Speciation of arsenic in a contaminated soil by solvent extraction

Soil collected from a disused cattle dip in northern New South Wales was studied with the aim of developing an inexpensive, yet effective method for quantitative determination of arsenic(III), arsenic(V) and total organic arsenic in a contaminated soil. Hydrochloric acid extractions were used as a method for removal of the arsenic from the soil in a form suitable for speciation. It was found that the extraction efficiency varied with the ratio of soil to acid, and the concentration of the acid. Arsenic(III), as arsenic trichloride, was selectively extracted into chloroform from a solution highly concentrated in hydrochloric acid. This was followed by back-extraction of the arsenic into water. Total inorganic arsenic was determined in a similar manner after the reduction of arsenic(V) to the trivalent state with potassium iodide. Arsenic(V) was determined by the difference between the results for arsenic(III) and total inorganic arsenic. All analyses for the various arsenic species were performed by hydride generation-atomic absorption spectroscopy; concentrations of total arsenic in the soil were confirmed using X-ray fluorescence spectrometry. It was found that all the arsenic in the soil was present as inorganic arsenic in the pentavalent state. This reflects the ability of arsenic to interchange between species, since the original species in cattle dipping solution is arsenic(III).     

Determination of trace arsenic in hydrofluoric acid by hydride generation and atomic absorption spectrometry

Practical procedures are given for determination of arsenic(III) and (V) in hydrofluoric acid by means of hydride generation and atomic absorption spectrometry. Arsenic(III) can be determined by direct generation of arsine with sodium borohydride in hydrochloric/hydrofluoric acid medium, arsenic(V) being only slightly reduced under the conditions used. For its determination, arsenic(V) has to be prereduced with potassium iodide, and even then its reduction to arsenic(III) and then arsine is far from complete. It is possible to determine it in presence of arsenic(III) by a difference method, but this is recommended only if the As(V)/As(III) ratio is greater than 1. Total arsenic can be determined after oxidation of As(III) and evaporation of most of the hydrofluoric acid. The limit of determination is 5 g/l for arsenic(III) and 0.25 g/l for total arsenic; the relative standard deviation is about 10%.     

Determination of As(III) and total inorganic arsenic by on-line pretreatment in hydride generation atomic absorption spectrometry

Summary A method for the on-line prereduction of As(V) was developed in order to determine As(III) and As(V) with the same sensitivity by continuous flow hydride generation. In this procedure, the sample is continuously mixed with concentrated hydrochloric acid and a potassium iodideascorbic acid solution, flows through a heated PTFE-tube and is determined by hydride generation atomic absorption spectrometry in a heated quartz cell. The selective analysis of As(III) is carried out by continuous mixing of the sample with acetic acid and hydride generation. The method allows the rapid determination of inorganic arsenic species at concentrations down to 1 g/l. A manual sample preparation is not required.     

A FIA-system for As(III)/As(V)-determination with electrochemical hydride generation and AAS-detection

A flow-injection system with electrochemical hydride generation and atomic absorption detection for As(III)/As(V) determination is described. A simple electrolytic flow-through cell has been developed and optimized. Several cathode materials like Pt, Ag, Cu, C and Pb have been tested. The influence of the electrolysis current, concentration of sulfuric acid, carrier stream, flow rate, sample volume and interferences by other metals on the arsenichydride generation have been studied. For the determination of total inorganic arsenic, As(V) is reduced to As(III) on-line by postassium iodide or L-cysteine at 95 degrees C. The influence of the temperature and the reduction medium on this pre-reduction step has been tested. The calibration curve is linear in the range of 5 to 50 microg/L for As(III) and total inorganic arsenic and shows a higher sensitivity than in case of reduction with sodium tetrahydroborate. The detection limit is 0.4 microg/L for As(III) and 0.5 microg/L for total inorganic arsenic at a sample volume of 1 mL.     

