Preconcentration of metal ions using an immobilized dialkyldithiocarbamate in a flow system

MICROCHEMICAL

JOURNAL

45, 210-218 (1992)

Preconcentration of Metal Ions Using an Immobilized Dialkyldithiocarbamate in a Flow System ALAN TOWNSHEND' AND KHALIL A.J. HABIB School of Chemistry,

University

of Hull, Hull HU6 7RX Great Britain

Received November 15, 1991; accepted November 18, 1991 Flow injection analysis with on-line preconcentration using a minicolumn loaded with dialkyldithiocarbamate immobilized on controlled pore glass is described for the detennination of Rh(III), Co’+, CL?+, H$+, and Hgz2+. The detection limits range from 0.05 ng ml-’ for Cu*+ to 50 ng ml-’ for Hg *+ for 5- or lo-ml samples, improvement of 2-3 orders of magnitude compared with direct injection. The operating conditions are optimized and the effects of interferents are studied. The capacity of the collector varied from 0.9 mmol g-’ for Rh(II1) to ca 4 mmol gg ’ (Co*+, Cu*+, Hg2’). 8 1~2 Academic mess, IIIC.

Preconcentration techniques based on chelating ion exchangers have proved to be very important for the determination of traces of metals, owing to their high degree of selectivity for certain metal ions (I). This paper describes the use of a chelating exchanger with dithiocarbamate groups. A polymeric chelating resin containing dithiocarbamate groups has been synthesized by the reaction of polyamine-polyurea with carbon disulfide (2,3) and its applicability determined for concentrating trace metals from aqueous media. Uptake of about 99% was achieved for Ag+ , Hg2+, Cu2+, Sb(III), Pb*+ , and Cd2+. The use of the combination of inductively coupled plasma-atomic emission spectrometry and the polydithiocarbamate chelating resin (70-80 mesh) to extend the limit of detection for trace elements in coal and oil samples has also been investigated (4). Chelating functional groups on silica and controlled pore glass (CPG) beads have been used to preconcentrate nanogram per milliliter concentrations of cations from aqueous solution (5, 6). Immobilized ethylenediamine and primary and secondary amines and their dithiocarbamates were prepared on such a support after the reaction of silica gel or CPG with various silylating reagents and treating the product with carbon disultide. It was found that the capacity for Zn2+ and Cu2+ ranged from 0.5 to 1.0 mmol g -’ for the different dithiocarbamates. Leyden ef al. (7) have also shown that traces of tungstate and molybdate can be extracted from aqueous sodium chloride solutions on a column packed with immobilized ethylenediamine itself. The detection limit was 18 and 32 ng ml-’ for molybdate and tungstate, when a l-liter sample was used. Preconcentration of heavy metals on dithiocarbamate-modified filter paper has been described (8). It was prepared by the silylation of silica gel contained in Whatman SG81 paper with N-2-aminoethyl-3-aminopropyltriethoxysilane and treatment of the product with carbon disulfide to give the dithiocarbamate. Best ’ To whom correspondence should be addressed. 210 0026-265x/92 $1.50 Copyright 6 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.

