Journal of Hazardous Materials 276 (2014) 138–148
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Synthesis and characterization of a novel hybrid material as amphoteric ion exchanger for simultaneous removal of cations and anions Brijesh Shah, Uma Chudasama ∗ Applied Chemistry Department, Faculty of Technology & Engineering, The M.S. University of Baroda, Vadodara 390 001, Gujarat, India
h i g h l i g h t s • • • • •
A novel hybrid exchanger ZrD (zirconium diethylene triamine) is synthesized for the first time. Characterization and structure elucidation reveals that ZrD exhibits amphoteric character. Amphoteric behaviour of ZrD is established by simultaneous removal of cations and anions. Cations are exchanged in ZrD through chelation with nitrogen as coordinating sites. ZrD can be regenerated and reused with not much decline in performance.
a r t i c l e
i n f o
Article history: Received 4 March 2014 Received in revised form 17 April 2014 Accepted 12 May 2014 Available online 24 May 2014 Keywords: Amphoteric exchangers Chelating exchangers Hybrid exchangers Inorgano-organic hybrid exchangers
a b s t r a c t A new hybrid chelating ion exchanger zirconium diethylene triamine (ZrD) has been synthesized by a simple sol–gel route using inexpensive and easily available chemicals. ZrD has been characterized for elemental analysis (ICP-AES, CHN analysis), TGA, FTIR, X-ray diffraction, SEM and EDX. Physical and ion exchange characteristics as well as chemical stability of the material in various media have been studied. Structural determination reveals that ZrD exhibits amphoteric character. Anion exchange capacity (AEC) for Cl− , Br− , Cr2 O7 2− , F− and AsO4 3− has been determined. Cations are exchanged through chelation where coordinating sites are offered by nitrogen atoms present in the amine groups of ZrD. Distribution coefficient Kd for Co2+ , Ni2+ , Cu2+ , Zn2+ (transition metal ions) and Hg2+ , Cd2+ , Pb2+ (heavy metal ions) has been evaluated by batch equilibration techniques in aqueous and various electrolyte media/concentrations. Based on ˛ the separation factor, a few binary separations have been performed on a chromatographic column packed with ZrD. The amphoteric behaviour of ZrD has been demonstrated by simultaneous exchange of Cu2+ and Cl− in CuCl2 . A study on the regeneration and reuse of ZrD indicates that it is effective upto four cycles without much decline in performance. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ion-exchange chromatography is a well-known and widely used technique for the separation and preconcentration of inorganic ions. However, the innumerable separation procedures published so far are almost exclusively based on the use of monofunctional ion exchangers, mostly strongly basic anion exchangers with
∗ Corresponding author at: Applied Chemistry Department, Faculty of Technology and Engineering, The M.S. University of Baroda, Vadodara 390 001, Gujarat, India. Tel.: +91 265 2434188x415; fax: +91 265 2423898. E-mail addresses: [email protected] (B. Shah), [email protected], [email protected] (U. Chudasama). http://dx.doi.org/10.1016/j.jhazmat.2014.05.031 0304-3894/© 2014 Elsevier B.V. All rights reserved.
quaternary ammonium groups and strongly acidic cation exchangers with sulphonic acid groups [1]. Amphoteric ion exchangers in contrast to conventional monofunctional exchangers contain anionic and cationic exchange sites, and under appropriate conditions, can exchange simultaneously anions and cations from external solutions [2]. Although it was surmised long ago [3], that simultaneous presence of anion and cation exchange groups may offer new interesting separation possibilities, very few attempts to exploit these possibilities in practice can be found in literature [2,3]. Some of these amphoteric ion exchangers are chelating resins with functional groups able to form complexes with several cations and are used for the preconcentration of trace elements [3–6], the sorptive effect of ions being based on the distribution of soluble ions between an aqueous solution and a reactive
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polymer/resin/material containing a selective ligand [2,7–9]. However, the most important influence on sorption is the internal structure of the exchangers and coordinating ability of ligand end groups, which coordinate or chelate metal ions [10–13]. Resins with nitrogen atom containing ligands have been extensively reported [13]. Very early reports reveal that amphoteric exchanger thorium triethanolamine was prepared by incorporation of triethanolamine group into the matrix of thorium oxide, where in nitrogen atom present in the amine group offers chelating sites for metal ions. Zirconium-bis(triethylamine) exhibiting amphoteric character that exchanges anions and adsorbs cations through chelation action has also been reported [14,15]. In the present endeavour, a new and novel hybrid chelating ion exchanger, zirconium diethylene triamine (ZrD), has been synthesized by a simple sol–gel route using inexpensive and easily available chemicals. ZrD has been characterized by instrumental methods of analysis, physical and ion exchange characteristics as well as chemical stability of the material in various acids, bases and organic solvent media have also been studied. Anion exchange capacity (AEC) for Cl− , Br− , Cr2 O7 2− , F− and AsO4 3− has been determined distribution coefficient Kd for Co2+ , Ni2+ , Cu2+ , Zn2+ (transition metal ions) and Hg2+ , Cd2+ , Pb2+ (heavy metal ions) has been evaluated by batch equilibration techniques in aqueous and various electrolyte media/concentrations. Based on ˛, the separation factor a few binary separations have been performed on a chromatographic column packed with ZrD. The amphoteric behaviour of ZrD has been demonstrated using CuCl2 and HgCl2 and amount of cation and anion simultaneously exchanged determined. The practical applicability of ZrD as an amphoteric exchanger has been further highlighted by performing a case study including regeneration and reuse of ZrD. 2. Experimental 2.1. Materials and methods Zirconium oxychloride (ZrOCl2 ·8H2 O) and diethylene triamine (DETA) (C4 H13 N3 : molecular weight = 103.17 and density = 0.955 g/mL) was procured from Loba Chemicals. All other chemicals and reagents used were of analytical grade. Doubledistilled water was used for all the studies. 2.2. Synthesis of ZrD The main objective was to obtain a material with maximum anion exchange capacity (AEC). Several samples of material were synthesized by sol–gel method varying several condition/parameters such as mole ratio of reactants, temperature, mode of mixing (metal salt solution to DETA solution or vice versa), pH and rate of mixing, in each case using AEC as the indicative tool. Table S1 (Supporting Information) describes optimization of parameters for synthesis of ZrD. 2.2.1. Synthesis of ZrD at optimized condition ZrD was prepared by mixing aqueous solutions of DETA (0.1 M, 50 mL) and ZrOCl2 (0.1 M, 50 mL) at room temperature, dropwise and with continuous stirring. A gelatinous precipitate was obtained, and solution along with precipitate further stirred for 1 h. The resulting gelatinous precipitate was allowed to stand for 15 h, then filtered, washed with double distilled water till removal of adhering ions, followed by drying at room temperature. The material was then broken down to the desired particle size 30–60 mesh (ASTM) by grinding and sieving. An yield of 35% was obtained. This material was used for all studies.
