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Article

The Role of Freezing Temperature in Modulating Chitosan Gel Structure and Evaporation Performance for Seawater Desalination

Textile Pollution Controlling Engineering Centre of Ministry of Ecology and Environment, College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Separations 2025, 12(8), 193; https://doi.org/10.3390/separations12080193
Submission received: 2 July 2025 / Revised: 22 July 2025 / Accepted: 23 July 2025 / Published: 24 July 2025

Abstract

Interfacial solar evaporation has emerged as a promising strategy for freshwater production, where 3D evaporators offer distinct advantages in heat management and salt rejection. Freeze–thaw cycling is a widely adopted fabrication method for 3D hydrogel evaporators, yet the impact of preparation conditions (e.g., freezing temperature) on their evaporation performance remains poorly understood, hindering rational optimization of fabrication protocols. Herein, we report the fabrication of chitosan-based hydrogel evaporators via freeze–thaw cycles at different freezing temperatures (−20 °C, −40 °C, and −80 °C), leveraging its low cost and environmental friendliness. Characterizations of crosslinking density and microstructure reveal a direct correlation between freezing temperature and network porosity, which significantly influences evaporation rate, photothermal conversion efficiency, and anti-salt performance. It is noteworthy that the chitosan hydrogel prepared at −80 °C demonstrates an excellent evaporation rate in high-salinity environments and exhibits superior salt resistance during continuous evaporation testing. Long-term cyclic experiments indicate that there was an average evaporation rate of 3.76 kg m−2 h−1 over 10 cycles, with only a 2.5% decrease observed in the 10th cycle. This work not only elucidates the structure–property relationship of freeze–thaw fabricated hydrogels but also provides a strategic guideline for tailoring evaporator architectures to different salinity conditions, bridging the gap between material design and practical seawater desalination.

1. Introduction

The shortage and uneven distribution of fresh water resources have become an increasingly serious problem, especially given the growing global demand for water. While the Earth’s surface is covered by a substantial amount of water, more than 97% of it is highly saline and cannot be directly utilized for drinking, irrigation, or most industrial purposes [1,2,3,4]. This stark reality necessitates the development of innovative technologies capable of efficiently purifying these abundant saline sources. Traditional desalination methods, such as reverse osmosis and thermal distillation, are widely deployed but suffer from significant drawbacks, including high energy consumption and environmental pollution stemming from their reliance on fossil fuels [2,5,6,7]. In contrast, solar-driven interfacial desalination (SDID) offers a sustainable alternative that not only addresses the immediate need for increased water supply but also does so without relying on traditional energy sources [8,9,10]. By directly harnessing solar energy, SDID addresses the urgent need for increased freshwater supply without depending on conventional energy sources [11,12]. This approach simultaneously contributes to mitigating the global energy crisis and minimizing the environmental footprint associated with conventional water treatment, representing a significant advancement towards sustainable water management and securing clean water for the future.
Within the SDID field, three-dimensional (3D) network-structured evaporators have emerged as a prominent research focus due to their demonstrated ability to significantly enhance evaporation efficiency and system stability [13,14]. Their unique spatial architecture provides a high specific surface area, which expands the gas–liquid interfacial contact area and optimizes internal mass transfer channels. Consequently, these structures can boost evaporation rates by an impressive 40–80% compared to traditional two-dimensional (2D) evaporators [15,16], effectively overcoming the issue of high mass transfer resistance inherent in planar liquid films. However, the construction of intricate 3D structures often entails increased material consumption, leading to a sharp rise in production costs [17,18]. This cost challenge is particularly acute when utilizing polymer composites or advanced nanomaterials, severely restricting the large-scale application potential of 3D evaporators in critical areas like seawater desalination, as highlighted in prior studies [19,20].
The quest for cost-effective 3D evaporator materials has increasingly turned towards natural biomass-based preparation strategies, which offer remarkable advantages in terms of sustainability and affordability [21,22,23]. Chitosan (CS), an abundant natural amino polysaccharide derived primarily from crustacean processing waste, exemplifies this potential with significant cost benefits [24,25]. Rich in hydroxyl and amino groups within its molecular chain, CS possesses excellent inherent hydrophilicity, ensuring a readily available water supply crucial for sustained evaporation [26,27]. Furthermore, the versatility of CS allows for the construction of tailored 3D network structures under mild conditions using either physical or chemical cross-linking methods. Critically, key structural parameters, such as pore size distribution and mechanical properties, can be precisely regulated by optimizing the processing conditions [28,29]. To achieve efficient SDID, however, these hydrophilic structural frameworks require integration with effective photothermal conversion materials [30,31,32,33], such as polypyrrole (PPy) [34,35,36], which is widely employed in SDID systems for its excellent light absorption and solar-to-thermal conversion properties. A growing body of research confirms that chitosan-based materials hold considerable promise for practical applications within the water treatment sector [37,38].
Building upon this foundation, our work specifically investigates the influence of different freezing-induced cross-linking conditions on the internal microstructure of chitosan gel evaporators. We employ precise control over freezing parameters to systematically fabricate distinct microarchitectures and rigorously analyze how these critical features govern overall evaporation performance. To complement the structural optimization and maximize solar energy utilization, we incorporate PPy as a photothermal layer, leveraging its capabilities to significantly enhance the light absorption and heat generation efficiency of evaporators. Through comprehensive characterization and performance testing, this study aims to elucidate the intricate relationships between the cross-linking conditions, the resulting water transport mechanisms, and the evaporation kinetics. Our findings are intended to provide fundamental insights and practical guidelines for the rational design and fabrication of high-efficiency, biomass-derived solar evaporators.

