Synthesis, characterization and evaluation of nano-zirconium vanadate ion exchanger by using three different preparation techniques

Materials Research Bulletin 46 (2011) 105–118

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Synthesis, characterization and evaluation of nano-zirconium vanadate ion exchanger by using three different preparation techniques M.M. Abd El-Latif *, M.F. Elkady 1 Fabrication Technology Department, Advanced Technology and New Materials Research Institute (ATNMRI), Mubarak City for Scientific Research and Technology Applications (MuCSAT), Alexandria, Egypt

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 May 2010 Received in revised form 31 August 2010 Accepted 23 September 2010 Available online 1 October 2010

Sol–gel, homogeneous precipitation and hydrothermal synthesis are three different preparation techniques have been used as an attempt to synthesize nano-zirconium vanadate with properties suitable to be used as ion exchangers. The impact of the synthetic preparation variables such as the reactant concentrations, reaction temperature and reaction time on the ion exchange capacity of the produced ion exchanger has been considered for each preparation technique. One sample from each preparation technique having the largest ion exchange capacity has been selected to be physically and chemically characterized using various analytical techniques such as XRD, TGA, DSC, pH titration, FTIR and SEM in order to determine the properties of the ion exchanger produced from each technique. For all the studied ZrV samples it can be presumed that they have the ion exchange affinity sequences for alkali metal ions K > Na > Li, the order for the alkaline earth metals is Ba > Ca > Mg and their affinity for radioactive metals follow Cs > Sr. Moreover, the prepared materials are of high thermal and radiation stabilities. Also they have high chemical stabilities toward wide concentration ranges of acid, basic as well as polar solvents. It has been deduced from the X-ray analysis that ZrV produced from the sol–gel technique has an amorphous structural. While those produced from the homogeneous precipitation and hydrothermal synthesis techniques, in the nano-scale have semi-crystalline structural. Furthermore, SEM confirms that particle size of the all studied prepared ZrV samples have nano-diameters of range 50–60 nm. Specific surface area of the three different prepared ion exchangers are found to be equal to 187, 192 and 320 m2/g for sol–gel, homogeneous precipitation and hydrothermal, respectively. A tentative structural formula of Zr(OH)2(HVO4)22H2O has been proposed for all studied samples on the basis of on FTIR, DSC and TGA results. ß 2010 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures B. Chemical synthesis B. Sol–gel chemistry C. X-ray diffraction C. Thermogravimetric analysis

1. Introduction Ion-exchange materials represent the essential constituents in a number of processes used to analysis, preconcentration and recovery of a number of ionic species from aqueous as well as from non-aqueous systems. These materials have also been employed in the preparation of various types of ion-exchange membrane, chemical sensors (ion-selective electrodes), etc. Ion-exchange materials have also been inducted to prepare ion-exchange fibers which will be a more suitable tool for various environmental problems [1]. Nowadays, the synthetic inorganic ion exchangers become much valuable than the organic resins because of their

* Corresponding author at: Advanced Technology and New Materials Research Institute, Mubarak City for Scientific Research and Technology Applications, New Borg El-Arab, P.O. Box: 21934 Alex, Alexandria, Egypt. Tel.: +20 03 4593414; fax: +20 03 4593414. E-mail addresses: [email protected] (M.M. Abd El-Latif), [email protected] (M.F. Elkady). 1 Tel.: +20 3 4599314; fax: +20 3 4593414. 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.09.032

characteristics [2]. Since, they have excellent stabilities towards thermal and radiation doses. Moreover, they often exhibit specificity towards certain metal ions [3]. Generally, there are three different techniques used for preparation nano-inorganic materials which are sol–gel precipitation, precipitation, and hydrothermal techniques. The sol–gel technique is one of the fastest growing fields of contemporary chemistry. The attractive feature of this technology is the fact that sol–gel materials can be obtained as bulks, thin films (on various supports) and (nano) powders [4]. On the other hand, the precipitation from a homogeneous solution is a technique in which a precipitating agent is generated in the reaction solution by slow chemical reaction. Homogeneously formed precipitates are usually better suited for analysis than the solid formed by the direct addition of a precipitating reagent [5]. Finally, the hydrothermal preparation technique which is usually refers to any heterogeneous reaction in the presence of aqueous solvents or mineralizers under high pressure and temperature conditions to dissolve and recrystallize materials that are relatively insoluble under ordinary conditions [6]. The literature survey illustrates that there are large number of the synthetic inorganic ion exchange materials that produced

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using sol–gel precipitation technique act as electron exchange material [2]. Electron exchange materials are solid oxidation and reduction agents. The advantages of electron exchangers over dissolved oxidizing and reducing agents is their insolubility in reaction medium and the ability of their are regeneration after use. Many synthetic inorganic ion exchangers have been used as electron exchangers [7]. Many studies have been done for their preparation, properties and analytical applications [8–13]. Amongst these materials, hydrous oxides, salts of heteropolyacids and insoluble ferrocyanides are worthy of mention [14]. A wide range of acidic salts of multivalent metals deserve special mention because of their unique ion exchange properties, along with several interesting applications [15,16]. They provide exchangeable hydrogen ions when immersed in aqueous solution, thus exhibiting cation exchange properties. The general formula of this type of ion exchanger is MIV(HXVO4)2nH2O (where M = tetravalent metal such as Zr(IV), Ti(IV), Sn(IV), etc. and X may be As, P, W, Sb, V, Se, etc.) [2,14]. Acidic salts of multivalent metals, prepared in combination with anions of phosphate, tungstate, arsenate, tellurate, etc. as two component ion exchangers have been studied most intensively [17,18]. Phosphate and arsenate of zirconium [18,19] show good thermal and chemical stabilities, so the preparation and evaluation of both poorly crystalline and amorphous compounds of these materials have been extensively studied. However, little attention has been paid to zirconium vanadate until now. Zirconium vanadate with good ion exchange properties and amorphous structure has been recently prepared using sol–gel method only [14,20–22]. It is worth mentioning that poly crystalline zirconium vanadate have been initially synthesized using new preparation technique that called homogeneous precipitation technique in our previous published research [23]. These different prepared zirconium vanadate materials have been proved to be effective as ion exchange materials for removing different ions such as cesium, uranium, strontium, and cobalt and nickel [20,24]. We are going to prepare nano-zirconium vanadate using the innovative hydrothermal preparation technique as a new trend. In this respect, the different factors that affect in the properties of the produced zirconium vanadate will be discussed. Also, these preparation factors will be applied on the other traditional preparation techniques that previously used for zirconium vanadate preparation, to the compare the chemical and physical properties of the synthesized zirconium vanadate using the three different techniques. 2. Materials and methods 2.1. Reagents and instrumentation The main reagents used for zirconium vanadate preparation using the three different techniques are zirconium oxy chloride (Avonchem, United Kingdom), sodium vanadate (Acros, USA), urea (Sisco Research Laboratories (Pvt. Ltd., India), and hydrochloric acid (Polskie Odczynniki Chemicze, Poland). All other chemicals and reagents used for properties identification of the prepared zirconium vanadate were of analytical reagent grade. pH measurements were performed using A single electrode digital pH-meter (Denver Instrument Co., USA). For hydrothermal preparation technique an Autoclave (Systec 3850-EL) has been used. Magnetic stirrer model-526 (J.P. Selecta Co., Spain) was used during the preparation procedure for both sol–gel and homogenous precipitation techniques with the aid of identical magnets. An electron microscope (JEOL JSM 6360LA, Japan) has been used for studying morphological properties of the synthesized materials. IR studies were made using an FTIR (Shimadzu FTIR-8400 S, Japan). For studying the thermal properties, a thermo gravimetric analyzer

