Environ. Sci. Technol. 1999, 33, 3686-3688
Evaluation of Arsine Generation in Arsenic Field Kit A. HUSSAM* Chemistry Department, George Mason University, Fairfax, Virginia 22030-4444 M. ALAUDDIN Chemistry Department, Wagner College, Staten Island, New York 10301 A. H. KHAN Department of Chemistry, University of Dhaka-1000, Dhaka, Bangladesh S. B. RASUL AND A. K. M. MUNIR Sono Diagnostics Center Environment Initiative, Courtpara, Kushtia, Bangladesh
The recent outbreak of arsenic in groundwater of Bangladesh has prompted the widespread use of arsenic field kits. The kit involves the generation of arsine (AsH3) from inorganic arsenic species by reduction with Zn and HCl. The arsine then reacts with a test strip containing HgBr2 to produce a color that is compared with a color scale for quantitation. It is known that arsine gas is one of the most toxic substances known to man. The objective of this work is to measure the concentration of ambient arsine produced during the test and suggest a safe handling procedure. The analytical method is based on integrated AsH3 measurement by a single-point arsine monitor. The method can be used to measure 4-50 ppb arsenic in water with 10% in precision and accuracy. Experiments show that a typical test kit produces arsine with a 90% efficiency. The concentration of arsine produced even at low level can be more than 9 times above the 50 ppbv threshold limiting value (TLV). Actual kit experiments show that 50% of the arsine escapes the reaction cell during the test. We estimate that the maximum arsine concentration in the immediate vicinity of the kit can be more than 35 times TLV of arsine from a single experiment with 100 ppb total arsenic in solution. Particularly, field workers performing a large number of tests in highly affected areas are exposed to a much higher level of arsine. We suggest that the tests should be performed in well-ventilated places and that the worker should be provided with a gas mask to minimize arsine inhalation.
Introduction Recently, a high concentration of arsenic was found in the groundwater of Bangladesh and neighboring India (1-3). Millions of people may have been affected by the present crisis because groundwater is the primary source of drinking water. According to World Health Organization (WHO), the maximum contamination level (MCL) of arsenic in drinking water is 50 µg/L (or ppb, parts per billion) and 10 µg/L as a provisional guideline value (4). WHO also reports that there are about 2.5 million tubewells (wells with a metal casing) * Corresponding author phone: (703)993-1085; fax: (703)993-1055; e-mail:
[email protected]. 3686
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 20, 1999
in Bangladesh, and more than 95% of the Bangladesh population of 120 million drinks well water. It is generally found that about 45% of the groundwater pumped out from shallow and deep tubewells may have concentrations higher than the MCL (5). It appears that more than 2 million tubewells have to be tested for arsenic contamination. To screen these tubewells from arsenic contamination, a large number of field kits are employed. In Bangladesh, more than 25 000 tests were performed with the kits (4). The field kits are based on the classical chemistry employing reduction of tri- and pentavalent arsenic species to arsine gas (AsH3) by Zn and HCl in a reaction vial. The arsine production is monitored by a color change of a piece of strip containing HgBr2 hanging in the headspace above the solution. The kits are semiquantitative above 100 ppb; however, they are often inaccurate due to visual color detection. Recent results also show very poor correlation (r 2 ) 0.2-0.5) between field kit and laboratory-based measurement of arsenic (6). Methods such as hydride generation atomic absorption spectroscopy (HGAAS) (7) and anodic stripping voltammetry (ASV) (8) are also used in a very limited scale in centralized laboratories. ASV and other electrochemical methods are less expensive, provide direct speciation information, and are environmentally safe (9-12). Both HGAAS and ASV require significant training and expertise to use. The present study is motivated by the need to develop a field-deployable analytical technique for the measurement of arsenic in drinking water at a relatively low cost. Devices such as these should be able to measure total arsenic concentration below the MCL with good precision and accuracy and preclude the need for a centralized laboratory. This paper deals with the measurement of arsine produced during the operation of a standard field kit. Because arsine gas is very toxic with a threshold limiting value (TLV) of 50 parts per billion by volume (ppbv) (13), the present study is also aimed at understanding the distribution of arsine in the immediate vicinity of the measurement kit and suggesting a safe handling procedure.
