Environ. Sci. Technol. 2000, 34, 5017-5022
Reductive Dechlorination of Tetrachloroethylene by Fe(II) in Cement Slurries INSEONG HWANG AND BILL BATCHELOR* Department of Civil Engineering, Texas A&M University, College Station, Texas 77843
Fe(II) is a potentially effective reductant for treating chlorinated organics in soils by degradative solidification/ stabilization (DS/S) process. DS/S is a modification of conventional S/S that promotes degradation of organic contaminants while immobilizing inorganic contaminants. In this study, degradation of tetrachloroethylene (PCE) by Fe(II) in the presence of cement hydration products was characterized by using batch slurry reactors. Cement has been found to catalyze or participate in the PCE degradation reactions over the pH range investigated (10.6-13.8), and the degradation kinetics can generally be described by a pseudo-first-order rate law. PCE degradation rate was greatest at pH ∼12.1 with a half-life of 4.1 days when [PCE]0 ) 0.245 mM and [Fe(II)]0 ) 39.2 mM. Under this condition, 98% of the PCE initially present in the system transformed to nonchlorinated products (acetylene, ethylene, ethane) with acetylene being predominant, implying that elimination pathways were favored in the cement systems at high pH. Production of chlorinated intermediates was minimal in Fe(II)/cement systems. Addition of Fe(III) to Fe(II)/ cement systems increased PCE degradation rates by factors of ∼2 to 3, suggesting Fe(II)-Fe(III) (hydr)oxides might have been reactive agents. Three hypotheses for the reaction mechanisms are discussed.
Introduction Solidification/stabilization (S/S) is the most commonly used remedial alternative at superfund sites in the United States (1). Degradative solidification/stabilization (DS/S) is a modification of conventional S/S processes that promotes degradation and containment of organic contaminants while also immobilizing inorganic contaminants. Destruction of organic contaminants can be facilitated by adding degradative reagents in the conventional S/S systems. Effective immobilization of contaminants in soils allows the use of degradative reactions that otherwise would be too slow. Portland cement (cement hereafter) is a common binder and can effectively immobilize contaminants (2). Hydration of cement produces calcium silicate hydrates (CaO-SiO2H2O), which are nonstoichiometric colloidal gels that principally control the chemical environment in hydrated cement (3). The most attractive degradative reaction for chlorinated hydrocarbons in DS/S is reductive dechlorination. A preliminary experiment was conducted in our lab that identified Fe(II) as the most effective of the five electron donors tested (sulfide, polysulfide, dithionite, pyrite, ferrous iron) in * Corresponding author phone: (979)845-1304; fax: (979)862-1542; e-mail:
[email protected]. 10.1021/es991377b CCC: $19.00 Published on Web 10/11/2000
2000 American Chemical Society
dechlorinating tetrachloroethylene (PCE) in systems containing cement. In this study, reductive dechlorination of PCE by Fe(II) was characterized further using cement slurry reactors. This characterization will allow quantitative prediction of the rates of degradation as a function of important system variables and result in more complete understanding of the degradation chemistry in the Fe(II)-based DS/S system. PCE was chosen as a model chlorinated hydrocarbon due to its widespread occurrence and toxicity (4). Fe(II) in the solid phase or Fe(II) bound on the surfaces is known to be a better reductant than Fe(II) in the solution (5, 6). Higher density of electrons in Fe(II) solids and lower reduction potential of the surface-bound Fe(II) relative to aqueous phase Fe(II) promote reductive transformation of organics. When Fe(II) is present in cementitious environments, high hydroxide ion concentration would be conducive to formation of iron solids such as Fe(II) hydroxide and Fe(II)Fe(III) (hydr)oxides. Green rust and magnetite are among the well characterized Fe(II)-Fe(III) (hydr)oxides in the environment and are known to reduce compounds such as nitrate, carbon tetrachloride, Cr(VI), and pertechnetate (710). These Fe(II)-Fe(III) (hydr)oxides are reported to form in concrete by corrosion of the embedded iron and can persist at pH up to ∼13 (11, 12). Fe(II) is markedly effective in reduction reactions when it is sorbed on iron (hydr)oxides such as hematite, magnetite, goethite, and lepidocrocite (13, 14). Thus, surfaces of Fe2O3 in the cement hydrates may provide reactive surfaces for dechlorination. The objective of this study was to characterize PCE degradation reactions by Fe(II) in the presence of cement hydration products. Kinetic experiments were conducted under various conditions to optimize PCE degradation reactions and to draw inferences on reaction mechanisms. Kinetic experiments examined the effects of Fe(II) dose, pH, and the addition of various solid phases, Fe(III), and anions. Degradation products of PCE were analyzed in selected experiments.
