Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Water – sewage – or other waste water
Reexamination Certificate
2001-12-31
2003-03-11
Phasge, Arun (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Water, sewage, or other waste water
C205S759000, C205S760000
Reexamination Certificate
active
06531050
ABSTRACT:
The present invention relates to a water purification process in which nitrate ions are removed from an aqueous solution thereof and to a process for the removal and destruction of nitrate ions from water such as ground water or surface water.
The recent widespread use of fertilizers has lead to an increase in the level of nitrates in water. Levels in excess of 50 ppm in drinking water have been linked to health problems such as “Blue Baby Syndrome” and possibly stomach cancer. Furthermore, nitrates are often present in effluent which can be discharged into the water system in concentrated form; such nitrate discharge has been identified as a major cause of algal “bloom” in reservoirs and also inland and coastal water eutrophication. This prevalence of nitrate in the environment has led to legislation limiting the permitted level of, nitrates in treated water and effluent.
Nitrates are currently commonly removed from solution either by ion exchange or reverse osmosis.
In an ion exchange process nitrate-containing solutions (typically containing calcium, magnesium and sodium cations of nitrate, sulphate, chloride and bicarbonate anions) are passed through a column containing an anion exchange resin. When the anion exchange resins are fully loaded with nitrate ions, the resin is regenerated, for example using a solution of brine (sodium chloride). Nitrate ions then exchange with chloride ions in the brine and the resulting sodium nitrate and brine mixture is then discharged as waste.
With reverse osmosis, nitrate solutions pass through a membrane which retains approximately 90% of the nitrate (and other) ions in, typically, 20% of the solution. The resultant concentrated solution of the retained ions must then be discarded.
Other technologies, such as bio-denitrification, are also available for removal of nitrate ions from solution.
A problem associated with known techniques of nitrate removal is that relatively concentrated nitrate solutions are discharged. Furthermore, in the case of ion exchange, fresh regenerative solutions may be required for subsequent use of the ion exchange resins, leading to significant running costs.
Removal of the nitrate ions using electrolysis is also known. For example EP-A-291,330 describes a process for treating ground water containing nitrates, which comprises contacting the water with a ion exchange resin and regenerating the resin with a regenerant, wherein the spent regenerant is subjected to electrolysis. The regenerant may, for example, comprise bicarbonate, chloride or sulphate ions. The electrolysis is carried out in an electrolytic cell containing an anode and a cathode. The material of each electrode is platinised titanium, nickel, stainless steel, copper or graphite. The nitrogen gas which in evolved can simply pass into the atmosphere.
U.S. Pat. No. 3,542,657 describes a method of converting an alkali metal nitrate to an alkali metal hydroxide by passing a solution of the nitrate through an electrolytic cell in which a direct current is imposed between the anode and cathodes in the cell, thereby producing oxygen gas at the anodes and alkali metal hydroxide at the cathodes. Nitrogen gas is also produced a the cathodes. A bipolar cell is preferably used in which the cathodes are copper, lead, tin, iron, silver, cadmium, platinum, cobalt, nickel and alloys thereof or coatings of these on the other metals.
The present invention seek to provide a further process for removing nitrate ions from an aqueous solution thereof using an electrochemical cell.
The present invention provides a process for removing nitrate ions from an aqueous solution thereof which comprises passing the solution through an electrochemical cell comprising at least one anode and at least one cathode and passing a current therebetween, wherein the cathode surface(s) comprise rhodium metal.
It has surprisingly been found that the electrical efficiency of the electrochemical cell wherein the cathode surface(s) comprise rhodium metal is surprisingly better than that of other cells containing cathode surfaces comprising, for example, platinum or nickel.
For example, it has been found that in a conventional bipolar electrical cell in which the anodes and cathodes are both made of titanium coated with a mixture of ruthenium dioxide and titanium dioxide, the electrical efficiency for destroying nitrate ions in a bicarbonate solution is about 40%, with about 12% of nitrate ions being reduced in a single pass through the cell. In the same cell fitted with anodes and cathodes made of nickel, the electrical efficiency is about 35%. However, if a cell comprising anodes made of titanium coated with a mixture of ruthenium dioxide and titanium dioxide and cathodes made of titanium electroplated with rhodium, the electrical efficiency is about 49% and about 24% of nitrate ions are reduced in a single pass through the cell. The exact values will, of course, depend on the cell dimensions and operating conditions.
The process of the present invention can be used to remove either partially or completely, nitrate ions from any aqueous solution thereof. However, it is preferred that the aqueous solution is one which has been obtained from the regeneration of an ion exchange column. The ion exchange column may, for example, have been used to purify water, especially ground water or surface water, which contains nitrate ions. The nitrate ions may be present in the solution treated by the ion exchange column in a concentration of, for example, from 15 to 1000 ppm, preferably 15 to 500 ppm. The ground water or surface water which may be treated can subsequently be used as drinking water. The maximum permitted nitrate level in drinking water is generally limited to 50 ppm (as nitrate) as a global standard.
The aqueous solution of nitrate ions treated in the electrochemical cell may also comprise further anions, for example, hydroxide ions, bicarbonate ions and chloride ions. It may also contain cations such as hydrogen, sodium or potassium. The electrochemical cell itself is well known and is described, for example, in U.S. Pat. No. 3,542,657. However, it is essential that the cathode surface(s) comprise rhodium retail.
The cathode may, for example, simply consist of rhodium metal, although this is expensive. Accordingly it is preferred to coat a cathode substance with rhodium metal, for example by electroplating. The thickness of the coating is desirably 0.1 &mgr;m to 0.75 &mgr;m, for example 0.5 &mgr;m to 0.75 &mgr;m.
The cathode substrate may, for example, comprise a metal such as titanium. It nay also comprise an intermediate coating layer under the rhodium metal coating, for example to facilitate the rhodium coating process and to reduce the amount of rhodium used in view of its expense. Thus, for example, the cathode substrate may comprise titanium or titanium coated with titanium dioxide, ruthenium dioxide, iridium. dioxide and/or gold. Many cathode substrates are commercially available.
The anode may be any appropriate anode. Suitable anodes are known to those skilled in the art. The anode surface may be coated with metals or metal oxides which promote the generation of chlorine over oxygen evolution. Thus, for example, the anode may comprise a metal such as titanium, optionally coated with a metal or metal oxide. Examples of metals are platinum, ruthenium and iridium. Examples of metal oxides are titanium dioxide, ruthenium dioxide, oxides of platinum and iridium and mixed oxides of these metals. Advantageously, the anode surface does not comprise rhodium metal so as to avoid undesirable back-reactions.
In a bipolar cell configuration one side of an intermediate electrode functions as a cathode, whereas the other side functions as an anode. In this case the cathode side is coated with the rhodium metal.
Desirably all of the cathode surfaces in the electrochemical cell comprise rhodium metal. However, this is not an essential feature and only some of the surfaces need comprise rhodium metal. Desirably at least 75% and preferably 100% of the cathodes comprise rhodium metal on their surfac
Doody Patrick A.
Hunton & Williams
Ionex Limited
Phasge Arun
LandOfFree
Water purification process does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Water purification process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Water purification process will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3042892