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Vol. 8. Issue 3.
Pages 2854-2864 (May - June 2019)
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Vol. 8. Issue 3.
Pages 2854-2864 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.04.023
Open Access
Role of preparation technique in the morphological structures of innovative nano-cation exchange
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Hassan Shokry Hassana,b
a Electronic Materials Research Department, Advanced Technology and New Materials Research Institute, City of Scientific Research and Technological Applications, New Borg El-Arab City, Alexandria 21934, Egypt
b Environmental Engineering Department, Egypt-Japan University of Science and Technology, New Borg El-Arab City, Alexandria, Egypt
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Tables (4)
Table 1. Effect of surfactant type onto the percentage material yield and IEC for all prepared samples produced from three different techniques.
Table 2. Effect of reactant molar ratio onto the percentage material yield and IEC for all prepared samples produced from three different techniques.
Table 3. Effect of reaction temperature onto the percentage material yield and IEC for all prepared samples produced from three different techniques.
Table 4. Effect of HCl concentration onto the percentage material yield and IEC for all prepared samples produced from three different techniques.
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Abstract

Nano-zirconium tungsto-vanadate cation exchange material was constructed in different morphological structures using sol–gel, hydrothermal and microwave techniques in presence of polymeric matrix as stabilizing agent. Polyvinyl alcohol was established as the optimum stabilizing agent. Influence of preparation parameters in presence of polyvinyl alcohol was examined as a function of ion exchange capacity and surface area for the prepared materials. The optimum reactant molar ratio was recorded as 1:0.5:1 for zirconium:vanadium:tungsten respectively. Properties of the most efficient ion exchange samples were inspected using X-ray diffractometer (XRD), thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM). The feasibility of the prepared materials as cation exchanger for lead and strontium decontaminations was compared. Microwave represented the most efficient technique for material production with highest surface area of 671m2/g and highest ion exchange capacity of 4.7meq/g and about 64.9 and 44.2mg/g lead and strontium sorption capacity respectively.

Keywords:
Zirconium tungsto-vanadate
Nanorod morphology
Nanomaterials architecture
Cation exchanger
Microwave technique
Full Text
1Introduction

The water pollution represents one of the most serious problems that facing the world now. Reliable access to clean and healthy water is considered one of the most essential human goals and remains a major global challenge for the 21st century [1,2]. Lead and strontium compounds are generally one of the most toxic pollutants in water; even at low concentrations, they are extremely toxic, causing brain damage in children [3]. Non-biodegradability and persistence of these pollutants in the environment is responsible for health hazards [4,5]. Among the various heavy metals, lead (II) is a well-known toxic metal that considered as priority pollutant and its adverse effects are well documented. It may cause a range of physiological disorders [6]. Lead metabolism can also closely mimic that of calcium, particularly at the receptor site of membranes, where it may replace calcium and thus negatively affect both neuromuscular and synaptic transmissions. Lead ions pollution may be derived from several sources, including lead in petrol, industrial effluents, and leaching of lead ions from the soil into lakes and rivers by acid rain. Furthermore, lead decontamination may be arising from smelting of ores, preparation of nuclear fuels, and electroplating. Despite many governments have enacted laws to hinder discharging heavy metals especially lead ions into water bodies and using toxic substances [7]. However, lead pollution still finds their way to water supplies.

On the other hand, respecting to the huge utilization of nuclear energy all over the world as an alternative energy resources, nowadays there are more than 400 reactors in operation at nuclear power plants around the world. These generate large amounts of nuclear wastes with low-level (LLW) that contain radioactive 137Cs, 90Sr and activation corrosion products mostly in solutions of low salt concentrations. All radioisotopes contained in the waste have high half-life-times, the time it takes for any radionuclide to lose half of its radioactivity. Eventually all waste decays into non-radioactive elements but after long time. Strontium emitted particles ionize or destabilize atoms as they pass through the body's cells damaging chromosomes, which can lead to cancer. Radioactive strontium can be taken up by bone, damaging bone marrow and reducing blood cell counts. Thus, it is of significance to propose highly efficient techniques to trap both toxic and radioactive metals from the contaminated waters [8].

