Preparation method of low-enthalpy high-salt-resistance hydrogel evaporator with core-shell structure

Abstract

The invention discloses a preparation method of a low-enthalpy and high-salt-resistance hydrogel evaporator with a core-shell structure, a reduced graphene oxide-ferroferric oxide (rGO-Fe3O4) photothermal conversion material is formed through one-step reduction of a solvothermal method of graphene oxide and ferric chloride, the formed hydrogel is of the core-shell structure, the interior of the hydrogel is a PAM / CMC composite skeleton, the shell of the hydrogel is PVA hydrogel, and the core-shell structure is a hollow structure. And additionally, an rGO-Fe3O4 photothermal conversion material is uniformly distributed in the whole structure, so that the PAM / CMC / PVA / rGO-Fe3O4 multi-network hydrogel is formed. The method can solve the problems of low preparation cost and material recovery when graphene is used for seawater desalination, and the problems of limited evaporation enthalpy reduction, poor mechanical strength, surface salting-out phenomenon in the seawater desalination process and the like of a traditional hydrogel evaporator.

Description

Technical Field

[0001] The invention belongs to the technical field of hydrogel evaporators, and in particular relates to a method for preparing a low-enthalpy and high-salt-resistance hydrogel evaporator with a core-shell structure. Background Art

[0002] Global water scarcity has become one of the most pressing challenges facing the world today. Rapid population growth, industrial expansion, and climate change are driving a significant increase in the demand for freshwater. Solar-driven interfacial evaporation has attracted widespread attention as an environmentally friendly and sustainable alternative for water purification, as it utilizes abundant solar energy to heat water and generate steam without requiring additional energy input. To achieve high water evaporation rates and optimal solar-to-steam conversion efficiency, solar evaporators must possess a broad light absorption range to fully utilize the majority of the solar spectrum. The evaporation rate relies heavily on photothermal materials, which efficiently absorb solar energy and convert it into heat, generating sufficient steam to drive water evaporation. To address the need for efficient water evaporation under natural sunlight, advanced photothermal materials, such as carbon-based materials, semiconductors, plasmonic metal particles, and polymers, have been rapidly developed. Graphene and its derivatives have been widely used in interfacial solar evaporation due to their exceptional light absorption, broad spectral response, and excellent chemical stability. However, the direct use of graphene as a photothermal conversion material presents challenges in terms of cost and material recovery.

[0003] In addition, hydrogel materials, due to their three-dimensional porous network structure, high hydrophilicity, and good water transport capacity, have demonstrated significant advantages in regulating the state of water molecules and reducing the evaporation enthalpy. Most researchers have already used them as substrate materials for photothermal evaporation. Some traditional hydrogel evaporators, such as those based on polyvinyl alcohol, cellulose, and polysaccharides, have demonstrated excellent performance in reducing the evaporation enthalpy. However, they suffer from poor mechanical strength and salt precipitation on their surfaces during the desalination process, resulting in reduced durability and an inability to meet the requirements for sustainable desalination applications. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a low-enthalpy, high-salt-resistance hydrogel evaporator with a core-shell structure. The method can solve the problems of preparation cost and material recovery when graphene is used for seawater desalination, as well as the problems of limited reduction of evaporation enthalpy, poor mechanical strength, and salting-out on the surface of traditional hydrogel evaporators during seawater desalination.

[0005] To achieve the above objectives, the present invention discloses a method for preparing a low-enthalpy, high-water-resistance hydrogel evaporator with a core-shell structure. The method comprises the following steps: a reduced graphene oxide-ferroferric oxide (rGO-Fe3O4) photothermal conversion material is formed by a solvothermal method using graphene oxide and ferric chloride to form a core-shell structure, wherein the internal structure is a PAM / CMC composite skeleton and the outer shell is a PVA hydrogel. The rGO-Fe3O4 photothermal conversion material is evenly distributed in the overall structure to form a PAM / CMC / PVA / rGO-Fe3O4 (abbreviated as PCPR) multi-network hydrogel.

