Characterization of complex mineral assemblages: Implications for contaminant transport and environmental remediation

Abstract

Surface reactive phases of soils and aquifers, comprised of phyllosilicate and metal oxohydroxide minerals along with humic substances, play a critical role in the regulation of contaminant fate and transport. Much of our knowledge concerning contaminant-mineral interactions at the molecular level, however, is derived from extensive experimentation on model mineral systems. Although these investigations have provided a foundation for understanding reactive surface functional groups on individual mineral phases, the information cannot be readily extrapolated to complex mineral assemblages in natural systems. Recent studies have elucidated the role of less abundant mineral and organic substrates as important surface chemical modifiers and have demonstrated complex coupling of reactivity between permanent-charge phyllosilicates and variable-charge Fe-oxohydroxide phases. Surface chemical modifiers were observed to control colloid generation and transport processes in surface and subsurface environments as well as the transport of solutes and ionic tracers. The surface charging mechanisms operative in the complex mineral assemblages cannot be predicted based on bulk mineralogy or by considering surface reactivity of less abundant mineral phases based on results from model systems. The fragile nature of mineral assemblages isolated from natural systems requires novel techniques and experimental approaches for investigating their surface chemistry and reactivity free of artifacts. A complete understanding of the surface chemistry of complex mineral assemblages is prerequisite to accurately assessing environmental and human health risks of contaminants or in designing environmentally sound, cost-effective chemical and biological remediation strategies.

The transport and fate of contaminants in soils and groundwater are highly coupled to the nature and relative abundance of the reactive mineral phases. Clay and oxide minerals, along with humified organic matter, comprise the surface reactive phases that are the primary controllers of sorption processes in soils, thus serving as important regulators of contaminant transport. Major challenges in understanding the processes controlling contaminant behavior in the environment include the complexity of the soil and aquifer matrix and the enormous spatial scales over which these processes occur.

Although it is well established that a fundamental understanding of molecular-level interactions is required to explain the underlying mechanisms controlling the fate and transport of solutes and contaminants in soils and subsurface environments, there has been limited success in translating molecular-level information to observations made at the larger scales. Although several explanations for this conundrum can be advanced, a prominent one is that much of our knowledge concerning the surface chemistry of clay and oxide minerals primarily is derived from experiments conducted on model mineral phases. These studies have established boundary conditions defining sorbate/mineral surface interactions and have identified the surface functional groups involved in surface complexation reactions, but they have produced little information that can be readily extrapolated to complex mineral assemblages typically present in heterogeneous soil and aquifer materials (1). Thus, utilization of bulk mineralogical data to represent predominant reactive phases in complex natural systems often has failed to reliably predict solute and contaminant behavior.

Reactive Mineral Phases in Soils: An Historical View

The pioneering work on reactive mineral phases in soils, which focused primarily on adsorption of group IA and IIA cations, has been comprehensively reviewed (2, 3) as has more recent work on specific sorption of metals and metalloids (1, 4).

The concept that surface reactive phases in soils are colloidal and comprised of Al(OH)₃, Fe(OH)₃, and SiO₂ hydrogels was proposed over a century ago (5). By the mid-1920s, a comprehensive understanding of the surface chemistry of Al, Fe, and Si colloids and their role in cation sorption was emerging, largely based on extensive investigations of Mattson (6–8). Mattson viewed reactive phases in soils as mixtures of Al₂O₃, Fe₂O₃, and SiO₂ colloids. Based on the observation that soils with a high SiO₂/Al₂O₃ + Fe₂O₃ ratio had higher cation exchange capacities (CEC) and that soils with a low SiO₂/Al₂O₃ + Fe₂O₃ ratios had high anion exchange capacities at low pH and higher CEC at high pH, Mattson concluded that the SiO₂ colloids were primarily responsible for the CEC of a soil and that the Al₂O₃ and Fe₂O₃ colloids were amphoteric in nature. Mattson’s compelling evidence for mixtures of positively and negatively charged colloids, based on careful cation/anion sorption experiments and electrophoresis, was largely disregarded as attention shifted to a new, rapidly emerging paradigm of reactive mineral phases predicated on the notion that soil clays were comprised primarily of crystalline phases.

