Iron Oxide

Feb 18, 2009 - ... 230031, Hefei, P.R. China, and Graduate School of Science and Technology, ... Percentage adsorption of Eu(III) on the magnetic comp...
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Environ. Sci. Technol. 2009, 43, 2362–2367

Europium Adsorption on Multiwall Carbon Nanotube/Iron Oxide Magnetic Composite in the Presence of Polyacrylic Acid C . L . C H E N , †,‡ X . K . W A N G , * ,† A N D M . N A G A T S U * ,‡ Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, 230031, Hefei, P.R. China, and Graduate School of Science and Technology, Shizuoka University, Hamamatsu, 432-8561, Japan

Received October 27, 2008. Revised manuscript received January 29, 2009. Accepted January 30, 2009.

This paper examines the interaction between Eu(III) and a multiwall carbon nanotube (MWCNT)/iron oxide magnetic composite in the absence and presence of poly(acrylic acid) (PAA). PAA was used as a surrogate for natural organic matter. The effects of pH, initial Eu(III) concentration, and PAA on Eu(III) adsorption on the magnetic composite were investigated using a batch technique. Percentage adsorption of Eu(III) on the magnetic composite increased with increasing pH and decreased with initial Eu(III) concentration. PAA adsorption on the magnetic composite decreased with increasing pH and was not obviously affected by the presence of Eu(III). The presence of PAA resulted in strong enhancement of Eu(III) adsorption belowpH4.5.However,abovepH5,anincreaseinsolubleEu-PAA complexes resulted in a decrease in Eu(III) adsorption on the magnetic composite. With increasing PAA concentrations, maximum adsorption of Eu(III) decreased and the adsorption “edge” shifted toward a lower pH range. Obvious difference of Eu(III)/PAA addition sequences on Eu(III) adsorption was observed above pH 4. The Freundlich model fitted Eu(III) adsorption isotherms very well in the absence and presence of PAA. These results are important for estimating and optimizing the removal of organic and inorganic pollutants by the magnetic composite.

Introduction Due to a large specific surface area and small, hollow, and layered structures, carbon nanotubes (CNTs) (1) have already been investigated as promising adsorbents for various organic pollutants and metal ions and radionuclides (2-13), and can be easily modified by chemical treatment to increase their adsorption capacity (3, 11-13). However, it is difficult to separate CNTs from the aquatic phase because of their small size. Traditional methods to recover CNTs from aqueous phase have included centrifugation and filtration. Centrifugation methods require very high speeds and traditional filtration methods are prone to filter blockages. The uncontrolled release of CNTs into the environment is of concern * Address correspondence to either author. Phone: +86-5515592788; fax: +86-551-5591310; e-mail: [email protected] (X.K.W.), [email protected] (M.N.). † Institute of Plasma Physics. ‡ Shizuoka University. 2362

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because of their small size (nanoscale particles); CNTs can enter cells, causing damage to plants, animals, and humans (14), and the toxicology of CNTs was highlighted recently (15). Thus, if the use of CNTs is not responsibly managed there is potential for CNTs to become another source of environmental contaminant if efficient methods for recovery are available. Compared with centrifugation and filtration methods, the magnetic separation method is considered a rapid and effective technique for separating nanoparticles from aqueous solution. The magnetic separation methods, which represent a group of techniques based on the use of magnetic or magnetizable adsorbents, carriers, and cells, have been used for many applications in biochemistry, microbiology, cell biology, analytical chemistry, mining ores, and environmental technologies (16-22). The formulation of magnetic nanoparticles is important for recovering nanomaterials from environmental applications. To facilitate the separation and recovery of CNTs from solution, the incorporation of magnetite (such as maghemite, magnetite) with CNTs may be a promising method. CNTs have cylindrically layered and hollow tubule nanostructures with high thermal and chemical stabilities, which allow them to function as supports for preparing nanosized metal and metal oxide particle catalysts and adsorbent composite materials (23-30). Removal of organic pollutants and heavy metal ions from wastewater solutions is an important environmental concern in wastewater management. In general, organic substances and metal ions (including radionuclides) exist simultaneously in solution and the strong complexation ability of organic substances with metal ions influences the removal of metal ions. The knowledge of simultaneous adsorptions of organic substances and metal ions on MWCNTs and their composites is crucial to understanding the application of MWCNTs and their composites in organic and inorganic pollutant cleaning. Although there are a large number of studies on the adsorption of organic substances or metal ions onto MWCNTs and their composites (2-13, 23-30), the investigation on the influence of organic substances on metal ion adsorptions on MWCNTs and their composites is still scarce. Research with surrogates for organic substances such as weak polyelectrolytes and simple organic acids has provided valuable insights into the behavior of natural organic substances in adsorption systems (31). Polyacrylic acid (PAA) is a polymeric substance containing carboxylic groups and linear CH2-CH2 chains. In earlier reports (31-34), PAA, being structurally simpler, has been used as a model compound for the study of natural biogeopolymers such as humic substances (HSs) (31). At pH ) 5, PAA has conditional stability constants similar to HSs (34). Yoon et al. (35) reported that Eu/PAA interactions were similar to Eu/HS interaction compared by time-resolved laser fluorescence spectroscopy (TRLFS). The objectives of this study were to (1) prepare MWCNT/ iron oxide magnetic composite and to characterize the magnetic composite by scan electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and X-ray photoelectron spectroscopy (XPS); (2) study the adsorption behavior of Eu(III) on MWCNT/iron oxide magnetic composite as a function of pH, initial Eu(III) concentration, and PAA; (3) investigate the influence of Eu(III)/PAA addition sequences on Eu(III) adsorption; and (4) simulate Eu(III) adsorption isotherms in the absence and presence of PAA. Europium is selected as both a fission product and a homologue of trivalent lanthanides and actinides, as it has adsorption properties similar to those of other trivalent lanthanides and actinides (36). 10.1021/es803018a CCC: $40.75

