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HCa2Nb3O10‚1.5H2O as an Ion Exchanger for NH4+ Ion Removal Ramesh Chitrakar,* Satoko Tezuka, Akinari Sonoda,* Hirotaka Kakita, Kohji Sakane, Kenta Ooi, and Takahiro Hirotsu Health Technology Research Center, National Institute of AdVanced Industrial Science and Technology, 2217-14 Hayashi-cho, Takamatsu 761-0395, Japan
The removal of NH4+ on protonated layered perovskite oxide HCa2Nb3O10‚1.5H2O from an aqueous solution at room temperature using a batch method was investigated. The protons situated between the layers of the material are exchangeable with cations in solution. Distribution coefficient values, Kd (cm3/g), of Na+, K+, and NH4+ cations at pH 2.2 from a mixed solution (NaCl, KCl, and NH4Cl) containing 1.0 mmol/dm3 of each ion were 1600 for K+, 2200 for Na+, and 10 500 for NH4+. The effect of solution pH on the ion exchange in aqueous solution (NH4Cl + HCl or NH4OH) showed a maximum uptake of 34 mg of NH4/g at around pH 6. The results indicate that the material is very effective for removing NH4+ at concentrations of less than 10 mg of NH4/dm3. Introduction
Materials and Methods
Both ammonium and nitrate ions are common contaminants that are introduced into the environment from fertilizer application, industrial wastewater, and disposal of human and animal wastes. Ammonia engenders eutrophication of lakes and rivers, dissolved oxygen depletion, and fish toxicity. Soluble ammonia and nitrate in drinking water are well-known to be toxic.1 Numerous reports have described ammonium ion removal from aqueous solutions using natural zeolites,2-9 sepiolite,10,11 volcanic tuff,12 ion exchange resins,13-15 and a clay based material, mesolite.16 Most of those studies used high concentrations of ammonium (10-200 mg of NH4/dm3 4-8 and 18-4000 mg of NH4/dm3 2,3,9). Clinoptilolite has been generally accepted as a material having a higher ion exchange affinity for NH4+ even at low concentrations. In nature, the exchangeable cations in clinoptilolite are Na+, K+, and Ca2+. Generally, a higher exchange capacity is obtained if the material is relatively pure and Na+-type. The reported ion exchange capacities are 1.22.0 mmol/g for Turkish clinoptilolite,8 0.65 mmol/g for Hungarian clinoptilolite,17 1.5 mmol/g for Australian clinoptilolite,6 and approximately 2.0 mmol/g for sodium pretreated Californian clinoptilolite.18 Modification of natural zeolites such as mordenite and clinoptilolite with NaOH solution under hydrothermal conditions enables more NH4+ uptake (34 mg of NH4/g) that is twice as high as that of the starting materials (18 mg of NH4/ g).9 Recently, Wang et al.19 carried out the removal of low concentrations of NH4+ in water by ion exchange using sodiummordenite. We are interested in the layered material HCa2Nb3O10‚1.5H2O as an ion exchanger for the removal of low concentrations of NH4+ from aqueous solutions. Ion exchange reactions of alkali metal cations20 and intercalation reactions21-24 with this layered material have been investigated. To our knowledge, no study has been investigated for the removal of NH4+ on HCa2Nb3O10‚1.5H2O from aqueous solutions at room temperature. In this study, we investigated the effectiveness of this protonated layered material for NH4+ removal from aqueous solution.
