Influence of Humic Acid on the Swelling Inhibition of Clay Minerals and Process Optimization

Abstract

Medium and heavy rare earths (REEs) are mainly from weathered crust elution-deposited rare earth ores (WREOs), where REEs are adsorbed in ionic form on the surface of clay minerals such as kaolinite, illite, halloysite, etc. REEs in WREOs are extracted through the in situ leaching process with (NH₄)₂SO₄ solution via ion exchange. However, this process often results in the swelling of clay minerals, subsequently destroying the ore body structure and causing landslides. This study investigated the inhibitory effects of humic acid (HA) on the swelling of primary clay minerals. An optimal inhibition on the swelling of clay minerals was demonstrated at 0.2 g/L. HA was mixed with 0.1 mol/L (NH₄)₂SO₄ solution at the solution pH of 6.8 and temperature of 25 °C. The swelling efficiency of kaolinite, illite, and halloysite in presence of HA decreased by 0.29%, 1.19%, and 0.19%, respectively, compared to using (NH₄)₂SO₄ alone. The surface hydration parameter of clay minerals was further calculated through viscosity theory. It was demonstrated that the surface hydration parameter of kaolinite and halloysite decreased nearly threefold, while that of illite decreased fivefold, demonstrating a desirable inhibition on clay swelling with HA. Viscosity theory offers valuable theoretical support for the development of anti-swelling agents.

1. Introduction

Rare earths (REEs) are vital strategic resources, and they are typically categorized into light rare earth elements (LREEs) and heavy rare earth elements (HREEs). The HREEs include elements ranging gadolinium to lutetium, along with yttrium [1]. These elements are mainly concentrated in weathered crust elution-deposited rare earth ores (WREOs), which are formed through the prolonged weathering of granite or volcanic rocks containing rare earth elements. These ores are characterized by their thin layer, loose texture, and fine particle size. The WREOs are primarily composed of clay minerals, with over 90% of REEs adsorbed onto their surface. The dominant clay minerals include kaolinite, illite, and halloysite. At present, the in situ leaching process is widely adopted to extract REEs from WREOs, which is considered as a green and effective extraction process because it avoids the need to strip the topsoil or excavate the ore, requiring only digging of injection wells on the surface of the mine area. As a result, it greatly reduces the impact on the topography and geomorphology of the mine area [2,3,4].

Common leaching agents for extracting REEs from Weathered Rare Earth Ores (WREOs) include inorganic salts such as NaCl, (NH₄)₂SO₄, NH₄Cl, and NH₄Ac [5]. During leaching, ammonium ions in the solution exchange with REE ions adsorbed onto the clay surfaces. This process desorbs the REEs from the ore matrix and transfers them into the leachate. The REEs are subsequently recovered from the leachate through purification and precipitation steps.

However, it has been observed that during in situ leaching, water molecules readily adsorb onto the clay mineral surfaces within the WREO body, forming a hydration layer [6]. This induces repulsive forces between clay particles, resulting in the swelling of the ore body. The resultant swelling compromises the structural integrity of the ore body, posing a significant risk of geological hazards (e.g., soil erosion and landslides) and adversely affecting the geomechanically stability of the mining site [7]. Therefore, inhibiting the hydration-induced swelling of clay minerals is one of the great challenges that need to be solved in the in situ leaching process.

In order to solve the problem of clay minerals’ hydration-induced swelling, two primary approaches have been identified to achieve anti-swelling effects: neutralizing the negative charge on the clay surface to inhibit swelling and altering the wettability on clay surface, thus reducing the thickness of the hydration layer [8]. Hu et al. [9] found that the compounding of magnesium sulfate and potassium chloride could effectively inhibit the swelling of clay minerals. Zhang et al. [10] found that the anti-swelling ability of the mixed ammonium salt solution was superior to that of a pure ammonium salt solution. Chen et al. [11] investigated the inhibitory effect of magnesium nitrate on clay swelling and found that a concentration of 0.2 mol/L with pH below 6.0 was favorable for suppressing the expansion of clay minerals. He et al. [12] used positively charged dimethyl diallyl ammonium chloride (DMDACC) to adsorb onto clay particles, neutralizing their negative charge distribution. This inhibited the electric double-layer effect and reduced the repulsive force between the clay particles, which ultimately suppressed the swelling of clay minerals. Yang et al. [13] found that a composite leaching agent composed of polyethyleneimine and (NH₄)₂SO₄ could neutralize the negative charge on the clay mineral surfaces, reducing the electrostatic repulsion and thereby weakening the hydration swelling. Sha et al. [14] proposed hydroxypropyl methylcellulose (HPMC) as a novel eco-friendly swelling inhibitor and demonstrated that its combination with ammonium sulfate ((NH₄)₂SO₄) served as an efficient composite leaching agent with significant suppression of clay mineral expansion. He et al. [15] evaluated the anti-swelling behavior of a composite solution consisting of 0.2 mol/dm³ magnesium sulfate and sodium citrate and demonstrated that this combination exerted a noticeable inhibitory effect on clay mineral expansion.

