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Mineral Surface Coatings: Environmental Records at the Nanoscale - Elements
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Mineral Surface Coatings: Environmental Records at the Nanoscale

Past and present (a)biotic soil processes can be preserved by mineral surface coatings, which can sequester contaminants in soils and sediments. The coatings can contain complex assemblages of nanometer-size minerals and organic components. The formation, composition, and morphology of these complex mineral assemblages depend on, and hence reflect, the mineralogical and chemical composition of the substrate they develop on and the environmental factors in the surrounding soils and sediments. Mineral surface coatings typically contain complex and variable porosities, many with regions of limited fluid flow. Low-flow conditions, combined with different nanometer-size phases in the interior of mineral surface coatings, allow coatings to sequester contaminant-bearing solutes, complexes, and nanoparticles.

DOI: 10.2113/gselements.13.3.159

Keywords: soils, geochemical records, contaminants, sorption, transport, microaerophilic conditions

Many chemical and mineralogical transformations in soils and sediments occur on the surface of colloids, mineral grains, and organic material. These soil constituents are often coated with secondary phases that form patchy to continuous mineral surface coatings at the nano- to micrometer scale. These coatings are not only part of the critical zone, the skin of our planet, they also share many of its attributes. Coatings can be a complex mixture of water, organic matter, and minerals, and chemical reactions proceed both abiotically and through catalysis by microorganisms. Minerals in the upper part of mineral surface coatings re-equilibrate with percolating fluids in soils and sediments to create environmental gradients between early and later-formed mineral assemblages. As a result, there is no representative naturally occurring mineral surface coating.

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Figure 1. (A) Transmission electron microscope image of goethite coating nanoparticles of crandallite [CaAl3(PO4)2(OH)5(H2O)] in surficial soils. Used with permission of the National Academy of Sciences USA from Bertsch and Seaman (1999). (B) Optical image of Fe-oxide-bearing nanocrystalline Al-silicate coatings (brown) on sand grains from an aquifer. Image by R. L. Penn. (C) Backscatter scanning electron microscope (SEM) image of Fe-hydroxide coatings on a chalcopyrite grain from acidic soils in Sudbury, Ontario (Canada). (D) Backscatter SEM image of a coating containing anglesite (PbSO4) on organic material in Pb-contaminated soils at Trail, British Columbia (Canada). Images for C and D by M. Schindler.

Coatings can occur in different environmental, mineralogical, and chemical settings. For example, goethite can coat nanoparticles of crandallite [CaAl3(PO4)2(OH)5(H2O)] in surficial soils (Fig. 1A); Fe-oxide-bearing nanocrystalline Al-silicate can coat sand grains in aquifers (Fig. 1B); Fe-hydroxide can coat Fe-sulfide grains in acidic soils (Fig. 1C); and anglesite [PbSO4] can coat organic material from Pb-contaminated soils (Fig. 1D). There are as yet no systematic studies on the abundance and composition of mineral surface coatings with depth or soil horizon. However, studies of surficial, highly weathered soils in Canada indicate that ~2/3 of all silt- to sand-size grains are coated with visible (micrometer-thick) material (Schindler et al. 2016).

Mineral surface coatings form through the influx of constituents from the dissolving underlying mineral and from fluids originating in the surrounding soils and sediments; constituent proportions in the coatings vary with the solubility and reactivity of the mineral and with the chemical and mineralogical environment. Mineral surface coatings that result from the weathering of primary aluminosilicate grains common in soils and sediments typically produce a thin, hydrous, patchy, natural coating of amorphous and crystalline secondary aluminosilicates that can range up to 10 nm in thickness. (Nugent et al. 1998; Zhu et al. 2006). These coatings often form from coupled dissolution–precipitation reactions at the interface between the mineral surface and the coating (Putnis and Ruiz-Aguda 2013) where the dissolution of the silicate results in the release of all incorporated elements and in the reprecipitation of a surface layer enriched in one of the major elements. Crystalline secondary phases, such as clays (e.g. kaolinite, smectite) on the surface of silicate minerals (Banfield and Egglegton 1990) and Fe-hydroxides on the surface of Fe-sulfide minerals (Huminicki and Rimstidt 2009), can form porous layers that are tens of micrometers in thickness. These thicker coatings do not commonly constitute a diffusion barrier for external solutions and, thus, do not prevent further weathering of the underlying mineral grain. Inclusion of external species, typically dominated by insoluble Fe(III), Al, and Mn(IV) oxy-hydroxides, may result in the formation of mineralogically complex surface coatings whose formation only partially depends on the underlying composition of primary and/or secondary phases. These coatings often contain confined pore spaces with adsorbed species, nanoparticles, and assemblages of nano-size minerals which are not in equilibrium with other constituents in the coatings or with pore solutions in the surrounding soils.

