1. Introduction
Food waste (FW) constitutes a significant portion of municipal solid waste (MSW), representing 30–45% by weight (w/w) of MSW in many countries [1]. In China, the annual production of FW is substantial, reaching approximately 195 million tons per year [2,3]. FW possesses a complex composition characterized by a high content of organic matter, which is highly prone to decay and deterioration [4,5]. The resultant leachate can infiltrate the subsoil and groundwater, leading to significant water resource contamination [6]. Additionally, this process can adversely affect soil structure and fertility [7] and pose risks to human health via the food chain [8]. Traditional food waste treatment methods, such as sanitary landfills, incineration, and composting, not only consume a significant amount of land resources but also contribute to over 8% of global greenhouse gas (GHG) emissions through incineration [9] and can result in severe environmental pollution issues [10]. Therefore, effectively managing and utilizing food waste has become a significant challenge that needs to be addressed in China.
In recent years, researchers have frequently utilized FW as a biomass resource to explore its potential for producing biogas [11], biomethane [12], bioethanol [13], and other applications [14]. However, during anaerobic fermentation of FW for biogas production, variations in the inoculum, process configuration, operating conditions, and inhibition parameters can affect product quality. Additionally, the process is complex and costly, characterized by low utilization of carbon components and the potential for secondary environment pollution. Furthermore, fermenting and distilling FW can produce bioethanol, a process that has been implemented on a large scale in China. However, this technology increases the likelihood of carbon components being released into the environment [15], thereby raising carbon emissions. The abundant carbon components within FW can form a carbon chain structure and carry functional groups such as C=C, –COOH, and –OH [16,17]. These components possess the property of free radical polymerization reaction, which can be utilized to polymerize and cross-link the main components of food waste, transforming them into larger molecules. This process enables the recycling and value-added utilization of the carbon-containing components in food waste [18].
Hydrogels are polymer-based three-dimensional network structures that can absorb and retain large amounts of water, exhibiting swelling, water retention, and adsorption properties. Their characteristic varies depending on the preparation conditions. Hydrogels are widely used in biomedicine, environmental protection, industry, personal care, and the food industry [19]. Queiroz et al. [20] demonstrated that the addition of cellulose not only enhances the biocompatibility and mechanical properties of hydrogels but also improves their swelling properties. Similarly, Manuel et al. [21] showed that the combination of starch and cellulose, both natural polymers, can form covalent bonds and establish a network structure. The hydrogels prepared from these components exhibit excellent water absorption ability. Nie et al. [22] extracted sugars from plants and used them as raw materials for hydrogel preparation. Their analysis showed that these sugars contain a number of cyclic structures, including –OH and –COOH groups, which can serve as a polymer flocculant to enhance the adsorption capacity of the hydrogels. The preparation of water retention hydrogels from carbon sources (e.g., food waste) can help address the global challenges of food waste management and resource recovery by converting waste into high-value products that can promote the resourceful use of waste [23].
In this study, FW was used as the raw material, AM as the monomer, MBA as the cross-linking agent, and SPS as the initiator. Homemade nano-CaO2 with high reactivity and good dispersibility was utilized to activate the carbon components within the food waste. The hydrogel was prepared through oxidative activation and radical graft polymerization. One-way and orthogonal optimization experiments were conducted to determine the optimal conditions for the hydrogel to achieve the best solubility performance. Subsequently, the morphology, structure, and thermal stability of hydrogel were characterized by SEM, XRD, and TGA. The swelling mechanism of the hydrogel was also analyzed. This study provides a novel approach for the high value-added utilization of food waste.
2. Experimental Materials and Methods
2.1. Experimental Reagents
Acrylamide (AM) and N,N′-methylene bisacrylamide (MBA) were selected as the primary chemical raw materials for the hydrogels in this study due to their excellent polymerization and cross-linking properties. AM forms a three-dimensional network structure through free radical polymerization, while MBA reacts with AM via its bifunctional groups to create a stable cross-linked structure, thereby significantly enhancing the mechanical strength and water absorption capacity of the hydrogels. However, AM and MBA are potentially carcinogenic in their monomeric states, with AM classified as a Group 2A carcinogen by the International Agency for Research on Cancer (IARC) [24,25]. Therefore, this study explicitly states that the hydrogels prepared are intended solely for use in controlled environments such as laboratories or environmental remediation scenarios and are not suitable for applications involving food, nutraceuticals, pharmaceuticals, or any products that directly or indirectly contact the human body. To minimize the risks associated with residual monomers, the study optimized reaction conditions (e.g., initiator concentration, reaction temperature, and time) to ensure complete polymerization and cross-linking while also providing a direction for future research to explore natural or biodegradable alternatives.
AM, MBA, and SPS were of analytical grade and sourced from Beijing Inokai Technology Co., Ltd. (Beijing, China) The equipment utilized in this study included a vacuum freeze dryer (LGJ-10, Beijing Yuecheng Jiaye Technology Co., Ltd.; Beijing, China), an artificial climate chamber (BIC-300, Shanghai Boxun Industrial Co., Ltd.; Shanghai, China), and a constant temperature magnetic water bath (Jiangsu Keranalytical Instrument Co., Ltd.; Jiangsu; China). Deionized water was employed throughout the experimental procedures.
2.2. Preparation of Nano-CaO2
The nano-calcium dioxide that excited the carbon fraction of the kitchen waste during the experiments was synthesized from previous experiments [26].
The preparation process of nano-CaO2 was conducted as follows: A specified amount of anhydrous CaCl2 was dissolved in deionized water at a solid-to-liquid ratio of 1:10. A dispersant was then added, and the pH of the solution was adjusted to a range of 9.5–11.0 using aqueous ammonia. The mixture was magnetically stirred for 30 min, after which a 30% H2O2 solution was introduced dropwise at a controlled rate via a constant-pressure dropping funnel while maintaining a stirring speed of 500 r/min. Upon completion of the addition, the reaction mixture was left undisturbed for 2 h to ensure the complete reaction between calcium chloride and H2O2. Subsequently, sodium hydroxide solution was added to induce the precipitation of CaO2. The resulting precipitate was collected using a Buchner funnel, thoroughly washed with anhydrous ethanol to remove impurities, and finally vacuum-dried to obtain nano-CaO2 powder. This material was determined to have particle sizes ranging from 40 nm to 310 nm, yielding elliptical or spherical nano-CaO2 particles with a purity of 81.32%. After comparative analysis, the properties of the material were found to be very close to those in the literature, indicating a successful synthesis.
2.3. Experimental Materials
The FW required for the experiment was sourced from the student dining hall of a university in Xinzheng City, Zhengzhou. The primary ingredients included rice, vegetables, pasta, oil, and water. Prior to the experiment, the FW was pretreated by sorting and removing impurities (e.g., plastic, bone, paper towel). The pretreated food waste was then mixed with distilled water at a 1:1 ratio and processed with a pulverized to form a food waste slurry (FWS). This slurry was filtered through a 0.150 mm (100 mesh) nylon sieve using distilled water to ensure the maximum removal of undissolved solids, resulting in a refined slurry. The filtered FWS allows the tiny particles to settle for further purification. The experimentally measured pH of the FWS was 5.87, and the conductivity was 6.83 μs/cm, the total solids content was 12.03%, and the volatile solids content was 10.64%.
