Research on the Preparation and Performance of Wood with High Negative Oxygen Ion Release Induced by Moisture

Abstract

With the growing severity of environmental pollution, people are paying increasing attention to their health. However, naturally occurring wood with health benefits and applications in human healthcare is still scarce. Natural wood exhibits a limited negative oxygen ion release capacity, and this release has a short duration, failing to meet practical application requirements. This study innovatively developed a humidity-responsive, healthy wood material with a high negative oxygen ion release capacity based on fast-growing poplar. Through vacuum cyclic impregnation technology, hexagonal stone powder was infused into the pores of poplar wood, endowing it with the ability to continuously release negative oxygen ions. The healthy wood demonstrated a static average negative oxygen ion release rate of 537 ions/cm³ (peaking at 617 ions/cm³) and a dynamic average release rate of 3,170 ions/cm³ (peaking at 10,590 ions/cm³). The results showed that the particle size of hexagonal stone powder in suspension was influenced by the dispersants and dispersion processes. The composite dispersion process demonstrated optimal performance when using 0.5 wt% silane coupling agent γ-(methacryloxy)propyltrimethoxysilane (KH570), achieving the smallest particle size of 8.93 μm. The healthy wood demonstrated excellent impregnation performance, with a weight gain exceeding 14.61% and a liquid absorption rate surpassing 165.18%. The optimal impregnation cycle for vacuum circulation technology was determined to be six cycles, regardless of the type of dispersant. Compared with poplar wood, the hygroscopic swelling rate of healthy wood was lower, especially in PEG-treated samples, where the tangential, radial, longitudinal, and volumetric swelling rates decreased by 70.93%, 71.67%, 69.41%, and 71.35%, respectively. Combining hexagonal stone powder with fast-growing poplar wood can effectively enhance the release of negative oxygen ions. The static average release of negative oxygen ions from healthy wood is 1.44 times that of untreated hexagonal stone powder, and the dynamic release reaches 2 to 3 times the concentration of negative oxygen ions specified by national fresh air standards. The water-responsive mechanism revealed that negative oxygen ion release surged when ambient humidity exceeded 70%. This work proposes a sustainable and effective method to prepare healthy wood with permanent negative oxygen ion release capability. It demonstrates great potential for improving indoor air quality and enhancing human health.

1. Introduction

Wood has the potential to positively impact human health and reduce harmful substances in the environment, thereby exhibiting health benefits. It can influence human visual and psychological responses, and the degree of these responses can be controlled by adjusting wood coverage, type, and application position [1]. Additionally, wood has a positive effect on human brain activity and the nervous system, helping to reduce stress and increase positive emotions [2]. The volatile organic compounds released by wood have health benefits for the human body and can also reduce the number of bacteria in the environment, demonstrating antibacterial properties [3]. Wood exhibits multiple beneficial functions such as temperature and humidity regulation, thermal insulation, and sound absorption, which are advantageous for humans [4,5,6]. However, globally, natural healthy wood resources are relatively limited, making it difficult to meet the enormous current demand for health benefits [7]. Therefore, it is hoped that through effective modification methods, a new type of functional healthy wood can be developed.

Healthy wood prepared through modification technology can effectively reduce the threats to human health posed by biological contamination (such as bacteria and mold), chemical contamination (such as formaldehyde), and physical contamination (such as electromagnetic radiation). Qi et al. treated the wood surface with a nanoscale AgCu alloy, achieving a 100% antibacterial rate against E. coli and Staphylococcus aureus, as well as a mold resistance efficiency of over 75% against Aspergillus niger, Penicillium citrinum, and Trichoderma viride on the wood surface [8,9]. Liu et al. used the sol–gel method to load a Bi₂O₃-doped silica–titanium composite film on the wood surface, achieving degradation rates of 96.0% for rhodamine B and 94.0% for gaseous formaldehyde [10]. Ba et al. treated wood with in situ aniline polymerization, achieving a specific electromagnetic shielding efficiency as high as 65.8 dB cm⁻³ g⁻¹ on the cross-section of the wood [11]. Wei et al. sprayed MXene sheets on the wood surface to prepare MXene-coated wood, which exhibited an excellent electromagnetic shielding effect of 31.1 dB in the frequency range of 8.2 to 12.4 GHz [12]. These studies demonstrate that endowing wood with healthcare functions through the introduction of modifiers can not only effectively mitigate the impact of environmental pollution on human health, but also provide multiple health benefits.

Hexagonal stone, as a novel modification material, exhibits both green/environmentally friendly properties and health-promoting effects. Mined from Inner Mongolia, research on hexagonal stone remains limited internationally due to geographical constraints. Containing Fe₂O₃ and trace TiO₂, this mineral exhibits permanent negative oxygen ion release capability [13]. Catalyzed by TiO₂, Fe²⁺ readily undergoes redox reactions, releasing electrons that are captured by water molecules to form negative oxygen ions [14]. At an ambient temperature of 20 °C, hexagonal stone emits far-infrared radiation with wavelengths of 8–15 μm [15]. Qin et al. developed health-promoting wood flooring by combining elm with hexagonal stone, demonstrating progressively enhanced antibacterial efficacy against E. coli and S. aureus with increasing mineral concentration [16]. Zhang et al. modified poplar wood with hexagonal stone, observing accelerated drying rates as water clusters absorbed the mineral’s persistent 8–15 μm far-infrared emission [17]. These studies demonstrate that leveraging the inherent properties of hexagonal stone is particularly crucial for advancing its applications in wood modification.