Building Complex Assemblages of Nano-size Minerals

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Figure 2. (A) Backscatter scanning electron microscope (SEM) image of mineral surface coating with multiple layers on an Fe-silicate mineral with a composition close to ferrosilite (FeSiO3). The area depicted in 2B is indicated with a square. (B) Higher-magnification SEM image of the square section indicated in 2A showing two distinct interfaces between the underlying grain (dashed lines) and two layers: Layer 1 and Layer 2. The location of a section lifted out with the focused ion beam technology is indicated with a rectangle. (C) Scanning transmission electron microscope image of the rectangular section depicted in 2B. The location of Layer 1 and Layer 2 and their interface (dashed line) are shown. The four numbers indicate examples of the major constituents: (1) locally formed crystals of magnetite and goethite; (2) precipitates of Fe-oxysalt minerals (jarosite) encapsulated in bands of amorphous silica; (3) petrified magnetotactic bacteria and magnetosomes (see also Fig. 5); (4) Fe-hydroxide matrix with pockets of encapsulated clay minerals (illite and clinochlore) and ternary Cu-phosphate adsorption complexes.

The formation of complex mineral assemblages in surface coatings (Fig. 2) is often the result of various chemical, physical, and biological processes, such as abiotic- and biotic-controlled dissolution–precipitation reactions, encapsulation of minerals and bacteria through silicification, and the aggregation of nanoparticles to larger particles and crystals [so called “crystallization through particle attachment” (De Yoreo et al. 2015)]. For example, recent nanometer-scale characterizations of coatings on sand grains (Fig. 1B) have shown the occurrence of crystalline Fe-oxide fragments in an amorphous/nanocrystalline matrix enriched in Si and Al relative to Fe (Penn et al. 2001). Mineral assemblages of even higher complexity can form in coatings when biogeochemical conditions in soils and sediments change dramatically over time (Schindler and Hochella 2015, 2016). For example, coatings formed in surficial soils that have been impacted by acidification and subsequent remediation can contain different generations of mineral assemblages:

  • A first generation of clay minerals formed during the weathering of the underlying mineral at near neutral pH condition and, thus, prior to the acidification of the soils.
  • Locally formed crystals of Fe-oxides (labelled “1” in Fig. 2C), precipitates of Fe-oxy-salt minerals and bands of amorphous silica (“2” in Fig. 2C) and aggregates of Ti- and Zr-oxide nanoparticles, all formed during soil acidification.
  • Petrified magnetotactic bacteria and preserved magnetosomes (see below), indicators of microbial activity and biotic-controlled dissolution reactions in the coatings (“3” in Fig. 2C).
  • A second generation of clay minerals within pore spaces (“4” in Fig. 2C) and on the surface of earlier-formed minerals that had formed during and after remediation of the soils.

These observations indicate that micrometer-thick coatings can preserve past and present environmental changes in local soils and sediments and that these changes can be deciphered through the study of nanometer-size mineral assemblages within these coatings.

Coatings as Contaminant Scavengers

Mineral surfaces in soils and sediments can directly affect water quality because these surfaces are able to scavenge contaminants such as metal-bearing complexes and nanoparticles, oxy-anions (phosphates and nitrates), organic matter and microorganisms (Chorover et al. 2007; Schindler et al. 2016). As such, the type and abundance of mineral surface coatings in soils and aquifers have a tremendous effect on the overall ability of the bulk material to sequester contaminants because mineral surface coatings can increase their surface area and modify their surface charges (Bertsch and Seaman 1999).

Scavenging and sequestration of contaminant-bearing complexes and nanoparticles by a mineral surface coating is principally controlled by two factors:

  • The occurrence of a porous matrix with spatial domains of limited flow potential (Singer et al. 2013; Zachara et al. 2016). These domains can exist as intra-aggregate and intragranular spaces, and over a wide range of spatial scales. In particular, pore spaces in the nanometer and sub-micron size domain may account for a large volume of reactive surface sites, despite accounting for only a small fraction of the total pore volume. With respect to contaminant transport, the fluids in these regimes are relatively immobile, and solute transport may be dominated by diffusion between extragranular and intragranular pore space, driven by concentration gradients. Furthermore, limited flow regimes can also hinder equilibration between phases of different generations and between such phases and current pore solutions in the surrounding soils and sediments (Wang et al. 2003; Singer et al. 2013).
  • Different generations and types of assemblages of nano-size phases in the interior of mineral surface coatings. These phases generate various types, abundances, and distributions of reactive surface sites and surface charges, which, in turn, control diffusion, adsorption and speciation of solutes and colloids (Singer et al. 2013; Schindler and Hochella 2015, 2016; Schindler et al. 2015, 2016).

