No abstract

A hybrid electrolysis and Pd-catalytic oxidation process is evaluated for degradation of trichloroethylene (TCE) in groundwater. A three-electrode, one anode and two cathodes, column is employed to automatically develop a low pH condition in the Pd vicinity and a neutral effluent. Simulated groundwater containing up to 5 mM bicarbonate can be acidified to below pH 4 in the Pd vicinity using a total of 60 mA with 20 mA passing through the third electrode. By packing 2 g of Pd/Al2O3 pellets in the developed acidic region, the column efficiency for TCE oxidation in simulated groundwater (5.3 mg/L TCE) increases from 44 to 59 and 68% with increasing Fe(II) concentration from 0 to 5 and 10 mg/L, respectively. Different from Pd-catalytic hydrodechlorination under reducing conditions, this hybrid electrolysis and Pd-catalytic oxidation process is advantageous in controlling the fouling caused by reduced sulfur compounds (RSCs) because the in situ generated reactive oxidizing species, i.e., O2, H2O2 and •OH, can oxidize RSCs to some extent. In particular, sulfite at concentrations less than 1 mM even greatly increases TCE oxidation by the production of SO4•−, a strong oxidizing radical, and more •OH.

[1] We present a novel approach to modeling stochastic multiphase flow problems, for example, nonaqueous phase liquid flow, in a heterogeneous subsurface medium with random soil properties, in particular, with randomly heterogeneous intrinsic permeability and soil pore size distribution. A stochastic numerical model for steady state water-oil flow in a random soil property field is developed using the Karhunen-Loeve moment equation (KLME) approach and is numerically implemented. An exponential model is adopted to define the constitutive relationship between phase relative permeability and capillary pressure. The log-transformed intrinsic permeability Y(x) and soil pore size distribution b(x) are assumed to be Gaussian random functions with a separable exponential covariance function. The perturbation part of these two log-transformed soil properties is then decomposed into an infinite series based on a set of orthogonal normal random variables {x n }. The phase pressure, capillary pressure, and phase mobility are decomposed by polynomial expansions and the perturbation method. Combining these expansions of Y(x), b(x) and dependent pressures, the steady state water-oil flow equations and corresponding boundary conditions are reformulated as a series of differential equations up to second order. These differential equations are solved numerically, and the solutions are directly used to construct moments of phase pressure and capillary pressure. We demonstrate the validity of the proposed KLME model by favorably comparing firstand second-order approximations to Monte Carlo simulations. The significant computational efficiency of the KLME approach over Monte Carlo simulation is also illustrated.

An effective approach is developed to regulate the generation of Fe 2+ from an iron cathode in a three-electrode system. The Fe 2+ is then used for the Pd-catalytic transformation of methyl tert-butyl ether (MTBE) in simulated groundwater. Under conditions of pH 3, a total current of 50 mA, and 1 g/L Pd/Al 2 O 3 , 20 mg/L MTBE was completely transformed within 60 min in an undivided electrolytic cell using the iron cathode, with 14 mg/L of accumulated Fe 2+. Fe 2+ accumulation follows pseudo-first-order kinetics and the rate is regulated by electric current and groundwater pH, giving the relation of k = 5.3 × 10 4 • 10-2pH-7.25 • I 2-8.8 × 10 11 • 10 2pH-28 , where k is the rate constant of Fe 2+ accumulation (min-1) and I is the current (mA). In a modified three-electrode column using iron as the first cathode, the localized acidic and oxidizing conditions developed automatically in the iron cathode zone by partitioning the current between the two cathodes, leading to controllable generation of Fe 2+ for MTBE transformation. The stable transformation of MTBE in a long-term study suggests that this method is a sustainable approach to supplying Fe 2+ for Pd-catalytic transformation of organic contaminants in groundwater.