Environ. Sci. Technol. 1994, 28, 178-185
Batch Experiments Characterizing the Reduction of Cr(V1) Using Suboxic Material from a Mildly Reducing Sand and Gravel Aquifer Linda Davis Anderson,. Douglas 6. Kent, and James A. Davis
Water Resources Division, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Batch experiments were conducted with sand collected from a shallow sand and gravel aquifer to identify the principal chemical reactions influencing the reduction of Cr(VI), so that field-observed Cr(V1) reduction could be described. The reduction appeared to be heterogeneous and occurred primarily on Fe(I1)-bearing minerals. At only 1wt % , the fine fraction (C64 pm diameter) of the sediments dominated the amount of aqueous Cr(V1) reduction because of its greater reactivity and surface area. Although reduction of Cr(V1) increased with decreasing pH, small variations in the abundance of fine fraction among the replicate samples obscured pH trends in the batch experiments. Consistent results could only be obtained by separating the fine material from the sand and running parallel experiments on each fraction. As pH decreased (6.4 to 4.5), Cr(V1) reduction increased from 30 to 50 nmol/m2 for the sand fraction (64-1000 pm) and from 130 to 200 nmol/m2 for the fine fraction. The amount of Cr(V1) reduced in both the sand-sized and fine material increased from 35 to 80 and from 130 to 1000 nmol/m2, respectively, for a 10-fold increase in Cr(VI)injtid. A consistent description of the rate data was achieved by assuming that intraparticle diffusion limited the observed rate of reduction.
Introduction Hexavalent chromium [Cr(VI)] is both toxic and mutagenic (1-3). Cr(II1) may have toxic effects as well but, because it reacts strongly with particles (4) and is very insoluble (5, 6 ) ,aqueous concentrations are usually well below water quality standards (7). Many industrial processes generate wastewaters containing Cr(V1) (7). Adequate protection of subsurface water supplies requires that the fundamental properties of Cr(V1) reactions are known. Adsorption of Cr(V1) on hydrous metal oxides decreases with increasing pH or concentrations of competing anions (8). Thus, Cr(V1) should be relatively mobile in near-neutral pH groundwater environments, particularly in systems with competing oxyanions (9,lO). Mowever, Cr(V1) reduction to Cr(II1) will immobilize Cr and can occur by reaction with inorganic (e.g., refs 11-14) or organic reductants (e.g., refs 15 andl6) or via biological processes (e.g., refs 17-20). Although there are some careful studies on natural materials (211, many investigators have failed to distinguish between Cr(V1) reduction by Fe(I1) in solution, Fe(11) associated with minerals, or organic reductants. It has often been assumed that if a positive correlation between organic matter and Crba associated with the solid exists within a Cr(V1)-contaminated soil, then the organic material reduced the Cr. It is as difficult to apply pure mineral-phase studies to field materials as it is problematic to discriminate and prioritize the role of the different components of a natural sample. However, to adequately assess chromium’s environmental hazards, it is necessary 178 Environ. Sci. Technoi., Vol. 28, NO. 1, 1994
to characterize natural samples and to incorporate reduction reactions in groundwater models. With redoxsensitive elements such as Cr, this includes identifying the primary oxidation-reduction reaction@) and determining a realistic kinetic model (15) for both laboratory and field-scale data, because thermodynamic equilibrium conditions can rarely be assumed (22). Cr(V1) tracer-injection tests have been carried out in the groundwater of a shallow, sand and gravel aquifer a t the U.S.Geological Survey Cape Cod Toxic Substances Hydrology Research Site (CCTSHRS) near Falmouth, MA (23). The results have shown that aquifer chemistry strongly affects the transport of Cr(V1) in the aquifer. A slight retardation of Cr(V1)was observed,but the dominant control of Cr transport within the suboxic zone was the reduction of Cr(V1) to Cr(II1) and the removal of Cr(II1) from solution (25). To gain a better understanding of the mechanisms of Cr(V1) reduction in the aquifer and the variables that influence the rate of the process, a set of batch experiments was performed using subsurface material from the suboxic, sewage-contaminated zone of the aquifer a t CCTSHRS. Additional effort was taken to assess the source of experimental variability, the relevant chemical reactions, and the rates of the processes controlling Cr(V1) reduction in this heterogeneous natural material, Experiments varying the initial Cr(V1) concentration, pH, and temperature were run to elucidate the kinetic components influencing the Cr(V1) reduction. In this paper, we describe the results of these experiments and discuss their significance in the context of modeling the transport of Cr(V1) in groundwater.
