CROSS-REFERENCE TO RELATED-APPLICATION(S) 0001 This is a continuation-in-part application of pending U.S. application Ser. No. 08/798,873, filed Feb. 11, 1997, scheduled to issue as U.S. Pat. No. 5,885,543 on Mar. 23, 1999, and entitled Method for Producing Chlorine Dioxide Using Calcium Chloride Impregnated Zeolite or Aqueous Calcium Chloride, inventor Fred Klatte.

FIELD OF THE INVENTION 0002 The invention relates to methods for producing chlorine dioxide, and to substances used in performing such methods. Each method produces chlorine dioxide by activating zeolite crystals (previously impregnated with a mixture of sodium chlorite and a water-retaining substance such as calcium chloride) with protons (from an acid or other proton generating species), or by activating an aqueous solution of a water-retaining substance (such as calcium chloride) and sodium chlorite with protons (from an acid or other proton generating species).

BACKGROUND OF THE INVENTION 0003 Zeolites are hydrated metal aluminosilicate compounds with well-defined (tetrahedral) crystalline structures. Because zeolite crystals (both natural and synthetic) have a porous structure with connected channels extending through them, they have been employed as molecular sieves for selectively absorbing molecules on the basis of size, shape, and polarity. 0004 Volumes packed with zeolite crystals (for example, small zeolite crystals chosen to have size in the range from 0.2 mm to one quarter inch) have been employed in air (or other gas) and water filtration systems to selectively absorb contaminants from a flowing stream of water or gas. 0005 U.S. Pat. No. 5,567,405, issued Oct. 22, 1996 (based on U.S. application Ser. No. 08/445,025, filed May 19, 1995), and U.S. Pat. No. 5,573,743, issued Nov. 12, 1996 (based on U.S. application Ser. No 08/445,076), teach methods for producing zeolite crystals impregnated with one or more of sodium chlorite, acetic acid, phosphoric acid, and citric acid, and methods for producing chlorine dioxide by moving a fluid (e.g., air or water) relative to a bed of zeolite crystals impregnated with sodium chlorite, and moving the fluid relative to another bed of zeolite crystals impregnated with one of the following: phosphoric acid, acetic acid and citric acid. The two beds can be physically mixed together, or the fluid can flow sequentially through distinct first and second beds. These references also teach a method for filtering a fluid by producing chlorine dioxide in the fluid (in the manner described in this paragraph) and then absorbing the chlorine dioxide from the fluid. 0006 U.S. patent application Ser. No. 08/704,086, filed Aug. 28, 1996, teaches a variation on the chlorine dioxide production method of U.S. Pat. No. 5,567,405, which includes the steps of moving a fluid through a first bed of impregnated zeolite crystals (impregnated with at least one of phosphoric acid, acetic acid, and citric acid) and then moving the fluid through a second bed of impregnated zeolite crystals (impregnated with sodium chlorite). 0007 Chlorine dioxide (ClO2) is useful for killing biological contaminants (such as microorganisms, mold, fungi, yeast and bacteria) and for oxidizing volatile organic chemicals which can contaminate fluid. 0008 It is known to produce chlorine dioxide by: activating a metal chlorite solution by adding an acid thereto, activating a powdered composition (or other dry composition) by adding water thereto, or preparing an activated dry composition which releases chlorine dioxide over time. U.S. Pat. No. 4,547,381 (issued Oct. 15, 1985) and U.S. Pat. No. 4,689,169 (issued Aug. 25, 1987) mention these three techniques for producing chlorine dioxide, and disclose in some detail one type of such an activated dry composition. They teach that this activated dry composition is a mixture of a dry inert diluent, a metal chlorite, and a dry agent capable of reacting with the metal chlorite in a dry state to produce chlorine dioxide. The metal chlorite can be sodium chlorite, and the dry agent can be a dry acid such as granular citric acid. The inert diluent can be diatomaceous earth, sodium chloride, sodium silicate, disodium sulfate, or magnesium chloride, or a combination of two or more thereof. The mixture releases chlorine dioxide over time until the rate of chlorine dioxide release becomes low, and the patents teach that the mixture can then be agitated for renewed generation of chlorine dioxide. 0009 However, an activated composition (such as that described in U.S. Pat. Nos. 4,547,381 and 4,689,169) is subject to undesirable storage and shipping losses, due to outgassing of chlorine dioxide before the time of intended use of the composition. 0010 Similarly, the inventor has found that sodium chlorite-impregnated zeolite crystals (of the type described in referenced U.S. Pat. Nos. 5,567,405 and 5,573,743) are also subject to undesirable storage and shipping losses, due to outgassing of chlorine dioxide therefrom before the time of their intended use (e.g., before fluid is caused to flow through both a bed of the sodium chlorite-impregnated zeolite crystals and a bed of acid-impregnated zeolite crystals). Also, efforts to activate sodium chlorite-impregnated zeolite crystals (of the type described in referenced U.S. Pat. Nos. 5,567,405 and 5,573,743) with acid at low temperature (below 40 degrees Fahrenheit) may fail in the sense that they will not result in release of sufficient amounts of chlorine dioxide. 0011 There are also disadvantages to use of conventional metal chlorite solutions (of the type mentioned in U.S. Pat. Nos. 4,547,381 and 4,689,169) to produce chlorine dioxide. For example, when such a conventional solution is activated (by adding an acid thereto) to release chlorine dioxide gas, it is difficult or impossible to control the rate of release of the chlorine dioxide gas. Overproduction of chlorine dioxide often results. 0012 There are also disadvantages to use of conventional powdered or dry compositions of the type activated by adding water thereto to release chlorine dioxide (as mentioned, for example, in U.S. Pat. Nos. 4,547,381 and 4,689,169). Masschelein, in the book Chlorine DioxideChemistry and Environmental Impact of Oxychlorine Compounds (published 1979 by Ann Arbor Science Publishers Inc., Ann Arbor, Mich.) at page 140, describes one such dry mixture comprising sodium chlorite, and a solid organic anhydride containing 2 to 20% of a desiccating product such as calcium chloride. When such a conventional dry composition is activated (by adding water thereto) to release chlorine dioxide gas, it is typically difficult or impossible to control the rate of release of the chlorine dioxide to achieve chlorine dioxide release rates useful for such applications as air or water filtration. Overproduction of chlorine dioxide often results. 0013 Until the present invention, it was not known how to produce a stable, unactivated substance (either in liquid or dry form) which does not release significant amounts of chlorine dioxide until activated by exposure to an acid, and which releases chlorine dioxide at a useful (and controllable) rate when exposed to an acid (even at temperatures below 40 degrees Fahrenheit). Nor had it been known to use such a stable, unactivated substance to produce controlled release of chlorine dioxide for filtering air or water.

