Environ. Sci. Technol. 2007, 41, 7851-7856
Bioaccessibility of Metals in Dust from the Indoor Environment: Application of a Physiologically Based Extraction Test ANDREW TURNER* AND KA-HEI IP School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK
A physiologically based extraction test, simulating sequential digestion in the stomach and intestine, has been applied to dust samples collected from various domestic and working settings to define bioaccessible concentrations of metals (Al, Ca, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sn, U, Zn) in the indoor environment. With the exception of Ca, Cd, and Zn in the stomach phase, mean bioaccessibilities (relative to respective total metal concentrations) were less than 50%. For a given metal, bioaccessibility in either phase was variable among samples but, in many cases, displayed an inverse dependence on total concentration. This suggests that, to a good approximation, variations in both metal contamination and accessibility in the indoor environment arise from variable proportions of metal-rich particulates of low digestibility. Compared with accessibility in the stomach, accessibility in the more alkaline, carbonaterich intestine was either lower (Al, Ca, Cd, Mn, Ni, Sn, Pb, Zn), similar (Co, Cu, Fe) or greater (Cr, U). We attribute these observations to precipitation and/or readsorption in the intestine, stabilization by complexation, or anion-like adsorption of negatively charged, polyatomic species, respectively.
Introduction Indoor dust is a heterogeneous assortment of particles derived from exfoliated skin, micoorganisms, human and animal hair, clothing and carpet fibers, paint chips, paper, food, rubber, cosmetic products, plastics, ash, building materials, cooking and heating activities, garden soil, road dust, vehicle exhausts, combustion products, and a multitude of more specific occupational practices (1-4). The chemical composition of dust reflects the makeup and relative abundance of these components and is, therefore, highly variable. With respect to heavy metals, like Pb, Cr ,and Cd, analysis of dust indicates concentrations that are well in excess of crustal and soil levels, at least when normalized by some geochemical means (1), but a high degree of variation among sample populations (5, 6). Despite obvious implications for human health, however, the bioaccessibility (or solubility in the gastrointestinal environment) of metals in indoor dust has received relatively little attention compared with other environmental solids like soils (7-11). First-order evaluations of bioaccessibility in dust have been based largely on extractability in dilute HCl, with or without the digestive enzyme, pepsin (6, 12, 13). However, such an approach * Corresponding author phone: +44 1752 233041; fax: +44 1752 233035; e-mail:
[email protected]. 10.1021/es071194m CCC: $37.00 Published on Web 11/13/2007
2007 American Chemical Society
appears to overestimate metal bioaccessibility because, first, the pH and time scale typically employed are at the lower and upper extreme, respectively, involved in digestion, and second, neutralization in the intestine, where metals are usually absorbed into the epithelium, is not considered. Application of an in vitro physiologically based extraction test (PBET), mimicking sequentially the chemical conditions in the gut and intestine, would overcome these limitations and provide valuable mechanistic information on metal mobilization from dust in the gastrointestinal tract. To our knowledge, however, such tests have only been applied to a very limited number of indoor samples and where either Pb or As has been the sole focus of study (14-17). The present study aims to further our general understanding of the exposure and risk imposed by metals in the indoor environment by applying a PBET that simulates human gastrointestinal physiology to a variety of dust samples. We examine metals that are intrinsically toxic (e.g., Cd, Cr, Pb), as well as those that are of more geochemical significance (e.g., Al, Ca, Fe) to elucidate potential sources and mechanisms of metal mobilization in the gastrointestinal tract.
