2302
Ind. Eng. Chem. Res. 2004, 43, 2302-2309
Equilibrium Studies on the Optimization of Solid-Phase Extraction of Metal Ions with Pyrogallol-Anchored Cellulose Synthesized by a New Method and Applications of the Extraction in Metal Enrichment, Removal, and Determination Vibha Gurnani and Ajai K. Singh* Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India
Pyrogallol is anchored onto cellulose via -NH-CH2-CH2-NH-SO2-C6H4-NdN- linker to prepare an effective sorbent for solid-phase extraction of Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), and Pb(II) from aqueous solutions. The modified cellulose is characterized by FT-IR, 13C CPMAS NMR, TGA, and elemental analyses. The conditions for effective sorption are optimized with respect to different experimental parameters in both batch and column processes. The feasibility of the sorption process was judged from both kinetic and thermodynamic points of view. The lower limit of detection as well as the enrichment factor for each metal ion was evaluated. Analytical utility of the solid-phase extraction is illustrated by successful determination of metal ions in natural water, pharmaceutical, and milk samples by flame AAS coupled with enrichment on cellulose loaded with pyrogallol. Introduction “Metal enrichment” essentially assists us in coping with the inadequate sensitivity and selectivity of analytical methods such as flame AAS or ICP-AES when applied to geochemical and environmental matrixes, which are typically complex. The determination of metal ions at low concentration (micro/trace) levels is very important at present, particularly in an area of increasing concern, environmental quality and public health. Among the different techniques known for separation and enrichment, solid-phase extraction1 is at present more favored. It is superior to the liquid-liquid extraction in terms of simplicity, rapidity, and the ability to obtain a high enrichment factor. Additionally, the ecofriendliness, freedom from generation of a large volume of waste, and possibility of multiple uses are also in its favor. The macromolecular chelators for solidphase extraction of metal ions have been found to be more selective than ion exchangers. They collect metal ions through chelation with the donor sites available on their surface. Silica gel,2-4 organic polymeric resins,5-7 and cellulose8-10 are the three main macromolecular systems which after ligand immobilization can function as macromolecular chelators and are promising for solidphase extraction of metal ions. Of these three support materials, cellulose is attractive due to its good porosity (fiber structure), hydrophilicity, and chemical reactivity, useful for its easy functionalization. The biodegradability of cellulose is an additional advantage. In addition, cellulose is renewable; due to its abundance in nature, the materials based on it are suitable for costeffective technologies. Some cellulose-based sorbents11-15,18-20,22 for enrichment of metal ions are known. We observed in the recent past6,7 that immobilization of ligands of small molecular size gives very efficient macromolecular chelators. Therefore, it was * To whom correspondence should be addressed. Tel.: 091011-2659 1379. Fax: 091-011-2686 2037. E-mail: aksingh@ chemistry.iitd.ac.in.
thought worthwhile to immobilize pyrogallol (a multidentate ligand of small molecular size) on cellulose via -NH-CH2-CH2-NH-SO2-C6H4-NdN- linker and explore the resulting matrix for the enrichment of Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), and Pb(II) including from water, pharmaceutical, and milk samples. The results of these investigations are reported in the present paper. Pyrogallol was immobilized earlier on polystyrene16 and Amberlite XAD-2,17 but the resulting chelators have not shown exceptional sorption capacity because of poor grafting. It was immobilized earlier on cellulose18 also but through a different linker. Moreover, maximum anchoring is not ensured in the earlier study and the macromolecular chelator is also not explored in detail for enrichment of the metal ions mentioned above. Materials and Methods (a) Instrumentation. The cross-polarization magic angle spinning (CPMAS) 13C NMR spectrum was recorded at 75.3 MHz on a Bruker (Fallenden, Switzerland) 300 spectrometer. The parameters of CPMAS used are number of scans 10 509, short width range 39 920.160 Hz, and acquisition time 0.0128 756 s. A flame atomic absorption spectrometer from Perkin-Elmer Instruments, Shelton, model Analyst 100, equipped with an air-acetylene flame was used for atomic absorption spectrometric measurements. The wavelengths (nm) used for monitoring Cu, Zn, Fe, Ni, Co, Cd, and Pb are 324.8, 213.9, 248.8, 231.1, 240.7, 228.8, and 217.0 nm, respectively. A Nicolet (Madison, WI) FT-IR spectrometer, model Prote´ge´ 460, was used to record IR spectra (in KBr) in the range 400-4000 cm-1. The pH was measured with a digital pH meter (Toshniwal Instruments, Ajmer, India). Thermogravimetric analyses were carried out on a Dupont (Wilmington, DE) 2100 thermal analyzer, and a Perkin-Elmer Series II C, H, N, S/O analyzer, model 2400, was used for elemental analyses. The flow of solution through the column was controlled using a peristaltic pump (Watson-Marlow model 101/