Among the techniques of water treatment, the ion exchange separation technique is characterized over the other separation techniques by its simplicity and because it is cheap and not energy consumable. Accordingly, the ion exchange process represents the most suitable technique for lead ion separation from polluted wastewaters. There are various types of ion exchange materials that are suitable to be utilized for the ion exchange process from either the organic or inorganic nature. Recently, synthetic inorganic ion exchangers have gained much attention owing to their high selectivity for certain elements, good sorption kinetics and specially their greater stability to heat, ionizing radiation and different chemical media. Since the ion exchange behavior, analytical application, and synthesis of several zirconium compounds such as zirconium (IV) tungstomolybdate, zirconium (IV) tungestoiodophosphate, zirconium (IV) iodovanadate, zirconium (IV) selenomolybdate, zirconium titanium phosphate, zirconium (IV) zirconium (IV) iodotungstate, zirconium (IV) antimonotungstate and zirconium (IV) tungsto-vanadate were investigated extensively [9–12]. These inorganic ion exchangers were characterized by their ion exchange capacity and selectivity and played a prominent role in water processing in the chemical and nuclear industries [13]. This new material investigated high ability, stability and high ion exchange capacity at raised temperature contrasted with the other formerly investigated zirconium-based hetero-polyacid cation exchange material such as zirconium tungstophosphate, zirconium tungstatephenolate, zirconium iodovanadate and zirconium arsenovanadate [13].

In this regard, zirconium tungsto-vanadate as a novel cation exchanger will be synthesized in different morphological nanostructures using sol–gel, hydrothermal and microwave techniques. To the best of my knowledge, this is the first research that deals with the zirconium tungsto-vanadate architecture at various morphological structure. The influence of preparation conditions variation on zirconium tungsto-vanadate particle size distribution and shape in presence of polymeric matrix as stabilizing agent will be monitored as an attempt to attain nanopowder cation exchange material characterized by its high ion exchange capacity and selectivity toward both the lead and strontium ions remediation from polluted water.

2Material and methods2.1Synthesis of nano-zirconium tungsto-vanadate via sol–gel, hydrothermal and microwave techniques

Nano-zirconium tungsto-vanadate in different morphological structures was synthesized via sol–gel, hydrothermal and microwave techniques. As predominated preparation methodology for the three different techniques, add simultaneously two different solutions from 75ml ammonium metavanadate and 75ml sodium tungstate drop wise into 150ml zirconium oxy chloride solution in different molar ratios in presence of HCl with continuous stirring at different reaction temperatures. In case of hydrothermal preparation technique; the reactants were directly mixed and reacted under fixed pressure at temperature of 100°C using autoclave (Systec 3850-EL) for 1h [14]. However, for the microwave technique, the solution reaction mixture aged into a microwave at different reaction temperatures (around 100°C) for 30min. The reaction mixture was diluted to 1l and allowed to settle for 24h for complete digestion. After the digestion period, the supernatant liquid was decanted, and gels were filtered and washed. The yielded slurry was dried at 60°C, then grounded and immersed in 1M nitric acid for 1 day to be transformed to its hydrogen form. As an attempt to attain nanorod or nanotube morphological structure for the prepared material, the influence of the stabilizing agents’ type that presence during the preparation process on both the material ion exchange capacity and morphological structure was examined. Polyvinyl alcohol (PVA), poly vinyl pyridine (PVP), tri-ethanol amine (TEA) and polyethylene glycol (PEG) are the selected stabilizing agents. The molarity of the utilized surfactant is equal to 1/10M from zirconium oxy chloride solution. Moreover, the effect of variation on the reactants molar ratios, the amount of HCL and reaction temperature on the material properties was determined. All prepared samples were screened based on samples ion exchange capacities to determine the most proper sample produced from each technique that poses the highest values. Also, the specific surface area of each prepared material was determined to confirm the estimated IEC value of the prepared materials.

