654
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
Flotation Separation and Atomic Absorption Spectrometric Determination of Selenium(1V) in Water Susumu Nakashima Institute for Agricultural and Biological Sciences, Okayama University, Kurashiki-shi, Okayama 7 10, Japan
A method is descrlbed for the flotation and determination of sub-microgram levels of selenium(I V ) In water. Selenium(IV) in a 1000-mL sample of water is coprecipitated with iron(II1) hydroxide at pH 4.0 f 0.2. The precipitate is floated with the ald of sodium lauryl sulfate and small air bubbles, then separated and dissolved in 4 M hydrochloric acid and diluted to 10 mL. Finally the selenium content Is determined by generatlon of hydrogen selenide using sodlum borohydride as a reductant and subsequent atomic absorption spectrophotometry with a long absorption cell (60 X 1.2 cm i.d.). Selenlum(1V) at the levels of 0.4 pg and 0.8 pg added to water (1000 mL) and to seawater could be determined with a recovery of above 97% and with a relatlve standard deviation within 2 % . The time required for the pre-concentration of selenium(1V) from a 1000-mL volume of solution was about 30 mln per sample.
In these days of concern over environmental pollution, there is a n increasing need for a simple, rapid, and precise method for determining low parts per billion (ppb) of selenium in water. In general, for an accurate determination of selenium at this concentration, pre-concentration is needed from water in order to achieve the necessary sensitivity for analytical methods. Iron(II1) hydroxide is well known as one of the most efficient collectors of trace metals in aqueous systems. As a preconcentration technique for the determination of selenium(1V) in water, coprecipitation with iron(II1) hydroxide is commonly used (1-3). This bulky amorphous precipitate, however, is difficult to filter, and centrifugation is cumbersome for larger volumes. In a previous paper ( 4 ) ,a flotation technique (5,6) in which the precipitate of iron(II1) hydroxide is floated with the aid of sodium oleate and small air bubbles was used for the pre-concentration of arsenic in natural nonsaline waters. As for the flotation of selenium(IV), Tzeng and Zeitlin (7) have reported the adsorption colloid flotation of selenium(IV1 added to a 500-mL seawater sample a t p H 3.5-5.3 using a ferric hydroxide collector, sodium dodecylsulfate (sodium lauryl sulfate) and nitrogen and the subsequent determination of selenium(1V) by the catalytic methylene blue method. However, in that investigation, the procedure used for the determination of selenium after separation is laborious and time-consuming. Recently for selenium, several publications have appeared (8-16) concerned with the development and application of the hydride generation technique using sodium borohydride as a reducing agent. However, a t a less than 1 hg/L level of selenium in water, a precise direct, determination is impracticable even by the highly sensitive atomic absorption spectrophotometry of hydrogen selenide unless a liquid nitrogen trap collection method is used (16). This paper describes the application of the above-mentioned arsenic separation technique ( 4 ) ,with suitable modifications for the pre-concentration of sub-microgram amounts of
selenium(1V) in water. T h e precipitate is readily separated from the mother liquor and then dissolved in dilute hydrochloric acid for the atomic absorption spectrophotometry by generation of hydrogen selenide using NaBH4 as a reductant. The various parameters that affect the flotation and determination of selenium(1V) were investigated. This method is simple and rapid, and applicable to the extraction of selenium(1V) a t low ppb levels from large volumes of water and seawater.
