GB2098974A - Iodine removal from a gas phase - Google Patents

Abstract

Iodine, e.g. radioactive iodine, present as one or more organic iodides, optionally with elemental iodine, in a gas phase (e.g. air) are removed by photochemically decomposing the organic iodides to elemental iodine, reacting the iodine produced, and any initially present with excess ozone, preferably photochemically produced in situ in the gas phase to produce solid iodine oxides, and removing the solid oxides from the gas phase. <IMAGE>

Description

SPECIFICATION Iodine removal from a gas phase This invention is directed to the removal of iodine, usually present at least partially as organic iodides, from various gas phases including air. The removal of radioactive iodine and iodides is of particular interest, but any isotope of iodine can be processed.

In some industrial operations, the removal of the halogen iodine in various forms from gaseous mixtures, is desirable. In the nuclear industry especially, this removal has become a necessary procedure.

Radioactive iodines (1291 and 1311) are produced in nuclear fuel by fission and by the decay of other fission products. These radionuclides are released, primarily as elemental iodine (12) and organic iodides (CH3l, C2Hsl, C6H51, etc.), in the off-gas streams of nuclear fuel recycle facilities. They can also be released in the off-gas streams of nuclear reactors from failed fuel elements, during routine operation or in an accident. In order to ensure that environmental release limits are satisfied, the radioiodines must be removed from the air, retained in stable chemical forms and disposed in safe environment until they become harmless by natural decay.

Various processes have been developed for the removal of gaseous radioiodines from air. The most important ones are: (i) adsorption on activated charcoals, (ii) scrubbing with caustic solutions, (iii) scrubbing with Hg(N03)2 solutions (MERCUREX) - see U.S. Patent 3,852,407, December 3, 1974, Schmitt et al, (iv) scrubbing with concentrated HNO3 solutions (IODOX) -- see U.S. Patent 3,752,876, August 1 4, 1973, Cathers et al, and (v) adsorption on silver loaded sorbents - see U.S. Patent 4,088,737, May 9, 1978, Thomas et al, and Canadian Patent 1 ,077,458.

Certain cross-linked anion exchange resins have been used also - see U.S. Patent 3,943,229, March 9, 1976, Keener et al. Although most of the above methods appear to meet the required decontamination factors, there are a number of disadvantages associated with each one of them. For instance, (i) and (ii) are ineffective in removing the organic forms, while (iii) and (v) can retain aliphatic iodides but not aromatic ones. The IODOX process requires highly concentrated No, which is difficult to handle. The efficiency of silver-loaded sorbents (v) is affected by impurities present in gas or air streams. In addition, silver-loaded sorbents are expensive. Finally, a general disadvantage of each of the above methods is the requirement of large volumes of solid sorbents, resins, or liquids with which to remove the radioiodines.These sorbents must either be recycled or disposed of as contaminated waste and either route adds to the process complexity and cost. Therefore, development of simpler and more selective methods of removal of gaseous radioiodines from air would be of benefit to the nuclear industry.

The iodine-ozone reaction, either in the vapour or in carbon tetrachloride, is known as a means for the preparation of 1409 and 1205 (G. Bauer, editor, "Handbook of Preparative Inorganic Chemistry" Vol.

1, 2nd ed., Academic Press, New York, 1963; H. J. Emeléus and A. G. Sharpe, editors, "Advances in Inorganic Chemistry and Radiochemistry", Vol. 5, Academic Press Inc., New York, 1963; and K. Selte and A. Kjeksus, "iodine Oxides Part II, On the System H30-1205,,, Acta Chem. Scand. 22, 3309, 1968).

Solid iodine oxides have also been observed in reactions of oxygen atoms with iodine (D. l. Walton and L. F. Phillips, "The Reaction of Oxygen Atoms with Iodine", J. Phys. Chem. 70, 1317, 1966). L. C.

Glasgow and J. E. Willard ("Reactions of Iodine Excited with 185-nm Radiation. lil. Reactions with Hydrogen, Methane, Trifluoromethane, Chloromethane, and Oxygen, Mechanistic Tests", J. Phys. Chem.

77, 1 585, 1 973) observed that, in the gas phase, approximately four ozone molecules were consumed per 12 molecule reacted and a solid yellow product was deposited on the walls of the reactor. Also, W. F.