Synthesis. KCa2Nb3O10 was synthesized as described.21 K2CO3 (3.90 g), CaCO3 (10.0 g), and Nb2O5 (20.0 g) (K/Ca/Nb ) 1.1:2:3 in molar ratios) were mixed, ground, and heated at 1000 °C in an aluminum crucible for 5 h in air. After cooling, the solid powder was further ground and heated at 1000 °C for 8 h in air. The solid powder KCa2Nb3O10 (4 g) was stirred in 6 mol/dm3 HCl (200 cm3) at 60 °C for 1 day. After repeating the acid treatment twice, the solid was filtered, washed with distilled water, and then dried in air (HCa2Nb3O10‚1.5H2O). Physical Measurements. X-ray diffraction analysis was carried out using an X-ray diffractometer (RINT 1200, Rigakutype) with a graphite monochromator. Chemical Analysis. Solutions were prepared for analysis of K, Ca, and Nb in materials as described.25 The K, Ca, and Nb contents in the solution were analyzed using an ICP atomic emission spectrometer (SPS 7800; Seiko Instruments Inc.). NH4+ Exchange Test. Experiments of NH4+ removal were performed using a batch method at room temperature. For the removal rate of NH4+, solid powder (0.25 g) was stirred in 500 cm3 of NH4Cl solution (9 mg of NH4/dm3). A small amount of the supernatant solution was sampled at different intervals and filtered using a membrane filter of 0.45 µm. The NH4+ concentration was determined using the indophenol method with a continuous-flow analyzer. The equilibrium pH was 3.0. All other batch experiments were performed for 1 day. For the removal of NH4+ at different initial concentrations of NH4Cl, solid powder (0.05 g) was stirred in 100 cm3 of NH4Cl, and the equilibrium pH was 3.0. For pH titration, solid powder (0.05 g) was stirred in 100 cm3 of NH4Cl solution (50 mg of NH4/dm3). The solution pH was adjusted by the addition of 1 mol/dm3 of HCl or 1 mol/dm3 of NH4OH solution. For the adsorption isotherm, solid powder (0.05 g) was stirred in 100 cm3 of NH4Cl solution with an initial concentration of 1-25 mg of NH4/dm3. Distribution Coefficient (Kd). Solid powder (0.25 g) was immersed in 25 cm3 of a solution containing 1 mmol/dm3 of each Na+, K+, and NH4+ cation with stirring for 1 day. After the attainment of equilibrium, the concentration of each cation in the supernatant solution was determined, and the pH of the solution was measured. The cation uptake was calculated relative to the initial concentration of the solution. The Kd value was calculated using the following formula:
* To whom correspondence should be addressed. E-mail: (R.C.)
[email protected] or (A.S.)
[email protected]. Tel.: +8187-869-3511. Fax: +81-87-869-3554.
10.1021/ie0704389 CCC: $40.75 © 2008 American Chemical Society Published on Web 11/28/2007
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Kd (cm3/g) ) cation uptake (mg/g of solid)/cation concentration (mg/cm3 of solution) Results and Discussion Characterization of Materials. In KCa2Nb3O10, the K+ ions are exchangeable with protons in HCl solution (6 mol/dm3) at 60 °C, resulting in the corresponding proton-type HCa2Nb3O10‚ 1.5H2O dried at room temperature (Figure 1). Protons and water molecules are situated between the layers; the protons are exchangeable with cations in solution.20,25,26 The X-ray diffraction pattern of synthesized KCa2Nb3O10 is shown in Figure 2. The observed diffraction lines can be indexed on a tetragonal unit cell with lattice parameters a ) 0.38 nm and c ) 1.48 nm, which show good agreement with those reported in a study that assessed KCa2Nb3O10 (a ) 0.39 nm and c ) 1.47 nm).25 The formation of HCa2Nb3O10‚ 1.5H2O was confirmed from X-ray diffraction analysis with a ) 0.38 nm and c ) 1.63 nm, which are also in good agreement with the reported values.25 Figure 3 shows TG-DTA curves of HCa2Nb3O10‚1.5H2O. The weight loss up to 100 °C with a sharp endothermic peak around 65 °C is due to the loss of interlayer water. The weight loss at 300-350 °C corresponds to the formation of the meta-stable compound Ca4Nb6O19.27 The SEM image of HCa2Nb3O10‚1.5H2O shows microcrystals of a plate-like morphology with different sizes (Figure 4). Chemical analysis data of KCa2Nb3O10 and its protonated material are shown in Table 1. The K+ content in the present protonated material was 0.33 wt %, which is much lower than the 1.2 wt % reported for HCa2Nb3O10.27 Kd. The distribution of cations between solution and solid ion exchanger reflects the selectivity of the ion exchanger under certain experimental conditions, such as cation concentration and pH. The selectivity is a characteristic property of the ion exchanger, which prefers one counterion to another in solution. Therefore, Kd values are useful to know how selective an ion exchange material is toward relevant ions in the presence of other ions in solution. Kd values of Na+, K+, and NH4+ ions at pH 2.2 from a mixed solution (NaCl, KCl, and NH4Cl) containing 1 mmol/dm3 of each cation were determined for the protonated material. The obtained Kd (cm3/g) values were 1600 for K+, 2200 for Na+, and 10 500 for NH4+ at an equilibrium pH of 2.2, indicating that the material is selective for NH4+ at low concentrations. The initial pH of the mixed solution was 5.2, and the pH was changed to 2.2 on addition of the material because of a cation/proton exchange reaction. The NH4+ cation resembles alkali metal ions such as Na+ and K+. It is possible that NH4+ cations in the interlayer stabilize the terminal oxygen atoms of the layer, resulting in a higher affinity than that of either Na+ or K+. NH4+ Exchange Test. The effect of contact time on NH4+ removal with an initial concentration of 9 mg of NH4/dm3 for a sample dose (0.5 g/dm3) is shown in Figure 5. The equilibrium was attained within 2 h, with a removal efficiency of 80%. The initial pH of the NH4Cl solution was 5.5, and the pH was changed to 3.0 with the addition of the material. The interlayer distance in HCa2Nb3O10‚1.5H2O was 0.48 nm estimated from the niobate layer thickness (1.15 nm)25 and basal spacing (1.63 nm). This interlayer distance was sufficiently large to accommodate NH4+ (ionic radius of 0.14 nm).14 The effect of initial NH4+ concentration on the removal efficiency was determined (Figure 6). The removal efficiency was 100% in concentrations of 1-5 mg of NH4/dm3 under a
Figure 1. Schematic structures of KCa2Nb3O10 and HCa2Nb3O10‚1.5H2O.
