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Adsorption behavior of some radionuclides on the Chinese weathered coal
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Applied Radiation and Isotopes 65 (2007) 901–909 www.elsevier.com/locate/apradiso
Adsorption behavior of some radionuclides on the Chinese weathered coal Jianfeng Wu, Qichu Xu, Tao Bai Northwest Institute of Nuclear Technology, P.O. Box 69-14, Xi’an, Shan’Xi 710024, China Received 14 September 2006; received in revised form 21 March 2007; accepted 4 April 2007
Abstract The equilibrium and kinetic properties of Am(III), Eu(III) and Cs(I) ions adsorption by three weathered coals (WCs) from China, have been investigated in batch stirred-tank experiments. The effects of contact time, solution acidity and initial sorbate concentration on the adsorption of Am(III), Eu(III) and Cs(I) by Yuxian(YX)¸Tongchuan (TC) and Pingxiang (PX) WC were evaluated. The radionuclide ions are able to form complex compounds with carboxylic and phenolic groups of WCs and they are also bounded with phenolic groups even at high acidity reaction solution (40.1 mol/L). Mechanisms including ion exchange, complexation and adsorption to the coal surface are possible in the sorption process. The acidity of the solution played an important role in the adsorption. Even acidity as high as 0.1 mol/L, 60% of Am(III) or Eu(III), 40% of Cs(I) were found to be sorbed on the YX WC, which had the best adsorption capacity for Am(III) and Eu(III). Our batch adsorption studies showed the equilibrium adsorption data fit the linear Langmuir and Freundlich adsorption isotherm. The maximum equilibrium uptake of Eu(III) were 0.412, 3.701, 5.446 mmol/g for JXWC, TCWC and YXWC, respectively. r 2007 Elsevier Ltd. All rights reserved. Keywords: Americium; Europium; Cesium; Adsorption; Weathered coals
1. Introduction For over 50 years, nuclear technologies have been supplying a better quality in different fields of our life. However, this has resulted in accumulation of substantial amounts of radioactive waste. Americium-241, europium152 and cesium-137, important sources of radioactivity in the waste, enter the environment mainly through fallout from nuclear weapon tests and controlled releases of industrial nuclear wastes. These radionuclides pose serious environmental threats and challenge remediation, so they are potentially very hazardous nuclides (Bostick et al., 2002). They are extremely long lived and do not undergo biodegradation. The half-life of 241Am, 152Eu, and 137Cs is 432.2, 13.55, and 30.07 year, respectively. Their solubility in aqueous solution makes them likely to migrate through ground water to the biosphere. Furthermore, 137Cs, because of its Corresponding author. Tel.: +86 029 84765269; fax: +86 029 84765333. E-mail address: [email protected] (J. Wu).
0969-8043/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2007.04.004
chemical similarity to K and Na, is readily assimilated by terrestrial and aquatic organisms(Balley et al., 1999). Many different processes have been investigated for removing these species from aqueous solutions(Kurbatov et al., 1945; Behrens et al., 1998; Lu et al., 1998; Clarke and Wai, 1998; Casnati et al., 2001; DePaoli et al., 2001). They vary from traditional methods such as precipitation, liquid extraction, ion exchange, and adsorption to relatively less conventional or new ones such as bioaccumulation (Venkatesan et al., 2000; Hassan and Usu, 2005; Walker et al., 2005, Bird et al., 1998), electric-field-assisted techniques (Brooks et al., 2006), and ion exchange fibers/membranes (Hegazy et al., 2004). The use of precipitation is limited by the effect of added reagents, which often results in increased pH of the wastewater. The inorganic materials have high chemical, thermal, radiation stabilities, and extremely high selectivity compared to conventional organic ion exchange resins. But the main obstacle in the wide use of synthetic ion exchangers is their cost and their need for periodic washings. Both sorption technology based on ion exchange processes and physical/chemical sorption have found increasing application in various fields in recent years.
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The use of sorption processes for removal of heavy metals from wastewater is especially promising. Adsorption can occur during metal removal processes, making its application in radioactive wastewater treatment possible. In several previous reports, various authors have documented the use of various materials and technologies for the removal of these metal ions. Silica (Delisee and Fromage, 1999), kaolinite (Jeong, 2001), zeolite (DePaoli et al., 2001), montmorillonite (Atun et al., 2005), and silicotitanites(Anthony et al., 1994; Mann and Todd, 2004) are the most common. However, other materials such as activated carbon (Paajanen et al., 1997), coal fly ash (Apak et al., 1996), and even magnetite (Delisee and Fromage, 1999; Matijevic and Meites, 1983), and fibers (Liu et al., 2003) have been studied. The use of peat for the removal of transition metals from aqueous systems was extensively studied and had already been recommended in the past and they were also rich in hydrophobic sites that could bind with transition metals (Carolyn et al., 2004; Hanzlka et al., 2004; Burns et al., 2004). Weathered coal (WC) is an immature coal with large amounts of humic substances (humic and fulvic acids). The primary use of WC is as a fertilizer and soil conditioner in agriculture. WC possesses a high content of oxygen, which are fixed in carboxyl and hydroxyl groups, these functional groups can form complexes with metal cations (e.g. Cu2+, Cd2+, Ba2+, Ni2+, Pb2+, Fe3+ and Al3+) (Mei, 2000). The infrared (IR) spectroscopic studies of metal–humic complexes suggested the participation of phenolicOH and COOH groups in binding of the metal ions followed C¼O groups. The reaction can be as follows: R2COOH þ Meþ 2R2COOMe þ Hþ , R2OH þ Meþ 2R2OMe þ Hþ . These groups are active centers of ion exchange. So, the WC-based materials can be used as an alternative cation exchanger. Carboxyl or hydroxyl groups are able to take part in the ion exchange. These properties enable WC to be used as a radioactive wastewater-treating medium to remove hazardous metals. The price of WC is only 1/20 of the activated coal and the availability of this material makes it a promising candidate for radioactive wastewater treatment. The present work was performed in order to investigate the effects of contact time, solution acidity and sorbate concentration on the adsorption behavior of several representative WCs toward 241Am, 152Eu, and 137Cs cations. 2. Experimental 2.1. Preparation of sorbents and stock solution WC samples were collected from the YuXian, TongChuan, and PingXiang reserve in northern, northwestern, and southern China, respectively. These materials were first
dried in an oven at 80 1C for 24 h, sieved to 80–100 meshes (ASTM), and then placed in an airtight container for future use. All adsorbents were dried overnight at 80 1C before the adsorption experiments. The radioactive solutions of Am(III), Eu(III) and Cs(I) solutions were prepared by dissolving the indicator in the deionized water containing a small quantity of nitric acid (the radioactive tracer 241Am, 152 Eu, and 137Cs of known purity was procured from China Institute of Atomic Energy, Beijing, China). An accurately weighed quantity of Eu2O3 and CsCl (purchased from Guoyao company, Shanghai, China) were dissolved in dilute nitric acid to prepare a stock solution (0.11 and 0.66 mol/L, respectively). Experimental solutions of the desired concentrations were obtained by successive dilutions. All primary chemicals used were of analytical reagent grade. A HPGe g detector, was used for the determination of radioactive intensity in the solutions. All experiments were performed in triplicate. An pHS-3CpH-meter, Leici Corp, ShangHai, was used for pH measurement, which was calibrated with standard pH-buffer from INOAB WTW Welheim. 2.2. Surface properties WC pH: A NaCl solution at a concentration of 0.1 mol/L was first prepared; its blank pH was found to be 5.65. A total of 0.2 g of WC sample was then added into 25 mL of the prepared NaCl solution. An pHS-3CpH-meter (Leici Corp, ShangHai, China) was used to measure the WC pH after the suspension was shaken for 48 h. Total acidity capacity (TAC): The WCs with a weight w(g) were titrated with sodium hydroxide solutions to determine the contents of acidic groups (mmol/g). The samples were shaken for 48 h in a sodium hydroxide solution that had a concentration CNaOH of 0.24 mol/L and a volume VNaOH of 10 mL in sealed polyethylene flasks. The solutions were then left for 6 h for settling of the coal particulates. Then the supernatant was filtered by a 0.45 mm Teflon membrane filter. A filtrate with a volume VS of 10 mL was added to a standardized HCl solution that had a concentration CHCl of 0.26 mol/L and a volume VHCl of 30 mL; excess HCl was determined by titration with standardized 0.1 mol/L NaOH ðC 0NaOH Þ. The volume of NaOH consumed was Vt. In the test, the sodium hydroxide solutions without WCs were referred to as blanks. Vs
V NaOH C NaOH ðTAC wÞ þ C 0NaOH V t ¼ C HCl V HCl . V NaOH
Total carboxylic groups capacity (TCC): The WCs with a weight w(g) were titrated with sodium hydroxide solutions to determine the contents of carboxylic groups (mmol/g). The samples were shaken for 48 h in a calcium acetate solution that had a concentration C CaAc2 of 0.5 mol/L and a volume V CaAc2 of 50 mL in sealed polyethylene flasks. The solutions were then left for 6 h for settling of the coal particulates. Then the supernatant was filtered by a 0.45 mm Teflon membrane filter. A filtrate with a volume Vs of
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50 mL was titrated with standardized 0.1 mol/L NaOH (CNaOH). The volume of NaOH consumed was Vt. In the test, the sodium hydroxide solutions without carbons were referred to as blanks. TCC w ¼ 2C CaAc2 V CaAc2 þ C NaOH V t . The total phenolic groups capactity (TPC) can be gained by subtracting TCC from TAC. 2.3. Equilibrium sorption experiments Batch sorption experiments were carried out by adding 20 mg of powdered YXWC, TCWC or PXWC to a series of round-bottomed plastic test-tube containing radioactive Am(III), Eu(III) and Cs(I) solutions prepared from the stock solution at 298 K and pH about 4.5. The mixture was stirred for 1 d on an orbital shaker at 300 rpm. Sufficient time (about 15 h) was allowed for the Am(III), Eu(III) and Cs(I) uptake process to reach equilibrium. The samples were withdrawn from the shaker at predetermined time intervals, and the sorbent was removed from the solution by centrifugation at 3000 rpm for 5 min. The residual Am(III), Eu(III) and Cs(I) concentrations in supernatant were measured by HPGe g detector. There were several parameters that affected the adsorption rate, including the stirring rate in the aqueous phase and the physical nature of sorbent (e.g. porosity and surface area). The pH of solution also determined the ionic species present in solution. Americium and europium were found in the form of cationic, hydroxide complexes (cationic or anionic), hydroxide precipitate and carbonate complexes, based on pH of solution. To evaluate the effect of acidity on the adsorption efficiency of the WCs, the acidity of 4 mL solutions of the Am(III), Eu(III) and Cs(I) ions was adjusted to between 0.1 and 4 mol/L with concentrated HNO3. The adsorbed quantities were then determined using the mass balance equation: q¼
ðA0 At Þ V Ce , A0 m
(1)
where q is the amount of europium (mol/g coal) adsorbed on the coal samples, m the weight of coal (g), V the volume of metal solution (L), A0 the radioactive intensity of initial radioactive solution, At the radioactive intensity of equilibrium solution, and Ce the initial metal concentration (mol/L). The percentage adsorption, P, was calculated from Kd by using the following equations: A0 At V , At m
(2)
100K d . K d þ V =W
(3)
Kd ¼
p¼
Amount adsorbed per unit weight of the WC, x/m, was calculated radiometrically from the initial (known metal concentrations) and final activities of the solutions.
