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Superhydrophilic and highly elastic monolithic sponge for efficient solar-driven radioactive wastewater treatment under one sun
Journal of Hazardous Materials 392 (2020) 122350
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Superhydrophilic and highly elastic monolithic sponge for efficient solardriven radioactive wastewater treatment under one sun
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Kaifu Yua, Pengfei Shaoa, Pengwei Menga, Tao Chena, Jia Leia, Xiaofang Yub, Rong Hea, Fan Yanga, Wenkun Zhua,*, Tao Duana,* a Nuclear Waste and Environmental Safety Key Laboratory of Defense, State Key Laboratory of Environment-friendly Energy Materials, National Co-innovation Center for Nuclear Waste Disposal and Environmental Safety, Sichuan Civil-military Integration Institute, School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang 621010, China b Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics, Mianyang 621900, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Danmeng Shuai
As an effective way to obtain solar energy and separate the soluble contaminants from water, solar-driven interfacial evaporation is used in desalination, wastewater treatment, electricity generation, and domestic water heating system. Herein, we demonstrate a monolithic sponge with three-dimensional porous structure as the solar-energy evaporator, which is composed of hydrophilic polymer (Konjac Glucomannan, KGM) and solar absorbent (reduced graphene oxide, rGO). Under one sun irradiation, the sponge achieves a rapid evaporation rate (1.60 kg m−2 h−1) and high interfacial water evaporation efficiency (92 %) due to its good absorption, photothermal, thermal insulation, and fast water transport properties. Meanwhile, the concentrations of radioactive elements (strontium, cesium, and uranium) in wastewater dropped from grams to micrograms after purification, even under radiation and acidic conditions. Additionally, the durability and repeatability of the sponge also have been verified. The results showed that solar-driven interfacial evaporation can effectively treat radioactive wastewater and enrich various radionuclides in a more energy-saving manner.
Keywords: Solar-driven interfacial evaporation Radioactive wastewater treatment Konjac Glucomannan Reduced graphene oxide
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Corresponding authors. E-mail addresses: [email protected] (W. Zhu), [email protected] (T. Duan).
https://doi.org/10.1016/j.jhazmat.2020.122350 Received 13 December 2019; Received in revised form 16 February 2020; Accepted 17 February 2020 Available online 19 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 392 (2020) 122350
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1. Introduction
prepared by Konjac Glucomannan (KGM) and reduced graphene oxide (rGO) composite. As a natural and renewable polymer, KGM contains a large number of hydroxyl groups, which provides hydrophilicity for the sponges (Zhu et al., 2018b). rGO, a widely used absorbent, enables the sponge to exhibit a broadband absorption over the full solar spectrum (Zhou et al., 2018). In addition, rGO interacts with KGM in a threedimensional network formed by freeze-drying, resulting in a black KGM/rGO sponge (KGS) with unique properties (Chen et al., 2018). Under one sun illumination, KGS achieved an evaporation rate of 1.60 kg m−2 h−1 and a photothermal efficiency up to 92 %. In addition, the concentrations of radioactive elements in wastewater dropped from grams to micrograms after purification, even after irradiation and 20 times cycle. It is revealed that the KGS possesses potent solar-driven interfacial evaporation capability, demonstrating excellent irradiation stability, photothermal conversion stability, and durability. And the KGS is expected to become a powerful solar-driven radioactive wastewater treatment material in practical application.
With the application of nuclear technology, radioactive wastewater is increasingly produced in various ways, including the operation of nuclear power plants, uranium mining and radiomedical research (Cao et al., 2016; Zhang et al., 2019a; Wang et al., 2019a). In order to protect the health of the human and dispose the hazard radioactive wastewater, new technologies have been developed, such as biological method (Shukla et al., 2017), chemical precipitation (Shao et al., 2016), photocatalytic (Chen et al., 2019a; Jiang et al., 2018), and adsorption method (Cai et al., 2017; Zou et al., 2016; Wang et al., 2019b). Nevertheless, these methods only focus on the enrichment of radionuclides. The volume reduction of the wastewater is also important for radioactive wastewater treatment. So, it is very urgent to meet the highquality requirements for both the volume reduction of the radioactive wastewater and the enrichment of radionuclides, which are the two most important points in the treatment of radioactive wastewater (Zhang et al., 2019a; Rahman et al., 2011). Furthermore, the advantages and disadvantages of various radioactive wastewater treatment methods in the real application were comprehensively compared in Supplementary Table S1 (Zhang et al., 2019a). In comparison, solardriven interfacial evaporation is not only a promising way to solve these problems, but also a low-carbon and energy-saving way (Zhu et al., 2019a; Li et al., 2018a). Solar-driven interfacial evaporation means that the thermal energy converted by sunlight is transmitted to water molecules at the interface of water and air, which is converted into steam continuously (Tao et al., 2018; Luo et al., 2019; Li et al., 2019a). As an effective way to obtain solar energy and separate the soluble contaminants from water, solardriven interfacial evaporation is used in desalination, wastewater treatment, electricity generation, and domestic water heating (Gao et al., 2019; Geng et al., 2019). For applications in wastewater treatment, solar-driven interfacial evaporation has been developed for heavy metal ion wastewater (Li et al., 2018a) and dye wastewater treatment (Lou et al., 2016; Higgins et al., 2018). Recently, varieties of materials, including black nano-metal oxides (Yi et al., 2017), plasmonic metal particles (Zhu et al., 2019b), and carbon-based solar absorber materials (Chen et al., 2017; Xu et al., 2017; Hou et al., 2019) have been be developed to enhance the solar conversion efficiently. Among these materials, carbon-based solar absorber materials has been widely applied for its intrinsic broadband light absorption, low cost, low thermal emission and low toxicity attributes (Li et al., 2019a; Zhu et al., 2018a). Up to now, typical carbon materials such as reduced graphene oxide (Hu et al., 2017; Fu et al., 2018), carbon nanotubes (Li et al., 2019b; Qin et al., 2019), and carbonbased hybrids (Zhou et al., 2018) have been reported, which can be used as interface photo-initiated vapor conversion absorbers. However, on the one hand, the hydrophobic nature of the carbon materials restrict the transport of water, thereby limiting the solar conversion efficiently (Zhou et al., 2018). Hu and his/her partners (Hu et al., 2017) have increased the solar conversion efficiently by adding sodium alginate to increase the hydrophilicity of reduced graphene oxide aerogels. Fu and his/her partners (Fu et al., 2018) used oxygen plasma treatment to increase the hydrophilicity of reduced graphene oxide aerogels to increase the efficiency. On the other hand, carbon nanostructures tend to aggregate easily in water and are vulnerable due to the stresses of volume-varying and the dynamic fluid (Zhu et al., 2018a). For the restricting service life and reusability of materials, it is difficult for carbon materials to be widely applied in various environment treatment. Additionally, solar absorber materials are often with membrane structure, which induces immense heat losses by thermal diffusion for its continual photothermal conversion surface and direct contacts with the bulk water (Chang et al., 2019; Li et al., 2016; Wang et al., 2018). Here, we develop an efficient solar-driven interfacial evaporation method for radioactive wastewater treatment, based on a superhydrophilic, highly underwater elastic, and low cost monolithic sponge
