ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
S. M. Warnick with the design of the signal processing electronics. LITERATURE CITED (1) T. Hirschfeld, Appl. Spectrosc., 31, 238 (1977). (2) C. Hu and J. R. Whinnery, Appl. Opt., 12, 72 (1973). (3) J. R. Whinnery, Acc. Chem. Res., 7 , 225 (1974). (4) A. Hordvik, Appl. Opt., 16, 2827 (1977). (5) T. D. Harris, Ph.D. Thesis, Purdue University, Lafayette, Ind., 1978, (6)J. P. Gordon. R. C. C. Leite, R. S. W e , S. P. S . Porto, and J. R. Whinnery, J . Appl. Phys., 36, 3 (1965). (7) F. W. Dabby, R. W. Boyko, C. V. Shank, and J. R. Whinnery, I€€€J. Quantum Electron., qe-5. 516 (1969). (8) J. H. Brannon and D. Magde, J. Phys. Chem., 8 2 , 705 (1978).
731
(9) M. E. Long, R. L. Swafford, and A. C. Albrecht, Science, 191, 183 (1976). (10) D. Solimini, J . Appl. Phys., 37, 3314 (1966). (11) S. A. Akhmanov, D. P. Krindach. A. V. Migulin, A. P. Sukhorukov. and R. V. Khokhlov, I€€€ J. Quantum Electron. qe-4, 568 (1968). (12) P. B. Sweetser and C. E. Bricker, Anal. Chem., 25, 253 (1953). (13) J. R. Arnaud, "Beam and Fiber Optics", Academic Press, New Ywk, 1976.
for review November 93 1978* Accepted January 12, 1979. This research was supported in part by the donors of the Petroleum Research Fund administered by the American Chemical Society, and by the University of Utah Research Fund.
Comparison of Digestion Methods for Determination of Organoarsenicals in Wastewater Charles E. Stringer Dallas Water Utilities, 1020 Sargent Road, Dallas, Texas 75216
Moses Attrep, Jr." Department of Chemistry, East Texas State University, Commerce, Texas 75428
Two digestive methods were investigated and compared as to their effectiveness In releasing arsenic from three organoarsenicals introduced into wastewater samples. The digestive methods utilized included a wet method employing hydrogen peroxide-sulfuric acid and ultraviolet photodecomposition. The organoarsenicals investigated were disodium methanearsonate, dlmethylarsinic acid, and triphenylarsine oxide. All the digestive methods gave quantitative arsenic recoveries for the three organoarsenic compounds when added to wastewater samples. The ultraviolet photodecomposition proved to be an effective digestive technique, requiring a 4-h irradiation to decompose a primary settled raw wastewater sample containing spiked quantities of the three organoarsenicals.
T h e knowledge and concern about the environmental impact and the ultimate fate of organoarsenicals applied to soils and other ecosystems is steadily growing. The extensive number of analytical procedures which have been reported in the literature would seem to indicate inherent difficulties in the analysis of arsenic. Most procedures intended to measure total arsenic incorporate some mode of wet or dry digestion to destroy any organically bound arsenic, in addition to any other organic constituents present in the sample. Probably the most frequently used method of digestion incorporates the use of nitric and sulfuric acids. Kopp (1) used this digestion method and experienced 91 to 114% recovery of arsenic trioxide added to deionized water and 86 to 100% recovery of the compound added to river water. Evans and Bandemer (2) recovered 87% of the arsenic trioxide added to eggs. By modifying the above digestive method by the addition of perchloric acid, Caldwell et al. ( 3 )observed 80 to 90% arsenic recovery with o-nitrobenzene arsonic acid, 85 to 94% arsenic recovery with 0-arsanilic acid, and 76.7'70 arsenic 0003-2700/79/035 1-0731$01.OO/O
