Determination of ultratrace dissolved arsenite in water--selective coprecipitation in the field combined with HGAFS and ICP-MS measurement in the laboratory

Because stabilization of arsenite in water samples during transit and storage is troublesome, this work deals with a method to prevent this by on-site selective coprecipitation of arsenite with dibenzyldithiocarbamate and recovery of the coprecipitate by filtration through a 0.45-microm membrane filter. In the laboratory arsenic on the filter is quantitatively released by oxidation of arsenite to arsenate with H2O2 (6%) in alkaline medium (8 mmol L(-1) NaOH) at elevated temperature (85 degrees C) for 30 min followed by ultratrace determination by routine HGAFS and ICP-MS. It is shown that arsenate contamination of the coprecipitate is so low that arsenate concentrations three orders of magnitude higher than the arsenite concentration do not interfere; this is essential, because arsenate is usually the dominant arsenic species in water. Because significant preconcentration can be achieved in the solution obtained from the leached filter (normally a factor 20 but easily increased to 100) very low detection limits can be obtained (only limited by the purity of the materials and the cleanliness of working); a realistic limit of determination is 0.01 microg L(-1) arsenite. The procedure was used for the determination of arsenite in two ground waters from an ash depository site in the Salek valley (Slovenia). The matrix contained some elements at very high levels but this did not impair the efficiency of arsenite coprecipitation. The results obtained by use of HGAFS and ICP-MS were not significantly different at the 5% level for sub-microg L(-1) arsenite concentrations.     

Determination of ultratrace dissolved arsenite in water – selective coprecipitation in the field combined with HGAFS and ICP–MS measurement in the laboratory

Because stabilization of arsenite in water samples during transit and storage is troublesome, this work deals with a method to prevent this by on-site selective coprecipitation of arsenite with dibenzyldithiocarbamate and recovery of the coprecipitate by filtration through a 0.45-μm membrane filter. In the laboratory arsenic on the filter is quantitatively released by oxidation of arsenite to arsenate with H₂O₂ (6%) in alkaline medium (8 mmol L^–1 NaOH) at elevated temperature (85?°C) for 30 min followed by ultratrace determination by routine HGAFS and ICP–MS. It is shown that arsenate contamination of the coprecipitate is so low that arsenate concentrations three orders of magnitude higher than the arsenite concentration do not interfere; this is essential, because arsenate is usually the dominant arsenic species in water. Because significant preconcentration can be achieved in the solution obtained from the leached filter (normally a factor 20 but easily increased to 100) very low detection limits can be obtained (only limited by the purity of the materials and the cleanliness of working); a realistic limit of determination is 0.01 μg L^–1 arsenite. The procedure was used for the determination of arsenite in two ground waters from an ash depository site in the ?alek valley (Slovenia). The matrix contained some elements at very high levels but this did not impair the efficiency of arsenite coprecipitation. The results obtained by use of HGAFS and ICP–MS were not significantly different at the 5% level for sub-μg L^–1 arsenite concentrations.     

Preservation of As(III) and As(V) in some water samples

This paper reports on the behavior of arsenite [As(III)] and arsenate [As(V)] in some water samples at storage under several conditions (pH=2/natural pH, 4°C/20°C). The investigation was carried out using⁷³As as a radiotracer for both forms and with the aid of earlier developed simple speciation methods for differentiation between arsenite and arsenate. Although arsenate is the thermodynamically stable arsenic form, it was observed that arsenate in deionized water is completely converted to the trivalent state; this phenomenon took place in about one week. By monitoring the radioactive As(III) and As(V) over a period of one month in two natural water samples, a fresh water and a sea water sample, it could be concluded that no adsorption occurs on the surface of polyethylene containers, independent of storage conditions. During that period, storage at natural pH values results for both water samples in a gradual oxidation of As(III); the oxidation rate is higher for storage at 20°C. At pH=2 As(III) is fairly stable in fresh water at both storage temperatures. However, in sea water a fast oxidation of As(III) is observed (complete oxidation within 3 d at both temperatures). As(V) is stable at all storage conditions studied.     

