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Soft Template Assisted Synthesis of Zirconium Resorcinol Phosphate Nanocomposite Material for the Uptake of Heavy-Metal Ions Arshid Bashir, Sozia Ahad, and Altaf Hussain Pandith* Department of Chemistry, University of Kashmir, Hazratbal, Srinagar-190006, Kashmir, India S Supporting Information *

ABSTRACT: Herein, we report the low temperature, template directed synthesis of zirconium resorcinol phosphate (ZrRP) nanocomposite material, within water-in-oil microemulsion with Tergitol-7 as a surfactant. The material was characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), simultaneous thermogravimetric analysis (TGA), differential thermal analysis (DTG), energy dispersive X-ray spectrometry (EDX), and specific area electron diffraction (SAED) studies. The powder X-ray diffraction studies revealed that the material is amorphous in nature with hardly developed crystallinity. SEM and TEM micrography studies showed that the ZrRP nanocomposite has nearly spherical morphology with average particle size of 30−40 nm. Viscoelastic behavior of ZrRP gel confirms its non-Newtonian behavior which is indicative of monodisperse nature of ZrRP nanoparticles. ZrRP possesses high ion exchange capacity of 2.9 mequiv g−1 for Sr2+ ion and can have potential application in radionuclide recovery from nuclear wastes. Distribution coefficients are high for Cd2+ ion (1.3 × 104 mL g−1) and Ni2+ ion (6.1 × 103 mL g−1) in 0.01 M perchloric acid medium. High distribution coefficient value of ZrRP toward Cd2+ ions was used for its quantitative separation from Zn2+ and Ni2+ ions. ion adsorbent21,22 and solid acid catalyst.23 With the advent of nuclear technology more focus was turned toward the synthesis of organic−inorganic hybrid materials which amalgamates the properties of both organic resins and inorganic based materials. This amalgamation aims in enhancing the adsorption behavior of hybrid materials such as their thermal stability, stability toward ionizing radiations, and better ion exchange properties. Zirconium phosphate proves to be the best host layered material for the synthesis of organic−inorganic ionexchange materials. A large number of zirconium phosphate based hybrid materials using organic as well as inorganic dopents such as poly o-toluidine,24 acrylonitrile,25 polyacrylamide,26 polyaniline,27,28 aluminum(III),29,30 sulfonic acid,31 polymethyl acrylate,32 nylon66,33 and L-(+)-phosphoserine34 have been reported in last two decades. These materials have been used in separation, purification and detection of heavy metal ions and environmentally hazardous dyes. Recently, many new adsorbent and ion exchange materials have been developed for the removal of heavy metal ions such as lead, cadmium, nickel and zinc from waste waters.35−37 However, to our best information, no major works has been reported in literature regarding the preparation of nano sized

1. INTRODUCTION Owning to the extremely small sizes and large specific surface area of nanoparticles, the physical and chemical properties of the nanoparticles are different from those of the bulk material. They have been widely used in the areas of catalysis,1,2 optoelectronic materials,3,4 high quality magnetic materials,5 biomedical engineering,6 and environmental remediation.7 Several methods have been used for the synthesis of nanomaterials such as, the hydrothermal method,8,9 solvothermal method,10 vapor phase synthesis,11 sol−gel method,12 pyrolysis by microwave,13 thermal decomposition method,14 and microemulsion method.15−17 However, water-in-oil reverse micelles, also called intelligent nanoreactors, are of particular interest in fabricating nanoparticles. The water-in-oil microemulsion consists of nanosized water droplets that are dispersed in a continuous oil medium and stabilized by a surfactant molecule giving rise to a reverse micelle with nano-sized aqueous core. These reverse micelles are indeed subjected to continuous Brownian motion, resulting in collisions and coalescence of droplets. This ensures the rapid exchange and redistribution of the solubilized water pool content among the reverse micelles. This dynamic process between droplets enables reverse micelles to be used as nanoreactors for synthesis of nanoparticles.18 So far, numerous zirconium based materials have been synthesized within water-in-oil reverse micelles like, zirconia,18 sulfated-zirconia,19 and inorganic zirconium phosphate.20 Zirconium phosphate nanocomposite acts as an efficient metal © 2016 American Chemical Society

Received: Revised: Accepted: Published: 4820

January 16, 2016 March 13, 2016 April 4, 2016 April 4, 2016 DOI: 10.1021/acs.iecr.6b00208 Ind. Eng. Chem. Res. 2016, 55, 4820−4829

