperature, and η is solvent viscosity.

For polydisperse systems, the measured intensity signal represents a weighted superposition of exponential decay functions from different size fractions. The electric field correlation function must then be expressed in integral form:

$$
g(\tau) = \int_0^\infty G(\Gamma) e^{-\Gamma\tau} d\Gamma
$$

where Γ = Dq², and G(Γ) is the decay rate distribution function satisfying the normalization condition ∫G(Γ)dΓ = 1. This study used the CONTIN algorithm built into the NanoBrook software to perform numerical inversion of equation (2), resolving the G(Γ) distribution to obtain hydrodynamic diameter distribution information. The "equivalent hydrodynamic diameter D_H (nm)" was used to measure the statistically averaged size of polydisperse bentonite colloid particles.

SAXS occurs as coherent scattering near the incident X-ray beam (originating from electron density fluctuations within the material) and is a non-destructive technique for studying nanoscale structures. By obtaining the relationship between scattering intensity (I) and q, information about nanoparticle size, morphology, and number density can be retrieved. For monodisperse, randomly oriented nanoparticle systems, scattering intensity is expressed as:

$$
I(q) = N V^2 \Delta\rho^2 P(q) S(q)
$$

where N is particle number density, V is single particle volume, Δρ is contrast (scattering length density difference) between particle and solvent, P(q) is the orientation-averaged form factor reflecting particle geometry, and S(q) is the structure factor describing spatial arrangement. For particle concentrations below 5%, interparticle interactions can typically be neglected (S(q) = 1).

For dilute solutions of randomly oriented sheet-like nanoparticles, where lateral dimensions far exceed thickness, scattering intensity can be expressed as:

$$
I(q) = N V_{\text{cylinder}}^2 \Delta\rho_{\text{cylinder}}^2 \frac{2}{q^2} \exp\left(-\frac{q^2 L^2}{12}\right)
$$

where V_cylinder is sheet particle volume, Δρ_cylinder is contrast, L is sheet thickness, and R is lateral radius. According to equation (4), when qR ≫ 1, I ∝ q⁻². Therefore, Kratky plots (Iq² vs. q) can determine whether colloidal particles possess ideal local lamellar structures. If the I–q curve shows no diffraction peaks and the Kratky plot approximates a horizontal line, the colloid has a monolayer structure.

If bentonite colloids form face-to-face stacked periodic structures, diffraction peaks appear in the high-q region of I–q curves. The interplanar spacing can be calculated from the Bragg peak position q_peak using d = 2π/q_peak. If edge-to-face aggregation occurs, scattering intensity increases in the low-q region, deviating from the I ∝ q⁻² relationship. Kratky plots clearly reveal the aggregation state of bentonite colloids.

Results and Discussion

Microstructure of Bentonite Colloids

The XRD pattern of sodium-based bentonite is shown in [FIGURE:1]. The main diffraction peaks belong to the montmorillonite phase, while the weak peak at 26.6° corresponds to quartz. The XRD pattern of powder obtained by drying bentonite colloid solution at 70°C shows all peaks belong to montmorillonite; some peaks disappeared due to preferred orientation during drying. The (001) plane spacing is 12.75 Å, consistent with literature values for sodium-based bentonite. These results demonstrate that the centrifugation-dialysis method yields high-purity bentonite colloids. In contrast, colloids prepared using GMZ bentonite typically contain cristobalite impurities.

SAXS data for bentonite colloid solution are presented in [FIGURE:2]. The Kratky plot (inset) shows a horizontal line in the 0.06–0.6 nm⁻¹ range, indicating I ∝ q⁻². Fitting experimental data using equation (4) shows good agreement between experimental and theoretical curves, yielding a bentonite colloid particle thickness L of 1.06 ± 0.01 nm. Combined SAXS and TEM results ([FIGURE:3]) confirm successful preparation of monolayer bentonite colloid suspensions.

Effect of Ca²⁺ on Bentonite Colloid Aggregation Reversibility

Hydrodynamic diameter distributions of bentonite colloids obtained by DLS are shown in [FIGURE:4]. In the 1–6 mmol/L Ca²⁺ concentration range, single-layer bentonite colloids underwent significant aggregation. After dialysis, aggregated particles partially disaggregated, but size distributions could not recover to the initial state. The equivalent hydrodynamic diameter D_H in ultrapure water was 261 ± 11 nm; at Ca²⁺ concentrations of 1, 2, and 6 mmol/L, D_H increased to (6.31 ± 1.08) × 10³ nm, (6.51 ± 1.01) × 10³ nm, and (6.53 ± 0.99) × 10³ nm, respectively. After dialysis, D_H decreased to (1.29 ± 0.15) × 10³ nm, (2.15 ± 0.32) × 10³ nm, and (2.20 ± 0.16) × 10³ nm. DLS results indicate that Ca²⁺-induced bentonite colloid aggregates (secondary structures) are partially reversible at the micrometer scale.