recovery with disodium methylarsenate. Two uncertainties seem to arise when reviewing the previous digestive methods using nitric and sulfuric acid. First, the addition of inorganic arsenic to an organic matrix and subsequent recovery of all the inorganic arsenic added is not definitive proof of total recovery of any organoarsenicals present. Secondly, the choice of o-nitrobenzene arsonic acid and 0-arsanilic acid seems unfortunate since both compounds represent arsenic attached to an aromatic ring which is atypical of cacodylic acid and disodium methyl arsonate, two widely used organoarsenicals. Aside from nitric and sulfuric acid, a relatively simple digestive method employing 30% hydrogen peroxide in the presence of sulfuric acid was reported by Kolthoff and Belcher ( 4 ) and subsequently used by Dean and Rues (5)to determine arsenic in triphenylarsine. The application of this digestive method to environmental samples including wastewater is the subject, in part, of this investigation. Armstrong e t al. (6) observed that organic matter in seawater could be oxidized to carbon dioxide on exposure t o sufficient ultraviolet radiation from a medium pressure mercury arc vapor lamp. Tam (7) also used this approach t o decompose organoarsenicals and observed 111% for 0-arsanilic acid, 97% for sodium cacodylate, and 108% for arsenazo. The feasibility of this digestive method t o organoarsenicals added to wastewater samples warrants further investigation. This research was undertaken to determine to what extent hydrogen peroxide-sulfuric acid and ultraviolet irradiated liberated inorganic arsenate from organoarsenicals added to treated wastewater samples. Arsenic determinations were performed by either the colorimetric procedure described by Kopp ( I ) or the arsine-atomic absorption method described by Manning (8).
EXPERIMENTAL Apparatus. All colorimetric determinations were made using a Bausch and Lamb Spectronic 20 colorimeter equipped with a 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
1-cm absorption cell a t a wavelength of 520 nm. Arsine was collected in a arsine generation apparatus, Fischer No. 1405. High sensitivity arsenic determinations were obtained on a Perkin-Elmer 403 Atomic Absorption Spectrophotometer equipped with deuterium background correction and a 10 mu Hitachi model 165 recorder. Samples were irradiated by a medium pressure 450-W mercury arc photochemical lamp (Hanovia lamp no. 67A0100) mounted vertically in the middle of an aluminum cylinder large enough in diameter to accommodate ten 1-inch diameter silica tubes which had been fashioned by sealing the ends of discarded germicidal ultraviolet lamps. A blower fan was attached to the bottom of the lamp housing and served as an extractor fan to provide cooling of the lamp and samples. Excessive cooling of the lamp and resulting dimming of the lamp discharge was alleviated by enclosing the lamp in a 2-inch diameter quartz liner which had been tapered a t one end to retard air flow. The silica tubes which contained the samples were approximately 20 cm long and set 2 cm parallel to the lamp. Reagents. The following arsenic compounds were used in this study without further purification: arsenic trioxide (99.9870 As203),Baker Reagent; triphenylarsine oxide (no purity listing), Research Organic/Inorganic Chemical Corporation; disodium methane arsonate (95%), Alpha Inorganics; and dimethylarsinic acid (no listing of purity), Research Organic/Inorganic Chemical Corporation. All other reagents were reagent grade or the highest purity obtainable. Analysis of Organoarsenicals. Quadruplicate samples of each of the three organoarsenicals were subjected t o vigorous digestion with hydrogen peroxide and sulfuric acid as described by Kolthoff and Belcher ( 4 ) . Arsenate, resulting from the decomposition of the organoarsenicals, was reduced to arsenite with hydrazine sulfate and titrated with potassium bromate. The yellow color of free bromine liberated by oxidation of bromide ion in the presence of excess bromate titrant served to distinguish the end point of the titration. Stock solutions of 1000 mg/L as As were prepared for each organoarsenical in addition to As203. Analytical Procedures. ( a ) Hydrogen Peroxide-Sulfuric Acid Digestion-AgDDC Analysis. Aliquots of each of the three organoarsenicals containing 5 yg of arsenic