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3. Results and discussion 3.1. Physicochemical properties Acid–base titration of carbons based materials is normally used for the characterization of surface functional groups. It is assumed for the surface functional groups in the WCs as a generalized amphoteric group (sometimes referred to as a weak acidic group), the parameters of the surface reactions can be determined by the titration. WCs pH and TAC can be used as a more straightforward approach to compare the chemical properties of WC. WC pH can be treated as an approximate measure of the pH at the point of zero charge (pHPZC). It is shown in Table 1 that the pH values for the WCs were different, ranging from 5.03 of JXWC to 6.65 of TCWC. The WCs pHPZC indicated that the surfaces were positively charged when the solution pH was below 5.03. Some of chemical characteristics of the studied WCs are also given in Table 1. IR spectra of three WCs and a ‘standard’ coal are depicted in Fig. 1. IR spectra were carried out with a Perkin Elmer BX Model FTIR Spectrometer. The region corresponding to high wave numbers showed a broadband centered at 3430 cm1. This band was generally attributed to the (O–H) stretching of carboxylic and alcoholic groups, indicating intermolecular hydrogen bonding of humic substance molecules. The content of carboxyl groups was determined by separation of the bands in the 1800–1500 cm1 spectral region. The band at 1610 cm1 was assigned to aromatic carbonyl and carbonyl motion in carboxyl groups. The band at around 1390 cm1 was assigned to symmetric COO-stretching motions and to the bending vibrations of aliphatic groups. The band appearing at approximately 1260 cm1 could be attributed to the C–O stretching of phenolic groups. Less intense band that appeared in these spectra at around 1035 cm1 could be attributed to the C–O stretching motions. YXWC showed peaks at 1105 cm1 and have more humic content than the other sorbents. However, compare the IR spectra of ‘standard’ coal with WCs, the ‘ standard’ coal only exhibits broad bands centered at 3440 and 1630 cm1 assigned to the (O–H) stretching of carboxylic and alcoholic groups, aromatic carbonyl and carbonyl motion in carboxyl groups, respectively. The IR spectroscopic examination showed that the IR spectra of ‘standard’ coal and WCs were different. It can be concluded that WC contain higher amount of oxygencontaining functional groups than ‘standard’ coal. Oxidation of coal increases CO groups associated with carboxylates and phenoxy structures. Pore volume and average pore diameter greater than 20 A˚ were determined by N2 adsorption at 77 K with an accelerated surface area and porosimeter (ASAP-2020, Micromeritics). The specific surface area was calculated from the isotherms by the Brunauer–Emmett–Teller (BET) equation. The Barrett–Joyner–Halenda (BJH) equation
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Table 1 Chemical and surface properties of various WC Properties
JXWC
YXWC
TCWC
pHPZC Acidic groups (mmol/g) Carboxyl groups (mmol/g) Phenolic hydroxyl (mmol/g) BET surface area (cm2/g) Total pore volume (cm3/g) Pore size (nm) IR spectra (cm1)
5.030 2.410 2.395 0.015 14.369 0.036 10.980 3392,2362,1616,1520,1405,1205
6.630 1.956 0.827 1.127 19.533 0.061 16.420 3401,2360,1608,1509,1265,1032
6.650 0.920 0.290 0.630 1.319 0.007 9.080 3398,2360,1612,1512,1288,1125
Fig. 1. IR spectra of the three WCs and a standard coal given in transmittance.
was used to calculate the pore volume from the N2 adsorption data. The detailed properties are shown in Fig. 2 and Table 1. Fig. 2 clearly indicates that the three WCs consisted mostly of primary mesopores and had an effective pore size distribution area within 3 and 20 nm, while TCWC had a narrower pore size distribution. The total pore volume of the TCWC, JXWC and YXWC were 0.007, 0.036 and 0.061 cm3 g1, respectively. 3.2. Speciation of radionuclides in solutions As investigated, in the adsorption of hydrated Eu3+ and Am3+ by WCs, americium or europium anions would not exchange with the WC. This species would remain in solution, so we must know the speciation of these radionuclides in the aqueous solution. The americium and europium ions speciations could be determined by batch ion exchange method developed by Schubert (152Eu 1 109 mol 11) in solution (Maes et al., 1988) or by time-resolved laser induced fluorescence (Plancque et al., 2003). Here, Am3+ and Eu3+ speciations were investigated using MINTEQA2, a geochemical equilibrium speciation code, assuming equilibrium with the atmosphere in the pH range 3–10. The formation of complexes with OH, CO2 and 3 2+ 2+ HCO was considered (Am(OH) , Eu(OH) log b ¼ 6.2, 3 1
Fig. 2. Pore size distribution of JXWC, TCWC and YXWC (DPV ¼ differential pore volume).
log b2 ¼ ll.8; Am(CO3)+ Eu(CO3)+ log b1 ¼ 8.26; AmCO+ 3 , log b2 ¼ 14; Am(CO3) log b2 ¼ 2.05) all at ionic 2 strength ¼ 0 (Pettit and Powell, 1993). The representative figures of the speciation distribution of Am (III) and Eu (III) at the concentration changing from 107 to 101 mol/L are depicted in Figs. 3 and 4, they were calculated by MINTEQA2 modeling. Hydrated Am3+ predominated at pH lower than 6, Am(OH)2+ was the primary species between pH 7 and 8, and Am(OH)+ 2 became the predominate at higher pH. Hydrated Eu3+ predominated up to pH 7, EuCO+ 3 became predominant between pH 7 and 8, and Eu(CO3) 2 took over at higher pH. The results showed that hydrated Am3+ or Eu3+ are dominant species at pH 5.1 (the speciation percent of hydrated Am3+ or Eu3+ all over 96%) independent of the increasing of the americium or europium concentration. In the following experiments, the pH of solution was all adjusted to close to 5.0 based on the gained results. This would insure the hydrated Am3+ and Eu3+ were the predominate species in the solution. 3.3. Effect of contact time Removal of Am(III), Eu(III) and Cs(I) by WC with time was carried out at pH close to 5.01 and the effect of contact time on the sorption of Am and Eu or Cs
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Fig. 3. (a) Speciation diagrams of Americium in water: [Am] ¼ 0.1 mg L–1 (E4.1 10–7 mol/L), I ¼ 0.1 mol L1 (pCO2 ¼ 3.16 10–4 atm). (b) Speciation diagrams of Americium in water: [Am] ¼ 24.1 g L–1 (E1times; 10–1 mol/L), I ¼ 0.1 mol L1 (pCO2 ¼ 3.16 10–4 atm).