SAXS data for Ca²⁺-containing bentonite colloid solutions are shown in [FIGURE:5]. At 1 mmol/L Ca²⁺, the I–q curve slope increased significantly, with a pronounced diffraction peak appearing in the high-q region (q_peak = 3.38 nm⁻¹), indicating formation of face-to-face stacked periodic structures with d-spacing of 1.86 nm. After dialysis, diffraction peak intensity and width remained unchanged, demonstrating that Ca²⁺-induced lamellar stacking structures (primary structures) are irreversible. Kratky plots ([FIGURE:5]b) clearly show reduced upturn in the low-q region after dialysis (indicated by arrows), likely arising from disaggregation of larger-scale secondary structures, consistent with DLS results.

Effect of Na⁺ on Bentonite Colloid Aggregation Reversibility

Hydrodynamic diameter distributions of bentonite colloids in Na⁺-containing solutions are shown in [FIGURE:6]. In the 10–50 mmol/L Na⁺ concentration range, single-layer bentonite colloids aggregated significantly; after dialysis, aggregated particles disaggregated and nearly returned to the initial state. At 10 mmol/L Na⁺, D_H increased to (1.19 ± 0.11) × 10³ nm; at 20 and 50 mmol/L Na⁺, D_H increased to ~(3.20 ± 0.20) × 10³ nm. After dialysis, D_H decreased to 273 ± 13 nm, 291 ± 13 nm, and 289 ± 12 nm, respectively—values very close to the initial D_H in ultrapure water.

SAXS data for Na⁺-containing bentonite colloid solutions are shown in [FIGURE:7]. At 50 mmol/L Na⁺, no diffraction peaks appeared in I–q curves, indicating no face-to-face stacked structures formed. The upturn in the Kratky plot suggests Na⁺ induced possible edge-to-face aggregation. After dialysis, the Kratky plot again shows a horizontal line in the low-q region (0.06–0.6 nm⁻¹), indicating disaggregation into monolayer structures. Both DLS and SAXS results demonstrate that Na⁺-induced bentonite colloid aggregation is reversible.

Aggregation and Dispersion Mechanisms

Data in [FIGURE:5] show that Ca²⁺ (1–6 mmol/L) induces irreversible primary aggregation structures through face-to-face periodic stacking (d = 1.86 nm) ([FIGURE:8]). This phenomenon can be understood from two perspectives. First, according to classic DLVO theory, divalent cations (Ca²⁺, Mg²⁺, etc.) have much stronger aggregation capability than monovalent cations (Na⁺, K⁺, etc.). Ca²⁺ introduction significantly increases solution ionic strength, reducing Debye screening length and compressing electrical double layers, thereby weakening electrostatic repulsion between colloidal particles. Second, the high charge density of Ca²⁺ forms a compact positive charge layer near bentonite colloid surfaces, reducing effective negative surface charge density and potentially causing local charge reversal. Ca²⁺ may generate short-range attractive forces between adjacent colloid surfaces through a "bridging" mechanism, effectively counteracting interlayer repulsion and ultimately inducing irreversible face-to-face aggregation. This mechanism also explains why calcium-based bentonite hardly swells to form stable colloids. SAXS results show an interlayer spacing d = 1.86 nm, equal to that of calcium-based bentonite after swelling and corresponding to the sum of monolayer thickness (~1 nm) and hydrated Ca²⁺ diameter (0.8–0.9 nm), further supporting this mechanism.

