Environ. Sci. Technol. 2000, 34, 2342-2347
Speciation of Submicrogram per Liter Levels of Arsenic in Water: On-Site Species Separation Integrated with Sample Collection X. CHRIS LE,* SERIFE YALCIN, AND MINGSHENG MA Environmental Health Sciences Program, Department of Public Health Sciences, Faculty of Medicine and Dentistry, 13-103 CSB, University of Alberta, Edmonton, Alberta, T6G 2G3 Canada
Speciation of arsenic is crucial for assessing health implications from arsenic ingestion and for effective removal of arsenic from water. We report a method for the speciation of submicrogram per liter levels of arsenic in water. The method incorporates water sample collection with on-site arsenic species separation. The method is based on selective retention of arsenic species on specific solidphase cartridges followed by selective elution and hydride generation atomic fluorescence analysis of the arsenic species. The use of a membrane filter, a resinbased strong cation-exchange cartridge, and a silica-based strong anion-exchange cartridge allows for the speciation of particulate arsenic and soluble arsenite, arsenate, monomethylarsonate, and dimethylarsinate species. Detection limit is on the order of 0.05 µg/L. The method is suitable for direct water sample collection and on-site separation of arsenic species by flowing a measured volume of water sample through the filter and cartridges connected in serial. A particular advantage of this approach is to maintain the integrity of original arsenic species in the sample. It overcomes the common problem of instability of arsenic species after water sampling and during sample storage and handling. Applications of the method are demonstrated to the speciation of arsenic in well water, raw (untreated) river water, bottled water, and a standard reference material (SRM 1643d). Results agree well with the certified values of the SRM.
Introduction Arsenic and some of its compounds are classified as human carcinogens (1-3). Evidence of chronic poisoning has been reported from many parts of the world. In India, Bangladesh, Inner Mongolia, and Taiwan, the high natural arsenic content of the drinking water has caused endemic, chronic arsenic poisoning (4-6). Epidemiological studies of populations (Taiwan, Argentina, and Chile) exposed to high levels of arsenic have demonstrated a relationship between elevated arsenic exposure via drinking water and the prevalence of skin, lung, and bladder cancers (7-12). While ingestion of high levels of arsenic (several hundred micrograms per liter) is believed to be a cause of certain cancers and noncancer effects, estimates of health risks resulting from the exposure to low levels of arsenic are controversial (13-18). * Corresponding author telephone: (780)492-6416; fax: (780)4920364; e-mail:
[email protected]. 2342
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Common routes of exposure to arsenic include ingestion and inhalation of arsenic compounds. Exposure to arsenic by the general population occurs mainly through ingestion of arsenic present in drinking water and food (1-3, 18, 19). The current maximum contaminant level (MCL) of arsenic in drinking water is 50 µg/L in the United States (18) and 25 µg/L in Canada (20). The safe Drinking Water Act Amendments of 1996 require that the U.S. Environmental Protection Agency (EPA) propose a standard for arsenic by January 2000 and promulgate a final standard by January 2001. The U.S. EPA is considering a reduction of the current MCL (16-19). Arsenic is present in the earth’s crust at an average concentration of 2-5 mg/kg and is associated primarily with igneous and sedimentary rocks in the form of inorganic arsenic compounds (21-25). It is found most frequently combined with sulfur, especially as arsenopyrite (FeAsS) and pyrite (FeS). Weathering of arsenic-containing rocks, which is considered to be the major natural source (21, 26-28), liberates arsenic in the form of inorganic compounds. The concentration of arsenic in natural waters of rivers, lakes, and oceans is not constant. In freshwater systems, the arsenic concentration varies considerably with the geological composition of the drainage area and the extent of anthropogenic input (21, 26, 29). Andreae et al. (29) reported a mean of 1.4 µg/L, with a range of 0.1-75 µg/L, of dissolved total arsenic in some European and American rivers. Others (30, 31) have reported similar large variations of arsenic concentration in freshwater systems. A concentration range of 0.4-80 µg/L is typical (21, 32). Arsenic concentrations in seawater tend to be less variable than in freshwater, although in surface seawater concentrations may be subject to some seasonal changes due to biological uptake, particularly in highly productive coastal regions (33, 34). An arsenic concentration range of 1.0-1.8 µg/L has been reported for deep Pacific and Atlantic waters (33, 34). In coastal waters a wider range is observed, but 1-3 µg/L is typical. As for speciation, arsenite [As(III)], arsenate [As(V)], monomethylarsonate (MMA), and dimethylarsinate (DMA) have been commonly found to be present in surface waters; their proportions vary with the extent of anthropogenic input and biological activities (31, 35-38). Methylarsenic(III) species (39-41) and