Hamilton et al ("Atmospheric Iodine Abates Smog Ozone", Science 140, 1 90, 1 963), interested in the application of the reaction in removing traces of ozone from aircraft cabins and other enclosed atmospheres, observed that low concentrations of iodine ( 3 x 10-9 mol/L) were effective in reducing about ~ 2 x 10-8 mol/L of 03 by a factor of nearly ten in a few minutes. These references make no suggestion that such a reaction could remove iodine from a gaseous mixture.

Recently a corona iodine scrubber (C.l.S.) method has been developed to remove radioactive iodine from air (D. F. Torgerson and I. M. Smith, "AECL Iodine Scrubbing Project", Proc. of 15th DOE Nuclear Air Cleaning Conference, August 1978, CONF-78081 9). In this CIS method the entire bulk of the air effluents containing radioactive iodines is subjected to a high voltage discharge and 1409 is formed along with other reaction products.

The present invention provides a method of removing iodine (12) from gaseous mixtures including same, comprising reacting 12 with ozone 03 to form solid iodine oxides, and separating the solid oxides, sufficient ozone being provided to react with all iodine present.

The invention includes a method of removing iodine from a gas phase which includes organic iodides, comprising: (a) irradiating the gas phase with radiation capable of photodecomposing the iodides present to elemental iodine; (b) providing sufficient ozone in the gas phase to react with all elemental iodine present to form solid iodine oxides, allowing this reaction to proceed, and (c) separating the solid iodine oxides and recovering the gas phase free of iodine.

The solid iodine oxide rapidly deposits out of the gas phase and can be recovered if necessary.

Where the gas phase contains oxygen, the ozone can be generated in situ photochemically.

Alternatively, the ozone can be generated elsewhere and fed to the gas phase for the step (b) reaction.

The elemental iodine released in step (a), and any elemental iodine already present reacts with ozone to yield solid iodine oxides which quickly separate from the gas phase.

Advantages of this method are: (i) It is applicable to the removal of all organic radioiodines as well as to the removal of elemental iodine.

(ii) A simple scrubber (no internal components), which is easily adaptable for remote operation in radioactive environments, can be designed.

(iii) It avoids the need of solid sorbents, liquid or other substrates which result in complex handling procedures, become poisoned by the accumulation of air impurities (NOx, H20, CO2, RH) and must be disposed, after a few regenerations, as contaminated nuclear waste.

The single figure of the accompanying drawing illustrates schematically the photochemical method. Ultraviolet light is used to selectively decompose the organic radioiodines (RI). The released elemental iodine (12) and the 12 already present in the air are reacted with 03 to form solid 1205 which deposits on the walls of the scrubber, leaving the air iodine-free. Scrubber &num;1 can be valved in to replace &num;2 when the latter is saturated with 1205, or &num;1 could be operated in parallel to increase throughput.

The gas phase to be treated can be any gas produced in industrial processes or present in nuclear reactor or nuclear fuel reprocessing environments. Usually the gas phase will comprise air but other gases such as NOX, H20, CO2, hydrocarbons and rare gases may be encountered.

Organic iodides such as methyl iodide, ethyl iodide, phenyl iodide, are present at least to some extent, and elemental iodine will also be present in most cases. Of most concern are radioactive gases where the radioactivity is at least partly due to radioactive isotopes of iodine, particularly 1291 and 1311.

The gas phase containing organic iodides (RI) is first exposed to radiation capable of photodecomposing the iodides present to elemental iodine (12). Ultraviolet radiation wavelengths less than 300 nm is sufficient. Preferably the radiation should be in the range of 220-300 nm, where air does not absorb, and it should be most intense near 260 nm where the organic iodide absorption is maximum. Any suitable source of such radiation may be used, e.g. mercury-vapour lamps or lasers. The extent of decomposition of the organic iodides is a function of the intensity of the radiation and of the residence time of the stream in the illuminated zone. In the presence of oxygen, as in air, the organic group (R) will be oxidized to the corresponding alcohols and aldehydes. The iodine atoms will form elemental iodine (12).The temperature for this reaction is not critical: room temperature normally is preferred.