2. Experimental section 2.1. Materials KGM (Mw∼980 000) was provided by Mianyang Haomao Konjac Food Co., Ltd (Mianyang, China) and used as received without further purification. Uranyl nitrate (UO2 (NO3)2·6H2O) was purchased from Hubei Chu enterprise Chemical Co. Ltd. Graphite powder (99 %), strontium chloride (SrCl2·6H2O), cesium chloride (CsCl) and other conventional reagents were purchased from Cheng Du Kelong Chemical Reagent Co., Ltd (Chengdu, China). All of these chemicals were analytical pure. Deionized water was used exclusively in this study. 2.2. Preparation of the KGS Graphene oxide (GO) was prepared by oxidizing natural graphite powders via a modified Hummers’ method. The KGS sample was prepared via freeze-drying and then hydrothermal treatment. Briefly, 0.1 g GO was dispersed into 100 mL deionized water with sonicated for 3 h to get a homogeneous suspension. Then, 1.0 g KGM was slowly dissolved in the GO suspension with agitate intensely at indoor temperature for 12 h to form a black paste. To obtain a desired pore structure, the mixture of GO and KGM was frozen into ice and dried in vacuum. To initiate the deacetylation, the dried sample was placed in reaction vessel with a mixture of 100 ml KOH solution (3.3 wt%) and 100 ml ethyl alcohol. Successively, reaction vessel was placed in oven at 80 °C and incubated 8 h to deacetylation. Finally, the obtained KGS samples were freeze-dried. 2.3. Evaluation of solar–driven vaporization in laboratory The experiments of solar-driven interfacial evaporation were performed in a glass beaker with the floating KGS composite sponge (∼1 cm thick) or 0.1 g GO at room temperature (25 °C in laboratory). For each test, the glass beaker was filled with 100 ml pure water or simulated radioactive wastewater. Before illuminating the set-up, the evaporation rate in dark conditions was measured for 1 h. A simulated sunlight with a radiation intensity of 1 kW m–2 (1 sun, 300 W xenon arc lamp) was used, which was measured by a Daystar Solar Meter. The water mass change was monitored by an electrical balance every 10 min. The infrared images were taken by a FLIR i50 infrared camera. The heat temperature profiles were measured under one sun irradiation for each min in the glass beaker. To investigate the solar wastewater treatment capability of KGS, simulated radioactive wastewater was prepared by dissolving UO2(NO3)2·6H2O (1.66 g), CsCl (1.27 g), and SrCl2·6H2O (3.04 g) in deionized water (1.0 L). For the test of effects of co-existing ions, the chlorides including NaCl (2.54 g), KCl (1.91 g), MnCl2·4H2O (3.60 g), 2
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the wall structure of the KGS without GO dose showed slippy internal surface (Supplementary Fig. S3). The addition of GO resulted in wrinkles on the internal surface of KGS (Fig. 1d). In order to analyze the chemical composition of the KGS, the Fourier-Transform infrared (FTIR) spectra of KGM, rGO, and the KGS are shown in Fig. 2a. For the KGM spectrum (red curve in Fig. 2a), the absorption signals at 875 cm−1 and 808 cm−1 are both attributed to characteristic vibrations of the mannose unit in KGM (Zhu et al., 2018c). For the rGO spectrum (blue curve in Fig. 2a), all the characteristic peaks of KGM and rGO were found in the FTIR spectra of KGS (black curve in Fig. 2a) without shifts, thus confirming the presence of rGO in the KGM network (Zhou et al., 2018). Futhermore, compared to the X-ray photoelectron spectroscopy (XPS) of KGS without GO, the obvious change in the KGS was only the increase in the relative area of the CeC peak (Supplementary Fig. S4). Considering the abundant oxygen-containing functional groups on the surface of KGM and rGO, the connection mode of KGM and rGO can be attributed to the function of hydrogen bond (Chen et al., 2018; Zhu et al., 2018c; Lei et al., 2019). As showed by the UV–vis-NIR spectra in Fig. 2b, the KGS (with a thickness of 1 mm) exhibits negligible optical loss with extremely small optical transmittance and reflectance, indicating the excellent solar absorption ability of KGS. This excellent absorption capability throughout the solar spectra owes to the optical absorption of rGO and the long optical path length of the light scattering from nanofibers. With such a large optical extinction and excellent photothermal characteristic, the KGS has potential to produce high photothermal efficiency. In addition, the results of thermogravimetric analysis (TGA) shows the weight loss changed with temperature from 25 to 400 °C (Supplementary Fig. S5), indicating the thermal stability of KGS under one sun. In order to analyze the surface wetting properties of the KGS, timedependent water contact angle measurements capturing the wetting process of a water droplet (Ma et al., 2019). As shown in Fig. 2c, under ambient conditions, a water droplet can quickly spread across the surface of KGS within only 126 ms, indicating its superhydrophilicity. Due to the capillary action, the superhydrophilicity of KGS can facilitate the transport of the internal water, which is from the bulk water body to the surface of the KGS. For the practical application, it is predictable that the structure of the photothermal materials will be structurally fragile due to dynamic fluid and volume variations stress (Zhao et al., 2019). In order to analyze the underwater mechanical capacity of the KGS, the KGS (a cylinder with a diameter of 4.0 cm) compressed by fingertips. As shown in Fig. 2d, it took only 10 s for the compressed KGS to recover to its original shape in the water (Supplementary Video S1). Further, the repeatable compressed-recovering process enabled a diameter of the KGS change between ca. 0.5 cm to ca. 4.0 cm (Supplementary Fig. S6). This highly underwater elastic of the KGS can be attributed to its unique three-dimensional network structure filled with small water masses in the water. This highly underwater elastic is critical to cushion the shape change under dynamic fluid and to prevent KGS pore collapse under pressure.
and MgCl2 (3.91 g) were added and dissolved to the as-prepared simulated radioactive wastewater (1.0 L). The evaporation rate can be obtained by Eq. (1)
v=
dm Sdt
(1)
Where v is the water evaporation rate, m is the mass of evaporated water, S is the cross sectional area of the photothermal sponge, and t is the time duration. According to the following Eq. (2), the photothermal efficiency of the device can be estimated.
η=
vHe Qs
(2)
Where η is the solar-driven interfacial evaporation efficiency, He is the total enthalpy of sensible heat and the phase change of liquid to water, v is the water evaporation rate and Qs is the power density of solar illumination. Considering the influence of natural water evaporation, all the solar energy conversion efficiencies were calculated by subtracting the evaporation rate (0.13 kg m−2 h−1) of pure water in the dark condition. 2.4. Cycle experiments After each cycle for simulated radioactive wastewater treatment, the samples were regenerated by squeezing and soaking with 0.1 mol/L HCl. Then, the samples were washed three times with pure water and squeezed dry for the next cycle. Furthermore, after 20 cycles, the samples were illuminated for a long time to collect condensed water. 2.5. Characterizations Surface morphologies and elemental composition of samples were examined by field-emission scanning electron microscopy (FE-SEM, 200 kV, ULTRA 55, Carl Zeiss, Germany) with Energy Dispersive Spectroscopy (EDS, Ultra 55, Carl Zeiss, Germany). The functional groups were estimated with Fourier transformed infrared spectroscopy (FTIR, PerkinElmer, USA). Water contact angles were measured on an OCA15 machine (Data-physics, Germany). The light transmittance and reflectance were measured by a UV–vis-NIR spectrophotometer (Perkin Elmer Lambda 950, USA). The concentration of ions was tracked by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 40 7500ce, USA). Thermogravimetric analysis (TGA) was obtained on a Discovery TGA 550 system with a heating rate of 10 ℃ min−1 in the air. Infrared images were captured using a FLIR i50 infrared camera. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MKII with Mg Kα (hv =21.22 eV) as the excitation source. 3. Results and discussion 3.1. Fabrication and characterization of the KGS We used the freeze-drying and then hydrothermal treatment method to prepare the multi-hole and deacetylated KGS samples (Fig. 1a). KGM and GO homogeneous suspension was freeze-dried to form a unique three-dimensional network structure. After that, the samples were heated in a mixture of ethanol and potassium hydroxide solution to prepare deacetylated KGM and reduced GO. In the meantime, KGM and rGO formed the cross interpenetrating networks, and the structure of the sample was also retained. The as-prepared KGS is black and stable in water compared with the KGS without deacetylation (Supplementary Figs. S1 and S2). As shows in Fig. 1b and c, the unique three-dimensional network structure of KGS indicated the interconnected capillary channels with a diameter of about 50 μm. These characteristics were not only providing a fast passage for water evaporation, but also imparting highly underwater elastic stability to the sponge. Furthermore,
3.2. Solar-driven interfacial vaporization under one sun In a typical solar-driven interfacial vaporization test, the beaker with pure water and floating KGS (via polystyrene foams controling) were placed under constant one sun irradiation (1 kW m−2) (Fig. 3a and Supplementary Fig. S7). In contrast, pure water and GO solution were conducted as control samples. As shown in the Fig. 3b, the temperature change and distribution of the system were visualized and recorded by an infrared camera, showing the excellent photothermal properties of KGS. Compared with pure water and GO solution under the same illumination (Supplementary Figs. S8 and S9), the KGS floating on the surface of water not only generated higher temperature, but also generated a local thermal region. As Fig. 3c shows, the 3
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Fig. 1. (a) Schematic diagram of preparation of KGS by freeze-drying followed by hydrothermal deacetylation. (b–d) SEM images with different magnification show the highly-porous tructure with uniform pores of KGS. Fig. 2. Physico-chemical Characterization of KGS. (a) FTIR spectra of KGM, rGO and KGS showing the chemical composition of the KGS. (b) UV–vis NIR spectra of KGM, rGO and KGS showing the excellent solar absorption ability of the KGS. (c) Time-dependent water contact angle of KGS showing the superhydrophilicity. (d) Photographs of a compressed KGS showing the highly underwater elastic. (For interpretation of the references to colour in this figure text, the reader is referred to the web version of this article.)
