Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Water – sewage – or other waste water
Reexamination Certificate
1997-10-14
2001-05-01
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Water, sewage, or other waste water
C205S758000, C205S759000, C204S275100, C204S277000
Reexamination Certificate
active
06224744
ABSTRACT:
The present invention relates to a process and equipment for the electrolytic purification of contaminated waters using oxygen cathodes for the electrochemical oxidation of pollutants, especially in wastewaters, using an oxygen diffusion cathode, in order to degrade said pollutants into innocuous, biodegradable products and products insoluble in the effluent. It also relates to equipment for carrying out said process and to the use of the process and of the equipment.
This is an alternative process to the conventional ones, permitting integration of the oxidizing power of oxygen and species derived therefrom into an electrochemical oxidation cell and permitting the treatment of wastewaters containing especially toxic, non-biodegradable substances or substances which, as is the case of many industrial effluents, cannot be treated using other oxidation methods.
Toxic pollutants pose serious problems due both to their special environmental impact and to the difficulty of treating them by biological treatment in wastewater-sewage treatment plants. Indeed, when the levels of certain substances such as heavy metals, hydrocarbons, cyanides, phenols or anilines exceed certain limits, biological treatment plants are obliged to close their gates in order to avoid destruction of the microbial “ecosystem” which permits them to operate.
BACKGROUND OF THE INVENTION
Electrolysis is a method widely used for the elimination of organic and inorganic pollutants in industrial effluents. From the point of view of electrochemical treatment of wastewaters, the processes of cathodic recovery of heavy metals, in use in many industries, is well known, though it is not mentioned explicitly herein because it is outside the scope of this invention.
The oxidation of waste substances by means of oxygen can be carried out directly in the electrolytic cell, as described in patent FR 227021 1, which uses the oxygen given off at the anode to produce a fast “biodegradation” of the organic compounds in septic tanks, with formation of a floating foam which is then removed.
Japanese patent JP 04197489 proposes treatment of the wastewater with a gas containing oxygen at approximately pH 10 and subsequent electrolysis of the treated water.
Patent JP 51019363 describes an electrolytic oxidation process in which different sugared compounds and lignin are totally oxidized to carbon dioxide and water on an anode, preferably of lead dioxide. In Swedish patent SU 962212, the wastewater is treated with a gas containing oxygen in a layer of granulated conductive material (for example, Ti and SiC) situated between two electrodes.
Patent SU 3200453 claims 98% purification of a vat-dyeing wastewater using a lead dioxide anode. In other cases, such a patent EP 231100, SU 966028 or JP 04118091, the current applied during the electrolytic treatment polarizes the pollutants and generates small gas bubbles which cause said residues to cluster together in lumps and rise to the surface, from which they are removed. This technique is termed electrofloating.
The electrochemical destruction of toxic organic pollutants in wastewaters has caused great interest in recent years. In particular, the studies undertaken on electro-oxidation of phenols [references 11-14 of the bibliography listed on pages 5 and 6] and anilines [14-19] in aqueous medium at different pHs has shown the formation of a wide variety of products (dimers, polymers, benzoquinone, maleic acid, etc.), depending on the electrolysis operating conditions and the chosen reaction medium. In all this works, special stress has been laid on the type of anode on which the electrochemical oxidation is carried out, reaching the conclusion that the best anodes are metals with high oxygen overtension. Platinum has undoubtedly been the anode most widely used in laboratory studies, due to its being a noble metal. Given the high price of platinum, however, alternative anodes have been sought, such as PbO
2
and doped SnO
2
. A recent study [13] on the electro-oxidation of phenol has revealed that under certain conditions doped SnO
2
is a more effective anode than PbO
2
, while the latter is actually better than platinum itself. Other types of electrodes, such as DSA® (Ti base), currently under full commercial development due to its great chemical inertia, are still not shown in the literature as anodes in the electro-oxidation processes of toxic pollutants such as phenols and anilines.
Rarely has account been taken of the effect of the cathode on the electro-oxidation of organic compounds, solid platinum or platinum deposited on a cheaper substratum such as titanium being used normally for this purpose.
Despite the work undertaken in the last few years on the electrochemical destruction of toxic organic pollutants in wastewaters, the bibliography contains no work in which their total elimination from the reaction medium has been achieved. In general, the use of anodes of Pt, PbO
2
, or SnO
2
and a platinum cathode has achieved complete destruction of the initial organic compounds (anilines and phenols, for example [11-19]), though complete elimination of their intermediate reaction products has not been achieved.
In accordance with the bibliography [1-3], oxygen in an aqueous medium is reduced on a cathode of gold, mercury or graphite to generate hydrogen peroxide according to the electrochemical reaction:
O
2
+H
2
O+2
e
−
→HO
2
−
+HO
−
(1)
The HO
2−
ion, conjugate base of the hydrogen peroxide, is a good oxidant and can react with intermediate products of the oxidation process of organic pollutants, boosting their complete degradation into water, carbon dioxide and other inorganic compounds of toxicity levels tolerable to microorganisms (NH
3
, HCl, etc.). The efficiency of the hydrogen peroxide is especially marked if it is subjected to UV radiation of about 254 nm, which in addition to its disinfectant effect, can decompose photolytically the hydrogen peroxide generated in reaction 1 into hydroxyl radicals (after fluorine, the most oxidizing species known), much more reactive than the H
2
O
2
itself [10]. Another by no means negligible way of boosting the oxidizing effect of this compound is the presence of Fe(II) as catalyst at a pH close to 3, which also produces hydroxyl radicals, according to the Fenton reaction. Silver or cobalt ions can also be used.
Although reaction 1 can be carried out on various cathodes, we have found that oxygen diffusion cathodes are the ones best suited for the present process.
Reaction 1 is faster and can be controlled better on oxygen diffusion electrodes than on a simple graphite electrode. Oxygen diffusion electrodes may be made of a mixture of carbon (very fine particle lampblack) and a water-repelling polymeric agglomerant (preferably polytetrafluoroethylene, PTFE), pressed at about 350-400° C. (pasty melting temperature of PTFE) on a metallic mesh generally of Ni, Ag or stainless steel) which acts as a current distributor [3-5]. The mission of the PTFE is to keep the carbon compact, with sufficient porosity to diffuse the oxygen gas and lend the whole a water-repellant character. Carbon-PTFE electrodes have been developed as components of fuel cells and some have been marketed as components of metal-air cells. Various patents have been filed over the last fifteen years describing different carbon-PTFE electrodes [2-4, 7] with applications as diverse as acting as cathodes in zinc-air cells [4-7] or being used as cathodes in electrolytic cells for the generation of base solutions of hydrogen peroxide [2,3].
Carbon-PTFE cathodes in alkaline medium are sensitive to the partial pressure of oxygen of the gas acting upon it, it having been found that the reaction speed (1) increases as the partial pressure of oxygen increases [8,9], so that circulation through them of current densities of up to 2 A/cm
2
can be achieved when operating with oxygen pressures of up to 5 atmospheres.
U.S. Pat
Bastida Bonany Rosa Maria
Brillas Coso Enrique
Casado Gimenez Juan
Vandermeiren Michel
Phasge Arun S,.
Sociedad Espanola de Carburos Metalicos S.A.
Steinberg & Raskin, P.C.
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