Nakashima, Susumu

Yagi, Masakazu

Okayama University Scientific Achievement Repository

Ber. Ohara Inst目 landw.Biol.， Okayama Univ. 19 : 49-56 (1986) RAPID FLOTATION SEPARATION AND ATOMIC ABSORPTION SPECTROMETRIC DETERMINATION OF ARSENIC IN NATURAL WATERS. Susumu N AKASHIMA and Masakazu Y AGI INTRODUCTION Arsenic is a naturally occurring element and can be found at the 1 μg/l level in natural water samples. It is important to develop a rapid and accurate method of determining arsenic (皿，V) in natural waters in va-rious fields including environmental chemistry， limnology， marine biolo-gy and agriculture owing to its severe toxicity. In general， for an accurate determination of arsenic in natural waters， preconcentration is needed in order to achieve the necessary sensitivity. In previous papers (Nakashima 1978， 1979 a)， a flotation technique (De Carlo and Zeitlin 1981; Hiraide and Mizuike 1977; Hiraide et al. 1976; Mizuike and Hiraide 1982) in which the precipitate of hydrated iron (田)oxide is floated with the aid of surfactant and small air bubbles was used for the preconcentration of arsenic in natural waters. Subsequently， the determination of arsenic by hydride generation and atomic absorption spectrometry was performed. In the procedure described previously (Nakashima 1978， 1979 a)， the preconcentration by flotation was carried out on aliquots of natural wa-ters， filtered through 0.45μm Millipore filters after the addition of 10ml of hydrochloric acid per 1000ml of sample immediately after collection. However， inthe procedure of the prefiltration of water samples， fil-tration for larger sample volumes is sometimes time-consuming because the filter is clogged with suspended solids present in natural waters. Therefore， for the more rapid determination of arsenic in natural wa・ters， the flotation cell used in the previous papers (Nakashima 1978， 1979 a) was improved. In the procedure of separation of arsenic from natural water samples by employing the new cell， arsenic coprecipitated with hy-drated iron (田)oxide was floated with suspended solids without prefiltra-tion of samples. As a result， good agreement has been obtained between Institute for Agricultural and Biological Sciences， Okayama University， Kurashiki Received January 20， 1986. • Data presented in this paper were published in Bunseki Kagaku， vol. 33， p.T1ー T3(1984)， in]apanese. EXPERIMENT AL 50 S. N akashima and M. Yagi the values obtained by this method and by the method previously de-scribed (the flotation method using natural water samples filtered through 0.45μm Millipore filters， after the addition of hydrochloric acid to the water samples immediately after collection). Therefore， itis clear that this method is more rapid than the method described previously (Nakashima 1978， 1979 a) for the determination of arsenic in natural water samples. This paper describes a rapid and convenient method for the separa-tion and determination of arsenic in natural waters. 1. Apparatω The flotation cells used in this work are shown in Fig. 1. The specific components and the schematic diagram of the experimental sys・F 、 、 ， ，???? ? ? ?Previous Cell (8) New Cell FIG. 1. Flotation cell for preconcentration of arsenic. A. Foam layer containing hydrated iron (阻)oxide and arsenic; B. Bubble; C. Sample solution; D. Cock; E. Drain pipe; F. Sintered.glass disk (porosity 4); G. Air; H. Milli. pore filter (pore size: 0.4μm; diameter: 47non); 1. Millipore stopper; J. Bubbler; K. Millipore clamp. Rapid Flotation Separation for Determination of Arsenic 51 tem employed in this work have been previously described (Nakashima 1978， 1979 a) except for the flotation cell (B) shown in Fig. 1. Figure 1 (A) shows the flotation cell described previously (Nakashima 1979b， c， 1980， 1981). The cell was composed of a glass cylinder， 40 X 6.5佃 i.d.， fitted with a sintered-glass filter (No. 4) to generate small air bubbles. A side arm was added near the bottom of the cell to drain the mother liquor rapidly after the flotation. Figure 1 (B) shows the new cell used in this work. 1t has a side arm near the bottom of the cell and a flange at the bottom of the cell. This cell is connected to the lower part of Millipore filter funnel assembly (fil-ter: 0.45μm pore size， 47mm diameter). Air is passed through a custom-made bubbler (No. 4 -sintered glass disk) which has been put into the flotation cell and the precipitate of the hydrated iron (田)oxide floats. 