Healthy wood can be engineered through modification technologies, owing to the negative oxygen ion release capability of hexagonal stone. Negative oxygen ions are hailed in the medical field as “vitamin oxygen” and as a “longevity factor.” They can alleviate allergic symptoms caused by dust, mold spores, and other allergens, and they can efficiently remove particulate matter (PM) from the air [18]. Zhou et al. found that terpenes and negative oxygen ions from camphor forests significant aid in regualting blood pressure and inflammation in elderly patients with hypertension [19]. A. P. Krueger et al. discovered that negative oxygen ions boost tracheal ciliary activity by directly acting on respiratory enzymes, functioning both in vitro and in vivo in mammals [20]. Moreover, negative oxygen ions have various positive effects, such as regulating human physiological states, improving skin quality, relieving fatigue, enhancing sleep quality, increasing appetite, and alleviating symptoms of cardiovascular diseases and cancer, as well as lowering blood pressure [18,21,22]. Xiao et al. found that negative oxygen ions may enhance amino acid metabolism through anti-inflammatory effects and by reducing inflammation and oxidative stress, thus benefiting overall health [23]. Given the multiple health benefits of negative oxygen ions, developing composite materials with negative oxygen ions release functionality via modification technologies has become a highly promising research direction. For example, Gao et al. (2017) prepared a composite material that releases negative oxygen ions through hydrothermal methods and the silver mirror reaction. After 60 min of visible light irradiation, the material released negative oxygen ions at a concentration of up to 1660 ions/cm³, and maintained a concentration above 1000 ions/cm³ for the next hour, meeting fresh air standards [24]. Chen et al. (2023) combined lignocellulose with tourmaline to create a composite material that releases negative oxygen ions and has anti-aging properties, promoting human health [25]. Wang et al. (2023) attached titanium dioxide/graphene oxide (TiO₂/GO) and silica nanoparticles (SiO₂) to wood, preparing a composite material that releases negative oxygen ions at a concentration of 1580 ions/cm³ under ultraviolet light, meeting the demand for healthy and environmentally friendly living [26]. Yang et al. (2024) developed a novel sustainable wood-based negative ion generator by fixing titanium dioxide and tourmaline inside wood, achieving a negative ion release concentration of 1590 ions/cm³ [27]. Han et al. (2025) used a tourmaline core-shell composite filler to enhance the wear resistance and negative oxygen ion release capabilities of wood decorative paper, achieving a negative oxygen ion release performance of 1000 ions/cm³ [28]. By utilizing the good permeability of poplar’s porous structure, hexagonal stone can enter the wood and exert its effect [29,30]. Hexagonal stone offers an innovative solution for developing functional healthy wood materials, thanks to its unique negative oxygen ion release properties.

In this study, we introduce a negative pressure impregnation modification method to develop healthy wood, addressing the limited variety and insufficient quantity of healthy wood in natural forests. We used hexagonal stone powder as the negative oxygen ion release material to modify poplar wood, using the preparation process illustrated in Figure 1. This method created a type of healthy wood that continuously releases negative oxygen ions. The study primarily investigates the effects of different dispersion systems and processes on the particle size of hexagonal stone powder, the influence of the impregnation process on the properties and structure of healthy wood, and the static and dynamic characteristics and environmental response mechanisms of negative oxygen ion release from healthy wood. The resulting healthy wood demonstrated an excellent weight gain rate, outstanding dimensional stability, and a high release capability of negative oxygen ions under moisture-induced conditions. This innovative method of combining hexagonal stone with poplar not only alleviates the supply pressure of healthy wood but also provides a theoretical foundation for its preparation, showing great potential for applications in human health.

2. Materials and Methods

2.1. Materials

In this study, γ-methacryloxypropyltrimethoxysilane (KH570), polyethylene glycol (PEG), and sorbitan monostearate (Span 60) were chosen as dispersants. Different dispersion processes and concentration gradients were employed to identify the optimal dispersion method for hexagonal stone powder. The dispersibility of the liquid phase system was assessed using a combination of particle size distribution analysis and microscopy to determine the best dispersion approach. Fourier-transform infrared spectrometer (FTIR) and X-ray diffractometry (XRD) were utilized to analyze and characterize the dispersed powder.

The reagents and equipment used are listed in

Table 1. Reagents.

Name	Specification	Manufacturer
Hexagonal stone	8000-mesh	Lingshou Yonghui Mineral Processing Factory (Shijiazhuang, China).
Sodium metasilicate	Na2SiO3·9H2O AR	Tianjin Fengchuan Chemical Reagent Technology Co., Ltd. (Tianjin, China).
3-(Trimethoxysilyl)propyl methacrylate	KH570 AR	Shandong Youso Chemical Technology Co., Ltd. (Linyi, China).
Sopropoxy tris(dioctyl phosphonate) titanate	KR-12 AR	Nanjing Chuangshi Chemical Auxiliary Co., Ltd. (Nanjing, China).
Aluminate coupling agent	UP-801 AR	Nanjing Youpu Chemical Co., Ltd. (Nanjing, China).
Polyethylene glycol	PEG AR	Fuchen (Tianjin) Chemical Reagent Co., Ltd. (Tianjin, China).
Cocoamidopropyl betaine	CAB-35 AR	Shandong Yousuo Chemical Technology Co., Ltd. (Linyi, China).
Sorbitan monostearate	Span60 AR	Wuxi Yatai United Chemical Co., Ltd. (Yixing, China).
Absolute alcohol 400	C2H6O AR	Tianjin Fengchuan Chemical Reagent Technology Co., Ltd. (Tianjin, China).
Deionized water	H2O AR	Self-made by Inner Mongolia Agricultural University
Populus bolleana Lauche	Free of skin defects, straight trunk, and a diameter of 35 cm at the chest; 20 mm × 20 mm × 20 mm	Saihan District, Hohhot City, Inner Mongolia Autonomous Region

and

Table 2. Instruments.

Instrument Name	Model	Manufacturer
Blast drying oven	101A-3B	Tianjin Hongnuo Instrument Co., Ltd. (Tianjin, China).
Ultrasonic cell disruptor	SM-1800D	Nanjing Shunma Instrument Equipment Co., Ltd. (Nanjing, China).
Scanning image grit size analyzer	BT-1700	Dandong Baite Instrument Co., Ltd. (Dandong, China).
Atmospheric negative (oxygen) ion detector	XDB-6400	Shenzhen New Landmark Environmental Technology Development Co., Ltd. (Shenzhen, China).
Fourier-transform infrared spectrometer	Nicolet Magna-IR 750	Thermo Nicolet Corporation (Madison City, State of Wisconsin, United States).
X-ray diffractometer	ESCA-14	Bruker Spectrum Instruments, Karlsruhe, Germany
X-ray fluorescence analyzer	EA1000VX	Hitachi, Tokyo, Japan
Scanning electron microscope	S-3400N	Hitachi, Tokyo, Japan
Miniature plant grinder	RS-FS1401	Hefei Rongshida Small Appliance Co., Ltd. (Hefei, China).
Type blast drying oven	101A-3B	Tianjin Hongnuo Instrument Co., Ltd. (Tianjin, China).
Vacuum drying oven	DZF-ZASB	Beijing Kewei Yongxing Instrument Co., Ltd. (Beijing, China).
Constant temperature and humidity oven	BPS-100CA	Tianjin Weiss Experimental Instrument Technology Co., Ltd. (Tianjin, China).
Type horizontal slicer	SM2010R	Leica Biosystems Nussloch GmbH (Nussloch, Germany).

.