Two classic papers by Pauling (9, 10) figured prominently in this paradigm shift. Shortly thereafter, soil chemists applied x-ray diffraction to soil clays and discovered the existence of, and delineated the structures for, the major classes of phyllosilicate clays commonly found in soils (11, 12). Soon after it was demonstrated that most phyllosilicates in soils had a permanent negative charge resulting from substitution of lower valence cations in both the tetrahedral and octahedral layers (13). For decades the surface chemistry of reactive phases in soils would be interpreted primarily according to this paradigm, i.e., that predominant reactive phases in soil were crystalline and comprised of negatively charged minerals of the phyllosilicate class.

Three rather fortuitous circumstances solidified this view of reactive mineral phases. First, free Fe oxides and organic macromolecules were removed from soil clay fractions via pretreatment to improve x-ray diffraction patterns by minimizing background scatter and improving preferred orientation of the phyllosilicate clay minerals. Second, most of the active soil mineralogy groups emerging during this period were limited to geographical areas characterized by young circumneutral soils having clay fractions dominated by 2:1 phyllosilicates; albeit, on a worldwide basis these soils were more of an exception. Finally, much of the experimentation during this period continued to involve the adsorption/exchange of class IA and IIA cations, both of which are relatively weakly bound and present at relatively high concentrations (an exception is K⁺, whose chemistry is controlled by a unique combination of cation size, low hydration energy, and structural properties of micaceous minerals and their weathering products). Thus, much of the data generated under these conditions was consistent with the phyllosilicate model, and the distribution of phyllosilicates within a given soil clay fraction generally could be used to predict observed cation exchange behavior for this limited range of extensively studied soils.

There continued to be prominent exceptions to this model that could be better interpreted according to a Mattson-like model of surface reactive phases (14, 15). Evidence for anion adsorption to soil clays having low SiO₂/Al₂O₃ + Fe₂O₃ ratios and slightly acidic pH continued to appear. Evidence for positively charged regions (edge sites) on phyllosilicate clays in slightly acidic suspensions appeared during this time (16–19). This model also was used to interpret anion adsorption and complex flocculation/dispersion behavior of kaolinite suspensions (20). Clearly, the phyllosilicate model of reactive mineral phases based largely on 2:1 minerals in soils of circumneutral pH was limited in its extent of applicability.

As mineralogical techniques improved and experimental approaches evolved, another very important body of literature on hybrid phyllosilicate-Al/Fe oxohydroxides emerged. The discovery (21) that 2:1 minerals in soils weathered from parent materials rich in mica schist were interlayered with nonexchangeable, positively charged hydroxo-Al polynuclear components stimulated a significant body of research that continues to this day and includes investigations on an important class of zeolite-like clay catalysts (22). Although this finding explained a number of properties related to the surface chemistry of many 2:1 soil clays, the research emphasis on the hydroxy-interlayered minerals largely focused on explaining the unique adsorption behavior of large weakly hydrated monovalent cations, such as K⁺, NH₄⁺, and Cs⁺, with less emphasis on anion sorption. Only many years later would the role of this complex mineral assemblage in the specific sorption of transition metals be considered (22).

Concurrent with these exciting developments, a new paradigm of surface reactive mineral phases was emerging. The structural aspects of important functional groups on oxide minerals were being unraveled (4, 23). The notion of surface structural hydroxyl groups having acid/base properties that could quantitatively explain the observed amphoteric behavior of oxides became firmly established (24). Thus, an accurate model of surface functional groups that could explain Mattson’s original observations was emerging, and a number of studies on anion adsorption to metal oxide surfaces followed quickly as did spectroscopic evidence for the proposed reactive surface hydroxyls (4). It was now established that solutes could interact with charged metal oxide surfaces via electrostatic (outer sphere) reactions or through specific ligand exchange reactions with the surface functional groups (inner sphere). The conceptual model of surface complexation to describe nonspecific and specific adsorption of anions and cations was advanced shortly thereafter by the classic work of Schindler and Gamsjager (25) and Stumm et al. (26). The surface complexation model has remained the basic framework for research on metal and anion sorption to metal oxide surfaces to the present time (1, 4), and many studies have demonstrated the importance of metal oxides as resident phases for a variety of metals and metalloids (1, 27).