SUMMARY OF THE INVENTION 0014 In some embodiments, the invention is a method for producing chlorine dioxide by activating zeolite crystals (previously impregnated with sodium chlorite and calcium chloride) with excess protons or activating an aqueous solution of sodium chlorite and calcium chloride with excess protons. Typically, the excess protons are produced by exposing the crystals to an acid (or other proton generating species), or exposing the aqueous solution of sodium chlorite and calcium chloride to an acid (or other proton generating species). Examples of proton generating species useful for such activation are acids such as acetic acid, phosphoric acid, citric acid, HCl, propionic acid, and sulfuric acid, and metal salts such as ferric chloride, ferric sulfate, ZnSO4, ZnCl2, CoSO4, CoCl2, MnSO4, MnCl2, CuSO4, CuCl2, and MgSO4. In some embodiments the proton generating species releases excess protons upon exposure to the impregnated zeolite crystals themselves. In other embodiments the proton generating species must itself be activated to cause it release excess protons, so that the protons can in turn activate the impregnated zeolite crystals. 0015 In accordance with the invention, the step of activating zeolite crystals with excess protons can be performed by causing a fluid (e.g., air, another oxygen-containing fluid, or water) to flow through a bed of zeolite crystals impregnated with sodium chlorite and calcium chloride, and a bed of zeolite crystals impregnated with the proton generating species. The two beds can be physically mixed together, or the fluid can be caused to flow sequentially through distinct first and second beds (preferably first through the bed containing proton generating species-impregnated zeolite, but alternatively first through the bed containing sodium chlorite and calcium chloride-impregnated zeolite). Alternatively, chlorine dioxide is produced using sodium chlorite and calcium chloride-impregnated zeolite crystals by immersing the impregnated zeolite in (or spraying the impregnated zeolite with) acid or another proton generating species, or otherwise exposing the impregnated zeolite to acid (preferably aqueous acetic, phosphoric, citric acid, HCl, sulfuric acid, propionic acid, or another acid, with a concentration of 0.025% to 0.5%, in the sense that the acid comprises 0.025% to 0.5% by weight of the combined impregnated zeolite and acid) or another proton generating species. 0016 To produce chlorine dioxide using the inventive aqueous solution of sodium chlorite and calcium chloride, the solution can be mixed (or otherwise combined) with acid. The liquid mixture can then be sprayed or coated on a surface (or the liquid mixture can be absorbed in a material such as a sponge, pad, mat, or the like, or simply placed in a reservoir, container, box, or the like) from which chlorine dioxide gas can escape at a desired rate. 0017 In any of the embodiments, the rate of chlorine dioxide release (following activation) can be controlled in any of several ways, including by appropriately selecting the concentration and amount of the activating acid (or other proton generating species), and using impregnated zeolite having appropriately selected weight ratios of metal chlorite (e.g., sodium chlorite) or sodium chlorate to zeolite, and deliquescent or water absorbing and retaining substance (e.g., calcium chloride) to zeolite. 0018 Other embodiments of the invention are substances useful for producing chlorine dioxide, which are stable until activated with excess protons (in the sense that they do not release chlorine dioxide gas in significant amounts until so activated with protons). One such embodiment is one or more zeolite crystals impregnated with sodium chlorite and calcium chloride. Preferably, the zeolite crystals are small (each of size in a range from 0.2 mm to one quarter inch). Also preferably, the crystals comprise 1%-6% sodium chlorite, 0.5%-2% calcium chloride, 0%-20% (4%-8% in some preferred embodiments) water, and 72%-98.5% (8%-94.5% in some preferred embodiments) zeolite by weight. Another such embodiment is an aqueous solution of sodium chlorite and at least one chemical selected from the group consisting of magnesium sulfate, potassium chloride, potassium hydroxide, and calcium chloride, preferably comprising 1%-6% of sodium chlorite, 0.5%-2% of said at least one chemical, and 92%-98.5% of water (by weight). Preferably, the solution comprises 1%-6% sodium chlorite, 0.5%-2% calcium chloride, and 92%-98.5% water (by weight). 0019 In variations on any embodiment of the invention, a water-retaining substance other than calcium chloride, such as magnesium sulfate (MgSO4), potassium chloride, or potassium hydroxide, is substituted for calcium chloride. For example, zeolite crystals impregnated with sodium chlorite and magnesium sulfate are within the scope of the invention. For another example, a method of producing chlorine dioxide by activating an aqueous solution of sodium chlorite and magnesium sulfate with an acid is also within the scope of the invention. 0020 In other variations on any embodiment of the invention, a metal chlorite other than sodium chlorite is substituted for sodium chlorite. For example, zeolite crystals impregnated with calcium chloride (or magnesium sulfate) and a metal chlorite other than sodium chlorite are within the scope of the invention. 