2.2. Preparation of Hexagonal Stone Dispersion by Chemical Dispersion Treatment

The chemical dispersion treatment employs dispersants with functional groups that bind to the surface of the powder or powder aggregates, modifying the wettability of the powder surface. This action enhances the dispersion performance of the mineral powder and reduces the particle size. For the experiment, 10 g portions of hexagonal stone powder and absolute ethanol as the dispersion medium were prepared with 200 mL of dispersion liquids with dispersant concentrations of 0.1 wt%, 0.25 wt%, 0.5 wt%, 1 wt%, and 1.5 wt%. During the experiment, the corresponding volumes of absolute ethanol and dispersant were added into a beaker, which was then placed on an electronic stirrer operating at 1000 r/min (Tianjin Taisite Instrument Co., Ltd., Jintan, China). and stirred at 25 °C for 60 min.

2.3. Preparation of Hexagonal Stone Dispersion by Physical Dispersion Treatment

The physical dispersion treatment employs ultrasound to fragment larger powder particles into smaller ones. It leverages the localized high temperature and pressure generated by cavitation to weaken the inter-particle interactions and reduce agglomeration. In the experiment, 10 g hexagonal stone powder was mixed with 200 mL of anhydrous ethanol dispersion liquid in a beaker. This suspension was then processed for 60 min using an ultrasonic cell breaker operating at 1350 W. To maintain the mixture’s temperature below 40 °C, an appropriate amount of ice cubes was added every 30 min.

2.4. Preparation of Hexagonal Stone Dispersion by Composite Dispersion Treatment

The composite dispersion treatment integrates the benefits of both physical and chemical dispersion methods. During the physical dispersion process, chemical dispersants are added to leverage ultrasound to reduce the particle size of hexagonal stone powder while the dispersants adhere to the powder surface. The process generates a new specific surface area due to powder fragmentation and produces a mechanical activation effect during the process, ultimately enhancing the dispersion performance of the powder [31]. In a beaker, 10 g of hexagonal stone powder and 200 mL of absolute ethanol as the dispersion medium were mixed with dispersant concentrations of 0.1 wt%, 0.25 wt%, 0.5 wt%, 1 wt%, and 1.5 wt%. The mixed solution was then processed for 60 min using an ultrasonic cell breaker operating at 1350 W.

2.5. Drying of Poplar Samples

The sample of poplar wood material (20 mm cubes) was placed in a convection oven at 60 °C for 2 h. Subsequently, the temperature was raised to 103 ± 2 °C until the sample reached absolute dryness. Finally, the mass of the dried sample was measured.

2.6. Preparation of Healthy Wood

The impregnation liquid was introduced into the wood via vacuum impregnation, employing a cyclic treatment process of impregnation, drying, and re-impregnation. Composite dispersion processes were utilized to prepare the hexagonal stone dispersion. In three separate beakers, 10 g of hexagonal stone powder was mixed with 200 mL of anhydrous alcohol dispersion liquid. Subsequently, dispersants—KH570 (0.5 wt%), PEG (0.5 wt%), and Span60 (1.0 wt%)—were added to each beaker, respectively. The mixtures were then processed for 60 min using an ultrasonic cell breaker operating at 1350 W. The fully dry sample was placed in a beaker containing the immersion liquid and covered with a plastic mesh to prevent floating and ensure effective impregnation. The vacuum pressure was maintained at −0.08 MPa, the temperature was 25 °C, and the single vacuum immersion cycle lasted 60 min under normal pressure. The specimen was immersed for 5 min, then placed in a convection oven at 103 ± 2 °C until completely dry, ensuring the absolute ethanol had fully evaporated. To maximize the penetration of the impregnation liquid into the wood, the impregnation treatment was repeated until the sample’s mass remained essentially unchanged after immersion and drying.

2.7. Particle Size Test

The BT-1700, manufactured by Dandong Baite Instrument company limited, was used to measure the particle size and distribution of the powder. The instrument has a particle size measurement range of 1 to 10,000 μm, a scanning diameter of 55 mm, a repeatability error of ≤1%, and an accuracy error was ≤1%. Before sampling, the experimenter stirred the dispersion with a glass rod for at least 30 seconds. Then, using a disposable rubber dropper, the dispersion was transferred to the center of a glass slide and allowed to dry naturally. Finally, the glass slide was placed on the microscope stage for particle size measurement and data recording.

2.8. SEM Characterization

The surface micro-morphology of the powder was analyzed using an S-3400N scanning electron microscope from Hitachi, Tokyo, Japan. The experimenter dried the powder to a dispersed state at 60 °C using a convection oven, then affixed an appropriate amount of dry powder firmly to a conductive tape. A metal film approximately 3 to 30 nm thick was then sprayed onto the powder surface. The sample was subsequently placed in the microscope for observation, with an appropriate magnification selected and relevant data recorded at a working voltage of 10 kV.

2.9. Monitoring of Negative Oxygen Ion Release from Hexagonal Stone

The XDB-6400 negative oxygen ion detector, with a resolution of 1 pcs/cm³, was used to monitor the static negative oxygen ion release of the hexagonal stone powder before and after dispersion, in accordance with the test method specified in the national standard GB/T 28628-2012 [32]. The laboratory conditions were maintained at a temperature of 23 ± 2 °C and a humidity of 30% to 40%. Testing was conducted in a relatively closed, constant temperature and humidity chamber measuring 500 mm × 300 mm × 500 mm. Prior to testing, the negative oxygen ion detection port was closed, the detector was zero-adjusted, the automatic detection mode was set, and data were updated and recorded every minute.

2.10. FT-IR Characterization of Hexagonal Stone Powder

The molecular composition and structure of the powder were analyzed using a Nicolet Magna-IR 750 spectrometer from Thermo Fisher Scientific (formerly Nicolet Company, United States). The experimenter prepared the sample by mixing an appropriate amount of dry, dispersed powder with potassium bromide using the pellet method and scanned in the spectral range of 450 to 4000 cm⁻¹.

2.11. XRD Characterization of Hexagonal Stone Powder

The crystal structure of the powder was characterized by ESCA-14 diffractometer from Bruker, Germany. An appropriate amount of the powder was dispersed after drying. The scanning was performed at a speed of 4°/min over a range of 5° to 90°.