EXPERIMENTAL Apparatus. A Nippon Jarrell-Ash, Model AA-1 Mark 11, atomic absorption spectrophotometer equipped with a Westinghouse selenium hollow-cathodelamp and a custom-made silica absorption cell (60 X 1.2 cm id.) was used with a Beckman bclrner supplied with nitrogen and hydrogen. The apparatus used for hydride generation was a modified Nippon Jarrell-Ash, Model ASD-1A, hydride measurement unit coupled to a custom-made hydride generating cell approximately 40 mL in volume. The schematic diagram of the analytical system was described previously ( 4 ) . All pH readings were made with a Hitachi-Horiba, Model M-5, pH meter, together with a combined glass electrode. The flotation and separation apparatus was similar to that described by Mizuike and co-workers (5, 6). The flotation cell was a glass cylinder, 40 X 6.5 cm i.d., which was fitted with a sintered-glass filter (No. 4)to generate small bubbles. A side arm was added near the bottom of the cell to drain the mother liquor rapidly after the flotation, as shown in Figure 1. Reagents. All reagents were of analytical-reagentgrade except for sodium lauryl sulfate. Aqueous reagents were prepared in deionized, distilled water. A selenium(1V) stock solution (1 mg Se/mL) was prepared from selenium dioxide and before each use a standard solution was freshly prepared by diluting this stock solution. An iron(II1) solution ( 5 mg Fe/mL, in 1M hydrochloric acid) was prepared from an ammonium iron sulfate solution. A sodium lauryl sulfate solution (1 mg/mL) was prepared by dissolving sodium lauryl sulfate (powder, extra-pure reagent, Wako Pure Chemicals, Co.) in 99.5% (v/v) ethanol. A 5% (w/v) NaBH4 solution in a 0.1 M sodium hydroxide solution was prepared. Procedures. Flotation Step. Place lo00 mL of acidified water in a 1000-mL beaker and add 2 mL of iron(II1) solution. Adjust the pH to 4.0 f 0.2 with aqueous ammonia solution to precipitate iron(II1) hydroxide, while stirring magnetically, for at least 10 min. Add 2 mL of sodium lauryl sulfate solution into the beaker. Transfer the contents of the beaker (excluding the stirring bar) to a flotation cell and wash the residue in the beaker into the cell by using three small portions of water. Pass air at a flow rate of 50 mL/min from the lower end of the cell for about 2 min, in order to obtain complete mixing and flotation of the precipitate. Drain most of the mother liquor from the side arm by opening the cock of the drain pipe. After closing the cock, suck off the residual mother liquor through the sintered-glass disk and wash the precipitate with 30 mL of water. Add 4 mL of 4 M hydrochloric acid to the cell to dissolve the precipitate, collect the filtrate by suction in a 10-mL calibrated flask, wash the sintered-glass disk with hydrochloric acid, add washings to the flask, and dilute to 10 mL with 4 M hydrochloric acid. Atomic Absorption Measurement. Transfer 1 mL of freshly prepared 5% (w/v) NaBH, solution into a hydride generating cell and attach the cell to the apparatus. Insert the needle of a plastic syringe containing 1 mL of sample solution containing less than 0.10 pg of selenium through the side-arm seal of the cell.
0003-2700/79/0351-0654$01.00/0Q 1979 American Chemical Society
-"I-7 NO. 6,
ANALYTICAL CHEMISTRY, VOL. 51, 1001
Foam layer containing iron(II1) hydroxide and selenium(1V)
Sintered-glass disc, porosity 4
655
MAY 1979
3
I 01
1
3
i
5
6
7
8
9
-
1
L
C
1
1
PH
Figure 1. Flotation cell for pre-concentration of selenium(1V)
Turn the four-way stopcock of the apparatus to the sweep position to introduce nitrogen into the system and inject the sample into the cell. Sweep the hydrogen selenide generated into the long absorption cell with nitrogen so that it is atomized in the nitrogen-hydrogen flame, and record the absorption signal on a recorder. Return the stopcock to the bypass position. Rinse the cell carefully with distilled water and recharge with NaBH, solution ready for the next sample. Construct a calibration curve using 4 M hydrochloric acid solutions containing 1 mg/mL of iron(II1) and 0-0.10 pg/mL of selenium(1V); this curve is nearly linear within the above range of selenium. The relative standard deviations were 1.5% and 1.4% in 10 replicate determinations of 0.04 pg and 0.08 pg of selenium(1V) (1-mL volume), respectively. The atomic absorption equipment was operated under the following conditions: wavelength, 196.0 nm; lamp current, 10 mA; gas flow rates, nitrogen 1.5,hydrogen 1.5, and auxiliary nitrogen 6 L/min; slit (spectral band width), 1 nm. The 60-cm tube system described in this paper cannot he used with most commercial atomic absorption spectrophotometer units. However, the selenium in a sample solution can also be determined by atomic absorption spectrophotometry using a hydride generation-electrically or flame-heated silica tube.