other unidentified arsenic species (42, 43) have also been observed. Inorganic As(V) is the major arsenic species in surface water; As(III), MMA, DMA, and the other arsenic species are present at lower levels. Inorganic As(III) and As(V) are the major arsenic species in groundwater (4, 21, 24, 31, 44). The devastating arsenic endemic episodes in India, Bangladesh, Taiwan, and Inner Mongolia (4-9) are attributed to the high levels of inorganic arsenic in well water. Changes of geochemical conditions due to drilling of wells result in the mobilization and dissolution of previously stable arsenic (e.g., arsenic pyrite) from the rock into the water. The current method of water analysis for arsenic at the routine laboratory level is reliable at a concentration of 4 µg/L or higher (45). This level of sensitivity is insufficient to comply with the more stringent MCL. Development of more sensitive, inexpensive, and fast methods is needed for routine water arsenic analysis. Toxicity differences between arsenic species require speciation of arsenic. Speciation of particulate arsenic and soluble As(III) and As(V) is also essential to improving and designing processes for removing arsenic from water (46). Furthermore, uncontrolled conversion between inorganic As(III) and As(V) species after sampling and prior to analysis 10.1021/es991203u CCC: $19.00
2000 American Chemical Society Published on Web 04/22/2000
gives rise to problems of losing original speciation information in water. Attempts to preserve inorganic arsenic species in water have shown little success or are not practical (18, 47). The objectives of this research were to develop more sensitive and rapid methods for trace arsenic speciation analysis in water and to overcome problems due to changes of arsenic speciation after water sampling. We have developed a method in which particulate arsenic, As(III), and As(V) were separated on-site by using disposable cartridges. Immediately after collection, a measured volume of a water sample was pumped through a 0.45-µm filter and a resin-based strong cation-exchange cartridge followed by a silica-based strong anion-exchange cartridge. The filter collected particulate. The cation-exchange cartridge retained DMA and allowed other arsenic species to pass through. The anion-exchange cartridge retained MMA and As(V); As(III) remained in the solution. The filters, cartridges, and solution containing separated arsenic species were brought to a laboratory for analysis. Suspended arsenic collected on the filter was eluted with 0.5 M HCl. The unretained arsenic in the solution was a measure of As(III) in the original water sample. Arsenic eluted from the cation-exchange cartridge with 1 M HCl was quantified for DMA. The anion-exchange cartridge was first eluted with 60 mM acetic acid for the determination of MMA and then with 1 M HCl to elute As(V). If only inorganic As(III) and As(V) were present as the major arsenic species, the procedure could be simplified by using only the filter and the anion-exchange cartridge.
Experimental Section Standards and Reagents. An atomic absorption arsenic standard solution (Sigma, St. Louis, MO) containing 1000.0 mg of As/L as arsenite in 2% KOH was used as the primary arsenic standard. Sodium arsenate, As(O)OH(ONa)2‚7H2O, and sodium cacodylate, (CH3)2As(O)ONa, were obtained from Sigma; monomethylarsonate, CH3As(O)OHONa, was obtained from Chem Service (West Chester, PA). Stock solutions (1000 mg of As/L) and standard solutions were prepared as described previously (48). A standard reference material (SRM), Trace Elements in Water 1643d, was obtained from National Institute of Standards and Technology (NIST, Gaithersburg, MD). L-Cysteine was obtained from Sigma. Samples. Raw water samples were obtained from a drinking water utility in Saskatchewan, Canada. Bottled waters were obtained from a local supermarket in Edmonton, Canada. Tap water samples were obtained from Edmonton, Canada. Several well water samples were collected from northern Alberta, Canada. For the on-site sampling and separation of arsenic species with cartridges, 15 mL of a water sample was allowed to pass through a set of cartridges and filter connected in serial. The flow rate was 1-2 mL/min. The effluent was collected in a 15-mL polyethylene tube. The filter and cartridges were disconnected and capped (sealed) separately. The cartridges, filters, and effluents were brought back to our laboratory for arsenic analysis. For comparison, water samples without passing through the cartridges were also collected from the same locations for later analysis. Selective Retention and Elution of Arsenic Species. A membrane filter, a resin-based cation-exchange cartridge, and a silica-based anion-exchange cartridge were connected in tandem, as shown in Figure 1. The filters (0.45 µm pore size, 13-mm, PVDF filter media) were obtained from Whatman (Clifton, NJ). Resin-based strong cation-exchange cartridges (500 mg sorbent of 35-75-µm particle size) were obtained from Alltech (Missisauga, ON, Canada). Silica-based anion-exchange cartridges (500 mg sorbent of 40-µm particle size and 60-Å pore size) were obtained from Supelco (Missisauga, ON, Canada). Cartridges were preconditioned with 50% methanol and deionized water before use.