10.1021/ie030453n CCC: $27.50 © 2004 American Chemical Society Published on Web 04/03/2004
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2303 Scheme 1
U/R, Falmouth, U.K.). The sorption-desorption studies of the metal ions on the chelating matrix were generally carried out using columns (Pharmacia, Bromma, Sweden) of 1 cm diameter and 10 cm length equipped with adjustable frits. A mechanical shaker equipped with an incubator (Hindustan Scientific, New Delhi, India) with a speed of 200 strokes min-1 was used for batch equilibration. (b) Reagents and Solutions. Microcrystalline cellulose (20 µm), N,N-dimethylformamide, ethylenediamine, thionyl chloride, and pyrogallol were all obtained from E. Merck (India). 4-Acetamidobenzenesulfonyl chloride was procured from Acros Organics, NJ. The metal salts and other chemicals used were of analytical reagent grade. The stock standard solutions21 of metal ions (1000 mg dm-3) were prepared by dissolving copper(II) sulfate pentahydrate, zinc(II) sulfate heptahydrate, ammonium iron(II) sulfate hexahydrate (followed by aerial oxidation), nickel(II) sulfate hexahydrate, cobalt(II) chloride hexahydrate, cadmium(II) acetate, and lead(II) acetate in an appropriate amount of doubly distilled water acidified with 5 cm3 of the corresponding acid. The pH adjustments were made with HCl, acetate-acetic acid, phosphate, and NH3NH4Cl buffer. The water samples from the Ganges River at Varanasi (India) and Musi River (Andhra Pradesh, India) were collected, acidified with 2% HNO3, filtered, and stored in glass bottles. The glassware was washed with chromic acid, soaked in 5% HNO3 overnight, and cleaned with doubly distilled water before use. Multivitamin tablets Polybion were procured from E. Merck (Mumbai, India), and milk powder Lactogen was obtained from Nestle´ (New Delhi, India). (c) Synthesis of Pyrogallol-Functionalized Cellulose. Pyrogallol was anchored onto cellulose via -NH-CH2-CH2-NH-SO2-C6H4-NdN- linker in a three-step process (Scheme 1). Synthesis of 1. A slurry of 5 g of ethylenediamine cellulose22 prepared from chlorodeoxycellulose23 was made in a mixture of tetrahydrofuran (150 cm3) and
distilled water (200 cm3) at room temperature. Aqueous NaOH (2.5 mol dm-3) was added dropwise to raise the pH to 11.5. 4-Acetamidobenzenesulfonyl chloride (ABSC) (15.8 g) was added as a solid in three equal portions to the slurry with rapid stirring and monitoring the pH. When the pH became steady (∼7.2) after the first addition, THF (30 cm3) and aqueous NaOH (2.5 mol dm-3) were added until the pH again became 11. The second portion of ABSC was added, and the pH similarly increased to 11. After addition of the third portion of ABSC, the pH dropped to 4-5 was brought back to 7.2 with aqueous NaOH. The mixture was further stirred for 4 h and filtered. The resulting solid (1) was washed with water and acetone and dried in vacuo. Synthesis of 2. 1 (2 g) was taken and mixed with HCl (0.75 mol dm-3) with stirring. The mixture was refluxed for 20 min, cooled, and filtered. The resulting solid 2 was washed with distilled water and dried in vacuo. Synthesis of 3. The suspension of 2 (2 g) in 1 mol dm-3 HCl (100 cm3) at 0 °C was reacted with 10% NaNO2 until the reaction mixture began to have a violet color with starch iodide paper and thereafter kept aside for 1 h at 0 °C. The solid was filtered at temperature below 5 °C and treated with pyrogallol (4 g dissolved in 50 cm3 of a 0.5 mol dm-3 aqueous solution of NaOH) at 0 to -5 °C for 6 h. The resulting brown powder was filtered, washed with water, and air-dried. Anal. Calcd for C20H24O9N4S‚3H2O: C, 43.60%; H, 5.40%; N, 10.1%. Found: C, 43.3%; H, 5.25%; N, 9.31%. The size of cellulose particles was found nearly unchanged after chemical modification. Synthesis of Model Compounds 4-6. To synthesize 4, solid ABSC (5 g) and 50 cm3 of ethylamine were mixed and heated on a steam bath for 30 min. The mixture was cooled in an ice bath, and 6 mol dm-3 H2SO4 was added to it in aliquots until it was acidic. On further cooling the reaction mixture in an ice bath, white needle-shaped crystals of 4 appeared, which were filtered, washed with cold water (1-4 °C), and air-dried. 4 (2 g) was refluxed in dilute HCl (50 cm3) for 30 min to obtain 5 in solution. The 10 cm3 portion of solution of 5 was taken, cooled to 0 °C, and treated with cold 5% NaNO2 solution until the reaction mixture began to give a violet color with starch iodide paper. The resulting solution was treated with pyrogallol at 0 °C (0.5 g dissolved in 20 cm3 of 0.5 mol dm-3 NaOH solution). A brown dye 6 was formed, which was filtered, washed with water, and air-dried.