Figure 2. X-ray diffraction patterns of KCa2Nb3O10 (a) and HCa2Nb3O10‚ 1.5H2O (b).
Figure 3. TG-DTA of HCa2Nb3O10‚1.5H2O.
Figure 4. SEM image of HCa2Nb3O10‚1.5H2O.
sample dose (0.5 g/dm3) at an equilibrium pH of 3. The removal efficiency decreased because of the saturation of the cation exchange sites in the material when the NH4+ concentration in the solution increased.
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Table 1. Chemical Analysis of Materials material
Ka
Caa
Nba H2Oa,b
KCa2Nb3O10 7.03 14.7 52.8 HCa2Nb3O10‚1.5H2O 0.33 14.9 53.9
0 4.51
formula K0.96Ca1.95Nb3.03O10 K0.04H0.96Ca1.94 Nb3.07O10‚1.30H2 O
a Wt %. b H O content was evaluated from weight loss at 200 °C in the 2 TG-DTA curve.
Figure 8. Effect of solution pH on NH4+ exchange on HCa2Nb3O10‚ 1.5H2O.
Figure 5. Rate of NH4+ removal on HCa2Nb3O10‚1.5H2O.
Figure 9. X-ray diffraction patterns of HCa2Nb3O10‚1.5H2O exchanged with different amounts of NH4+ : 0 mg of NH4/g (a), 14 mg of NH4/g (b), and 34 mg of NH4/g (c).
Figure 6. Removal of NH4+ at different initial NH4+ concentrations on HCa2Nb3O10‚1.5H2O.
Figure 7. Langmuir isotherm plot (a) and Freundlich isotherm plot (b) for NH4+ exchange on HCa2Nb3O10‚1.5H2O.
Adsorption Isotherm. The adsorption isotherms of NH4+ are presented in Figure 7. Langmuir and Freundlich isotherm models were examined for analyzing the NH4+ exchange equilibrium on the material. The linear form of the Langmuir equation is written as [Ce/qe ) 1/(qmKL) + Ce/qm], where Ce is the equilibrium NH4+ concentration in solution (mg of NH4/
dm3), qe is the amount of NH4+ exchanged on the material (mg of NH4/g), qm is the monolayer capacity of the material (mg of NH4/g), and KL is the Langmuir constant (dm3/mg). The linear form of the Freundlich equation is written as [ln qe ) ln KF + (1/n)ln Ce], where KF and n are the Freundlich constants. Correlation coefficients, R2, were determined from the linear regression analysis. The experimental equilibrium data were well-fitted to the Langmuir isotherm equation (R2 ) 0.998) as compared to the Freundlich equation (R2 ) 0.783). The material shows a high affinity for NH4+ ions, even at a NH4+ concentration of less than 1 mg of NH4/dm3. The maximum NH4+ uptake calculated from the Langmuir model was 18 mg of NH4/g, which is approximately the experimental value at pH 3. Effect of Solution pH on the Ion Exchange Reaction. The effect of pH on NH4+ uptake with the protonated material was studied at a pH of 2.2-8.5, as shown in Figure 8. The NH4+ uptake is almost constant at pH 2.2-4; it then increases up to 34 mg of NH4/g at around pH 6. Then, the uptake decreases to 24 mg of NH4/g at pH 8.5. The presence of more protons at pH < 4 suppresses the uptake of NH4+ by ion exchange. On the other hand, at pH > 8, the formation of more free NH3 species inhibits the uptake of NH4+ by ion exchange. Thus, the maximum uptake of NH4+, 34 mg of NH4/g, appears at a pH of around 6. The NH4+ cations enter the interlayer space by ion exchange with protons. X-ray diffractions of NH4+ exchanged phases show that (00l) peaks are shifted to higher angles with an increasing amount of NH4+ in the interlayer space (Figure 9). A decrease in the c value (1.51 nm) is observed upon NH4+ exchange. This decrease in the c value is consistent with K+ exchange on HCa2Nb3O10 (c ) 1.51 nm).20 The a value of the material is constant irrespective of NH4+ exchange (0.39 nm). Comparison of NH4+ Uptake with Other Materials. The NH4+ uptake on HCa2Nb3O10‚1.5H2O is shown for comparison with published data on zeolites and other materials in Table 2. Of those materials, HCa2Nb3O10‚1.5H2O shows the highest NH4+ uptake at a low equilibrium concentration (10 mg of NH4/