The iodine from the photodecomposition, plus any elemental iodine present initially, is reacted with ozone in step (b). The ozone can be generated in a separate generator and fed to the gas phase.

Preferably, ozone can be fed continuously to a gas stream moving through a reaction zone. Where the gas phase contains oxygen, (as in air), the ozone can be generated in situ by irradiating with ultraviolet radiation less than 220 nm in wavelength. The iodine reacts with ozone to yield solid iodine oxide which deposit out of the gas phase. The initial oxide formed is believed to be 1405, but if this oxide is heated to, or the reaction carried out at, about 1 20--1 40 OC then 1205 is believed to be formed exclusively. The ozone is provided in excess to assure reaction with all of the iodine.

Reaction (a), ozone generation, and reaction (b) can be carried out concurrently by providing that oxygen is present in the gas phase and irradiating with radiation comprising e.g. both of.the UV wavelengths about 254 nm and about 185 nm, normally obtained from low-pressure mercury-lamps.

The combined reaction can be carried out at room temperature, in which case the 1405 is formed, or in the temperature range of 1 00#2000 C, in which case the 1205 is formed. Fixation of the iodine in the more stable 1205 form is preferred.

The deposits of the iodine oxides form on any surfaces in the reaction zone. The deposits can be removed by physical or chemical means, such as by heating to 3000C or more, in which case the iodine is recovered as elemental iodine (I,), or by washing with water, in which case the iodine will be recovered as aqueous 103. Where the iodine oxides are radioactive, the concentrated forms recovered above can be disposed of by a number of methods which are beyond the scope of this invention.

The following examples are illustrative.

EXAMPLE I It is shown in this example that elemental iodine (12) reacts with ozone (03) to yield solid 1405. This reaction can be used per se to remove elemental iodine from air or other gas, but it is also considered to be a key reaction in the abatement of organic iodine from air by the photochemical method disclosed herein.

The i2~03 reaction was studied in a flow system using oxygen as the carrier gas at room temperature (20-250C), a pressure of 100 kPa and flow rates in the range of 1 to 17 cm3(NTP)#sec-1.

The reaction vessel was made out of glass in the form of a cylinder 50 cm long and 2.5 cm I.D. The concentration of iodine in the reaction vessel was controlled in the range of 10-6 to 10-5 mol/L by varying the temperature of a fine iodine saturator, and also, by varying the oxygen flow rate through the system. Ozone was generated separately in oxygen by a corona discharge, and entered the reactor through a 1 -mm diameter nozzle at concentrations of 10-5 to 10-4 mol/L. The concentration of ozone was monitored immediately past the reaction vessel by its absorption at the wavelength 253.7 nm using an on-line spectrophotometer.

The efficiency of iodine removal by reaction with ozone was determined by condensing the unreacted iodine in a trap located past the ozone detector. The trap was packed with 2-mm diameter glass beads and was cooled to -780C with a dry-ice and acetone bath. The accumulated iodine was subsequently dissolved in CCI, and analysed spectrophotometrically in the visible absorption band.

The faint violet colour of gaseous iodine was observed to disappear on reaction with ozone immediately past the ozone nozzle and a visible lemon-yellow powder was observed to form in the gas phase and settle on the walls of the reaction vessel. When heated to between 1 000C and 2000C with a heat gun, the solid deposit released iodine and turned white. The remaining white solid on the reaction vessel walls decomposed entirely to its constituents (13 and 02) when heated in the range of 400 to 5000C with an oxygen-natural gas flame. The latter procedure was routinely employed for the removal of the iodine oxide deposits. Data relating to the efficiency of iodine removal and to the reaction rate, are given in Table 1.

TABLE 1 REACTION RATE DATA*

03 Concentration 12 Concentration Reaction (yL/L) (yL/L) Time Initial/Final Initial/Final (s) 23.2/8.4 10.3/2.6 21 21.5/9.3 10.7/3.1 21 34.0/20.5 10.8/1.6 21 60.4/41.3 10.8/0.04 21 82.4/67.2 7.5/0.15 24 60.5/40.0 7.4/0.15 23 44,0/23.2 7.2/0.19 24 101/78.5 16/ < 0.03 66 156/ ** 18/ < 0.09 100 * These data were obtained in a flow system using oxygen as the carrier gas, at room temperature (#250 C), a total pressure of 100 kPa and flow rates in the range of 3-1 7 cm3 (NTP)~s-' ** Not measured.