2. Materials and Methods

2.1. Materials

Chitosan (CS, with a deacetylation degree of ≥95%) and pyrrole (Py) were supplied by Aladdin Life Science Technology Co., Ltd., Shanghai, China. Glutaraldehyde (GA) was obtained from Macklin Biochemical Co., Ltd., Shanghai, China. Ferric chloride hexahydrate (FeCl3·6H2O), sodium chloride (NaCl), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were sourced from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.

2.2. Preparation of Crosslinking Solution

CS powder was dissolved in 1 wt% acetic acid under constant stirring at room temperature for 4 h to form a transparent solution. Next, 0.8061 g of ferric trichloride hexahydrate was added to the solution, which was then stirred in an ice-water bath for 10 min, resulting in a brown transparent solution. Subsequently, 0.21 mL of pyrrole solution was introduced. The mixture was continuously stirred in the ice-water bath for an additional 9 h, yielding a black solution.

2.3. Preparation of CPG Gel Evaporators

A certain amount of the above mixed solution was placed in a container, and a certain amount of GA solution was added under stirring. After continuous stirring for 3 min, the mixture was left to stand for 2 h. The preliminarily cross-linked gel was subjected to freeze cross-linking at different temperatures (−20 °C, −80 °C, and −196 °C). Specifically, it was frozen for 24 h at −20 °C and −80 °C, and for 10 min at −196 °C. Subsequently, the frozen gel was allowed to thaw at 4 °C for 12 h. This constituted one freeze–thaw cycle. After three cycles, the gel was lyophilized in a freeze dryer for 24 h (Figure 1a). Finally, the evaporators were named CPG-20, CPG-80, and CPG-196 according to the freezing temperature.

2.4. Preparation of CPG Gels and PPy Particles

The CG gel was prepared following the procedures for CPG-80. CS solution was transferred into a container, and GA solution was added dropwise under continuous stirring. After uniform mixing, the mixture was left to crosslink statically. Finally, the container was placed in a −80 °C environment for freeze–thaw cycling to complete the preparation. For PPy particles, Py solution was dissolved in deionized water. After thorough stirring, ferric chloride hexahydrate was added to the solution, which was then continuously stirred at 4 °C. The resulting black solution was frozen and lyophilized to obtain PPy particles for subsequent characterization.