based on KGS is 1.60 kg m−2 h−1, about 4.0 times that of pure water (0.40 kg m−2 h−1) and 2.1 times that of GO solution (0.77 kg m−2 h−1). To optimize the solar evaporation conditions, the effects of GO dose in the preparation of the KGS on the water evaporation rate were investigated (Fig. 3e). Due to the KGS without GO can not guarantee enough energy input (it is white), the evaporation rate is only 0.49 kg m−2 h−1. And the water evaporation rate increases
temperature related to irradiation time of pure water, GO solution, and KGS were measured. After 10 min of light exposure, the KGS reached a steady state temperature of 40.4 °C, which was 16 °C higher than that of pure water and 12 °C higher than that of GO solution. In addition, after the temperature reaches steady state, changes in total mass were recorded to indicate the amount of evaporated water. As shown in Fig. 3d, the rate of solar-driven interfacial vaporization 4
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Fig. 3. Solar-driven interfacial vaporization under 1 sun. (a) The sketch map showing the typical solar-driven interfacial vaporization test. (b) The infrared images showing the temperature distribution under 1 sun after irradiation times of 0, 10 and 60 min. (c) Plot showing the temperature of pure water, GO solution, and KGS under 1 sun relative to irradiation time. (d) Plot showing the mass change of pure water, GO solution, and KGS under 1 sun relative to irradiation time (after 10 min). (e) The influence of GO dose of KGS on evaporation rate. The illustrations are the photo of KGS without GO and the contact angle photo of KGS with 1000 mg GO dose without KGM.
significantly from 0.49 kg m−2 h−1 to 1.60 kg m−2 h−1 with increase of GO dose at a range from 0 to 100 mg. However, excessive addition of GO led to the decrease of evaporation rate. Under extreme conditions, the amount of GO increased to 1000 mg without KGM, and the evaporation rate dropped to 1.07 kg m−2 h−1. This can be attributed to the gradual change of KGS from hydrophilic to hydrophobic (the contact angle was 133°), hindering waterway transportation (Hu et al., 2017; Fu et al., 2018). It can be calculated that the energy efficiency for KGS was 92 % under constant one sun irradiation (Supplementary Fig. S10), which was superior to that of most other reported materials (Supplementary Table S2). Also, the calculated energy efficiency of pure water is 17 %, and that of graphene oxide solution is 40 %. Compared with pure water and GO solution, the high efficiency of KGS is mainly attributed to the unique photovapor conversion properties of the KGS. The strong and broadband solar absorption of KGS ensures the efficient input of solar energy (Zhu et al., 2018a). The KGS converts absorbed solar energy into heat and confines it to the surface for evaporation (Li et al., 2016). Due to the unique pore structure and hydrophilicity of KGS, bulk water is efficiently transported to the surface of KGS by capillary effect and evaporated (Zhou et al., 2018).
evaporation of KGS, we tested the purification effect of KGS on artificial radioactive wastewater. As a demonstration, the artificial radioactive wastewater has common radioactive elements (Sr, Cs, and U) (Wang et al., 2015; Zheng et al., 2017; Zhang et al., 2019b,c). Evaporative condensate based on KGS was collected and its ion concentration was measured by inductively coupled plasma mass spectrometry (ICP-MS). As shown in Fig. 4a, in solar-driven interfacial evaporation, the concentrations of Sr, Cs, and U in the artificial radioactive wastewater decreased from 1 g/L to 0.265, 2.459 and 1.585 μg/L respectively. Considering that the evaporated solute originates from the entrainment of small water droplets during the vaporization process, the solute with low solubility in 100 °C water is easier to carry out (Tao et al., 2018). Thereby, the phenomenon of the highest removal efficiency of Sr is likely accounting for the fact that the solubility of SrCl2 (0.38 mol) is much less than that of CsCl (1.62 mol) and UO2(NO3)2 (1.20 mol) (https://en.m.wikipedia.org/wiki/Solubility_tabl). The energy dispersive spectroscopy (EDS) element mapping of the KGS surface after solar-driven radioactive wastewater treatment are shown as Fig. 4e, illustrating radioactive elements were fixed to the KGS after treatment. Futhermore, XPS spectra of KGS before and after treatment also showed the characteristic peaks of Sr 3d (134.18 and 135.93 eV), Cs 3d (738.32 and 724.42), and U 4f (381.63 and 392.56), illustrating the adsorption of KGS (Supplementary Fig. S11) (Wang et al., 2019c; Chen et al., 2019b; Liu et al., 2019; Wang et al., 2020). The source of a small amount of U(IV) can be interpreted as the reduction of U(VI) due to the weak reduction properties of hot electrons (Cortes et al., 2017). Compared with the O 1s and C 1s XPS spectra of KGS before treatment, the relative peak areas and binding energy of eOH were decreased after treatment, indicating that eOH plays a major role in adsorption (Supplementary Fig. S11) (Cai et al., 2017). Therefore, the purification mechanism can be attributed to the deposition by evaporation, the adsorption of KGS, and the weak reduction of U(VI) by hot electrons (Li et al., 2018a). At the same time, we proved that the pH value of artificial
3.3. Laboratory solar-driven artificial radioactive wastewater treatment In the traditional treatment of radioactive wastewater, the traditional evaporation has been widely used due to its high decontamination factor, high volume reduction factor, and feasibility for a variety of radionuclides. However, the problems of corrosion, high operating costs, and high energy demand, restrict the further application of the traditional evaporation method (Rahman et al., 2011). In our work, the KGS based on solar-driven interfacial evaporation not only retains the advantages of traditional evaporation, but also overcomes its disadvantages. After proving the high efficiency of solar-driven interfacial 5
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Fig. 4. Efficient solar-driven radioactive wastewater treatment under 1 sun. (a) Concentrations of Sr, Cs, and U before and after treatment. (b) Concentrations of Sr, Cs, and U after treatment under different pH conditions. (c) Concentrations of Sr, Cs, and U after treatment by KGS with different irradiation dose. (d) Evaporation rates of KGS with different irradiation dose under 20 cycles. (e) The EDS element mapping of the KGS surface after solar-driven radioactive wastewater treatment.
were well maintained after 20 cycles (Supplementary Fig. S15).
