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Influence of Dissolved Organic Matter on the Environmental Fate of Metals, Nanoparticles, and Colloids George R. Aiken,*,‡ Heileen Hsu-Kim,§ and Joseph N. Ryan^ ‡
U.S. Geological Survey, 3215 Marine Street, Boulder, Colorado 80303, United States Department of Civil and Environmental Engineering, Duke University, Box 90287, Durham, North Carolina 27708, United States ^ Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, Colorado 80309, United States §
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e have known for decades that dissolved organic matter (DOM) plays a critical role in the biogeochemical cycling of trace metals and the mobility of colloidal particles in aquatic environments. In recent years, concerns about the ecological and human health effects of metal-based engineered nanoparticles released into natural waters have increased efforts to better define the nature of DOM interactions with metals and surfaces. Nanomaterials exhibit unique properties and enhanced reactivities that are not apparent in larger materials of the same composition1,2 or dissolved ions of metals that comprise the nanoparticles. These nanoparticle-specific properties generally result from the relatively large proportion of the atoms located at the surface, which leads to very high specific surface areas and a high proportion of crystal lattice imperfections relative to exposed surface area. Nanoscale colloids are ubiquitous in nature,2 and many engineered nanomaterials have analogs in the natural world. The properties of these materials, whether natural or manmade, are poorly understood, and new challenges have been presented in assessing their environmental fate. These challenges are particularly relevant in aquatic environments where interactions with DOM are key, albeit often overlooked, moderators of reactivity at the molecular and nanocolloidal scales. Interest in understanding in greater detail the chemistry and importance of DOM interactions with metals and mineral colloids has been motivated, in large part, by two developments. First, a better understanding of the environmental factors that control metal bioavailability and reactivity is sorely needed as society strives to manage resources, restore ecosystems, and ameliorate r 2011 American Chemical Society
the effects of metal pollutants such as mercury (Hg) or copper (Cu). For instance, resource management efforts in the Florida Everglades3 and the Sacramento-San Joaquin Delta in California4 have identified the need to elucidate the behavior of ecologically active metals as a means for attaining long-term success in restoration efforts. In these systems, management decisions often hinge on the answers to key questions that involve chemically complex environments at aqueousgeologicalmicrobial interfaces. The processes of interest are often kinetically limited and not simply controlled by equilibrium-based drivers.5 The second wave of interest arises from concerns about the ecological and human health consequences stemming from exposure to engineered nanoparticles that have proliferated in commercial products over the past decade.6 The major concern is that engineered nanoparticles uniquely interact with organisms owing to their minute size and enhanced reactivity relative to larger particles. To respond to potential environmental, ecological, and health concerns, there is a concerted effort to evaluate potential risks in parallel with developments in nanotechnology applications.7 A first step in delineating potential ecological and human health hazards of nanoparticles is to determine the means of exposure after release to the environment. Thus, an important research need is to assess and predict the influences of DOM on the transport, transformations, bioavailability, and toxicity of nanoparticles in the natural environment. The DOM Connection. Dissolved organic matter consists of a complex, heterogeneous continuum of high- to low-molecular weight species exhibiting different water solubilities and reactivities. Historically, aquatic organic matter has been arbitrarily divided into dissolved and particulate organic matter based on filtration, generally through a 0.45-μm filter. No natural cutoff exists between these two fractions and the distinction is operational. Based on recent advances in the analysis of DOM, many thousands of molecules, most of which are relatively low molecular weight (less than 2000 Da), are known to contribute to the composition of DOM in a given water sample.8 DOM is, therefore, depicted in Figure 1 to be smaller than 10,000 Da. Ultimately, DOM composition between samples varies as a function of source materials, biogeochemical processes, and hydrology. Because of their ubiquity in natural waters, the molecules comprising Special Issue: Nanoscale Metal-Organic Matter Interaction Published: March 15, 2011 3196
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Figure 1. Typical length scales corresponding to dissolved organic matter, dissolved metals and metalligand complexes, polynuclear metal clusters, colloids (including nanoparticles), and microorganisms. Operationally defined size ranges have been employed to study these constituents in the aquatic environment (modified from ref 17).