[0006] As a further solution of the present invention: the preparation method comprises the following steps:

[0007] Step 1: Preparation of rGO-Fe3O4, including the following steps:

[0008] The rGO-Fe3O4 nanocomposite was synthesized and reduced by a solvothermal method. The process flow is as follows: 30 wt% GO was dispersed in ethylene glycol, stirred at 70°C for 1 hour and ultrasonicated for 1 hour to fully disperse it; FeCl3·6H2O and sodium acetate were then added in sequence, each stirring for 0.5 hour to obtain a uniform brown-yellow precursor solution; the precursor solution was transferred to a polytetrafluoroethylene-lined hydrothermal reactor and reacted at 200°C for 12 hours to allow Fe3O4 to grow in situ on the rGO surface; after natural cooling, the solution was washed multiple times with deionized water under a magnetic field and finally dried in a vacuum at 50°C for 48 hours to obtain rGO-Fe3O4 composite powder;

[0009] Step 2: Preparation of PAM / CMC / PVA / rGO-Fe3O4 precursor solution, including the following steps:

[0010] First, sodium carboxymethyl cellulose was gradually added to deionized water and stirred at 40°C for 15 minutes until completely dissolved. Subsequently, acrylamide was added to the solution and stirred for another 5 minutes. At the same time, PVA was added to another portion of deionized water and stirred at 70°C for 15 minutes until dissolved. The resulting PVA solution was then added to the aforementioned CMC-AM mixed solution and stirred thoroughly to obtain a uniform mixture.

[0011] Then, polyvinyl pyrrolidone, sodium lauryl sulfate, rGO-Fe3O4 composite material and N,N'-methylenebisacrylamide were added in sequence, and the uniformity of the system was ensured under continuous stirring. Finally, APS was added as an initiator, and the precursor solution was prepared.

[0012] Step 3: Preparation of PAM / CMC / PVA / rGO-Fe3O4 evaporator, including the following steps:

[0013] The evenly mixed precursor solution from step 2 was poured into a culture dish to form a hydrogel film with a certain thickness; subsequently, it was frozen with liquid nitrogen for 15 minutes for preliminary shaping; then, it was placed in a 70°C vacuum oven for cross-linking reaction for 2 hours until the hydrogel was completely formed; finally, the hydrogel sample was immersed in a 3wt% concentration of borax solution for physical cross-linking for 1 hour, eventually forming a core-shell structure hydrogel.

[0014] As a further solution of the present invention: in the step 1, the amount of ethylene glycol is 40 mL, the amount of FeCl3·6H2O and the amount of sodium acetate are 1.35 g and 2.87 g, respectively, and the amount of polytetrafluoroethylene is 80 mL; in the step 2, the amount of sodium carboxymethyl cellulose is 0.4 g, the amount of deionized water is 10 mL, the amount of acrylamide is 2 g, the amount of PVA is 0.4 g, the amount of polyvinyl pyrrolidone, the amount of sodium lauryl sulfate, the amount of rGO-Fe3O4 composite material, and the amount of N,N'-methylenebisacrylamide are 0.1 g, 0.04 g, 50 mg, and 0.04 g, respectively, and the amount of APS is 0.04 g; in the step 3, the thickness of the hydrogel film is 4 mm.

[0015] As a further solution of the present invention: the core-shell structured hydrogel is a composite structure, including C, O, N, Na, and Fe elements, and is evenly distributed throughout the entire area.

[0016] As a further solution of the present invention: when the content of rGO-Fe3O4 is 50mg, the evaporation rate of the PAM / CMC / PVA / rGO-Fe3O4 evaporator reaches 1.9kg·m -2 ·h -1 , the evaporation efficiency is 80.14%.