The determination of arsenic by electrothermal atomic absorption spectrometry with a graphite furnace: Part 2. Determination of arsenic(III) and arsenic(V) after extraction

A procedure is described for the sequential determination of arsenite and arsenate in samples of natural waters. It is based on the extraction of arsenic(III) with ammonium sec-butyl dithiophosphate and measurement, after re-extraction into water, by graphitefurnace atomic absorption spectrometry. Reduction of arsenic(V) allows its subsequent determination. The method is applied to fresh and sea water samples. The detection limit is 6 ngl^-1.     

Competitive sorption of carbonate and arsenic to hematite: combined ATR-FTIR and batch experiments

The competitive sorption of carbonate and arsenic to hematite was investigated in closed-system batch experiments. The experimental conditions covered a pH range of 3-7, arsenate concentrations of 3-300 μM, and arsenite concentrations of 3-200 μM. Dissolved carbonate concentrations were varied by fixing the CO(2) partial pressure at 0.39 (atmospheric), 10, or 100 hPa. Sorption data were modeled with a one-site three plane model considering carbonate and arsenate surface complexes derived from ATR-FTIR spectroscopy analyses. Macroscopic sorption data revealed that in the pH range 3-7, carbonate was a weak competitor for both arsenite and arsenate. The competitive effect of carbonate increased with increasing CO(2) partial pressure and decreasing arsenic concentrations. For arsenate, sorption was reduced by carbonate only at slightly acidic to neutral pH values, whereas arsenite sorption was decreased across the entire pH range. ATR-FTIR spectra indicated the predominant formation of bidentate binuclear inner-sphere surface complexes for both sorbed arsenate and sorbed carbonate. Surface complexation modeling based on the dominant arsenate and carbonate surface complexes indicated by ATR-FTIR and assuming inner-sphere complexation of arsenite successfully described the macroscopic sorption data. Our results imply that in natural arsenic-contaminated systems where iron oxide minerals are important sorbents, dissolved carbonate may increase aqueous arsenite concentrations, but will affect dissolved arsenate concentrations only at neutral to alkaline pH and at very high CO(2) partial pressures.     

Arsenic speciation in xylem sap of cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.)

Flow injection analysis (FIA) and high-performance liquid chromatography double-focusing sector field inductively coupled plasma mass spectrometry (HPLC-DF-ICP-MS) were used for total arsenic determination and arsenic speciation of xylem sap of cucumber plants (Cucumis sativus L.) grown in hydroponics containing 2 μmol dm⁻³ arsenate or arsenite, respectively. Arsenite [As(III)], arsenate [As(V)] and dimethylarsinic acid (DMA) were identified in the sap of the plants. Arsenite was the predominant arsenic species in the xylem saps regardless of the type of arsenic treatment, and the following concentration order was determined: As(III) > As(V) > DMA. The amount of total As, calculated taking into consideration the mass of xylem sap collected, was almost equal for both treatments. Arsenite was taken up more easily by cucumber than arsenate. Partial oxidation of arsenite to arsenate (<10% in 48 h) was observed in the case of arsenite-containing nutrient solutions, which may explain the detection of arsenate in the saps of plants treated with arsenite.     

High-performance liquid chromatography with hydride generation/atomic absorption spectrometry for the determination of arsenic species with application to some water samples

High-performance liquid chromatography (h.p.l.c.) is used for separation of arsenite, arsenate, monomethylarsinate (MMA) and dimethylarsonate (DMA) followed by continuous sodium tetrahydroborate reduction and atomic absorption spectrometric detection. Sample preconcentration, offering improved detection limits for the individual species and the removal of matrix interferences, is achieved with a pellicular anion-exchange column. The arsenic species are then separated on a strong anion-exchange column placed in series with the preconcentration column. Detection limits of 2 ng (as arsenic) for arsenite, arsenate and MMA, and 1 ng for DMA. Results for arsenic species in soil waters and commercial bottle waters are given.     