Although extensive modeling efforts have demonstrated reasonable success for predicting metal and metalloid sorption to model monomineralic metal oxide phases, applications to natural systems have been less than satisfying (1). A major challenge in extending such results to complex mineral assemblages typically found in nature has been the identification and quantification of the primary reactive phase and associated surface functional groups. High surface area, low abundance metal oxohydroxide phases, and organic materials can be coassociated with more prominent mineral grains as armoring agents or as surface coatings. The term surface coating as used here does not imply the presence of a uniform gel-like phase as is often envisioned. Rather, it is used to describe domains of crystalline or noncrystalline components coassociated with well-defined mineral grains. The complex nature of the electrostatic and van der Waals interactions between fine-grained crystalline and poorly ordered phases with mixed surface-charge properties has hampered the development of suitable models to represent surface reactive functional groups in mixed mineral assemblages. Adsorption studies using binary mixtures of model mineral phases have demonstrated remarkable complexity, with adsorption generally being very poorly predicted by considering a weighted sum of individual mineral components (1, 28).

Mixed Mineral Assemblages in Natural Systems

It is becoming increasingly clear that many natural mineral phases possess different surface chemical properties than their model mineral analogues. Zachara and others (29–31) have provided compelling evidence suggesting that the small crystallite size of soil smectites enhances the importance of edge site aluminol (Al-OH) functional groups imparting an oxide-like behavior compared with the widely used Source Clay Repository, SWy-1 montmorillonite. Other studies have suggested that organic or metal oxide minerals may be the primary reactive phases in soils and sediments even at relatively low abundance (32–37). A major theme that emerges from these investigations is that surface modifiers in the form of organic/metal oxohydroxide armoring agents or coatings, rather than bulk mineralogical composition per se, control the surface chemistry of reactive phases in soils and aquifers. For example, it has been demonstrated that organic constituents coassociated with variable charge minerals significantly alter the point of zero net charge (pznc, the pH at which the cation and anion exchange capacities are equal), shifting the pznc to significantly lower pH values (32, 33, 35–37). Conversely, Fe and Al oxohydroxide phases coassociated with quartz, and permanent charge phyllosilicate minerals have been found to shift the pznc to higher pH values (38–40).

A number of recent studies have focused on the surface chemistry of mineral assemblages isolated from natural systems (33–35, 37, 38, 40–42). Characterizing natural mineral assemblages is challenging, because it has been virtually impossible to isolate them free of artifacts. In fact, the methods used to isolate and concentrate clay minerals involve dispersion of the clay fraction via treatment with harsh reagents designed to significantly alter surface charge properties and destroy complex mineral assemblages present in the original material. Recently, however, collection and examination of complex mineral assemblages has been achieved in a different context: that dealing with the transport of colloidal phases through porous media. The past decade has witnessed great interest in the generation and transport of mineral colloidal phases through natural porous media (33, 34, 40, 42–47). Interest in this subject has paralleled evidence that colloidal minerals are important vectors for facilitating the transport of contaminants in certain environments (43, 47–50).

Mobile colloids can be generated by a number of mechanisms, including precipitation of colloidal size phases, dissolution of cementation agents composed of fine-grained crystalline and poorly crystalline secondary minerals, and release from soil and aquifer materials via physicochemically controlled dispersion processes. Transport of the mineral colloids also depends on a number of factors, including fluid flow rate, electrostatic and van der Waals forces between colloids and between colloids and matrix minerals, and physical factors related to the relative size of the colloids and pores and pore throats. Recent evidence has indicated that mobile colloids comprised of minerals and complex mineral assemblages can be generated via dispersion processes and transported through many soils and groundwater systems with relatively minor changes in solute chemistry of the invading fluid, thereby avoiding the serious artifacts typically encountered in the isolation of complex mineral assemblages.

