Environ. Sci. Technol. 1996, 30, 392-399
Mineralogic Constraints on the Bioavailability of Arsenic in Smelter-Impacted Soils ANDY MARK GARY PAUL
D A V I S , * ,† M I C H A E L V . R U B Y , ‡ BLOOM,‡ ROSALIND SCHOOF,§ FREEMAN,| AND D. BERGSTROM⊥
Geomega, 2995 Baseline Road, Suite 202, Boulder, Colorado 80303, PTI Environmental Services, 4940 Pearl Circle East, Boulder, Colorado 80301, PTI Environmental Services, 15375 SE 30th Place, Suite 250, Bellevue, Washington 98007, Battelle, 505 King Avenue, Columbus, Ohio 43201, and Atlantic Richfield Company, 555 17th Street, Denver, Colorado 80202
Superfund risk assessments and the resulting soil arsenic (As) cleanup levels selected for mining sites are currently based on the toxicity of soluble As in drinking water. However, Anaconda soils and house dusts contain less soluble smelter-related As phases, consisting primarily of metal-arsenic oxides and phosphates. If accidentally ingested, As bioaccessibility is restricted by the sparingly soluble nature of As-bearing phases, the prevalence of authigenic carbonate and silicate rinds, the kinetic hindrance to dissolution, and the inaccessibility of encapsulated As. These limitations to As disolution explain the lower bioavailability factors observed for Anaconda Asbearing soils.
Introduction For arsenic (As), estimates of carcinogenic and noncarcinogenic risk derived from ingestion of As in soil currently assumes that soil As will be equally bioavailable as As in drinking water. However, recent animal studies in Cynomolgus monkeys using smelter-impacted soils and house dusts from Anaconda, MT, a smelting site from 1860 to 1980, have demonstrated that relative As bioavailability in these matrices is considerably lower (20-28%, respectively, based on urinary data) than from water (1, 2). These data help explain the lack of significant As impact on Anaconda children from soil exposure (3). While these and other investigations (4, 5) provide a compelling argument for reassessment of As toxicity, our investigation tested the hypothesis that arsenic bioavailability may explain the lower than anticipated As exposure from smelter-impacted soils. We hypothesized that reduced As bioavailability in mine and smelter wastes compared to an equivalent mass of As * Author to whom correspondence should be addressed; fax: 303938-8123. † Geomega. ‡ PTI Environmental Services, Boulder. § PTI Environmental Services, Bellevue. | Battelle. ⊥ Atlantic Richfield Company.
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dissolved in water was due to the lower solubility of As in soils and house dust and the kinetic constraints on dissolution of these phases during transit through the stomach [total residence time in humans of approximately 1-2 h; (6)] and small intestine, the primary site of As absorption [transit time in humans of 3-5 h; (7)]. The mineralogy and theoretical solubility of As phases in Anaconda soil and house dust are described in this paper, while the dissolution rate of As minerals in the GI tract and development of an in vitro screening test for As bioaccessibility are presented in a companion paper (8). Arsenic-bearing phases in Anaconda soils probably resulted from process wastes generated during historical smelting of copper sulfide ore in Anaconda (Figure 1). The ore contained ancillary As in enargite (Cu3AsS4), luzonite (Cu3AsS4), goldfieldite [Cu12(Te,Sb,As)4S13], tennantite (Cu12As4S13), and arsenopyrite (FeAsS) (9). Following crushing, ore was segregated into coarse and fine fractions (>0.2 and metal-arsenic sulfide > ironarsenic sulfate ) metal-arsenic silicate (Table 3). Both iron-arsenic oxide and manganese-arsenic oxide contained other metals, with relative abundances of Fe > Pb > Cu > As and Mn > Pb > Cu > As, while the arsenic phosphate phase consisted of a chemistry similar to
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FIGURE 3. Photomicrographs of characteristic mineralogy of residential soils, showing (a) a liberated Fe-arsenic oxide grain and slag particle containing 0.9 and 0.12 wt % arsenic, respectively, and (b) an FeSi core surrounded by hydrated FeSi containing 0.72% As.
FeAlPbAs(PO4)(SO4). The metal-arsenic oxides were composed mainly of Cu, Fe, and Pb in association with As in a silicate matrix, while the metal-arsenic sulfides generally
TABLE 4
House Dust Arsenic Mineralogy phase
I
II
III
sample IV
V
VI
VII
cumulative mineralogya
metal-arsenic oxideb metal-arsenic sulfideb metal-arsenic silicateb arsenic phosphateb iron-arsenic oxideb metal-arsenic sulfateb slagb
38 0 14 6 12 1 29
31 0 16 5 16 3 29
27 7 11 6 37 1 11
80 0 2 1 5 1 10
45 0 1 3 27 1 23
14 7 5 16 29 0 29
23 0 8 1 34 8 24
51 1 6 5 16 2 19
no. of particles counted arsenic concn (mg/kg)
163 115
117 585
53 320
228 338
100 171
136 146
85 187
a
Represents the As mineralogy of the seven house dust sets in aggregate.
b
Values represent percent of arsenic mass in arsenic-bearing phases.
FIGURE 4. Arsenic particle associations in Anaconda soil and house dust.
