METAL INTERFERENCES AND THEIR REMOVAL PRIOR TO THE DETERMINATION OF As(T) AND As(III) IN ACID MINE WATERS BY HYDRIDE GENERATION ATOMIC ABSORPTION SPECTROMETRY
By R. Blaine McCleskey, D. Kirk Nordstrom, and James W. Ball
Water-Resources Investigations Report 03-4117
U.S. Department of the Interior U.S. Geological Survey
Boulder, Colorado 2003
CONTENTS
Abstract.......................................................................................................................................................2 Introduction.................................................................................................................................................2 Methods of investigation.............................................................................................................................3 Sample collection and preservation.....................................................................................................3 Reagents .............................................................................................................................................3 Analytical apparatus ............................................................................................................................3 Analytical procedures.........................................................................................................................3 Determination of arsenic(T) .........................................................................................................4 Determination of arsenic(III)........................................................................................................4 Removal of interfering metals prior to arsenic(III) determination ...............................................4 Results and discussion ................................................................................................................................5 Selective reduction of arsenic(III).......................................................................................................5 Potential metal interferences on the determination of arsenic(T) .......................................................5 Potential metal interferences on the determination of arsenic(III)......................................................6 Cation exchange separation of iron(III) and copper............................................................................8 Accuracy and time stability of arsenic redox species in acid mine waters .........................................8 Accuracy of arsenic(T) determinations ........................................................................................8 Accuracy of arsenic(III) determinations.....................................................................................11 Time stability of arsenic redox species.......................................................................................11 Summary ...................................................................................................................................................12 References cited ........................................................................................................................................12
FIGURES
1-13. Graphs showing
1. Effect of NaBH4 concentration on the calibration curve in solutions containing only As(III)........5 2. Effect of NaBH4 concentration on the reduction of As(V) to As(III) in solutions containing 10 μg/L As(III) and varying As(V) concentrations .........................................................................5 3. As(T) determinations in the presence of potential interfering metals..............................................6 4. As(T) determinations in the presence of hydride-forming metals ...................................................6 5. As(III) determinations in the presence of potential interfering metals ............................................7 6. As(III) determinations in the presence of interfering metals ...........................................................7 7. As(III) determinations in the presence of hydride-forming metals .................................................8 8. Additive interference in the determination of As(III) in a solution containing 10 micrograms per liter As(III) and 5 milligrams per liter Cu(II) ............................................................................8 9. Spike recovery determinations of As(T) using hydride generation atomic absorption spectrometry....................................................................................................................................9 10. As(T) determinations by inductively coupled plasma-optical emission spectrometry and hydride generation atomic absorption spectrometry for 20 samples collected from Summitville Mine, Colo., 6 samples collected from Richmond Mine, Calif., and 3 samples collected from the Penn