Effects of pH and competing anions on the speciation of arsenic in fixed ionic strength solutions by solid phase extraction cartridges
Introduction
Arsenic contamination in the environment has received increased attention in recent years due to the manifestations of As poisoning in drinking water sources in Bangladesh [1], [2] and Viet Nam [3]. The US EPA has lowered the regulatory limit for As in drinking water from 50 μg L−1 to 10 μg L−1 (due to take effect in 2006). This change stems from research on increased cancer risk due to As in drinking water [4]. In addition to cancer risks, chronic exposure to As can cause health problems for humans with symptoms ranging from severe discomfort to death [5]. The different chemical species or forms of As exhibit different degrees of toxicity. It is difficult to compare with absolute certainty the differing toxicities of As species. Different organisms respond to As with a high degree of variability. Generally, the inorganic species (commonly found in the environment as arsenates H2AsO4− and HAsO42− and arsenite H3AsO3) are more toxic than the organic forms (e.g. monomethylarsonate [MMA] or dimethylarsinate [DMA]). Inorganic species have been estimated to be 25–100 times [6], [7] more toxic than organic forms. Ferguson and Gavis [8] estimated the toxicity of As(III) as being 60 times that of As(V).
The speciation of inorganic As can also affect the overall mobility of As in the environment. In the pH range 4–9, As(V) is negatively charged while As(III) is theoretically uncharged. Thus, the charged As(V) tends to react with cationic sorption sites on Fe-oxide surfaces such as goethite [9], clays [10], and Al-oxides [11]. This is a major consideration for As treatment in water. The presence of As(III) requires the use of oxidation processes such as ozonation [12] or reaction with MnO2 [13].
In summary, knowledge on the speciation of As in a particular system is important for assessing toxicity risks and mobility. Many methodologies exist for the speciation of As. Generally, As speciation methods consist of a separation step followed by detection. In the early seventies, a spectrophotometric method was developed for differentiation of inorganic As [14]. This was followed by arguably the most influential research on As speciation procedures which allowed the determination of not only the inorganic species but MMA and DMA as well [6], [15]. This method and modifications of this method are based on the pH-selective generation of arsines from different species of As. The arsines are cold-trapped and selectively volatilized from the cold trap by increasing temperature. The modified methods generally differ in the detection of the volatilized forms. Detection methods for selectively volatilized As species include graphite furnace atomic absorption spectrometry (AAS) [16], [17], flame AAS [18], [19], [20], and inductively coupled plasma (ICP) mass spectrometry (MS) [21].
The separation step may also be accomplished by ion chromatography (IC). Very sensitive analyses (detection limits (DLs) on the order of ng L−1) of As species have been conducted using IC separation followed by analysis via ICP-MS [22], [23]. Chromatographic separation followed by analysis via differential pulse polarography resulted in DLs for As(V), As (III), MMA, and DMA on the order of 5–20 μg L−1 [24], [25]. High performance liquid chromatography for species separation followed by flame AA analysis has been used effectively (0.8 μg L−1) [26]. More thorough reviews of arsenic speciation techniques can be found in the literature [7], [27].
Anion exchange resins (AERs) offer an economical alternative to other As separation methods. Similar in theory to As separation by IC columns, they are relatively easy to employ and suitable for use in the field. The use of AERs minimizes the adverse effects that sample preservatives can have on As species in environmental samples as the separation can be performed instantly. Many of the speciation methods using AERs are based on a procedure by Ficklin [28] where a 100–200 mesh Dowex 1×8 AER separates the inorganic As species. The procedure is based on the retention of charged As(V) by the AER while uncharged As(III) passes through. Edwards et al. [29] modified the Ficklin procedure by using an AER with a larger particle size (50–100 mesh). Cation exchange resins (CER) have also been employed for As separations [30], [31]. Various combinations of AERs and CERs have been utilized for As speciation [25], [32], [33]. Solid phase extraction cartridges (SPECs) have also been employed. A comparison of several SPECs was performed by Yalcin and Le [34].
