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TECHNICAL REPORT
Heavy Metals in the Environment

Fractionation of Arsenic in Soil by a Continuous-Flow Sequential Extraction Method

^a Dep. of Chemistry, Mahidol Univ., Rama VI Road, Bangkok 10400, Thailand
^b Centre for Soil and Environmental Quality, Soil and Physical Sciences Group, P.O. Box 84, Lincoln Univ., Canterbury, New Zealand

* Corresponding author (scysw{at}mahidol.ac.th)

Received for publication October 23, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Batch sequential extraction techniques for fractionating metals or metalloids in soils are time consuming and subject to several potential errors. The development of a continuous-flow sequential extraction method for soil As is described and assessed, having the benefits of simplicity, rapidity, less risk of contamination, and less vulnerability to changes in extraction conditions compared with traditional batch methods. The validated method was used to fractionate soil As using water, NaHCO₃, NaOH, and HCl, followed by digestion of the residue with HNO₃ and HF acids. The extracts and digests were analyzed for As by graphite furnace atomic absorption spectrometry. Good recoveries of total soil As (97–115%) were obtained and fractionation data generally comparable with those obtained using conventional batch techniques. Soils from a tin-mining area in Thailand and soils from As-contaminated cattle (Bos taurus) dip sites in Australia were used to test the applicability of the method, and to demonstrate the usefulness of the extractogram obtained. The ability to produce detailed extractograms for As and other elements (Al, Fe, and Ca) enabled an examination of elemental associations in individual fractions. With the exception of As extracted with HCl, the extractograms generally support previous suggestions of the likely forms or associations of As present in the different soil fractions.

Abbreviations: CRM, certified reference material • GFAAS, graphite furnace atomic absorption spectrophotometry • SRM, standard reference material

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
MINERALS, metals, or metalloids, toxic or essential, are present in soils or sediments in various forms with varying bioavailability, toxicity, and mobility. Determination of total concentrations of these elements in solid materials is therefore considered to be of limited use in assessing potential environmental impacts. In order to assess their actual behavior, role, and impact, a good understanding of the chemical forms of the elements of interest is required. The use of sequential extraction techniques to fractionate metals in solid materials, and evaluate their potential effects, has become widely used and well recognized (Tessier et al., 1979). However, there are relatively few reports on the fractionation of metalloids such as As. Some reports have employed the extraction schemes originally designed for metals to fractionate both metals and metalloids. For example, Száková et al. (1999) used the European Community Bureau of Reference (BCR EUR) method (Ure et al., 1993) to evaluate the mobility of As, Cd, and Zn in soil, and Carbonell-Barrachina et al. (1999) have used the Tessier et al. (1979) scheme to fractionate As in sludge-amended soils. Other investigators believe that because As and P form similar oxyanions in the +5 oxidation state in soils (O'Neill, 1990), soil extraction schemes that have long been employed for P fractionation are more suitable for As (Jacobs et al., 1970; Woolson et al., 1971; McLaren et al., 1998).

Chemical fractionation (operationally defined chemical speciation by sequential extraction) has been widely accepted and applied. However, the technique has been questioned because of poor selectivity of reagents toward the targeted solid materials. Therefore, it is important that reagents are selected carefully considering the types of solid materials and the elements of interest being examined. Sequential extraction can also suffer from inaccuracy due to operational inconsistency during extraction and solid–liquid phase separation procedures (Sahuquillo et al., 1999; Quevauviller et al., 1993). Errors resulting from readsorption during extraction have also been reported (Kheboian and Bauer, 1987; Shan and Chen, 1993; Tu et al., 1994; Raksasataya et al., 1996; Lo and Yang, 1998).