METAL

ION

PRECONCENTRATION

IN

A FLOW

SYSTEM

211

recoveries of Fe(III), Cu*+ , Ni*+ , Zn*+ , and Pb*+ were obtained in the pH range 5-6 using acetate buffer. Ryan and Weber (9) investigated the use of certain immobilized chelating agents for recovery and preconcentration of trace Cu*+ from media containing natural complexing agents. The immobilized chelating agents used included N-propylethylenediamine and its bis(dithiocarbamate). In addition immobilized 8quinolinol and Chelex-100 (iminodiacetate function) were tested. It was found that Chelex-100 gave the poorest results, removing only 62% of Cu*+. The immobilized 8-quinolinol is probably the most suitable of the materials tested for preconcentration work. A back titration of 8quinolinol with CU*+ monitored by ion-selective electrode indicated a conditional stability constant pK = 7.9, which is larger than the corresponding value for the complex with nonimmobilized 8-quinolinol under the same conditions. Dialkyldithiocarbamate-silica gel has been made by the silylation of silica with N-methyl-3-aminopropyltriethoxysilane followed by treatment with carbon disulfide. It has been used to bind copper ions and then enrich some amino acids chosen as Lewis base model compounds (20). The choice of this system (copper/ amino acid) is based on a knowledge of behavior in homogeneous solution. Moreover, ligand exchange chromatography has been extensively studied for the separation of amino acids on copper-loaded silicas. After the enrichment step, the amino acids tested were eluted directly onto an analytical column, separated, and detected by fluorescence after postcolumn derivatization with o-phthalaldehyde. This paper describes the preparation of a dialkyldithiocarbamate from N-methyl-3-aminopropyltriethoxysilane bound to CPG, and its use in a flow injection system for preconcentration of several metal ions prior to their determination by flame atomic absorption spectrometry (AAS). CPG was chosen because immobilization of 8-quinolinol-5-sulfonic acid thereon had produced a collector with metal ion capacity as high as 8 mmol gg ’ (II). EXPERIMENTAL

Reagents All chemicals used were of analytical grade, unless otherwise stated. Distilled/ deionized water was used throughout. Standard solutions of Cu*+ and Co*+ were made up by suitable dilutions of 1000 pg ml- ’ aqueous standard solutions for AAS (Spectrosol grade, BDH). A 1000 kg ml-’ Hg,*+ solution was prepared by dissolving 0.1398 g of Hg2(N0,),*2H20 (AnalaR,BDH) in 100 ml of 1% HNO, (Spectrosol). Mercury(I1) solution (1000 kg ml- ‘) was prepared by dissolving 0.1357 g of HgCl, (AnalaR) in 100 ml of deionized water. A stock solution of Rh(II1) was prepared by dissolving 2.558 g of RhCl,*3H20 (AnalaR) in 100ml of 1% nitric acid. N-Methyl-3-aminopropyltriethoxysilane was obtained from Fluorochem Ltd. (Derbyshire, England). The controlled pore glass was 22.6 nm pore diameter, 80-120 mesh (Sigma Chemical Co.). pH dependence was studied using 0.1 M sodium hydrogen orthophosphate and Tris buffers. Apparatus An atomic absorption spectrometer (Varian AA 775) was used with an air/ acetylene flame, and connected to a chart recorder (Chessell BD 4040). A peri-

212

TOWNSHEND

AND

HABIB

staltic pump (Ismatec Minipuls SA 8031) and a rotary Teflon valve (Rheodyne 5020) were also used, together with connecting tubing of 0.8 mm i.d. Teflon. The chelating CPG was held in a linear glass tube (4 cm long, 2.5 mm i.d.). The flow injection manifold used is shown in Fig. 1. Preparation of Immobilized N,N-Dialkyldithiocarbamate(5) A 5-ml portion of a 10% (v/v) solution of N-methyl-3-aminopropyltriethoxysilane in dry toluene was added to 1 g of CPG (activated with 10 ml of 5% nitric acid for 30 min). The mixture was stirred for 15 min and placed in an oven to remove the solvent for 12 h at 80°C. The silylated CPG was washed with water (50 ml) and allowed to air dry. The dithiocarbamate was made by stirring 1 g of silylated CPG with 10 ml of benzene, 5 ml of 2-propanol, 5 ml of carbon disulfide, and 1 ml of a 10% (w/v) solution of tetramethylammonium hydroxide in methanol for 15 min. The product was filtered off, washed with 2-propanol, allowed to air dry, and stored in a refrigerator (4°C). The reaction stages are shown in Fig. 2. Exchange Capacity Determination A solution (25 ml) of 0.1 M metal ion (Co*+ or Cu2+) in Tris buffer, pH 8.5 or 7.5, respectively, or Rh(II1) or H8+ in phosphate buffer, pH 9.0 or 7.0, respectively, was added to the CPG-dithiocarbamate (0.2 g). This mixture was allowed to equilibrate for 16 h with stirring at room temperature (25”(Z), after which the solid was filtered off. The capacity was determined from the decrease in metal ion concentration of the filtrate measured by injection into the manifold shown in Fig. 3 (see below). The capacity (C) was calculated according to the following equation: C = (Ci - Cf)VlW, where ci is the initial concentration of the metal M before equilibration, cf is the concentration of M after equilibration, w is the weight (g) of the exchanger, and V is the volume (ml) of the solution (22, 23). Direct InjectionlAAS The single stream manifold used is shown in Fig. 3. Determinations were carried out by AAS at 343.5, 240.7, 324.8, and 253.7 nm for Rh, Co, Cu, and Hg, 2ml min-1 Metal