Speciation of As(III), As(V), MMA and DMA in contaminated soil extracts by HPLC-ICP/MS

A method to separate and quantify two inorganic arsenic species As(III) and As(V) and two organic arsenic species, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), by HPLC-ICP/MS has been developed. The separation of arsenic species was achieved on the anionic exchange column IonPac AS11 (Dionex) with NaOH as mobile phase. The technique was successfully applied to analyze extracts of two contaminated soils, sampled at a former tannery site (soil 1) and a former paint production site (soil 2). The soils were extracted at pH values similar to the natural environment. Extractions were performed at different pH values with 0.3 M ammonium oxalate (pH = 3), milli-Q water (pH = 5.8), 0.3 M sodium carbonate (pH = 8) and 0.3 M sodium bicarbonate (pH = 11). No organically bound arsenic was found in the extracts. As(V) was the major component. Only up to 0.04% of the total arsenic contained in soil 1 were mobilized. The highest amount of extracted arsenic was found at the highest pH. In the milli-Q water extract of soil 1 As(III) and As(V) were found. High amounts of As(V) were found in the extracts of soil 2. Up to 20% of the total arsenic bound to soil 2 constituents were released. The results show that the mobilization of arsenic depended on the pH value of the extraction solution and the kind of extracted soil. Dramatic consequences have to be expected for pH changes in the environment especially in cases where soils contain high amounts of mobile arsenic.     

On‐Line Determination of Arsenic Species in Sea Water by Selective Hydride Generation Coupled with Inductively Coupled Plasma — Mass Spectrometry

A method for direct de termination of total in organic arsenic (III+V), arsenic (III) and dimethylarsinate (DMA) in sea water was developed by combining continuous‐flow selective hydride generation and inductively coupled plasma mass spectrometry (ICP‐MS) is presented. The principle underlying selective hydride generation is based on proper control of the reaction conditions for achieving separation of the respective arsenic species. The effects of pH and composition of reaction media on mutual interference between the arsenic species were investigated in detail. The results indicate that the appropriate media for the selective determination of total in organic arsenic, DMA and As(III) are 6 M HNO₃, acetate buffer at pH = 4.63 and citrate buffer at pH = 6.54, respectively. The concentrations of total inorganic arsenic species, As(III+V), and As(III) were respectively deter mined and that of As(V) was obtained by the difference between them. As to the concentration of DMA, it was obtained after correction from the interference caused by As(III) and As(V). By following the established procedure, the detection lim its (as based on 3‐sigma criterion) for As(III+V), As(III) and DMA were 0.050, 0.009, and 0.002 ng/mL, respectively. There liability of the pro posed method was evaluated in terms of precision and spike addition. The results indicated that the precision of better than 3% and spike recovery of 95 to 105% for all the arsenic species tested in the natural sea water samples can be obtained.     

Extraction and speciation of arsenic in plants grown on arsenic contaminated soils

A sequential arsenic extraction method was developed that yielded extraction efficiencies (EE) that were approximately double those using current methods for terrestrial plants. The method was applied to plants from two arsenic contaminated sites and showed potential for risk assessment studies. In the method, plants were extracted first by 1:1 water-methanol followed by 0.1 M hydrochloric (HCl) acid. Total arsenic in plant and soil samples collected from contaminated sites was mineralized by acid digestion and detected by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and hydride generation-atomic absorption spectrometry (HG-AAS). Arsenic speciation was done by high performance liquid chromatography coupled with HG-AAS (HPLC-HGAAS) and by HPLC coupled with ICP-mass spectrometry (HPLC-ICP-MS). Spike recovery experiments with arsenite (As(III)), arsenate (As(V)), methylarsonic acid (MA) and dimethylarsinic acid (DMA) showed stability of the species in the extraction processes. Speciation analysis by X-ray absorption near edge spectroscopy (XANES) demonstrated that no transformation of As(III) and As(V) occurred due to sample handling. Dilute HCl was efficient in extracting arsenic from plants; however, extraction and determination of organic species were difficult in this medium. Sequential extraction with 1:1 water-methanol followed by 0.1 M-HCl was most useful in extracting and speciating both organic and inorganic arsenic from plants. Trace amounts of MA and DMA in plants could be detected by HPLC-HGAAS aided by the process of separation and preconcentration of the sequential extraction method. Both organic and inorganic arsenic compounds could be detected simultaneously in synthetic gastric fluid extracts (GFE) but EEs by this method were lower than those of the sequential method. The developed sequential method was shown to be reliable and applicable to various terrestrial plants for arsenic extraction and speciation.     