radioactive wastewater had no effect on the purification effect and the evaporation rate (Fig. 4b and Supplementary Fig. S12). This result can be interpreted as the combined effect of the stability of KGS under different pH conditions and the inherent properties of evaporation. Considering that there are a large number of co-existing ions in the actual radioactive wastewater, the further tests were carried out in the artificial radioactive wastewater added with 1 g/L M ions (herein, M = Na+, K+, Mn2+ and Mg2+) (Jiang et al., 2018). Compared with the treatment without the effects of co-existing ions, the results showed that co-existing ions have little effect on the purification effect of the KGS on artificial radioactive wastewater (Supplementary Fig. S13). Considering the radioactivity of actual radioactive wastewater, KGS were irradiated by 60Co γ-ray irradiation facility with the total irradiation doses of 100 kGy (KGS-100kGy), 200 kGy (KGS-200kGy) and 300 kGy (KGS-300kGy), respectively (Zhu et al., 2017; Li et al., 2018b). After γ-ray irradiation, the structure of the irradiated KGS had not been destroyed (Supplementary Fig. S14). As shown in Fig. 4c, compared with unirradiated KGS, the concentrations of Sr, Cs, and U after treatment by the irradiated KGS had not changed much, indicating that the purification effect of the irradiated KGS had not been damaged. As the stability and reusability are vital properties for real-life applications, the cycling stability performance of the KGS for water evaporation was evaluated (Sun et al., 2019; Yang et al., 2019). As shown in Fig. 4d, it is proved that KGS can produce clean water stably and efficiently through solar-driven interfacial vaporization in 20 cycles (one hour each cycle). Furthermore, the effect of decontamination of KGS and irradiated KGS
4. Conclusions In summary, we have demonstrated the preparation of a superhydrophilic and highly underwater elastic monolithic sponge via a simple process for efficient solar-driven radioactive wastewater treatment. The enhanced water evaporation efficiency is due to the good absorption, photothermal, thermal insulation and fast transport properties of the KGS. After evaporation, the concentration of radioactive elements in radioactive wastewater decreased significantly. Even under radiation and acidic conditions, the KGS can maintain high evaporation rate and purification efficiency for radioactive wastewater. After 20 cycles, the performance of the KGS did not decrease significantly, which indicated its stability and durability. The results showed that the KGS based on solar-driven interfacial evaporation can effectively treat radioactive wastewater and enrich various radionuclides in a more energy-saving manner. CRediT authorship contribution statement Kaifu Yu: Conceptualization, Writing - original draft, Methodology. Pengfei Shao: Investigation, Validation. Pengwei Meng: Investigation, Validation. Tao Chen: Resources, Data curation. Jia Lei: Resources. Xiaofang Yu: Supervision. Rong He: Conceptualization, Writing - review & editing, Formal analysis. Fan Yang: Writing - review & editing. 6
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Wenkun Zhu: Conceptualization, Project administration, Supervision, Funding acquisition. Tao Duan: Project administration, Supervision.
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Declaration of Competing Interest There are no conflicts to declare. Acknowledgements This work was supported by NSFC (Nos. 21601147, 21902130, and 21976147), China Postdoctoral Fund (No. 2018m630715), Sichuan Science and Technology Program (Nos. 2019JDRC0118, 2019YFN0125, 2019YFS0469, 2019YFS0503, 2019YFS0461, 2019YFG0433, and 2019YFG0434), Education Department of Sichuan Province (Nos. 17zd1131 and 18ZA0494), Sichuan’s Training Program of Innovation and Entrepreneurship for Undergraduate (No. S201910619101), Plan Projects of Mianyang Science and Technology (No. 18YFZJ003), Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory (No. 18kfhk03), the Project of State Key Laboratory of Environment-friendly Energy Materials in SWUST (No. 18fksy0218), Research fund of SWUST for PhD (No. 18zx7149), Longshan academic talent research supporting program of SWUST (Nos. 17LZX526, 18LZXT04 and 18LZX420), and Postgraduate Innovation Fund Project by SWUST (No. 19ycx0036). And thanks to Li Zhou, Renhao Tan, Wencai Bai, Xin Yuan, Changxue Dong, Ruihao Ai, Chenxi Hou and others for their help during the revision. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2020.122350. References Cao, J., Cohen, A., Hansen, J., Lester, R., Peterson, P., Xu, H., 2016. China-US cooperation to advance nuclear power. Science 353, 547–548. Zhang, X., Gu, P., Liu, Y., 2019a. Decontamination of radioactive wastewater: state of the art and challenges forward. Chemosphere 215, 543–553. Wang, X., Chen, L., Wang, L., Fan, Q., Pan, D., Li, J., Chi, F., Xie, Y., Yu, S., Xiao, C., Luo, F., Wang, J., Wang, X., Chen, C., Wu, W., Shi, W., Wang, S., Wang, X., 2019a. Synthesis of novel nanomaterials and their application in efficient removal of radionuclides. Sci. China-Chem. 62, 933–967. Shukla, A., Parmar, P., Sarar, M., 2017. Radiation, radionuclides and bacteria: an inperspective review. J. Environ. Radioact. 180, 27–35. Shao, L., Wang, X., Ren, Y., Wang, S., Zhong, J., Chu, M., Tang, H., Luo, L., Xie, D., 2016. Facile fabrication of magnetic cucurbit 6 uril/graphene oxide composite and application for uranium removal. Chem. Eng. J. 286, 311–319. Chen, T., Zhang, J., Ge, H., Li, M., Li, Y., Liu, B., Duan, T., He, R., Zhu, W., 2019a. Efficient extraction of uranium in organics-containing wastewater over g-C3N4/GO hybrid nanosheets with type-II band structure. J. Hazard. Mater. 121383. Jiang, X., Xing, Q., Luo, X., Li, F., Zou, J., Liu, S., Li, X., Wang, X., 2018. Simultaneous photoreduction of Uranium(VI) and photooxidation of Arsenic (III) in aqueous solution over g-C3N4/TiO2 heterostructured catalysts under simulated sunlight irradiation. Appl. Catal. B-Environ. 228, 29–38. Cai, Y., Wu, C., Liu, Z., Zhang, L., Chen, L., Wang, J., Wang, X., Yang, S., Wang, S., 2017. Fabrication of a phosphorylated graphene oxide-chitosan composite for highly effective and selective capture of U(VI). Environ. Sci.-Nano 4, 1876–1886. Zou, J., Liu, H., Luo, J., Xing, Q., Du, H., Jiang, X., Luo, X., Luo, S., Suib, S.L., 2016. Threedimensional reduced graphene oxide coupled with Mn3O4 for highly efficient removal of Sb(III) and Sb(V) from water. ACS Appl. Mater. Interfaces 8, 18140–18149. Wang, K., Ma, H., Pu, S., Yan, C., Wang, M., Yu, J., Wang, X., Chu, W., Anatoly, Z., 2019b. Hybrid porous magnetic bentonite-chitosan beads for selective removal of radioactive cesium in water. J. Hazard. Mater. 362, 160–169. Rahman, R.O.A., Ibrahium, H.A., Hung, Y.-T., 2011. Liquid radioactive wastes treatment: a review. Water 3, 551–565. Zhu, L., Gao, M., Peh, C.K.N., Ho, G.W., 2019a. Recent progress in solar-driven interfacial water evaporation: advanced designs and applications. Nano Energy 57, 507–518. Li, X., Lin, R., Ni, G., Xu, N., Hu, X., Zhu, B., Lv, G., Li, J., Zhu, S., Zhu, J., 2018a. Threedimensional artificial transpiration for efficient solar waste-water treatment. Sci. Rev. 5, 70–77. Tao, P., Ni, G., Song, C., Shang, W., Wu, J., Zhu, J., Chen, G., Deng, T., 2018. Solar-driven interfacial evaporation. Nat. Energy 3, 1031–1041. Luo, Y., Fu, B., Shen, Q., Hao, W., Xu, J., Min, M., Liu, Y., An, S., Song, C., Tao, P., Wu, J., Shang, W., Deng, T., 2019. Patterned surfaces for solar-driven interfacial evaporation. ACS Appl. Mater. Interfaces 11, 7584–7590.
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S., Maier, S.A., 2017. Plasmonic hot electron transport drives nano-localized chemistry. Nat. Commun. 8, 14880. Zhu, L., Sheng, D., Xu, C., Dai, X., Silver, M.A., Li, J., Li, P., Wang, Y., Wang, Y., Chen, L., Xiao, C., Chen, J., Zhou, R., Zhang, C., Farha, O.K., Chai, Z., Albrecht-Schmitt, T.E., Wang, S., 2017. Identifying the recognition site for selective trapping of (TcO4-)-Tc99 in a hydrolytically stable and radiation resistant cationic metal-organic framework. J. Am. Chem. Soc. 139, 14873–14876. Li, J., Dai, X., Zhu, L., Xu, C., Zhang, D., Silver, M.A., Li, P., Chen, L., Li, Y., Zuo, D., Zhang, H., Xiao, C., Chen, J., Diwu, J., Farha, O.K., Albrecht-Schmitt, T.E., Chai, Z., Wang, S., 2018b. (TcO4-)-Tc-99 remediation by a cationic polymeric network. Nat. Commun. 9, 3007. Sun, Y., Gao, J., Liu, Y., Kang, H., Xie, M., Wu, F., Qiu, H., 2019. Copper sulfide-macroporous polyacrylamide hydrogel for solar steam generation. Chem. Eng. Sci. 207, 516–526. Yang, Y., Que, W., Zhao, J., Han, Y., Ju, M., Yin, X., 2019. Membrane assembled from anti-fouling copper-zinc-tin-selenide nanocarambolas for solar-driven interfacial water evaporation. Chem. Eng. J. 373, 955–962.