However, research on the swelling inhibition mechanism of inhibitors on clay minerals remains limited. Although the effect of swelling inhibitors on clay mineral hydration could be evaluated by measuring surface water layer thickness, the study of hydration mechanisms is complicated by the uneven thickness and varying sizes of the lamella of tiny clay mineral particles in the solution during the rare earth leaching process. During hydration, water molecules are adsorbed onto the surface of clay mineral particles and gradually form structured hydration layers. In these layers, water molecules are no longer present as free water but are transformed into combined water, with highly ordered molecular arrangements influenced by surface polarity. This combined water has significantly higher density and viscosity than free water, which reduces the amount of free water in the slurry and increases interparticle resistance, ultimately leading to a gradual increase in the viscosity of the clay suspension [6]. Therefore, viscosity measurement becomes an effective method to indirectly respond to the hydration mechanism. This approach not only proves changes in the particle dispersion state in the system but also reflects the effect of solution structure on the formation of the particle hydration membrane. By integrating colloidal stability theory, electric double layer theory, and interfacial adsorption theory, the swelling inhibition mechanism of the inhibitor could be further explained from the perspectives of particles interaction and solution structure.

Humic acid (HA) is primarily composed of carbon, oxygen, nitrogen, and sulfur and exhibits properties such as acid–base reactivity, colloidal behavior, adsorption, and ion-exchange capabilities [16,17]. As a complex organic matter, HA contains active functional groups, including hydroxyl, phenolic hydroxyl, and carboxyl groups. Lu et al. [18] highlighted that these active hydrogen ions could undergo ion exchange with metal ion and minerals, facilitating the extraction of rare earth ions. Additionally, HA could adsorb onto the surface of clay mineral particles [19], forming a stable adsorption layer that prevents water molecules from penetrating the clay interlayer space and inhibits swelling [20]. Moreover, the ability of HA to neutralize the negative charge distribution on the surface of clay minerals further suppresses hydration-induced swelling. HA also could moderate the alkalinity introduced by ammonium sulfate during in situ leaching, which reduces potential environmental disturbance. This dual function not only enhances its performance as a swelling inhibitor but also contributes to the ecological restoration of WREO deposits. To better illustrate the advantages of HA as a swelling inhibitor compared to other common inhibitors, a performance comparison table is presented in

Table 1. Comparison of swelling inhibitors for clay minerals and performance.

Inhibitor Type	Swelling Efficiency	Advantages and Disadvantages
HA	1.54%~2.51%	Environmentally friendly, cost-effective, relatively stable
Hexadecyl trimethyl ammonium bromide [21]	0.45–0.82%	Strong inhibition, high cost, high toxicity, unfavorable for ecological restoration.
Ammonium chloride [22]	2.50%	Soluble, economical, corrosive
Ammonium acetate [11]	2.705%	Effective under specific conditions and low in price, unstable at high pH

:

In this study, HA was used as swelling inhibitor to investigate its synergistic effect with (NH₄)₂SO₄ on suppressing the swelling of the main minerals in WREOs, including kaolinite, illite, and halloysite. Linear swelling experiments were conducted to determine the optimal inhibition conditions, including concentration, pH, and temperature. Additionally, viscosity experiments were carried out to measure the surface hydration characteristics of clay mineral particles, providing a deeper understanding of the inhibition mechanism. The results from the viscosity experiments further validate the findings of the linear swelling experiments, confirming the feasibility of HA in reducing the swelling of clay minerals.

2. Materials and Methods

2.1. Materials

Ammonium sulfate (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was used as the leaching agent for extracting rare earth from clay minerals in WREOs. Humic acid (Aladdin Biochemical Technology Co., Ltd., Shanghai, China) served as the anti-swelling agent. Hydrochloric acid (Kaifeng Dongda Chemical Co., Ltd., Kaifeng, China) and sodium hydroxide (Sinopharm Chemical Reagent Co., Ltd., China) were utilized as pH regulators. All reagents were of analytical grade. The single minerals, kaolinite, and illite, were purchased from Sinopharm Chemical Reagent (China), and halloysite was obtained from Guzhang County Shanlin Stone Language Mineral Products (Guzhang County, Hunan, China). The single minerals were then pressed into flake samples or processed into powder for subsequent analyses. All clay minerals were analyzed by X-ray diffraction, and the results are shown in Figure 1.