Speciation of As(III), As(V), MMA and DMA in contaminated soil extracts by HPLC-ICP/MS

A method to separate and quantify two inorganic arsenic species As(III) and As(V) and two organic arsenic species, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), by HPLC-ICP/MS has been developed. The separation of arsenic species was achieved on the anionic exchange column IonPac^®AS11 (Dionex) with NaOH as mobile phase. The technique was successfully applied to analyze extracts of two contaminated soils, sampled at a former tannery site (soil 1) and a former paint production site (soil 2). The soils were extracted at pH values similar to the natural environment. Extractions were performed at different pH values with 0.3 M ammonium oxalate (pH = 3), milli-Q water (pH = 5.8), 0.3 M sodium carbonate (pH = 8) and 0.3 M sodium bicarbonate (pH = 11). No organically bound arsenic was found in the extracts. As(V) was the major component. Only up to 0.04% of the total arsenic contained in soil 1 were mobilized. The highest amount of extracted arsenic was found at the highest pH. In the milli-Q water extract of soil 1 As(III) and As(V) were found. High amounts of As(V) were found in the extracts of soil 2. Up to 20% of the total arsenic bound to soil 2 constituents were released. The results show that the mobilization of arsenic depended on the pH value of the extraction solution and the kind of extracted soil. Dramatic consequences have to be expected for pH changes in the environment especially in cases where soils contain high amounts of mobile arsenic.     

Indirect atomic-absorption determination of ppM levels of arsenic by combustion of an MIBK extract of arsenomolybdic acid

An indirect atomic-absorption method for arsenic has been developed. Arsenic(III) is oxidized to arsenic(V) by iodine, then arsenomolybdic acid is formed and extracted into MIBK from 0.2-1.6M hydrochloric acid. Excess of molybdate is scrubbed from the organic phase, and then the molybdenum in the heteropoly acid is determined by its atomic absorption at 313.3 nm. Silicate and phosphate interfere. A procedure is described for determination of ppM levels of arsenic in water.     

Separation of arsenic(III) and arsenic(V) in ground waters by ion-exchange

The predominant species of arsenic in ground water are probably arsenite and arsenate. These can be separated with a strong anion-exchange resin (Dowex 1 x 8; 100-200 mesh, acetate form) in a 10 cm x 7 mm column. Samples are filtered and acidified with concentrated hydrochloric acid (1 ml per 100 ml of sample) at the sample site. Five ml of the acidified sample are used for the separation. At this acidity, As(III) passes through the acetate-form resin, and As(V) is retained. As(V) is eluted by passage of 0.12M hydrochloric acid through the column (resulting in conversion of the resin back into the chloride form). Samples are collected in 5-ml portions up to a total of 20 ml. The arsenic concentration in each portion is determined by graphite-furnace atomic-absorption spectrophotometry. The first two fractions give the As(III) concentration and the last two the As(V) concentration. The detection limit for the concentration of each species is 1 mug l .     

Flow potentiometric and constant-current stripping analysis for arsenic(V) without prior chemical reduction to arsenic(III): Application to the Determination of Total Arsenic in Seawater and Urine

Arsenic(V) is reduced to elemental arsenic on a gold-coated platinum-fibre electrode at electrolysis potentials below ?1.60 V vs. Ag/AgCl and subsequently re-oxidized, either by means of a constant current, or chemically, with gold(III) as oxidant. Total arsenic in acidified seawater can be determined by means of electrolysis for 60 s at ?1.80 V vs. Ag/AgCl and subsequent stripping in 4 M hydrochloric acid containing 2.5 M calcium chloride. The detection limit obtained after 60 s of electrolysis (ca. 0.1 μg1^?1) is about ten times lower than that obtained by the electrochemical stripping methods for arsenic(III) reported hitherto. Total arsenic in urine is determined after digestion with nitric acid and hydrogen peroxide.     