0021 The inventor has found that the presence (in a sufficient amount) of calcium chloride (or another water-retaining substance such as magnesium sulfate, potassium chloride, or potassium hydroxide) in the unactivated composition of the invention reduces the rate of chlorine dioxide outgassing to no more than a negligible amount at times prior to activation of the composition with protons, and yet allows release of chlorine dioxide at a desired rate following activation of the composition with protons. One of the reasons for chlorine dioxide outgassing (prior to activation with excess protons) from zeolite impregnated with metal chlorite (but not impregnated with a water-retaining substance such as calcium chloride) is migration of protons in the aluminosilicates comprising the zeolite. 0022 In other embodiments, the invention is a method for producing chlorine dioxide, including the steps of providing a zeolite crystal mixture, wherein the mixture comprises zeolite crystals impregnated with sodium chlorate and zeolite crystals impregnated with an oxidizer, and activating the mixture with excess protons, and a zeolite crystal mixture for use in performing such method. In other embodiments, the invention is a method for producing chlorine dioxide, including the steps of providing a zeolite crystal mixture, wherein the mixture comprises zeolite crystals impregnated with a proton generating species and zeolite crystals impregnated with an oxidizer (or sodium chlorate), and causing the zeolite crystal mixture to come in contact with sodium chlorate solution (or an oxidizer, where the mixture includes zeolite crystals impregnated with sodium chlorate but not an oxidizer), and a zeolite crystal mixture for use in performing such method.

BRIEF DESCRIPTION OF THE DRAWINGS 0023 FIG. 1 is a cross-sectional view of a zeolite crystal impregnated with a metal chlorite, and one of calcium chloride (or other deliquescent or water absorbing and retaining substance), magnesium sulfate, potassium chloride, and potassium hydroxide. 0024 FIG. 2 is a cross-sectional view of a zeolite crystal impregnated with a proton generating species, or another substance or mixture of substances in accordance with any of the embodiments of the invention. 0025 FIG. 3 is a cross-sectional view of a mixture of impregnated zeolite crystals (prepared in accordance with any embodiment of the invention) in a sealed container. 0026 FIG. 4 is a cross-sectional view of a mixture of impregnated zeolite crystals (prepared in accordance with the invention) surrounded by a barrier permeable to chlorine dioxide gas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 0027 In one class of embodiments, the invention is a process for impregnating zeolite crystals with sodium chlorite and calcium chloride, and the product of such process. The zeolite crystals can have size (i.e., largest dimension) equal (or approximately equal) to 0.125 inch, 0.25 inch, 0.50 inch, or 0.75 inch, or size in the range from 0.2 mm to several millimeters, or size in the range from 0.2 mm to 0.25 inch, or the zeolite crystals can have dimensions equal or substantially equal to 0.25 inch0.167 inch, 0.125 inch0.10 inch, 0.25 inch0.125 inch, 0.125 inch0.50 inch, or 0.50 inch0.75 inch. The impregnation process produces zeolite crystals uniformly impregnated (throughout the volume of each crystal) with sodium chlorite and calcium chloride. 0028 Preferred embodiments of this impregnation process employ, as input material, zeolite crystals whose moisture content has been reduced (substantially below an initial moisture content) to a low level (preferably about 5%). Such input material is preferably produced by mining zeolite, crushing the mined mineral into appropriately sized zeolite crystals (having a natural moisture content substantially above 5%, and then dehydrating the zeolite crystals until their moisture content is reduced to about 5%. 0029 The dehydrated zeolite crystals are then immersed in (or sprayed with) an aqueous solution of sodium chlorite and calcium chloride at high temperature (e.g., in the range from 120 F. to 190 F.), and the resulting slurry is thoroughly mixed. Then, the mixed slurry is air dried (or allowed to equilibrate) to the desired moisture level (typically 0-20%) to produce impregnated zeolite crystals. Alternatively, the air drying step can be avoided by calculating the amount of aqueous impregnating material needed to achieve the desired final moisture level (e.g., 15%) and adding this amount to the dehydrated zeolite at the time of impregnation. 0030 Alternatively, the dehydrated zeolite crystals are immersed in (or sprayed with) an aqueous solution of sodium chlorite at high temperature (e.g., at least 190F.), and the resulting slurry is thoroughly mixed. Then, the mixed slurry is air dried (or allowed to equilibrate) to the desired moisture level to produce sodium chlorite-impregnated zeolite crystals. Alternatively, one calculates the amount of aqueous sodium chlorite needed to achieve the desired moisture level (e.g., a desired level in the range 15%-20%) and adds this amount to the dehydrated zeolite at the time of impregnation. Then, the sodium chlorite-impregnated zeolite crystals are immersed in (or sprayed with) an aqueous solution of calcium chloride at high temperature, and the resulting slurry is thoroughly mixed. Then, the mixed slurry is air dried (or allowed to equilibrate) to the desired final moisture level (typically 0-20%) to produce zeolite crystals impregnated with both sodium chlorite and calcium chloride. Alternatively, air drying is avoided by calculating the amount of aqueous calcium chloride needed to achieve the desired final moisture level (e.g., a desired level in the range 15%-20%) and adding this amount to the sodium chlorite-impregnated zeolite at the time of calcium chloride impregnation. 