139
2.3. Characterization Physical characteristics such as appearance, percentage moisture content, apparent density, true density, void volume fraction, concentration of fixed ionogenic groups and volume capacity were studied according to literature methods [16–18]. A study on pH titration curve, chemical stability, anion exchange capacity (AEC) and effect of calcination on AEC are described below. Further, the material has been characterized by Instrumental methods. Zr was analyzed by ICP-AES performed on Labtam, 8440 Plasma lab. While C, H, N analysis was performed on Perkin Elmer-2400. TGA was performed on a thermal analyzer Shimadzu (model TGA-50) at a heating rate of 10 ◦ C/min. FTIR spectra was obtained using KBr pellet on a Shimadzu (model 8400S). 1 H NMR spectra was obtained using NMR spectrometer (Bruker 500 MHz) in a D2 O solvent. UVDRS was obtained using Shimadzu (Model UV-DRS 2450). X-ray diffractogram was obtained on X-ray diffractometer (Brucker AXS D8) with Cu K␣ radiation with nickel filter. SEM and EDX of the material were obtained on Jeol JSM-5610-SLV scanning electron microscope. 2.3.1. pH titration curve The pH titration of ZrD was performed by Topp and Pepper method [16–18]. 500 mg of exchanger was treated with 50 mL 0.01 M HCl solution with intermittent shaking. The solution along with exchanger was allowed to equilibrate and pH noted when the value was constant. This was the initial reading. This solution mixture was now titrated with 0.01 M NaCl solution. After addition of every 0.5 mL of titrant, sufficient time was provided for establishment of equilibrium between the ion exchanger and the solution. A pH titration curve was obtained by plotting pH vs. volume of NaCl as depicted in Fig. 2. 2.3.2. Chemical stability The chemical stability of ZrD in various media – acids (HCl, H2 SO4 and HNO3 ), bases (NaOH and KOH) and organic solvents (ethanol, benzene, acetone and acetic acid) was studied by taking 0.5 g of ZrD in 50 mL of the particular medium and allowed to stand for 24 h. The change in colour, nature, weight, solubility, metal washout and particle size, etc. was observed. To confirm the solubility of exchanger in a particular medium, supernatant liquid was checked qualitatively for respective elements of exchanger. The chemical stability is expressed as maximum tolerable limits evaluated in a particular medium. 2.4. Anion exchange properties of ZrD 2.4.1. Determination of anion exchange capacity (AEC) Two grams of the exchanger was treated with NaCl/KBr/K2 Cr2 O7 (0.2 M, 20 mL) for 30 min in a conical flask with continuous shaking and material then separated from solution by decantation. This process was repeated at least five times. The material was finally washed with double distilled water for removal of any adhering ions and dried at room temperature. 0.5 g of this material [(ZrD) exchanged (Cl− /Br− /Cr2 O7 2− )2 ] was placed in a glass column [30 cm × 1 cm (internal diameter)], double distilled water was poured onto column with a flow rate adjusted to 0.5 mL min−1 to wash the column. A 1.0 M, 250 mL sodium nitrate solution was now passed through the column. The effluent containing (NaCl/KBr) was titrated against 0.1 M AgNO3 for determination of chloride and bromide, while the effluent containing (K2 Cr2 O7 ) was titrated against 0.1 M FeSO4 (NH4 )2 SO4 ·6H2 O solution for determination of dichromate. AEC for Cl− , Br− and Cr2 O7 − was determined using the formula aV/W, where a is molarity, V the amount of titrant used during titration, and W is the weight of the exchanger. The AEC values for F− and AsO4 3− was
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Table 1 Physical and ion exchange characteristics of ZrD. Physico-chemical and ion exchange characterization Characteristics
Observation
Appearance Glassy transparent granules 250–590 m Particle size (range) 16.49% % Moisture content True density 1.46 0.53 Apparent density 0.65 Void volume fraction 3.13 Concentration of fixed ionogenic groups Volume capacity of resin 1.07 Nature of exchanger Mono functional Anion exchange capacity − Cl 1.91 Br− 0.96 0.92 Cr2 O7 2− 0.94 F− 2.10 AsO4 3− Effect of temperature on anion exchange capacity for Cl− AEC (RT) 1.91 1.15 100 ◦ C ◦ 200 C 1.09 0.98 300 ◦ C 0.90 400 ◦ C 0.84 500 ◦ C Maximum tolerable limits Chemical stability 18 N H2 SO4 , 8 N HNO3 , 5.6 N HCl (i) Acids 2 N NaOH, 5 N KOH (ii) Bases (iii) Organic solvents Ethanol, benzene, acetone, acetic acid
determined by a colorimetric method [19,20]. Arsenic, based on the formation of molybdoarsenate with subsequent reduction to heteropoly blue and fluoride, was determined by SPANDS ((2-(psulphophenylazo)1,8-dihydroxynaphthalene-3,6-disulphonic acid trisodium salt)) method [19,20]. A detailed procedure has been provided in Supporting Information section. 2.4.2. Effect of calcination on AEC The effect of calcination on AEC (Table 1)(in case of Cl− ) was studied by calcining several 1 g portions of the material for 2 h in the temperature range 100–500 ◦ C at 100 ◦ C intervals in a muffle furnace, cooling them to room temperature, and determining AEC by column method discussed above. 2.5. Cation exchange properties of ZrD 2.5.1. Equilibrium time determination Ten milliliter metal ion (0.014 M) solution was shaken with 0.1 g of exchanger ZrD in stoppered conical flasks at room temperature for different time intervals (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 7.0 h). The supernatant liquid was removed after every prescribed time interval, and the metal ion concentration evaluated by EDTA titration. From the plot of fractional attainment of equilibrium U(t) vs. time (t), maximum equilibrium time was determined. 2.5.2. Effect of metal ion concentration on distribution coefficient Effect of metal ion concentration on distribution coefficient (Kd ) for Co2+ , Ni2+ , Cu2+ , Zn2+ (transition metals) and Cd2+ , Hg2+ , Pb2+ (heavy metals) was determined by batch method. 0.1 g of ZrD was equilibrated with 20 mL of varying metal ion concentration (0.002–0.02 M with interval of 0.002 M) for 6 h (maximum equilibrium time) at room temperature. The metal ion concentration before and after exchange was determined by EDTA titration.
(heavy metal ions) was added and pH (1–7) adjusted using dilute HNO3 /NaOH and the mixture shaken for 30 min. The supernatant liquid was used to determine the metal ion concentration by EDTA titrations. Uptake of metal ions (%) has been calculated using formula, [(C0 − Ce )/C0 ] × 100, where C0 = initial concentration of metal ion in mg L−1 and Ce = final concentration of metal ion in mg L−1 . 2.5.4. Distribution study (Kd ) at optimized condition Kd has been evaluated by batch method at optimum condition (optimum metal ion concentration, pH of maximum adsorption and maximum equilibrium time – 6 h) using 0.1 g of the exchanger in aqueous as well as various electrolyte media like HNO3 , citric acid and HClO4 of 0.02 and 0.2 M concentration at room temperature. The supernatant liquid was used to determine the metal ion concentration by EDTA titrations. Kd was evaluated using the expression, Kd = [(I − F)/F] × V/W (mL g−1 ), where I = total amount of the metal ion in the solution initially, F = total amount of metal ions left in the solution after equilibrium, V = volume of the metal ion solution, and W = weight of the exchanger in grams. 2.6. Elution and separation studies For elution studies (single metal), 0.5 g ZrD was packed in a glass column (30 cm × 1 cm), washed thoroughly with double distilled water, and flow rate adjusted to 0.5 mL min−1 . The metal ion solution (0.014 M, 10 mL) was loaded onto the column. The metal ion loaded was eluted with reagents like HNO3 , citric acid and HClO4 , of 0.02 and 0.2 M concentration. For binary separation the mixture of the metal ions (0.014 M, 10 mL of each metal ion) to be separated was loaded on column. The separation was achieved by passing suitable eluent through the column. In all cases, the metal ion concentration was determined quantitatively by EDTA titration. For every experimental point in graphs, two identical sets are prepared to compare/verify the obtained values. Reproducibility in values for same experimental point was assessed by again preparing two identical sets. The percentage metal recovered (metal eluted) was calculated in terms of % elution expressed as, % E = (Ce /C0 ) × 100 where Ce is the concentration of the metal ion in the eluted solution and C0 is the concentration of metal ion loaded onto the column. 2.7. Amphoteric behaviour of the material Amphoteric behaviour of the ZrD was studied by batch method. 0.1 g of ZrD was treated with CuCl2 /HgCl2 and allowed to stand for 24 h. The supernatant solution was titrated with EDTA for determination of cations and argentometric titration for determination of anions, respectively. The exchanger was collected for EDX analysis for confirming the adsorption of cations and anions on exchanger surface. 2.8. Case study The practical applicability of ZrD has been demonstrated in a case study using tap water sample from our institute. The tap water sample was passed through a column packed with 0.5 g ZrD (prepared as above) in 10 mL fractions. The initial sample as well as eluent collected were analyzed for the presence of respective cations before and after exchange by EDTA titration and argentometric titration for determination of anions and it is further confirmed by EDX analysis. 2.9. Regeneration and reusability of ion exchanger
2.5.3. Effect of pH on adsorption of metal ions To 0.1 g of exchanger, ZrD, 10 mL of 0.014 M metal ion solution CO2+ , Ni2+ , Cu2+ , Zn2+ (transition metal ions) and Cd2+ , Hg2+ , Pb2+
Regeneration and reuse of ion exchanger ZrD was performed in the case of Copper ion by batch method. 0.014 M (Optimum
B. Shah, U. Chudasama / Journal of Hazardous Materials 276 (2014) 138–148
O
O
Zr
Zr
O
N
N H
Zr H N
N
(OH)n
O
Zr N
Zr
Zr
%T
N
55 50 45 40 35 30 25 20 15 10 5 0 4000
O
O
141
1410 1580
Cu-ZrD 1360 1580 ZrD 3500
3000
2500
2000
1500
1000
500
wavelenght (cm-1)
n
Fig. 4. FTIR of ZrD and Cu2+ treated ZrD. Fig. 1. Proposed structure of ZrD gel.
Kd(R) = Kd(C) /Kd × 100 where Kd = initial value obtained, Kd(C) = Kd determined in each subsequent cycle.
6
pH
5 4
3. Results and discussion
3
3.1. Structural studies
2
ZrD was obtained as hard glassy transparent granules. A gel is formed when an aqueous solution of DETA is added to aqueous solution of ZrOCl2 (pH = 1). The solution first turns neutral (pH = 7) and then basic (pH = 9). The gel formed is entirely different as compared to addition of an inorganic base to ZrOCl2 . The gel formation probably takes place in two steps: (a) formation of hydrous ZrO2 gel [21,22] followed by (b) surface interaction and polymerization of amine molecules involving weak and strong bonding, resulting in the formation of a macro-particle [21,22] and can be formulated as [(ZrO)n (DETA)n ](OH)n (Fig. 1). Since the unit Zr-DETA in ZrD is positively charged, it therefore behaves as an anion exchanger where OH part of ZrD behaves as the anion exchange unit. The pH titration curve (Fig. 2) of ZrD exhibits mono functional behaviour [14,15,21]. When ZrD was kept in contact with a solution of sodium nitrate, no release of H+ was observed. However, the presence of NaOH was tested by phenolphthalein indicator (which turns the solution pink) confirming OH as anion exchange sites. It is quite clear that the component [Zr(DETA)] contains free amino groups, offering chelating sites to metal ions and behaves as the cation exchange unit. Due to lone pair of electrons present on the nitrogen atom of amine groups (Fig. 1). ZrD can act as chelating exchanger. The chelating property of ZrD was studied by a batch process. When
1 0
0
2
4
6
8
Volume of 0.01M NaCl Fig. 2. pH titration curve.
concentration) Cu2+ solution was treated with 0.1 g of ZrD and kept for 6 h (maximum equilibrium time) after which metal ion concentration was determined by EDTA titration and Kd value determined. The Cu2+ exchanged ZrD was treated with HNO3 (1 M, 50 mL) for 30 min with occasional shaking. The sample ZrD was separated from acid by decantation and treated with double distilled water to remove adhering acid. This process was repeated at least five times to ensure complete removal of Cu2+ from exchanger. Kd values were determined using this regenerated ZrD. This process was repeated till wide variation in Kd values was observed. % retention in Kd values Kd(R) was determined using the expression,
Fig. 3. EDX of (A) ZrD and (B) Cu2+ exchanged ZrD.