ion solution

HN03 P

I

water 2 FIG. 1. Schematic diagram of the FM/MS system for on-line preconcentration using an immobilized N,N-dialkyldithiocarbamate column. (1) Three-way valve, (2) peristaltic pump, (3) injection valve, (4) column of immobilized N,N-dialkyldithiocarbamate, (5) atomic absorption spectrometric detector; the tube length between injection valve and nebulizer is 20 cm.

METAL

ION PRECONCENTRATION

IN A FLOW

SYSTEM

213

CS2/0H15 min

FIG. 2. Synthesis of immobilized NJ-substituted

dithiocarbamate.

respectively. The flame conditions and other parameters were: fuel:oxidant ratio, 8:40; burner height, 12 mm; no background correction. PreconcentrationlAAS For on-line preconcentration, a three-way valve with a single key and the minicolumn filled with the CPG-dialkyldithiocarbamate were added to the above manifold, as shown in Fig. 1. Metal ions were absorbed from volumes of solution up to 10 ml. The column was washed with water (ca. 10 ml), and the accumulated metal ions released by injection of acid (see below) for transport to the spectrometer. RESULTS AND DISCUSSION Direct Injection The calibration graphs and figures of merit for direct determination of Cu2+ and Hg2+ in this system have been discussed (II). The correlation coefficients, limits 2ml min-1

FIG. 3. Single line FIA/AAS manifold where the carrier solution (C) is deionized water, sample volume is 100 ~1, and the tube length between the injection valve and nebulizer is 20 cm. (1) Peristaltic pump, (2) injection valve; W is to waste.

214

TOWNSHEND

AND

HABIB

of detection, sampling frequencies, and the relative standard deviations (RSDs) for eight replicate injections for Rh(III), Co2+, and Hg2’+ are shown in Table 1. The combination of a continuous flow system with the conventional instrumental method was used with a 60 p,g ml- ’ Rh(II1) solution for estimation of dispersion. The dispersion coefficient was found to be 7.2,3.7, and 2.7 for 20-, lOO-, and 150~~1samples, respectively. Flow InjectionlPreconcentration The effect of pH on the absorption of metal ions was investigated to find the conditions necessary for their most effective removal from solution. The data from these experiments are presented in Fig. 4. These data show that maximum chelation of Rh(III), Co2+, Cu2+, Hg2+, and Hg22+ occurs at pH 10.0, 9.0, 7.0, 7.0, and 7.0, respectively, when using phosphate buffer. The results of Leyden and Luttrell (5) for the same ligand attached to silica gel are somewhat different; Co2+, Cu2+, and Hg2+ were recovered maximally at pH 7, 6.5, and 6, respectively. The effect of column length on metal ion preconcentration was also studied. The results obtained showed that change in column length over the range 2-5 cm had little effect on the peak height. This also indicates that the contribution of the column to dispersion was negligible compared to the dominating effect of the nebulizer in this respect (24). The effect of concentration of nitric or hydrochloric acid injected to elute the metal ion is shown in Fig. 5. At each corresponding acid level 100 p,l of acid was used. It was found that complete elution was attained with 0.5 M HNO, for Rh(II1) and Co2+, 0.1 M HNO, or 0.5 M HCl for Cu2+, and 0.1 it4 HCl or HNO, for Hg2 + and Hg22+ . The signals of H8+ and Hg22+ with thiourea were 4 times larger than that without thiourea. It was necessary to include thiourea in the 0.1 M HCl or HNO, for the elution of Hg2+ and Hg22f because it forms sufficiently strong complexes with mercury (15) to release it from the immobilized reagent. It was found that 0.05 M and 0.025 M thiourea were suitable for Hg2+ and Hgz2+, respectively. For calibration purposes, using the optimum conditions established above, a TABLE 1 Analytical Performance for the Flow Injection/AAS System with Direct Injection of Sample (100 ~1) Parameter

