Research PaperFacile fabrication of Cu-exchanged ZnS nanoadsorbents for highly efficient removal of contaminants
Graphical abstract
Introduction
Nanotechnology has hold enormous potential in environmental remediation and wastewater treatment because nanomaterials have substantially large surface areas to provide enhanced adsorption capability and affinity to pollutants compared to bulk-sized materials [1], [2], [3], [4]. Especially, adsorption, membrane separation, coagulation/flocculation, photocatalysis, and disinfection have been investigated broadly using nanomaterials for the treatment of wastewater [1], [2]. However, most methods mentioned above are not very successful due to many restrictions as follows: high energy and high cost in photocatalysis [5], [6], [7], high generation of sludge in coagulation/flocculation [1], [2], [8], [9], and less effectiveness in disinfection [10]. In contrast, adsorption has been one of the most preferred ways to deal with pollutants owing to simplicity and cheapness as well as high efficiency and adsorbent diversity [2], [3], [4], [11], [12], [13], [14], [15], [16], [17], [18].
Most extensively used are carbon-based adsorbents, which are suitable for the elimination of traditional pollutants such as dyes, phenols, organic acids, and heavy metals [15], [16], [17], [18]. In particular, graphene-derivatives like carbon nanotubes and graphene oxides have shown considerably stronger binding affinity with organic pollutants and heavy metals via combination of van der Walls forces, electrostatic attraction, and π-π stacking interactions [17], [18], [19], [20], [21]. However, these designed and engineered carbon-based materials require high cost in the fabrication process and suffer from the low dispersibility of water [19], [20]. Moreover, they are costly in regeneration and non-selective for the removal of ionic pollutants [20]. Thus, to overcome these drawbacks, alternative materials such as organic matters, natural and synthetic polymers, and inorganic solids have been reported [3], [4], [22], [23], [24], [25], [26].
Metal sulfides as adsorbents have been hardly studied yet [12], [27]. However, micro- and nano-sized metal sulfides can be utilized adequately to manage organic contaminants because their fabrication process is cheap and simple [27], [28], [29], [30]. Furthermore, they are abundantly available and environmentally friendly, and they can be easily modified with other materials via being doped with metals and non-metals or via being coupled with other semiconductors [5], [26], [27], [28], [29], [30], [31], [32], [33]. Among metal sulfides, wurtzite ZnS has been investigated primarily due to not only its great potential in photocatalysis and opto-electronics but also its high thermodynamical stability [34], [35]. It is also easily size-controllable, highly mass-producible, relatively inexpensive, and less toxic. One-dimensional (1D) nanostructures such as nanorods and nanobelts have been employed usefully for the purpose of wastewater treatment due to their peculiar structures having nano-micrometer scales, allowing them to be easily separated from water by a simple sedimentation process [34], [35], [36], [37]. Furthermore, 1D nanostructures consisting of micro- and mesopores also have large pore volumes and surface areas, which will be also beneficial for efficient liquid transport as well as effective contact with pollutants in water [5], [31], [37], [38], [39]. Moreover, theses nano-micro scaled 1D nanostructures can serve as good templates for the formation of hybrid materials that may not only inherit the advantages of their parent materials but also possess the synergistic effect, further enhancing the water-treatment performances of their parent materials [37], [38], [39].
In this work, we have fabricated Cu(I)-exchanged ZnS nanoadsorbents through facile cation exchange of ZnS nanorods, nanobelts, or nanosheets as templates [12], [28], [29], [30]. Their surface properties such as surface charges and areas have been controlled by adjusting the molar ratio of Cu to Zn (RCu/Zn). The adsorption efficiency of Cu-exchanged ZnS nanoadsorbents is highest at a RCu/Zn value of 0.4 because the net surface charges of the nanocomposites are electronically most negative. The kinetics and isotherms of adsorption and the effects of various parameters such as concentration, pH, and time on the adsorption process have been investigated in detail. We have found that the high removal efficiencies of cationic organic pollutants and heavy metals result from the increased negative surface charges as well as the enhanced surface areas and the total pore volumes of nanocomposites. Overall, our prepared Cu-exchanged ZnS nanoadsorbents can be applied potentially for the treatment of wastewater containing cationic organic dyes and heavy metals.
Section snippets
Materials
Ethylenediamine (en, 99%) and N2H4·H2O (98%) from Alfa Aesar, ZnCl2 (>98%), CuCl (>99.995%), HCl (37%), NiCl2·6H2O (>99.999%), and CoCl2·H2O (>99.999%) from Sigma-Aldrich, sulfur (>99.0%), ethanol (>99.0%), methyl orange (MO), methylene blue (MB), light green sf yellowish (LGY), and NAOH (>97%) from Daejung Chemicals, and rhodamine B (RhB) from Wako Pure Chemical were used as received without further purification. Deionized water (>15 MΩ cm) from an Elga PURELAB system was used throughout the
Results and discussion
In order to investigate the morphologies and structures of Cu-exchanged ZnS (RCu/Zn = 0.4) nanosheets, nanobelts, and nanorods, SEM and TEM images have been measured (Fig. 1). Note that RCu/Zn is the molar ratio of Cu to Zn in Cu-exchanged ZnS nanocomposites. As-prepared nanosheets have two dimensional (2D) morphologies with an average width of 500 ± 150 nm and an average length of 1.1 ± 0.3 μm (Fig. 1a) while nanobelts have 1D morphologies with an average width of 130 ± 40 nm and average length of 4.6 ± 1.3
Conclusions
Cu(I)-exchanged ZnS nanoadsorbents have been fabricated through facile cation exchange of ZnS nanorods, nanobelts, or nanosheets as templates. Their surface properties such as surface areas and charges have been controlled by adjusting the value of RCu/Zn. The adsorption efficiencies of Cu-exchanged ZnS nanoadsorbents have been found to be highest at a RCu/Zn value of 0.4 because the net surface charges of Cu-exchanged ZnS nanocomposites resulting from the substitution of Cu(I) ions for Zn(II)
Acknowledgements
This work was supported by research grants through the National Research Foundation (NRF) of Korea funded by the Korean government (2017-006153 and 2015-051798).
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