FIGURE 1. Schematic diagrams showing (a) a combination of cartridges and a filter in tandem for selective retention of arsenic species and (b) flow injection hydride generation for arsenic analysis. F, filter (0.45 µm); C, resin-based strong cation-exchange cartridge; A, silica-based strong anion-exchange cartridge; CE, cartridge effluent; P, peristaltic pump; S, sample; R, 0.05 M HCl; R1, 1.3% NaBH4 in 0.1 M NaOH; R2, 1.2 M HCl; I, sample injector; GLS, gas-liquid separator; Ar, argon gas; D, AFS or AAS detector; W, waste. A 10-20-mL sample was allowed to flow through the serially connected filter and ion-exchange cartridge set at 1-2 mL/min flow rate. In the laboratory setting, a peristaltic pump (Gilson, Minipuls 3) was used to deliver the sample. In the case of field sampling, disposable syringes were used to push water samples through the filter/cartridge set. Particulate was collected on the filter. DMA was retained on the resin-based strong cation-exchange (SCX) cartridge, while As(V) and MMA were retained on the silica-based strong anion-exchange (SAX) cartridge. As(III) was not retained on either cartridge and was collected in the effluent solution, which was kept for subsequent analysis. After the sample was passed through the tandem cartridges, the cartridges were detached and separately eluted with 3-5 mL of eluting buffers. Another 3-5 mL of eluting buffer solutions were allowed to flow through each of the cartridges for a second time to verify a complete elution of arsenic species. Particulate arsenic collected on membrane filters was eluted with 0.5 M HCl for three times (3, 3, and 6 mL) at a flow rate of 1 mL/min. Each eluent fraction was analyzed for arsenic separately. Most of the arsenic was found in the first two fractions. The sum of arsenic determined in the three eluents divided by the volume of original water sample that was passed through the filter was reported as particulate arsenic (µg/L). DMA retained on the resin-based strong cation-exchange cartridge was eluted with 1.0 M HCl. MMA and As(V) were sequentially eluted from the silica-based strong anionexchange cartridge by using 60 mM acetic acid (for MMA) and 1.0 M HCl (for As(V)). Arsenic species eluted from the corresponding filters and cartridges were determined by using flow injection hydride generation atomic absorption spectrometry (FIA-HGAAS) or flow injection hydride generation atomic fluorescence spectrometry (FIA-HGAFS). The amount of arsenic present in the effluent and eluents was quantified relative to arsenic signal in standard solutions. To eliminate inconsistent signal intensities in different acid media, standard solutions were prepared in the corresponding eluting buffer solutions. Hydride Generation Atomic Absorption and Atomic Fluorescence. An atomic absorption spectrophotometer (model SpectrAA-5, Varian, Victoria, Australia) was used in conjunction with hydride generation. An arsenic lamp (193.7 nm) was operated at 10 mA using an external control module (Varian). The spectral bandwidth was 0.5 nm. A T-shaped quartz absorption tube (Varian) heated to 925 °C with a temperature controller module (Varian, model ETC-60) was used as the atomization cell. A computer with a Varian Star Workstation ADC board and software was used to record VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Comparison of arsenic species in a tap water sample that was spiked with 5 µg/L each of As(III) and As(V). HPLCHGAFS analyses of the same sample were carried out immediately (a) and 2 days after (b) sample collection.