are presented in Figs. 5–7. As may be seen from Fig. 5, the binding of Am(III)/Eu(III)/Cs(I) on JXWC increased very rapidly with agitation time. These studies further indicated that the binding of both Am(III) and Eu(III) was about 80% and 70%, respectively. In the first 1 h, the percent adsorption of Cs(I) reached 60%. Later, the percentage sorption of Am and Eu reached about 90% and 80%, respectively, and attained equilibrium by the end of a contact period of 10 h. The further increase of contact time from 8 to 15 h had no significant effect on the percentage sorption of Cs(I), percentage sorption of Am(III) and Eu(III) had no obvious increase from 10 to 15 h. Fig. 6 gives the effect of contact time on the adsorption of Am(III)/Eu(III)/Cs(I) by YXWC. The rapid increase of percent adsorption of Am/Eu were found within the preliminary 0.5 h, and the percent adsorption of Am(III) and Eu(III) reached 95%. Later, equilibrium adsorption of Am and Eu was established. But the Cs(I) required 400 min to attain equilibrium. Fig. 7 shows high adsorption rates of Am(III)/Eu(III)/Cs(I) by TCWC were observed at the beginning, but eventually became constant (i.e. adsorption
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Fig. 4. (a). Speciation diagrams of Europium in water: [Eu] ¼ 0.1 mg L–1 (E6.6 10–7 mol/L), I ¼ 0.1 mol L1 (pCO2 ¼ 3.16 10–4 atm). (b) Speciation diagrams of Europium in water: [Eu] ¼ 15.2 g L–1 (E1 10–1 mol/ L), I ¼ 0.1 mol L1 (pCO2 ¼ 3.16 10–4 atm).
Fig. 5. Adsorption rate of Am(III), Eu(III) and Cs(I) ions by JXWC from aqueous solution at pH: 5.01. Adsorption conditions: amount of coal, 0.02 g; volume of adsorption medium, 30 mL; temperature, 298 K.
equilibrium) within about 15 h. The percent adsorption of Am(III) and Eu(I) reached about 85%, only 45% Cs was adsorbed.
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Fig. 6. Adsorption rate of Am(III), Eu(III) and Cs(I) ions by YXWC from aqueous solution at pH: 5.03. Adsorption conditions: amount of coal, 0.02 g; volume of adsorption medium, 30 mL; temperature, 298 K.
increasing acidity, as shown in Fig. 8. Both Kd and percent adsorption (P) decreased with an increase in acidity within the acidity range studied. A significant decrease of Kd was seen with the increase of acidity from 0.001 to 1 mol/L, however, a much lower decrease in Kd values was observed when acidity of the suspension changed from 1 to 4 mol/L. At high acidity the lower Kd values were due to a relatively small number of available sites. This was because of the solubility of different constituents of WC in acidic solutions; therefore relatively smaller number of active sites, which would be available for the sorption of radionuclides. At higher acidity, excess hydrogen ions could compete effectively with Am(III)/Eu(III)/Cs(I) for binding sites, resulting in a low level of Am(III)/Eu(III)/ Cs(I) sorption. Interactions of metals with the coal surface functional groups are complex, probably simultaneously dominated by adsorption, ion exchange and chelation. In addition, metal ions may also be adsorbed on sandy sediment grains, by exchange with protons, which are present in the outer part of the electric double layer. The extent of sorption will, therefore, depend upon experimental parameters that could affect the surface charge. Similar results had been reported for sorption of metal ions on wollastonite (Dakiky et al., 2002). Even at acidity as high as 0.1 mol/L, 60% of Am and Eu, and 40% of Cs were found to be sorbed on the YXWC. The maximum sorption of Am(III)/Eu(III)/Cs(I) was observed at an initial acidity (0.001 mol/L) and at this acidity, almost 95% of both Am and Eu, 50% of Cs was found to be sorbed. 3.5. Effect of sorbate concentration
Fig. 7. Adsorption rate of Am(III), Eu(III) and Cs(I) ions by TCWC from aqueous solution at pH: 5.08. Adsorption conditions: amount of coal, 0.02 g; volume of adsorption medium, 30 mL; temperature, 298 K.
The sorption of europium as a function of its concentration was studied at 298 K at solution to clay ratio being 200:1 (i.e. 4 mL:0.02 g) by varying the metal concentration from 107 to 101 mol/L. The results are shown in Fig. 9. Both Kd and P values decreased with increasing europium concentration. The results in Fig. 9 suggest that at least two
Initial rapid binding of these radionuclides occured initially due to easily available exchangeable sites located on surface of the WCs. Subsequent slow process suggested that intrapore diffusion be also involved in the sorption. It was stated that the plateau portion of the curve corresponded to pore diffusion and the linear portion of the curve reflected surface layer diffusion. The faster adsorption equilibrium of YXWC may be attributed to its high surface area and larger pore size. Therefore, a contact time of 15 h was used for all other subsequent experiments. 3.4. Effect of acidity of solution The effect of acidity on sorption of spiked Am(III)/ Eu(III)/Cs(I) on WC was studied at 298 K at solution to clay ratio (i.e, 200:1) by varying the acidity of the solution–coal suspension from 1 103 to 4 mol/L. The sorption of the three radionuclides on WC decreased with
Fig. 8. Effect of acidity on the adsorption of Am(III)/Eu(III)/Cs(I) by YXWC, Am(’), Eu(K),Cs(m), temperature at 298K, solution/weathered coal ratio, 200:1.
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types of phenomena (i.e., adsorption as well as exchange) are taking place in the range of Eu(III) concentrations studied. The removal of Eu by the latter process might be due to isomorphous replacement or by ion exchange. The log Kd values for YXWC and TCWC remained almost constant at low Eu loadings up to 105 mol/L, while at loadings between 104 and 10–1 mol/L, log Kd values decreased significantly. A dramatic decrease in P for JXWC, TCWC, and YXWC as the concentration exceeded about 104 mol/L was found. Furthermore, the initial concentrations were increased up to 0.224 mol/L for Eu(III) in order to reach the plateau values, which represented the saturation of the active points which were available for interaction with Eu(III) on the coal samples. The maximum adsorption capacities were 0.412, 3.701 and 5.446 mmol/g of JXWC, TCWC and YXWC, respectively. This suggests that energetically less favorable lattice positions or exchange sites become involved with increasing Eu(III) concentration to 104 mol/L.
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The amount of Eu(III) adsorbed by per unit mass of coal (i.e. adsorption capacity) decreased with increasing initial carrier concentration of ions, as expected. At low concentrations, the ratio of the initial number of moles of metal ion to the available surface area is large and subsequently the fractional adsorption is more or less independent of initial concentrations. However, at higher concentrations, the number of available sites for adsorption is relatively fewer and hence the percentage of removal of metal ions depends upon the initial concentration. 4. Adsorption isotherms Several models have been proposed in the literature to describe experimental data of adsorption isotherms. The Langmuir and Freundlich models are the most frequently employed models. In this work, both models were used to describe the relationship between the amount of Eu3+ adsorbed and its equilibrium concentration in solution (see Figs. 10 and 11). Although the Freundlich isotherm does not take the sorbents finite capacity for sorption at high concentrations into account, it often describes sorption of trace amounts of sorbing species satisfactorily. The linear form of the Freundlich isotherm model can be represented by using the equation below: log x=m ¼ log K þ 1=n log C e ,
(4)
where x/m is the amount adsorbed per gram of the adsorbent (mol/g), Ce the equilibrium concentration (mol/L), K and 1/n are constants. The equilibrium concentration of europium studied was in the range of 1.12 107–1.12 103 mol/L. A plot of log x/m against log Ce gave a straight line, the slope and intercept of which corresponds to 1/n and log K, respectively. Freundlich plots for europium sorption on WC are shown in Fig. 10. The coefficient of relationship (R) for the Freundlich plot is 1, 1, and 0.99757 for JXWC, TCWC, and YXWC,
Fig. 9. (a) Effect of europium concentration on its sorption on weathered coal percentage adsorption (a) YXWC(,), JXWC(n), TCWC(J), solution/weathered coal ratio, 200:1, pH 5.05, temperature: 298K. (b) Effect of europium concentration on its sorption on weathered coal Kd (b) YXWC(m), JXWC(K), TCWC(’), solution/weathered coal ratio, 200:1, pH 5.05, temperature, 298K.
Fig. 10. The Freundlich isotherm of three weathered coal, TCWC(’), YXWC(K), JXWC(m), solution/weathered coal ratio, 200:1, pH 5.05, temperature, 298K.
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respectively. The Freundlich constant K indicated the sorption capacity of the sorbent and the value of K (mmol/g) is 60.3, 199.5 and 198.4 for JXWC, TCWC and YXWC, respectively. Furthermore, the values of the Freundlich constant ‘1/n’ are 0.9181, 1.0001 and 0.9997 for JXWC, TCWC and YXWC, respectively. Linear regression of Freundlich plots (Fig. 10) gave slopes equal to 1, indicating a concentration-independent (i.e., linear) sorption of europium in the concentration range used. Langmuir’s adsorption equation was also used for the sorption of europium on WC. This model assumes uniform energies of adsorption onto the surface with no transmigration of adsorbate in the plane of the surface. The linear form of Langmuir isotherm is given by the following equation: C e x=m ¼ 1=bQ0 þ C e =Q0 ,
of Eu sorption were calculated from the binding constant K obtained from Langmuir’s equation by using the following relation: DG ¼ RT In K.
