Methods Utilised for the Speciation of Arsenic in Soils

One of the earliest know methods used for the speciation of arsenic in soils was reported by Forehand et al. [261] in 1976. The method used in this work is based on a method reported some years earlier [262] that utilized benzene as a highly selective and sensitive extractant for AsIII _{(from a HCl medium) to separate arsenic from antimony} and bismuth. In the Forehand et al. [261] method the soil sample is digested in HCl and this is followed by the reduction of AsV to AsIII using SnCl2 and KI. Benzene is used to extract the AsIII which is then back extracted into water and analysed using AAS. The method relies on the premise that AsIII is selectively extracted into an organic phase (i.e. benzene) from a strongly acidic phase that can then be back extracted into water for analysis. In the presence of excess HCl, chlorination of the arsenic will occur, resulting in arsenic trichloride and arsenic pentachloride. It has been shown that the arsenic trichloride is a covalent molecule while arsenic pentachloride most likely exists as ion complexes [263]. The arsenic trichloride is extracted selectively into the organic phase while arsenic pentachloride is excluded due its ionic properties. The arsenic trichloride contained in the organic phase can be recovered by back extraction with water, since arsenic is most stable in solution in its hydrolysed form [264-265].

Takamatsu et al. [29] in 1982 used almost the same method as Forehand et al. [261] to determine arsenic species in a contaminated soil. In that study, an analytical technique that consists of a sequential extraction using HCl, HCl/KI, benzene and H2O/H2O2 followed by anion-exchange chromatography and final determination of arsenic by

flameless AAS is reported. Studies have been completed by others [32,42] who have utilized a similar speciation method to that of Forehand et al. [261] and Takamatsu et al.

[29] however in some cases the solvent benzene has been substituted with chloroform.

In 1984, Maher [266] reported a speciation method to determine the arsenic species in a marine sediment. The method involves extracting the arsenic from the solid phase using HCl and NaOH/NaCl solution. The extracted arsenic species are then separated by solvent extraction and ion exchange chromatography and determined by AAS. The sediment extract has been found to contain 45-90% inorganic arsenic (i.e. AsIII _and AsV), 4-39% monomethyl arsenic species and 4-22% as dimethyl arsenic species. However, the method does not enable the elucidation of whether the methylated species were due to material entering the sediments in this form (i.e. from anthropogenic inputs or from natural marine plankton or other organic remains) or whether they were

produced by chemical reactions within the sediment [28].

In a study conducted in 1992 [267], arsenic speciation analyses were conducted on soil samples that had been contaminated with an arsenic wood preservation material. A portion of each soil sample was extracted with water using ultrasonic treatment for a duration of 60 min. The establishment of the treatment was not stated in the reference. The mixture was filtered under pressure and then injected without further purification into a HPLC system for separation. Seven arsenic species were separated isocratically by anion- and cation- exchange HPLC in less than 4 min. Arsenic detection was carried out using an arsenic-specific FAAS detector with a hydrogen-argon flame that was optimized and modified for best achievable detection limits, and was interfaced on-line to the HPLC system. The advantage of this particular set-up is its simplicity and the common, moderately-priced equipment of which it is comprised. The detection limits obtained with the HPLC-FAAS system are sufficiently low for selected applications, includingarsenic speciation in contaminated soils. The arsenic speciation results obtained using the HPLC-FAAS were compared with results obtained from the same HPLC separation system, this time, interfaced to the more powerful ICP-MS detector. The comparison confirmed the data obtained with the HPLC-FAAS system, but also emphasized that the latter system can only be applied to certain practical arsenic speciation problems.

In some cases the analysis of solid extraction residues by X-ray absorption fluorescence spectrometry (XAFS) has also been used for arsenic speciation analysis [268]. The XAFS technique has also been used to determine arsenic oxidation states, local

coordination, and the relative proportion of different arsenic species in three California mine wastes [269]. X-ray absorption near edge structure (XANES) analysis indicated that AsV was the dominant oxidation state in the mine samples, but mixed oxidation states (nominally As0 and AsV) were observed in one of the three wastes [269].

In the late 1990s, a reported procedure for the extraction and separation of alkylarsenic species from Buffalo River Sediment standard reference material was developed [230]. The sample was sonicated in methanol-hydrochloric acid-water mixture (50:10:40% v/v) and filtered. The arsenic species in the extract were separated by anion exchange HPLC, using phosphate (pH = 6.0) and citrate (pH = 6.0) buffers as eluents, and detected by direct aspiration into an ICP-MS. Helgesen and Larsen [21] also used HPLC coupled with on-line ICP-MS to separate and selectively detect arsenic species in food and environmental samples (including soil samples) at their naturally occurring concentrations [270]. However, in this study, the soil samples were extracted using a calcium nitrate solution in which the samples were soaked for 1 h with gentle

mechanical shaking at room temperature. The supernatant was separated from the soil solids by centrifugation prior to injection into the HPLC-ICP-MS system for arsenic speciation determination. The anion-exchange HPLC column was eluted isocratically using an ammonium carbonate solution containing 97% v/v water and 3% v/v methanol (at pH = 10.3) as the mobile phase. The eluate from the HPLC system was

continuously introduced into the ICP-MS instrument that enabled the determination of AsIII, AsV, MMA and DMA.