Materials and Methods Chemicals. The chemicals used were as follows: tetrachloroethylene (99.9+%, HPLC grade, Aldrich), trichloroethylene (99.5+%, Fisher Scientific), 1,1-DCE (99%, Lancaster), cisDCE (97%, Aldrich), trans-DCE (99%, Chem Service), vinyl chloride (30 000 µg/mL in methanol, Supelco), dichloroacetylene (synthesized (15)), chloroacetylene (synthesized (15)), toluene (99.9%, glass distilled, EM), methanol (99.8%, HPLC grade, EM), 1% ethylene in helium (Scott Specialty Gas), 1000 ppm acetylene in helium (Scott Specialty Gas), 99% ethane (Scott Specialty Gas), pentane (99.9%, HPLC grade, EM), ferrous chloride (99+%, tetrahydrate, Sigma), ferrous sulfate (99+%, heptahydrate, Sigma), ferric chloride (97.0-102%, hexahydrate, Sigma), ferric sulfate (21-23%, Sigma), cement (type I, Capitol Cement), calcium hydroxide (Fisher Scientific), and tricalcium silicate (monoclinic, Construction Technology Laboratories). The specific surface area and Fe(III) content of the cement, calcium hydroxide, and tricalcium silicate are measured by the manufacturers and are presented in Table 1. Deoxygenated deionized water was prepared by sparging the water purified by the Barnstead Nanopure system for at least 12 h with the atmosphere of an anaerobic chamber (Coy Laboratory Products) containing 95% N2/5% H2. All of the aqueous stock solutions were prepared using the deoxygenated deionized water in the anaerobic chamber. Methanolic stock solutions of PCE were prepared daily. Stock solutions of Fe(II) and Fe(III) were also prepared daily by dissolving chloride or sulfate salts of iron VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Specific Surface Area and Fe(III) Content for Solids surface
specific surface area (m2/g)
Fe(III) (wt %)
Portland cementa calcium hydroxide (Ca(OH)2) tricalcium silicate ((CaO)3SiO2)
0.358b n.a.c
1.76 0.063
0.355b
0.050
a 94.8% passing 325 mesh (0.45 µm). b Measured by the Blaine method (2). c Not available. 99% passing 325 mesh (0.45 µm).