were added to 125-mL Erlenmeyer flasks which also served as arsine generators. To each organic compound and a complete set of arsenic trioxide standards, ranging from reagent blank to 6 pg of arsenic, 5 mL of concentrated sulfuric acid and 5 mL of 30% hydrogen peroxide were added. Samples and standards were boiled off and fumed for an additional 2 min. The samples were cooled and to each flask the following were added in succession: 25 mL of water, 7 mL of HCl (concentrated), 5 mL of 15% KI, and after 10 min, five drops of 40% SnC12. The samples were refrigerated for 20 rnin to allow for reduction to occur. Arsine was generated into an absorption tube containing 4 mL of the AgDDC reagent for 1 h. The applicability of this digestive procedure to real samples was investigated by adding known quantities of the three organoarsenic compounds to wastewater samples and observing the arsenic recovery after digestion with sulfuric acid-hydrogen peroxide. The samples chosen for spiking with organoarsenicals were a primary settled raw sewage sample being fed into an activated sludge plant (Aeration Feed) and the effluent (SBE) from the discharge of this process. These samples were taken from the Dallas Water Utilities Pilot Plant facility. This portion of the study was accomplished by dividing each sewage sample into four separate subsamples of 100 mL volume and spiking separately three of the four subsamples with each of the three organoarsenicals equivalent to 5 kg arsenic. The samples and accompanying standards were then subjected to hydrogen peroxidesulfuric acid digestion with analysis to AgDDC. ( b ) Hydrogen Peroxide-Sulfuric Acid-Analysis by A A . Aliquots of each of the three organoarsenicals containing 0.50 kg of arsenic were added to 125-mL Erlenmeyer flasks which were also to serve as arsine generators. The samples were digested in an analogous manner as described in the previous digestion. To the samples and standards, ranging from reagent blank to 1.0 pg of arsenic, 25 mL of water, 10 mL of concentrated hydrochloric acid, and 5 mL of 15% KI were added. After 10 min, five drops of 40% stannous chloride were added and the samples were allowed to react at room temperature for 20 min prior to analysis.
Each flask, after addition of a stirring bar, was connected to the dosing column by means of a Teflon seal and purged of air with argon. The two-way purge valve was then closed and 2 g of arsenic-free zinc was delivered through the dosing stopcock to the flask. A magnetic stirrer was used to stir the contents of the flask during the 240 s the arsine and hydrogen were being generated and collected in a ballon reservoir for analysis by atomic absorption. ( c ) Ultraviolet Decomposition-AgDDC Analysis. Samples were irradiated by a medium pressure 450-W mercury arc photochemical lamp (Hanovia lamp no. 679A0100) mounted vertically in the middle of an aluminum cylinder large enough in diameter to accommodate ten 1-inch diameter silica tubes which had been fashioned by sealing the ends of discarded germicidal ultraviolet lamps. A 12-V blower fan was attached to the bottom of the lamp housing and served as an extractor fan to provide cooling of the lamp and samples. Excessive cooling of the lamp and resulting dimming of the lamp discharge was alleviated by enclosing the lamp discharge in a 2-inch diameter quartz liner which was tapered at one end to retard air flow. The silica tubes which contained the samples were approximately 20 cm long and set 2 cm parallel to the lamp. An assessment of the length of irradiation time necessary to decompose photochemically the three organoarsenicals was investigated. This was accomplished by placing into each of ten silica tubes 50 mL of cacodylic acid equivalent to 50 yg of arsenic and removing each tube from the unit after irradiation for a predetermined exposure up to 3.5 h. Triphenylarsine oxide and DSMA were experimentally treated in a analogous