Optimization of the solubilization, extraction and determination of inorganic arsenic [As(III) + (As(V)] in seafood products by acid digestion, solvent extraction and hydride generation atomic absorption spectrometry

A method for the selective quantitative determination of inorganic arsenic [As(III) + As(V)] in seafood was developed. In order to do so, various procedures for the solubilization and extraction of inorganic arsenic quoted in the literature were tested. None provided satisfactory recoveries for As(III) and As(V) in real samples. Consequently, a methodology was developed which included solubilization with HCl and subsequent extraction with chloroform. The arsenic was solubilized in 9 mol l-1 hydrochloric acid. After reduction by hydrobromic acid and hydrazine sulfate, the inorganic arsenic was extracted into chloroform, back-extracted into 1 mol l-1 HCl, dry-ashed, and quantified by hydride generation-atomic absorption spectrometry (HG-AAS). The analytical features of the method are as follows: detection limit, 3.07 ng g-1 As (fresh mass); precision (RSD), 4.0%; recovery, As(III) 99%, As(V) 96%. In the optimized conditions, other arsenic species--dimethylarsinic acid (DMA), arsenobetaine (AB), arsenocholine (AC) and tetramethylarsonium-ion (TMA+)--were not co-extracted. However, different percentages of minor species were extracted with chloroform: monomethylarsonic acid (MMA) 100%, and trimethylarsine oxide (TMAO) 3-10%. Real samples and reference materials of seafood (DORM-1, DORM-2, TORT-2, CRM-278 and SRM-1566a) were analyzed. The analysis of DORM-1 provided an inorganic arsenic value of 124 +/- 4 ng g-1 As, dry mass (dm), which is very close to the value obtained by other authors using high performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) and ionic chromatography-hydride generation-atomic absorption spectrometry (IC-HG-AAS).     

Determination of inorganic arsenic(III) in ground water using hydride generation coupled to ICP-AES (HG-ICP-AES) under variable sodium boron hydride (NaBH4) concentrations

The determination of inorganic arsenic species in ground water matrices using hydride generation coupled online to ICP-AES (HG-ICP-AES) is suggested on the fact that the As(III)-species shows significantly higher signal intensities at low sodium boron hydride (NaBH₄) concentrations than the As(V)-species. The sodium boron hydride concentration used for the determination of As(III) without any considerable interferences of As(V) was at 13.2 mmol/L NaBH₄ (0.05 wt/v%), whereas the concentration for the total As determination was at 158.4 mmol/L NaBH₄ (0.6 wt/v%). The interferences of As(V) during the As(III) measurements were very small: at concentrations below 100 μg/L of total arsenic, the interferences of As(V) were smaller than 2%. An amount of As(III) higher than 10% of the total As amount could be determined exactly and reliably. The total amount of arsenic is measured after reducing the sample with 20 mmol/L L-cysteine (C₃H₇NO₂S). Finally, the amount of the As(V)-species is calculated by the difference between the As(III)-species and the total arsenic. Therefore, this analytical method requires the absence of organic arsenic species, but if they still appear, they could be frozen out with liquid nitrogen after the hydride generation system. The linearity of calibration reaches from 2 μg/L up to 1000 μg/L with a detection limit routinely of about 1 μg/L for each species. The advantages of this method in comparison to AAS measurements are the higher extent of the linear calibration range (3 orders of magnitude) and a higher sensitivity. Additional merits of the method developed are easy handling and high sampling rates.     

A FIA-system for As(III)/As(V)-determination with electrochemical hydride generation and AAS-detection

A flow-injection system with electrochemical hydride generation and atomic absorption detection for As(III)/As(V) determination is described. A simple electrolytic flow-through cell has been developed and optimized. Several cathode materials like Pt, Ag, Cu, C and Pb have been tested. The influence of the electrolysis current, concentration of sulfuric acid, carrier stream, flow rate, sample volume and interferences by other metals on the arsenichydride generation have been studied. For the determination of total inorganic arsenic, As(V) is reduced to As(III) on-line by postassium iodide or L-cysteine at 95° C. The influence of the temperature and the reduction medium on this pre-reduction step has been tested. The calibration curve is linear in the range of 5 to 50 g/L for As(III) and total inorganic arsenic and shows a higher sensitivity than in case of reduction with sodium tetrahydroborate. The detection limit is 0.4 g/L for As(III) and 0.5 g/L for total inorganic arsenic at a sample volume of 1 mL.     

