Reactivity of Zerovalent Metals in Aquatic Media: Effects of Organic

Chapter 18

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Reactivity of Zerovalent Metals in Aquatic Media: Effects of Organic Surface Coatings Paul G. Tratnyek,1,* Alexandra J. Salter-Blanc,1 James T. Nurmi,1 James E. Amonette,2 Juan Liu,2 Chongmin Wang,3 Alice Dohnalkova,3 and Donald R. Baer3 1Division

of Environmental and Biomolecular Systems, Oregon Health & Science University, 20000 NW Walker Road, Beaverton, OR 97006 2Fundamental and Computational Sciences Directorate, 3Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352 *[email protected]

Granular, reactive zerovalent metals (ZVMs)—especially iron (ZVI)—form the basis for model systems that have been used in fundamental and applied studies of a wide variety of environmental processes. This has resulted in notable advances in many areas, including the kinetics and mechanisms of contaminant reduction reactions, theory of filtration and transport of colloids in porous media, and modeling of complex reactive-transport scenarios. Recent emphasis on nano-sized ZVI has created a new opportunity: to advance the understanding of how coatings of organic polyelectrolytes—like natural organic matter (NOM)—influence the reactivity of environmental surfaces. Depending on many factors, organic coatings can be activating or passivating with respect to redox reactions at particle-solution interfaces. In this study, we show the effects of organic coatings on nZVI vary with a number of factors including: (i) time (i.e., “aging” is evident not only in the structure and composition of the nZVI but also in the interactions between nZVI and NOM) and (ii) the type of organic matter (i.e., suspensions of nZVI are stabilized by NOM and the model polyelectrolyte carboxymethylcellulose (CMC), but NOM stimulates redox reactions involving nZVI while CMC inhibits them). © 2011 American Chemical Society In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Introduction The redox reactivity of zerovalent metals (ZVMs) in aquatic media is relevant in a diverse array of contexts, ranging from corrosion of ferruginous metals in all sorts of structures (bridges, ships, nuclear reactors, etc.) to activity of catalysts for organic synthesis (1–4), bioavailability of nutritional supplements (5, 6), weathering of chondritic meteorites (7, 8), metal/oxide cycling for solar hydrogen production (9, 10), lowering carbon emissions from fuel combustion (11, 12), and the recently proposed role of blood-borne metal nanoclusters as nucleating sites for a variety of physiological phenomena (13, 14). However, the context with the most direct relevance to environmental science and engineering is the use of ZVMs (mainly zerovalent iron, ZVI) for water treatment. This technology takes several forms, the most important involving filters for removing metals (where the process is sometimes referred to as “cementation” (15, 16) or “electrocoagulation” (17)) and reactive treatment zones for remediation of contaminated groundwater (typically called permeable reactive barriers (18–21)). There is also a wide range of variations on these technologies where ZVI is used, including reactive impermeable barriers (22), mechanically-mixed soils and sediments (23, 24), constructed wetlands (25, 26), reactive caps for sediments (27, 28), and many combined or sequential remedies (e.g., (29, 30)). Starting in the early 1990s, interest in ground water remediation applications of ZVMs grew rapidly, and the technology soon became well established (31). Simultaneously, a large body of scientific literature developed on many aspects of remediation with ZVMs (32), and some of these papers have earned exceptionally high numbers of citations. The quantity—and especially the high citation impact—of this research suggests significance that extends beyond the practical applications of ZVMs to remediation of contamination. The main reason for this is that granular ZVI in aquatic media has become a preferred model system for investigating aspects of many processes related to contaminant fate and redox processes in the aquatic environment. Some of these studies have produced significant advances, and these have fueled further interest in what can be learned using ZVI model systems. A prominent example of a fundamental advance from early work done using ZVI model systems is the determination of how branching between hydrogenolysis and reductive-elimination pathways of dechlorination determine the final distribution of products from this important contaminant degradation process (33–35). The data originally used to demonstrate the dynamics of this complex set of reactions were obtained with batch model systems containing ZVI or zerovalent zinc (ZVZ), but the conceptual model is now used widely in interpreting the outcome of dehalogenation in other systems (36–40). Another fundamental advance that derived from early work on dechlorination with ZVMs concerns the relationship between dechlorination rates and the structure of chlorinated aliphatic parent compounds. Using rate constants from batch experiments performed with ZVI, the first quantitative structure-activity relationships (QSARs) were derived for this contaminant degradation pathway (41), which has lead to numerous efforts to obtain additional or alternative QSARs for dechlorination (42–47). Other areas where studies using ZVM model 382 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