[0017] Compared with the existing technology, the method of the present invention uses reduced graphene oxide formed by reducing graphene oxide to replace graphene, which has little effect on the photothermal conversion performance, but greatly reduces the preparation cost. At the same time, the introduction of magnetic ferroferric oxide makes the composite material recyclable. The hydrogel evaporator prepared by the method of the present invention has a flexible PVA shell and a dense PAM / CMC skeleton. It can effectively regulate the internal water state, achieve an intermediate water to free water ratio of 1.60, and significantly reduce the evaporation enthalpy of water from 2442 J·g -1 Reduced to 1450 J·g -1 Under one sun irradiation, the hydrogel showed excellent evaporation performance, with an evaporation rate of 1.90 kg·m -2 h-1, and no salting out occurs in high-salinity environments. Furthermore, the hydrogel can effectively purify fuel-contaminated wastewater, demonstrating excellent water purification capabilities and structural stability. The hydrogel evaporator prepared by the present invention has excellent mechanical properties, can be bent and knotted, is not prone to breakage, and has good structural stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram of the principle of reducing evaporation enthalpy using the PAM / CMC / PVA / rGO-Fe3O4 hydrogel evaporator of the present invention.

[0019] Figure 2 (a) is the overall macroscopic morphology of the PAM / CMC / PVA / rGO-Fe3O4 hydrogel evaporator of the present invention; (b) is a cross-sectional view of the freeze-dried PAM / CMC / PVA / rGO-Fe3O4 hydrogel of the present invention; (c) is a cross-sectional SEM image of the hydrogel of the present invention.

[0020] Figure 3 It is the Fourier transform infrared spectrum of PVA, CMC, PAM and PCPR hydrogel.

[0021] Figure 4 This is the EDS spectrum of PCPR, where (a) is the electron microscope image; (b) is the EDS image; (c) is the carbon element distribution; (d) is the oxygen element distribution; (e) is the nitrogen element distribution; (f) is the sodium element distribution; and (g) is the iron element distribution.

[0022] Figure 5 (a) is the evaporation rate of pure water and various hydrogel samples under light-free conditions; (b) is the calculated results of the equivalent evaporation enthalpy of pure water, PAM, PC, PCP-1, PCP-2, PCP-3 and PCPR; (c)–(f) are the Raman spectral peak separation results of PC, PCP-1, PCP-2 and PCP-3 hydrogel samples, respectively, showing the fitting peaks corresponding to intermediate water (IW) and free water (FW).

[0023] Figure 6 (a) is the ultraviolet-visible-near-infrared (UV–Vis–NIR) absorption spectra of different freeze-dried hydrogel samples, showing the effect of rGO-Fe3O4 addition on light absorption performance; (b) is the surface temperature change curve of pure water and rGO-Fe3O4 hydrogel samples with different mass fractions (PCPR-10, PCPR-30, PCPR-50, PCPR-80) under sunlight; (c) is the mass change curve of the corresponding samples; (d) is the evaporation rate and evaporation efficiency of different samples.

[0024] Figure 7 The results of seawater desalination, durability research and wastewater treatment are shown in Figure 1, where (a) shows the PCPR hydrogel under sunlight (1kW / m 2) is the mass loss and surface temperature change curve after 1 hour of irradiation; (b) is the surface temperature change of PCPR hydrogel under different salt concentrations after 1 hour of solar irradiation; (c) is the evaporation rate and efficiency of PCPR hydrogel in salt water with different salt concentrations after 1 hour of solar irradiation; (d) is the evaporation rate and efficiency of PCPR hydrogel under 1kW / m 2 The evaporation rate and efficiency after 20 cycles of testing under light show its durability. (e) and (f) are the UV-visible absorption spectra of methyl blue (MB) and rhodamine B (RhB) solutions before and after treatment, respectively. The inset in (e) is the MB solution before purification, and the inset in (f) is the RhB solution before purification.

[0025] Figure 8 This is a graph of ambient temperature, relative humidity, and solar radiation intensity during the outdoor evaporation test of PCPR hydrogel. DETAILED DESCRIPTION

[0026] The present invention will be further described below with reference to the accompanying drawings.

