Qingmio Wng ,Yi Qin ,Feifei Ji ,c,*,Shoxin Song ,c,Ynmei Li
a Hubei Key Laboratory of Mineral Resources Processing and Environment,Wuhan University of Technology,Luoshi Road 122,Wuhan,Hubei Province,430070,China
b Department of Mines,Metallurgy and Geology Engineering,University of Guanajuato,Av.Benito Ju′arez 77,Zona Centro,Guanajuato,36000,Mexico
c School of Resources and Environmental Engineering,Wuhan University of Technology,Luoshi Road 122,Wuhan,Hubei Province,430070,China
Abstract Volumetric solar evaporations by using light-absorbing nanoparticles suspended in liquids (nanofluids) as solar absorbers have been widely regarded as one of the promising solutions for clean water production because of its high efficiency and low capital cost compared to traditional solar distillation systems.Nevertheless,previous solar evaporation systems usually required highly concentrated solar irradiation and high capital cost,limiting the practical application on a large scale.Herein,for the first time in this work,polydopamine(PDA)-capped nano Fe3O4(Fe3O4@PDA) nanofluids were used as solar absorbers in a volumetric system for solar evaporation.The introduction of organic PDA to nano Fe3O4 highly contributed to the high light-absorbing capacity of over 85%in wide ranges of 200–2400 nm because of the existence of numerous carbon bonds and pi(π)bonds in PDA.As a result,high evaporation efficiency of 69.93%under low irradiation of 1.0 kW m-2 was achieved.Compared to other nanofluids,Fe3O4@PDA nanofluids also provided an advantage in high unit evaporation rates.Moreover,Fe3O4@PDA nanofluids showed excellent reusability and recyclability owing to the preassembled nano Fe3O4,which significantly reduced the material consumptions.These results demonstrated that the Fe3O4@PDA nanofluids held great promising application in highly efficient solar evaporation.
Keywords: Fe3O4@Polydopamine;Nanofluids;Volumetric solar evaporation;Recyclability
To deal with the global freshwater scarcity,numerous technologies such as multiple stage flash (MSF) ,multiple-effect distillation (MED) ,reverse osmosis (RO),capacitive deionization (CDI) [4,5] and adsorption [6,7],etc.have been applied to produce clean water from seawater/brackish water or polluted water.However,these conventional methods usually face the same shortages of high energy consumption and high capital cost,which might fail to meet the growing demand for clean freshwater,particularly in underdeveloped countries and areas,resulting in the urgent development for novel desalination technology.Recently,by the utilization of the inexhaustible solar energy,solar evaporation has been widely regarded as one of the new approaches to generate water vapor and produce freshwater [9–11].In general,solar evaporation systems mainly consist of the volumetric evaporation system,the interfacial evaporation system,and the isolation system[12,13].Generally,the interfacial solar evaporation system is an effective strategy that can localize the solar heat and steam generation at the water–air interface using a nonsubmersible solar absorber [14,15],while the volumetric evaporation is a straightforward strategy for photothermal evaporation by direct nanostructure solar energy absorption.Among these three systems,the volumetric evaporation system with the help of nanofluids turned out to be one of the efficient and promising designs because of its highefficiency and low-cost existing systems [16,17].For instance,by using Au nanofluids to directly generate water vapors,the device efficiency of 24% was achieved under highly concentrated solar irradiation .Evaporation efficiency of 69% was reached under the solar power of 10 sun(1 sun=1 kW m-2) by using graphitized carbon blackbased nanofluids as the solar receiver .Under highintensity lasers of 106W m-2,it was reported that light trapping-induced localized heating of nanoparticles in aqueous solutions mainly accounted for the low-temperature light-induced steam generation,which was also consistent with classical heat transfer .
Based on the previous studies,some progress still should be made in volumetric solar evaporation:(1) solving the poor recoverability of nanomaterials derived from their micro sizes to achieve green production and cut down the capital cost,(2) increasing the solar evaporation efficiency to enable this method more competitive with other steam generation techniques,and (3) developing novel materials or system designs that can efficiently utilize lower solar power(≤3.0 kW m-2) than previous work to further minimize operation cost and achieve its commercial application.