Experimental Section Sampling. Samples were collected from seven private households (PH-1 to PH-7) within a 10 km radius of the city center of Plymouth, UK (population 250 000), and from four buildings on the University of Plymouth Campus; namely, a carpeted library (LY), a modern, carpeted administrative building (AB), and two uncarpeted (mainly linoleum-based) technical buildings housing laboratories for teaching and research purposes (TB-1 and TB-2). To obtain sufficient material for characterization, composite samples from each location were collected by the different participants using their own vacuum cleaner fitted with a fresh bag or a precleaned cylinder (18). Samples were subsequently sieved through a 63 µm Nylon mesh with the aid of a Nylon brush and the fine fractions (ranging from about 0.7 to 3 g) stored in individual zip-lock bags under desiccation. The cutoff employed is close to the average value of the various upperbound size definitions of dust (19), and is consistent with a particle size cutoff commonly adopted in geochemistry (20). Sample Digestion. All glassware and plasticware were soaked in 10% HCl for at least 24 h and rinsed with copious quantities of Milli-Q (Millipore) water before being used. Reagents were purchased from Merck or Sigma and were of analytical grade or equivalent. (i) Total Digestion. Total acid-extractable metal concentrations in the samples were determined after digestion in aqua regia (6, 14). Thus, portions of about 0.05 g were accurately weighed into 50 mL Pyrex beakers to which 10 mL of a 3:1 mixture of HCl:HNO3 were added. Each beaker was covered with a watch glass and heated to 80 °C for 3 h on a hot plate before being allowed to cool. Digests were then transferred to 25 mL Pyrex volumetric flasks and diluted to mark with Milli-Q water. Procedural controls were performed likewise in the absence of solids. Metal extraction efficiency was evaluated by digesting a certified reference soil (SRM 2711; National Institute of Standards and Technology, Gaithersburg, MD) in triplicate as above. Accuracy was estimated from triplicate digestions of a river sediment sample certified for metal concentrations available to aqua regia (LGC 6187; Laboratory of the Government Chemist, Teddington, UK). (ii) PBET. The physiologically based extraction test employed in this study is based on the analog for the human VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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gastrointestinal tract developed by Ruby et al. (7), and as subsequently modified for practical purposes by Rodriguez et al. (9) and Cave et al. (10). The gastric solution was prepared by dissolving 1.25 g of pepsin A (from porcine stomach mucosa), 0.5 g of sodium malate, 0.5 g of sodium citrate, 420 µL of lactic acid, and 500 µL of acetic acid in 1 L of Milli-Q water whose pH was subsequently adjusted to 2.5 by dropwise addition of HCl. The working pH represents “average” gastric conditions, or a nutritional status intermediate between fasting and fed states (7). Digestion of dust in the stomach was simulated by accurately weighing about 0.5 g of sample into a 125 mL screw-capped, wide-necked HDPE bottle to which 100 mL of gastric solution were added. The contents were then placed on an end-over-end shaker within a water bath at 37 °C for 2 h. Subsequently, a 5 mL aliquot, whose pH differed by no more than 0.2 units from the original value, was pipetted into a 60 mL polypropylene centrifuge tube and the contents centrifuged at about 3500 rpm (or 2100g) for 10 min. The supernatant was transferred to a 25 mL Pyrex volumetric flask before 5 mL of Milli-Q water were added to the residue and the contents recentrifuged. The second supernatant was added to the flask which was then diluted to mark with Milli-Q water and labeled as stomach phase (SP). Meanwhile, to simulate digestion in the small intestine, the remaining contents of the HDPE bottle were titrated to pH 7 with saturated sodium bicarbonate solution before 175 mg of bile salts and 50 mg of pancreatin (both porcine) were added. Following incubation at 37 °C for 2 h, a 5 mL aliquot was removed, centrifuged, and transferred to a volumetric flask (as above) before being acidified with a few drops of HCl and labeled as intestinal phase 1 (IP1). To check for any kinetic effects, the remaining contents of the bottle were incubated for a further 2 h before an additional aliquot was processed likewise and labeled as intestinal phase 2 (IP2). Procedural controls for each phase were performed by triplicate, sequential digestions in the absence of solids. Metal Analysis. Concentrations of Ag, Cd, Co, Ni, Cu, Pb, Sn, and U were determined in all digests by inductively coupled plasma mass spectrometry using a Plasma Quad PQ2+ (Thermoelemental, Winsford, UK) fitted with an Ebdon nebulizer. The instrument was calibrated with mixed standards in either 0.3 M HNO3 (aqua regia digests) or simulated gastric solution (PBET digests), and 115In and 193Ir were added to all samples and standards to correct for detector drift and variations in plasma conditions. A standard was inserted after every ten samples as a check, and the instrument was flushed with 0.3 M HNO3 between samples and standards. Concentrations of Al, Ca, Cr, Fe, Mn, and Zn were determined in all digests by inductively coupled plasma atomic emission spectrometry using a Varian 725 ES (Mulgrave, Australia) calibrated with mixed standards prepared in 0.3 M HNO3. Metal concentrations were detectable (greater than three standard deviations of concentrations in the respective procedural controls) in all cases with the exception of Cd, Co, Cr, Cu, Ni, Sn, and U in some and Ag in all PBET digests. Precision, based on analyses of triplicate digestions of selected samples (i.e., those where sufficient mass was retrieved), was better than 15% in aqua regia digests, but was highly variable in the PBET digests (between about 5 and 40%, depending on the sample and metal). Accuracy was generally better than 5%, and extraction efficiency from the reference soil was greater than 80% for all certified metals except Al (25%) and Fe (60%). Presumably, the relatively low recovery of these metals reflects their association with refractory mineral phases (e.g., aluminosilicates and crystalline oxides) in soil particles; that said, however, indoor dust is typically composed of a greater proportion of more soluble, Ca-rich minerals (14, 21), and metals are likely to be extracted to a greater extent by aqua regia from such a matrix. 7852
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CHN Analysis. Total concentrations of C, H, and N were determined in triplicate, 2 mg aliquots of each sample by flash combustion using a Carlo Erba EA 1110 elemental analyzer calibrated with EDTA standards. Precision was better than 10% for each element, and accuracy, based on analysis of C in a variety of certified soils and sediments, was greater than 95%.