2. Reagents All reagents were of analytical-reagent grade except for sodium dodecyl sulfate (sudium lauryl sulfate) and sodium oleate. Aqueous re-agents were prepared in deionized， distilled water. Stock solutions ( 1 mgIml) of As (圃)and As (V) were prepared from diarsenic trioxide (AS203) and sodium arsenate (N a2HAs04・7H20)， respectively， and arsenic stan-dard solutions were freshly prepared by diluting stock solutions before use. An iron (皿)solution ( 5町 Fe/ml)was prepared from ammonium iron (皿)sulfate. A sodium dodecyl sulfate solution ( 1 mgIml) was pre-pared by dissolving sodium dodecyl sulfate (powder， extra-pure reagent， Wako Pure Chemicals， Osaka) in 99.5% (v/v) ethanol. A sodium oleate solution ( 1町Iml)was prepared by dissolving sodium oleate (powder， ex-tra-pure reagent， Wako Pure Chemicals) in 99.5% (v Iv) ethanol with magnetic stirring. A potassium iodide solution (20%w/v) was prepared by dissolving in water. A sodium tetrahydroborate solution ( 5 %w/v) in a O.lM sodium hydroxide solution was freshly prepared. 3. Procedure for the flotation stet The procedure for the separation using the flotation cell shown in Fig. 1 (B) is as follows: Ten ml of hydrochloric acid per liter of natural water samples was added immediately after collection in the field. The 1000・mlsample of fresh water (500ml for sea water) was placed in a beak-er without prefiltration， and 2 ml of iron (困)solution was added. The pH of the solution was adjusted to 8.0・8.5(or 3.8-4.2) with aqueous ammo-nia to precipitate hydrated iron (皿)oxide， and the mixture was stirred for about 15min. One ml ( 2 ml for sea water) of sodium doclecyl sulfate solu-tion and 1 ml of sodium oleate solution were added to the beaker. The 52 S. Nakashima and M. Yagi contents of the beaker (excluding the stirring bar) were transferred to the flotation cell and the residue in the beaker was washed into the cell us・ing three small portions of water. Air was passed through at 20mllmin from the sintered-glass disk (No. 4) of the bubbler for about 2 min to obtain complete mixing and to float the precipitate. Most of the mother liquor was drained from the side arm by opening the cock on the drain pipe. After the cock had been closed， the residual mother liquor was suck-ed off through the Millipore filter. The precipitate adhering to the wall of the cell and the bubbler was washed down by spraying with water， and then the precipitate was washed with 30ml of water. A 8-ml portion of 6 M hydrochloric acid was added to the cell to dissolve the precipitate. The filtrate was collected by suction in a 20・mlvolumetric flask， the Millipore filter was washed with water， the washings were added to the flask， and the mixture was diluted to 18ml with water. One ml of potas-sium iodide solution was added into the flask prior to analysis and the mixture was diluted to volume (20ml) with water. 4. Procedure lor the atomic absorpti.on measurement The procedure for the atomic absorption spectrometric determination of arsenic after the separation was almost the same as the determination of arsenic described previously (Nakashima 1979 a). A calibration curve using 2.4M hydrochloric acid solutions containing 1町Imlof iron (町)，1 % of potassium iodide and 0-0.10μg/ml of arsenic (困)is linear within the above range of arsenic concentration. The atomic absorption equip-ment was operated under the following conditions: wavelength， 193. 7nm; lamp current， 14mA; gas-flow rates， nitrogen 1. 0， hydrogen 1. 