Mine, Calif. ...............................................................................................9 11. Spike recovery determinations using hydride generation atomic absorption spectrometry for As(III) in samples separated by cation exchange and not separated .............................................11 12. As(III) determinations using hydride generation atomic absorption spectrometry for samples collected from the Summitville Mine ............................................................................................11 13. Time stability of As(III) for 45 surface and ground water samples from Yellowstone National Park, Wyo.; Questa Mine site, N. Mex.; Summitville Mine site, Colo.; Richmond Mine, Calif.; Penn Mine, Calif.; Ester Dome, Alaska; Fallon, Nev.; Mojave Desert, Calif.; and Kamchatka, Russia reanalyze 3 to 15 months after the initial determination. The curved line is a Gaussian Fit .............................................................................................................................12
TABLES
1. Cation exchange separation of Cu and Fe(III)..................................................................................9 2. Composition of acid mine water samples collected from Summitville Mine, Colo.; Richmond Mine, Calif.; and Penn Mine, Calif. ............................................................................10 3. As(T) determinations in U.S. Geological Survey standard reference water samples.....................10
--- (not analyzed or not measured) °C (degrees Celsius) AAS (atomic absorption spectrometry) As(III) (arsenic(III) or arsenite) As(V) (arsenic(V) or arsenate) As(T) (total arsenic (As(III) plus As(V)) cm (centimeter) conc. (concentration) EDL (electrodeless discharge lamp) FIAS (flow injection analysis system) g (gas) HCl (hydrochloric acid) HGAAS (hydride generation-atomic absorption spectrometry) HNO3 (nitric acid) KI (potassium iodide) ICP-OES (inductively coupled plasma-optical emission spectrometry) ISE (ion-selective electrode) μg/L (micrograms per liter) M (moles per liter) meq/L (milliequivalents per liter) meq/mL (milliequivalents per milliliter) mg/L (milligrams per liter) min. (minute) mL (milliliters) mv (millivolt) MPV (most probable value) μL (microliter) μm (micrometer) μS/cm (microsiemens per centimeter at 25 degrees Celsius) n (number of analyses) NaBH4 (sodium borohydride) nm (nanometer) RSD (relative standard deviation) s (standard deviation) SRWS (standard reference water sample) v/v (volume per volume) w/v (weight per volume) w/w (weight per weight)
Hydride generation atomic absorption spectrometry (HGAAS) is a sensitive and selective method for the determination of total arsenic (arsenic(III) plus arsenic(V)) and arsenic(III); however, it is subject to metal interferences for acid mine waters. Sodium borohydride is used to produce arsine gas, but high metal concentrations can suppress arsine production. This report investigates interferences of sixteen metal species including aluminum, antimony(III), antimony(V), cadmium, chromium(III), chromium(IV), cobalt, copper(II), iron(III), iron(II), lead, manganese, nickel, selenium(IV), selenium(VI), and zinc ranging in concentration from 0 to 1,000 milligrams per liter and offers a method for removing interfering metal cations with cation exchange resin. The degree of interference for each metal without cation-exchange on the determination of total arsenic and arsenic(III) was evaluated by spiking synthetic samples containing arsenic(III) and arsenic(V) with the potential interfering metal. Total arsenic recoveries ranged from 92 to 102 percent for all metals tested except antimony(III) and antimony(V) which suppressed arsine formation when the antimony(III)/total arsenic molar ratio exceeded 4 or the antimony(V)/total arsenic molar ratio exceeded 2. Arsenic(III) recoveries for samples spiked with aluminum, chromium(III), cobalt, iron(II), lead, manganese, nickel, selenium(VI), and zinc ranged from 84 to 107 percent over the entire concentration range tested. Low arsenic(III) recoveries occurred when the molar ratios of metals to arsenic(III) were copper greater than 120, iron(III) greater than 70, chromium(VI) greater than 2, cadmium greater than 800, antimony(III) greater than 3, antimony(V) greater than 12, or selenium(IV) greater than 1. Low recoveries result when interfering metals compete for available sodium borohydride, causing incomplete arsine production, or when the interfering metal oxidizes arsenic(III). Separation of interfering metal cations using cation-exchange prior to hydridegeneration permits accurate arsenic(III) determinations in acid mine waters containing high concentrations of interfering metals. Stabilization of the arsenic redox species for as many as 15 months is demonstrated for samples that have been properly filtered and acidified with HCl in the field. The detection limits for the method described in this report are 0.1 micrograms per liter for total arsenic and 0.8 micrograms per liter for arsenic(III).
