Environ. Sci. Technol. 2000, 34, 4407-4412
Adsorption/Reduction Reactions of Trichloroethylene by Elemental Iron in the Gas Phase: The Role of Water SIBEL ULUDAG-DEMIRER* AND ALAN R. BOWERS Civil and Environmental Engineering Department, Vanderbilt University, Nashville, Tennessee 37235
The interactions of TCE with “clean” and “partially oxidized” Fe0 were investigated in the gas phase. The removal of TCE from the gas phase proceeded by adsorption onto the Fe0 surface or by dechlorination reactions with Fe0 depending on the moisture content of the reaction medium. Unlike the aqueous phase, water was a critical variable competing with TCE for adsorption sites and, above a critical limit, serving as a proton donor for the hydrogenolysis of TCE. The adsorption of water (relative humidity, RH ) 6-97%) was fit to a multilayer BET isotherm. The extent of water adsorption was nearly identical for both forms of Fe0 on a surface area basis, i.e., mol/m2. The adsorption of TCE decreased by more than 1 order of magnitude from 6% to 10% RH, after which a more stable region (100% conditions. There is no previous study on the adsorption of TCE by Fe0 under varying RH conditions. Therefore, an analogy is established here between the adsorption of TCE by soil mineral surfaces and Fe0. A relationship between the RH of the gas phase and the partition of organics, such as benzene, n-octane, chlorobenzene, naphthalene, ethanol, trichloroethylene, and anisole, onto soil minerals has been found to be exponential at RH levels less than 100%, or (12, 22, 24, 26, 28-30):
Kads ) Ae(B(RH))
(10)
where RH is relative humidity (%), A and B are empirical model parameters. The parameter A (L/m2) is the natural logarithm of Kads at RH ) 0%, and the parameter B, RH coefficient, is the extent of change in the magnitude of Kads with RH, i.e., as B increases, adsorption of TCE increases. Goss (29) reported that eq 10 is valid between RH conditions above the H2O monolayer coverage of the surface and below 100% RH of the gas phase and that the model parameters A and B could be predicted from the physical and chemical properties of the organic compounds adsorbed on similar minerals. While parameter A is sorbent dependent, parameter B (above monolayer coverage for H2O) is reportedly compound specific due to the loss of direct contact with the surface after the water monolayer has been exceeded or (29)
B ) -0.05β - 0.0007mR - 0.0041µ + 0.00061 (11) where β is a measure of the hydrogen-bonding potential of an organic sorbate (hydrogen bond acceptor), mR is the polarizability of the compound (molar refraction), and µ is the measure of the dipole-dipole interactions (dipole moment). Adsorption of TCE decreased dramatically with an increase in RH in the lower RH range (6-10%) and continued to decline, but with at smaller rate from 10% to 97% (Figure 6). The sharp decline in TCE adsorption rates in low RH range was attributed to the competition of H2O molecules for the adsorption sites below the water monolayer coverage of the surface. Following the monolayer coverage of the surface by H2O molecules (RH = 26%, calculated from BET isotherm equation fit), TCE adsorbed onto ice-like structured H2O molecules and did not have direct contact with the Fe0 surface site. Similar mechanisms of TCE adsorption on Fe0 surface at low and high RH conditions were also observed in the studies of TCE adsorption on mineral surfaces (12, 22, 24,
FIGURE 7. Equilibrium adsorption coefficient of TCE on acid-washed and unamended Fe0 as a function of RH at 24 °C. The solid lines are linear regression fit to the data. Model applies above H2O monolayer coverage only. 26). The byproducts of TCE hydrogenolysis (DCE isomers, VC, and ETH) were not detected in the headspace of the reactors after 30 days from the injection of TCE into the reactors. Pentane extraction allowed 70-86% recovery of the initially loaded TCE. The GC analyses of the pentane solutions also confirmed the absence of the byproducts in the reactors by the method of residence time. Assuming that the adsorption isotherms of TCE onto acidwashed and unamended Fe0 surface are linear on the basis of very low (10-4-10-3) partial pressures of TCE in the gas phase, weight-based linear partition coefficients, Kads, could be calculated for other RH levels by using eq 9. The