Elsevier

Chemical Engineering Journal

Volume 402, 15 December 2020, 126180
Chemical Engineering Journal

Introduction of vacancy capture mechanism into defective alumina microspheres for enhanced adsorption of organic dyes

https://doi.org/10.1016/j.cej.2020.126180Get rights and content

Highlights

  • DF-AlOx has abundant oxygen vacancies and coordinatively unsaturated Al-species.

  • The DF-AlOx exhibits enhanced adsorption performance for CR, MB and MO.

  • A vacancy capture mechanism for enhanced adsorption performance was proposed.

  • Oxygen vacancies can be easily generated on the DF-AlOx compared with Al2O3.

  • Oxygen vacancies mainly act as capture centers and facilitate dyes adsorption.

Abstract

Introducing defects into metal oxides is considered as an effective method to improve their performance. However, to fully understand the role of oxygen vacancies in the adsorption of organic dyes is still a challenging target. Herein, defective alumina (DF-AlOx) microspheres were synthesized and used as adsorbents for removing organic dyes. In contrast to ordinary Al2O3, the DF-AlOx has an amorphous structure. The DF-AlOx possesses abundant coordinatively unsaturated Al-species, oxygen vacancies and oxygen-containing groups. The DF-AlOx exhibited excellent adsorption performance for dyes compared to Al2O3. The maximum adsorption capacities for congo red (CR), methylene blue (MB) and methyl orange (MO) estimated from the Langmuir model are 1156.5, 349.5 and 1270.8 mg/g, respectively. Based on the experimental data and density functional theory (DFT) calculations, a vacancy capture mechanism for enhanced adsorption of dyes was proposed. Compared with Al2O3, oxygen vacancies can be easily generated on the DF-AlOx surface, which mainly act as the capture centers and facilitate the adsorption of dyes. This work may shed light on designing high-efficiency adsorbents and further investigation of adsorption mechanism for organic dyes removal.

Introduction

Water contaminated by organic dyes is becoming a potential threat to human health and environment, because a majority of them are carcinogenic, teratogenic and genotoxic [1], [2], [3], [4]. Therefore, there is an urgent need to develop effective methods to remove organic dyes from water. Adsorption is one of the most effective technologies that have been widely adopted to achieve this purpose [5], [6], [7], [8], [9]. However, the practical application of traditional absorbents such as activated carbon is often limited by their low adsorption capacity [10], [11], [12], [13], [14]. Hence, it is high demand for developing high-efficiency adsorbents to remove organic dyes.

Alumina-based materials with hierarchical structure have attracted considerable attention owing to its low cost and environmentally benign nature [15], [16], [17]. Compared with traditional bulk adsorbents, the hierarchical structured alumina-based materials exhibited enhanced removal performance in water pollutants. For instance, Al2O3@ZnO core–shell microfibers [18], γ-Al2O3 hollow microspheres [19] and nanoparticles [20] showed improved adsorption performance for congo red. For spindle like γ-Al2O3 [21], it has been used to remove congo red (176.7 mg/g), phenol (21.0 mg/g), and Cd(II) (10.1 mg/g) in water. However, the adsorption capacities of these alumina-based materials are still not satisfactory, and it is imperative to design efficient alumina-based adsorbents to remove organic dyes.

Recently, creating defects (e.g., oxygen vacancies) in material has proved to be a promising approach to improve its performance owing to great number of surface active sites and high surface energy [22], [23], [24], [25], [26], [27]. More specifically, introducing oxygen vacancies into catalysts not only change coordination structure, but also regulate the surfaces electronic state, which in turn influence the intrinsic reactivity [28]. For instance, increasing the abundance of oxygen vacancies to photocatalysts, such as TiO2−x [29], ZnO1−x [30], can greatly improve their activity by narrowing bandgap and extending visible light absorption. In some case, the presence of oxygen vacancies can obviously reduce the resistance of charge-transfer and lower the adsorption energy of H2O molecules, and thus improving the electrocatalytic activity [31], [32]. In addition, oxygen vacancies formed played a critical role in promoting photoinduced charge separation, leading to an increased photocatalytic activity towards the reduction of CO2 [33]. Similarly, the key roles of oxygen vacancies in promoting the adsorption and activation of reactant molecules have been fully demonstrated [34], [35], [36]. Upon contact, oxygen vacancies can change the state of the reactants (eg, bond length, bond angle, coordination pattern, or intermediates), thereby increasing the affinity for the reactants. Zhang et al. [25] reported the high adsorption affinities of oxygen vacancy-rich WOx/C nanowires for Pb2+ and methylene blue. Although the relationship between defects and catalytic performance has been widely investigated in heterogeneous catalysis. The specific role of defects in adsorption performance of alumina-based materials for organic dyes have not been identified and clarified to date. Therefore, it is very meaningful to adjust the adsorption performance of materials with defects.

Our previous research found that the microsphere-shaped materials generally have larger specific surface area, well-developed pore structure and abundant functional groups [37], [38], [39], [40]. This allows such materials to expose more active sites, thus showing better adsorption performance. Therefore, in this work, we designed alumina into a spherical shape and use it to remove organic dyes. The aims of this work are to develop high-efficiency adsorbents for dye removal, and then give detailed analysis about the role of the defects in dye removal, finally discuss adsorption mechanism for dye on the defective alumina microspheres (DF-AlOx).

Section snippets

Synthesis of materials

The chemical reagents used were purchased from Sinopharm Chemical Regent Beijing Co., Ltd., including aluminum nitrate (Al(NO3)3·9H2O), acetic acid (HAc), isopropanol (IPA), congo red (CR, C32H22N6Na2O6S2), methyl blue (MB, C37H27N3Na2O9S3) and methyl orange (MO, C14H14N3SO3Na). The chemical structure of CR and MB are displayed in Fig. S1.

The preparation process for the DF-AlOx is as follows. Firstly, 30 mmol Al(NO3)3·9H2O was placed in a mixed solution (60 mL) containing HAc and IPA, and then

Characterization of materials

The as-obtained DF-AlOx was successfully synthesized via a solvothermal reaction by using Al(NO3)3·9H2O as metal salt, HAc and IPA acetic acid as solvent, as demonstrated in Fig. 1a. Firstly, the Al(NO3)3·9H2O, HAc and IPA were hydrothermally treated at 200 °C for 2 h to form the precursor of the DF-AlOx (step I). Subsequently, the resulting precursor was calcinated under N2 atmosphere to obtain the final product (step II). The as-obtained DF-AlOx was characterized using XRD and SEM to verify

Conclusions

In this work, defective alumina microspheres were prepared and used as adsorbent to remove organic dyes from aqueous solution. The DF-AlOx microspheres have amorphous structure and present yellow colour, which are different from the ordinary alumina. The DF-AlOx microspheres possess coordinatively unsaturated Al-species, large surface area, abundant oxygen-containing groups and oxygen vacancies. Due to its structural characteristics, the DF-AlOx microspheres exhibit excellent adsorption

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.

Acknowledgments

This work was supported by National Natural Science Foundation of China (51808037), Fundamental Research Funds for the Central Universities (FRF-TP-19-020A2) and Natural Science Foundation of Guangdong Province (2020A1515011197).

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