TGA (Shimadzu TGA-50, Japan) and DSC (Shimadzu DSC-60A, Japan) were used. X-ray diffractometer (Schimadzu-7000, USA) was used for determining the X-ray diffraction pattern of the prepared materials. Thermal stabilities of the different prepared materials have been studied using muffle furnace (Carbolite, Aston Lane, England). 2.2. Synthesis of zirconium vanadate using sol–gel precipitation technique A sodium vanadate solution of different molarities was added drop wise into a solution of 0.1 M zirconium oxychloride in the presence of hydrochloric acid with constant stirring. After the addition was complete a fine yellow precipitate appeared. The reaction mixture was diluted to 1 L and allowed to settle for 24 h for complete digestion [21,22]. The supernatant liquid was decanted and gels were filtered by suction and the excess acid was washed thoroughly with hot water. The washed precipitate was then dried by gentle heating at 40 8C. Then the product is grounded and immersed in 1 M nitric acid for 1 day with gentle stirring in order to transform the ion exchanger to its hydrogen form. The effect of hydrochloric acid concentration (0–2 M), gelation temperature (25–100 8C) and sodium vanadate concentration (0.1–0.8 M) were studied. Hence, a number of samples of zirconium vanadate ion exchanger were prepared using this technique, on the basis of Na+ ion exchange capacity one sample was selected to be characterized. 2.3. Synthesis of zirconium vanadate using homogenous precipitation technique The material was synthesized by adding a solution of sodium vanadate of different molarities to the mixture of 0.1 M of zirconium oxychloride that dissolved in an aqueous hydrochloric (0.04 mol/L) and 1.5 g of urea. The resulting solution was heated to 90 8C on an electrical plate with continuous stirring to decompose the precipitating agent, leading to the formation of zirconium vanadate. After precipitation, the material was allowed to rest at 90 8C for another hour [23]. The produced precipitate was filtered and washed repeatedly with distilled water for the removal of chloride traces, and then dried at 40 8C for 24 h. The product was then ground and transformed to its hydrogen form as mentioned before [5]. The effect of urea precipitating agent amounts (0.5–6 g), preparation temperature (50–120 8C), hydrochloric acid concentration (0–2 M) and sodium vanadate concentration (0.1–0.8 M) were studied. A large number of zirconium vanadate samples were produced; the most proper prepared sample which has the largest ion exchange capacity, IEC, will be selected in order to studying its properties. 2.4. Synthesis of zirconium vanadate using hydrothermal technique Yellow precipitate of zirconium vanadate can be obtained when mixing a solution of 0.1 M of ZrOCl28H2O, dissolved in an aqueous hydrochloric acid (0.5 mol/L), with a solution of sodium vanadate. The mixture was heated under pressure at different temperatures using Autoclave by varying the heating times. The reaction mixture was diluted to 1 L and allowed to settle for 24 h for complete digestion. The precipitate was washed thoroughly and dried by gentle heating. Then the product is grounded and transformed to its hydrogen form as mentioned previously. The effect of heating time (20 min–4.5 h), preparation temperature (60–120 8C), hydrochloric acid concentration (0–2 M), and sodium vanadate concentration (0.1–0.8 M) were studied. In order to determine the chemical and physical properties of the prepared zirconium vanadate using this technique, only one sample that has the highest IEC was selected for details characterization.

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2.5. Chemical characterization of the prepared nano-zirconium vanadate ion exchangers 2.5.1. Ion exchange capacity (IEC) The ion exchange capacities of different prepared ion exchangers were determined by acid–base titration [25]. The weighted sample of the ion exchanger in its H+ form were soaked in 50 mL of 1 M NaCl solution for at least 12 h with shaking at ambient temperature to exchange protons with sodium ions. The ion exchanged solution was titrated to the phenolphthalein end point (2 drops of ph.ph indicator, 1% ph.ph in ethanol) with a NaOH solution of 0.1 M concentration. The ion exchange capacity (IEC) was calculated using the following equation: IEC ðmequiv:=gÞ ¼ V NaOH

C NaOH Wd

where VNaOH, CNaOH and Wd are the volume of NaOH consumed in titration, the concentration of NaOH solution, and the weight of the dry sample, respectively. 2.5.2. Effect of eluant concentration on IEC In order to determine the optimum concentration of the eluant for complete elution of H+ ions, 0.5 g of ion exchanger was shaken for 12 h with 50 mL sodium chloride solution with concentrations range from 0.2 M to 2 M with step difference of 0.2 M. The elution curves of the hydrogen ion were found for different NaCl concentrations [26]. 2.5.3. Effect of contact time on IEC Half a gram of ion exchanger was shaken with 50 mL sodium chloride and the amount of librated H+ ions was titrated against a standard alkali NaOH solution after every 1-h interval. In order to determine the optimum shaking time of ion exchanger with sodium chloride solution for complete elution of H+ ions. 2.5.4. Elution behavior The elution of the prepared ion exchangers is performed using the optimum concentration of NaCl. A glass column having an outer diameter (o.d.) 12 mm fitted with glass wool support at the bottom containing 1 g of the exchanger in its H+ form was eluted with NaCl solution in different 10 mL fractions with minimum flow rate 0.5 mL/min and each fraction of 10 mL effluent was titrated against a standard NaOH solution for the H+ ions eluted out [27]. 2.5.5. pH titration Half a gram of the exchanger in each of several conical flasks is mixed with equimolar solutions of alkali metal chlorides and their hydroxides in different volume ratios. The final volume is 50 mL to maintain the ionic strength constant (0.05 M). The pH of the solution was recorded every 24 h until equilibrium was attained which needed 7 days. The pH at equilibrium was plotted against the millimoles of OH ions added [28]. 2.5.6. Ion exchange capacity for different metal ions The ion exchange capacity of different prepared zirconium vanadate for alkali and alkaline earth metal ions was determined by equilibrating nearly 0.5 g of the exchanger with 50 mL of 2 M solution of different metal salts. The librated acid was measured by titration with standard alkali solution as described above. 2.5.7. Thermal effect on IEC The effect of temperature on the ion exchange capacity for the different prepared ion exchangers was determined by heating a sample of 0.5 g from the ion exchange materials at various temperatures in muffle furnace for 1 h. The Na+ ion exchange

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capacity after cooling the samples at room temperature was determined as mentioned in the ion exchange capacity technique [29]. 2.5.8. Radiation effect on IEC To determine the radiation stability of ion exchange capacity of the prepared ion exchangers, the synthetic zirconium vanadate ion exchanger was irradiated with different radiation doses from 2.5 Gy to 100 kGy at a rate of 0.471 Gy/min using 60Co gamma ray source. Then the ion exchange capacity of each irradiated sample was determined as mentioned in the ion exchange capacity technique [30]. 2.5.9. Chemical composition The chemical composition of the ion exchanger materials was determined by dissolving 200 mg of the sample in 20 mL of hot concentrated H2SO4. The material was analysed for zirconium, vanadium and sodium using inductive coupled plasma mass spectrophotometer (ICP-AES) [31]. On the other hand, the composition (atomic ratios) of the prepared samples was examined by energy dispersive X-ray spectroscopy (EDAX) analysis that combined with the scanning electron microscope using liquid nitrogen in order to determine the O2 contents of the prepared exchangers. 2.5.10. Chemical stability Two hundred and fifty milligrams portion of the ion exchanger in H+ form were treated with 20 mL each of different acids such as HCl, HNO3, H2SO4, different bases such as NaOH, NH4OH, and organic solvents such as dimethyl sulphoxide (DMSO), acetone, ethanol, glycerol and also with double distilled water (DMW) for 24 h with occasional shaking [27]. 2.6. Physical characterization of the differed prepared nanozirconium vanadate ion exchangers 2.6.1. X-ray diffraction (XRD) X-ray diffractometer with Cu Ka radiation beam (l = 0.154060 nm) was used to determine the structure of the ion exchangers. The finely powdered samples of the ion exchanger were packed in to a flat aluminum holder, where the X-ray source was a rotating anode operating at 30 kV and 30 mA with a copper target. Data were collected between 108 and 808 in 2u. 2.6.2. Thermal analysis (TGA/DSC) The measurements of the thermal analysis of the prepared samples were carried out using thermal gravimetric analysis (TGA) and differential scanning calorimeter (DSC) with a heating rate 20 8C/min under flow of N2 to avoid thermal oxidation of the powder samples, starting from ambient condition up to 800 8C. 2.6.3. Infrared spectroscopy (FTIR) The I.R. spectrum of the ion exchanger was measured. The disc technique using KBr as a matrix was found to be suitable. In this concern, the ion exchanger was thoroughly mixed with KBr and the mixture was ground and then pressed with a special press to give a disc of standard diameter. The I.R. spectrum was scanned through a wave length range of 600–4000 cm1. 2.6.4. Scanning electron microscope (SEM) The grounded prepared samples were scanned to identify their structure and estimate the particle diameter at different magnifications. The mean diameter of the grains was determined from the SEM pictures by measuring at least 5 crystals for each formulation using the software Image tool.