FIGURE 2. Typical traces from the FIA-HGAFS analysis of As(III) in deionized water (0.05-2 µg/L). and process signals from the atomic absorption detector. Peak areas in chromatograms were used for quantitation. A hydride generation atomic fluorescence spectrometer (HGAFS) (model Excalibur 10.003, PS Analytical, Kent, U.K.) was also used for detection of arsenic hydrides. The atomic fluorescence detector consisted of an excitation source, an atom cell, fluorescence collection optics, a photomultiplier tube, and a data collection unit as described previously (49). Cysteine was added to effluents, eluents, and water samples to a concentration of 2% prior to analysis. The sample was injected using a six-port sample injection valve (Rheodyne) with either a 200- or a 500-µL sample loop. The sample, carried by 0.05 M HCl (Figure 1, R), met with the continuous flow of sodium borohydride (NaBH4) solution (Figure 1, R1). We found that, in the presence of 2% cysteine, the optimum reagent concentrations for the generation of arsenic hydride are 0.05 M HCl and 1.3% NaBH4. In the case of AFS detection, the hydrogen produced from the reaction of NaBH4 with the acid sustained a hydrogen diffusion flame. However, the 0.05 M HCl was too low to produce sufficient amount of hydrogen for the flame. To solve this problem, we introduced another flow of 1.2 M HCl (Figure 1, R2) after the sample already reacted with 0.05 M HCl and 1.3% NaBH4. This additional HCl did not affect the hydride generation. It reacted with excess NaBH4 to produce the required hydrogen. To confirm the results, water samples without any treatment were also analyzed for arsenic species by using HPLC separation with HGAFS detection, a method described previously (50). A reversed-phase C18 column (ODS-3, 150 mm × 4.6 mm, 3-µm particle size. Phenomenex, Torrance, CA) was used for separation. A solution (pH 5.8) containing 5 mM tetrabutylammonium hydroxide, 4 mM malonic acid, and 5% methanol was used as the HPLC mobile phase at a flow rate of 1.5 mL/min.
Results and Discussion Determination of Arsenic Using Hydride Generation Atomic Fluorescence Spectrometry (HGAFS). Figure 2 shows typical traces from the analysis of arsenic (0.05-2 µg/L) using HGAFS. 2344
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A 200-µL sample was injected for each analysis. Detection limit, based on 3σ, is estimated to be 0.05 µg/L. The excellent detection limit is achieved with the combination of atomic fluorescence detection and hydride generation processes. Molecular fluorescence has demonstrated ultimate singlemolecule detection limit (51, 52). Conventional atomic fluorescence based on liquid sample nebulization, however, suffers from interference due to light scattering and background noise from sample matrix. Hydride generation is a chemical derivatization process that converts certain chemical species in solution to gaseous hydride. Thus, the analyte of interest, such as arsenic, is separated from the sample matrix. When combined with atomic fluorescence, hydride generation dramatically reduces the interference and improves the detection limit. In addition, the formation of arsenic hydride is very efficient, resulting in an enhancement of sensitivity by 10-100-fold as compared with direct liquid sample nebulization. The improved detection limit is adequate to meet the needs for direct analysis of arsenic in water to comply with the more stringent drinking water guidelines. The current interim maximum contaminant level (MCL) for arsenic in the United States is 50 µg/L, and this is expected to be reduced to a lower level (16-18). Instability of Arsenic Species in Water Samples. A crucial requirement for chemical speciation is to maintain the integrity of the original chemical species in the sample before analysis. However, trace inorganic As(III) and As(V) species in water samples are not stable and undergo rapid conversion (18, 24, 47, 53, 54). Figure 3a shows a chromatogram obtained from a tap water sample analyzed immediately after being spiked with 5 µg/L As(III) and As(V). The same sample analyzed 2 days after storage at 4 °C is shown in Figure 3b. Clearly, a portion of inorganic As(III) is oxidized to As(V) after 2 days. Detailed studies of the stability of arsenic species in water showed that As(III) and As(V) were not stable and that the conversion between these two species was dependent on sample matrix (data not shown). Although As(V) is thermodynamically favored and was predominant in most cases, it was found to be reduced to As(III) in some water samples during storage. One approach to overcoming the problem of altering the chemical species after sample collection is to separate the chemical species on-site and keep them separate for laboratory analysis. This led us to further develop speciation techniques that are suitable for field applications. Speciation of Arsenic Using Disposable Solid-Phase Cartridges. In a detailed study of arsenic behavior on various disposable solid phase cartridges, we have observed selective
