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Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes
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JECE 593 1–8 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
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Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes P. Ilaiyaraja a, * , Ashish Kumar Singha Deb b , D. Ponraju c , B. Venkatraman a a b c
Radiological Safety Division, Radiological Safety and Environmental Group, Kalpakkam 603102, India Chemical Engineering Division, Bhabha Atomic Research Centre, Mumbai, India Safety Engineering Division, Reactor Design Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 December 2014 Accepted 4 March 2015
Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand, first of its kind, has been synthesized from hydroxyl terminated poly(amido)amine dendrimer. Studies on performance of XFPD in removal of Cu2+ and Eu3+ metal ions were carried out from aqueous solution. Investigation on quantitative removal of Cu2+ and Eu3+ metal ions at various pH revealed that XFPD effectively precipitates the metal ions at pH > 4 with settling time of about 3 h. In case of Cu–XFPD complex at pH > 6, settling requires the addition of coagulating agent like aluminium sulphate. The loading capacity of XFPD for Cu2+ and Eu3+ metal ions was estimated to be 0.48 and 0.95 g g1, respectively. The XFPD chelating ligand deployed for treatment of radioactive liquid waste (RLW) showed that percentage removal of radionuclides were in the following order; 95Zr 154Eu 60Co (>99.8) > 144Ce (98.8) > 125Sb (83.3) > 106Ru (79.4) > 55Mn (54.3) > 137Cs (24.0). It has been demonstrated that XFPD chelating ligand has potential application in effective removal of various radionuclides from aqueous waste. ã 2015 Published by Elsevier Ltd.
Keywords: Xanthate Dendrimer Precipitation Radioactive waste Heavy metals
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Introduction
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Removal of radionuclides from aqueous waste has received renewed interest due to concern over human health and environmental hazards. Radioactive wastes in general are being generated in various processes such as mining and milling of ore, nuclear fuel fabrication, reprocessing of nuclear spent fuel and use of radioactive materials for research, medicine and industrial applications. Based on their radioactivity, the wastes are classified into three categories (i) low level waste (LLW), (ii) intermediate level waste (ILW) and (iii) high level waste (HLW) [1]. Storage of large volume of radioactive waste generated in nuclear plants and by other means is more risky and expensive [2]. Hence, these waste streams has to be treated for reducing their activity to a level at which they are permitted to be discharged as per national regulations. HLW are managed by using vitrification process following safe disposal in deep geological repository [3]. The strategy for management of LLW and ILW involves development of innovative processes with volume reduction as one of the important criteria. In general, treatment involves several processes such as filtration, precipitation, sorption, ion exchange, steam/
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* Corresponding author. Tel.: +91 44 27480500 23427; fax: +91 44 27480235. E-mail address: [email protected] (P. Ilaiyaraja).
solar evaporation and membrane separation [4]. Among these, precipitation is best suited for treatment of large volume of liquid waste containing relatively low concentration of radioactive elements. In this process both radioactive and non-radioactive metal ions are removed by using various precipitants such as hydroxides, carbonates, peroxides, sulfides, sulphates, phosphates, copper/nickel ferrocyanides etc. [5,6]. These precipitants are Q3 effective in removal of radionuclides at pH 8. Organic xanthate (R-OCS2) chelating agents form stable water insoluble complexes with various metal ions at pH > 3 [7,8]. Owing to low solubility product and high stability constant of metal–xanthate complex, the xanthate chelating agent has been used for removal of heavy metal ions from wastewater [9]. Removal of metal ions such as copper, nickel, cobalt, iron, platinum, palladium, chromium, zinc, arsenic, mercury, antimony, bismuth and lead from waste water have been already reported [10–20]. There are many advantages of using xanthate for waste treatment which includes (i) high percentage of metal removal, (ii) less sensitivity to pH variation, (iii) less sensitivity to coexisting complexing agent, (iv) improved sludge dewatering property and (v) capability for selective removal of metals [8]. Also, metal xanthates are extensively used as fungicides, pesticides, catalyst, corrosion inhibitors, and agricultural reagents [21,22]. Organic molecules with single and multiple xanthate groups are widely studied for removal of heavy metal ions. Molecules with
http://dx.doi.org/10.1016/j.jece.2015.03.013 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: P. Ilaiyaraja, et al., Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.03.013
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multiple xanthate group exhibit excellent binding characteristics and better settling performance [7,23]. Hence, it is desirable to increase the number of xanthate functional groups in a molecule to enhance binding and precipitating abilities. Recently, xanthate functional groups are introduced into macromolecules to increase its capacity and removal efficiency [24,25]. In the present work, the xanthate groups were introduced in dendrimer macromolecules. Dendrimers are a new class of polymeric materials which consists of a core, regularly branched dendron from the core and peripheral reactive functional groups [26]. PAMAM (Polyamidoamine) dendrimers as described by Tomalia et al. [27] are relatively easy to synthesize with desired number of reactive functional groups. The xanthate functionalized PAMAM dendrimer (XFPD) chelating ligands were synthesized for treatment of radioactive wastewater. Precipitation of Eu3+ and Cu2+ metal ions from aqueous solution using XFPD chelating ligand were studied. Europium and copper were used as homologue solutes for fission and corrosion activation products present in low level radioactive wastes produced in nuclear industries and different research activities. Effect of pH, ionic strength and settling behaviour of precipitate were investigated. Decontamination studies carried out with simulated nuclear liquid waste (SNLW) and radioactive liquid waste (RLW) are discussed.
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Experimental
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Materials and instruments
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All the chemicals used were of analytical reagent grade. Ethylenediamine was purified by distillation over calcium hydride and all other chemicals were used as such without further purification. Eu(NO3)35H2O (99.9%) (Sigma–Aldrich), CuCl22H2O (98%) (Loba Chemie) were used for preparation of standard solution. Millipore water (resistivity 18.2 MV cm) was used in all experiments. The pH of solution was measured using Eco-testr pH 2 m (Eutech make). Distillation was carried out using Equitron rotary evaporator. Whatman 542 hardened ashless filter paper was used for filtration. Metal complexes were dried using vacuum oven (SEMCO make). HACH turbidity meter with working range of 0–1000 NTU was used for measuring residual turbidity of the solution.
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Analytical instrumentation
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FT-IR spectra were recorded within a scanning range of 500–4000 cm1 at 16 cm1 resolution by mounting KBr pellet of the compound in (ABB) MB 3000 spectrometer. 1H and 13C NMR spectra were recorded using Bruker 400 MHz spectrometer. Mass spectra were recorded using Voyager-DETM PRO matrix assisted laser desorption ionization-time of flight (MALDI-TOF) and Thermo DSQ-II quadruple mass spectrometers. Philip Model XL30 scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) was used for in-situ chemical analysis of the metal precipitate. Jobin Yvow-Spex Fluorolog fluorimeter was used for measuring fluorescence emission spectra. VGEscalab MKII X-ray photoelectron spectrometer (XPS) was used to examine the oxidation state of copper and europium in Cu–XFPD and Eu–XFPD complexes. An Inductive coupled plasma optical emission spectrometer (ICP-OES) and Analytica, Janan NovAA300 atomic absorption spectrometer (AAS) were used for estimation of metal ion concentrations.