Procedure for Sorption/Enrichment and Monitoring of Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), and Pb(II). The sorption behavior of the metal ions on pyrogallol-anchored cellulose was studied under both dynamic and static conditions. The operational
2304 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 1. Optimum Experimental Conditions for Sorption and Desorption of Metal Ions metal ion experimental parameter
Cu(II)
Zn(II)
Fe(III)
Ni(II)
Co(II)
Cd(II)
Pb(II)
pH HCl concentration for desorption (mol dm-3) flow rate (cm3 min-1) sorption capacity of resin (µmol g-1) average recovery (%) standard deviationb relative standard deviation (%)
4.0-7.0 0.3-1.0 2.0-7.0 202.2 99.6 0.02 1.83
5.0-6.0 0.3-1.0 2.0-5.0 123.3 98.2 0.02 2.32
3.0-5.0 0.4-1.0 2.0-5.0 160.7 98.2 0.02 2.09
6.0-8.0 0.5-1.0 2.0-4.0 148.2 98.4 0.02 2.10
7.0-9.0 0.5-1.0 2.0-4.0 168.6 98.0 0.02 2.04
6.0-8.0 0.4-1.0 2.0-6.0 99.1 99.0 0.03 2.76
4.0-5.0 0.5-1.0a 2.0-5.0 92.4 98.6 0.03 3.01
a
Only HNO3 was used for desorption. b For five extractions of 0.25 µg cm-3 (0.5 µg cm-3 for Pb).
variables were optimized. Sorption capacity was determined by batch method, and for kinetics of sorption this process was used also. To determine the influence of sample volume and flow rate in column operation, the dynamic method was used. The column method was used for application to water, pharmaceutical, and milk samples. Column Process. Pyrogallol-modified cellulose (0.5 g) swollen fully in doubly distilled water for 12 h was packed in a glass column C10/10 (Pharmacia, size 10 cm × 10 mm) by the slurry method using frits. The column was thereafter treated with 50 cm3 of 1.0 mol dm-3 HCl and washed with doubly distilled water until free from acid. A suitable aliquot (depending upon concentration level) of the solution containing Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), or Pb(II) in the concentration range 0.0033-1.0 µg cm-3 was passed through this column after adjusting its pH to an optimum value (Table 1) at a flow rate of 2-7 cm3 min-1 controlled with the peristaltic pump. The column was washed with distilled water to remove free metal ions. The bound metal ions were desorbed from the matrix bed with HCl or HNO3 (10-15 cm3) of optimum concentration (Table 1), and the eluates were aspirated into the flame of a standardized flame AAS instrument. The concentrated eluates were suitably diluted with doubly distilled water before aspiration. Batch Process. A suitable aliquot (depending upon the concentration level) of solution containing 0.1-13 µg cm-3 of Cu(II), Zn(II), Fe(III), Co(II), Ni(II), Pb(II), or Cd(II) was placed in a glass stoppered bottle (250 cm3) after adjusting its pH to the optimum value (Table 1). Pyrogallol-modified cellulose (0.05 g) was added to it, and the bottle was stoppered and shaken for 40 min. The solid was filtered, washed with doubly distilled water, shaken with 1 mol dm-3 HCl or HNO3 (10-15 cm3) for 20 min, and again filtered. The filtrate was aspirated into the flame of a prestandardized flame AAS after suitable dilution if required. Results and Discussion Synthesis and Characterization of PyrogallolModified Cellulose. The temperature of the chlorination reaction (with SOCl2) of cellulose carried out to prepare chlorodeoxycellulose22 was restricted to 