2.2. Experimental Methods

2.2.1. Linear Swelling Experiment

HA concentrations of 0 g/L, 0.05 g/L, 0.1 g/L, 0.2 g/L, 0.3 g/L, 0.4 g/L, and 0.5 g/L were prepared and mixed with 0.1 mol/L (NH₄)₂SO₄ solutions, respectively.

A total of 6 g of clay minerals was pressed to prepare a flake sample using a HY-12 tablet press under a pressure of 12 MPa for 3 min. The height of each flake sample was measured with a vernier caliper and recorded in the setup of a PCY dilatometer. Then the flake samples were placed into the measuring chamber of the PCY dilatometer, and solutions at different concentrations were injected for each experiment. Under atmospheric pressure, the swelling efficiency of the samples was measured at 25 °C over 3 h. After identifying the optimal concentration, the temperature was set to 25 °C, 40 °C, 60 °C, and 80 °C to determine the optimal temperature. Finally, at the optimal concentration, the temperature and the pH of the mixed solutions were adjusted to 4, 5, 6, 7, and 8, respectively, and the swelling efficiencies were measured. All experiments were repeated three times to make sure that the results were reliable.

In this study, “swelling” referred to the volume change or particle dispersion caused by adsorption and hydration of water molecules on the mineral surface.

The final swelling efficiency δ for the expansion process of the mineral samples was calculated by the following equation:

δ = (ΔH/H₀) × 100%

(1)

where ΔH is the change in height of the mineral sample after swelling; and H₀ is the original height of the flake sample.

2.2.2. Viscosity Measurement

The HA-(NH₄)₂SO₄ solution was first prepared, and its pH was adjusted to the desired value using HCl or NaOH. Then, 50 mL of the prepared solution was transferred to a 100 mL beaker. The clay mineral powders were weighed precisely to obtain the following masses corresponding to the target volume fractions in a 50 mL solution: 0.2% (0.1 g), 0.4% (0.2 g), 0.6% (0.3 g), 0.8% (0.4 g), and 1.0% (0.5 g). The powders were added to the solution, and the suspensions were stirred using a magnetic stirrer at 300 rpm for a sufficient time to ensure complete dispersion of the clay minerals. These suspensions were subsequently used for viscosity measurements.

The viscosities of these suspensions were subsequently measured using a digital viscometer. The relationship between relative viscosity and volume fraction was calculated, and the resulting data were used to plot fitted curves. The slope K was determined and corrected based on Einstein’s viscosity equation. After correction, the hydration parameters of clay mineral particles under different chemical environments were measured using the viscosity method to evaluate the degree of hydration on the surfaces of clay mineral particles.

In a low-concentration dispersion system, the Einstein’s viscosity equation effectively describes the relationship between the volume fraction of rigid spherical particles and the viscosity of the dispersion system, which is defined as

η = η₀ (1 + 2.5ϕ)

(2)

where η/η₀ is the relative viscosity, η is the viscosity of the dispersion system, η₀ is the viscosity of the solvent, and ϕ is the volume fraction of solid particles.

Since the hydration layer was not rigid, the Einstein viscosity equation needed to be modified. It was assumed that the contribution of the hydration layer to the viscosity increase in the dispersion system was g times that of an equivalent volume of solid particles. Then the equation became

η/η₀ = 1 + 2.5(ϕ_p + γϕ₁)

(3)

ϕ_p = V_p/V

(4)

ϕ₁ = V₁/V

(5)

where V_p and V₁ are the volumes of solid particles and the hydration layer, respectively, and V is the total volume of the dispersion system.

After substitution and transformation, the following equation could be obtained:

η/η₀ = 1 + 2.5(1 + γV₁/V_p) ϕ_p

(6)

The slope k of a straight line could be expressed as

k = 2.5(1 + γV₁/V_p)

(7)

Define the hydration parameter as

f = γV₁/V_p

(8)

Substituting Equation (8) into Equation (7) gives

f = 0.4k − 1

(9)

2.2.3. X-Ray Diffraction Analysis

The mineralogical characterization of the clay minerals was conducted using X-ray diffraction (XRD) with Cu Kα radiation under the following operational conditions: a step size of 0.02°, an operating voltage of 50 kV, a scanning speed of 2°/min, and a scanning range of 5° to 85°.