2.12. Weight Gain Rate and Fluid Absorption Rate of Healthy Wood

The evaluation indices of the impregnation effect mainly include WPG and LAR. WPG refers to the percentage increase in the absolute dry mass of the sample after impregnation, which directly indicates the retention percentage of modified substances within wood cells, and which essentially reflects the nature of chemical immersion. LAR, the wet weight gain percentage, refers to the percentage increase in the mass of the impregnation liquid absorbed by the sample after impregnation, implying that the sample contains a significant amount of dispersion medium. To a certain extent, LAR reflects the rate of impregnation [33]. The weight percentage gain (WPG) and liquid absorption percentage (LAR) of the impregnated modified wood were calculated according to Equations (1) and (2), respectively [34].

$$WPG={\displaystyle \frac{m_a-m_0}{m_0}}\times 100\%$$

(1)

$$LAR={\displaystyle \frac{m_b-m_0}{m_0}}\times 100\%$$

(2)

where m₀ is the total dry mass of the sample (g), m_a is the total dry mass after impregnation treatment (g), and m_b is the wet weight of absorbed impregnation solution after impregnation treatment (g).

2.13. Dimensional Stability

Dimensional stability is defined in accordance with LY/T2490-2015 [35] “Test method for dimension stability of modified wood”. The data measured in the test are calculated using Equations (3)–(7):

(The linear hygroscopic expansion percentage is given by

$${\alpha }_L={\displaystyle \frac{L_W-L_0}{L_0}}\times 100\%$$

(3)

where α_L is the percentage change in size of the modified material relative to its initial size (%), L_W is the size of the material after high-humidity treatment (mm), and L₀ is the initial size of the material (mm).

The volumetric hygroscopic expansion percentage is given by

$${\alpha }_V={\displaystyle \frac{V_W-V_0}{V_0}}\times 100\%$$

(4)

where α_V is the relative percentage change in the volume of the initial material and modified material, V_W is the volume of the material after high-humidity treatment (mm³), and V₀ is the initial volume (mm³).

The linear resistance to hygroscopic expansion is calculated as

$${\beta }_L={\displaystyle \frac{L_C-L_T}{L_C}}\times 100\%$$

(5)

where β_L is the anti-hygroscopic expansion ratio of the modified wire (%), L_C is the hygroscopic expansion ratio of the unmodified material (%), and L_T is the hygroscopic expansion ratio of the modified material (%).

The volume resistance to hygroscopic expansion is

$${\beta }_V={\displaystyle \frac{V_C-V_T}{V_C}}\times 100\%$$

(6)

where β_V is the volume hygroscopic expansion percentage of the modified material (%), V_C is the volume hygroscopic expansion percentage of the original material (%), and V_T is the volume hygroscopic expansion percentage of the modified material (%).

The moisture resistance is given by

$$\epsilon ={\displaystyle \frac{M_C-M_T}{M_C}}\times 100\%$$

(7)

where ε is the moisture absorption resistance of the modified material (%), M_C is the stable moisture content of the unmodified material (%), and M_T is the stable moisture content of the modified material (%).

2.14. SEM Characterization

The surface micro-morphology of the samples was analyzed using an S-3400N scanning electron microscope from Hitachi, Japan. The raw and modified materials were prepared by cutting 60 μm thick slices from the core layer of the sample along the radial, chord, and lateral directions. The test piece was placed in a convection oven until it was completely dry, and then a gold film approximately 3 to 30 nm thick was sprayed on the surface. The observations were made at a working voltage of 10 kV with an appropriate magnification selected.

2.15. Monitoring of Negative Oxygen Ion Release from Healthy Wood

An XDB-6400 negative oxygen ion detector, with a resolution of 1 pcs/cm³, was used to monitor the release of negative oxygen ions from materials and modified materials, in accordance with the national standard GB/T28628-2012 [32], titled “Test method for the amount of air ions induced by materials”.

2.16. FT-IR Characterization of Healthy Wood

The molecular composition and structure of the sample were analyzed using a Nicolet Magna-IR 750 spectrometer from Thermo Fisher Scientific (formerly Nicolet Company, USA). After the sample was completely dry, it was cut into small wooden strips, pulverized, and the powder passed through a 100 mesh sieve. An appropriate amount of powder was then dried in a convection oven at 60 °C and prepared using the potassium bromide pellet method. The sample was scanned in the spectral range of 450 to 4000 cm⁻¹.

2.17. XRD Characterization of Healthy Wood

The crystal structure of the powder was characterized using an ESCA-14 diffractometer from Bruker, Germany. An appropriate amount of powder was dried in a convection oven at 60 °C to disperse it. The scanning was performed at a speed of 4°/min over a range of 5° to 90°.

3. Results

3.1. Study on the Dispersion Process of Hexagonal Stone Powder

Due to the limitations of the wood’s pore structure, mineral-modified poplar is typically modified using an impregnation method, which requires the minerals to be prepared in powder form. The particle size distribution of the mineral powder not only affects the dispersion of the suspension but also influences the modification effect. The median particle size (D50 value) is commonly used to evaluate the particle size distribution of the mineral powder. The D50 value is the particle size at which the cumulative particle size distribution of the powder reaches 50% [36]. A smaller median particle size in the powder generally results in better dispersibility, with little to no powder agglomeration.

One of the factors affecting the particle size of the powder in the hexagonal suspension is the dispersant. As shown in Figure 2a, at a dispersant concentration of 1 wt%, the particle size of the hexagonal powder in suspensions prepared with different types of dispersants varies. The D50 values for PEG, KH570, and Span60 formulations were 11.42, 11.53, and 12.27 μm, respectively, indicating excellent dispersing effects on hexagonal powder. The mechanisms by which these three dispersants influence the D50 value of the hexagonal powder in the suspension are different. According to the covalent bond adsorption theory, KH570 undergoes hydrolysis upon contact with water molecules. The hydrolysis products react with the functional groups on the surface of the hexagonal powder to form hydrogen bonds. After dehydration and condensation, these hydrogen bonds transform into covalent bonds, forming a new interface region, which ultimately allows the hexagonal powder to disperse effectively [37]. PEG, due to its numerous hydrophilic and hydrophobic groups, exhibits good surface activity and promotes the wetting effect of the dispersing medium on the hexagonal powder, thereby enhancing dispersion [38]. Span60 interacts with the hexagonal powder through its hydrophilic groups, demonstrating good emulsifying and dispersing effects on the powder.

The particle size of the mineral in the hexagonal stone suspension is also influenced by the powder dispersion process. After chemical, physical, and composite dispersion treatments, the measured D50 values of powders treated with various concentrations and types of dispersants are shown in Figure 2b. The average D50 value of the powder after physical dispersion is 14.21 μm. For the hexagonal stone suspension treated by chemical dispersion, the optimal particle size is reduced, and it is further reduced when treated by the composite dispersion process. At the same dispersant concentration, the smallest particle sizes in the hexagonal stone suspension treated by chemical and composite dispersion processes are achieved with PEG and KH570 (except for 1 wt%), with the composite dispersion process yielding even smaller particle sizes. When the same dispersant is used at different concentrations, the particle size in the hexagonal stone suspension treated by chemical and composite dispersion processes initially decreases and then increases with increasing dispersant concentration. This indicates that the particle size of the hexagonal stone suspension is influenced by factors such as the type of dispersant, dispersant concentration, and powder dispersion process. The relative effectiveness of the three treatment processes on the dispersion effect of hexagonal stone powder can be ranked as follows: composite process > chemical process > physical process.