[0027] A method for preparing a low-enthalpy, high-resistance saltwater gel evaporator with a core-shell structure, wherein a reduced graphene oxide-ferroferric oxide (rGO-Fe3O4) photothermal conversion material is formed by a solvothermal reduction method of graphene oxide and ferric chloride. The reduced graphene oxide formed by the reduction of graphene oxide instead of graphene has a smaller impact on the photothermal conversion performance, but the preparation cost is greatly reduced. At the same time, the introduction of magnetic ferroferric oxide makes the composite material recyclable. Figure 1 As shown, the hydrogel has a core-shell structure, with a PAM / CMC composite skeleton inside and a PVA hydrogel shell. The rGO-Fe3O4 photothermal conversion material is evenly distributed throughout the structure, forming a PAM / CMC / PVA / rGO-Fe3O4 (abbreviated as PCPR) multi-network hydrogel. Specifically, the following steps are involved:

[0028] Step 1: Preparation of rGO-Fe3O4, including the following steps:

[0029] Firstly, the improved Hummers method was used to oxidize the flake graphite to prepare graphene oxide (GO); then, the reduced graphene oxide-ferroferric oxide (rGO-Fe3O4) nanocomposite was synthesized by a solvothermal method, and the process flow was as follows: 30wt% GO was dispersed in 40 mL ethylene glycol, stirred at 70°C for 1 h and ultrasonically for 1 h to make it fully dispersed; then, 1.35 g FeCl3·6H2O and 2.87 g sodium acetate were added in turn, and each was stirred for 0.5 h to obtain a uniform brownish yellow precursor solution; it was transferred to an 80 mL polytetrafluoroethylene-lined autoclave, and Fe3O4 was grown in situ on the surface of rGO at 200°C for 12 h; after natural cooling, it was washed with deionized water under a magnetic field for several times, and finally dried at 50°C under vacuum for 48 h to obtain rGO-Fe3O4 composite powder.

[0030] Step two: preparation of PAM / CMC / PVA / rGO-Fe3O4 precursor solution, including the following steps:

[0031] Firstly, 0.4 g of sodium carboxymethyl cellulose (CMC) was gradually added to 10 mL of deionized water, and stirred at 40°C for 15 minutes until completely dissolved. Then, 2 g of acrylamide (AM) was added to the solution, and continued to stir for 5 minutes. At the same time, 0.4 g of polyvinyl alcohol (PVA) was added to another 10 mL of deionized water, and stirred at 70°C for 15 minutes until dissolved. The obtained PVA solution was then added to the aforementioned CMC-AM mixed solution (marked as solution A), and stirred thoroughly to obtain a uniform mixture.

[0032] Then, 0.1 g of polyvinylpyrrolidone, 0.04 g of sodium dodecyl sulfate, 50 mg of reduced graphene oxide-ferroferric oxide (rGO-Fe3O4) composite material, and 0.04 g of N,N'-methylenebisacrylamide (MBA) were added in turn, and the system was ensured to be uniform under continuous stirring. Finally, 0.04 g of ammonium persulfate (APS) was added as an initiator, and at this time the preparation of the precursor solution was completed.

[0033] Step three: preparation of PAM / CMC / PVA / rGO-Fe3O4 evaporator, including the following steps:

[0034] The uniformly mixed precursor solution in step two was poured into a culture dish with a diameter of 56 mm to form a hydrogel film with a thickness of about 4 mm. Then, it was preliminarily shaped by liquid nitrogen freezing for 15 min. Then, it was placed in a vacuum oven at 70°C for crosslinking reaction for 2 hours until the hydrogel was completely formed. Finally, the hydrogel sample was immersed in a 3wt% concentration of borax solution for physical crosslinking for 1 h, and finally a hydrogel with a core-shell structure was formed.