2.2Ion exchange capacity (IEC) of the prepared nano-zirconium tungsto-vanadate

The hydrogen contents in each prepared zirconium tungsto-vanadate material were evaluated using ion-exchange capacity methodology. This method is based on acid-base titration through mixing 0.5g from the prepared material in its H+ form with 50ml of 1M NaCl solution under vigorously stirring at room temperature overnight. The material ion exchange protons will be exchanged with sodium ions and liberated in the solution. The solution mixture was titrated against a standard solution of 0.1M NaOH using phenolphthalein as indicator to determine the liberated hydrogen ions from the material. The ion exchange capacity (IEC) can be calculated using the following formula:

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.

2.3Characterization of the prepared nano-zirconium tungsto-vanadate

To select the most proper prepared material produced from each preparation technique, the specific area values of the different prepared materials were determined. The crystalline, thermal and the morphological structures of the most proper prepared materials produced from each preparation technique were established.

2.3.1Surface area (BET)

One of the most important nanomaterials characteristics especially for the ion exchange materials is their specific surface area. Accordingly, the different prepared zirconium tungsto-vanadate samples will be screened based on their surface area and ion exchange capacity to select the most proper prepared sample produced from each technique. Whereas the material specific surface area is related to its particle size, so, based on the nitrogen chemisorption and physisorption analyzer (Beckman Coulter AS3100, USA), the different materials surface areas were determined after samples out gassed at 200°C for 180min. The BET surface area was calculated from the adsorption isotherm data.

2.3.2X-ray diffraction (XRD)

XRD patterns of best prepared cation exchange material produced from each technique were recorded according to step screening procedures using Schimadzu-7000 diffractometer with CuKα radiation beam (λ=0.154060nm) to determine the crystalline structure of the materials. The powdered samples were packed into a flat aluminum sample holder, where the X-ray can be operated at 30kV and 30mA with a copper target. Scans are performed at 4°min−1 for 2θ values between 10 and 80°.

2.3.3Thermal properties (TGA)

The thermal stability of the most proper three prepared samples was compared using thermal gravimetric analysis (Shimadzu TGA-50 instrument). The TGA curves of the prepared materials as a function of temperature were acquired through adding the predetermined weight from the dried powder material at the aluminum pan and subjected to thermal heating rate of 20°C /min that started from ambient condition up to 800°C under N2 atmosphere.

2.3.4Morphological characterization (SEM)

The prepared powder materials were stocked onto holder and were gold sputtered prior their examination. The samples were scanned to identify morphology of the prepared samples and estimate the particle size at different magnifications 500,00× and 10,000×.

2.3.5Assessment of the prepared nano-zirconium tungsto-vanadate materials for water remediation

The affinity of the optimum three zirconium tungsto-vanadate nanomaterials for lead and strontium ions decontamination from aqueous solutions was compared. 100ml solutions at various initial concentrations from synthetic waste solution contaminated with either lead ions or strontium ions were agitated vigorously with 0.2g from the selected prepared cation exchange material in its H+ form for 2h. The reaming concentration of ions contamination (lead or strontium) presence at the waste solution after the treatment process was measured using inductive coupled plasma mass spectrophotometer (ICP-AES). The removal efficiency was estimated from the following equation:

where Ci is the initial metal ion concentration (ppm) and Ce is the concentration of metal ions (ppm) at equilibrium.

3Results and discussion3.1Influence of surfactant type onto zirconium tungsto-vanadate properties

As an attempt to prepare nano-zirconium tungsto-vanadate in different morphological structures, the influence of different surfactant polymers onto the ion exchange properties of the produced material will be tested at the predetermined optimum conditions for each technique. Different zirconium tungsto-vanadate samples were prepared using 1M ZrOCl2, 0.5M sodium tungstate and 1M ammonium metavanadate in presence of 0.01M HCl at 25°C in presence of different stabilizing agents (PVA, PVP, PEG and TEA). Table 1 investigated the sample production yield and its ion exchange capacity.

Table 1.

Effect of surfactant type onto the percentage material yield and IEC for all prepared samples produced from three different techniques.