0031 In variations of any of the described zeolite impregnation processes, a water-retaining substance such as magnesium sulfate (MgSO4), potassium chloride, or potassium hydroxide is substituted for calcium chloride as an impregnating agent. 0032 In other variations on the described zeolite impregnation processes, a metal chlorite other than sodium chlorite is substituted for sodium chlorite. 0033 FIG. 1 represents one impregnated zeolite crystal, having channels uniformly impregnated with mixture 2 of metal chlorite and a water-retaining substance. The water-retaining substance is preferably calcium chloride, but alternatively is magnesium sulfate, potassium chloride, potassium hydroxide, or another water-retaining substance. The metal chlorite is preferably sodium chlorite. 0034 The rate at which impregnated zeolite crystals of the type shown in FIG. 1 (e.g., zeolite crystals uniformly impregnated sodium chlorite and calcium chloride) react with in the presence of excess protons to release chlorine dioxide can be controlled (reduced or increased to a desired level) by varying the relative amounts (by weight) of the different impregnating agents therein. 0035 In some embodiments of the invention, the proton generating species is in the form of zeolite crystals impregnated with one or more metal salts (preferably one or more of ferric chloride, ferric sulfate, ZnSO4, ZnCl2, CoSO4, CoCl2, MnSO4, MnCl2, CuSO4, CuCl2, and MgSO4). Upon exposure of the metal salt or salts to water (either liquid water or a moisture-containing gas, e.g. atmospheric moisture), excess protons are generated. 0036 In other embodiments of the invention, the proton generating species is activated (other than by exposure to water) to release excess protons. The proton generating species is activated in such a manner that impregnated zeolite crystals are exposed to excess protons, thereby initiating a reaction resulting in release of chlorine dioxide gas. In some embodiments, the proton generating species releases excess protons upon exposure to the impregnated zeolite crystals themselves (the impregnated zeolite crystals together with the proton generating species does not need to be further activated to cause the proton generating species to release excess protons). 0037 In some embodiments of the invention, the proton generating species is in the form of zeolite crystals impregnated with at lease one acid (preferably one or more of acetic acid, citric acid, phosphoric acid, HCl, propionic acid, and sulfuric acid). Above-referenced U.S. Pat. Nos. 5,567,405 and 5,573,743 disclose methods for producing such acid-impregnated zeolite crystals. Briefly, the acid impregnation step is preferably performed by immersing dehydrated zeolite crystals in (or spraying zeolite crystals with) an aqueous solution of one or more of the acids at high temperature, thoroughly mixing the resulting slurry, and finally air drying (or allowing the slurry to equilibrate) to the desired moisture level (e.g. 0%-20%) the mixed slurry to produce the acid-impregnated zeolite crystals. FIG. 2 represents one such impregnated crystal, having channels uniformly impregnated (throughout the volume of the crystal) with chemical 10, where chemical 10 is acetic acid, citric acid, phosphoric acid, HCl, propionic acid, or sulfuric acid (or another acid), or a mixture of two or more of such acids, or another proton generating species, or another substance or mixture of substances in accordance with any of the embodiments of the invention. 0038 In general, the acid or acids employed (for activation of impregnated zeolite crystals) in accordance with the invention can be impregnated in zeolite crystals, mixed in an aqueous solution (including a metal chlorite and a water-retaining substance such as calcium chloride), or added to (e.g., sprayed on) zeolite crystals that have been impregnated with a metal chlorite and a water-retaining substance such as calcium chloride. 0039 With reference again to zeolite crystals that have been impregnated in accordance with the invention with sodium chlorite and calcium chloride, such crystals are preferably small (each of size in a range from 0.2 mm to one quarter inch), and each crystal preferably comprises 1%-6% sodium chlorite, 0.5%-2% calcium chloride, 0%-20% (4%-8% in some preferred embodiments) water, and 72%-98.5% (84%-94.5% in some preferred embodiments) zeolite by weight. The inventor has found that-the presence of calcium chloride (preferably in the preferred amount noted above) in the unactivated impregnated zeolite reduces the rate of chlorine dioxide outgassing to no more than a negligible amount until activation of the composition with acid, and yet allows release of chlorine dioxide at a desired rate following activation of the composition with acid. This finding was unexpected, since mixing of sodium chlorite and calcium chloride with water would have been expected to produce chlorine dioxide as a result of the following sequence of reactions: 0040 1. 2NaClO22H2OCaCl20041 2. Ca(OH)22HCL2NaClO20042 3. HCLNaClO2NaClClO2protons. 0043 Instead, the unexpected finding that the rate of chlorine dioxide outgassing is substantially reduced is believed to result from the following reaction: 0044 1. 