As shown in Figure 2b, the optimal D50 values of the powder in the hexagonal stone suspension are 8.93 μm, 9.21 μm, and 9.89 μm, corresponding to the dispersants KH570, PEG, and Span60, with the respective dispersant concentrations of 0.5 wt%, 0.5 wt%, and 1.0 wt%, respectively. This indicates that for the same dispersant under identical dispersion process conditions, there is an optimal concentration that minimizes the D50 value of the powder. At this concentration, the dispersant achieves maximum wettability on the hexagonal stone powder, forming a stable monomolecular adsorption layer on the powder surface [39]. This results in the most stable dispersion liquid and the best dispersion effect. When the dispersant concentration is too low, insufficient surface coverage of the powder hampers the dispersant’s ability to penetrate the hexagonal stone powder surface, leading to suboptimal dispersion. Conversely, when the dispersant concentration is too high, an oversaturated adsorption layer forms on the powder surface, reducing its hydrophilicity and hindering dispersion.

The infrared spectra (FT-IR) of the hexagonal powder before and after treatment with KH570, PEG, and Span60 are shown in Figure 2c. The infrared spectra of the hexagonal stone before and after the dispersion treatment are essentially consistent, indicating that the chemical composition remains unchanged after dispersion treatment. Comparison of the spectra reveals that the powders dispersed by KH570 and PEG exhibit very prominent -CH3 and -CH2 stretching vibration absorption peaks at 2975 cm⁻¹, 2900 cm⁻¹ and 2974 cm⁻¹, 2900 cm⁻¹. This indicates that new modified groups were introduced to the surface of the hexagonal powder, and some chemical bonding and coating occurred on its surface [40,41]. The broad peaks at 3350 cm⁻¹ and 3348 cm⁻¹ are attributed to the antisymmetric stretching vibrations of structural water -OH. He et al. used KH570 to hydrophobically modify the surface of diatomite [42]. Compared with its infrared spectrum, there is no obvious C=O absorption peak in KH570-modified spectrum, which is also observed in many studies [43,44]. This is because the amount of coupling agent added in the dispersion process is far less than the powder content, so the infrared spectrum fails to reflect the obvious C=O characteristic absorption peak [45,46]. The spectra for PEG and Span60 can be explained similarly.

Figure 2d shows the X-ray (XRD) diffraction patterns of the hexagonal powder before and after treatment with KH570, PEG, and Span60. The peak shapes of the XRD diffraction peaks of the hexagonal powder before and after treatment with the dispersant are identical in shape, indicating that the crystal structure of hexagonal powder remains essentially unchanged after dispersion treatment, and its physical properties are preserved. Meanwhile, it is evident from Figure 2d that the intensity of the diffraction peaks has been enhanced to some extent after dispersion treatment. This enhancement is attributed to the composite treatment process used in the dispersion of the powder; the particle size of the ultrasonically treated powder becomes smaller, and the powder becomes finer, leading to the increased intensity of the diffraction peaks.

In conclusion, the type, concentration, and dispersion process of dispersants affect the particle size of hexagonal stone powder. KH570, PEG, and Span60 were selected as dispersants, with concentrations of 0.5 wt%, 0.5 wt%, and 1.0 wt%, respectively. After treatment with the composite dispersion process, the D50 value of hexagonal stone was minimized. New functional groups were introduced on the surface of the hexagonal stone powder after treatment, while the crystal structure remained unchanged, and the ability of hexagonal stone to release negative oxygen ions was unaffected.

3.2. The Effect of Dispersion Treatment on the Microstructure and Negative Oxygen Ion Release Capability of Hexagonal Stone Powder

The microstructure of the hexagonal stone dispersion liquid powder treated with different dispersion processes and dispersants is shown in Figure 3, and the particle size of the powder is listed in

Table 3. Powder particle size characteristics of different dispersants.

Dispersant	Maximum (μm)	Minimum (μm)	Average (μm)
Untreated	25.04	2.12	18.37
KH570	19.32	0.24	8.93
PEG	20.05	0.35	9.21
Span60	18.97	0.19	9.89

. The boundaries between untreated hexagonal stone particles are indistinct, with significant agglomeration, and the overall particles are relatively large, making it difficult to enter wood pores. After treatment with dispersants, the overall particle size of the hexagonal stone powder is reduced, the particle morphology becomes clear, the particle size distribution is uniform, and no severe agglomeration occurs. The dispersibility of the powder is significantly improved. This is because when the surface of the hexagonal stone powder bonds with the silane coupling agent, the surface tension between particles is reduced through hydrolysis and condensation reactions, and steric hindrance increases, thereby preventing particle agglomeration. This allows the powder to disperse more effectively and avoids the formation of large agglomerates [47,48]. When the concentration of KH570 exceeds 0.5 wt%, the dispersion of hexagonal stone deteriorates because the dispersant itself tends to aggregate [49]. PEG can adsorb onto the surface of hexagonal stone particles through hydrogen bonding, reducing the surface tension and inducing a steric hindrance effect, which enables the dispersion of the hexagonal stone powder. When the concentration of PEG exceeds 0.5 wt%, the density of the dispersion liquid increases, making the dispersion of hexagonal stone powder more difficult [50,51,52]. Span60 coats the surface of hexagonal stone powder through hydrogen bonding or ionic interactions with hydroxyl groups or metal ions on the hexagonal stone surface, effectively preventing nanoparticle collisions and thus suppressing agglomeration. The hydroxyl groups of Span60 adsorb onto the hexagonal stone surface, while its hydrophobic alkyl chains enhance the hydrophobicity of the hexagonal stone. When the Span60 concentration exceeds 1.0 wt%, the increased hydrophobicity of hexagonal stone induces hydrophobic interactions among particles, promoting their aggregation and reducing dispersibility [53,54]. As shown in Table 3, after treatment with dispersants, the maximum, minimum, and average particle sizes of the hexagonal stone powder decrease. The extent of reduction depends on the type of dispersant. The average particle size of the hexagonal stone powder treated with KH570 is the smallest, while the maximum and minimum particle sizes of the hexagonal stone powder treated with Span60 are the smallest, as shown in Table 3. The minimum particle size of the hexagonal stone powder has reached the nanometer level, which is suitable for entering the wood cell wall. The proportion of nanometer-sized hexagonal stone powder is related to the type of dispersant used.