2.2.4. Zeta Potential Measurement

Zeta potential measurements were conducted using a Malvern Nano-ZS90 zeta potential analyzer. Suspensions of three types of clay minerals were prepared to examine the zeta potential distribution.

(NH₄)₂SO₄ solution was mixed with HA at concentrations ranging from 0 to 0.5 g/L. Clay minerals were ground and passed through a 200-mesh sieve. Then, 0.5 g of the sieved clay was added to 30 mL of the prepared solution in a 50 mL centrifuge tube. Each sample was shaken at 250 rpm for 6 h using a rotary shaker. After shaking, the suspensions were allowed to stand for 24 h.

Zeta potential was measured using the zeta potential analyzer on 3 mL of supernatant collected from each sample. Each concentration was tested nine times, and the average value was calculated based on valid measurements.

2.2.5. Infrared Spectrum Measurement

Infrared spectrum analysis was conducted using a Fourier Transform Infrared Spectrometer (Thermo Nicolet NEXUS 470, Thermo Nicolet Corporation, San Jose, CA, USA). The scanning wavenumber range was from 500 cm⁻¹ to 4000 cm⁻¹.

3. Results and Discussion

3.1. The Swelling Efficiency of Clay Minerals in Presence of HA

3.1.1. Effect of HA Concentration on Clay Minerals Swelling

Mixed solutions of 0.1 mol/L (NH₄)₂SO₄ and HA at different concentrations (0%~0.05%) were used as the leaching agent. The effect of the HA concentration on the swelling inhibition of clay minerals was studied under conditions of 25 °C and pH = 6.80, and the results are shown in Figure 2.

The swelling efficiencies of the three clay minerals increased rapidly within the first 20 min under different HA concentration conditions, slowing down afterwards and eventually stabilizing.

For kaolinite, the minimum swelling efficiency was 1.24% at a HA concentration of 0.1 g/L, followed by 1.26% and 1.47% at HA concentrations of 0.05 g/L and 0.2 g/L, respectively (Figure 2a). Compared to the condition without HA, the swelling efficiency of kaolinite was reduced by 0.53%, 0.51%, and 0.29% for the three different HA concentrations, respectively. The lowest swelling efficiency was observed at 0.1 g/L HA mixed with (NH₄)₂SO₄, indicating that this concentration inhibited surface hydration. At 0.05 g/L, the swelling efficiency was also reduced, though the inhibitory effect was slightly weaker, possibly due to an insufficient dosage resulting in incomplete surface coverage. The performance at 0.2 g/L was better than that of the solution without HA and those with higher HA concentrations (e.g., 0.3, 0.4, and 0.5 g/L), suggesting that though not the optimal, this intermediate concentration could still form a stable adsorption layer and inhibit swelling. The swelling efficiency increased at concentrations above 0.3 g/L, possibly due to the aggregation among excess HA molecules, leading to an increase in swelling efficiency, as reflected in the experimental results.

The swelling efficiency of illite reached a minimum of 2.51% when the HA concentration was 0.1 g/L. This represented a reduction of 1.57% compared to the condition using (NH₄)₂SO₄ alone. At HA concentrations of 0.5 g/L and 0.2 g/L, the swelling efficiency decreased by 1.27% and 1.19%, respectively (Figure 2b). The experimental results for illite differ from those for kaolinite. At a HA concentration of 0.1 g/L, the sample exhibited the lowest swelling efficiency, indicating that an appropriate amount of HA is beneficial for forming a compact adsorption layer. This possibly due to the formation of a monolayer of HA on the illite surface, with hydrophobic groups aligned outward, shielding hydration effects and thereby inhibiting swelling. However, compared to kaolinite, illite still shows a relatively low swelling efficiency at a HA concentration of 0.5 g/L, even lower than that observed at 0.2 g/L. This could be attributed to the stronger layer charge and greater adsorption capacity of illite, which allowed more HA molecules to be adsorbed without leading to flocculation. At a HA concentration of 0.2 g/L, the system may be in a transitional state between monolayer adsorption and colloid formation, where the inhibitory effect was retained but less obvious than 0.1 g/L. When the HA concentration increased further to 0.3–0.4 g/L, it likely exceeded the monolayer adsorption capacity, resulting in the exposure of hydrophilic groups such as carboxyl at the interface. This increased the surface hydrophilicity and partially offset the hydrophobic inhibition of mineral swelling.

