Evenly distribution of amorphous iron sulfides on reconstructed Mg-Al hydrotalcites for improving Cr(VI) removal efficiency
Graphical abstract
Introduction
Elevated hexavalent chromium (Cr(VI)) concentration in industrial water is a significant threat to human health [1], [2], [3]. In the aquatic system, toxic Cr(VI) exists as the hydrophilic oxyanions that is easy to migrate and diffuse, while the low-toxic trivalent chromium (Cr(III)) is always insoluble in neutral and alkaline conditions [4], [5]. Therefore, the reduction of Cr(VI) to Cr(III) is a traditional and effective decontamination way for chromium in water [6], [7], including the main reduction techniques: electrocatalytic, photocatalytic, and chemical reduction [8], [9].
Various reducing agents have been utilized for the reduction of Cr(VI) to Cr(III) including ferrous ions, metallic iron, metallic sulfides, and organic complexes [10], [11], [12], [13]. Iron sulfides (FeS) have attracted great attention because they hold significant advantages of containing Fe(II) and S(-II) species that both can serve as reducing substance [14]. Compared with natural iron sulfides, nanoscale FeS is particularly attractive owing to their extremely large specific surface areas and potentially greater reactivity. However, nanoscale FeS is very easy to form agglomeration and loses its advantage on surface area. To suppress the aggregation, porous materials such as biochar, natural silica sand, activated carbon have been applied to distribute FeS, but the weak chemical bonding between FeS and the supporting materials might increase leakage risk during practical application [15], [16], [17]. Especially, the high concentration of Cr(VI) could acidify water, which would increase the secondary pollution risk due to the decomposition of FeS. Thus, improving the acid resistance has become the key to improve the applicability of FeS-based adsorbents in a wide range of pH conditions.
Hydrotalcite-like compounds (HTs), also known as anionic clays or layered double hydroxides (LDHs), is one of 2-dimensional (2D) layered nanomaterials with a general formula: (Anionn−)x/n·mH2O. As a kind of nanosheet material, HT-like compounds combine the advantages of the large specific surface area of nanomaterials and the layered structure of 2D materials. Mg-Al-CO32--hydrotalcite, the most popular HT-like compounds, can be redesigned to improve basic properties by calcination and rehydration utilizing the “memory effect” of HTs [18]. Additionally, a large number of hydroxide groups on the surface can help resist the strong acid environment and expand applicable pH range of adsorbents by buffering solution pH to neutral or alkaline [19]. It has been proved that the surface of HT is always positively charged due to the protonation of the surface hydroxide groups. The positive surface charge enhances the electrostatic attraction between anions and HTs [20], which benefits the adsorption of Cr(VI) anions. However, simple adsorption by anion exchange or electrostatic attraction cannot reduce the toxicity of adsorbed Cr(VI), and the risk of desorption still exists. In recent researches, many HT-containing hybrids have been investigated for their adsorption performance, but the diffusion, penetration and transportation of Cr(VI) from the surface of host layers to the interlayer area greatly limited the adsorption speed and capacity [21]. Hence, a powerful surface treatment of HT might achieve a boost in adsorption performance of Cr(VI) [22], [23], [24].
Inspired by the reducing properties of FeS and abundant sorption sites of HTs, we believe that combining FeS and HT together can take the advantages of both materials. However, to the best of our knowledge, no efforts have been reported about the removal of Cr(VI) by FeS/HTs composite materials. Here, we synthesized a novel FeS/HTs composite adsorbent via “calcination-reconstruction-calcination” synthesis method, which showed good adsorption-coupled reduction property to Cr(VI). Meanwhile, the removal mechanism of Cr(VI) by FeS/HTs was systematically investigated.
Section snippets
Chemicals
All the reagents used in this study were commercially available analytical grade and applied without any further purification. FeCl2·4H2O (98%, Inno Chem), thioacetamide (TAA, ≥ 99%, Aladdin Chemical), and Hydrotalcites (HT, synthetic, Sigma-Aldrich) were used to prepare the FeS/HTs. K2Cr2O7 (>99.8%, Sinopharm Chemical Reagent) were used to prepare the solutions for batch removal experiments. Ultrapure water (UP) (18.2 MΩ*cm) was used in all experimental processes.
Preparation of FeS/HTs
Because of the “memory
Morphology of as-prepared FeS/HTs
To understand the effect of calcination and FeS/HT mass ratios on the microscopic structures of materials, the morphologies of these materials were observed through SEM and TEM. As shown in Fig. 1a, the original HT was a brucite-like layer material with smooth surface. However, the HT particles stacked together after 500 °C calcination and the surface showed a reconstructed thin nanosheet-like structure, which might offer more modification sites for the afterward loading of Fe ions (Fig. 1b).
Conclusions
This work developed a novel FeS/HTs adsorbent which can achieve an effective removal of Cr(VI) in swage. Fe atoms successfully replaced Mg atoms in mineral sheets in HT, and the typical hierarchical structure of HT remained unchanged after low-temperature sulfidation. In addition, the crystallinity of FeS was effectively controlled by the mass ratio of FeS/HT, the amorphous iron sulfide could be achieved when the proportion of HT was relatively high. In this work, FeS/HT-1/2 exhibited a better
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.
Acknowledgement
This work was supported by the National Key R&D Program of China (No.2018YFC1800600); the National Key Research and Development Program (No.2017YFE0127100); the National Natural Science Foundation of China (NSFC 21773155, 42007125); the Shanghai Sailing Program (19YF1422200); Engineering Research Center for Nanophotonics & Advanced Instrument, the Ministry of Education, East China Normal University (No.202001), and JST SICORP Grant Number JPMJSC18H1, Japan.
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