60 °C as at higher temperatures degradation of cellulose was noticed. The refluxing of 1 in HCl (0.75 mol dm-3) for 20 min to obtain 2 did not result in its degradation. IR spectral data of model compounds 4-6 authenticate the immobilization of pyrogallol onto cellulose through -NH-CH2-CH2-NH-SO2-C6H4-NdN- linker as there is similarity in the IR bands of pairs 1 and 4 and 3 and 6 (Table 2). Assignment of the band around 1567 cm-1 to a -NdN- stretching vibration (Table 2) is on the basis of a report by Fevre and O’Dwyer.24 1 was
Table 2. IR Bands (cm-1) of 1, 3, 4, and 6 bands/ vibration phenyl νasymm(SdO) νsymm(SdO) C-S -CONH -NdNpyrogallol
1
3
4
6
1593, 1539, 1496 1374 1157 749 1683
1593 1353 1159 746
1596, 1544, 1497 1367 1158 773 1683
1592 1375 1159 780
1567 1521
1561 1521
authenticated by the 13C CPMAS NMR spectrum, which has signals due to six carbon atoms of the glucose unit at δ 106.8, 90.4, 76.3, 73.4, and 66.9 ppm. Additional signals consistent with the solution 13C{1H} NMR spectrum of the 4-CH3CONHC6H5SO2- group are observed at δ 174.4, 145.6, 135.8, 131.8, and 122 ppm in the 13C CPMAS spectrum of 1. They may be assigned to the italicized carbon atoms of CO, ArC-SO2, ArCNHCOCH3, Ar-C ortho to the -SO2- group, and Ar-C meta to the -SO2- group, respectively. Moreover, the signal appearing at 26 may be assigned to the methyl carbon of the acetamido group. The decomposition (other than water loss) of pure cellulose when recording the TGA in a nitrogen atmosphere starts at about 320 °C, and rapid weight loss occurs at 350-380 °C, whereas pyrogallol-loaded cellulose shows a rapid weight loss at 243.5-344 °C. This decreasing of the temperature of decomposition is consistent with earlier observations.25 The weight loss in the TGA at 150 °C suggests the presence of ∼3.0 water molecules per repeat unit of the polymer, which is supported by elemental analyses. Optimization of Operational Parameters for Enrichment of Metal Ions. Metal ions are sorbed due to chelation via the -NdN- group and OH group ortho to it as well as through two other OH groups which are ortho to each other. In view of steric considerations, it is largely through two OH groups. The column method was optimized for quantitative sorption (pH and flow rate) and desorption (concentration and volume of eluent) of Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), and Pb(II). The multivariate approach was used for this purpose. Each optimum condition was established by repeated trials; others were kept at the optimum value and rechecked after standardizing those remaining. The pH of the medium is one of the most important factors controlling the limit of extractability of metal ions and an important criterion for application to real samples. The protons present in solution compete with metal ions for interacting with donor atoms of the chelating ligand. Consequently, at low pH values, metal-ligand interaction and thus metal extraction is hampered. At high pH values, the hydrolysis of metal ions reduces the extraction. A typical process for optimizing the pH value for Co is as follows. A set of solutions (volume 100 cm3)
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2305
Figure 1. Effect of pH on the sorption of Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), and Pb(II).