[0035] As Figure 2As shown in (a), the overall macroscopic morphology of the PAM / CMC / PVA / rGO-Fe3O4 hydrogel evaporator is disc-shaped. Figure 2 As shown in (b), after freeze-drying and brittle fracture of the hydrogel, a clear layered structure can be seen from the cross section, confirming the formation of a core-shell structure. SEM testing was further used to characterize the microscopic morphology of the hydrogel cross section, as shown in Figure 2 As shown in (c), a dense porous structure can be observed, proving the dense water transport channels inside the hydrogel.

[0036] In order to analyze the chemical composition of the hydrogel, the present invention shows the FTIR spectra of PVA, CMC, PAM, and PCPG. Figure 3 As shown, CMC has a stretching vibration at 3431 cm due to the –OH group. -1 The broad absorption peak at 2914 cm -1 , 1632cm -1 , 1421cm -1 The bands at 1268 cm-1 correspond to the stretching vibration of C–H and the asymmetric and symmetric stretching of COO–, respectively. -1 and 1059cm -1 The peak at 3432 cm is attributed to the stretching vibration of the C–O–C bond, and the peak at 3432 cm -1 The broad absorption peak at 2929cm is the stretching vibration of -OH. -1 , 1447cm -1 , 1096cm -1 The characteristic peak at 3447 cm is attributed to the asymmetric stretching of C–H bonds, bending vibration of –CH2 and stretching vibration of C–O. -1 The broad peak at 2936 cm corresponds to the stretching vibration of NH in -CONH2 in PAM; -1 The absorption peak at 1632 cm corresponds to the stretching vibration of the C-H bond, while the absorption peak at 1632 cm -1 and 1599cm -1 The double absorption peaks at 3348 cm-1 are typical amide I band (C=O stretching vibration) and amide II band (NH bending vibration). The appearance of these characteristic peaks strongly proves the formation of PAM. In the FTIR spectrum of PCPR composite hydrogel, multiple typical absorption peaks were observed, indicating that there is a significant interaction between the components. First, at 3348 cm-1, the peak at 3348 cm-1 is the peak at 3348 cm-1. -1 The broad absorption peaks in the region are attributed to O–H and N–H stretching vibrations, which are direct evidence of the formation of a hydrogen bond network between hydroxyl and amide groups in CMC, PVA, and PAM. The enhancement and slight red shift of this peak indicate the formation of a stable hydrogen bond interaction between the components. -1 The C–H stretching vibration peak at 1680 cm indicates that the PVA and CMC skeleton structures are still retained.-1 C=O stretching vibration (amide I band) and 1616 cm -1 The N–H bending vibration (amide II band) corresponds to the amide group of PAM, proving the existence of PAM, and the shift and absorption spectrum width change are related to the weak coordination or hydrogen bonding after the introduction of rGO. -1 and 1322cm -1 The COO-symmetric and bending vibration peaks appearing nearby are derived from the carboxylate groups in CMC, which complex with the metal ions in Fe3O4 and enhance the network stability of the hydrogel. -1 The C–O–C and C–O stretching vibrations of CMC and PVA are derived from the main chain structure, indicating that the backbone structure remains intact after compounding. -1 The Fe–O characteristic absorption peak appeared in the region, confirming the successful introduction of Fe3O4 nanoparticles and their participation in the construction of the network structure. To further analyze the elemental composition of PCPR hydrogel, energy spectrum analysis (EDS) was used to perform internal element analysis. Figure 4 As shown, the elements C, O, N, Na, and Fe are uniformly distributed throughout the entire region. N, Na, and Fe represent the presence of PAM, CMC, and rGO-Fe₃O₄, respectively, while PVA can be identified by the non-overlapping distribution of C and O with the other three elements. These results are consistent with the elemental composition of the hydrogel and verify its composite structure.