Mineralogy and Surface Chemistry of Complex Mineral Assemblages Isolated From Soils and Aquifer Materials

Although several studies have demonstrated the enhanced mobility of contaminants in the presence of mobile colloids, far fewer have focused on characterizing the mineralogical composition and surface charge properties of the mineral and organic-mineral assemblages comprising the mobile phase. Over the past decade, our investigations have focused on providing evidence for the facilitated transport of contaminants associated with mobile colloidal phases and in defining the mechanisms leading to the generation and transport of mobile colloidal phases and solutes (33, 34, 40, 46, 49, 51–53). These studies have examined surface chemical controls on colloid generation and of colloid and solute migration in surface and subsurface highly weathered oxide-rich systems having similar bulk clay mineralogy.

The samples examined are coarse textured (≥85% sand; <9% clay), have varying quantities of Fe-oxide and organic carbon, a predominance of exchangeable Al, low pH, low pore water ionic strength, and similar bulk clay mineralogies (Table 1). They are also poorly structured with little or no evidence for meso/macropore development, thus minimizing complications involving preferential flow. They both have silt and sand fractions composed entirely of quartz, thus minimizing artifacts resulting from dissolution of less stable minerals that contribute solutes and complicate solution chemistry. The properties of these highly weathered soils and aquifer materials have important commonalities with those widely distributed in humid tropics, which play an important role in global geochemical cycles (41).

Table 1.

Chemical and mineralogical characteristics of a surface soil (Orangeburg Series) and three subsurface sediments representative of the Tobacco Road (TR) formation from the Atlantic Coastal Plain

	Orangeburg series	TR1	TR2	TR3
pH	4.61	5.37	5.33	5.18
Extractable Fe (mg kg−1)
CDB*	15.0	73.5	111.0	359.4
NH4-oxalate	3.1	1.4	1.2	2.3
Clay mineralogy	k, HIV, gibb	k, m, g	k, m, g	g, k, m

k, kaolinite; HIV, hydroxy-interlayered vermiculite; m, mica (illite); gibb, gibbsite; g, goethite. 

^*Citrate dithionite extraction. 

Our results have demonstrated that colloids mobilized from surface soils have high negative electrophoretic mobilities (−2.5 to −3.5 μm cm s⁻¹⋅V⁻¹), inconsistent with mineral composition of the assemblages and pore water pH. The mobile colloids generally are enriched in kaolinite and Al- and Fe-oxohydroxide phases in the ≈ 200-nm size range and more dilute in quartz and hydroxy-interlayered vermiculite (≈700 nm to 1 μm) relative to the bulk clay mineralogy of the soil horizons where the colloids were generated. More colloids were generated in surface soils having slightly elevated pH (≥5.5) and low total electrolyte concentrations (<2 mol_c m⁻³). The mobile colloidal phase also was found to have high concentrations of surface-associated organic C, which explained the anomalous high negative electrophoretic mobilities (33, 34). As much as 50% of the total organic C in the surface soil horizons was associated with the water-dispersible clay, a fraction of the total clay thought to be an indicator of the potential for mineral colloids to become mobilized under physicochemical perturbation (34, 44). These results suggest that, consistent with previous investigations on model organic-clay mineral suspensions and on natural mineral assemblages, the surface charging mechanism of both the mobile and immobile reactive phases in these surface soils was dominated by organic-mineral interactions or surface coatings, which lower the pznc of the mineral assemblage compared with the model mineral phases of similar composition. Surprisingly, the generation and transport of mobile colloidal phases and the composition of these phases were extremely sensitive to minor changes in pore water chemistry and to a narrow range in pore water flow velocities (33, 34, 44). Thus, the generation and transport of colloids from managed surface soil systems, such as might be experienced in remediation of contaminated soils or the reclamation of highly disturbed sites, is an extremely dynamic and complex process.