2.4. Preparation of Kitchen Hydrogels
The hydrogel was synthesized through oxidative excitation and free radical polymerization [27]. The preparation principle and process of the hydrogel are illustrated in Figure 1.
Take 50 mL of food waste slurry (FWS) in a beaker and place it in a constant-temperature magnetic water bath at 70 °C for preheating for 20 min. During this process, CaO2 is added to generate hydroxyl radicals (∙OH) (Equation (1)) through its decomposition in water:
(1)
Simultaneously, sodium persulfate (SPS) acts as an initiator and thermally decomposes at elevated temperatures, producing sulfate radicals (SO4−), and the equation is shown in Equation (2).
(2)
These radicals, particularly –OH, exhibit strong oxidizing properties that promote the release and decomposition of organic components in food waste, providing more reactive sites for graft copolymerization with AM. This reaction initiates the formation of a denser three-dimensional network structure. After preheating for 20 min, AM is added, and the carbon–carbon double bond (C=C) in its molecular structure breaks under the initiation of –OH and SO4−, facilitating the polymerization reaction.
After 20 min of reaction, a specified amount of MBA is added as a cross-linking agent. MBA reacts with polymer chains through its two functional amide (–CONH2) groups, forming covalent bonds that interconnect various polymer branches, further enhancing the structural integrity of the hydrogel. The reaction mixture is allowed to proceed for another 5 min before being poured into molds. The molds are placed in a water bath for approximately 30 min to allow the hydrogel to solidify. Once cooled and removed from the molds, the food waste hydrogel (FWH) is obtained.
2.5. Analysis of the Swelling Properties of Hydrogels
The dissolution rate is a key response value to characterize hydrogel properties. A certain mass of FWH samples is taken after vacuum freeze-drying for 24 h and placed in deionized water in an artificial climate chamber set at 25 °C with 55% humidity for the dissolution experiment. Every 12 h, the samples are removed, residual water is removed from the surface of the hydrogel with absorbent paper, and the mass is measured to determine the extent of dissolution. This process is repeated until the dissolution equilibrium is reached. The results are averaged over three groups of data. The hydrogel swelling capacity calculation equation is shown in Equation (3):
(3)
where Sc is the hydrogel swelling capacity (%); Wt is the mass of the hydrogel after vacuum freeze-drying (g); Wc is the mass of the hydrogel at swelling equilibrium (g).2.6. One-Factor Experimental Setup
This study is based on the free radical cross-linking polymerization reaction. AM was used as the monomer, SPS as the initiator, and MBA as the cross-linking agent. Under the excitation of FW carbon components by CaO2, the synthesis conditions of FWH were preliminarily optimized using a one-factor experimental method at different polymerization temperatures (T). Subsequently, four-factor, three-level orthogonal experiments were conducted to further optimize the preparative parameters. The one-factor factorial level settings are shown in Table 1.
2.7. Orthogonal Experimental Setup
Four factors, nano-CaO2 dosage, MBA dosage, reaction temperature, and SPS dosage, were selected as the variables to investigate. The hydrogel swelling capacity was used as the index to design the L9 (34) orthogonal test, which aimed to explore the effect of these preparation parameters on the swelling capacity of FWH. Orthogonal tests were carried out using orthogonal experimental design software 3.1, and the results are shown in Table 2.
2.8. Characterization of Morphology and Composition of Hydrogels
To comprehensively analyze the structure and properties of the hydrogels, advanced characterization techniques were employed. The surface morphology was examined using a scanning electron microscope (FEI-Quanta 250, Hillsboro, OR, USA) at 10–15 kV, revealing the microstructure and surface folding. The phase composition was analyzed via X-ray diffraction (D8 ADVANCE-BRUKER, Billerica, MA, USA) over a scanning range of 5–75° (2θ) to identify crystalline features. The thermal stability of food waste (FW) and hydrogels (FWH) was evaluated using a simultaneous thermal analyzer (STA 449 F3-NETZSCH, Selb, Bavaria, Germany) under nitrogen from room temperature to 800 °C, combining TG and DSC to study thermal decomposition.
3. Results
3.1. Determination of AM Dosage
AM forms a covalent cross-linked network structure under the initiation of –OH. The amount of AM affects the porosity of the hydrogel network structure, which in turn influences the swelling performance of FWH. Therefore, the effect of AM dosage (2.00 g, 2.50 g, 3.00 g, 3.50 g, 4.00 g) on the swelling performance of FWH was investigated. As shown in Figure 2a, the swelling capacity of FWH samples increased in the first 12 h and then stabilized or slightly decreased, indicating that the dissolution equilibrium was gradually reached. As shown in Figure 2b, when the amount of AM is not greater than 3.0 g, the SR increases with the increase in AM. This is because the AM molecular chain contains many hydrophilic groups, such as –OH, –COOH, and –NH2, which enhance the reaction system. These hydrophilic groups contribute to a certain pore structure through covalent bonding and intermolecular hydrogen bonding, improving hydrophilicity and forming a better hydrogel network [27,28,29,30]. However, when the AM dosage exceeds 3.0 g, too many hydrophilic groups are introduced into the hydrogel. The interactions between these groups generate numerous intermolecular hydrogen bonds [31], leading to an increase in medium viscosity and cross-linking density. This hinders the binding of monomers and free radicals, affecting the formation of three-dimensional network structures and ultimately inhibiting the swelling behavior of hydrogels [32,33].
3.2. Determination of Nano-CaO2 Dosage
The amount of nano-CaO2 dosage is closely related to the amount of –OH in the reaction system, and the strong oxidizing effect of ·OH contributes to the release and decomposition of the organic components in the kitchen waste, thereby affecting the swelling performance of FWH. Therefore, the amounts of nano-CaO2 were set at 0.02 g, 0.04 g, 0.06 g, 0.08 g, and 0.10 g to investigate its effect on the dissolution performance of FWH. As shown in Figure 3a,b, the SR initially increased with the increase in CaO2 dosage, reached a maximum at 0.04 g, and then decreased. This is because nano-CaO2 undergoes the reaction shown in Equation (2) in solution [34]. Additionally, there is an O–O bond in H2O2, and due to the instability of this bond, it breaks periodically, producing ·OH. Meanwhile, SPS decomposes under certain temperature conditions to produce SO4−, which can react with the OH− produced in Equation (4), as shown in Equation (5) [35].