RESULTS AND DISCUSSION Optimum Conditions for the Determination of Selenium(1V). T h e effect of NaBH, concentration on the evolution of hydrogen selenide was investigated. As a result, it was found t h a t the use of more than 2 % (w/v) of the reductant concentration (1-mL volume) can quantitatively reduce u p t o 0.10 pg of selenium(1V) t o hydrogen selenide. In this work, 1 mL of 5% (w/v) solution was used. T h e optimum condition for sensitivity of selenium was dependent on the flow rate of auxiliary nitrogen. T h e absorption of selenium increased with each increase in the flow rate of auxiliary nitrogen. T h e optimum absorption was obtained with the flow rate of carrier gas between 6 and 8 L/min when both flow rates of nitrogen and hydrogen were 1.5 L/min; a flow rate of auxiliary nitrogen of 6.0 L/min was used throughout. T h e effect of hydrochloric acid concentration was studied. T h e optimal concentration of hydrochloric acid was between 2-5 M; the acidity of hydrochloric acid was maintained a t 4 M in further work. The presence of up to a t least 2.5 mg/mL of iron(II1) added as a collector did not affect the generation of hydrogen selenide. Optimum Conditions for the Flotation of Selenium(IV). Effect o f p H . T h e effect of the p H of the 1000-mL solution containing 0.8 pg of selenium(IV), 10 mg of iron(III), and 2.0 mg of surfactant on the coprecipitation of selenium(1V) was studied. Hydrochloric acid and aqueous ammonia solution were used to adjust the p H to values within the range 3.5-10.5. As shown in Figure 2, satisfactory recoveries of selenium(1V) were obtained in the p H range 3.5-8.5. At p H values above
Figure 2. Coprecipitation of selenium(1V) with iron(II1) hydroxide as a function of pH. Solution contained 0.8 pg of Se(IV) and 10 mg of Fe(II1); volume, 1000 mL
Table I. Permissible Amounts of Foreign Ions for Determination of Seleniumu ion Na
+
K' Ca2+ Mg"
c1-
NO,-
so,*-
limit, [ion]/[Se]
1
10000 10000 10000 10000 10000 b
+
+
C d Z + 1000' Zn2+ Mn2' ~
1
3
+
Cr3-
10000
Cr6+
1000
Mo6 Pb2' Hg2
1000 1000 1000
1000
co*
1000
+
+
+
V5' PO, 3 NiZ cu2+ Bi3+ +
10000
Si0,2- 1 0 0 0 0 Sr2 Ba2
limit, ion [ion]/[Se] ion
Sb3+ As3+ As5 +
Sn4+ Te3
+
limit, [ion]/[Se] 1 I O0o0o0 800 800 400 100 100 100 10 10
t
Solution contained 1.0 pig of SP(IV)and 1 0 mg of Fe(II1); volume, 1000 mL. b Maximum concentrations tested.