For halloysite, the most significant inhibition of swelling was also achieved at a HA concentration of 0.1 g/L, with the swelling efficiency reduced to 0.43%. The swelling efficiency decreased by 0.68% compared with using (NH₄)₂SO₄ alone (Figure 2c). Market swelling inhibition was also observed at HA concentrations of 0.3 g/L and 0.2 g/L, with differences were 0.45% and 0.19%, respectively. At a HA concentration of 0.1 g/L, the swelling efficiency was the lowest, indicating that HA molecules were able to adsorb more uniformly and sufficiently onto the mineral surface, forming a stable shielding layer that prevented water from entering the layered structure, thus achieving the optimal swelling inhibition. At a concentration of 0.3 g/L, the inhibitory effect slightly decreased, possibly due to near-saturation adsorption without significant aggregation, resulting in an uneven molecular distribution and partial surface exposure. Although the concentration at 0.2 g/L was higher than 0.1 g/L, the continuity of the adsorption layer was reduced, leading to a weakening inhibitory effect, which suggests that adsorption efficiency did not increase linearly with concentration. At HA concentrations of 0.4 and 0.5 g/L, self-aggregation among HA molecules began to occur and, although occupying the mineral surface, failed to effectively inhibit water penetration, further diminishing the swelling inhibitory effect. At 0.05 g/L, the low dosage led to insufficient surface coverage, resulting in relatively poor inhibition. Nevertheless, compared with the 0 g/L HA, all concentrations of HA treatment showed the swelling inhibition effect to a certain extent.

It was seen that the combination of 0.1 g/L HA with (NH₄)₂SO₄ solution resulted in the lowest swelling efficiencies for all three clay minerals, indicating relatively strong swelling inhibition. However, 0.2 g/L HA exhibited consistently good performance across all minerals, with stronger stability. It avoided the limitations of insufficient surface coverage at low concentrations and structural disruption at high concentrations. Moreover, a previous study has demonstrated that a mixture of 0.2 g/L HA and (NH₄)₂SO₄ yielded the optimal rare earth leaching efficiency [20]. Therefore, this study selected 0.2 g/L HA combined with (NH₄)₂SO₄ for subsequent experiments.

3.1.2. Effect of Temperature on Clay Minerals Swelling

The effect of temperature on the swelling inhibition of three clay minerals at pH = 6.80 and a HA concentration of 0.2 g/L were investigated, and the results are shown in Figure 3.

Figure 3 clearly shows that the swelling efficiency of all three clay minerals increased rapidly in the initial stage and then gradually stabilized over time until reaching equilibrium. A comparison of swelling efficiencies at different temperatures revealed a similar trend among the three minerals: swelling efficiency increased with rising temperature in the order of 80 °C > 60 °C > 40 °C > 20 °C. At 25 °C, the mixed solution of HA and (NH₄)₂SO₄ exhibited the most effective swelling inhibition, with swelling efficiencies of 1.47%, 2.90%, and 0.92% for kaolinite, illite, and halloysite, respectively.

The weakened swelling inhibition at higher temperatures could be due to enhanced molecular thermal motion, which reduced the stability of interparticle interactions and the stability of the electrical double layer, leading to increased surface hydration and thus greater swelling.

3.1.3. Effect of Solution pH on Clay Minerals Swelling

The effect of solution pH on the swelling efficiencies of kaolinite, illite, and halloysite was investigated, and the results are shown in Figure 4. The clay minerals were mixed with 0.2 g/L HA and 0.1 mol/L (NH₄)₂SO₄ at 25 °C.

For the three clay minerals, their swelling efficiencies initially increased rapidly and then gradually stabilized over time. With increasing pH, the swelling efficiencies first decreased and then increased, reaching their lowest values at pH = 6.80.

The swelling efficiency of kaolinite reached a minimum of 1.47% when the solution pH = 6.80. At pH values ranging from 4 to 6.8, the increase in pH led to the neutralization of negative surface charges on the mineral particles by NH₄⁺, which compressed the electrical double layer and, to some extent, inhibited the adsorption of water molecules. Meanwhile, partially dissociated carboxyl groups from HA could adsorb onto positively charged edge sites of the minerals through electrostatic interactions, forming a hydrophobic membrane and simultaneously suppressing surface hydration. This is reflected in the data as a decrease in swelling efficiency. However, when the pH further increased to 8, part of the NH₄⁺ converted to NH₃, weakening the compression of the electrical double layer. In addition, the dissociated carboxyl group of HA became deprotonated to form a large number of —COO⁻ groups, which were electrostatically repelled by the negatively charged kaolinite surface. The increased particle dispersion enhanced surface hydration, resulting in a rise in swelling efficiency as observed in the experimental data.