Preparation technique  Sample  Surfactant type  % production yield  IEC (meq/g) 
Sol–gelS1  PVA (Mwt.72000)  57.94  3.02 
S2  PVP (Mwt.35000)  52.93  2.52 
S3  TEA (Mwt.149)  50.38  1.32 
S4  PEG (Mwt.400)  51.71  1.46 
HydrothermalH1  PVA (Mwt.72000)  58.83  3.43 
H2  PVP (Mwt.35000)  53.78  3.27 
H3  TEA (Mwt.149)  52.67  3.18 
H4  PEG (Mwt.400)  51.83  3.22 
MicrowaveM1  PVA (Mwt.72000)  67.83  3.62 
M2  PVP (Mwt.35000)  65.78  3.38 
M3  TEA (Mwt.149)  64.67  3.31 
M4  PEG (Mwt.400)  53.47  2.97 

It was indicated from Table 1 that all microwave prepared samples recorded the highest ion exchange capacities and production yield compared with sol–gel and hydrothermal techniques. This result is in accordance with the expected results, where, it is well known that microwave dielectric heating not only enhances the rate of nanomaterials formation, it also enhances the material homogeneity production and its size distributions [15].

Fig. 1 that illustrates the surface area of the prepared materials confirms the previously stated results, where the microwave prepared samples pose the highest specific areas compared with the other two preparation techniques. Generally, for each preparation technique, it was evident that the samples prepared in presence of polyvinyl alcohol (S1, H1 and M1) pose high ion exchange capacities compared with that prepared with other stabilizing polymers at the same preparation conditions. These materials ion exchange capacities are related to their specific surface area that illustrated in Fig. 1. Where, it was evident from this figure that highest surface area values were demonstrated at 516, 473 and 446m2/g for samples M1, H1 and S1 respectively. These results may be returned to the high molecular weight of the utilized poly vinyl alcohol (PVA) as stabilizing agent that act as a good dispersing agent during zirconium tungsto-vanadate synthesis that improves the surface area of the prepared material through the reduction of its nanosize. Accordingly, PVA was the optimum stabilizing polymer for nano-zirconium tungsto-vanadate production using any studied preparation technique. Moreover, the microwave production technique is most proper technique for nano-zirconium tungsto-vanadate production rather than sol–gel and hydrothermal techniques [16].

Fig. 1.

Effect of stabilizing agent type on specific surface area of zirconium tungsto-vanadate.

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3.2Influence of reactants molar ratio onto zirconium tungsto-vanadatetungsto-vanadate properties

The influence of reactant molar variation onto zirconium tungsto-vanadate production yields, IEC and surface area in presence PVA as the optimum predetermined stabilizing agent was investigated in Table 2 and Fig. 2. It was obvious for the three preparation techniques that only the increment in zirconium concentration in the reaction mixture enhances both the ion-exchange capacity and the specific surface area of the prepared materials. In contrast to the increment at both vanadate and tungstate reactant concentrations that have negative impact on the IEC of the prepared samples [17]. However, the material production yield was improved with the increment at zirconium, tungsten or vanadate precursor's concentrations. These results may be owed to for high tungstate and vanadate concentrations studied, the equivalent amounts from both tungstate and vanadate were reacted with zirconium precursor to produce zirconium tungsto-vanadate molecules. The remaining unreacted excess sodium vanadate and sodium tungstate salts have tendency to react with the presence PVA stabilizing agent according to the following formulas [18].

Table 2.

Effect of reactant molar ratio onto the percentage material yield and IEC for all prepared samples produced from three different techniques.

Preparation technique  Sample  Reaction molar ratio (Zr:V:W)  % production yield  IEC (meq/g) 
Sol–gelS5  1:1:1  61.283  2.6 
S6  1:0.5:1  57.94  3.02 
S7  0.5:1:1  57.965  1.83 
S8  1:1:2  72.458  1.21 
S9  1:2:1  70.140  0.92 
HydrothermalH5  1:1:1  63.444  2.81 
H6  1:0.5:1  58.83  3.43 
H7  0.5:1:1  57.239  1.32 
H8  1:1:2  71.579  1.46 
H9  1:2:1  74.11  1.27 
MicrowaveM5  1:1:1  78.93  2.5 
M6  1:0.5:1  67.83  3.62 
M7  0.5:1:1  70.38  1.47 
M8  1:1:2  83.71  1.44 
M9  1:2:1  82.019  1.7 
Fig. 2.