The free energy of europium sorbed on the three WCs at 298 K is also listed in Table 2. The thermodynamic equilibrium constant determined in these experiments was used to calculate other thermodynamic parameters. The Gibbs free energy for the adsorption process was obtained at 25 1C using Eq. (7). The Gibbs free energy indicates the degree of driving force of the adsorption process, where more negative values reflect a more energetically favorable adsorption process. The negative DG1 values obtained in this study for YXWC, TCWC, and JXWC confirmed the feasibility of the adsorption. From Table 2, the Langmuir adsorption capacity Qmax (mmol/g) for europium on JXWC, TCWC, and YXWC is 9.47 102, 8.48 101, and 1.34 mmol/g, respectively, and the equilibrium constant b (L/mg) is 3219.6, 316.8, and 169.5, respectively. The maximum europium adsorption capacity (Qmax) at constant pH 5.05 followed a descending order of YXWC4TCWC4JXWC. This is consistent with the experimental observation mentioned in Section 3.5. In general, this model was more suitable to describe the experimental equilibrium data for the three WC samples. Comparison of Qmax values indicated that the YXWC, which had the most content of phenolic groups (Table 1), had the maximal europium adsorption capacity. This might be due to the reactions between the organic functional groups and europium, and the phenolic hydroxyl was the most effective functional group under the sorption condition.
(5)
where x/m is the amount adsorbed by per unit mass of adsorbent (mol/g), Ce the equilibrium concentration of adsorbate in solution (mol/L), Q0 the monolayer capacity (mol/L), and b the binding constant that is related to the heat of adsorption by the equation K ¼ K 0 expðq=RTÞ,
(7)
(6)
where q is the heat of adsorption. The equilibrium concentration of europium was used as mentioned above. A straight line was obtained by plotting Ce/x/m against Ce (Fig. 11). The constants involved in the equation are given in Table 2. Thermodynamic parameters such as free energy
5. Conclusion WC has been proved to be an useful and inexpensive sorbent for Am(III) and Eu(III) and Cs(I) from aqueous solutions. When the sorption equilibrium is known precisely, it is possible to predict the optimum conditions for a pre-concentration or removal of metal ions from radioactive wastewater. This study showed that the adsorption of Eu(III) was dependent on time of contact, solution acidity and the initial concentrations of sorbate. The YXWC had higher adsorption efficiency than the TCWC and JXWC. The acidity of the solution played an important role in the
Fig. 11. The Langmuir isotherm of three weathered coal, TCWC(’), YXWC(K), JXWC(m), solution/weathered coal ratio, 200:1, pH 5.05, temperature, 298K.
Table 2 Values of Freundlich and Langmuir constants for europium sorption on WC Sample
JXWC TCWC YXWC
Freundlich isotherm
Langmuir isotherm 2
Kf (mmol /g)
1/n
R
60.3 199.5 198.4
0.9181 1.0001 0.9997
1 1 0.9976
b (L/mg)
Q0 (mmol/g)
R2
DG0 (kJ/mol)
3219.6 316.8 169.5
9.47 102 8.48 101 1.34
0.998 0.9998 0.9996
20.0114 14.2664 12.7164
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adsorption distribution. At very low pH or high acidic functional oxidized groups (hydroxyl, carboxyl, phenol, etc.) of HAs contained in the WC were protonated. It was observed that the removal percentage decreased sharply with the increase of acidity of solution. But even at acidity as high as 0.1 mol/L, 60% of Am(III) and Eu(III), and 40% of Cs(I) were found to be sorbed on the YXWC. The maximum sorption of Am(III)/Eu(III)/Cs(I) was observed at an initial acidity (0.001 mol/L) and at this acidity, almost 95% of both Am(III) and Eu(III), and 50% of Cs(I) were found to be sorbed. The Eu(III) adsorption by three WCs had been described by the Langmuir and Freundlich isotherm model. Although the adsorption capacities of WCs obtained in this study, 0.412 mmol/g for Eu(III) of JXWC, 3.701 mmol/ g for TCWC and 5.446 mmol/g for YXWC, were lower in comparison to those of synthetic ion exchange resins, substantially lower cost, easy availability of low-rank coal in China indicates they can be used by small-scale industries having low-level radioactive wastewater. It is necessary to note that the weathered coals surface can be modified to develop desirable physico-chemical properties by adequate choice of activation procedures. This effect will be studied further. References Anthony, R.G., Dosch, R.G., Gu, D., 1994. Use of silicotitanates for removing cesium and strontium from defense waste. Ind. Eng. Chem. Res. 33, 2702–2705. Apak, R., Atun, G., Gu¨cu¨lu¨, K., Tu¨tem, E., 1996. Sorptive removal of cesium-137 and strontium-90 from water by unconventional sorbents. II. Usage of coal fly ash. J. Nucl. Sci. Technol. 33, 396. Atun, G., Bilgin, B., Mardinli, A., 2005. Sorption of cesium on montmorillonite and effects of salt concentration. J. Radioanal. Nucl. Chem. 211 (2), 435–442. Balley, S., Olin, Bricka, R.M., et al., 1999. A review of potentially low-cost sorbents for heavy metals. Water Res. 33 (11), 2469–2479. Behrens, E.A., Sylvester, P., Clearfield, A., 1998. Assessment of a sodium nonatitanate and pharmacosiderite-type ion exchangers for strontium and cesium removal from DOE waste simulants. Environ. Sci. Technol. 32 (1), 101–107. Bird, G.A., Hesslein, R.H., Mills, K.H., Schwartz, W.J., Turner, M.A., 1998. Bioaccumulation of radionuclides in fertilized Canadian shield lake basins. Sci. Total Environ. 218, 67. Bostick, B.C., Vairavamurthy, M.A., Karthikeyan, K.G., Chorover, J., 2002. Cesium adsorption on clay minerals: an EXAFS spectroscopic investigation. Environ. Sci. Technol. 36 (12), 2670–2676. Brooks, K.P., Augspurger, B.S., Blanchard, D.L., et al., 2006. Hydraulic testing of ion exchange resins for cesium removal from Hanford tank waste. Sep. Sci. Technol. 41 (11), 2391–2408. Burns, C.A., Boily, J.F., Crawford, R.J., 2004. Cd(II) binding by particulate low-rank coals in aqueous media: sorption characteristics and NICA–Donnan models. J. Colloid Interface Sci. 278, 291–298. Carolyn, A.B., Jean-Franc- ois, B., Russell, J., Crawford, Ian H.H., 2004. Cd(II) binding by particulate low-rank coals in aqueous media: sorption characteristics and NICA–Donnan models. J. Colloid Interface Sci. 278, 291–298.
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Casnati, A., Barboso, S., Rouquette, H., Schwing-Weill, M.-J., 2001. New efficient calixarene amide ionophores for the selective removal of strontium ion from nuclear waste: synthesis, complexation, and extraction properties. J. Am. Chem. Soc. 123 (49), 12182–12190. Clarke, T.D., Wai, C.M., 1998. Selective removal of cesium from acid solutions with immobilized copper ferrocyanide. Anal. Chem. 70 (17), 3708–3711. Dakiky, M., Khamis, M., Manassra, A., Mer’eb, M., 2002. Selective adsorption of chromium(VI) in industrial wastewater using low-cost abundantly available adsorbents. Adv. Environ. Res. 6, 533–540. Delisee, M.N., Fromage, A., 1999. Surface complexation modeling of Yb(III) and Cs(I) sorption on silica. J. Colloid Interface Sci. 212, 228. DePaoli, S.M., Bostick, D.T., Lucero, A.J., 2001. Analysis of breakthrough profiels based on gamma-ray emission along loaded packed bed column: comparative evaluation of ionsiv@ IE-911 and chabazite zeolite for the removal of radiostrontium and cesium from groundwater. Sep. Sci. Technol. 36 (5–6), 941–957. Hanzlka, J., Wkaa, J.J., Sebek, O., 2004. Multi-component adsorption of Ag(I), Cd(II) and Cu(II) by natural carbonaceous materials. Water Res. 38 (8), 2178–2184. Hassan, N.M., Usu, K.A., 2005. Cesium removal from Hanford tank waste solution using resorcinol-formaldehyde resin. Sol. Extra. & Ion Exch. 23 (3), 375–389. Hegazy, E.S., Abdel, H.K., et al., 2004. Removal of 134Cesium-and 60 Cobalt with radiation-grafted copolymers from their liquid wastes. J. Appl. Poly. Sci. 95 (4), 936–945. Jeong, C.H., 2001. Mineralogical and hydrochemical effects on adsorption removal of 137Cesium and 90Strontium by kaolinite. J. Environ. Sci. Health, Part A. 36 (6), 1089–1099. Kurbatov, J.D., Kulp, J.L., Edward, M.J., 1945. Adsorption of strontium and barium ions and their exchange on hydrousferric oxide. J. Am. Chem. Soc. 67 (11), 1923–1929. Liu, C., Huang, Y., Naismith, N., Economy, J., Talbott, J., 2003. Novel polymeric chelating fibers for selective removal of mercury and cesium from water. Environ. Sci. Technol. 37, 4261–4268. Lu, N., Kung, K.S., Mason, C.F.V., Triay, I.R., Cotter, C.R., 1998. Removal of 239Plutonium and 241Americium from rocky flats soil by leaching. Environ. Sci. Technol. 32 (3), 370–374. Maes, A., Brabandere, D., Cremers, J., 1988. A modified Schubert method for the measurement of the stability of europium humic. acid complexes in alkaline conditions. Radiochim. Acta 44/45, 51. Mann, N.R., Todd, T.A., 2004. Removal of Cesium from acidicradioactive tank waste by using ions IE-911. Sep. Sci. Technol. 39 (10), 2351–2357. Matijevic, T.H., Meites, E., 1983. Adsorption of Co2+ ions on spherical magnetite particles J. Colloid. Interface. Sci. 92, 303. Mei, J.T., 2000. The adsorption and desorption of Pb2+, Cu2+, Ni2+ and Zn2+ in the waste plating water by weathered coal. Mater. Protec. 33 (6), 15–17. Paajanen, A., Lehto, J., Santapakka, T., et al., 1997. Sorption of cobalt on activated carbons from aqueous solution. Sep. Sci. Technol. 32, 813. Pettit, L.D., Powell, H.K.J., 1993. IUPAC Stability Constant Database. Academic Software, UK. Plancque, G., Moulin, V., Moulin, V., 2003. Europium speciation by time-resolved laser-induced fluorescence. Analy. Chim. Acta. 478 (1), 11–22. Venkatesan, K.A., Selvam, G.P., Vasudeva, P.R., 2000. Sorption of strontium on hydrous zirconium oxide. Sep. Sci. Technol. 35 (14), 2343–2357. Walker, D.D., Norato, M.A., Campbell, S.G., et al., 2005. Cesium removal from savannah river site radioactive saste using the causticSide Solvent Extraction (CSSX) Process Sep. Sci. Technol. 40 (1–3), 297–309.