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Fig. 5. SEM of (A) ZrD and (B) Cu2+ exchanged ZrD.
Fig. 6. UV-DRS spectra of ZrD and Cu2+ exchanged ZrD.
ZrD was treated with Cu2+ salt solution it turns to a deep blue colour confirming the chelation of Cu2+ with ZrD. Elemental analysis shows % of Zr = 60.80, % of C = 9.52, % of N = 8.18 which matches well with formula [(ZrO)6 (C8 H18 N6 )](OH)4 . Further, the presence of Zr and N is confirmed by EDX (Fig. 3A). Based on the above discussion, in FTIR spectrum of ZrD, sharp bands are expected in the region ∼3300 cm−1 ( NH stretching frequency) and at ∼3400 cm−1 ( OH stretching frequency), indicating presence of free NH2 groups and OH (anion sites) in ZrD. However, the FTIR (Fig. 4) spectrum of ZrD shows a broad band in the region ∼3500–3200 cm−1 which is probably due to merging of OH and
NH stretching frequency. Band at ∼1580 cm−1 is attributed to C C stretching vibration, while, band at ∼1360 cm−1 is attributed to C N bending. The presence of Cu2+ on exchanger was confirmed by scanning an FTIR spectrum of Cu2+ exchanged ZrD, in which the ∼1360 cm−1 band shifts to ∼1410 cm−1 due to coordination of Cu2+ with nitrogen atom of amine group. This is further, confirmed by SEM (Fig. 5) and EDX (Fig. 3B) of ZrD and Cu2+ exchanged ZrD. Fig. 6 presents UV-DRS spectra for ZrD, before and after Cu2+ chelation. In the present study Cu2+ chelated ZrD exhibits peak around 500 nm which could be attributed to d–d transition band of a Cu2+ ion centred in pseudo-octahedral environment [23]. Fig. 7(A and B) presents NMR spectra of ZrD before and after Cu2+ chelation, respectively. In Fig. 7A, the peak at 3.04 corresponds to (multiplate methylene group –Zr–N–CH2 –) and peak at 2.42 corresponds to (multiplate methylene group CH2 –N–CH2 ). In Fig. 7B, these bands shift to 4.02 and 3.05, respectively, attributed to copper chelated cation. Accordingly, Cu2+ chelated ZrD could be depicted as proposed (Fig. 8). Based on formulation, [Zr(DETA)]2 (OH)2 anion exchange capacity was determined for Cl− , Br− , Cr2 O7 2− , F− and As2 O4 3− . The AEC values in mequiv. g−1 are presented in (Table 1). TGA (Fig. 9) of ZrD exhibits two regions of weight loss. The first weight loss in the temperature range 30–150 ◦ C could be attributed to loss of moisture/hydrated water. A second weight loss in the temperature range 150–500 ◦ C, is attributed to dehydroxylation, and decomposition of the organic moiety. The thermal behaviour of the materials is further supported by the effect of calcination on AEC. As observed from (Table 1), AEC values decrease as calcination temperature increases. Decrease in AEC with increasing temperature could be attributed to dehydroxylation and decomposition of organic moiety. The absence of sharp peaks in the XRD (Fig. 10) of ZrD indicates amorphous nature of the materials. When using ion exchangers, it
Fig. 7. NMR spectra of (A) ZrD and (B) Cu2+ exchanged ZrD.
B. Shah, U. Chudasama / Journal of Hazardous Materials 276 (2014) 138–148
O
Zr
Zr N H
N O
1.2
Zr
N
Cu2+
(OH)n
N
H N
N
O
Zr
Zr
Zr
Fractional attainment of equilibrium U(t)
O
1 0.8 0.6 0.4
0.2 0
O
O
143
0
n
100
200
300
400
Time (min) Fig. 8. Proposed structure of Cu2+ exchanged ZrD. Fig. 11. Fractional attainment of equilibrium U(t) vs. time (t).
10 3.2. Cation exchange properties of ZrD
8 2 nd weight loss
6 mg
1 st weight loss
4 2 0 0
250 500 Temperature (ºC )
750
Fig. 9. TGA of ZrD.
has to be subjected to various environments and chemical media. A study on the chemical stability of ZrD shows that it is stable in acid, base and organic solvent media (Table 1). No transmetalation or Zirconium metal washout is also observed, confirming the ZrD moiety to be the rigid framework. Physical and ion exchange characteristics of ZrD are presented in (Table 1).
9000
A plot of fractional attainment of equilibrium U(t) vs. time (t) indicates that maximum equilibrium time for ZrD is 6 h (Fig. 11). Initially, rate of adsorption is faster due to availability of a large number of chelating sites [24,25]. A maximum removal of Cu2+ is observed at 6 h. Subsequently with decrease in number of chelation sites rate of metal uptake decreases [26]. It is observed that with increase in metal ion concentration, Kd values increase. Above a particular concentration, Kd values are constant which could be explained to be due to the fact that at lower concentrations, almost all the ions are exchanged due to availability of exchangeable sites, which are not available at higher concentrations. A plot of Kd values vs. metal ion concentration (Fig. 12) shows optimum concentration for all metal ions. It is observed that at pH values less than ∼3, very less sorption has been observed for all metal ions. The lack of sorption at low pH could be attributed to high concentration of hydrogen ions competing with the metal ions for sorption/exchange sites. Amongst transition metal ions maximum sorption is observed at pH 5 in case Cu2+ and Co2+ , percentage uptake being 94.76 and 82.11, respectively. In case of Ni2+ and Zn2+ maximum sorption is observed at pH 4, percentage uptake being 73.58 and 78.82, respectively. Amongst heavy metal ion maximum sorption is observed at pH 4, percentage uptake being 89.69 and 64.44 for Hg2+ and Cd2+ , respectively. In case of Pb2+ maximum sorption take place at pH-3 and percentage uptake being 94.55. The distribution coefficient (Kd ) values evaluated for the metal ions under study at optimum conditions using ZrD have been
8000
6000 5000 4000 3000 2000
1000 900 800 700 600 500 400 300 200 100 0
Cu2+ Ni2+ Zn2+ Co2+ Pb2+ Cd2+
Distribution coefficient(kd)
Intensity(CPS)
7000
10
20
30
40
50
60
2 Theta Scale Fig. 10. XRD of ZrD.
70
80
90
0
0.005 0.01 0.015 0.02 Concentration of Metal ions
Fig. 12. Distribution coefficient for different metal ions at different concentration.
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Table 2 Distribution coefficient (Kd ) (mL g−1 ) at optimum condition in aqueous and various electrolyte media/concentration. Metal ions
Aqueous
0.02 M HNO3
0.2 M HNO3
0.02 M citric acid
0.2 M citric acid
0.02 M HClO4
0.2 M HClO4
Co2+ Ni2+ Cu2+ Zn2+ Pb2+ Cd2+ Hg2+
1647 402 2876 301 1019 374 1980
317 204 306 397 551 265 609
256 115 94 298 540 200 536
231 232 616 32 319 151 754
212 163 584 18 238 70 657
446 59 911 56 466 306 446
354 30 434 41 325 249 256
Maximum deviation in Kd values = ±3. Table 3 Percentage elution of various metal ions in different electrolyte media using ZrD. Metal ions 2+
Co Ni2+ Cu2+ Zn2+ Pb2+ Cd2+ Hg2+
0.02 M HNO3
0.2 M HNO3
0.02 M citric acid
0.2 M citric acid
0.02 M HClO4
0.2 M HClO4
91 83 85 91 84 91 86
93 96 88 99 89 96 88
78 88 84 88 92 85 82
86 89 85 90 93 89 82
89 95 86 90 89 85 83
92 96 89 92 92 86 87
Eluent volume = 60 mL and 50 mL for 0.02 M and 0.2 M electrolytes, respectively. Maximum deviation in % elution of metal ions = ±2. Table 4 Binary separations of metal ions using ZrD. Separation achieved
Eluent
Metal ion loaded (mg)
Metal ion eluted (mg)
Elution (%)
2+
Zn –Co
0.2 M citric acid (Zn2+ ) 0.2 M HNO3 (Co2+ )
9.15 8.25
8.21 6.55
89 82
Zn2+ –Cu2+
0.2 M citric acid (Zn2+ ) 0.2 M HNO3 (Cu2+ )
9.15 8.89
8.89 6.13
97 68
Ni2+ –Cu2+
0.2 M HClO4 (Ni2+ ) 0.2 M HNO3 (Cu2+ )
8.21 8.89
7.88 6.19
96 69
Co2+ –Cu2+
0.2 M citric acid (Co2+ ) 0.2 M HNO3 (Cu2+ )
8.25 8.89
6.60 6.05
80 68
Cd2+ –Hg2+
0.2 M citric acid (Cd2+ ) 0.2 M HNO3 (Hg2+ )
17.36 28.00
14.32 18.06
82 64
Cd2+ –Pb2+
0.2 M HClO4 (Cd2+ ) 0.2 M citric acid (Pb2+ )
17.36 29.00
15.71 22.18
90 74
2+
Maximum deviation in % elution = ±2%.