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Synthesis and characterization of XFPD
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The detailed synthesis and characterization of XPFD is given in Supplementary data. The synthesis involves three distinct steps
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(Fig. S1): (1) Michael addition of methyl acrylate with ethylenediamine, (2) amidation of ester terminated dendrimer (PAMAMG0.5COOMe) with tris(hydroxymethyl)aminomethane resulting in formation of hydroxyl terminated PAMAM dendrimers (PAMAMG1OH) and (3) reaction between hydroxyl group of PAMAMG1OH and carbon disulfide leading to formation of xanthate functionalized PAMAM dendrimer (PAMAMG1OCS2Na) chelating ligand (XFPD). The formation of PAMAMG0.5COOMe and PAMAMG1OH were confirmed by FT-IR, 1H and 13C NMR and mass spectroscopic studies. The presence of xanthate functional group in XFPD was confirmed by UV, FT-IR spectral studies and total number of xanthate group was estimated by acid–base titration.
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Removal of Cu2+ and Eu3+ metal ions from aqueous solution
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Stock solutions of Cu2+ and Eu3+ metal ions (1 g l1) and XFPD (2.7860 mM) chelating agent were prepared. Precipitation studies were carried out individually with various concentrations of Cu2+ and Eu3+ metal ions (25, 50, 100, 250 and 500 mg l1) by adding 0.5 ml of 2.7860 mM XFPD ligand. The resultant precipitate was filtered through whatman 542 ashless filter paper and washed with water. The metal–XFPD precipitate was dried at 343 K in a vacuum oven and then characterized by using FT-IR, XPS and SEM-EDS.
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Effect of pH on formation and settling of metal–XFPD complex
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Effect of pH on removal of Cu2+ and Eu3+ metal ions were studied at pH between 3.0 and 9.0. Either 0.1 M HNO3 or 0.1 M NaOH solutions were used for adjusting the pH of feed solution. The concentration of Cu2+ and Eu3+ metal ions in filtrate was determined by using ICP-OES. The metal ion removal efficiency for Cu2+ and Eu3+ metal ions in term of percentage removal (% R) was calculated by using Eq. (1). C f C fi (1) 100 %R¼ Cf
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where, Cf is the concentration of metal ion in feed and Cfi is the concentration of metal ion in filtrate. Metal ion removal efficiency depends on both effective complexation and its settling. Suspended particles of metal–XFPD complex could be removed from aqueous solution by centrifugation or filtration or sedimentation process. Sedimentation process is generally used for solid–liquid separation which involves settling of suspended particles. The settling behaviour of XFPD complexes of Cu and Eu was studied by measuring residual turbidity of the solution as a function of time.
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Effect of ionic strength on metal ion removal
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Considering the coexistence of various electrolytes in nuclear waste and environmental samples, the influence of electrolytes such as Ca(NO3)2 and NaNO3 on quantitative removal of metal ions was investigated. Experiments were carried out by varying the concentrations of electrolyte from 0.01 to 0.5 M with constant metal ion to XFPD mole ratio.
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Extent of binding of Eu3+ and Cu2+ metal ions
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The extent of binding (EOB) of metal ions (Cu2+ and Eu3+) with XFPD was studied as a function of metal ion concentration. About 9 ml of metal ion (Cu2+/Eu3+) solution was mixed with 1 ml of 2.786 mM XFPD solution in 50 ml centrifuge tube. The resultant precipitate was centrifuged, washed with water, dissolved in concentrated HNO3 and made up to 25 ml in a standard flask. The solution was analysed to estimate the loaded metal ion on XFPD
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ligand. The EOB was calculated by using Eq. (2) CM EOB ¼ C XFPD
(2)
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where, [CM] is the concentration of metal ion (Cu2+/Eu3+) complexed with XFPD (mol l 1) and [CXFPD] is the concentration of XFPD in solution (mol l 1).
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Removal of metal ions from liquid wastes
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Studies on removal of metal ions from simulated nuclear liquid waste (SNLW) and radioactive liquid waste (RLW) were carried out using XFPD ligand. SNLW represents liquid waste arising from nuclear fuel reprocessing, decontamination of nuclear components and active laboratories. The composition of SNLW is given in Table 1. Precipitation studies were carried out by adding 2.5 ml of 2.786 mM XFPD ligand into 20 ml of SNLW taken in a 50 ml centrifuge tube. The resultant precipitate was filtered and the metal ions concentration in the filtrate was determined using ICP-OES and AAS. RLW represents the composition of high level waste (HLW) arising from reprocessing of mixed carbide fuel, (U0.3Pu0.7)C, irradiated to the burn-up of 155 GWd/Te in fast breeder test reactor (FBTR) situated at Kalpakkam in India. The activity of each radionuclides RLW is given in Table 1. Precipitation studies were carried out with 10 ml of RLW (feed solution) taken in a 20 ml glass tube and added 0.25 ml of 2.786 mM XFPD ligand. The resultant suspended particles were filtered and the residual activity in the filtrate was measured by using HPGe detector. The percentage decontamination (% D) for each metal ion was calculated using Eq. (3):
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%D¼
Ai Af 100 Ai
(3)
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where Ai and Af are activities (dps) of metal ion in feed solution and filtrate, respectively.
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Results and discussion
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Precipitation of copper with XFPD ligand
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A dark-brown precipitate was obtained by the addition of XFPD ligand to Cu2+ solution. FTIR spectrum of Cu–XFPD complex is shown in Fig. 1(a). The peaks appear at 1049 cm1 and 1197 cm1 are attributed to asymmetric and symmetric vibration of —CS2 group. The value of Dy[y(CS2)asym y(CS2)sym] can illustrate the coordination fashion of the complex. The Dy value [y(CS2)asym y(CS2)sym] is 148 cm1, almost similar to the corresponding sodium xanthate value (Fig. S3) (Dy = 139 cm1), which
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Table 1 Composition of liquid wastes. SNLW
RLW
Metal ion
Conc. (mg l1)
Metal ion
Activity (dps)
La Ce Nd Gd Sm Mn Co Ni Zn U Th Ba Sr Cs
68 66 64 78 72 25 30 42 37 103 104 70 66 70
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21.3 1.14 104 1.05 103 45.3 – 21.39 2.46 – – – – 3.95 102 – 1.13 104
Mn 144 Ce 106 Ru 154 Eu Nd 95 Zr 60 Co Ni Zn U Th 125 Sb 90 Sr 137 Cs
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Fig. 1. FTIR Spectra of (a) Cu–XFD (b) Eu–XFD complexes.