Similar to the surface soils, minor differences in solute chemistry were found to have a profound influence on colloidal generation and transport in the subsurface aquifer materials. In contrast to the results obtained from surface soils, those derived from the organic poor subsurface materials indicate a completely different surface charging mechanisms despite the similar mineralogy of the bulk clay fractions. Dynamic column transport experiments with simulated groundwater and various salt solutions (K⁺, Na⁺, Mg²⁺, and Ca²⁺; Cl⁻ and SO₄²⁻) at concentrations ranging from 1 × 10⁻⁴ to 0.1 M were used to examine the processes regulating surface chemistry of the mixed mineral systems. Information gleaned from trends in colloid transport, effluent pH, and solute leaching histories, combined with characterization of the mobile colloidal phases indicate a complicated surface charging mechanism in these mixed mineral systems. The generation and transport of mobile colloids was found to be minimal in the presence of groundwater or Na⁺ solutions, but significantly enhanced on introduction of dilute Ca²⁺ solutions, a result at odds with the well-established empirical observations of dispersion and flocculation behavior of soil colloids (Fig. 1; refs. 40 and 46). Effluent pH was found to decrease between 1 and 1.5 units concurrent with the introduction of the dilute Ca²⁺ salt solution, which contrasts to the groundwater and Na⁺ salt systems, where pH remained relatively constant. Electrophoretic mobility measurements of the colloids generated in the Ca²⁺ transport studies reveal a net positive surface charge, strongly suggesting reversal of the net matrix mineral assemblage charge from slightly negative to strongly positive on the introduction of the dilute Ca²⁺ salt solutions. Additional evidence that the immobile reactive mineral assemblages possessed a net positive charge is the significant retention of anionic tracers observed in transport experiments (see below).

Figure 1.

Influence of treatment solution (1 × 10⁻³ and 0.1 M NaCl; 5 × 10⁻⁴ M CaCl₂; 5 × 10⁻⁴ M CaCl₂, pH 3.00) on effluent turbidity (a measure of colloid concentration) (a), pH (b), and the electrophoretic mobility (c) of mobile colloids from sample TR1 (Table 1). Columns were leached with a given influent for 10 pore volumes at a Darcy velocity of ≈ 0.72 m d⁻¹ followed by several pore volumes of deionized (DI) water.

A major feature of the mobile colloid elution profile is the concentration maximum at ≈2.5 pore volumes of CaCl₂ injectate that was followed by a rapid decrease in colloid concentration between ≈2.5 and 5 pore volumes because of destabilization of the suspended colloids as the ionic strength of the efluent approaches that of the influent. Evidence to support this explanation is provided in the column colloid transport histories, which reveal a second colloid concentration maximum when deionized water was introduced as the influent solution after 10 pore volumes of the Ca²⁺ solution.

To explain the complex surface charging processes observed in this mixed mineral system, we propose a mechanism involving the strong coupling between surface exchange reactions on permanent negatively charged phyllosilicate minerals (kaolinite and mica) and subsequent protonation of the variable-charge Fe-oxohydroxide mineral components (Fig. 2). According to this model, Ca²⁺ undergoes exchange with native cations associated with the negatively charged phyllosilicate minerals, of which Al occupies ≈85% of the exchange phase. Evidence for ion exchange of Ca²⁺ with native cations is provided by examining the effluent discharge concentrations of major cations from the column, where the delay in Ca²⁺ transport is accompanied by the breakthrough of Mg²⁺ and trace levels of Na⁺ (data not shown). The model also suggests that the observed decrease in effluent pH is a result of exchanged Al undergoing hydrolysis reactions. Consistent with this model, we were unable to detect elevated Al in the effluents but could measure a decrease in cation exchangeable Al. Dilute Na⁺ influent solutions were ineffective at displacing native exchangeable cations associated with the phyllosilicate minerals, explaining the differences between the Na⁺ and Ca²⁺ systems in effluent pH and colloid transport histories. This mechanism was confirmed in transport experiments with more concentrated Na⁺ solutions (0.1 M) where native cations, including Al, were displaced, as evidenced by elevated effluent cation concentration and depressed pH. Mobile colloids were not detected in these high Na⁺ systems, however, because of ionic strength destabilization. Consistent with the proposed mechanism, introduction of an acidified CaCl₂ solution enhanced positive surface charge development as evidenced by the high mobile colloid yields throughout the leaching event, i.e., higher positive surface charging countered the ionic strength destabilization mechanism.

