preservation of inorganic arsenic species in environmental water samples for reliable speciation...

Trends Trends in Analytical Chemistry, Vol. 29, No. 10, 2010

Preservation of inorganic arsenicspecies in environmental watersamples for reliable speciationanalysisA. Ramesh Kumar, P. Riyazuddin

Preservation of arsenic species in environmental water samples is important to produce representative species-distribution data.

However, published literature on preserving arsenic speciation is confusing and contradictory, predominantly because of the

reactions of the Fe(II)/Fe(III) redox system and the microbe-mediated arsenic-species transformations. Field filtration, refriger-

ation and storage in the dark are prerequisites for stabilization of As(III)/As(V). Filtration removes suspended matter and most

microbes, refrigeration suppresses most biotic and abiotic reactions, and storage in the dark avoids photochemical reactions of

Fe(III) and As(III). There are reagents that inhibit the oxidation of Fe(II), but the efficiency of preservation depends on the sample

matrix and its response to these methods of preservation.

ª 2010 Elsevier Ltd. All rights reserved.

Keywords: Acid-mine-drainage sample; Arsenic-field-speciation method; EDTA preservation; Environmental water; HCl preservation; H3PO4

preservation; Inorganic arsenic species; Preservation; Natural waters; Speciation analysis

Abbreviations: AAS, Atomic absorption spectrometry; AcH, Acetic acid; AFS, Atomic fluorescence spectrometry; AMD, Acid-mine drainage; BDL,

Below detectable limit; DMA, Dimethylarsinic acid; DO, Dissolved oxygen; EDTA, Ethylenediamine tetraacetic acid; Eh, Redox potential; HFO,

Hydrated ferric oxide; HPLC, High-performance liquid chromatography; HG, Hydride generation; IC, Ion chromatography; ICP-MS, Inductively

coupled plasma mass spectrometry; MMA, Monomethylarsonic acid; ND, Not detected; NOM, Natural organic matter; TAs, Total arsenic; WHO,

World Health Organization

A. Ramesh Kumar

Chemical Laboratory,

Central Ground Water Board,

E1;Rajaji Bhavan,

Besant Nagar,

Chennai 600 090,

India

P. Riyazuddin*

Department of Analytical

Chemistry,

University of Madras,

Guindy Campus,

Chennai 600 025,

India

*Corresponding author.

Tel.: +91 44 22351269;

Fax: +91 44 22353309;

E-mail: [email protected]

1212

1. Introduction

Groundwater contamination of arsenic hasbeen acknowledged as a major publichealth issue by the World Health Organi-zation (WHO) based on its prevalenceworldwide [1]. In natural waters, arsenicexists as arsenite [As(III)], arsenate [As(V)],monomethylarsonic acid (MMA) anddimethylarsinic acid (DMA), and as variousorganoarsenicals (Fig. 1). Among these,As(III) and As(V) are the predominantspecies in most natural waters. MMA andDMA occur at levels of a few lg/L in somegroundwaters and surface waters [2].However, in reducing environments and incontaminated sites, MMA and DMA occurat levels above those of inorganic species. Inseawater, besides inorganic and methyl-ated species, various organoarsenicals exist[3]. In geothermal waters, besides arsenic

0165-9936/$ - see front matter ª 2010 Elsev

oxyions, thioarsenic anions and theirmethylated species have been reported [4].

Among inorganic forms, As(III) is moretoxic than As(V), while the toxicities oforganoarsenicals are comparatively lower,so, for accurate risk assessment of arsenic inthe environment, it is necessary to deter-mine the various arsenic species ratherthan total arsenic concentrations. Thepentavalent forms (H3AsO4, H2AsO4

�,HAsO4

2� and AsO43�) are typically pre-

dominant in oxidizing environments,whereas the trivalent forms (H3AsO3,H2AsO3

�, HAsO32� and AsO3

3�) aremostly found in reducing environments. Inthe pH range of most natural waters (6.5–8.5), As(III) occurs as uncharged species,whereas As(V) occurs as oxyanionic species[2].

As most of the trace elements cannot bedetermined on site for various reasons, it is

ier Ltd. All rights reserved. doi:10.1016/j.trac.2010.07.009

mailto:[email protected]

http://dx.doi.org/10.1016/j.trac.2010.07.009

Figure 1. Arsenic species generally found in natural waters.

Trends in Analytical Chemistry, Vol. 29, No. 10, 2010 Trends

general practice to collect a representative sample and topreserve it until analysis in the laboratory. The preser-vation procedure aims to maintain the distributions ofnatural species and their concentrations. Preservation ofwater samples aims to retard redox, co-precipitation,hydrolysis, adsorption, biodegradation and photochemi-cal reactions that may occur during storage (Fig. 2). It istherefore important to consider the influence of thesefactors on the species stability to get both a representa-tive analytical speciation of the studied sample, and anaccurate assessment of the environmental problem. Aplethora of methods is available for arsenic-speciationanalysis [5], but the ability to preserve the native point ofsampling distribution of As(III) and As(V) until thesample can be analyzed in the laboratory is a somewhatmore challenging task. Several studies have beenreported on arsenic-species stability in water samples,but with contradictory conclusions concerning filtration,acidification, storage temperature and type of matrix [6].

In this article, we review these studies, and attempt toexplain the sources of instability in the light of aquaticchemistry reactions. Also, on the basis of publishedliterature and our experience, we offer suggestions forselecting appropriate preservation strategies to preserveAs(III) and As(V) species in different environmentalwater samples.