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2.6.5. Surface area (BET) The specific surface area of the three different prepared ESC was determined by nitrogen Chemisorption Physisorption analyzer (Beckman Coulter AS3100, USA) after the samples were outgassed at 200 8C for 180 min. The BET surface area was calculated from the adsorption branches. 3. Result and discussion Results obtained from zirconium vanadate ion exchange preparations using the three different techniques have been studied. Factors affecting the preparation of zirconium vanadate using each technique discretely are discussed in details. The impact of these factors on the ion exchange capacity of the produced zirconium vanadate ion exchanger is also considered. 3.1. Preparation using sol–gel precipitation technique Different factors that affecting on the preparation process are examined and discussed below. 3.1.1. Effect of hydrochloric acid concentration Eight samples of zirconium vanadate ion exchanger were prepared using 0.1 M ZrOCl2 and 0.2 M sodium vanadate at different hydrochloric acid molarities. The ion exchange capacities of these yellow produced ion exchangers were tabulated in Table 1. It was observed that the ion exchange capacity of the produced ion exchanger doubled when using 0.2 M HCl in the preparation media. This may be due to the increase in the replacement hydrogen ions present inside the exchanger structure, which are responsible mainly for the exchange process inside the exchanger according to the following reaction [32]: RH þ Mþ ðaq:Þ @ RM þ Hþ ðaq:Þ On the other hand, a further increase in acid molarity than 0.2 M decreases the IEC of the produced ion exchanger until 0.5 M, when no precipitate has formed. Therefore, it is clear that the increase in the acid concentration in the preparation media dissolve some of the formed ion exchanger until 0.5 M, all produced exchanger dissolved. Experimentally the dissolution of formed precipitate takes place immediately as soon as they formed. The corresponding decrease in IEC with the increase in acid concentration may be attributed to dissolution of the vanadate groups present inside the prepared exchanger, which are associated with the exchangeable hydrogen ions (as will be shown in the characterization part of the exchanger). This is by its role decreases IEC of the prepared ion exchanger. 3.1.2. Effect of gelation temperature The most proper acid concentration has been determined to be 0.2 M, so the effect of gelation temperature was determined using 0.1 M ZrOCl2 and 0.2 M sodium vanadate in presence of 0.2 M HCl at different temperatures. Table 1 shows that there exists a slight increase in IEC of the prepared exchanger as the preparation temperature increased. This may be due to the enhancement of the surface properties of the prepared exchanger caused by the elevation of temperature. But the improvement in the IEC is not high enough compared with the studied range of temperature elevation. That may be returned to the enhancement in the ion exchange properties due to the increase in temperature that disregarded by the water evaporation. That will decrease the replacement hydrogen ion present inside the exchanger structure and accordingly decrease their IEC. So the preparation temperature has a negligible effect on the IEC of the prepared ion exchanger using sol–gel technique.

3.1.3. Effect of sodium vanadate concentration It had been observed before [33] that the increase of anionic part inside the inorganic ion exchanger contributed towards higher ionexchange capacity as the replaceable groups are attached to this group. Accordingly, our study was designed to illustrate the impact of vanadate groups presence in zirconium vanadate on IEC, that was prepared using 0.1 M ZrOCl2 and 0.2 M HCl at different sodium vanadate concentrations. The results presented in Table 1 show that their exist, an increase in vanadate groups inside the precipitated exchanger took place as the sodium vanadate concentration increased until 0.4 M amplifying IEC with 2.5 times than the initial one. Nevertheless, further increase in sodium vanadate concentration beyond 0.4 M reduces the IEC of the prepared exchanger and returns it back to its initial value. This may be due to the depletion of the hydrochloric acid present in the preparation media as the sodium vanadate concentration increased. This by its role decreased hydrogen ions associated with the vanadate group. From this study 0.2 M sodium vanadate was taken as the most proper concentration because further increase in IEC is not proportional to the increase in sodium vanadate concentration. Finally from the previous studies for the sol–gel technique, sample S15 which have the highest IEC and was prepared using 0.1 M ZrOCl2 and 0.2 M sodium vanadate in presence of 0.2 M HCl at 25 8C, was selected for chemically and physically characterization. 3.2. Preparation using homogeneous precipitation technique Powdered zirconium vanadate was prepared by homogenous precipitation using urea hydrolysis. Corresponding to the applied preparation conditions both the physical appearance and ion exchange capacity varied for the produced ion exchanger as will be discussed. 3.2.1. Effect of hydrochloric acid concentration The ion exchange capacity and physical appearance of six samples of zirconium vanadate exchanger, prepared using 0.1 M ZrOCl2, 0.2 M sodium vanadate and 1.5 g urea at different hydrochloric acid moralities are given in Table 2. The results show that the amount of replaceable hydrogen ions present in the zirconium vanadate products increased with increase in molarities of acid up to 0.04 M. But further increase in acid concentration decreased IEC of the produced ion exchanger and with acid

Table 1 Effect of different studied preparation parameters that affect on the ion-exchange capacity (IEC) of zirconium vanadate prepared using sol–gel technique. Sample no.

Preparation parameter

HCl concentration (M) S1 0 S2 0.1 S3 0.2 S4 0.3 S5 0.4 S6 0.5 S7 1 S8 2 Temperature (8C) S9 25 S10 40 S11 60 S12 80 S13 100 Sodium vanadate concentration (M) S14 0.1 S15 0.2 S16 0.4 S17 0.6 S18 0.8

IEC (mequiv./g) 0.2 0.29 0.4 0.36 0.28 No precipitate No precipitate No precipitate 0.4 0.44 0.44 0.48 0.5 0.2 0.4 0.5 0.34 0.22

M.M. Abd El-Latif, M.F. Elkady / Materials Research Bulletin 46 (2011) 105–118 Table 2 Effect of different studied preparation parameters that affect on the ion-exchange capacity (IEC) and color of zirconium vanadate prepared using homogeneous precipitation technique. Sample no.

Preparation parameter

HCl concentration (M) P1 0 P2 0.02 P3 0.04 P4 0.5 P5 1 P6 2 Precipitating agent amount (g) P7 0.5 P8 1.5 P9 3 P10 6 Sodium vanadate concentration (M) P11 0.1 P12 0.2 P13 0.4 P14 0.6 P15 0.8 Precipitating temperature (8C) P16 50 P17 70 P18 90 P19 100 P20 120

Color appearance

IEC (mequiv./g)

Greenish yellow White brown Brownish yellow Yellow Yellow –

0.16 0.3 0.82 0.4 0.36 No precipitate

Yellow Yellow Yellow White yellow

0.64 0.82 0.76 0.46

Brownish yellow Brown Brownish red Brown Brown

0.42 0.82 1.26 0.64 0.48

Dark yellow Brownish orange Brownish red Brownish red Brown

0.85 1.1 1.24 0.9 0.5

concentration 2 M, no precipitate was formed. This may be attributed to, as previously mention in sol–gel preparation technique, the increase in the acid concentration in the preparation media increased the replaceable hydrogen ion associated with vanadate group inside the produced ion exchanger. But further increase in the acid cause dissolution in the produced ion exchanger; especially dissolution of vanadate groups present inside the prepared exchanger, which by its rule decrease IEC of prepared exchanger. Further increase in acid concentration in the preparation media dissolves all produced precipitate and at 2 M HCl no ion exchanger was formed. 3.2.2. Effect of precipitating agent amounts In order to determine the most proper amount of the precipitating agent (urea) to yield ion exchanger with high IEC, four samples were prepared using 0.1 M ZrOCl2, 0.04 M HCl and 0.2 M sodium vanadate at different amounts of urea related to the weight of zirconium oxy chloride used. It is evident, as shown in Table 2, that the physical appearance of the produced exchanger has no relation with the change of urea amount. However, exchange capacity increases as the amount of urea increases then tends to decrease. This may be due to the presence of carbonate precipitate from urea hydrolysis mixed with the produced ion exchanger, which by its role decreased the exchange capacity of the exchanger. It was mentioned before [20] mentioned that an excess of precipitating agent should be avoided in the precipitation processes in order to avoid contamination of prepared exchanger with carbonate, produced from the hydrolysis of urea in an initially acidic solution as follows: ðNH2 Þ2 CO þ 3H2 O ! CO2 þ 2NH4 þ þ 2OH This hydrolytic reaction takes place slowly and occurs at temperatures just below the boiling point of water, in order to generate hydroxyl ions, leading to the precipitation of zirconium vanadate with different characteristics [20]. So, as the amount of urea increases the chance for contamination of the prepared exchanger with carbonate increases, which by its role decrease the exchange capacity of the synthesized exchanger.