Experimental Section The Merck field kit (Arsenic Quant Test Strips, Alfa-Aesar, USA) for semiquantitative determination of arsenic ions was used to generate arsine under normal laboratory conditions. The reagents provided with the field kit, Zn dust and 32% HCl, were used throughout the experiment. Ambient AsH3 gas concentration was measured by a commercial singlepoint arsine monitor (SPM, Hydride Chemcassette Detection System by Zellweger Analytics, Inc., Licolnshire, IL). The instrument works by measuring the change in color of a proprietary reagent (silver nitrate as the main component) on a paper tape by reflectance photometry. As an ambient arsine monitor, the instrument pumps in air through an inlet at 350 mL/min. The monitor has two alarm levels at 25 and 50 ppbv. Although the alarms can be disabled, we decided to leave them on in order to simulate a toxic hazardous condition. The two experimental setups used are shown in Figure 1a,b. Figure 1a shows the setup to measure in situ concentration of arsine produced in the reaction cell. The setup involves an 80-mL three-neck thermostated sample cell fitted with Teflon septa-backed screw caps. Teflon tubings are used as the outlet for arsine gas and the inlet for air. The middle cap was used to add arsenic solution and reagents. The dead volume of the cell and tubing is about 100 mL. The cell content was mixed on a magnetic stirrer when necessary. Figure 1b 10.1021/es9901462 CCC: $18.00
1999 American Chemical Society Published on Web 09/04/1999
FIGURE 2. Measurement of AsH3 gas produced by using experimental setup in Figure 1a at 25 °C: ([) measured ambient value, (0) cumulative value.
Zn(s) + 2HCl(aq) f Zn2+(aq) + 2Cl-(aq) + H2(g)
(1)
HAsO42-(aq) + 4H2(g) + 2H+(aq) f AsH3(g) + 4H2O (2) FIGURE 1. (a) Setup for the measurement of in situ arsine generation: 1, magnetic stirrer; 2, three-neck reaction cell thermostated at 25 °C; 3, air inlet tube; 4, reagent inlet port; 5, SPM arsine monitor; 6, arsine trap. (b) Setup for the measurement of ambient arsine produced by the arsenic test kit: 7, arsine generation cell for field kit; 8, test strip with HgBr2 hanging in the headspace. shows the second setup. The glass reaction vessel of the arsenic kit was used as the generator according to kit instructions. The inlet Teflon tubing of the arsine meter was placed about 5 cm from the generator. To minimize arsine emission in the room, the monitor outlet Teflon tube was placed in a 6 M HCl solution trap. Primary standards for As(V) were prepared by dissolving Na2HAsO4 in deionized water acidified with trace metal grade HCl (Fisher Scientific). Generally, kit instructions were followed for arsine generation and detection except 10.0 mL of standard solution instead of 5.0 mL solution was transferred to the cell. Then one measuring spoonful (about 0.4 g) of Zn dust was added and mixed followed by 10 drops of 32% HCl. A larger solution volume was used to ensure complete immersion of Zn dust at the start of the experiment. The test strip containing the HgBr2 reagent was used only in the second setup. Test strips are only sensitive above 100 µg/L arsenic as indicated by the manufacturer color chart. The arsine meter continuously displayed the arsine concentration in ppbv and was recorded as a function of time. Except for the kit experiment with low level arsenic (14 ppb), all other experiments were performed under in situ conditions as shown in Figure 1a. Tubewell water samples were collected from western Bangladesh (Kushtia) in 250-mL acid-prewashed high-density polyethylene bottles containing 1 mL of HCl (Analar) without headspace. The wells were allowed to drain for 10 min before sample collection. Experimental setup shown in Figure 1a was used for this part of the experiment. A Perkin-Elmer model 5100 Zeeman-effect atomic absorption spectrometer with a graphite furnace (Z-AASGF) and model A-60 autosampler were used to validate the well water arsenic concentration.