2. Arsenic-speciation changes in unacidifiedspiked solutions

Standard solutions of As(III) and As(V) prepared indeionized/distilled water should be stable for a reason-able period, because they are used for calibration pur-poses. Tallman and Shaikh found that lg/L levels ofinorganic arsenic solutions in distilled water, as well asin the presence of common redox agents (O2, Fe(III),H2S), were stable for at least 3 weeks without addingpreservatives [7]. This implies that redox reactions ofarsenic species in natural waters occur at rates suffi-

ciently slow to enable water samples to be collected andanalyzed without significant changes in species distri-bution. McClesky et al. reported that As(III)-spikeddouble-distilled water was stable for at least 16 days [6].

However, several studies report that lg/L levels ofarsenic species in de-ionized water undergo redoxtransformations if not preserved. For example, at roomtemperature, deionized water spiked with As(III) andAs(V) (0.5–20 lg/L) was reduced to As(III) within a fewdays [8]. Bender et al. observed that unpreserved solu-tions of As(III) and As(V) are stable for only 24 h [9].Feldman reported complete oxidation of 1–10-lg/L levelsof As(III) within 4 days [10].

An experience of Francesconi and co-workers wassurprising! Within 36 h at room temperature, As(III)standard solutions were completely oxidized, and As(V)standard solutions were completely reduced [3].

3. Stability of arsenic species in synthetic solutionsand natural waters with acid preservation

Acidification of water sample to pH <2 minimizes pre-cipitation, adsorption and microbial activity. Table 1summarizes representative studies carried out on spikeddeionized/natural waters with acid preservation. As canbe seen, acidification with HCl [6], HNO3 [8], H2SO4 [11]and H3PO4 [12] increases the stability of As(III)/As(V)species compared to unacidified samples.

Cherry et al. reported a 1% oxidation of As(III) after78 days in a sample adjusted to pH 2 with HCl [13].Also, the As(III) � As(V) transformation was suffi-ciently low even in the presence of other redox species(O2, Fe(III), H2S) commonly found in natural waters. Asa rare case, Polya et al. observed acidified As(III) stan-dard solutions to be stable for 3 years [14].

Comparison of the studies presented in Table 1 alsoreveals that the rate of arsenic-speciation changesduring storage, is highly variable and can be sufficientlyhigh to invalidate even the most precise laboratory

http://www.elsevier.com/locate/trac 1213

Analysis

Stage Changes/processes affecting stability of arsenic speciation

Loss of equilibrium reactions, change of pH, Eh, precipitation of FeOOH and coprecipitation of arsenic species

l reactions, coprecipitation, Adsorption of species on container wall, Microbialactivity, Photochemicareactions between analyte and matrix components

Filtration, acidification, species interconversions, analyte loss due to various manipulations

Incomplete recovery due to various analytical and sretemaraplatnemurtsni

Sampling

Sample preparation

Sample storage

Figure 2. Possible sources of species interconversion or loss during various stages of sampling and analysis.


analysis. For example, HCl-preserved groundwatersamples were stable for 90 days [15], but a 15-daystability period was also reported [8]. HCl with ascorbicacid preserved both the species for 28 days [16]. Oxida-tion of As(III) in spiked rainwater was observed within3 days, but acidification with 0.2% H2SO4 preservedarsenic species for 125 days at room temperature [16].Bender et al. reported that significant oxidation of As(III)occurred in HCl-preserved reagent water within 2 days[9]. The efficiency of these acids to preserve As(III)/As(V)varied considerably {i.e. HCl: 90 days; H2SO4: 125 days;HNO3: 15 days; H3PO4: 9 days [6,8,11,12]}.

The wide variation noticed in the stabilization period isdue to the nature of sample matrix, microbial activityand other preservation strategies. For example, syntheticsolutions prepared in double-distilled water are stable forlong periods, because distillation inactivates mostmicroorganisms [7,13]. However, the ion-exchangeresin used for deionization could contribute microor-ganisms capable of oxidizing As(III) [8,9]. Also, it isimportant to note that most arsenic-preservation pro-cedures used unrealistic background deionized waterwithout typical background matrix.

4. Processes changing arsenic speciation

The major causes of arsenic-speciation changes duringsampling and storage are:

1) precipitation of HFOs and adsorption of arsenic spe-cies;

2) photochemical reactions between Fe(III)/Fe(II) andAs(III)/As(V); and,

3) microbial activity.

1214 http://www.elsevier.com/locate/trac

4.1. As(III)/As(V) stability in the presence ofFe(II)/Fe(III)Iron is a common element present in arsenic-contami-nated waters. Mobilization of arsenic in groundwater in-volves desorption of arsenic from ferric oxide precipitatesof aquifer sediments and/or oxidation of arsenopyrites(FeAsS), so As and Fe occur together in most groundwa-ters [2]. Mine waters contain high concentrations of Feand other metals besides arsenic [23]. Depending on pHand Eh, Fe exists as Fe(II) or Fe(III). Once groundwater ispumped to surface, the dissolved Fe(II) reacts with atmo-spheric oxygen, resulting in the precipitation of Fe as HFOsand provides sorption sites for dissolved As species. SinceAs(V) exists as anionic species, it is co-precipitated pref-erentially [24]. Co-precipitation of arsenic also occursonto MnO2 by a similar mechanism. In addition, MnO2

could oxidize As(III) [25].Rapid oxidation of As(III) to As(V) in the presence of

Fe(III) was observed in synthetic solutions preservedwith HCl and light exposure [26]. At a solution pH of1.5, the hydrolysis of Fe(III) produces mainly FeOH2+. Inthe presence of chloride ions, a more photoactive species,FeCl2+, is generated. These species produce OH� and �Cl2

�

radicals on photolysis.