When the pH ranged from 4.0 to 6.8, HA complexed with Al³⁺ or Fe³⁺ at the edges of illite through carboxyl groups, forming stable surface complexes that hindered the surface hydration. As a result, the swelling efficiencies of illite decreased with increasing pH, reaching a minimum of 2.89% at pH = 6.8. However, when the pH increased from 6.8 to 8.0, the increased negative charge of HA caused electrostatic repulsion with the negatively charged illite surface. This repulsion disrupted the adsorbed HA layer, promoted surface hydration, and increased particle dispersion, leading to a rise in the swelling efficiencies of illite under alkaline conditions.

When the pH ranged from 4.0 to 6.8, HA interacted with Si–OH groups on the surface of halloysite through hydrogen bonding, forming a dense adsorbed layer that hindered the hydration and expansion of the mineral. As a result, the swelling efficiencies of halloysite decreased with increasing pH, reaching a minimum of 0.92% at pH 6.8. However, as the pH approached 8.0, deprotonation of surface Si–OH groups generated a large number of Si–O⁻ sites, which experienced electrostatic repulsion with the –COO⁻ group of HA. This repulsion caused partial outward expansion of the tube walls.

This indicates that the HA-(NH₄)₂SO₄ solution exhibited a synergistic inhibition of clay minerals swelling under these conditions. Additionally, the pH of the solution of 0.2 g/L HA mixed with 0.1 mol/L (NH₄)₂SO₄ was measured to be 6.80, demonstrating that clay minerals swelling could be suppressed without further pH adjustment.

In conclusion, at an experimental temperature of 25 °C, with a HA concentration of 0.2 g/L and pH = 6.80, the HA-(NH₄)₂SO₄ mixed solution demonstrated strong inhibition of the hydration swelling of clay minerals.

3.2. The Surface Hydration of Clay Minerals in Presence of HA

3.2.1. Effect of HA Concentration on Surface Hydration of Clay Mineral

The concentration of the solution influenced the hydration on the surfaces of clay mineral particles, which affected the dispersion of the suspension and subsequently its viscosity. Seven concentrations of HA (0 g/L, 0.05 g/L, 0.1 g/L, 0.2 g/L, 0.3 g/L, 0.4 g/L, and 0.5 g/L) were adjusted to investigate the effect of HA concentration on the hydration of kaolinite, illite, and halloysite particles. The variation in surface hydration of the clay minerals was determined by measuring the viscosity of these suspensions under experimental conditions of 25 °C and pH = 6.80. The results are as follows:

Figure 5 illustrates the effect of HA concentration on the surface hydration of the three clay minerals. The viscosity curves of the mineral suspension at various volume fractions were fitted, and the relationships between them are shown in Figure 5, respectively.

Since the hydration parameter is associated with the slope of the fitted curve, the surface hydration parameters of clay mineral particles under different HA concentrations could be calculated using Equation (9).

For instance, at a HA concentration of 0.2 g/L, by fitting the relative viscosity data, we obtained the equation for relative viscosity as η/η₀ = 1 + 17 ϕ_p, where ϕ _p is the volume fraction. The slope k of the curve is 17. Using the definition of the hydration parameter f = 0.4k − 1, the surface hydration parameter f can be calculated as f = 0.4 × 17 − 1 = 5.800.

The calculated results are shown in

Table 2. Hydration degree of kaolinite particles under different HA concentrations.

Suspension	HA Concentration	Slope k	Surface Hydration Parameter f
Kaolinite	0 g/L	29.636	10.854
	0.05 g/L	14.910	4.964
	0.1 g/L	10.910	3.364
	0.2 g/L	17	5.800
	0.3 g/L	24.182	8.673
	0.4 g/L	21.910	7.764
	0.5 g/L	19.273	6.709
Illite	0 g/L	40.364	15.146
	0.05 g/L	17.545	6.018
	0.1 g/L	9.818	2.927
	0.2 g/L	15.455	5.182
	0.3 g/L	27.545	10.018
	0.4 g/L	18.909	6.564
	0.5 g/L	13	4.200
Halloysite	0 g/L	45.182	17.073
	0.05 g/L	44	16.6
	0.1 g/L	15	5
	0.2 g/L	22.091	7.836
	0.3 g/L	25	9
	0.4 g/L	30.636	11.254
	0.5 g/L	44.364	16.746

.

