Environ. Sci. Technol. 1996, 30, 2470-2477
Effect of Temperature, Ionic Strength, Background Electrolytes, and Fe(III) on the Reduction of Hexavalent Chromium by Soil Humic Substances PAUL R. WITTBRODT AND CARL D. PALMER* Department of Environmental Science and Engineering, Oregon Graduate Institute of Science & Technology, P.O. Box 91000, Portland, Oregon 97291-1000
The rate of hexavalent chromium reduction by soil humic substances (SHSs) was investigated in aqueous solutions where the temperature, ionic strength, background electrolyte, [Fe(III)], and [Cr(III)] were independently varied. Rate experiments were conducted with an excess of SHS over Cr(VI). An Arrenhius plot for the reduction of Cr(VI) by a soil fulvic acid and a soil humic acid indicates that the activation enthalpies for oxidation of these substances are nearly the same (63 ( 1 and 61 ( 3 kJ mol-1, respectively) and the activation entropies are significantly different (-160 ( 5 and -203 ( 9 J mol-1 K-1, respectively). Rates of reduction are not significantly altered due to changes in either background electrolyte or ionic strength. The presence of Cr(III) slightly inhibits the rate of reduction by soil humic acid, but not that of soil fulvic acid. Ferric iron increases the rate of Cr(VI) reduction, even when only a small amount of Fe(III) is added to the system. Fe may enhance the reduction of Cr(VI) by being alternately reduced by the SHS and then oxidized by the Cr(VI) as part of a redox cycle. The reduction of FeCrO4+ complexes via a parallel reaction pathway may also enhance Cr(VI) reduction in the Cr-Fe-SHS system.
Introduction Numerous industrial activities, including metallurgy, leather tanning, electroplating, lumber treating, and electricity generation produce significant quantities of chromium wastes. Leakage, unsuitable storage, or improper disposal practices have led to many instances where chromium has been released to the subsurface (1-3). Because chromium can be acutely toxic, carcinogenic, and teratogenic (4-8), it is necessary to understand the processes governing the transport and transformation of chromium in the environ* Corresponding author telephone: 503-690-1197; fax: 503-6901273; e-mail address:
[email protected].
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ment. Since both the mobility and toxicity of Cr depend on its oxidation state, redox reactions involving Cr are extremely important in determining its fate in the environment and its potential risk to human health. Chromium is a redox active element with oxidation states from -2 to +6 (9), but only the +3 and +6 states are prevalent in the environment. The hexavalent form is more toxic (10) and generally more mobile than trivalent chromium (11). Chromium(VI) species occur as the oxyanions chromate (CrO42-), bichromate (HCrO4-), and dichromate (Cr2O72-) and are not strongly sorbed to many soils under alkaline to slightly acidic conditions (12), thus they can be very mobile in subsurface environments. In contrast, Cr(III) does not readily migrate in many environments because it precipitates as Cr(OH)3 or as the solid solution FexCr1-x(OH)3 (13, 14) or because it is chelated by organic molecules that are adsorbed to mineral surfaces (15, 16). Thus, the transformation of Cr(VI) to Cr(III) by reductants commonly found in subsurface environments has received a fair amount of attention because of the potential for immobilization of chromium in the subsurface. Such natural attenuation of hexavalent chromium can potentially impact remediation goals and strategies (17). Hexavalent chromium is a strong oxidant that can be reduced to the trivalent form in soils by redox reactions with aqueous ions, electron transfers at mineral surfaces, reaction with simple organic molecules, or reduction by soil humic substances (3). In a study of chromate reduction by various soils, Eary and Rai (18) concluded that Cr(VI) reduction by organic matter and Fe(II) were roughly equivalent. Humic substances are present in virtually all soils and constitute the major dissolved and particulate organic fraction in most soils. Soil humic substances (SHSs) thus may represent a significant reservoir of electron donors for Cr(VI) reduction. Humic substances are operationally defined in terms of their acid-base solubility (19). Fulvic acid (FA) is soluble in both acid and base and has lower molecular weights, carbon contents, and aromaticity than other humic fractions. Humic acid (HA) is soluble in base but not acid. Reported standard redox potentials of 0.7 V. for HA (20, 21) and 0.5 V. for FA (22) suggest that FA is a better reducing agent than HA. The rates of Cr(VI) reduction by soil fulvic acid (SFA) and soil humic acid (SHA) were examined by Wittbrodt and Palmer (23, 24). Reaction of Cr(VI) and excess SHS is characterized by a nonlinear loss of Cr(VI) with time. The rate of Cr(VI) reduction decreases with time due to the depletion of Cr(VI) and the reduced reactivity of the SHS as it becomes oxidized. Empirical rate equations for SFA and SHA were developed in which the rate of reaction was dependent on the equivalent fraction of humic substance oxidized. These rate equations were generated from experiments in which the H+, Cr(VI), and SHS concentrations were independently varied and the first 5% of the SHS was oxidized. In additional experiments, the rate equations and the rate coefficients were demonstrated to be applicable to the first 20% of the oxidizable SHS (23, 24). The rate equations were developed over a pH range of 1-7 for SFA and 2-7 for SHA. As is the case with many organic Cr(VI) reactions, the rate of reduction dramatically increases
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SHA have been studied under conditions where the independent variables were different than in those studied by Wittbrodt and Palmer (23, 24). The objectives of these experiments were to determine the variability of Cr(VI) reduction rates due to changes in ionic strength, background electrolyte, and temperature. Additionally, experiments were conducted to determine the effects on Cr(VI) reduction rates due to the presence of Cr(III) or Fe(III). All of the experiments reported here were conducted at pH 2. Such pH values are found at plating operations where chromic acid has been spilled.