FIGURE 4. Comparison of signals from arsenic in standard solutions, eluents, and waste from cartridges. S, standard containing 10 µg/L of arsenic as one of the four arsenic species; S2, standard containing 2.5 µg/L of arsenic in the form of As(III); E, eluent (5 mL); CE, cartridge effluent from a sample being passed through the cartridges. retention of arsenic species on certain types of cartridges. The difference in retention and elution properties of these arsenic species enabled us to develop a simple method for their speciation. The method was based on the following observations: (a) a strong cation-exchange cartridge was able to quantitatively retain only DMA while allowing MMA, As(V), and As(III) to pass through; (b) MMA and As(V) were completely retained on a strong anion-exchange cartridge; and (c) As(III) was not retained on either cation or anionexchange cartridges and was only retained on alumina cartridges. Examples of selective retention and elution of arsenic species on cation- and anion-exchange cartridges are depicted in Figure 4. A 20-mL solution containing 2.5 µg/L As(III) was passed through both strong cation-exchange (SCX) and strong anion-exchange (SAX) cartridges linked in tandem. The effluent was collected for analysis of unretained arsenic. As(III) was not retained on either cartridge and was recovered in the waste as demonstrated by the same signal intensity between the cartridge effluent (CE) and the 2.5 µg/L standard (S2) (Figure 4a). When a 20-mL solution containing 2.5 µg/L DMA was passed through the SCX cartridge, no arsenic was detectable in the waste solution, indicating that DMA was completely retained on the cartridge. A 5-mL solution of 1.0 M hydrochloric acid was used to elute the DMA from the cartridge and the eluent analyzed (Figure 4b). As the final solution volume was reduced from the initial 20-mL sample solution to the 5-mL eluent, the concentration of DMA was expected to be 10 µg/L for a quantitative recovery. The same peak intensity between the eluent (E) and the 10 µg/L standard (S) confirmed that DMA was quantitatively retained and eluted (Figure 4b). When a solution (20 mL) containing both As(V) and MMA (each 2.5 µg/L) was passed through the anion-exchange cartridge, no arsenic was detectable in the waste solution, indicating that As(V) and MMA were retained on the cartridge. A 60 mM acetic acid (5 mL) sample was used to selectively elute MMA from the cartridge, and a quantitative recovery of MMA was obtained as shown by the same signal intensity
between the eluent (E) and the standard (S) (Figure 4c). Subsequent elution with 1.0 M hydrochloric acid (5 mL) resulted in the quantitative recovery of As(V) as shown in Figure 4d. Similar results were obtained when the arsenic species were spiked into river water and bottled water samples. All four arsenic species were quantitatively recovered. Separation of the four arsenic species could be achieved by using cation- and anion-exchange cartridges linked in tandem (Figure 1). When a sample is passed through the strong cation-exchange cartridge, DMA is captured. As the solution flows through the anion-exchange cartridge, MMA and As(V) are retained. As(III) remains in the solution and is collected for further analysis. The sequence of the cationexchange cartridge followed by the anion-exchange cartridge is important because only DMA can be retained on the cationexchange cartridge while MMA, As(V), and DMA can be retained on the anion-exchange cartridge. DMA is separated from MMA and As(V) by using the cation-exchange cartridge. Application of the technique was demonstrated through the speciation of arsenic in several types of water samples, including a standard reference material (SRM 1643d) river water, a bottled water, four well water samples, and a raw water sample from a water treatment plant. The same samples were also analyzed by using the HPLC-HGAFS method for comparison. Results are summarized in Table 1. No methylated arsenic species were detected in these samples. The only detectable arsenic species were inorganic As(III) and As(V). The results obtained from the two methods are in good agreement. Results from the analysis of the SRM water sample are also in good agreement with the certified value of 56.02 ( 0.73 µg/L. There is no speciation information available from the SRM. Figure 5 shows typical traces obtained from the analysis of three well water samples. Only As(III) and As(V) were quantified because an HPLC-HGAFS analysis confirmed that these were the only detectable arsenic species. For the speciation of these two arsenicals in water, only a silicabased strong anion-exchange cartridge was needed to retain As(V). As(III) was not retained and was detected in the effluent solution. Results from the analyses of the three well water samples are also summarized in Table 1. To estimate the capacity of the cartridge, 250 mL of a deionized water solution containing 2000 µg/L As(V) was passed through an anion-exchange cartridge at a flow rate of 2 mL/min. The effluent was continuously monitored for