FIGURE 5. Particle size distribution of As-bearing phases in soil and house dust.
contained Cu, Fe, and Pb. Relative contributions (by mass) of As sources follow the order: smelting-derived (arsenic oxides and slag) > soil alteration phases (arsenic phosphate and ferromanganese oxides) > remnant ore concentrate (e.g., enargite).
TABLE 5
Using the classification scheme of Link et al. (30), the majority of As phases occur as liberated, cemented, and rimmed associations, with a high percentage of the ironarsenic oxide phases occurring as liberated particles (Figure 4). Encapsulation of Cu-As-Fe-S within slag and, at higher As concentrations, encapsulation of enargite (Cu3AsS4) within quartz were frequent, while authigenic weathering reactions consisted of arsenic phosphate and amorphous ferric hydroxide after pyrite and arsenic-iron oxide. Pyrite was also observed altering to As-bearing iron silicate, sulfate, and oxide, with As contributed either from surface-bound or sparingly soluble As-bearing phases. These associations, and their cemented occurrence, suggest that manganesearsenic oxide, iron-arsenic oxide, and arsenic phosphate represent continued evolution of As minerals in these soils, due to neutral pH values (6.0-7.4) and the presence of Fe, Mn, and P (Table 3), while the presence of enargite and metal-arsenic sulfides (frequently encapsulated in quartz) is due to residual process wastes. House Dust Mineralogy. The majority of As-bearing particles in the seven house dust samples were slag > ironarsenic oxide > metal-arsenic silicate > arsenic phosphate > metal-arsenic sulfate > metal-arsenic sulfide (Table 4). The sample blended for the As bioavailability study had a slightly different mineralogy (i.e., more As mass in the sulfide and less in slag and iron-arsenic oxide) than the average of the seven house dusts (Tables 4 and 5). Similarly, the soil blended for the As bioavailability study contained more As in the metal-arsenic silicate and less in the iron-arsenic oxide and arsenic phosphate than the average Anaconda soil (Tables 3 and 5). Primate Fecal Mineralogy. The mineralogy of the preingested composite soil and house dust samples were
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FIGURE 6. Fecal mineralogy showing alteration as a result of passage through the GI tract of (a) house dust, (b) house dust, (c) soil, and (d) soil.
similar (Table 5), explaining their similar absolute As bioavailability (between 20 and 28% expressed relative to soluble As in urine) when ingested by Cynomolgus monkeys (2). In both materials, the major As-bearing phase was liberated metal-arsenic oxide, with the cation consisting primarily of Cu, Fe, and Zn in varying proportions. Based on soil and fecal mineralogy, there was little change in the mass-based As phase distribution after transit through the GI tract, although the relative proportion of metal-arsenic sulfides, primarily enargite, increased slightly in both fecal soil and house dusts. Despite the small particle size of the As-bearing phases in both materials, many dust particles passed through the GI tract. These grains retained definitive weathering features, including botryoidal phosphate precipitates (Figure 6a), while other amorphous oxides retained much of their As content (Figure 6b). A similar lack of reaction was apparent in fecal soil where amorphous ferric hydroxide containing peripheral As (Figure 6c) and unreacted arsenic phosphate rinds (Figure 6d) were common. These data suggest that As bioavailability is primarily due to desorption and solubilization of the surface-bound fraction and not to dissolution of any discrete As mineral phase. The alternative hypothesis, that all As phases are dissolving at equivalent rates, is untenable due to the
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differing solubilities of the various As forms. The first hypothesis is also supported by the morphology of the fecal As-bearing particles, which remained resistant to dissolution under GI tract conditions (Figures 6a-d) and to the theoretical insolubility of the mineral phases (Figure 7). Solubility of As Phases in the GI Tract. Under atmospheric conditions, the experimental solubility of enargite at pH 3.5 has been measured at 32 mg/L, dissolving at a nearly linear rate over a 21-day period (31), which suggests limited bioaccessibility during its residence time in the human stomach (1-2 h) and small intestine (3-5 h). These data suggest that the bioaccessibility of liberated As-bearing phases is constrained by both kinetic factors and their relative solubility (Figure 7). The theoretical solubility of As in sulfides (enargite), complex phosphates, and slag (the latter due to encapsulation within a silicate matrix) is low (Figure 7). The good agreement between predicted and observed dissolution behavior in these studies suggests that the solid solution method is generally applicable to diverse glass compositions including slags and may be used to estimate their solubility when the bulk composition is known. Other As minerals exhibit amphoteric As solubility with minima from pH 3 to pH 7. Both arsenic phosphate (mimetite-pyromorphite) and oxide [(Pb,Cu)3(AsO4)2] are
FIGURE 7. Solubility of metal arsenates, mimetite-pyromorphite solid solution, enargite, scorodite, and slag as a function of pH. The hatched area between Cu3(AsO4)2 and Pb3(AsO4)2 represents the solubility range of a solid solution between these end members. Mineral formulas for these phases are provided in Table 1.
theoretically soluble at the pH of the stomach; however, kinetic constraints (i.e., 2-h stomach residence time) preclude ready dissolution of these phases based on fecal soil and house dust mineralogy (Figures 6a-d). In the alkaline small intestinal environment, no As phases exceed a theoretical solubility of 10-4 mg/L As (i.e.,