[0037] To verify the effect of PAM-based hydrogels on the evaporation enthalpy of water, the evaporation rates of pure water, PAM, PAM / CMC (PC), PAM / CMC / PVA-x (PCP-x, where x represents the PVA mass fraction of 1wt%, 2wt% and 3wt% respectively) and PAM / CMC / PVA / rGO-Fe3O4 (PCPR, PVA content is 2wt%, rGO-Fe3O4 addition amount is 50mg) were measured under light-free conditions. In the experiment, the evaporator was placed in a container filled with water, which was then placed in a dark environment with a temperature of approximately 25°C and a relative humidity of approximately 40%. The mass loss of the system was recorded within 1 hour, and the equivalent evaporation enthalpy of the evaporator was calculated according to the following formula:

[0038] m0H0=m e H equ (1)

[0039] Among them, m0 (kg·m -2 ·h -1 ) represents the evaporation rate of pure water under no light conditions, H0(J·g -1 ) is the latent heat of vaporization of water under standard conditions, usually taken as 2442 J·g-1 , represents the evaporation rate of the evaporator under no light conditions, H equ It represents the equivalent evaporation enthalpy of the evaporator.

[0040] like Figure 5 As shown in (a), the evaporation rates of the samples are 0.103, 0.135, 0.152, 0.166, 0.175, 0.169 and 0.176 kg·m -2 ·h -1 The enthalpy of evaporation of water at 25°C is 2442 J·g -1 The calculated equivalent evaporation enthalpies of PAM, PC, PCP-1, PCP-2, PCP-3, and PCPR are approximately 1893, 1682, 1541, 1462, 1508, and 1450 J·g, respectively. -1 ,like Figure 5 (b) shown.

[0041] To further explore the effect of polyvinyl alcohol (PVA) content on the IW ratio in hydrogels, 1wt%, 2wt% and 3wt% PVA were added to the PAM / CMC system. The ratio of free water (FW) to intermediate water (IW) in each sample was calculated by Gaussian fitting and peak separation of the Raman spectra. Figure 5 As shown in (c) and (d), the introduction of PVA significantly increased the IW / FW ratio in the hydrogel, increasing the IW:FW ratio from 0.72 before the introduction of PVA to 1.55. Since IW has a lower evaporation enthalpy, this result shows that the addition of PVA helps to achieve efficient water evaporation. As the proportion of PVA added increases, the IW:FW value shows a trend of first increasing and then decreasing, as shown in Figure 5 As shown in (e) and (f), the 2 wt% PVA sample exhibits the lowest evaporation enthalpy value.

[0042] Figure 6 (a) Ultraviolet-visible-near-infrared (UV–Vis–NIR) absorption spectra of different freeze-dried hydrogel samples, demonstrating the effect of rGO-Fe₃O₄ addition on light absorption properties. To further investigate the effect of varying rGO-Fe₃O₄ photothermal conversion material content on evaporation rate and efficiency, PCPR-x hydrogels with varying rGO-Fe₃O₄ contents (where x represents the mass of rGO-Fe₃O₄, 10, 30, 50, and 80 mg, respectively) were prepared. All other process conditions were identical to those in steps 2 and 3. Figure 6(b), (c) and (d) respectively show the surface temperature, evaporation mass and evaporation efficiency of the hydrogel under different rGO-Fe3O4 contents. It can be found that the increase of rGO-Fe3O4 content leads to higher light absorption intensity, resulting in the increase of the surface temperature of the evaporator. However, the evaporation rate of the evaporator shows a trend of first increasing and then decreasing, and when the content of rGO-Fe3O4 is 50 mg, the evaporation rate of the evaporator reaches 1.9 kg·m -2 ·h -1 under a certain intensity of sunlight, and the evaporation efficiency is 80.14%. Although more capture of light can provide more energy for water evaporation, thereby increasing the water evaporation rate. However, when the surface temperature of the evaporator is too high, the water supply rate does not match the too fast evaporation rate, resulting in insufficient water storage on the surface of the evaporator, and the heat generated by the photothermal material is used for invalid heating, thereby causing the water evaporation rate to decrease. Therefore, when the content of rGO-Fe3O4 is 50 mg, the evaporation performance of the evaporator is optimal.