Understanding Solute Transport Through Complex Porosity

Solute reactions and transport within mineral coatings occur in a physical environment with inherently complex porosity. Pore-scale simulations have revealed the importance of coupled mass transport and chemical reactions in both intragranular and intergranular domains: these can influence metal-complexation rates in sediments, both spatially and temporally (Liu et al. 2013). Further, these pore-scale simulations also reveal that the rate of coupled diffusion and molecular surface complexation reactions in the intragranular porous domains were slower than either individual process alone. A critical component of building predictive models of contaminant transport is, therefore, to determine the specific controls on metal sorption and diffusion in porous material. Using model materials can aid in these studies.

Porous materials are broadly classified according to IUPAC nomenclature as microporous (<2 nm), mesoporous (2–50 nm), and macroporous (>50 nm) (Rouquerol et al. 1994). Synthetic porous materials are ideal models for studying naturally occurring porous earth materials under variable conditions because they can be tailored to have a simplified pore geometry, a well-constrained pore-size distribution, and reproducible surface properties (Selvam et al. 2001). Mesoporous materials represent a pore-size regime that can account for a significant fraction of the reactive surface area of porous earth materials and mineral surface coatings (Sammartino et al. 2003). Porous silica is an important constituent in these settings, both as quartz and amorphous silica gels, playing an important role in surface environments by attenuating contaminant transport by sorption and precipitation reactions (Chen and Hayes 1999; Ford et al. 2001). These materials are also relevant to metal transport phenomena in the weathering and degradation of glass and other amorphous materials used in nuclear waste forms (Greaves et al. 1989). Mesoporous silica is, therefore, a useful and relevant model for these materials, while allowing for a high degree of control over the physical and chemical properties of the sorbent.

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Figure 3. (A) Schematic of uranium (U) interacting with mesoporous silica. As U diffuses into the pore channels, sorption along the pore walls leads to polymerization and will prevent U from diffusing more deeply into the channels. As the concentration gradient near the channel openings increases, polymerization at these corner sites lead to a precipitation front that moves deeper into the channels along the pore walls through the polymerized surface species. (B) Transmission electron microscope image of washed, unreacted mesoporous silica. Inset: cross section of a particle exhibiting the regular channel array structure. (C) Mesoporous silica reacted with 10 mM U at pH 4.0 with no carbonate and no calcium present. The white arrows point towards some of the precipitates. Modified with permission of Elsevier from Singer et al. (2014).

A fundamental understanding of solute diffusion into micro- and mesoporous materials remains uncertain. The complex relationship between solute transport and reactivity in these systems is exemplified by the sorption of hexavalent uranium [U(VI)]. Under acidic and oxidizing conditions, U exists as uranyl hydrolysis products, and readily forms aqueous complexes with carbonate (CO3) and calcium (Ca) at circumneutral and alkaline pH values (Singer et al. 2014). All of these U species have significantly different solute properties and sorption behaviors. Uptake of U by porous silica compared to bulk material can result in different sorption reaction products, slower desorption kinetics, enhanced nucleation and precipitation, and the capacity of colloidal transport, all of which are dependent on the pore structure and size, as well as the type and density of terminating functional groups at pore surfaces (Štamberg et al. 2003; Vidya et al. 2004; Wang et al. 2013; Singer et al. 2014). One key finding is that rates of desorption from small pores are not only scale-dependent, but they may be concentration- and speciation-dependent due to polymerization and precipitation (Singer et al. 2014). This latter process can result in a recalcitrant pool of ions that are sequestered in deep internal pore spaces (Fig. 3). This highlights the need of future work aimed at elucidating the relationship between coupled geochemical processes occurring in nano-size pore spaces and solute transport in confined pore spaces.

Solute Transport and Reaction in Confined Pore Spaces

Pore spaces in mineral surface coatings with limited flow potential often create microaerophilic conditions: aerobic environments with oxygen at lower concentration than in the atmosphere. These conditions can affect the speciation of contaminants in pore spaces and promote the formation of minerals and the activity of bacteria that do not commonly occur in upper soil horizons due to high O2 fugacity (Figs. 4 and 5). In one study aimed at determining the role of nano-size pore spaces on limiting contaminant transport, Singer et al. (2013) examined mineral surface coatings from a well-characterized field site to determine the rate-limiting causes of arsenic (As) sorption and redox processes within mineral coatings (Fig. 4). Sediments were obtained from the US Geological Survey’s field research site at Cape Cod (Massachusetts, USA) and these sediments were exposed to synthetically contaminated groundwater solutions. Uptake of As(III) and As(V) into the (pre-existing) coatings was studied with a combination of high-resolution transmission electron microscopy and synchrotron techniques to assess concentration gradients and reactive processes, including electron transfer reactions. Arsenic was primarily associated with micron- to submicron aggregates of Mn-bearing nano-size goethite. Oxidation of As(III) by goethite was spatially limited to the exterior of the mineral coatings where goethite grains had exposed surface area. Little to no oxidation of As(III) to As(V) occurred deeper in the coating (Fig. 4). This work showed that microaerophilic conditions in the interior of mineral surface coatings can prevent oxidation of redox-sensitive elements and that mineral surface coatings are potentially both sinks and sources of contaminants (depending on the history of a contaminated site) and may need to be included explicitly in reactive transport models.