2.5. Characterization of Gel Evaporators

The chemical characteristics of the samples were analyzed via Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Nicolet iS20, Waltham, MA, USA) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha, Waltham, MA, USA). The internal structure of the gel was observed using a scanning electron microscope (SEM, Hitachi SU8600, Tokyo, Japan, Vacc = 5.0 kV). Infrared images and water contact angle measurements were captured with a thermal camera and a contact angle goniometer, respectively. UV-visible-near-infrared spectroscopy (UV-Vis-NIR, Shimadzu UV-3600i Plus, Kyoto, Japan) was employed to measure the absorbance in the wavelength range of 250–2500 nm. A water contact angle instrument was used to investigate the hydrophilicity (Kino SL200KS, Boston, MA, USA). The ionic concentrations in the solutions were determined by an inductively coupled plasma emission spectrometer (ICP-OES, Agilent 7500, Santa Clara, CA, USA). The differential scanning calorimeter (DSC, CLARUS SQ8-STA8000, Bülach, Switzerland) was used to calculate the enthalpy of evaporation. A xenon lamp with a solar filter was used to simulate sunlight (YM-GHX-XE-300, Shanghai, China), and the specific irradiance was measured with an irradiance meter (HANDY FZ-A, Beijing, China). Finally, the degradation process of the gel was investigated by thermogravimetric analysis (CLARUS SQ8-STA8000, Bülach, Switzerland), and the influence of different temperatures on the embedding degree of PPy was explored. In a nitrogen atmosphere of 20 mL/min, the temperature gradually increases from 30 °C to 800 °C at a rate of 10 °C/min.

2.6. Evaporation Test

To evaluate the evaporation capacity, we measured the mass change of the solution during evaporation. Under identical environmental conditions, the evaporator was embedded in foam and placed in deionized water or saltwater of varying concentrations. The evaporation test was then performed using simulated sunlight, with an electronic balance used to record the real-time mass loss of the solution. To minimize experimental errors, protective devices were installed around the setup to shield against external light sources and airflow, ensuring consistent test conditions. Large-diameter containers were employed to prevent a decrease in light intensity that could result from falling liquid levels due to water evaporation. The evaporation rate was calculated using the formula below:
v = d m A d t
where v is the evaporation rate, dm is the mass change of the solution, A is the evaporation area, and dt is the corresponding irradiation time.

2.7. Swelling Test

The gel evaporator was vertically placed in deionized water, ensuring only the bottom of the gel was in contact with the water and the evaporator remained upright throughout the test. After a certain period, the gel was taken out of the water. Excess surface moisture was removed, and it was weighed again. Water absorption was considered complete when the weight measurements were consistent for three consecutive trials. The saturated water content (QS) can be calculated using the following formula:
Q S = m S m d m d
where mS is the mass of the gel after full swelling, and md is the mass of the dry gel. The water transport rate was used to evaluate the kinetic analysis of the gel during the swelling process. This rate can be described by the half-swelling rate (V):
V = Q S 2 t
where t is the time required for half-swelling.

2.8. Light Absorption Test

The absorption (A) was determined from the reflectance (R) and transmittance (T) measurements, taken over the 250–2500 nm wavelength range using a UV-Vis-NIR Spectrometer (Shimadzu UV-3600i Plus, Tokyo, Japan) with an integrating sphere. The absorption is calculated according to the following equation:
A = 1 R T

2.9. Long-Term Evaporation Test

The evaporator with the most outstanding performance in saltwater was employed for long-term evaporation of saline water (salinity: 10 wt%). The daily evaporation duration was 8 h under one sun illumination intensity. After evaporation, the system was kept stationary for 16 h, followed by supplementation with saline water of the same concentration to continue cyclic evaporation.

