Enhanced uranium(VI) adsorption by chitosan modified phosphate rock

https://doi.org/10.1016/j.colsurfa.2018.02.043Get rights and content

Highlights

  • A high-performance chitosan modified natural phosphate rock was successfully synthesized.

  • The adsorption capacity for uranium(VI) was remarkably enhanced by chitosan modification.

  • New functional groups and crystalline phases were determined for the adsorption mechanism.

Abstract

The interactions between uranium(VI) and phosphate are known to be critical in governing the mobility of uranium(VI). However, only using phosphate compounds to remove uranium(VI) in aqueous solution is often unsatisfactory owning to insufficient active sites on their surface. The enhancement of the adsorption ability of phosphate compounds is timely important. In this study, we successfully modified a typical type of natural phosphate rock with chitosan and applied this product to remove uranium(VI). The effects of initial uranium(VI) concentration, contact time and adsorbent dosage on uranium(VI) removal behavior were systematically investigated via a series of batch experiments. The uranium(VI) adsorption on chitosan modified phosphate rock can reached equilibrium after 5 h at acidic pH (pH 2.5). The uranium(VI) removal efficiency was greatly enhanced after phosphate rock was modified by chitosan. Adsorption mechanism of uranium(VI) on modified phosphate rock was proved by FT-IR, XRD, and SEM. After modification, a new group (–NH2) and a crystalline phase of ammonium calcium hydrogen phosphate (NH4CaHP2O7) were generated. Being adsorbed uranium(VI), the morphology of adsorbents (i.e., chitosan modified phosphate rock and phosphate rock) changed markedly. XRD results showed that certain diffraction peaks for newly generated crystal structures after chitosan modification, besides these diffraction peaks became weakened and even disappeared after uranium(VI) adsorption. As to the adsorption mechanism of this modify adsorbent, results from isotherm and kinetic analyses demonstrated that uranium(VI) adsorption was more related to Freundlich model and pseudo second order equation.

Graphical abstract

The adsorption of uranium(VI) by chitosan modified natural phosphate rock before and after treatment.

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Introduction

Uranium is an important industrial material that widely used in nuclear power production. The release of radioactive wastewater containing uranium from uranium mining and nuclear facilities has become a severe environmental problem [[1], [2], [3]]. Uranium presents mainly in the forms of uranium(IV) and uranium(VI). Among them, uranium(VI) often attracts a greater attention because of its high toxicity and mobility [4,5]. Uranium(VI) can easily accumulate in human body via food chains [6]. In order to cope with the radioactive wastewater containing uranium(VI), many efforts on the development of treatment technologies have been performed and these technologies included chemical precipitation, ion-exchange, biological and extraction [[7], [8], [9], [10]]. However, these methods commonly have many inconvenient short comings such as unstable, uncertainty, difficult in operation, high cost, etc. [11,12]. Alternatively, adsorption offers a cost-effective treatment approach that could effectively remove contaminants from aqueous solution at low cost [13,14].

It has been found that many natural minerals show good environmental compatibility and excellent removal capability. For example, clay minerals and iron ore slimes showed a high sorption capacity and efficiency for metal contaminants in wastewater [15,16]. As one of the natural mineral, phosphate rock (PR) exists widely in nature and commonly being accompanied by organic matter, clay minerals and pyrite [17]. Phosphate rock shows a great potential to be one of efficient adsorbents [18]. Previous study has demonstrated that phosphate rock has the maximum sorption capacity to 7.63 mg/g for Ni2+ [19] and 101.13 mg/g for methylene blue [20] from aqueous solution. But when chemical synthesis and modifies was added hydroxyl function, a greater adsorption capability ions can be achieved 46.17 mg/g for Ni2+ [21] and 405.4 mg/g for methylene blue [22]. Therefore, it can be inferred that hydroxyl apatite is a huge potential adsorbent for both heavy metal ions and organic pollutants from wastewater stream. Of note, there is also some other evidence illustrate hydroxyl apatite also can effectively remove uranium(VI) from aqueous solution [[23], [24], [25]]. And due to the strong uranium(VI)-phosphate interactions, the adsorbed uranium(VI) on phosphate compounds is not easily desorbed, displaying irreversible tendency and thus achieving the aim of immobilization and adsorption on the surface of absorbent [[26], [27], [28]].

In addition, amino belong a kind of common functional group which can react with hexavalent uranium, their importance are cannot be ignored [29]. Therefore, chitosan as one of the most prevalent modifiers that contains both amino and hydroxyl functional groups, it also exhibits a good affinity for uranium and thus can effectively adsorb uranium(VI) [[30], [31], [32]].

Moreover, in our previous work, we have demonstrated that natural phosphate rock can remove uranium(VI) from wastewater but the adsorption capability of uranium(VI) still can be improved [33]. To further enhance the adsorption capability for uranium(VI), we will modify the natural phosphate rock with chitosan. This study will investigate the enhanced performance of chitosan modified phosphate rock on the removal of uranium(VI) with the combination of both the advantages of phosphate rock and chitosan. The processing parameters (initial uranium concentration, reaction time and adsorbent dosage) for removal of uranium(VI) by chitosan modified phosphate rock will be studied in detail. Adsorption isotherm and kinetic studies would conduct as well. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques were performed for the characterization of adsorbent.

Section snippets

Materials

Phosphate rock (PR) used in this study was collected from Hebei Fanshan phosphate mine, China. The collected PR was screened and only PR with a finer fraction (<150 μm) was used. The chemical and mineralogical compositions of PR have been determined and illustrated in our previous work [33]. For the raw PR, the major phases are P2O5 and SiO2.

The simulated radioactive wastewater was dissolved by solid U3O8 in the nitric acid at first, and used the sodium hydroxide aqueous solution to adjust

Characterization of PR and CPR

The SEM micrographs of the phosphate rock (PR) and chitosan modified phosphate rock (CPR) are shown in Fig. 1. Many irregular flaky structures could be observed on the surface of natural PR and these structures scattered everywhere (Fig. 1a and c). As to CPR, its particle size was much bigger than the natural PR. The surface of CPR become furry, indicating the surface of CPR has been successfully modified with chitosan (Fig. 1b and d). Fig. 1e and f shows the SEM images of natural PR and CPR

Conclusions

In this study, the uranium(VI) in radioactive wastewater was effectively adsorbed on phosphate rock and chitosan modified phosphate rock. Results showed that CPR possessed a better uranium(VI) adsorption performance rather than unmodified PR. The calculate results about isotherm is more fit to Freundlich model. The kinetic studies pointed out that the equilibrium data agreed well with the pseudo second order model. However, the other interference of uranium adsorption by chitosan modified

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos., U1501231, 51708143, 51508116), the Project of Guangdong Key Laboratory of Radioactive Contamination Control and Resources (No. 2017B030314182), and the Science and Technology Programs of Guangzhou (</GN3>201804020072, 201804010366</GN3>).

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