Minerals formed under microaerophilic conditions in mineral surface coatings have been identified by Schindler and Hochella (2015). For example, single crystals of fougèrite {[Fe2+6(1−x)Fe3+6xO12H14−6xCO3](H2O)3}, which is also known as green rust, can occur in nano-size pore spaces of coatings from the upper 5 cm of surficial soils (Fig. 5A), despite fougèrite most commonly being formed in wet soils under reducing conditions. Similarly, petrified magnetotactic bacteria occur in proximity to earlier-formed jarosite [KFe3+3(OH)6(SO4)2] in the same mineral surface coatings, despite such bacteria being commonly active under reducing to microaerophilic conditions (Kopp and Kirschvink 2008) (Figs. 2, 5B, 5C).

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Figure 4 (A) Fe micro-X-ray fluorescence (micro-XRF) tri-colored map of characteristic coatings on primary quartz grains in a polished thin section. The map shows the distribution of silicon (green), iron plus manganese (red), and arsenic (blue). The white-dashed rectangle indicates the area shown in 4B. (B) Fe micro-XRF map of the rectangular section shown in 4A. Arrowed white-framed boxes indicate the amount of remaining As(III) in a Mn-bearing Fe-rich phase (likely nano-size goethite) determined from micro-X-ray absorption near-edge structure (m-XANES) spectra shown in 4C. The amount of As(III) remaining is related to the extent of available surface area in contact with the exterior of the mineral coating (the blackish area is epoxy and denotes the original grain surface/solution contact). This sample was exposed to As(III) for 30 minutes. (C) First derivative As K-edge m-XANES spectra of the approximate location where the arrows point in 4B. Sodium arsenite and sodium arsenate were used as As(III) and As(V) standards, respectively.

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Figure 5. Microaerophilic conditions (aerobic environments with oxygen at lower concentration than in the atmosphere) in confined pore spaces in mineral surface coatings within the upper 5 cm of surficial soils can be identified on minerals and bacteria that form under wet and reducing conditions. (A) Transmission electron microscope (TEM) image of elongated crystals of green rust. Crystals of green rust commonly form in wet soils under reducing conditions but are observed in mineral surface coatings formed in the upper 5 cm of a soil profile. (B) TEM image of magnetosomes (chains of small cubes; indicated with arrow) in proximity to petrified magnetotactic bacteria. Also, spherical particles of magnetite on the surface of jarosite, likely of abiotic origin (indicated with arrow). (C) A plot with the concentration of sulfide and oxygen versus depth. Higher concentrations of oxygen occur at shallower levels near the surface and higher concentration of sulfide occur at deeper levels. Note that the depth scale is undefined because it depends on the porosity of the material; it can be on the nanoscale in mineral surface coatings and on the centimeter- to meter-scale in soils. Magnetite-producing bacteria are only active at low concentrations of O2 and sulfide. After Kopp and Kirschvink (2008).

The above findings may be relevant for identifying and understanding potential storage places for carbon in surficial soils. The carbon budget of soils is controlled by the stability of natural organic material, which is commonly stabilized against microbial degradation in two ways: first, by adsorption on clay minerals and poorly crystalline Al and Fe oxides; second, by enclosure into micrometer-size pore spaces (Brantley et al. 2007; Chorover et al. 2007). On the basis of the observations made by Schindler and Hochella (2015), pore spaces in mineral surface coatings should be able to prevent the degradation of encapsulated carbon species, making such pore spaces potential locations for the long-term storage of carbon species in surficial soil horizons.

A New Dimension in Earth Sciences

Mineral surface coatings are structurally and compositionally complex and heterogeneous. They must be probed across a range of scales in order to determine their roles in the sequestration of carbon and in controlling contaminant transport in soils and aquifers. Chemical, mineralogical and biological processes within pore spaces of mineral surface coatings are an underexplored domain in understanding earth processes at the nanoscale. Future nano-mineralogical and geochemical studies of synthetic porous media , soils, and sediments will most likely discover many unexpected features in terms of adsorption, attachment and formation of contaminant-bearing aqueous species, colloids and mineral assemblages.

Acknowledgments

We thank Ronald Dorn, Friedhelm von Blankenburg and the reviewers Jon Chorover and Eugene Ilton for their constructive comments. MS thanks NSERC and the Nano Earth Program at Virginia Tech for financial support and Michael Hochella Jr. for his enthusiasm to study processes in soils at the nanoscale. DMS thanks James Davis (LBNL) for the opportunity to study natural and synthetic porous materials.

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