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Industrial & Engineering Chemistry Research

the capacity for heavy metal up-take in different solvent systems.

hybrid ion-exchangers as adsorbents by reverse microemulsion route, for selective removal of heavy toxic metal ions from wastewater. Cadmium is considered to be highly toxic element and responsible for several cases of poisoning through water, food, and smoking. When excessive amounts of cadmium are ingested, it replaces zinc at key enzymatic sites, causing kidney damage, renal dysfunction, anemia, hypertension, bone marrow disorders, cancer, and toxicity to aquatic biosystems. Cadmium is released into the atmosphere from smelters and factories processing cadmium and also from the incineration or disposal of cadmium-bearing products. Cadmium enters natural water through industrial discharges mainly from electroplating industry and nickel−cadmium galvanized water pipes. Cadmium is, therefore, a potential pollutant in the environment. Many researchers have developed materials for the selective removal of Cd(II)38,39 from wastewater. The utility of hybrid composite ion exchangers has been explored for adsorption and the quantitative separation of Cd(II) from some binary mixtures on its column. The present study aims at the synthesis, characterization, and employment of zirconium resorcinol phosphate (ZrRP) nanocomposite material for the removal of heavy metal ions viz., Cd(II), Ni(II), Pb(II), and Zn(II) from aqueous solution. We have employed the facile room temperature sol−gel synthetic route for intercalation of resorcinol within the matrix of inorganic zirconium phosphate with an objective to prepare a more efficient and ion-selective ion-exchange material, incorporating the best qualities of both the species, for useful applications in environmental analysis. The material was characterized by using Fourier transform infrared (FT-IR), powder X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDX), thermal analysis (TGA-DTG), scanning electron microscopy (SEM), transmission electron microscopes (TEM), and specific area electron diffraction (SAED) to identify various functional groups, surface morphology, thermal stability, particle size, and ion-exchange sites present in the material. Distribution studies were performed on the synthesized material so as to determine the selectivity and

2. MATERIALS AND METHODS 2.1. Reagents and Chemicals. The main reagents used for the synthesis ZrRP nanocomposite were zirconium oxychloride (Merck Germany), orthophosphoric acid (Merck Germany), resorcinol (Fischer Scientific UK), cyclohexane (Spectrochem India), 1-octanol (Spectrochem India), and Tergitol-7[C11−15H23−31O(CH2CH2O)xH] (Himedia India) . All the chemicals were of AR grade and were used as purchased. 2.2. Synthesis of ZrRP Nanocomposite Using Tergitol-7 Based Microemulsion. At room temperature ZrRP nanocomposite was synthesized by using microemulsion method with Tergitol-7 as the nonionic surfactant, 1-octanol as cosurfactant, cyclohexane as continuous organic oil phase, and a nano-sized water pool as site for the reaction. ZrRP nanoparticles were synthesized by rapid mixing of three separate microemulsion designated as A, B, and C. Each microemulsion contained 16.7 mL of Tergitol-7, 13.9 mL of 1-octanol, and 59 mL of cyclohexane. Then, 10.5 mL of 0.1 M aqueous zirconium oxychloride (prepared in 0.01 M HCl), 10.5 mL of 0.1 M aqueous orthophosphoric acid, and 10.5 mL of 0.05 M aqueous resorcinol were solublized, respectively, in the three microemulsions, maintaining the chemical composition of reverse microemulsion as 16.7:13.9:59:10.5 for Tergitol:1-octanol:cyclohexane:water pool content, respectively.40 In this synthesis, reverse micelle acts as soft templating agents for the synthesis of ZrRP nanocomposite. The final clear mixture was aged at room temperature without stirring for 24 h; during this process the droplets continuously collide which result in exchange of reactants inside the nanoreactor and hence formation of ZrRP nanocomposite. The product was separated from microemulsion by centrifugation and then the sample is washed several times with 1:1 mixture of acetone and ethanol. The sample was then dried in oven at 50 °C for 2 h. The material was converted into its H+ form by treating it with 1 M HNO3 for 24 h and was finally washed with demineralized water (DMW) and dried at 50 °C. Schematically the synthesis of ZrRP nanocomposite is shown in (Scheme 1).