containing 0.25 µg cm-3 of Co was taken. The pH of the solutions of the set was adjusted to different values in the range 2.0-10.0 with 0.01 mol dm-3 HCl, acetateacetic acid, phosphate, or NH3-NH4Cl buffer, and the solutions were passed through the column at a flow rate of 3 cm3 min-1 controlled by a peristaltic pump. The bound metal ions were eluted from the solid matrix with 25 cm3 of 1 mol dm-3 HCl after washing off free metal ions from the column with doubly distilled water, and their concentration was determined with flame AAS. The optimum pH range for maximum recovery of each metal ion is given in Table 1. The recovery profiles for all seven cations as a function of pH is shown in Figure 1. The effect of pH on sorption was also studied using the recommended batch method, and the results were found to be consistent with those of the column method. The use of 4-8 cm3 of acetate, phosphate, and ammonia buffer to adjust pH does not affect the sorption behavior. At pH 3.0, there is a huge difference between the % extraction of Fe and those of Co and Ni, which can be exploited for their separations at micro and submicro levels. The flow rate of solution through a column influences the metal ion sorption on the material packed in it. This parameter is thus important if the material is to be used for an on-line metal separation/enrichment system. It depends on the stability of metal complex and characteristics of material controlling diffusion through it. Thus, the amount of metal ions sorbed on a pyrogallolimmobilized cellulose column was studied at different flow rates (2-10 cm3 min-1) controlled with the aid of a peristaltic pump. The Cu was quantitatively sorbed (∼99.6%) even at a flow rate of 7 cm3 min-1. All the metals were quantitatively retained on the column at a flow rate 2-4 cm3. Flow rates lower than 2 cm3 min-1 were not used to avoid long extraction times. Similarly, for desorption, a flow rate of 2.0-4.0 cm3 min-1 was found to be sufficient. The optimum acid concentration for elution of sorbed metal ions was determined by eluting the metal ions from the modified cellulose column with 25 cm3 of HCl/ HNO3 of varying concentration (0.001-1.0 mol dm-3), keeping an optimum flow rate. It was found that 0.3 mol dm-3 HCl was sufficient for Cu and Zn, 0.4 mol dm-3 for Fe and Cd, and 0.5 mol dm-3 for Ni and Co. The 0.5 mol dm-3 HNO3 was sufficient for quantitative recovery (98.6%) of Pb. As the recovery for all the metals was found to be quantitative with 0.5 mol dm-3 HCl/ HNO3, further studies on the column were carried out with 0.5 mol dm-3 HCl/HNO3. The optimum acid
concentrations for maximum and instantaneous recovery (98.0-99.6%) are given in Table 1. The elution with acid also regenerates the chelating matrix which can be reused. The efficacy of the eluent (0.5 mol dm-3 HCl/ HNO3) was studied by taking its different volume (325 cm3) and applying the recommended column procedure. It was found that 8 cm3 of 0.5 mol dm-3 HCl was sufficient for quantitative recovery of Cu, Zn, and Fe, 10 cm3 for Cd, and 15 cm3 for Ni and Co. For lead, 8 cm3 of 0.5 mol dm-3 HNO3 gave 98.6% recovery. The metal ions were not significantly desorbed ( 0.99), where Cs is the concentration of metal ion in solution at equilibrium (mg dm-3) and Nf is the concentration of metal ions sorbed per gram of the matrix (mg g-1). This suggests that sorption process conforms to the Langmuir model, which assumes uniform distribution of sorption sites and sorption energies without interactions between the sorbed molecules, which is described by eq 1
Cs Cs 1 ) + Nf Ns Nsb
(1)
The adsorption coefficient Ns is the maximum amount of solute sorbed per gram of surface (mg g-1), corresponding to a condition in which all available sites are filled. The coefficient b is related primarily to the net enthalpy of adsorption. From linear sorption isotherm
∆G ) -RT ln KC
(3)
where R (J K-1 mol-1) is the universal gas constant and T (K) is the absolute temperature. The negative values of ∆G suggest the sorption process is spontaneous and favorable thermodynamically (Table 3). The difference in the value of Ns and b for different metal ions (Table 3) suggests that the size of the metal ion influences the extent of sorption. The fitting of sorption with the Langmuir model further corroborates the conclusion that chelation is the major reason for sorption of metal ions. Once the all chelation sites are bonded to metal, nonspecific sorption is minimal. Moreover, out of the two chelation possibilities, only one at a time appears to be operative. In view of steric considerations, it is most probably through two OH groups. Preconcentration Limit, Enrichment Factor, and Limit of Detection. The enrichment factor was determined by increasing the dilution of metal ion solution while keeping the total amount of loaded metal ion fixed at 10 µg for Cu, Zn, Fe, Cd, or Pb and 15 µg for Co or Ni and applying the recommended column procedure (see Experimental Section). The maximum preconcentration factors achieved, corresponding lowest concentration (below which recoveries become nonquantitative), and final volume for elution are given in Table 4. The feed volume, i.e., maximum volume of solution passed, and the recoveries at the lowest concentration are also reported. The limit of detection (LOD) values (defined as (blank + 3s) where s is the standard
Figure 3. Kinetics of metal ion sorption on pyrogallol-loaded cellulose.