3. Results and Discussion

3.1. Characterization of CPG-X Gel Evaporators

CS cross-linked with glutaraldehyde under freezing conditions. The low temperature restricted molecular motion, increased contact opportunities, and promoted cross-linking. Different freezing temperatures significantly affected ice crystal formation, leading to varying internal gel structures. SEM observations revealed that during freezing cross-linking, ice crystals of different sizes and growth rates formed, creating a cross-linked network with diverse pores. The SEM images showed that as the cross-linking temperature decreased (Figure 1b), the internal network of gel became more compact, with smaller pore sizes [39,40]. Additionally, the cross-linking temperature also influenced the chemical reaction (Figure 2a). As shown in Figure 2b, the FTIR spectra of the various gels exhibit a distinct C=N stretching vibration peak near 1600 cm−1, which confirms the successful formation of a Schiff base linkage (C=N) through the reaction between the -NH2 groups of chitosan and the –CHO groups of glutaraldehyde [24,29]. Notably, the CPG gel series exhibited peak broadening and shifting in this region, primarily attributed to π–π stacking or hydrogen-bonding interactions between the C=C structure in PPy and the Schiff base within the cross-linked network. Comparison of gels cross-linked at different temperatures showed variations in the peak intensity of the characteristic band at approximately 1050 cm−1, corresponding to C–O–C stretching vibrations [41]. The gel cross-linked at −196 °C exhibited a markedly enhanced peak intensity at this band, suggesting that lower temperatures enhanced hydroxyl group reactions, generating more ether bonds and strengthening the cross-linking. Furthermore, the broad peak of CPG gels between 3100 and 3300 cm−1 is associated with the overlap of –OH from chitosan and N–H bonds from PPy [27]. The broad peak near 2180 cm−1 is attributed to the deep doping of PPy in the protonated chitosan environment [42]. The intensity of this peak increased from −20 °C to −196 °C, accompanied by a slight blue shift, indicating an enhancement in both the doping degree and content of PPy with decreasing temperature. XPS analysis offered further insights. Relative to CG gels, CPG gels displayed characteristic peaks for Fe and Cl ions. In the C1s spectrum, peaks at 284.8 eV, 283.2 eV, and 286.9 eV were assigned to C–C, C=C, and C–OH/C–N, respectively. The C=C peak confirmed the successful incorporation of PPy (Figure 2d–f). In the N1s spectrum (Figure 2g–i), peaks at 400.2 eV and 398.4 eV corresponded to –NH2 and C=N, with the latter further confirming the successful cross-linking [27,41]. Combining FTIR and XPS data, variations in peak area ratios were mainly attributed to changes in C–N and C=N bonds. The C1s peak area for C–OH/C–N increased with decreasing temperature, primarily due to enhanced C–N, indicating stronger PPy integration with the gel matrix and higher overall content, consistent with FTIR results [27,42]. The increased C=N peak intensity in the N1s spectrum was mainly attributed to higher PPy content. Lower cross-linking temperatures slowed network curing, providing more diffusion channels and reaction time for PPy molecules to penetrate and anchor within the network. This significantly improved PPy loading in the gel. To further verify this hypothesis, gels prepared under different conditions were subjected to thermogravimetric analysis. Figure S1 shows that in the temperature range of PPy decomposition (400–600 °C), CPG-196 exhibits the slowest degradation rate, confirming a more optimal PPy doping degree in this gel. In addition, both the decomposition process of the chitosan main chain (200–400 °C) and the amount of final residue are affected by the presence of PPy; generally, gels with higher cross-linking degree and higher PPy content exhibit a slower main chain decomposition rate and higher residual mass [43,44,45]. In conclusion, adjusting the freezing cross-linking temperature is an effective method for achieving efficient PPy loading and stable doping in chitosan gels, establishing a foundation for subsequent photothermal conversion applications.

3.2. Water Transport and Evaporation Enthalpy

The chitosan skeleton, abundant in hydrophilic groups including hydroxyl (–OH) and amino (–NH2) groups, provides the material with continuous water absorption capability while facilitating upward water transport. FTIR and XPS analyses confirm that the –OH and –NH2 groups serve as the primary contributors to the gel hydrophilicity, as evidenced by the direct correlation between their relative content and material hydrophilic performance. By comparing the relative peak areas in the spectra of different gels, it was found that CPG-20 and CPG-80 exhibited higher hydrophilic group content than CPG-196. The water contact angle measurements corroborated this trend: CPG-20 exhibited the lowest contact angle, thus demonstrating the strongest hydrophilicity among all samples, consistent with spectral observations (Figure 3a). Swelling experiments further elucidated differences in water retention and transport properties (Figure 3b and Table S1). CPG-20 achieved the highest saturated water content due to its relatively loose network structure, which provides expanded water storage capacity (35.55 g g−1). Regarding water transport rate, however, CPG-80 demonstrated optimal performance (16.76 mg s−1). This advantage stems from its balanced pore architecture: compared to the looser CPG-20 network, the moderately smaller pores of CPG-80 generate enhanced capillary forces for rapid water intake; whereas compared to the highly dense CPG-196 structure, CPG-80 exhibits reduced pore tortuosity, significantly shortening the water molecule transmission path (Figure 3b).
The binding states of water molecules in different gels were also characterized by DSC analysis after water absorption (Figure 3c). The calculated evaporation enthalpies (ΔHvap) were all significantly lower than the theoretical value of pure water (2450 J·g−1), confirming effective regulation of water phase transition behavior by these evaporators. Variations in phase transition peak positions indicated differential binding forces exerted on water molecules across gel networks. This discrepancy primarily results from the differences in network pore structures and internal stresses formed at different cross-linking temperatures: smaller pore sizes (e.g., CPG-196) bind water molecules more strongly, requiring higher energy for evaporation (larger ΔHvap). The magnitude of evaporation enthalpy is mainly influenced by the state of water molecules in the gel: gels with larger pore sizes (e.g., CPG-20) typically accommodate more free water, leading to lower energy requirements for evaporation (smaller ΔHvap) [46]. Furthermore, CPG-80 possesses a higher hydrophilic group content than CPG-196, intensifying hydration interactions with water molecules. This enhanced binding force contributes to the observed increase in evaporation enthalpy for CPG-80 relative to CPG-20.