In a more recent study completed by Bissen and Frimmel [24] the speciation of AsIII, AsV_{, MMA and DMA were reported by extracting the arsenic species from soil}

samples. The samples were sequentially extracted with 0.3 M ammonium oxalate (pH = 3, adjusted with 0.3 M oxalic acid), milli-Q water (pH = 5.8), 0.3 M sodium bicarbonate (pH = 8) and 0.3 M sodium carbonate (pH = 11). After the extraction procedure, the samples were filtered, diluted if necessary, and analyzed immediately by HPLC-ICP- MS. The separation of arsenic species was performed by an anionic exchange HPLC system containing a microporous resin bed consisting of ethylvinylbenzene crosslinked with 55% divinylbenzene having alkanol quaternary ammonium groups on latex

particles. The HPLC system was coupled with an ICP-MS detector (using NaOH as the mobile phase).

In 2001, a study [271] was conducted to determine the AsIII, AsV, MMA and DMA concentrations in three reference materials, namely river sediment, agricultural soil, sewage sludge, that had been certified for their total arsenic content. The analytical method included an ion exchange liquid chromatography separating device coupled on- line to a HGAFS detector. Prior to analysis, the arsenic species were extracted from the soils using orthophosphoric acid that was chosen as a “soft” extractant able to dissolve arsenic species without modifying them [272]. The efficiency of this extraction

procedure was studied in detail and was found to be more dependent upon the nature of the material analysed than on the acid concentration. Recoveries of 90–100% of total arsenic were obtained for the sediment and sludge reference materials whereas a 62% recovery was obtained for the soil reference materials. It was found that the method provides very low detection limits and has a high sensitivity for both the analysis of arsenic-poor samples and the dilution of arsenic-rich extracts. It was concluded that the proposed method has very good potential as a routine speciation analysis procedure for arsenic speciation studies in environmental solids. In a slightly earlier study, Thomas et al. [273] also used a phosphoric acid extraction method to speciate arsenic in soil and sediment samples. The extracts were then analysed using HPLC coupled to ICP-MS.

Another method reported for the speciation of arsenic in soils is based on the extraction of arsenic using phosphoric acid and ascorbic acid [274]. The arsenic extract is then analysed using a multiple hyphenated technique that includes the coupling of liquid chromatography (LC), ultraviolet (UV) irradiation, HG and ICP-MS. This speciation method has been applied to several contaminated soils. It showed that AsV is the main species in the soils and that in some samples AsIII and methylated species could also be detected. The same study also compared results obtained using the LC-UV-HG-ICP- MS speciation technique with those obtained using a varied speciation technique, LC- UV-HG-AFS. The findings revealed that LC-UV-HG-AFS is adequate for arsenic speciation in soils to detect both inorganic and organic species. However, the findings also indicated that the ICP-MS coupled detection system is much more sensitive.

In 2003, an alternative study [275] was reported that utilized HPLC coupled to ICP-MS. In this study the efficiency of consecutive extractions using several individual

extractants or solvent mixtures was evaluated. The extractants included: water, methanol/water and phosphoric acid for arsenic species extracted from rice, fish,

chicken tissue and soil samples. The analysis revealed the presence of AsIII, AsV, MMA and DMA in the soil samples and that 1 M phosphoric acid was found to be the most efficient extractant. It was also revealed that MMA and DMA are stable in 1 M phosphoric acid extracts from soils whereas AsIII gradually oxidizes to AsV.

Overall, it is evident that most speciation analyses are limited by the soil extraction procedure. The soil extractants used are not necessarily capable of extracting all the arsenic specifically from the soil and, even more importantly, there can be no certainty that the arsenic species remain in their indigenous form (i.e. extractants most likely alter the natural species in the soil upon extraction). More research is required for many of these extractants to validate their use in arsenic speciation analyses.

It would also be of great value to adopt a (validated) standard arsenic extraction method that can be used nationally or, possibly, globally for soil analyses. Currently, there are numerous extraction methods being used that have not been appropriately validated and so the results obtained may not be entirely reliable. It is clear from the review that, in terms of analytical instrumentation, the current technology is adequate for the speciation of arsenic extracts from soil samples (i.e. there is sufficient sensitivity and acceptable detection limits) however this may not the case for water samples and other food samples. Even though current instrumentation provides sufficient sensitivity and detection levels for soil arsenic extract analyses, most of the reviewed analytical instruments are expensive to purchase, maintain and run.