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a Institute of Geography and Natural Resources, Chinese Academy of Sciences, Beijing 100101, China
b Soil and Water Science Dep., Univ. of Florida, Gainesville, FL 32611-0290
Corresponding author (mchen{at}gnv.ifas.ufl.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DDW, deionized, distilled water • GFAAS, graphite furnace atomic absorption spectrophotometer • ICP-OES, inductively coupled plasma optical emission spectrometer • NIST, National Institute of Standards and Technology • RSD, relative standard deviation • SRM, standard reference material
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The conventional aqua regia digestion procedure consists of digesting soil samples on a hotplate with a 3:1 mixture of HCl and HNO3 (Nieuwenhuize et al., 1991). The nitric acid reacts with concentrated HCl to form aqua regia: 3 HCl + HNO3 
2 H2O + NOCl + Cl2. This digestion procedure is so widely used that the European Community Bureau of Reference has certified several soil and sludge samples based on it, in addition to the total elemental concentrations (Vercoutere et al., 1995; Quevauviller et al., 1993). In many countries (excluding the USA), this procedure is required by regulations to estimate the impact of soil amendments such as sewage sludge on the environment (Krause et al., 1995; Marr et al., 1995; Vercoutere et al., 1995). However, this procedure is very tedious and time-consuming (Siaka et al., 1998). In addition, if open systems are used during digestion, there are risks of atmospheric contamination and volatilization losses of volatile compounds during the oxidation of organic substances of the soils (Kubrakova, 1997; Nieuwenhuize et al., 1991; Quevauviller et al., 1993).
Since the 1980s, microwave-assisted sample digestion techniques have become popular and are widely used (Chen and Ma, 1998; Quevauviller et al., 1993; Smith and Arsenault, 1996). Microwave-assisted aqua regia digestion using a Teflon bomb is considered a rapid sample digestion method (Marr et al., 1995; Nieuwenhuize et al., 1991; Paudyn and Smith, 1992; Rantala and Loring, 1989). This technique provides a rapid, safe, and efficient digestion and is not susceptible to losses of volatile metals. However, the aqua regia digestion failed to accurately quantify more than 20 elements in some sediments (Krause et al., 1995), especially K and Al, which are part of some clay mineral structures. Berrow and Stein (1983) reported that aqua regia extracted <70% of Cd, Mn, and Ni from some sediments. Recovery of 43 to 77% for Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn from AGAL-11 reference sediment was also reported by Siaka et al. (1998). Paudyn and Smith (1992) found that the typical elemental recovery in ashes and sediments using aqua regia digestion was 
80%, though the recovery was higher for many elements (e.g., Al, Fe, Mg) using microwave rather than hotplate digestion.
Nadkarni (1984) reported that a mixture of aqua regia, HF, and H3BO3 provides satisfactory precision and accuracy for dissolving silica matrices. The following reactions occur during digestion: 6HF + SiO2 H2SiF6 + 2 H2O (Wu et al., 1996) and H3BO3 + 3HF HBF3OH + 2H2O, and HBF3OH + HF HBF4 + H2O (Ryss, 1956). Adding boric acid in the second stage of the digestion not only complexes the free fluoride ions in the solution, but also facilitates the dissolution of the precipitated fluorides (Wu et al., 1996). However, when ICP-OES is used for analysis, adding boric acid can create a matrix effect and was reported to result in a 20% decrease in sensitivity for Mo, Ni, Pb, Sb, Se, and Sn, and a 50% and 70% decrease in sensitivity for P and S, respectively (Paudyn and Smith, 1992). This procedure was modified by Ammons et al. (1995) for analyzing metals in soils with >45% sand. In the modified method, HF was added to the solid 16 h before the aqua regia addition and the mixture was then subjected to microwave heating.