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3.2.3. Effect of sodium vanadate concentration From the previous studied preparation factors; it was observed that the effect of sodium vanadate concentration could be investigated through zirconium vanadate preparation by addition 0.04 M HCl and 1.5 g urea to 0.1 M ZrOCl2 at different sodium vanadate concentrations. It was concluded from Table 2 that increasing the concentration of sodium vanadate increases the ion exchange capacity up to 0.4 M, a further increase in vanadate concentration IEC tends to decrease. This may be returned to the responsibility of the vanadate group to the H+ exchanging inside the exchanger due to its structure as will be mentioned in the characterization part. So, as the number of vanadate groups increase inside the exchanger the IEC of the produced exchanger increases. This suggestion is in agreement with Nabi [33], who concluded that anionic part inside the prepared Zr(IV) tungstomolybdate contributed towards ion-exchange capacity as the replaceable H+ groups are attached to this group. Also, this result matching those obtained before by Nilchi et al. [34]. However, the increase in the vanadate concentration (beyond 0.4 M) shows a decrease in IEC of the prepared exchanger, which may be due to the consumption of the HCl present in the preparation media. As mentioned above, the concentration of HCl decreases the IEC of the produced exchanger decrease. So, the concentration of HCl should be increased in order to save H+ ions associated with vanadate group as the reacted concentration of sodium vanadate increased. 3.2.4. Effect of precipitating temperature In order to consider effective hydrolysis of urea [35–37], so IEC for zirconium vanadate ion exchanger precipitated at various temperatures was studied using 0.1 M ZrOCl2, 0.04 M HCl, 1.5 g urea and 0.4 M sodium vanadate. It is clear from the results that the best precipitation temperature that produces ion exchanger with maximum IEC is 90 8C. This result supports the observation of Silva et al. [5] regarding the hydrolytic reaction of urea which takes place slowly and occurs at temperatures just below the boiling point of water. Also, this result is in accordance with the fact that the decomposition rate of urea is strongly depends on the temperature [38], the rate become constant and then increasing by a factor of about 200 as the temperature increases from 60 to 100 8C [38,39]. But the decrease in the IEC at 100 8C may be returned to the increase in urea dissociation which produces carbonate ions that contaminate the prepared exchanger with carbonate and decrease the exchange capacity of the synthesized exchanger. Furthermore, a sharp decrease in IEC is noticed as the precipitation temperature is elevated to 120 8C. This may be attributed to the boiling of the synthesized ion exchanger at 120 8C for 1 h, which might causes loss of its structure which is responsible for the exchange process and hence decrease its IEC. Finally from the previously studies preparation parameters using homogenous precipitation technique, sample P18 that has the highest IEC prepared using 0.1 M ZrOCl2, 0.04 M HCl, 1.5 g urea and 0.4 M sodium vanadate at 90 8C was selected to be chemically and physically characterized. 3.3. Preparation using hydrothermal synthesis technique Hydrothermal synthesis is defined as a process that utilizes single or heterogeneous phase reactions at temperatures >25 8C and elevated pressures >100 kPa to synthesize materials directly from solutions [40]. Also, it can be defined as the synthesis process that involves H2O both as a catalyst and occasionally as a component of solid phases in the synthesis at elevated temperature (>100 8C) and pressure (>a few atmosphere) [41]. The impact of the synthesis parameters such as sodium vanadate and hydrochloric acid concentrations, preparation temperature, and

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Table 3 Effect of different studied preparation parameters that affect on the ion-exchange capacity (IEC) of zirconium vanadate prepared using hydrothermal technique. Sample no.

Preparation parameter

HCl concentration (M) H1 0 H2 0.2 H3 0.4 H4 0.5 H5 0.7 H6 1 H7 2 Sodium vanadate concentration (M) H8 0.1 H9 0.2 H10 0.4 H11 0.6 H12 0.8 Reaction temperature (8C) H13 60 H14 80 H15 100 H16 120 H17 140 Reaction time H18 10 min H19 30 min H20 1h H21 1.5 h H22 3h H23 5h

IEC (mequiv./g) 0.4 0.48 0.62 0.8 0.68 No precipitate No precipitate 0.5 0.64 0.8 0.92 1 No precipitate No precipitate 0.64 0.8 0.9 0.3 0.38 0.5 0.8 1.2 1.8

reaction time on the ion exchange capacity of the produced zirconium vanadate using the hydrothermal synthetic technique have been investigated. 3.3.1. Effect of hydrochloric acid concentration It was confirmed after a set of runs at high-temperature (120 8C) for 90 min for the reaction of a 0.1 M ZrOCl2 and 0.4 M sodium vanadate at different hydrochloric acid molarities under a high pressure of 500 kPa, that 0.5 M is the most proper acid concentration to yield zirconium vanadate with the highest IEC as indicated from Table 3. At acid concentrations less than 0.5 M the possibility of incorporation of H+ species into zirconium vanadate gel is believed to be low. However, at higher acid concentration the formed gel goes to dissolution by the excess acid present in the preparation media. 3.3.2. Effect of sodium vanadate concentration Five samples of the hydrothermally synthesized zirconium vanadate ion exchanger were prepared using 0.1 M ZrOCl2 and 0.5 M HCl at different sodium vanadate concentrations at 120 8C and 500 kPa for 90 min. Table 3 indicates that the increment in the vanadate concentration increases the ion exchange capacity of the produced ion exchanger. This result is in accordance with that reported by Inoue [26] for preparation of stannic phosphate. Inoue observed that the exchange capacity is strikingly dependent upon the ratio of PO43/Sn4+ in the product; namely, the greater the ratio, the higher the capacity. On the other hand, this trend is not achieved for the effect of sodium vanadate concentration using others two techniques sol–gel or homogeneous precipitation. This may be due to the role of water in the hydrothermal synthetics technique, that acts as a catalyst where, it hydrothermally cracked producing H+ that compensate the decrease in the hydrochloric acid in the preparation media as the vanadate concentration is increased. The enlargement in the IEC of the produced exchanger is not proportional to the increase in sodium vanadate concentration and because of the high cost of sodium vanadate, 0.4 M sodium vanadate will be selected to synthesized zirconium vanadate using this technique.

3.3.3. Effect of reaction temperature The temperature feasibility for synthesis of zirconium vanadate has been studied for the reaction of 0.1 M ZrOCl2 and 0.4 M sodium vanadate in presence of 0.5 M HCl for 90 min at 500 kPa using different temperatures. It is clear from Table 3 that zirconium vanadate began to precipitate only at 100 8C and a slight increase in the IEC of the produced ion exchanger was noticed as the temperature increased. This may be due to the fact that the increase of the temperature of hydrothermal synthesis favors the evolution and crystallization of the powdered phase produced [42,43]. 3.3.4. Effect of reaction time The reasonable reaction time using the previously mentioned hydrothermal synthesis conditions at 120 8C was determined. From the tabulated results (Table 3) a significant increase in IEC was observed as the holding time increased from 10 min to 5 h. This is due to the fact that as the time of crystal growth increased through the increase in the holding time, the evolution and crystallization of the synthesized powdered would be promoted [43]. Finally from the previously studied conditions for the hydrothermal synthesis technique, sample H23 having the highest IEC and which was prepared using 0.1 M ZrOCl2 and 0.5 M HCl and 0.4 M sodium vanadate at 120 8C and 500 kPa for 5 h, was selected for full characterization. Generally, for the three different preparation techniques the ion exchange capacities for the selected samples produced from each technique were compared with the other ion exchanger capacities for the different inorganic ion exchange materials that reported in literatures and illustrated in Table 4 [2,7,44–46]. 3.4. Chemical characterization of the synthesized ion exchangers The selected samples were chemically characterized in terms of their chemical and thermal stability as well as their elution behavior, pH titration, capacity for different ions and composition. First of all it is important to carefully determine the optimum eluant concentration and elution time in order to obtain the most precision results. 3.4.1. Effect of eluant concentration and elution time on ion exchange capacity The elution of zirconium vanadate ion exchanger is governed by both eluant concentration and elution time, which was found as a usual behavior for these types of inorganic ion-exchange materials. The minimum molar concentration of NaCl as eluent and the minimum elution times for the ZrV samples prepared using sol–gel precipitation, homogeneous precipitation and hydrothermal synthesis were found to be 0.6 M, 4 h and 1 M, 5 h and 1.2 M, 10 h, respectively for maximum release of H+ ions from 1.0 g of the