Accurate determination of the redox state of dissolved As is important for interpreting its toxicity and mobility in the environment. Dissolved As can form aqueous species in several oxidation states, but in natural waters occurs dominantly as the inorganic oxyanions As(III) and As(V). Several analytical methods use hydride generation to selectively measure As redox species, but high metal concentrations can interfere with the determination of As(T) (As(III) plus As(V)) and As(III) (Smith, 1975; Welz and Melcher, 1984; Welsch and others, 1990; Creed and others 1996; Hageman and Welsch, 1996). Accurate determination of arsenic redox species in acid mine waters is challenging because of elevated metal concentrations frequently found in this type of water (Nordstrom and Alpers, 1999). Sixteen metal species including Al, Cd, Co, Cr(III), Cr(VI), Cu(II), Fe(II), Fe(III), Mn, Ni, Pb, Sb(III), Sb(V), Se(IV), Se(VI), and Zn were evaluated as possible interferents on the determination of As(T) and As(III). Because As(III) and As(V) exist as oxyanions in solution, interfering metal cations can be removed from solution using cation-exchange while maintaining the existing As(III) and As(T) concentrations. Pre-analysis separation of Fe(III) using cation-exchange resin for the determination of As redox species by ion chromatography in iron sulfate-sulfuric acid media has been successfully performed (Tan and Dutrizac, 1985). Cyanide also has been used as a complexing agent to eliminate interferences by metals in the determination of As using hydride-generation (Jamoussi and others, 1996). Interferences usually associated with atomic absorption analysis are negligible because arsine (AsH3(g)) is separated from the sample matrix. Results of accurate and precise analytical methods can be invalidated by deteriorated samples. Proper filtration and preservation are critical for maintaining the existing As(III)/As(T) ratio prior to analyses. Wing and others (1987) observed that filtration (0.1-μm or 0.45-μm), acidification with HCl, and storage in the dark at 4°C effectively preserved the As redox species for up to 8 months in ground waters collected near Fallon, Nev. Except for Fe(III) and hydrogen sulfide (H2S), laboratory studies demonstrate that rates of oxidation of As(III) and reduction of As(V) in aqueous solution with redox agents common to natural waters are slow (Cherry and others, 1979). Filtering the sample removes colloidal material and bacteria that can affect the dissolved As(III)/As(T) ratio (Gihring and others, 2001; Wilkie and Hering, 1998). Acidification prevents precipitation of Fe hydroxides that can coprecipitate or adsorb As (Gao and others, 1988; Wilkie and Hering, 1996). Excluding light inhibits potential photochemical reactions (Emett and Khoe, 2001; Hug and others, 2001) and storing the samples at 4°C slows chemical reactions.
To address the need for more accurate determinations of As(T) and As(III) by hydride generation atomic absorption spectrometry (HGAAS), the U.S. Geological Survey (USGS) evaluated metal interferences and their removal in acid mine waters. This report (1) describes sample collection, preservation, and analytical procedures for the accurate determination of As(T) and As(III) using HGAAS; (2) identifies metals and the concentration at which each interferes in the determination of As(T) and As(III); (3) demonstrates that Cu(II) and Fe(III), both of which interfere in the determination of As(III) by HGAAS, can be removed from solutions using cation-exchange while maintaining existing As(III)/As(T) ratios; (4) applies the method to acid mine waters collected from the Summitville Mine, Colo.; Penn Mine, Calif.; and Richmond Mine, Calif.; and (5) presents As redox species time stability data for 45 surface- and ground-water samples collected during 1999-2002 from Yellowstone National Park, Wyo.; Questa Mine site, N. Mex.; Summitville Mine site, Colo.; Richmond Mine, Calif.; Penn Mine, Calif.; Ester Dome, Alaska; Fallon, Nev.; Mojave Desert, Calif.; and Kamchatka, Russia.
Water samples collected in the field were pumped from the source through a 0.1 μm tortuous-path filter, acidified to pH less than 2 with hydrochloric acid (typically 2 mL 6-M HCl per 250 mL sample), and stored in acidwashed opaque bottles at 4°C (To and others, 1999).