Experimental Section FIGURE 1. Reduction of Cr(VI) by soil fulvic acid (open symbols) and soil humic acid (closed symbols) at pH ) 2, 4, and 6. [Cr(VI)]0 ) 0.02 mM, [SHS]0 ) 100 mg/L, I ) 0.1 M LiCl. The lines represent the best fit to the integrated form of the rate equations for soil fulvic acid (dashed line) and soil humic acid (solid line) using eqs 3 and 4, respectively.
as the pH decreases (Figure 1). The empirical rate equations for SFA and SHA are similar. In both cases, the rate of reduction was found to vary with [HCrO4-]0.5 (23, 24). By treating the SHS as a continuum of reactive groups, the rate of reduction should also be a function of the equivalent fraction of SHS that is oxidized (Xe). Wittbrodt and Palmer (23, 24) found that the rate of reduction of Cr(VI) by both SFA and SHA varies with Xe-1. The rate of reduction of Cr(VI) by SFA, RSFA, is
RSFA ) -kSFA[H+]0.45[HCrO4-]0.5[SFA]Xe-1
(1)
Similarly, the rate equation for the reduction of Cr(VI) by SHA (23) is
RSHA ) -(k0 + kSHA[H+]0.5)[HCrO4-]0.5[SHA]0.5Xe-1 (2) SFA reduces Cr(VI) more rapidly than SHA at the same pH (Figure 1). The rate of reduction of Cr(VI) by two other humic acids was also found to vary with [HCrO4-]0.5 (23). The rate equations are written in terms [HCrO4-] because it is believed to the reactive species in many Cr(VI) organic reactions SHS (23, 24). Protonation of SHS are not explicitly included in the rate equations but are indirectly included in the empirical approach used to define the rate of Cr(VI) reduction as a function of [H+]. This approach is justified given the difficulties and lack of agreement in representing the general acid-base chemistry of humic substances and particularly the specific moities involved in Cr(VI) reduction. The general applicability of these empirically derived rate equations remains questionable primarily because the experimental conditions under which they were derived are not likely to be encountered at many sites. For example, the background electrolyte for experiments used to develop the rate equation was LiCl. Li+ was used as the background cation because it does not exhibit site-specific binding with humic acid (25); however, Na, Ca, and Mg are more likely to be the dominant cations in many natural waters. Similarly, all of the experiments were conducted at 25 °C. In addition, other ions in solution may impede or enhance the reduction of Cr(VI) by SHSs. Conformational changes in the SHS due to the presence of Cr(III) may be expected to inhibit the rate of Cr(VI) reduction In this paper, we describe a series of laboratory experiments in which the rate of Cr(VI) reduction by SFA and
Materials. Soil fulvic acid and soil humic acid were obtained from the International Humic Substances Society (IHSS) collection of reference humic materials (ref nos. 1R102F and 1R102H) and have been extensively characterized by other researchers (26). The elemental compositions of the SFA and SHA used in the experiments are reported in Wittbrodt and Palmer (23). Solutions were prepared with ultrapure (∼18 MΩ‚cm) water from a Barnstead Nanopure water purification system. Chemicals were all of reagent grade. The glass reaction vessels were foilwrapped to prevent photoreactions. Analytical and Experimental Methods. The reduction of Cr(VI) by a SHS was determined by measuring the timedependent concentration of Cr(VI) in batch reaction vessels. Most of these experiments were performed at room temperature (25 ( 2 °C) in triplicate. These reduction experiments were conducted with excess SHS over chromium as determined