Table 4 Comparison between the three different prepared zirconium vanadate ion exchangers and those cation exchangers reported in literatures. Ion exchange materials

Na+ ion exchange capacity (IEC)

Zirconium vanadate (S15) (present study) Zirconium vanadate (P18) (present study) Zirconium vanadate (H23) (present study) Titanium(IV) iodovanadate [2] Titanium(IV) tungstosilicate [7] Zirconium oxide [44] Titanium(IV) tungstophosphate [7] Titanium(IV) molybdosilicate [45] Zirconium iodovanadate [46]

0.4 1.24 1.8 1.7 0.44 0.5 0.8 0.74 2.2

[()TD$FIG]

[()TD$FIG]

M.M. Abd El-Latif, M.F. Elkady / Materials Research Bulletin 46 (2011) 105–118

111

H+ ions excahnged (meq)

1.5 1 0.5 0 2

Concentration of NaCl (M) Sol-gel precipitation(S15) Homogeneous precipitation(P18) Hydrothermal synthesis(H23)

0.2

0.1

3.4.3. pH titration The pH titration curve (Fig. 4) of selected ZrV samples and their composites shows the same behavior as monofunctional acids. Generally the ion exchange materials releases H+ ions easily on addition of NaCl solution to the system in neutral medium (no OH added), which is indicated by low initial pH values that ranged between 1.5 and 3.6 of the solutions. As the volume of NaOH added

S15

P18

H23

Fig. 3. Elution behavior of inorganic zirconium vanadate prepared using the three different techniques.

to the system is increased, more OH ions are consumed suggesting that the increase in the ion exchange rate in the basic medium may be due to the removal of H+ ions from the external solution. Respecting to the monofunctional behavior of the prepared ion exchangers, the titration curves show only one inflexion point indicating that selected ZrV samples prepared by homogeneous precipitation, hydrothermal synthesis and sol–gel precipitation behave as monofunctional ion exchanger [7]. These cation-exchangers may be strong acid cation-exchangers because the pH-titration curve showed only one step edge for each cationexchanger that is, the –H functional groups on the cationexchanger were depleted and replaced with Na+ ions at that point. Thus, theoretical ion-exchange capacity of these cationexchangers may be considered at the inflexion point for each one as 2.5, 3.5, 1 mmol g1 of ZrV prepared by homogeneous precipitation, hydrothermal synthesis and sol–gel precipitation, respectively. After that point, when more NaOH are added, the equilibrium pH further increases but more slowly. This slow increase of pH–titration curve after the inflexion point is only due to surface precipitation [50]. X-ray examination of the solid

[()TD$FIG]

12 10

[()TD$FIG]

8

2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

pH

IEC (meq/g)

160

Volume of Effluent (ml)

cation-exchangers as shown in Figs. 1 and 2. As a result, the exchange capacity of the selected samples was determined using 1.5 M salt solution and samples are equilibrated for 12 h to confirm that all H+ ions are released from the prepared zirconium vanadate. 3.4.2. Elution behavior of zirconium vanadate ion exchanger Earlier observations of Nilchi et al. [47–49] have shown that 250 mL of a 1 M NaNO3 solution is sufficient to completely elute H+ ions from a column of the ion-exchange material (1 g), out of which the first 100 mL effluent contained the major amount (70–90%). Therefore, 150 mL was selected for eluting the H+ ions from various ion-exchange columns. The elution behavior (Fig. 3) depicts that the exchange is quite fast at the beginning and maximum H+ ions are eluted out in the first 100 or 150 mL of the effluent by using 1.5 M NaCl solution for the different inorganic ZrV samples. It is worthy to mention that for the same preparation technique, all the inorganic ZrV have the same elution behavior where, the 100 mL of the eluted exchangeable H+ ions from the different prepared ZrV is collected within 24 h. This may be due to the small particle size of the powdered ZrV that will clog the channels for passing the treated liquid through the bed resulting in extremely high column pressure drops which decrease the liquid flow.

140

Fig. 1. Exchange capacity of zirconium vanadate prepared using the three different techniques as a function of eluent concentration.

120

0

0 100

1.5

80

1

60

0.5

0.3

40

0

0.4

20

IEC (meq/g)

2

6 4

2 0 0

2

4

6

8

10

12

14

Elution time (hour) Sol-gel precipitation(S15)

0

1

2

3

4

5

m moles OH ions added/g of ion exchanger

Homogeneous precipitation(P18)

Hydrothermal synthesis(H23)

Fig. 2. Effect of elution time on ion exchange capacity of zirconium vanadate prepared using different techniques.

S15

P18

H23

Fig. 4. pH titration curve of inorganic zirconium vanadate prepared using the three different techniques and their composites.

M.M. Abd El-Latif, M.F. Elkady / Materials Research Bulletin 46 (2011) 105–118

112

Table 5 Effect of size and charge of the exchanged ion on the exchange capacity of inorganic zirconium vanadate prepared using the three different techniques. Exchanging ions +

Li Na+ K+ Mg2+ Ca2+ Ba2+ Sr2+ Cs+

Ionic radii (A˚) [51]

Hydrated ionic radii (A˚) [51]

IEC of S15 (mequiv./g)

IEC of P18 (mequiv./g)

IEC of H23 (mequiv./g)

0.4 0.5 0.8 0.45 0.7 1.05 0.85 1.05

3 2.25 1.5 4 3 2.5 2.5 1.25

0.3 0.4 0.5 0.16 0.2 0.6 0.64 0.7

0.7 1.24 1.3 0.4 0.62 1.68 1.42 2.4

1 1.8 1.9 0.54 0.9 2.3 2 3

Fig. 5. From the comparative study of the heating effect on Na ionexchange capacity of the selected ZrV samples, it was found that all samples possess higher thermal stability, as the samples prepared using sol–gel, homogeneous precipitation and hydrothermal synthesis maintained about 63, 69, and 74%, respectively of their initial masses by heating up to 700 8C. However, with respect to ion exchange capacity, these materials were found stable up to 200 8C, where they retained about 50, 55, and 68% of their IEC for samples prepared using sol–gel, homogeneous precipitation and hydrothermal synthesis, respectively. Also from the remarkable results shown in Fig. 5, it is interesting to note that according to the semicrystalline structural of the samples prepared by homogeneous precipitation and hydrothermal synthesis they hold their structural water into their crystals, and the chance for releasing their structural water becomes more difficult than the amorphous one, prepared by sol–gel. As to the important role of the structural water in the ion exchange process as will be discussed, the percentage of IEC retention of these samples were higher than the amorphous one.

exchangers indeed shows that partial solid solution occurs with the composition range increasing with decreasing crystallinity. Thus the degree of order in the solid is reflected in the extent of solid solution and hence the titration curve. According to this viewpoint, it is thus possible to obtain a measure of the degree of crystallinity of the selected ZrV samples from their titration behaviors. The titration curves that exhibit a positive slope indicating that the restriction on zero degrees of freedom, that presence in case of fully crystalline samples, was removed. Thus as the titration curve slope tends to zero, the degree of freedom decreases and the material crystallinity increases [32]. Accordingly we may predict from the titration curves that ZrV prepared using sol–gel technique has amorphous structure. However, both the other two preparation techniques produce ion exchangers with semi-crystalline structures, and the degree of crystallinity for that prepared using hydrothermal technique is more crystalline than that prepared by homogenous precipitation.