suggests that XFPD ligand adopts a unidentate coordination mode (Fig. 2(a)) in this complex [28,29]. EDS analysis of Cu–XFPD complex indicates the presence of copper and sulphur (Fig. 3) with weight and atom percentages of Cu – 23.2%, S – 22.4% and Cu – 6.7%, S – 12.8%, respectively. From the above values, the calculated mole ratio of S to Cu is about 2:1 which indicates coordination of copper to two sulphur atoms. XPS analysis of Cu–XFPD precipitate showed the existence of copper in +1 oxidation state. Fig. 4 shows the XPS spectrum of Cu2p from Cu–XFPD complex. Two major peaks with binding energy of 934 and 954 eV are the characteristics of cuprous [9] and the absence of cupric (the nonexistence of satellite structure at binding energy of 944 and 963 eV in Fig. 4) confirms that cuprous form of xanthate complex (Cu–XFPD) is precipitated. Hence, the mechanism of complexation is explained by the following Eqs. (4) and (5):
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i) Cu2+ is reduced to Cu+ ion by xanthate group with the formation
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of dixanthogen (PAMAMG1(OCS2)2). ii) Formation of cuprous XFPD complex.
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PAMAMG1OCS2 + Cu2+ ! PAMAMG1(OCS2)2 + Cu+
(4)
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PAMAMG1OCS2 + Cu+ ! PAMAMG1OCS2Cu
(5)
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Precipitation of europium with XFPD ligand
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A pale yellow precipitate was obtained by the addition of XFPD ligand to Eu3+ solution. FTIR spectrum of Eu–XFPD complex is shown in Fig. 1(b) . The presence of xanthate group in europium
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Fig. 2. Coordination pattern of xanthate ligand. (a) unidentate (b) anisobidentate (c) coordination between two hetero atom.
Please cite this article in press as: P. Ilaiyaraja, et al., Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.03.013
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Fig. 3. EDS spectra of Cu–XFD and Eu–XFD complexes.
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complex was confirmed from the appearance of vibrational peaks at 1197 and 998 cm1 corresponding to asymmetric and symmetric stretching vibrations of —CS2 group (Fig. 1(b)). The Dy[y(CS2)asym y(CS2)sym] value (199 cm1) is much larger than the values corresponding to sodium xanthate (Dy = 139 cm1) and hence, xanthate groups may coordinate with Eu3+ either in anisobidentate state or by coordination with two hetero atom (Fig. 2(b) and (c)) [28,29]. Europium undergo hydrolysis leading to the formation of various species such as Eu(OH)2+ and Eu(OH)2+ at pH > 3. In Fig. 1 (b), the appearance of strong vibrational bands at 3419 and 1420 cm1 are due to O—H stretching and bending vibrations suggesting the presence of hydrolysed europium species in the complex. EDS analysis of Eu–XFPD complex shows the presence of europium and sulphur (Fig. 3). The weight and atom percentages of europium and sulphur in Eu–XFPD complex are found to be Eu – 56.84%, S – 3.12% and Eu – 10.91%, S – 2.84%, respectively. The weight percentage of sulphur in Eu–XFPD complex is reduced due to elimination of carbon disulfide (CS2) from XFPD ligand during complex formation. According to Hard Soft Acid Base (HSAB) principle, the hard acid Eu3+ ion interacts strongly with hard base donor ‘oxygen’ as compared to soft base donor ‘sulphur’. Hence, the coordination ability of europium with oxygen is stronger than that of sulphur atom [30]. Due to this oxophilic character of Eu3+ ion, some of the xanthate groups in XFPD ligand dissociate with the
elimination of CS2 group resulting in existence of few xanthate coordinations in Eu–XFPD complex. The following possible complexation reactions (Eqs. (6)–(9)) are predicted based on the above results.
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PAMAMG1(OCS2)3 + Eu3+ ! PAMAMG1(O2EuCS2) + 2SC2
(6)
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PAMAMG1(OCS2)2 + Eu(OH)2+ ! PAMAMG1(O2Eu(OH)) + 2SC2 (7)
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PAMAMG1(OCS2) + Eu(OH)2+ ! PAMAMG1(OEu(OH)2) + SC2
(8)
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PAMAMG1(OCS2) + Eu(OH)2+ ! PAMAMG1(OCS2Eu(OH)2)
(9)
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XPS study supports the above complexation mechanism. The influence of chemical environment on BE values for core level photoelectron peaks of O1s and Eu4d is shown in Fig. 5. The shape of O1s signal is quite different for Eu–XFPD, when compared to other Eu(III) compounds reported by Mercier et al. [31]. An intense peak appears at lower BE (531.1 eV) and a shoulder at high BE (531.6 eV) depicts the existence of various chemical environment of O1s in Eu–XFPD complex. The shoulder peak at high BE (531.6 eV) is due to C—O of alcoholic group coordinated to Eu(III) and the peak at lower BE (531.1 eV) is attributed to C—O—C of xanthate group coordinated to Eu(III). Similar trend is observed for Eu4d peak (Fig. 5) and the type of coordination of Eu(III) in Eu– XFPD complex has an influence on BE values of Eu4d. Occurrence of peaks at 135.2 and 136.3 eV shows characteristic BE of Eu(III) coordinated to xanthate ((OCS2–Eu(OH)) and alcoholic (C—O—Eu (OH)) group, respectively. Thus, simultaneous study of O1s and Eu4d core level photoelectron peaks allows to distinguish between various types of Eu(III) coordinations in the Eu–XFPD complex. The complexation of europium with XFPD ligand was also confirmed by fluorimetric study. Fluorescence emission spectra of free Eu3+ and Eu–XFPD complex is shown in Fig. 6. The emission spectra clearly shows well-known bands of europium luminescence corresponding to 5D0 ! 7F0 (580 nm), 5D0 ! 7F1 (593 nm) and 5 D0 ! 7F2 (617 nm) transitions. 5D0 ! 7F2 transition is very sensitive to the surrounding microenvironment of Eu3+ ion and its intensity enhances due to complex formation with ligands [32]. Electric dipole transition (5D0 ! 7F2) at 617 nm is observed to be stronger than magnetic dipole transition (5D0 ! 7F1) at 593 nm.
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Fig. 4. XPS spectrum of Cu2p from Cu–XFD complex.
Please cite this article in press as: P. Ilaiyaraja, et al., Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.03.013
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Fig. 6. Fluorescence emission spectra of Eu3+ and Eu–XFD complex.
Fig. 5. XPS spectra of O1s and Eu4d from Eu–XFD complex. 288 289
Thus, the strong emission band at 617 nm (5D0 ! 7F2) clearly indicates that europium is complexed with XFPD ligand.
