Environ. Sci. Technol. 1997, 31, 2028-2033
Changes in Transition and Heavy Metal Partitioning during Hydrous Iron Oxide Aging R O B E R T G . F O R D , †,‡ P A U L M . B E R T S C H , * ,† A N D KEVIN J. FARLEY§ Savannah River Ecology Laboratory, University of Georgia, Aiken, South Carolina 29802, and Department of Environmental Engineering, Manhattan College, Riverdale, New York 10471
The fate and transport of metal ions in soils and sediments may be controlled by sorption to the metastable iron (hydr)oxide, ferrihydrite. The reversibility of metal partitioning to ferrihydrite can be significantly influenced by its transformation to more thermodynamically stable structures such as goethite or hematite. We studied changes in metal partitioning during aging of coprecipitates of ferrihydrite containing Cd(II), Mn(II), Ni(II), or Pb(II) at pH 6 and temperatures of 40 or 70 °C and as a function of metal surface loading. Aqueous metal concentrations as well as the fraction extracted by 0.2 M ammonium oxalate were continuously monitored. At the end of aging, solids were characterized by thermogravimetric analysis and X-ray diffraction. Prior to aging, the extent of metal sorption decreased in the order Pb(II) >> Ni(II) > Mn(II) = Cd(II). However, with ferrihydrite transformation, the extent of sorption increased and apparent sorption reversibility decreased significantly for Mn(II) and Ni(II). Both Pb(II) and Cd(II) demonstrated net desorption with aging, and sorption reversibility remained essentially unchanged. These differences in metal behavior are consistent with structural incorporation of Mn(II) and Ni(II) into the goethite or hematite structure and minimal incorporation of Cd(II) and Pb(II) within these crystalline products at pH 6.
Introduction Metal concentrations in aqueous environments are often controlled by partitioning to iron (hydr)oxides (1-3). The iron (hydr)oxides most commonly identified in soils and sediments are ferrihydrite, goethite, and hematite (4, 5). Ferrihydrite, a poorly ordered (hydr)oxide and a common primary mineral phase precipitated at locations of high metal activity, acts as a very efficient sink for trace metals due to its high specific surface area (6-8). However, this mineral is metastable with respect to either goethite or hematite and, depending on the prevailing solution conditions, transforms to these more highly ordered phases over time (4, 9). These transformation reactions can lead to structural incorporation of sorbed metals, resulting in nonreversible partitioning (1014). There are several possible processes that can lead to apparent nonreversible metal partitioning to ferrihydrite. * Corresponding author e-mail:
[email protected]; phone: (803)7255637; fax: (803)725-3309. † University of Georgia. ‡ Current address: Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19717. § Manhattan College.
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Metals initially sorbed to surface sites at the solid-water interface may diffuse to internal sorption sites, which are not readily accessible by the bulk solution. This behavior has been observed in long-term sorption studies with goethite, where slow desorption was attributed to metal migration within the internal pore structure (15). Alternatively, at high metal sorption densities, surface precipitation may be catalyzed leading to a new solid phase that is less readily dissolved or desorbed. However, due to the high surface sorption capacity of ferrihydrite, surface precipitation would only be expected at very high surface loadings. Finally, since ferrihydrite will undergo structural reordering to more crystalline forms, partitioned metals may be incorporated into crystalline structural sites forming a solid solution (16). In theory, the solubility of the substituted metal would be controlled by its mole fraction within the bulk solid, and partitioning would in effect be nonreversible without dissolution of the host mineral (17). Many experimental studies have addressed the potential for metal substitution into goethite and hematite formed from metal-ferrihydrite coprecipitates (9, 11-13, 18-20). This research has been designed primarily to establish the solubility limits for metal substitution within the structure of these two minerals, and it has provided indirect evidence for observed metal distributions within these minerals in highly weathered soils (21, 22). However, these experimental studies have focused on crystallization to a mono-mineralic end point (e.g., ferrihydrite transformation to goethite) that necessitated the use of pH g 11 to direct transformation (e.g., refs 18-20). Such high pH values are unrealistic in most aqueous environments and lie well above the pH sorption edge observed for most trace metals (5). Thus, observed metal sorption is typically much higher than expected, and results from these studies may lead to an overestimation of the potential for solid solution formation, as noted by Gerth (18). The influence of ferrihydrite aging at circumneutral pH on metal