Table 6 Effect of temperature on the physical appearance of inorganic zirconium vanadate prepared using the three different techniques. Heating temp. (8C)

Appearance color of S15

Appearance color of P18

Appearance color of H23

40 100 150 200 300 500 700

Yellow Yellow Yellow Yellow Brown Dark brown Black

Brownish red Brownish red Brownish red Brownish red Green Dark green Black

Dark yellow Dark yellow Dark yellow Dark yellow Light gray Gray Gray

40

100

35 80 30 25

60

20 40

15 10

%Weight loss

3.4.5. Thermal effects on ion exchange capacity It was observed from the obtained results data that when heating selected ZrV samples prepared using the three different techniques to different temperatures for 1 h, the mass, physical appearance and ion-exchange capacity of the dried samples changed as the temperature increased as shown in Table 6 and

[()TD$FIG]

3.4.6. Radiation effect on ion exchange capacity Table 7 shows the effect of radiation on capacity of zirconium vanadate produced from the three different preparation techniques. It can be seen from the table that, the capacity of all synthesized ZrV ion exchangers does not change significantly upon irradiation with a dose of 25 kGy. There exists an exception for the samples produced by the hydrothermal synthesis, where some fluctuation is observed with maximum capacity loss 10%. When these exchangers irradiated by dose greater than 25 kGy their ion exchange capacities show a small extent of descending that increase in the case of ion exchanger that produced from hydrothermal synthesis. In spite of this descending in the capacity,

%Retention of IEC

3.4.4. Ion exchange capacity for different metal ions Table 5 shows the ion exchange capacities of the inorganic zirconium vanadate prepared using the three different techniques (S15, P18 and H23) for alkali and alkaline earth metal ions. Generally they show maximum ion exchange capacities for Cs+ ion that equal to 0.7, 2.4, and 3 mequiv./g for samples prepared by sol– gel, homogeneous precipitation and hydrothermal synthesis, respectively. Also, their affinity sequences for alkali metal ions was K > Na > Li, on the other hand, for alkaline earth metals Ba > Ca > Mg and for the radioactive metals Cs > Sr. The size and charge of the exchanging ion affects the ion exchange capacity of the prepared zirconium vanadate. This sequence is in accordance with the size of the hydrated radii of the exchanging ions [51]. Ions with the smaller radii easily enter the pore of the exchanger, resulting in greater IEC. These results were also similar to that reported by Nabi et al. [46] and Nilchi et al. [34].

20 5 0

0 30

130

230

330

430

530

630

730

Heating temperature (ºC) IEC ofS15 IEC of H23 Wt loss of P18

IEC of P18 Wt loss of S15 Wt loss of H23

Fig. 5. Thermal stability of inorganic zirconium vanadate prepared using the three different techniques.

M.M. Abd El-Latif, M.F. Elkady / Materials Research Bulletin 46 (2011) 105–118

Radiation dose (Gy)

0 2.5 10 10,000 25,000 50,000 100,000

%Retention IEC

Table 9 Elemental analysis of inorganic zirconium vanadate prepared using the three different techniques using ICP.

Sol–gel precipitation

Homogeneous precipitation

Hydrothermal synthesis

100 100 100 100 100 98.2 95

100 100 100 100 96.7 88 84.6

100 100 100 94.8 92.5 85.5 79.7

3.4.7. Chemical composition The chemical composition detected by (EDX) analysis (Table 8) for ZrV prepared using the three different techniques reveals that the ratio of Zr:V is approximately 1:2 for both exchangers that prepared using homogeneous precipitation and hydrothermal synthesis. However, this ratio become 1:1.5 for that prepared using sol–gel precipitation. This may be due to the amount of sodium vanadate used in the sol–gel preparation technique is half that used for others two preparation techniques. These results are in accordance with that obtained from the elemental analysis of the prepared ion exchangers using ICP. Table 9 indicates that ZrV prepared by sol–gel precipitation contains 26.4% Zr and 21.8% V corresponding to an approximate Zr:V ratio of 1:1.5. However, the others exchangers that produced from the homogeneous precipitation technique contains 24.6% Zr and 28.8% V and that from the hydrothermal synthesis contains 22.7% Zr and 25.9% V which nearly corresponds to the ratio Zr:V of 1:2.

Elements (wt%) Zr

V

Na

Cl

Prepared by sol–gel precipitation (S15) Prepared by homogeneous precipitation (P18) Prepared by hydrothermal synthesis (H23)

26.4 24.6 22.7

21.8 28.8 25.9

0 0 0

0 0 0

10

45

9

40

HCl(V) HNO3(V) H2SO4(V) Ethanol(V)

8

35

7 30 6 25 5 20 4 15 3

Acetone(V)

Zirconium released (mg/g)

the maximum capacity loss is 20% at the maximum radiation dose (100 kGy). So we can conclude that all prepared zirconium vanadate ion exchangers are highly resistant to the radiation.

[()TD$FIG]

ZrV ion exchanger

Vanadium released (mg/g)

Table 7 Effect of radiation on ion-exchange capacity of ZrV prepared using the three different techniques.

113

Glycerol(V) ammonia(V) DMS(V) NaOH(V) Ethanol(Zr) Ammonia(Zr) DMS(Zr) HCl(Zr)

HNO3(Zr)

10 2

H2SO4(Zr) Acetone(Zr) Glycerol(Zr)

5

1

NaOH(Zr)

0

0 0

0.5

1

1.5

2

Solvent concentration (M) Fig. 6. Chemical stability of ZrV sample prepared by homogeneous precipitation.

3.4.8. Chemical stability The solubility of the prepared ion exchangers at ambient conditions, in different concentrations of solvent solutions was determined in terms of zirconium and vanadium ion released. Figs. 6–8 show the amount released for each ion from one gram of the prepared ion exchangers versus the solvent concentration. Generally, it can be concluded that all selected samples of prepared ZrV are highly stable up to 1 M concentrations of the polar solvents such as ethanol, acetone, DMS and ammonia, but they tend to bleed very small amounts of vanadium as solvent concentrations increased. However, for acidic solvents, their solubility increased as the acidity increased from 0.1 to 2 M for both zirconium and vanadium ions. Furthermore, their solubility in the alkaline NaOH solvent shows nearly the same behavior as the acidic solvents. Comparing the largest amounts of zirconium and vanadium ions released from ZrV prepared by homogeneous precipitation is equal to 40 and 7.4 mg/g, respectively with 118 and 1.85 mg/g released from that prepared by sol–gel precipitation and 126 and 2.1 mg/g released from that prepared by hydrothermal synthesis, so ZrV prepared by homogeneous precipitation is more stable than the others.

Finally, in spite of the amount of released vanadium and zirconium increased for the high acidity and alkalinity solvent solutions compared with that released on using polar solvents, but their values are not significantly high for one gram of zirconium vanadate. So, we can conclude that all prepared zirconium vanadate using different preparation techniques are stable for a wide concentration range of acid, basic as well as polar solvents. 3.5. Physical characterization of the synthesized ion exchangers It is rather constructive to learn about the structure of the prepared zirconium vanadate ion exchangers to assess the sorption and fixed bed results obtained. In this concern, measurements of the infrared spectra (IR), thermal gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopic (SEM) and surface area (BET) were carried out. 3.5.1. X-ray diffraction (XRD) Fig. 9 represents the (XRD) patterns of the prepared ZrV ion exchangers produced from the three different preparation

Table 8 EDX analysis of inorganic zirconium vanadate prepared using the three different techniques. ZrV ion exchanger

Elements (at%) Zr

V

O

Na

Cl

Prepared by sol–gel precipitation (S15) Prepared by homogeneous precipitation (P18) Prepared by hydrothermal synthesis (H23)

12.71 11.88 10.51

18.23 24.2 21.96

68.9 63.68 67.39

– – –

0.16 0.24 0.14

[()TD$FIG]

[()TD$FIG]

M.M. Abd El-Latif, M.F. Elkady / Materials Research Bulletin 46 (2011) 105–118

114

140

2.5

HCl(V) HNO3(V)

120

H2SO4(V) Ethanol(V)

100

1.5 80

60 1

Zirconium released (mg/g)

Vanadium released (mg/g)

2

Acetone(V) Glycerol(V) ammonia(V) DMS(V) NaOH(V) HCl(Zr) HNO3(Zr) H2SO4(Zr) Ethanol(Zr)

40

Acetone(Zr)

0.5

Glycerol(Zr)

20

Ammonia(Zr)

Fig. 9. X-ray diffraction pattern of zirconium vanadate prepared using the three different preparation techniques.