Figure 2.

Schematic representation of the coupled mechanisms controlling effluent pH and the generation of positive surface charge on the mobile colloids and the stationary matrix.

Examination of the mobile colloids by transmission electron microscopy (EM) and selected area electron diffraction reveals that they are comprised primarily of aggregates of microcrystalline Al-substituted goethite along with complex mineral assemblages of goethite-armored kaolinite and crandallite (Ca Al₃[PO₄]₂[OH]₅⋅H₂O) in the 200- to 300-nm size range (Fig. 3). Examination of mobile colloids by scanning EM reveals that the phyllosilicates and phosphate minerals present in the mobile phase are extensively armored in all instances and generally fall in the 100- to 300-nm size range. Examination of isolated bulk clay reveals that the kaolinite and crandallite are present in the bulk clay fraction in two major size populations; the 100- to 300-nm size class as observed in the mobile phase and the 700-nm to 1-μm size class, which is predominant and presumably more representative of the reactive minerals comprising the immobile matrix. The former size class also contains the micaceous minerals found in the bulk clay mineral fraction. Scanning EM images of the bulk clay suggest that even these larger mineral grains of kaolinite and micaceous minerals are partially armored along the negatively charged basal surfaces with goethite crystallites and that these assemblages often are coassociated with larger quartz grains as has been observed in at least one other natural subsurface system (38).

Figure 3.

Transmission EM and selected-area electron diffraction pattern (Inset) for (A) crandallite and (B) kaolinite armored with fine-grained goethite and (E) goethite aggregates generated during dynamic transport experiments with 5 × 10⁻⁴ M CaCl₂ solutions. Transmission EM image of crandallite (C) and kaolinite (D) from bulk clay with Fe-oxides removed by dithionite extraction. Electrophoretic mobility behavior of (F) complex mineral aggregates generated during reactive transport experiments when repeatedly analyzed by laser doppler velocimetry: (a) initial mobility distribution, (b) third consecutive, (c) fifth consecutive, (d) sixth consecutive mobility distribution, respectively, and (e) typical mobility distribution observed after sample relaxation (t ≈ 5 min). (G) Average electrophoretic mobility of colloidal suspension as a function of consecutive analyses.

Thus, in these oxide-rich subsurface systems, Fe-oxohydroxide surface modifiers increase the pznc of the complex mineral assemblages, resulting in surface reactivity that is controlled by the development of a net positive surface charge. Based on electrophoretic mobility measurements of stable colloidal suspensions generated in the column transport experiments we conclude that the complex mineral assemblages observed in the EM images are not artifacts of sample isolation and preparation, i.e., surface armoring of negatively charged basal surfaces with Fe-oxohydroxide crystallites in the 10- to 30-nm size range is required to fully explain the observed high surface positive charge of the mineral aggregates. Further evidence for complex mineral aggregates comprised of a phyllosilicate/phosphate mineral core and a Fe-oxohydroxide veneer is provided on longer-term exposure of the assemblages to fluctuating electric fields. During consecutive electrophoretic mobility measurements we observed evidence that the complex mineral assemblages were disaggregating in the electric field, resulting in a suspension having two primary charged populations, one positively charged and one negatively charged (Fig. 3). The strong bias for enhanced scattering intensity observed for the negatively charged colloid population can be explained by the relative size and thus scattering by the 100–300 nm phyllosilicate and phosphate minerals dominates over the 10–30 nm goethite crystallites. When the disaggregated mineral mixture is allowed to reequilibrate in the absence of an electric field the system is found relax to the original state, i.e., the mixed mineral assemblage again is formed and the net positive charge reestablished.

