Environ. Sci. Technol. 1993, 27, 1415-1425
Micromlneralogy of Mine Wastes in Relatlon to Lead Bioavailability, Butte, Montana Andy Davls,'*t John W. Drexler,* Michael V. Ruby,t and Andrew Nicholsont PTI Environmental Services, 2995 Baseline Road, Suite 202,Boulder, Colorado 80303,and Department of Geological Sclences, Unlverslty of Colorado, Boulder, Colorado 80309
Electron microprobe analysis of soil and waste rock mineralogy helps explain the low blood P b levels observed in young children living in Butte, MT. The sulfide/sulfate assemblageconsists primarily of galena, anglesite, and lead jarosite,while the oxide/phosphate assemblage was principally manganese lead oxide, lead phosphate solid-solution series, and lead oxide. Modeled paragenetic sequences initiated using galena and lead oxide demonstrated that both primary phases weather to less soluble anglesite and pyromorphite end members, respectively. If these soils were ingested, P b solubility would be constrained by alteration and encapsulation of the Pb-bearing minerals, which limits the available Pb-bearing surface area. This premise is supported by the results of the Butte-Silver Bow environmental health P b study, which indicate that no statistical relation exists between blood-Pb levels and the presence of individual Pb-bearing phases.
Introduction One of the primary exposure pathways suspected of contributing to elevated blood lead (Pb) levels in young children is the incidental ingestion of soil containing Pb. However,recent research has demonstrated that the bloodPb levels in children living in urban areas (e.g., Cincinnati, OH), smelter communities (e.g., East Helena, MT), and mining communities (e.g., Butte, MT) vary considerably, even when soil P b concentrations are comparable (1,Z). Several variables cause these differences, including the source of Pb in the soil, forms of available Pb, housing age, and concentration of P b in house paint and house dust (1-4). Recently, it has been recognized that the mineralogic form of P b is also an important factor controlling P b bioavailability at mining sites. The solubility of ingested P b phases is dependent on mineralogy, encapsulation of P b phases within alteration and precipitation products, and slow rates of P b dissolution (kinetic constraints)that limit P b availability as the Pb-bearing solids traverse the gastrointestinal (GI) tract (5-7). To date, investigations of P b bioavailability in Butte, MT, have been specific to the sulfitic P b mineral assemblage, determined by electron microprobe analysis to be predominantly galena (PbS) and its oxidation product anglesite (PbSOr) (6,B). These studies suggest that surface precipitation of jarosite and plumbojarosite on the exterior of Pb-bearing grains acts to reduce Pb solubility in soils impacted by mine wastes, while encapsulation in latestage pyrite and quartz prevents dissolution of galena and anglesite over the residence times of these soils in the GI tract. From a mining perspective, the mineralogy of the Butte ore body, hosted by the Boulder Batholith Butte quartz
* To whom correspondence should be addressed.
t PTI Environmental Services.
t University of Colorado. 0013-936X193/0927-1415$04.0010
0 1993 American Chemical Soclety
monzonite, has been studied extensively (9-1 I),resulting in the identification of a limited suite of Pb-bearing minerals consisting of galena, anglesite, cerussite, and plumbojarosite (12). This assemblage is comparable with that found in Pb-bearing ores of the Tri-State District of Missouri, Kansas, and Oklahoma; the Coeur d' Alene district in Idaho; and Leadville, CO (13). However, no morphological interpretation or description has been published describing the weathering of Pb phases in soil nor of the genetic relations (or interactions) between P b phases at the micron scale in the ore body or in association with mining or residential media. The theoretical geochemistry controlling chemical alteration and dissolution of P b in soils has been described for common forms of Pbminerals, e.g., lead oxides, sulfides, sulfates, carbonates, phosphates, and silicates (14,15). However, the application of mineralogicaland geochemical data to the assessment of P b bioavailability has been limited to studies in Butte, MT, and Derbyshire, England (5, 6 ) . Further, only limited investigation has been conducted to determine the successive order in which associated