DOM frequently control metal speciation,9 alter the surface charge of particles,10 sorb to mineral surfaces, interfere with mineral dissolution/precipitation reactions,11,12 and drive redox13 and photochemical14,15 reactions. DOM also plays significant roles in the kinetics of environmental reactions and the availability of metals to exposed organisms.16 Much of the early work to elucidate DOMmetalcolloid chemistry addressed interactions with colloids of larger size and under environmentally unrealistic conditions because of analytical limitations. With the development of new analytical approaches and a growing need to understand the biogeochemistry of metals and nanoparticles, there has been a resurgence of research into the effects of DOM on their behavior. MetalDOM Binding. Trace metal complexation by DOM in aquatic environments has been studied for decades, but even today, determination of binding constants is hampered by the intrinsic complexity of DOM, the lack of stoichiometric information, and analytical limitations.18 Metal binding by DOM remains poorly defined at the molecular scale under environmentally relevant conditions such as low concentrations of metals relative to DOM. Under these conditions, metal binding is often driven by functional group chemistry and structural constraints not reflected in the bulk-scale properties often used to describe DOM chemistry. Examples of bulk-scale properties include aromaticity, elemental composition, and major functional group content, such as carboxyl and hydroxyl groups. The strong binding affinities by low-abundance functional groups, including nitrogen (N) and reduced sulfur (S) groups (i.e., thiols, organic sulfides) is now clearly recognized for many trace metal pollutants (e.g., refs 1922). Measuring the presence and stereochemistries of these types of binding sites remains an analytical challenge. Furthermore, the mechanisms that allow these groups to persist in unexpected settings (i.e., well-oxygenated water where sulfur oxidizes) remain poorly understood. Recent work suggests that metal complexation, itself, may provide a protective effect against oxidation of reduced-sulfur groups in DOM.23
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Another major challenge remains in developing generalized speciation models that can adequately account for poorly defined macromolecular ligands that make up the bulk of the DOM. Dissolved organic matter contains metal-binding functional groups (such as carboxylates, phenols, amines, thiols) with binding affinities and ligand densities spanning many orders of magnitude. The challenge in accounting for all binding states has been addressed by quantifying metalDOM binding in experiments that include a wide range of conditions (e.g., pH, ionic strength, metal-to-DOM ratio, other competing ions). The results are incorporated into models that portray the binding interactions with continuous or discrete distribution of complexation equilibrium constants.9 Such models can be specific to the DOM tested or averaged over many types of DOM. This approach largely ignores the molecular-scale basis and kinetics of such interactions; nevertheless, it has been incorporated into equilibrium-based models, such as the biotic ligand model (Figure 2a), for predictions of acute toxicity and development of water quality standards by regulatory agencies. The incorporation of metal complexation by DOM into the biotic ligand model represented a major advance in the determination of metal toxicity standards.24 However, over the past decade, further advances in the understanding of metal binding by DOM and awareness of the potential role of nanoparticles for metal bioavailability provide the basis for improvements in the next generation of models. Cluster Stabilization and Heterogeneous Precipitation. Beyond serving as a ligand, organic matter also alters interactions between metals and inorganic ligands, especially as the transformations involving metalinorganic ligand compounds progress from complexes to clusters to colloids under conditions of mineral supersaturation. As depicted in Figure 2b, organic matter alters the kinetics of reactions involved in the heterogeneous precipitation of a mineral. In studies that employed low molecular weight organic acids, surfactants, and DOM isolates, organic matter has been shown to interfere with the precipitation of calcium carbonate (CaCO3),25 iron oxyhydroxides,26 and metal sulfides.12,27 The polymerization or growth of new solid phases under kinetically controlled conditions typically involves metastable amorphous nanoparticle or cluster compounds generated during the initial stages of heterogeneous precipitation reactions.28,29 In environmental scenarios where dissolved DOM exceeds metal ion concentrations in water, DOM binds to dissolved metal ions, resulting in a decrease of the mineral saturation index (the driving force for