Herein,nanofluids containing PDA-capped Fe3O4(Fe3O4@PDA)nanospheres were used as photothermal materials for solar evaporation for the first time.Fe3O4@PDA nanospheres were synthesized by controlling the self-polymerization time of dopamine monomers on the outer surfaces of Fe3O4nanospheres.With the nano Fe3O4serving as the core,the solar absorbers could be readily separated from aqueous solutions by an external magnet,laying the foundation for their excellent recyclability.With the incorporation of hydrophilic PDA shell,Fe3O4@PDA nanospheres exhibited excellent colloidal stability and high solar absorption capacity,leading to the achievement of high evaporation efficiencies of 69.93%–85.47%in the volumetric system under low solar densities of 1–3.0 kW m-2.Moreover,the concept of unit evaporation rate (UER) was used as a comprehensive indicator to further evaluate the evaporation performance for the first time.Fe3O4@PDA nanofluids outperformed most of the advanced nanofluids in terms of unit evaporation rates,demonstrating that they were excellent and cost-effective solar absorbers for highly efficient volumetric solar evaporation.This work demonstrated a nanofluid with high economical effectiveness and high evaporation performance for clean water production,which might contribute to mitigating the terrible global water scarcity.
Ethanol (C2H6O),tris(hydroxymethyl)aminomethane(C4H11NO3,Tris),Dopamine hydrochloride (C8H11NO2∙HCl,DA) was obtained from Shanghai Aladdin Bio-Chem Technology Co.,Ltd.FeCl3∙6H2O,FeSO4∙7H2O,citric acid monohydrate and NH4OH solution (25%) were originated from Sinopharm Chemical Reagent Co.,Ltd (Shanghai).Hydrochloric acid(HCl)was purchased from Merck Pty.Ltd.All chemicals were of analytical purity and used without further purification.Ultra-pure water (18.2 MΩ cm) originated from Milli-Q instrument(Millipore Corporation)was used in all the experiments.
2.2.Preparation of Fe3O4@polydopamine (PDA)nanocomposites
Firstly,Fe3O4nanospheres were prepared by a modified method based on the reported work :200 mL of the solution containing 6.0 g of FeCl3∙6H2O and 3.1 g of FeS-O4∙7H2O was mechanically stirred at 400 r min-1for 10 min.After the rapid addition of 20 mL of NH4OH (25%) solution,7.0 g of citric acid monohydrate was added into the mixture and then stirred for another 1.0 h.Afterwards,the resultant black product was repeatedly washed with deionized water with the help of a magnet for at least six times and naturally dried at ambient temperature.Secondly,0.1 g of the resultant Fe3O4was added into 200 mL,10 mmol L-1Tris buffer and sonicated at 150 W for 10 min.Subsequently,0.4 g of DA was added into the solution and then mechanically shaken at 250 r min-1for 4 h at room temperature.Thirdly,the final product was washed with deionized water for at least six times with the help of a magnet and dried at 60°C in a drying oven.In comparison,the products originated from the mixture suffered the mechanical shaking for 8,12,24 at room temperature were also prepared in the similar way.
The structure information was recorded on the X-ray diffraction spectroscope (XRD,PIXcel-Empyrean).The morphology and chemical composition of samples were obtained from the scanning electron microscope(SEM,Hitachi S-4700) and the transmission electron microscope (TEM,Jeol 2100F).The magnetic property of the samples was measured with a VSM(PPMS-9T,Quantum Design).The Fourier transform infrared(FT-IR)spectra were examined using the Fourier transform infrared spectrometer (Nicolet6700,Thermo Scientific) The solar absorption capacity was measured by an ultraviolet-visible-near infrared diffuser spectrophotometer(UV–V-NIR,LAMBDA950).Thermalgravimetric analysis(TGA)curves were recorded on the thermal analyzer(TGA-50/50H)with a heating rate of 10°C min-1in air atmospheres.
