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A TEM study of samples from acid mine drainage systems: metal-mineral association with implications for transport

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A TEM study of samples from acid mine drainage systems: metal-mineral association with implications for transport
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  PII S0016-7037(99)00260-4 A TEM study of samples from acid mine drainage systems:Metal-mineral association with implications for transport M ICHAEL  F. H OCHELLA , J R .* ,1 J OHNNIE  N. M OORE , 2 U TE  G OLLA , 1 and A NDREW  P UTNIS 1 1 Institu¨t fu¨r Mineralogie and Interdisciplinary Centre for Electron Microscopy and Microanalysis, Universita¨t Mu¨nster,Corrensstr. 24, D-48149, Mu¨nster, Germany 2 Department of Geology, University of Montana, Missoula, MT, 59812 USA(  Received October   15, 1999;  accepted in revised form April  29, 1999) Abstract —Transmission electron microscopy (TEM), with energy dispersive X-ray (EDX) analysis andenergy filtered transmission electron microscopy/electron energy loss spectroscopy (EFTEM/EELS), as wellas powder X-ray diffraction (XRD) and scanning electron microscopy (SEM), have been used to study bedsediments from two acid mine drainage (AMD) sites in western Montana, USA. TEM and associatedtechniques, including sample preparation via epoxy impregnation and ultramicrotome sectioning, afford theopportunity to better interpret and understand complex water-rock interactions in these types of samples. Forthe sample taken from the first site (Mike Horse mine), ferrihydrite is the dominant phase, Si and Zn are themost abundant elements sorbed to ferrihydrite surfaces, and Pb is notably absent from ferrihydrite association.Three additional important metal-containing phases (gahnite, hydrohetaerolite, and plumbojarosite), that werenot apparent in the powder XRD pattern because of their relatively low concentration, were identified in theTEM. The presence of these phases is important, because, for example, gahnite and plumbojarosite act as sinksfor Zn and Pb, respectively. Therefore, the mobility of Pb from this part of the drainage system depends onthe stability of plumbojarosite and the ability of ferrihydrite to sorb the released Pb. From thermodynamic datain the literature, we predict that Pb will be released by the dissolution of plumbojarosite above a pH of 4 to5, but it will then be recaptured by ferrihydrite if the pH continues to rise to 5.5 and higher, irrespective of competition effects from other metals. Therefore, only a relatively narrow pH window exists in which Pb canescape this portion of the system as an aqueous species. For the sample taken from the other site included inthis study (the Carbonate mine), jarosite and quartz are the dominant phases. Interestingly, however, the jarosites are both Pb-poor and Pb-enriched. In addition, TEM reveals the presence of microcrystalline hematitewith Si, S, and P sorbed to its surfaces, a nearly pure amorphous Si, Al oxyhydroxide, and an amorphous silicaphase containing minor amounts of Al, Ca, and Fe. Pb will probably be released from these mixed K-Pb jarosites above pH 4 to 5, but the Pb may be retarded by the strongly adsorbing microcrystalline hematite inthis pH range. The sink for Al in this system is the amorphous Si, Al oxyhydroxide, not Al(OH) 3  which istypically used in AMD modeling schemes.  Copyright © 1999 Elsevier Science Ltd  1. INTRODUCTION Mineral-water interfacial interactions associated with workingand abandoned sulfide-bearing mines and mining wastes areamong the most complex, dynamic, and environmentally im-portant of all near-surface rock-water systems (e.g., Jambor andBlowes, 1994; Alpers and Blowes, 1994). These mining sites,numbering approximately 200,000 in the United States alone,typically release large amounts of metals into the environmentranging up to hundreds of kilometers along hydrologic gradi-ents in relatively short times. Contamination occurs when highsolute concentrations of iron form during the weathering of pyrite and associated metal sulfides. Acidic effluent from mineentrances and waste piles mix with air and oxygenated surfacewater and precipitate iron oxides, oxyhydroxides, and/or hy-droxysulfates. These phases form