precipitation). Dissolved organic matter also sorbs to the surface of nucleated particles, thereby altering the interfacial properties of new phases that form from solution. Such reactions are controlled, in part, by the metal/DOM concentration ratio (Figure 3). For example, the precipitation kinetics of amorphous ferric hydroxides in DOMrich water is a balance between complexation of dissolved Fe by DOM and hydrolysis reactions of ferric iron.26,30 Similarly, sulfhydryl-containing ligands are capable of decreasing precipitation rates of metal sulfides,27 presumably by complexing metal ions or by sorbing to the surface of nucleated nanoclusters. Much is unknown regarding the polynuclear clusters or nanoparticles that are formed during heterogeneous precipitation with DOM. Such nanoscale materials could potentially exhibit solubility and bioavailability properties that are different from larger, more crystalline materials of similar composition. Mineral Colloid Stability and Dissolution. At the larger end of the size scale, DOM has long been known to influence the 3197
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Figure 2. (a) In the conventional view, dissolved organic matter (DOM) influences metal bioavailability by decreasing the amount of metal ion available for the biotic ligand. (b) In real aquatic environments, DOM reacts with a continuum of dissolved metals, polynuclear clusters, nanoparticles, and colloids through heterogeneous precipitation of metastable clusters and nanoparticles, sorption of DOM to mineral particle surfaces, and reactions at colloidwater interfaces that control aggregation, deposition, and dissolution kinetics of these colloids. Whereas bioavailability of metals depends on the type of organism and route of exposure, dissolved metal speciation and the solubility of particles (both outside and inside the organism) are two properties that influence accessibility to exposed organisms.
behavior of colloid-size materials with respect to surface chemistry, aggregation, and dissolution (Figure 2b). Mineral surfaces readily adsorb DOM, resulting in neutralization, and even reversal, of the surface charge of colloids.10 Charge reversal means that organic matter binds to mineral surfaces as specific surface complexes, even in the face of negative charge on the mineral surfaces. The study of particle aggregation and deposition kinetics in DOM-containing water has applied mostly to water treatment applications where flocculation and bed-filtration processes actively induce DOM interactions with aluminum and iron oxides as a means for removing DOM from the water (e.g., ref 32). In natural settings, these interactions have been shown to stabilize particles and colloids in fresh waters and even seawater.33 Sorption of DOM to mineral colloids can also enhance or inhibit dissolution, depending on the mechanism of DOM sorption and solution composition.11 Challenges remain in understanding the geochemistry and bioavailability of stabilized
clusters and small colloids under commonly encountered conditions where the colloids of interest are present in much lower concentrations than those in the studies cited above. The incorporation of macromolecular DOM in surface interaction models also remains a challenge and an ongoing area of research.34 Engineered Nanomaterials. New challenges are posed by the anticipated proliferation of engineered nanomaterials including metallic nanoparticles (e.g., iron [Fe], silver [Ag], and gold [Au]), metal oxides (e.g., titanium dioxide (TiO2), zinc oxide [ZnO]), metal sulfides (e.g., cadmium sulfide [CdS]), and quantum dots (e.g., cadmium selenide [CdSe]). Key questions for this challenge relate to DOM reactivity with manufactured nanoparticles and potential differences with their naturally occurring nanoparticle analogs. Much of the recent work involving DOM and engineered nanomaterials (e.g., refs 3537) has shown that DOM slows aggregation of nanoparticle suspensions and inhibits the deposition of nanoparticles in saturated porous 3198
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photoreactive species to include unique structural aspects related to bonding (e.g., thiols and methylmercury [CH3Hgþ]47) or photoreactive microregions within DOM macromolecules.14 Dissolved organic matter itself is susceptible to light-induced transformation15 caused by photoreactive intermediates generated from the metal and nanoparticles. Nanoparticles of titanium dioxide (TiO2), zinc oxide (ZnO), and silicon dioxide (SiO2) are known for their ability to catalyze photolysis reactions owing to their generation of reactive oxygen species upon irradiation by ultraviolet light or sunlight.48 The effects of photolysis on natural organic matter (NOM) adsorbed to the surface of these metal oxide nanoparticles, and the resulting effect on the surface charge and transport of the nanoparticles, have not been investigated.