manner. Prior to irradiation, each sample was acidified with three drops of nitric acid and also three drops of 30% hydrogen peroxide. Following irradiation, each sample was transferred to a 100-mL volumetric flask and made to volume. Then, 10 mL of the 100 mL was volumetrically transferred to a 125-mL Erlenmeyer flask, which also served as the arsine generator. The inorganic arsenic content was determined colorimetrically with AgDDC and the extent of photodecomposition based on the experimentally observed quantity over the 5-pg arsenic total in the 100-mL volume taken for analysis. The applicability of ultraviolet decomposition to wastewater samples was investigated by spiking the same samples which had previously been evaluated using the hydrogen peroxide-sulfuric acid digestion with the organoarsenicals. The aerator feed and settling basin effluent samples were handled separately. The aerator feed sample contained a substantial amount of particulate matter and was blended for approximately 5 min beforehand. After blending, 50 mL of the aerator feed sample was volumetrically pipetted into each of four silica tubes. To three of the four tubes, 1-yg quantities of the three organoarsenicals were added. Sufficient nitric acid was added to bring the pH to 2 and three drops of 30% hydrogen peroxide were added. The samples and a 50-mL arsenic trioxide standard containing 10 yg of arsenic were irradiated for 4 h. After irradiation, the aerator feed samples were transferred t o 100-mL volumetric flasks and made to volume. Arsenic analysis was carried out on 25-mL aliquots using atomic absorption. The analysis of the effluent sample (SBE) was performed in a similar manner, except that 25-mL sample volumes spiked with 0.5 yg of the three organoarsenicals were utilized in the study. In an effort to shorten the time of irradiation, the smaller sample volumes were used in conjunction with 2-h irradiations.
RESULTS Since reagent grade purities of the three organoarsenicals utilized in this study were not available, it was necessary t o get a comparison of the arsenic content of each compound by several different methods. T h e arsenic content of the three organoarsenicals was determined by three independent methods of analysis including bromate titration of arsenious acid, neutron activation, and X-ray fluorescence. T h e arsenic content for triphenylarsine oxide revealed arsenic percentages of 22.0, 22.3, and 27.2% arsenic (as As) by each of the above methods, respectively. Dimethylarsinic acid was observed t o contain 34.6, 34.4, and 34.9%, respectively. Disodium methanearsonate was found t o contain 25.6% arsenic by
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979 I
Table 1. Recovery of Arsenic from Triphenylarsine Oxide, Disodium Methanearsonate, and Dimethylarsinic Acid Employing Wet Digestion with 5 m L of 30% Hydrogen Peroxide and 5 mL of Sulfuric Acid with Analyses by AgDDC and Atomic Absorption recovery, %a atomic AgDDC absorption compound 100.8 ( 2 ) triphenylarsine 101.0 2 5.2 ( 4 ) oxide disodium 103.5 z 1 . 2 ( 4 ) 100.8 ( 2 ) methanearsonate dimeth ylarsinic 99.9 I2.3 ( 3 ) 99.2 ( 2 ) acid a Each sample contained 5 p g As. Number in parentheses indicates the number of samples run. Table 11. Recovery of Arsenic from Triphenylarsine Oxide, Disodium Methanearsonate, and Dimethylarsinic Acid Spiked into AF and SBE Sample with Wet Digestion Employing 1 5 mL of 30% Hydrogen Peroxide and 5 mL of Sulfuric Acid with Analysis b y AgDDC' recovery, . . %b -
sample 100 mL AF 100 mL AF + triphenylarsine oxide 1 0 0 mL AF + disodium methanearsonate 1 0 0 mL AF + dimethylarsinic
sample 2
sample 1 (9.7 pg A s L ) 92.4 90.4 96.0
89.3 90.2
acid
1 0 0 m L SBE 100 mL SBE
+
(22.6 p g As/L) (18.5 pg As/L) 100.6 101.1
triphenylarsine oxide 100 mL SBE + 98.1 disodium methanearsonate 100 mL SBE + 89.4 dimethylarsinic acid a All 5 - p g weights are as arsenic. amount of arsenic initiallv present.