From the results in Table 2, it can be observed that the surface hydration parameter of kaolinite, illite, and halloysite particles was relatively low when the HA concentration is within the range of 0.1–0.2 g/L. Compared to using (NH₄)₂SO₄ alone, the surface hydration parameter of the three mineral particles decreased by nearly three times, indicating that its surface hydration was the weakest under this condition. These values are consistent with the experimental results in Section 3.1.1.

3.2.2. Effect of Solution pH on the Surface Hydration of Clay Minerals

The pH of the solution influenced the charge state on the surface of both HA and clay mineral particles [23,24], which in turn altered the inter-particle interactions and the degree of surface hydration. Consequently, this affected the viscosity of the clay mineral suspension. To investigate the effect of pH on surface hydration, HA-(NH₄)₂SO₄ mixed solutions with varying pH values were prepared and used to form suspensions with clay minerals. Under the condition of a HA concentration of 0.2 g/L and a temperature of 25 °C, the effect of pH on the surface hydration of clay minerals was examined. The results are as follows.

Figure 6 illustrates the effect of pH on the surface hydration of clay minerals. The calculated results are presented in

Table 3. Hydration degree of clay mineral particles under different pH.

Suspension	pH	Slope k	Surface Hydration Parameter f
Kaolinite	4.02	19.727	6.891
	5.02	18.636	6.454
	6.00	18.181	6.272
	6.80	17	5.800
	8.00	28.727	10.491
Illite	4.06	22.727	8.091
	5.10	20.182	7.073
	6.08	19.091	6.636
	6.80	15.455	5.182
	8.02	28.909	10.564
Halloysite	4	36.636	13.654
	5	33.909	12.564
	6	31.273	11.509
	7	22.091	7.836
	8	24.091	8.636

.

As shown in Table 3, the hydration parameter of the three clay mineral particles initially decreased and then increased with rising pH, reaching its minimum values of 5.800, 5.182, and 7.836, respectively, when the pH value was close to 6.8. This indicated that the mixed solution achieved optimal inhibition of mineral surface hydration under this condition. The results are consistent with the conclusions drawn in Section 3.1.3, further confirming that HA inhibited clay minerals’ surface hydration and suppressed swelling.

3.2.3. Effect of Temperature on the Surface Hydration of Clay Minerals

As the temperature increased, the motion of HA molecules accelerated, facilitating their more effective adsorption onto clay mineral surfaces and inhibiting hydration. However, due to the relatively low stability of HA, excessively high temperatures may have caused structural changes in HA molecules [25], diminishing its effectiveness in inhibiting hydration. Furthermore, temperature variations have a more pronounced impact on the diffusion of water molecules compared to cation diffusion [26]. This disparity further affected the ability of HA to neutralize the negative charge on the surface of clay minerals, altering its anti-swelling efficacy.

To investigate the effect of temperature on the surface hydration of kaolinite, illite, and halloysite, experiments were conducted with 0.2 g/L HA at pH = 6.80, and the temperatures were set to 25 °C, 40 °C, 60 °C, and 80 °C. The optimal temperature for inhibiting the surface hydration of these three clay minerals was identified. The variation behaviors of surface hydration of clay minerals are shown as follows.

Figure 7 illustrates the effect of different temperatures on the surface hydration of clay minerals. The calculated results are presented in

Table 4. Hydration degree of clay minerals particles under different temperature.

Suspension	Temperature	Slope k	Surface Hydration Parameter f
Kaolinite	25 °C	17	5.800
	40 °C	25.364	9.146
	60 °C	27.818	10.127
	80 °C	33.636	12.454
Illite	25 °C	15.455	5.518
	40 °C	20.091	7.036
	60 °C	23.818	8.527
	80 °C	26.545	9.618
Halloysite	25 °C	22.091	7.836
	40 °C	30.273	11.109
	60 °C	37	13.8
	80 °C	40.818	15.327

.

As seen from the results in Table 4, the surface hydration parameter of the three clay minerals reached minimum values of 5.800, 5.518, and 7.836 at 25 °C, respectively, corresponding to the weakest hydration of kaolinite particles. As temperature increased, the diffusion of water molecules was enhanced, leading to a corresponding increase in the surface hydration parameter. This trend is consistent with the experimental findings presented in Section 3.1.2 of this study. The positive effect of temperature on molecular diffusion in kaolinite has also been demonstrated in other studies [27]. The results show that HA achieved the strongest inhibition of clay mineral surface hydration at 25 °C.

