Enhanced separation performance of Hg2+ in desulfurization wastewater using a tannin acid reduced graphene oxide membrane
Introduction
Mercury has been reported to be one of the most toxic heavy metals in the environment [1]. It may accumulate in the human body through the food chain, causing irreversible effects on the nervous system, kidneys, and liver [2]. Mercury pollution is closely related to anthropogenic activities and can be mainly ascribed to fossil fuel combustion in the coal-fired power plant [3]. The mercury and its compounds existed in the coal are likely to release in the combustion process. The elemental mercury (Hg0), oxidized mercury (Hg2+), and particle bound-mercury (HgP) compose the fundamental forms of mercury in fuel gas. The oxidized mercury (Hg2+) could be easily removed by the wet flue gas desulfurization (WFGD) system due to its high-water solubility. However, the large number of Hg2+ transferred into desulfurization slurry, which would deteriorate the performance of the WFGD system [4]. In addition, the Hg2+ could be reduced into Hg0, and escape from desulfurization wastewater [5], [6]. Therefore, it is necessary to remove Hg2+ from the desulfurization wastewater to prohibit this trend.
The membrane separation method, which offers low secondary pollution and effective separation performance, can be used in desulfurization wastewater treatment. Na et al. introduced an integrated membrane system, which consisted of the ultrafiltration ceramic membrane for desulfurization wastewater purification [7]. Zhang found a new way to treat desulfurization wastewater via bipolar membrane electrodialysis hybrid technology, and the duplicate cycles were also investigated [8]. Han investigated the separation performance of Hg2+ from wastewater using the FeS ultrafiltration membrane, and the mechanism was further studied [9].
Recently, Graphene-based membranes have attracted much attention, own to their unique functional structures [10], [11]. Graphene oxide, as a derivative of graphene, was a promising membrane for water treatment [12], [13]. However, the hydrogen-bonding interactions between functional groups and water molecules may reduce water flux [14]. Thus, the high separation barrier needs to be overcome for water transport, limiting the application in practical applications [15], [16].
To meet the requirement of practical application, the structure of the GO membrane should be further modified. At present, there were mainly two ways to improve the flux of membranes. On the one hand, some macromolecular materials can be used to insert into the membrane layers to expand the spacing of water channels. Chen et al. intercalated the well-dispersed carbon nanotubes to membranes to enlarge the water channel, making the membrane with a high-water flux [17]. Huang fabricated an ultrafiltration membrane, which contained numerous nano-channels with diameters of 3–5 nm [18]. However, the mostly designed membranes suffer from a low rejection rate and may not be a good choice for modifying membranes. On the other hand, the reduction of the GO membrane could have considerable potential [19]. The reduced GO membrane is obtained by removing the part of the epoxy and hydroxyl groups. As a result, the pristine graphitic sp2 domains in the GO membrane were recovered [15]. Thus, the reduced GO membrane always possesses a lower barrier to water transport. Huang reported a reduced graphene oxide membrane, which has a high flux and rejection rate towards Na+ and Cl− [19]. Liu prepared the freestanding ultrathin rGO membrane, and the flux of the membrane was significantly increased while still keeping an excellent separation efficiency [20]. However, the reduction process typically involves highly toxic reducing agents and harms the water body [21]. Moreover, the reduction process was intense, and the reduction degree cannot be easily controlled, leading to the aggregation of rGO in the water.
Tannic acid (TA), a low-cost and environmentally friendly polyphenol, has been regarded as a novel reductant [22]. It was reported that the TA could effectively reduce the GO, and the reduction degree could be easily adjusted [23], [24]. Singhal developed the tannic acid linked with graphene oxide for oil-contaminated water treatment. The results indicated that the TA could improve the separation performance of the GO membrane, and about 91.3% of total dissolved solids could be removed [25]. Luo investigated the effect of TA addition on GO reduction, and the results showed that the appropriate degree of reduction could improve the separation performance towards organic solvent from the water [26]. Lim fabricated the tannic acid functionalized GO membrane, and the rejection rate for NaCl and MgSO4 was 66.3% and 82.3%, respectively [27]. In addition, it was proved that TA could effectively bind with Hg2+ due to its abundant adjacent phenolic hydroxyls, and the high electrophilic reaction activities could improve the separation performance [28]. Thus, it is believed that the rGO-TA membrane could have an excellent potential for Hg2+ separation in wastewater treatment.
In this study, a novelty rGO-TA membrane was fabricated, which exhibited an excellent separation performance towards Hg2+ in wastewater treatment. The rGO-TA membrane was fabricated by the vacuum-assisted filtration method. This method was easy to operate, and it could ensure the membrane structure was highly uniform. After that, the rGO-TA membrane was optimized under different reduction degrees and tested in the lab-scale membrane evaluation system. The influences of several parameters, such as rGO-TA loading, initial Hg2+ concentration, operating temperature, and regeneration performance, were investigated. Moreover, the separation mechanism of the rGO-TA membrane was further discussed.
Section snippets
Materials
Tannic acid was purchased from Nanjing Chemical Reagent Limited Ltd. Graphene oxide, which was brought from Tanfeng Technology Co., Ltd (Suzhou, China), was obtained via hummers methods. The supporting substance used in this experiment was the Polyethersulfone (PES) flat sheet microfiltration membrane with a pore size of 0.1 µm. The PES membrane was known as high water flux and excellent mechanical properties due to its porous structures inside, which made it an ideal candidate for supporting
Characterization
Fig. 2 depicts the aqueous dispersion of GO, rGO-TA, and H-rGO. It can be observed that the color changed from brown to black after 12 h TA reduction process, indicating that the GO has been reduced by TA. It should be noted that the rGO-TA exhibited excellent stability in the water, and there was no significant aggregation or cluster. As compared, the H-rGO aqueous dispersion tended to precipitate in the water.
The TEM was carried out to analyze the structure and stability of GO-based
Conclusion
In this study, the controllable reduction degree of the rGO membrane was prepared by a facile and green method, and the reduction degree of the rGO-TA membrane could be adjusted by the addition of TA. In typical experimental conditions, the rGO reduced by TA retains the remarkable dispersion in water and could form the uniform membrane structure. More importantly, it recovered the graphitic sp2 domains and decreased the hydrogen-bonds force between functional groups and water molecules,
CRediT authorship contribution statement
Heng Chen: Investigation, Formal analysis, Writing - original draft. Mingyu Wang: Investigation, Formal analysis. Leilei Wang: Formal analysis, Methodology. Mengchang Zhou: Software, Investigation. Hao Wu: Writing - review & editing, Supervision. Hongmin Yang: Conceptualization, Supervision.
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 project is supported by the National Natural Science Foundation of China (No. 51676101 and 51806107)
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