systems have produced notable advances of fundamental significance include diagnosing mechanisms of dechlorination from carbon isotope fractionation (36) and molecular modeling (48), kinetic modeling of complex surface reactions (49), and reactive transport in heterogeneous media (50). Since the late 1990s, much of the research on ZVMs has focused on nano-sized materials (e.g., nZVI). Model systems based on nZVI have lead to new emphasis on particle-size dependent effects, and significant advances have been made as a result of this work. The most prominent example of this is advancement of classical filtration theory to accommodate a wider range of particle interactions (51–54). Among the particle interaction effects that have proven to be most important to the colloidal behavior of nZVI are those mediated by organic surface coatings. In addition to the effects of organic surface coatings on nZVI aggregation, attachment, and transport (55–60), they influence reactivity. This chapter provides a perspective on results regarding the effects of organic matter on reactivity of nZVI and other particulate ZVMs.

Background Organic coatings can influence the reactivity of surfaces in many ways that could be significant under environmental, aquatic conditions. With respect to reactivity of the granular ZVI that traditionally has been used for groundwater remediation, four types of effects on contaminant degradation have been proposed (61–63). The first is enhanced solubilization of hydrophobic organic contaminants by organic polyelectrolytes, making the contaminants more mobile and possibly more available for reaction with the particle surface. Second, enhanced sorption might result if the organic polyelectrolyte coats the particle surface first, making sorption of the contaminant to the modified surface more favorable. Third, competitive sorption may arise if the organic coating that forms on the particle surface inhibits reaction with organic contaminants. And, fourth, mediated electron transfer could be important if the organic polyelectrolyte (either as a surface coating or as a solute) serves as a catalyst by shuttling electrons between the ZVM and the organic contaminant. Recently, additional studies have added further evidence that both inhibition and acceleration of contaminant removal are possible, depending on various operational factors (64–66). The four-part conceptual model described above was formulated for systems where reaction occurs on the surface of ZVM particles that are large relative to other features of the system. Allowing for nano-sized ZVMs—and micelles or other relatively large features formed from amphiphilic organic polyelectrolytes—introduces additional considerations (67, 68). As the ZVM particles become small relative to the organic polyelectrolyte, their relationship evolves from molecules of adsorbed organic polyelectrolyte distributed on a ZVM particle surface, to a thin film of organic polyelectrolyte coating ZVM particles, an aggregate of nZVI bound together by an organic polyelectrolyte phase, and, finally, a particle composed primarily of organic polyelectrolyte that is embedded or encrusted with nZVI. The organic coating conceptual model is prototypical and illustrated in Fig. 1., although the interpretation of real experimental data (such as 383 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


presented below) usually requires accommodation for non-uniform or incomplete surface coatings, complex and heterogeneous aggregate compositions, etc.

Figure 1. Organic coating model for the effects of organic polyelectrolytes on the structure and properties of nano ZVMs that have core-shell structure consisting of metal and metal oxide. With respect to nZVI, the organic coatings of primary significance are those that are deliberately added to impart properties to nZVI that make the material more suitable for groundwater remediation applications. The main concern here has been with making nZVI more mobile in porous geological materials by decreasing its tendency to aggregate with itself and other colloids or to stick to the mineral grains of the aquifer matrix. This has been achieved by coating the particles with a variety of organics, including anionic polyelectrolytes like polyacrylate, polyaspartate, and carboxymethyl cellulose (56, 58, 69–78); other polysaccharides like guar, xanthan gum, and cyclodextrins (74–77, 79–82); and synthetic surfactants or water-soluble polymers like the triblock copolymers and polyvinylpyrrolidone (55, 75, 83–85). Coatings of this type are now part of almost all formulations of nZVI that are used in field scale remediation applications (69). Of the alternative materials for coating nZVI, carboxymethyl cellulose (CMC) probably has been used in the widest range of laboratory and field studies of nZVI behavior. In addition to being inexpensive, nontoxic, and biodegradable, CMC is highly effective at controlling the aggregation and sedimentation of nZVI (58, 76–78, 86–88). Synthesizing nZVI in the presence of CMC can produce primary particles with Fe0/Fe-oxide core-shell structure that are uniform, spherical, and

Reactivity of Zerovalent Metals in Aquatic Media: Effects of Organic

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