P b minerals are formed (i.e., the paragenetic sequence) in soil containing Pb-bearing mine wastes. Such an analysis is critical when assessing the risks associated with the ingestion of soil containing mine-waste P b by individuals who live in current or former mining communities. Weathering (Authigenic) Reactions in Soils. In Butte, the orebody was formed in equilibrium withsulfaterich hydrothermal solutions. When exposed to moist, cool, and oxygenated surface conditions after mining, these minerals are thermodynamically unstable and alter to more stable forms as minerals react with and dissolve in infiltrating rainwater. In general, silicate minerals will weather to clay minerals and quartz, while suIfide minerals will alter to hydrated oxides, sulfates, phosphates, and carbonates, depending on the soil pH and availability of soil anions (16). Because many transformation reactions in soils are kinetically controlled (e.g., sulfide minerals start to oxidize in 10 pg/ dL (19samples) or where residential soils contained >2000 mg/kg of Pb (3 samples). This subset included samples from play areas (15samples), house perimeters (5samples), and garden soils (2 samples). The 38 soil samples analyzed, both play areas and perimeter soils, were collected from 34 individual residences. Sampling Method, Play area samples from the first subset were collected after identifying five areas at each residence where children’s play activities were most likely to occur (e.g., sandbox, swing set, bare areas), based on conversations with parents and locations of play articles. Subsamples of 200 g each were collected from the top 5-cm soil horizon of each area using a stainless steel spoon and were homogenized in a stainless steel mixing bowl. Samples were collected in prelabeled, l-L polyethylene sample bottles. Mine-waste and garden soil samples from the first sample set were composites of five subsamples from the top 15cm of waste rock piles and garden soils, respectively. House perimeter samples were composited from five 5-cm subsamples located 1 m out from the house exterior. In the event that bare areas were not available for perimeter sampling, a section of grass was removed, and subsod samples were collected at a depth where native soils were present. The mine-waste,garden soil, and house-perimeter samples were collected using the same protocol as the play area samples. Sample collection methods for play area and garden soil samples from the second sampling set (22 samples) are described in detail in the Butte-Silver Bow Environmental Health Lead Study (3). Sample Preparationand Analysis. Bulk soil samples were dried at 80 OC for 48 h and sieved to C250 pm using 1418 Envlron. Scl. Technol., Vol. 27, No. 7, 1993
an 8-in. stainless steel sieve and a RoTap sieve shaker. The C250-pm size fraction was chosen, because the smaller particles adhere to children’s hands and may be ingested (20). Total Pb, As, P, Mn, Ca, and Fe concentrations were determined for the C250-pm size fraction using a KEVEX 0700 XRF spectrophotometer with instrument operating parameters set as recommended by the manufacturer (21). Blood-Pb concentrations in children (6-72 months old) were determined during the Butte-Silver Bow study (3), and soil pH values were determined by the saturated paste method (22). Polished sample pucks were prepared for electron microprobe analysis by embedding 4 g of sample in epoxy within a sample mold, setting the molds to cure at room temperature, and grinding a flat surface on the sample side to expose as much sample as possible. Successive polishing steps employed a 600-gritwet/dry abrasive paper stretched across a glass plate, 15-pm and 6-pm diamond on a cloth pad fixed to a steel lap, and finally 0.1-pm diamond on a felt pad fixed to a steel lap. All polishing steps used kerosene to avoid dissolution of water-soluble Pb phases, and all polishing was performed at low speeds to avoid plucking of the sample grains. Finally, sample pucks were cleaned in an ultrasonic cleaner with isopropyl alcohol, air-dried, and placed in a carbon coater, where a thin layer of carbon was sputtered onto the surface of each puck. Electron microprobe analysis (EMPA) was conducted at the Laboratory for Geological Studies, University of Colorado, Boulder, CO, on a JEOL 8600 electron microprobe operating at 15 kV with a 20-nA specimen current and a l-pm beam. Quantitative data