(4)
(5)
With the increase in nano-CaO2 dosage, the amount of ·OH produced in the reaction system increased. Due to its highly oxidizing characteristics, the FW component could be completely decomposed during the reaction process. This increased the released carbon component in the system, facilitating the graft copolymerization reaction and effectively promoting the formation of the hydrogel’s three-dimensional network. However, when the amount of nano-CaO2 exceeded 0.04 g, although the content of released carbon components increased, the ratio of cross-linking agent to initiator was insufficient to promote the full conversion of the released carbon components. Excessive nano-CaO2 would introduce too much Ca2+ into the reaction system, resulting in agglomeration of FW, which affected the formation of the hydrogel’s three-dimensional network and led to a decrease in its swelling performance [36].
3.3. Determination of MBA Dosage
The dosage of MBA affects the number of effective chemical cross-linking sites and the cross-linking density in the covalent cross-linking reaction. The state of the three-dimensional cross-linked network hydrogel depends on the cross-linking density. Therefore, this section examines the effect of MBA dosage (0.04 g, 0.05 g, 0.06 g, 0.07 g, 0.08 g) on the dissolution properties of FWH. As shown in Figure 4, the SR exhibited a regular change with the increase in MBA dosage, peaking at 614.5% when the MBA dosage was 0.07 g. This was attributed to the fact that the MBA contains two –CONH2 groups. The polymer can improve the 3D spatial mesh structure and the cross-linking density within the gels in the presence of the –CONH2 group. It also promotes the formation of more network nodes [37,38], resulting in an increase in the molecular weight of the branched chain of the graft copolymerization product, which enhances the swelling capacity of hydrogels [39].
However, when the MBA dosage exceeded 0.07 g, the SR showed a significant decreasing trend. Excessive MBA led to an excessive enhancement of cross-linking within the hydrogel network structures, which greatly limited the expansion and stretching of the network, resulting in a dense and compact state with a gradual reduction in free space [40]. Additionally, excessive MBA might exhibit a tendency for self-polymerization in the reaction system, and these combined factors ultimately led to a decrease in the swelling performance of FWH [41].
3.4. Determination of Reaction Temperature
FWH is a hydrogel that completes the cross-linking polymerization reaction at a certain temperature to form a three-dimensional mesh structure, resulting in a solid phase. The temperature affects the mechanical properties of the food-based hydrogel. Therefore, this section examines the effect of polymerization temperature changes (60 °C, 65 °C, 70 °C, 75 °C, 80 °C) on the hydrogel’s swelling properties. Figure 5 shows the effect of different polymerization temperatures on the swelling properties of hydrogels. It is worth noting that SPS undergoes a decomposition reaction under heating conditions, and the amount of SO4−· generated from the thermally activated decomposition of SPS increases as the reaction temperature rises, which accelerates the polymerization reaction rate and promotes the formation of the hydrogel’s network structure, resulting in a gradual increase in the swelling capacity. However, when the reaction temperature was higher than 70 °C, the amount of SO4−· increased further, intensifying the reaction degree. The violent reaction process affected the stability of the hydrogel network structure, resulting in a sharp contraction of the hydrogel, which inhibited the swelling capacity of the hydrogel [42,43].
3.5. Determination of SPS Dosage
The free radicals formed by SPS dissolved in water will affect the double bonds present in the AM and MBA molecules to a certain extent. The polymer chains in the hydrogel molecules will be intertwined through a covalent bonding manner to form long polymer chains, constituting the basic structure of the hydrogel. The use of the initiator affects the swelling performance of the hydrogel. Therefore, this section examines the effect of SPS dosage (0.30 g, 0.40 g, 0.50 g, 0.60 g, 0.70 g) on the swelling properties of FWH.
As shown in Figure 6, the Sc exhibited a tendency to increase and then decrease with the increase in SPS dosage, reaching a maximum of 703.1% when the dosage of SPS was 0.6 g. The Sc of the monomers in the reaction system was 703.1%. The addition of initiator SPS introduces many active radicals into the reaction system, significantly improving the efficiency of graft copolymerization with monomer [44]. The reaction formulas are shown in Equations (6) and (7).
(6)
(7)
When the SPS dosage was low, the number of macromolecular chains formed was limited, resulting in a low cross-linking density of the hydrogel network. As the initiator dosage increased, the number of network structures and the active centers of the polymerization reaction increased, improving the swelling properties of the hydrogel [45]. However, when the initiator dosage exceeded 0.6 g, the Sc showed a decreasing trend. This could be attributed to the increase in polymer cross-linking density, which made it difficult for water molecules to enter the interior of the hydrogel. Additionally, an excessively high concentration of SPS and overactivation might destroy the integrity of other radicals in the reaction system, thereby disrupting the three-dimensional mesh structure of the hydrogel and ultimately leading to a decrease in the hydrogel’s swelling properties [46].
3.6. Orthogonal Experiment Optimization
The experimental results analyzed through the orthogonal test are presented in Table 3. In orthogonal experimental design, K represents the number of different levels a factor can take, while R refers to the extreme difference (Range), which is a crucial statistical metric used to analyze the experimental results. By calculating the range, the relative impact of each factor on the experimental outcomes can be determined. The orthogonal analysis in this study revealed that the factors influencing the hydrogel swelling capacity are ranked in the following order: SPS dosage > MBA dosage > reaction temperature > Nano-CaO2 dosage.
Based on the variance analysis (Table 4), the SPS dosage (A) significantly impacts the swelling performance, showing the highest F-value. As an initiator, SPS plays a key role in generating free radicals, thereby greatly promoting polymerization and cross-linking reactions. This establishes SPS dosage as the dominant factor affecting the hydrogel’s swelling capacity. The MBA dosage (B), with an F-value of 4.53605 (p = 0.180), did not show a statistically significant impact but still contributes to the cross-linking density and cannot be overlooked. Reaction temperature (C) and Nano-CaO2 dosage (D) had smaller effects on the experimental results, with F-values of 1.000 and 0.406 and p-values of 0.500 and 0.711, respectively, indicating that their influence on the hydrogel swelling capacity was not statistically significant under the given experimental conditions.
Integrating the findings from both orthogonal analysis and variance analysis, the study identified the main contributing factors and optimal preparation conditions for hydrogel synthesis. The optimal parameters were determined as follows: SPS dosage of 0.6 g, MBA dosage of 0.08 g, reaction temperature of 65 °C, and Nano-CaO2 dosage of 0.04 g. Under these conditions, the hydrogel demonstrated the highest swelling performance. Furthermore, the results highlight the critical role of SPS as the primary initiator, while the influence of Nano-CaO2 dosage was relatively minimal. These findings provide a solid foundation for further optimization and refinement of hydrogel preparation processes.
3.7. SEM Characterization Results
Analyzing Figure 7a,b, the organic matter on the surface of FW showed obvious agglomeration, leading to the change in its original lamellar structure (square and dotted square). Compared to the sparse lamellar structure in Figure 7c, the three-dimensional mesh structure inside the hydrogel is more evident. This indicates that the cross-linking is highly complex during the transformation from FW to hydrogel. The hydrogel surface exhibits many folds, grooves, and rough micro-surface morphology; this is due to the existence of the cross-linking phenomenon of functional groups within the kitchen waste under the condition of chemical triggering [47], the rough micro-surface morphology of the hydrogel and the numerous folds and groove’s structure increase its specific surface area, which greatly increases the adsorption performance of the hydrogel [7].