3.3. Photothermal Conversion Performance and Evaporation Performance

Efficient light-harvesting capability is crucial for excellent solar water evaporation performance. As shown in Figure 4a, the CPG series gels exhibited remarkable light absorption across 250–2500 nm, with CPG-196 showing the strongest full-band absorption. This mainly stems from optimized PPy dispersion in the gel network during low-temperature cross-linking (−196 °C). The low temperature retards the curing kinetics of the gel network, thereby enabling PPy nanoparticles sufficient time for diffusion and penetration to achieve uniform dispersion and maximum loading within the chitosan matrix [27]. Consequently, the material exhibits a significant enhancement in photon capture efficiency. To quantitatively evaluate the photothermal conversion performance, the evaporators were irradiated under simulated sunlight at 1.0 kW·m−2 (Figure 4b), and thermocouples were used to monitor the temperature changes of the evaporation surface in real time. After 1 h of illumination, the photothermal conversion process of each evaporator tended to stabilize, and the surface temperature change rate significantly decreased. During this process, the evaporation surface temperature of CPG-196 reached the peak first and continuously maintained the highest value (52.6 ± 2.8 °C, n = 3), indicating that it had the optimal photothermal conversion efficiency. This phenomenon originates from the synergistic effect between the existence state of PPy and the internal structure of the gel: on the one hand, its strong full-band light absorption capacity ensures that incident photons are efficiently converted into localized heat; on the other hand, the highly densified network structure effectively suppresses heat convection losses caused by water flow, while the internal micro-nano pores form a thermal barrier layer—this mechanism is confirmed by infrared thermal imaging (Figure 4d shows that the water temperature rise at the bottom of CPG-196 is significantly lower than that of other groups), which significantly reduces the longitudinal dissipation of heat to the bulk water and achieves excellent thermal localization management.
Further quantitative analysis of clean water evaporation rate via real-time mass loss measurement (Figure 4c and Table S2) and surface thermal distribution recorded by an infrared camera (Figure 4d) revealed that CPG-20 exhibited the highest clean water evaporation flux. Its superior performance is attributed to the synergistic optimization of water transport and phase transition kinetics by the structure: the high content of hydrophilic groups (–OH/–NH2) and loose hierarchical pores endow the material with strong capillary water transport capacity and saturated water content, ensuring continuous water supply to the evaporation interface. Meanwhile, the lower evaporation enthalpy (ΔHvap) significantly reduces the energy barrier for phase transition, greatly promoting the vaporization process. Thus, although CPG-196 maintained the highest surface temperature, CPG-20 achieved optimal evaporation performance through the synergistic effect of continuous water supply, effective thermal management, and a low phase transition energy barrier. Notably, there was a slight discrepancy between the temperature distribution recorded by the infrared thermal imager and the fixed-point measurement data from thermocouples. This deviation primarily arises from the local occlusion effect of the thermocouple probe on the evaporation surface, leading to an approximately 5–8% loss in light flux in this region. Additionally, during continuous illumination, the water temperature rise at the bottom of CPG-196 was significantly lower than that of other evaporators, providing direct experimental evidence for the thermal localization mechanism of CPG-196. Its dense network structure suppresses thermal convection, while micro-nano pores hinder heat conduction, effectively curbing the longitudinal dissipation of thermal energy to the bulk water.