Scheme 1. Template Assisted Synthesis of Zirconium Resorcinol Phosphate Inside Reverse Micelle

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DOI: 10.1021/acs.iecr.6b00208 Ind. Eng. Chem. Res. 2016, 55, 4820−4829

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Industrial & Engineering Chemistry Research 2.3. Characterization Methods. FT-IR spectra of the sample were recorded by KBr disc method by using PerkinElmer (1730, USA) spectrometer. Crystallinity and structure were determined by powder X-ray diffraction (XRD) using Bruker AXS D8 Advance diffractometer with Cu Kα radiation. Simultaneous TGA-DTG curves were recorded using Exstar 6000 TGA-DTG from SIINT, Japan, with a heating rate of 10 °C min−1 under nitrogen flow. Elemental analysis of sample was done by energy dispersive X-ray spectrometry (EDX) operated at 200 kV using a JEM 2100F (UHR) instrument. Surface morphology of exchange material was characterized by scanning electron microscopy (SEM; Hitchi-S3000H), transmission electron microscopy (TEM; JEOL-JEM-2100F) of the material was carried out to determine the particle size, and selected area electron diffraction (SAED; JEOL-JEM-2100F) was carried out to determine the crystallinity of the material. The Brunauer−Emmett−Teller (BET; Micromeritics ASAP −2020) method was utilized to evaluate the surface area of the ZrRP nanoparticles. 2.4. Rheological Characterization of ZrRP Dispersion. The rheological characterization of the dispersions was performed with an Anton Paar Rheometer MCR102 at three different time intervals of 10 min, 6 h, and 12 h at 25 ± 0.01 °C. Viscosity, storage modulus and loss modulus were measured by cone and-plate system with a diameter of 50 mm and a cone angle of 1.006°. Non-Newtonian behavior of the ZrRP nano dispersion was confirmed from shear stress and steady shear viscosity as a function of shear rate. In oscillatory experiments, viscoelastic property was determined by investigating the influence of angular frequency on storage and loss modulus. 2.5. Ion Exchange Properties. The ion exchange capacity is the measure of the number of replaceable H+ ions per unit mass of the exchanger. The ion exchange capacity was determined by the usual column process by taking 1 g of the exchanger (H+ form) in a column of internal diameter −1 cm, fitted with sintered disc at the bottom. A 250 mL NaNO3 (1 M) solution was passed through it, and a very slow flow rate was maintained (0.5 mL min−1). The effluent was titrated against a standard 0.01 M NaOH solution using phenolphthalein as indicator to determine the total H+-ions liberated during the ion exchange process 2.6. Elution Behavior. The column containing 1 g of the ZrRP in the H+ form was eluted out with 1 M NaNO3 solution (100 mL), having a standard flow rate of 0.5 mL min−1. Several 10 mL fractions of the effluent were collected and checked for H+ ion concentration tritrimetrically using standard alkali solution. Almost all of the H+ ions were found to be released by the column within the first 70 mL of the effluent. The results are shown in Figure S1 (Supporting Information) 2.7. Regeneration of the Ion Exchange Material. The material was regenerated by keeping it overnight in 1 M HNO3. It was washed with demineralized water until it attained neutral pH, and the ion exchange capacity was determined by the same procedure as described above. 2.8. Distribution Studies. Distribution studies were carried out by batch method, in which 200 mg of the ion exchange material was taken in each flask to which 30 mL of different metal nitrate solutions in the required medium were added. After continuous shaking for 6 h, the flasks were kept for 24 h in a temperature controlled incubator shaker at 25 ± 1 °C to attain equilibrium. The amount of metal ions, Ni(II), Cd(II), Pb(II), and Zn(II), in the solution before and after equilibrium was determined by titrating against standard 0.001 M EDTA

solution. Distribution coefficients (Kd) is the measure of the fractional uptake of metal ions competing for H+ ions from a solution by an ion exchange material. Distribution coefficients may be calculated as the ratio of the amount of specific metal ions in the exchanger phase and in the solution phase, by using the formula as given bellow: Kd =