presented in (Table 2). Kd values in aqueous medium follows the order: Cu2+ > Co2+ > Ni2+ > Zn2+ amongst the transition metal ions, Hg2+ > Pb2+ > Cd2+ amongst the heavy metal ions. 3.3. Elution and separation studies using ZrD The elution behaviour of transition metal ions and heavy metal ions (under study) have been carried out using different electrolytes such as HNO3 , citric acid and HClO4 , of 0.02 and 0.2 M concentration and results presented in (Table 3). The % metal eluted in all cases is in the range 79–97%. Good elution is observed due to presence of single metal ion and non-interference of elements. Typical elution curves for Cu2+ ions with different electrolytes has been presented in Fig. 13. Higher concentration of eluent and acids in general, are better eluents. Order of % metal eluted amongst transition metal ions is Zn2+ (99%) > Ni2+ (96%) > Co2+ (93%) > Cu2+ (88%) and amongst heavy metal ions is Cd2+ (96%) > Pb2+ (89%) > Hg2+ (88%). This observation is in keeping with the fact that metal ions with high Kd values are less eluted and vice versa. Separation factor ˛, the rate at which two constituents separate on a column, given by, ˛ = Kd1 /Kd2 , where Kd1 and Kd2 are the distribution coefficients of the two constituents being separated, provides a guideline for metal separation. The greater the deviation of ˛ from unity, the better is the separation. The efficiency of
an ion exchange separation depends on the condition under which ˛ has a useful value, or influencing in a direction favourable to separation. For a given metal ion pair, the electrolyte media in which the separation factor is the highest, is selected as the eluent. Thus, a study on distribution behaviour of metal ions in various electrolyte media gives an idea about the eluents that can be used for separation. The separation factor (˛) of different metal ions pairs are presented in (Fig. 14). The separation factor (˛) values are very high indicating efficient separations can be achieved. Binary separations for following metal ion pairs (under study) Zn2+ –Co2+ , Zn2+ –Cu2+ , Ni2+ –Cu2+ , Co2+ –Cu2+ , Cd2+ –Hg2+ , Cd2+ –Pb2+ have been performed using concept of high separation factor in a particular medium as discussed earlier in the text. In binary separations (Fig. 15), separation efficiency is in the range 68–96% amongst transition metal ions and 64–90% amongst heavy metal ions (Table 4). In all cases of binary separation, irrespective of metal ion pair, maximum % metal eluted is Zn2+ (97%), Ni2+ (96%), Co2+ (82%), Cu2+ (68%) (amongst transition metal ions) and Cd2+ (90%), Pb2+ (74%), Hg2+ (64%), (amongst heavy metal ions). This observation is in keeping with separation factor (˛) and Kd values of metal ions. % metal eluted decreases with decreasing separation factor and increases with increasing separation factor and as explained earlier, metal ions with high Kd values are less eluted and vice versa. From the above
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40
0.02M HNO3
0.2M HNO3 % Elution
% Elution
30 25 20 15 10 5 0
30 20 10 0
0
0
10 20 30 40 50 60
20
40
A 50
50 0.2M HClO4
40 % Elution
% Elution
B 0.02M HClO4
40
60
Volume of eluant (ml)
Volume of eluant (ml)
30 20 10
30 20 10
0 0
0
10 20 30 40 50 60
0
Volume of eluant (ml)
10 20 30 40 50 60 Volume of eluant (ml)
D
C 30 20 10
% Elution
40 0.02M Citric Acid
% Elution
145
0.2M Citric Acid
30 20 10 0
0
0 0
10 20 30 40 50 60 Volume of eluant(ml)
10
20
Fig. 13. Single elution curves for Cu citric acid using ZrD.
3.4. Amphoteric behaviour of ZrDETA On treating a ZrD packed column with CuCl2 /HgCl2 simultaneous exchange of cations and anions occurred. Respective cations
F
and anions present before and after exchange are presented in Table 5. Further, the presence of exchanged Cu2+ , Hg2+ and Cl− on ZrD is confirmed by EDX (Fig. 16A and B). 3.5. Case study Practical utility of ZrD has been demonstrated in a case study. Simultaneous exchange of cations and anions occurred on treating Table 5 Simultaneous uptake Cu2+ , Hg2+ and Cl− using ZrD.
20 16 Zn2+-Cu2+ Separation factores
50
ion in different electrolytes: (a) 0.02 M HNO3 , (b) 0.2 M HNO3 , (c) 0.02 M HClO4 , (d) 0.2 M HClO4 , (e) 0.02 M citric acid and (f) 0.2 M
studies, it could be concluded that ZrD is an effective chelating ion exchanger.
Cations and anions
12 8
40
Volume of eluant (ml)
E 2+
30
Ni2+-Cu2+ Co2+-Cu2+Hg2+-Cd2+ 2+ 2+ Cd -Pb Zn2+-Co2+
4 0 Metal ion separaion Pairs
Fig. 14. Proposed metal ion separation pairs based on separation factor (˛).
Amount of cation/anion in solution Amount of cation/anion after exchange in solution Amount of cation/anion on exchange Percentage uptake of cation/anion
Cu2+
Cl−
Hg2+
Cl−
1.27
1.41
1.28
1.41
0.99
0.07
4.00
0.09
0.28
1.34
1.28
1.31
22.97
95.03
32.00
92.90
Maximum deviation in percentage uptake of cation and anion = ±1%.
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40
25 H2O
20 15 10
Zn2+
Co2+
5
30 25
H2O
20 15 10
Zn2+
Cu2+
5
0
0 0
20
40 60 80 100 120 140 160 Volume of Eluant (ml)
0
20
40 60 80 100 120 140 160 Volume of Eluant (ml)
45 40 35 30 25 20 15 10 5 0
(II)
0.2M HClO4
0.2M HNO3 % Eluon
% Eluon
(I)
H2O
Ni2+ Cu2+ 0
20
40
60
80
45 40 35 30 25 20 15 10 5 0
0.2M Citric acid
Cu2+
Co2+ 0
100 120 140
20
40
30
0.2M HNO3
25
15 10
0.2M HClO4
25 % Eluon
H2O
20
80 100 120 140
(IV)
0.2M Citric acid
30
60
Volume of Eluant(ml)
(III) 35
0.2M HNO3
H2O
Volume of Eluant(ml)
% Eluon
0.2M HNO3
0.2M Citric acid
35 % Eluon
% Eluon
30
40
0.2M HNO3
0.2M Citric acid
35
0.2M Citric acid
20 H2O
15 10
Hg2+
Cd2+
5
5
Cd2+
Pb2+
0
0 0
20 40 60 80 100 120 140 160 Volume of Eluant (ml)
(V)
0
20 40 60 80 100 120 140 160 Volume of Eluant (ml)
(VI)
Fig. 15. Binary separation of transition metal ions: (I) Zn2+ –Co2+ (II) Zn2+ –Cu2+ (III) Ni2+ –Cu2+ (IV) Co2+ –Cu2+ (V) Cd2+ –Hg2+ and (VI) Cd2+ –Pb2+ .
Fig. 16. EDX of ZrD (A) treated with CuCl2 and (B) treated with HgCl2 .
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147
high solubility in acids or bases and poor performance in column operations. Succinylated mercerized cellulose modified with triethylenetetramine was used for adsorption of Cu2+ , Cd2+ and Pb2+ from aqueous single metal solutions which involves tedious synthesis strategies [29]. In the present endeavour, a new and novel amphoteric ion exchanger, ZrD, has been synthesized by a simple sol–gel route using inexpensive and easily available chemicals. ZrD exhibits good affinity for anions (Cl− , Br− , F− and AsO4 3− ) as well as cations (Cu2+ , Hg2+ , Pb2+ ). The practical applicability of ZrD as an amphoteric exchanger has been well established through case studies including regeneration and reuse of ZrD upto four cycles without much decline in performance. The study reveals the promising use of ZrD as an amphoteric exchanger.
Acknowledgement Fig. 17. EDX of tap water sample.
One of the authors (BS) is thankful to The M.S. University of Baroda, Vadodara for providing a Research Fellowship.
100 % Retention in Kd Values
90 80
Appendix A. Supplementary data
70 Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2014.05.031.
60 50 40 30
References
20 10 0 1
2
3 4 5 Number of cycles
6
7
Fig. 18. % retention in Kd values vs. number of cycles using ZrD.
a ZrD packed column using tap water sample from our institute region. Analysis of water sample by EDX (Fig. 17) in wt.% – Mg2+ = 1.93, Ca2+ = 26.76, Cu2+ = 1.09 and Cl− = 18.85, respectively. Since, concentration of Cu2+ and Mg2+ is very less, we have focused towards Ca2+ and Cl− in the present study. The initial concentration of Ca2+ and Cl− in tap water sample was found to be 300 mg L−1 and 289 mg L−1 , respectively, whereas eluent (after passing through column containing ZrD) contains 151 mg L−1 and 94 mg L−1 of Ca2+ and Cl− , respectively. The water samples were analyzed for Ca2+ and Cl− by EDTA titration and argentometric titration, respectively, before and after passing through the column containing ZrD. 3.6. Regeneration and reusability of ion exchanger A study on regeneration and reuse was performed as described in Section 2.1. It is observed that the exchanger once used can be converted back to its original form by desorption of the metal ions with concentrated nitric acid. A plot of % retention in Kd values vs. number of cycles is presented in Fig. 18, which shows that % retention in Kd values is almost the same upto four cycles, indicating that ZrD could be regenerated and reused without much decline in performance. 4. Conclusion Oxides and hydrous oxides of Zr, Ti, Fe, Al, Nb, Cr, Th, etc. exhibit amphoteric behaviour, exchanging anions in acidic medium and cations in basic medium [27,28]. However, the major drawback of oxides and hydrous oxides as amphoteric exchangers is their
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Synthesis and characterization of a novel hybrid material as amphoteric ion exchanger for simultaneous removal of cations and anions Brijesh Shah, Uma Chudasama ∗ Applied Chemistry Department, Faculty of Technology & Engineering, The M.S. University of Baroda, Vadodara 390 001, Gujarat, India
h i g h l i g h t s • • • • •
A novel hybrid exchanger ZrD (zirconium diethylene triamine) is synthesized for the first time. Characterization and structure elucidation reveals that ZrD exhibits amphoteric character. Amphoteric behaviour of ZrD is established by simultaneous removal of cations and anions. Cations are exchanged in ZrD through chelation with nitrogen as coordinating sites. ZrD can be regenerated and reused with not much decline in performance.