Ind. Eng. Chem. Res., Vol. 47, No. 1, 2008 179 Table 2. NH4+ Uptake from NH4Cl Solution on Different Ion Exchangers material HCa2Nb3O10‚1.5H2O Sepiolite Zeolite natural zeolite (unmodified) natural zeolite (modified) New Zealand zeolite volcanic tuff a
max uptake uptake (mg of NH4/g)a (mg of NH4/g) 18 4 3 9 18 5 6
34 63 7 16 35 14 19
ref this study 10 9 9 9 5 12
Uptake at a 10 mg NH4/dm3 equilibrium concentration.
dm3), which is comparable to that of modified natural zeolite. The calculated cation exchange capacity of the protonated material is 1.8 mmol/g (34 mg of NH4/g), as compared to 1.9 mmol/g (35 mg of NH4/g) of modified zeolite.9 Conclusion Removal of NH4+ using HCa2Nb3O10‚1.5H2O proceeds very rapidly through an ion exchange mechanism requiring 2 h to attain equilibrium. Distribution coefficient values, Kd (cm3/g), of NH4+, Na+, and K+ in mixed solution show that the material has a higher affinity for NH4+ than the other cations. The material is effective in removing NH4+ at a low concentration of less than 10 mg of NH4/dm3. The isotherm of NH4+ uptake is well-fitted to the Langmuir model. The effect of solution pH on the ion exchange reaction shows a maximum NH4+ uptake of 34 mg of NH4/g around pH 6. The material is promising as an ion exchanger for the removal of NH4+ ions in solutions of various concentrations but particularly for those of less than 1 mg of NH4/dm3. Literature Cited (1) Kurama, H.; Poetzschke, J.; Haseneder, R. The Application of Membrane Filtration for the Removal of Ammonium Ions from Potable Water. Water Res. 2002, 36, 2905. (2) Watanabe, Y.; Yamada, H.; Kokusen, H.; Tanaka, J.; Moriyoshi, Y.; Komatsu, Y. Ion Exchange Behavior of Natural Zeolites in Distilled Water, Hydrochloric Acid, and Ammonium Chloride Solution. Sep. Sci. Technol. 2003, 38, 1519. (3) Bernal, M. P.; Lopez-Real, J. M. Natural Zeolites and Sepiolite as Ammonium and Ammonium Adsorbent Materials. Bioresour. Technol. 1993, 43, 27. (4) Inglezakis, V. J.; Hadjiandreou, K. J.; Loizidou, M. D.; Grigoropolou, H. P. Preparation of Natural Clinoptilolite in a Laboratory-Scale Ion Exchange Packed Bed. Water Res. 2001, 35, 2161. (5) Weatherley, L. R.; Miladinovic, N. D. Comparison of Ion Exchange Uptake of Ammonium Ion onto New Zealand Clinoptilolite and Mordenite. Water Res. 2004, 38, 4305. (6) Cooney, E. L.; Booker, N. A.; Shallcross, D. C.; Stevens, G. W. Ammonia Removal from Wastewaters Using Natural Australian Zeolite. I. Characterization of Zeolite. Sep. Sci. Technol. 1999, 34, 2307. (7) Sprynsky, M.; Lebedynets, M.; Terzyk, A. P.; Kowalczyk, P.; Namiesnik, J.; Buszewski, B. Ammonium Sorption from Aqueous Solutions by the Natural Zeolite Transcarpathian Clinoptilolite Studied under Dynamic Conditions. J. Colloid Interface Sci. 2005, 284, 408.