3.4. Seawater Evaporation Performance

The application of CPG gel evaporators in evaporation experiments with saltwater of different concentrations showed that CPG-20 exhibited a higher evaporation rate in low-concentration saltwater, but its performance gradually lagged behind other gels as salinity increased. When the saltwater concentration reached 20 wt%, CPG-196 achieved the highest evaporation rate (Figure 5a and Table S2). This discrepancy originated from the synergistic effect between the material structure and the physicochemical properties of salt solutions. In low-concentration systems, the physicochemical properties of saltwater did not significantly hinder water transport, allowing the loose network structure and high exposure rate of hydrophilic groups in CPG-20 to fully utilize its superior water transport capacity and saturated water retention. In high-concentration saltwater, however, ionic hydration disrupted the gel-water hydrogen bond network, and the weak thermal management capability of CPG-20 led to salt crystallization deposition inside pores, thereby blocking water transport pathways. The high evaporation rate of CPG-196 in high-salt environments was attributed to a dual mechanism: the confinement effect generated by its dense nanopores suppressed salt ion penetration [40], while its excellent thermal management performance caused salts to preferentially form a porous crystalline layer at the evaporation interface [47], preventing pore blockage. Notably, interactions between high-concentration salt ions and the conjugated structure of PPy enhanced photothermal conversion efficiency, synergistically promoting evaporation rate improvement.
To further validate evaporator salt tolerance, continuous evaporation tests in 20 wt% saltwater were conducted using various gels. Both mass changes of saltwater and surface salt crystallization were monitored throughout the process (Figure 5b,d). Results indicated that CPG-80 maintained a consistent mass reduction trend. Moreover, salt deposition on its evaporation surface occurred later than on other materials and accumulated minimally. After crystallized evaporators rested for an identical duration, salt deposits on the surface of CPG-80 nearly disappeared completely, demonstrating excellent recyclability for repeated evaporation cycles. This superior salt tolerance can be attributed to a well-balanced combination of efficient water transport and anti-salt crystallization performance, enabled by moderate pore structure and effective thermal management capability. Such characteristics offer significant potential for practical applications. Based on these advantages, CPG-80 underwent long-term saltwater evaporation cycle tests and real seawater desalination experiments. Long-term cyclic tests revealed that CPG-80 maintained a stable evaporation rate over ten consecutive cycles, confirming high operational feasibility (Figure 5e). Ion concentration analysis of collected seawater and condensed water further verified its excellent desalination performance for freshwater collection.

4. Conclusions

In this study, gel evaporators were prepared through simple chemical cross-linking and freeze–thaw cycles, demonstrating the design and performance of a chitosan-based CPG gel evaporator. This evaporator features a customized network structure for efficient solar-driven water evaporation. The crosslinking temperature significantly affects the pore structure of the gel, the distribution of hydrophilic groups, and the photothermal conversion ability. CPG-80 has a moderate pore structure and thermal management, balancing water transport and salt crystallization resistance, and maintaining stable performance in long-term cycling tests. The application in seawater desalination has confirmed that CPG-80 can effectively remove salt ions and produce high-quality condensate water. These findings emphasize the crucial role of structure–performance synergy in optimizing solar evaporators with different brine treatment schemes, providing a promising material strategy for actual freshwater production.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/separations12080193/s1, Figure S1: The TGA test curve of CPG gel; Table S1: Transmission performance of different evaporators; Table S2: Evaporation performance of different evaporators.

Author Contributions

Conceptualization, J.C., Y.B. and F.L.; methodology, J.C. and Y.B.; experiments and characterizations, J.C. and Y.B.; Original draft, J.C.; reviewing and editing, J.C., Y.B. and F.L.; data curation, J.C., Y.B. and F.L.; funding acquisition, F.L.; J.C. and Y.B. contributed equally to this paper. All authors have read and agreed to the published version of the manuscript.

Funding

The work has been supported by the Fundamental Research Funds for the Central Universities (No. 2232025A-11).

Data Availability Statement

Data are available on request from the authors.

