Elsevier

Chemosphere

Volume 280, October 2021, 130618
Chemosphere

g-C3N4/MoS2 based floating solar still for clean water production by thermal/light activation of persulfate

https://doi.org/10.1016/j.chemosphere.2021.130618Get rights and content

Highlights

  • A g-C3N4/MoS2 based floating solar still (CM-FSS) was fabricated.

  • The CM-FSS was applied for water evaporation based on interfacial solar heating.

  • VOCs were reduced by g-C3N4/MoS2 photocatalysis and thermal/light activation of PS.

  • The quality of condensed freshwater below the limit values of WHO, EPA and China.

Abstract

Currently, seawater desalination based on air-water interface solar heating has triggered significant research interests because it effectively makes use of the solar energy and avoids fossil fuel consumption. However, to prevent the volatile organic compounds (VOCs) from volatilizing with water vapor which later will liquefy and enter the condensed freshwater is still a challenge. In this work, a g-C3N4/MoS2 based floating solar still (CM-FSS) combined with thermal/light activation of persulfate (PS) at air-water interface was applied for clean freshwater production for the first time. The CM-FSS was composed of a g-C3N4/MoS2 top layer for solar absorption, simultaneous thermal/light activation of PS and then VOCs degradation at air-water interface, a floating layer of expandable polyethylene (EPE) foam for heat isolation, and a transport channel of air-laid paper (ALP) for seawater and PS solution delivery. The water evaporation rate of the CM-FSS was measured at 1.23 kg m−2 h−1 under 1 kW m−2, which is 4.09 times higher than that of pure water without an evaporator. With the assistance of g-C3N4/MoS2 photocatalytic degradation and thermal/light activation of PS at the air-water interface, a high removal efficiency of a selected model VOCs pollutant of nitrobenzene (NB) could reach to 98.2% in condensed freshwater. Finally, when real seawater samples were employed as source water for solar distillation, the typical water-quality indices such as salinity, turbidity, anions, cations and organics of the condensed freshwater were below the limit values of the Standards for Drinking Water Quality in WHO, US EPA and China.

Graphical abstract

A g-C3N4/MoS2 based floating solar still combined with heat/light activated persulfate for continuous clean water production based on air-water interface solar heating.

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Introduction

Recently, seawater desalination based on air-water interfacial solar heating has triggered significant research interests due to its low energy consumption, high evaporation efficiency, simple operation, low cost, and etc. (Chen et al., 2017a). However, the temperature of the air-water interface can reach to 40–72 °C under 1 sun irradiance which will lead to the evaporation of the organic compounds (Ghasemi et al., 2014; Li et al., 2016; Wang et al., 2016b, 2017; Zhou et al., 2016; Chen et al., 2017a, 2017b; Shi et al., 2019), especially the volatile organic compounds (VOCs), together with the water vapor and the organic compounds can enter into the condensed freshwater if polluted source water was applied. Moreover, the Henry's constant of organic compounds increases 1.88 times as much as the original value for every 10 °C rise in temperature (Staudinger and Roberts, 2001). The higher Henry's constant results in the more volatility of the organic compounds. To date, most of the previous clean water quality research focused on the salinity, turbidity, cations (such as Na+, K+, Ca2+, Mg2+) and anions (such as F, Cl, NO3, SO42−) of the freshwater while few works focused on the organic compounds (Liu et al. (2015); Lou et al., 2016; Liu et al., 2017; Hao et al., 2018; Liu et al., 2018; Gan et al., 2019). Hao et al. (2018) have reported a multifunctional cotton fabric with deposition of titanium dioxide nanoparticles for both efficient steam generation and methyl orange removal. Liu et al. (2015) have reported a membrane consisting of three layers consist of TiO2 nanoparticles, Au NPs, and anodized aluminum oxide for steam generation and rhodamine B removal. It is well-known that the seawater distillation can directly leave behind the non-volatile compounds such as anions, cations, organic dyes, and simultaneously generate the clean water vapor. It is essential to pay more attention to the removal of VOCs during the solar distillation process, as these have not been thoroughly investigated.