Results and Discussion Sample Composition. The elemental composition of the dust samples and the geometric mean concentration of each constituent are given in Table 1. The elements analyzed account for between about 30 and 50% of the total mass of sample in each case. Concentrations of C, H, and N in the domestic samples are similar to those reported in a previous study (6), and are consistent with a dust matrix highly enriched in organic matter. Carbon and H concentrations exhibit relatively little variation among all samples, with an interquartile range relative to the geometric mean of less than 0.5. In contrast, concentrations of N are highly variable, with a relative interquartile range of about 1.5. Specifically, N concentrations are significantly lower (and C/N significantly greater) in the working environment than the domestic environment. One of the principal components of domestic dust is protein-rich material derived from exfoliated skin and hair, and the low C/N ratio of protein ()2-3; 6) is reflected in a C/N ratio below ten in this setting. In the working environment, therefore, we infer that there is a greater proportion of nonproteinaceous components in the dust matrix. These components may be derived externally, due to the greater mobility and throughput of its occupants, or internally from, for example, the maintenance and running of office or laboratory equipment. Metal concentrations in the domestic samples exhibit considerable variation (in excess of an order of magnitude in some cases), reflecting the inherent heterogeneity of indoor solids and, perhaps partly, differences in the sampling units employed. Overall, geometric mean metal concentrations are consistent with equivalent values reported in other recent studies (2, 5, 6). Concentrations in the library and administration building fall within the range of concentrations in the domestic environment. However, for many metals (e.g., Fe, Ag, Cd, Co, Mn, Zn), concentrations are higher in the technical buildings, reflecting the presence of metallic particles derived from various laboratory equipment and practices (e.g., worn metallic constructions, electrical winding, plated, and galvanized surfaces, oils, chemical reagents). Significant correlations among the different constituents of the samples are identified in Table 2. Metal concentrations were not correlated with concentrations of C, H, or N, but some metals were correlated with the C/N ratio, presumably because the latter affords a better measure of the source apportionment of dust particles than individual elements (see above). Significant correlations were evident between pairs of some metals (most commonly involving Ag, Cd, Co, Cr, and Pb). However, closer inspection of the data revealed that such relationships were often controlled by elevated metal concentrations of these metals in a few samples (e.g., TB-1 and TB-2), and that concentrations in the domestic samples alone were poorly correlated. Metal Bioaccessibility. The bioaccessibility of metals in indoor dust is exemplified for four samples in Table 3. Bioaccessibility in the present study is defined as the concentration of metal solubilized, sequentially, in the stomach phase, [Me]SP, and the two intestinal phases, [Me]IP1 and [Me]IP2, respectively, relative to the concentration of metal digested in boiling aqua regia, [Me]AR, and is expressed on a percentage basis. Note, however, that since kinetic effects in the intestine were not evident (deviations between metal concentrations solubilized in the first and second intestinal
a Errors reported for C, H, and N in all samples and metals in samples LY, TB-1, and TB-2 represent the standard deviation about the mean of three independent experimental determinations. Data in bold represent the highest concentration (or value) determined among the samples. Sample code: LY ) library; AB) administration building; TB) technical building; PH ) private house. GM represents the geometric mean of all results for each element.