2NaClO2H2OCaCl20045 2. H2OCaClO22NaCl. 0046 The products of this reaction, when activated with excess protons (e.g., by exposure to acid or a metal salt) are converted to C102 and other substances including excess protons. In another embodiment, when sodium chlorite and ferric chloride react with water to produce chlorine dioxide gas, it is believed that the chlorine dioxide is produced as a result of the following reaction: 3NaClO23H2OFeCl33ClO2(Fe) (OH)33NaCl. 0047 In another class of embodiments, the invention is an aqueous solution of sodium chlorite and calcium chloride (or sodium chlorite and a water-retaining substance other than calcium chloride). This solution can be activated by exposure to protons (e.g., as a result of exposure to a proton generating species such as acetic acid, phosphoric acid, citric acid, or another acid) so as to release chlorine dioxide in a controlled manner. Preferably, the solution comprises 1%-6% sodium chlorite, 0.5%-2% calcium chloride, and 92%-98.5% water (by weight). Also preferably, the solution is activated by being mixed (or otherwise combined) with aqueous acetic acid, phosphoric acid, citric acid, HCl, propionic acid, or sulfuric acid (or another acid) having a concentration of 0.025% to 0.5% (by weight of the total mixture). An example of such an aqueous acid is produced by adding 1k (by weight of the total mixture) of a 3% aqueous acetic acid solution. The activated liquid mixture (of aqueous sodium chlorite, calcium chloride, and acid) can then be sprayed or coated on a surface (or the liquid mixture can be absorbed in a material such as a sponge, pad, mat, or the like, or simply placed in a reservoir) from which chlorine dioxide gas can escape at a desired rate. 0048 The inventor has found that the presence of diluted calcium chloride or one of the alternative water-retaining substances (preferably in the above-noted preferred amount of 0.5%-2% by weight) in the unactivated solution reduces the rate of chlorine dioxide outgassing to no more than a negligible amount at times prior to activation with an acid, and yet allows release of chlorine dioxide at a desired rate upon activation of the solution with acid. The unexpected finding that the rate of chlorine dioxide outgassing (prior to activation) is substantially reduced is believed to result from the following reaction in the unactivated solution: 0049 1. 2NaClO2H2OCaCl20050 2. H2OCaClO22NaCl. The products of this reaction, when activated with excess protons (e.g., by addition of acid) are converted to ClO2 and other substances including excess protons. 0051 The impregnated zeolite crystals of the invention (crystals impregnated with a metal chlorite and calcium chloride, or a metal chlorite and a water-retaining substance other than calcium chloride) are useful in a class of methods for producing chlorine dioxide (ClO2). Such methods will be described in preferred embodiments in which calcium chloride is an impregnating agent and the metal chlorite is sodium chlorite, but it should be understood that a water-retaining substance (such as magnesium sulfate, potassium chloride, or potassium hydroxide) can be substituted for calcium chloride as an impregnating agent in the methods (and that a metal chlorite other than sodium chlorite can be substituted for sodium chlorite). 0052 In one chlorine dioxide-producing method in accordance with the invention, a fluid (preferably a fluid containing oxygen, such as air or water) is caused to move relative to a first bed (i.e., the fluid flows through the first bed or the crystals comprising the first bed move through the fluid) of zeolite crystals impregnated with: (1) phosphoric acid (H3PO4), (2) acetic acid (CH3COOH), (3) citric acid, (4) ferric chloride (FeCl3), (5) ferric sulfate, or (6) another metal salt, such as ZnSO4, ZnCl2, CoSO4, CoCl2, MnSO4, MnCl2, CuSO4, CuCl2, or MgSO4, or (7) an acid other than phosphoric, acetic, or citric acid, or another proton generating species suitable for the particular application. Then, the fluid is caused to move relative to a second bed (i.e., the fluid flows through the second bed, or the crystals comprising the second bed move through the fluid) of zeolite crystals which are impregnated with a mixture of sodium chlorite (NaClO2) and calcium chloride (CaCl2). Upon moving the fluid relative to the second bed, chlorine dioxide is released (due to contact with acid or other proton generating substance transferred from the first bed). It is believed that the chlorine dioxide release occurs as a result of the following reaction, in the case of a first bed impregnated with acetic acid:

CH3COOHCaClO22NaClH2OClO2Ca(CH3COO)2excess protons,

0053 where Ca(CH3COO)2 is calcium acetate. 0054 As discussed above, CaClO2, 2NaCl, and H2O are believed to be present in the channels of the zeolite crystals of the first bed as a result of reaction of the impregnating agents NaClO2 and CaCl2, and H2O, in the channels of the zeolite crystals of the first bed. 0055 In variations on this chlorine dioxide production method, the first bed can include a mixture of phosphoric acid-impregnated zeolite crystals and acetic acid-impregnated zeolite crystals, or a mixture of phosphoric acid-impregnated zeolite crystals and citric acid-impregnated zeolite crystals, or a mixture of acetic acid-impregnated zeolite crystals and citric acid-impregnated zeolite crystals, or a mixture of all three types of acid-impregnated zeolite crystals, or a mixture of zeolite crystals impregnated with one of these three types of acids and zeolite crystals impregnated with another acid (e.g., HCl or H2SO4). 