Figure 3. Concentration of the metal relative to natural organic matter will control the extent to which DOM alters heterogeneous mineral precipitation kinetics and subsequent reactivity of the metal/mineral and bioavailability of the metal to aquatic organisms (modified from ref 31).
media—results that are similar to studies of DOM interactions with larger colloids. Few environmental studies have considered interfacial reactions with nanoscale-dependent characteristics such as sorption of solutes3840 and mineral dissolution.41 Nanospecific reactions between DOM and nanoparticles need to be considered in future work. Also, engineered nanomaterials may typically be released into the environment with polymer or surfactant coatings applied to control the size and stability of the nanoparticles. The role of DOM for the “weathering” or modification of these coatings is not clear. The alteration of interfacial chemistry by DOM and the kinetics of nanoparticle aggregation, deposition, and dissolution (Figure 2b) are relevant for most pollution scenarios where DOM greatly exceeds nanoparticle concentration. All of these interactions will collectively determine the persistence of the nanoparticles in the environment and their potential toxicity to exposed organisms. For instance, sorption of DOM to ZnO42 and nanoparticulate zerovalent iron (Fe0, ZVI)43 has been shown to reduce toxicity to microbes, possibly by decreasing direct interaction of the nanoparticles with microbial cells.43 Moreover, DOM binds to dissolved metals released from the surfaces of quantum dots44 and Ag nanoparticles,45 thereby ameliorating metal toxicity to microbes in ways consistent with the biotic ligand model. Phototransformations. A critical area for future advances is related to photoreactivity of metals, nanoparticles, and DOM. Interactions of DOM with sunlight are important drivers for a number of reactions in surface waters that control metal speciation and surface chemistry of colloids. In these reactions, DOM acts as a primary chromophore (light-absorbing entity) that can influence photochemical transformations either directly through bonding interactions (e.g., serving as a ligand or sorbed to the surface of a nanoparticle) or indirectly by generating highly reactive intermediates (e.g., organic radicals) and reactive oxygen species (e.g., singlet oxygen, hydrogen peroxide [H2O2], superoxide [O2], hydroxyl radicals [•OH]). Sunlight-induced reactivity of DOM is critical for the transformations of key elements in the environment (e.g., iron, mercury46,47). In all cases, the photochemical reactions of DOM, metals, and nanoparticles are complex and poorly understood, particularly with regard to their effects on the surface chemistry of engineered nanoparticles. The complexity extends beyond the role of DOM as a generator of a
’ FUTURE DIRECTIONS AND NEW TOOLS TO ADDRESS THESE QUESTIONS Our recent focus on nano- and molecular-scale structure is largely driven by the application of new tools and novel adaptations of older approaches that enable the study of environmentally relevant compounds and materials at this scale. With these new methodologies, we are at the cusp of uncovering hidden processes that can explain the interactions among dissolved organic matter, metals, and minerals. In addition to more sophisticated tools for characterizing DOM composition,8 new applications of synchrotron-based methods and other spectroscopic approaches enable investigations of element-specific or functional groupspecific speciation (e.g., refs 12,49). Whereas these tools are generally used for bulk scale characterization and lack sufficient sensitivity for measurements at low constituent concentrations, they are especially powerful for elucidating surface composition and surface transformation of nanoscale materials. Time-resolved techniques such as photon or neutron scattering enable kinetics studies of particle transformations while they are in aqueous suspension. Electron, X-ray, and atomic force microscopy techniques (and associated spectroscopic devices) allow for spatially resolved speciation data of nanoparticles. When combined with more conventional research approaches that consider metal complexation and mineral solubility, these tools provide the possibility for revealing complex reactions and transformations that govern DOMmetalmineral interactions. Interest in the behavior of nanoparticles in natural systems has brought about a new appreciation of the roles these materials play in fundamental processes at the watermineralcell interface, and has opened new avenues of inquiry in the geosciences, ecotoxicology, risk assessment, environmental engineering, and soil science. Our understanding of the global cycling of major and trace elements can be informed by ongoing advances in delineating the interactions between DOM and nanoparticles and the consequences of these interactions for metal speciation and bioavailability in natural waters. Likewise, assessments on the environmental implications of nanomaterials need to consider new and decades-old lessons learned from research on metals and mineral colloids. Along with these advances, an improved comprehension of DOM composition and reactivity may evolve, and this will provide insights to the roles played by DOM in ecological and geochemical processes. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 3199
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’ BIOGRAPHY George Aiken is a senior research scientist at the U.S. Geological Survey in Boulder, Colorado. Heileen Hsu-Kim is an assistant professor in the Department of Civil and Environmental Engineering at Duke University. Joe Ryan is a professor in the Department of Civil, Environmental, and Architectural Engineering at the University of Colorado, Boulder.