All three digestion methods, that is, hotplate aqua regia and microwave aqua regia with or without HF, are commonly used as total or pseudo-total soil digestion methods by different researchers in environmental studies, especially in European countries. However, those methods are not widely used in the USA for analyzing soils and NIST (National Institute of Standards and Technology, Gaithersburg, MD) standard reference materials (SRMs), partially due to the establishment of the USEPA methods (Chen and Ma, 1998). The accuracy and precision of those methods in determining elemental concentrations in soils might vary with element, soil properties, digestion method, as well as origin of the soils (anthropogenic vs. natural deposits). For example, aqua regia digestion might give close results for the maximum levels of polluting metals such as Cd, Cu, Pb, and Zn in soils (Marr et al., 1995), while metals like Ba, Cr, and Ni could be efficiently recovered only by using HF digestion (Sawhney and Stilwell, 1994). Florida soils were formed primarily from sandy marine sediments and typically consisted of >870 g kg-1 sand (Table 1). These soils are prone to leaching and have extremely low baseline metal concentrations compared with soils from other parts of the world (Chen et al., 1999). Thus, great elemental recovery should be sought during method selection (Chen and Ma, 1998). The objectives of this study were (i) to compare the accuracy and precision of the three above-mentioned digestion methods in determining concentrations of different elements using three SRMs, and (ii) to study the variations in soil properties using 20 Florida sandy soils. It is hoped that such information will help to determine how the European regulatory methods work in analyzing trace metals in American soils and NIST SRMs. It will also assist scientists worldwide to choose the most appropriate technique for analyzing trace-metal concentrations in soils.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Microwave digestions were performed in a CEM MDS-2000 microwave oven (Matthews, NC). For aqua regia digestion, a sample to be analyzed needs to be randomly drawn from a well-mixed sample of 0.5000 g and weighed into a 120-mL Teflon-PFA microwave digestion vessel, then 12 mL of freshly prepared aqua regia solution (1/3 HNO3–HCl, v/v) was added. The samples were digested at 0.69 x 106 Pa for 5.5 min, then filtered through Whatman no. 42 filters, transferred to 100-mL volumetric flasks, and diluted with deionized, distilled water (DDW). For aqua regia + HF digestion, 4 mL of concentrated HF were added to the soil sample and were allowed to react overnight at room temperature. Following the initial reaction period, 12 mL of aqua regia were added to the sample. The samples were then digested at 0.83 x 106 Pa (120 psi) for 20 min. After the digestion, 2 g of boric acid were added to neutralize excess HF before analysis. The samples were then filtered through Whatman no. 42 filters and diluted to 100 mL with DDW.
Conventional aqua regia digestion was performed in 250-mL glass beakers covered with watch glasses. A well-mixed sample of 0.5000 g was digested in 12 mL of aqua regia on a hotplate for 3 h at 110°C. After evaporation to near dryness, the sample was diluted with 20 mL of 2% (v/v with H2O) nitric acid and transferred into a 100-mL volumetric flask after filtering through Whatman no. 42 paper and diluted to 100 mL with DDW.
Nitric acid, HCl, HF, and H3BO3 of trace-metal grade and standard solutions of Ca, Mg, K, P, Al, Fe, As, Ba, Cd, Cr, Cu, Pb, Mn, Ni, Se, and Zn were obtained from Fisher Scientific (Fair Lawn, NJ). Working standards were prepared daily by dilution from the stock solutions with digestion blanks. Instrument calibration standards were obtained from Spex Industries (Edison, NJ). Aluminum, Ba, Ca, Fe, K, Mg, Mn, P, and Zn were analyzed on a multi-channel Jarrell-Ash ICP-OES 61-E unit. Arsenic, Cd, Cr, Cu, Ni, Pb, and Se were determined on a Perkin-Elmer SIMAA6000 GFAAS unit. Each NIST SRM was digested in four replicates. In the stage of digestion, four quality-assurance samples (a blank, a duplicate, a spike, and an SRM) were included with every 20 soil samples (Chen, 1997). More (30%) quality-control samples (analytical blanks and calibration standards) were included in the determination of elemental concentrations in the digestion solution using either an ICP-OES or a GFAAS (Chen, 1997).