0056 It is preferable for the fluid to flow through a first bed of proton generating species-impregnated (e.g., acid-impregnated) zeolite crystals before the fluid flows through a second bed containing sodium chlorite and calcium chloride-impregnated zeolite crystals, since this sequence will result in protons entering the fluid (due to interaction of the fluid with the acid or other proton generating species in the crystals of the first bed), and since the presence of the hydrogen ions in the fluid will enhance chlorine dioxide production when the hydrogen ion-containing fluid interacts with the impregnating chemical in the crystals of the second bed. 0057 While the foregoing process for producing ClO2 has been described with reference to two distinct (first and second) beds of impregnated zeolite crystals, a single bed containing a mixture of crystals can be used as long as the mixed bed contains both impregnated zeolite crystals from the first bed described above and zeolite crystals from the second bed described above. As an example, a mixed bed of zeolite crystals (zeolite crystals impregnated with sodium chlorite and calcium chloride mixed with zeolite crystals impregnated with phosphoric acid) can be used to produce chlorine dioxide (e.g., by flowing a fluid through the mixed bed). 0058 Alternatively, chlorine dioxide can be produced by adding aqueous acid to a bed of zeolite crystals impregnated with a metal chlorite (e.g., sodium chlorite) and a water-retaining substance (e.g., calcium chloride). In these alternative embodiments, the rate of release of chlorine dioxide can be controlled by varying the amount and concentration of the added acid. 0059 In alternative embodiments, chlorine dioxide is produced with a reversed sequence of distinct first and second beds, as follows. A fluid (preferably a fluid containing oxygen such as air) is caused to move relative to a first bed (i.e., the fluid flows through the first bed, or the crystals comprising the first bed move through the fluid). The first bed comprises zeolite crystals impregnated with sodium chlorite (NaClO2) and calcium chloride (CaCl2). Then, the fluid is caused to move relative to a second bed (i.e., the fluid flows through the second bed, or the crystals comprising the second bed move through the fluid) of zeolite crystals which are impregnated with: (1) phosphoric acid, (2) acetic acid, (3) citric acid, (4) ferric chloride (FeCl3), (5) ferric sulfate, or (6) another metal salt, such as ZnSO4, ZnCl2, CoSO4, CoCl2, MnSO4, MnCl2, CuSO4, CuCl2, or MgSO4, or (7) an acid other than phosphoric, acetic, or citric acid, or another proton generating species suitable for the particular application. Upon moving the fluid relative to the second bed, chlorine dioxide is released. As with other embodiments described herein, a water-retaining substance such as magnesium sulfate, potassium chloride, or potassium hydroxide can be substituted for the calcium chloride (and a metal chlorite other than sodium chlorite can be substituted for the sodium chlorite). 0060 In another class of embodiments of the inventive chlorine dioxide production method, zeolite crystals impregnated with sodium chlorite and calcium chloride are activated by being immersed in (or sprayed with) aqueous acetic acid, phosphoric acid, citric acid, HCl, or sulfuric acid, with a concentration of 0.025% to 0.5% (in the sense that the acid comprises 0.025% to 0.5% by weight of the combined impregnated zeolite and acid). Alternatively, another acid or other proton generating species suitable for the particular application can be used to immerse (or spray) the impregnated zeolite crystals as a substitute for the aqueous acetic acid, phosphoric acid, citric acid, HCl, or sulfuric acid. 0061 While chlorine dioxide can kill microorganisms in the fluid undergoing treatment, and can oxidize volatile organic chemicals which contaminate the fluid undergoing treatment (as described above), chlorine dioxide itself is a contaminant. Therefore, it is sometimes desirable to remove chlorine dioxide from the fluid stream after the chlorine dioxide has performed purification (including biological purification) of the fluid stream. Another aspect of the present invention relates to a multi-step filtration process in which chlorine dioxide is first produced in a fluid stream and, subsequently, removed from the fluid stream. 0062 First, a fluid is caused to be moved relative to a primary bed (or primary beds) of zeolite crystals (i.e., the described first and second beds, or the described mixed bed) which will cause chlorine dioxide to be released, as described above. While the fluid moves relative to (e.g., while the fluid flows through) the primary bed or beds, chlorine dioxide is released (generally as a gas). The released chlorine dioxide will kill biological contaminants in the fluid and will oxidize volatile organic chemicals which contaminate the fluid. 0063 Then, the fluid is moved relative to a secondary bed (filter) of zeolite crystals impregnated with one of the following: (1) potassium hydroxide (KOH), (2) sodium sulfite, (3) sodium bisulfite, and (4) ferrous sulfate (i.e., the fluid flows through the secondary bed or the crystals comprising the secondary bed move through the fluid). The zeolite crystals of the secondary bed react with the chlorine dioxide to remove the chlorine dioxide from the fluid. Some chemical equations describing possible reactions in the secondary bed follow. 0064 For a secondary bed of potassium hydroxide:

2KOH2ClO2KClO2KCLO4H2O

0065 For a secondary bed of sodium sulfite:

ClO2Na2SO3H2OS2O5H3ClO3

0066 Note that H from the acids involved in the previous reactions cancel this reaction. 0067 For a secondary bed of sodium bisulfite:

ClO2Na2SO5H2OS2O5H3ClO3

0068 Note that H from the acids involved in the previous reactions cancel this reaction. 0069 The preferred composition of the secondary bed is ferrous sulfate. For a secondary bed of ferrous sulfate:

ClO2FeSO4FeClClO2FeCl3 (primarily).