Combined with Figure 7d and the XRD analysis, the clusters concentrated on the surface are identified as residual Na2SO4 crystals resulting from the participation of Na2S2O4 in the cross-linking polymerization reaction [48,49,50]. The cross-linking and polymerization of multiple functional groups (e.g., C = C, –COOH, and –OH) in the kitchen waste driven by chemical reaction significantly increase the specific surface area of the hydrogel and enhances the density of the hydrogel network, greatly improving the swelling properties of the hydrogel [50].
3.8. XRD Characterization Results
Figure 8 shows the X-ray diffraction (XRD) characterization patterns of the kitchen ingredients and hydrogel samples. There are two broad peaks in the FW ingredients between 2θ = 15 and 25°, and these two characteristic peaks disappeared from the FWH spectra, indicating a certain microcrystalline structure inside the hydrogel [51,52].
Moreover, the FWH spectrum shows three weak diffraction peaks at 2θ = 22.89°, 29.37°, and 31.71°, corresponding to the (012), (104), and (006) crystal planes of Na2SO4, respectively. This indicates that the crystalline structure of the hydrogel surface is due to the reaction of SPS forming Na2SO4 crystals, confirming that SPS was successfully involved in the synthesis reaction of the hydrogel [53].
3.9. TG Characterization Results
Figure 9 shows the TGA characterization profiles of FW raw material and FWH. From Figure 9, the weight loss temperature interval for both FW raw material and FWH is roughly from 100 to 500 °C. Due to the difference in their structural properties, the weight loss rate of FW raw material is higher than that of the hydrogel. The mass loss of FWH in this temperature interval is approximately 52.4%, while the mass loss of FW feedstock is about 52.7%.
As analyzed in Figure 10, the maximum mass loss rate of FW occurred at 85.6 °C, 210.0 °C, and 288.9 °C, respectively. In these three temperature ranges, the process of physical evaporation loss of water mainly occurred before 210.0 °C, involving the free and bound water in the internal components of FW. The decomposition of macromolecules, such as cellulose and hemicellulose, mainly occurred in the subsequent temperature intervals [54,55,56]. Compared with the weight loss of FW feedstock, the temperature point of faster weight loss for FWH showed a general trend of shifting to higher temperatures, With the maximum weight loss rate occurring at 355.0 °C. This significant increase in the thermal decomposition temperature indicates that FWH has better thermal stability compared to FW and can facilitate better application in subsequent industrial, agricultural, and environmental fields.
4. Performance Comparison
By comparing the relevant references in recent years in Table 5, the present study demonstrated significant innovation and application potential. Firstly, in terms of swelling performance, the swelling rate of hydrogels prepared in this study reached 703.1%, which was significantly higher than that of conventional hydrogels (e.g., 2.34% by Liu et al., 2024 and 6.90% by Sabzi et al., 2020) and close to or even exceeded that of some high-performance hydrogels (e.g., 607.72% by Matar et al., 2022 and 850% by Kumar et al., 2023). Secondly, this study uses kitchen waste as a carbon source and combines green material design concepts to achieve high-value utilization of waste and low-cost preparation of hydrogels, which demonstrates strong environmental sustainability and contrasts with most of the studies relying on traditional chemical raw materials. In addition, by introducing Nano-CaO2 to enhance the cross-linking performance and combining it with orthogonal tests to optimize the preparation conditions, this study further enhanced the structure regulation ability and material properties of hydrogels. This study combines the optimization of swelling performance with the resourceful utilization of waste, which provides a new idea and technical support for the development of environmentally friendly high-performance hydrogels.
5. Conclusions
This study successfully demonstrated the preparation of hydrogels from kitchen waste-derived carbon sources using calcium peroxide (CaO2) as an oxidant. The hydrogel preparation process was optimized through orthogonal tests, yielding the following optimal conditions: SPS dosage of 0.60 g, MBA dosage of 0.07 g, reaction temperature of 70 °C, and CaO2 dosage of 0.04 g. Under these conditions, the hydrogel achieved a maximum swelling rate of 703.1%.
Characterization techniques, including SEM, XRD, and TGA, revealed that the hydrogels’ swelling properties are closely associated with their interwoven, reticulated folded structure and the microcrystalline formations induced by the incorporation of organic matter. This study proposes a novel approach for the high-value utilization of kitchen waste, offering a sustainable solution for waste resource recovery and carbon emission reduction.
Furthermore, hydrogels derived from food waste demonstrate significant potential in agricultural applications. These hydrogels can store water and nutrients and release them gradually to plants, thereby enhancing water use efficiency and reducing the need for frequent irrigation.
Y.L. and Y.H.: Conceptualization, methodology, research, writing—original draft, visualization, and obtaining funding; X.Z. and D.S.: methodology, research, writing—reviewing and editing; H.Q. and J.Z.: Conceptualization, writing—review, and supervision; S.X., T.D., M.Q. and L.D.: writing—original draft, writing—review and editing. All authors have read and agreed to the published version of the manuscript.
Due to technical limitations and resource constraints, data that may have been analyzed or generated during the course of this study were not publicly archived. We recognize the importance of data sharing to improve the reproducibility and transparency of scientific research. However, in this instance, the lack of data is not due to privacy or ethical constraints, but rather to the nature of the study itself. We apologize for any inconvenience this may have caused and encourage interested parties to contact the corresponding author for further inquiries about the study’s methodology and findings.
No potential conflict of interest was reported by the authors.
| FW | Food waste |
| AM | Acrylamide |
| MBA | N,N′-methylene bisacrylamide |
| SPS | Sodium persulfate |
| FWH | Food waste hydrogel |
| FWS | Food waste slurry |
| SEM | Scanning electron microscopy |
| XRD | X-ray diffractometer |
| TGA | Thermogravimetric analysis |
| MSW | Municipal solid waste |
| GHG | Greenhouse gas |
Footnotes
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Enlarge this image.Figure 2. (a) Variation in hydrogel swelling capacity over time. (b) Effect of AM dosage on hydrogel swelling capacity.
Enlarge this image.Figure 3. (a) Variation in hydrogel swelling capacity over time. (b) Effect of nano-CaO2 dosage on hydrogel swelling capacity.
Enlarge this image.Figure 4. (a) Variation in hydrogel swelling capacity over time. (b) Effect of MBA dosage on hydrogel swelling capacity.
Enlarge this image.Figure 5. (a) Variation in hydrogel swelling capacity over time. (b) Effect of temperature on hydrogel swelling capacity.
Enlarge this image.Figure 6. (a) Variation in hydrogel swelling capacity over time. (b) Effect of SPS dosage on hydrogel swelling capacity.
Enlarge this image.Figure 7. (a) SEM image of FW raw material. (b–d) SEM image of FWH at different magnifications.