0， and au-xiliary nitrogen 6.0 1 Imin; spectral bandwidth 1 nm. The long tube system of 60αn was used in the atomic absorption units. RESUL TS AND DISCUSSION The recovery rates for arsenic using this preconcentration method were investigated. Solutions (l000ml) at pH 3.8-4.2 or pH 8.0-8.5 con-taining lOmg of iron (皿)， 1 ml of sodium dodecyl :;ulfate solution， 1 ml of sodium oleate solution， andO.2-1.0μg of arsenic (圃)were analyzed by the proposed method using the flotation cell (B) shown in Fig. 1. The results are shown in T able 1. Recoveries of the arsenic added were greater than 95% in al instances. The blank value throughout the whole analytical process was less than 5 ng AsI 1 . It is clear that arsenic in 1000 ml of water samples can be quantitatively recovered by the proposed method using the flotation cell (B) newly developed. To investigate the applicability of this method to the separation and Rapid Flotation Separation for Determination of Arsenic T ABLE 1. Recovery of arsenic added to water. pH of sample solution for coprecipitation and flotation 3.8-4.2 8.0-8.5 Added. • ( ，Ls) 0.2 0.4 0.6 0.8 1.0 0.4 0.6 1.0 Recovery (%) 95， 1∞ 96， 95 99. 97 98， 99 98， 96 98， 95 97， 1∞ 98， 98 These recoveries were examined by the flotation separation method using the flotation cell (8). • Solution containing 10町 ofiron (国);volume. 10∞ml •• Added as As (町).53 determination of arsenic in natural waters， the analyses of arsenic in tap， well， river and sea water samples were carried out by the proposed method without prefiltration and the previous method with prefiltration. As can be seen in Table 2 ， good agreement of the arsenic results was obtained irrespective of whether the samples were prefiltered through 0.45μm Millipore filters， or not. The pervious method described the procedures for the flotation separation and determination of arsenic by atomic absorption spec-trometry following arsine generation. However， ithad a disadvantage of T ABLE 2. Determination of arsenic in natural water samples Sample SS・Sample volume (明11)pH. • (ml) Tap water 10∞ く0.5 8.0-8.5 Well water 1α)0 く0.5 8.0-8.5 River water 1 1α)0 7.2 8.0-8.5 River water 2 l∞o 6.1 3.8-4.2 8.0-8.5 Sea water 1 500 1.2 8.0-8.5 Sea water 2 500 8.2 8.0-8.5 • Suspended solids remained 00 a 0.45μm Millipore filter. •• pH of sample solution for coprecipitation and flotation. ・ Valuesobtained by this method. Arsenic content (μg/l) Without With prefiltration. •• prefiltration.... 1.04， 1.07 1.06， 1.11 0.22， 0.22 0.22， 0.20 1.14， 1.16 1.07， 1.1 1.20， 1.22 1.20， 1.16 1.23， 1.20 1.22， 1.21 1.64， 1.60 1. 50， 1.59 1.66， 1.68 1.62， 1.64 ・ Valuesobtained by flotation separation method using the flotation cell (A) and water samples filtered through 0.45μm Millipore filters. 54 S. Nakashima and M. Yagi being time-consuming for the prefiltration process of larger volumes of natural water samples. In the present method， the procedure for the flota-tion separation can be carried out without prefiltration of natural water samples. Therefore， this method is more rapid and convenient than the prevlOus one. The use of mixed surfactant (sodium dodecyl sulfate and sodium ole-ate) was more effective for the flotation of the precipitate in water， parti-cularly in sea water， than that of sodium dodecyl sulfate or sodium oleate alone (Nakashima 1980， 1981). Moreover， when natural water samples were treated， the precipitate of hydrated iron (m) oxide added to samples containing suspended solids floated more easily than the precipitate added to samples prefiltered through 0.45μm Millipore filters. The arsenic contents obtained by the procedure of the coprecipita-tion-flotation separation at pH 3.8-4.2 showed good agreement with at pH 8.0-8.5. The flotation of hydrated iron (皿)oxide at pH 3.8-4.2 offers a useful procedure for the simultaneous separation of hydride-forming elements such as arsenic (m， V)， selenium (町)， tin (n ，N)， bismuth (皿)，and antimony (m ，V) from a large volume of fresh water and sea water， and subsequent atomic absorption measurement in a long absorption tube of the hydrides generated from hydride-forming elements is found to be an accurate method for the determination of above elements. In addition， the advantage of the proposed method at pH 3.8-4.2 is the satisfactory isolation of hydride-forming elements from interfering ions such as nickel ( n) and copper ( n) present in water for the atomic absorption determina-tion after the separation (Nakashima 1979 b， c， 1980， 1981). On the other hand， from the viewpoint of adjusting the pH of sample solutions， itis easier to adjust the pH of the sample to 8.0-8.5 than to 3.8-4.2. Therefore， itis preferable to adjust the pH of the solution to 8.