Chemistry: analytical and immunological testing – Determination of water
Reexamination Certificate
2000-12-15
2004-08-03
Warden, Jill (Department: 1743)
Chemistry: analytical and immunological testing
Determination of water
C436S073000, C436S041000, C436S084000, C436S083000, C210S719000, C210S094000
Reexamination Certificate
active
06770483
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to new and useful improvements in a determination for the presence of water-soluble toxic reducible metals and metal salts in water and a method for removal therefrom and, more particularly, to a method for determining the presence of toxic metal salts and a system for the removal of such metal salts from drinking water which relies upon a reduction of the metal salt in a reduction/oxidation reaction to a different valence state and which would enable determination of the presence and allow for removal of the contaminant.
2. Brief Description of the Related Art
It is well established that water and, particularly, municipal drinking water, as well as water from aquifers and wells, presently contain trace amounts of highly toxic metals, including, for example, arsenic, mercury and chromium. Each of these metals are usually present in the form of various metal salts in different oxidized states and are known to be either carcinogens or otherwise known to present significant health risks.
Chromium is an example of a metal present as a metal salt contaminant found in many water sources. Two of the most common forms of chromium are trivalent chromium (Cr
+3
), and hexavalent chromium (Cr
+6
), although chromium can exist in other valence states, such as Cr
+5
, Cr
+4
and Cr
+2
. It is also known that salts containing metals, such as Cr
+3
are relatively harmless while salts of Cr
+6
are highly toxic. Many of these metals and metal salts, such as Cr
+6
and Cr
+6
compounds are not normally present as natural constituents of environmental media, and their presence is almost always the result of human activity, including commercial and industrial processes, which generate Cr
+6
and its salts and release them into the environment. As an example, cooling towers and ancillary equipment, catalysts used in the cracking and refining of petroleum products, tanning, textile dying, etc., are some of the commercial and industrial processes which give rise to these oxidized metals and their salts, such as those of Cr
+6
.
It is also known that many of these metals and their salts, such as Cr
+6
, which may be airborne, can induce lung cancer through inhalation. The presence of Cr
+6
is not only recognized as a toxic substance, but its presence in drinking water is limited by current U.S. government standards with a maximum contaminant level of 0.1 milligrams per liter (100 parts per billion). In the State of California, USA, the standard for chromium in drinking water is 50 parts per billion. A present proposal suggests that even these standards are too lax and that the maximum allowable amount of total chromium present should be no more than 2.5 parts per billion. There have even been studies which suggest that the maximum amount of Cr
+6
which should be allowable in drinking water should be no more than 0.2 parts per billion.
The seriousness of the health consequences of these oxidized metals and metal salts, when present in drinking water have been studied. It has been found that Cr
+6
, for example, can be distributed throughout the body and accumulates in the kidney, spleen and pancreas. Uptake of Cr
+6
into the liver is 40 to 90 times that found in other organs. While Cr
+3
does not readily enter the cells of these organs, Cr
+6
does so. Within cells, Cr
+6
is reduced stepwise to Cr
+5
, Cr
+4
, and Cr
+3
. During this process, aberrant forms of oxygen, including hydroxyl free radicals, OH, and the superoxide anion, O
2
−
, are produced as potent toxins. These potent toxins can cause chemical changes in cellular DNA, i.e., mutations, leading to severe alterations in cell functions and carcinogenic effects. Other effects of these Cr
+6
-induced toxins result in a potent genotoxic agent. Still other effects of these Cr
+6
-induced toxins result in compromising the body's immunoprotective systems and they can act as neurotoxins. In addition, they can cause developmental and reproductive damage, not to mention other adverse conditions and maladies caused by their presence.
Although it would be desirable to advise the public of the presence of metals and their metal salts in water, and particularly those which are toxic, no convenient test is currently available to detect their presence. Many of these oxidized metals are not visible at low concentrations, cannot be tested directly and easily and are otherwise sensibly indeterminable. Nevertheless, determination of the presence of metal toxic substances in water by the public in general would be desirable.
The prior art has usually involved the removal of Cr
+6
and other toxic oxidizable metals by providing a reducing agent or reductant as a source of electrons. These agents reduce these metals to a lower valence state, one which is often non-toxic or less toxic. However, precipitation of Cr
+6
, for example, by control of pH alone is insufficient to remove chelated or complexed forms of Cr
+6
or other heavy metals. There have been attempts to use controlled pH methods supplemented with flocculents or precipitants to allow for the removal of Cr
+6
through flocculation, precipitation and settling out followed by filtration.
Reduction of Cr
+6
to Cr
+3
does eliminate the toxic hexavalent form of the metal by converting it to the essentially non-toxic form of Cr
+3
by the reaction:
Cr
VI
+
3
e
′
s
reduction
⇐
oxidation
⇒
Cr
III
1
)
Conversion of the Cr from a higher valence state to a lower valence state is accompanied by oxidation of the reducing agent or reductant from a corresponding lower valence state to a higher valence state. The combination of these two reactions constitutes a coupled redox reaction.
There have been several attempts and proposals for removal of some of these oxidized metal contaminants from water including, for example, U.S. Pat. No. 4,149,953 to Rojo, which relies upon an electrolytic cell to remove impurities. An anode of this cell containing aluminum particles and a cathode containing iron particles operates with the water serving as an electrolyte. The aluminum and iron particles which enter the water function as flocculents and adsorb impurities in the water. The flocculated materials are then separated from the water by conventional means. Application of this type of system to waste, process or drinking water for removal of Cr
+6
or other oxidized metals is limited because it may bring down the chromium in the flocculate without necessarily reducing the metal. Moreover, this type of process would be difficult to implement, would not be efficient and would be costly to operate. Thus, the desirable conversion of Cr
+6
to Cr
+3
and subsequent removal of the latter as a precipitate, may not result.
U.S. Pat. No. 4,693,798 to Gale and O'Donnell discloses use of an electrolytic cell for generation of Fe
+2
ions in an acidified medium. The Fe
+2
interacts with Cr
+6
ions reducing them to Cr
+3
ions. However, a part of this stream is bled off into the contaminated stream containing Cr
+6
ions. This must be followed by alkalinization of the treated stream of water to a pH in excess of 7.5 to allow for coprecipitation of Cr
+3
and Fe
+3
hydroxides.
Another system for waste water treatment is described in U.S. Pat. No. 4,923,599 to Bowers. In the system described in the Bowers patent, a controlled volume of waste water containing heavy metals and including, for example, Cr
+6
, is treated by optimizing and controlling pH to cause precipitation of some of the heavy metal contaminants. This is followed by filtration and monitoring of the samples' turbidity to determine the amount of additional precipitating agent which may still be needed. Although a claimed advantage of this method is
Siefke Sam P.
Warden Jill
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