Single factor level setting.
| Parameters | Units | Levels | ||||
|---|---|---|---|---|---|---|
| Level 1 | Level 2 | Level 3 | Level 4 | Level 5 | ||
| AM | g | 2.00 | 2.50 | 3.00 | 3.50 | 4.00 |
| Nano-CaO2 | g | 0.02 | 0.04 | 0.06 | 0.08 | 0.10 |
| MBA | g | 0.04 | 0.05 | 0.06 | 0.07 | 0.08 |
| SPS | g | 0.30 | 0.40 | 0.50 | 0.60 | 0.70 |
| Temperatures | °C | 60.0 | 65.0 | 70.0 | 75.0 | 80.0 |
Orthogonal test factor level design.
| Factors | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
| Nano-CaO | 0.03 | 0.04 | 0.05 |
| MBA Dosage/g | 0.06 | 0.07 | 0.08 |
| SPS Dosage/g | 0.50 | 0.60 | 0.70 |
| Temperatures/°C | 65.0 | 70.0 | 75.0 |
Analysis of orthogonal experiment results.
| Number | A (g) | B (g) | C (°C) | D (g) | Y (%) |
|---|---|---|---|---|---|
| SPS Dosage | MBA Dosage | T | Nano-CaO2 Dosage | Sc | |
| 1 | 0.5 | 0.06 | 65 | 0.03 | 514.6 |
| 2 | 0.5 | 0.07 | 70 | 0.04 | 577.5 |
| 3 | 0.5 | 0.08 | 75 | 0.05 | 560.3 |
| 4 | 0.6 | 0.06 | 70 | 0.05 | 695.2 |
| 5 | 0.6 | 0.07 | 75 | 0.03 | 700.1 |
| 6 | 0.6 | 0.08 | 65 | 0.04 | 759.1 |
| 7 | 0.7 | 0.06 | 75 | 0.04 | 511.0 |
| 8 | 0.7 | 0.07 | 65 | 0.05 | 544.8 |
| 9 | 0.7 | 0.08 | 70 | 0.03 | 584.9 |
| k1 | 550.800 | 573.60 | 606.167 | 599.867 | |
| k2 | 718.133 | 607.48 | 619.200 | 615.867 | |
| k3 | 546.900 | 634.78 | 590.467 | 600.100 | |
| R | 171.233 | 61.16 | 28.733 | 16.000 |
ANOVA for orthogonal experiments.
| Number | df | SS | MS | F | p |
|---|---|---|---|---|---|
| A | 2 | 28,668.25 | 46.16 | 0.0212 | Significance |
| B | 2 | 2816.80 | 4.54 | 0.1806 | / |
| C | 2 | 620.98 | 1.00 | 0.4999 | / |
| D | 2 | 252.32 | 0.41 | 0.7111 | / |
| Error | 2 | 620.98 |
Comparison of hydrogel swelling properties.
| Type of Hydrogel | Sc/% | Author(s) |
|---|---|---|
| Carrageenan-based pH-responsive hydrogels | 233.3 | Azeem et al. [ |
| PVAxHA-Cay hydrogels | 120.0 | Wu et al. [ |
| Palygorskite/soluble starch/sodium alginate hydrogel | 18.6 | Gao et al. [ |
| Polyacrylamide hydrogel | 2.3 | Liu et al. [ |
| poly(2-methacryloyloxyethyl phosphate) hydrogels | 400.0 | Leng et al. [ |
| HP-β-CD/agarose-g-poly(MAA) hydrogel | 95.0 | Rehman et al. [ |
| Poly(vinyl alcohol) (PVA)/nano silver hydrogels | 6.9 | Sabzi et al. [ |
| Etophylline-loaded CMTKG/PAM/PEG hydrogel | 850.0 | Kumar et al. [ |
| Locust bean gum–polyvinyl alcohol hydrogels | 607.7 | Matar et al. [ |
| pH and glucose-responsive silk fibroin hydrogels | 345.0 | Tao et al. [ |
References
1. Zhao, J.; Wang, Z.; Li, J.; Yan, B.B.; Chen, G.Y. Pyrolysis of food waste and food waste solid digestate: A comparative investigation. Bioresour. Technol.; 2022; 354, 127191. [DOI: https://dx.doi.org/10.1016/j.biortech.2022.127191] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35447328]
2. Ng, H.S.; Kee, P.E.; Yim, H.S.; Chen, P.T.; Wei, Y.H.; Lan, J.C.W. Recent advances on the sustainable approaches for conversion and reutilization of food wastes to valuable bioproducts. Bioresour. Technol.; 2020; 302, 122889. [DOI: https://dx.doi.org/10.1016/j.biortech.2020.122889] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32033841]
3. Srivastava, N.; Srivastava, M.; Abd-Allah, E.F.; Singh, R.; Hashem, A.; Gupta, V.K. Biohydrogen production using kitchen waste as the potential substrate: A sustainable approach. Chemosphere; 2021; 271, 129537. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2021.129537] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33450424]
4. Dutta, S.; He, M.J.; Xiong, X.N.; Tsang, D.C.W. Sustainable management and recycling of food waste anaerobic digestate: A review. Bioresour. Technol.; 2021; 341, 125915. [DOI: https://dx.doi.org/10.1016/j.biortech.2021.125915]
5. O’Connor, J.; Hoang, S.A.; Bradney, L.; Dutta, S.; Xiong, X.N.; Tsang, D.C.W.; Ramadass, K.; Vinu, A.; Kirkham, M.B.; Bolan, N.S. A review on the valorisation of food waste as a nutrient source and soil amendment. Environ. Pollut.; 2021; 272, 115985. [DOI: https://dx.doi.org/10.1016/j.envpol.2020.115985]
6. Chu, Z.J.; Fan, X.H.; Wang, W.N.; Huang, W.C. Quantitative evaluation of heavy metals’ pollution hazards and estimation of heavy metals’ environmental costs in leachate during food waste composting. Waste Manag.; 2019; 84, pp. 119-128. [DOI: https://dx.doi.org/10.1016/j.wasman.2018.11.031]
7. Zhou, T.; Zhao, M.H.; Zhao, X.; Guo, Y.Y.; Zhao, Y.C. Simultaneous remediation and fertility improvement of heavy metals contaminated soil by a novel composite hydrogel synthesized from food waste. Chemosphere; 2021; 275, 129984. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2021.129984]
8. Essien, J.P.; Ikpe, D.I.; Inam, E.D.; Okon, A.O.; Ebong, G.A.; Benson, N.U. Occurrence and spatial distribution of heavy metals in landfill leachates and impacted freshwater ecosystem: An environmental and human health threat. PLoS ONE; 2022; 17, e0263279. [DOI: https://dx.doi.org/10.1371/journal.pone.0263279]
9. He, M.; Zhu, X.; Dutta, S.; Khanal, S.K.; Lee, K.T.; Masek, O.; Tsang, D.C. Catalytic co-hydrothermal carbonization of food waste digestate and yard waste for energy application and nutrient recovery. Bioresour. Technol.; 2022; 344, 126395. [DOI: https://dx.doi.org/10.1016/j.biortech.2021.126395]
10. Jeswani, H.K.; Figueroa, T.G.; Azapagic, A. The extent of food waste generation in the UK and its environmental impacts. Sustain. Prod. Consump.; 2021; 26, pp. 532-547. [DOI: https://dx.doi.org/10.1016/j.spc.2020.12.021]
11. Świechowski, K.; Matyjewicz, B.; Telega, P.; Białowiec, A. The influence of low-temperature food waste biochars on anaerobic digestion of food waste. Materials; 2022; 15, 945. [DOI: https://dx.doi.org/10.3390/ma15030945] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35160890]
12. Keerthana, D.M.; Manikandan, S.; Senthil, K.P.; Yaashikaa, P.R.; Oviyapriya, M.; Rangasamy, G. A comprehensive review on current trends and development of biomethane production from food waste: Circular economy and techno economic analysis. Fuel; 2023; 351, 128963.