Molybdenum disulfide (MoS2), an excellent light absorption material in the visible and infrared regions (Chou et al., 2013; Lei et al., 2018), has been used as photothermal material for solar distillation. Though the adsorption and photocatalytic activity of MoS2 is not high, it has been applied for the removal of organic pollutants in aqueous solution (Wang et al., 2018; Chen et al., 2019), To improve its photocatalytic activity, MoS2 has been coupled with graphitic carbon nitride (g-C3N4) due to its low toxicity and visible light response (Lu et al., 2016; Yu et al., 2017; Wang et al., 2019; Zhu et al., 2019). Given this, it is rational to expect g-C3N4/MoS2 to ideally act as a bifunctional material for both solar evaporation and VOCs removal during the solar distillation process. In addition, it has been widely reported that the PS can be activated by heat and ultraviolet (UV) to generate free radicals to remove organic pollutants (Dominguez et al., 2020). Given this, it is also reasonable to think that PS can be used to degrade VOCs during the solar distillation process because it can be activated by the high temperature of the air-water interface as well as the sunlight during evaporation.

In this work, g-C3N4/MoS2 was used as a photothermal material for both water evaporation and a photocatalyst for organic pollutants removal. For further enhancing the water evaporation efficiency, the g-C3N4/MoS2 was combined with air-laid paper (ALP) and expanded polyethylene (EPE) foam to form a floating solar still (denoted as CM-FSS). Nitrobenzene (NB) was chosen as a model VOCs pollutant to investigate the contaminant removal property of CM-FSS. In addition, certain amounts of PS were added into the impaired source water for the first time due to its activation by the high temperature of the air-water interface as well as the sunlight during evaporation to further enhance the NB removal rate. Finally, to explore the potential application of CM-FSS combined with PS in practical setting, a real seawater sample was employed as source water for solar distillation and the quality of the condensed freshwater was evaluated. The purposes of this study are: (i) to provide a g-C3N4/MoS2 based floating solar still for freshwater production based on interfacial solar heating; (ii) to reduce the volatile organic pollutants into condensed freshwater with the assistance of g-C3N4/MoS2 photocatalysis and thermal/light activation of PS.

Section snippets

Materials

All chemicals were of an analytical grade and used as received without any further purification. Sodium molybdate (VI) dihydrate (Na2MoO4·2H2O), thiourea (CS(NH2)2), and dicyandiamide (C2H4N4) were obtained from Sinopharm Chemical Reagent Co., China. Sodium persulfate (PS) and nitrobenzene (NB) were purchased from the Shanghai Aladdin Industrial Co., China. Air-laid paper (ALP) was purchased from Kimberly-Clark Co. USA. Expandable polyethylene (EPE) foam was supplied by Zibo Baisheng Packing

Characterization of the CM-FSS

The production process of CM-FSS can be two steps. First, the g-C3N4/MoS2 hybrids with different MoS2 depositions were fabricated via a facile ultrasonic adhering approach (Wang et al., 2018). The SEM images (Fig. 1a and b) show that the morphology of the MoS2 was flowerlike in shape with an average diameter of about 500 nm while the morphology of the g-C3N4 was in layered structure. With the deposition of MoS2, the surface of the g-C3N4 was covered with a large number of small particles (Fig. 1

Conclusion

In summary, CM-FSS was prepared and employed for seawater desalination via air-water interface solar heating. The water evaporation rate could be significantly enhanced to 1.23 kg m−2 h−1 under 1 sun irradiance (1 kW m−2) by CM-FSS due to its high optical absorbance of g-C3N4/MoS2, the insulation layer of EPE foam and rapid water delivery channel of the ALP, which is 4.09 times higher than that of pure water without an evaporator. Also, a high removal efficiency of the selected model VOCs

Supporting information

Digital photo of condensed freshwater collection device; The proportion of light absorption in different spectrum regions; Summary of representative photothermal materials under solar irradiance; The typical water-quality ions of the seawater before and after desalination Energy balance analysis.

Credit author statement

Qimao Gan and Yangyi Xiao contributed equally to this work, Qimao Gan: Investigation, Data curation, Writing – original draft, Yangyi Xiao: Investigation, Data curation, Writing – original draft, Chenxing Li: Investigation, Data curation, Huan Peng: Investigation, Tuqiao Zhang: Supervision, Funding acquisition, Miaomiao Ye: Conceptualization, Supervision, Writing- Reviewing and Editing, Project administration

Declaration of competing 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.

Acknowledgements

The present work was financially supported by the National Natural Science Foundation of China (No. 52070161).

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