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Effect of pH on complexation and metal ions removal efficiency
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XFPD ligand could remove metal ions from aqueous solution almost quantitatively. The metal ion removal efficiency depends on formation of metal–XFPD complex followed by its settling. The pH of feed solution plays an important role on both formation of complex as well as settling of precipitate. The variation in percentage removal of Eu3+ and Cu2+ ions as a function of pH is shown in Fig. 7. At pH < 3, the ligand undergoes decomposition by acid hydrolysis which suggests that pH of the feed solution must be maintained above 3. An increase of pH from 3 to 5 leads to increase in percentage removal of both Eu3+ and Cu2+ ions beyond which it remains almost constant. At pH > 3, xanthogenic acid groups of XFPD ligand ionizes to form xanthogenates and thus effectively coordinates with Eu3+ and Cu2+ which in turn increases its percentage removal. The removal efficiency of XFPD for Eu3+ and Cu2+ are found to be 98.6% and 99.5%, respectively at pH 5. Beyond pH 8, precipitation of Eu3+ ions as europium hydroxide (Eu(OH)3) is predominant compared to that of formation of Eu–XFPD complex. This suggests that the effective removal of metal ions by using XFPD ligand shall be carried out between pH 3 and 8.
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Settling behaviour of Cu–XFPD and Eu–XFPD precipitates
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The settling behaviour monitored by measuring the residual turbidity of Eu–XFPD complex shows that it possesses a favourable settling rate with reduction in turbidity of the solution by a factor of 14.6 within an hour (Fig. 8). In case of Cu–XFPD complex, the settling time is highly pH dependent. At pH > 6, a colloidal suspension of Cu–XFPD complex neither settles by gravity nor aggregates into larger particles for a reasonable length of time due to similar charge associated with the complex [18]. But it is observed that the addition of 2 mM aluminium sulphate as coagulating agent at pH greater than 6 makes the suspended particles to settle and the turbidity of the solution is reduced by a factor of 7.2 within an hour. Hence, settling of metal complexes occurs within 2–3 h due to addition of coagulating agent.
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Extent of binding of Eu3+ and Cu2+ metal ions
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The extent of binding of Eu3+ and Cu2+ metal ions with XFPD ligand as a function of initial metal ions concentration is shown in Fig. 9. It is observed that loading of both europium and copper ions increases with increasing concentration of metal ions and gets saturated at mole ratios of 10.4 and 12.3, respectively. From the above determined mole ratio, a typical loading capacity of XFPD for
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Fig. 7. Effect of pH on removal of Eu and Cu metal ions.
Please cite this article in press as: P. Ilaiyaraja, et al., Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.03.013
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Fig. 10. Effect of NaNO3 concentration on removal of Eu & Cu metal ions. Fig. 8. Residual turbidity of Eu–XFD and Cu–XFD complexes as a function of time.
Removal of metal ions from simulated nuclear liquid waste (SNLW) and decontamination of radioactive liquid waste (RLW)
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Eu3+ and Cu2+ metal ions are estimated as 0.94 g and 0.48 g g1 of XFPD, respectively. The higher loading capacity of XFPD ligand may be due to coordination of these metal ions with amine, amide and xanthate groups present in the XFPD ligand.
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Effect of ionic strength on removal of metal ions
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Experiments were carried out by varying the concentration of electrolyte from 0.01 to 0.5 M with constant metal ion to XFPD mole ratio. It is observed that the presence of sodium does not exhibit any significant effect on removal of Eu3+ ions but there is slight enhancement in the removal of Cu2+ ions (Fig. 10). In the presence of divalent cations (Ca2+), percentage removal of Cu2+ ions increases gradually with increase in concentration of calcium metal ion and levelled off after 0.1 M (Fig. 11). The enhancement in percentage removal of Cu2+ is due to the coagulation of suspended particles caused by Ca2+ ion. In case of Eu3+, the percentage removal increased initially up to 0.1 M and then decreased gradually. This decrease in percentage removal of Eu3+ at higher concentration of Ca2+ is due to the competing interaction of Ca2+ ions with Eu3+ to the amine, amide and xanthate functional groups of XFPD ligand.
The percentage removal of various metal ions from SNLW by xanthate process is shown in Fig. 12. It is observed that except cesium, all other metal ions are effectively removed. The percentage removal of lanthanides (Ce, Gd, La, Nd, Sm) and actinides (Th & U) are observed to >99% whereas for Co, Ni and Zn it is greater than 90%. The precipitation of lanthanides and transition metal ions are mainly due to metal–xanthate complex formation whereas the precipitation of actinides is due to their hydroxide formation. Hydroxides of uranium and thorium were formed due to the increase in pH of the solution during the addition of XFPD. Though alkaline earth metal ions (Ba & Sr) does not form any precipitate with XFPD ligand, their percentage removal is observed to be about 50–70%. The removal of Ba & Sr metal ions could be explained by considering their role as coagulating agent. Experiments performed for decontamination of RLW using XFPD ligand shows that its removal efficiency is high for almost all radionuclides and poor in case of cesium. The percentage decontamination determined for various radionuclides (Fig. 13) is in the following order: Zr Eu Co (>99.8) > Ce (98.8) > Sb (83.3) > Ru (79.4) > Mn (54.3) > Cs (24.0).
Fig. 9. Extent of binding of Eu and Cu metal ions on XFD ligand.
Fig. 11. Effect of Ca(NO3)2 concentration on removal of Eu & Cu metal ions.
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Please cite this article in press as: P. Ilaiyaraja, et al., Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.03.013
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Fig. 12. Removal of metal ions from SNLW.
Fig. 13. Decontamination of radionuclides from RLW. 373
Conclusion
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A new xanthate functionalized PAMAM dendrimer (XFPD) ligand has been synthesized for effective removal of Cu2+ and Eu3+ metal ions from aqueous solution. XFPD form water insoluble complexes with Cu2+ and Eu3+ metal ions and pH of aqueous solution plays an important role on both complexation and settling of suspended metal–xanthate complexes. Removal of metal ions are effective in the pH range of 3–8 and settling of suspended metal –xanthate complexes at pH > 6 is achieved by the addition of aluminium sulphate as coagulating agent. The presence of alkaline earth metal enhances removal of radionuclides due to coagulation process. It is demonstrated that xanthate process is effective for decontamination of various radionuclides from RLW. Hence, XFPD ligand possesses potential application in total decontamination of radioactive wastes.
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Acknowledgements
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The authors express their sincere gratitude to Dr P.R. Vasudeva Rao, Director, IGCAR for constant encouragements. We thank Dr K. Sivasubramanian, Head, RDS, Shri H. Krishnan, Shri Shailesh Joshi, Shri B.N. Mohanty and Smt. O. Annalakshmi, RSD for their help during the experiments. We also thank Shri Swpan Kumar Mahato,
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CG for EDS analysis and Smt S. Annapoorni, ACSS, CG, IGCAR for ICP-OES analysis.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jece.2015.03.013.