Zhang, J., Chen, L., Dai, X., Zhu, L., Xiao, C., Xu, L., Zhang, Z., Alekseev, E.V., Wang, Y., Zhang, C., Zhang, H., Wang, Y., Diwu, J., Chai, Z., Wang, S., 2019c. Distinctive twostep intercalation of Sr2+ into a coordination polymer with record high Sr-90 uptake capabilities. Chem 5, 977–994. Wang, H., Guo, H., Zhang, N., Chen, Z., Hu, B., Wang, X., 2019c. Enhanced photoreduction of U(VI) on C3N4 by Cr(VI) and bisphenol A: ESR, XPS, and EXAFS investigation. Environ. Sci. Technol. 53, 6454–6461. Chen, S., Hu, J., Shi, J., Wang, M., Guo, Y., Li, M., Duo, J., Deng, T., 2019b. Composite hydrogel particles encapsulated ammonium molybdophosphate for efficiently cesium selective removal and enrichment from wastewater. J. Hazard. Mater. 371, 694–704. Liu, Z., Zhou, Y., Guo, M., Lv, B., Wu, Z., Zhou, W., 2019. Experimental and theoretical investigations of Cs+ adsorption on crown ethers modified magnetic adsorbent. J. Hazard. Mater. 371, 712–720. Wang, S., Ning, S., Zhang, W., Zhang, S., Zhou, J., Wang, X., Wei, Y., 2020. Synthesis of carboxyl group functionalized silica composite resin for strontium removal. Mater. Des. 185, 108224. Cortes, E., Xie, W., Cambiasso, J., Jermyn, A.S., Sundararaman, R., Narang, P., Schlueker,
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Superhydrophilic and highly elastic monolithic sponge for efficient solardriven radioactive wastewater treatment under one sun
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Kaifu Yua, Pengfei Shaoa, Pengwei Menga, Tao Chena, Jia Leia, Xiaofang Yub, Rong Hea, Fan Yanga, Wenkun Zhua,*, Tao Duana,* a Nuclear Waste and Environmental Safety Key Laboratory of Defense, State Key Laboratory of Environment-friendly Energy Materials, National Co-innovation Center for Nuclear Waste Disposal and Environmental Safety, Sichuan Civil-military Integration Institute, School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang 621010, China b Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics, Mianyang 621900, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Danmeng Shuai
As an effective way to obtain solar energy and separate the soluble contaminants from water, solar-driven interfacial evaporation is used in desalination, wastewater treatment, electricity generation, and domestic water heating system. Herein, we demonstrate a monolithic sponge with three-dimensional porous structure as the solar-energy evaporator, which is composed of hydrophilic polymer (Konjac Glucomannan, KGM) and solar absorbent (reduced graphene oxide, rGO). Under one sun irradiation, the sponge achieves a rapid evaporation rate (1.60 kg m−2 h−1) and high interfacial water evaporation efficiency (92 %) due to its good absorption, photothermal, thermal insulation, and fast water transport properties. Meanwhile, the concentrations of radioactive elements (strontium, cesium, and uranium) in wastewater dropped from grams to micrograms after purification, even under radiation and acidic conditions. Additionally, the durability and repeatability of the sponge also have been verified. The results showed that solar-driven interfacial evaporation can effectively treat radioactive wastewater and enrich various radionuclides in a more energy-saving manner.
Keywords: Solar-driven interfacial evaporation Radioactive wastewater treatment Konjac Glucomannan Reduced graphene oxide
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Corresponding authors. E-mail addresses: [email protected] (W. Zhu), [email protected] (T. Duan).
https://doi.org/10.1016/j.jhazmat.2020.122350 Received 13 December 2019; Received in revised form 16 February 2020; Accepted 17 February 2020 Available online 19 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.
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1. Introduction
prepared by Konjac Glucomannan (KGM) and reduced graphene oxide (rGO) composite. As a natural and renewable polymer, KGM contains a large number of hydroxyl groups, which provides hydrophilicity for the sponges (Zhu et al., 2018b). rGO, a widely used absorbent, enables the sponge to exhibit a broadband absorption over the full solar spectrum (Zhou et al., 2018). In addition, rGO interacts with KGM in a threedimensional network formed by freeze-drying, resulting in a black KGM/rGO sponge (KGS) with unique properties (Chen et al., 2018). Under one sun illumination, KGS achieved an evaporation rate of 1.60 kg m−2 h−1 and a photothermal efficiency up to 92 %. In addition, the concentrations of radioactive elements in wastewater dropped from grams to micrograms after purification, even after irradiation and 20 times cycle. It is revealed that the KGS possesses potent solar-driven interfacial evaporation capability, demonstrating excellent irradiation stability, photothermal conversion stability, and durability. And the KGS is expected to become a powerful solar-driven radioactive wastewater treatment material in practical application.
With the application of nuclear technology, radioactive wastewater is increasingly produced in various ways, including the operation of nuclear power plants, uranium mining and radiomedical research (Cao et al., 2016; Zhang et al., 2019a; Wang et al., 2019a). In order to protect the health of the human and dispose the hazard radioactive wastewater, new technologies have been developed, such as biological method (Shukla et al., 2017), chemical precipitation (Shao et al., 2016), photocatalytic (Chen et al., 2019a; Jiang et al., 2018), and adsorption method (Cai et al., 2017; Zou et al., 2016; Wang et al., 2019b). Nevertheless, these methods only focus on the enrichment of radionuclides. The volume reduction of the wastewater is also important for radioactive wastewater treatment. So, it is very urgent to meet the highquality requirements for both the volume reduction of the radioactive wastewater and the enrichment of radionuclides, which are the two most important points in the treatment of radioactive wastewater (Zhang et al., 2019a; Rahman et al., 2011). Furthermore, the advantages and disadvantages of various radioactive wastewater treatment methods in the real application were comprehensively compared in Supplementary Table S1 (Zhang et al., 2019a). In comparison, solardriven interfacial evaporation is not only a promising way to solve these problems, but also a low-carbon and energy-saving way (Zhu et al., 2019a; Li et al., 2018a). Solar-driven interfacial evaporation means that the thermal energy converted by sunlight is transmitted to water molecules at the interface of water and air, which is converted into steam continuously (Tao et al., 2018; Luo et al., 2019; Li et al., 2019a). As an effective way to obtain solar energy and separate the soluble contaminants from water, solardriven interfacial evaporation is used in desalination, wastewater treatment, electricity generation, and domestic water heating (Gao et al., 2019; Geng et al., 2019). For applications in wastewater treatment, solar-driven interfacial evaporation has been developed for heavy metal ion wastewater (Li et al., 2018a) and dye wastewater treatment (Lou et al., 2016; Higgins et al., 2018). Recently, varieties of materials, including black nano-metal oxides (Yi et al., 2017), plasmonic metal particles (Zhu et al., 2019b), and carbon-based solar absorber materials (Chen et al., 2017; Xu et al., 2017; Hou et al., 2019) have been be developed to enhance the solar conversion efficiently. Among these materials, carbon-based solar absorber materials has been widely applied for its intrinsic broadband light absorption, low cost, low thermal emission and low toxicity attributes (Li et al., 2019a; Zhu et al., 2018a). Up to now, typical carbon materials such as reduced graphene oxide (Hu et al., 2017; Fu et al., 2018), carbon nanotubes (Li et al., 2019b; Qin et al., 2019), and carbonbased hybrids (Zhou et al., 2018) have been