Our previous work on development of a continuous-flow extraction system for sequential extraction has manifested many advantages compared with using a batch method (Shiowatana et al., 2001). For example, the flow system has the benefits of simplicity, rapidity, less risk of contamination, and less vulnerability to changes in extraction conditions. In a continuous-flow system, because elements brought into solution are continuously being removed from the system, there should be less opportunity for readsorption to occur. The system also has an additional advantage in that the extractograms obtained can provide useful information on the association of elements in each solid phase. In this study, the continuous-flow extraction system was applied to the sequential extraction of As using an extraction scheme modified from the work of McLaren et al. (1998). This scheme was selected because it was developed specifically for As fractionation and we had access to soil samples previously analyzed using this scheme. Graphite furnace atomic absorption spectrometry (GFAAS) was used to determine the concentration of As in the extracts and digests. As for extracting conditions, it has been shown (Maiz et al., 1997; Sahuquillo et al., 1999; Shiowatana et al., 2001) that reagent concentration can affect extractability for some elements and should be selected carefully. Recommended reagent concentrations are therefore strictly followed in this study. Studies of the effect of sample weight to extraction volume ratio on the extractability and speed of extraction were also carried out in this work to obtain optimal extraction conditions. Soil and sediment certified reference materials (CRMs) were used for the validation of the method. Comparisons of the sum of all fractions with the results of analysis of total concentrations, and with the certified values, were made.

The overall objective of this study was to examine the potential for using a continuous extraction technique for soil As in comparison with conventional batch extraction procedures. A secondary objective was to examine the use of the continuous-flow extraction technique for interpretation of elemental associations in the various soil fractions determined.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Continuous-Flow Extraction
The Extraction Chamber
An extraction chamber was designed to allow containment and stirring of a weighed sample of soil, and through which extractants could flow sequentially and leach As from the targeted phases. Chambers and their covers (Fig. 1) were constructed from borosilicate glass to have a capacity of approximately 10 mL. The outlet of the chamber was furnished with a filter (Whatman [Maidstone, UK] glass microfiber filter GF/B, 1-µm particle retention) to allow dissolved matter to flow through. Extractant was pumped through the chamber using a peristaltic pump (Ismatec SA [Glattbrugg-Zürich, Switzerland] MS-CA4/620C) at its highest rate, and using silicone tubing of 2 mm inner diameter.

View larger version (24K): [in this window] [in a new window]   Fig. 1. The continuous-flow extraction system.

Step 1: ultrapure water (water-soluble As).

Step 2: 0.5 M NaHCO₃ (surface-adsorbed As).

Step 3: 0.1 M NaOH (Fe- and Al-associated As).

Step 4: 1 M HCl (carbonate-bound As).

For the final step (residual As), the residue was removed from the chamber and transferred to a digestion vessel (UNISEAL Co., Haifa, Israel) of 20 mL capacity, and 5 mL HNO₃ and 5 mL HF were added. The vessels were then tightly sealed and heated in an oven at 150°C for 16 h. After cooling, the digested clear solutions were made up to volume in a 50-mL volumetric flask. Total As in separate samples (0.25 g) was determined using the same digestion procedure.

Batch Sequential Extraction
Step 1 (Water Soluble–Extractable Arsenic)
A sample of soil (1.00 g) was suspended in 30 mL of deionized water in a Nalgene polypropylene centrifuge tube (Nalge Nunc International, Rochester, NY) and shaken for 16 h. The soil suspension was centrifuged using a Hettich-Zentrifugen (Tuttlingen, Germany) Model D 7200 centrifuge at 3000 rpm for 15 min and the supernatant solution filtered through Whatman No. 40 filter paper.

Step 2 (Sodium Bicarbonate–Extractable Arsenic)
The soil residue from Step 1 was washed with 30 mL of water and centrifuged, and the washing discarded by decantation. The residue was resuspended in 30 mL of NaHCO₃ (0.5 M) and shaken for 16 h. The soil was then centrifuged as in Step 1 and the supernatant solution filtered through Whatman No. 40 filter paper.

Step 3 (Sodium Hydroxide–Extractable Arsenic)
The soil residue from Step 2 was washed with 30 mL of water and centrifuged, and the washing discarded by decantation. The residue was resuspended in 30 mL of NaOH (0.1 M) and shaken for 16 h. The soil was then centrifuged as in Step 1 and the supernatant solution filtered through Whatman No. 40 filter paper.

Step 4 (Hydrochloric Acid–Extractable Arsenic)
The soil residue from Step 3 was washed with 30 mL of water, centrifuged, and the washing discarded by decantation. The residue was resuspended in 30 mL of HCl (1 M) and shaken for 16 h. The soil was then centrifuged as in Step 1 and the supernatant solution filtered through Whatman No. 40 filter paper. The soil residue was oven-dried at 60°C for 48 h and then finely ground in an agate mortar.