24.0 2.99 1.86 12.99 0.64 6.94 1.11 1.54 1.95 5.9 84.3 162 484 43.4 115 27.5 0.83 763 26.8 ( 0.2 3.75 ( 0.07 5.12 ( 0.08 5.24 ( 0.08 0.90 5.85 1.00 1.44 0.96 4.9 129 222 391 45.9 120 135 0.67 1140 27.3 ( 0.1 4.05 ( 0.01 5.02 ( 0.03 5.43 ( 0.01 0.92 2.86 1.48 2.84 4.76 5.5 59.6 114 687 51.9 77.2 45.5 0.33 460 29.7 ( 0.7 4.18 ( 0.17 3.69 ( 0.21 8.05 ( 0.27 0.74 8.76 0.65 3.45 5.03 8.0 139 389 428 69.0 156 31.3 0.91 742 27.8 ( 0.2 3.67 ( 0.02 3.74 ( 0.04 7.45 ( 0.03 0.67 8.56 0.89 0.96 1.36 3.0 77.5 130 362 18.1 195 20.6 0.40 782 13.0 ( 0.3 1.60 ( 0.12 1.40 ( 0.08 9.35 ( 0.39 0.80 6.71 1.01 0.95 0.87 5.9 57.7 758 396 114 123 195 1.11 464 23.7 ( 0.3 3.38 ( 0.12 2.84 ( 0.08 8.35 ( 0.15 0.37 12.6 0.59 0.38 1.08 3.1 15.6 54.2 285 13.4 26.7 3.2 0.39 98 32.0 ( 0.8 3.97 ( 0.11 3.70 ( 0.15 8.67 ( 0.14 0.50 10.4 0.65 0.72 1.02 6.2 57.6 73.8 346 38.2 227 12.3 0.50 1330 22.3 ( 0.2 2.48 ( 0.06 0.60 ( 0.06 37.14 ( 3.42 1.01 ( 0.03 11.10 ( 0.24 2.51 ( 0.45 9.04 ( 3.93 9.89 ( 1.37 29.5 ( 2.2 231 ( 8.6 320 ( 26.5 814 ( 16.7 84.3 ( 11.4 468 ( 36.6 59.2 ( 3.6 0.76 ( 0.10 3090 ( 20.7 25.5 ( 0.4 3.00 ( 0.10 1.53 ( 0.04 16.61 ( 0.19 0.40 3.89 0.40 0.63 0.80 2.3 99.6 41.4 222 26.7 50.3 6.9 0.33 563 16.6 ( 1.3 1.63 ( 0.12 0.54 ( 0.05 31.03 ( 1.94 0.57 ( 0.03 6.07 ( 0.28 0.82 ( 0.04 0.88 ( 0.60 0.81 ( 0.15 4.2 ( 0.3 99.7 ( 4.1 81.9 ( 8.0 462 ( 18.4 39.4 ( 3.7 62.3 ( 3.9 10.2 ( 1.1 0.47 ( 0.12 696 ( 46.3 C, % H, % N, % C/N Al, % Ca, % Fe, % Ag, µg g-1 Cd, µg g-1 Co, µg g-1 Cr, µg g-1 Cu, µg g-1 Mn, µg g-1 Ni, µg g-1 Pb, µg g-1 Sn, µg g-1 U, µg g-1 Zn, µg g-1
27.4 ( 0.3 2.79 ( 0.11 0.35 ( 0.06 80.04 ( 14.67 0.53 ( 0.03 6.23 ( 0.07 11.75 ( 0.06 6.85 ( 0.92 8.31 ( 3.55 14.5 ( 1.8 156 ( 4.4 489 ( 51.0 2510 ( 32.7 67.7 ( 10.2 156 ( 27.4 52.6 ( 8.7 0.64 ( 0.10 2280 ( 39.1
GM PH-7 PH-6 PH-5 PH-4 PH-3 PH-2 PH-1 TB-2 TB-1 AB LY element
TABLE 1. Elemental Composition of the Fractionated ( Cd, Mn > Al, Cu, Co, Cr, Ni, Pb > Fe, Sn, U. However, it must be appreciated that, for some metals (e.g., Al, Fe), recovery by aqua regia may not have been complete and measures of bioaccessibility should be regarded as upper estimates. Conversely, in a few cases, the bioaccessibility of Ca, Cd, and Zn was close to or in excess of 100%, suggesting that the simulated gastric solution was at least as efficient in solubilizing these metals as aqua regia, and/or that subsamples displayed some heterogeneity. With these exceptions, however, measures of metal bioaccessibility in the present work are considerably lower than those reported for domestic dusts in previous studies employing simple surrogates for the human stomach. For example, Turner and Simmonds (6) found that the availability of Al, Cu, Ni, and Pb to pepsin in 0.075 M HCl averaged between about 60 and 100%, whereas Rasmussen (13) showed that Ni and Pb were, respectively, about 30-40% and 55-75% available to 0.07 M HCl. Discrepancies are most likely related to differences in experimental pH, which was 2.5 in the stomach phase of the present PBET (representative of an “average” gastric state), but closer to 1 in dilute HCl (more representative of transient, fasting conditions). Differences in pH of this order can significantly affect the degree of metal mobilization from a variety of contaminated solids (7, 22, 23). For most metals, bioaccessibility in the stomach was inversely related to [Me]AR, as exemplified in Figure 2 for Co, Cu, and Zn. This implies that both metal