0070 When using a secondary bed of ferrous sulfate a color change, from white to brown, takes place. This is an indicator of neutralization. 0071 Various modifications and variations of the described methods and compositions of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. For example, a carrier other than zeolite crystals or water (e.g., pumice, diatomaceous earth, bentonite, or clay) can be used to carry the metal chlorite and water-retaining substance (and optionally also the acid or other proton generating species) of the invention. 0072 We next describe other classes of embodiments of the invention. 0073 In one such class of embodiments, the invention is a method for producing chlorine dioxide by activating a mixture comprising zeolite crystals impregnated with sulfuric acid (or other proton generating species), zeolite crystals impregnated with sodium chlorate, zeolite crystals impregnated with an oxidizer, and optionally also zeolite crystals impregnated with calcium chloride (or another deliquescent or water absorbing and retaining substance). The oxidizer is a substance (or mixture of substances) which reacts (in the presence of water absorbed by the deliquescent or water absorbing substance) to release another substance which in turn reacts with one or more of the other impregnating substances to produce chlorine dioxide. In some of these embodiments, the oxidizer is ferric chloride or ferric sulfate, or a mixture of ferric chloride and ferric sulfate. Although each of these iron salts desirably absorbs water in addition to functioning as an oxidizer, ferric chloride is preferably used (rather than ferric sulfate, or a mixture of ferric chloride and ferric sulfate) since it is the most reactive during performance of the inventive method. Where sodium chlorate, ferric chloride, and sulfuric acid are present upon activation, the chlorine dioxide-producing reaction is believed to be: 2Na(ClO3)FeCl3H2SO43H2ONa2SO4Fe(OH)33HCl2H2O2ClO2. In any of this class of embodiments, the activation can be accomplished by exposing the mixture to a moisture-containing gas (e.g., air including water vapor), or (to achieve a higher chlorine dioxide production rate) causing water or a moisture-containing gas to flow through the mixture (or by causing the mixture to move through the gas). When activation is accomplished using a flowing fluid, all the impregnated zeolite crystals can be physically mixed together in a single bed, or the gas can be caused to flow sequentially through distinct beds of subsets of the zeolite crystals. 0074 In other embodiments, chlorine dioxide is produced by activating (with excess protons) a zeolite crystal mixture comprising zeolite crystals impregnated with sodium chlorate, zeolite crystals impregnated with at least one oxidizer (e.g., hydrogen peroxide, ferric chloride, or peracetic acid), and optionally also zeolite crystals impregnated with calcium chloride. Preferably, the mixture comprises equal (or substantially equal) amounts of crystals impregnated with each of the four species. The activation can be accomplished by exposing the mixture to a moisture-containing gas (e.g., air including water vapor), or (to increase the chlorine dioxide production rate) causing water or a moisture-containing gas (e.g., air including water vapor) to flow through the mixture. When activation is accomplished using flowing fluid, all impregnated zeolite crystals can be physically mixed together in a single bed, or the fluid can be caused to flow sequentially through distinct beds of subsets of the zeolite crystals. In embodiments in which the impregnated crystal mixture comprises distinct beds of different subsets of zeolite crystals (e.g., a bed of crystals impregnated with a first substance, and another bed of crystals impregnated with a second substance mixed with crystals impregnated with a third substance), the fluid must flow sequentially through the beds to accomplish activation. 0075 Where sodium chlorate, hydrogen peroxide, and sulfuric acid are present upon activation, the chlorine dioxide-producing reaction is believed to be 2Na(ClO3)H2O2H2SO42ClO2Na2SO4H2O. 0076 In variations on the embodiments described in the previous paragraph, zeolite crystals impregnated with sodium metabisulfite (or sodium bisulfite) are used rather than zeolite crystals impregnated with at least one of hydrogen peroxide, ferric chloride, and peracetid acid. Activation of the mixture by absorbed water (water absorbed by the deliquescent or water absorbing and retaining substance) causes a reaction of the sodium metabisulfite (or sodium bisulfite) which releases sulfur dioxide (SO2). The SO2 then reacts to form chlorine dioxide gas. Each of sodium metabisulfite and sodium bisulfite is considered an oxidizer in the sense that the latter expression is used herein, since each reacts (in the presence of the absorbed water) to release sulfur dioxide which in turn reacts with one or more of the other impregnating substances to produce chlorine dioxide. 0077 In other embodiments, chlorine dioxide is produced by activating a zeolite crystal mixture comprising zeolite crystals impregnated with sodium chlorate, zeolite crystals impregnated with sulfuric acid (or another proton generating species, such as ferric chloride, ferric sulfate, ZnSO4, ZnCl2, CoSO4, CoCl2, MnSO4, MnCl2, CuSO4, CuCl2, and MgSO4 or another metal salt), zeolite crystals impregnated with an oxidizer, and optionally also zeolite crystals impregnated with a deliquescent (or water absorbing and retaining substance). As in other embodiments, activation of the zeolite crystal mixture can be accomplished by exposing the mixture to a moisture-containing gas (e.g., air including water vapor), or (in order to increase the chlorine dioxide production rate) by causing water or a moisture-containing gas to flow through the mixture (or causing the mixture to move through such fluid). When activation is accomplished using flowing fluid, the impregnated zeolite crystals can be physically mixed together in a single bed or the fluid can be caused to flow sequentially through distinct beds of subsets of the zeolite crystals. In some of these embodiments, the deliquescent (or water absorbing and retaining substance) is magnesium sulfate (MgSO4), potassium chloride, or magnesium chloride (MgCl). 