Bull. Environm. Contam. Toxicol. 21, 37-42 (1979)

A Simple Method of Arsenic Speciation Edward J. Brown and Don K. Button 1 Institute of Water Resources 1, Institute of Marine Science, University of Alaska, Fairbanks, Ala. 99701

Arsenic is known for its biological toxicity (FEINGLASS 1973). Arsenate [As(V)] uncouples oxidative p h o s p h o r y l a t i o n reactions (LEHNINGER 1972) and competes directly with phosphate during m i c r o b i a l (BUTTON et al. 1973) and tissue cell (HILBORN 1976) phosphate transport. A r s e n i t e [As(III)] is also toxic but m o s t likely reacts d i f f e r e n t l y than As(V) in biological systems. For example, As(III) has been shown to affect m i c r o b i a l growth by inhibiting a r e c o m b i n a t i o n repair d e p e n d e n t step in the repair of damaged DNA (ROSSMAN et al. 1975). M e t h y l a t e d forms of As(III) are even more b i o l o g i c a l l y toxic than As(V) or As(III) (WOOD 1974). The exact m e c h a n i s m of all forms of acute arsenic poisoning, however, is still not clear. Perhaps even more significant from an e n v i r o n m e n t a l standpoint is that the long term effects of exposure to low levels of arsenic are not known, p a r t i c u l a r l y now that there is indirect evidence that arsenic may be a human carcinogen (SUNDERMAN 1975). A n a l y t i c a l d i f f i c u l t y in speciation of arsenic at low levels (part per billion concentrations) in water, soils, sediments, and b i o l o g i c a l tissues has contributed to the present lack of understanding. The m o s t sensitive methods for arsenic d e t e c t i o n (atomic absorption; E D I G E R 1975 and d i s t i l l a t i o n AMERICAN PUBLIC HEALTH A S S O C I A T I O N 1971) require transformation of the arsenic to a single chemical form so that only total arsenic can be quantified. Potentially useful atomic absorption methods of arsenic speciation are d i s c u s s e d in the work of CLEMENT AND FAUST (1973). In this study we have used a m o d i f i c a t i o n of the p h o s p h a t e - d e t e c t i o n procedure (MURPHY and RILEY 1962) to d i f f e r e n t i a t e between dissolved As(V) and d i s s o l v e d As(III) in water samples. This simple m e t h o d is based on the fact that arsenate, like phosphate and silicate, forms an alcohol extractable m o l y b d a t e complex (blue colored when reduced) while arsenite does not (JOHNSON 1971). The results show that this procedure used prior to total arsenic analysis will enable speciation of arsenic to the d e t e c t i o n limits of the common methods used for total arsenic analysis.

0007-4861/ 79/0021-0037 $01.20 9 1979 Springer-Verlag New York Inc.