reported, which can be used as interface photo-initiated vapor conversion absorbers. However, on the one hand, the hydrophobic nature of the carbon materials restrict the transport of water, thereby limiting the solar conversion efficiently (Zhou et al., 2018). Hu and his/her partners (Hu et al., 2017) have increased the solar conversion efficiently by adding sodium alginate to increase the hydrophilicity of reduced graphene oxide aerogels. Fu and his/her partners (Fu et al., 2018) used oxygen plasma treatment to increase the hydrophilicity of reduced graphene oxide aerogels to increase the efficiency. On the other hand, carbon nanostructures tend to aggregate easily in water and are vulnerable due to the stresses of volume-varying and the dynamic fluid (Zhu et al., 2018a). For the restricting service life and reusability of materials, it is difficult for carbon materials to be widely applied in various environment treatment. Additionally, solar absorber materials are often with membrane structure, which induces immense heat losses by thermal diffusion for its continual photothermal conversion surface and direct contacts with the bulk water (Chang et al., 2019; Li et al., 2016; Wang et al., 2018). Here, we develop an efficient solar-driven interfacial evaporation method for radioactive wastewater treatment, based on a superhydrophilic, highly underwater elastic, and low cost monolithic sponge
2. Experimental section 2.1. Materials KGM (Mw∼980 000) was provided by Mianyang Haomao Konjac Food Co., Ltd (Mianyang, China) and used as received without further purification. Uranyl nitrate (UO2 (NO3)2·6H2O) was purchased from Hubei Chu enterprise Chemical Co. Ltd. Graphite powder (99 %), strontium chloride (SrCl2·6H2O), cesium chloride (CsCl) and other conventional reagents were purchased from Cheng Du Kelong Chemical Reagent Co., Ltd (Chengdu, China). All of these chemicals were analytical pure. Deionized water was used exclusively in this study. 2.2. Preparation of the KGS Graphene oxide (GO) was prepared by oxidizing natural graphite powders via a modified Hummers’ method. The KGS sample was prepared via freeze-drying and then hydrothermal treatment. Briefly, 0.1 g GO was dispersed into 100 mL deionized water with sonicated for 3 h to get a homogeneous suspension. Then, 1.0 g KGM was slowly dissolved in the GO suspension with agitate intensely at indoor temperature for 12 h to form a black paste. To obtain a desired pore structure, the mixture of GO and KGM was frozen into ice and dried in vacuum. To initiate the deacetylation, the dried sample was placed in reaction vessel with a mixture of 100 ml KOH solution (3.3 wt%) and 100 ml ethyl alcohol. Successively, reaction vessel was placed in oven at 80 °C and incubated 8 h to deacetylation. Finally, the obtained KGS samples were freeze-dried. 2.3. Evaluation of solar–driven vaporization in laboratory The experiments of solar-driven interfacial evaporation were performed in a glass beaker with the floating KGS composite sponge (∼1 cm thick) or 0.1 g GO at room temperature (25 °C in laboratory). For each test, the glass beaker was filled with 100 ml pure water or simulated radioactive wastewater. Before illuminating the set-up, the evaporation rate in dark conditions was measured for 1 h. A simulated sunlight with a radiation intensity of 1 kW m–2 (1 sun, 300 W xenon arc lamp) was used, which was measured by a Daystar Solar Meter. The water mass change was monitored by an electrical balance every 10 min. The infrared images were taken by a FLIR i50 infrared camera. The heat temperature profiles were measured under one sun irradiation for each min in the glass beaker. To investigate the solar wastewater treatment capability of KGS, simulated radioactive wastewater was prepared by dissolving UO2(NO3)2·6H2O (1.66 g), CsCl (1.27 g), and SrCl2·6H2O (3.04 g) in deionized water (1.0 L). For the test of effects of co-existing ions, the chlorides including NaCl (2.54 g), KCl (1.91 g), MnCl2·4H2O (3.60 g), 2
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the wall structure of the KGS without GO dose showed slippy internal surface (Supplementary Fig. S3). The addition of GO resulted in wrinkles on the internal surface of KGS (Fig. 1d). In order to analyze the chemical composition of the KGS, the Fourier-Transform infrared (FTIR) spectra of KGM, rGO, and the KGS are shown in Fig. 2a. For the KGM spectrum (red curve in Fig. 2a), the absorption signals at 875 cm−1 and 808 cm−1 are both attributed to characteristic vibrations of the mannose unit in KGM (Zhu et al., 2018c). For the rGO spectrum (blue curve in Fig. 2a), all the characteristic peaks of KGM and rGO were found in the FTIR spectra of KGS (black curve in Fig. 2a) without shifts, thus confirming the presence of rGO in the KGM network (Zhou et al., 2018). Futhermore, compared to the X-ray photoelectron spectroscopy (XPS) of KGS without GO, the obvious change in the KGS was only the increase in the relative area of the CeC peak (Supplementary Fig. S4). Considering the abundant oxygen-containing functional groups on the surface of KGM and rGO, the connection mode of KGM and rGO can be attributed to the function of hydrogen bond (Chen et al., 2018; Zhu et al., 2018c; Lei et al., 2019). As showed by the UV–vis-NIR spectra in Fig. 2b, the KGS (with a thickness of 1 mm) exhibits negligible optical loss with extremely small optical transmittance and reflectance, indicating the excellent solar absorption ability of KGS. This excellent absorption capability throughout the solar spectra owes to the optical absorption of rGO and the long optical path length of the light scattering from nanofibers. With such a large optical extinction and excellent photothermal characteristic, the KGS has potential to produce high photothermal efficiency. In addition, the results of thermogravimetric analysis (TGA) shows the weight loss changed with temperature from 25 to 400 °C (Supplementary Fig. S5), indicating the thermal stability of KGS under one sun. In order to analyze the surface wetting properties of the KGS, timedependent water contact angle measurements capturing the wetting process of a water droplet (Ma et al., 2019). As shown in Fig. 2c, under ambient conditions, a water droplet can quickly spread across the surface of KGS within only 126 ms, indicating its superhydrophilicity. Due to the capillary action, the superhydrophilicity of KGS can facilitate the transport of the internal water, which is from the bulk water body to the surface of the KGS. For the practical application, it is predictable that the structure of the photothermal materials will be structurally fragile due to dynamic fluid and volume variations stress (Zhao et al., 2019). In order to analyze the underwater mechanical capacity of the KGS, the KGS (a cylinder with a diameter of 4.0 cm) compressed by fingertips. As shown in Fig. 2d, it took only 10 s for the compressed KGS to recover to its original shape in the water (Supplementary Video S1). Further, the repeatable compressed-recovering process enabled a diameter of the KGS change between ca. 0.5 cm to ca. 4.0 cm (Supplementary Fig. S6). This highly underwater elastic of the KGS can be attributed to its unique three-dimensional network structure filled with small water masses in the water. This highly underwater elastic is critical to cushion the shape change under dynamic fluid and to prevent KGS pore collapse under pressure.
and MgCl2 (3.91 g) were added and dissolved to the as-prepared simulated radioactive wastewater (1.0 L). The evaporation rate can be obtained by Eq. (1)
v=
dm Sdt
(1)
Where v is the water evaporation rate, m is the mass of evaporated water, S is the cross sectional area of the photothermal sponge, and t is the time duration. According to the following Eq. (2), the photothermal efficiency of the device can be estimated.