104.4 96.6
Corrected for
bromate titration and 24.5% As by neutron activation. T h e values of arsenic obtained by t h e volumetric determinations were used in the preparation of the lo00 mg/L stock solutions of the three organoarsenicals. As an additional check on each stock solution, 50 m L of each of the three solutions was placed into separate flasks, digested with sulfuric acid and hydrogen peroxide, and titrated with 1.0 N potassium bromate. Each sample required 13.3 m L of 0.1 N potassium bromate which is equivalent to 49.8 mg of the 50 mg of arsenic known to be present in the sample. A final check was made on the stock solutions by atomic absorption. A lo-, 50-, and 100-mg per liter solution was made from each organoarsenic stock solution and compared to arsenic trioxide solutions of t h e same concentrations. Each sample was aspirated into a hydrogen-argon flame a n d observed to give identical signals-any one compound could be used as a standard for t h e others. T h e Perkin-Elmer 403 Atomic Absorption Spectrophotometer was operated with deuterium background correction. The percent recoveries obtained by the digestion procedure using 5 mL of 30% hydrogen peroxide and 5 mL of sulfuric acid with AgDDC analysis and atomic absorption analysis are given in Table I. T h e recovery results obtained by this digestion method applied to the aeration feed and settling
I
,
,
,
I
I
I
7
733 1
1 0
20
40
60
80
100
IRRADIATION
120
140
160
l?C
200
TIME (min)
Figure 1. Recovery of arsenic after ultraviolet exposure as a function of time. 0 , triphenylarsine oxide; . , disodium methanearsonate; A, dimethylarsinic acid
Table 111. Recovery of Arsenic from Triphenylarsine Oxide, Disodium Methanearsonate, and Dimethylarsinic Acid Spiked into Aerator Feed ( A F ) and Settling Basin Effluent (SBE) with Digestion by UV and Analysis by High Sensitivity Atomic Absorption recovery, %a aerator aerator settling feed feed basin compound sample 1 sample 2 effluent triphen ylarsine oxide disodium methanearsonate dimethylarsinic acid Corrections were made ples.
110.0
100.0
102.8
100.0
102.4
102.8
100.0
109.8
88.6
for arsenic present in the sam~~
Table IV. Chemical Analysk-of the Aerator Feed Sample and the Settling Basin Effluent Sample Taken at Dallas samples aerator settling feed basin
parametera pH @ 25 " C 7.3 total alkalinity as CaCO, 216.0 ammonia nitrogen 12.7 organic nitrogen 12.8 nitrite-nitrate nitrogen 0.2 nitrite nitrogen 0.01 total phosphorus as P 5.5 chemical oxygen demand 222.2 total organic carbon as C 57.0 total inorganic carbon as C 50.9 total carbon as C 107.0 suspended solids 132 All values reported in mg/L units.
7.6 160.0 1.93
3.70 1.7 0.08 6.5 42.65 12.0 33.0 45.0 21
basin effluent samples spiked with each of the three organoarsenicals equivalent t o 5 pg of arsenic are presented in Table 11. T h e effects of ultraviolet irradiation as a function of time for triphenylarsine oxide, disodium methanearsonate, and dimethylarsinic acid are illustrated in Figure 1. T h e extent of arsenic recovery using photo-oxidation in conjunction with high sensitivity arsenic analysis when applied to the aerator feed and settling basin effluent samples is shown in Table 111. A chemical analysis of the aerator feeding and settling basin effluent samples is provided in Table IV in order to illustrate the overall water quality of t h e samples used in this study.