Rh(II1)

co2+

I-k,* +

Linear range (pg ml-‘) Correlation coefficient” Sampling rate (h- ‘) Detection limitb (pg ml-‘) RSD (%)

O-80

040

O-160

0.9983

0.9992

150 1 2.8 (60)

158 1 1.5 (30)

0.9998

138 3

1.5 (80)

“n=6. ’ 2x noise.

c For eight replicate injections of the specified concentrations in p.g ml- ’ given in parenthesis.

METAL ION PRECONCENTRATION

215

IN A FLOW SYSTEM

PH

FIG. 4. Metal uptake as a function of pH: (0) Rh(III), 0.4 pg ml-‘; (0) Co*+, 0.5 pg ml-‘; (X) CU*+, 0.12 pg ml-‘; (0) Hg*+, 1.5 )Lg ml-‘; (s) Hg,*+, 4 p,g ml-‘.

series of standard metal ion solutions (5 ml) was passed through the column for preconcentration. The metals were eluted with acid of the concentration specified above directly into the nebulizer of the atomic absorption spectrometer. Linear calibration graphs are shown in Fig. 6. Typical peaks for copper are also shown in

0.25 Concentration

0.5

,, 1 .o

I 2.0

of HNOB (M)

FIG. 5. Effect of acid eluent concentration on release of metal ion: (0) Rh(III), 0.4 pg ml-‘; (0) CO*+, 0.8 pg III-‘; (-) Cu*+, 0.12 p,g ml-‘; (0) Hg*+, 2 p.g ml-‘; (*) Hg2*+, 4 pg ml-‘.

216

TOWNSHEND

AND

HABIB

150.

130 110

r

Concentration

of metal ion (ng ml -‘)

FIG. 6. Calibration by a series of metal ion standards as obtained with the manifold shown in Fig. 1. The peaks for copper are also shown, with original concentrations in ng ml-’ (S-ml sample): (0) Cu’+; (0) Co’+ x 10; (x) Rh(III) x 5; (0) H$+ x 40. Co*+ X 10 means the X-axis is multiplied by 10.

the figure. Figures of merit are given in Table 2. The limits of detection are improved by 2-3 orders of magnitude compared with direct injection. Capacity

The metal uptake capacity of the collector was calculated as described above. The results obtained are shown in Table 3. The capacities for the metal ions studied are considerably greater than the values obtained for the same ligand immobilized on silica gel (ca. 0.5 mmol g-‘) for Cu*+ and Zn2+ (5), and are about half of the values found for 8-quinolinol-Ssulfonic acid immobilized on CPG (II). TABLE 2 Analytical Performance of the Flow Injection/AAS System with On-Line Preconcentration of Sample (Sampling Rate 20 h-l) Parameter

Rh(II1)

co2+

cu2+

I-W+

Hgz2+

Linear range (pg ml-‘) Correlation” coefficient Detection limitb (ng ml - ‘)

O-O.6 0.9966

O-O.14 0.999 0.05 (10 ml)

O-2.5 0990

ck5 0.9991

h-4

O-l 0.9987 2 (5 ml)

(2)

(Pl,

(3 ml)

(fi ml)

RSD (%)= (Z2)

(Z)

“n=6. b As Table 1, volume of sample given in parenthesis. c For six replicate readings of the concentration in pg ml-’ given in parenthesis.