To investigate the change in the release amount of negative oxygen ions from hexagonal powder before and after dispersant treatment, the static release amount was monitored under visible light. The release of negative oxygen ions from the untreated hexagonal powder was highly unstable, with the release amount fluctuating within the range of 200-700 pcs/cm³. After treatment with PEG, the variation range of the release amount narrowed, and both the average and peak release amount of negative oxygen ions reached their highest values, as shown in Figure 4. Analysis of Figure 2b and

Table 4. Negative oxygen ion release.

Dispersant	Average Values/ (pcs/cm3)	Highest Values/ (pcs/cm3)	Lowest Values/ (pcs/cm3)
Untreated	352	650	143
KH570	374	689	176
PEG	415	701	182
Span60	398	699	189

indicates that hexagonal stone with smaller D50 values has a stronger capacity to release negative oxygen ions. Furthermore, based on the analysis of Table 3, Figure 3, and the XRD results, it can be concluded that the crystal structure of hexagonal stone remains unchanged before and after dispersion treatment, while the particle size distribution range of the hexagonal stone powder is reduced. This suggests that the release of negative oxygen ions is related to the particle size of hexagonal stone and the type of dispersant used.

Thus, following treatment, the average particle size of the hexagonal stone powder is reduced, the particle size distribution range is narrowed, the release amount of negative oxygen ions is increased, and the stability of negative oxygen ion release is improved.

3.3. Preparation and Characterization of Healthy Wood

Poplar wood is characterized by its low density, hierarchical porous structure, and good permeability. These properties enable it to absorb hexagonal powder through impregnation modification, thereby endowing the wood with the ability to release negative oxygen ions. The number of impregnation cycles significantly affects the impregnation efficacy of the hexagonal stone dispersion liquid in poplar wood. As the number of impregnation cycles increases, the average weight gain rate and average liquid absorption rate of poplar wood initially rise rapidly before gradually leveling off, as shown in Figure 5a. When the number of impregnation cycles is below 4, the average weight gain rate increases swiftly, and then the growth rate decelerates progressively until it stabilizes. The average liquid absorption rate follows a similar trend, indicating an upper limit to the number of impregnation cycles and a finite capacity for the dispersion liquid’s effective components to enter the wood. This is because with increased impregnation cycles, the hexagonal stone powder, along with the dispersion liquid, fills the wood pores. As the filling level rises, the absorption of the dispersion liquid by the poplar wood slows down. Once the amount of hexagonal stone powder and dispersant in the wood pores reaches a certain threshold, the hexagonal stone powder can no longer enter the wood through the pores. Therefore, the optimal impregnation cycle for hexagonal stone/poplar wood modified materials is determined to be six cycles. This process involves a single vacuum impregnation for 60 min, followed by atmospheric pressure impregnation for 5 min. After drying to absolute dryness, the impregnation treatment is repeated six times. The hexagonal stone dispersion liquid primarily enters the poplar wood through the vessels, as shown in Figure 5b.

As the impregnation time increases, both the average weight gain rate and the average liquid absorption rate of healthy wood rise, as illustrated in Figure 5c,d. When KH570 is used as the dispersant, the average weight gain rate of poplar wood is the highest, and when Span60 is used, the average liquid absorption rate is the highest. This suggests that the impregnation rate of hexagonal stone dispersion liquid is not directly proportional to the weight gain rate. According to Table 3, when KH570 is used as the dispersant, the D50 value of the hexagonal stone powder is the smallest, and when Span60 is used, the D50 value is the largest. This indicates that the particle size of powder affects the average weight gain rate of poplar wood. Specifically, the smaller the particle size of the hexagonal stone powder, the more powder can enter the poplar wood during impregnation under the same conditions. Conversely, the larger the powder particle size, the fewer particles that can enter the wood. Additionally, at a Span60 concentration of 1 wt%, and PEG and KH570 concentrations of 0.5 wt%, the hexagonal stone dispersion liquid with Span60 enters the poplar wood more easily. The average liquid absorption rates of the hexagonal stone dispersions with PEG and KH570 are similar, indicating that the average liquid absorption rate is positively correlated with the dispersant concentration. In summary, the D50 value affects the weight gain rate of healthy wood, while the dispersant concentration affects the liquid absorption rate of healthy wood.

The infrared spectra (FT-IR) of poplar and healthy wood are shown in Figure 5e. Comparing the infrared spectra before and after modification, it is evident that the unimpregnated poplar wood exhibits a broad and strong -OH stretching vibration peak at 3348 cm⁻¹, indicating a high content of -OH groups on its surface. However, after impregnation with KH570, PEG, and Span60 dispersion, the -OH stretching vibration peaks appear at 3351 cm⁻¹, 3351 cm⁻¹, and 3343 cm⁻¹, respectively. The peak positions are shifted to some extent, likely due to the addition of the dispersion liquid, which forms intermolecular hydrogen bonds with the wood -OH groups. Additionally, the vibration peaks after modification are noticeably narrowed and weakened. This is because the -OH groups on the wood surface are replaced by hydrophobic groups from the wood dispersant and hexagonal stone powder, which reduces the absorption peaks of -OH in the cellulose and hemicellulose within the wood’s own structure [55]. Moreover, the modified wood shows a distinct O-H stretching characteristic peak of metal oxide at 3351 cm⁻¹, confirming the successful introduction of hexagonal powder into the wood.

In contrast, the infrared spectra (FT-IR) of poplar and healthy wood show very distinct C-H stretching vibration absorption peaks for -CH₃ and -CH₂ groups at 2974 cm⁻¹ and 2900 cm⁻¹, respectively. Characteristic absorption peaks of xylan C=O [56], C=O, and aromatic ring skeleton vibration appear at 1594 cm⁻¹. The characteristic peak of the carbon skeleton vibration of the benzene ring is one important indicator of the existence of aromatic rings and the result of the vibration of the benzene skeleton in lignin. Additionally, the characteristic peaks of cellulose appear at 1158 cm⁻¹ and 881 cm⁻¹. Compared with the poplar and the healthy wood, these three characteristic peaks have not changed significantly, indicating that there is no chemical change between lignin and cellulose after modification.