Materia Is Core Material. Cores described in this paper are solely from the suboxic zone ( 0 2 C 30pM), where groundwater, depleted in oxygen, shows an increase in manganese concentrations, but the conditions are not sufficiently reducing to observe iron reduction (25). All cores were collected in 1.5-msections using a modified wire-line piston corer with no drilling mud (26). Water was not introduced to clear the core barrel during coring to minimize any compromising of the suboxic cores by the introduction of oxygenated water. Cores were maintained in an upright position to avoid mixing of sediment layers and were capped immediately. Most of the cores were taken as close to each other as possible (within about a 3-m radius). Cores were either used immediately for experiments or frozen upright within 2 h of collection; cores were opened and processed in a glovebag purged with nitrogen. The top and bottom 5 cm of the cores were removed to minimize potential sources of air-contaminated sand. The median grain size of the sediments is 0.6 mm, and the fine fraction (C64 pm) ranges from 0.1 to 2.4% of the total weight. Specific surface areas were calculated from single-point measurements of N2 adsorption a t a relative pressure of 0.3 using the single-point BET equation (27).
This article not subject to U.S. Copyright. Published 1993 by the American Chemical Society
Specific surface area for the sand was 0.4 mz/g and, on the fine material, was 11.9 mZ/g. Pore volumes in the size range o where [Crlo' is the Cr(V1) concentration after the initial, rapid phaseof the reaction. The reaction stops when either Cr(V1) or the reducing agent is consumed, [Cr]; and k were determined from the logarithmic regression of the experimental data after 2 h of reaction (Tables 1and 3). Most of the experimental observations were consistent with this model. Reasonable fits to the experimental data Environ. Sci. Technol., Vol. 28,
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were obtained (Figure 4). Values of k determined from experiments with sand-sized material over the temperature range of 10-35 "C yielded an activation energy of 4.5 kcal/ mol (Figure 5), which is in the proper range for diffusioncontrolled processes (2.5-5 kcal/mol; 56). Furthermore, diffusion coefficients, estimated from the Stokes-Einstein equation, show a strong linear relationship with the estimated ks (Figure 5, insert). Values of k determined from experiments with fines were greater than those from experiments with sand-sized material (Tables 1 and 3), consistent with the greater abundance of reducing agent and smaller particle size in the fine fraction. Experiments conducted with air-exposed or acid-leached aquifer solids showed a lag in the onset of reduction (Figure 6). This is consistent with the proposed model because readily available reduction sites on the solids would be lost due to oxidation by atmospheric oxygen during prolonged air exposure or leached from the solids during exposure to acid. The aquifer material is heterogeneous, and it is likely that a variety of mechanisms actually contributed to the observed rate of reduction. The qualitative success of this simple model suggests that diffusion control accounts for most of observed reduction at room temperature and pH 6. However, reduction via other mechanisms such as reduction of Fe(II1) to Fe(I1) (39,40,45-49), dissolution of Fe(I1) (14, and increased surface adsorption of Cr(V1) (57,581could contribute to the observed rate. Acceleration of these reactions as conditions were varied could account for the rate changes observed a t lower pH values and higher temperatures (Table 1, Figure 5). 184
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Figure 6. Demonstrationof the time lag in reduction when the sand is exposed to air for 24 h or treated with a weak acid (pH = 3). No treatment and air-treated sand are at pH = 6.8, g of sand/ L = 900, and Cr(VI)lnMHal= 20 pM; the acid-treated sand is Cr(VIhM, = 20 pM, pH = 6.3, g of sand/L = 830. The comparison is not meant to imply similar experimental conditions, but is meant to demonstrate the dlfference in the initiation of Cr(V1) reduction.