2.4.Solar evaporation test
The key experimental set-up used in this work for the solar evaporation was displayed in Fig.S1.The critical components included the solar simulator (Perfectsolar M300),the volumetric solar receiver(acrylic beaker),and the electric balance(Shanghai Yueping,YP6002,accuracy:0.01 g) which was connected with a computer for the online measurements of the water mass changes.The designed solar receiver consisted of an inner acrylic beaker (Inner diameter:48 mm,height:30 mm,thickness:3 mm),an outside acrylic beaker (Inner diameter:70 mm,height:40 mm,thickness:3 mm) and conical cap (diameter:64 mm,cone angle:120°).The inner acrylic beaker contained a constant volume (～40 mL) of nanofluids for solar evaporation.The probe of an electronic temperature logger (Jingchuang GSP-6),which had been connected with the computer,was inserted into the center of the inner acrylic beaker with underwater depth of～10 mm to record the temperature vibration of the nanofluids.The height of nanofluids was kept at 20 mm in every single experiment by the peristaltic pump to obtain the fast temperature rises.All experiments of solar evaporation had been finished in Wuhan city,Hubei province,China from March 05,2019,to May 25,2019,during which the ambient temperature had been kept at 22±3°C by the air conditioner and the ambient humidity had been 30 ± 5%.
3.Results and discussion
The XRD patterns of the as-synthesized materials were shown in Fig.1(a).The characteristic peaks at 30.1°,43.1°,54.4°,57.2°,and 62.6°were indexed and assigned to the(220)(400) (422) (511) and (440) planes of the cubic lattice of Fe3O4,respectively,according to a standard pattern JCPDS No.89–0691 .The XRD patterns of both stand-alone Fe3O4and Fe3O4@PDA showed similar characteristics,demonstrating the unchanged crystalline phase of Fe3O4after the coating of PDA shell.Fig.1(b) displayed the thermogravimetric curve of Fe3O4@PDA,which was recorded from 30°C to 1000°C in the air atmosphere.The first-stage weight loss of Fe3O4@PDA nanocomposites from room temperature to 150°C could be ascribed to the separation of absorbed water molecules from the nanocomposites.Furthermore,the second-stage weight loss of Fe3O4@PDA nanocomposites occurred from 100°C to 450°C was derived from the decomposition of PDA polymers,while the final stage of weight loss occurred from 450°C to 600°C was because of the removal of impurities .The final total weight loss of the Fe3O4@PDA nanocomposites was around 31.5%,indicating that the mass of Fe3O4comprised around two-third of the Fe3O4@PDA nanocomposites.Such a high content of Fe3O4contributed to the easy separation of the nanocomposites from aqueous solutions by an external magnet.The FTIR spectra of stand-alone Fe3O4and Fe3O4@PDA nanocomposites were displayed in Fig.1(c).The peaks at 3423 cm-11626 cm-1were attributed to the stretching vibrations of hydroxyl groups (-OH) or/and water molecules on the materials’ surfaces .Moreover,the strong peaks at 567 cm-1were originated from the characteristic vibrations of Fe–O .Compared to stand-alone Fe3O4,the newly emerging peaks at 1488 cm-1and 1302 cm-1of Fe3O4@PDA were assigned to the vibrations of C=C and amine groups of the PDA,respectively,demonstrating the successful introduction of PDA onto the surface of Fe3O4nanoparticles.Fe3O4@PDA nanospheres exhibited excellent colloidal stability and dispersion property in aqueous solutions since only an ignorable decrease in concentration (8%) of the nanofluids was found even stound still for 7 days (Fig.S2,Supporting Information).This was because that the introduced PDA with abundant functional groups could highly contribute to the colloidal stability.Note that the concentration of Fe3O4@PDA dispersions was determined based on the Beer–Lambert law,which declared that the absorbance of a dilute solution was proportional to its solute concentration.In addition,the introduction of organic PDA might also contribute to the absorption of optical energy and the conversion of solar energy to heat through lattice vibration because of the equivalent introduction of numerous carbon bonds and pi(π)bonds.