XRD was used to investigate the crystallinity and crystal structure of wood fibers modified by the impregnation solution, as well as the interaction between modifiers and wood fibers. The X-ray diffraction patterns are shown in Figure 5f. This figure reveals that the characteristic peaks appearing near the diffraction angles of 16° and 22° for all four groups of samples correspond to the (101) and (002) crystal planes of wood cellulose, respectively. The positions of these peaks remain unchanged, indicating that the impregnation treatment did not destroy the crystalline structure of cellulose. According to the Segal method [57], the relative crystallinity of the untreated material is 40.22%. The crystallinity of the materials modified with KH570, PEG, and Span60 are 40.93%, 41.22%, and 46.34%, respectively. These values are 1.73%, 2.49%, and 15.22% higher than those of the untreated materials. The increase in crystallinity may be attributed to the rearrangement of the molecular chains into a more regular crystalline structure during the modification process, as well as the removal of some amorphous pectin [58,59]. Additionally, hydrogen bond rearrangement within cellulose could lead to recrystallization [60]. Moreover, the three groups of modified materials exhibit distinct characteristic diffraction peaks of hexagonal stone at the diffraction angles of 30.94°, 33.54°, 37.36°, 41.14°, 44.94°, 50.56°, and 51.08°, respectively.

3.4. Microscopic Structure of Healthy Wood

The microstructure of poplar wood and healthy wood is shown in Figure 6. The surface of poplar wood not been impregnated with hexagonal stone is smooth, with no impurities in the vessels and cell cavities, as shown in Figure 6A. After modification, hexagonal stone powder is distributed within the vessels and cell cavities of poplar wood, appearing as irregular granules attached to the cell walls. The modification process did not alter the original pore structure of the poplar wood, as shown in Figure 6B. After treatment with a dispersant, the amount of hexagonal stone in some vessels of the healthy wood significantly increased, with the powder adhering to the vessel walls and pore edges. In some areas, powder accumulation occurred, reducing the wood’s permeability, as shown in Figure 6C–H. The dispersant used was PEG, which allowed the hexagonal stone powder to uniformly adhere within the wood cell cavities. Due to the swelling effect of PEG, the wood cell walls thickened, as shown in Figure 6E,F. Figure 6 indicates that hexagonal stone powder can extensively adhere to wood tissues, providing a theoretical basis for the long-term sustained release of negative oxygen ions [61].

3.5. Changes in the Properties of Modified Healthy Wood and the Effect of Moisture on Negative Oxygen Ion Release Capacity

Wood is a porous material with a honeycomb-like pore structure. Studies have shown that 50%–80% of the volume of wood is occupied by pores [62]. It is precisely due to the presence of this pore structure that wood has a large amount of moisture-absorbing space. In addition, the cell wall of wood is primarily composed of natural macromolecular compounds containing a large number of hydrophilic hydroxyl groups, which contribute to the strong hygroscopic ability of wood [63]. After moisture absorption, the microfibrils in the wood undergo a swelling reaction, causing the cell walls to expand and deform. At the same time, wood exhibits anisotropy, and as the moisture content changes, uneven expansion and contraction occur in the tangential, radial, and longitudinal directions. This leads to defects such as deformation, cracking, and warping, which affect the dimensional stability of the wood and restrict its processing and utilization [64].

The tangential, radial, longitudinal, and volumetric hygroscopic swelling rates of the healthy wood are all reduced to varying extents compared to the poplar wood. As can be analyzed from Figure 2b, Figure 5c,d and Figure 6, hexagonal stone and dispersants fill the pore structure of poplar wood, blocking the channels of water molecules by filling and clogging the wood vessels, fiber lumens, and bordered pit channels. When the wood is in a high-humidity environment, the water absorption of healthy wood is reduced, thereby lowering its hygroscopic swelling rate [65]. Among them, the samples treated with PEG show the greatest reduction in hygroscopic swelling rates, which decrease by 70.93%, 71.67%, 69.41%, and 71.35%, respectively, compared to the poplar wood, as shown in Figure 7a. This is because after impregnation modification, hexagonal stone powder and dispersants are present in the wood pores, blocking the entry of water molecules from the air. Additionally, the dispersants and smaller particle sizes of the hexagonal stone powder fill and block the gaps between wood fibers, hindering the penetration of water molecules into the fibers, thus reducing the hygroscopic swelling rate of the wood [65].

The anti-hygroscopic swelling rate is a key indicator for assessing the stability of wood: the higher the anti-hygroscopic swelling rate, the better the dimensional stability of the wood. The average volumetric anti-hygroscopic swelling rate of the healthy wood follows the order PEG > Span60 > KH570 treatment, as shown in Figure 7b. After PEG treatment, the tangential, radial, longitudinal, and volumetric anti-hygroscopic swelling rates of the impregnated samples are 70.90%, 71.59%, 69.25%, and 71.31%, respectively. This is because PEG has good compatibility with wood and can easily diffuse into it, replacing moisture and adhering to the wood surface, thereby improving the wood’s dimensional stability [66]. The enhanced dimensional stability of healthy wood stems from the combination of modifiers with the hydroxyl groups of wood and the increased crystallinity of wood cellulose during the modification process. Repeated swelling and shrinking of wood under humidity fluctuations can lead to cracking and deformation (such as floor warping and loose joints). The increased dimensional stability of wood helps to reduce the damage caused by repeated swelling and shrinking, thereby extending the service life of wooden products and components. This not only improves the physical properties of wood but also broadens its application scenarios, providing an advanced engineering material with high precision, long service life, and multifunctional applications. This study utilized natural and biodegradable materials such as wood and ethanol and employed environmentally friendly materials like KH570, PEG, and Span60 as dispersants to conduct impregnation modification under closed conditions, developing healthy wood that combines excellent properties with sustainability. Moreover, the enhanced dimensional stability of healthy wood reduces wood deformation and damage, extends the service life of wood, and further decreases the consumption of wood resources. Meanwhile, the combination of wood with hexagonal stone increases the release of negative oxygen ions and endows the material with long-lasting negative oxygen ion release capability, contributing to sustainability. Compared with the findings of Zhang et al. [67] and Yang et al. [68], the healthy wood in this study demonstrates superior dimensional stability.

Figure 7c shows the change in the amount of negative oxygen ions released from the healthy wood changes with the air humidity. When the air humidity rises to about 70% or higher, the release of negative oxygen ions from the healthy wood increases sharply. Conversely, when the ambient humidity decreases, the concentration of negative oxygen ions also drops. This indicates that its release mechanism is related to water, suggesting a water response mechanism. Specifically, moisture in the air enters the healthy wood and reacts with the hexagonal stone to produce negative oxygen ions, as shown in Figure 7d. The hexagonal stone has a nanopore structure. The variable valency Fe²⁺ around it generates a micro-electric field. When water molecules in the air enter this field, they are ionized into positively charged hydrogen ions and negatively charged hydroxide ions. The positively charged hydrogen ions combine to form hydrogen gas, which is released into the air. The negatively charged hydroxide ions combine with water to form negative ion water, which is released into the air as a gas [14]. When the humidity is low, water molecules in the air are more likely to bond with the wood’s hydroxyl groups, resulting in fewer negative oxygen ions being released by the healthy wood. In contrast, when the humidity is high, water molecules form multilayer adsorption in the wood, creating a high-humidity environment. This facilitates the entry of water molecules into the hexagonal stone’s micro-electric field, leading to a sharp increase in the release of negative oxygen ions from the healthy wood. However, when the environmental humidity is excessively high, water molecules occupy the large pore structures such as vessels and cell cavities, blocking the channels for negative oxygen ions. As a result, the release of negative oxygen ions from the healthy wood decreases [69].