desorption hysteresis has been addressed in a limited number of studies (23, 24). Schultz et al. (23) examined the reversibility of Cu, Ni, Pb, Zn, and Cr sorption to ferrihydrite and observed non- or slowly-reversible sorption after prolonged contact times. Increases in the fraction of nonreversible partitioning appeared to correlate with increasing sorption pH, which ranged from 7 to 11.5. Faster rates of ferrihydrite crystallization are anticipated with increasing pH (25), leading to a greater potential for metal incorporation within a crystalline iron (hydr)oxide. Ainsworth et al. (24) determined that a fraction of Co and Cd (but not Pb) was irreversibly sorbed to ferrihydrite after equilibration for periods up to 86 weeks at pH 7. Examination of select samples during and at the end of aging by selective extraction with ammonium oxalate and X-ray diffraction demonstrated a degree of transformation to goethite and hematite. Nonreversible sorption was attributed to formation of a solid solution within these iron (hydr)oxides (24). Due to limited solid-phase characterization, it is not clear if differences in the magnitude of this nonreversible metal fraction were due solely to ion size, as argued by the authors. Alternatively, retardation of ferrihydrite crystallization as a function of metal surface stability constants (Pb > Cd > Co) could also explain their observations (5, 26). We attempted to address this issue in our research by examining changes in metal partitioning as they were linked to concurrently observed transformations in the solid assemblage. We employed selective extraction procedures in conjunction with physical characterization of the iron (hydr)oxides during aging to provide a more direct link between crystallization reactions and observed nonreversible metal
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sorption. The objectives of this work were to (1) examine the influence of iron (hydr)oxide aging on metal sorption in closed systems, (2) evaluate the reversibility of partitioning using selective extraction methods, and (3) establish a relationship between apparent nonreversible metal sorption and incorporation into a crystalline iron (hydr)oxide solid phase.
Materials and Methods Precipitation and Aging. Coprecipitates of each metal with Fe(III) (10 mM total) were generated by the addition of 1 M low-CO2 NaOH to acidified solutions of the metal salts with a Radiometer automatic titration system (TTT 80 titrator unit, ABU 80 autoburet with 10 mL buret). In all cases, starting solutions consisted of 20 mL of 0.1 M Fe(NO3)3, 140 mL of 0.1 M NaNO3, and 20 mL of Me(NO3)2 or Mn(Cl)2 solutions to give a final metal concentration of 0.05 or 0.5 mM. All stock metal solutions were made up in 0.1 M HNO3 and NaNO3. Base was added to these starting solutions (pH 1.6-1.7) over a period of 1 h to an end point of pH 6. The resultant precipitate slurries were held at pH 6 for an additional 2 h to ensure near-equilibrium sorption of coprecipitated metals and complete hydrolysis of solution Fe. After this period, the slurries were placed in a water bath at 40 or 70 °C (time zero) and allowed to age for periods of 2-6 weeks. Changes in pH were mitigated by addition of either MES or PIPES as a buffer (5 mM). Aging in the presence of a buffer did not significantly influence metal sorption to ferrihydrite or alter the rate of crystallization. However, the presence of MES increased the relative fraction of hematite in the crystallization products, although to a lesser extent than observed for organic acids possessing carboxylic functional groups (27). For select systems (0.5 mM Ni or Pb, 0.05 mM Cd), duplicate runs with manual pH adjustment (0.1 M NaOH) indicated that MES resulted in > Ni(II) > Mn(II) = Cd(II) (Table 1), which was consistent with surface complexation predictions using the diffuse layer model (5). Included in Table 1 is the predicted pH for 50% metal sorption (pH50), indicating the important role of pH toward overall metal partitioning (e.g., see results for Cd(II) below). Aging resulted in observable changes in overall metal sorption, as followed by solution-phase measurements and extraction of partitioned metals. In the data presented below, these fractions are referred to as ‘aqueous’ and ‘extractable’ Me(II), respectively. The extraction techniques employed solubilized metals associated with ferrihydrite (which was dissolved) as well as those sorbed to the surface of goethite or hematite (10, 32). The fraction of metal remaining was either coprecipitated within the crystalline iron (hydr)oxide phase(s) or precipitated as a separate non-extractable (residual) solid phase. Aging at 70 °C, 0.5 mM TOTMe. In Figures 2 and 3, we compare the partitioning behavior of the four metals at the highest metal loading studied and aging at 70 °C. In these and subsequent figures, we have plotted aqueous, extractable, and residual Me(II) fractions as well as changes in the residual Fe fraction. In all cases except for the Ni(II) system, the fraction of residual Fe increased