C0 − Ce V (mL g −1) C0 M

(1) −1

where Kd is the distribution coefficient (mL g ), C0 is the total amount of the metal ions in the solution initially, Ce is the amount of the metal ions finally left in the solution after attainment of equilibrium, V is the volume of the solution (mL), and M is the mass of the exchange material in grams. 2.9. Quantitative Separations of Metal Ions in Synthetic Binary Mixtures. Binary separations of Ni(II)−Cd(II) and Zn(II)−Cd(II) was achieved on column loaded with ZrRP. 1.0 g of exchanger in H+ form was packed in a glass column (0.6 cm internal diameter) with sintered disc supported at the end. The column was washed thoroughly with demineralized water, and the mixtures of two metal ions were loaded on it and were allowed to pass through the column at a flow rate of 2−3 drops min−1. The process was repeated three times for each run in order to ensure the reproducibility of the results. The separation of metal ions was achieved by collecting the effluent in 10 mL fractions and treated against the standard solution of 0.01 M disodium salts of EDTA.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization. The synthesized nanocomposite material was characterized by various techniques. In order to elucidate the structural architecture and various functionalities of the composite material, we recorded its FT-IR spectrum in solid phase and at room temperature. From the FT-IR spectrum shown in Figure 1a, the peaks are indicative of the presence of the external water molecules in addition to the O−H groups of resorcinol moiety and the metal oxides present internally in the material. The FT-IR spectrum of ZrRP nanocomposite material shows obvious sharp absorption bands at 517 and 608 cm−1 which can be attributed to Zr−O vibrations.41 The absence of a band at 750 cm−1 suggests nonexistence of P−O−P (polyphosphate) like groups in ZrRP.42 An intense band at 1056 cm−1 is attributed to the symmetric stretching vibration of P−O bonds of PO43− moiety.43,44 The absorption band at 1637 cm−1 is assigned to CC stretch of benzene ring.45 The broad band at 3438 cm−1 is ascribed to the stretching mode of O−H bonds due to presence of hydrogen bonded external water molecules and resorcinol moiety.46,47 The weak band at 2927 cm−1 is assigned to C−H stretching vibration of adsorbed surfactant.48 The elemental compositions of as-prepared ZrRP nanocomposite was examined by EDX as shown in Figure 1b . The EDX spectrum of ZrRP nanocomposite material reveals the existence of C, O, P, and Zr elements with the mass percent as 37.18, 35.32, 19.72, and7.78, respectively. These results show that ZrRP were successfully synthesized49 using the proposed facile soft template assisted route. Moreover, no peak related with any impurity or other element is found in the spectrum which is indicative of the purity of synthesized ZrRP nanocomposite.50 In order to assess the thermal stability of the nanocomposite, simultaneous TGA-DTG of the ZrRP was carried out under 4822

DOI: 10.1021/acs.iecr.6b00208 Ind. Eng. Chem. Res. 2016, 55, 4820−4829

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Figure 1. (a) FT-IR spectrum and (b) EDX spectrum of nanocomposite zirconium resorcinol phosphate material.

Figure 2. (a) TGA-DTG curves and (b) X-ray diffraction spectrum (inset: SAED) pattern of nanocomposite zirconium resorcinol phosphate material.

nitrogen atmosphere with a constant heating rate of 10 °C min−1 in the temperature range of 20−700 °C, using state-of-the-art instruments. Figure 2a shows the TGA-DTG trace of ZrRP composite. Two distinct stages of weight loss were observed in the TGA-DTG curve of ZrRP composite. The first weight loss of 16.9% accompanied by an endothermic peak around 44.5 °C can be assigned to the loss of adsorbed water.51,52 A second weight loss of 22.2% has been observed up to 465.4 °C with a sharp endothermic peak at 465.4 °C. This weight loss observed subsequent to the dehydration of adsorbed water can be attributed to the decomposition of organic species.53 After temperature of 465.4 °C, small weight loss occurs due to condensation of phosphate groups in pyrophosphate groups until it becomes slowly constant at 670 °C, which marks the completion of the process of zirconia (ZrP2O7) formation.51,52 XRD analysis was carried out with a purpose of determining the crystal structure of the composite material and the powder diffraction pattern was recorded in the 2θ range of 10−100°, as shown in Figure 2b. The diffraction peaks at 2θ value of 22.90, 33.70, 41.94, 53.04, 60.70, and 71.23 were observed in the spectrum which can be indexed as (112), (201), (228), (−219), (322), and (334) planes of ZrRP.54 Two diffraction peaks at 2θ = 22.90 and 33.70 were consistent with the earlier