Mechanism of removal of arsenic by bead cellulose loaded with iron oxyhydroxide (beta-FeOOH): EXAFS study

Bead cellulose loaded with iron oxyhydroxide (BCF) with 47 mass% Fe content was prepared and was successfully applied to the elimination of arsenic from aqueous solutions. A clearer understanding of the arsenic removal mechanism will provide accurate prediction of the arsenic adsorptive properties of the new adsorbent. To study the mechanism of the adsorption process, we measured the extended X-ray absorption fine structure (EXAFS) spectra of arsenite and arsenate sorbed onto the adsorbent with different surface coverages. Both arsenite and arsenate were strongly and specifically adsorbed by akaganéite adsorptive centers on BCF by an inner-sphere mechanism. There was no change in oxidation state following interaction between the arsenic species and the BCF surface. The dominant complex of arsenic species adsorbed on akaganéite was bidentate binuclear corner-sharing ((2)C) between As(V) tetrahedra (or As(III) pyramids) and adjacent edge-sharing FeO(6) octahedra. On the basis of the results from EXAFS spectra, the adsorptive characteristics of arsenic, such as the effects of pH and competing anions, were satisfactorily interpreted.     

Arsenic sorption and redox transformation on iron‐impregnated ordered mesoporous carbon

Ordered mesoporous carbon has been actively investigated for its potential applications as catalyst supports, electrochemical materials and gas separation media. In this study, we tested an iron‐modified ordered mesoporous carbon (FeOMC) for its ability to adsorb arsenic from the aqueous phase. The FeOMC synthesis involved the preparation of an ordered silica template SBA‐15, in situ polymerization of acrylic acid in the template, carbonization and template removal to obtain the ordered mesoporous carbon, and iron impregnation. Batch experiments showed that the pH level of the solution had a major impact on arsenic sorption. Further, we found that the presence of anions (i.e. PO₄^3? and SiO₃^2?) could significantly decrease the sorption of both arsenate and arsenite. Arsenite oxidation to arsenate was observed in alkaline solutions, with or without anions being present. The oxidation of arsenite was attributed to both direct and catalytic reactions with the surface functional groups on the ordered mesoporous carbon. Adsorption of arsenic on FeOMC could be well explained by the surface complexation model. Copyright © 2007 John Wiley & Sons, Ltd.     

Arsenic speciation by capillary gas-liquid chromatography

Specific environmentally significant arsenic compounds are determined by capillary gas-liquid chromatography. Inorganic (arsenite, arsenate) and organic (monomethylarsonate, dimethylarsinate) arsenicals are measured as the corresponding methylthioglycolate derivatives, which are simultaneously separated on wide-bore borosilicate glass and fused-silica columns under conditions of temperature programming. Inorganic arsenate and arsenite cannot be differentiated by the derivatization technique. Flame-ionization and electron-capture detection are evaluated. A simple and rapid sample preparation procedure is used for water, urine, blood, and tissue.     

Determination of arsenite, arsenate and monomethylarsonic acid in aqueous samples by gas chromatography of their 2,3-dimercaptopropanol (bal) complexes

Procedures are described for the determination of arsenite, arsenate and monomethylarsonic acid in aqueous samples. The arsenicals (after reduction of arsenic to the tervalent state) readily react with 2,3-dimercaptopropanol (BAL) to yield their BAL complexes. The products are extracted with benzene and introduced into a gas Chromatograph equipped with a flame-photometric detector for sulphur. One aliquot of sample is treated with stannous chloride solution and potassium iodide solution to reduce arsenate and monomethylarsonic acid, then BAL is added and the complexes are extracted with benzene. The extract is analysed for total inorganic As plus monomethylarsonic acid. Magnesia mixture and phosphate solution are added to another aliquot to remove arsenate by co-precipitation with magnesium ammonium phosphate. The precipitate is filtered off and arsenite determined in the filtrate. The detection limits are 0.02 ng of As for arsenate and arsenite and 0.04 ng of As for monomethylarsonic acid.     