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2307 Table 3. Sorption and Thermodynamic Parameters (298 K) parameter metal ion
KC
-∆G (kJ mol-1)
Ns (mg g-1)
B (dm3 mg-1)
R2
sorption capacity (mg g-1)
Cu(II) Zn(II) Fe(III) Ni(II) Co(II) Cd(II) Pb(II)
15.00 ( 0.28 2.64 ( 0.17 2.87 ( 0.21 3.29 ( 0.33 1.16 ( 0.08 7.00 ( 0.32 15.00 ( 0.33
6.71 ( 0.38 2.41 ( 0.16 2.61 ( 0.18 2.95 ( 0.27 0.37 ( 0.18 4.82 ( 0.56 6.71 ( 0.53
13.11 ( 0.28 8.18 ( 0.14 8.94 ( 0.11 8.76 ( 0.27 10.15 ( 0.17 11.40 ( 0.23 20.37 ( 0.47
1.59 ( 0.13 0.78 ( 0.03 0.41 ( 0.03 0.57 ( 0.04 0.14 ( 0.01 0.7 ( 0.09 1.29 ( 0.21
0.9995 0.9973 0.9904 0.9862 0.9902 0.9957 0.9988
12.84 8.06 8.97 8.70 9.94 11.14 19.15
Table 5. Tolerance Limit of Electrolytes, Ca(II), Mg(II), and Other Species in Enrichment tolerance limit (mol dm-3/aµg cm-3/bmmol dm-3) species NaNO3 NaCl NaBr NaI Na3PO4 Na2SO4 humic acida EDTAb citric acid ascorbic acid sodium tartarate Ca(II) Mg(II) Figure 4. Sorption isotherm of Cu on cellulose anchored with pyrogallol. Table 4. Enrichment Factors and Enrichment Limits of Metal Ions metal ion
total volume (cm3)
concn (ng cm-3)
final volume (cm3)
recovery %
preconcn factor
Cu Zn Fe Ni Co Cd Pb
3000 2000 2500 2000 2000 2500 2000
3.3 5.0 4.0 7.5 7.5 4.0 5.0
10 10 10 15 15 10 10
99.6 ( 0.5 99.3 ( 0.3 99.9 ( 0.4 98.3 ( 0.5 98.3 ( 0.4 98.6 ( 0.2 99.0 ( 0.2
300 200 250 133 133 250 200
deviation of the blank determination) are 0.52, 3.67, 4.82, 2.14, 1.78, 1.05, and 4.39 µg dm-3 for Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), and Pb(II), respectively, and the corresponding limit of quantification (blank + 10s) values are 0.62, 3.84, 4.98, 2.34, 1.92, 1.16, and 4.59 µg dm-3, respectively. They were determined with the column procedure given in the Experimental Section, using distilled water in place of metal ion solution. Effect of Electrolytes, Cations, and Organic Species on Sorption. The chelation process on immobilized ligand for the removal of soluble metals has become an important option in the integrated approach to aqueous waste treatment. The effluents discharged from various industries and atomic energy plants may contain various complexing agents such as aminopolycarboxylic acids including EDTA, diethylenetriaminepentaacetic acid, and nitrilotriacetic acid (NTA). In addition, the chelating matrixes are commonly used to enrich metal ions from water samples. Chloride, nitrate, sulfate, and phosphate ions present in natural water have the capability to complex with many metal ions. Therefore, in the presence of anions and complexing agents, the efficiency of the immobilized ligand to bind
Cu(II) Zn(II) Fe(III) Co(II) Ni(II) Cd(II) Pb(II) 1.0 0.8 0.8 0.03 0.04 0.4 30 0.01 0.04 1.0 0.40
0.2 0.3 0.3 0.04 0.02 0.3 25 0.003 0.04 0.75 0.08
0.8 0.6 0.6 0.1 0.002 0.2 15 0.002 0.03 0.50 0.30
0.2 0.04
0.01 0.08 0.008 0.01
0.8 0.2 0.4 0.1 0.006 0.1 8 0.001 0.05 0.40 0.40
0.3 0.1 0.2 0.2 0.004 0.1 20 0.001 0.04 0.50 0.07