Reagents
All reagents were of purity at least equal to the reagent-grade standards of the American Chemical Society. Double-distilled water and re-distilled or trace metal grade acids were used in all preparations. The following reagents were used for the As(T) and As(III) HGAAS procedure: 10 percent (w/v) KI from Aldrich; 10 percent (w/v) LAscorbic Acid from Aldrich; NaOH from Fisher; NaBH4 from Fisher; trace metal grade HCl from Fisher, Assay w/w 35 to 38 percent; 1,000 mg/L As(III) from High Purity Standards; 1,000 mg/L As(V) from High Purity Standards; and cation-exchange resin, AG 50W-X8, 20-50 mesh, H+ form from Bio- Rad. The following single-element standards (1 to 10,000 mg/L) were prepared for interference studies: CuSO4⋅5H2O was used to prepare Cu(II); FeSO4⋅7H2O was used to prepare Fe(II); Fe2(SO4)3 was used to prepare Fe(III); Zn(NO3)2⋅6H2O was used to prepare Zn(II); MnSO4⋅H2O was used to prepare Mn(II); Al(NO3)3⋅9H2O was used to prepare Al(III); Ni(SO4)⋅6H2O was used to prepare Ni(II); CrCl3⋅6H2O was used to prepare Cr(III); K2Cr2O7 was used to prepare Cr(VI); CoCl2⋅6H2O was used to prepare Co(II); Cd powder was used to prepare Cd(II); NaSeO4 was used to prepare Se(VI); Sb2O3 was used to prepare Sb(III); SbCl5 was used to prepare Sb(V); and Pb metal was used to prepare Pb. The Se(IV) standard was prepared by adding an equal volume of concentrated HCl to a portion of Se(VI) standard and heating at 90°C for 20 minutes.
An atomic absorption spectrophotometer (Perkin-Elmer (PE) – AAnalyst 300) with an electrically heated quartz cell having a path length of 15-cm inline with a flow injection analysis system (FIAS; PE – FIAS 100), an autosampler (PE – AS90), and an arsenic electrodeless discharge lamp (EDL) attached to an EDL power supply (PE – EDL System 2) were used. The following spectrometer parameters were used: EDL current: 380 mv; wavelength: 193.7 nm; slit: 0.7 nm. Peak height was used for data processing. The following FIAS parameters were used: carrier gas: Ar; cell temperature: 900°C; sample loop: 500 μL; carrier solution: 10 percent (v/v) HCl; reducing agent: 0.25 percent (As(T)) or 0.03 percent (As(III)) NaBH4 in 0.05 percent NaOH; carrier solution flow rate: 10 mL/min; reductant flow rate: 5 mL/min. Sodium borohydride was prepared daily and filtered through a 0.45 μm polyvinylidene fluoride filter membrane using a vacuum pump.
Hydride formation occurs when As(III) in an acidic solution reacts with NaBH4 according to the following reaction: 3NaBH4 + 4H3AsO3 → 4AsH3(g) + 3H3BO3 + 3NaOH (1) For the determination of As(T), As(V) is prereduced to As(III) using KI and L-ascorbic acid. All water samples and standards were prepared in 25-mL volumetric flasks. The method detection limits of the HGAAS analytical procedure used at the USGS National Research Laboratory in Boulder, Colo., are 0.1 μg/L for As(T) and 0.8 μg/L for As(III).
Determination of Arsenic(T)
1. A solution containing 10 percent (w/v) KI and 10 percent (w/v) L-ascorbic acid was prepared. 2. To each 25-mL volumetric flask, 5 mL concentrated HCl and 2.5 mL of the KI−L-ascorbic acid solution prepared from step 1 were added. 3. The As(III) standard stock solution was used to prepare standards ranging in concentration from 0 to 10 μg/L As. 4. To demonstrate completeness of reduction, synthetic samples containing varying As(III)/As(V) ratios were prepared. 5. A volume of sample (up to 16.5 mL) that will yield 1 to 10 μg/L As(T) when diluted to volume was transferred to a 25-mL volumetric flask. 6. Appropriate spike-recovery samples were prepared by adding both As(III) and As(V) to selected samples to demonstrate reduction of As(V) and AsH3(g) production. 7. The HGAAS analyses were performed a minimum of 30 minutes after sample preparation.