in water. The base solution of 5.3 N KOH and the acid solution of 5 N HCl were sparged with the chamber atmosphere for ∼36 h and used for pH control. Experimental Procedures. Clear borosilicate glass vials with volumes ∼24 mL were used as batch slurry reactors. Closures of the vials were designed to minimize intrusion of oxygen and volatilization losses of PCE and its degradation products. Closures consist of three-layer sealing components: Teflon lined silicon septum, lead foil tape (3M, adhesive backed), and Teflon tape (Norton Performance Plastics Co., nonadhesive, 2 mil thick). All samples were prepared in the anaerobic chamber. Controls were prepared in duplicate, and all other reactive samples were prepared in triplicate. Controls contained the water and PCE. The mass ratio of the solid to solution was 0.1. The uptake of water by the cement in cement experiments resulted in approximately 2% decrease of the aqueous phase volumes over the reaction period of the experiments (∼90 days). Errors due to these changes in aqueous phase volumes were neglected because they were considered within the experimental and analytical errors. To prepare samples, vials were filled with aliquots of the water, stock solutions, and the acid or base solution, as needed. Headspace volumes of the vials were minimized (0.3-0.6 mL). Fe(II) doses of the kinetic experiments ranged from 9.8 mM to 196 mM. The solids used in this study were able to buffer the pH of 10% slurries around 12.6 in the experiments with Fe(II) doses ranging from 0 to 98 mM. For other experiments, to control the pH, appropriate aliquots of 5 N HCl or 5.3 N KOH solution were added to samples when they were prepared. Experiments were initiated by spiking 10 µL of the methanolic PCE stock solution into the slurries, to yield an initial PCE concentration of 0.245 mM, except for the experiment to investigate chloride balance (0.483 mM). After PCE spiking, the vials were capped rapidly using three-layer seals. Then the vials were placed in a tumbler that provided end-over-end rotation at 7 rpm at the room temperature (19.3 ( 0.68 °C). The vial containers were covered with an aluminum foil to exclude photochemical effects. At each sampling time, duplicate or triplicate sample vials were sacrificed for the analysis of PCE and its degradation products. The vials containing solids were centrifuged at 582g for 2 min. Appropriate aliquots (5-10 mL) of the aqueous sample were rapidly withdrawn from the vials with a 10-mL gastight syringe while injecting 5-15 mL of air using a 30-mL gastight syringe. The aqueous samples were then transferred to 20mL vials carrying 5 mL of a pentane extractant containing toluene as an internal standard. To determine the extent of PCE sorption onto the solids and iron precipitates, ∼10 mL of aqueous samples were withdrawn from selected vials, and the remaining slurries were extracted using 5 mL of the pentane extractant. The vials containing the extractant were shaken at 250 rpm for 30 min using an orbital shaker. After extraction, ∼1 mL of the extractant layer was transferred to 1-mL autosampler vials for GC analysis. A kinetic experiment was conducted to measure Clproduced in an Fe(II)/cement system. Sampling for PCE was conducted as described previously. At every sampling time, 5018
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∼7 mL of aqueous sample was withdrawn from the vials and filtered through a 0.45-µm nitrocellulose membrane for IC analysis of Cl-. Analytical Procedures. PCE and its chlorinated degradation products, TCE, 1,1-DCE, c-DCE, t-DCE, vinyl chloride, dichloroacetylene, and chloroacetylene, were analyzed by a Hewlett-Packard G1800A GCD equipped with a DB-VRX column (J&W Scientific), 60 m × 0.25 mm i.d., with a film thickness of 1.8 µm and operated with electron impact ionization and a mass selective detector. Extractants were injected using an autosampler with a split ratio of 30:1 at 230 °C. Helium was used as a carrier gas at 1 mL/min. The oven temperature program was as follows: 80 °C for 8 min, ramp 20 °C/min to 160 °C, and hold for 2 min. The temperature of the detector was 300 °C. Nonchlorinated products (acetylene, ethylene, ethane) were analyzed for the selected samples that showed complete degradation of PCE. The retrieved vials were centrifuged at 582g for 3 min. Then ∼10 mL of aqueous samples were rapidly transferred to 20-mL vials, and the vials were capped by closures with Teflon lined septa. The vials were shaken at 250 rpm for 5 min by using the orbital shaker, followed by 5-h equilibration of the solution and gas phases. Headspace samples were analyzed by a Hewlett-Packard 6890 GC equipped with a GS-Alumina column (J&W Scientific), 30 m × 0.53 mm i.d., and a flame ionization detector. Using a gastight syringe, 100 µL of headspace samples were manually injected with a split ratio of 10:1. The oven and injector temperature