MATERIALS AND METHODS As(V) (Na2HAsO4.7H20) and As(III) (NaAsO 2) standards were made by serial dilutions of stock solutions containing 20 m i c r o g r a m s (~g) As per m i l l i l i t e r (ml) of distilled water. The usual MURPHY and RILEY (1962) phosphate reagent was used (ammonium molybdate, 0.9 g/l; p o t a s s i u m antimony tartarate, 0.02 g/l; ascorbic acid, 0.02 g/l) except that the final concentration of H~SO~ in the r e a g e n t - s a m p l e m i x t u r e was 0.5N instead of-0.4N. For s p e c t r o p h o t o m e t r i c assays, i0 ml of reagent were added to 40 ml of each standard as contained in carefully w a s h e d 50 ml v o l u m e t r i c reaction flasks. The a r s e n o - m o l y b d a t e complex (blue color) was allowed to develop for 90 minutes at room temperature (JOHNSON 1971). The absorbance of an aliquot was m e a s u r e d at an optimized w a v e l e n g t h of 712 nanometers in a s p e c t r o p h o t o m e t e r cell with a path length of one centimeter. Six ml of isoamyl alcohol were then added to a 4 ml aliquot of the sample and the m i x t u r e was shaken vigorously. The absorbance of both the alcohol phase (one extraction) and the aqueous phase was measured. For the isotopic assays, carrier-free 74As(V) was added to q u a d r u p l i c a t e standard solutions of arsenate as contained in a 50 ml v o l u m e t r i c reaction flask. Five ml of sulfide reducing agent (JOHNSON 1971~4were a d d ~ to duplicates of each standard to reduce " As(V) to As(III). After 15 minutes, i0 ml of the m o d i f i e d MURPHY and RILEY (1962) reagent were added to both the reduced and n o n - r e d u d e d standard solutions. The a r s e n o - m o l y b d a t e complex was then allowed to develop for 90 minutes. Two ml aliquots of each standard were placed in glass or p o l y s t y r e n e test tubes and extracted 3-5 times with 0.5 ml water saturated isoamyl alcohol. After the last e x t r a c t i o n 1 ml of ethanol was added to the 2 ml aliquot to ensure complete recovery of the r a d i o a c t i v i t y in the aqueous phase. Each extract (alcohol phase) and the 2 ml aliquot with ethanol wash (aqueous phase) were added to separate vials containing i0 ml of toluene Triton-X 100 s c i n t i l l a t i o n cocktail. The r a d i o a c t i v i t y in each sample was then d e t e r m i n e d by liquid scintillation spectrometry.

RESULTS AND D I S C U S S I O N Table 1 shows that as expected dissolved As(V) will form a b l u e - c o l o r e d a r s e n o - m o l y b d a t e complex while dissolved As(III) does not. This fact is the

38

basis for the reduction methods developed to eliminate arsenate interference in the detection of phosphate in soils (VON SCHOUWENBURG and WALINGA 1967) and natural waters (JOHNSON 1971). Table 1 also shows that the blue arseno-molybdate complex can be extracted with isoamyl alcohol in the presence of 0.5N H2SO 4. However, the results in Table 1 do not ensure that dissolved As (III) is completely isoamyl alcohol insoluble or that the As(V) derived compound is completely isoamyl alcohol soluble. TABLE 1 Extraction of Arseno-molybdate Complex with Isoamyl Alcohol as Measured b Y Absorbance

As(V) concentration

H^O-phase Zbefore extraction

Absorbance* Alcohol phase (i extraction)

H20-phase after extraction

~gAs/50 ml

milli-o.d,

milli-o.d,

milli-o.d.

0

3

2

--

20

60

56

12

40

141

120

18

60

240

185

34

i00

290

242

15

3

4

--

i00"*

The radioisotope studies were done to examine these possibilities. The results in Fig. 1 show that after 3 extractions As(V) (non-reduced samples) can be essentially completely removed from the aqueous phase (clear areas of bar-graph) and that As(III) (reduced samples) remains predominantly in the aqueous phase (hatched areas of bar graph). The relatively constant 4% As(III)-alcohol phase carryover with each extraction (Fig. i) was indicative of arsenic partitioning between the alcohol and aqueous phases.

*Mean of duplicate values corrected for blank. **Contained i00 ~g As(III)/50 ml (NaAsO2).

39

100

"/4///~

z~ 11/

75. }'//,

E o. (J

11. i..

50iiii rill r/ I.,

=-

r/v

N

'1

r,..

25-

0

F//I

5 4 3 2 1 A B O*

3 2 A

B 10

N

A B 20

2

A B 40

A

I B

H B

I00

Added carrier As (V), IZg A s / 5 O m l .

FIGURE i. 74As-molybdate Extractions of Non, reduced (A) and Reduced (B) Standard Solutions. Hatch Marks Represent % cpm in Aqueous Phase and the Clear Areas Represent % cpm in Each (numbered) Isoamyl Alcohol Extract (Mean Values of Duplicates). Table 2 shows the effect of increasing isoamyl alcohol concentration on water solubility of As(III). The As(III) partition ratios for several alcohol:water systems reflect the slight solubility of arsenite in isoamyl alcohol. Thus while three alcohol extractions are recommended for As(V) derived arseno-molybdate removal from the aqueous phase, the alcohol addition should never exceed 25% of the sample volume. Because As(III) is slightly soluble in isoamyl alcohol, carryover of As(III) in each alcohol fraction must be corrected for according to the partition ratios given in Table 2. *Reaction conducted

in a polystyrene vessel.