13. Konti, A.; Kekos, D.; Mamma, D. Life cycle analysis of the bioethanol production from food waste-a review. Energies; 2020; 13, 5206. [DOI: https://dx.doi.org/10.3390/en13195206]
14. Li, X.X.; Tong, J.J.; Yuan, M.M.; Song, M.; Gao, J.S.; Zhu, J.; Liu, Y.P. Demonstrating the application of batch anaerobic digestion recirculating slurry inoculation of food waste engineering from a microbiological perspective. Renew. Energy; 2023; 217, 119216. [DOI: https://dx.doi.org/10.1016/j.renene.2023.119216]
15. Sridhar, A.; Kapoor, A.; Kumar, P.S.; Ponnuchamy, M.; Balasubramanian, S.; Prabhakar, S. Conversion of food waste to energy: A focus on sustainability and life cycle assessment. Fuel; 2021; 302, 121069. [DOI: https://dx.doi.org/10.1016/j.fuel.2021.121069]
16. Zeng, M.X.; Ge, Z.F.; Ma, Y.N.; Zha, Z.T.; Wu, Y.Q.; Hou, Z.H.; Zhang, H.Y. Hydrothermal carbonization coupled with gasification for collaborative disposal of kitchen waste and yard waste. Energy Convers. Manag.; 2023; 283, 116864. [DOI: https://dx.doi.org/10.1016/j.enconman.2023.116864]
17. Zhang, Y.S.; Khademhosseini, A. Advances in engineering hydrogels. Science; 2017; 356, eaaf3627. [DOI: https://dx.doi.org/10.1126/science.aaf3627]
18. Yan, J.; Luo, F.; Wu, L.Y.; Ou, Y.L.; Gong, C.C.; Hao, T.W.; Huang, L.; Chen, Y.H.; Long, J.Y.; Xiao, T.F. et al. Cost-effective desulfurization of acid mine drainage with food waste as an external carbon source: A pilot-scale and long-term study. J. Clean. Prod.; 2022; 361, 132174. [DOI: https://dx.doi.org/10.1016/j.jclepro.2022.132174]
19. Liu, J.J.; Qu, S.X.; Suo, Z.G.; Yang, W. Functional hydrogel coatings. Natl. Sci. Rev.; 2021; 8, nwaa254. [DOI: https://dx.doi.org/10.1093/nsr/nwaa254]
20. Queiroz, B.G.; Ciol, H.; Inada, N.M.; Frollini, E. Hydrogel from all in all lignocellulosic sisal fibers macromolecular components. Int. J. Biol. Macromol.; 2021; 181, pp. 978-989. [DOI: https://dx.doi.org/10.1016/j.ijbiomac.2021.04.088]
21. Manuel, M.; Jennifer, A. A review on starch and cellulose-enhanced superabsorbent hydrogel. J. Chem. Rev.; 2023; 5, pp. 183-203.
22. Nie, L.; Chang, P.B.; Liang, S.; Hu, K.H.; Hua, D.L.; Liu, S.L.; Sun, J.F.; Sun, M.; Wang, T.C.; Okoro, O.V. et al. Polyphenol rich green tea waste hydrogel for removal of copper and chromium ions from aqueous solution. Clean. Eng. Technol.; 2021; 4, 100167. [DOI: https://dx.doi.org/10.1016/j.clet.2021.100167]
23. Yu, I.K.M.; Wong, K.H. Food waste—Derived 3D printable materials: A carbon neutral solution to global foodloss. Trends Food Sci. Technol.; 2023; 137, pp. 156-166. [DOI: https://dx.doi.org/10.1016/j.tifs.2023.05.014]
24. Bridgeman, L.; Juan, C.; Berrada, H.; Juan-García, A. Effect of Acrylamide and Mycotoxins in SH-SY5Y Cells: A Review. Toxins; 2024; 16, 87. [DOI: https://dx.doi.org/10.3390/toxins16020087] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38393165]
25. Sebastià, A.; Fernández-Matarredona, C.; Castagnini, J.M.; Barba, F.J.; Berrada, H.; Moltó, J.C.; Ferrer, E. Acrylamide content in popcorn from Spanish market: Risk assessment. Food Chem. Toxicol.; 2024; 196, 115145. [DOI: https://dx.doi.org/10.1016/j.fct.2024.115145]
26. Li, Y.L.; Liu, L.; Xiong, M.X.; Li, K.P.; Xie, J.P.; Wang, Z.J.; Zhang, X. Deep dewatering of municipal sludge by combining alternating electric field electro-osmosis and nano-CaO2 oxidation. Chem. Eng. J.; 2024; 490, 151769. [DOI: https://dx.doi.org/10.1016/j.cej.2024.151769]
27. Hamada, Y.; Togawa, S.; Shintani, R. Radical stitching polymerization and its alternating copolymerization. J. Am. Chem. Soc.; 2024; 146, pp. 19310-19316. [DOI: https://dx.doi.org/10.1021/jacs.4c05094]
28. Son, D.; Hwang, H.; Fontenot, J.F.; Lee, C.; Jung, J.P.; Kim, M. Tailoring physical properties of dual-network acrylamide hydrogel composites by engineering molecular structures of the cross-linked network. ACS Omega; 2022; 7, pp. 30028-30039. [DOI: https://dx.doi.org/10.1021/acsomega.2c03031]
29. Liu, Y.; Wang, J.P.; Chen, H.Y.; Cheng, D.D. Environmentally friendly hydrogel: A review of classification, preparation and application in agriculture. Sci. Total Environ.; 2022; 846, 157303. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2022.157303]
30. Zhang, M.; Hou, S.Y.; Li, Y.; Hu, S.W.; Yang, P. Swelling characterization of ionic responsive superabsorbent resin containing carboxylate sodium groups. React. Polym.; 2022; 170, 105144. [DOI: https://dx.doi.org/10.1016/j.reactfunctpolym.2021.105144]
31. Sayed, M.M.E. Production of polymer hydrogel composites and their applications. J. Polym. Environ.; 2023; 31, pp. 2855-2879. [DOI: https://dx.doi.org/10.1007/s10924-023-02796-z]
32. Sennakesavan, G.; Mostakhdemin, M.; Dkhar, L.K.; Seyfoddin, A.; Fatihhi, S.J. Acrylic acid/acrylamide based hydrogels and its properties—A review. Polym. Degrad. Stab.; 2020; 180, 109308. [DOI: https://dx.doi.org/10.1016/j.polymdegradstab.2020.109308]