was 100 °C and the detector temperature was 150 °C. The flow rate of the carrier gas (N2) was 6.3 mL/min. Quantification was conducted using the gas standards of acetylene, ethylene, and ethane (Scott Specialty Gas). Aqueous phase concentrations of the nonchlorinated products were calculated using the dimensionless Henry’s law coefficients at 25 °C obtained from literature (16, 17). Chloride ion was analyzed by a Dionex DX-500 IC/HPLC equipped with a self-regenerating suppressor, a CD-20 conductivity detector, and an IonPac AS4A-SC analytical column, 250 mm × 4 mm i.d. The eluent was 1.8 mM Na2CO3/1.7 mM NaHCO3 and the flow rate was 3 mL/min. The filtered solution was manually injected into the column through a 25-µL sample loop. Evaluation of Kinetic Data. A pseudo-first-order rate law was used to describe PCE degradation kinetics in this study. Assuming instantaneous gas-solid-aqueous phase equilibrium, partitioning of PCE into the gas and solid phases can be incorporated in an equation for the pseudo-firstorder rate law as follows
dCl )dt
k k Cl ) - Cl ) -kappCl (1) Vg P 1+H + Ks Vl
(
)
where Cl represents PCE concentration in the aqueous phase, k is the corrected pseudo-first-order rate constant, H is the dimensionless Henry’s law constant for PCE (0.525 (18)), Vg and Vl are volumes of the gas and aqueous phases, Ks is the solid-phase partition coefficient of PCE (ratio of mass of PCE in the solid phase to mass of PCE in the aqueous phase), P is the partitioning factor ) 1 + HVg/Vl + Ks, and kapp is the apparent pseudo-first-order rate constant. The value of kapp was obtained by conducting nonlinear regression of PCE concentration in the aqueous phase using the SAS System for Windows (Release 6.12, SAS Institute Inc.). Corrected pseudo-first-order rate constants (k) (pseudo-first-order rate constants hereafter) were obtained by multiplying appropriate partitioning factors (P) times apparent pseudo-first-order rate constants (kapp).
FIGURE 1. Kinetics of PCE reduction by Fe(II) in 10% cement slurries. The symbols represent the results from experiments on Fe(II) dose effect (exp. 1, 3, and 4 in Table 2). The error bars are ranges of measured PCE concentrations. Some error bars are smaller than the symbols. Lines represent first-order fits. [PCE]0 ) 0.245 mM. The extent of PCE partitioning into the solid phases of various systems was examined by the total extraction procedure as described in the experimental procedures. Solidphase partition coefficients (Ks) for various systems were calculated by averaging at least eight measurements of Ks for each experiment. Statistical comparisons of Ks data show that Ks for the controls and those of different systems containing solids and iron were identical at a significance level of 5%, except for the Fe(II)/cement system at pH 10.6. This implies that PCE sorption was mostly due to the Teflon lining material of the closures and the reactor wall and that the sorption onto the solids was negligible. The Ks value and partitioning factor (P) for the system at pH 10.6 were 0.15 and 1.16, respectively. For all other systems, Ks value of 0.12 and the partitioning factor of 1.13 were used. In the pH-effect experiments, zero-order dependence on the substrate concentration was observed in some experiments. Zero-order rate constants for the selected experiments were also obtained using the nonlinear regression procedure and corrected by the pertaining partitioning factors for the experiments.
Results and Discussion Figure 1 shows results from a typical PCE degradation experiment. Figure 1 shows that first-order kinetics can reasonably describe PCE reduction kinetics for three selected conditions (Fe(II) dose). Table 2 shows the pseudo-first-order rate constants of PCE for various experiments conducted at pH 12.6 ( 0.05. For most of the systems containing cement, PCE concentrations decreased below the detection limit (0.066 mg/L) during the reaction periods (∼13 to 40 days). A supplemental experiment was conducted to determine if PCE degradation could occur by base-catalyzed hydrolysis at pH ∼12.6. In this experiment, PCE concentration in 10% cement slurries in the absence of Fe(II) was monitored over time. The results showed that decrease of the PCE concentration was not significant over the time span of the current study (control 2 in Figure 1). Therefore, PCE degradation observed in the cement systems is attributable to reductive dechlorination reactions by Fe(II) or Fe(II)-containing species. Effect of Fe(II) Dose. The effect of Fe(II) dose in cement systems was investigated over a range of 9.8-196 mM Fe(II). Pseudo-first-order rate constants for these experiments are presented in Table 2 (exp. 1-5) and are plotted against Fe(II) dose in Figure 2(a). As shown in Figure 2(a), the rate constant increased nearly linearly with Fe(II) dose at low values and approached a maximum at high doses.