mineral/rock coatings andiron-rich sediments that may contain very high concentrationsof associated metals (up to percent levels of, for example, Cu,Zn, and Pb).Field and laboratory studies suggest that both the growth anddissolution of discrete metal-bearing precipitates and adsorp-tion/desorption reactions are the most important processes inthe attenuation or release of toxic metals. However, despitemineralogical (e.g., XRD) and wet chemical (e.g., extraction)techniques, coupled with thermodynamic predictions (e.g., sol-ubility products, saturation indices), many of the phases thatcontain Fe and toxic metals of interest have not been identifiedwith certainty (e.g., Jambor, 1994; McCarty et al., 1998; Web-ster et al., 1998), although important progress is being made(e.g., Bigham et al., 1994; Schwertmann et al., 1995; Websteret al., 1998). Sometimes phases are presumed to be present,usually via thermodynamic arguments, because direct evidenceis lacking. In other cases, the thermodynamic database is in-sufficient to reasonably presume anything. A much better un-derstanding of the minerals and mineral coatings present, aswell as their composition, degree of crystallinity, and micro-fabric, are all critical in understanding toxic metal partitioning(including sorption/desorption reactions) under various Eh/pHenvironments in these complex natural systems. Further, mod-eling the formation of acid mine drainage (AMD) environ-ments, and the transport of toxic metals through and away fromthese sites, would be greatly aided by a much more completeknowledge of the bulk and interface phases that contain thesemetals. *Author to whom correspondence should be addressed: M. F. Hochella,Department of Geological Sciences, Virginia Polytechnic Institute andState University, Blacksburg, VA 24061, USA (hochella@vt.edu). Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 19/20, pp. 3395–3406, 1999Copyright © 1999 Elsevier Science LtdPrinted in the USA. All rights reserved0016-7037/99 $20.00  .00 3395  This study was conceived in hopes of beginning to addresssome of these questions, that is: Is it possible to remove someor all of the uncertainty in determining how metals are tied upin complex, heterogeneous samples taken from acid minedrainage systems? The transmission electron microscope(TEM) was chosen as an excellent way to proceed in such astudy, given its high-resolution imaging, diffraction, and ana-lytic capability. The problem, as is not uncommon in TEMstudies, is sample preparation. How is it possible to prepareheterogeneous precipitates, which might be sensitive to vacuumand beam exposure, for TEM study, while assuring that anunbiased sampling of the particles within the collected sampleis maintained? Although there is no perfect solution to this task,we chose the method of embedding the samples in epoxy resinand using a diamond-knife ultramicrotome to cut cross-sectionsof the resulting block. This method not only allows one toobserve an unbiased sampling of material from a complex andfine-grained sample, but the encapsulation also helps to stabi-lize delicate phases in the TEM environment. Within this study,this combination of sample preparation (embedding/cutting)and probing (analytical TEM) has allowed us to demonstratethat complex trace metal distributions from acid mine drainagesamples can be determined with a high degree of reliability.This in turn allows for a better understanding of toxic metaltransport in these systems, as explored at the end of this article.The samples chosen for this study were associated with theeffluent coming from the Mike Horse and Carbonate mines,located within the upper drainage basin of the Blackfoot Riverin western Montana, USA. Silver, gold, zinc, and copper weremined from sulfide-bearing vein deposits exposed within thesemines starting about 100 years ago. Although mining ceased atthese two sites some time ago, and both have been recentlyremediated, waters that drained from waste rock piles adjacentto the mines, as well as the main mine entrances, were acidicwith elevated concentrations of several transition metals andsulfate for a number of decades. Abundant iron oxide coatingsformed on existing rock and drainage surfaces near the mines.Precipitates formed within these drainages, several kilometersfrom the mine site. Elevated metal solute concentrations couldbe measured along hydrologic gradient for tens of kilometers.This situation is typical of many AMD environments. 