4o

TABLE 2 P a r t i t i o n i n g of As(III) Between Isoamyl Alcohol and Water*

Alcohol phase As(III) Alcohol:Water in e x t r a c t i o n no.: ratio of sample 1 2 3

Dissolved As(III) alcohol:water p a r t i t i o n ratio

100(cpm extracted/remaining) 2:1

22.3

21.6

21.4

1:4.5

i:i

11.7

10.5

ii.i

1:9

1:2

5.8

5.5

5.7

1:18

1:4

4.0

3.1

2.5

1:36

As with phosphate (RYDEN et al. 1972) arsenate sorption to glassware must also be c o n s i d e r e d at low concentrations. When carrier-free As(V) was reacted with m o l y b d a t e reagent in acid w a s h e d glass tubes, only 35% could be removed in three alcohol extractions while 93% was removed when the reaction was carried out in polystyrene tubes (Fig. i). Thus specially treated glassware or plastic ware m u s t be used for samples containing low levels of As(V). The implication of these results is that low concentrations of As(V) can easily and routinely be separated from As(III) in water, soil, sediment and b i o l o g i c a l samples using procedures analogous to well established phosphorus m e t h o d o l o g y (STRICKLAND and PARSONS 1968). Once separated total As(III) or As(V) can then be d e t e r m i n e d by atomic absorption or distillation allowing arsenic speciation at part per billion levels.

ACKNOWLEDGMENT The authors wish to thank Dr. D. B. Hawkins and Dr. P. B. Reichardt for their help and suggestions for this study. Support for this investigation was supplied by the National Science F o u n d a t i o n International Decade of Oceanic E x p l o r a t i o n Grant DCE 75-036772A02 *Each sample contained 74As(V) with i00 ~g As(V)/1 carrier before reduction to As(III) by methods described in the text.

41

REFERENCES AMERICAN PUBLIC HEALTH ASSOCIATION: Standard methods for the e x a m i n a t i o n of water and wastewater, 13th ed. New York A m e r i c a n Public Health Assoc. Inc.: 1971. BUTTON, D. K., S. S. DUNKER, and M. L. MORSE: J. Bacteriol. 113, 599 (1973). CLEMENT, W. H., and B.D. FAUST: Letters 5, 155 (1973).

Environmental

EDIGER, R. D.: Atomic Absorp. News Letter 14, 127 (1975). FEINGLASS, E. J.: (1973). HILBORN,

D. A.:

JOHNSON, D. L.: (1971). LEHNINGER, A. L.: W o r t h 1972.

N e w Engl. J. Med.

J. Cell. Physiol. Environ.

288, 828

8_~7, iii

(1976).

Sci. Technol. 5, 411

Biochemistry.

MURPHY, J. and J. P. RILEY: 31 (1962).

6th ed. New York:

Anal.

Chem. Acta 27,

ROSSMAN, T., M. S. MEYN, and W. TROLL: 30, 157 (1975).

Mutat,

Res.

RYDEN, J. C., J. K. SYERS, and R. F. HARRIS: A n a l y s t 97, 903 (1972). STRICKLAND, J. D. H., and T. R. PARSONS: Fish. Res. Board Can. Bull. 125, 43 (1965). SUNDERMAN, F. W., JR.: VON SCHOUWENBURG,

J. C.,

Chim. Acta 37, 271 WOOD, J. M.:

Prev. Med. 5, 279 and I. WALINGA:

(1967).

Science 183, 1049

42

(1974).

(1976). Anal.

A simple method of arsenic speciation.

Bull. Environm. Contam. Toxicol. 21, 37-42 (1979) A Simple Method of Arsenic Speciation Edward J. Brown and Don K. Button 1 Institute of Water Resour...
289KB Sizes 0 Downloads 0 Views