Applied Radiation and Isotopes 65 (2007) 901–909 www.elsevier.com/locate/apradiso
Adsorption behavior of some radionuclides on the Chinese weathered coal Jianfeng Wu, Qichu Xu, Tao Bai Northwest Institute of Nuclear Technology, P.O. Box 69-14, Xi’an, Shan’Xi 710024, China Received 14 September 2006; received in revised form 21 March 2007; accepted 4 April 2007
Abstract The equilibrium and kinetic properties of Am(III), Eu(III) and Cs(I) ions adsorption by three weathered coals (WCs) from China, have been investigated in batch stirred-tank experiments. The effects of contact time, solution acidity and initial sorbate concentration on the adsorption of Am(III), Eu(III) and Cs(I) by Yuxian(YX)¸Tongchuan (TC) and Pingxiang (PX) WC were evaluated. The radionuclide ions are able to form complex compounds with carboxylic and phenolic groups of WCs and they are also bounded with phenolic groups even at high acidity reaction solution (40.1 mol/L). Mechanisms including ion exchange, complexation and adsorption to the coal surface are possible in the sorption process. The acidity of the solution played an important role in the adsorption. Even acidity as high as 0.1 mol/L, 60% of Am(III) or Eu(III), 40% of Cs(I) were found to be sorbed on the YX WC, which had the best adsorption capacity for Am(III) and Eu(III). Our batch adsorption studies showed the equilibrium adsorption data fit the linear Langmuir and Freundlich adsorption isotherm. The maximum equilibrium uptake of Eu(III) were 0.412, 3.701, 5.446 mmol/g for JXWC, TCWC and YXWC, respectively. r 2007 Elsevier Ltd. All rights reserved. Keywords: Americium; Europium; Cesium; Adsorption; Weathered coals
1. Introduction For over 50 years, nuclear technologies have been supplying a better quality in different fields of our life. However, this has resulted in accumulation of substantial amounts of radioactive waste. Americium-241, europium152 and cesium-137, important sources of radioactivity in the waste, enter the environment mainly through fallout from nuclear weapon tests and controlled releases of industrial nuclear wastes. These radionuclides pose serious environmental threats and challenge remediation, so they are potentially very hazardous nuclides (Bostick et al., 2002). They are extremely long lived and do not undergo biodegradation. The half-life of 241Am, 152Eu, and 137Cs is 432.2, 13.55, and 30.07 year, respectively. Their solubility in aqueous solution makes them likely to migrate through ground water to the biosphere. Furthermore, 137Cs, because of its Corresponding author. Tel.: +86 029 84765269; fax: +86 029 84765333. E-mail address: [email protected] (J. Wu).
0969-8043/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2007.04.004
chemical similarity to K and Na, is readily assimilated by terrestrial and aquatic organisms(Balley et al., 1999). Many different processes have been investigated for removing these species from aqueous solutions(Kurbatov et al., 1945; Behrens et al., 1998; Lu et al., 1998; Clarke and Wai, 1998; Casnati et al., 2001; DePaoli et al., 2001). They vary from traditional methods such as precipitation, liquid extraction, ion exchange, and adsorption to relatively less conventional or new ones such as bioaccumulation (Venkatesan et al., 2000; Hassan and Usu, 2005; Walker et al., 2005, Bird et al., 1998), electric-field-assisted techniques (Brooks et al., 2006), and ion exchange fibers/membranes (Hegazy et al., 2004). The use of precipitation is limited by the effect of added reagents, which often results in increased pH of the wastewater. The inorganic materials have high chemical, thermal, radiation stabilities, and extremely high selectivity compared to conventional organic ion exchange resins. But the main obstacle in the wide use of synthetic ion exchangers is their cost and their need for periodic washings. Both sorption technology based on ion exchange processes and physical/chemical sorption have found increasing application in various fields in recent years.
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The use of sorption processes for removal of heavy metals from wastewater is especially promising. Adsorption can occur during metal removal processes, making its application in radioactive wastewater treatment possible. In several previous reports, various authors have documented the use of various materials and technologies for the removal of these metal ions. Silica (Delisee and Fromage, 1999), kaolinite (Jeong, 2001), zeolite (DePaoli et al., 2001), montmorillonite (Atun et al., 2005), and silicotitanites(Anthony et al., 1994; Mann and Todd, 2004) are the most common. However, other materials such as activated carbon (Paajanen et al., 1997), coal fly ash (Apak et al., 1996), and even magnetite (Delisee and Fromage, 1999; Matijevic and Meites, 1983), and fibers (Liu et al., 2003) have been studied. The use of peat for the removal of transition metals from aqueous systems was extensively studied and had already been recommended in the past and they were also rich in hydrophobic sites that could bind with transition metals (Carolyn et al., 2004; Hanzlka et al., 2004; Burns et al., 2004). Weathered coal (WC) is an immature coal with large amounts of humic substances (humic and fulvic acids). The primary use of WC is as a fertilizer and soil conditioner in agriculture. WC possesses a high content of oxygen, which are fixed in carboxyl and hydroxyl groups, these functional groups can form complexes with metal cations (e.g. Cu2+, Cd2+, Ba2+, Ni2+, Pb2+, Fe3+ and Al3+) (Mei, 2000). The infrared (IR) spectroscopic studies of metal–humic complexes suggested the participation of phenolicOH and COOH groups in binding of the metal ions followed C¼O groups. The reaction can be as follows: R2COOH þ Meþ 2R2COOMe þ Hþ , R2OH þ Meþ 2R2OMe þ Hþ . These groups are active centers of ion exchange. So, the WC-based materials can be used as an alternative cation exchanger. Carboxyl or hydroxyl groups are able to take part in the ion exchange. These properties enable WC to be used as a radioactive wastewater-treating medium to remove hazardous metals. The price of WC is only 1/20 of the activated coal and the availability of this material makes it a promising candidate for radioactive wastewater treatment. The present work was performed in order to investigate the effects of contact time, solution acidity and sorbate concentration on the adsorption behavior of several representative WCs toward 241Am, 152Eu, and 137Cs cations. 2. Experimental 2.1. Preparation of sorbents and stock solution WC samples were collected from the YuXian, TongChuan, and PingXiang reserve in northern, northwestern, and southern China, respectively. These materials were first
dried in an oven at 80 1C for 24 h, sieved to 80–100 meshes (ASTM), and then placed in an airtight container for future use. All adsorbents were dried overnight at 80 1C before the adsorption experiments. The radioactive solutions of Am(III), Eu(III) and Cs(I) solutions were prepared by dissolving the indicator in the deionized water containing a small quantity of nitric acid (the radioactive tracer 241Am, 152 Eu, and 137Cs of known purity was procured from China Institute of Atomic Energy, Beijing, China). An accurately weighed quantity of Eu2O3 and CsCl (purchased from Guoyao company, Shanghai, China) were dissolved in dilute nitric acid to prepare a stock solution (0.11 and 0.66 mol/L, respectively). Experimental solutions of the desired concentrations were obtained by successive dilutions. All primary chemicals used were of analytical reagent grade. A HPGe g detector, was used for the determination of radioactive intensity in the solutions. All experiments were performed in triplicate. An pHS-3CpH-meter, Leici Corp, ShangHai, was used for pH measurement, which was calibrated with standard pH-buffer from INOAB WTW Welheim. 2.2. Surface properties WC pH: A NaCl solution at a concentration of 0.1 mol/L was first prepared; its blank pH was found to be 5.65. A total of 0.2 g of WC sample was then added into 25 mL of the prepared NaCl solution. An pHS-3CpH-meter (Leici Corp, ShangHai, China) was used to measure the WC pH after the suspension was shaken for 48 h. Total acidity capacity (TAC): The WCs with a weight w(g) were titrated with sodium hydroxide solutions to determine the contents of acidic groups (mmol/g). The samples were shaken for 48 h in a sodium hydroxide solution that had a concentration CNaOH of 0.24 mol/L and a volume VNaOH of 10 mL in sealed polyethylene flasks. The solutions were then left for 6 h for settling of the coal particulates. Then the supernatant was filtered by a 0.45 mm Teflon membrane filter. A filtrate with a volume VS of 10 mL was added to a standardized HCl solution that had a concentration CHCl of 0.26 mol/L and a volume VHCl of 30 mL; excess HCl was determined by titration with standardized 0.1 mol/L NaOH ðC 0NaOH Þ. The volume of NaOH consumed was Vt. In the test, the sodium hydroxide solutions without WCs were referred to as blanks. Vs