2. SAMPLES AND METHODS 2.1. Geologic Setting and General Mine Descriptions The Mike Horse and Carbonate mines are both within the Hed-dleston District of west-central Montana, hosted within the SpokaneShale of the Belt Supergroup. The district consists of red and greenargillites intruded by diorite sills and quartz monzonite porphyry in-trusives of Tertiary age. Economic mineralization in these rocks in-clude Cu-Mo porphyrys and Ag-Pb-Zn veins. Veins are typically0.5–5 m thick and tens of meters long. Hydrothermal alteration of wallrocks adjacent to veins has changed the diorite and quartz monzonite toa rock composed of sericite, clay minerals, carbonates, and pyrite.Mineralized veins within the Mike Horse mine consisted principally of quartz, pyrite, galena, sphalerite, chalcopyrite and an unidentifiedCa-Mn carbonate mineral. Mineralized veins within the Carbonatemine consisted primarily of quartz, pyrite, galena, and sphalerite. 2.2. Sample Collection and Handling The solid samples used in this study, in both the Mike Horse andCarbonate mines, were collected from the top 1–2 cm of bed sedimentwithin 20 m of the main mine adits (horizontal entrances). In the caseof the Mike Horse mine, our sample came from a small channelcarrying water directly from the main adit. The channel was heavilycoated with a reddish-orange mud, generally 1–5 cm thick. The Car-bonate mine sample came from a small and shallow pond in front of theadit. The bed sediment was bright yellowish-orange and very fine, andprobably contained sediment from a nearby tailings pile.All samples were collected as follows: The wet sediments weresieved in the field with ambient water to assure that no chemicalchanges would take place during the sieving process. We used 63   mplastic sieves set in a plastic Bruckner funnel. (Sieving in this sizerange helps remove detrital material, and allows for more uniformcomparison with other studies.) The sieved samples were collected in250 ml plastic bottles which were packed in ice and transported to thelab for immediate processing. In order to remove as much interstitialwater as possible, the samples were centrifuged at 2000 rpm for 15 minand the clear supernatant poured off. The open bottles were then placedin a vented 60°C oven and removed when the samples were fully dry,within 24 hr. Finally, each sample was powdered without grinding bygently mashing the sediment cake with a clean glass rod in the bottles.The clots of sample separated very easily using this method.Drying samples of this nature at temperatures above ambient aregenerally not recommended due to possible mineralogical changesduring heating (e.g., Bigham, 1994). Nevertheless, in this study, wechose the 60°C drying for the following reasons. First, in past work onsamples similar to these, we have noticed no difference in extractionresults for samples that were dried in air, at 60°C, or not at all (analyzedwet). Second, metastable two-line ferrihydrite, shown below to be theprincipal component of our Mike Horse mine sample, is likely to bemore stable when dried quickly at warm temperatures. Air-dried two-line ferrihydrite has the potential to slowly convert to goethite orhematite (Cornell and Schwertmann, 1996). Reduced water activity hasbeen shown to retard or block such transformations (e.g., Torrent et al.,1982). Third, and perhaps most important for this particular study, oursamples are from the surface of bed sediments associated with minedrainage where flow is not continuous. Therefore, our samples havebeen wetted and dried many times previously in their natural setting.Further, in a direct, hot sun, surface temperatures can easily exceed50°C.Samples of the water associated with the recovered bed sedimentswere also taken. These samples were collected by filtering the water inthe field through a 0.45   m membrane (polyethersulfone) filter with a60 cc syringe into an acid washed and DI rinsed plastic bottle. Beforethe final samples were collected, approximately 50 mL of the samplewater was passed through the filter and used to rinse the bottles. At eachlocation, one sample was collected for metal analyses and one