FeOH2þ þ hv! FeðIIÞ þ OH�

FeCl2þ þ hv! FeðIIÞ þ Cl�

Cl� þ Cl� ! �Cl2�

The OH� and �Cl2� radicals produced in the initiation

reactions react rapidly with As(III) to produce anintermediate As(IV), which then reacts with Fe(III) toproduce Fe(II) and As(V).


AsðIIIÞ þ OH� ! AsðIVÞ þ OH�

AsðIIIÞ þ Cl2�� ! AsðIVÞ þ 2Cl�

AsðIVÞ þ FeðIIIÞ ! AsðVÞ þ FeðIIÞ

In the presence of DO, the oxidation is further facili-tated as:

AsðIVÞ þ O2 þ Hþ ! AsðVÞ þ HO2�

The rate of As(III) oxidation by Fe(III) increases as theratio Fe(III)/As(III) increases and as the pH is lowered, soAs(III) present in iron-rich samples [e.g., acid-minedrainage (AMD)] undergoes oxidation if preserved withHCl and light exposure. However, in the absence of light,oxidation is almost inhibited [26].

The oxidation of As(III) could be inhibited by thepresence of Fe(II). The intermediate As(IV) speciesformed from the oxidation of As(III) preferentially reactswith Fe(II) and is reduced to As(III). The reaction ofAs(IV) intermediate with Fe(II) and Fe(III) is a com-petitive one, as the rate constants of the two reactionsare similar, so the ratio Fe(II)/Fe(III) has a profoundinfluence on the oxidation of As(III) {i.e. higher con-centration of Fe(II) inhibits As(III) oxidation [26]}.Successful preservation of As(III) in iron-rich ground-water samples has been achieved by adding Fe(II) at pH<2 [27]. The presence of sulfate could also inhibit theoxidation of As(III) by complexing Fe(III) as FeSO4

+,which does not produce free radicals required for theoxidation [26]. Typically, natural waters containinghigh Fe(III) concentrations also contain high Fe(II) andSO4

2� concentrations, so the fast oxidation of As(III) byFe(III) in synthetic solutions is not observed in naturalwaters.

4.2. As(III)/As(V) stability in the presence of naturalorganic matter (NOM)NOM is ubiquitous in natural waters and possesses acombination of functional groups, including carboxylic,esteric, phenolic, quinine, amino, nitroso, sulfhydryl,hydroxyl and other moieties [28]. The hydroquinonemoieties within NOM are redox active and may reactwith arsenic species or compete with them for oxidantsor reductants. The reducing property of NOM is wellknown, so NOM-rich waters generally contain As(III) asthe dominant arsenic species [29]. Also, NOM maydecrease the Fe(III) oxidation of As(III) by complexingFe(II) [30].

The reduction of As(V) standard solutions prepared indeionized water is attributed to the release of organicmatter from ion-exchange resins [19]. Reduction ofAs(V) due to NOM in EDTA-preserved rainwater andsoil-pore water has been reported [31]. Drastic reductionin the stabilization period has been observed after spikingAs(III)/As(V) solutions with NOM [16]. Contrarily,oxidation of As(III) due to NOM has also been reported[30].

4.3. As(III)Ð As(V) transformations due to microbialactivityMicroorganism-mediated arsenic transformations,including redox, methylation and demethylation, arewell documented [32]. Microorganisms may oxidize orreduce arsenic species over a wide range of temperature,pH, and chemical composition of water.

Sealed water samples that contain dissolved organiccarbon utilizable as a food source may promote As(V)reduction during storage, if not preserved properly [6].Oxic water samples can also contain microbes thatreduce As(V) [33].

Mixed microbial cultures were found to oxidize As(III)and to reduce As(V) [34]. Rapid As(III) oxidation bymicrobes common to hot springs was reported [35].

In geothermal stream waters of Nevada, rapid oxida-tion of As(III) to As(V) was observed with conservationof total arsenic values [36]. Samples collected directlyfrom the hot spring did not show any oxidation, becausemicrobes did not survive at the very high temperature ofthe source (�200�C).

Samples filtered through sterile filters and treated withantibiotics [36] or formaldehyde [35] did not undergooxidation. In hot spring pool waters, temperaturedependence of As(III) oxidation was observed, becausemicrobial survival decreased with temperature [36].

Unlike abiotic oxidation of As(III) that subsequentlyremove a portion of As(V), microbe-mediated oxidationconserves total arsenic concentrations, so arsenic-speciestransformations reported in most synthetic waters andstandard solutions can be attributed to microbial activ-ity, as evidenced by the conservation of total arsenic.Also, sterile water samples have been observed to be lesssusceptible to speciation changes than non-sterilesamples [34]. A detailed account of As(III)-noxidizingmicrobes was given by Salamassi et al. [37].

5. General procedures for preservation of arsenicspecies

5.1. FiltrationFiltration through an unreactive 0.45-lm membranefilter is standard practice in trace-elemental analysis ofwater to differentiate particulate bound and dissolvedfraction of an element. In arsenic-speciation analysis,filtration suppresses speciation changes partly throughthe removal of hydrated ferric oxides, and partly throughthe removal of microorganisms. Many bacteria havedimensions <0.45 lm, hence filtration using 0.1-0.2 lmfilters has been reported to improve species stability[38].