η=
vHe Qs
(2)
Where η is the solar-driven interfacial evaporation efficiency, He is the total enthalpy of sensible heat and the phase change of liquid to water, v is the water evaporation rate and Qs is the power density of solar illumination. Considering the influence of natural water evaporation, all the solar energy conversion efficiencies were calculated by subtracting the evaporation rate (0.13 kg m−2 h−1) of pure water in the dark condition. 2.4. Cycle experiments After each cycle for simulated radioactive wastewater treatment, the samples were regenerated by squeezing and soaking with 0.1 mol/L HCl. Then, the samples were washed three times with pure water and squeezed dry for the next cycle. Furthermore, after 20 cycles, the samples were illuminated for a long time to collect condensed water. 2.5. Characterizations Surface morphologies and elemental composition of samples were examined by field-emission scanning electron microscopy (FE-SEM, 200 kV, ULTRA 55, Carl Zeiss, Germany) with Energy Dispersive Spectroscopy (EDS, Ultra 55, Carl Zeiss, Germany). The functional groups were estimated with Fourier transformed infrared spectroscopy (FTIR, PerkinElmer, USA). Water contact angles were measured on an OCA15 machine (Data-physics, Germany). The light transmittance and reflectance were measured by a UV–vis-NIR spectrophotometer (Perkin Elmer Lambda 950, USA). The concentration of ions was tracked by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 40 7500ce, USA). Thermogravimetric analysis (TGA) was obtained on a Discovery TGA 550 system with a heating rate of 10 ℃ min−1 in the air. Infrared images were captured using a FLIR i50 infrared camera. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MKII with Mg Kα (hv =21.22 eV) as the excitation source. 3. Results and discussion 3.1. Fabrication and characterization of the KGS We used the freeze-drying and then hydrothermal treatment method to prepare the multi-hole and deacetylated KGS samples (Fig. 1a). KGM and GO homogeneous suspension was freeze-dried to form a unique three-dimensional network structure. After that, the samples were heated in a mixture of ethanol and potassium hydroxide solution to prepare deacetylated KGM and reduced GO. In the meantime, KGM and rGO formed the cross interpenetrating networks, and the structure of the sample was also retained. The as-prepared KGS is black and stable in water compared with the KGS without deacetylation (Supplementary Figs. S1 and S2). As shows in Fig. 1b and c, the unique three-dimensional network structure of KGS indicated the interconnected capillary channels with a diameter of about 50 μm. These characteristics were not only providing a fast passage for water evaporation, but also imparting highly underwater elastic stability to the sponge. Furthermore,
3.2. Solar-driven interfacial vaporization under one sun In a typical solar-driven interfacial vaporization test, the beaker with pure water and floating KGS (via polystyrene foams controling) were placed under constant one sun irradiation (1 kW m−2) (Fig. 3a and Supplementary Fig. S7). In contrast, pure water and GO solution were conducted as control samples. As shown in the Fig. 3b, the temperature change and distribution of the system were visualized and recorded by an infrared camera, showing the excellent photothermal properties of KGS. Compared with pure water and GO solution under the same illumination (Supplementary Figs. S8 and S9), the KGS floating on the surface of water not only generated higher temperature, but also generated a local thermal region. As Fig. 3c shows, the 3
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Fig. 1. (a) Schematic diagram of preparation of KGS by freeze-drying followed by hydrothermal deacetylation. (b–d) SEM images with different magnification show the highly-porous tructure with uniform pores of KGS. Fig. 2. Physico-chemical Characterization of KGS. (a) FTIR spectra of KGM, rGO and KGS showing the chemical composition of the KGS. (b) UV–vis NIR spectra of KGM, rGO and KGS showing the excellent solar absorption ability of the KGS. (c) Time-dependent water contact angle of KGS showing the superhydrophilicity. (d) Photographs of a compressed KGS showing the highly underwater elastic. (For interpretation of the references to colour in this figure text, the reader is referred to the web version of this article.)
based on KGS is 1.60 kg m−2 h−1, about 4.0 times that of pure water (0.40 kg m−2 h−1) and 2.1 times that of GO solution (0.77 kg m−2 h−1). To optimize the solar evaporation conditions, the effects of GO dose in the preparation of the KGS on the water evaporation rate were investigated (Fig. 3e). Due to the KGS without GO can not guarantee enough energy input (it is white), the evaporation rate is only 0.49 kg m−2 h−1. And the water evaporation rate increases
temperature related to irradiation time of pure water, GO solution, and KGS were measured. After 10 min of light exposure, the KGS reached a steady state temperature of 40.4 °C, which was 16 °C higher than that of pure water and 12 °C higher than that of GO solution. In addition, after the temperature reaches steady state, changes in total mass were recorded to indicate the amount of evaporated water. As shown in Fig. 3d, the rate of solar-driven interfacial vaporization 4
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Fig. 3. Solar-driven interfacial vaporization under 1 sun. (a) The sketch map showing the typical solar-driven interfacial vaporization test. (b) The infrared images showing the temperature distribution under 1 sun after irradiation times of 0, 10 and 60 min. (c) Plot showing the temperature of pure water, GO solution, and KGS under 1 sun relative to irradiation time. (d) Plot showing the mass change of pure water, GO solution, and KGS under 1 sun relative to irradiation time (after 10 min). (e) The influence of GO dose of KGS on evaporation rate. The illustrations are the photo of KGS without GO and the contact angle photo of KGS with 1000 mg GO dose without KGM.
significantly from 0.49 kg m−2 h−1 to 1.60 kg m−2 h−1 with increase of GO dose at a range from 0 to 100 mg. However, excessive addition of GO led to the decrease of evaporation rate. Under extreme conditions, the amount of GO increased to 1000 mg without KGM, and the evaporation rate dropped to 1.07 kg m−2 h−1. This can be attributed to the gradual change of KGS from hydrophilic to hydrophobic (the contact angle was 133°), hindering waterway transportation (Hu et al., 2017; Fu et al., 2018). It can be calculated that the energy efficiency for KGS was 92 % under constant one sun irradiation (Supplementary Fig. S10), which was superior to that of most other reported materials (Supplementary Table S2). Also, the calculated energy efficiency of pure water is 17 %, and that of graphene oxide solution is 40 %. Compared with pure water and GO solution, the high efficiency of KGS is mainly attributed to the unique photovapor conversion properties of the KGS. The strong and broadband solar absorption of KGS ensures the efficient input of solar energy (Zhu et al., 2018a). The KGS converts absorbed solar energy into heat and confines it to the surface for evaporation (Li et al., 2016). Due to the unique pore structure and hydrophilicity of KGS, bulk water is efficiently transported to the surface of KGS by capillary effect and evaporated (Zhou et al., 2018).
evaporation of KGS, we tested the purification effect of KGS on artificial radioactive wastewater. As a demonstration, the artificial radioactive wastewater has common radioactive elements (Sr, Cs, and U) (Wang et al., 2015; Zheng et al., 2017; Zhang et al., 2019b,c). Evaporative condensate based on KGS was collected and its ion concentration was measured by inductively coupled plasma mass spectrometry (ICP-MS). As shown in Fig. 4a, in solar-driven interfacial evaporation, the concentrations of Sr, Cs, and U in the artificial radioactive wastewater decreased from 1 g/L to 0.265, 2.459 and 1.585 μg/L respectively. Considering that the evaporated solute originates from the entrainment of small water droplets during the vaporization process, the solute with low solubility in 100 °C water is easier to carry out (Tao et al., 2018). Thereby, the phenomenon of the highest removal efficiency of Sr is likely accounting for the fact that the solubility of SrCl2 (0.38 mol) is much less than that of CsCl (1.62 mol) and UO2(NO3)2 (1.20 mol) (https://en.m.wikipedia.org/wiki/Solubility_tabl). The energy dispersive spectroscopy (EDS) element mapping of the KGS surface after solar-driven radioactive wastewater treatment are shown as Fig. 4e, illustrating radioactive elements were fixed to the KGS after treatment. Futhermore, XPS spectra of KGS before and after treatment also showed the characteristic peaks of Sr 3d (134.18 and 135.93 eV), Cs 3d (738.32 and 724.42), and U 4f (381.63 and 392.56), illustrating the adsorption of KGS (Supplementary Fig. S11) (Wang et al., 2019c; Chen et al., 2019b; Liu et al., 2019; Wang et al., 2020). The source of a small amount of U(IV) can be interpreted as the reduction of U(VI) due to the weak reduction properties of hot electrons (Cortes et al., 2017). Compared with the O 1s and C 1s XPS spectra of KGS before treatment, the relative peak areas and binding energy of eOH were decreased after treatment, indicating that eOH plays a major role in adsorption (Supplementary Fig. S11) (Cai et al., 2017). Therefore, the purification mechanism can be attributed to the deposition by evaporation, the adsorption of KGS, and the weak reduction of U(VI) by hot electrons (Li et al., 2018a). At the same time, we proved that the pH value of artificial
3.3. Laboratory solar-driven artificial radioactive wastewater treatment In the traditional treatment of radioactive wastewater, the traditional evaporation has been widely used due to its high decontamination factor, high volume reduction factor, and feasibility for a variety of radionuclides. However, the problems of corrosion, high operating costs, and high energy demand, restrict the further application of the traditional evaporation method (Rahman et al., 2011). In our work, the KGS based on solar-driven interfacial evaporation not only retains the advantages of traditional evaporation, but also overcomes its disadvantages. After proving the high efficiency of solar-driven interfacial 5
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Fig. 4. Efficient solar-driven radioactive wastewater treatment under 1 sun. (a) Concentrations of Sr, Cs, and U before and after treatment. (b) Concentrations of Sr, Cs, and U after treatment under different pH conditions. (c) Concentrations of Sr, Cs, and U after treatment by KGS with different irradiation dose. (d) Evaporation rates of KGS with different irradiation dose under 20 cycles. (e) The EDS element mapping of the KGS surface after solar-driven radioactive wastewater treatment.