0.4 0.5 0.4 0.3 0.006 0.4 25 0.001 0.02 0.45 0.08
0.5 0.01 0.2 0.2 0.03 0.3 30 0.002 0.10 1.00 0.50
0.002 0.02 0.02 0.03 0.002 0.002 0.001 0.001
with metal ions may be hampered, resulting in reduction of overall extraction, as the anions and complexing agents form complexes of reasonably good stability with metal ions which in turn may not bind with the donor sites present on the cellulose. Thus, the effect of NaCl, NaBr, Na3PO4, NaNO3, NaI, Na2SO4, humic acid, citric acid, sodium tartrate, and EDTA on the sorption efficiency of Cu(II), Zn(II), Fe(III) Ni(II), Co(II), Cd(II), and Pb(II) on the functionalized cellulose was studied by column process (as for cleanup it is most likely to be used). The effect of bivalent cations such as Ca(II) and Mg(II) (added as chloride and sulfate, respectively) was also investigated. The tolerance limit of electrolytes, Ca(II), and Mg(II) evaluated in each case are given in Table 5. A 3% lower recovery in comparison with the value observed in the absence of interfering species was used as a criterion of interference. Each reported tolerance/interference is in the preconcentration and not in the determination by flame AAS, as checked with the help of reagent-matched standard solutions. The pyrogallol-immobilized cellulose shows good tolerance toward most of the electrolytes. It was observed that NaNO3, NaCl, NaBr, and Na2SO4 do not interfere with the enrichment of metal ions up to a concentration level of 0.1 mol dm-3, except in the case of Pb. Humic acid is tolerable up to 8 µg cm-3. Ascorbic acid is tolerable up to 0.4 mmol dm-3, whereas citric acid and sodium tartrate are tolerable with the metal ions up to a concentration level of 0.02-0.10 and 0.07-0.50 mmol dm-3. Regeneration and Stability of the Sorbent. The matrix was found to be stable up to 516 K, as evident from TGA analysis. The capacity of the matrix was also found to be practically constant (variation 2.5-4%) after its repeated use for more than 20 times, indicating that its repeated use is feasible as leaching of the ligand from the matrix or its deterioration with use is insignificant. The matrix showed no signs of degradation on treatment with HCl/HNO3 up to 2.5 mol dm-3.
2308 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 6. Determination of Metal Ions in Water Samples metal ion (µg dm-3 ) origin of sample
method
Cu(II)
RSD (%)
Zn(II)
RSD (%)
Fe(III)
RSD (%)
Musi River, Andhra pradesh Ganges River, Varanasi tap water, Ghaziabad
direct SA direct SA direct SA
16.80 16.20 11.35 10.90 13.95 13.65
4.1 6.6 4.6 3.1 4.1 3.8
129.4 130.2 15.10 15.05 29.50 29.15
2.3 2.2 5.1 3.0 3.8 1.3
43.75 43.85 106.80 106.60 81.90 81.35
5.5 3.1 1.9 1.2 4.5 2.8
a
Ni(II)
RSD (%)
6.75 6.55
Co(II)
RSD (%)
3.30 3.05 4.25 4.10 7.10 6.95
6.3 3.7 5.9 3.3 4.0 7.0
7.0 5.0
Cd(II))
RSD (%)
Pb(II)
RSD (%)
54.55 55.30 19.40 19.05 38.70 38.10
4.4 2.9 6.0 4.3 6.0 5.6
Direct: recommended procedure is directly applied. SA: standard addition method RSDs for five determinations.