were collected using wavelength dispersive spectrometers and mineral standards and were corrected using Phi-Rho-2 parameters. The Pb-bearing particles were identified using a combination of energy dispersive detection (EDS), wavelength dispersive detection (WDS), and backscatter electron image detection (BEI). Initially, spectra are generated for each grain, which allows identification of all elements with an atomic mass greater than or equal to carbon. Subsequently, the elemental proportions are quantified using standards, and the mineral is identified based on the equivalent weight of the oxide. Therefore, the identifications provide quantitative stoichiometric ratios (e.g.,for the phosphates, Table I) from which the mineral identity may be calculated. The relations between Pb-bearing phases were established from BE1 images and WDS/EDS analyses as necessary. Representative BE1photomicrographs of identified phases and their associations were produced, with scale bar, magnification, sample identification, and phase identification recorded on each photomicrograph. Individual Pb-bearing particles were analyzed (representing one point count each) until a minimum of 100 particles had been evaluated or 5 h of machine time had been spent on the analysis. Point counts were made by traversing each sample from left to right and top to bottom in a grid pattern, with each vertical displacement moving only to the adjacent field of view. Magnification settings of 40-1OOX and 300-600X were used; the latter magnification allowed analysis of the smallest identifiable (1-2 pm) phases. The grain size of each P b carrier was determined by measuring the dimension of the long axis. Frequency of occurrence of Pb phases in each sample were
Table I. Representative Quantitative Analyses of Lead Phosphates in Butte Soils sample code PbO" MW5 PA14 PA14 PA15 PA16
PA18 PA20 PA21 PA22 PA22 PA22 PA22 651 651 651 652 652 652
54 32 70 76 66 62 58 75 57 57 57 49 66 69 54 68 29 30
P205 so4
19 13 10 15 16 15 20 13 16 17 16 20 18 16 17 15 15 25
total Fez03 Si02 A203 C1 CaO wt % b
1 10 1
0 1 1 1 0 2 2
0 1 1 0 1 0 15 2
0 33 1 1 1 3 1 0 2 2 1
2 1 0 0 2 0 1
0 1 2 0 2 0 0 0 2 2 4 0 0 0 0 1 0 0
0 3
0 0 0 1 0 0 0 1 1 1 0 0 0 0 33 29
2 0 1 2 2 2 2 1 2 2 2 2 2 1 1 2 0 0
12
0 3 3 5 3 10 5 4 5 6 12 7 5 12 4 5 2
88 92 88 97 93 87 92 94 85 88 87 87 95 91 85 92 97 89
a Values indicate the weight percent of each constituent as an oxide as determined by WDS analysis. b Water of hydration indicated by cumulative weight percent lead phosphates > lead oxides > iron lead oxides > anglesite > lead silicates > slag > lead jarosite (Figure 1).The relative
Table 11. Chemical Formulas and Solubility Products of Pb Phases in Butte Soils Pb phase lead phosphates pyromorphite corkite drugmanite hinsdalite plumbogummite manganese lead oxides cesarolite coronadite magnetoplumbite senaite iron lead oxides plumboferrite general P b phases anglesite galena cerussite plumbojarosite wulfenite lead oxides (e.g., litharge, plattnerite) lead silicates (e.g., alamosite) slag paint
log %pa
chemical formula Pba(POrhC1 PbFes(PO@O4(OH)6 Pb2Fe(P04)2(0H).H20 PbAls(P04)S04(OH)s PbAls(PO4)z(OH)aHzO PbMnsO6(OH)z PbMnsOl6 Pb(Fe,Mn)lzOls Pb(Ti,Fe,Mn)21038
-84.4d -112.6d A
-99. I d -99.3d -C -C
A -C
PbFe401
-c
PbSO4 PbS PbCOs P~F~~(SO~)~(OH)IZ PbMoOi PbO, Pb(OH)2, Pb205 PbSiOs, PbBiO4 Pb, Cu, Zn, S(FeCaSiO4) PbO, 2PbCOrPb(OH)z, PbCrO4
-7.7b -27.5e -12Ab -12.6" -16.0' 33.1 for PbOf -27.5 for PbzSiO48 -C
33.1 for PbOf/-12.8 for PbCO3
a These data pertain to the stoichiometric ionization of the mineral. b From ref 42. Not available. d From ref 15. e From ref 14. f Calculated from data in ref 26.8 Calculated from data in ref 14. Calculated from data in ref 43.
Envlron. Sci. Technol., VoI. 27, No. 7, 1993 1417
A P
Anglesite FePb oxides Pb oxides 1 Pb phosphates I MnPb oxides I 5
0
15
10
20
25
Frequency of Occurrence in Butte Soils Fbun 1. Frequency of Pb phases occunlng In the Butte solls. The percent Occurrence of each phase was calculated by summlng the slze 01 "long dlmenslon" of all grains counted for each phase.
1
01
0
--20
30
40
50
60
70
80
90
100
Frequency OxideIPhosphate Fbwo 2. Dlsirlbutlon of oxkllzed and reduced mineral assemblages In Bune ~011s.The reduced assemblage Includes galena. anglesite. and Impure lead sulfates.while the oxidized assemblage includes lead phosphates, lead fenomanganeseoxides, wulfenne. and lead oxides.