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References
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Contents lists available at ScienceDirect
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Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes P. Ilaiyaraja a, * , Ashish Kumar Singha Deb b , D. Ponraju c , B. Venkatraman a a b c
Radiological Safety Division, Radiological Safety and Environmental Group, Kalpakkam 603102, India Chemical Engineering Division, Bhabha Atomic Research Centre, Mumbai, India Safety Engineering Division, Reactor Design Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 December 2014 Accepted 4 March 2015
Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand, first of its kind, has been synthesized from hydroxyl terminated poly(amido)amine dendrimer. Studies on performance of XFPD in removal of Cu2+ and Eu3+ metal ions were carried out from aqueous solution. Investigation on quantitative removal of Cu2+ and Eu3+ metal ions at various pH revealed that XFPD effectively precipitates the metal ions at pH > 4 with settling time of about 3 h. In case of Cu–XFPD complex at pH > 6, settling requires the addition of coagulating agent like aluminium sulphate. The loading capacity of XFPD for Cu2+ and Eu3+ metal ions was estimated to be 0.48 and 0.95 g g1, respectively. The XFPD chelating ligand deployed for treatment of radioactive liquid waste (RLW) showed that percentage removal of radionuclides were in the following order; 95Zr 154Eu 60Co (>99.8) > 144Ce (98.8) > 125Sb (83.3) > 106Ru (79.4) > 55Mn (54.3) > 137Cs (24.0). It has been demonstrated that XFPD chelating ligand has potential application in effective removal of various radionuclides from aqueous waste. ã 2015 Published by Elsevier Ltd.
Keywords: Xanthate Dendrimer Precipitation Radioactive waste Heavy metals
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Introduction
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Removal of radionuclides from aqueous waste has received renewed interest due to concern over human health and environmental hazards. Radioactive wastes in general are being generated in various processes such as mining and milling of ore, nuclear fuel fabrication, reprocessing of nuclear spent fuel and use of radioactive materials for research, medicine and industrial applications. Based on their radioactivity, the wastes are classified into three categories (i) low level waste (LLW), (ii) intermediate level waste (ILW) and (iii) high level waste (HLW) [1]. Storage of large volume of radioactive waste generated in nuclear plants and by other means is more risky and expensive [2]. Hence, these waste streams has to be treated for reducing their activity to a level at which they are permitted to be discharged as per national regulations. HLW are managed by using vitrification process following safe disposal in deep geological repository [3]. The strategy for management of LLW and ILW involves development of innovative processes with volume reduction as one of the important criteria. In general, treatment involves several processes such as filtration, precipitation, sorption, ion exchange, steam/
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* Corresponding author. Tel.: +91 44 27480500 23427; fax: +91 44 27480235. E-mail address: [email protected] (P. Ilaiyaraja).
solar evaporation and membrane separation [4]. Among these, precipitation is best suited for treatment of large volume of liquid waste containing relatively low concentration of radioactive elements. In this process both radioactive and non-radioactive metal ions are removed by using various precipitants such as hydroxides, carbonates, peroxides, sulfides, sulphates, phosphates, copper/nickel ferrocyanides etc. [5,6]. These precipitants are Q3 effective in removal of radionuclides at pH 8. Organic xanthate (R-OCS2) chelating agents form stable water insoluble complexes with various metal ions at pH > 3 [7,8]. Owing to low solubility product and high stability constant of metal–xanthate complex, the xanthate chelating agent has been used for removal of heavy metal ions from wastewater [9]. Removal of metal ions such as copper, nickel, cobalt, iron, platinum, palladium, chromium, zinc, arsenic, mercury, antimony, bismuth and lead from waste water have been already reported [10–20]. There are many advantages of using xanthate for waste treatment which includes (i) high percentage of metal removal, (ii) less sensitivity to pH variation, (iii) less sensitivity to coexisting complexing agent, (iv) improved sludge dewatering property and (v) capability for selective removal of metals [8]. Also, metal xanthates are extensively used as fungicides, pesticides, catalyst, corrosion inhibitors, and agricultural reagents [21,22]. Organic molecules with single and multiple xanthate groups are widely studied for removal of heavy metal ions. Molecules with
http://dx.doi.org/10.1016/j.jece.2015.03.013 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: P. Ilaiyaraja, et al., Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.03.013
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multiple xanthate group exhibit excellent binding characteristics and better settling performance [7,23]. Hence, it is desirable to increase the number of xanthate functional groups in a molecule to enhance binding and precipitating abilities. Recently, xanthate functional groups are introduced into macromolecules to increase its capacity and removal efficiency [24,25]. In the present work, the xanthate groups were introduced in dendrimer macromolecules. Dendrimers are a new class of polymeric materials which consists of a core, regularly branched dendron from the core and peripheral reactive functional groups [26]. PAMAM (Polyamidoamine) dendrimers as described by Tomalia et al. [27] are relatively easy to synthesize with desired number of reactive functional groups. The xanthate functionalized PAMAM dendrimer (XFPD) chelating ligands were synthesized for treatment of radioactive wastewater. Precipitation of Eu3+ and Cu2+ metal ions from aqueous solution using XFPD chelating ligand were studied. Europium and copper were used as homologue solutes for fission and corrosion activation products present in low level radioactive wastes produced in nuclear industries and different research activities. Effect of pH, ionic strength and settling behaviour of precipitate were investigated. Decontamination studies carried out with simulated nuclear liquid waste (SNLW) and radioactive liquid waste (RLW) are discussed.
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Experimental
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Materials and instruments
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All the chemicals used were of analytical reagent grade. Ethylenediamine was purified by distillation over calcium hydride and all other chemicals were used as such without further purification. Eu(NO3)35H2O (99.9%) (Sigma–Aldrich), CuCl22H2O (98%) (Loba Chemie) were used for preparation of standard solution. Millipore water (resistivity 18.2 MV cm) was used in all experiments. The pH of solution was measured using Eco-testr pH 2 m (Eutech make). Distillation was carried out using Equitron rotary evaporator. Whatman 542 hardened ashless filter paper was used for filtration. Metal complexes were dried using vacuum oven (SEMCO make). HACH turbidity meter with working range of 0–1000 NTU was used for measuring residual turbidity of the solution.
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Analytical instrumentation
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FT-IR spectra were recorded within a scanning range of 500–4000 cm1 at 16 cm1 resolution by mounting KBr pellet of the compound in (ABB) MB 3000 spectrometer. 1H and 13C NMR spectra were recorded using Bruker 400 MHz spectrometer. Mass spectra were recorded using Voyager-DETM PRO matrix assisted laser desorption ionization-time of flight (MALDI-TOF) and Thermo DSQ-II quadruple mass spectrometers. Philip Model XL30 scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) was used for in-situ chemical analysis of the metal precipitate. Jobin Yvow-Spex Fluorolog fluorimeter was used for measuring fluorescence emission spectra. VGEscalab MKII X-ray photoelectron spectrometer (XPS) was used to examine the oxidation state of copper and europium in Cu–XFPD and Eu–XFPD complexes. An Inductive coupled plasma optical emission spectrometer (ICP-OES) and Analytica, Janan NovAA300 atomic absorption spectrometer (AAS) were used for estimation of metal ion concentrations.