However, the dissolved arsenic concentration deter-mined after 0.2-lm filtration was significantly lowerthan that of 0.45-lm filtered samples [39], due to thearsenic being sorbed onto colloidal particles sized in the


Table 1. Typical studies reported on the acid preservation of inorganic arsenic species in synthetic solutions and in natural waters

Nature of watersample

As species concentrationrange (lg/L)

Preservation Observations of changes in speciation/comments

Ref.

Synthetic samples As(III) and As(V) pH <2, 4�C in dark Stable for 12 months. Storage in light andincrease in temperature decrease stability time

[17]

Sediment interstitial As(III):1–4 Filtered onsite, acidified Stable up to 6 weeks. The rate of As(III)oxidation depends on sample

[18]

Waters As(V):1–4 with HCl (pH <2), �0�C matrixSynthetic solutions As(III)): 4

As(V): 4Stable up to 28 days with HCl as well as withascorbic acid. In the presence of NOM, stabilitydecreased to 8 days. Waterworks samplespreserved with ascorbic acid were stable for8 days.

[16]

Synthetic solutions As(III) :1–10 Acidified with H2SO4 Stable for 125 days. [11]River water As(V): 1–10 (0.2%), room temperatureRiver water - Filtered onsite, acidified

with HCl(pH <2), 4�C Stable for 33 days at 4 and 20�C. [19]Synthetic solutions,river water

As(III):0.5 and 5.0 Filtered onsite, acidified Stable for 15 days for both HCl and HNO3

acids. HNO3 showed higher[8]

As(V):0.5 and 5.0 with HCl(0.1%) or degree of oxidation at 0.4% concentration.Storage at 5�C did not improve

HNO3(0.1%), 5 and 22�C species stability in river waterSynthetic solutions As(III):10 20�C, light and dark HCl oxidizes As(III) within a day; HNO3

preserved the species for[9]

As(V): 10 HCl(0.06 mol/L),HNO3(0.08 mol/L),H2SO4(0.09 mol/L)

5 days only in the dark. In presence of lightoxidation was observed within a day; H2SO4

preserved the species for 5 days both inpresence and absence of light

Synthetic solutions,geothermal and

As(III): 20 HCl (pH <2) Stable for a period of 45 days in both light anddark conditions

[6]

As(V): 20 light and darkAs(III) �15–260 Filtered onsite, acidified Stable for 5-19 months. Samples had a wide

range of pH,[6]

AMD, surface andgroundwater

(HCl pH <2), stored in Fe(II)/Fe(III), As(III)/As(V) and SO4

concentrationsTAs 6-33000 opaque bottles

Groundwater TAs 30.1–278 Filtered onsite, acidified (HCl pH<2), stored in dark

Stable for 3 months. Oxidation of As(III) wasobserved after 3 months. TAs stable for12 months.

[15]

Geothermal waters As(III): >1000 Filtered onsite, acidified Stable for more than 6 months. Unacidifiedsamples preserved under the

[14]

(HCl pH <2), stored in same conditions showed similar results. Riverwater preserved under the

4�C dark same conditions were not stable and oxidationof As(III) was noticed

Groundwater As(III): 3.5–710 Filtered onsite, acidified Stable for 15 weeks. Samples were collectedfrom arsenic affected area of

[20]

As(V): 1.0–53 (HCl 24 mmol/L), stored West Bengal. Samples were alkaline (pH 7.0-8.5), low Eh (-11-233 mV)

in opaque container Fe 0-14 mg/L. As(III) is the predominant speciesGroundwater and As(III): nd-9.6 Filtered onsite, acidified Stable for 40 days. [21]Polluted river water As(V): 0.5–11.3 (HCl pH <2), stored in

4�C darkSpiked mine water As(III) :500 Filtered, acidified with Stable for 6 days. Samples having higher Fe

require higher acid[12]

As(V): 500 H3PO4 (0.01%), 6�C (10 m mol/L) concentrations for complexation.Spiked raw and treatedwastewater

As(III) :8 Prefiltered with 1.2 lm, As(III) was stable for 4 and 2 months in treatedwastewater stored at 4 and

[22]

As(V): 15 filtered with 0.45 lm, acidified topH 1.6 and in dark at 4, 20 and40�C

40�C, respectively in unacidified (pH 7.27)samples. As(III) in untreated wastewatercontaining more organic matter was oxidized toAs(V).




range 0.2–0.45 lm. Filtration should be done on siteand delays in filtration as small as a few hours couldinfluence the aqueous vs. particulate-bound arsenicproportion [40].

The duration of filtration time should be kept minimum(i.e. �<10 min.), especially if the sample pH is >8 andcontains dissolved iron [41]. If more time is expected,filtration in an N2 atmosphere is recommended.

Loss of arsenic due to HFOs deposited on filters is acommon problem in some field-speciation methods [16].

Some authors recommend application of positivepressure for filtration, rather than suction, which createsmore disturbances in the sample. However, the authorshave not compared the changes effected by both filtra-tion techniques [42].