Effect of pH. The effect of pH was studied over the pH range typically observed in solidification/stabilization systems. Solution pH values of the experiments at nominal pH values of 10.6, 11.5, and 12.1 were maintained within (0.3 pH unit. Measured values of pH of the experiments at nominal pH values of 12.6 and 13.8 did not vary more than (0.05 pH unit. The lower pH systems are more susceptible to pH changes because they have lower buffering capacities due to lower amounts of lime (Ca(OH)2) and hydroxide ion (19). The first-order model could not adequately describe PCE reduction kinetics in some experiments, so a zeroth-order rate law was tested. Table 3 presents the zero-order rate constants obtained as well as the pseudo-first-order rate constants. As uncertainties in Table 3 imply, the zero-order model provides substantially better fits than the first-order model for the data from experiments conducted at nominal pH values of 11.5, 12.1, and 13.8. This suggests that the surface might have been saturated with PCE in these experiments. The pseudo-first-order rate constants for the five pHeffect experiments were plotted against pH in Figure 2(b). This figure shows that PCE reduction reactions in cement systems are strongly dependent on pH. An optimum pH was found around 12.1. A normal distribution function fits the data reasonably well. Effect of Solids. Table 2 shows that PCE degradation was minimal with a half-life of 260 days when 39.2 mM Fe(II) was present without addition of a solid (exp. 6). The effect of solids other than cement on PCE degradation was investigated by using the solids that have similar characteristics as cement such as tricalcium silicate and calcium hydroxide. Tricalcium silicate is a major component of the cement (60.6% of the cement used in the current study (20)), and its hydration products, calcium silicate hydrates, govern surface characteristics of hydrated cement due to their large surface area (21). Specific surface area of the cement is reported to increase by as much as 2-3 orders of magnitude when hydrated (2). The surface area of the tricalcium silicate system should be comparable to that of the cement system or even could be higher because the tricalcium silicate forms only calcium silicate hydrates upon hydration, whereas calcium silicate hydrates occupies 60-70% of the cement hydrates (21). On the other hand, calcium hydroxide accounts for 15-25% of the cement hydration products (2, 22). Table 2 shows that the reaction rates for the systems containing the tricalcium silicate or calcium hydroxide were much lower than those for the cement systems (exp. 7, 8 compared to exp. 3). This suggests that some component of the cement catalyzed or participated in PCE reduction reactions. As shown in Table 1, the cement contains substantially more Fe(III) than the other solids. This motivated experiments to investigate whether Fe(III) in cement systems could play a role in PCE reduction reactions by forming Fe(II)-Fe(III) (hydr)oxides. Effect of Fe(III) and Anion. Experiments were conducted to investigate the effect of ferric chloride and ferric sulfate additions on PCE degradation kinetics. Different anions were evaluated to determine if they could affect PCE degradation, possibly by promoting formation of chloride-containing green rust (FeII3FeIII(OH)8Cl)(23)) or sulfate-containing green rust (FeII4FeIII2(OH)12SO4)(8)). The results are presented in Table 2. In the case of noncement systems, the addition of Fe(III) did not substantially change the reaction rates compared to the systems with only Fe(II) (exp. 9, 10, and 12 compared to exp. 6). However, the reaction rates increased by factors of 1.8 and 3.1 when FeCl3 and Fe2(SO4)3 were added to systems with cement (exp. 11 and 13 compared to exp. 3). However, the reactivities of the cement systems containing FeCl2 and FeSO4 without additional Fe(III) were similar (exp. 3, 14, 5, 15). Degradation Products and Reaction Pathways. TCE was the only chlorinated intermediate product observed in this VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Pseudo-First-Order Rate Constants for PCE in Various Systems Containing Different Types or Amounts of Solids, Fe(II) and Fe(III)a exp.