Ion-chromatographic screening method for monitoring arsenate and other anionic pollutants in ground waters of Northern Italy

A novel, rapid ion-chromatographic method for screening anionic pollutants in ground water, based on both conductivity and postcolumn spectrophotometric detection, has been developed. A relatively rapid separation of more than ten inorganic and polarizable anions was achieved by coupling an high capacity, hydroxide selective anion-exchange columns (Dionex IonPac AS16) supplied with an electrolytic eluent generator operating in gradient mode. The good control of the selectivity allowed the determination of polarizable anions including arsenate, thiocyanate, thiosulfate and perchlorate without interference from major components present at levels greater than 100 mg l⁻¹. This method was applied to the determination of arsenate in ground water samples collected in industrial and agricultural zones of Lombardia (Northern Italy). No traces of arsenate were detected in any sample, but added arsenate cannot be revealed by chromatographic analyses. This fact can be attributed to different causes, from reduction to the more reduced arsenic form to precipitation or dissolution in organic or inorganic based colloids. Oxidation with hydrogen peroxide seems to be useful for a partial recovery of added arsenate, but a stronger oxidation method, compatible with chromatographic separation, must be studied.     

Speciation of arsenic in water samples by high-performance liquid chromatography-hydride generation-atomic absorption spectrometry at trace levels using a post-column reaction system

Anion-exchange HPLC has been combined with hydride generation - atomic absorption spectrometry (HG-AAS) for the routine speciation of arsenite, arsenate, monomethylarsenic acid and dimethylarsinic acid. The sensitivity of the AAS-detection was increased by a post-column reaction system to achieve complete formation of volatile arsines from the methylated species and arsenate. The system allows the quantitative determination of 0.5 microg/l of each arsenic compound in water samples. The stability of synthetical and natural water containing arsenic at trace levels was investigated. To preserve stored water samples, a method for quantitative separation of arsenate at high pH-values with the basic anion-exchange resin Dowex 1x8 was developed.     

Arsenic speciation in soil-pore waters from mineralized and unmineralized areas of south-west England

Arsenale, arsenite and monomethylarsonic acid (MMAA) have been characterized in soil-pore waters extracted from soils in mineralized and unmineralized areas. Special attention has been paid to collection and storage of the samples. The dominant arsenic species in aerobic soils was arsenate, with small quantities of arsenite and MMAA in mineralized areas. In anaerobic soils arsenite was found to be the major soluble species. The analysis was done with an HPLC anion-exchange column combined with continuous-flow hydride-generation and atomic-absorption spectrometry. A preconcentration column was incorporated to increase the sensitivity.     

Arsenite oxidation and arsenate determination by the molybdene blue method

Based on the similarity in properties of arsenate and phosphate, the colorimetric method using the molybdene blue complex was tested in order to determine low As(V) concentration in waters. The influence of complex formation time, daylight, temperature and competitive anions (silicate and sulphate) upon complex formation was determined. Optimal complex formation was reached in 1 h at 20±1 °C and was slightly favoured when developed in daylight. The formation rate declined with decreasing reaction temperature and no influence of any of the competitive anions tested (at concentrations usually found in natural waters of granitic areas) was noted. The detection limit of this method was 20 μg As(V) l⁻¹. This simple, fast and sensitive arsenic determination method is suitable for field analysis, especially for waters containing low levels of phosphate and organic matter. Through arsenate determination, this colorimetric method allowed the arsenite oxidation efficiency of five common industrial oxidants to be compared. H₂O₂ and MnO_2(s) were not considered as effective oxidants as a high excess was necessary to ensure As(III) oxidation. NaOCl and KMnO₄ were promising oxidants as they allowed complete arsenite oxidation with a small excess for NaOCl or even less than the electron stoichiometric ratio in the case of KMnO₄. FeCl₃ was the most effective oxidant among the reagents tested here.     

Continuous Hydride Generation with Direct Current Plasma Emission Spectroscopic Detection for Total Arsenic Determinations (HY-DCP)

Abstract

Total arsenic determinations in complex food matrices can be performed with a high degree of accuracy and precision using an initial hydride formation step followed by direct current plasma (DCP) emission spectroscopic detection¹. The hyphenated technique, HY-DCP, uses a continuous flow hydride formation step with dual mixing of the hydride forming reagents, followed by on-line, continuous introduction of the aqueous arsenic sample. The final aqueous solution of arsine, excess sodium borohydride, and sample components, is directly introduced into the conventional spray chamber of the DCP instrument. Calibration plots for both arsenate and arsenite have been determined, together with linearities and minimum detection limits (MDLs) The overall methods for total arsenic determination have been applied to spiked water and tunafish samples. Accuracy and precision determinations have been performed for these total arsenic analyses, and compared with continuous hydride formation-flame atomic absorption (FAA) detection, as well as sequential hydride formation-FAA methods. All of these results are then compared, with the individual advantages and disadvantages o f each approach summarized.     