Other evidence of the rather fragile nature of the complex mineral assemblages in these natural systems was found in an attempt to conduct conventional batch flocculation/dispersion and critical coagulation concentration experiments. On either air drying or physical disturbance of the sample, the surface chemistry of the mineral assemblages was found to be highly biased toward the permanently negatively charged mineral components in the mineral mixture, an observation consistent with several previous studies (56, 57). Even drying minimally disturbed columns via purging with Ar before conducting the dynamic transport experiments resulted in the complete loss of colloid generation and transport as observed for identically prepared columns that were maintained in the field moist condition.

These observations have significant ramifications for how experiments are designed to examine surface chemical properties of mixed mineral assemblages. Furthermore, they provide some insight into the electrostatic forces involved in the stabilization of the complex mineral assemblages, although major challenges remain for developing methods for isolating, examining, and quantitatively describing the electrostatic/van der Waals forces involved in their stabilization.

Surface Chemistry of Mixed Mineral Assemblages and the Implication for Solute Transport

Surface charge reversal of reactive mineral phases according to the proposed model should be manifested in the reactive transport behavior of anionic solutes. This phenomenon was examined by investigating Br⁻ breakthrough behavior in additional dynamic transport experiments. Consistent with the proposed model based on colloid generation and transport behavior, Br⁻ displayed significant retardation when referenced to the truly conservative ³H₂O (Fig. 4). Also consistent with the proposed model was the greater retardation of Br⁻ with increased abundance of Fe-oxohydroxide minerals. The fact that the surface sample displays enhanced Br⁻ transport, compared with the ³H₂O tracer, suggests anion exclusion, consistent with the proposed role of organic matter on surface charge characteristics of the mineralogically similar surface soil and, furthermore, demonstrates the sensitivity of dynamic transport compared with batch experiments for investigating subtle changes in surface charge characteristics of mixed mineral systems. Also the fact that Br⁻ transport was more retarded in samples with higher pH (because of the greater abundance of Fe oxohydroxide minerals) provides additional evidence for the coupling of the surface reactions between the phyllosilicates and the Fe-oxohydroxides. The use of Mg²⁺ compared with K⁺ as the counter ion resulted in a greater decrease in effluent pH and greater Br⁻ retardation, an observation also consistent with the proposed coupled reaction model (53). As with the colloid generation and transport behavior, disturbance of the samples for use in batch experiments or air drying before dynamic transport experiments greatly reduced or eliminated observed anion retardation (54). Finally, dynamic transport experiments conducted over a wide range of Br⁻ concentrations revealed nonlinear sorption and at high concentrations of influent Br⁻ salts (≥ 0.1 M) a nearly complete masking of the anion retardation behavior, with breakthrough appearing to be conservative (53, 54). This observation is critical because column transport experiments are commonly conducted to calibrate physical parameters for field scale tracer experiments. The concentrations used in these column experiments are typically 2–3 orders of magnitude higher than those used in the subsequent field scale tracer experiments. Thus, sorption reactions in the field typically would be misinterpreted as having physical significance (i.e., mixing, flow rate, permeability, regions of immobile/mobile water, stratification, etc.).

Figure 4.

Bromide breakthrough (A) and effluent pH (B) for 10⁻³ M KBr tracer solutions in columns packed with materials described in Table 1. Column pore volumes were calibrated based on tritium breakthrough (adapted from ref. 54).

Surface Chemistry of Mixed Mineral Assemblages and the Implication for Contaminant Transport and Subsurface Remediation

There are a number of important implications of this work to the modeling of contaminant transport and to the environmental remediation of contaminated aquifers. In oxide-rich, organic-poor subsurface environments, typical of many aquifer systems, underestimation of contaminant transport distances can result from both an overestimation of contaminant sorption to reactive mineral phases that are assumed to be related to mineral abundance and assumed to posses a static surface chemistry, and by a misunderstanding of the primary mechanisms leading to the generation and transport of mineral colloids when predominantly negative charge surfaces are assumed to control the surface chemical behavior. For example, previous modeling efforts on coarse-textured highly weathered sediments similar to those studied here have considered quartz and kaolinite as the primary reactive phases and predicted limited metal and actinide mobility from an acidic plume (51). However, metal transport distances from the source plume were found to be significant and the primary mechanisms for this apparent enhanced transport were found to be a charge reversal on the matrix mineral phases leading to limited sorption reactions and the transport of trace levels of actinides and other metals specifically sorbed to colloids. Other studies also have demonstrated that the surface chemical properties of aquifer materials are dominated by high surface area phases of relatively low abundance (38). Greater emphasis must be placed on the identification and surface chemical characterization of complex mineral assemblages in natural systems to accurately define the mechanisms controlling solute and contaminant transport.