5.2. AcidificationAcidification of aqueous samples is a widely-practicedprotocol aimed at suppressing the precipitation of Fe andMn by increasing their solubility. Acidification also helpsin reducing microbial activity. HCl is commonly used topreserve arsenic species [6]. HNO3 is an oxidizing agentand does not preserve arsenic species due to photore-duction of HNO2, which oxidizes As(III) to As(V). H2SO4

has been used by some authors [11], but it is notdesirable because it is difficult to purify. Further, it is notgenerally used for trace-metal preservation, because ofthe possibility of BaSO4 precipitation. H3PO4 has beenknown to preserve As speciation by complexing Fe andlowering pH [12]. AMD and groundwater sampleshaving Fe and Mn up to 15 mg/L and total arsenicconcentration of 1–2 mg/L were successfully preservedup to 6 days using H3PO4 (0.01 mol/L). Increasing acidconcentration to 10 mol/L sequestered up to 100 mg/Lof Fe and Mn for 3 months, but higher acid concentra-tions may damage ion-chromatographic columns. Also,the formation of insoluble phosphates of Fe (i.e. strengiteFe3(PO4)2) may limit its use in high iron samples (e.g.,AMD waters) [43]. An important drawback of theaddition of H2SO4, and H3PO4 is that they may increasethe solution Eh, particularly in oxic water samples (Eh�>400 mV), resulting in the oxidation of As(III) [44].

5.3. Type of containerGenerally, glass and various kinds of plastics (polyeth-ylene, polypropylene, polycarbonate) containers are usedfor sample storage. The container material should notcontribute towards adsorption loss and should not leachany constituent from it. Glass containers prewashed withHNO3 are shown to leach out arsenic [31]. The loss ofarsenic in samples stored in plastics containers wasattributed to the volatility and escape through the poresof plastics containers [42]. However, such an assump-tion was not supported by the literature. Some of theearlier studies also observed loss of arsenic species storedin plastics containers [10,11]. It can be inferred that

most of these losses could be due to photochemicalreactions. Plastics containers are generally translucent,so some authors preferred opaque glass containers [45].The results of a BCR study showed that both types ofcontainer are equally good for storage of samples forarsenic-speciation analysis. Based on these observations,it appears that both glass and plastics-containermaterials do not influence the As(III) � As(V) inter-conversions, so both types of container can be used.

5.4. Refrigeration and storage in dark conditionsRefrigeration of samples is recommended by most of theauthors in order to reduce biotic and abiotic processesthat change arsenic-species interconversion. Samplestorage at 4�C has been recommended by many authorsfor arsenic-species stabilization [15,17,18]. Biotic andabiotic oxidation rates of As(III) increase with tempera-ture, so the stabilization period would be less if samplesare stored at room temperature [40,22]. Storage at�20�C (freezing) is useful for Fe-poor samples [46], butAs(III) � As(V) interconversions were noticed inFe-rich waters [12,31]. Precipitation of Fe and Mn wasnoticed in interstitial water samples of sediment due tofreezing, which was not reversible upon thawing [18].However, flash-freezing of geothermal waters andthawing under oxygen-free atmosphere was reported topreserve arsenic speciation [47].

6. Preservation of arsenic species in AMD sampleswith EDTA

Oxidation of pyrite ores is the major cause of AMDwaters. The low pH (�1–4) of AMD favors dissolution ofFe(III) and other secondary minerals, thereby increasingmetal load to drainage. In contrast to natural waters,the predominant form of Fe in AMD waters is Fe(III), dueto its solubility at low pH [23]. The Eh-pH diagramshows, that AMD with pH >�3.5 is susceptible to Fe(II)oxidation, resulting the precipitation of HFOs, and As(V)is preferentially coprecipitated [23]. Studies show thatcoprecipitation would be more effective, when the HFOsare formed in situ from Fe(II) oxidation [26], so As(III) isthe predominant species of such AMD waters. However,if pH is <3.5, then a substantial amount of Fe(III) will bein the dissolved form, which then oxidizes As(III) toAs(V).

To stabilize arsenic species in their original state, theredox activity of the coexisting Fe(III) and Fe(II) shouldbe minimized. The formation of metal complexes canhelp to decrease the free metal-ion concentration,thereby inhibiting the arsenic-species interconversion.Gallagher et al. preserved lg/L levels of arsenic speciescontaining 7 mg/L of Fe(III) for 14 days, by the additionof 500 mg/L of EDTA after adjusting sample pH to 3.3using acetic acid [48]. Sample acidification is necessary


Table 2. Typical studies reported on the preservation of inorganic arsenic species using EDTA/EDTA-AcH in iron-rich samples

Nature of watersample

As speciesconcentrationrange (lg/L)

Preservation Observations of changes in speciation/comments Ref.

Groundwater As(III): 0–720 0.45 lm syringe filtration, As(III) and As(V) were preserved for the studied3 months period.

[9]

As(V): 0.1–1080 1.25 mmol/L EDTA, opaquepolyethylene bottle storage atroom temperature

Field speciation was carried out using ionexchange resin on EDTA preserved samples.EDTA was necessary to prevent HFOprecipitation. Excellent agreement between fieldand laboratory speciated results.

AMD As(III): 11–12800 0.45 lm syringe filtration,As(V): 180–7050 2.5 mmol/L EDTA, opaque

polyethylene bottle storage atroom temperature

As(III) and As(V) were preserved for the studied3 months period.Precipitation of HFOs on filters were observeddue insufficient EDTA.Excellent agreement between field and laboratoryspeciated results.