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Synthesis and characterization of XFPD
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The detailed synthesis and characterization of XPFD is given in Supplementary data. The synthesis involves three distinct steps
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(Fig. S1): (1) Michael addition of methyl acrylate with ethylenediamine, (2) amidation of ester terminated dendrimer (PAMAMG0.5COOMe) with tris(hydroxymethyl)aminomethane resulting in formation of hydroxyl terminated PAMAM dendrimers (PAMAMG1OH) and (3) reaction between hydroxyl group of PAMAMG1OH and carbon disulfide leading to formation of xanthate functionalized PAMAM dendrimer (PAMAMG1OCS2Na) chelating ligand (XFPD). The formation of PAMAMG0.5COOMe and PAMAMG1OH were confirmed by FT-IR, 1H and 13C NMR and mass spectroscopic studies. The presence of xanthate functional group in XFPD was confirmed by UV, FT-IR spectral studies and total number of xanthate group was estimated by acid–base titration.
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Removal of Cu2+ and Eu3+ metal ions from aqueous solution
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Stock solutions of Cu2+ and Eu3+ metal ions (1 g l1) and XFPD (2.7860 mM) chelating agent were prepared. Precipitation studies were carried out individually with various concentrations of Cu2+ and Eu3+ metal ions (25, 50, 100, 250 and 500 mg l1) by adding 0.5 ml of 2.7860 mM XFPD ligand. The resultant precipitate was filtered through whatman 542 ashless filter paper and washed with water. The metal–XFPD precipitate was dried at 343 K in a vacuum oven and then characterized by using FT-IR, XPS and SEM-EDS.
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Effect of pH on formation and settling of metal–XFPD complex
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Effect of pH on removal of Cu2+ and Eu3+ metal ions were studied at pH between 3.0 and 9.0. Either 0.1 M HNO3 or 0.1 M NaOH solutions were used for adjusting the pH of feed solution. The concentration of Cu2+ and Eu3+ metal ions in filtrate was determined by using ICP-OES. The metal ion removal efficiency for Cu2+ and Eu3+ metal ions in term of percentage removal (% R) was calculated by using Eq. (1). C f C fi (1) 100 %R¼ Cf
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where, Cf is the concentration of metal ion in feed and Cfi is the concentration of metal ion in filtrate. Metal ion removal efficiency depends on both effective complexation and its settling. Suspended particles of metal–XFPD complex could be removed from aqueous solution by centrifugation or filtration or sedimentation process. Sedimentation process is generally used for solid–liquid separation which involves settling of suspended particles. The settling behaviour of XFPD complexes of Cu and Eu was studied by measuring residual turbidity of the solution as a function of time.
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Effect of ionic strength on metal ion removal
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Considering the coexistence of various electrolytes in nuclear waste and environmental samples, the influence of electrolytes such as Ca(NO3)2 and NaNO3 on quantitative removal of metal ions was investigated. Experiments were carried out by varying the concentrations of electrolyte from 0.01 to 0.5 M with constant metal ion to XFPD mole ratio.
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Extent of binding of Eu3+ and Cu2+ metal ions
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The extent of binding (EOB) of metal ions (Cu2+ and Eu3+) with XFPD was studied as a function of metal ion concentration. About 9 ml of metal ion (Cu2+/Eu3+) solution was mixed with 1 ml of 2.786 mM XFPD solution in 50 ml centrifuge tube. The resultant precipitate was centrifuged, washed with water, dissolved in concentrated HNO3 and made up to 25 ml in a standard flask. The solution was analysed to estimate the loaded metal ion on XFPD
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ligand. The EOB was calculated by using Eq. (2) CM EOB ¼ C XFPD
(2)
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where, [CM] is the concentration of metal ion (Cu2+/Eu3+) complexed with XFPD (mol l 1) and [CXFPD] is the concentration of XFPD in solution (mol l 1).
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Removal of metal ions from liquid wastes
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Studies on removal of metal ions from simulated nuclear liquid waste (SNLW) and radioactive liquid waste (RLW) were carried out using XFPD ligand. SNLW represents liquid waste arising from nuclear fuel reprocessing, decontamination of nuclear components and active laboratories. The composition of SNLW is given in Table 1. Precipitation studies were carried out by adding 2.5 ml of 2.786 mM XFPD ligand into 20 ml of SNLW taken in a 50 ml centrifuge tube. The resultant precipitate was filtered and the metal ions concentration in the filtrate was determined using ICP-OES and AAS. RLW represents the composition of high level waste (HLW) arising from reprocessing of mixed carbide fuel, (U0.3Pu0.7)C, irradiated to the burn-up of 155 GWd/Te in fast breeder test reactor (FBTR) situated at Kalpakkam in India. The activity of each radionuclides RLW is given in Table 1. Precipitation studies were carried out with 10 ml of RLW (feed solution) taken in a 20 ml glass tube and added 0.25 ml of 2.786 mM XFPD ligand. The resultant suspended particles were filtered and the residual activity in the filtrate was measured by using HPGe detector. The percentage decontamination (% D) for each metal ion was calculated using Eq. (3):
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%D¼
Ai Af 100 Ai
(3)
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where Ai and Af are activities (dps) of metal ion in feed solution and filtrate, respectively.
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Results and discussion
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Precipitation of copper with XFPD ligand
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A dark-brown precipitate was obtained by the addition of XFPD ligand to Cu2+ solution. FTIR spectrum of Cu–XFPD complex is shown in Fig. 1(a). The peaks appear at 1049 cm1 and 1197 cm1 are attributed to asymmetric and symmetric vibration of —CS2 group. The value of Dy[y(CS2)asym y(CS2)sym] can illustrate the coordination fashion of the complex. The Dy value [y(CS2)asym y(CS2)sym] is 148 cm1, almost similar to the corresponding sodium xanthate value (Fig. S3) (Dy = 139 cm1), which
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Table 1 Composition of liquid wastes. SNLW
RLW
Metal ion
Conc. (mg l1)
Metal ion
Activity (dps)
La Ce Nd Gd Sm Mn Co Ni Zn U Th Ba Sr Cs
68 66 64 78 72 25 30 42 37 103 104 70 66 70
54
21.3 1.14 104 1.05 103 45.3 – 21.39 2.46 – – – – 3.95 102 – 1.13 104
Mn 144 Ce 106 Ru 154 Eu Nd 95 Zr 60 Co Ni Zn U Th 125 Sb 90 Sr 137 Cs
3
Fig. 1. FTIR Spectra of (a) Cu–XFD (b) Eu–XFD complexes.