The initial reaction product was shown by elemental analysis to have the stoichiometric composition of 1405. Its behaviour as a function of temperature was stuidied by thermogravimetric analysis. It was shown that 1405 solid decomposed to 13 and 1205 (also a solid) when heated to more than 1000C, according to equation (1).

The 1205 subsequently decomposed to 12 and 02 when heated about 3000C.

The stoichiometry of the l2#03 reaction was obtained by measuring the amount of ozone consumed per iodine molecule removed from the gas-phase. This was done under conditions of excess ozone and after allowing sufficient reaction time for more than 99% removal of the initial iodine. It was thus shown that 3.7 + 0.1 molecules of ozone were consumed per iodine molecule fixed.

Experiments to determine the rate of the i2~03 reaction were carried out as follows. A flow system at room temperature and total pressure of 100 kPa, with nitrogen: oxygen 2:1 as the carrier gas, was used. A cylindrical reaction vessel (55 cm long and 2.2 cm I.D.) and ozone analysis cell of 10.9 cm path length were used with other details as in Example I. Rate measurements were done with initial I pressures of 2 to 10 Pa, and initial 03 pressures of 20 to 100 Pa. Reaction times ranges from 32 to 120 seconds.

A simplified form of the rate law in integrated form, when ozone is in excess, can be written

where t = vol. of scrubber/flow of gas during time t, k = rate constant, DF = decontamination factor, and subscripts i and t refer to initial and final (at time t) concentrations.

Data were obtained from measurements of the rate of 12 consumption (and also from rate of 03) consumption). The rate constant was calculated to be (1.5 + 0.1) x 103 dm3 mol-"s-'. This value fork can be used to estimate decontamination factors for various situations.

EXAMPLE II Abatement of both elemental and organic iodine from air by photochemical means is demonstrated in this example. Under illumination with ultraviolet radiation (comprising 254 nm) the CH3l present decomposed to iodine and methyl radicals. In the presence of ~10~5 mol/L of ozone, which was generated by the absorption of ultraviolet radiation (comprising 185 nm) by the oxygen in air, the iodine liberated from the CM31, and any other elemental iodine, reacted at a temperature of 120-1 400C to yield 1205, according to the discussion in Example I. The CH3 radicals were oxidized in air to form paraformaldehyde primarily.

Dry air was used as the carrier gas at flow rates in the range of 8-45 cm3 (STP) s-#. Iodine and methyl iodide concentrations in the range of 1~50yL/L of air were obtained by saturation of separate air streams which mixed with the main carrier stream before the scrubber. The total pressure in the scrubber was 100 kPa. The CH31 concentration was monitored initially with an on-line quadrupole mass spectrometer which was later substituted with a gas chromatograph equipped with an electron capture detector. Elemental iodine and ozone were monitored as described in Example I. A Westinghouse [ trademarki Model G37T6VH 39 W mercury lamp was used as a source of ultraviolet radiation. This lamp was tubular (79 cm in length, 1.6 cm O.D.) and emitted radiation at 254 nm which photodissociated the organic iodide, and also radiation at 1 85 nm which was absorbed by the oxygen in air to generate ozone.

TABLE 2 Photochemical Decontamination Data for CH3l and 12

Before Scrubber After Scrubber Decontamination Factor CH31 12 CH31 12 CH3l 12 (microliters per liter of air) 0 12.7 0 < 0.05 - > 250 0 25.4 0 < 0.05 - > 500 0 27.3 0 0.12 - 230 0 39.0 0 0.29 - 135 4.87 0 0.17 < 0.05 29 > 50 9.85 0 0.22 < 0.05 44 1100 8.0 0 0.13 - 62 9.5 0 0.073--0.125 - (76-130) 10.6 0 0.71 - 150 20.0 0 0.067-0.087 - (230-300) 23.5 0 O 0.10 230 - 32.0 0 0.076 - 420 40.0 0 0.053-0.077 - (520-750) 2.42 7.17 0.008 < 10-03 290 > 280 5.14 25.5 0.10 0.245 110 10.4 23.6 0.21 0.12 49 240 (a) These data were obtained at a total flow-rate of 42 cm3 (STP)s-', a total pressure of 100 kPa and a temperature of 120-1400C.