(ii’

METAL

ION

PRECONCENTRATION

IN

A FLOW

217

SYSTEM

TABLE 3 Metal Ion Capacity of the Collector Metal ion

pH of buffer

Metal uptake capacity (mmol g-l)

Rh(II1) co2+ cl12+ Hg2+

9.0 (phosphate) 8.5 (Tris) 7.5 (Tris) 7.0 (phosphate)

0.88 4.3 4.6 3.9

The relatively smalI value for rhodium(III) probably reflects a slow reaction with the dithiocarbamate . The column of immobilized dialkyldithiocarbamate functioned satisfactorily for at least 1 month of continuous use, during which time the peak height response was constant. The addition of acid has little effect on the lifetime of the column. The recovery of metal ion was determined by comparing the signal for injection of metal ion with and without the column. The enrichment factor, defined as the ratio of peak heights obtained after, and prior to, preconcentration, was measured by comparing the signal for aspiration of metal ion and after preconcentration of 5 ml of solution of the same concentration. The term “concentration efficiency” which was defined as the product of enrichment factor (EF) and the sampling frequency (number of samples analyzed per minute), is used to compare different on-line column preconcentration systems (16). The results obtained for recoveries, enrichment factors, and concentration efficiencies for the metal ions are presented in Table 4. Zntetierences The effect of various other ions (Cd*+, Cu*+, Pb*+, Co*+, K+, Rh(III), and Hg*+) up to 50 pg ml-’ on the signal of the analyte was studied. It was found that there were little or no effects up to the specified concentration studied, as would be expected from the high capacity of the collector and the lack of interelement effects in AAS. Conclusions The flow injection technique has much to offer in improving the efficiency and in the automation of various separation and preconcentration processes. On-line TABLE 4 Recoveries, Enrichment Factors, and Concentration Efticiencies of the Metal Ions Metal ion

Recovery @/o)

Enrichment factor (fold)

Concentration efficiency”

Rh(III) co2+ cl12+ Hg2+ I-h,’ +

46 100 100 74 38

8 12 18 8 6

2.4 3.6 5.4 2.4 1.8

a Concentration efficiency = EF min- ‘.

218

TOWNSHEND

AND HABIB

operation often dramatically decreases sample consumption and minimizes the risks of contamination. The possibilities of using immobilized complexing and chelating groups on silanes chemically bound to controlled pore glass were investigated for their usefulness as a preconcentration aid for on-line flow injection atomic absorption spectrometry. These studies indicate their properties are more than adequate for this purpose with respect to rate of metal uptake and capacity. They have potential not only in the area of AAS, but also in the other areas where quantitative analytical separations are advantageous. ACKNOWLEDGMENT K. A. J. Habib is grateful to the Iraqi Government (Ministry of Higher Education and Scientific Research), University of Basrah, for financial support.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Mukhejee, A.; Mandal, S. K. Taluntn, 1984, 31, 195. Dingman, J. F., Jr.; Gloss, K. M.; Milano, E. A.; Siggia, S. Anal. Chem., 1974, 46, 774. Dingman, J. F., Jr.; Siggia, S.; Barton, C.; Hiscock, K. Anal. Chem., 1972, 44, 1353. Mahanti, H. M.; Barnes, R. M. Anal. Chim. Actu, 1983, 149, 395. Leyden, D. E.; Luttrell, G. H. Anal. Chem., 1975, 47, 1612. Leyden, D. E.; Luttrell, G. H. Anal. Chim. Acta, 1976, 84, 97. Leyden, D. E.; Steele, M. L.; Jablonski, B. B. Anal. Chim. Acta, 1978, 100, 545. Linder, H. R.; Schreiber, B.; Frei, R. W. Znt. J. Environ. Anal. Chem., 1977, 5, 63. Ryan, D. K.; Weber, J. H. Talanta, 1985, 32, 859. Veuthey J. R.; Haerdi, W. Anal. Chim. Acta, 1989, 225, 303. Devi, S.; Habib, K. A. J.; Townshend, A. Quim. Anal., 1989, 8, 159. Marshall, M. A.; Mottola, H. A. Anal. Chem., 1983, 55, 2089. Berge, D. E.; Going, J. E. Anal. Chim. Acta, 1981, 123, 19. Tyson, J. F.; Appleton, J. M. H.; Idris, A. B. Anal. Chim. Actu, 1983, 145, 159. Kotrly, S.; Sucha, L. Handbook of Equilibria in Analytical Chemistry, Wiley, New York, 1985. Fang, Z.; Ruzicka, J.; Hansen, E. H. Anal. Chim. Acta, 1984, 164, 23.