An additional implication of these results relates to remediation of contaminated oxide-rich, organic-poor subsurface environments. Understanding the surface chemical controls of the reactive mineral phases has facilitated the development of an enhanced groundwater remediation technology, which is predicated on the selective mobilization through surface chemical manipulation of the highly reactive Fe-oxohydroxide phases and Fe-oxohydroxide armored minerals, which are the primary resident phase for both inorganic and organic contaminants (58).

Conclusions

Investigations of complex mineral assemblages in a highly weathered coarse-textured system have demonstrated that the surface chemistry of these assemblages is influenced by complex physicochemical interactions between natural organic constituents, phyllosilicate, and Fe-oxohydroxide phases. In surface soil environments containing as little as 1% organic matter, the surface chemistry was found to be controlled by organic constituents coating the phyllosilicate and Al- and Fe-oxohydroxide clay minerals, resulting in a much higher negative charge and a pznc shifted to much lower values than predicted based on bulk mineralogical composition. The surface charge modification by organic constituents was found to control the flocculation/dispersion processes of clay mineral assemblages in the surface soils, as well as the transport of mineral colloidal phases through soils.

The complex mineral assemblages isolated from subsurface environments were found to be comprised of Fe-oxohydroxide phases partially or totally armoring the more abundant phyllosilicate minerals present. Unperturbed, these systems appear to be near the pznc; however, minor changes in solute chemistry can induce surface charge reversal (from slightly net negative to strongly net positive) through an elaborate coupled reaction between the permanent negatively charged and variable charged surfaces leading to the dispersion and transport of Fe-rich colloidal mineral phases. The surface charge reversal of these systems also is manifested by a significant retention of anions, such as Br⁻, which traditionally have been considered as conservative tracers in hydrological investigations of these systems.

Defining the surface chemical properties of these complex mineral assemblages has a number of important implications for solute and contaminant transport. The development of robust predictive models describing solute and contaminant transport requires a thorough understanding of physical transport parameters (generally derived from solute tracer experiments) and solute/contaminant-mineral surface interactions. Most reactive transport modeling efforts define reactive mineral phases based on the relative abundance of the clay minerals present and surface chemistry defined by studies with model minerals. That the surface chemistry of reactive phases in aquifers may be controlled by minerals of relatively low abundance and more importantly, by complex physicochemical interactions occurring between individual components in complex mineral assemblages, suggests that we need less emphasis on studies of model minerals and more research on mineral assemblages isolated from natural systems. Clearly, the results of this and other investigations demonstrate that information based entirely on mineral abundance is insufficient for predicting the surface charge characteristics of natural mixed mineral systems.

An important finding of these investigations is that the surface charge behavior defined in dynamic transport experiments could not be reproduced with conventional batch experiments. Both air drying or significant physical manipulation of the aquifer materials apparently results in a disruption of the complex mineral assemblages that appear to be primarily composed of a phyllosilicate core with a partial or total Fe-oxohydroxide veneer. Batch experiments were found to produce results strongly biased toward the permanent negative-charged components in the complex mineral assemblages and could not be used to predict solute transport or flocculation/dispersion behavior observed in both column and field scale transport experiments. These observations raise important questions concerning methods used for determining surface chemistry of complex mineral assemblages and pose significant challenges for designing isolation techniques so that complex mineral assemblages can be investigated in a meaningful way. It is clear that a detailed understanding of reactive mineral phases in soils and aquifers is necessary to accurately evaluate environmental and human health risks associated with contaminants and to design technologies for the protection or remediation of soil and water resources.

ABBREVIATIONS

pznc

point of zero net charge

EM

electron microscopy