To further confirm the structures,the morphology of the samples was further determined.Fig.1(d) shows the TEM image of stand-alone Fe3O4,which demonstrated that the nano Fe3O4all were in sphere-like architectures with a narrow diameter distribution of～20 nm.After the coating of PDA shell,the nanocomposites remained the nanosphere-shapes according to the SEM images as given in Fig.1(e).To obtain a more specific and clearer morphonology of the nanocomposites,the high-resolution TEM images were displayed in Fig.1(f).The spherical particles showed the lattice fringe spacing of 0.25 nm,which was in accordance with the (311) plane of Fe3O4.Furthermore,the average diameters of the core Fe3O4of the nanocomposites were further proved to be around 20 nm,and the out-shell PDA layer was around 10 nm.Such tiny particle sizes of the core–shell nanocomposites ensured the nanocomposites be well dispersed in water solutions.
It was known that the polymerization time played one of the most critical roles in controlling the formation of the PDA shell on substrates .In this case,the effect of different polymerization time of Fe3O4@PDA nanocomposites on their magnetic and optical properties,which were two of the most significant properties of Fe3O4@PDA nanocomposites serving as volumetric solar absorbers,were also investigated.Displayed in Fig.2(a)were the magnetic hysteresis loops of both stand-alone nano Fe3O4and various Fe3O4@PDA nanocomposites at room temperature.Accordingly,the saturation magnetizations of various Fe3O4@PDA nanocomposites ranged from 42.20 emu g-1to 54.57 emu g-1.Although a reduced saturation magnetizations were found in Fe3O4@PDA nanocomposites compared to the stand-alone Fe3O4(60.02 emu g-1),such saturation magnetizations of Fe3O4@PDA nanocomposites were still larger than that of most reported magnetic composites (28.7–43.0 emu g-1)[20,27,28],meeting the needs of recyclability in practical application.In addition,no hysteresis loops of Fe3O4@PDA nanocomposites were found,suggesting its superparamagnetism .Both the high magnetization and the superparamagnetic property of Fe3O4@PDA nanocomposites were beneficial for their fast recyclability by an external magnetic field(Set example of Fe3O4@PDA(4 h)in Video S1,Supporting Information).
Fig.1.(a) XRD pattern of stand-alone Fe3O4 and Fe3O4@PDA nanocomposites.(b) TGA curve of Fe3O4@PDA nanocomposites in air atmosphere.(c) FTIR of stand-alone Fe3O4 and Fe3O4@PDA nanocomposites.(d) TEM image of stand-alone Fe3O4 nanospheres.(e) The SEM images and (f) the high-resolution TEM images of Fe3O4@PDA nanocomposites.
Fig.2.(a)Magnetic property of the stand-alone Fe3O4 and various Fe3O4@PDA nanospheres.(b)UV-Vis-NIR spectroscopy of stand-alone Fe3O4 and various Fe3O4@PDA nanospheres.(c) UV–Vis-NIR spectra of Fe3O4@PDA nanofluids at different concentrations.(d) Absorbance of Fe3O4@PDA nanofluids at five different wavelengths (260 nm (UV),500 nm (Vis),850 nm,1400 and 1900 nm (NIR).
Supplementary data related to this article can be found at https://doi.org/10.1016/j.gee.2020.07.020.
Furthermore,the UV-Vis-NIR spectra of the various samples were shown in Fig.2(b)to investigate their light-absorbing capability.It was found that Fe3O4@PDA nanocomposites showed great enhancement in light-absorbing capability compared to stand-alone Fe3O4nanospheres.This was mainly because the outside PDA shell was conductive polymer,which would be polarized under electromagnetic fields when exposed to external solar radiation,resulting in the additional electronic oscillations .In addition,the light-absorbing capability of Fe3O4@PDA nanocomposites at 808 nm increased with the increase of polymerization time(namely the increasing thickness of PDA shell),which was consistent with the results in the previous work[30,31].However,the total light-absorbing capability of various Fe3O4@PDA nanocomposites came to a similar level.Moreover,the high light-absorbing capability that more than 85% of the solar power in wideband of 200–2400 nm was absorbed by the Fe3O4@PDA nanocomposites,laying the foundation for the high solar conversion property of the nanofluids.Note that the solar energy absorbed by Fe3O4@PDA was converted into the thermal energy throughout its vibrational modes of the phonons and the atomic lattices,and subsequently the thermal energy was diffused into the surrounding medium,resulting in the macroscopic temperature rise of the nanofluids.Considering the magnetic property and light absorption capacity,4 h turned out to be the best the polymerization time for synthesizing the Fe3O4@PDA nanocomposites with well both photothermal conversion and separation performance.Moreover,the UV–Vis-NIR spectra of Fe3O4@PDA(4 h)nanofluids with different concentrations were further determined,and the results were displayed in Fig.2(c)and(d).In the UV wavelength range(200–400 nm),Fe3O4@PDA nanofluids with different concentrations exhibited similar absorbances of light.Within the wavelengths of visible light (400–780 nm) and short-wave infrared light (780–1100 nm),higher concentration of nanofluids contributed to the higher solar absorbance.And in the UV wavelengths of long-wave infrared light (1100–2200 nm),nanofluids with concentrations of 0.5,1.0,1.5 and 2.0 g L-1exhibited similar solar absorbances which much outperformed that of 0.2 g L-1.