solidb
Fe(II) dose (mM)c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
cement cement cement cement cement none tricalcium silicate calcium hydroxide calcium hydroxide tricalcium silicate cement calcium hydroxide cement cement cement
9.8 19.6 39.2 98 196 39.2 39.2 39.2 39.2 39.2 39.2 39.2 39.2 39.2 193
source of Fe(III)d
FeCl3 FeCl3 FeCl3 Fe2(SO4)3 Fe2(SO4)3
k (day-1)e
nf
0.028 ((9.6%) 0.079 ((6.2%) 0.10 ((5.4%) 0.16 ((6.5%) 0.13 ((8.9%) 0.0027 ((51%) 0.0066 ((13%) 0.0038 ((37%) 0.00090 ((100%) 0.0077 ((14%) 0.18 ((8.1%) 0.0025 ((37%) 0.31 ((12%) 0.11 ((5.4%) 0.11 ((13.4%)
20 25 29 23 29 18 16 17 12 18 17 17 18 29 32
a Initial concentration of PCE was 0.245 mM, except for exp.15, where the concentration was 0.483 mM. b Mass ratio of solid to solution was 0.1 except for exp. 6. c FeCl2 was added except for exp. 12, 13, 14, and 15 where FeSO4 was used. d Fe(III) was added only in exp. 9-13 with a dose of 13.07 mM. e Uncertainties represent 95% confidence limits expressed in % relative to estimates for k. (( (asymptotic standard error) × t(n - 2, 0.025)/estimate for k × 100). f Number of samples used in the nonlinear regression. For some sampling points, only duplicate samples were available due to loss of samples, etc.
TABLE 3. Pseudo-First-Order Rate Constants and Zero-Order Rate Constants for PCE in Fe(II)/Cement Systems at Various pHa exp.
pH
16 17 18 3 19
10.6 11.5 12.1 12.6 13.8
first-order fit k (day-1)b zero-order fit k0 (mM/day)b nc 0.0048 ((27%) 0.064 ((32%) 0.17 ((17%) 0.10 ((5.4%) 0.017 ((13%)
0.00072 ((28%) 0.0092 ((11%) 0.020 ((5.1%) 0.0080 ((16%) 0.0020 ((9.1%)
24 20 26 29 22
a PCE and FeCl concentrations were 0.245 mM and 39.2 mM, 2 respectively. Mass ratio of cement/solution was 0.1. b Uncertainties represent 95% confidence limits expressed in % relative to estimates for k and k0. (( (asymptotic standard error) × t(n - 2, 0.025)/estimate for k × 100). c Number of samples used in the nonlinear regression. For some sampling points, only duplicate samples were available due to loss of samples, etc.
FIGURE 2. (a). Dependence of pseudo-first-order rate constant on Fe(II) dose. The symbols represent the rate constants for exp. 1-5 in Table 2. The error bars are 95% confidence intervals. The solid line represents a fitting model: k ) rmax [Fe(II)]0/b + [Fe(II)]0, where rmax is the maximum pseudo-first-order rate constant, [Fe(II)]0 is the Fe(II) dose, and b is the constant. rmax is 0.614 day-1 and b is 0.0415. (b). Dependence of pseudo-first-order rate constant on pH. The error bars for k are 95% confidence intervals. The error bars for pH are ranges of measured pH values. Some error bars are smaller than the symbols. The solid line represents a Gaussian model fitting. The model is k ) a exp{-(pH - b)2/(2c2)} (a ) 0.170; b ) 12.13; c ) 0.453). study. Appearance of TCE in cement systems was transitory with the amount being less than 1.6% of the initial concentration of PCE on a molar basis. This absence of degradation products indicates that PCE may be completely transformed into nonchlorinated compounds in cement systems. To 5020
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FIGURE 3. Formation of Cl- from dechlorination of PCE by Fe(II) in a cement system. The error bars are ranges of measured concentrations. The dotted line represents the initial amount of the total Cl- in the system (1.932 mM). [PCE]0 ) 0.483 mM, [Fe(II)]0 ) 193 mM. demonstrate this, a chloride balance experiment was conducted for a cement system. Figure 3 shows disappearance of PCE and production of Cl- in a 10% cement system (exp. 15) containing 193 mM FeSO4. Concentrations of PCE and Cl- are presented in terms of Cl- concentration in Figure 3. Chloride ion concentration measured in the solution was approximately 50-70% of the amount of Cl- that would be produced assuming complete