DMS(Zr) NaOH(Zr)

0

0

0

0.5

1

1.5

2

Solvent concentration (M) Fig. 7. Chemical stability of ZrV sample prepared by sol–gel precipitation.

techniques. We can deduced that ZrV produced from the sol–gel technique have an amorphous structural, where they show weak diffraction lines which can be indexed with those of ZrO2 with crystal system [52]. However, a small peak at 328 began to appear for that prepared from the homogeneous precipitation technique which indicates some degree of crystallinity and the compound can be considered as poly-crystalline in nature. This degree of crystallinity may be due to the heating that takes place at the preparation at 90 8C for 1 h. The average diameter of this exchanger was found to be 53 nm, which calculated from the full width at half-maximum of the peak using Debye Scherrer’s equation [53]:

D¼

[()TD$FIG]

0:89l B2 u cos umax

2.5

140 HCl(V) HNO3(V)

120

H2SO4(V)

2

1.5 80

60 1

40

Zirconium released (mg/g)

Vanadium released (mg/g)

Ethanol(V)

100

Acetone(V) Glycerol(V) ammonia(V) DMS(V)

HCl(Zr) HNO3(Zr) H2SO4(Zr) Ethanol(Zr)

Glycerol(Zr)

20

Ammonia(Zr) DMS(Zr) NaOH(Zr)

0

0 0

0.5

1

1.5

3.5.2. Morphological characterization (SEM) Morphological characterization of zirconium vanadate ion exchangers produced from the three different preparation techniques was performed using SEM. It can be seen from Fig. 10 that all studied ZrV samples show particle size in range of 50–60 nm. Thus, all the prepared materials particle size produced in the nano-range. These results support the previously calculated particle diameter using the Scherrer’s equation from the X-ray patterns of the prepared semi-crystalline ion exchangers, which confirm that the prepared ZrV ion exchangers were produced in the nano-scale. Additionally, an acceptable homogenous spherical morphology was noticed for both ZrV prepared using sol–gel precipitation and that produced from the homogeneous precipitation. On the other hand, the morphological structural of the exchanger produced from the hydrothermal synthesis shows a spherical and needle particle shapes inside its structural.

NaOH(V)

Acetone(Zr)

0.5

where D is the average crystal size in nm, l is the characteristic wavelength of X-ray used (1.5406 A˚), u is the diffraction angle, and B2u is the angular width in radians at intensity equal to half of the maximum peak intensity [54]. While the degree of the crystallinity increased in case of preparation using the hydrothermal synthesis than homogeneous precipitation technique as two others small peaks at 478 and 638 appears in its pattern. This may be due to the preparation conditions of this exchanger that take place at 500 kPa pressure and 120 8C for 5 h. The average diameter of the exchanger produced from hydrothermal synthesis that calculated using the Scherrer’s equation was found to be 61 nm. Finally, from these results, it can be concluded that both ZrV prepared using the homogeneous precipitation and hydrothermal synthesis techniques produced in the nano-scale with semicrystalline structural.

2

Solvent concentration (M) Fig. 8. Chemical stability of ZrV sample prepared by hydrothermal synthesis.

3.5.3. Infrared spectroscopy (IR) Fig. 11 represents the FTIR spectrum of the cation exchange material zirconium vanadate produced from the three different preparation techniques. All IR spectra show a strong and broad peak around 3200 cm1 corresponds to the presence of interstitial water and hydroxyl groups [55,56]. A sharp peak at 1617 cm1 corresponds to the deformation vibration of free water molecules. However, the peak at 1000 cm1 shows the presence of vanadate ion. Finally, a broad peak in the region around 730 cm1 was due to metal–oxygen bond [33]. The peak at 1250 cm1 present in the prepared ZrV ion exchangers may be ascribed to water bonded with ZrO2 [52]. Hence, from the IR analysis, it can be deduced that H2O, –OH, vanadate ion and Zr–O–H are present.

[()TD$FIG]

[()TD$FIG]

M.M. Abd El-Latif, M.F. Elkady / Materials Research Bulletin 46 (2011) 105–118

Fig. 10. Scanning electron microphotographs (SEM) of zirconium vanadate produced from (a) sol–gel preparation, (b) homogeneous precipitation and (c) hydrothermal synthesis.

3.5.4. Differential scanning calorimeter (DSC) The DSC patterns of zirconium vanadate ion exchangers produced from the three different preparation techniques are illustrated in Fig. 12. From this figure, it was indicated that all prepared ZrV samples show the same behavior, where they show two main endothermic peaks. The first broad endothermic peak, at temperature around 100 8C, represents the heat losses due to the gradual loss of external water molecules from the materials. This is followed by a broad shoulder which is terminates in another small inflexion at temperature around 350 8C, which may be resulting from the loss of structural water. An exothermic peak, noticed only in the cases of homogeneous precipitation and hydrothermal synthesis preparation techniques around 450 8C, may be caused by

115

Fig. 11. Infrared spectroscopy (IR) of zirconium vanadate produced from (a) sol–gel preparation, (b) homogeneous precipitation and (c) hydrothermal synthesis.

physical transitions such as some degree of crystallization occurring during heating [57]. 3.5.5. Thermo gravimetric analysis (TGA) The thermograms of the different prepared zirconium vanadate ion exchangers illustrated in Fig. 13 are verifying the previously discussed DSC results, where, the curves patterns of the three main ZrV samples prepared show two main degradation or weight losses steps that confirm the previous DSC behavior. The first degradation step for all samples representing the weight loss of the ion exchanger with a maximum at 160 8C is due to the removal of free external water molecules [58]. While, the second gradual loss in the samples weight that began from temperatures above 260 8C is due to the removal of interstitial water molecules by condensation of exchangeable hydroxyl groups(–OH) from the material, which is

[()TD$FIG]

116

[()TD$FIG]

M.M. Abd El-Latif, M.F. Elkady / Materials Research Bulletin 46 (2011) 105–118

a

7.6 7.4 7.2 7.0

TGA (mg)

6.8 6.6 6.4 Start= 424.67ºC

6.2 6.0

End= 467.48ºC Weight Loss= 4.17%

5.8

Start= 33.5ºC

5.6

End= 148.25ºC Weight Loss= 10.05%

5.4 5.2 5.0

0

200

400

600

800

1000

Temperature(ºC)

b

6.0 5.8 5.6

TGA (mg)

5.4 5.2

Start= 260.52ºC

5.0

End= 424.75ºC Weight Loss= 9.583%

4.8

Start= 30.67ºC

4.6

End= 124.36ºC Weight Loss= 9.097%

4.4 4.2 4.0

0

200

400

600

800

1000

Temperature(ºC)

c

13.8 13.2 12.6

Fig. 12. Differential scanning calorimeter (DSC) of zirconium vanadate produced from (a) sol–gel preparation, (b) homogeneous precipitation and (c) hydrothermal synthesis.

TGA (mg)

12.0 11.4 10.2 9.6 9.0 8.4

characteristic of synthetic inorganic ion exchangers. There is no significant weight loss after 500 8C for all samples, which indicates that no structural changes can occur in the materials. This suggests that all prepared the ion exchanger is stable up to 1000 8C. Furthermore, it is interesting to observe that the ion exchange capacities of the prepared exchangers could be monitored through their structural water contents. It is clear from Fig. 13 that the structural water contents of the prepared ZrV exchanger follow the order sequence of hydrothermal (H23) > homogeneous precipitation (P18) > sol–gel (S15). Also, from the preparation part, it was noticed that the ion exchange capacities of the selected ZrV samples produced from the three different preparation techniques follow the same sequence. This observation confirms that the ion exchange process takes place inside the exchanger through its water structure content. According to the obtained IR, DSC and TGA results, all the prepared zirconium vanadate ion exchanger are assumed to have the molecular formula Zr(OH)2(HVO4)2nH2O. Where, the value of ‘‘n’’, the external water molecules, can be calculated using Alberti’s equation [59].

Start= 316.9ºC

10.8

End= 370.8ºC Weight Loss= 13.57%

Start= 47.22ºC End= 157.77ºC Weight Loss= 8.607%

7.8 7.2

0

200

400

600

800

1000

Temperature(ºC) Fig. 13. Thermal gravimetric analysis (TGA) of zirconium vanadate produced from (a) sol–gel preparation, (b) homogeneous precipitation, (c) hydrothermal synthesis.

18n ¼ X

  M þ 18n 100

where X is the percent weight loss of the exchanger by heating up to 160 8C and (M + 18n) is the molecular weight of the material. Thus, the values of X and n are illustrated in Table 10, it is clear that the external water molecules for all prepared zirconium vanadate ion exchangers range between 1.7 and 1.9. Finally, as a result of the above findings, a tentative formula of Zr(OH)2(HVO4)22H2O has been assigned to the selected studied samples of zirconium vanadate ion exchanger.

