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Organic dye removal from aqueous solutions by hierarchical calcined Ni-Fe layered double hydroxide: Isotherm, kinetic and mechanism studies

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Abstract

Hierarchically porous nickel–iron-layered double hydroxide (NiFe-LDH) with a Ni2+/Fe3+ molar ratio of 3 was successfully synthesised through a simple hydrothermal route. After calcination at 400 °C, NiFe-LDH transformed into nickel–iron-layered double oxides (NiFe-LDO). The as-prepared samples were characterised through X-ray powder diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and nitrogen adsorption. The calcined and uncalcined NiFe-LDH was used as adsorbents to remove Congo red (CR) dye in an aqueous solution. The equilibrium adsorption data of NiFe-LDH and NiFe-LDO samples were well fitted to Langmuir model and were characterised by excellent adsorption capacities of 205 and 330 mg/g, respectively. Pseudo-second-order kinetic and intra-particle diffusion models indicated that CR was well adsorbed on the adsorbent. The underlying adsorption mechanism was investigated and observed as anion exchange and reconstruction.

Introduction

Organic dyes have been widely used in various industrial processes, including textiles, leather, printing, dyes and plastics [1], [2], [3]. Dye pollutants discharged into water released from these industries present a serious threat to human health and the ecosystem because of their high toxicity, carcinogenicity and mutagenicity [4], [5], [6]. Therefore, rational and efficient methods should be developed to remove dyes from wastewater. Various wastewater treatment techniques, such as adsorption [7], coagulation [8], flocculation [9], electrochemical method [10], photocatalysis [11], ion exchange [12], and biodegradation [13], have been applied to eliminate dye effluents from water. Among these techniques, adsorption is the most promising strategy because of its high efficiency, simple operation and low energy requirement [14]. An appropriate adsorbent is central to the adsorption method. Various materials, such as activated carbon [15], fly ash [16], clay minerals [17], silicon nanomaterials [18], and metal oxides [7], [19], have been used as adsorbents. However, the application of these adsorbents is limited by different factors, such as low adsorption capability, high cost and difficult regeneration. Therefore, new adsorbents with excellent adsorption capability and low production costs should be developed for theoretical research and practical applications.

Layered double hydroxides (LDHs, [M1−x2+Mx3+(OH)2Ax/nn·mH2O]) as a class of anionic clays [20] have been extensively investigated because of their tunable charge density and wide application prospects [21], [22], such as catalysts [23], [24], [25], biological agents [26], energy storage and conversion [27], [28], [29]. LDHs have been considered excellent adsorbent materials for wastewater treatment because of their layered structure, high surface area and interlayer ion exchange [22], [30], [31], [32]. Easily prepared LDHs, such as ZnAl-LDH [22], MgAl-LDH [4], MgFe-LDH [32] and CuAl-LDH [33], have been applied to remove dyes. These LDHs provide advantages over traditional adsorbents in terms of high adsorption capacity, low cost and non-toxicity. Thus, the utilisation of LDHs can provide substantial economic and environmental benefits to wastewater treatment. LDH microspheres with hierarchically porous structures have also been widely explored because of their excellent surface area and structural stability. LDH microspheres have also been utilised for adsorption. For instance, Lin et al. [30] synthesised ZnAl-LDH microspheres through co-precipitation and compared their adsorptive and removal properties for methyl orange. Ahmed et al. [32] prepared Mg–Fe–CO3-LDH to adsorb anionic reactive dye. Sun et al. [34] synthesised hierarchically porous NiAl-LDHs via a hydrothermal method and investigated their capacity to adsorb p-nitrophenol from water.

Congo red (CR) as a typical anionic diazo dye is usually chosen as a model pollutant because of its toxicity, carcinogenicity and poor degradation. In this work, a simple hydrothermal method was used to prepare nickel–iron-layered double hydroxide and oxide with a hierarchical porous structure and a large surface area. These compounds were utilised as adsorbents to remove CR dye from water.

Section snippets

Materials

All reagents, such as Ni(NO3)2·6H2O, FeSO4·7H2O, urea and CR (Shanghai Chemical Industrial Company) were of analytical grade and used as received without further purification. Distilled water was utilised in synthesis and treatment processes.

Preparation of samples

Hierarchical porous nickel–iron-layered double hydroxide (NiFe-LDH) was prepared hydrothermally. In a typical synthesis process, 3 mmol Ni(NO3)2·6H2O, 1 mmol FeSO4·7H2O and 12 mmol urea were dissolved in 60 mL of deionised water. The resulting solution was

Phase structure and morphology

Fig. 1 demonstrates the XRD patterns of the NiFe-LDH and NiFe-LDO samples. The NiFe-LDO samples after CR was adsorbed (Fig. 1). Nine major peaks of the NiFe-LDH sample were located at approximately 11.4°, 23.0°, 34.4°, 39.0°, 45.98°, 59.9°, 61.3°, 65.1° and 71.3°. These peaks can be assigned to the (0 0 3), (0 0 6), (0 1 2), (0 1 5), (0 1 8), (1 1 0), (1 1 3), (1 1 6) and (1 1 9) planes of Ni0.75Fe0.25(CO3)0.125(OH)20·38H2O (JCPDS Data Card No. 40-0215) [35]. After calcination occurred, the layered structure of

Conclusions

In summary, NiFe-LDH with hierarchical porous structures was successfully synthesised through a facile hydrothermal route. After the sample was calcined at 400 °C, NiFe-LDH transformed into NiFe-LDO. The BET surface area increased from 60 m2/g to 121 m2/g. The maximum adsorption capacities of the NiFe-LDH and NiFe-LDO samples of CR calculated from the Langmuir isotherm model were 205 and 330 mg/g, respectively. The experimental data of the samples well fitted to the Langmuir isotherm model and

Acknowledgements

This study was partially supported by the National Natural Science Foundation of China (NSFC) (51208068), Social Development Project of Jiangsu Province (BE2015670, BY2016029-20).

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