1. Twenty-five mL of concentrated HCl was added to each 25-mL volumetric flask. 2. As(III) standards ranging in concentration from 0 to 20 μg/L were prepared. 3. To demonstrate that As(V) was not reduced, synthetic samples were prepared containing several As(III)/As(V) ratios. 4. A volume of sample (up to 24.75 mL) that will yield 2 to 20 μg/L As(III) when diluted to volume was transferred to a 25-mL volumetric flask. 5. Appropriate spike-recovery samples were prepared by adding both As(III) and As(V) to demonstrate that As(V) is not reduced and that adequate AsH3(g) production occurs. 6. The HGAAS analysis was performed immediately.
Removal of Interfering Metals Prior To Arsenic(III) Determination
For samples having a Cu(II)/As(III) molar ratio greater than 120, Fe(III)/As(III) molar ratio greater than 70, Cd/As(III) molar ratio greater than 800, or for samples with low As(III) spike recoveries, the sample was mixed with cation-exchange resin (AG 50WX8, 20-50 mesh, H+ form) to remove interfering metals prior to determination of As(III). The following procedure was used to remove interfering metals and regenerate the resin: 1. Approximately 100 grams dry resin was transferred to a 250-mL polyethylene bottle. The resin was washed 2 times with 100 mL 2-M HCl followed by 5 washes with distilled water. The excess water was decanted. 2. The resin was dried in an oven at 100°C. 3. To a 50-mL polyethylene bottle, 12.5 grams dried resin and 25 mL sample were added. The mixture was periodically shaken for 30 minutes. 4. The sample was separated from the resin by decanting it into an acidwashed 15-mL polyethylene bottle. This sample was used to determine the As(III) concentration.
During the As(III) determination, As(V) can be reduced to AsH3(g) by the NaBH4 reductant causing an overestimation of As(III) concentration. At a constant carrier concentration and FIAS flow rate, increasing the NaBH4 concentration increases the sensitivity of the As(III) determination (fig. 1); however, when the NaBH4 concentration exceeds 0.0625 percent, As(V) is reduced, causing an overestimation of As(III) concentration (fig. 2). To ensure selective reduction of As(III), the NaBH4 concentration is maintained at 0.03 percent for the As(III) determination for analyses described in this report. Potential Metal Interferences on the
Determination of Arsenic(T)
Sixteen metal species including Al, Cd, Co, Cr(III), Cr(VI), Cu(II), Fe(III), Fe(II), Mn, Ni, Pb, Sb(III), Sb(V), Se(IV), Se(VI), and Zn were evaluated as possible interferents on the determination of As(T). The interference of each metal was evaluated by spiking synthetic samples containing 6 μg/L As(III) and 6 μg/L As(V) with 0.002 to 1,000 mg/L of the potential interfering metal and then determining the As(T) concentration using the HGAAS procedure (without cation exchange treatment). Arsenic(T) recoveries ranged from 92 to 102 percent for individual solutions spiked with Al, Cd, Co, Cr(VI), Cr(III), Cu(II), Fe(II), Fe(III), Mn, Ni, or Zn (fig. 3). For solutions spiked with a combination of metals including Al, Cd, Co, Cr(III), Cr(VI), Cu(II), Fe(II), Fe(III), Mn, Ni, and Zn, each up to a concentration of 500 mg/L, As(T) recoveries ranged from 97 to 102 percent. Dissolved Cu(II) was reduced to Cu metal during the pre-reduction step. The precipitated metal is visible at Cu concentrations greater than 100 mg/L and the liquid phase was decanted and analyzed with little change in As(T) concentration. Antimony(III) and Sb(V) were the only hydride-forming species found to interfere with the formation of AsH3(g) (fig. 4). Antimony(III) and Sb(V) substantially suppressed AsH3(g) formation by reacting with the NaBH4 when the Sb(III)/As(T) molar ratio exceeded 4 or the Sb(V)/As(T) molar ratio exceeded 2. In most waters, concentrations of Sb(III) and Sb(V) are much lower than As(T) concentrations and are likely to be diluted (As(T) less than 10 μg/L) to a concentration that will not interfere with the As(T) determination. Alkali and alkaline earth elements do not interfere with the As(T) determination (Smith, 1975).
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