Precision was defined as relative standard deviation (RSD), which was calculated as a percentage using the standard deviation divided by the mean of replicated samples. Accuracy was determined by comparing the measured concentration with the certified value and was expressed as percentage recovery (%R). Based on a Florida Department of Environmental Protection-approved research quality-assurance plan, satisfactory precision and accuracy were required to be within 
20% and 80 to 120% for all elements, respectively, which corresponded to the uncertainty of the NIST-certified values, according to a 95% confidence interval for the true values (Chen, 1997). In the present study, both accuracy and precision for all elements analyzed by the ICP-OES and GFAAS were acceptable, with matrix spike recovery being 90–110% and precision being <15%.
All statistical analyses were performed using a PC version of the SAS program (ASA Institute, 1987). Analysis of variance was used to assess significant differences between treatments using a simple paired t-test at a confidence level of 
= 0.05. Simple correlation analysis was used to determine method correlation coefficients for each element at probability levels of = 0.1, 0.01, and 0.001, respectively.
Percent biases were used to compare magnitude and consistency of these methods for analyzing individual metals. The bias between hotplate and microwave aqua regia digestion methods was defined as the concentration difference between the two methods divided by the concentration of the hotplate digestion. As such, a positive bias between the two methods implied that the concentration obtained from the microwave aqua regia digestion was greater than that from the hotplate aqua regia digestion. The bias between the microwave aqua regia and aqua regia + HF digestion was defined similarly.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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20% (Table 2). Poor precision for Se in SRM 2704 may be attributed to low Se concentration (1.16 mg kg-1) in the sample (Berrow and Stein, 1983; Binstock et al., 1990), which is 19.3x the detection limit for Se (0.06 mg kg-1) using GFAAS. This will possibly make a relatively large contribution to imprecision of the analysis. Also, chemical interference due to volatilization losses for highly volatile elements include Pb, Cd, Zn, As, and Se is problematic for GFAAS (Sawhney and Stilwell, 1994), and is possibly another reason that contributed to the poor precision of As and Se in SRM 2704. For both digestion methods, four elements (Al, K, Cd, and Ni) showed relatively large RSD% (>10%) in addition to As and Se, whereas seven elements, Ca, Fe, Mg, P, Mn, Pb, and Zn, showed excellent precision (<5%) for all three SRMs. This is generally consistent with results reported by Nieuwenhuize et al. (1991), who indicated that RSD values for Fe, Mn, Pb, and Zn were <12% when using microwave aqua regia digestion.
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Satisfactory precision was achieved for all elements in all three NIST SRMs by using the microwave aqua regia + HF digestion, except Cd in SRM 2709 (22% RSD) (Table 2), which had much lower certified value (0.35 mg kg-1) than SRMs 2704 (3.45 mg kg-1) and 2711 (41.7 mg kg-1) (Table 3). The low Cd concentration plus possible volatilization losses of Cd by the GFAAS determination (Sawhney and Stilwell, 1994) would make SRM 2709 perform differently from the other two SRMs. The precision for all elements was close to 5%, except for Cd, Ni, and Se, which were analyzed by GFAAS. High precision for analyzing As, Al, Ca, Cr, Cu, Fe, Mg, Mn, P, Pb, and Zn in marine sediments using this method has been reported by Paudyn and Smith (1992). Compared with the microwave aqua regia digestion procedure, the microwave aqua regia + HF procedure improved precision for more than one-half of the 48 analyses (16 elements x 3 RSMs) (Table 2). The former method provided better precision for Mg, Ni, and Zn, and the latter for Al, K, Ba, and Pb. In addition, the average precision for the microwave aqua regia + HF digestion over 16 elements (3.7%) was better than the microwave aqua regia (5.2%) and the hotplate digestion method (4.6%, Table 2). This is consistent with the results of Marr et al. (1995) and Kackstaetter and Heinrichs (1997). They reported that in most cases the RSD values for both aqua regia digestion procedures were quite low, rarely >6% (Marr et al., 1995). Exceptions were observed for Pb, with 7% error, and As, with >12% error, due to their volatile characteristics (Kackstaetter and Heinrichs, 1997).