Results and Discussion The arsine generation in aqueous acidic solution is governed by the following reactions
Figure 2 shows a typical result of the concentration of arsine recorded as a function of time with setup in Figure 1a. It shows that, in the beginning of the experiment when no acid was added, the ambient arsine concentration was 44 ppbv. This was due to the acidic sample starting to react with Zn in the cell, and reaction 2 was initiated. The remaining data were obtained after the addition of acid. By using the integrated area (6680 ppbv-s) and the intake flow rate of 350 mL/min, we calculate that 1.59 nmol of arsine is produced from 1.85 nmol of As(V) (13.75 µg/L As(V) in 10.1 mL solution) at NTP. Based on the arsine generation reaction, the efficiency of the system is found to be 86%. Three such experiments showed an integrated area 7433 ( 866 ppbv-s and 96 ( 11% in conversion efficiency. Table 1 shows a series of such measurements with standards and tubewell water samples. The concentrations found by the arsine monitor and Z-AASGF are in very good agreement except for the highest value. Since the well water samples were diluted 5-fold, the effective concentration range monitored by the arsine monitor was 4-60 ppb. The average arsine generation efficiency was about 90%. It is known that the well water samples contained more than 40% of the total inorganic arsenic as As(III) (14). It is also known that As(III) remains stable at pH 2 with HCl for at least 2 months (15). Therefore, the arsine generation is not measurably affected by the presence of arsenic in different oxidation states. A major source of error in the experiment is the nonuniform flow of arsine through the sensor as observed by the nonuniform color development on the paper tape. At high concentration, the sensor saturation (above 150 ppbv arsine) and the inherent slow tape movement are the limiting factors. Despite these limitations, the experiment demonstrates that a very low level of inorganic arsenic can be measured in aqueous solution with good precision and accuracy. At its present configuration, the instrument is not suitable for field use, but the basic detection and measurement principle can be used to develop a portable instrument. Another aspect of this work is to warn the level of toxic arsine present in the ambient atmosphere during the experiment. All experiments produced higher than the alarm level (50 ppbv) of arsine more than half the time. The average cumulative arsine concentration for the first standard in Table 1 was 456 ( 66 ppbv with 90 ( 13% conversion efficiency. Clearly, the value increases in direct proportion to the initial concentration of arsenic in solution. The range shows 9-187 VOL. 33, NO. 20, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3687
TABLE 1. Measurement of Tubewell Water for Arsenic by Arsine Monitor and Zeeman-Effect Atomic Absorption Spectrometer with a Graphite Furnace (Z-AASGF)
sample As(V) standard As(V) standard E5 E6 E7 P1 WW1
integrated cumulative concn AsH3, found Z-AASGFc dilution area (ppbv)a (ppb)b factor (ppbv-s)a (ppb) 1 5 5 5 5 5 5
7433 28143 9825 19433 32763 76621 160553
456 1721 655 1285 2234 4196 9355
13.7 50 18 22 ( 4 35 40 ( 4 58 58 ( 4 136 127 ( 4 285 320 ( 10
a Integrated area is calculated as described in the text and corrected for sample dilution. b First two standards and zero blank were used to calibrate the arsine monitor. The calibration slope ) 564.6 ppbv-s/ppb and intercept ) -139 ppbv-s were used to calculate arsenic concentration in field samples. c Experimental conditions: wavelength, 193.7 nm; slit, 0.7 low; integration time, 6 s; and three replicates for standards and samples. Linear concentration calibration range: 0-40 ppb arsenic, slope ) 0.0039, intercept ) 0, and r 2 ) 0.99979.
experiment above, if 50% of arsine escaped, the ambient concentration of arsine could be 230 ppbv. Figure 3 shows that the measured cumulative arsine concentration is 207 ppbv before dismantling the cell. The value could be higher if the sample cell is exposed to air after the experiment or the plastic cell top is not secured. The level of ambient arsine will also increase proportionately with the increase of arsenite and arsenate in water. It is therefore possible that a field worker using the kit measuring 100 ppb arsenic can be exposed to a high level of toxic arsine (35 times the TLV). Clearly, a worker performing 10 such experiments a day keeping a watchful eye on the kit can get an alarmingly high level (350 times TLV) of arsine unless appropriate safety measures are taken. Because the exposed area near the test site is not known, we consider these to be the maximum possible values. The arsenic calamity of Bangladesh presents a challenge in field analytical methodology for rapid and accurate measurement of inorganic arsenic in drinking water. The kit is not only inadequate to screen water samples containing less than 100 ppb of arsenic, it also produces toxic arsine gas that may be a health hazard. We believe the kit experiment should be performed either under a working fume hood or in the open field with good airflow. The later may cause unacceptable results as discussed earlier. Particularly, workers performing a large number of tests in highly affected areas should be provided with a gas mask to minimize arsine inhalation.