V NaOH C NaOH ðTAC wÞ þ C 0NaOH V t ¼ C HCl V HCl . V NaOH
Total carboxylic groups capacity (TCC): The WCs with a weight w(g) were titrated with sodium hydroxide solutions to determine the contents of carboxylic groups (mmol/g). The samples were shaken for 48 h in a calcium acetate solution that had a concentration C CaAc2 of 0.5 mol/L and a volume V CaAc2 of 50 mL in sealed polyethylene flasks. The solutions were then left for 6 h for settling of the coal particulates. Then the supernatant was filtered by a 0.45 mm Teflon membrane filter. A filtrate with a volume Vs of
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50 mL was titrated with standardized 0.1 mol/L NaOH (CNaOH). The volume of NaOH consumed was Vt. In the test, the sodium hydroxide solutions without carbons were referred to as blanks. TCC w ¼ 2C CaAc2 V CaAc2 þ C NaOH V t . The total phenolic groups capactity (TPC) can be gained by subtracting TCC from TAC. 2.3. Equilibrium sorption experiments Batch sorption experiments were carried out by adding 20 mg of powdered YXWC, TCWC or PXWC to a series of round-bottomed plastic test-tube containing radioactive Am(III), Eu(III) and Cs(I) solutions prepared from the stock solution at 298 K and pH about 4.5. The mixture was stirred for 1 d on an orbital shaker at 300 rpm. Sufficient time (about 15 h) was allowed for the Am(III), Eu(III) and Cs(I) uptake process to reach equilibrium. The samples were withdrawn from the shaker at predetermined time intervals, and the sorbent was removed from the solution by centrifugation at 3000 rpm for 5 min. The residual Am(III), Eu(III) and Cs(I) concentrations in supernatant were measured by HPGe g detector. There were several parameters that affected the adsorption rate, including the stirring rate in the aqueous phase and the physical nature of sorbent (e.g. porosity and surface area). The pH of solution also determined the ionic species present in solution. Americium and europium were found in the form of cationic, hydroxide complexes (cationic or anionic), hydroxide precipitate and carbonate complexes, based on pH of solution. To evaluate the effect of acidity on the adsorption efficiency of the WCs, the acidity of 4 mL solutions of the Am(III), Eu(III) and Cs(I) ions was adjusted to between 0.1 and 4 mol/L with concentrated HNO3. The adsorbed quantities were then determined using the mass balance equation: q¼
ðA0 At Þ V Ce , A0 m
(1)
where q is the amount of europium (mol/g coal) adsorbed on the coal samples, m the weight of coal (g), V the volume of metal solution (L), A0 the radioactive intensity of initial radioactive solution, At the radioactive intensity of equilibrium solution, and Ce the initial metal concentration (mol/L). The percentage adsorption, P, was calculated from Kd by using the following equations: A0 At V , At m
(2)
100K d . K d þ V =W
(3)
Kd ¼
p¼
Amount adsorbed per unit weight of the WC, x/m, was calculated radiometrically from the initial (known metal concentrations) and final activities of the solutions.
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3. Results and discussion 3.1. Physicochemical properties Acid–base titration of carbons based materials is normally used for the characterization of surface functional groups. It is assumed for the surface functional groups in the WCs as a generalized amphoteric group (sometimes referred to as a weak acidic group), the parameters of the surface reactions can be determined by the titration. WCs pH and TAC can be used as a more straightforward approach to compare the chemical properties of WC. WC pH can be treated as an approximate measure of the pH at the point of zero charge (pHPZC). It is shown in Table 1 that the pH values for the WCs were different, ranging from 5.03 of JXWC to 6.65 of TCWC. The WCs pHPZC indicated that the surfaces were positively charged when the solution pH was below 5.03. Some of chemical characteristics of the studied WCs are also given in Table 1. IR spectra of three WCs and a ‘standard’ coal are depicted in Fig. 1. IR spectra were carried out with a Perkin Elmer BX Model FTIR Spectrometer. The region corresponding to high wave numbers showed a broadband centered at 3430 cm1. This band was generally attributed to the (O–H) stretching of carboxylic and alcoholic groups, indicating intermolecular hydrogen bonding of humic substance molecules. The content of carboxyl groups was determined by separation of the bands in the 1800–1500 cm1 spectral region. The band at 1610 cm1 was assigned to aromatic carbonyl and carbonyl motion in carboxyl groups. The band at around 1390 cm1 was assigned to symmetric COO-stretching motions and to the bending vibrations of aliphatic groups. The band appearing at approximately 1260 cm1 could be attributed to the C–O stretching of phenolic groups. Less intense band that appeared in these spectra at around 1035 cm1 could be attributed to the C–O stretching motions. YXWC showed peaks at 1105 cm1 and have more humic content than the other sorbents. However, compare the IR spectra of ‘standard’ coal with WCs, the ‘ standard’ coal only exhibits broad bands centered at 3440 and 1630 cm1 assigned to the (O–H) stretching of carboxylic and alcoholic groups, aromatic carbonyl and carbonyl motion in carboxyl groups, respectively. The IR spectroscopic examination showed that the IR spectra of ‘standard’ coal and WCs were different. It can be concluded that WC contain higher amount of oxygencontaining functional groups than ‘standard’ coal. Oxidation of coal increases CO groups associated with carboxylates and phenoxy structures. Pore volume and average pore diameter greater than 20 A˚ were determined by N2 adsorption at 77 K with an accelerated surface area and porosimeter (ASAP-2020, Micromeritics). The specific surface area was calculated from the isotherms by the Brunauer–Emmett–Teller (BET) equation. The Barrett–Joyner–Halenda (BJH) equation
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Table 1 Chemical and surface properties of various WC Properties
JXWC
YXWC
TCWC
pHPZC Acidic groups (mmol/g) Carboxyl groups (mmol/g) Phenolic hydroxyl (mmol/g) BET surface area (cm2/g) Total pore volume (cm3/g) Pore size (nm) IR spectra (cm1)
5.030 2.410 2.395 0.015 14.369 0.036 10.980 3392,2362,1616,1520,1405,1205
6.630 1.956 0.827 1.127 19.533 0.061 16.420 3401,2360,1608,1509,1265,1032
6.650 0.920 0.290 0.630 1.319 0.007 9.080 3398,2360,1612,1512,1288,1125
Fig. 1. IR spectra of the three WCs and a standard coal given in transmittance.
was used to calculate the pore volume from the N2 adsorption data. The detailed properties are shown in Fig. 2 and Table 1. Fig. 2 clearly indicates that the three WCs consisted mostly of primary mesopores and had an effective pore size distribution area within 3 and 20 nm, while TCWC had a narrower pore size distribution. The total pore volume of the TCWC, JXWC and YXWC were 0.007, 0.036 and 0.061 cm3 g1, respectively. 3.2. Speciation of radionuclides in solutions As investigated, in the adsorption of hydrated Eu3+ and Am3+ by WCs, americium or europium anions would not exchange with the WC. This species would remain in solution, so we must know the speciation of these radionuclides in the aqueous solution. The americium and europium ions speciations could be determined by batch ion exchange method developed by Schubert (152Eu 1 109 mol 11) in solution (Maes et al., 1988) or by time-resolved laser induced fluorescence (Plancque et al., 2003). Here, Am3+ and Eu3+ speciations were investigated using MINTEQA2, a geochemical equilibrium speciation code, assuming equilibrium with the atmosphere in the pH range 3–10. The formation of complexes with OH, CO2 and 3 2+ 2+ HCO was considered (Am(OH) , Eu(OH) log b ¼ 6.2, 3 1
Fig. 2. Pore size distribution of JXWC, TCWC and YXWC (DPV ¼ differential pore volume).