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Materials and Methods Chemicals. All reagents were of analytical reagent grade and used as received. Milli-Q water (resistivity of 18.2 MΩ · cm) was used in all experiments. Eu(III) stock solution at 0.1 mol/L was prepared from Eu2O3 (Purity, 99.99%) after dissolution, evaporation, and redissolution in 10-3 mol/L perchloric acid. The radiotracer 152 + 154Eu(III) was used in all batch adsorption experiments. PAA at 5000 Da (Aldrich) was used as received as a gift from SUBATECH laboratory (France). The proton exchange capacity of PAA was 11.3 ( 0.4 meq/g (32, 33). Preparation and Characterization of MWCNT/Iron Oxide Magnetic Composite. Magnetic composite was synthesized from suspension oxidized MWCNTs (1.0 g) in a 150 mL solution of 2.98 g FeCl3 · 6H2O and 1.53 g FeSO4 · 7H2O at 70 °C under N2. The preparation of oxidized MWCNTs iss described in detail in Supporting Information SI-1. SEM image and FT-IR spectrum of oxidized MWCNTs are shown in Figures S1 and S2, respectively. The preparation process of MWCNT/iron oxide magnetic composite is described in detail in SI-2 and Figure S3. Figure S3 shows that oxidized MWCNTs are functionalized with negative carboxylic and hydroxylic groups (14, 15), which have a potential ability to bind metal ions. In this work, positive ferrous and ferric ions are attached on oxidized MWCNTs due to the coordination reaction between ferrous and ferric ions and carboxylic and hydroxylic groups in the wet impregnation process. The specific surface area of the prepared MWCNT/iron oxide magnetic composite was measured using the N2-BET method. The magnetic composite was characterized by SEM, XRD, VSM, and XPS. The morphological structures of the magnetic composite were determined by SEM using a field emission scanning electron microscope (FEI-JSM 6320F). Magnetic curves were obtained using a model 155 VSM at room temperature, and its measurement range is 0 to (20.0 KOe. XRD measurements were performed by D/Max-2400 Rigaku X-ray powder diffractometer operated in the reflection mode with Cu KR (λ ) 0.15418 nm) radiation. XPS data were obtained with a Thermo ESCALAB 250 electron spectrometer from VG Scientific using 150 W Al KR radiation. The XPS photoelectron binding energies (BE) of the adventitious carbon species, i.e., the C 1s line at 284.6 eV was used to correct the observed binding energies for surface charging. Batch Adsorption Experiment. The adsorption of Eu(III) was investigated by using batch adsorption experiments in 15 mL polyethylene centrifuge tubes sealed with screw caps under N2 at T ) 25 ( 2 °C in the presence of 0.1 mol/L NaClO4. NaClO4 was usually chosen as background electrolyte, due to the ClO4- noncomplexing behavior with metal ions and numerous adsorbent surfaces. The stock suspensions of adsorbent and NaClO4 were pre-equilibrated for 24 h, and then PAA (or Eu(III) stock solution and radiotracer152 + 154Eu(III)) was added to achieve the desired concentration of the different components, and finally HClO4 or NaOH was added to adjust pH. The test tubes were shaken for 2 d to obtain equilibrium (preliminary experiments found that this was adequate for the suspension to obtain equilibrium). Subsequently an aliquot of the suspension (2.0 mL) was removed and the total activity of 152 + 154Eu(III) (Atot) was determined. The adsorbent in remaining suspension was separated by centrifugation at 18000 rpm for 30 min or by a magnetic process using a permanent magnet made of Nd-Fe-B for analysis of the supernatant solutions (AL). The efficiency of the separation by centrifuge was identical to that of magnetic separation under the experimental conditions employed herein. 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