from the modified Walkley-Black tests reported by Wittbrodt and Palmer (23, 24). Aliquots (20 mL) of a stock solution containing 100 mg/L SHS were equilibrated for 24 h at pH 2.0 before being spiked with K2CrO4. Changes in Cr(VI) concentrations were measured over time by mixing a 0.5-mL sample with 0.1 mL of a diphenylcarbazide (DPC) chromatogen, then adding 2.0 mL of 0.1 N H2SO4 (27). The solution was then filtered through a 0.1-µm polysulfonate filter. Ten minutes after the addition of DPC, the absorbance at 540 nm was measured. The Fe(II) concentration was measured in the experiments spiked with Fe(III) using a 1,10-phenanthroline method (27, 28) by adding 33 µL of 0.36 M H2SO4, 67 µL of NH4F, 67 µL of 1% 1,10-phenanthroline solution, and 100 µL of ammonium acetate buffer to 0.5 mL of sample and measuring the absorbance at 510 nm. Several sets of rate experiments were conducted to determine the effects on the Cr(VI) reduction rate due to various solution parameters. The pH in all of the experiments was 2.0 ( 0.05. The effects of varying the background electrolyte were studied using both mono- and divalent cations (Li+, K+, Ca2+, and Mg2+). For the remaining experiments, those with various ionic strengths, temperatures, or trivalent cations, the background electrolyte was LiCl. The rate of Cr(VI) reduction as a function of ionic strength was studied in the range I ) 0.05-0.5 M. The rate of reduction as a function of temperature was studied in the range of 4-55 °C. The rate of Cr(VI) reduction by SHSs in the presence of Cr(III) and Fe(III) was examined. Rate experiments were conducted in the Cr(III) concentration range of 0.0-0.4 mM in 0.1 M LiCl. The rate of Cr(VI) reduction was studied in the presence of two concentrations of FeCl3‚6H2O (2 and 120 µM) to determine how the reduction rate changes in the presence of an ion known to be reduced by SHS and
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FIGURE 3. Influence of reaction temperature on the observed rate coefficients for Cr(VI) reduction by SHS. Plots of ln (kexp/T) versus 1/T for 4, 15, 25, 35, and 55 °C. Curves fitted by a weighted leastsquares linear regression. Heats (∆Hq) and entropies (∆Sq) of activation computed from slopes and intercepts, respectively. Rate coefficients at 4 °C based on data for time >3000 min. Curves are the linear regression lines.
further simplified to
ySHA ) -kSHA[H+]0.5ξ0.5[SHA]0.5t
FIGURE 2. Reduction of Cr(VI) by (A) soil fulvic acid and (B) soil humic acid at various temperatures (4, 15, 25, 35, and 55 °C). Plots of log relative Cr(VI) concentration versus log time. [Cr(VI)]0 ) 0.02 mM, [SHS]0 ) 100 mg/L, I ) 0.1 M, pH ) 2.0. The solid lines are the best fit to eq 3 (soil fulvic acid) and eq 4 (soil humic acid). Data labeled 4 °C were not maintained at that temperature for time 3000 min. The calculations of kSFA and kSHA at 4 °C were calculated from the slopes of the ySFA and ySHA versus t curves for t > 3000 min. Activation enthalpy and entropy (∆Hq and ∆Sq) were calculated using
kexp )
kbT ∆Sq/R -∆Hq/RT e e h
(9)
(29) where the enthalpy (∆Hq) and entropies (∆Sq) of activation were calculated from the slopes and intercepts of plots of ln (kexp/T) versus 1/T (Figure 3, Table 1). T is the temperature in kelvin, kb is Boltzman’s constant (1.38
TABLE 1
Activation Data for Cr(VI) Reduction by IHSS Reference Soil Humic Acid and Soil Fulvic Acid at pH 2a SFA SHA a
∆Hq (kJ mol-1)
∆Sq (J mol-1 K-1)
Ea (kJ mol-1)
63 ( 1 61 ( 3
-160 ( 5 -203 ( 9
111 ( 1 122 ( 3
[Cr(VI)]0 ) 0.02 mM, [SHS]0 ) 100 mg/L, pH ) 2, T ) 4-55 °C.