33. Kang, B.; Lang, Q.L.; Tu, J.; Bu, J.; Ren, J.J.; Lyu, B.; Gao, D.G. Preparation and properties of double network hydrogel with high compressive strength. Polymer; 2022; 14, 966. [DOI: https://dx.doi.org/10.3390/polym14050966] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35267788]
34. Wang, K.; Yan, T.; Li, R.K.; Pan, W.G. A review for Ca(OH)2/CaO thermochemical energy storage systems. J. Energy Storage; 2022; 50, 104612. [DOI: https://dx.doi.org/10.1016/j.est.2022.104612]
35. Ruan, S.Y.; Deng, J.; Cai, A.H.; Chen, S.N.; Cheng, Y.Q.; Li, J.; Li, Q.S.; Li, X.Y. Improving dewaterability of waste activated sludge by thermally-activated persulfate oxidation at mild temperature. J. Environ. Manag.; 2021; 281, 111899. [DOI: https://dx.doi.org/10.1016/j.jenvman.2020.111899]
36. Li, Y.L.; Huang, Y.; Liu, L.; Liu, H.Z.; He, H.Y.; Lu, D.X.; Dong, T.T. Nano-CaO2 promotes the release of carbon sources from municipal sludge and the preparation of double-network hydrogels with high swelling ratios. J. Renew. Mater.; 2023; 11, pp. 1237-1253. [DOI: https://dx.doi.org/10.32604/jrm.2022.022972]
37. Kowalski, G.; Ptaszek, P.; Kuterasiński, L. Swelling of hydrogels based on carboxymethylated starch and poly (acrylic acid): Nonlinear rheological approach. Polymer; 2020; 12, 2564. [DOI: https://dx.doi.org/10.3390/polym12112564]
38. Siddiqua, A.; Ranjha, N.M.; Rehman, S.; Shoukat, H.; Ramzan, N.; Sultana, H. Preparation and characterization of methylene bisacrylamide crosslinked pectin/acrylamide hydrogels. Polym. Bull.; 2022; 79, pp. 7655-7677. [DOI: https://dx.doi.org/10.1007/s00289-021-03870-z]
39. Pille, A.; Dumont, M.J.; Tavares, J.R.; Roy, R. The effect of foaming additives on acrylic acid/acrylamide hydrogels. J. Environ. Chem. Eng.; 2022; 10, 107310. [DOI: https://dx.doi.org/10.1016/j.jece.2022.107310]
40. Hu, Z.H.; Fan, X.T.; Xiang, Z.Y.; Wang, L.L.; Wang, S.D.; He, C.; Zhang, X.R.; Liu, Y.S. A one-pot strategy for preparation of carboxymethyl xylan-based hydrogels with antibacterial property. Ind. Crops Prod.; 2024; 210, 118171. [DOI: https://dx.doi.org/10.1016/j.indcrop.2024.118171]
41. Kumar, N.; Gusain, R.; Pandey, S.; Ray, S.S. Hydrogel nanocomposite adsorbents and photocatalysts for sustainable water purification. Adv. Mater. Interfaces; 2023; 2, 2201375. [DOI: https://dx.doi.org/10.1002/admi.202201375]
42. Wang, C.; Yuan, Z.Y.; Wang, A.; Qu, J.L.; Fang, Z.; Wen, Y.B. Ultraviolet light enhanced sodium persulfate oxidation of cellulose to facilitate the preparation of cellulose nanofibers. Cellulose; 2020; 27, pp. 2041-2051. [DOI: https://dx.doi.org/10.1007/s10570-019-02941-9]
43. Li, G.; Varga, I.; Kardos, A.; Dobryden, I.; Claesson, P.M. Nanoscale mechanical properties of core-shell-like poly-NIPAm microgel particles: Effect of temperature and cross-linking density. J. Phys. Chem. B; 2021; 125, pp. 9860-9869. [DOI: https://dx.doi.org/10.1021/acs.jpcb.1c04173] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/34428041]
44. Jiang, T.Y.; Chen, F.Y.; Duan, Q.F.; Bao, X.Y.; Jiang, S.; Liu, H.S.; Chen, L.; Yu, L. Designing and application of reactive extrusion with twice initiations for graft copolymerization of acrylamide on starch. Eur. Polym. J.; 2022; 165, 111008. [DOI: https://dx.doi.org/10.1016/j.eurpolymj.2022.111008]
45. Yan, Y.Q.; Wei, Z.S.; Duan, X.G.; Long, M.C.; Spinney, R.; Dionysios, D.D.; Xiao, R.Y.; Alvarez, P.J.J. Merits and limitations of radical vs. nonradical pathways in persulfate-based advanced oxidation processes. Environ. Sci. Technol.; 2023; 57, pp. 12153-12179. [DOI: https://dx.doi.org/10.1021/acs.est.3c05153]
46. Zhou, B.B.; Kang, W.L.; Jiang, H.Z.; Yang, H.B.; Li, Z.; Lv, Z.Q.; Xu, Z.; Ning, C.Y.; Wang, H.Z.; Xie, S.T. Preparation and crosslinking mechanism of delayed swelling double-crosslinking nano polymer gel microsphere for anti-CO2 gas channeling. J. Pet. Sci. Eng.; 2022; 219, 111122. [DOI: https://dx.doi.org/10.1016/j.petrol.2022.111122]
47. Sun, S.Z.; Yu, B.C.; Shen, Y.M.; Liu, Y.; Sun, H.M.; Bie, X.; Wu, M.N.; Xu, Y.Q.; Wu, C.F.; Zhou, H. Promoting proximity to enhance Fe-Ca interaction for efficient integrated CO2 capture and hydrogenation. Sep. Purif.; 2025; 357, 130227. [DOI: https://dx.doi.org/10.1016/j.seppur.2024.130227]
48. Zhang, L.; Ge, J.; Zhuang, T.; Ding, X.; Zheng, X. Enhanced photocatalytic nitrogen fixation performance of g-C₃N₄ under the burning explosion effect. React. Kinet. Mech. Catal.; 2021; 132, pp. 1211-1224. [DOI: https://dx.doi.org/10.1007/s11144-021-01947-4]
49. Bashir, S.; Hina, M.; Iqbal, J.; Rajpar, A.H.; Mujtaba, M.A.; Alghamdi, N.A.; Wageh, S.; Ramesh, K.; Ramesh, S. Fundamental concepts of hydrogels: Synthesis, properties, and their applications. Polymers; 2020; 12, 2702. [DOI: https://dx.doi.org/10.3390/polym12112702]
50. Zhao, X.; Niu, L.; Hao, Z.; Long, X.; Wang, D.; Li, G. Sodium persulfate promoted interzeolite transformation of USY into SSZ-13 via a solid-state grinding route and its enhanced catalytic lifetime in the methanol-to-olefins reaction. React. Kinet. Mech. Catal.; 2021; 134, pp. 837-849. [DOI: https://dx.doi.org/10.1007/s11144-021-02105-6]