Acknowledgments

The authors gratefully acknowledge the funding support from the Fundamental Research Funds for the Central Universities (No. 2232025A-11).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Preparation process of gel precursor solution and the detailed preparation process of evaporators; (b) SEM images of evaporators under different preparation conditions.
Figure 1. (a) Preparation process of gel precursor solution and the detailed preparation process of evaporators; (b) SEM images of evaporators under different preparation conditions.
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Figure 2. (a) Reaction process diagram of key components in the evaporator; (b) FTIR spectra of CG, CPG-20, CPG-80, CPG-196, and PPy; (c) XPS spectra of CG, CPG-20, CPG-80, and CPG-196; (di) XPS of C1s and N1s for CPG-20, CPG-80, and CPG-196.
Figure 2. (a) Reaction process diagram of key components in the evaporator; (b) FTIR spectra of CG, CPG-20, CPG-80, CPG-196, and PPy; (c) XPS spectra of CG, CPG-20, CPG-80, and CPG-196; (di) XPS of C1s and N1s for CPG-20, CPG-80, and CPG-196.
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Figure 3. (a) Water contact angles for CPG-20, CPG-80, and CPG-196 (the arcs are marked as prominent water forms on the gel surface); (b) saturated water content and the calculated water transport rate of CPG-20, CPG-80, and CPG-196; (c) DSC curves of evaporation of water in CPG-20, CPG-80, and CPG-196 (corresponding evaporation enthalpy).
Figure 3. (a) Water contact angles for CPG-20, CPG-80, and CPG-196 (the arcs are marked as prominent water forms on the gel surface); (b) saturated water content and the calculated water transport rate of CPG-20, CPG-80, and CPG-196; (c) DSC curves of evaporation of water in CPG-20, CPG-80, and CPG-196 (corresponding evaporation enthalpy).
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Figure 4. (a) Absorption spectra of CPG-20, CPG-80, and CPG-196 in the entire solar spectrum (250–2500 nm); (b) temperature of CPG-20, CPG-80, and CPG-196 evaporating surfaces as a function of time; (c) mass changes and evaporation rates of CPG-20, CPG-80, and CPG-196 in pure water under 1-sun solar irradiation for 2 h; (d) infrared images and temperature changes on the surface of different evaporators after 1 h of irradiation under one sun.
Figure 4. (a) Absorption spectra of CPG-20, CPG-80, and CPG-196 in the entire solar spectrum (250–2500 nm); (b) temperature of CPG-20, CPG-80, and CPG-196 evaporating surfaces as a function of time; (c) mass changes and evaporation rates of CPG-20, CPG-80, and CPG-196 in pure water under 1-sun solar irradiation for 2 h; (d) infrared images and temperature changes on the surface of different evaporators after 1 h of irradiation under one sun.
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Figure 5. (a) Evaporation rates of CPG-20, CPG-80, and CPG-196 in brines of different concentrations under 1-sun solar irradiation for 8 h; (b) mass changes of CPG-20, CPG-80, and CPG-196 in 20 wt% NaCl solution under 1-sun solar irradiation for 8 h; (c) four major ions concentrations (Na+, Mg2+, Ca2+, and K+) in the seawater (from the East China Sea) and its distilled water; (d) surface salt crystallisation of CPG-20, CPG-80, and CPG-196 in a 20% NaCl solution during 8 h of illumination with 1 sun; (e) the long-term stability of CPG-80 in a 10% NaCl solution under 1-sun solar irradiation (the same color is the average evaporation rate per 2h in a cycle).
Figure 5. (a) Evaporation rates of CPG-20, CPG-80, and CPG-196 in brines of different concentrations under 1-sun solar irradiation for 8 h; (b) mass changes of CPG-20, CPG-80, and CPG-196 in 20 wt% NaCl solution under 1-sun solar irradiation for 8 h; (c) four major ions concentrations (Na+, Mg2+, Ca2+, and K+) in the seawater (from the East China Sea) and its distilled water; (d) surface salt crystallisation of CPG-20, CPG-80, and CPG-196 in a 20% NaCl solution during 8 h of illumination with 1 sun; (e) the long-term stability of CPG-80 in a 10% NaCl solution under 1-sun solar irradiation (the same color is the average evaporation rate per 2h in a cycle).
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Cai, J.; Bai, Y.; Li, F. The Role of Freezing Temperature in Modulating Chitosan Gel Structure and Evaporation Performance for Seawater Desalination. Separations 2025, 12, 193. https://doi.org/10.3390/separations12080193

AMA Style

Cai J, Bai Y, Li F. The Role of Freezing Temperature in Modulating Chitosan Gel Structure and Evaporation Performance for Seawater Desalination. Separations. 2025; 12(8):193. https://doi.org/10.3390/separations12080193

Chicago/Turabian Style

Cai, Jiaonan, Yong Bai, and Fang Li. 2025. "The Role of Freezing Temperature in Modulating Chitosan Gel Structure and Evaporation Performance for Seawater Desalination" Separations 12, no. 8: 193. https://doi.org/10.3390/separations12080193

APA Style

Cai, J., Bai, Y., & Li, F. (2025). The Role of Freezing Temperature in Modulating Chitosan Gel Structure and Evaporation Performance for Seawater Desalination. Separations, 12(8), 193. https://doi.org/10.3390/separations12080193

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