a r t i c l e
i n f o
Article history: Received 4 March 2014 Received in revised form 17 April 2014 Accepted 12 May 2014 Available online 24 May 2014 Keywords: Amphoteric exchangers Chelating exchangers Hybrid exchangers Inorgano-organic hybrid exchangers
a b s t r a c t A new hybrid chelating ion exchanger zirconium diethylene triamine (ZrD) has been synthesized by a simple sol–gel route using inexpensive and easily available chemicals. ZrD has been characterized for elemental analysis (ICP-AES, CHN analysis), TGA, FTIR, X-ray diffraction, SEM and EDX. Physical and ion exchange characteristics as well as chemical stability of the material in various media have been studied. Structural determination reveals that ZrD exhibits amphoteric character. Anion exchange capacity (AEC) for Cl− , Br− , Cr2 O7 2− , F− and AsO4 3− has been determined. Cations are exchanged through chelation where coordinating sites are offered by nitrogen atoms present in the amine groups of ZrD. Distribution coefficient Kd for Co2+ , Ni2+ , Cu2+ , Zn2+ (transition metal ions) and Hg2+ , Cd2+ , Pb2+ (heavy metal ions) has been evaluated by batch equilibration techniques in aqueous and various electrolyte media/concentrations. Based on ˛ the separation factor, a few binary separations have been performed on a chromatographic column packed with ZrD. The amphoteric behaviour of ZrD has been demonstrated by simultaneous exchange of Cu2+ and Cl− in CuCl2 . A study on the regeneration and reuse of ZrD indicates that it is effective upto four cycles without much decline in performance. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ion-exchange chromatography is a well-known and widely used technique for the separation and preconcentration of inorganic ions. However, the innumerable separation procedures published so far are almost exclusively based on the use of monofunctional ion exchangers, mostly strongly basic anion exchangers with
∗ Corresponding author at: Applied Chemistry Department, Faculty of Technology and Engineering, The M.S. University of Baroda, Vadodara 390 001, Gujarat, India. Tel.: +91 265 2434188x415; fax: +91 265 2423898. E-mail addresses: [email protected] (B. Shah), [email protected], [email protected] (U. Chudasama). http://dx.doi.org/10.1016/j.jhazmat.2014.05.031 0304-3894/© 2014 Elsevier B.V. All rights reserved.
quaternary ammonium groups and strongly acidic cation exchangers with sulphonic acid groups [1]. Amphoteric ion exchangers in contrast to conventional monofunctional exchangers contain anionic and cationic exchange sites, and under appropriate conditions, can exchange simultaneously anions and cations from external solutions [2]. Although it was surmised long ago [3], that simultaneous presence of anion and cation exchange groups may offer new interesting separation possibilities, very few attempts to exploit these possibilities in practice can be found in literature [2,3]. Some of these amphoteric ion exchangers are chelating resins with functional groups able to form complexes with several cations and are used for the preconcentration of trace elements [3–6], the sorptive effect of ions being based on the distribution of soluble ions between an aqueous solution and a reactive
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polymer/resin/material containing a selective ligand [2,7–9]. However, the most important influence on sorption is the internal structure of the exchangers and coordinating ability of ligand end groups, which coordinate or chelate metal ions [10–13]. Resins with nitrogen atom containing ligands have been extensively reported [13]. Very early reports reveal that amphoteric exchanger thorium triethanolamine was prepared by incorporation of triethanolamine group into the matrix of thorium oxide, where in nitrogen atom present in the amine group offers chelating sites for metal ions. Zirconium-bis(triethylamine) exhibiting amphoteric character that exchanges anions and adsorbs cations through chelation action has also been reported [14,15]. In the present endeavour, a new and novel hybrid chelating ion exchanger, zirconium diethylene triamine (ZrD), has been synthesized by a simple sol–gel route using inexpensive and easily available chemicals. ZrD has been characterized by instrumental methods of analysis, physical and ion exchange characteristics as well as chemical stability of the material in various acids, bases and organic solvent media have also been studied. Anion exchange capacity (AEC) for Cl− , Br− , Cr2 O7 2− , F− and AsO4 3− has been determined distribution coefficient Kd for Co2+ , Ni2+ , Cu2+ , Zn2+ (transition metal ions) and Hg2+ , Cd2+ , Pb2+ (heavy metal ions) has been evaluated by batch equilibration techniques in aqueous and various electrolyte media/concentrations. Based on ˛, the separation factor a few binary separations have been performed on a chromatographic column packed with ZrD. The amphoteric behaviour of ZrD has been demonstrated using CuCl2 and HgCl2 and amount of cation and anion simultaneously exchanged determined. The practical applicability of ZrD as an amphoteric exchanger has been further highlighted by performing a case study including regeneration and reuse of ZrD. 2. Experimental 2.1. Materials and methods Zirconium oxychloride (ZrOCl2 ·8H2 O) and diethylene triamine (DETA) (C4 H13 N3 : molecular weight = 103.17 and density = 0.955 g/mL) was procured from Loba Chemicals. All other chemicals and reagents used were of analytical grade. Doubledistilled water was used for all the studies. 2.2. Synthesis of ZrD The main objective was to obtain a material with maximum anion exchange capacity (AEC). Several samples of material were synthesized by sol–gel method varying several condition/parameters such as mole ratio of reactants, temperature, mode of mixing (metal salt solution to DETA solution or vice versa), pH and rate of mixing, in each case using AEC as the indicative tool. Table S1 (Supporting Information) describes optimization of parameters for synthesis of ZrD. 2.2.1. Synthesis of ZrD at optimized condition ZrD was prepared by mixing aqueous solutions of DETA (0.1 M, 50 mL) and ZrOCl2 (0.1 M, 50 mL) at room temperature, dropwise and with continuous stirring. A gelatinous precipitate was obtained, and solution along with precipitate further stirred for 1 h. The resulting gelatinous precipitate was allowed to stand for 15 h, then filtered, washed with double distilled water till removal of adhering ions, followed by drying at room temperature. The material was then broken down to the desired particle size 30–60 mesh (ASTM) by grinding and sieving. An yield of 35% was obtained. This material was used for all studies.