Relationship with Field-Observed Cr(V1) Reduction. Kent et al. (25) observed significant differences in the amount of reduction a t different strata within the aquifer. Data from batch experiments indicate that the variations in the fine fraction could account for the differences. The pH may affect variability in fieldobserved reduction, but there was no significant changes in pH with depth that could explain the observed differences in Cr reduction (25). Thermodynamic modeling of redox-sensitive elements in general poses problems. A realistic evaluation of pe of field waters is difficult to acquire because redox couples in natural waters are generally not a t equilibrium (22). The dissolved species present a t the CCTSHRS site show considerable disequilibrium among the redox sensitive species within the suboxic portion of the aquifer (23).The -log of the electron activity ranges from 14.1 to 5.5 (calculated for pH = 6) and thus the thermodynamically predicted oxidation state of Cr could be either Cr(V1) or Cr(II1). This makes the identification of the dominant redox couple(s) influencing Cr(V1) reduction critical to describing its field behavior.
Concluding Remarks In this study, Cr(V1) reduction appeared to occur on the mineral surface. There was insufficient dissolved Fe(11)to explain the amount of Cr(V1) reduction observed. Experiments with bipyridine and phenanthroline, which block potential reducing sites, suggest that Fe(I1)-containing minerals are the primary reductant within these sediments. Other investigations looking specificallya t Cr(VI) reduction by organic compounds (41-441, in addition to our study, show that organic compounds do not directly reduce Cr(V1)very rapidly at;pHs greater than 2. However, organic compounds, even in small amounts, may influence the availability of Fe(I1) by reducing Fe(II1). It is difficult to determine the redox state in the field because redox-active elements are in low abundance and are often not in thermodynamic equilibrium. This appears to be the case a t the CCTSHRS field site (25). The laboratory investigations are useful in identifying the appropriate redox couple to apply to the aquifer a t CCTSHRS and suggest that reaction-based modeling may be better applied to this field site than more general thermodynamic models. Furthermore, in batch experiments Cr(V1) reduction appears to be diffusion controlled.
Variability in the amount of fine material significantly affected the amount of reduction in the laboratory experiments despite being a minor weight percentage of the total sediment. Variability in the observed field reduction may be accounted for by the heterogeneous distribution of the fine-grained material.
Acknowledgments The authors would like to thank D. Waite, A. Stone, A. White, C. Fuller, and J. Coston for respective fruitful discussions; D. Waite for help with the marathon ‘freshcore’ batch experiments; the USGS crew a t Cape Cod for providing us with space and equipment to run the ‘freshcore’ batch experiments; M. Kohler for running the BET and porosity measurements on the sand and fines; A. White et al. for enduring numerous hours of L.D.A. using the glovebox in their lab; J. Friedly for providing the simplifying assumptions that allowed the kinetic evaluation of the data; and three anonymous reviewers for their constructive comments. Literature Cited (1) Turner, M. A,; Rust, R. H. Soil Sci. SOC.Am. Proc. 1971, 35, 755. (2) Venitt, S.; Levy, L. S. Nature 1974,250, 4. (3) Ajmal, M.;Nomani,A.A.;Ahmad,A. Water,Air,SoilPollut. 1984, 23, 119. (4) Leckie, J. 0.;Appleton, A. R.; Ball, N. B.; Hayes, K. F.;
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Received for review September 13, 1993. Accepted September
Proceedings of the 7thInternational Symposium on Water-
1993.
20, 1993. *
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* Abstract published in Advance ACSAbstracti November 1, Envlron, Scl. Technol., Vol. 28, No. 1, 1994
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