Use of perchloric acid as a reaction medium for speciation of arsenic by hydride generation-atomic absorption spectrometry

Simple and inexpensive methods for the speciation of arsenite, arsenate, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in environmental water samples were developed. In these methods a hydride generation-atomic absorption spectrometry (HG-AAS) technique was employed and perchloric acid (as a reaction medium), L-cysteine (as a pre-reducing agent for a certain contact time between its addition and analysis) and sodium tetrahydroborate(III) (NaBH4, as a reducing agent) were used. The use of L-cysteine greatly enhances the absorption signals of all four arsenic species at low acid concentration (0.001-0.04 M). The methods developed for the determination of total arsenic and total inorganic arsenic and speciation of the four arsenic species in environmental water samples are as follows. (i) DMA: 0.005 M acid and 0.04% NaBH4 in the absence of L-cysteine. DMA can also be speciated in the presence of L-cysteine as follows: 2 M acid, 2.5% L-cysteine after a contact time of approximately 5 min and 0.6% NaBH4. (ii) As(III): 5 M acid and 0.08% NaBH4 in the absence of L-cysteine. (iii) Total inorganic arsenic (As(III) + As(V)]: 8 M acid and 0.6% NaBH4 in the absence of L-cysteine. (iv) Total arsenic: 0.01 M acid, 5% L-cysteine after a contact time of 5 min and 2% NaBH4. (v) MMA: 8 M acid, 3% L-cysteine after a contact time of 50 min and 0.6% NaBH4. (vi) As(V): by difference. Detection limits and recoveries of added spikes for all analyses were found to be 0.5-1.7 ppb and 90-112% respectively.     

Speciation of arsenic by hydride generation-atomic absorption spectrometry (HG-AAS) in hydrochloric acid reaction medium

The effects on the absorbance signals obtained using HG-AAS of variations in concentrations of the reaction medium (hydrochloric acid), the reducing agent [sodium tetrahydroborate(III); NaBH(4)], the pre-reducing agent (l-cysteine), and the contact time (between l-cysteine and arsenic-containing solutions) for the arsines generated from solutions of arsenite, arsenate, monomethylarsonic acid (MMA), and dimethylarsenic acid (DMA), have been investigated to find a method for analysis of the four arsenic species in environmental samples. Signals were found to be greatly enhanced in low acid concentration in both the absence (0.03-0.60 M HCl) and the presence of l-cysteine (0.001-0.03 M HCl), however with l-cysteine present, higher signals were obtained. Total arsenic content and speciation of DMA, As(III), MMA, and As(V) in mixtures containing the four arsenic species, as well as some environmental samples have been obtained using the following conditions: (i) total arsenic: 0.01 M acid, 2% NaBH(4), 5% l-cysteine, and contact time<10 min; (ii) DMA: 1.0 M acid, 0.3-0.6% NaBH(4), 4.0% l-cysteine, and contact time <5 min; (iii) As(III): 4-6 M acid and 0.05% NaBH(4) in the absence of l-cysteine; (iv) MMA: 4.0 M acid, 0.03% NaBH(4), 0.4% l-cysteine, and contact time of 30 min; (v) As(V): by difference. Detection limits (ppb) for analysis of total arsenic, DMA, As(III), and MMA were found to be 1.1 (n=7), 0.5 (n=5), 0.6 (n=7), and 1.8 (n=4), respectively. Good percentage recoveries (102-114%) of added spikes were obtained for all analyses.     

A novel method for the estimation of arsenic(V) in organic compounds

A novel method for the determination of arsenic(V) in organic compounds has been developed by reducing combined arsenic(V) to arsenic(III) in aqueous acetic acid medium with zinc dust. In some cases, addition of ethyl alcohol is necessary to dissolve the compound and to keep the arsenic(III) compound in solution. The arsenic(III) is titrated with iodine and the end-point is detected visually with starch as indicator or potentiometrically.     