9, the efficiency of coprecipitation decreased considerably. T h e most stable layer of surface foam supporting the precipitate of iron(II1) hydroxide using sodium lauryl sulfate was formed within the p H range 3.5-7.0; in all further work, the p H of 4.0 i 0.2 was used for the precipitation and flotation. At a p H above 7 , a stable surface-foam layer was obtained by using sodium oleate as a surfactant. Amounts of Iron(Il1) a n d Surfactant. T h e recovery of selenium(1V) was investigated as a function of the amount of iron(II1) added to the solution. Quantitative recoveries of selenium(1V) were obtained above 5.0 mg of iron(II1). In this work, 10 mg of iron(II1) were added to lo00 mL of the solution. The amount of sodium lauryl sulfate required for complete flotation of the precipitate was investigated. Quantitative recoveries of selenium(1V) were obtained between 0.5-8.0 mg of sodium lauryl sulfate, and 2.0 mg in 1000 mL of solution was adopted in further work. Stirring Time. The relation between stirring time and recovery of selenium(1V) was investigated. As a result, coprecipitation was quantitative over the range 5-40 min. In practice, stirring for 15 min was used. Effect o f Foreign Ions. By following the proposed procedure, the effect of various ions on the separation and determination of selenium(1V) was investigated. Table I shows permissible amounts of foreign ions within 10% negative error for the determination of 1.0 pg of selenium(1V) in 1000-mL solutions with 10 mg of iron(II1) added. As can be seen, most foreign ions hardly interfere with the determination of selenium(1V). When coprecipitation a t p H 4.0 0.2 was used, suppressive effects by diverse ions, especially copper(II), nickel(II), and mercury(I1) were found to be largely eliminated in comparison with those which occur when selenium is directly determined without previous concentration. The values of permissible amounts (ion/Se in weight) of coexisting ions in the proposed method increased from 10, 20, and 40 to 800,
*
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ANALYTICAL CHEMISTRY, VOL. 51,
NO.6,
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Table 11. Recovery of Selenium(1V) Added to Natural SeawateP
added p g
selenium, p g found p g
none
0.023
?
0.400 0.800
0.409 0.805
i: i.
0.005& 0.007c
0.013c
mean re-
covery, recovered
%
0.386 0.782
97 98
a Volume of sample, 1000 mL. Mean value of four measurements. Mean value of seven measurements.
800, and 1000 for copper(II), nickel(II), and mercury(II), respectively, compared with the values permissible in direct determination. However, hydride-forming elements such as tellurium(IV), tin(IV), arsenic(II1, V) and antimony(II1) are coprecipitated with iron(II1) hydroxide in the same way as selenium(1V) and had a relatively strong effect on the generation of hydrogen selenide. Recovery of Seleniurn(1V). Solutions (1000 mL) at p H 4.0 f 0.2 containing 10 mg of iron(III), 2.0 mg of sodium lauryl sulfate, and 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0, 15.0, and 20.0 pg of selenium(1V) were analyzed by the described procedure. Recoveries of the selenium that had been added were greater than 96% in all instances. The blank value throughout the whole analytical process was less than 5 ng Se/L. Proposed conditions, therefore, appear to be optimum for 1000-mL volumes of solutions containing up to a t least 20 pg of selenium(1V). The relative standard deviations of 10 analyses of solutions containing 0.4 and 0.8 Kg of selenium(1V) per 1000 mL were 1.8% and 1.570,respectively. Application t o the Analysis of Selenium(ZV) Added t o Seawater. T o investigate the applicability of this method to the separation and determination of selenium(1V) in seawater, recoveries of known amounts of selenium(1V) added to natural seawater samples were examined by the above procedure. The analyses were carried out on 1000-mL aliquots of clear uncontaminated seawater, filtered through 0.45-pm Millipore filters after t h e addition of 10 mL of hydrochloric acid per loo0 mL of sample immediately after collection at Shibukawa, Okayama Prefecture, Japan. Table I1 presents recovery results of selenium(1V). Recovery results indicate that the analytical system could be successfully applied for the separation and determination of the selenium(1V) at the levels of 0.4 and 0.8 pg in seawater containing large amounts of salt matrix.
As for the selenium content in seawater, Schutz and Turekian (17) found 0.05-0.12 kg S e / L (average 0.09 pg/L) in deep ocean waters using a neutron activation procedure in a worldwide survey. According to recent reliable data (3, 16, 18), the concentrations of selenium(1V) in uncontaminated natural fresh and seawaters are quite low,