Step 5 (Residual Arsenic)
Arsenic in the dried residue from Step 4 was determined using the HNO₃–HF digestion procedure as described above.

Analysis of Extracts and Digests
Arsenic, Fe, Al, and Ca concentrations in extracts and digests were determined using GFAAS and flame atomic absorption spectrometry (FAAS). The GFAAS measurements were performed with a PerkinElmer (Norwalk, CT) Analyst 100 equipped with a deuterium-arc background corrector and HGA-800 heated graphite atomizer. The sample was introduced to the atomizer using an AS-72 autosampler. For the measurement of As, an electrodeless discharge lamp operated at 300 mA and the 193.7 nm wavelength was used. The spectral bandwidth used was 0.7 nm. For Al and Fe, hollow cathode lamps were used and the wavelengths monitored were 309.3 and 248.3 nm respectively. Concentrations of metals were obtained by the standard calibration technique. Operating conditions for the GFAAS for As, Fe, and Al are shown in Table 1.

Preparation of Standard Solutions and Glassware
The chemical reagents in this work were of analytical grade. Ultrapure water from a MilliQ water purification unit (Millipore, Bedford, MA) was used throughout. All glassware used was previously cleaned and soaked in 10% HNO₃ and rinsed with ultrapure water before use. Standard metal solutions (1000 µg mL^-1) were purchased from Merck (Darmstadt, Germany) or prepared in-house from pure metals. Working standard solutions were prepared by diluting the stock solution with ultrapure water or with extracting reagents before use.

Certified Reference Materials (CRMs)
The CRMs were purchased from the National Institute of Standards and Technology (Gaithersburg, MD). Standard reference material (SRM) 2704 is a sediment (particle size 38–150 µm) from the Buffalo River; SRM 2710 and SRM 2711 are soils (particle size <74 µm) from Montana. The CRMs are carefully prepared to achieve a high degree of homogeneity and certified total elemental concentrations of a number of trace elements are provided. Therefore, they are suitable for using to validate the proposed procedure. Furthermore, the results obtained can be useful for future comparisons. The CRMs are guaranteed for homogeneity provided that analyses are performed using a minimum sample size of 250 mg.

Soil Samples
Soil samples were obtained from two locations. The first location was Ronphiboon, Thailand, which is a district well known for an As poisoning problem. The samples from Ronphiboon were sampled from a residential area and from a tin-mining factory. They were of sandy loam texture. The soils were separated into particle sizes of <106, 106 to 150, and >150 µm by sieving. The other location was Australia, at cattle dip sites where sodium arsenite had been used for many years to control cattle ticks (McLaren et al., 1998). Properties of soils from the cattle dip sites have been published previously (McLaren et al., 1998). All soils were stored in a desiccator until required.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Evaluation of the Continuous-Flow Sequential Extraction System for Arsenic
A typical extractogram for the four-step sequential extraction of As using the continuous-flow system is shown in Fig. 2 . The extractogram is a graphical plot of the As concentration in a subfraction versus the corresponding subfraction number. Ideally, a decrease in As concentration to zero by the end of each step would be indicative of the completeness of extraction. In practice, this did not always occur and for some fractions small As concentrations were still being detected when the following step was initiated. However, since these baseline concentrations of As were generally much lower than the peak concentrations, the resulting error in the concentration of As in the fractions would be relatively small. Figure 2 also shows the time elapsed between each subfraction (right-hand scale). It is obvious that Steps 1, 2, and 4 required less than 1 h for each step while Step 3 (0.1 M NaOH leaching) needed a considerably longer time of extraction. This was because the flow of the extractant decreased as a result of blockage of the filter following dispersion of the soil during the NaOH extraction. A similar problem of soil dispersion occurred in the batch system described by McLaren et al. (1998) and required a much coarser filter than that used for all other steps. In the current study, the speed of this rate-determining step depended very much on the type of sample being processed. Sample SRM 2711, shown in Fig. 2, was extremely slow compared with most other samples examined. Table 2 shows a comparison of the times required for extraction for two soils of different textures, and different particle-size fractions separated from the soils. The time required for the sequential extraction, particularly the time required for Step 3, clearly decreased with a decrease in particle size. This undoubtedly represents a potential problem for the continuous-flow system in that it counteracts one of the perceived benefits of the system (i.e., speed). It also means that extraction condition (contact time) will vary between samples. However, varied contact time due to varying flow rates to the degree experienced in this study did not appear to affect the amounts of As extracted (see next section). In the current study, total extraction times varied from 3 to 10 h.