contamination and accessibility in dust are, to a first approximation, controlled by the relative abundance of metal-rich (or metallic) particles which have a low accessibility (or rate of mobilization) in the gastric medium. Significantly, however, Al and U, proxies for external geochemical material in the indoor environment, displayed no such dependence on [Me]AR (see also Figure 2), suggesting that metal-rich particles are largely derived internally in these samples. Bioaccessibility in the Intestinal Phase. Also shown in Figure 1 are (arithmetic) mean bioaccessibilities of metals in the intestinal phase. As with accessibility in the stomach, metals exhibit considerable variation among the samples. However, intermetal correlations were more abundant in the intestinal phase (see Table 2), and the rank order of bioaccessibility is different. Thus, compared with the stomach, mean accessibility in the intestine is either lower (Al, Ca, Cd, Mn, Ni, Pb, Sn, Zn), similar (Fe, Co, Cu) or enhanced (Cr, U). For a given metal, we determined whether concentrations were statistically greater or smaller in the intestine by applying a paired, one-tailed t test to all samples in which metal was detectable in both phases. The results, shown in Table 4, indicate that differences were most significant (p < 0.05) for Al, Ca, Cd, Mn, Pb, U, and Zn, of lower significance (p < 0.1) for Ni and Sn, and insignificant (p > 0.1) for Fe, Co, and Cu. Mobilization in the intestine relative to the stomach was quantified for each metal by applying the following model:
[Me]IP/[Me]AR ) m[Me]SP/[Me]AR + c where m and c are constants. Results, derived from linear regression analysis of the data, are also shown in Table 4, VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Correlation Matrix Identifying Significant Relationships (p < 0.05 or, in Bold, p < 0.01) between Different Metals (and C/N) Available to Aqua Regia (AR), or Mobilized in the Stomach Phase (SP) or Intestinal Phase (IP) (n Ranges from 7 to 11) Al Ca Fe Ag Cd Co Cr Cu Mn Ni Pb Sn U Zn
C/N
Al
Ca
AR AR AR
SP,IP
Fe
Ag
Cd
AR AR AR
AR,SP AR
AR
AR
AR
IP
AR
AR
AR AR IP
SP IP
AR
SP,IP IP
AR
SP,IP
AR
AR,SP
Co
AR SP,IP IP IP AR,SP,IP IP AR,SP
Cr
AR
Cu
AR, IP SP,IP AR,SP AR,IP
AR
Mn
Ni
Pb
Sn
U
IP AR,IP AR SP
AR AR
TABLE 3. Stomach Phase (SP) and Intestinal Phase (IP) Bioaccessibility of Metals (as a Percentage of Total Acid-Extractable Concentration) in Four Dust Samplesa AB
TB-2
PH-5
PH-7
metal
SP
IP1
IP2
SP
IP1
IP2
SP
IP1
IP2
SP
IP1
IP2
Al Ca Fe Cd Co Cr Cu Mn Ni Pb Sn U Zn
12.1 100 6.6 100 24.7 nd 31.9 64.5 20.6 36.3 7.0 nd 130
9.0 91.8 5.3 68.4 18.2 nd 35.0 44.5 15.8 22.0 3.1 nd 105
9.3 93.6 5.8 52.2 17.1 nd 29.9 48.1 11.0 13.5 6.8 32.1 100
12.4 66.7 12.4 25.2 7.5 4.5 10.3 38.1 10.8 13.0 4.2 7.6 63.8
10.3 59.5 10.5 14.1 10.9 2.9 8.8 31.7 8.7 9.2 2.5 nd 41.9
11.1 63.5 11.0 21.6 11.5 3.7 11.4 35.1 8.1 11.9 2.8 20.7 41.9
20.4 78.0 6.2 13.4 5.6 9.6 5.0 53.8 6.6 11.6 1.7 32.2 81.3
18.1 68.3 6.0 12.6 10.7 18.5 8.8 42.1 11.5 9.4 2.2 nd 40.3
16.7 66.1 6.0 11.3 9.5 15.8 8.3 40.4 9.0 4.5 1.5 26.9 47.2
9.0 73.7 1.3 17.6 13.1 19.3 10.5 30.7 53.5 16.7 0.7 2.2 59.8
7.1 56.2 2.0 9.1 10.1 25.2 10.6 22.8 3.2 2.0 0.6 45.0 39.3
7.1 55.5 2.0 9.1 11.0 27.2 13.6 24.0 6.0 2.0 0.5 45.0 40.5
a Note that accessible Ag was not detected (nd) in these samples. Numbers in bold represent the highest accessibility for each sample phase. For sample code, see caption to Table 1.