Effect of reactant molar ratio on specific surface area of zirconium tungsto-vanadate.

(0.11MB).

where the dissolved vanadate and tungstate ions in solution serve as a cross linking agents for PVA generating new gel polymeric structure matrices as indicated from the reaction equations. The suggested mechanism for the gel polymer may be regarded to the hydrolysis of Na2VO4 and Na2WO4 in the solution to generate V(OH)4− and W(OH)4− ions respectively. These ions will be reacted with the OH groups on PVA to generate gel polymeric structure matrices [19]. Accordingly, the produced material in case of high tungstate and vanadate concentrations is composed from a mixture of zirconium tungsto-vanadate and the gel polymeric matrices that explain the reason for the decline at IEC of the produced materials at these reaction conditions. Moreover, this explanation investigates the main reason for increasing the material production yield with the improvement at both tungstate and vanadate reactants concentrations. Where, as the tungstate and vanadate reactant concentrations incremented, the amount of produced polymeric matrices that mixed with the produced zirconium tungsto-vanadate was increased that increase the overall production yield.

Accordingly, the most proper samples from each preparation techniques are S6, H6, M6, which are produced at the optimum reaction molar ratios of 1:0.5:1 of zirconium:vanadium:tungsten. Furthermore, Table 2 evident that the microwave prepared sample M6 attains the highest IEC value. This result was compatible with Fig. 2, where the highest surface area was recorded for microwave prepared sample compared with the hydrothermal and sol–gel prepared samples. Finally, it was induced the high impact of production zirconium tungsto-vanadate using microwave technology.

3.3Influence of reaction temperature onto zirconium tungsto-vanadate properties

The effect of gelatin temperature on the material properties was determined using the predetermined optimum reactants amounts of 1M ZrOCl2, 0.5M sodium tungstate and 1M ammonium metavanadate in presence of 0.2M HCl and PVA as stabilizing agent maintaining heating for 90min. Table 3 explored that both the materials production yield and the IEC of the produced ion exchangers were increased with increasing reaction temperature. The improvement at the materials ion exchange capacity is followed by the increment at their surface area with increasing reaction temperature as indicated from Fig. 3.

Table 3.

Effect of reaction temperature onto the percentage material yield and IEC for all prepared samples produced from three different techniques.

Preparation technique  Sample  Reaction temperature (°C)  % production yield  IEC (meq/g) 
Sol–gelS10  25  57.94  3.02 
S11  40  62.31  3.28 
S12  60  63.9  3.44 
S13  75  67.15  3.51 
S14  85  69.12  3.66 
HydrothermalH11  25  58.83  3.43 
H12  40  62.71  3.61 
H13  60  64.12  3.78 
H14  75  69.9  3.82 
H15  80  70.14  3.9 
MicrowaveM11  25  67.83  3.62 
M12  40  70.31  3.9 
M13  60  74.28  4.26 
M14  75  76.1  4.6 
M15  80  80.2  4.67 
Fig. 3.

Effect of reaction temperature on specific surface area of zirconium tungsto-vanadate.

(0.11MB).

These results may be returned to the scientific fact of improving the reaction temperature which accomplished with increasing the rate of reactant particles diffusion that by its role increases the material production yield. Furthermore, the increment at the reaction temperature increases the momentum force for each reacted molecule and improves the dispersion degree of produced zirconium tungsto-vanadate particle onto the PVA chains that act as stabilizing agents and decline the nanosize of the produced particles [20]. Moreover, it was evident from Table 3 that the increment at the reaction temperature above 60°C is not compensated by the IEC improvement at the materials for the three studied reaction techniques. Accordingly, respecting to the economical wise for nano-zirconium tungsto-vanadate production, the accomplishment of nano-zirconium tungsto-vanadate production process at 60°C is favorable to produce ion exchange material with high IEC and surface area [21]. Finally, the microwave production technique was proved to be the better process for nano-zirconium tungsto-vanadate production either with or without heating.