arsenic, and it was found to be undetectable. Thus, the capacity of the anion-exchange cartridge for As(V) is greater than 500 µg. Arsenic species captured on the cartridges can be quantitatively recovered for up to 4 weeks after the cartridges were stored at 4 °C in a refrigerator. Incorporating the Tandem Cartridge Method to OnSite Water Sampling. The cartridge method was applied to direct sampling of water in the field. A measured volume of water sample (typically 15 mL) was directly passed through a 0.45-µm membrane filter and a strong anion-exchange cartridge, which were connected in serial. The effluent, which contained As(III), was collected in a clean tube. The filter and the cartridge were disconnected, capped tightly, and brought to the laboratory for arsenic speciation analyses. Table 2 shows the results from speciation analyses of soluble As(III) and As(V) and particulate arsenic in several well water samples collected from northern Alberta, Canada. These are shown as mean ( 1 SD (n ) 6) from duplicate sample preparations and triplicate analyses of each sample preparation. It is important to note the substantial amount of particulate fraction of arsenic. In two well water samples, the particulate arsenic is the major contributor to the total arsenic levels in water. The information on particulate fraction VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Arsenic Species in Water Samples (µg/L)a As(III) samples
cartridge method
SRM-1643d bottled water well 30 well 31 well 32 well 27 raw water
18.0 ( 0.2 nd 0.6 ( 0.1 0.7 ( 0.1 0.6 ( 0.1 13.0 ( 0.3 0.2 ( 0.1
As(V)
HPLC-HGAFS
cartridge method
22.7 ( 0.3 nd 1.0 nd nd 20 nd
29.9 ( 0.8 8.9 ( 0.3 0.3 ( 0.1 0.5 ( 0.1 4.2 ( 0.1 7.0 ( 0.2 0.3 ( 0.1
sum of As(III) and As(V)
total As
HPLC-HGAFS
cartridge method
HPLC-HGAFS
FIA-HGAAS method
31 ( 2 8.7 ( 0.6 nd nd 4.8 8.9 nd
47.9 ( 0.8 8.9 ( 0.3 0.9 ( 0.1 1.2 ( 0.1 4.8 ( 0.1 20.0 ( 0.3 0.5 ( 0.1
54 ( 2 8.7 ( 0.6 1.0 nd 4.8 28.9 nd
56.02 ( 0.73b 9.2 ( 0.3 0.8 ( 0.1 1.1 ( 0.1 4.9 ( 0.1 21.7 ( 0.3 -
a Results of cartridge method are mean ( 1 SD (n ) 6) from duplicate samples passing through the cartridge followed by triplicate analyses of each effluent and eluent. Results of HPLC-HGAFS are from triplicate analyses or from a single analysis when no standard deviation is given. FIA-HGAAS data are mean ( 1 SD (n ) 3) from triplicate analysis of the samples. nd, not detectable (concentration below detection limit); -, analysis not performed. b Certified value.
analysis. As(III) and As(V) species in water undergo rapid conversion after sample collection, leading to the loss of reliable speciation information (18, 24, 47, 53, 54). Attempts to preserving arsenic species in water by using various additives have not been proven to be successful or practical. The method described here using a solid-phase cartridge to separate As(III) and As(V) species on-site combined sample collection and species preservation in one step. Analysis of arsenic retained on the anion-exchange cartridge [As(V)] and arsenic remaining in solution [As(III)] provides original arsenic speciation in the water sample. The method is highly sensitive (detection limit 0.05 µg/L), simple, and inexpensive (each cartridge costs less than $2). On-site testing of the method demonstrated its applicability to water arsenic speciation analysis.
Acknowledgments We thank Dr. Joe Bergman of Buffalo Pound Water Administration Board, Saskatchewan, Canada, for providing the raw water samples. We also acknowledge the American Water Works Association Research Foundation for its financial, technical, and administrative assistance in funding and managing the project through which this information was discovered.
Literature Cited FIGURE 5. FIA-HGAFS traces from the analysis of As(III) and As(V) standard solutions and arsenic in water samples. Each well water sample was passed through a separate strong anion-exchange cartridge. As(V) was retained on the cartridge and eluted with 1 M HCl for HGAFS analysis. As(III) was not retained and was directly determined in the waste solution.
TABLE 2. Concentration of Arsenic Species (µg/L) in Three Well Water Samplesa
sample
soluble As(III)
soluble As(V)
total soluble As
particulate As
total As
well 1 well 2 well 3
12 ( 1 6.3 ( 0.2 63 ( 2
20 ( 1 6.2 ( 0.2 11 ( 1
32 ( 1 13 ( 1 74 ( 2
33 ( 1 47 ( 1 59 ( 1
65 ( 2 60 ( 1 133 ( 3
a Results are mean ( 1 SD (n ) 6) from duplicate samples passing through the filter and the cartridge followed by triplicate analyses of each effluent and eluent.
of arsenic is important because particulate can be readily removed by filtration, thereby reducing arsenic level (46). The separation of particulate arsenic, soluble As(III), and As(V) directly on-site using cartridges is particularly useful for the preservation of arsenic species before laboratory 2346
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Received for review October 20, 1999. Revised manuscript received February 15, 2000. Accepted March 13, 2000. ES991203U
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