and indicate that relationships were significant for all metals except Fe, Cr, Ni, and U, and that, with the further exception of Co and Cu, values of m differed significantly from unity. These observations generally confirm the statistical differences ascertained from t testing above, but also suggest that, for most metals, a common sequence of (e.g., metal-specific) reaction mechanisms takes place from stomach to intestine among the different samples. Mechanisms of Metal Mobilization. Ultimately, the precise bioaccessibility of a metal in the stomach depends on the mineralogical, biogenic, and artificial phases that the metal exists in or is encapsulated by, and the kinetics of release from such under acidic and enzymatic conditions and in the presence of potential inorganic and organic complexants. In the higher pH, carbonate-rich environment of the intestine, metals may be stabilized in solution by complexation, undergo readsorption to preexistent or altered sites at the particle surface, or precipitate as relatively insoluble compounds. The results reported above are consistent with a multitude of metal-specific mechanisms and rates of release. However, the broad findings can be reasonably well-understood in terms of general chemical and mineralogical considerations. Thus, Ca and Zn are most accessible overall and accessible fractions in either phase are highly correlated (Table 2) because calcium-bearing minerals are readily solubilized in dilute HCl and evidence suggests that the majority of Zn in house dust is associated with these minerals (13, 21). In contrast, the relatively low accessibilities of Al, Fe, and Sn can be attributed to their association with more refractory 7854
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minerals (e.g., silicates, crystalline oxides, and cassiterite, respectively) or particulates in the household environment (1). Many metals studied undergo a reduction in accessibility from stomach to intestine (and in the approximate order: Sn, Cd > Al > Ca, Ni, Pb, Zn > Mn; see Table 4). This effect has been reported previously for Pb in both soil and house dust (14, 24) and attributed to readsorption or to precipitation as lead phosphates, such as chloropyromorphite [Pb5(PO4)3Cl(s)], in the intestine. Although phosphate was not a component of the present PBET, it is likely that quantities sufficient to effect precipitation were released from the dust samples. More generally, we surmise that both precipitation of inorganic compounds (including hydroxides and carbonates) and readsorption of metallic cations to deprotonated surfaces proceed simultaneously (and to relative extents that are metal-dependent). Metals that display statistically indistinguishable accessibilities in the stomach and intestine (Fe, Co, Cu) are presumably stabilized in solution to a greater extent by complexation with available organic ligands as the pH rises. Candidate complexants include the malate ion, derived from malic acid added as a surrogate for digested sugars in the PBET, anions of bile acids, such as chenodeoxycholate and hyodeoxycholate, and components of organic matter digested from the dust matrix. Consistent with this assertion, therefore, available constants defining metal binding by malate suggest that Fe and Cu are likely to undergo greater complexation than Al, Ca, Cd, Ni, and Zn (25). With respect to bile acids, polarographic measurements indicate that Cu complexes and
TABLE 4. Results of Regression Analysis and t Testing of Metal Accessibility in the Stomach Phase and Metal Accessibility in the Intestinal Phasea [Me]IP/[Me]AR ) m[Me]SP/[Me]AR + c metal Al Ca Fe Cd Co Cr Cu Mn Ni Pb Sn U Zn
[Me]SP/[Me]AR * [Me]IP/[Me]AR
n
m
c
r2
p
p (m * 1)
p
11 11 11 8 9 3 9 11 9 11 6 6 10
0.341 0.589 0.572 0.284 0.864 1.53 0.721 0.644 0.577 0.613 0.223 0.262 0.591
5.71 15.3 2.69 12.9 1.74 -1.43 3.86 5.46 3.11 -2.26 1.29 19.4 3.18
0.402 0.929 0.247 0.865 0.799 0.964 0.661 0.882 0.159 0.726 0.298 0.009 0.697
0.036