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Low Cr recovery from soil and other materials has been attributed to the presence of insoluble Cr minerals such as chromspinels and chromite (FeCr2O4), and aqua regia was inefficient in dissolving these minerals (Liu et al., 1996; Medved et al., 1998). Low Cr recovery from river sediments AGAL-10 and AGAL-11 by both hotplate (36%) and microwave (43%) aqua regia digestions has been reported (Siaka et al., 1998). Low Cr recoveries (23–74%) from standard and sewage-sludge-amended soils have also been reported (Heydron and Damsgaard, 1995).
Compared with hotplate aqua regia digestion, microwave aqua regia digestion increased elemental recovery for all elements except Cr, Ni, and Se (Table 3). Such improvement was especially obvious for Al, K, Ba, Cd, Cu, and Pb as reflected by their relatively large positive biases (10–136%). In addition, the average recovery over 16 elements for the microwave aqua regia (79.9%) was slightly greater than that of the hotplate aqua regia (74.3%) digestion procedure (Table 3). The high efficiency of the sample digestion using the microwave method has been attributed to fragmentation of the solid materials due to the high pressures, which exposed fresher and finer portions of the sample to acid attack (Nadkarni, 1984). Significantly higher values were found by Nieuwenhuize et al. (1991) for Cr and Fe in SRMs after microwave aqua regia digestion than hotplate aqua regia digestion. Paudyn and Smith (1992) noted that for many nonvolatile elements, the two methods provide similar recovery except for Al, whose recovery was greater using the microwave digestion, which is consistent with our result (Table 3).
Accurate results were obtained for all elements except Cr and Ni in three SRMs using the microwave aqua regia + HF digestion (Table 3). Low Cr recoveries in NIST SRMs 2709, 2710, and 2711 (75–89%) using microwave HNO3–HF digestion (Wu et al., 1996) and in USGS standard rocks using microwave aqua regia + HF digestion have been reported (Nadkarni, 1984; Lamothe et al., 1986). A sequential extraction study on industrially contaminated soil indicated that both Cr and Ni were mainly associated with the residual fractions (Davisson et al., 1998). Medved et al. (1998) found that Cr is bound to insoluble minerals such as chromspinels and chromite in soils. As indicated by Marr et al. (1995), even aqua regia + HF digestion was inefficient in dissolving these minerals. However, complete recoveries for Cr and Ni from marine and river sediments by microwave aqua regia + HF digestion have been reported (Rantala and Loring, 1989) and were attributed to a strong dissolution technique being adopted (i.e., more acid and longer heating times) (Bettinelli et al., 1987). Similar results for Cr from NIST SRM 2704 were reported when using microwave HNO3–HF–HClO4 digestion (Liu et al., 1996). The authors noted that satisfactory performance could be achieved if the element was present in a relatively labile form, which was consistent with our results in the case of river sediment NIST SRM 2704 (89% for Cr and 91% for Ni respectively, Table 3). For the other two NIST SRMs (2709 and 2711), however, incomplete dissolution of Cr (66–77%) and Ni (79–80%) minerals was obtained (Table 3), which implies that Cr and Ni in the two standard soils may be present as insoluble minerals.
Compared with the microwave aqua regia digestion (Table 3), microwave aqua regia + HF digestion improved accuracy for Al, Ca, K, As, Ba, Cr, Cu, Se, and Zn in the three SRMs, as reflected by their large positive biases and the closeness to the certified values (Table 3). These results are consistent with a sequential extraction study of Ca, Fe, Cr, Mn, Pb, and Zn in sediments by Mahan et al. (1987), who indicated that an overall superior recovery of metals was obtained by using the aqua regia + HF digestion. Among the three digestion procedures, as expected, microwave aqua regia + HF digestion achieved the overall best average elemental recovery (94%) for 16 elements in three SRMs followed by the microwave (80%) and hotplate (74%) aqua regia digestions (Table 3). Our results are consistent with those of Kokot et al. (1992) who demonstrated statistically that aqua regia + HF was the preferable procedure for digesting NIST SRM 2704. Among the three SRMs tested, SRM 2711 showed the lowest average recovery (79%) over 16 elements using three digestion methods, whereas SRMs 2704 (85%) and 2709 (84%) showed comparable recovery (Table 3).