from between 0 and 0.05 to about 0.8 over a period of ∼380 h, demonstrating that ferrihydrite was undergoing crystallization. In the presence of Ni(II), the fraction of residual Fe increased to only 0.6 by the end of aging. Thus, crystallization was retarded relative to the other systems. However, examination of Me(II) distributions during aging revealed that the metal partitioning behavior was very different in each case. For Mn(II), the aqueous and residual fraction changed dramatically during the course of aging (Figure 2A). The aqueous fraction decreased from 0.9 to 0.05 over a period of 380 h, and this behavior was paralleled by an equivalent increase in the residual or crystalline fraction. The extractable Mn fraction increased from 0.1 to 0.2 over a 75-h period, and then it fell to near zero by the end of aging. Thus, the first part of aging was dominated by a continuous transfer of Mn(II) from solution to ferrihydrite and crystalline iron (hydr)oxides, while the final stages of aging were dominated by transfer of ferrihydrite-associated Mn into more crystalline iron (hydr)oxides. Based on the extraction results, the rate of transfer of Mn into the crystalline product(s) was more rapid than for Fe. This trend suggests that small degrees of iron (hydr)oxide crystallization may lead to significant levels of irreversible sorption for metals that are easily incorporated into the
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FIGURE 2. Changes in solution and solid-phase Me(II) partitioning and solid-phase Fe(III) partitioning during aging of coprecipitates with 0.5 mM (A) Mn(II) and (B) Ni(II) at pH 6 and 70 °C. Experimental conditions: 0.01 M Fe(III), 0.14 M NaNO3, 5 mM MES buffer. crystalline iron (hydr)oxide structure (see results for Mn at 40 °C, below). The behavior of Ni(II) was similar to that of Mn(II), though stabilization within a crystalline phase was not as dramatic. The aqueous Ni(II) fraction decreased from 0.8 to 0.5 during aging, and this increase in sorption was again paralleled by an increase in the residual fraction from 0 to 0.4 (Figure 2B). Extractable Ni(II) fractions remained higher than for Mn(II), but the same trend of an initial rapid increase followed by a gradual decrease was observed during aging. Values for the residual Ni(II) fraction remained lower than for Fe. Thus, the crystallization of Ni(II) and Fe was not a congruent process, and Ni associated with the solid phase appears to have been concentrated within ferrihydrite. Similar behavior has been observed at higher Ni loadings for systems aged at pH > 11, where substitution into goethite was observed (12). Characterization of the solids at the end of aging by XRD and HRTGA showed that hematite was the primary crystalline iron (hydr)oxide (data not shown). A change in crystallization end point and higher extractable Fe values show that the overall crystallization process was retarded at 0.5 mM TOTNi. Thus, stabilization of Ni(II) within a crystalline iron (hydr)oxide occurred at a slower rate than for Mn(II). However, the levels of residual Ni(II) support that partitioning to a crystalline iron (hydr)oxide was occurring, but the approach to an equilibrium end point was slower than in the presence of Mn(II). Comparison of the behavior of Pb(II) and Cd(II) to that of Mn(II) or Ni(II) revealed that these two metals were not stabilized as significantly within a crystalline product (Figure 3, panels A and B). At the end of aging, the fraction of residual Pb(II) only reached a value of 0.1. A high fraction of extractable
FIGURE 3. Changes in solution and solid-phase Me(II) partitioning and solid-phase Fe(III) partitioning during aging of coprecipitates with 0.5 mM (A) Pb(II) and (B) Cd(II) at pH 6 and 70 °C. Experimental conditions: 0.01 M Fe(III), 0.14 M NaNO3, 5 mM PIPES (for Pb(II)) or 5 mM MES (for Cd(II)). Pb was apparently due to the inability to substitute the sorbed species into a crystalline iron (hydr)oxide structure, since Fe crystallization was not retarded. Approximately 20% of TOTPb actually desorbed from the solid phase and re-entered solution over the course of aging, accounting for a significant portion of the decrease in extractable Pb(II). Metal desorption is most likely explained by a loss of available sorption sites due to surface area reductions occurring with crystallization (33, 34). Examination of the Cd(II) results also shows little metal stabilization within a crystalline fraction; however, the reason for this behavior is different. Solution measurements show that the aqueous fraction remained constant at a value of ∼0.95 during aging. The extractable fraction decreased from a value of 0.1 to 0.05, again suggesting a net desorption from the iron (hydr)oxide surface. No apparent substitution occurred within a crystalline iron (hydr)oxide because the system pH was below or near the low end of the pH adsorption edge for Cd(II) (5, 24). Thus, the formation of a surface chemical bond to the host solid surface is a prerequisite for formation of a solid solution. Aging at 70 °C, 0.05 mM TOTMe. To examine the influence of metal surface loading on metal partitioning, experiments were conducted at TOTMe loadings that were an order of magnitude lower for Ni(II), Pb(II), and Cd(II). As shown in Figure 4A, the partitioning behavior of Ni(II) showed a dramatic change and resembled that for Mn(II) at 0.5 mM TOTMn (Figure 2A). The aqueous Ni(II) fraction fell to zero after aging for 150 h. This was accompanied by a rapid increase in the residual fraction to a value of ∼0.95 and by