adsorption coefficients, Kads, for TCE with acid-washed and unamended Fe0 are presented in linear form in accordance with eq 10 in Figure 7, indicating good agreement above monolayer coverage (26-97% RH). Clearly, the unamended Fe0 shows a higher affinity for TCE, A ) -2.66 (unamended) versus -3.10 (acid-washed). However, the slope is similar, B ) -0.021 (unamended) versus -0.023 (acid-washed), indicating dependence on the compound, i.e., TCE in this case, rather than the surface characteristics. The values for the adsorption parameter for TCE, B, are similar to the value calculated using eq 11 [B ) -0.0229 using β ) 0.05; mR ) 25.32; µ ) 0.80 (29)] and the value reported by Goss (29) for TCE adsorption on polar mineral surfaces (B ) -0.0223). This supports the use of eq 9 to model the linear partition coefficient of TCE onto Fe0 (clean and partially oxidized surfaces) for lower TCE concentrations and suggests that the adsorbability of other compounds could be estimated from eq 10 as well. Critical RH for TCE Reduction. The concentrations of the TCE reduction byproducts were determined in the gas samples taken from the reactors containing acid-washed and unamended Fe0. At low RH levels, no byproducts were observed indicating that adsorption of TCE occurred without chemical reaction on the surface. The first appearance of the byproducts (1,2-cis-DCE, 1,1-DCE, VC, and ETH) occurred at 72% RH for acid-washed and 92% RH for unamended Fe0. These RH levels were considered to be the critical values (RHc) for the TCE reduction via hydrogenolysis. The concentrations of the byproducts were above their detection limits [detection limits are as follows: 1,2-cis-DCE ) 0.042 mg/L; 1,2-trans-DCE ) 0.038 mg/L; 1,1-DCE ) 0.087 mg/L; VC ) 0.09 mg/L; ETH ) 0.0075 mg/L (31)] at 4 days after the initial injection of TCE into the reactors. The total conversion of TCE to the byproducts was 18.0% and 9.8% in the acidVOL. 34, NO. 20, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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washed and unamended Fe0 containing reactors respectively at 30-day reaction time. The TCE recovery by pentane extraction ranged between 78 and 84% of the initially loaded TCE accounting the byproducts formed. The gap in the TCE mass recovery is attributed to the formation of other byproducts, which were not identified in this study, e.g., chloroacetylene, acetylene, ethane. The difference between the critical RH for the acid-washed versus the unamended Fe0 surface may point out possible effects of the oxide layer on the water distribution on the surfaces, i.e., the water may reside in the outer regions of the oxide layer that are nonreactive. From the water adsorption isotherms for Fe0 (see Figures 2 and 3), the critical RH level (RHc), 72% and 92%, corresponds to moisture contents (θwc, %) of 0.28% and 0.66% for acidwashed and unamended Fe0, respectively. Very low RHc levels required for TCE reduction is not expected to cause difficulty under most natural soil conditions in terms of the moisture content required for TCE reduction, except for the first few centimeters, where the moisture content of soil is at equilibrium with the atmospheric humidity level (11). In addition, it is envisioned that such a system in the field could be irrigated to maintain sufficient water levels for TCE reduction to occur. According to our results, a conventional reactive barrier technology for the remediation of groundwater systems can be extended in its use by isolating the contamination resource and contaminated sites from the atmosphere and surrounding subsurface environment to achieve complete cleanup of the site. However, with the TCE reduction kinetics under water-limiting conditions, the effects of the oxygen on the reaction mechanism and rate should be investigated to apply a reactive barrier technology for the in-situ remediation of the soil.
Acknowledgments We thank the Chemistry Department of Vanderbilt University for the surface area measurements and Ken Debelak at the Chemical Engineering Department of Vanderbilt University for making the TGA available for the water adsorption analyses.
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Received for review February 18, 2000. Revised manuscript received July 21, 2000. Accepted July 27, 2000. ES001017K