Comparison of Three Digestion Procedures for Analyzing 16 Elements in Florida Soils
Average Elemental Concentrations Based on 20 Florida Soils
Greater values of all elements except for Ca, P, Cd, and Se were obtained in the 20 Florida soils using microwave rather than hotplate aqua regia digestion. The most notable were K, Cr, Cu, and Ni as reflected by their large positive biases (>116%, Table 4). However, significant improvement in digestion efficiency was only observed for Al, Fe, K, Mg, Ba, Pb, and Zn, when using a simple t-test at = 0.05. This indicates that with the help of the microwave procedure, aqua regia was more effective in dissolving these elements from Florida surface soil horizons than was open digestion. Smaller, lower volatilization losses of elements with the microwave technique than the hotplate is possibly another reason. In addition, analytical results for the GFAAS determination of As, Cd, Cr, Cu, Ni, Pb, and Se provided low acid blank values for Cd and Pb. This is consistent with Chakraborty et al. (1996), who reported that the microwave digestion procedure should be used when the risk of environmental pollution is high. Similarly, significantly greater recoveries of Cr and Fe from Dutch soils were obtained using the microwave technique than using the hotplate aqua regia digestion (Nieuwenhuize et al., 1991). The biases for concentrations of Ca and Se between the two digestion procedures were negligible (<10%) (Table 4).
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Significant positive correlation coefficients were found for all elements except for Ni in 20 Florida soils between the two aqua regia digestion procedures and between the two microwave digestion procedures (Table 4). This suggests that similar biases between these methods would be expected if they were used to determine concentrations of these 16 elements in other Florida soils (except for Ni). For example, on average, 21% more Ba was released by the microwave than the hotplate aqua regia digestion and 62% more K was released by the microwave aqua regia + HF than the aqua regia digestion for Florida soils (Table 4). The discrepancy in elemental recovery of Ni among the three methods may be attributed to certain soil matrices, such as chromite, aluminosilicate minerals, and organic matrix, which cannot be decomposed by either aqua regia or aqua regia + HF (Wen et al., 1997; Wu et al., 1996). However, further investigation is needed to identify the possible soil matrices that exist in Florida soils.
The biases between hotplate and microwave aqua regia digestion procedures for the three NIST SRMs, compared with the microwave aqua regia and aqua regia + HF digestion procedures for 20 Florida soils for the 16 elements, were obviously different. Greater biases were obtained between the first two methods for elements Fe, Mg, As, Cr, Cu, Mn, Ni, Pb, and Zn and lower biases for elements Al and Ba in the 20 Florida soils than the three NIST SRMs (Tables 3 and 4). On the other hand, greater biases were obtained between the second two methods for elements Cr, Cu, Ni, Se, and Zn and lower biases for elements Al, K, Ba, and Pb in the 20 Florida soils than in the three SRMs (Tables 3 and 4). The NIST SRMs were ground finer (100 mesh for SRM 2704 and <200 mesh for SRMs 2709 and 2711) than the Florida soils (60 mesh), and that probably improved the overall precision of the analysis for those materials. In addition, soil properties are important in determining elemental recovery during digestion. Elemental recovery may vary from soil to sediment and other material, even using the same digestion procedure. Otherwise, biases between different digestion procedures in NIST SRMs and Florida soils should be comparable.
Average Elemental Concentrations of 20 Florida Soils in Five Groups
To better understand how elemental concentrations vary with soil properties and digestion method, concentrations of 16 elements in 20 soils of five great groups were determined using the three aqua regia digestion methods (Table 5). Based on how elemental concentrations changed with the three digestion methods and soil properties, the 16 elements can be divided into four groups.