for anionanalyses. The samples used for metal analyses were acidified in thefield with nitric acid (trace metal grade). All samples were transportedback to the lab packed in ice. Analysis of metals was performed byinductively coupled plasma-optical emission spectroscopy (TJAATOM COMP 800); analysis of anions was performed by ion chro-motography (DIONEX 2000I).In order to obtain the total concentration of each metal in oursamples, minus what is tied up within the silicates, a portion of the twodried powders were digested in concentrated aqua-regia in a microwaveoven. This process removes the metals sorbed to silicates, and dissolveseverything else. The resulting solutions were analyzed as indicatedabove. 2.3. Transmission Electron Microscopy 2.3.1. Sample preparation and mounting A portion of dried sediment for each sample was added to a glasscentrifugation tube filled with an epoxy resin (we used 5 parts Buehlerepoxide resin #20-8130 mixed with 1 part Buehler epoxide hardener#20-8132). The sediment was spun to the bottom of the tube for asufficiently long time and high speed to reasonably compact the ma-terial, but not enough to density-stratify it. After curing, the tube wassectioned with a diamond saw. These sections were cemented withepoxy to glass slides and carefully polished to a thickness of between10 and 20   m. A second glass slide was cemented to the top of thepolished section with acetone-soluble Crystalbond 509 ® to form a3396 M. F. Hochella et al.  sandwich, and the srcinal glass slide was then ground and polishedaway. The once-again exposed polished section was covered with a thinlayer of epoxy, and after curing, the section/epoxy was finally releasedfrom the second glass slide with acetone. This assembly was embeddedwith more epoxy into the end of a pre-formed epoxy block prism(approximately 4    4    15 mm in size with a cut slit to receive thesection). After trimming the end of the block with a razor blade, it wassectioned with a Leica Ultracut UCT ultramicrotome. Relatively thick500 nm sections were first cut with glass knives prepared using aTaab-Pyper Mk II knifemaker. Final cutting was performed using adiamond knife from Micro Star (#5560 CW). We found that the optimalthickness (in terms of cutting quality) for our samples was approxi-mately 100 nm. This thickness was verified by the interference color(gold) of the epoxy film floating on the water in the trough of thediamond knife holder. The floating films were recovered onto 200- to400-mesh copper, nickel, and/or beryllium TEM grids. The grid mate-rial was chosen according to X-ray interferences from the grid (in thecase of copper and nickel) that could not be tolerated during EDXanalysis. Due to the reactivity of metallic beryllium in water, and itshigh toxicity, use of these grids in this study was limited to a fewsections and is not recommended in general. Finally, all sections/gridswere lightly carbon coated using standard carbon evaporation tech-niques before insertion into the TEM. 2.3.2. Microscopy and chemical analysis A JEM-3010 (JEOL) transmission electron microscope was used inthis study at a beam energy of 300 kV and beam currents between 115and 119   A generated from a LaB 6  filament. Sample grids were loadedinto a JEOL double-tilting, analytical specimen holder for observation.Energy dispersive X-ray (EDX) analysis was performed with the at-tached Oxford Link ISIS system (model 6636) equipped with anatmosphere thin window for light element detection. Semi-quantitativedetermination of key metal concentration ratios were performed usingthe Cliff-Lorimer ratio technique and k AB -factors published in Wil-liams and Carter (1996).Energy-filtered TEM/electron energy loss spectroscopy (EFTEM/ EELS) was performed at Graz University of Technology (Austria) tobetter obtain spatially resolved information about elemental distribu-tion. The images were recorded using a Phillips CM20/STEM equippedwith a Gatan Imaging Filter (GIF) and a slow scan CCD camera. Allimages were processed with Gatan’s Digital Micrograph software andare corrected for dark current and gain variations. Drift between suc-cessive images was corrected using a cross-correlation algorithm avail-able in Digital Micrograph. We acquired jump ratio images (division of post-edge image by a pre-edge