Applications of the Enrichment Process. (i) Determination of Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), and Pb(II) in River Water and Tap Water Samples. Cellulose functionalized with pyrogallol was used to enrich Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), and Pb(II) ions in water samples collected from the Ganges River (Varanasi, India), the Musi River (Andhra Pradesh, India), and tap water (Ghaziabad, India). For this purpose the samples were subjected to the recommended column procedure (with and without standard addition) after adjusting their pH to an optimum value. The seven metal ions were determined with flame AAS. When applying the standard addition (SA) method, 1000 cm3 of water sample was spiked with 50-100 µg of each of the seven metal ions before subjecting it to the recommended column procedure. The results are given in Table 6 and reflect the suitability of the chelating matrix for water clean up and analysis. The values reported under SA method are after subtracting the amount added. The closeness of results of direct and SA method indicates the reliability of the present results of metal analyses in water samples. (ii) Determination of Co in Pharmaceutical Samples. Cellulose functionalized with pyrogallol was used for determining cobalt in multivitamin tablets. Ten tablets (weighing 3.28 g) of Polybion (E. Merck, Mumbai, India) were digested in a beaker containing 20 cm3 of concentrated HNO3 by slowly increasing the temperature of the mixture to 400 K. The mixture was further heated until a solid residue was obtained. It was allowed to cool and dissolved in 25 cm3 of concentrated HNO3. The solution was gently evaporated on a steam bath until a residue was left again. It was mixed with 50 dm3 of distilled water, and concentrated HNO3 was added dropwise until a clear solution was obtained on gentle heating. The pH of the solution was adjusted to 7.0, and the concentration of cobalt was estimated by the recommended column procedure using flame AAS. The average (four determinations) amount of cobalt was found to be 1.96 µg g-1 of tablet with a RSD of 2.37%. The reported value of cobalt in the tablet is 1.99 µg g-1. (iii) Analysis of Synthetic Water Samples. To check the validity and accuracy of the present matrix coupled with FAAS for metal ion monitoring, the recommended column procedure was applied to determine copper and iron content in synthetic water samples (1000 cm3) having a composition similar to those of certified water samples SLRS-4 (National Research Council, Ottawa, Ontario, Canada). The average values of three determinations of copper and iron were found to be 1.79 and 101.7 µg dm-3 with RSD values 1.96% and 2.05%, respectively. Amounts (µg dm-3) present in the synthetic certified sample are Cu ) 1.81 and Fe ) 103. (iv) Determination of Zinc in a Milk Sample. A sample of powdered milk (1.0 g) was digested in a
beaker with an acid mixture containing 20 cm3 of concentrated sulfuric acid and 5 cm3 of concentrated of nitric acid until a clear solution was obtained. It was allowed to cool, and most of the acid was neutralized with sodium hydroxide. The pH of the solution was adjusted to optimum value (Table 1), and the volume was 500 cm3. The concentration of zinc was estimated by the recommended column procedure using flame AAS. The average (four determinations) amount of zinc was found to be 38.5 µg g-1 of a milk sample with a RSD of 2.05%. The reported value of zinc in the sample is 38 µg g-1. Conclusion A new chelating matrix having pyrogallol anchored onto cellulose via -NH-CH2-CH2-NH-SO2-C6H4NdN- linker has been synthesized and characterized by FT-IR, 13C CPMAS NMR, TGA, and elemental analyses. The new matrix is an effective sorbent for solid-phase extraction of Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II), and Pb(II) from aqueous solutions and has been applied, after standardization of various parameters of extraction, to enrich metal ions from natural water, pharmaceutical, and milk samples. Acknowledgment The authors thank the Department of Atomic Energy (India) and the Council of Scientific and Industrial Research (India) (Project 01(1575)/99 EMR-II) for financial support. Literature Cited (1) Simpson, N. J. K. Solid-Phase Extraction, Principles, Techniques and Applications, 1st ed.; Marcel Dekker: New York, 2000. (2) Sarkar, M.; Datta, P. K.; Das, M. Equilibrium Studies on the Optimization of Solid-Phase Extraction Using Modified Silica Gel for Removal, Recovery and Enrichment Prior to the Determination of Some Metal Ions from Aqueous Samples of Different Origin. Ind. Eng. Chem. Res. 2002, 41, 6745. (3) Shirashi, Y.; Nishimura, G.; Hirai, T.; Komasawa, I. Separation of Transition Metals Using Inorganic Adsorbents Modified with Chelating Ligands. Ind. Eng. Chem. Res. 2002, 41, 5065. (4) Arakaki, L. N. H.; Sousa, A. N.; Espinola, J. G. P.; Oliveira, S. F.; Airoldi, C. Chemisorption and Thermodynamic Data of the Interaction between a Chelate Free Acidic Centre with Basic Groups Attached to Grafted Silicas. J. Colloid Interface Sci. 2002, 249, 290. (5) Li, A.; Long, C.; Sun, Y.; Zhang, Q.; Liu, F.; Chen, J. A New Phenolic Hydroxyl Modified Polystyrene Adsorbent for the Removal of Phenolic Compounds from Their Aqueous Solutions. Sep. Sci. Technol. 2002, 37, 3211. (6) Kumar, M.; Rathore, D. P. S.; Singh, A. K. Metal Ion Enrichment with Amberlite XAD-2 Functionalized with Tiron: Analytical Applications. Analyst 2000, 125, 1221. (7) Tewari, P. K.; Singh, A. K. Thiosalicylic Acid-Immobilized Amberlite XAD-2: Metal Sorption and Applications in Estimation
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Received for review May 27, 2003 Revised manuscript received January 12, 2004 Accepted January 20, 2004 IE030453N