(8) Karadag, D.; Koc, Y.; Turan, M.; Armagan, B. Removal of Ammonium Ion from Aqueous Solution Using Natural Turkish Clinoptilolite. J. Hazard. Mater. 2006, 136, 604. (9) Watanabe, Y.; Yamada, H.; Tanaka, J.; Moriyoshi, Y. Hydrothermal Modification of Natural Zeolites To Improve Uptake of Ammonium Ions. J. Chem. Technol. Biotechnol. 2005, 80, 376. (10) Balci, S.; Dincel, Y. Ammonium Ion Adsorption with Sepiolite: Use of Transient Uptake Method. Chem. Eng. Process. 2002, 41, 79. (11) Balci, S. Nature of Ammonium Ion Adsorption by Sepiolite: Analysis of Equilibrium Data with Several Isotherms. Water Res. 2004, 38, 1129. (12) Maranon, E.; Ulmanu, M.; Fernandez, Y.; Anger, I.; Castrillon, L. Removal of Ammonium from Aqueous Solutions with Volcanic Tuff. J. Hazard. Mater. 2006, 137, 1402. (13) Chen, J. P.; Chua, M.-L.; Zhang, B. Effect of Competitive Ions, Humic Acid, and pH on Removal of Ammonium and Phosphorus from the Synthetic Industrial Effluent by Ion Exchange Resins. Waste Manage. 2002, 22, 711. (14) Jorgensen, T. C.; Weatherley, L. R. Ammonia Removal from Wastewater by Ion Exchange in the Presence of Organic Contaminants. Water Res. 2003, 37, 1723. (15) Lin, S. H.; Wu, C. L. Ammonia Removal from Aqueous Solution by Ion Exchange. Ind. Eng. Chem. Res. 1996, 35, 553. (16) Thornton, A.; Pearce, P.; Parson, S. A. Ammonium Removal from Digested Sludge Liquors Using Ion Exchange. Water Res. 2007, 41, 433. (17) Jorgensen, S. E.; Barkacs, K. Ammonia Removal by Use of Clinoptilolite. Water Res. 1976, 10, 213. (18) Pabalan, R. T. Thermodynamics of Ion Exchange Between Clinoptilolite and Aqueous Solutions of Na+/K+ and Na+/Ca2+. Geochim. Cosmochim. Acta 1994, 58, 4573. (19) Wang, Y.; Kmiya, Y.; Okuhara, T. Removal of Low-Concentration Ammonia in Water by Ion-Exchange Using Na-Mordenite. Water Res. 2007, 41, 269. (20) Jacobson, A. J.; Lewandowski, J. T.; Johnson, J. W. Ion Exchange Reactions of the Layered Solid Acid HCa2Nb3O10 with Alkali Metal Cations. Mater. Res. Bull. 1990, 25, 679. (21) Jacobson, A. J.; Johnson, J. W.; Lewandowski, J. T. Interlayer Chemistry between Thick Transition-Metal Oxide Layers: Synthesis and Intercalation Reactions of K[Ca2Nan-3NbnO3n+1] (3 e n e 7). Inorg. Chem. 1985, 24, 3727. (22) Xu, F. F.; Bando, Y.; Ebina, Y.; Sasaki, T. Modification of Crystal Structures in Perovskite-Type Niobate Nanosheets. Philos. Mag. A 2002, 82, 2655. (23) Gao, L.; Gao, Q.; Wang, Q.; Peng, S.; Shi, J. Immobilization of Hemoglobin at the Galleries of Layered Niobate HCa2Nb3O10. Biomaterials 2005, 26, 5267. (24) Han, Y.-S.; Park, I.; Choy, J.-H. Exfoliation of Layered Perovskite, KCa2Nb3O10, into Colloidal Nanosheets by a Novel Chemical Process. J. Mater. Chem. 2001, 11, 1277. (25) Jacobson, A. J.; Lewandowski, J. T.; Johnson, J. W. Ion Exchange of the Layered Perovskite KCa2Nb3O10 by Protons. J. Less-Common Met. 1986, 116, 137. (26) Dion, M.; Ganne, M.; Tournoux, M. Nouvelles Familles de Phases MIM2IINb3O10 a Feuillets “Perovskites”. Mater. Res. Bull. 1981, 16, 1429. (27) Fang, M.; Kim, C. H.; Mallok, T. E. Dielectric Properties of the Lamellar Niobates and Titanoniobates AM2Nb3O10 and ATiNbO5 (A ) H, K and M ) Ca, Pb) and Their Condensation Products Ca4Nb6O19 and TiNb2O9. Chem. Mater. 1999, 11, 1519.
ReceiVed for reView March 26, 2007 ReVised manuscript receiVed September 13, 2007 Accepted October 22, 2007 IE0704389