determined patterns of zirconium phosphate.55 However, the broad diffraction peaks of low intensity is consistent with amorphous nature of ZrRP with hardly developed crystals of ZrRP.56,57 The broad diffraction maxima of low intensity can also be related to the reflection broadening by the extremely small particle size of ZrRP.58 Furthermore, the corresponding SAED pattern, shown as inset in Figure 2b indicates the existence of amorphous ZrRP material.59 Surface area of the composite particles plays an important role in the adsorption of heavy metal ions on such materials. In order to have an insight about the scope of adsorption on this material, we evaluated the surface area of the ZrRP nanocomposite by nitrogen adsorption−desorption method at 77 K. Figure S2 (see the Supporting Information) displays the N2 adsorption−desorption isotherm of ZrRP nanocomposite with a prominent hysteresis loop in the P/P0 range of 0.40 to 0.80, indicating the presence of mesoporous structures.60 The BET surface area was calculated and found to be 413 m2 g−1, indicating that the sample has a relatively high surface area for efficient adsorption characteristics Electron microscopic analysis of solid ZrRP and ZrRP gel was used to study the surface morphology, average particle size and growth of ZrRP composite. Pertinent to the understanding and illustrations of the dynamic process of the composite 4823

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Figure 3. SEM images of the ZrRP dispersion at three different time interval after the mixing of microemulsion indicating coalescing of loaded micelle droplets, and growth of ZrRP within the micelle: (a) after 10 min, (b) after 6 h, and (c and d) after 12 h.

Figure 4. (a and b) SEM and (c and d) TEM images of solid sample of nanocomposite zirconium resorcinol phosphate material.

droplets coalesce and develops intoa pollen tube like structure (Figure 3a) for the fast exchange of chemical species as shown by yellow colored arrow in Figure 3(b and c). This fast exchange of chemical species ensures the incorporation of resorcinol within the matrix of zirconium phosphate for the growth of ZrRP nanocomposite.61 At 10 min of time from microemulsion mixing, few droplets were concentrated with ZrRP nanocomposite (shown in red color) .With the passage of

formation and growth of ZrRP within reverse micelle during the process of gelation, we performed scanning electron microscopy of the ZrRP dispersion/gel at three time intervals of 10 min, 6 h, and 12 h which are shown in Figure 3. From careful examination of the SEM micrographs it appears that nearly spherical shaped micellar droplets loaded with the chemical species have evolved immediately upon microemulsion mixing. It is intriguing to see that these micellar 4824

DOI: 10.1021/acs.iecr.6b00208 Ind. Eng. Chem. Res. 2016, 55, 4820−4829

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Figure 5. Plot depicting flow characteristics of zirconium resorcinol phosphate dispersion. (a) Viscosity as a function of shear rate. (b) Shear stress as a function of shear rate.

time at 6 and 12 h, micellar droplets becomes more and more concentrated with the composite material which assist in faster growth15 of ZrRP. Up to 12 h of time nearly all the micellar droplets got saturated with ZrRP due to increased growth rate. This increased growth within reverse micelle result in agglomerates of ZrRP.59 Interestingly, the resulting ZrRP composite assumes nearly spherical shape of the micellar droplet with porous structure, shown inset in Figure 3d. A closer look at the SEM images taken from solid ZrRP composite material suggests that the nanosized particles possess nearly spherical shape and smooth microporous surface characteristics (see Figure 4a and b). The TEM image depicted in Figure 4c and d reveals that ZrRP composite possess characteristic layered structure with average particle size ranging from 30 to 40 nm. 3.2. Rheological Studies on ZrRP Loaded Microemulsion. 3.2.1. Steady Flow Properties of ZrRP Microemulsion Gel. The shear stress and steady shear viscosity as a function of shear rate for the ZrRP dispersion at different time intervals after the mixing of microemulsions were shown in Figure 5a and b. It is evident that after the time of 10 min the solution shows Newtonian behavior at higher shear rate, where viscosity is independent of shear rate. This Newtonian behavior has been attributed to the homogeneity of the microemulsion. After the time lapse of 6 and 12 h, the flow curves show nonNewtonian behavior where viscosity depends strongly on shear rate, suggesting the presence of agglomerates of ZrRP due to thickening of sol or gel within reverse micelle, which can also be inferred from SEM images of the ZrRP nanocomposite material as in Figure 3d. The flow curves also show a clear shear thinning behavior, which suggests that at higher shear rate the agglomerates of ZrRP breakdown into primary particles of ZrRP.62,63 Shear thinning phenomenon becomes more significant as the concentration of ZrRP nanocomposite increases within reverse micelle, and it does so with increase in gelation time. 3.2.2. Dynamic Viscoelastic Behavior of ZrRP Microemulsion Gel. Oscillatory shear measurements were performed for the study of viscoelastic behavior of the ZrRP dispersion within the microemulsion. The results of the oscillatory measurements for the dispersions of ZrRP within reverse micelle at 25 °C at three time intervals of 10 min, 6 h, and 12 h are shown in Figure 6. Typical results of storage modulus (G′) and loss modulus (G″) as a function of angular