Speciation analysis of inorganic arsenic by a multisyringe flow injection system with hydride generation-atomic fluorescence spectrometric detection

In this study, a new technique by hydride generation-atomic fluorescence spectrometry (HG-AFS) for determination and speciation of inorganic arsenic using multisyringe flow injection analysis (MSFIA) is reported. The hydride (arsine) was generated by injecting precise known volumes of sample, a reducing sodium tetrahydroborate solution (0.2%), hydrochloric acid (6 M) and a pre-reducing solution (potassium iodide 10% and ascorbic acid 0.2%) to the system using a multisyringe burette coupled with one multi-port selection valve. This solution is used to pre-reduce As(V) to As(III), when the task is to speciate As(III) and As(V). As(V) is determined by the difference between total inorganic arsenic and As(III). The reagents are dispensed into a gas-liquid separation cell. An argon flow delivers the arsine into the flame of an atomic fluorescence spectrometer. A hydrogen flow has been used to support the flame. Nitrogen has been employed as a drier gas (Fig. 1).Several variables such as sample and reagents volumes, flow rates and reagent concentrations were investigated in detail. A linear calibration graph was obtained for arsenic determination between 0.1 and 3 μg l⁻¹. The detection limit of the proposed technique (3σ_b/S) was 0.05 μg l⁻¹. The relative standard deviation (R.S.D.) of As at 1 μg l⁻¹ was 4.4 % (n = 15). A sample throughput of 10 samples per hour was achieved. This technique was validated by means of reference solid and water materials with good agreement with the certified values. Satisfactory results for speciation of As(III) and As(V) by means of the developed technique were obtained.     

Speciation of Arsenic by Potentiometric Stripping Analysis Using Gold(III) Solution as Chemical Reoxidant and a Wall‐Jet Flow Cell

A simple procedure is described for the potentiometric stripping of arsenic with a wall‐jet cell by means of potentiostatic co‐deposition of gold and arsenic at a glassy‐carbon electrode and subsequent chemical stripping with Au(III). Optimum medium containing 160 mg L^?1 of Au(III) in HCl 0.1 M, where it is possible to speciate As(III) and As(V). As(V) was electrodeposited directly without prior chemical reduction at working electrode. As(III) was first determined at an electrodeposition potential of ?0.1 V. Afterwards, total arsenic was determined by an electrodeposition potential of ?0.7 V, from the area of peak obtained of the differential stripping potentiogram by using the standard addition method. The original As(V) concentration in the sample was calculated by difference. The possibilities of the optimized method were demonstrated by determinations of As(III), As(V) and total arsenic in samples of polluted water.     

Differential determination of arsenic (III) and total arsenic with L-cysteine as prereductant using a flow injection non-dispersive atomic absorption device

Speciation of arsenic in environmental samples gains increasingly importance, as the toxic effects of arsenic are related to its oxidation state. A method was developed for the determination of trace amounts of arsenic (III) and total arsenic by flow injection hydride generation coupled with an in-house made non-dispersive AAS device. The total arsenic is determined after prereduction of arsenic (V) to arsenic (III) with L-cysteine in a low concentration of hydrochloric, acetic or nitric acid. The conditions for the prereduction, hydride generation and atomization were systematically investigated. A quartz tube temperature of 800 degrees C was found to be optimum in view of peak shape and baseline stability. Pb(II), Ni(II), Fe(III), Cu(II), Ag(I), Al(III), Ga(II), Se(IV), Bi(III) were checked for interfering with the 2 microg/L As(V) signal. A serious signal depression was only observed for Se(IV) and Bi(III) at a 150-fold excess. With the above system, arsenic was determined at a sampling frequency of about 1/min with a detection limit (3sigma) of 0.01 microg/L using a 0.5 mL sample. The reagent blank was 0.001+/-0.0003 absorbance units and the standard deviation of 10 measurements of the 2 microg/l As signal was found to be 1.2%. Results obtained for standard reference materials and water samples are in good agreement with the certified values and those obtained by ICP-MS