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Elemental concentrations in the second group of elements, including Al, Ca, Fe, Mg, and Ba, generally increase as the three digestion methods became more aggressive in four groups of soils (the Alaquods and the Quartzipsamments) but not in the Paleudults. For the Paleudults, elemental concentrations of the second group increased as digestion methods changed from hotplate to microwave aqua regia; however, they decreased unexpectedly as the digestion methods changed from microwave aqua regia to aqua regia + HF (Table 5). The difference between the latter two digestion procedures was the addition of HF, with a longer reaction time and a higher temperature, which was intended to aid in the decomposition of Al–Si minerals (Nadkarni, 1984; Quevauviller et al., 1993). The effectiveness of HF in decomposing Al–Si minerals was demonstrated in the three SRMs as reflected by the mean bias of +79% in Al concentrations between the microwave aqua regia and aqua regia + HF digestion methods (Table 3). In addition, the mean total Al concentrations in the three SRMs (67.1 mg g-1, Table 3) were approximately 28 times greater than the mean total Al concentrations in the 20 Florida soils (2.44 mg g-1, Table 4). The extremely high quartz contents (identified by x-ray diffraction) in the 20 Florida soils probably made them behave differently from the three SRMs. Paleudults contained relatively high Al concentrations (Table 5) compared with the other four soil groups and extremely high quartz contents compared to the three SRMs, which may have caused significant Al concentration reductions in the presence of HF. It was possible that a mixture of Al–Si–F minerals precipitated during the digestion process (Hingsten et al., 1972). It was also possible that Ca, Fe, Mg, and Ba coprecipitated with the Al–Si–F minerals, thus reducing their concentrations (Table 5). However, no minerals except quartz were identified by x-ray diffraction in the residuals of the Paleudults after microwave aqua regia + HF digestion (data not shown).
Lead is the only element of the third group. Its concentrations in all soil groups increased as the digestion method changed from hotplate to microwave aqua regia digestion. But it decreased as the digestion method changed from microwave aqua regia to aqua regia + HF (Table 5). It seemed that total decomposition using microwave aqua regia + HF method would underestimate total Pb concentrations in Florida soils. This was also confirmed by the paired t-tests at = 0.05 (Table 4). Yet such a pattern was not observed for the three SRMs (Table 3). It is speculated that formation of Pb2SiO4 may have caused the reduction in Pb concentrations using the microwave aqua regia + HF digestion. Generally speaking, Pb2SiO4 is a relatively soluble Pb mineral and thus is not normally present in soils (Lindsay, 1979). However, in the presence of HF and under high temperature and pressure, larger amounts of SiO2 may have been dissolved to form SiO4-4 to react with Pb2+ to form Pb2SiO4. Since Pb concentrations in Florida soils were <4 mg kg-1 (Table 4), it was not possible to identify this mineral in the residual fraction after digestion. Wen et al. (1997) suggested that the low Pb recovery from sediment SRM 280 after aqua regia + HF digestion is possibly due to PbCl2 formation, which also had a low solubility. No pattern was observed in concentration changes with the three digestion methods for elements in the fourth group including P, As, Cd, and Se. There is possibly the problem of the detection limits and sensibility of the instruments used to determine the elemental concentrations (P, As, and Se). At a low concentration range (<50x the detection limits), it is hard to determine the improvement of either precision or accuracy of the analysis by varying digestion procedures.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In terms of method correlation coefficients and elemental recoveries of the target elements in 20 Florida surface soils, microwave aqua regia improved digestion efficiency for Al, Fe, K, Mg, Ba, Pb, and Zn, when using t-tests at = 0.05. Microwave aqua regia + HF improved results (at the = 0.05 level) only for K, Ba, Cu, and Zn, but not for the other 12 elements. However, according to the digestion efficiency in 20 different Florida soils, the elements were separated into four groups: Group 1 (K, Cr, Cu, Ni, Mn, and Zn), Group 2 (Al, Ca, Fe, Mg, and Ba), Group 3 (Pb), and Group 4 (P, As, Cd, and Se). The microwave aqua regia + HF procedure underestimated total concentrations of Group 3 in all 20 Florida soils and total concentrations of Group 2 in four Florida Paleudults.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication September 14, 1999.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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