Comparison of Three Aqua Regia Digestion Methods for Twenty Florida Soils

soil and sludge samples based on it, in addition to the total elemental concentrations (Vercoutere et al., 1995; It is important to evaluate different digestion methods to accurately Quevauviller et al., 1993). In many countries (excluding determine elemental concentrations in soils. Three commonly used digestion procedures, hotplate aqua regia, microwave aqua regia, and the USA), this procedure is required by regulations to microwave aqua regia 1 HF, were compared for digestion of three estimate the impact of soil amendments such as sewage standard reference materials (SRMs 2704, 2709, and 2711) and 20 sludge on the environment (Krause et al., 1995; Marr Florida soils (three groups of Quartzipsammemts, one group of Alaet al., 1995; Vercoutere et al., 1995). However, this proquods, and one group of Paleudults). Sixteen elements (Al, Ba, Ca, cedure is very tedious and time-consuming (Siaka et Fe, K, Mg, Mn, P, Zn, As, Cd, Cr, Cu, Ni, Pb, and Se) were analyzed al., 1998). In addition, if open systems are used during using either an inductively coupled plasma optical emission spectromdigestion, there are risks of atmospheric contamination eter (ICP-OES) or a graphite furnace atomic absorption spectrophoand volatilization losses of volatile compounds during tometer (GFAAS). Precise analysis was achieved for all elements the oxidation of organic substances of the soils (Kubraexcept As (1.0–25%), Cd (1.8–22%), and Se (4.1–22%) in the SRMs kova, 1997; Nieuwenhuize et al., 1991; Quevauviller et using all three procedures, with the microwave aqua regia 1 HF procedure having slightly better precision (3.7%) averaged across all al., 1993). elements. Compared with the elemental recoveries in SRMs by the Since the 1980s, microwave-assisted sample digestion microwave aqua regia digestion (80%), microwave aqua regia 1 HF techniques have become popular and are widely used digestion achieved greater accuracy (94%), whereas the hotplate aqua (Chen and Ma, 1998; Quevauviller et al., 1993; Smith regia digestion achieved less accuracy (74%). In general, the microand Arsenault, 1996). Microwave-assisted aqua regia wave aqua regia 1 HF digestion was overall the best procedure for digestion using a Teflon bomb is considered a rapid determining concentrations of most metals in SRMs and Florida soils, sample digestion method (Marr et al., 1995; Nieuwenhufollowed by the microwave aqua regia and the hotplate aqua regia ize et al., 1991; Paudyn and Smith, 1992; Rantala and digestion. However, this so-called total–total digestion procedure (miLoring, 1989). This technique provides a rapid, safe, and crowave aqua regia 1 HF) may underestimate total Pb concentration efficient digestion and is not susceptible to losses of in Florida soils and total Al, Ca, Fe, Mg, and Ba concentrations in the Florida Paleudults. 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 A determination of potentially toxic trace and Stein (1983) reported that aqua regia extracted metals in soils is important for cleaning up contam,70% of Cd, Mn, and Ni from some sediments. Recovinated soils and monitoring land application of metalery of 43 to 77% for Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, bearing nonhazardous materials (Chen and Ma, 1998; and Zn from AGAL-11 reference sediment was also Chen et al., 1999). Sample digestion is often a necessary reported by Siaka et al. (1998). Paudyn and Smith (1992) step before determining metal concentrations in soils. found that the typical elemental recovery in ashes and The aqua regia (3:1, v/v, HCl to HNO3) digestion procesediments using aqua regia digestion was ≈80%, though dure (ISO standard 11466) is considered adequate for the recovery was higher for many elements (e.g., Al, analyzing total-recoverable heavy metals in soils of cerFe, Mg) using microwave rather than hotplate digestion. tain regions and is used to estimate the maximum eleNadkarni (1984) reported that a mixture of aqua rement availability to plants (Vercoutere et al., 1995). gia, HF, and H3BO3 provides satisfactory precision and Residual elements that are not released by aqua regia accuracy for dissolving silica matrices. The following reacdigestion are mostly bound to silicate minerals and are tions occur during digestion: 6HF 1 SiO2 → H2SiF6 1 2 considered unimportant for estimating the mobility and H2O (Wu et al., 1996) and H3BO3 1 3HF → HBF3OH 1 behavior of the elements (Niskavaara et al., 1997). 2H2O, and HBF3OH 1 HF → HBF4 1 H2O (Ryss, 1956). The conventional aqua regia digestion procedure conAdding boric acid in the second stage of the digestion sists of digesting soil samples on a hotplate with a 3:1 not only complexes the free fluoride ions in the solution, mixture of HCl and HNO3 (Nieuwenhuize et al., 1991). but also facilitates the dissolution of the precipitated The nitric acid reacts with concentrated HCl to form fluorides (Wu et al., 1996). However, when ICP-OES aqua regia: 3 HCl 1 HNO3 → 2 H2O 1 NOCl 1 Cl2. This is used for analysis, adding boric acid can create a matrix digestion procedure is so widely used that the European effect and was reported to result in a 20% decrease in Community Bureau of Reference has certified several sensitivity for Mo, Ni, Pb, Sb, Se, and Sn, and a 50% M. Chen, Institute of Geography and Natural Resources, Chinese and 70% decrease in sensitivity for P and S, respectively Academy of Sciences, Beijing 100101, China; L.Q. Ma, Soil and Water Science Dep., Univ. of Florida, Gainesville, FL 32611-0290. Approved Abbreviations: DDW, deionized, distilled water; GFAAS, graphite for publication as Florida Agricultural Experiment Station Journal furnace atomic absorption spectrophotometer; ICP-OES, inductively Series no. R-06802. Received 14 Sept. 1999. *Corresponding author coupled plasma optical emission spectrometer; NIST, National Insti([email protected]). tute of Standards and Technology; RSD, relative standard deviation; SRM, standard reference material. Published in Soil Sci. Soc. Am. J. 65:491–499 (2001).