DISCUSS I ON T h e digestive method employing hydrogen peroxide and sulfuric acid with analysis by AgDDC gave extremely good
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
results with arsenic recoveries ranging from a low of 98.5 to a high of 104.9%. This same digestive method when applied to a primary settled raw sewage sample gave arsenic recoveries ranging from 89.1 to 96.0%. Arsenic recoveries of 89.4 to 104.4% were experienced from a n activated sludge effluent sample. T h e same digestive method Pmploying the high sensitivity arsenic analysis by atomic absorption resulted in arsenic recoveries of 99.2 to 100.8%. T h e hydrogen peroxide-sulfuric acid digestion seemed to provide t h e most consistent and complete recoveries of any of the wet digestive procedures previously examined. Previous studies using nitric-sulfuric acid, with and without ammonium oxalate, revealed arsenic recoveries ranging from 80.2 to 105.9% and from 100.5 to 110.5%, respectively. This digestion procedure coupled with the AgDDC colorimetric analysis resulted in quantitative arsenic recoveries from wastewater samples. T h e hydrogen peroxide-sulfuric acid digestion combined with t h e high sensitivity arsenic analysis gave acceptable recoveries of arsenic and should be considered a viable technique. The most encouraging digestive technique examined during t h e course of this investigation was the photodecomposition approach using ultraviolet light. The time-dependent study of the digestive effects of ultraviolet irradiation of the three organoarsenicals resulted in some interesting observations. A 15-min exposure of triphenylarsine oxide resulted in greater than 99% photodecomposition of the compound. The monoalkylated arsenic compound reacted much slower than triphenylarsine oxide, requiring 2 h for complete decomposition. The application of ultraviolet photodecomposition with high sensitivity atomic absorption analysis was demonstrated on t h e same set of wastewater samples used in the hydrogen peroxide-sulfuric acid digestion. Four-hour irradiation of the aerator feed samples spiked with the three organoarsenicals resulted in arsenic recoveries of 100 to 110%. During the 4 h of irradiation, the total organic carbon (TOC) content of the sample was diminished from an initial value of 57 mg/L carbon to 1 mg/L carbon. T h e total carbon content was reduced from 107 mg/L carbon to 5 mg/L carbon-a drastic
reduction in both organic and total carbon content. 'The settling basin effluent sample showed 102.8% recoveries for both disodium methanearsonate a n d triphenylarsine oxide. An 88.6% recovery of dimethylarsinic acid added to SBE indicated that 2-h irradiation was insufficient for all of this compound to photodecompose in the presence of other organic matter. T h e total organic carbon content was reduced from a n initial value of 12 mg/L carbon to 6 mg/L carbon-a 50% reduction. There were several observations made concerning the arsenic analysis by both the AgDDC technique and the high sensitivity atomic absorption procedure during the course of this investigation. Turbidity problems were initially encountered in the AgDDC colorimeteric procedure. Following the generation and collection of arsine into the chloroform solution containing I-epedrine and AgDDC, a precipitate would form which would seriously interfere with the absorbance measurements. T h e apparent cause of this problem was traced to the AgDDC. Of three different lots of AgDDC on hand, all three lots were distinctively different in color. The colors ranged from brown to pale green with the brown compound apparently responsible for the difficulty. In conclusion, it is felt that the data support the premise that the recovery of arsenic from the organoarsenic compounds studied-triphenylarsine oxide, disodium methanearsonate, and dimethylarsenic acid-is complete by wet digestion using hydrogen peroxide-sulfuric acid and by photodecomposition using ultraviolet light.
LITERATURE CITED (1) J. F. Kopp, Anal. Chem., 45, 1789 (1973). (2) R. J. Evans and S. L. Bandemer, Anal. Chem., 26, 595 (1954). (3) J. S. Caldwell, R. L. Lishka, and E. F. McFarren, J . A m . Wafer Works ASSOC.,65, 731 (1973). (4) I. M. Kotthoff and R. Belcher, "Volumetric Analysis", Vol. 3, Interscience Publishers, New York. 1957, pp 511-13. (5) J. A. Dean and R . E. Rues, Anal. Lett., 2 , 105 (1969). (6) F. A. J. Armstong, P.M. Williams, and J. D. H. Sbickbnd, Nature(Londonl. 211, 418 (1966). (8) D. C. Manning, A f . Absorpt. News/., 10, 6 (1971).
RECEIVED for review June 5,1978. Accepted January 26,1979.
