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Heavy metals are potentially toxic to human life and the environment. Metal toxicity depends on chemical associations in soils. For this reason, determining the chemical form of a metal in soils is important to evaluate its mobility and bioavailability. Sequential extraction was used to fractionate four heavy metals (Cd, Cu, Ni, and Zn) from nine contaminated soils into six operationally defined groups: water soluble, exchangeable, carbonate, Fe‐Mn oxide, organic, and residual. The residual fraction was the most abundant pool for all four metals examined. A significant amount (2.4–44%) of Zn was present in the potentially available fraction: nonresidual fraction. A major portion (40–74%) of Cu was associated with the organic, Fe‐Mn oxide, and carbonate fractions in most of the soils. Contamination of Cd and Ni in these soils was not as severe as Zn and Cu. Assuming that mobility and bioavailability of these metals are related to their solubility and geochemical forms, and that they decrease in the order of extraction sequence, the apparent mobility and potential bioavailability for these four metals in the soils were: Zn > Cu > Cd > Ni. Metal distributions in different chemical fractions in these soils depended on respective total metal concentrations, except for Zn.

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 ϩ 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 ϩ 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 ϩ 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 ϩ 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 (mi-Loring, 1989). This technique provides a rapid, safe, and crowave aqua regia ϩ 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. * Significant at the 0.05 probability level. ** Significant at the 0.01 probability level. *** Significant at the 0.001 probability level. † Arithmetic mean Ϯ standard deviation. ‡ Paired t-test (least significant difference) at ␣ ϭ 0.0...

Summary• Pteris vittata is the first plant reported to be a hyperaccumulator of arsenic (As), and little is known about the mechanisms of As hyperaccumulation in this plant.• Arsenic distribution at the whole plant (fronds) and cellular level was investigated using chemical analyses and energy dispersive X-ray microanalyses (EDXA). Speciation of As in the fronds was determined using X-ray absorption near edge spectroscopy (XANES) analyses.• The majority of As was found in the pinnae (96% of total As). The concentration of As in pinnae decreased from the base to the apex of the fronds. Arsenic concentrations in spores and midribs were much lower than in the pinnae. EDXA analyses revealed that As was compartmentalized mainly in the upper and lower epidermal cells, probably in the vacuoles. The distribution pattern of potassium was similar to As, whereas other elements (Ca, Cl, K, Mg, P and S) were distributed differently.• XANES analyses showed that approximately 75% of the As in fronds was present in the As(III) oxidation state and the remaining as As(V).