The adsorption of uranium (VI) onto colloidal TiO2, SiO2 and carbon black

https://doi.org/10.1016/j.colsurfa.2016.04.003Get rights and content

Highlights

  • Adsorption isotherms of U(VI) onto colloidal carbon, titanium dioxide and silica were measured over a pH range of 2–10.

  • Electrophoretic-pH characteristics of colloidal carbon, titanium dioxide and silica were determined in the presence of U(VI).

  • The solution species of U(VI) adsorbing onto the different colloidal substrates was identified.

Abstract

The adsorption behavior of uranium (VI) onto colloidal hydrophobic carbon black and the hydrophilic, hydrous metal oxides, silica and titanium dioxide, was studied as a function of pH using quantitative adsorption isotherms and electrokinetic measurements of the suspended particles. Adsorption isotherms and micro-electrophoresis results were used in conjunction with uranium (VI) hydrolysis equilibrium data to identify the metal ion species adsorbing onto the various substrate surfaces as a function of pH. The study focuses on the solution conditions of metal ion concentration and pH, below the precipitation edge of solid uranium hydroxide. The adsorption of the uranium significantly modifies the electrokinetic properties of the colloidal adsorbent. The initial uranium (VI) species adsorbing at low pH on the different adsorbent surfaces is the uranyl ion (UO22+). For all adsorbents, the adsorption of the uranium (VI) renders the surfaces more positive compared to the native surface, over the pH range examined.

Introduction

The last decade has seen a growing dependence on nuclear-generated power, as a reliable source of energy. Uranium is to date by far the most efficient nuclear fuel being used in the production of nuclear electricity. Environmental contamination from highly toxic uranium as the result of leaching from ore deposits, mining and fuel processing has unrelentingly raised global concerns. The ability to extract or concentrate any soluble uranium from sources such as sea water, ground water, soils and even nuclear waste may provide an alternative uranium supply to the demanding nuclear power industry and the establishment of an efficient and safe management plan for nuclear waste generated during the production process.

Many techniques, including ion exchange and solvent extraction, have been thoroughly investigated for the removal of uranium in the form of stable uranyl ions (UO22+). Among these techniques, the adsorption method using an efficient adsorbent has been considered most feasible. It has been shown that adsorption is not only a removal process, but it also allows enrichment and pre-concentration of uranium. The uptake of uranium on various solids such as activated carbon [1], montmorillonite [2], kaolinite [3], zeolite [4], titanium nanotubes [5], hydrous titanium oxide [6], gibbsite [7], zirconium titanium oxide [8], anion exchange resins (mainly for uranyl carbonate complexes) [9], [10], [11] and more recently on complex mixed material substrates, such as graphene oxide-supported iron nanoparticles [12] and Fe3O4@TiO2 [13], has been studied.

The effects of adsorption time, amount of adsorbent, solution pH and initial uranyl ion concentration on the uranium adsorption process are among some of the parameters that have been extensively examined [14]. In recent years, spectroscopic techniques, such as extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) have also been applied in the study of the surface interaction between uranium and its adsorbent at a molecular level [15]. However, no studies have provided direct comparison between the adsorption behavior and ensuing electrokinetic characteristics of uranium adsorbing onto various adsorbents with different surface functional groups.

The purpose of this study was not only to investigate the adsorption properties of uranium (VI) species, but also to present a detailed description of the electrokinetic behavior of UO22+ and its mononuclear and poly-nuclear hydroxo complexes on three commonly used adsorbents, colloidal titanium (VI) oxide, silica and carbon black. Hydrous titanium (VI) oxide is considered as a desirable adsorbent because of its higher selectivity and adsorption capacity. The surface of colloidal silica is negatively charged over the entire pH range being studied and so offers a strong electrostatic factor in the adsorption of positive uranium species in solution. Carbon black is generally considered a cost effective adsorbent and widely used in commercial metal ion extraction, hence its interest in this study. Adsorption isotherms and electrophoresis measurements, under essentially similar experimental conditions, have been used in conjunction with hydrolysis equilibrium data to allow the identification of the uranium (VI) species adsorbing onto the various substrates as a function of pH well before bulk precipitation of UO2(OH)2.

Section snippets

Materials

High purity (99%) uranium nitrate from Alfa Products was used as supplied. Aqueous uranium (VI) solutions at pH 3.0 ± 0.5 at a known ionic strength (10−3 M) were prepared freshly prior to each set of experiments. The procedure used in present study was based on the standard method developed for the study of hydrolysable metal ion adsorption onto the substrates (adsorbents) [16], [17], [18], [19].

Solids

The TiO2 was obtained from Tioxide International Limited, UK. The sample was 99.9% TiO2, with the major

Preparation of colloidal dispersions

Reproducible colloidal dispersions of the hydrophobic carbon black were prepared using a method that has been previously described in detail [24]. Titanium dioxide and silica dispersions were prepared by sonicating the required mass of finely ground samples in 20 mL of the uranium (VI) aqueous solution for 10 min.

pH measurements

The pH of the aqueous uranium (VI) solutions and the various aqueous colloidal dispersions were measured with a Titron combination calomel-glass electrode. The electrode was calibrated

Adsorption measurements

The hydrolysis behavior of uranium (VI) at different concentrations is illustrated in Fig. 1. Adsorption results, expressed as a percentage of uranium (VI) adsorbed onto TiO2, SiO2 and carbon black, as a function of pH are shown in Fig. 2, Fig. 3, Fig. 4 for each respective colloidal material. This allows a direct comparison between the adsorption and hydrolysis behavior of uranium (VI). These solids were studied at two different added metal ion concentrations, 1 × 10−5 M and 5 × 10−4 M. A higher

Adsorption measurements

The adsorption isotherms presented in Fig. 2, Fig. 3, Fig. 4 reveal quite distinctly that, of the substrates studied, uranium (VI) adsorbs most strongly on TiO2 colloids. However, in all cases, provided that sufficient uranium (VI) is present in solution, there is more than monolayer coverage of uranium (VI) on the substrate surfaces. Approximately 7 × 10−5 M of uranium (VI) is required to give monolayer coverage of a colloidal dispersion containing 50 m2 L−1 of solids. The monolayer level is taken

Summary and conclusions

There is a strong specific adsorption of the positive uranium (VI) species, predominantly UO22+, onto positively charged TiO2 and carbon black surfaces, resulting in an increase in their isoelectric points. At high pH, particularly above the precipitation pH of uranium (VI), the electrophoretic mobilities of the TiO2/U(VI) and carbon black/U(VI) particles closely approach that of colloidal UO2(OH)2. The mobility curve of SiO2, upon adsorption of uranium (VI) species, becomes more positive until

References (39)

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    The PSM had an isoelectric point at pH 4.61 (Fig. S2). In the studied pH range from 4.5 to 9, the PSM existed mostly in its anionic form, and the charge density on the PSM surface changed with solution pH. At pH > 4.61, the PSM was negatively charged due to a large number of deprotonated hydroxyl groups that were gradually converted to SiO− groups at the higher end of the pH range; while at pH < 4.61, the PSM was positively charged due to protonated hydroxyl groups converting to Si-OH2+ groups at low pH (Lamb et al., 2016). While LZ (pI 11) was positively charged over a pH range from 4.5 to 9.

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