3.4Influence of hydrochloric acid concentration onto zirconium tungsto-vanadate properties

Concerning the vital role of hydrochloric acid presence at the reaction media during zirconium tungsto-vanadate preparation, where HCl is responsible for the amounts of replacement hydrogen ions (H+) inside the produced cation exchange matrices. Accordingly, the influence of variation HCl concentration at the reaction media using the predetermined optimum preparation conditions onto the ion exchange capacity, material production yield and surface area of prepared zirconium tungsto-vanadate was indicated (Table 4 and Fig. 4) respectively.

Table 4.

Effect of HCl concentration onto the percentage material yield and IEC for all prepared samples produced from three different techniques.

Preparation technique  Sample  HCl concentration (M)  % production yield  IEC (meq/g) 
Sol–gelS15  61.7  3.2 
S16  0.01  63.9  3.44 
S17  0.05  64.2  3.54 
S18  0.1  63.8  3.6 
S19  0.2  63.72  3.64 
HydrothermalH15  60.89  3.36 
H16  0.01  64.12  3.78 
H17  0.05  65.7  3.95 
H18  0.1  65.9  4.1 
H19  0.2  64.8  4.34 
MicrowaveM15  66.94  3.6 
M16  0.01  74.28  4.26 
M17  0.05  75.1  4.7 
M18  0.1  75  4.82 
M19  0.2  74.7  4.97 
Fig. 4.

Effect of HCl concentration on specific surface area of zirconium tungsto-vanadate.

(0.11MB).

It was established from the table that the ion exchange capacity and the surface area of the produced ion exchanger materials were affected strongly with the HCl concentration especially for the hydrothermal and microwave preparation techniques. These results were confirmed from Fig. 4 that investigated the high impact of HCl concentration on the produced materials surface area for these two preparation techniques. However, the HCl concentration showed mild influence on both the IEC and surface area of produced materials using sol–gel technique. This may be regarded to the reaction at sol–gel technique is takes place under electrical heating only, however, at the hydrothermal technique the reaction is subjected to both heating and pressure.

Moreover, at the microwave preparation technique the reaction is occurred under the microwave radiation that enhances the influence of heating conditions in presence of HCl. Generally, there is no tangible variation change at the production yield of materials with the variation at HCl concentration for the three studied preparation techniques. Nevertheless, it was evident from Table 4 that as HCl concentration increased; the IEC of the produced cation exchange material from each preparation technique was improved.

This behavior agrees with the excepted results, where, the presence of HCl contributed in increasing the replacement hydrogen ions that present inside the produced ion exchangers structure [22]. Despite the IEC of the prepared materials is proportional to the increment at the HCl concentration. However, the surface area of the prepared materials is non-convenient with the increment at the HCl concentration as evident from Fig. 4. For all studied preparation techniques, it was indicated that the increase at HCl concentration up to 0.05M is associated with harmony improvement at both the materials IEC and surface area. As the HCl concentration increased above 0.05M, only the materials IEC were improved comparatively to their surface area that showed a dramatically decrease with HCl concentration. The decline at the materials surface area with the increment at HCl concentration above 0.05M may be owed to the presence of PVA at the reaction media as stabilizing agent. The excess of HCl above 0.05M may tend to react with PVA and destroy its chain. The decline at the PVA amounts presence at the reaction media will increase the tendency of formed zirconium tungsto-vanadate particles for aggregation that increase its particle size and decrease the material surface area [23].

The highest IEC value for prepared zirconium tungsto-vanadate produced from each preparation technique was recorded using 0.05M HCl in the preparation media. Microwave preparation technique represents the promising technique for nano-zirconium tungsto-vanadate production with the highest surface area of 671m2/g and IEC value equivalent to 4.7mg/g.

3.5Characterization of the most proper prepared nano-zirconium tungsto-vanadate produced from each technique3.5.1Crystalline structure of the prepared materials (XRD)

X-ray diffraction patterns of the most proper prepared cation exchange materials (S17, H17 and M17) produced from the three studied preparation techniques that attain high ion exchange capacity values and specific surface areas were recorded at Fig. 5. The X-ray diffraction pattern of the prepared ion-exchange material from sol–gel technique (S17) shows a limited number of characteristics peaks at different 2θ values of 17.7° and 43.1°. The intensities of these peaks are extremely low; these give prediction that the sol–gel prepared material may have semi-crystalline structure. However, the number of the materials characteristic peaks was improved for both the hydrothermal and microwave prepared samples (H17 and M17) to be attributed as a polycrystalline material compared with the sol–gel prepared sample. Where, their XRD patterns exhibited strong diffraction peaks at 43.1°, 51.6°, 65.97° and 78.67° that indicating well defined zirconium tungsto-vanadate.