abundance8 indicate that the oxide/phosphate mineral assemblage predominates over the galena/anglesite mineral assemblage (Figure 2). No obvious differences were observed between the P b minerals present in the minewaste samples and those in the soil samples (play area, garden, and perimeters;Table 111). However, pyrite (FeS,) was more abundant in mine-waste samples, the oxidation of which results in decreased sample pH values. Sulfide/Sulfate Assemblage. Galena and anglesite make up 1% and 7 %,respectively, of the total Pb mineral stoichiometry in the Butte samples (Figure 1). Therefore, the sulfidelsulfate mineral assemblage is unlikely to be the dominant factor controlling overall P b bioavailability from Butte soils, except in certain discrete soil samples (e.g., PA6, Table 111). However, a discussion on the mechanisms by which observed mineral phases may have evolved serves to elucidate their spatial relations. Galena is unstable under oxidizingconditions and reacts with atmospheric oxygen to form a rind of anglesite around a galena core (eq 1):
+
PbS 20, t PbSO, (1) Therefore, weathering of galena results in armoring of the primary mineral grain by a secondary reaction product. Concurrently, the microbially mediated dissolution of pyrite, and subsequent oxidation of ferrous to ferric iron, results in the generation of acidic interstitial water in the 1418 Envirn.
Scl. Technd.. VoI. 27. NO. 7.
+ 6H,O F? KFe3(S04),(OH)6+ 6H+ (2)
~~
10
lead
waste rock pile (23,24). Where pyrite is ahundant, the waste rock is generally acidic (pH < 51, while soils are pH neutral where pyrite is minimal or absent (Tahle 111). Subsequent dissolution of feldspar provides a potassium source for jarosite and plumbojarosite precipitation, both of which are stable in acidic pH environments (25) and whichmayformarindon theexteriorofpb-bearinggrains, e.&: K+ + 3Fe3++
. -
~
Figure 3. Photomicrograph showing aneration of lead oxide to phosphate, with silicate partially rinding the grain exterior.
1993
for jarosite and
+
+
Pb2+ 6Fe3+ 4SO4% + 12H,O
t
PbFe6(SO4),(0H),, + 12H' (3) for plumbojarosite. Generally, plumbojarosite is not rimmed by later Pbbearing phases and is, therefore, considered a mature P b phase. The average soil pH for the eight samples that contained 10% or more anglesite was 4.6 (Table III), supporting the pH dependence of the proposed sulfide/ sulfate paragenetic sequence. Photomicrographic evidence for the galena-to-anglesite alteration mechanism is presented in ref 6. Consequently, galena, with a Kap of -27.5 (a large negativeK, indicates a low theoreticalsolubility inneutral pH fluid) oxidizes to sparingly soluble anglesite (K,, = -7.7) and forms lead jarosite (Kap= -12.6; Table 11)on the exterior of the P b grains. Oxide/Phosphate Assemblage. Although a few soils are representative of the reduced assemblage (e&, PA6, Table III), the majority of the soils collected in Butte are dominated by the phosphate/oxide assemblage (Figure 2). In the pH-neutralphosphate/oxideassemblage, amore complex paragenetic model is necessary to describe the evolution of soluble PhO (log Ksp= 33.1,26) to the less soluble phosphate assemblage (log K, = -84 to -113,15), an alteration sequence demonstrated in Figure 3. Although log KSpvalues for pyromorphite [PbdPO4)&11 reported in the literature range from -34 (27) to -84 (15, 28),the majority of the reported solubility products are < 4 7 (29), indicating that Pbs(P04)$1 is essentially insoluble. A suite of lead oxides, including PbO, PbzOa, and multimetal lead oxides, is present in some soils, altering
Flgure 4. Photomicrographof iron ieadoxidegrain withFeO precipitate
on the grain periphery. Lighter areas represent higher Pb concen-
Flgure 6. Photomicrograph of manganese lead oxide grain with authigenic silicate rind
trations.
.