139
2.3. Characterization Physical characteristics such as appearance, percentage moisture content, apparent density, true density, void volume fraction, concentration of fixed ionogenic groups and volume capacity were studied according to literature methods [16–18]. A study on pH titration curve, chemical stability, anion exchange capacity (AEC) and effect of calcination on AEC are described below. Further, the material has been characterized by Instrumental methods. Zr was analyzed by ICP-AES performed on Labtam, 8440 Plasma lab. While C, H, N analysis was performed on Perkin Elmer-2400. TGA was performed on a thermal analyzer Shimadzu (model TGA-50) at a heating rate of 10 ◦ C/min. FTIR spectra was obtained using KBr pellet on a Shimadzu (model 8400S). 1 H NMR spectra was obtained using NMR spectrometer (Bruker 500 MHz) in a D2 O solvent. UVDRS was obtained using Shimadzu (Model UV-DRS 2450). X-ray diffractogram was obtained on X-ray diffractometer (Brucker AXS D8) with Cu K␣ radiation with nickel filter. SEM and EDX of the material were obtained on Jeol JSM-5610-SLV scanning electron microscope. 2.3.1. pH titration curve The pH titration of ZrD was performed by Topp and Pepper method [16–18]. 500 mg of exchanger was treated with 50 mL 0.01 M HCl solution with intermittent shaking. The solution along with exchanger was allowed to equilibrate and pH noted when the value was constant. This was the initial reading. This solution mixture was now titrated with 0.01 M NaCl solution. After addition of every 0.5 mL of titrant, sufficient time was provided for establishment of equilibrium between the ion exchanger and the solution. A pH titration curve was obtained by plotting pH vs. volume of NaCl as depicted in Fig. 2. 2.3.2. Chemical stability The chemical stability of ZrD in various media – acids (HCl, H2 SO4 and HNO3 ), bases (NaOH and KOH) and organic solvents (ethanol, benzene, acetone and acetic acid) was studied by taking 0.5 g of ZrD in 50 mL of the particular medium and allowed to stand for 24 h. The change in colour, nature, weight, solubility, metal washout and particle size, etc. was observed. To confirm the solubility of exchanger in a particular medium, supernatant liquid was checked qualitatively for respective elements of exchanger. The chemical stability is expressed as maximum tolerable limits evaluated in a particular medium. 2.4. Anion exchange properties of ZrD 2.4.1. Determination of anion exchange capacity (AEC) Two grams of the exchanger was treated with NaCl/KBr/K2 Cr2 O7 (0.2 M, 20 mL) for 30 min in a conical flask with continuous shaking and material then separated from solution by decantation. This process was repeated at least five times. The material was finally washed with double distilled water for removal of any adhering ions and dried at room temperature. 0.5 g of this material [(ZrD) exchanged (Cl− /Br− /Cr2 O7 2− )2 ] was placed in a glass column [30 cm × 1 cm (internal diameter)], double distilled water was poured onto column with a flow rate adjusted to 0.5 mL min−1 to wash the column. A 1.0 M, 250 mL sodium nitrate solution was now passed through the column. The effluent containing (NaCl/KBr) was titrated against 0.1 M AgNO3 for determination of chloride and bromide, while the effluent containing (K2 Cr2 O7 ) was titrated against 0.1 M FeSO4 (NH4 )2 SO4 ·6H2 O solution for determination of dichromate. AEC for Cl− , Br− and Cr2 O7 − was determined using the formula aV/W, where a is molarity, V the amount of titrant used during titration, and W is the weight of the exchanger. The AEC values for F− and AsO4 3− was
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Table 1 Physical and ion exchange characteristics of ZrD. Physico-chemical and ion exchange characterization Characteristics
Observation
Appearance Glassy transparent granules 250–590 m Particle size (range) 16.49% % Moisture content True density 1.46 0.53 Apparent density 0.65 Void volume fraction 3.13 Concentration of fixed ionogenic groups Volume capacity of resin 1.07 Nature of exchanger Mono functional Anion exchange capacity − Cl 1.91 Br− 0.96 0.92 Cr2 O7 2− 0.94 F− 2.10 AsO4 3− Effect of temperature on anion exchange capacity for Cl− AEC (RT) 1.91 1.15 100 ◦ C ◦ 200 C 1.09 0.98 300 ◦ C 0.90 400 ◦ C 0.84 500 ◦ C Maximum tolerable limits Chemical stability 18 N H2 SO4 , 8 N HNO3 , 5.6 N HCl (i) Acids 2 N NaOH, 5 N KOH (ii) Bases (iii) Organic solvents Ethanol, benzene, acetone, acetic acid
determined by a colorimetric method [19,20]. Arsenic, based on the formation of molybdoarsenate with subsequent reduction to heteropoly blue and fluoride, was determined by SPANDS ((2-(psulphophenylazo)1,8-dihydroxynaphthalene-3,6-disulphonic acid trisodium salt)) method [19,20]. A detailed procedure has been provided in Supporting Information section. 2.4.2. Effect of calcination on AEC The effect of calcination on AEC (Table 1)(in case of Cl− ) was studied by calcining several 1 g portions of the material for 2 h in the temperature range 100–500 ◦ C at 100 ◦ C intervals in a muffle furnace, cooling them to room temperature, and determining AEC by column method discussed above. 2.5. Cation exchange properties of ZrD 2.5.1. Equilibrium time determination Ten milliliter metal ion (0.014 M) solution was shaken with 0.1 g of exchanger ZrD in stoppered conical flasks at room temperature for different time intervals (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 7.0 h). The supernatant liquid was removed after every prescribed time interval, and the metal ion concentration evaluated by EDTA titration. From the plot of fractional attainment of equilibrium U(t) vs. time (t), maximum equilibrium time was determined. 2.5.2. Effect of metal ion concentration on distribution coefficient Effect of metal ion concentration on distribution coefficient (Kd ) for Co2+ , Ni2+ , Cu2+ , Zn2+ (transition metals) and Cd2+ , Hg2+ , Pb2+ (heavy metals) was determined by batch method. 0.1 g of ZrD was equilibrated with 20 mL of varying metal ion concentration (0.002–0.02 M with interval of 0.002 M) for 6 h (maximum equilibrium time) at room temperature. The metal ion concentration before and after exchange was determined by EDTA titration.
(heavy metal ions) was added and pH (1–7) adjusted using dilute HNO3 /NaOH and the mixture shaken for 30 min. The supernatant liquid was used to determine the metal ion concentration by EDTA titrations. Uptake of metal ions (%) has been calculated using formula, [(C0 − Ce )/C0 ] × 100, where C0 = initial concentration of metal ion in mg L−1 and Ce = final concentration of metal ion in mg L−1 . 2.5.4. Distribution study (Kd ) at optimized condition Kd has been evaluated by batch method at optimum condition (optimum metal ion concentration, pH of maximum adsorption and maximum equilibrium time – 6 h) using 0.1 g of the exchanger in aqueous as well as various electrolyte media like HNO3 , citric acid and HClO4 of 0.02 and 0.2 M concentration at room temperature. The supernatant liquid was used to determine the metal ion concentration by EDTA titrations. Kd was evaluated using the expression, Kd = [(I − F)/F] × V/W (mL g−1 ), where I = total amount of the metal ion in the solution initially, F = total amount of metal ions left in the solution after equilibrium, V = volume of the metal ion solution, and W = weight of the exchanger in grams. 2.6. Elution and separation studies For elution studies (single metal), 0.5 g ZrD was packed in a glass column (30 cm × 1 cm), washed thoroughly with double distilled water, and flow rate adjusted to 0.5 mL min−1 . The metal ion solution (0.014 M, 10 mL) was loaded onto the column. The metal ion loaded was eluted with reagents like HNO3 , citric acid and HClO4 , of 0.02 and 0.2 M concentration. For binary separation the mixture of the metal ions (0.014 M, 10 mL of each metal ion) to be separated was loaded on column. The separation was achieved by passing suitable eluent through the column. In all cases, the metal ion concentration was determined quantitatively by EDTA titration. For every experimental point in graphs, two identical sets are prepared to compare/verify the obtained values. Reproducibility in values for same experimental point was assessed by again preparing two identical sets. The percentage metal recovered (metal eluted) was calculated in terms of % elution expressed as, % E = (Ce /C0 ) × 100 where Ce is the concentration of the metal ion in the eluted solution and C0 is the concentration of metal ion loaded onto the column. 2.7. Amphoteric behaviour of the material Amphoteric behaviour of the ZrD was studied by batch method. 0.1 g of ZrD was treated with CuCl2 /HgCl2 and allowed to stand for 24 h. The supernatant solution was titrated with EDTA for determination of cations and argentometric titration for determination of anions, respectively. The exchanger was collected for EDX analysis for confirming the adsorption of cations and anions on exchanger surface. 2.8. Case study The practical applicability of ZrD has been demonstrated in a case study using tap water sample from our institute. The tap water sample was passed through a column packed with 0.5 g ZrD (prepared as above) in 10 mL fractions. The initial sample as well as eluent collected were analyzed for the presence of respective cations before and after exchange by EDTA titration and argentometric titration for determination of anions and it is further confirmed by EDX analysis. 2.9. Regeneration and reusability of ion exchanger
2.5.3. Effect of pH on adsorption of metal ions To 0.1 g of exchanger, ZrD, 10 mL of 0.014 M metal ion solution CO2+ , Ni2+ , Cu2+ , Zn2+ (transition metal ions) and Cd2+ , Hg2+ , Pb2+
Regeneration and reuse of ion exchanger ZrD was performed in the case of Copper ion by batch method. 0.014 M (Optimum
B. Shah, U. Chudasama / Journal of Hazardous Materials 276 (2014) 138–148
O
O
Zr
Zr
O
N
N H
Zr H N
N
(OH)n
O
Zr N
Zr
Zr
%T
N
55 50 45 40 35 30 25 20 15 10 5 0 4000
O
O
141
1410 1580
Cu-ZrD 1360 1580 ZrD 3500
3000
2500
2000
1500
1000
500
wavelenght (cm-1)
n
Fig. 4. FTIR of ZrD and Cu2+ treated ZrD. Fig. 1. Proposed structure of ZrD gel.
Kd(R) = Kd(C) /Kd × 100 where Kd = initial value obtained, Kd(C) = Kd determined in each subsequent cycle.
6
pH
5 4
3. Results and discussion
3
3.1. Structural studies
2
ZrD was obtained as hard glassy transparent granules. A gel is formed when an aqueous solution of DETA is added to aqueous solution of ZrOCl2 (pH = 1). The solution first turns neutral (pH = 7) and then basic (pH = 9). The gel formed is entirely different as compared to addition of an inorganic base to ZrOCl2 . The gel formation probably takes place in two steps: (a) formation of hydrous ZrO2 gel [21,22] followed by (b) surface interaction and polymerization of amine molecules involving weak and strong bonding, resulting in the formation of a macro-particle [21,22] and can be formulated as [(ZrO)n (DETA)n ](OH)n (Fig. 1). Since the unit Zr-DETA in ZrD is positively charged, it therefore behaves as an anion exchanger where OH part of ZrD behaves as the anion exchange unit. The pH titration curve (Fig. 2) of ZrD exhibits mono functional behaviour [14,15,21]. When ZrD was kept in contact with a solution of sodium nitrate, no release of H+ was observed. However, the presence of NaOH was tested by phenolphthalein indicator (which turns the solution pink) confirming OH as anion exchange sites. It is quite clear that the component [Zr(DETA)] contains free amino groups, offering chelating sites to metal ions and behaves as the cation exchange unit. Due to lone pair of electrons present on the nitrogen atom of amine groups (Fig. 1). ZrD can act as chelating exchanger. The chelating property of ZrD was studied by a batch process. When
1 0
0
2
4
6
8
Volume of 0.01M NaCl Fig. 2. pH titration curve.
concentration) Cu2+ solution was treated with 0.1 g of ZrD and kept for 6 h (maximum equilibrium time) after which metal ion concentration was determined by EDTA titration and Kd value determined. The Cu2+ exchanged ZrD was treated with HNO3 (1 M, 50 mL) for 30 min with occasional shaking. The sample ZrD was separated from acid by decantation and treated with double distilled water to remove adhering acid. This process was repeated at least five times to ensure complete removal of Cu2+ from exchanger. Kd values were determined using this regenerated ZrD. This process was repeated till wide variation in Kd values was observed. % retention in Kd values Kd(R) was determined using the expression,
Fig. 3. EDX of (A) ZrD and (B) Cu2+ exchanged ZrD.