AMD As(III):22.9–39.6As(V): 33.6–49.6

Field filtration (0.2 lm) 0.25 mol/L EDTA, storage at �18, 4 and20�C

Samples stored at 4 and 20�C were stable for only12 hrs. Samples had high concentrations of Fe,Cu, Pb, Al and Zn.

[49]

Creek water As(III):BDL-45.2As(V): 372-3667

Surface water - Field filtration, 12.5 mmol/LEDTA, room temperature anddark conditions

Samples were alkaline, Ca-Mg rich and had Fe<10 mg/L. 1.25 m mol/L

[50]

Groundwater - of EDTA did not preserve As species, becauseCa-Mg ions were also complexed by EDTA.

Groundwater As(III): 3.9–72.7 1 mmol/L EDTA + 0.08 mol/L As(III) and As(V) were stable for 30-85 days.Compared EDTA,

[51]

As(V): 2.1–25.2 acetic acid, room temperatureand dark conditions

EDTA-AcH, H3PO4 and H2SO4 preservatives.Except EDTA-AcH preservative, others did notprevent formation of HFOs from samples had3 mg/L of Fe(II) and at pH 8.3–3.0.

Groundwater As(III): 4.1–88.1As(V): 0.4–23.7

EDTA-AcH preservation, opaquepolyethylene bottle and roomtemperature storage

As(III) and As(V) were stable up to 85 days. Lightexposure causes instability of Fe-EDTA complexesresulting oxidation of As(III). Excellent agreementbetween field and laboratory speciated results.

[44]

Groundwater As(III):4.6–88.1As(V):0.7–18.3

0.5 mmol/LEDTA + 0.01 mol/LAcH, room temperature and darkconditions

Compared field speciation methods using HCl,H2SO4, EDTA and EDTA-AcH pretreatments forsamples having pH 2 and 8.3 and Fe 3.0 mg/Lwith chloride and acetate forms of IC resins. HCland H2SO4 deplete the exchange capacity ofresins. EDTA-AcH worked well under allinvestigated conditions. Excellent agreementbetween field and laboratory speciated results.

[52]

Groundwater As(III)and As(V) Field filtered using 0.45 and 0.2lm filters, 50 mg/LEDTA + 0.08 mol/L AcH, storedin dark at 20�C

As(III) and As(V) were stable for 80 days. AcH isrequired to minimize precipitation of Fe ofsamples having pH >7.0. Storage at 5�Cminimized As(III) oxidation relative to that at20�C.

[41]

Groundwater As(III):3.5–710 0.45 lm filtration, Significant oxidation of As(III) was observed.Possible reasons were

[20]

As(V): 1.0–53 10 mmol/LEDTA 1) instability of Fe-EDTA complexes, 2) high pH ofsamples and 3) failure to inhibit microbial activityby EDTA

Groundwater Total As:19.3–511.0

Field filtered using 0.45 and 0.2lm filters, pH adjusted to 3.3with AcH+500 mg/LEDTA

As species were stable for 10 days. Precipitationof HFOs were observed without pH adjustment to3.3.

[48]


to bring all Fe(III) to solution. Bender et.al [9] used1.25 mmol/L and 2.5 mmol/L of EDTA to preservegroundwater, and AMD samples for 3 months without


adding acetic acid. However, other studies have shownthat EDTA alone cannot preserve arsenic speciation, andpH adjustment to 3.2 is necessary to inhibit oxidation of


Fe(II) [44]. Bender et al. [9] were successful becausetheir samples were highly acidic (pH 2.8–5.8). Moreexamples of EDTA preservation are given in Table 2.

6.1. EDTA vs. HCl preservationEDTA is not a particularly selective chelating agent, andwill form stable complexes with calcium and magnesiumfound in natural waters, so higher concentration ofEDTA is required to stabilize arsenic species, especially ingroundwater samples [50]. Addition of higher concen-tration of EDTA (i.e. 10 mmol/L) increases the pH due tothe formation of protonated metal-EDTA complexes [20].The oxidation of Fe(II) to Fe(III) is pH dependent {i.e.positively dependent on the concentration of [OH�]}, sothe higher the amount of EDTA added, the larger will bethe pH rise, resulting in increased oxidation of Fe(II) toFe(III) and coprecipitation. In addition, Fe(III)-EDTAcomplexes are more stable than Fe(II)-EDTA, and thiswill also favor oxidation of Fe(II) to Fe(III), resulting in ahigher concentration of Fe(III) potentially capable ofoxidizing As(III). Lower EDTA concentration(1.25 mmol/L) is therefore ineffective in preservinggroundwater samples characterized by high Fe(II), Ca,and Mg [20]. However, higher concentration of EDTA(10 mmol/L) results in increased pH, which in turnincreases the rate of oxidation of Fe(II), so bothconditions do not preserve arsenic species in such typesof groundwater. Also, in some types of AMD sample,EDTA stabilizes As(III)/As(V) species only for 3 h [49].

However, HCl could preserve arsenic species ofgroundwater samples with high iron content for 6 days[20]. The successful preservation by HCl is due to sup-pression of Fe(II) oxidation at low pH. Also, a high Fe(II)/Fe(III) ratio retards oxidation of As(III) [26]. Some HCl-preserved AMD samples were stable for 2 days, evenafter the removal of the metallic ions by cation-exchangeresin [49]. This apparent inefficiency of EDTA and HClpreservation procedures was due to the differencesbetween the metallic matrices of the AMD samples. Theoxidation of Fe(II) observed in EDTA-preserved ground-water samples by Gault et al. [20] might have been dueto the considerable time taken to filter samples. Benderet al. [9] utilized an in-line syringe filter, in which theatmospheric contact was very minimal. Other workersalso observed precipitation of Fe(III) during field filtrationdue to exposure to atmospheric oxygen [51,52].