’ ACKNOWLEDGMENT We thank M.-N. Croteau, R. Harvey, A. Slowey, L. Larsen, and three anonymous reviewers for providing valuable comments to the manuscript. H.H. was supported by the U.S. Department of Energy, Department of Defense, and the Center for Environmental Implications of Nanotechnology (funded by the National Science Foundation and the U.S. Environmental Protection Agency). J.R. was funded by the National Science Foundation and the U.S. Department of Energy. ’ REFERENCES (1) Auffan, M.; Rose, J.; Bottero, J. Y.; Lowry, G. V.; Jolivet, J. P.; Wiesner, M. R. Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective. Nat. Nanotechnol. 2009, 4 (10), 634–641. (2) Hochella, M. F.; Lower, S. K.; Maurice, P. A.; Penn, R. L.; Sahai, N.; Sparks, D. L.; Twining, B. S. Nanominerals, mineral nanoparticles, and Earth systems. Science 2008, 319, 1631–1635. (3) Science Plan in Support of Ecosystem Restoration, Preservation, and Protection in South Florida; U.S. Department of the Interior: Washington, DC, 2005; p 151. (4) Wiener, J. G., Gilmour, C. C., Krabbenhoft, D. P. Mercury Strategy for the Bay-Delta Ecosystem: A Unifying Framework for Science, Adaptive Management, and Ecological Restoration, 2004; http://calwater. ca.gov/Programs/Science/adobe_pdf/MercuryStrategy_FinalReport_1-12-04.pdf. (5) Buffle, J.; Wilkinson, K. J.; van Leeuwen, H. P. Chemodynamics and bioavailability in natural waters. Environ. Sci. Technol. 2009, 43 (19), 7170–7174. (6) Wiesner, M. R.; Lowry, G. V.; Alvarez, P.; Dionysiou, D.; Biswas, P. Assessing the risks of manufactured nanomaterials. Environ. Sci. Technol. 2006, 40 (14), 4336–4345. (7) NSETS. Environmental, Health, and Safety Research Needs for Engineered Nanoscale Materials; Nanoscale Science, Engineering, and Technology Subcommittee, Committee on Technology, National Science and Technology Council: Washington, DC, 2006; p 62. (8) Sleighter, R. L.; Hatcher, P. G. The application of electrospray ionization couples to high resolution masss spectrometry for the molecular characterization of natural organic matter. J. Mass Spectrom. 2007, 42, 559–574. (9) Tipping, E. Cation Binding by Humic Substances; Cambridge University Press: Cambridge, UK, 2002; p 434. (10) Tiller, C. L.; O’Melia, C. R. Natural organic matter and colloidal stability: Models and measurements. Colloids Surf., A 1993, 73, 89–102. (11) Brantley, S. L. Kinetics of Mineral Dissolution. In Kinetics of Water-Rock Interaction; Brantley, S. L., Kubicki, J. D., White, A. F., Eds.; Springer: New York, 2008; pp 151210. (12) Slowey, A. J. Rate of formation and dissolution of mercury sulfide nanoparticles: The dual role of natural organic matter. Geochim. Cosmochim. Acta 2010, 74 (16), 4693–4708. (13) Borch, T.; Kretzschmar, R.; Kappler, A.; Cappellen, P. V.; Ginder-Vogel, M.; Voegelin, A.; Campbell, K. Biogeochemical redox processes and their impact on contaminant dynamics. Environ. Sci. Technol. 2009, 44 (1), 15–23.
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’ NOTE ADDED AFTER ASAP PUBLICATION The incorrect unit was given in the description of Figure 1 in the version published on March 15, 2011. The correct version published March 17, 2011.
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