Figure 6. Plot showing viscoelastic behavior of zirconium resorcinol phosphate dispersion gel by plotting storage modulus (G′) and loss modulus (G″) versus angular frequency at three different time intervals of 10 min (G1′, G1″), 6 h (G2′, G2″), and 12 h (G3′, G3″).

frequency (ω) can be seen from Figure 6. Initially within the mixing duration of microemulsion, storage modulus was higher than the loss modulus (G′ > G″) at higher frequency, this indicates that flocculation occurs at high frequencies as gelation process becomes more effective indicating predominant elastic behavior. At low frequencies the loss modulus was higher than storage modulus (G′ < G″), suggesting that the microemulsion predominantly is in sol phase indicating predominantly viscous behavior (sol behavior).63,64 Interestingly Figure 6 displays distinct sol−gel transition at an angular frequency value of 29.42 rad s−1, at which G′ = G″.63 With the passage of time, at 6 and 12 h from microemulsion mixing, the reverse micelles become more and more concentrated with the ZrRP nanocomposite and hence storage modulus (G′) becomes greater than loss modulus (G″) at all frequencies and the critical frequency value is not observed. At this point of time, the system behaves as a stable gel, as can be clearly seen in Figure 6. Furthermore, at a time interval of 12 h, the values of G′ and G″ are higher than at time interval of 6 h, indicating the presence of more concentration of ZrRP nanocomposite within the gel. 3.3. Ion Exchange Capacity and Elution Behavior. In order to explore the working capacity of the nanocomposite polymer material as an ion-exchanger, the ion exchange capacities of some monovalent and divalent cations were investigated 4825

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Table 2. Kd Values of Some Heavy Toxic Metal Ions on Nanocomposite Zirconium Resorcinol Phosphate Materials in Aqueous, Nitric Acids and Perchloric Acid Media

Table 1. Ion Exchange Capacity of Nanocomposite Zirconium Resorcinol Phosphate for Different Metal Ions s. no. cation 1 2 3 4 5

Na+ K+ Mg2+ Ca2+ Sr2+

salt used NaNO3 KNO3 Mg(NO3)2 Ca(NO3)2 Sr(NO3)2

ionic Pauling hydrated ionic IEC radii (A0) radii (A0) radii (A0) (mequiv g−1) 0.97 1.33 0.78 0.94 1.06

0.97 1.33 0.65 0.99 1.13

2.76 2.32 7.00 6.30 6.10

metal ion

1.90 2.20 2.36 2.55 2.90

DMW

0.1 M HNO3

0.01 M HNO3

0.01 M HClO4

9500 3500 3943 5213

5800 1250 1600 1900

7996 2833 3133 4500

13006 4500 4500 6166

2+

Cd Zn2+ Pb2+ Ni2+

in Table 2. The calculated distribution coefficient (Kd) for four metal ions follows the trend Cd(II) > Ni(II) > Pb(II) > Zn(II). Figure 7b showed that Cd(II) ion exhibited maximum distribution coefficient in perchloric acid medium. This high selectivity of the nanocomposite material makes it suitable for the removal of Cd(II) from aqueous systems. High selectivity of ZrRP nanocomposite toward Cd(II) is most probably attributed to the smaller hydrated radii of Cd(II), which best match with the interlayer layer distance and pore size of resorcinol intercalated zirconium phosphate.47 This allows the easy accessibility of Cd(II) toward ionogenic sites of ZrRP, which assists in quick exchange of Cd(II) from aqueous medium with H+ ions of exchanger. This behavior can be useful for the uptake of Cd(II) ion from the industrial and laboratory wastes and, also, for the chemical analysis of Cd(II) present in water bodies. On the basis of Kd values of various metal ions, separation factor68,69 can be used as a better indicative parameter. Separation factor is defined as