Literature Cited
FIGURE 3. Measurement of AsH3 gas produced by using experimental setup in Figure 1b under normal laboratory condition: ([) measured ambient value, (9) cumulative value. times TLV of arsine. This is the maximum possible cumulative arsine obtained from undiluted groundwater samples. Unless arsine is properly vented or removed by absorption in a suitable media, the immediate work environment may remain a health hazard. Figure 3 shows the result based on the experimental setup in Figure 1b. Because most field workers are using this kit, the measurement of ambient arsine concentration is essential to understand the health hazard of the kit. The results show that arsine concentration dropped off nearly to zero at 500 s. The integrated area shows 4435 ppbv-s arsine, which corresponds to 57% of the total arsine that came out of the reaction vessel. The plastic top of the arsenic kit has a slot and two small orifices at each side of the slot. The arsine gas came out of these orifices under the influence of hydrogen gas generated inside the sample cell. At the end of 600 s, the plastic top was dismantled, and the residual arsine gas increased to 70 ppbv and went above the alarm level until the reaction vessel was removed. A repeat of this experiment for a shorter duration (300 s) produced an integrated area of 3680 ppbv-s, which corresponds to 48% of arsine escaped from the reaction vessel. These results are not significantly different considering the fact that the arsine collection tube is outside the sample cell and room air draft can cause irregular mass flow to the sensor. The nonuniform mass flow inside and outside the cell could be a reason for inaccuracies of the kit. On the basis of the data obtained from the controlled 3688
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 20, 1999
(1) Das, D.; Chatterjee, A.; Mandal, B. K.; Samanta, G.; Chakraborti, D.; Chanda, B. Analyst 1995, 120, 917-924. (2) International Conference on Arsenic Pollution of Ground Water in Bangladesh: Causes, Effects and Remedies. Abstracts; LGED Auditorium, Dhaka, Bangladesh, February 8-12, 1998. (3) Lepkowski, W. Chem. Eng. News 1998, 16, 27-29. (4) Arsenic in Drinking Water; Fact Sheet 210; WHO’s Office of Public Information: Geneva, February 1999. (5) Chakraborti, D.; Dhar, R. K.; Biswas, B.; Roy, S.; Kabir, S.; Arif, A. I. Abstracts; International Conference on Arsenic Pollution of Ground Water in Bangladesh: Causes, Effects and Remedies, February 8-12, 1998. (6) Groundwater Studies for Arsenic Contamination in Bangladesh, Phase I: Rapid Investigation Phase; Government of the People’s Republic of Bangladesh, Ministry of Local Government, Rural Development and Cooperative: Dhaka, Bangladesh, October 1998. (7) Eaton, A. J. Am. Water Works Assoc. 1994, February, 100-114. (8) U.S. EPA Method 7063: Arsenic in Aqueous Samples and Extracts by Anodic Stripping Voltammetry (ASV); December 1996. (9) Sun, Y.-C.; Mierzwa, J.; Yang, M.-H. Talanta 1997, 44, 13791387. (10) Davis, P. H.; Dulude, G. R.; Griffin, M. R.; Matson, W. R.; Zink, E. W. Anal. Chem. 1978, 50, 137-143. (11) Aldstadt, J. H.; Martin, A. F. Analyst 1996, 121, 1387-1391. (12) Huiliang, H.; Jagner, D.; Renman, L. Anal. Chim. Acta 1988, 207, 37-46. (13) Butler, J. D. Air Pollution Chemistry; Academic Press: New York, 1979; pp 6-12. TLV is the maximum toxic level that should be tolerated assuming an 8-h exposure per day for a 40-h working week. The TLV value shown above is recommended by USAOSHA. (14) Rasul, S. B.; Ahmed, N.; Munir, A. K. M.; Washe, S.; Khaliquzzaman, M.; Khan, A. H.; Hussam, A. Abstracts; International Conference on Arsenic in Bangladesh Ground Water: World’s Greatest Arsenic Calamity, Wagner College, New York, February 27-28, 1999. (15) Elteren, J. T. V. Analytical Aspects of the Speciation of Arsenic in the Aquatic Environment. Doctoral Dissertation, University of Utrecht, Utrecht, The Netherlands, September 1991.
Received for review February 10, 1999. Revised manuscript received July 12, 1999. Accepted July 29, 1999. ES9901462