3.2.Solar evaporation performance
As shown in Fig.3(a),the mass changes of the produced water over time under illumination of 1.0 kW m-2by using the Fe3O4@PDA nanofluids with different concentrations as the solar absorbers were recorded.The mass change increased with the increase of solar absorbers’ mass concentration from 0 to 0.5 g L-1,and came to the plateau from 0.5 g L-1to 2.0 g L-1.The IR images were also used to measure the final stable temperature of Fe3O4@PDA nanofluids in the top section.As shown in Fig.3(b),if the concentration of Fe3O4@PDA nanofluids increased from 0 to 0.5 g L-1,the final stable temperature in top section of nanofluids increased visibly from 37.0°C to 47.0°C,while the concentration increased from 0.5 to 2.0 g L-1the final stable temperature merely increased from 47.0°C to 47.6°C.The evaporation efficiency (η) is further used to evaluate the evaporation performance,and it is defined as:
Based on the mass change over illumination time as given in Fig.3(a),the evaporation efficiencies and evaporating rates were displayed in Fig.3(c).Under the same solar illumination of 1.0 kW m-2,with the concentration of Fe3O4@PDA nanofluids increased from 0 to 2.0 g L-1,the evaporation rates increased from 0.104 kg m-2h-1to 1.002 kg m-2h-1and leveled off at～1.05 kg m-2h-1,while the evaporation efficiency increased from 7.2% to 69.93% and reached a plateau of～71%.Namely,within a certain concentration range,high Fe3O4@PDA concentrations resulted in a high solar absorption as aforementioned in Fig.2(c) and (d),which then resulted in a high evaporation efficiency.Beyond that range,nanofluids with higher concentration would lead to the easier agglomeration and precipitation of Fe3O4@PDA nanospheres and more inaccessible of sufficient light to the deep fluids,and then the maximum and stable evaporation rates and evaporation efficiency were reached.Thereby,nanofluids containing 0.5 g L-1of Fe3O4@PDA were selected as the working concentration in order to obtain the best evaporation performance with the least consumption of solar absorbers.
Effective solar density also played an important role in affecting the temperature of the working fluid.In this case,the mass changes of water over illumination time under various illumination were displayed in Fig.3(d),based on which the corresponding evaporation rates and evaporation efficiencies were displayed in Fig.3(f).Also,the temperature distribution of nanofluids in the top section under different solar irradiation were recorded by the IR camera were displayed in Fig.3(e).The temperatures increased by Fe3O4@PDA nanofluids could reach from 47.0°C under 1.0 kW m-2to 52.8°C under 3.0 kW m-2,much higher than that caused natural evaporation of the nanofluids.With the solar power increasing from 1.0 kW m-2to 3.0 kW m-2,the evaporation rates increased from 1.002 kg m-2h-1at 1.0 kW m-2to 3.652 kg m-2h-1at 3.0 kW m-2,and the evaporation efficiencies increased from～69.93% at 1.0 kW m-2to 85.47% at 3.0 kW m-2.