7. Preservation of arsenic species in seawater

Arsenic species in seawater could not be stabilized byacidification or by any other preservative due to theinterconversions resulting from microbial and plank-tonic activities, salinity, and the presence of iodide[19,53]. Upon acidification, I�is converted to I2, whichthen oxidizes As(III) to As(V) [54]. However, MMA and

DMA are quite stable in acidified seawater samples [55].Though earlier reports recommended quick freezing forpreserving As(III) and As(V) [55,56], current knowledgenow considers this invalid. In spite of this, severalresearchers still use acidification to preserve inorganicarsenic species in seawater [57]. On several cruises,on-board species separation using ion-exchange resinswere used for As(III) and As(V) [54].

8. Preservation of arsenic species in sulfidic waters

Under anaerobic conditions, sulfate and organic sulfurcompounds present in groundwaters are reduced bymicroorganisms to sulfide. As(V) is unstable in sulfidicwater, and is rapidly reduced to As(III), which formssoluble thioarsenite complexes, depending on pH andsulfide concentrations [58]. Preservation of arsenicspecies in sulfidic waters is rather difficult, becauseacidification (HCl) leads to precipitation of realgar(As2S3). However, acidification using HNO3 and lightexposure preserve total arsenic [47]. HNO3 is particu-larly useful here because the photochemical reaction ofHNO3 prevents precipitation of As2S3.

For total arsenic determinations, samples are madealkaline (1 mol/L NaOH), treated with H2O2 to convertAs(III) to As(V), and then preserved with HCl to pH<2[59]. Flash-freezing of geothermal waters, thawing inoxygen-free atmosphere and separation of species byanion-exchange column have been reported [47]. Field-speciation methods {e.g., acidification and N2 spargingfor total arsenic, As(III) removal by selective arsinegeneration and N2 sparging at pH 6 for As(V)} havebeen proposed [35].

9. Field-speciation methods

In view of the numerous possible influences on As spe-ciation during sampling and storage, several authorshave separated As(III) and As(V) species on site usingion-exchange cartridges or resins (Table 3). Some of thepopular approaches are the Edwards, Ficklin and Cliffordmethods [16,60,61].

Since As(III) exists as neutral species and As(V) asanion, separation could easily be achieved by using astrong anion-exchange resin. Uncharged As(III) remainsin the sample and elutes from the cartridge, whereasanionic As(V) is retained on the cartridge. As(V) is elutedfrom the cartridge with acid in the laboratory andanalyzed as total As. The ion-exchange resin may con-veniently be used in the form of disc or cartridge in-linewith a syringe. If the sample contains dissolved iron,oxidation of Fe(II) on exposure to the atmosphere andsubsequent adsorption is possible. Acidification with HCl[61] or H2SO4 [16] prior to ion exchange prevents Fe(II)


Table 3. Examples of field speciation methods for the separation of arsenic species

Nature ofwater sample

As speciesconcentrationrange (lg/L)

Samplepretreatment

Ion-exchange columncharacteristics

Comments Ref.

Groundwater As(III) and As(V) Samples wereacidified with 1%HCl

Glass column: id 0.7 cm,length:10 cmResin type: chloride form

Interference due to 40Ar35Cl+ species ifICP-MS is used for quantification; HClincreases anion load; pH <2.0 may resultin H3AsO4, which cannot be exchanged.

[60]

Syntheticgroundwater

As(III) and As(V) Acidification to pH3 with HCl, ifsample contains Fe

Glass column: id 0.8 cm,length:10 cmResin volume: 5 mL, chloride

form;

HCl increases anion load; pH <2.0 mayresult in H3AsO4, which cannot beexchanged.

[61]

Flow rate: 1–10 mL/minGroundwater As(III) and As(V) Filtered with 0.45

lmPolypropylene column: id 1.5 cm, Particulate bound As represented 0-53%

of total[16]

Filter, thenacidified with

length:10 cm; acetate form As. Acidification is necessary to inhibitFe(II)

H2SO4 (0.05%) oxidation H2SO4 may increase an ionload.Filtration may increase Fe(II) oxidation.

Groundwater As(III):4.6–88.1 0.5 mmol/LEDTA+

Glass column: id 1 cm, length:10 cm

Successful for the separation of As speciesin Fe

[52]

As(V):0.7–18.3 0.01 mol/L AcH Resin capacity, volume and type: containing waters. High exchangecapacity of

8-14 meq/g;10 mL, chloride and resin overcomes the effects of Cl� andSO42-

acetate forms. Samplerequirement:

EDTA complexed Fe and the pH �3.2

�60 mL. Flow rate: 20–150 mL/min

ensures all As(V) are in ionic form.

Groundwater As(III): 0–720 0.45 lm syringefiltration,

SPE cartridge; capacity: 0.2 meq/g Samples having high anion and Feconcentrations

[9]

and As(V): 0.1–1080 1.25 mmol/LEDTA,

chloride and acetate forms.Sample

should be diluted to overcome resincapacity.