[0043] In view of the excellent photothermal evaporation performance of PCPR-50 hydrogel shown in the previous experiment, the application further applies it to the desalination experiment of seawater and different salt concentration brine driven by sunlight. The seawater is collected from the Yellow Sea of China, and the salt concentration of the synthetic brine is set to be 5wt%, 7.5wt% and 10wt% respectively. Figure 7 (a) is that the PCPR-50 hydrogel can reach thermal equilibrium on the surface within 30 minutes of sunlight irradiation under all test conditions, and the surface temperature fluctuates little. Figure 7 (b) and (c) are the evaporation mass change diagram and the column chart of evaporation rate and evaporation efficiency of PCPR-50 hydrogel under the light irradiation condition of 1 sun intensity. The evaporation rates of PCPR-50 hydrogel in 2.5wt%, 5wt%, 7.5wt%, 10wt% brine are 1.82 kg·m -2 ·h -1 , 1.78 kg·m -2 ·h -1 , 1.61 kg·m -2 ·h -1 and 1.52 kg·m -2 ·h -1 respectively. During the whole experiment, there is no obvious salt crystallization on the surface of the evaporator, indicating that PCPR can effectively inhibit the performance decline caused by salt accumulation. Further, the durability of the evaporator in seawater is tested, as shown in Figure 7 (d), the results show that the PCPR-50 hydrogel evaporator always maintains a high stability of evaporation rate in 20 continuous cycles, and the maximum standard deviation is controlled within ±1.9%. Such excellent stability is attributed to the excellent physical performance of the PCP substrate.

[0044] Furthermore, methylene blue (MB, 100 mg·L -1 ) and rhodamine B (RhB, 100 mg·L -1 ) Two organic dyes were used as pollutant models to test the wastewater purification ability of the evaporator. The UV-visible absorption spectra before and after purification are shown in Figure 2. Figure 7 As shown in (e) and (f), the MB solution and RhB solution before purification have strong absorption peaks at 667nm and 553nm, respectively. After purification, the absorption peaks of the above solutions disappear. Figure 7 The illustrations in (e) and (f) are photographic comparisons of MB solution and RhB solution before and after purification, respectively. The colorless and transparent one is the purified solution; this fully proves that the condensed water obtained by the evaporator does not contain organic molecules, thus achieving wastewater purification.

[0045] Furthermore, after a comprehensive evaluation of the evaporation performance, an outdoor experiment was conducted to verify the practical application potential of the evaporator. The experiment was carried out using a special device. During the experiment, the solar radiation intensity, ambient temperature and relative humidity were monitored in real time from 8:30 am to 5:30 pm. The relevant data are as follows: Figure 8 As shown. The solar radiation intensity is between 38 and 83.1 mW·cm -2 The ambient temperature fluctuated between 26.8°C in the morning and 36.1°C at its highest, and the relative humidity varied between 28.6% and 45.1%. In the outdoor evaporation experiment, two small PCPR-50 evaporators collected a total of 31.8g of condensed water, with a theoretical water production of 11.83kg·m -2 , enough to meet the daily drinking water needs of about five adults.

[0046] The present invention effectively regulates the internal water state through the flexible PVA shell and the dense PAM / CMC skeleton, achieves an intermediate water to free water ratio of 1.60, and significantly reduces the evaporation enthalpy of water from 2442 J·g -1 Reduced to 1450 J·g -1 Under 1 sun illumination, the hydrogel showed excellent evaporation performance, with an evaporation rate of 1.90 kg·m -2 ·h-1, and can also maintain 1.82kg·m-1 in real seawater -2 h-1, and exhibited no salt precipitation in high-salinity environments. Furthermore, the hydrogel was able to effectively purify dye-contaminated wastewater, demonstrating excellent water purification capabilities and structural stability. These results suggest that this hydrogel evaporator has broad application prospects in practical applications such as seawater desalination and wastewater treatment.