0078 In other embodiments, the invention is a method for producing chlorine dioxide by activating an impregnated zeolite crystal mixture with an oxidizer (such as ferric chloride solution (preferably) or liquid hydrogen peroxide or liquid peracetic acid). The zeolite crystal mixture comprises zeolite crystals impregnated with sodium chlorate, zeolite crystals impregnated with sulfuric acid (or another proton generating species), and optionally also zeolite crystals impregnated with calcium chloride (or another deliquescent or water absorbing and retaining substance). 0079 In other embodiments, the invention is a method for producing chlorine dioxide by activating an impregnated zeolite crystal mixture with excess protons, such as by exposing it to liquid sulfuric acid (or another proton generating species). The zeolite mixture comprises zeolite crystals impregnated with sodium chlorate, zeolite crystals impregnated with an oxidizer (e.g., at least one of hydrogen peroxide, ferric chloride, and peracetic acid), and optionally also zeolite crystals impregnated with calcium chloride (or another deliquescent or water absorbing and retaining substance). 0080 In other embodiments, the invention is a method for producing chlorine dioxide by activating an impregnated zeolite crystal mixture with sodium chlorate solution. The zeolite mixture comprises zeolite crystals impregnated with sulfuric acid (or another proton generating species), zeolite crystals impregnated with an oxidizer (e.g., at least one of hydrogen peroxide, ferric chloride, and peracetic acid), and optionally also zeolite crystals impregnated with calcium chloride (or another deliquescent or water absorbing and retaining substance). 0081 Also within the scope of the invention are methods in which mixtures of two or more of the above-described impregnated zeolite mixtures are activated to produce chlorine dioxide. An example is a method for producing chlorine dioxide by activating a zeolite crystal mixture comprising zeolite crystals impregnated with sulfuric acid, zeolite crystals impregnated with sodium chlorate, zeolite crystals impregnated with at least one of hydrogen peroxide, ferric chloride, and peracetic acid, zeolite crystals impregnated with calcium chloride, and zeolite crystals impregnated with sodium metabisulfite (or sodium bisulfite). 0082 In any embodiment of the invention, the rate of chlorine dioxide release (upon activation of the zeolite crystal mixture) can be controlled in any of several ways, including by appropriately selecting the concentration and amount of an activating liquid (e.g., where the activating liquid is sodium chlorate solution or liquid sulfuric acid), using impregnated zeolite crystals having an appropriately selected weight ratio of one or more of the impregnating substances (e.g., iron salt) to zeolite, and selecting an appropriate method for activating the mixture. For example, activation by exposing the mixture to moisture-containing gas typically results in a low chlorine dioxide release rate, and activation by flowing water or moisture-containing gas through the mixture typically results in a higher chlorine dioxide release rate. Prior to activation, the mixture of impregnated crystals should be as dry as possible. Preferably, the mixture (e.g., mixture of impregnated zeolite crystals 40 of FIG. 3) is sealed within a dry, air-tight capsule or other container (e.g., container 41 of FIG. 3), and the container is unsealed to expose the mixture to the activating fluid shortly before activation. 0083 In yet another class of embodiments, the invention is a mixture of impregnated zeolite crystals which can be activated (by any of the above-described liquids or by water or moisture-containing gas in accordance with the invention) to perform any embodiment of the inventive chlorine dioxide production method. 0084 The chlorine dioxide produced in accordance with the invention can be used to kill microorganisms in the moisture-containing gas which activates the crystal mixture, and can oxidize volatile organic chemicals which contaminate the moisture-containing gas. 0085 In another class of embodiments, the invention is composition of matter which carries sufficient amounts of chemicals to be capable of releasing chlorine dioxide to a target region upon activation by excess protons, said composition of matter comprising a quantity of impregnated zeolite crystals including zeolite crystals impregnated with a metal chlorite and zeolite crystals impregnated with a proton generating species, wherein the quantity is capable of being activated by excess protons to produce chlorine dioxide. Zeolite crystals 30 of FIG. 4 are such a quantity of impregnated zeolite crystals. Preferably, those of crystals 30 which are impregnated with a metal chlorite are also impregnated with at least one of a deliquescent and a water absorbing and retaining substance (e.g., they are impregnated with calcium chloride) in a concentration which limits (to a predetermined maximum level) the rate of release of chlorine dioxide to the target region in response to activation of the quantity of crystals with excess protons. For example, zeolite crystals 30 can include zeolite crystals impregnated with metal chlorite, a water absorbing and retaining substance, and an acid (or other proton generating species). Such crystals will be activated by liquid water or atmospheric moisture (which will cause production of excess protons, which in turn will cause chlorine dioxide production). The relative concentrations of impregnating chemicals will control the rate at which excess protons are produced (and thus the rate at which chlorine dioxide is produced) in response to a specific amount of water. 0086 For some applications, the quantity of crystals is separated by a barrier (e.g., barrier 50 of FIG. 4) from the target region. The barrier should be permeable to flow of an activating fluid (e.g., water) from the target region such that the activating fluid will interact with the quantity of impregnated zeolite crystals in a manner resulting in production of excess protons (which in turn results in production of chlorine dioxide). The barrier should also be permeable to flow of the chlorine dioxide to the target region. Preferably, the permeability of the barrier is such that the quantity of zeolite crystals is exposed to no more than predetermined maximum flow rate of the activating fluid (e.g., the barrier is permeable to flow of the activating fluid from the target region at up to a predetermined maximum flow rate), so that the barrier limits to a predetermined maximum level the rate of release of chlorine dioxide to the target region. 0087 Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.