Figure 5. Photomicrograph Of iron lead oxide phase forming on, and paRiaily encapsulated in, a quartz matrix
to the hydrated form: PbO + H,O e Pb(OH), (4) Dissolution of minerals such as apatite [(Cas)P04)3(F,C1,OH)I and biotite (MgAlFe silicate) provides a source of PO& Fe, and A1 for alteration of the lead oxides to pyromorphite, plumbogummite [Pb&(P04)~(0H)sH~01, hmsdalite [Pb&(PO4)SO4(OH)61, corkite [Pb,Fe33+(P04)(SO4)(OH)el,and drugmanite [Pb2Fe3+(P04)2(0H)Ithat may also precipitate directly from solution, i.e.: 5Pb2++ CI- + 3H2PO;
= Pb,(P04),C1+
6H+ ( 5 )
These lead phosphates, however, are not end members, because quantitative analyses of 2651ead phosphate grains indicate substitution of Ca, Mn, and other elements into the crystal lattice, i.e.: Pb,(P04)3CI + 4Ca2+s (Pb, 2Ca+8)5(P04)3Cl+ 4Pb2+ (6) resulting in a complex set of solid-solution series, that based on log Kspdata, are apparently more stable than pyromorphite, inferred previously to be the most stable lead phosphate (5). M a n g a n e s e Iron Lead Oxides. If lead phosphates are absent, possiblydue toinsufficient soil PO4,manganese
lead oxide and iron lead oxide are formed by a combination of geochemical or bacterially mediated reactions. The manganese lead oxides are the most prevalent Pb-bearing phase in Butte soils (24%). while iron lead oxides are the fourth most prevalent (9%) after lead phosphates and lead oxides (Figure 1). These data are consistent with soil/sediment P b mineral assemblages near Leadville, CO, where P b is also associated with iron and manganese oxides (30). The manganese lead and iron lead oxides are generally present in three forms: as discrete grains (Figure 4), as coatings on non-Pb-bearing phases (Figure 5), and as alteration rinds on iron oxide grains. In addition, a coating of authigenic silicate (Figure 6) was occasionally observed on some lead manganese oxide grains. The mineral grain sizes, chemical composition, and rounded nature of the lead manganese oxide phases in Butte soils are similar to those observed on sediment casts (31).desert varnish (32),and marine nodules (33),indicating that these mineral phases are common under a wide variety of environmental conditions. Manganese lead oxides are more common than iron lead oxides a t Butte, even when whole-rock Mn/Fe ratios are 0.014.06. McKenzie (34) also demonstrated that P b adsorbs preferentially to synthetic manganese oxides over iron oxides by a factor of 40, while P b has also been demonstrated to accumulate in the manganese oxide soil fraction (35, 36). Ferromanganese oxide grains are characteristically porous and lamellar, with fluctuating P b concentrations in cross section. However, they are not rimmed by other Pb-bearing phases and are, therefore, considered mature. Quantitative microprobe analyses show three general populationsof ferromanganese oxides (Figure 7). The first is characterized by compositions similar to the minerals cesarolite [PbMn34+O&H)21 and coronadite [Pb(Mn4++,Mn2+)sOl6l,withMn:Pbratiosof31and8:1,respectively. The second is composed of iron lead oxides and exhibits amore restricted compositionalfield,similartothemineral plumboferrite (PhFe43+07,Figure 7). Thethird population of ferromanganese oxides suggests limited solid solution along the Mn/Fe join and is compositionally similar to senaite [Pb(Ti4+,Fe3+,Mn3+)210~sl and magnetoplumbite [Pb(Fe3+,Mn3+)120191. The formation of lead ferromanganese oxides may be due to a combination of processes, including (1) the bioaccumulation of Mn in soils by bacteria, followed by Environ. Sci. Technoi.. Voi. 27. No. 7.
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Table 111. Butte Soil Mineralogy, Chemistry, and Blood Pb Levels: Values Associated with Mineral Phases Indicate Frequency of Occurrence in Sample sample code lead phosphates MnPb oxides FePb oxides anglesite galena cerussite lead jarosite wulfenite lead oxides' Pb(OH)z Pb metallicsb Pb silicatesc impure lead sulfated native Pb solder slag paint Pbe presence of pyritd grains counted Pb conc, ppm As conc, ppm P conc, ppm Mn conc, ppm Fe,wt % Ca, wt % soil pH blood lead 2nd child (3rd)
mining waste soils play area soils MW1 MW2 MW3 MW4 MW5 PA1 PA2 PA3 PA4 PA5 PA6 PA7 PA8 4 48
1
7 30
Tr
7 31 29 4 Tr
1
24 Tr
37 7 6 10
38 25
1
5
60 32
18 Tr 58
8 2
7
Tr 3
15 8
63
12
5 2 11
7 2
59 15 7
15 18 21 T r 1 1 1 6 4 4
1
39
43
14 5 19 4 6
98 2
PA9 PA10 PA11 PA12 PA13
28
44
1
36 2 Tr 9 3
1
4
8
2 Tr
71
17 15 14 9
23 16 3 37
45
6
3
2
Tr
1 18 1
15 65 16 Tr 2
Tr
3
Tr
10 15
Tr
3
Tr
C 159 630 c4oe 1490 2700 4.0 2.4 7.0 22.5
M
Tr 4
Tr 2
5 44
2
1
Tr 2
I
Tr
98
M
M 49 260 310 2280 800 3.0 9.8 7.8
A 74 1540 2500 1480 2200 8.0 6.6 7.5
A 159 1030 620 8500 2000 12.3 2.9 2.8
A 144 119 7540 1790 1180 420 370