× 1023 J K-1), h is Planck’s constant (6.63 × 10-34 J s/-1), and R is the gas constant (8.314 J K-1 mol-1). Because of the empirical manner in which the rate coefficients were derived, the activation parameters must be taken as composite values for all reactions up to and including the rate-limiting step. As such, they do not necessarily describe the activation energy of an elementary reaction. The computed activation enthalpy for SFA and SHA are not statistically different at the 90% confidence level (t ) 0.639; df ) 3) as calculated from a Student t test. This could imply that the rate-limiting step is the same for both SFA and SHA. The computed enthalpies of activation are 1.1-2.3 times larger than those reported for Cr(VI) reduction reactions by simple organic compounds with hydroxyl or sulfhydryl functional groups (e.g., ref 30). In contrast, the entropy of activation (∆Sq) for SFA is significantly greater than ∆Sq for SHA at the 95% significance level (t ) 4.109; df ) 3). ∆Sq is often used as an indicator of the configuration of the activated complex. A large negative number, such as those calculated for the SHSs, indicates that the reactant molecules are separated by short bonds, thus the decrease in entropy is large and the preexponential factor [(kbT/h) exp(∆Sq/R)] is small. The calculated ∆Sq for the reduction of Cr(VI) by SFA (-160.1 ( 4.6 J K-1 mol-1) and SHA (-203.1 ( 9.4 J K-1 mol-1) are similar to those found for the reduction of Cr(VI) by 4-methylphenol (30) (-181 to -185 J K-1 mol-1), benzyl alcohol (31) (-180.8 J K-1 mol-1), and glutathione (32) (-167 J K-1 mol-1). The activation energies, Ea, for SFA and SHA were computed from
Ea ) ∆Hq- ∆SqT
(10)
The activation energy for SHA at 298 K (121.8 ( 2.8 kJ mol-1) is significantly greater than the activation energy for SFA at 298 K (111.1 ( 1.4 kJ mol-1) at the 5% significance level (t ) 3.418; df ) 3). These activation energies are much greater than the 21 kJ/mol for diffusion-controlled processes in aqueous systems. Effect of Varying Ionic Strength and Background Electrolyte. Changes in ionic strength of the solution or the valence of the background electrolyte can cause conformational modifications in the SHS (33). We suspected that these conformational changes would make the reactive functional groups either more or less accessible to Cr(VI), thereby altering the rate of reduction. Cr(VI) reduction by SFA and SHA was studied in the ionic strength range 0.05-0.5 M. Plots of [Cr(VI)] versus time do not display a great difference in rates of reduction as the ionic strength is varied over the range studied (Figure 4). The logarithms of the rate coefficients for SFA and SHA with the data pooled over the replicates are summarized in Table 2. Analysis of variance indicates that the variation
FIGURE 4. Reduction of Cr(VI) by (A) soil fulvic acid and (B) soil humic acid in solutions of various ionic strengths in the range I ) 0.05-0.5 M. Points represent means of triplicate measurements. [Cr(VI)]0 ) 0.02 mM, [SHS]0 ) 100 mg/L, pH ) 2.0. TABLE 2
Log k Values for Experiments with Different Ionic Strengthsa ionic strength
log k
n
r2
0.05 0.10 0.20 0.50
Soil Fulvic Acid -6.765 ( 0.046 -6.698 ( 0.018 -6.698 ( 0.055 -6.751 ( 0.112
27 33 27 24
0.950 0.990 0.929 0.783
0.05 0.10 0.20 0.50
Soil Humic Acid -8.660 ( 0.025 -8.652 ( 0.015 -8.662 ( 0.023 -8.599 ( 0.021
27 39 27 27
0.985 0.992 0.987 0.989
a Experiments conducted in 0.1 M LiCl, [Cr(VI)] ) 0.02 mM, [SHS] 0 0 ) 100 mg/L, pH ) 2.
between the replicates for the SFA experiments is not different than the variation between the various ionic strengths [F(3,8) ) 1.011] at the 5% significance level. For SHA, the variance between the ionic strengths is greater than the variance of the replicates at the 5% significance level [F(3,8) ) 7.460]. There is no apparent trend in the log k with ionic strength. The largest and smallest log k are significantly different at the 5% significance level (t ) 3.193; df ) 4); however, the difference is only 0.073 unit. The dependence of reduction rates on the background electrolyte was investigated using two monovalent (Li+, K+) and two divalent (Ca2+, Mg2+) cations. Plots of [Cr(VI)] versus time (Figure 5) show very little variation in Cr(VI) reduction rates by SFA or SHA as the background cation changes. The logarithm of the rate coefficients (Table 3)
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mean log k between the experiments conducted in KCl and the group of experiments conducted with the other electrolytes is 0.14 unit. If the ionic strengths or background electrolytes tested in these experiments have any effect of the rate of reduction of Cr(VI) by SHSs, the effect is relatively small (