View larger version (15K): [in this window] [in a new window]   Fig. 2. Continuous-flow As extractogram for standard reference material (SRM) 2711 (solid line). Elapsed time shown as dotted line; subfraction volume = 30 mL.

Effect of Sample Size
The effect of sample size was investigated because the extractability of soil As in batch extractions has been reported to be dependent on sample to extractant ratio (Davidson et al., 1999; Maiz et al., 1997). Using the 10-mL chamber, the fractionation of As in sample SRM 2704 was carried out using sample sizes ranging from 0.067 to 0.25 g. The results are shown in Table 3 and indicate that the sample to extractant ratio did not affect the total amount of arsenic extracted. Our previous study of the continuous-flow extraction of metals also showed that extractability was unaffected by the sample to extractant ratio or sample to chamber volume ratio (Shiowatana et al., 2001). This differs from the results observed using the batch technique (e.g., Davidson et al., 1999; Maiz et al., 1997) and emphasizes one of the benefits of the continuous-flow technique. In the batch system, extractions depend on the establishment of equilibrium between the soil and the extractant solution, and are quite likely to be influenced by the soil weight to extractant volume ratio. In such a system, as pointed out by some authors (e.g., Hall et al., 1996), complete extraction of the element of interest is unlikely without repeated equilibration with the same reagent. The flow system is not subject to the same constraint since the extracted elements are continuously being removed from the system. From the practical point of view, use of higher sample to chamber volume ratios can decrease the flow rate of the extractant and the extraction takes a longer time to complete. Conversely, decreasing the sample to chamber volume ratio can reduce the extraction time. However, if the sample weight used is too low, sample non-homogeneity can affect the analytical precision. On the other hand, using a larger chamber would result in high reagent consumption. For this reason, 0.25 g was selected as an optimum sample size for a 10-mL chamber volume, enabling reasonable total extraction times without substantially affecting analytical precision.

View larger version (36K): [in this window] [in a new window]   Fig. 3. Comparison of As fraction distribution in certified reference materials (CRMs) as determined by batch and continuous-flow techniques.

Clearly, although the total recovery of As using the flow system was good, further work is required to fully understand the reasons for the differences in As fractional distribution compared with results from the batch system.

Arsenic Fractionation of Contaminated Soils from Various Sources
The continuous-flow sequential extraction was applied to fractionate As in contaminated soil samples from two different locations. The samples were from Ronphiboon, an old tin-mining site in Thailand, and from cattle dip sites in Australia. The results of the arsenic fractionation of soil samples from the cattle dip sites using a batch system have been taken from an earlier report by McLaren et al. (1998). The results of As fractionation of samples from the two locations using both batch and flow methods are shown in Fig. 4 and 5 .

View larger version (49K): [in this window] [in a new window]   Fig. 4. Comparison of As fraction distribution in soils from Ronphiboon, Thailand as determined by batch and continuous-flow techniques.

View larger version (44K): [in this window] [in a new window]   Fig. 5. Comparison of As fraction distribution in soils from cattle dip sites in Australia as determined by batch and continuous-flow techniques.

Cattle Dip Sites, Australia
The results of the batch method for soil samples from cattle dip sites in Australia have been taken from earlier work of one of the authors (McLaren et al., 1998). Although the extraction scheme was slightly different from that used in the current flow method, the results can be compared by considering resin-extractable As in the batch method to be equivalent to water-soluble As in the flow method. Similarly, NaOH-extractable As plus sonicated NaOH-extractable As in the batch method can be considered as equivalent to the single NaOH-extractable As fraction in the flow method. On this basis, although there were clearly differences with some individual fractions, the flow and batch methods give generally similar fractional distributions of As (Fig. 5). This occurred in spite of the differences discussed above and the fact that analytical detection techniques differed between the two schemes, and the two sets of data were obtained by different personnel working in different laboratories. Only for soils from the two Lindendale sites was there a substantial difference in the total As recovery between the two methods.