Disadvantages of the use of AERs for inorganic As speciation include false-positive results for As(III) due to incomplete retention of As(V), false-positive results for As(III) when As(V) associated with particulates passes through the resin, and false-negative results for As(III) due to As(III) retention by the AER. Of course, a false positive/negative for one species will have a concurrent, opposite effect on the other species using this scheme. Arsenic associated with particulates has been reported as the most problematic aspect [29]. Furthermore, when assessing inorganic As only, significant error may occur with the presence of large concentrations of DMA and MMA. These organic species contain As in the +5 valence state [35]. In one procedure, DMA and MMA passed through the AER along with As(III) yielding false-positive results for As(III) [36]. In SPECs containing silica-based AERs examined by Yalcin and Le [34] the exact opposite occurred. The DMA and MMA species were retained by the resins which led to false-positive values for As (V). In most environments, the inorganic As forms dominate the overall As chemistry. Possible exceptions include samples high in organic matter [37], samples with a high amount of biological activity [38], and samples with contamination from arsenic-containing pesticides which may be indistinguishable from biologically methylated species [6].
While the use of AERs for As speciation has received much attention, there is a lack of information concerning the effects of competing anions and pH on the efficacy of AERs for use in As speciation. Furthermore, the focus of most studies has been on speciation in water samples with no consideration of ionic strength effects. It is quite common for laboratories dealing with sorption/desorption reactions in sediments and soils to utilize background electrolyte (BG) solutions. Therefore it is imperative to optimize the inorganic speciation procedure in fixed ionic strength solutions that contain potentially interfering anions in a variety of pH environments.
Section snippets
Objectives
The objectives of this study were to: (1) Assess two SPECs for As speciation in solutions with ionic strengths that relate to freshwaters and soil pore waters ( M), (2) Assess the effects of initial solution pH and competing anions on the speciation procedure based on breakthrough of As(V), (3) Calculate DLs in spiked BG solutions and report spike recoveries from actual samples. The ideal material will be usable directly from the manufacturer with no conversion of functional groups.
Materials and methods
All chemicals were ACS reagent grade (Fisher Scientific, Fair Lawn, NJ) unless otherwise noted. The 0.01 mol L−1 NaNO3 BG was made in 18 MΩ cm deionized (DI) H2O. All pH measurements were made by an Orion Model 350 pH meter interfaced with PerpHect pH probe (ThermoOrion, Beverly, MA) and pH adjustments were made with 0.1 mol L−1 NaOH or 0.1 mol L−1 HNO3. Two cartridges containing different AERs were selected for examination based on results from previous work by Yalcin and Le [34]. The LC SPEC contains
Results and discussion
Preliminary results on the retention of As(III) indicated that the lowest retention of As(III) occurred with the “dry” (no pre-wetting or conversion of functional groups) QMA cartridge. The percent recoveries for all of the combinations tested were: QMA dry—98.0±2.7%, QMA pre-wetted—89.4±4.1%, LC pre-wetted—83.6±3.4%, LC dry—74.2±3.4% (n=5 for each combination). A comparison of the means (by two sample t-test) showed that the means within the cartridge type (i.e. pre-wetted vs. dry) were
Conclusions
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The methodology using the QMA SPEC for As speciation in soil/sediment pore water surrogate extracts (I=0.01 M) is inexpensive ($2 sample−1) and simple.
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Sulfate and phosphate had the greatest effect on As(V) retention.
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For As(V) and As(III), the MDLs were 0.004 mg L−1. The average recoveries from spiked mine waste extracts were 107% for As(V) and 94% for As(III).
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Concentrations of the potentially competing anions examined in this study are typically much lower in fresh surface waters. Thus, this
Acknowledgements
The author thanks Collin A. Richardson for his laboratory work, which was made possible by the USEPA High School Apprenticeship Program. Although the research in this paper has been undertaken by the US Environmental Protection Agency, it does not necessarily reflect the policy of the Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. Existing data not generated by EPA employees for informational purposes were not subjected to EPA's
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