Fig. 5.

XRD patterns of zirconium tungsto-vanadate cation exchange material prepared using sol–gel (S17), hydrothermal (H17) and microwave (M17) techniques.

(0.2MB).

All characteristics peaks of the three prepared materials are in good agreement with the standard spectrum (JCPDS no.: 01-087-1528 and 01-088-0586). These results elucidated that the three prepared materials are composed from zirconium tungsten oxide and zirconium vanadium oxide mixture with cubic crystal configurations but with different degree of crystallinity. As evident from Fig. 5, the crystallinity degree of zirconium tungsto-vanadate samples produced from both hydrothermal and microwave techniques is larger than the comparable sample produced from sol–gel technique. This behavior may be regarded to the harsh preparation conditions of the high reaction temperature and pressure at the hydrothermal technique and the effect of the microwave radiation at the microwave technique that improves the degree of crystallinity and orientation of the samples of H17 and M17 respectively compared with sample S17 that produced at the mild conditions [24].

3.5.2Morphological structure of the prepared materials (SEM)

As an attempt to architecture zirconium tungsto-vanadate at different morphological structures, the SEM photographs of the most proper prepared cation exchanger samples were compared at Fig. 6. This figure indicates the essential role of variation the preparation technique in presence of PVA as stabilizing agent at zirconium tungsto-vanadate architecture. It was investigated that the sol–gel prepared sample (S17) was architecture the material at nanospherical particles with average diameter approximately 60nm. However, the other two preparation techniques of hydrothermal and microwave (H17 and M17) were architecture the material at nanorod morphological structures. The nanorod morphological structure of microwave prepared sample is more uniform and homogeneous compared with the hydrothermal prepared sample. Where, it was indicated from Fig. 6 that the hydrothermal prepared sample was produced as a mixture of nanorods and spherical nanoparticles morphological structures. Moreover, the average aspect ratio of microwave prepared sample that equal to 12 is larger than that prepared from the hydrothermal technique. This improvement at the nanorod aspect ratio may be owed to the action of microwave radiation in presence of the high molecular weight PVA at the preparation media that act as stabilizing agent. Where, the microwave radiation beams may force the initiated nano-zirconium tungsto-vanadate molecules to be arranged and propagated at the nanorod morphological structure with the assistance of the high molecular weight PVA matrix [25]. These results are in accordance with the materials surface area results, where the sequence of the materials specific surface area follows the order sol–gel<hydrothermal<microwave. This order is adequate to the materials morphological structures.

Fig. 6.

SEM images of zirconium tungsto-vanadate cation exchange material prepared using sol–gel (S17), hydrothermal (H17) and microwave (M17) techniques.

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3.5.3Thermal properties of the prepared materials (TGA)

In order to compare the thermal properties of the most proper three prepared zirconium tungsto-vanadate nanomaterials; Fig. 7 investigated their thermal profiles over the studied temperature range 25–800°C. It was indicated that the three thermo-graphs accomplish three main degradation steps. The first degradation step at the three samples that almost ended around 150°C represents the surface water adsorbed onto the material [26]. The average samples weight losses at this temperature range for the three prepared nanomaterials not exceeded than 16% from the material weight. The second thermal degradation step at the three samples that began above 150°C and ended around 185°C may be stand for the PVA degradation [27]. The maximum samples weight losses regarding to this thermal degradation step for the three studied samples is 6% from the initial material weight. This result gives prediction about the remaining of PVA matrix as very small amounts at the prepared zirconium tungsto-vanadate. The third and final thermal degradation step at the three prepared samples that started above 150°C up to 450°C is owed to the interstitial water molecules at the materials that give idea about the material ion exchange capacity [28].

Fig. 7.