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Fig. 5. SEM of (A) ZrD and (B) Cu2+ exchanged ZrD.
Fig. 6. UV-DRS spectra of ZrD and Cu2+ exchanged ZrD.
ZrD was treated with Cu2+ salt solution it turns to a deep blue colour confirming the chelation of Cu2+ with ZrD. Elemental analysis shows % of Zr = 60.80, % of C = 9.52, % of N = 8.18 which matches well with formula [(ZrO)6 (C8 H18 N6 )](OH)4 . Further, the presence of Zr and N is confirmed by EDX (Fig. 3A). Based on the above discussion, in FTIR spectrum of ZrD, sharp bands are expected in the region ∼3300 cm−1 ( NH stretching frequency) and at ∼3400 cm−1 ( OH stretching frequency), indicating presence of free NH2 groups and OH (anion sites) in ZrD. However, the FTIR (Fig. 4) spectrum of ZrD shows a broad band in the region ∼3500–3200 cm−1 which is probably due to merging of OH and
NH stretching frequency. Band at ∼1580 cm−1 is attributed to C C stretching vibration, while, band at ∼1360 cm−1 is attributed to C N bending. The presence of Cu2+ on exchanger was confirmed by scanning an FTIR spectrum of Cu2+ exchanged ZrD, in which the ∼1360 cm−1 band shifts to ∼1410 cm−1 due to coordination of Cu2+ with nitrogen atom of amine group. This is further, confirmed by SEM (Fig. 5) and EDX (Fig. 3B) of ZrD and Cu2+ exchanged ZrD. Fig. 6 presents UV-DRS spectra for ZrD, before and after Cu2+ chelation. In the present study Cu2+ chelated ZrD exhibits peak around 500 nm which could be attributed to d–d transition band of a Cu2+ ion centred in pseudo-octahedral environment [23]. Fig. 7(A and B) presents NMR spectra of ZrD before and after Cu2+ chelation, respectively. In Fig. 7A, the peak at 3.04 corresponds to (multiplate methylene group –Zr–N–CH2 –) and peak at 2.42 corresponds to (multiplate methylene group CH2 –N–CH2 ). In Fig. 7B, these bands shift to 4.02 and 3.05, respectively, attributed to copper chelated cation. Accordingly, Cu2+ chelated ZrD could be depicted as proposed (Fig. 8). Based on formulation, [Zr(DETA)]2 (OH)2 anion exchange capacity was determined for Cl− , Br− , Cr2 O7 2− , F− and As2 O4 3− . The AEC values in mequiv. g−1 are presented in (Table 1). TGA (Fig. 9) of ZrD exhibits two regions of weight loss. The first weight loss in the temperature range 30–150 ◦ C could be attributed to loss of moisture/hydrated water. A second weight loss in the temperature range 150–500 ◦ C, is attributed to dehydroxylation, and decomposition of the organic moiety. The thermal behaviour of the materials is further supported by the effect of calcination on AEC. As observed from (Table 1), AEC values decrease as calcination temperature increases. Decrease in AEC with increasing temperature could be attributed to dehydroxylation and decomposition of organic moiety. The absence of sharp peaks in the XRD (Fig. 10) of ZrD indicates amorphous nature of the materials. When using ion exchangers, it
Fig. 7. NMR spectra of (A) ZrD and (B) Cu2+ exchanged ZrD.
B. Shah, U. Chudasama / Journal of Hazardous Materials 276 (2014) 138–148
O
Zr
Zr N H
N O
1.2
Zr
N
Cu2+
(OH)n
N
H N
N
O
Zr
Zr
Zr
Fractional attainment of equilibrium U(t)
O
1 0.8 0.6 0.4
0.2 0
O
O
143
0
n
100
200
300
400
Time (min) Fig. 8. Proposed structure of Cu2+ exchanged ZrD. Fig. 11. Fractional attainment of equilibrium U(t) vs. time (t).
10 3.2. Cation exchange properties of ZrD
8 2 nd weight loss
6 mg
1 st weight loss
4 2 0 0
250 500 Temperature (ºC )
750
Fig. 9. TGA of ZrD.
has to be subjected to various environments and chemical media. A study on the chemical stability of ZrD shows that it is stable in acid, base and organic solvent media (Table 1). No transmetalation or Zirconium metal washout is also observed, confirming the ZrD moiety to be the rigid framework. Physical and ion exchange characteristics of ZrD are presented in (Table 1).
9000
A plot of fractional attainment of equilibrium U(t) vs. time (t) indicates that maximum equilibrium time for ZrD is 6 h (Fig. 11). Initially, rate of adsorption is faster due to availability of a large number of chelating sites [24,25]. A maximum removal of Cu2+ is observed at 6 h. Subsequently with decrease in number of chelation sites rate of metal uptake decreases [26]. It is observed that with increase in metal ion concentration, Kd values increase. Above a particular concentration, Kd values are constant which could be explained to be due to the fact that at lower concentrations, almost all the ions are exchanged due to availability of exchangeable sites, which are not available at higher concentrations. A plot of Kd values vs. metal ion concentration (Fig. 12) shows optimum concentration for all metal ions. It is observed that at pH values less than ∼3, very less sorption has been observed for all metal ions. The lack of sorption at low pH could be attributed to high concentration of hydrogen ions competing with the metal ions for sorption/exchange sites. Amongst transition metal ions maximum sorption is observed at pH 5 in case Cu2+ and Co2+ , percentage uptake being 94.76 and 82.11, respectively. In case of Ni2+ and Zn2+ maximum sorption is observed at pH 4, percentage uptake being 73.58 and 78.82, respectively. Amongst heavy metal ion maximum sorption is observed at pH 4, percentage uptake being 89.69 and 64.44 for Hg2+ and Cd2+ , respectively. In case of Pb2+ maximum sorption take place at pH-3 and percentage uptake being 94.55. The distribution coefficient (Kd ) values evaluated for the metal ions under study at optimum conditions using ZrD have been
8000
6000 5000 4000 3000 2000
1000 900 800 700 600 500 400 300 200 100 0
Cu2+ Ni2+ Zn2+ Co2+ Pb2+ Cd2+
Distribution coefficient(kd)
Intensity(CPS)
7000
10
20
30
40
50
60
2 Theta Scale Fig. 10. XRD of ZrD.
70
80
90
0
0.005 0.01 0.015 0.02 Concentration of Metal ions
Fig. 12. Distribution coefficient for different metal ions at different concentration.
144
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Table 2 Distribution coefficient (Kd ) (mL g−1 ) at optimum condition in aqueous and various electrolyte media/concentration. Metal ions
Aqueous
0.02 M HNO3
0.2 M HNO3
0.02 M citric acid
0.2 M citric acid
0.02 M HClO4
0.2 M HClO4
Co2+ Ni2+ Cu2+ Zn2+ Pb2+ Cd2+ Hg2+
1647 402 2876 301 1019 374 1980
317 204 306 397 551 265 609
256 115 94 298 540 200 536
231 232 616 32 319 151 754
212 163 584 18 238 70 657
446 59 911 56 466 306 446
354 30 434 41 325 249 256
Maximum deviation in Kd values = ±3. Table 3 Percentage elution of various metal ions in different electrolyte media using ZrD. Metal ions 2+
Co Ni2+ Cu2+ Zn2+ Pb2+ Cd2+ Hg2+
0.02 M HNO3
0.2 M HNO3
0.02 M citric acid
0.2 M citric acid
0.02 M HClO4
0.2 M HClO4
91 83 85 91 84 91 86
93 96 88 99 89 96 88
78 88 84 88 92 85 82
86 89 85 90 93 89 82
89 95 86 90 89 85 83
92 96 89 92 92 86 87
Eluent volume = 60 mL and 50 mL for 0.02 M and 0.2 M electrolytes, respectively. Maximum deviation in % elution of metal ions = ±2. Table 4 Binary separations of metal ions using ZrD. Separation achieved
Eluent
Metal ion loaded (mg)
Metal ion eluted (mg)
Elution (%)
2+
Zn –Co
0.2 M citric acid (Zn2+ ) 0.2 M HNO3 (Co2+ )
9.15 8.25
8.21 6.55
89 82
Zn2+ –Cu2+
0.2 M citric acid (Zn2+ ) 0.2 M HNO3 (Cu2+ )
9.15 8.89
8.89 6.13
97 68
Ni2+ –Cu2+
0.2 M HClO4 (Ni2+ ) 0.2 M HNO3 (Cu2+ )
8.21 8.89
7.88 6.19
96 69
Co2+ –Cu2+
0.2 M citric acid (Co2+ ) 0.2 M HNO3 (Cu2+ )
8.25 8.89
6.60 6.05
80 68
Cd2+ –Hg2+
0.2 M citric acid (Cd2+ ) 0.2 M HNO3 (Hg2+ )
17.36 28.00
14.32 18.06
82 64
Cd2+ –Pb2+
0.2 M HClO4 (Cd2+ ) 0.2 M citric acid (Pb2+ )
17.36 29.00
15.71 22.18
90 74
2+
Maximum deviation in % elution = ±2%.