AMD As(III):11–12800

0.45 lm syringefiltration,

requirement: �10 mL;

As(V):180–7050

2.5 mmol/L EDTA, Flow rate: 1–2 drops/s

Groundwater As(III): 0.6–13 0.45 lm filtration An integrated 0.45 lm filter, cation This method separated particulate, DMA,MMA,

[63]

As(V): 0.5–8.9 and exchange cartridges were usedfor

As(V) and As(III) species. The cartridgescan be

Particulate As:33–59

the retention of DMA, and As(V)and

operated in tandem with HG-AAS/AFS.

MMA, respectively. Samplevolume

Applicability of the method wasdemonstrated

and flow rate: 20 and 1–2 mL/min with limited samplesSynthetic As(III): 20 Total As: Treated

withGlass column: id 1 cm, length:10 cm

Separates As(III) and thioarsenic species.Study

[59]

groundwater thioarsenicspecies

NaOCl andpreserved with

Resin volume and type: 7 mL and indicated potential error due to thesorption of

HNO3 or EDTA-AcH

chloride thioarsenic species on IC if watercontains

As(III): Eluatepreserved

S2- �1 mg/L But, real samples were notanalyzed.

with EDTA-AcHGroundwater As(III):

40.8–67.9As(V): 2.2–11.2

pH adjusted to 4-6 Polyethylene column: id 0.8 cm,length: 20 cm; chloride form resin.Sample requirement: 50 mL; flow

rate: 2-2.7 mL/min

Sample pH was adjusted to 4-7 with HClto form neutral As(III) species, howeverthe effect of acidification was notinvestigated.

[64]

AMD As(III):500–3001

0.45 lm filtration, Syringe type, resin type andvolume:

Good separation was achieved in the pHrange 7–9. SO4

2� concentration up to1100 mg/L can be tolerated.

[65]

As(V):700–4001

2.5 mmol/L EDTA, chloride, and 8 mL;

pH adjusted to 7–9 flow rate: 4 mL/min



Figure 3. General approach for the preservation of As(III)/As(V) in environmental water samples.


oxidation. However, these acids increase the anionconcentration that interferes in the quantification (ifICP-MS is used), and decrease the exchange capacity ofresins. These problems could be overcome if EDTA orEDTA-AcH is used. In general, excellent agreement hasbeen reported between field and laboratory methods forspeciation [51,52].

Samples with high anion (i.e. SO42�, Cl�) concentra-

tion or with MMA and DMA must be diluted or a largercapacity cartridge used [62]; however, matrix dilutioncan be a disadvantage, because the analyte concentra-tions are also diluted. The As(III)-sulfide species, found insome strongly reducing waters, could also be exchangedin an anion-exchange column with As(V) [63].

Purging samples with N2 can convert As(III)-sulfidespecies to As(III). However, separation of As(III) andthioarsenic species could be achieved by an ion-exchange method because As(V) does not exist in suchreducing waters [63].

10. Generalized treatment of preservationof arsenic species

The failure and the success of the various preservationstrategies led to the conclusion that the major causes ofAs(III)/As(V) interconversions are:(1) the Fe(II)/Fe(III) system and the influence of pH, DO

and light on the Fe redox couple; and,(2) microbe-mediated conversions.

Successful preservation is therefore achieved bysequestering Fe(II)/Fe(III) and/or delaying the redox

transformations, in addition to the general procedures offiltration, refrigeration and storage in dark conditions.These general procedures help to suppress the microbe-mediated redox reactions.

Based on the literature, Fig. 3 gives a generalizedapproach to preserving As(III)/As(V) in differentenvironmental water samples. Please note that thesegeneralizations can only be used as a guideline. HClpreservation could be used for a wide range of surface-water and groundwater samples containing dissolvedFe(II). If Fe(II) concentration is more than �0.5 mg/L,field filtration should be done as quickly as possible.However, EDTA is useful to preserve highly acidic AMDand other Fe(III)-rich samples. The EDTA-AcH combi-nation is more effective in preserving arsenic speciesthan EDTA alone because acidification is required toinhibit the oxidation of Fe(II).

Selection of a proper preservation strategy for arsenicspecies also depends on the analytical methodology to befollowed. HCl preservation is better suited if hydride-generation (HG) atomic spectrometric techniques areused, because the sample matrix is similar to the HGcarrier stream. However, for samples with greatermetallic content (e.g., AMD and geothermal waters),EDTA is the preferred preservative with HPLC/IC sepa-ration and ICP-MS determination.

11. Conclusions

Knowledge of the various processes that influence theinterconversion of arsenic species is necessary to produce



representative data on species distribution. Filtration,refrigeration at 4�C and storage in dark conditions areprerequisites in arsenic-species stabilization. Preparationof standards, preferably in distilled-deionized water,could minimize microbe-mediated species interconver-sions. The efficiency of preservatives (e.g., HCl, H2SO4,EDTA and EDTA-AcH) in As(III)/As(V) stabilizationdepends on sample matrix and microbial activity, andtheir response to these preservation methods.

In summary, there is no universal preservative that iseffective for all water samples, so individual samplematrices will need to be tested for species stability beforea particular preservative can be employed. Preservationof arsenic species also depends on the analytical tech-nique to be used. HCl is better suited to a wide variety ofnatural water samples if HG atomic spectrometric tech-niques are used. For IC/HPLC-ICP-MS applications,EDTA-AcH preservation is preferred.

AcknowledgmentsThe first author thanks B.M. Jha, Chairman, CGWB,S. Kunar, Member (SAM), CGWB, Faridabad, India, andD.S.C. Thambi, Regional Director, CGWB, SECR,Chennai, India, for granting permission to publish thisarticle.

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