This paper describes the results obtained from an electrokinetic treatment of a real soil polluted by lead acetate. Powdered soil samples were pressed with a consolidometer till 100 kPa to obtain cylindrical specimens with the same characteristics as a subsoil. Tests were carried out in a Perspex electrochemical cell where soil specimens were introduced with a hollow punch. A low intensity direct current was applied in order to remove contaminants, due to electrophoresis and electroosmosis phenomena. The water flow, conductivity, apparent electroosmotic coefficient, as well as other characteristic parameters, were measured throughout the test. The water content and degree of pore saturation were estimated at the beginning and end of the test. The soil slab was divided into four slices and the Pb concentration profile determined. The main factor governing the extraction of contaminant was found to be the pH in the acidic range. Indeed, under these conditions high removal efficiencies could be reached. These results could lead to the design of a new electrochemical treatment cell equipped with a cationic membrane to expand the region of favourable pH within the soil. Nomenclature a Generic ionic species c a a species molar concentration, [mol m -3 ] D Relative electrical permeativity, dimensionless D a a species diffusion coefficient, [m 2 s -1 ] D* Effective molecular diffusion coefficient, [m 2 s -1 ] F Faraday constant, [C mol -1 ] H Hydraulic head, [m] j a1 Electromigrative flux, [mol m -2 s -1 ] j a2 Electroosmotic flux, [mol m -2 s -1 ] j a3 Diffusive flux, [mol m -2 s -1 ] j a4 Hydraulic flux, [mol m -2 s -1 ] K eo Electroosmotic coefficient, [m 2 s -1 V -1 ] K h Hydraulic permeability, [m s -1 ] M Generic metal M n+ Generic metal ion n Volumetric porosity, dimensionless R Gas universal constant, [8.314 J mol -1 K -1 ] T Absolute temperature, [K] T* Tortuosity tensor (scalar in isotropic conditions), dimensionless u a Ionic mobility of a species, [m 2 V -1 s -1 ] V Potential value, [V] z a Ionic valence, dimensionless e o Void dielectric constant, [C V -1 m -1 ] f Zeta potential, [V] g Dynamic viscosity of the fluid phase, [N s m -2 ] s Tortuosity factor, dimensionless
A pilot plant has been built for the regeneration of humidity condensates during shuttle missions. The plant is an automated experimental apparatus which includes an electrochemical cell, two reservoirs and a control system to maintain the temperature within a chosen range, so that the ionic exchange membranes are not damaged. The electrodes used to determine catalytic efficiency were: platinised titanium with ruthenium oxide and Sb and Sn coatings deposited on a Ti substrate, prepared with two different techniques, spray/brush coating and electrodeposition. The Ti/SnO 2 -Sb 2 O 5 coating was characterized through the following techniques: X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS) and linear and cyclic voltammetry (CV). Among the different contaminant dissolved in the contaminant solution, urea was chosen to carry out the electrochemical tests. Several regenerative treatment runs were performed with the solution simulating the shuttle condensate water to determine the evolution, during the tests, of the total organic carbon (TOC) concentration, the nitrate and nitrite concentrations, the ammonia concentration, the pH and the conductivity. These runs were carried out at different current intensities to understand the influence of the anodic current on the organic decomposition rates, in particular of the urea in presence of an amount of sodium chloride. The contaminant removal efficiencies in the electro-oxidation test appeared to be depressed for both electrodes at high current density, whereas the inversion of the electrode polarity seemed to offer a further advantage. The best result reached in the direct electro-oxidation was the 24 ppm as TOC with Ti/SnO 2 -Sb 2 O 5 at 0.2 A. This value is far from the limit concentration (0.5 ppm), which is the standard level for drinking water in space appliances. During the tests, a decrease in the pH was observed, due to the formation of refractory substances such as organic acids.
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