Fig.3.(a)Mass change over time for different concentrations of Fe3O4@PDA under illumination of 1.0 kW m-2.(b)IR images of stable temperature distributions in top section of the solar receivers at irradiation time of 50 min.(c)The corresponding evaporation rates and evaporation efficiency.(d)Mass change over time for 0.5 g L-1 Fe3O4@PDA under various illumination of 0,1.0,1.5,2.0 and 3.0 kW m-2.(e)IR images of stable temperature distributions in top section of the solar receivers at irradiation time of 50 min.(f) The corresponding evaporation rates and evaporation efficiencies.
If the difference of the designed evaporation systems were ignored,two factors influenced the evaluation of the evaporation performance of the various nanofluids:the concentration of the used nanofluids and the incident light density.In general,less concentration of the solar absorbers in nanofluids and less incident light density used in a volumetric evaporation process meant the less operation and capital cost of such materials.In this case,a comprehensive indicator-unit evaporation rate (UER,defined by Eq.(2)),which means the evaporation rate per unit mass per unit incident light density,was used for the first time to evaluate the evaporation performance of different nanofluids.
whereUERdenotes the unit evaporation rate,m3(kW h)-1,andCis the concentration of nanofluids used for evaporation,g L-1or kg m-3,respectively.Thus,such a factor can be directly used to assess the steam production performance of different nanofluids.As given in Table 1,Fe3O4@PDA nanofluids and numerous advanced materials such as reduced graphene oxide (rGO)-based and carbon nanotubes (CNT)-based nanofluids were assessed.It could be concluded that:(1)Fe3O4@PDA nanofluids outperformed most of the advanced nanofluids in terms of unit evaporation rates.(2) higher solar irradiation contributed to the higher unit evaporation rates of nanofluids.Namely,although the work mainly paid attention to a low solar density range of 1.0–3.0 kW m-2,Fe3O4@PDA nanofluids provided advantages in both evaporation performance and capital cost compared to the reported advanced solar absorbers.These results demonstrated Fe3O4@PDA nanofluids an excellent solar absorber for high-performance volumetric solar evaporation.
Table 1 Comparison of various solar absorbers in the volumetric systems.
Fig.4.(a) The maximum,minimum,and average weight change as a function of illumination time using 0.5 g L-1 Fe3O4@PDA nanofluids under solar illumination of 1.0 kW m-2.(b) The total weight change as a function of cycle numbers during the illumination time of 20–60 min in each cycle (Average value:0.673 kg m-2).
To investigate the reusability,the evaporation experiment was cycled 15 times under the same conditions.In each cycle,the evaporation rate of Fe3O4@PDA nanofluids was Fe3O4@PDA nanofluids (0.5 g L-1) was illuminated under the solar density of 1.0 kW m-2for 60 min.Afterwards,Fe3O4@PDA nanofluids were recollected by a permanent magnet (PC-0127,N33 nickel plated NdFeB)for the next cycle.The maximum,minimum,and average weight changes of water as function of illumination time were displayed in Fig.4(a).Furthermore,the weight change of evaporated water during 30–60 min versus cycle number was displayed in Fig.4(b).No obvious decrease of weight change was found even after 15 successive cycles of reuse,demonstrating the excellent reusability of Fe3O4@PDA nanofluids as solar absorbers for volumetric solar evaporation.
The conclusions of this work mainly lied in:
(1) The PDA-capped Fe3O4hybrid nanocomposites were successfully fabricated by the facile solvothermal process.
(2) 4 h turned out to be the best polymerization time for synthesizing the Fe3O4@PDA nanocomposites with excellent colloidal stability,easy separation performance and light absorption capacity.
(3) Owing to the high light-absorbing capacity,Fe3O4@PDA nanofluids serving as solar absorbers provided a high evaporation efficiency of 69.93% and a high unit evaporation rate of 2.004 m3(kW h)-1under low irradiation of 1 kW m-2,which outstood most of the recent advanced nanomaterials.
Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No.51704220,No.51974216 and No.51674183) for this work.Wang gratefully acknowledges the Consejo Nacional de Ciencia y Tecnología of Mexico for offering him the scholarship under the grant No.868976 during his Ph.D.studies.
Appendix A.Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2020.07.020.
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