TGA profiles of zirconium tungsto-vanadate cation exchange material prepared using sol–gel (S17), hydrothermal (H17) and microwave (M17) techniques.

(0.4MB).

Comparing the percentage material losses respecting to this degradation step, it was evident that the values are almost equivalent to the materials ion exchange capacity values. The percentage weight losses follow the order sol–gel prepared sample (4.27%)<hydrothermal prepared sample (5.29%)<microwave prepared sample (6.14%). This sequence is obeyed the materials IEC values of sol–gel prepared sample (3.54meq/g)<hydrothermal prepared sample (3.95meq/g)<microwave prepared sample (4.7meq/g). Generally, the TGA thermographs of the three-prepared zirconium tungsto-vanadate nanomaterials confirm their thermal stabilities up to 800°C. However, the sol–gel prepared sample showed very small degradation step above 450°C, this step give prediction about its less thermal stability compared to the hydrothermal and microwave prepared samples. This may be regarded to its semi-crystalline structure compared with the crystalline structures of the other remaining samples.

3.5.4Assessment of the prepared nano-zirconium tungsto-vanadate materials for water remediation

The ion exchange efficiency of the most efficient prepared samples produced from each preparation technique, which are pose the highest IEC values and specific surface areas, toward lead and strontium ions sorption was tested separately (each ion presence as individual waste pollutant at the waste solution) over a wide range concentration from the two ions. It was indicated from Fig. 8 that the microwave prepared sample (M17) poses the highest lead and strontium sorption capacities compared with both the sol–gel and hydrothermal samples at all studied initial ions concentrations. Where, the microwave prepared sample (M17) recorded 64.9mg/g lead sorption capacity compared with 44mg/g for the sol–gel prepared sample (S17) at 100ppm initial lead concentration. Respecting to the strontium ion sorption onto the prepared zirconium tungsto-vanadate samples, it was indicated that sample M17 poses 44.2mg/g strontium sorption capacity compared with 35mg/g accomplished using sample S17 at 100ppm initial strontium concentration. Accordingly, the most proper prepared zirconium tungsto-vanadate sample is represented as the microwave produced sample. This result agrees with the expected results. Where, zirconium tungsto-vanadate produced from the microwave technique in nanorod morphological structure was characterized by its high IEC and surface area. Moreover, it was evident that all the three studied samples attain higher lead sorption affinity compared with their strontium sorption affinity. Where, the microwave prepared sample poses 64.9mg/g lead sorption capacity compared with 44.2mg/g strontium sorption capacity for metal ion sorption with initial concentrations of 100ppm at the same contact time of 120min.

Fig. 8.

Sorption behavior of lead and strontium ions onto three different zirconium tungsto-vanadate samples.

(0.21MB).

Accordingly, it was obvious that the prepared zirconium tungsto-vanadate materials attain higher lead ions up take than strontium ions. This result may be owed to the correlation between the hydrated ionic radii of the polluted ions either lead or strontium and the material pore sizes [29].

4Conclusions

Zirconium tungsto-vanadate in nanorod morphological structure was prepared successfully at reactant molar ratio equal to 1:0.5:1 for zirconium:vanadium:tungsten respectively at 60°C using microwave technique in presence of 0.05M HCl and PVA as stabilizing agent. However, zirconium tungsto-vanadate with nano-spherical structure was produced using sol–gel technique at the preparation conditions. The microwave prepared sample characterized by its high specific surface value that equivalent to 671m2/g compared with both the hydrothermal and sol–gel prepared samples, which attain 545 and 476m2/g respectively. Regarding to the high surface area of microwave prepared sample, it recorded the highest ion exchange capacity value of 4.7meq/g compared with the other two prepared samples. XRD results evident that the crystallinity degree of the microwave and hydrothermal prepared samples is larger than the sol–gel prepared sample that produced has polycrystalline structure. The thermal analysis results of the three prepared samples produced from each technique indicated their thermal stability up to 800°C. Accordingly, it was concluded that the microwave technique is much favored over the hydrothermal and sol–gel techniques for production nano-zirconium tungsto-vanadate cation exchanger with good properties.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgement

This work was supported by the Egyptian Science and Technology Development Fund (STDF) (grant no. 30735).

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