dechlorination of PCE. This deficit was found to be mainly due to immobilization of Cl- by the cement hydrates. Chloride ion in the cement slurries is known to be sorbed by cement hydrates or to form chloroaluminate precipitates (24, 25). Thus, experiments were conducted to characterize the kinetics and extent of Cl- immobilization by the cement hydrates. The results and a method to interpret the Cl- data of the Cl- balance experiment are available in the Supporting Information. The amount of immobilized Cl- at each sampling time was estimated from the Cl- immobilization experiments (Supporting Information) and was used to calculate total Cl- concentration in the system as follows
[Cl-]total ) 4[PCE]t + [Cl-]aq + [Cl-]im
(2)
where [Cl-]total is the total Cl- concentration in the system, [Cl-]aq is the chloride concentration in the aqueous phase, [Cl-]im is the chloride concentration in all immobile (nonsolution) phases, and [PCE]t is the total PCE concentration ) PCE concentration in the aqueous phase × (1 + HVg/Vl + Ks). Figure 3 shows that chloride balance is near 100% up to 13 days and decreases to 77% on 25th day. Lower recoveries after 13 days might be attributable to underestimation of the immobilized Cl- in the later stages of the reaction. The chloride balance data generally support a complete dechlorination of PCE that produced no detectable amount of the intermediate products during the reaction. Minimal accumulation of intermediate products can be explained by either of two hypotheses on the reaction mechanism. The first hypothesis assumes that PCE is initially bound to reactive Fe(II) surfaces and sequentially dechlorinated to nonchlorinated products while still bound to the surfaces, and the final products are released to the aqueous phase. The second hypothesis assumes that intermediate degradation products of PCE are released to the aqueous phase and that they are dechlorinated at very high rates so as not to be observed in the aqueous phase. To make the second hypothesis possible, degradation rates for the products of PCE should be much higher than that of PCE. For example, in the case of pH 12.6 experiment (exp. 3), the degradation rate of the first product of PCE (TCE or dichloroacetylene) should be approximately 100 times higher than that of PCE. The reduction rate of dichloroacetylene has been reported to be 83 times the rate of PCE (26), which could explain the lack of accumulation of chlorinated acetylenes and would be consistent with the hypothesis that PCE degradation products desorbed and degraded rapidly in solution. Final products of selected experiments were analyzed to gain some insights on reaction pathways and to provide additional data on the extent of PCE transformation. Acetylene, ethylene, and ethane analyses were conducted for the samples retrieved from the experiments that already showed complete removal of PCE (exp. 17, 18). The detailed results are presented in the Supporting Information. The carbon recovery of the pH 12.1 experiment was 98%, indicating that Fe(II)/cement systems can convert PCE almost completely to nonchlorinated products that can easily be attenuated in natural environments. The product analyses show that acetylene was the primary nonchlorinated product rather than ethylene or ethane. Acetylene accounted for 94.5% and 82.8% of the final products for the pH 11.5 and pH 12.1 experiments, respectively. Dominance of acetylene among the degradation products of PCE implies that PCE reductions in the Fe(II)/cement systems occurred primarily via reductive elimination pathways (27, 28). Reaction Mechanisms. Based on the results from various kinetic experiments, three hypotheses for the reaction mechanisms are evaluated.