Claims

1. A method for preparing a hydrogel evaporator with a core-shell structure, characterized in that: Reduced graphene oxide-ferroferric oxide (rGO-Fe3O4) photothermal conversion material is formed by solvothermal reduction of graphene oxide and ferric chloride in one step. The formed hydrogel has a core-shell structure, with a PAM / CMC composite skeleton inside and a PVA hydrogel outer shell. The rGO-Fe3O4 photothermal conversion material is evenly distributed in the overall structure, forming a PAM / CMC / PVA / rGO-Fe3O4 (abbreviated as PCPR) multi-network hydrogel.

2. The preparation method according to claim 1, characterized in that The preparation method comprises the following steps: Step 1: Preparation of rGO-Fe3O4, including the following steps: The rGO-Fe3O4 nanocomposite was synthesized and reduced by a solvothermal method. The process flow is as follows: 30 wt% GO was dispersed in ethylene glycol, stirred at 70°C for 1 hour and ultrasonicated for 1 hour to fully disperse it; FeCl3·6H2O and sodium acetate were then added in sequence, each stirring for 0.5 hour to obtain a uniform brown-yellow precursor solution; the precursor solution was transferred to a polytetrafluoroethylene-lined hydrothermal reactor and reacted at 200°C for 12 hours to allow Fe3O4 to grow in situ on the rGO surface; after natural cooling, the solution was washed multiple times with deionized water under a magnetic field and finally dried in a vacuum at 50°C for 48 hours to obtain rGO-Fe3O4 composite powder; Step 2: Preparation of PAM / CMC / PVA / rGO-Fe3O4 precursor solution, including the following steps: First, sodium carboxymethyl cellulose was gradually added to deionized water and stirred at 40°C for 15 minutes until completely dissolved. Subsequently, acrylamide was added to the solution and stirred for another 5 minutes. At the same time, PVA was added to another portion of deionized water and stirred at 70°C for 15 minutes until dissolved. The resulting PVA solution was then added to the aforementioned CMC-AM mixed solution and stirred thoroughly to obtain a uniform mixture. Then, polyvinyl pyrrolidone, sodium lauryl sulfate, rGO-Fe3O4 composite material and N,N'-methylenebisacrylamide were added in sequence, and the uniformity of the system was ensured under continuous stirring. Finally, APS was added as an initiator, and the precursor solution was prepared. Step 3: Preparation of PAM / CMC / PVA / rGO-Fe3O4 evaporator, including the following steps: The evenly mixed precursor solution from step 2 was poured into a culture dish to form a hydrogel film with a certain thickness; subsequently, it was frozen with liquid nitrogen for 15 minutes for preliminary shaping; then, it was placed in a 70°C vacuum oven for cross-linking reaction for 2 hours until the hydrogel was completely formed; finally, the hydrogel sample was immersed in a 3wt% concentration of borax solution for physical cross-linking for 1 hour, eventually forming a core-shell structure hydrogel.

3. The preparation method according to claim 2, characterized in that In the step 1, ethylene glycol is 40 mL, FeCl3·6H2O and sodium acetate are 1.35 g and 2.87 g respectively, and polytetrafluoroethylene is 80 mL; in the step 2, sodium carboxymethyl cellulose is 0.4 g, deionized water is 10 mL, acrylamide is 2 g, PVA is 0.4 g, polyvinyl pyrrolidone, sodium lauryl sulfate, rGO-Fe3O4 composite material and N,N'-methylenebisacrylamide are 0.1 g, 0.04 g, 50 mg and 0.04 g respectively, and APS is 0.04 g; in the step 3, the thickness of the hydrogel film is 4 mm.

4. The preparation method according to claim 2, characterized in that The core-shell hydrogel is a composite structure, including C, O, N, Na, and Fe elements, which are uniformly distributed throughout the entire area.

5. The preparation method according to claim 2, characterized in that The PAM / CMC / PVA / rGO-Fe3O4 evaporator, when the content of rGO-Fe3O4 is 50 mg, has an evaporation rate of 1.9 kg·m under one sun intensity. -2 ·h -1 , the evaporation efficiency is 80.14%.

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