suggests that XFPD ligand adopts a unidentate coordination mode (Fig. 2(a)) in this complex [28,29]. EDS analysis of Cu–XFPD complex indicates the presence of copper and sulphur (Fig. 3) with weight and atom percentages of Cu – 23.2%, S – 22.4% and Cu – 6.7%, S – 12.8%, respectively. From the above values, the calculated mole ratio of S to Cu is about 2:1 which indicates coordination of copper to two sulphur atoms. XPS analysis of Cu–XFPD precipitate showed the existence of copper in +1 oxidation state. Fig. 4 shows the XPS spectrum of Cu2p from Cu–XFPD complex. Two major peaks with binding energy of 934 and 954 eV are the characteristics of cuprous [9] and the absence of cupric (the nonexistence of satellite structure at binding energy of 944 and 963 eV in Fig. 4) confirms that cuprous form of xanthate complex (Cu–XFPD) is precipitated. Hence, the mechanism of complexation is explained by the following Eqs. (4) and (5):
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i) Cu2+ is reduced to Cu+ ion by xanthate group with the formation
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of dixanthogen (PAMAMG1(OCS2)2). ii) Formation of cuprous XFPD complex.
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PAMAMG1OCS2 + Cu2+ ! PAMAMG1(OCS2)2 + Cu+
(4)
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PAMAMG1OCS2 + Cu+ ! PAMAMG1OCS2Cu
(5)
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Precipitation of europium with XFPD ligand
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A pale yellow precipitate was obtained by the addition of XFPD ligand to Eu3+ solution. FTIR spectrum of Eu–XFPD complex is shown in Fig. 1(b) . The presence of xanthate group in europium
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Fig. 2. Coordination pattern of xanthate ligand. (a) unidentate (b) anisobidentate (c) coordination between two hetero atom.
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Fig. 3. EDS spectra of Cu–XFD and Eu–XFD complexes.
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complex was confirmed from the appearance of vibrational peaks at 1197 and 998 cm1 corresponding to asymmetric and symmetric stretching vibrations of —CS2 group (Fig. 1(b)). The Dy[y(CS2)asym y(CS2)sym] value (199 cm1) is much larger than the values corresponding to sodium xanthate (Dy = 139 cm1) and hence, xanthate groups may coordinate with Eu3+ either in anisobidentate state or by coordination with two hetero atom (Fig. 2(b) and (c)) [28,29]. Europium undergo hydrolysis leading to the formation of various species such as Eu(OH)2+ and Eu(OH)2+ at pH > 3. In Fig. 1 (b), the appearance of strong vibrational bands at 3419 and 1420 cm1 are due to O—H stretching and bending vibrations suggesting the presence of hydrolysed europium species in the complex. EDS analysis of Eu–XFPD complex shows the presence of europium and sulphur (Fig. 3). The weight and atom percentages of europium and sulphur in Eu–XFPD complex are found to be Eu – 56.84%, S – 3.12% and Eu – 10.91%, S – 2.84%, respectively. The weight percentage of sulphur in Eu–XFPD complex is reduced due to elimination of carbon disulfide (CS2) from XFPD ligand during complex formation. According to Hard Soft Acid Base (HSAB) principle, the hard acid Eu3+ ion interacts strongly with hard base donor ‘oxygen’ as compared to soft base donor ‘sulphur’. Hence, the coordination ability of europium with oxygen is stronger than that of sulphur atom [30]. Due to this oxophilic character of Eu3+ ion, some of the xanthate groups in XFPD ligand dissociate with the
elimination of CS2 group resulting in existence of few xanthate coordinations in Eu–XFPD complex. The following possible complexation reactions (Eqs. (6)–(9)) are predicted based on the above results.
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PAMAMG1(OCS2)3 + Eu3+ ! PAMAMG1(O2EuCS2) + 2SC2
(6)
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PAMAMG1(OCS2)2 + Eu(OH)2+ ! PAMAMG1(O2Eu(OH)) + 2SC2 (7)
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PAMAMG1(OCS2) + Eu(OH)2+ ! PAMAMG1(OEu(OH)2) + SC2
(8)
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PAMAMG1(OCS2) + Eu(OH)2+ ! PAMAMG1(OCS2Eu(OH)2)
(9)
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XPS study supports the above complexation mechanism. The influence of chemical environment on BE values for core level photoelectron peaks of O1s and Eu4d is shown in Fig. 5. The shape of O1s signal is quite different for Eu–XFPD, when compared to other Eu(III) compounds reported by Mercier et al. [31]. An intense peak appears at lower BE (531.1 eV) and a shoulder at high BE (531.6 eV) depicts the existence of various chemical environment of O1s in Eu–XFPD complex. The shoulder peak at high BE (531.6 eV) is due to C—O of alcoholic group coordinated to Eu(III) and the peak at lower BE (531.1 eV) is attributed to C—O—C of xanthate group coordinated to Eu(III). Similar trend is observed for Eu4d peak (Fig. 5) and the type of coordination of Eu(III) in Eu– XFPD complex has an influence on BE values of Eu4d. Occurrence of peaks at 135.2 and 136.3 eV shows characteristic BE of Eu(III) coordinated to xanthate ((OCS2–Eu(OH)) and alcoholic (C—O—Eu (OH)) group, respectively. Thus, simultaneous study of O1s and Eu4d core level photoelectron peaks allows to distinguish between various types of Eu(III) coordinations in the Eu–XFPD complex. The complexation of europium with XFPD ligand was also confirmed by fluorimetric study. Fluorescence emission spectra of free Eu3+ and Eu–XFPD complex is shown in Fig. 6. The emission spectra clearly shows well-known bands of europium luminescence corresponding to 5D0 ! 7F0 (580 nm), 5D0 ! 7F1 (593 nm) and 5 D0 ! 7F2 (617 nm) transitions. 5D0 ! 7F2 transition is very sensitive to the surrounding microenvironment of Eu3+ ion and its intensity enhances due to complex formation with ligands [32]. Electric dipole transition (5D0 ! 7F2) at 617 nm is observed to be stronger than magnetic dipole transition (5D0 ! 7F1) at 593 nm.
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Fig. 4. XPS spectrum of Cu2p from Cu–XFD complex.
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Fig. 6. Fluorescence emission spectra of Eu3+ and Eu–XFD complex.
Fig. 5. XPS spectra of O1s and Eu4d from Eu–XFD complex. 288 289
Thus, the strong emission band at 617 nm (5D0 ! 7F2) clearly indicates that europium is complexed with XFPD ligand.