3.6. Release Amount of Negative Oxygen Ions from Healthy Wood

To investigate the changes in the release of negative oxygen ions from healthy wood before and after modification, the static release of healthy wood was monitored under visible light. Additionally, dynamic negative oxygen ion release monitoring was conducted. To verify the recyclability of the modified material in generating negative oxygen ions, a cycle test lasting up to 120 min was performed on the sample under visible light. The amount of negative oxygen ions released from the poplar wood and healthy wood is shown in Figure 8. The monitoring results show that the amount of negative oxygen ions released by the material is 75 pcs/cm³ in the static state and 71 pcs/cm³ in the dynamic state, with almost no difference. The trace amount of negative oxygen ions can essentially be regarded as the negative oxygen ions present in the air.

Under static conditions, the release of negative oxygen ions from healthy wood is comparable, and significantly higher than that from poplar wood and hexagonal stone powder. Under dynamic conditions, the release of negative oxygen ions from the healthy wood increases, reaching 2 to 3 times the national fresh air standard for negative oxygen ion content, as shown in

Table 5. Modified material negative oxygen ion release values.

	KH570 (pcs/cm3)	PEG (pcs/cm3)	Span60 (pcs/cm3)
Average static	507	531	537
Highest values	551	563	617
Lowest values	463	462	420
Average dynamic	2780	3170	3171
Highest values	10,590	9580	10,510
Lowest values	831	909	1037

[70]. Although the release of negative oxygen ions from the healthy wood still shows some fluctuations; these fluctuates are less pronounced compared to those of hexagonal stone powder itself, resulting in a more stable release, as shown in Figure 8. The release of negative oxygen ions from healthy wood is influenced by the D50 value of hexagonal stone (Figure 2b), the weight gain rate (Figure 5c), and the hydrophilic and hydrophobic properties of the dispersant. The smaller the D50 value and the higher the weight gain rate, the greater the content of hexagonal stone powder in healthy wood, and the stronger its ability to release negative oxygen ions. Healthy wood prepared with PEG and Span60 dispersants releases more negative oxygen ions than that with KH570. This is because KH570 is hydrophobic, while PEG and Span60 are hydrophilic. The negative oxygen ion release capacity of hexagonal stone powder is enhanced after dispersion treatment compared to that before treatment. Moreover, healthy wood exhibits an even stronger ability to release negative oxygen ions than the treated hexagonal stone powder, indicating that wood as a carrier is conducive to improving the negative oxygen ion release capacity of hexagonal stone powder. As can be analyzed from Figure 5e,f, Figure 6 and Figure 7c, during the modification process, the number of hydroxyl groups in wood decreases (FT-IR), the crystallinity increases (XRD), and the hygroscopic swelling rate of wood is reduced. These changes decrease the adsorption sites for water molecules and endow wood with hygroscopic and desorption properties, which increase the water molecule content around the hexagonal stone and thereby promote the release of negative oxygen ions. Compared with the work of Weng et al. [71] and Zhou et al. [72], who utilized high-pressure methods to generate negative oxygen ions, the healthy wood prepared in this study not only releases negative oxygen ions but also does not produce toxic ozone. Compared with the methods of Liu et al. [73] and Gao et al. [24], the preparation method of healthy wood in this study is simpler. Innovatively, we have combined hexagonal stone with fast growing poplar wood to develop a new type of functional material that has both a high negative oxygen ion release performance and excellent dimensional stability. Moreover, this constitutes a straightforward approach to enhancing the negative-ion emissions of hexagonal stone.

4. Conclusions

This study introduces hexagonal stone powder into poplar wood using a vacuum cyclic impregnation technique, successfully preparing healthy wood with negative oxygen ion release capability. The main conclusions are as follows:

After different dispersion processes, the D50 values of the hexagonal stone powder follow this order: physical dispersion process > chemical dispersion process > composite dispersion process. The composite dispersion process yields the smallest particle size of hexagonal stone powder, making it the optimal powder dispersion method. At the optimal concentration of 0.5 wt%, the minimum D50 values of KH570 and PEG are 8.93 μm and 9.21 μm, respectively; at the optimal concentration of 1.0 wt%, the minimum D50 value of Span60 is 9.89 μm. After treatment with dispersants, the composition and crystal structure of the hexagonal stone remain unchanged. The negative oxygen ion release capability of the hexagonal stone powder is enhanced after treatment with dispersants.

The optimal impregnation cycle for the healthy wood is six cycles. Each cycle consists of 60 min of vacuum impregnation, 5 min of atmospheric pressure impregnation, and absolute drying.

Compared to untreated poplar wood, the healthy wood treated with PEG shows the greatest reduction in hygroscopic expansion rate. The radial, tangential, longitudinal, and volumetric hygroscopic expansion rates are reduced by 70.93%, 71.67%, 69.41%, and 71.35%, respectively. Hexagonal stone powder treated with dispersants is more easily absorbed into the poplar wood and adheres to the vessel walls and cell walls. The static and dynamic negative oxygen ion release of untreated poplar wood does not differ significantly. In contrast, the average static negative oxygen ion release of the healthy wood reaches 537 pcs/cm³, with the highest static release reaching 617 pcs/cm³. The average dynamic negative oxygen ion release reaches 3171 pcs/cm³, with the highest dynamic release reaching 10,590 pcs/cm³, showing significantly improved stability. The negative oxygen ion release capability of the healthy wood is affected by environmental humidity, demonstrating a moisture-responsive mechanism.

The detection duration of negative oxygen ions in this study was 120 min. The wood samples selected for the experiment did not contain high levels of extractives. Future research will focus on the following three directions: (1) developing new functional modification technologies to significantly improve the release efficiency of negative oxygen ions from healthy wood; (2) breaking through the technical bottleneck of negative oxygen ion release in low-humidity environments (RH < 70%); (3) constructing a multi-scale synergistic enhancement system to simultaneously optimize the dimensional stability and functional durability of wood.