Mechanistic modeling of arsenic retention on natural red earth in simulated environmental systems

Arsenic retention on natural red earth (hereafter NRE) was examined as a function of pH, ionic strength, and initial arsenic loading using both macroscopic and spectroscopic methods. Proton binding sites on NRE were characterized by potentiometric titrations yielding an average pH(zpc) around 8.5. Both As(III)- and As(V)-NRE surface configurations were postulated by vibration spectroscopy. Spectroscopically, it is shown that arsenite forms monodentate complexes whereas arsenate forms bidendate complexes with NRE. When 4相似文献   

Quantitative trace-level speciation of arsenite and arsenate in drinking water by ion chromatography

We describe an improved method for the determination of inorganic arsenic in drinking water. The method is based on comprehensive optimization of the anion-exchange ion chromatographic (IC) separation of arsenite and arsenate with post-column generation and detection of the arsenate-molybdate heteropoly acid (AMHPA) complex ion. The arsenite capacity factor was improved from 0.081 to 0.13 by using a mobile phase (2.0 mL min(-1)) composed of 2.5 mM Na2CO3 and 0.91 mM NaHCO3 (pH 10.5). A post-column photo-oxidation reactor (2.5 m x 0.7 mm) was optimized (0.37 microM potassium persulfate at 0.50 mL min(-1)) such that arsenite was converted to arsenate with 99.8 +/- 4.2% efficiency. Multi-variate optimization of the complexation reaction conditions yielded the following levels: 1.3 mM ammonium molybdate, 7.7 mM ascorbic acid, 0.48 M nitric acid, 0.17 mM potassium antimony tartrate, and 1.0% (v/v) glycerol. A long-path length flow cell (Teflon AF, 100-cm) was used to measure the absorption of the AMHPA complex (818 +/- 2 nm). Figures of merit for arsenite/arsenate include: limit of detection (1.6/0.40 microg L(-1)): standard error in absorbance (5.1 x 10(-3)/3.5 x 10(-3)); and sensitivity (2.9 x 10(-3)/2.2 x 10(-3) absorbance units per ppb). Successful application of the method to fortified surface and ground waters (100 microL samples) is also described.     

Separation of organic and inorganic arsenic species by capillary zone electrophoresis

Summary Capillary zone electrophoresis has been used to separate arsenite, arsenate, dimethylarsinic acid, and phenyl-,p-aminophenyl-, ando-aminophenylarsinic acids. Identification and quantification of the arsenic species at mg L⁻¹ levels was possible by use of direct UV detection at 200 nm. The relative standard deviation (n=7) ranged from 0.97 to 1.52% for migration times and from 2.08 to 4.31% for peak areas. A method for rapid separation of inorganic arsenic species was also developed; by use of this method arsenite and arsenate could be separated within 2 min. Presented at Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

Hydride generation-flame atomic-absorption spectrometry as an arsenic detector for high-performance liquid chromatography

Hydride generation-flame atomic-absorption spectrometry (HG-FAAS) was used as a continuous detection system for arsenic in the eluate from high-performance liquid chromatography (HPLC). Four arsenic species (arsenite, arsenate, monomethylarsonate and dimethylarsinate) were detected separately with the HPLC-HG-FAAS system equipped with an anion-exchange column. When hijiki (Hizikia fusiforme) extract was examined, arsenate was found predominantly and arsenite and dimethylarsinate were also detected. Liver supernatant fraction obtained from mice administered orally with arsenite was also studied with the HPLC-HG-FAAS system equipped with a gel permeation column. In addition to free or low-molecular-weight ligand-bound arsenic, high-molecular-weight protein-bound arsenic fractions were also detected.