and the results are summarized in Table 1. The material shows greater affinity for K+ ions than Na+ among the alkali metals. For alkaline earth metals, the exchange affinity is in order as Sr2+ > Ca2+> Mg2+. It clearly indicates that ion exchange capacity increases with decrease in the hydrated ionic radii of the in-going metal ions Figure 7a, which is a commonly observed phenomenon for such class of materials toward alkali and alkaline earth metals.37,65−67 It has been reported earlier as well that the size and charge of the exchanging ions affects the ion exchange capacity of these hybrid composite materials. The reason is that the ions with smaller hydrated radii easily enter the pores of the exchanger, resulting in greater ion exchange capacity. Since, the hydrated radii of ions decrease down the group, the ion exchange capacity increases down the group. Furthermore, the entropy contributions to the free energies of exchange are favorable to the uptake of cations with smaller hydrated radii. Highest ion-exchange capacity (IEC) to the tune of 2.9 mequiv g−1 was observed for strontium ion in aqueous medium. This ionexchange behavior of ZrRP nanocomposite could be utilized for the uptake of strontium ions from nuclear wastes. A part from good ion exchange capacity, ZrRP nanocomposite has been successfully regenerated by keeping it overnight in 1 M HNO3. The calculated Sr2+ ion exchange capacity over column loaded with regenerated ZrRP nanocomposite was found to be 2.90 ± 0.5 mequiv g−1. Elution behavior on a 1 g column bed showed that the exchange is quite fast at the beginning of the process, as most of the exchangeable H+ ions are eluted out in the first 50 mL of the 1 M NaNO3 effluent (see the Supporting Information, Figure S1). 3.4. Distribution Studies and Quantitative Binary Separation of Heavy Metal Ions. For assessment of the potential applications of this nanocomposite material in the separation of metal ions, distribution studies for four metal ions were performed in aqueous and acidic media (HNO3 and HClO4) by usual batch method and the results are summarized

SF =

KdA KdB

(2)

where A and B are the two metal ions of interest. The SF is a scale used to know whether a medium may be used to separate out two metal ions from one another. The higher the values of separation factor, the more efficient the separation. In 0.01 M HNO3 and 0.01 M HClO4, we obtain separation factor to the tune of 4.64 for the Cd(II)−Zn(II) mixture and 2.49 for the Cd(II)−Ni(II) mixture. On the basis of this, the Cd(II) ion has been found to be efficiently separable from Zn(II) and Ni(II) ions over the column loaded with ZrRP nanocomposite material (see Supporting Information, Table S1). The binary chromatograms, for Zn(II)−Cd(II) and Ni(II)−Cd(II) separations, are shown in Figure 8a and b. A schematic representation of separation of Zn(II) and Cd(II) over column loaded with nanocomposite zirconium resorcinol phosphate material is also shown in Figure 8c.

Figure 7. Bar graph showing (a) ion exchange capacity and (b) Kd values in 0.01 M HClO4 of nanocomposite zirconium resorcinol phosphate material. 4826

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Figure 8. Binary separation plots of (a) Zn(II)−Cd(II) and (b) Ni(II)−Cd(II) metal ions. (c) Schematic representation of separation of Zn(II) and Cd(II) over column loaded with nanocomposite zirconium resorcinol phosphate material.

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4. CONCLUSION Novel zirconium resorcinol phosphate (ZrRP) nanocomposite material were synthesized by water in oil (W/O) microemulsion method, and the results revealed that nanocomposite possess novel characteristics as a new material, having efficient ion-exchange and adsorptive properties. The material is highly selective for Cd(II) and can be used for wastewater treatment. The selectivity of the material for Cd(II) has been successfully employed for some binary separations of Cd(II) mixtures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00208. Figure S1: Histogram showing elution behavior of nanocomposite zirconium resorcinol phosphate material with 1 M sodium nitrate. Figure S2: Nitrogen adsorption− desorption isotherm of ZrRP composite recorded at 77K. Table S1: Quantitative separation of metal ions from a binary mixture using nanocomposite zirconium resorcinol phosphate cation exchanger columns at room temperature and results (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +91-194-2424900 (Office); +91-9906424293 (Mobile). Fax: +91-194-2414049. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of Dr. Ajaz Ahmad Dar, Associate professor, Department of Chemistry, University of Kashmir, for providing rheological facilities for carrying out this work. We are also thankful to Professor Abdullah G. Al-Sehemi, Dean of Scientific Research, King Khaled University, Abha, Saudi Arabia, for his valuable suggestions. A.H.P thanks the University Grants Commission (UGC), Government of India, for research grant [F. No. 42-305/2013(SR)]. A.B. would like to thank CSIR for financial assistance in the form of a junior research fellowship. 4827

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