log b2 ¼ ll.8; Am(CO3)+ Eu(CO3)+ log b1 ¼ 8.26; AmCO+ 3 , log b2 ¼ 14; Am(CO3) log b2 ¼ 2.05) all at ionic 2 strength ¼ 0 (Pettit and Powell, 1993). The representative figures of the speciation distribution of Am (III) and Eu (III) at the concentration changing from 107 to 101 mol/L are depicted in Figs. 3 and 4, they were calculated by MINTEQA2 modeling. Hydrated Am3+ predominated at pH lower than 6, Am(OH)2+ was the primary species between pH 7 and 8, and Am(OH)+ 2 became the predominate at higher pH. Hydrated Eu3+ predominated up to pH 7, EuCO+ 3 became predominant between pH 7 and 8, and Eu(CO3) 2 took over at higher pH. The results showed that hydrated Am3+ or Eu3+ are dominant species at pH 5.1 (the speciation percent of hydrated Am3+ or Eu3+ all over 96%) independent of the increasing of the americium or europium concentration. In the following experiments, the pH of solution was all adjusted to close to 5.0 based on the gained results. This would insure the hydrated Am3+ and Eu3+ were the predominate species in the solution. 3.3. Effect of contact time Removal of Am(III), Eu(III) and Cs(I) by WC with time was carried out at pH close to 5.01 and the effect of contact time on the sorption of Am and Eu or Cs
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Fig. 3. (a) Speciation diagrams of Americium in water: [Am] ¼ 0.1 mg L–1 (E4.1 10–7 mol/L), I ¼ 0.1 mol L1 (pCO2 ¼ 3.16 10–4 atm). (b) Speciation diagrams of Americium in water: [Am] ¼ 24.1 g L–1 (E1times; 10–1 mol/L), I ¼ 0.1 mol L1 (pCO2 ¼ 3.16 10–4 atm).
are presented in Figs. 5–7. As may be seen from Fig. 5, the binding of Am(III)/Eu(III)/Cs(I) on JXWC increased very rapidly with agitation time. These studies further indicated that the binding of both Am(III) and Eu(III) was about 80% and 70%, respectively. In the first 1 h, the percent adsorption of Cs(I) reached 60%. Later, the percentage sorption of Am and Eu reached about 90% and 80%, respectively, and attained equilibrium by the end of a contact period of 10 h. The further increase of contact time from 8 to 15 h had no significant effect on the percentage sorption of Cs(I), percentage sorption of Am(III) and Eu(III) had no obvious increase from 10 to 15 h. Fig. 6 gives the effect of contact time on the adsorption of Am(III)/Eu(III)/Cs(I) by YXWC. The rapid increase of percent adsorption of Am/Eu were found within the preliminary 0.5 h, and the percent adsorption of Am(III) and Eu(III) reached 95%. Later, equilibrium adsorption of Am and Eu was established. But the Cs(I) required 400 min to attain equilibrium. Fig. 7 shows high adsorption rates of Am(III)/Eu(III)/Cs(I) by TCWC were observed at the beginning, but eventually became constant (i.e. adsorption
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Fig. 4. (a). Speciation diagrams of Europium in water: [Eu] ¼ 0.1 mg L–1 (E6.6 10–7 mol/L), I ¼ 0.1 mol L1 (pCO2 ¼ 3.16 10–4 atm). (b) Speciation diagrams of Europium in water: [Eu] ¼ 15.2 g L–1 (E1 10–1 mol/ L), I ¼ 0.1 mol L1 (pCO2 ¼ 3.16 10–4 atm).
Fig. 5. Adsorption rate of Am(III), Eu(III) and Cs(I) ions by JXWC from aqueous solution at pH: 5.01. Adsorption conditions: amount of coal, 0.02 g; volume of adsorption medium, 30 mL; temperature, 298 K.
equilibrium) within about 15 h. The percent adsorption of Am(III) and Eu(I) reached about 85%, only 45% Cs was adsorbed.
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Fig. 6. Adsorption rate of Am(III), Eu(III) and Cs(I) ions by YXWC from aqueous solution at pH: 5.03. Adsorption conditions: amount of coal, 0.02 g; volume of adsorption medium, 30 mL; temperature, 298 K.
increasing acidity, as shown in Fig. 8. Both Kd and percent adsorption (P) decreased with an increase in acidity within the acidity range studied. A significant decrease of Kd was seen with the increase of acidity from 0.001 to 1 mol/L, however, a much lower decrease in Kd values was observed when acidity of the suspension changed from 1 to 4 mol/L. At high acidity the lower Kd values were due to a relatively small number of available sites. This was because of the solubility of different constituents of WC in acidic solutions; therefore relatively smaller number of active sites, which would be available for the sorption of radionuclides. At higher acidity, excess hydrogen ions could compete effectively with Am(III)/Eu(III)/Cs(I) for binding sites, resulting in a low level of Am(III)/Eu(III)/ Cs(I) sorption. Interactions of metals with the coal surface functional groups are complex, probably simultaneously dominated by adsorption, ion exchange and chelation. In addition, metal ions may also be adsorbed on sandy sediment grains, by exchange with protons, which are present in the outer part of the electric double layer. The extent of sorption will, therefore, depend upon experimental parameters that could affect the surface charge. Similar results had been reported for sorption of metal ions on wollastonite (Dakiky et al., 2002). Even at acidity as high as 0.1 mol/L, 60% of Am and Eu, and 40% of Cs were found to be sorbed on the YXWC. The maximum sorption of Am(III)/Eu(III)/Cs(I) was observed at an initial acidity (0.001 mol/L) and at this acidity, almost 95% of both Am and Eu, 50% of Cs was found to be sorbed. 3.5. Effect of sorbate concentration
Fig. 7. Adsorption rate of Am(III), Eu(III) and Cs(I) ions by TCWC from aqueous solution at pH: 5.08. Adsorption conditions: amount of coal, 0.02 g; volume of adsorption medium, 30 mL; temperature, 298 K.
The sorption of europium as a function of its concentration was studied at 298 K at solution to clay ratio being 200:1 (i.e. 4 mL:0.02 g) by varying the metal concentration from 107 to 101 mol/L. The results are shown in Fig. 9. Both Kd and P values decreased with increasing europium concentration. The results in Fig. 9 suggest that at least two
Initial rapid binding of these radionuclides occured initially due to easily available exchangeable sites located on surface of the WCs. Subsequent slow process suggested that intrapore diffusion be also involved in the sorption. It was stated that the plateau portion of the curve corresponded to pore diffusion and the linear portion of the curve reflected surface layer diffusion. The faster adsorption equilibrium of YXWC may be attributed to its high surface area and larger pore size. Therefore, a contact time of 15 h was used for all other subsequent experiments. 3.4. Effect of acidity of solution The effect of acidity on sorption of spiked Am(III)/ Eu(III)/Cs(I) on WC was studied at 298 K at solution to clay ratio (i.e, 200:1) by varying the acidity of the solution–coal suspension from 1 103 to 4 mol/L. The sorption of the three radionuclides on WC decreased with
Fig. 8. Effect of acidity on the adsorption of Am(III)/Eu(III)/Cs(I) by YXWC, Am(’), Eu(K),Cs(m), temperature at 298K, solution/weathered coal ratio, 200:1.
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types of phenomena (i.e., adsorption as well as exchange) are taking place in the range of Eu(III) concentrations studied. The removal of Eu by the latter process might be due to isomorphous replacement or by ion exchange. The log Kd values for YXWC and TCWC remained almost constant at low Eu loadings up to 105 mol/L, while at loadings between 104 and 10–1 mol/L, log Kd values decreased significantly. A dramatic decrease in P for JXWC, TCWC, and YXWC as the concentration exceeded about 104 mol/L was found. Furthermore, the initial concentrations were increased up to 0.224 mol/L for Eu(III) in order to reach the plateau values, which represented the saturation of the active points which were available for interaction with Eu(III) on the coal samples. The maximum adsorption capacities were 0.412, 3.701 and 5.446 mmol/g of JXWC, TCWC and YXWC, respectively. This suggests that energetically less favorable lattice positions or exchange sites become involved with increasing Eu(III) concentration to 104 mol/L.