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Effect of pH on complexation and metal ions removal efficiency
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XFPD ligand could remove metal ions from aqueous solution almost quantitatively. The metal ion removal efficiency depends on formation of metal–XFPD complex followed by its settling. The pH of feed solution plays an important role on both formation of complex as well as settling of precipitate. The variation in percentage removal of Eu3+ and Cu2+ ions as a function of pH is shown in Fig. 7. At pH < 3, the ligand undergoes decomposition by acid hydrolysis which suggests that pH of the feed solution must be maintained above 3. An increase of pH from 3 to 5 leads to increase in percentage removal of both Eu3+ and Cu2+ ions beyond which it remains almost constant. At pH > 3, xanthogenic acid groups of XFPD ligand ionizes to form xanthogenates and thus effectively coordinates with Eu3+ and Cu2+ which in turn increases its percentage removal. The removal efficiency of XFPD for Eu3+ and Cu2+ are found to be 98.6% and 99.5%, respectively at pH 5. Beyond pH 8, precipitation of Eu3+ ions as europium hydroxide (Eu(OH)3) is predominant compared to that of formation of Eu–XFPD complex. This suggests that the effective removal of metal ions by using XFPD ligand shall be carried out between pH 3 and 8.
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Settling behaviour of Cu–XFPD and Eu–XFPD precipitates
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The settling behaviour monitored by measuring the residual turbidity of Eu–XFPD complex shows that it possesses a favourable settling rate with reduction in turbidity of the solution by a factor of 14.6 within an hour (Fig. 8). In case of Cu–XFPD complex, the settling time is highly pH dependent. At pH > 6, a colloidal suspension of Cu–XFPD complex neither settles by gravity nor aggregates into larger particles for a reasonable length of time due to similar charge associated with the complex [18]. But it is observed that the addition of 2 mM aluminium sulphate as coagulating agent at pH greater than 6 makes the suspended particles to settle and the turbidity of the solution is reduced by a factor of 7.2 within an hour. Hence, settling of metal complexes occurs within 2–3 h due to addition of coagulating agent.
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Extent of binding of Eu3+ and Cu2+ metal ions
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The extent of binding of Eu3+ and Cu2+ metal ions with XFPD ligand as a function of initial metal ions concentration is shown in Fig. 9. It is observed that loading of both europium and copper ions increases with increasing concentration of metal ions and gets saturated at mole ratios of 10.4 and 12.3, respectively. From the above determined mole ratio, a typical loading capacity of XFPD for
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Fig. 7. Effect of pH on removal of Eu and Cu metal ions.
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Fig. 10. Effect of NaNO3 concentration on removal of Eu & Cu metal ions. Fig. 8. Residual turbidity of Eu–XFD and Cu–XFD complexes as a function of time.
Removal of metal ions from simulated nuclear liquid waste (SNLW) and decontamination of radioactive liquid waste (RLW)
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Eu3+ and Cu2+ metal ions are estimated as 0.94 g and 0.48 g g1 of XFPD, respectively. The higher loading capacity of XFPD ligand may be due to coordination of these metal ions with amine, amide and xanthate groups present in the XFPD ligand.
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Effect of ionic strength on removal of metal ions
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Experiments were carried out by varying the concentration of electrolyte from 0.01 to 0.5 M with constant metal ion to XFPD mole ratio. It is observed that the presence of sodium does not exhibit any significant effect on removal of Eu3+ ions but there is slight enhancement in the removal of Cu2+ ions (Fig. 10). In the presence of divalent cations (Ca2+), percentage removal of Cu2+ ions increases gradually with increase in concentration of calcium metal ion and levelled off after 0.1 M (Fig. 11). The enhancement in percentage removal of Cu2+ is due to the coagulation of suspended particles caused by Ca2+ ion. In case of Eu3+, the percentage removal increased initially up to 0.1 M and then decreased gradually. This decrease in percentage removal of Eu3+ at higher concentration of Ca2+ is due to the competing interaction of Ca2+ ions with Eu3+ to the amine, amide and xanthate functional groups of XFPD ligand.
The percentage removal of various metal ions from SNLW by xanthate process is shown in Fig. 12. It is observed that except cesium, all other metal ions are effectively removed. The percentage removal of lanthanides (Ce, Gd, La, Nd, Sm) and actinides (Th & U) are observed to >99% whereas for Co, Ni and Zn it is greater than 90%. The precipitation of lanthanides and transition metal ions are mainly due to metal–xanthate complex formation whereas the precipitation of actinides is due to their hydroxide formation. Hydroxides of uranium and thorium were formed due to the increase in pH of the solution during the addition of XFPD. Though alkaline earth metal ions (Ba & Sr) does not form any precipitate with XFPD ligand, their percentage removal is observed to be about 50–70%. The removal of Ba & Sr metal ions could be explained by considering their role as coagulating agent. Experiments performed for decontamination of RLW using XFPD ligand shows that its removal efficiency is high for almost all radionuclides and poor in case of cesium. The percentage decontamination determined for various radionuclides (Fig. 13) is in the following order: Zr Eu Co (>99.8) > Ce (98.8) > Sb (83.3) > Ru (79.4) > Mn (54.3) > Cs (24.0).
Fig. 9. Extent of binding of Eu and Cu metal ions on XFD ligand.
Fig. 11. Effect of Ca(NO3)2 concentration on removal of Eu & Cu metal ions.
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Please cite this article in press as: P. Ilaiyaraja, et al., Xanthate functionalized PAMAM dendrimer (XFPD) chelating ligand for treatment of radioactive liquid wastes, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.03.013
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Fig. 12. Removal of metal ions from SNLW.
Fig. 13. Decontamination of radionuclides from RLW. 373
Conclusion
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A new xanthate functionalized PAMAM dendrimer (XFPD) ligand has been synthesized for effective removal of Cu2+ and Eu3+ metal ions from aqueous solution. XFPD form water insoluble complexes with Cu2+ and Eu3+ metal ions and pH of aqueous solution plays an important role on both complexation and settling of suspended metal–xanthate complexes. Removal of metal ions are effective in the pH range of 3–8 and settling of suspended metal –xanthate complexes at pH > 6 is achieved by the addition of aluminium sulphate as coagulating agent. The presence of alkaline earth metal enhances removal of radionuclides due to coagulation process. It is demonstrated that xanthate process is effective for decontamination of various radionuclides from RLW. Hence, XFPD ligand possesses potential application in total decontamination of radioactive wastes.
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Acknowledgements
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The authors express their sincere gratitude to Dr P.R. Vasudeva Rao, Director, IGCAR for constant encouragements. We thank Dr K. Sivasubramanian, Head, RDS, Shri H. Krishnan, Shri Shailesh Joshi, Shri B.N. Mohanty and Smt. O. Annalakshmi, RSD for their help during the experiments. We also thank Shri Swpan Kumar Mahato,
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CG for EDS analysis and Smt S. Annapoorni, ACSS, CG, IGCAR for ICP-OES analysis.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jece.2015.03.013.
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References
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