FIGURE 4. Changes in solution and solid-phase Me(II) partitioning and solid phase Fe(III) partitioning during aging of coprecipitates with 0.05 mM (A) Ni(II), (B) Pb(II), and (C) Cd(II) at pH 6 and 70 °C. Multiple data points are for single extractant duplicates, and error bars are for multiple extractant analyses. Experimental conditions: 0.01 M Fe(III), 0.14 M Na NO3, 5 mM MES buffer. dynamic changes in the extractable fraction, which mimicked Mn(II) behavior (Figure 2A). Iron (hydr)oxide crystallization proceeded more rapidly than at the higher TOTNi loading, and a greater fraction of goethite was identified by XRD in the crystalline product (data not shown). Nickel also entered the crystalline product at a rate faster than that of Fe, indicating that Ni(II) partitioning to a crystalline iron (hydr)oxide was more favorable at the lower TOTNi loading. Thus, metal partitioning to a crystalline product became more significant with decreasing ferrihydrite surface loading. Examination of Pb(II) and Cd(II) partitioning at 0.05 mM TOTMe showed minimal change from behavior at the higher loading. While Pb(II) predominantly remained partitioned to a solid phase, little of it was transferred into a crystalline fraction (Figure 4B). At the end of aging, only 10% of TOTPb had entered a residual phase. Approximately 3% of TOTPb re-entered solution as compared to 20% at 0.5 mM TOTPb. Thus, within the range of metal concentrations studied, the ability to incorporate Pb(II) into a crystalline iron (hydr)oxide seems to have been influenced more by the physicochemical properties of this cation than by surface loading. This argument also applies to the behavior of Cd(II), which again demonstrated minimal interaction with the solid phase(s) when compared to Mn(II) and Ni(II) (Figure 4C). The fraction of aqueous Cd(II) remained high (0.83-0.9), but there was a measurable increase in the fraction of residual Cd(II) by the end of aging. Thus, the dominant factor limiting Cd(II) stabilization was the low system pH.
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FIGURE 6. Unit cell c-dimension, co, as a function of Mn mol fraction for Mn-goethites aged for 1000 h at pH 6 (5 mM MES buffer) and 40 °C. Closed symbols are from this study, and the regression line and open symbols are from ref 13.
FIGURE 5. Changes in solution and solid-phase Me(II) partitioning and solid-phase Fe(III) partitioning during aging of coprecipitates with (A) 0.5 mM Mn(II), (B) 0.05 mM Mn(II), and (C) 0.5 mM Cd(II) at pH 6 and 40 °C. Error bars are for multiple extractant analyses. Experimental conditions: 0.01 M Fe(III), 0.14 M NaNO3, 5 mM MES buffer. Aging at 40 °C, Mn and Cd. We also examined the potential role temperature may play in directing partitioning for a select set of systems. The primary focus of this stage of the research was to establish whether observed irreversible partitioning due to ferrihydrite crystallization would occur at temperatures more representative of terrestrial environments. In all cases, the fraction of residual Fe increased to 0.2 over ∼1000 h of aging as compared to 0.8 at 70 °C. Again, Mn(II) partitioning into a crystalline iron (hydr)oxide (predominantly goethite) was significant at both surface loadings (Figure 5A,B). In both systems, the decrease in solution concentration was mirrored by a near-equivalent increase in the residual fraction. The extractable Mn(II) fraction remained nearly constant with a slight maximum observed for the 0.5 mM TOTMn system. The overall rate for Mn(II) redistribution was slower, but significant stabilization was still achieved within a relatively short time period. Partitioning in the Cd(II) system was similar to experiments at 70 °C (Figure 5C). There appeared to be an increase in the residual Cd(II) fraction toward the end of aging, but the kinetics of the process was much slower when compared to the Mn(II) system.
Discussion The selective extraction results suggest that all metals were partitioned to a residual phase to a certain degree under the conditions of our experiments. For Pb(II) and Cd(II), the percentage of this fraction represented a minor proportion of the total mass of metal present at any given time. However,
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for Mn(II) and Ni(II), the potential for incorporation into a crystalline iron (hydr)oxide appeared significant. Unfortunately, it was impossible to unequivocally verify that Ni(II) was substituted into either goethite or hematite by XRD since the total level of substitution was low (