image; most useful in cases of lowsignal-to-noise) and images based on the three window method (peakminus background, where background fitting is calculated from twopre-edge windows) (see e.g., Golla and Kohl, 1997, for an explanationof these data collection techniques).No beam or vacuum induced sample degradation was observedduring extended viewing and analysis of our samples in the TEMexcept for jarosite and plumbojarosite. The diffraction patterns of thesephases degraded with time (over several minutes), but not fast enoughto prevent the collection of useful diffraction patterns and EDX spectra.In an attempt to observe a representative portion of material at bothcollection sites with the TEM, we studied ten sections of the MikeHorse mine sample and five sections of the Carbonate mine sample. Inboth cases, the last few sections that were studied showed nothing new. 2.4. Other Methods The samples used in this work were also studied by optical micros-copy both in reflected and transmitted light, as well as by scanningelectron microscopy (SEM, JEOL models JSM-840A and JSM-6300F)and attached EDX systems. X-ray diffraction (XRD) analyses of thesamples were performed on a Philips X’Pert diffractometer and in aDebye-Scherrer camera (Fe radiation to prevent Fe fluorescence). 3. RESULTS 3.1. Mike Horse Mine Sample Analysis of the water collected with this sample is shown inTable 1, and the total metals content of the solid sample (minuswhat is tied up in, but not on the surface of, the silicates) isshown in Table 2. The principal metals present in the solidsample are Fe  Zn  Cu  Al  Mn  Pb.The XRD patterns of this sample are nearly featureless. Thepowder XRD pattern using Cu radiation (Fig. 1) shows a weakline at 27.67°2    (corresponding to 3.34 Å), the principal line of quartz, and a diffuse hump with a maximum at about 35°2   (2.6 Å) which is most likely from ferrihydrite or schwertman-nite. On the film from the Debye-Scherrer camera using FeX-rays, only two very weak but sharp lines are observedrepresenting the most intense quartz lines (d  4.25 and 3.34Å). There are also two very weak, diffuse lines present, oneat 14.5°2    (7.6 Å, srcin unknown), and one at 44.7°2    (2.5Å), the latter the same spacing seen on the pattern collectedwith Cu radiation.Transmitted light microscope examination of the Mike Horsemine sample shows rounded grains in the range of 10–60   m.All grains in normal light appear reddish-brown (rusty) incolor. The interior of the grains are mottled, but any more detailis beyond the resolution of the optical microscope.The SEM reveals a very complicated grain character (Fig. 2).Primary grain size appears to be in the range of 1–60   m.Ultrafine particles, with sizes ranging down to 0.1   m, adhere Table 1. Water chemistry at time of sample collection. (All concen-trations in mg/L).Mike Horse Mine Carbonate MinepH 6.2–6.6 5.3–6.1Mg 207 6Ca 310 12Mn 37 1Fe 49 5Zn 57 1SO 4  1550 50Table 2. Metals released during aqua-regia microwave digestion.(All concentrations in ppm).Mike Horse Mine Carbonate MineA1 9,500 1,400As 200 160Ca 5,700 300Cd 330   1Co 46   3Cu 12,000 734Fe 330,000 140,000K   300 6,100Mg 1,100 37Mn 7,990 22Na   10 660Ni 98   2P 900 400Pb 3,500 14,000Ti 24 55Zn 153,000 1913397A TEM study of samples from acid mine drainage systems  to all of these grains. The ultrafines are dispersed over thelarger grains so that EDX analyses of the larger grains cannotbe obtained without including the ultrafines. All SEM/EDXspectra from this sample, no matter what grain one tries toprobe, shows essentially the same apparent composition, that ismajor O and Fe, followed in abundance by Zn and Si, withlesser amounts of Cu, Ca, Al, Mn, and S.TEM/EDX and EFTEM/EELS reveals the true and highlycomplex nature of this AMD sample. The grains seen in opticalmicroscopy and SEM are actually aggregates of much smallerparticles. Roughly 85–90% (by volume) of the many TEMsections studied of this sample consists of rounded masses of ferrihydrite(nominally5Fe 2 O 3   9H 2 O)between10and100nmindiameter (Figs. 3a–d, 4a, 5a, 6), although occasionally massesa few times larger than this are observed. All of these massesproduce a diffuse but distinct two-ring electron diffractionpattern (maxima at 2.55 and 1.50 Å), corresponding to two-lineferrihydrite (Figs. 4b, c). TEM/EDX and EFTEM/EELS spectraof the ferrihydrite in these samples show it to be principallycomposed of Fe and O, as expected, but also with majoramounts of Zn (EDX indicates a Fe:Zn atomic ratio of up toapproximately 3:1 in some samples, up to 6:1 in others), andsignificant amounts of Si (Fe:Si approximately 6:1) and Al(Fe:Al approximately 10:1). Smaller amounts (on the order of a few percent or less) of Ca and Cu are also present. Mg and Pare sometimes present at the few percent level, but most oftenare not detectable.In addition to the detrital quartz (approximately 5% byvolume), we identified three additional phases, these account-ing for the remaining 5–10% of these samples. They are gahnite(ZnAl 2 O 4 ), hydrohetaerolite (Zn 2 Mn 4 O 8    H 2 O), and plumbo- jarosite (Pb 0.5 Fe 3 (SO 4 ) 2 (OH) 6 ).The gahnite is generally found in rod-like groups, some of which seem to be aggregates of gahnite crystals 5 to 10 nm indiameter (Figs. 3b–d, 5a, b). These groups are typically foundin separate (from the ferrihydrite) and very loose (open) groups,and also often individually within ferrihydrite clumps. EDXspectra show only Zn, Al, and O, with the Zn:Al ratio of approximately 1:2 as expected. EFTEM/EELS also shows thepresence of Zn, Al, and O (Fig. 3b–d). Selected area diffractionpatterns (Fig. 5b) of individual aggregates show ring patternsformed by relatively sharp spots. These rings match the mostprominent powder diffraction lines of gahnite (220, 311, 400,422, 511, 440; JCPDS 5-669). Fig. 1. Powder X-ray diffraction pattern of the Mike Horse mine sample. The data are not noisy, but the vertical scalehas been exaggerated to help bring out the weak features in the pattern.Fig. 2. Representative SEM images of the Mike Horse mine sampleused for TEM sample preparation. (a) Scale bar    10   m. (b) Scalebar  1   m.3398 M. F. Hochella et al.  The hydrohetaerolite is found as minute fibers associated(protruding from) the larger clumps of ferrihydrite (Fig. 4).Hydrohetaerolite is a rare mineral, but it has been describedpreviously to occur in a fibrous form where the fibers mayappear slightly twisted along their long axis (McAndrew,1956). The fibers in our samples are often bent or twisted, andare typically 50–100 nm in length and only a few to 10 nmwide. EDX spectra from these fibers show Zn, Mn, and O, asexpected. Lesser amounts of Fe are present in the spectra, butthis results from the X-ray emission of the intimately associatedferrihydrite that is either in the EDX sampling volume, or fromscattered electrons that generate X-rays from the nearby ferri-hydrite. Positive identification of the hydrohetaerolite comesfrom its electron diffraction pattern (Fig. 4d). Selected areadiffraction patterns obtained from groups of fibers gives dif-fuse, spotted ring patterns. The rings match the d-spacings forthe 112, 103, 211, 220, 231, 224, and 413 lines of hydrohetae-rolite representing 7 of the 9 most intense lines reported fromX-ray diffraction (JCPDS 9-459).The plumbojarosite is found as anhedral to subhedral blockycrystals as small as 100 nm and as large as 1  m in their largestdimension (Fig. 6). The EDX spectra show Pb, Fe, and a smallamount of K (substituting for Pb). The principal S line (K  )overlaps the Pb M line so that it cannot be individually ob-served. Positive identification comes from a number of singlecrystal, selected area diffraction patterns obtained from thesegrains. Diffraction maxima generated by the hkl’s 012, 202,116, 0114, and 306 have been observed; these represent themajority of the most intense lines observed in X-ray diffractionanalysis of plumbojarosite (JCPDS 39-1353). Fig. 3. EFTEM/EELS images of the Mike Horse mine sample. (a) Fe image. The bright circular areas (high Feconcentration) are individual ferrihydrite aggregates. (b) O image. Ferrihydrite particles can again be seen, as in (a). Thebright stringers are gahnite aggregates, as shown in (c) and (d). (c) Al image. The gahnite shows the highest concentrationof Al, although the ferrihydrite has an appreciable amount as well. (d) Zn image. The gahnite aggregates clearly stand out,and the Zn sorbed to the ferrihydrite can also be seen.3399A TEM study of samples from acid mine drainage systems
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