3.3. Inhibition Mechisom of HA on the Swelling of Clay Minerals

3.3.1. XRD Analysis of Clay Minerals in the Presence of HA

To investigate the effect of HA adsorption on the structure of clay minerals, XRD patterns of the three clay minerals were obtained in the presence of 0.2 g/L HA, and the results are presented in Figure 8. The analysis indicated that no significant changes were observed in any of the three clay minerals following HA adsorption. The positions of the characteristic peaks remained unchanged, suggesting that the interlayer distance was not altered, consistent with Bragg’s law (2d sinθ = nλ). These results further support that the swelling of clay minerals can be attributed to the hydration layer adsorbed on their surface rather than interlayer expansion.

3.3.2. Effect of HA Concentration on Zeta Potential of Clay Minerals

The zeta potential after adsorption of HA was investigated, and the results are shown in Figure 9. For kaolinite, illite, and halloysite, the absolute value of the zeta potential increased with rising HA concentration and became stable at 0.2 g/L, indicating surface coverage saturation. Linear swelling experiments and viscosity experiments showed consistent results: the lowest swelling efficiencies and hydration parameters occurred at 0.1–0.2 g/L HA. This suggests that HA at 0.1–0.2 g/L effectively adsorbs onto the mineral surfaces, forming a stable layer. The adsorption of HA changes the surface charge distribution, which reduces the attraction of water molecules to the clay surfaces. At HA concentration of 0.1–0.2 g/L, the saturated surface charge and the stable adsorption layer hinder water interacted with mineral surface particles, thereby limiting swelling and controlling the thickness of the hydration layer.

3.3.3. FTIR Analysis of Clay Minerals in Presence of HA

The FTIR spectra of kaolinite, illite, and halloysite before and after treatment with HA are shown in Figure 10. All three clay minerals displayed three principal characteristic peaks: the H-O stretching vibration at 3600–3700 cm⁻¹, the Al-OH bending vibration near 910 cm⁻¹; and the Si-O stretching vibration around 1030 cm⁻¹. Following treatment with humic acid, the positions of these characteristic peaks shifted, suggesting adsorption of humic acid onto the clay mineral surfaces. Humic acid contains functional groups such as carboxyl (-COOH) and phenolic hydroxyl (-OH), which can form hydrogen bonds with surface hydroxyl groups or oxygen atoms of the clay minerals, resulting in the observed shifts in the characteristic absorption bands. It should be noted that the weak absorption bands observed in the 3000–2800 cm⁻¹ region are not intrinsic structural features of the clay minerals but are more likely attributable to trace adsorption of external organic matter during the experimental process. Nevertheless, these peaks did not affect the identification or analysis of the main characteristic absorption bands.

4. Conclusions

In this study, the inhibition effect of HA on the swelling of kaolinite, illite, and halloysite was investigated using viscosity theory. Linear swelling and viscosity experiments revealed that HA concentration, pH, and temperature influenced the swelling and hydration of kaolinite, illite, and halloysite. The optimal anti-swelling and hydration inhibition effects were observed at HA concentrations of 0.2 g/L, pH = 6.80, and 25 °C, where the swelling efficiency of kaolinite, illite, and halloysite decreased by 0.29%, 1.19%, and 0.19%, respectively, compared with using (NH₄)₂SO₄ solution alone. Under these conditions, the hydration parameters of the three minerals were also reduced. In addition, zeta potential measurements showed that the absolute value of the zeta potential initially increased with increasing HA concentration and then became stable at approximately 0.2 g/L, indicating that the surface charge of the particles had approached saturation. The charge saturation and the formation of a stable adsorption layer contributed to the inhibition of hydration and swelling of the clay minerals. XRD patterns of the three clay minerals before and after HA treatment showed no significant changes in peak positions, indicating that the crystallographic structures of the minerals were not affected and the swelling of clay minerals resulted from the hydration layer absorbed on their surfaces rather than interlayer expansion.

Compared with other anti-swelling agents, HA is environmentally friendly, making it more attractive for practical applications. The experimental results confirmed that HA could suppress the hydration-induced swelling of clay minerals under the optimal conditions. Viscosity theory provided a theoretical foundation for the development of anti-swelling agents, and this study proposed a new direction for selecting anti-swelling agents for clay minerals, which provides new insights for the green development of WREOs.