187 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7

Determination of Some Trace Elements in Sea-water by Atomic Absorption Spectrometry After Concentration With Modified Silicas*

Suleyman Akman, Hurrem h c e and Unel Koklu Istanbul Teknik Universitesi, Fen-Edebiyat Fakiiltesi, Kimya Boliimii, 80626 Maslak-Istanbul, Turkey

Silica gels modified with 3-aminopropyltriethoxysilane or 3-mercaptopropyltrimethoxysilane groups have been developed for the preconcentration of copper and cadmium prior to their determination by flame atomic absorption spectrometry. The surface areas of the modified silica gels were determined by the Brunauer- Emmett-Teller (BET) method to be 290 m2 g-l of the amino-modified silica and 410 m2 g-1 of the thiol-modified silica. Batch and column methods were used for the separation and concentration of copper and cadmium. These metals were quantitatively retained on both the modified gels in slightly acidic media. In the batch method, the effects of pH, shaking time and type of buffer on the adsorption of copper and cadmium were investigated. In the batch and column procedures both copper and cadmium adsorbed on the silicas were quantitatively recovered (relative standard deviation of 2-6%) using 2 mol dm-3 hydrochloric acid even in the presence of sodium chloride up to a concentration of 1 .O%. Keywords: Adsorption; preconcentration; determination of copper and cadmium; modified silica; flame atomic absorption spectrometry

Pure silica has some ion-exchange properties. However, for most metal ions, the interactions with the silica surface are rather weak, because of the low acidity of the silanol groups as well as the less pronounced donor properties of the surface oxygen atoms. By modifying the silica surface with organo-functional silanes, other groups such as - NH2, -SH, -CN, -PR2 (where R is an alkyl group) can be introduced. Therefore, the modification of silica gel with inorganic and particularly organo-functional groups has been a subject of interest. Terada and co-workers, for example, described the preparation and characteristics of 2- mercaptobenzothiazolel and pdimethylaminobenzylidene- rhodamine2 supported on silica gel and successfully used them for the preconcentration of some transition elements in water samples. Tong et a1.j proposed the use of 3-methyl- 1 -phenyl-4-stearoyl-5-pyrazolone supported on silica gel as a chelating agent for the preconcentration and separation of copper, nickel and cobalt in sodium chloride and tap water.

Koklu and c o - ~ o r k e r s ~ - ~ determined the equilibrium constants for the adsorption of zinc, cadmium, mercury, iron, cobalt, nickel, copper, silver and gold on silicas modified with 3-aminopropyltriethoxysilane and 3-mercap- topropyltrimethoxysilane and investigated some properties of the modified silicas. However, these adsorbents were not used for the preconcentration of trace elements prior to determination by atomic absorption spectrometry (AAS).

In this study, silica gel is modified with 3-aminopropyltri- ethoxysilane, (C2H50)Si(CH2)3NH2, and 3-mercaptopro- pyltrimethoxysilane, (CH30)Si(CH2)3SH, to obtain amino- (A-S) and thiol- (T-S) modified silicas, respectively. The characteristics of these modified silicas and their applica- tions to the preconcentration separation of copper and cadmium prior to determination by AAS are described.

Experimental Apparatus and Chemicals A Beckman Model 1272 flame atomic absorption spectro- meter was used for the determination of copper and cadmium. Instrumental parameters were adjusted accord- ing to the manufacturer's recommendations. All chemicals were of analytical-reagent grade. Standbrd stock solutions

* Presented at the XXVII Colloquium Spectroscopicum Interna- tionale (CSI), Bergen, Norway, June 9- 14, 199 l .

of copper and cadmium (1000 mg 1 - * ) were prepared from Titrisol concentrates (Merck). Reference solutions were prepared as required by further dilution with distilled water.

Preparation of Thiol- and Amino-modified Silica Gels At first, silica gel was digested with dilute hydrochloric acid (1 + 1 ) and washed with distilled water. The cleaned silica gel was dried at 150 "C for 24 h. The silanes, (C2H50)3- SiCH2CH2NH2 or (CH30)3SiCH2CH2CH2SH were added to a silica-methanol suspension (1 mmol of silane per gram of silica) and the methanol was boiled off under vacuum. The residual silica was washed until the washings contained no dissolved materials and then dried. The surface area of the modified silica gels were determined by the Brunauer- Emmett-Teller (BET) method (290 m2 g-' of the amino- modified silica and 410 m2 8-I of the thio-modified silica).

Procedure for Batch Method Ten millilitres of a standard metal solution (2 ppm of Cu and 0.5 ppm of Cd), 5 ml of a buffer solution and 0.2,0.3 or 0.4 g of the modified silica were mixed together in a small conical flask. The mixture was shaken, using an electric shaker, for 30 min (optimum). The supernatant was filtered into a measuring flask and the volume was adjusted to 25 ml. The concentration of copper and cadmium was measured using a flame atomic absorption spectrometer.

Procedure for Column Method A burette was filled with 7 g of dried A-S or T-S. A given volume of an aqueous sample, was adjusted to a suitable pH, then percolated through the column at a flow rate of about 3 ml min-'. After washing the column with distilled water, copper and cadmium were eluted with 10 ml of hydro- chloric acid (2 mol drn-j), again at an elution rate of 3 ml min-l. The emuent was collected in a measuring flask, and diluted to 25 ml with distilled water; this volume may vary depending on the concentration of the analyte. The concentration of copper and cadmium was measured using a flame atomic absorption spectrometer.