Based solely on the similarity of results obtained using the two methods, as shown in Fig. 5, it is clearly impossible to show superiority of one method over the other. However, some of the benefits of using the flow system as discussed above, and the ability to produce detailed extractograms for As and other elements, may be a considerable advantage.

Association of Elements in Soil Fractions
One of the potential problems with any sequential extraction is the lack of specificity of extractants towards particular elemental forms or associations. Although most current fractionation schemes are operationally defined, it is still of benefit to have some appreciation of the "nominal" forms of elements likely to be present in each fraction. The extractograms produced using the continuous-flow fractionation system enable an examination of possible associations between elements, and therefore provide an insight into the sources of elements present in each fraction. Such detailed comparisons are not possible using batch fractionation data alone, where the only possible comparison is between the total concentration of elements extracted in a particular fraction. With the continuous-flow system, there is also the ability to compare peak positions and shapes between elements. Arsenic is known to be strongly associated with Fe and Al oxide materials in soils (e.g., Huang, 1975; Anderson et al., 1975; Goldberg, 1986), and by analogy with phosphate, and possibly with Ca. Figure 6 shows the extractograms for As, together with the corresponding ones for Fe, Al, and Ca for three different soil samples. These extractograms are representative of those observed with all the other samples analyzed in this study.

View larger version (29K): [in this window] [in a new window]   Fig. 6. Extractograms for As, Ca, Fe, and Al for Lindendale (20–40 cm) and Benaud (0–10 cm) cattle dip soils, and soil from a tin-mining factory.

	SUMMARY AND CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
The results obtained from this study demonstrate that the proposed continuous-flow extraction technique has the potential to be an effective and accurate method for fractionating As in soil samples. In many cases, the results obtained using the continuous-flow system are comparable with those determined using more conventional batch techniques, and good total recoveries of As are possible. Clearly, further investigation is required to determine the reasons why, with some samples, there is substantial variation between the results obtained using the two methods. Benefits of using the continuous-flow system compared with the batch system include the removal of errors associated with repeated centrifugation, filtration, and washing and minimization of potential re-adsorption problems. However, until flow rate conditions can be stabilized, exact replication of extraction conditions between samples will not be possible. However, varying extraction flow rates as a result of varying sample to chamber volume ratios do not have a marked influence on the concentration of As extracted. Variation in extraction flow rate resulting from particle-size differences is significant and its influence on the concentration of As extracted remains to be determined.

The time taken to process a single sample is much shorter than that required for batch systems; however, to obtain detailed extractograms, considerably more analytical determinations are required. It could be argued that the large number of analyses required for a full extractogram could result in the accumulation of analytical errors. However, in the current study, this did not appear to be a problem, with good agreement between the sum of As concentration in all fractions and certified total As concentration. Clearly, if the full extractograms were not required, a single analysis for each fraction would be sufficient. However, for detailed investigations, the full extractograms, particularly with interelement comparisons, can provide useful information not possible with batch techniques. The coincidence of extractogram peaks is much more revealing than a simple comparison of total amounts of elements present in a particular fraction. In this study, with the exception of As extracted with HCl, the extractograms obtained generally support previous suggestions of the likely forms or associations of As present in the different soil fractions. In the case of HCl, it would appear that the As extracted with this reagent is just as likely, or more likely, to be associated with Fe than with Ca.

	ACKNOWLEDGMENTS

 
Financial support from the Thailand Research Fund and partial support from the Postgraduate Education and Research Development Program in Chemistry, Higher Education Development Project, Ministry of University Affairs, are gratefully acknowledged. Comments from Ron Beckett (Monash University, Australia) and anonymous reviewers are appreciated. The authors would like to thank the Division of Soil Science, Department of Agricultural Science, Ministry of Agriculture and Cooperatives, and the Department of Biology, Faculty of Science, Mahidol University for provision of soil samples used in this study.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (20) Citing Articles via Google Scholar Google Scholar Articles by Shiowatana, J. Articles by Samphao, A. Search for Related Content PubMed PubMed Citation Articles by Shiowatana, J. Articles by Samphao, A. GeoRef GeoRef Citation Agricola Articles by Shiowatana, J. Articles by Samphao, A. Related Collections Heavy Metals Soil Pollution Soil Analysis