51. Sun, S.; Xu, Y.; Zhou, H.; Wu, C.; Sun, H.M.; Zhao, X.T.; Zhang, Y.R.; Yang, X.X.; Bie, X.; Wu, M.N. et al. Ni-functionalized Ca@Si yolk-shell nanoreactors for enhanced integrated CO2 capture and dry reforming of methane via confined catalysis. Appl. Catal. B-Environ.; 2024; 348, 123838. [DOI: https://dx.doi.org/10.1016/j.apcatb.2024.123838]
52. Wang, M.Q.; Chen, B.; He, Y.; Li, Z.L.; Ge, Y.Y. Accelerated synthesis of TS-1 zeolite for epoxidation of 1-hexene under ambient environment: Role of sodium persulfate in inhibiting the formation of anatase species. Appl. Catal. A General.; 2024; 679, 119730. [DOI: https://dx.doi.org/10.1016/j.apcata.2024.119730]
53. Yang, S.; Liu, D.; Li, Y.; Yang, C.; Yang, Z.; Chen, X.; Tang, Z. Experimental study on the oxidative dissolution of carbonate-rich shale and silicate-rich shale with H2O2, Na2S2O8 and NaClO: Implication to the shale gas recovery with oxidation stimulation. J. Nat. Gas Sci. Eng.; 2020; 76, 103207. [DOI: https://dx.doi.org/10.1016/j.jngse.2020.103207]
54. Wang, L.T.; Yang, J. Zirconia-doped methylated silica membranes via sol-gel process: Microstructure and hydrogen permselectivity. Nanomaterials; 2022; 12, 2159. [DOI: https://dx.doi.org/10.3390/nano12132159]
55. Wang, S.E.; Shi, Y.; Chen, S.J.; Zhu, C.Y.; Wang, X.; Zhou, T.; Su, L.H.; Tan, C.Q.; Zhang, L.J.; Xiang, H.M. Porous biochars with nitrogen defects prepared from hydrogel template-modified food waste for high-performance supercapacitors. J. Energy Storage; 2023; 72, 108720. [DOI: https://dx.doi.org/10.1016/j.est.2023.108720]
56. Sulaeman, A.P.; Gao, Y.; Dugmore, T.; Remón, J.; Matharu, A.S. From unavoidable food waste to advanced biomaterials: Microfibrilated lignocellulose production by microwave-assisted hydrothermal treatment of cassava peel and almond hull. Cellulose; 2021; 28, pp. 7687-7705. [DOI: https://dx.doi.org/10.1007/s10570-021-03986-5]
57. Azeem, M.K.; Islam, A.; Rizwan, M.; Rasool, A.; Gul, N.; Khan, R.U.; Rasheed, T. Sustainable and environment-friendlier carrageenan-based pH-responsive hydrogels: Swelling behavior and controlled release of fertilizers. Colloid Polym. Sci.; 2023; 301, pp. 209-219. [DOI: https://dx.doi.org/10.1007/s00396-023-05054-9]
58. Wu, X.; Yang, Q.; Zhang, X.; Tsou, C.; De Guzman, M.R.; Li, X.; Gao, C. A tough conductive hydrogel with triple physical cross-linking, pH-responsive swelling behaviors, and excellent strain sensitivity. Polymer; 2023; 273, 125887. [DOI: https://dx.doi.org/10.1016/j.polymer.2023.125887]
59. Gao, J.; Fan, D.; Song, P.; Zhang, S.; Liu, X. Preparation and application of pH-responsive composite hydrogel beads as potential delivery carrier candidates for controlled release of berberine hydrochloride. R. Soc. Open Sci.; 2020; 7, 200676. [DOI: https://dx.doi.org/10.1098/rsos.200676]
60. Liu, Z.; Basem, A.; Mostafa, L.; Jasim, D.J.; Al-Rubaye, A.H.; Salahshour, S.; Esmaeili, S. Investigating the effect of pH on the swelling process, mechanical and thermal attributes of polyacrylamide hydrogel structure: A molecular dynamics study. Case Stud. Therm. Eng.; 2024; 55, 104148. [DOI: https://dx.doi.org/10.1016/j.csite.2024.104148]
61. Leng, Y.; Britten, C.N.; Tarannum, F.; Foley, K.; Billings, C.; Liu, Y.; Walters, K.B. Stimuli-responsive phosphate hydrogel: A study on swelling behavior, mechanical properties, and application in expansion microscopy. ACS Omega; 2024; 9, pp. 37687-37701. [DOI: https://dx.doi.org/10.1021/acsomega.4c02475] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/39281925]
62. Rehman, U.; Sarfraz, R.M.; Mahmood, A.; Hussain, Z.; Thu, H.E.; Zafar, N.; Batool, N. Smart pH-responsive co-polymeric hydrogels for controlled delivery of capecitabine: Fabrication, optimization and in vivo toxicology screening. Curr. Drug Deliv.; 2021; 18, pp. 1256-1271. [DOI: https://dx.doi.org/10.2174/1567201818666210212085912] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33583374]
63. Sabzi, M.; Afshari, M.J.; Babaahmadi, M.; Shafagh, N. pH-dependent swelling and antibiotic release from citric acid crosslinked poly(vinyl alcohol) (PVA)/nano silver hydrogels. Colloids Surf. B; 2020; 188, 110757. [DOI: https://dx.doi.org/10.1016/j.colsurfb.2019.110757] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31887648]
64. Kumar, K.K.; Meena, P.; Warkar, S.G. Development and characterization of pH-responsive CMTKG/PAM/PEG hydrogel for oral administration of etophylline. Colloid Polym. Sci.; 2023; 301, pp. 1313-1323.
65. Matar, G.H.; Andac, M.; Elmas, A. Locust bean gum-polyvinyl alcohol hydrogels: Synthesis, characterization, swelling behaviors, and mathematical models. J. Appl. Polym. Sci.; 2022; 139, 51498. [DOI: https://dx.doi.org/10.1002/app.51498]
66. Tao, X.; Jiang, F.; Cheng, K.; Qi, Z.; Yadavalli, V.K.; Lu, S. Synthesis of pH- and glucose-responsive silk fibroin hydrogels. Int. J. Mol. Sci.; 2021; 22, 7107. [DOI: https://dx.doi.org/10.3390/ijms22137107]
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