presented in (Table 2). Kd values in aqueous medium follows the order: Cu2+ > Co2+ > Ni2+ > Zn2+ amongst the transition metal ions, Hg2+ > Pb2+ > Cd2+ amongst the heavy metal ions. 3.3. Elution and separation studies using ZrD The elution behaviour of transition metal ions and heavy metal ions (under study) have been carried out using different electrolytes such as HNO3 , citric acid and HClO4 , of 0.02 and 0.2 M concentration and results presented in (Table 3). The % metal eluted in all cases is in the range 79–97%. Good elution is observed due to presence of single metal ion and non-interference of elements. Typical elution curves for Cu2+ ions with different electrolytes has been presented in Fig. 13. Higher concentration of eluent and acids in general, are better eluents. Order of % metal eluted amongst transition metal ions is Zn2+ (99%) > Ni2+ (96%) > Co2+ (93%) > Cu2+ (88%) and amongst heavy metal ions is Cd2+ (96%) > Pb2+ (89%) > Hg2+ (88%). This observation is in keeping with the fact that metal ions with high Kd values are less eluted and vice versa. Separation factor ˛, the rate at which two constituents separate on a column, given by, ˛ = Kd1 /Kd2 , where Kd1 and Kd2 are the distribution coefficients of the two constituents being separated, provides a guideline for metal separation. The greater the deviation of ˛ from unity, the better is the separation. The efficiency of
an ion exchange separation depends on the condition under which ˛ has a useful value, or influencing in a direction favourable to separation. For a given metal ion pair, the electrolyte media in which the separation factor is the highest, is selected as the eluent. Thus, a study on distribution behaviour of metal ions in various electrolyte media gives an idea about the eluents that can be used for separation. The separation factor (˛) of different metal ions pairs are presented in (Fig. 14). The separation factor (˛) values are very high indicating efficient separations can be achieved. Binary separations for following metal ion pairs (under study) Zn2+ –Co2+ , Zn2+ –Cu2+ , Ni2+ –Cu2+ , Co2+ –Cu2+ , Cd2+ –Hg2+ , Cd2+ –Pb2+ have been performed using concept of high separation factor in a particular medium as discussed earlier in the text. In binary separations (Fig. 15), separation efficiency is in the range 68–96% amongst transition metal ions and 64–90% amongst heavy metal ions (Table 4). In all cases of binary separation, irrespective of metal ion pair, maximum % metal eluted is Zn2+ (97%), Ni2+ (96%), Co2+ (82%), Cu2+ (68%) (amongst transition metal ions) and Cd2+ (90%), Pb2+ (74%), Hg2+ (64%), (amongst heavy metal ions). This observation is in keeping with separation factor (˛) and Kd values of metal ions. % metal eluted decreases with decreasing separation factor and increases with increasing separation factor and as explained earlier, metal ions with high Kd values are less eluted and vice versa. From the above
B. Shah, U. Chudasama / Journal of Hazardous Materials 276 (2014) 138–148
40
0.02M HNO3
0.2M HNO3 % Elution
% Elution
30 25 20 15 10 5 0
30 20 10 0
0
0
10 20 30 40 50 60
20
40
A 50
50 0.2M HClO4
40 % Elution
% Elution
B 0.02M HClO4
40
60
Volume of eluant (ml)
Volume of eluant (ml)
30 20 10
30 20 10
0 0
0
10 20 30 40 50 60
0
Volume of eluant (ml)
10 20 30 40 50 60 Volume of eluant (ml)
D
C 30 20 10
% Elution
40 0.02M Citric Acid
% Elution
145
0.2M Citric Acid
30 20 10 0
0
0 0
10 20 30 40 50 60 Volume of eluant(ml)
10
20
Fig. 13. Single elution curves for Cu citric acid using ZrD.
3.4. Amphoteric behaviour of ZrDETA On treating a ZrD packed column with CuCl2 /HgCl2 simultaneous exchange of cations and anions occurred. Respective cations
F
and anions present before and after exchange are presented in Table 5. Further, the presence of exchanged Cu2+ , Hg2+ and Cl− on ZrD is confirmed by EDX (Fig. 16A and B). 3.5. Case study Practical utility of ZrD has been demonstrated in a case study. Simultaneous exchange of cations and anions occurred on treating Table 5 Simultaneous uptake Cu2+ , Hg2+ and Cl− using ZrD.
20 16 Zn2+-Cu2+ Separation factores
50
ion in different electrolytes: (a) 0.02 M HNO3 , (b) 0.2 M HNO3 , (c) 0.02 M HClO4 , (d) 0.2 M HClO4 , (e) 0.02 M citric acid and (f) 0.2 M
studies, it could be concluded that ZrD is an effective chelating ion exchanger.
Cations and anions
12 8
40
Volume of eluant (ml)
E 2+
30
Ni2+-Cu2+ Co2+-Cu2+Hg2+-Cd2+ 2+ 2+ Cd -Pb Zn2+-Co2+
4 0 Metal ion separaion Pairs
Fig. 14. Proposed metal ion separation pairs based on separation factor (˛).
Amount of cation/anion in solution Amount of cation/anion after exchange in solution Amount of cation/anion on exchange Percentage uptake of cation/anion
Cu2+
Cl−
Hg2+
Cl−
1.27
1.41
1.28
1.41
0.99
0.07
4.00
0.09
0.28
1.34
1.28
1.31
22.97
95.03
32.00
92.90
Maximum deviation in percentage uptake of cation and anion = ±1%.
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40
25 H2O
20 15 10
Zn2+
Co2+
5
30 25
H2O
20 15 10
Zn2+
Cu2+
5
0
0 0
20
40 60 80 100 120 140 160 Volume of Eluant (ml)
0
20
40 60 80 100 120 140 160 Volume of Eluant (ml)
45 40 35 30 25 20 15 10 5 0
(II)
0.2M HClO4
0.2M HNO3 % Eluon
% Eluon
(I)
H2O
Ni2+ Cu2+ 0
20
40
60
80
45 40 35 30 25 20 15 10 5 0
0.2M Citric acid
Cu2+
Co2+ 0
100 120 140
20
40
30
0.2M HNO3
25
15 10
0.2M HClO4
25 % Eluon
H2O
20
80 100 120 140
(IV)
0.2M Citric acid
30
60
Volume of Eluant(ml)
(III) 35
0.2M HNO3
H2O
Volume of Eluant(ml)
% Eluon
0.2M HNO3
0.2M Citric acid
35 % Eluon
% Eluon
30
40
0.2M HNO3
0.2M Citric acid
35
0.2M Citric acid
20 H2O
15 10
Hg2+
Cd2+
5
5
Cd2+
Pb2+
0
0 0
20 40 60 80 100 120 140 160 Volume of Eluant (ml)
(V)
0
20 40 60 80 100 120 140 160 Volume of Eluant (ml)
(VI)
Fig. 15. Binary separation of transition metal ions: (I) Zn2+ –Co2+ (II) Zn2+ –Cu2+ (III) Ni2+ –Cu2+ (IV) Co2+ –Cu2+ (V) Cd2+ –Hg2+ and (VI) Cd2+ –Pb2+ .
Fig. 16. EDX of ZrD (A) treated with CuCl2 and (B) treated with HgCl2 .
B. Shah, U. Chudasama / Journal of Hazardous Materials 276 (2014) 138–148
147
high solubility in acids or bases and poor performance in column operations. Succinylated mercerized cellulose modified with triethylenetetramine was used for adsorption of Cu2+ , Cd2+ and Pb2+ from aqueous single metal solutions which involves tedious synthesis strategies [29]. In the present endeavour, a new and novel amphoteric ion exchanger, ZrD, has been synthesized by a simple sol–gel route using inexpensive and easily available chemicals. ZrD exhibits good affinity for anions (Cl− , Br− , F− and AsO4 3− ) as well as cations (Cu2+ , Hg2+ , Pb2+ ). The practical applicability of ZrD as an amphoteric exchanger has been well established through case studies including regeneration and reuse of ZrD upto four cycles without much decline in performance. The study reveals the promising use of ZrD as an amphoteric exchanger.
Acknowledgement Fig. 17. EDX of tap water sample.
One of the authors (BS) is thankful to The M.S. University of Baroda, Vadodara for providing a Research Fellowship.
100 % Retention in Kd Values
90 80
Appendix A. Supplementary data
70 Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2014.05.031.
60 50 40 30
References
20 10 0 1
2
3 4 5 Number of cycles
6
7
Fig. 18. % retention in Kd values vs. number of cycles using ZrD.
a ZrD packed column using tap water sample from our institute region. Analysis of water sample by EDX (Fig. 17) in wt.% – Mg2+ = 1.93, Ca2+ = 26.76, Cu2+ = 1.09 and Cl− = 18.85, respectively. Since, concentration of Cu2+ and Mg2+ is very less, we have focused towards Ca2+ and Cl− in the present study. The initial concentration of Ca2+ and Cl− in tap water sample was found to be 300 mg L−1 and 289 mg L−1 , respectively, whereas eluent (after passing through column containing ZrD) contains 151 mg L−1 and 94 mg L−1 of Ca2+ and Cl− , respectively. The water samples were analyzed for Ca2+ and Cl− by EDTA titration and argentometric titration, respectively, before and after passing through the column containing ZrD. 3.6. Regeneration and reusability of ion exchanger A study on regeneration and reuse was performed as described in Section 2.1. It is observed that the exchanger once used can be converted back to its original form by desorption of the metal ions with concentrated nitric acid. A plot of % retention in Kd values vs. number of cycles is presented in Fig. 18, which shows that % retention in Kd values is almost the same upto four cycles, indicating that ZrD could be regenerated and reused without much decline in performance. 4. Conclusion Oxides and hydrous oxides of Zr, Ti, Fe, Al, Nb, Cr, Th, etc. exhibit amphoteric behaviour, exchanging anions in acidic medium and cations in basic medium [27,28]. However, the major drawback of oxides and hydrous oxides as amphoteric exchangers is their
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