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The amount of Eu(III) adsorbed by per unit mass of coal (i.e. adsorption capacity) decreased with increasing initial carrier concentration of ions, as expected. At low concentrations, the ratio of the initial number of moles of metal ion to the available surface area is large and subsequently the fractional adsorption is more or less independent of initial concentrations. However, at higher concentrations, the number of available sites for adsorption is relatively fewer and hence the percentage of removal of metal ions depends upon the initial concentration. 4. Adsorption isotherms Several models have been proposed in the literature to describe experimental data of adsorption isotherms. The Langmuir and Freundlich models are the most frequently employed models. In this work, both models were used to describe the relationship between the amount of Eu3+ adsorbed and its equilibrium concentration in solution (see Figs. 10 and 11). Although the Freundlich isotherm does not take the sorbents finite capacity for sorption at high concentrations into account, it often describes sorption of trace amounts of sorbing species satisfactorily. The linear form of the Freundlich isotherm model can be represented by using the equation below: log x=m ¼ log K þ 1=n log C e ,
(4)
where x/m is the amount adsorbed per gram of the adsorbent (mol/g), Ce the equilibrium concentration (mol/L), K and 1/n are constants. The equilibrium concentration of europium studied was in the range of 1.12 107–1.12 103 mol/L. A plot of log x/m against log Ce gave a straight line, the slope and intercept of which corresponds to 1/n and log K, respectively. Freundlich plots for europium sorption on WC are shown in Fig. 10. The coefficient of relationship (R) for the Freundlich plot is 1, 1, and 0.99757 for JXWC, TCWC, and YXWC,
Fig. 9. (a) Effect of europium concentration on its sorption on weathered coal percentage adsorption (a) YXWC(,), JXWC(n), TCWC(J), solution/weathered coal ratio, 200:1, pH 5.05, temperature: 298K. (b) Effect of europium concentration on its sorption on weathered coal Kd (b) YXWC(m), JXWC(K), TCWC(’), solution/weathered coal ratio, 200:1, pH 5.05, temperature, 298K.
Fig. 10. The Freundlich isotherm of three weathered coal, TCWC(’), YXWC(K), JXWC(m), solution/weathered coal ratio, 200:1, pH 5.05, temperature, 298K.
ARTICLE IN PRESS J. Wu et al. / Applied Radiation and Isotopes 65 (2007) 901–909
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respectively. The Freundlich constant K indicated the sorption capacity of the sorbent and the value of K (mmol/g) is 60.3, 199.5 and 198.4 for JXWC, TCWC and YXWC, respectively. Furthermore, the values of the Freundlich constant ‘1/n’ are 0.9181, 1.0001 and 0.9997 for JXWC, TCWC and YXWC, respectively. Linear regression of Freundlich plots (Fig. 10) gave slopes equal to 1, indicating a concentration-independent (i.e., linear) sorption of europium in the concentration range used. Langmuir’s adsorption equation was also used for the sorption of europium on WC. This model assumes uniform energies of adsorption onto the surface with no transmigration of adsorbate in the plane of the surface. The linear form of Langmuir isotherm is given by the following equation: C e x=m ¼ 1=bQ0 þ C e =Q0 ,
of Eu sorption were calculated from the binding constant K obtained from Langmuir’s equation by using the following relation: DG ¼ RT In K.
The free energy of europium sorbed on the three WCs at 298 K is also listed in Table 2. The thermodynamic equilibrium constant determined in these experiments was used to calculate other thermodynamic parameters. The Gibbs free energy for the adsorption process was obtained at 25 1C using Eq. (7). The Gibbs free energy indicates the degree of driving force of the adsorption process, where more negative values reflect a more energetically favorable adsorption process. The negative DG1 values obtained in this study for YXWC, TCWC, and JXWC confirmed the feasibility of the adsorption. From Table 2, the Langmuir adsorption capacity Qmax (mmol/g) for europium on JXWC, TCWC, and YXWC is 9.47 102, 8.48 101, and 1.34 mmol/g, respectively, and the equilibrium constant b (L/mg) is 3219.6, 316.8, and 169.5, respectively. The maximum europium adsorption capacity (Qmax) at constant pH 5.05 followed a descending order of YXWC4TCWC4JXWC. This is consistent with the experimental observation mentioned in Section 3.5. In general, this model was more suitable to describe the experimental equilibrium data for the three WC samples. Comparison of Qmax values indicated that the YXWC, which had the most content of phenolic groups (Table 1), had the maximal europium adsorption capacity. This might be due to the reactions between the organic functional groups and europium, and the phenolic hydroxyl was the most effective functional group under the sorption condition.
(5)
where x/m is the amount adsorbed by per unit mass of adsorbent (mol/g), Ce the equilibrium concentration of adsorbate in solution (mol/L), Q0 the monolayer capacity (mol/L), and b the binding constant that is related to the heat of adsorption by the equation K ¼ K 0 expðq=RTÞ,
(7)
(6)
where q is the heat of adsorption. The equilibrium concentration of europium was used as mentioned above. A straight line was obtained by plotting Ce/x/m against Ce (Fig. 11). The constants involved in the equation are given in Table 2. Thermodynamic parameters such as free energy
5. Conclusion WC has been proved to be an useful and inexpensive sorbent for Am(III) and Eu(III) and Cs(I) from aqueous solutions. When the sorption equilibrium is known precisely, it is possible to predict the optimum conditions for a pre-concentration or removal of metal ions from radioactive wastewater. This study showed that the adsorption of Eu(III) was dependent on time of contact, solution acidity and the initial concentrations of sorbate. The YXWC had higher adsorption efficiency than the TCWC and JXWC. The acidity of the solution played an important role in the
Fig. 11. The Langmuir isotherm of three weathered coal, TCWC(’), YXWC(K), JXWC(m), solution/weathered coal ratio, 200:1, pH 5.05, temperature, 298K.
Table 2 Values of Freundlich and Langmuir constants for europium sorption on WC Sample
JXWC TCWC YXWC
Freundlich isotherm
Langmuir isotherm 2
Kf (mmol /g)
1/n
R
60.3 199.5 198.4
0.9181 1.0001 0.9997
1 1 0.9976
b (L/mg)
Q0 (mmol/g)
R2
DG0 (kJ/mol)
3219.6 316.8 169.5
9.47 102 8.48 101 1.34
0.998 0.9998 0.9996
20.0114 14.2664 12.7164
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adsorption distribution. At very low pH or high acidic functional oxidized groups (hydroxyl, carboxyl, phenol, etc.) of HAs contained in the WC were protonated. It was observed that the removal percentage decreased sharply with the increase of acidity of solution. But even at acidity as high as 0.1 mol/L, 60% of Am(III) and Eu(III), and 40% of Cs(I) were found to be sorbed on the YXWC. The maximum sorption of Am(III)/Eu(III)/Cs(I) was observed at an initial acidity (0.001 mol/L) and at this acidity, almost 95% of both Am(III) and Eu(III), and 50% of Cs(I) were found to be sorbed. The Eu(III) adsorption by three WCs had been described by the Langmuir and Freundlich isotherm model. Although the adsorption capacities of WCs obtained in this study, 0.412 mmol/g for Eu(III) of JXWC, 3.701 mmol/ g for TCWC and 5.446 mmol/g for YXWC, were lower in comparison to those of synthetic ion exchange resins, substantially lower cost, easy availability of low-rank coal in China indicates they can be used by small-scale industries having low-level radioactive wastewater. It is necessary to note that the weathered coals surface can be modified to develop desirable physico-chemical properties by adequate choice of activation procedures. This effect will be studied further. References Anthony, R.G., Dosch, R.G., Gu, D., 1994. Use of silicotitanates for removing cesium and strontium from defense waste. Ind. Eng. Chem. Res. 33, 2702–2705. Apak, R., Atun, G., Gu¨cu¨lu¨, K., Tu¨tem, E., 1996. Sorptive removal of cesium-137 and strontium-90 from water by unconventional sorbents. II. Usage of coal fly ash. J. Nucl. Sci. Technol. 33, 396. Atun, G., Bilgin, B., Mardinli, A., 2005. Sorption of cesium on montmorillonite and effects of salt concentration. J. Radioanal. Nucl. Chem. 211 (2), 435–442. Balley, S., Olin, Bricka, R.M., et al., 1999. A review of potentially low-cost sorbents for heavy metals. Water Res. 33 (11), 2469–2479. Behrens, E.A., Sylvester, P., Clearfield, A., 1998. Assessment of a sodium nonatitanate and pharmacosiderite-type ion exchangers for strontium and cesium removal from DOE waste simulants. Environ. Sci. Technol. 32 (1), 101–107. Bird, G.A., Hesslein, R.H., Mills, K.H., Schwartz, W.J., Turner, M.A., 1998. Bioaccumulation of radionuclides in fertilized Canadian shield lake basins. Sci. Total Environ. 218, 67. Bostick, B.C., Vairavamurthy, M.A., Karthikeyan, K.G., Chorover, J., 2002. Cesium adsorption on clay minerals: an EXAFS spectroscopic investigation. Environ. Sci. Technol. 36 (12), 2670–2676. Brooks, K.P., Augspurger, B.S., Blanchard, D.L., et al., 2006. Hydraulic testing of ion exchange resins for cesium removal from Hanford tank waste. Sep. Sci. Technol. 41 (11), 2391–2408. Burns, C.A., Boily, J.F., Crawford, R.J., 2004. Cd(II) binding by particulate low-rank coals in aqueous media: sorption characteristics and NICA–Donnan models. J. Colloid Interface Sci. 278, 291–298. Carolyn, A.B., Jean-Franc- ois, B., Russell, J., Crawford, Ian H.H., 2004. Cd(II) binding by particulate low-rank coals in aqueous media: sorption characteristics and NICA–Donnan models. J. Colloid Interface Sci. 278, 291–298.
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