M.M. Abd El-Latif, M.F. Elkady / Materials Research Bulletin 46 (2011) 105–118 Table 10 External water molecules associated with the inorganic zirconium vanadate prepared using the three different techniques. ZrV ion exchanger

Percent weight loss

External water molecule, n

Prepared by sol–gel precipitation (S15) Prepared by homogeneous precipitation (P18) Prepared by hydrothermal synthesis (H23)

10.05

1.9

9.09

1.8

8.6

1.7

3.5.6. Surface area (BET) The nitrogen adsorption–desorption isotherms of the three different prepared ZrV ion exchangers indicated that all synthesized materials have a high surface area. The sample produced from sol–gel precipitation (S15) has specific surface area equal to 187 m2/g and that from homogeneous precipitation (P18) equal to 192 m2/g, moreover the material produced using hydrothermal technique (H23) has 320 m2/g. For comparison it is pointed out that the surface area of the three different ion exchangers were proportional to their IEC. Whereas, the specific surface area increased, the available active sites for ion exchange process increase and subsequently the IEC was increased. 4. Conclusions Three different preparation techniques sol–gel precipitation, homogeneous precipitation and hydrothermal synthesis are used for the preparation of zirconium vanadate ion exchangers that have appropriate ion exchange properties. The ion exchange properties of the synthesized materials were controlled by systematic changes in the synthetic variables of each preparation technique, in order to improve the properties of the exchangers and their performance. With respect to the sol–gel precipitation method, it has been deduced that the gelation temperature does not substantially affect the exchanging properties of the produced zirconium vanadate gel. However, the increment in both the hydrochloric acid and sodium vanadate concentrations in the reactant media leads to improve the IEC of the produced exchanger, while further increase in sodium vanadate deplete the IEC of the produced exchangers, on the other hand, the increase in the hydrochloric acid above 0.5 M dissolve the formed ion exchanger in the reaction media, resulting in no zirconium vanadate gel produced. Out of eighteen different samples of zirconium vanadate prepared using this technique, sample S15 that prepared using 0.1 M ZrOCl2 and 0.2 M sodium vanadate in presence of 0.2 M HCl at room temperature was selected as the most proper prepared ion exchanger using this technique for detailed characterization. On the other hand, the impact of all studied preparation factors corresponding to the homogeneous precipitated technique follow the same behavior, where the improvement in either the precipitation temperature or the hydrochloric acid, sodium vanadate and precipitating agent concentrations in the reaction media enhance the ion exchange capacities of the produced ion exchangers till a limit, then further increase in any of the these factors decline the IEC of the yielded ion exchangers. Twenty samples were produced from this technique at the various preparation conditions, sample P18 that prepared using 0.1 M ZrOCl2, 0.04 M HCl, 1.5 g urea and 0.4 M sodium vanadate at 90 8C was appropriate to be chemically and physically characterized. Finally, regarding the hydrothermal synthesis technique, the same observation was concluded for the effect of the hydrochloric acid concentration as the two previous preparation techniques. However, the continuous increase in sodium vanadate concentration in reaction media

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enlarges the IEC of the produced exchanger. Moreover, the elevation in both the temperature and the holding time of the hydrothermal reaction enhance the ion exchange capacities of the produced zirconium vanadate ion exchangers. From the 23 samples prepared using this technique, due to the sample H23 that prepared using 0.1 M ZrOCl2 and 0.5 M HCl and 0.4 M sodium vanadate at 120 8C and 500 kPa for 5 h shows higher ionexchange capacity and was selected for complete characterization. The feasibility of these selected prepared zirconium vanadate samples for the ion exchange performance was recognized through their full characterizations. The pH titration curve reveals that most of selected prepared ion exchangers were behaves as monofunctional acids. Moreover, all selected prepared inorganic ZrV samples have the ion exchange affinity sequences for alkali metal ions K > Na > Li, however the order for the alkaline earth metals is Ba > Ca > Mg, finally their affinity for radioactive metals follow Cs > Sr. The selected ion exchangers have reasonable physical and chemical stabilities towards both the heat and radiation effects as well as for wide concentration ranges of acid, basic as well as polar solvents. X-ray spectra indicate that ZrV produced from the sol–gel technique has an amorphous structural. Nevertheless, samples produced from the homogeneous precipitation and hydrothermal synthesis techniques, where they were produced in the nano-scale have semicrystalline structural. Furthermore, SEM confirms that particle size of the studied prepared ZrV samples have nano-diameters in range of 50–60 nm. Finally, specific surface area of the three different prepared ion exchangers was found to be equal to 187, 192 and 320 m2/g for sol–gel, homogeneous precipitation and hydrothermal, respectively. References [1] A.P. Gupta, G.L. Verma, S. Ikram, React. Funct. Polym. 43 (2000) 33–41. [2] S.A. Nabi, M. Naushad, Chem. Eng. J. 158 (2) (2009) 100–107. [3] C.B. Amphlett, in: Proceedings of 2nd Conference Peaceful uses of Atomic Energy, Geneva, 1958. [4] C.J. Brinker, G.W. Sherer, Sol–Gel Science, Academic Press, San Diego, 1990, pp. 160–174.. [5] G.L.J.P. Silva, M.L.C.P. Silva, T. Caetano, Mater. Res. 5 (2002) 149–153. [6] K. Byrappa, M. Yoshimura, Handbook of Hydrothermal Technology, William Andrew Publishing, LLC Norwich, NY, USA, 2001. [7] P. Singh, J.P. Rawat, N. Rahman, Talanta 59 (2003) 443–452. [8] M. Qureshi, K.G. Varshney, Inorganic Ion Exchangers in Chemical Analysis, CRC Press, Boca Raton, FL, USA, 1991. [9] K.G. Varshney, S.M. Maheshwari, Ecotoxicol. Environ. Saf. 18 (1) (1989) 1–10. [10] M. Qureshi, J.P. Gupta, H. Khan, A. Ahmad, Bull. Chem. Soc. Jpn. 61 (6) (1988) 2181–2184. [11] K.V.S. Nath, S.N. Tandon, Can. J. Chem. 68 (2) (1990) 346–349. [12] A.P. Gupta, Renuka, Ind. J. Chem. 36 (1997) 1073–1974. [13] Z.R. Turel, S.S. Narkhade, J. Ind. Chem. Soc. 75 (1998) 172–177. [14] K. Roya, D.K. Pala, S. Basua, D. Nayakb, S. Lahiri, Appl. Radiat. Isot. 57 (2002) 471– 474. [15] V. Lobo, Z.R. Turel, J. Radioanal. Nucl. Chem. 247 (2001) 221–227. [16] L.F. Chekomova, N.V. Cherednichenko, Analiticheskoj. Khimii. 53 (1998) 1032– 1039. [17] V. Vesley, V. Pekarek, Talanta 19 (1972) 219–262. [18] A. Clearfield, Chem. Rev. 88 (1988) 125–148. [19] Y. Kanzaki, M. Abe, in: Proceedings of the International Conference on Ion Exchange ICIE’91, Tokyo, 1991. [20] K. Roya, P.K. Mohapatrab, N. Rawatb, D.K. Pala, S. Basua, V.K. Manchanda, Appl. Radiat. Isot. 60 (2004) 621–624. [21] S. Lahiri, K. Roy, S. Bhattachary, S. Maji, S. Basu, Appl. Radiat. Isot. 63 (2005) 293– 297. [22] K. Roy, S. Basu, A. Ramaswami, S. Lahiri, Appl. Radiat. Isot. 59 (2003) 105–108. [23] M.M. Abd El-Latif, M.F. El-Kady, J. Appl. Sci. Res. 4 (1) (2008) 1–13. [24] M.M. Abd El-Latif, M.F. El Kady, Desalination 255 (2010) 21–43. [25] M. Rikukawa, D. Inagaki, K. Kaneko, Y. Takeoke, I. Ito, Y. Kanzaki, K. Sanui, J. Mol. Struct. 739 (2005) 153–161. [26] Y. Inoue, J. Inorg. Nucl. Chem. 26 (1964) 2241–2253. [27] A.A. Khan, A. Khan, Inamuddin, Talanta 72 (2007) 699–710. [28] N.E. Topp, K.W. Pepper, J. Chem. Soc. (1949) 3299–3303. [29] Inamuddin, S.A. Khan, W.A. Suiddiqwi, A.A. Khan, Talanta 71 (2007) 841–847. [30] L. Susanta, R. Kamalika, B. Soumya, M. Samir, S. Basu, Appl. Radiat. Isot. 63 (2005) 293–297. [31] W.A. Siddiqui, S.A. Khan, Inamuddin, Colloids Surf. 295 (2007) 193–199.

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