1. Reduction by Fe(II) Solids. In the experiment with 39.2 mM Fe(II) without addition of a solid (exp. 6), substantial amounts of Fe(OH)2 should form. The minimal reactivity shown in the experiment suggests that Fe(OH)2 was not very active in degrading PCE. The aqueous phase in cementitious systems contains a substantial amount of silicate ion (29). Thus, formation of ferrous silicate can be expected in cement or tricalcium silicate systems. The rate constant of the system with tricalcium silicate (exp. 7) was an order of magnitude lower than those of cement systems. This suggests that reactive ferrous silicates did not form in either of these systems. However, a reactive compound containing silica and other compounds provided by cement could have formed. 2. Reduction by Fe(II)-Fe(III) (hydr)oxides. The increases in PCE reduction rates by adding Fe(III) in cement systems (exp. 11, 13) support the hypothesis that Fe(II)-Fe(III) (hydr)oxides are reactive reductants for PCE. The procedure of adding Fe(III) salts to alkaline solutions containing Fe(II) salts with an Fe(II)/Fe(III) ratio of 3 is very similar to the one used to synthesize green rusts (23). However, the formation of magnetite cannot be ruled out in this case, because it can also be precipitated by mixing salts of Fe(II) and Fe(III) (14). Different reactivities shown between chloride- and sulfatecontaining systems (exp. 11, 13) suggest that the reactive agents might be green rusts and that sulfate-containing green rust might be a better reductant for PCE than chloridecontaining green rust. There are several observations that may repute the presence of green rusts or Fe(II)-Fe(III) (hydr)oxides in cement systems. Similar reactivities of the chloride- and sulfate-containing systems in the absence of additional Fe(III) (exp. 3, 5, 14, 15) are not consistent with what was observed in the presence of additional Fe(III) (exp. 11, 13). Low reactivities shown in noncement systems with additional Fe(III) may suggest that Fe(II)-Fe(III) (hydr)oxides did not precipitate in the absence of cement or if they did precipitate, they were not very active in degrading PCE in those systems (exp. 9, 10, 12). This also may mean that reactive Fe(II)Fe(III) (hydr)oxides formed in this study required component(s) specific to cement such as aluminum. 3. Reduction by Sorbed Fe(II). The saturation type of behavior shown in the experiments on the Fe(II) dose effect (Figure 2(a)) can be explained by a sorbed Fe(II) mechanism, i.e., PCE was reduced by Fe(II) sorbed on cement hydration products. Under this hypothesis, it can be argued that the amount of Fe(II) sorbed on the surfaces approached a maximum value at the Fe(II) dose of around 98 mM. However, substantial differences in reactivities shown between the cement and tricalcium silicate systems may contradict the sorbed Fe(II) mechanism because the surface areas of two systems would have been similar. Nevertheless, the sorbed Fe(II) hypothesis cannot be totally excluded because Fe(II) could have been sorbed onto solid phases other than calcium silicate hydrates that would be present in the cement systems but not in the tricalcium silicate systems. The sorbed Fe(II) mechanism cannot explain increases in reaction rates with the addition of Fe(III) and anions in cement systems. Fe(III) and anions could have competed for the sorption sites with Fe(II), resulting in less sorption of Fe(II), which could have led to lower reaction rates. The experimental results so far appear to provide more evidence in favor of the Fe(II)-Fe(III) (hydr)oxides hypothesis than the other hypotheses. Research is currently underway to identify reactive iron species in the DS/S system. With the reactive agent being identified, the Fe(II)-based DS/S technology can be more optimized, and another process could possibly be developed that uses the synthesized reactive agent as an additive rather than Fe(II). VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Acknowledgments This project has been funded entirely with funds from the United States Environmental Protection Agency (US EPA) as part of the program of the Gulf Coast Hazardous Substance Research Center. The contents do not necessarily reflect the views and policies of the U.S. EPA nor does the mention of trade names or commercial products constitute endorsement or recommendation for use.
Supporting Information Available The results and data analysis of the chloride immobilization experiments and a table presenting final products analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review December 14, 1999. Revised manuscript received June 19, 2000. Accepted August 29, 2000. ES991377B