In bentonite colloid solutions with 10–50 mmol/L Na⁺, loose edge-to-face aggregation induced by Na⁺ shows complete reversibility ([FIGURE:7]). In this concentration range, Na⁺ compresses colloid surface electrical double layers, promoting electrostatic attraction between positively charged edges and negatively charged basal surfaces of montmorillonite colloids to form loose "card-house" structures. After dialysis reduces Na⁺ concentration, increased Debye length makes electrostatic repulsion dominant, causing "card-house" disaggregation and recovery to initial monolayer structures. Under coexisting Ca²⁺ and Na⁺ conditions, Ca²⁺-Na⁺-bentonite colloids show identical scattering features to Ca²⁺-bentonite colloids ([FIGURE:8]), indicating Ca²⁺ dominates aggregation behavior. Xu et al. studied bentonite colloid migration in quartz porous media through column experiments, finding both Ca²⁺ and Na⁺ caused aggregation and retention, but only Na⁺-induced aggregates could be eluted with pure water. This study further elucidates the microscopic mechanism: Ca²⁺ induces stable periodic structures that cause retention in quartz media and resist elution and disaggregation. The aggregation and dispersion processes induced by Ca²⁺ and Na⁺ are schematically illustrated in [FIGURE:9].

Significance for High-Level Radioactive Waste Geological Disposal

DLS is suitable for analyzing size changes of bentonite colloids from hundreds of nanometers to micrometers, while SAXS excels at resolving local aggregation structures from 0.5 to 200 nm. This work combined both techniques to resolve primary and secondary aggregation structures across nanometer to micrometer scales. Synchrotron SAXS data, in particular, revealed micro- and meso-scale aggregation behavior, promising strong technical support for safety assessment of high-level radioactive waste geological disposal.

Beishan groundwater in Gansu is primarily Cl·SO₄-Na and SO₄·Cl-Na type (7.5 < pH < 8.5). Besides Ca²⁺ and Na⁺, it contains considerable amounts of K⁺ and Mg²⁺ with typical concentration ranges of 0.1–1 mmol/L and 0.5–3 mmol/L, respectively. Taking deep groundwater from borehole BS06 as an example, Ca²⁺, Na⁺, Mg²⁺, and K⁺ concentrations are 6.24, 30.22, 0.85, and 0.17 mmol/L, respectively. [FIGURE:10] shows SAXS data for bentonite colloid solutions in 2× and 10× diluted BS06 groundwater. Based on scattering curve characteristics analyzed previously, we can infer that divalent cations such as Ca²⁺ and Mg²⁺ induce face-to-face stacking of bentonite colloids in BS06 groundwater. This stable aggregation structure likely inhibits radionuclide migration in near-field environments through deposition, filtration, and clogging mechanisms.

Conclusions

This study combined DLS and SAXS techniques to investigate the effects of Ca²⁺ and Na⁺ on bentonite colloid aggregation structures and reversibility in the Beishan groundwater environment, explaining aggregation mechanisms and deepening understanding of bentonite colloid environmental behavior. The main conclusions are:

(1) Ca²⁺ induces formation of stable face-to-face stacked periodic structures (d = 1.86 nm) in bentonite colloids. After dialysis, partial disaggregation of micrometer-scale secondary structures occurs while periodic structures remain stable, demonstrating irreversibility. Na⁺ induces "card-house" structures that completely disaggregate after dialysis, showing reversible characteristics. In mixed systems, aggregation behavior is dominated by Ca²⁺.

(2) Ca²⁺ likely generates short-range attractive forces between adjacent colloid surfaces through a "bridging" mechanism, effectively counteracting interlayer electrostatic repulsion and inducing irreversible aggregation. Na⁺ compresses electrical double layers to form loose "card-house" structures; after dialysis, increased Debye length makes electrostatic repulsion dominant, causing structural dissociation back to monolayer states.

(3) Beishan groundwater contains substantial alkaline earth cations (Ca²⁺, Mg²⁺, etc.) that cause bentonite colloids to remain in irreversible aggregated states (sedimented) for extended periods, which may positively contribute to inhibiting bentonite colloid-radionuclide migration and enhancing repository safety.

(4) Synchrotron SAXS is an important technique for in-situ analysis of weakly scattering colloidal systems and has broad application prospects in studying bentonite colloid formation and migration mechanisms.

Acknowledgments

We thank the staff at beamline BL19U2 (https://cstr.cn/31129.02.NFPS.BL19U2) of the National Facility for Protein Science in Shanghai (https://cstr.cn/31129.02.NFPS) for technical support and assistance in data collection and analysis.

Author Contributions

DU Jin and ZHU Yi contributed equally: responsible for experimental work, data analysis, and manuscript writing. LI Jiebiao: responsible for resource integration and manuscript review. TIAN Qiang: responsible for experimental design, technical support, and manuscript review. LIU Chunli: responsible for project management and manuscript review.

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