Results and Discussion The capacities of A-S and T-S for the adsorption of copper were measured by the batch method. Ten millilitres of the

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188 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7

100 - (4

80 -

60-

40 -

4- - (b ) a" 100

80 -

60-

40 -

0 2 4 6 8 10 PH

0 2 4 6 8 10 PH

Fig. 1 Effect of pH on the retention of copper using (a) an A-S and (6) a T-S column: A, 100; By 50; and C, 67 pg of copper per gram of silica

copper solution (75 ppm), 5 ml of buffer solution (pH 7) and 0.3 g of A-S or T-S were mixed for 30 min. The supernatant was filtered and the copper concentration was measured using an atomic absorption spectrometer. The capacities of A-S and T-S were calculated to be about 0.032 and 0.022 mmol of copper per gram of silica, respectively. The capacities of both the modified silicas are large enough to collect and concentrate copper and all the other heavy metals from a large volume of sea-water. In general, the total heavy metal ion concentration of sea-water is not more than approximately 3 x l 0-4 mmol dm-3, which is compat- ible with the capacity of 0.02 g and 0.03 g of A-S and T-S, respectively. On the other hand, when high concentration factors are necessary, a column process rather than a batch method should be used, e g . , a concentration factor of around 200-300 may be necessary for the determination of copper and cadmium in sea-water. For samples which need only a 10-fold preconcentration step, a batch method could be used.

Retention of Copper and Cadmium at Various pH Values The retention of copper and cadmium on A-S and T-S for various pH values and for various analyte masses per gram of silica are given in Figs. 1 and 2. The pH of the solutions was adjusted using the following buffers: sodium acetate- hydrochloric acid for pH 1-5; potassium dihydrogen phosphate-sodium hydroxide for pH 6-7; and boric acid- potassium chloride-sodium hydroxide for pH 8- 10. The type of buffer is very important if quantitative retention is to be achieved. Citrate, tartrate and ammonia buffers

Fig. 2 Effect of pH on the retention of cadmium using (a) an A-S and (b) a T-S column: A, 25; By 17; and C, 12 pg of cadmium per gram of silica

prevent quantitative adsorption. Copper is retained quanti- tatively on A-S and T-S at pH 6.5 and 3.0, respectively. Cadmium shows similar behaviour and its quantitative retention is reached after 30 min. This value is almost independent of the pH of the solution and the ratio of analyte to silica. With respect to reproducibility, retention values have a standard deviation of about 5%, which represents the sum of the uncertainties in the measurement of absorbance by flame AAS, modification of the analyte (leading to non-uni formi t y), weighing, adjusting solution volumes and the irreproducibility of blanks (in spite of close control of impurities). Reproducibility is better at high retentions especially at, 100% retention. When using the column technique, the results are much more reprodu- cible because the experiments are performed with the same silica, so some of the uncertainties in the batch method, which cannot be controlled, e.g., imperfection of modifica- tion, uncertainties in the weighing and adjusting the volumes, are removed. Both copper and cadmium were quantitatively released from the modified silicas with 2 mol dm-3 hydrochloric acid.

Determination Using the Column Method When high concentration factors are necessary, e.g., in sea- water analysis, a column method is required. To 500 ml of sodium chloride solution containing 0.5 and 1.0% of salt, 100 pg of copper and 200 pg of cadmium standards were added. After adjusting the pH to 7 with 30 ml of buffer, the solutions were passed through the A-S and T-S columns and the metals were subsequently eluted with 2 mol dm-3

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JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7 189

~

Table 1 Effect of sodium chloride on the determination of copper and cadmium

T-S A-S

Sample Found*/pg Recoveryt (O/O) Found*/pg Recoveryt (Oh)

100 pg of Cu in 0.5% NaCl I06 + 2 98 91 + 3 91 I00 pg of Cu in 1 .O% NaCl 11523 95 9 0 2 4 90 200 pg of Cd in 0.5% NaCl 200 * 5 90 185k5 90 200 pg of Cd in 1.0% NaCl 22026 89 220 * 6 I10

*Results are given as the average of three determinations with the relative standard deviation (Yo). t Recoveries are calculated by comparing the averages of standards prepared directly from stock solutions and those of samples passed

through the column.

hydrochloric acid. The flow and elution rates were about 3 ml min-l in every case. Standards and blanks were treated in a similar way. The effect of sodium chloride on the determination of copper and cadmium is shown in Table 1. Each metal is recovered satisfactorily in the presence of sodium chloride, which shows that the proposed method could be used for the determinaton of trace metals in environmental water samples, e.g., sea-water.

The 3-aminopropyltriethoxysilane or 3-mercaptopropyl- trimethoxysilane supported on silica gel show an excellent adsorption capacity for copper and cadmium in acidic media. Because of this fast absorption, a short shaking time for the batch process or a high flow rate for the column process is needed in order to obtain quantitative retention. The other attractive feature of these materials is the ease of preparation.

References 1

2

3

4 5

6

7

Terada, K., Inoue, A., Inamura, J., and Kiba, T., Bull. Chem. Soc. Jpn., 1977, 50, 1060. Terada, K., Morimoto, K., and Kiba, T., Anal. Chim. Acta, 1980, 116, 127. Tong, A., Akama, Y., and Tanaka, S., Analyst, 1980, 115, 947. Koklii, U., Chim. Acta Turc., 1984, 12, 265. Tavcioglu, S., and Koklii, U., Chim. Acta Turc., 1986, 14, 387. Aydin, A., Koklu, U., and Tavcioglu, S., Chim. Acta Turc., 1987, 15, 91. Koklii, U., and Tavcioglu, S., Chim. Acta Turc., 1986, 10 283.

Paper 1 /03025F Received June 17, 1991

Accepted December 3, 1991

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