← 8.5 when determining arsenic in natural waters as routine work. The time required for the preconcentration of arsenic from a 10∞・mlvolume of solution is about 3O-35min per sample including 15min of stir-ring. However， during the preconcentration period of 15ー20minafter stir-ring， stirring of the next sample solution can be carried out. Therefore， in practice， the preconcentration of arsenic from a 1000ml volume of solu-tion can be made at intervals of 15-20min per sample when a number of samples is treated. The 60cm tube system described in this paper cannot be used with most commercial atomic absorption spectrophotometers. However， the arsenic in a sample solution can also be determined by hydride genera-tion-atomic absorption spectrometry using an electrically or flame-heated silica tube. Rapid Flotation Separation for Determination of Arsenic 55 SUMMARY A rapid and accurate method is described for the separation and de-termination of arsenic in natural waters by using the improved flotation cell. A sub-microgram amount of arsenic (皿，V) in natural water samples (1000ml for fresh water， 500ml for sea water) is coprecipitated with hy-drated iron (皿)oxide at pH 8.←8.5 or 3.8-4.2 without prefiltration through membrane filters. The precipitate is floated with the aid of mixed surfactant (sodium oleate and sodium dodecyl sulfate) and small air bub-bles， then separated and dissolved in dilute hydrochloric acid. The arse-nic content is determined by arsine generation using sodium tetrahy-droborate and atomic absorption spectrometry. In the analytical process， mean recoveries of 95-100% for arsenic added to water were obtained. Good agreement of the arsenic values was obtained irrespective of whether the samples were prefiltered through 0.45μm Millipore filters， or not. The time required for the preconcentration was 15ー20minafter 15min stunng. Acknowledgem.ent One of the authors (S. N.) thanks the ]apanese Minis-try of Education， Science and Culture for financial support through a Grant-in-Aid for Scientific Research (No. 56350053). REFERENCES DeCarlo， E. H. and Zeitlin， H.1981. Sirnultaneous separation of trace levels of gerrnaniurn， antirnony， arsenic， and seleniurn frorn an acid rnatrix by adsorbing colloid f1otation. Anal. Chern. 53: 1104-1107. Hiraide， M. and Mizuike， A. 1977. Flotation of iron (皿)hydroxide precipitate. Bunseki Kagaku. 26 : 47ー 50.Hiraide， M.， Yoshida， Y.and Mizuike， A.1976. Flotation of traces of heavy rnetals copre・cipitated with alurninurn hydroxide frorn water and sea water. Anal. Chirn. Acta. 81 : 185-189. Mizuike， A. and Hiraide， M. 1982. Separation and preconcentration of trace substances.田・Flotationas a preconcentration technique. Pure and Appl. Chern. 54 : 1556ーー1563.Nakashirna， S. 1978. Flotation of sub-rnicrograrn arnounts of arsenic coprecipitated with iron (皿)hydroxide frorn natural waters and deterrnination of arsenic by atornic-absorption spectrophotometry following hydride generation. Analyst (London). 103 1031 -1036 Nakashirna， S.1979 a. Arsenic separation frorn sea water by f1otation and deterrnination by atomic-absorption spectrophotornetry following arsine generation. Ber. Ohara Inst. landw. Biol. 17: 179-186. Nakashima， S. 1979 b. Flotation separation and atornic absorption spectrornetric deternina-56 s. Nakashima and M. Yagi tion of selenium (lV) in water. Anal. Chem. 51 : 654ー-656.Nakashima， S. 1979 c. The separation of tin from water and sea-water by flotation and the determination of tin by atomic absorption spectrophotometry following stannane gen-eration. Bull. Chem. Soc. Jpn. 52 : 1844-1848. Nakashima， S. 1980. Determination of sub.microgram amounts of bismuth in water by atomic absorption spectrophotometry following flotation separation. Fresenius' Z. Anal. Chem. 303 : 10-ー13.Nakashima， S. 1981. The determination of antimony in water by atomic-absorption spec-trophotometry following flotation separation. Bull. Chem. S∞. Jpn. 54 : 291-292. 

RAPID FLOTATION SEPARATION AND ATOMIC ABSORPTION SPECTROMETRIC DETERMINATION OF ARSENIC IN NATURAL WATERS*

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