were well maintained after 20 cycles (Supplementary Fig. S15).
radioactive wastewater had no effect on the purification effect and the evaporation rate (Fig. 4b and Supplementary Fig. S12). This result can be interpreted as the combined effect of the stability of KGS under different pH conditions and the inherent properties of evaporation. Considering that there are a large number of co-existing ions in the actual radioactive wastewater, the further tests were carried out in the artificial radioactive wastewater added with 1 g/L M ions (herein, M = Na+, K+, Mn2+ and Mg2+) (Jiang et al., 2018). Compared with the treatment without the effects of co-existing ions, the results showed that co-existing ions have little effect on the purification effect of the KGS on artificial radioactive wastewater (Supplementary Fig. S13). Considering the radioactivity of actual radioactive wastewater, KGS were irradiated by 60Co γ-ray irradiation facility with the total irradiation doses of 100 kGy (KGS-100kGy), 200 kGy (KGS-200kGy) and 300 kGy (KGS-300kGy), respectively (Zhu et al., 2017; Li et al., 2018b). After γ-ray irradiation, the structure of the irradiated KGS had not been destroyed (Supplementary Fig. S14). As shown in Fig. 4c, compared with unirradiated KGS, the concentrations of Sr, Cs, and U after treatment by the irradiated KGS had not changed much, indicating that the purification effect of the irradiated KGS had not been damaged. As the stability and reusability are vital properties for real-life applications, the cycling stability performance of the KGS for water evaporation was evaluated (Sun et al., 2019; Yang et al., 2019). As shown in Fig. 4d, it is proved that KGS can produce clean water stably and efficiently through solar-driven interfacial vaporization in 20 cycles (one hour each cycle). Furthermore, the effect of decontamination of KGS and irradiated KGS
4. Conclusions In summary, we have demonstrated the preparation of a superhydrophilic and highly underwater elastic monolithic sponge via a simple process for efficient solar-driven radioactive wastewater treatment. The enhanced water evaporation efficiency is due to the good absorption, photothermal, thermal insulation and fast transport properties of the KGS. After evaporation, the concentration of radioactive elements in radioactive wastewater decreased significantly. Even under radiation and acidic conditions, the KGS can maintain high evaporation rate and purification efficiency for radioactive wastewater. After 20 cycles, the performance of the KGS did not decrease significantly, which indicated its stability and durability. The results showed that the KGS based on solar-driven interfacial evaporation can effectively treat radioactive wastewater and enrich various radionuclides in a more energy-saving manner. CRediT authorship contribution statement Kaifu Yu: Conceptualization, Writing - original draft, Methodology. Pengfei Shao: Investigation, Validation. Pengwei Meng: Investigation, Validation. Tao Chen: Resources, Data curation. Jia Lei: Resources. Xiaofang Yu: Supervision. Rong He: Conceptualization, Writing - review & editing, Formal analysis. Fan Yang: Writing - review & editing. 6
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Wenkun Zhu: Conceptualization, Project administration, Supervision, Funding acquisition. Tao Duan: Project administration, Supervision.
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Declaration of Competing Interest There are no conflicts to declare. Acknowledgements This work was supported by NSFC (Nos. 21601147, 21902130, and 21976147), China Postdoctoral Fund (No. 2018m630715), Sichuan Science and Technology Program (Nos. 2019JDRC0118, 2019YFN0125, 2019YFS0469, 2019YFS0503, 2019YFS0461, 2019YFG0433, and 2019YFG0434), Education Department of Sichuan Province (Nos. 17zd1131 and 18ZA0494), Sichuan’s Training Program of Innovation and Entrepreneurship for Undergraduate (No. S201910619101), Plan Projects of Mianyang Science and Technology (No. 18YFZJ003), Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory (No. 18kfhk03), the Project of State Key Laboratory of Environment-friendly Energy Materials in SWUST (No. 18fksy0218), Research fund of SWUST for PhD (No. 18zx7149), Longshan academic talent research supporting program of SWUST (Nos. 17LZX526, 18LZXT04 and 18LZX420), and Postgraduate Innovation Fund Project by SWUST (No. 19ycx0036). And thanks to Li Zhou, Renhao Tan, Wencai Bai, Xin Yuan, Changxue Dong, Ruihao Ai, Chenxi Hou and others for their help during the revision. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2020.122350. References Cao, J., Cohen, A., Hansen, J., Lester, R., Peterson, P., Xu, H., 2016. China-US cooperation to advance nuclear power. Science 353, 547–548. Zhang, X., Gu, P., Liu, Y., 2019a. Decontamination of radioactive wastewater: state of the art and challenges forward. Chemosphere 215, 543–553. Wang, X., Chen, L., Wang, L., Fan, Q., Pan, D., Li, J., Chi, F., Xie, Y., Yu, S., Xiao, C., Luo, F., Wang, J., Wang, X., Chen, C., Wu, W., Shi, W., Wang, S., Wang, X., 2019a. Synthesis of novel nanomaterials and their application in efficient removal of radionuclides. Sci. China-Chem. 62, 933–967. Shukla, A., Parmar, P., Sarar, M., 2017. Radiation, radionuclides and bacteria: an inperspective review. J. Environ. Radioact. 180, 27–35. Shao, L., Wang, X., Ren, Y., Wang, S., Zhong, J., Chu, M., Tang, H., Luo, L., Xie, D., 2016. Facile fabrication of magnetic cucurbit 6 uril/graphene oxide composite and application for uranium removal. Chem. Eng. J. 286, 311–319. Chen, T., Zhang, J., Ge, H., Li, M., Li, Y., Liu, B., Duan, T., He, R., Zhu, W., 2019a. Efficient extraction of uranium in organics-containing wastewater over g-C3N4/GO hybrid nanosheets with type-II band structure. J. Hazard. Mater. 121383. Jiang, X., Xing, Q., Luo, X., Li, F., Zou, J., Liu, S., Li, X., Wang, X., 2018. Simultaneous photoreduction of Uranium(VI) and photooxidation of Arsenic (III) in aqueous solution over g-C3N4/TiO2 heterostructured catalysts under simulated sunlight irradiation. Appl. Catal. B-Environ. 228, 29–38. Cai, Y., Wu, C., Liu, Z., Zhang, L., Chen, L., Wang, J., Wang, X., Yang, S., Wang, S., 2017. Fabrication of a phosphorylated graphene oxide-chitosan composite for highly effective and selective capture of U(VI). Environ. Sci.-Nano 4, 1876–1886. Zou, J., Liu, H., Luo, J., Xing, Q., Du, H., Jiang, X., Luo, X., Luo, S., Suib, S.L., 2016. Threedimensional reduced graphene oxide coupled with Mn3O4 for highly efficient removal of Sb(III) and Sb(V) from water. ACS Appl. Mater. Interfaces 8, 18140–18149. Wang, K., Ma, H., Pu, S., Yan, C., Wang, M., Yu, J., Wang, X., Chu, W., Anatoly, Z., 2019b. Hybrid porous magnetic bentonite-chitosan beads for selective removal of radioactive cesium in water. J. Hazard. Mater. 362, 160–169. Rahman, R.O.A., Ibrahium, H.A., Hung, Y.-T., 2011. Liquid radioactive wastes treatment: a review. Water 3, 551–565. Zhu, L., Gao, M., Peh, C.K.N., Ho, G.W., 2019a. Recent progress in solar-driven interfacial water evaporation: advanced designs and applications. Nano Energy 57, 507–518. Li, X., Lin, R., Ni, G., Xu, N., Hu, X., Zhu, B., Lv, G., Li, J., Zhu, S., Zhu, J., 2018a. Threedimensional artificial transpiration for efficient solar waste-water treatment. Sci. Rev. 5, 70–77. Tao, P., Ni, G., Song, C., Shang, W., Wu, J., Zhu, J., Chen, G., Deng, T., 2018. Solar-driven interfacial evaporation. Nat. Energy 3, 1031–1041. Luo, Y., Fu, B., Shen, Q., Hao, W., Xu, J., Min, M., Liu, Y., An, S., Song, C., Tao, P., Wu, J., Shang, W., Deng, T., 2019. Patterned surfaces for solar-driven interfacial evaporation. ACS Appl. Mater. Interfaces 11, 7584–7590.
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