The scrubber was made entirely of quartz by glass-blowing concentrically two quartz tubes. The inner one was 2.2 cm O.D. and 2.0 cm l.D., and the outer one was 8.0 cm O.D. and 7.5 cm l.D. The overall length of the scrubber was 85 cm. The lamp, in this case, fitted into the cavity of the inner tube and thus was not in contact with the gases inside the annular volume of the scrubber. The scrubber was heated to a temperature of 120-1 400C with electrical tape wound around the outer surfaces of the scrubber.

Representative data on the removal of 12, CH3 I and mixtures of the two from air, obtained from this system, are given in Table 2. The decontamination factor is defined as the concentration of the species before the scrubber divided by the concentration of the species after the scrubber. No volatile iodine products, other than residual CH3 and 12, were detected in the scrubber effluents. Instead, solid deposits of 1205 were observed to plate out on the walls of the scrubber. These deposits were removed following each run by washing with water.

The stability of the iodine oxide solids, with respect to release of 12, following deposition in the scrubber was also tested. After the deposition of 1.1 x 10-4 mol 12 over a period of 44 min, the I flow and the light source were turned off and the air flow through the scrubber was maintained at 40 cm 3 (STP) ~ s~1. Over a period of 16.7 h, a total of ~ 3 x 1 mol 12 (s 0.2% of the deposited i2) was collected downstream of the scrubber. The small amount of 12 collected may have been due to the degradation of the deposits or to unreacted 12 which was adsorbed on the surfaces of the scrubber. Even if the release was entirely due to the degradation of the deposits, it is too low to affect the separation of 12 from air by this method.

Nitrogen dioxide (NO2), which is a common impurity in the off-gas streams of nuclear fuel recycle facilities, reacts fast with 03 to form N205. It consumes one 03 molecule for every two NO2 molecules and thus impedes the fixation of 12 with 03. It was, however, shown in this study that as long as sufficient excess 03 is maintained in the system, the efficiency of the 12~03 fixation step remains unabated.

The only effect of H20 vapour predicted and also confirmed in this study is the conversion of the 1205 to its hydrated forms (Hl03, He308) which are also non-volatile products.

While study of the detailed mechanism of operation of the photochemical scrubber is beyond the scope of this invention, it is instructive to comment on the basic reactions thought to be responsible for its operation. The following non-limiting mechanism is, therefore, proposed.

It has already been shown in Example I that elemental iodine reacted stoichiometrically with ozone to yield 1409 and that 1409 decomposed to 1205 and 12 at temperatures in excess of 1 000C. Thus the net reaction in excess ozone, in the range of 120-1 400 C, is the conversion of 12 to 1205.

The conversion of CH31 to elemental iodine and the oxidized form of the methyl radical (paraformaldehyde) must be the result of the photo-dissociation of the CH3l following absorption of 254 nm radiation which is not absorbed by any of the other components (N2, 02, CO2) in air. The CH3l also absorbs the 185 nm radiation, however, this radiation is absorbed strongly by the oxygen in air, therefore, this radiation is not expected to contribute significantly to the CH31 photodecomposition at the low partial pressures of CH31 employed here.

Following dissociation of CH3l to Cm, and I, the iodine atoms, which do not react with 02, recombine to give 12 and the CH3 radicals react with oxygen to yield paraformaldehyde as the final product. In the presence of ozone generated by the 1 85 nm radiation the 12 is fixed as 1205 which is quite stable, as discussed above.

It should be observed that the efficiencies of removal of CH31 and 12 from air as well as the scale of operation were intended for demonstration of the concept only and not for optimum scavenging. Higher removal efficiencies can be achieved by increasing the intensity of ultraviolet radiation, such as by using more lamps or a more powerful lamp in the scrubber, and/or by using more then one scrubber in series.

Similarly, the scale of the operation can be amplified, e.g. with higher ultraviolet light intensities and/or by running several such scrubbers in parallel. Other scale-up options and other variations will be known to those skilled in this art. Furthermore, it is obvious that although the examples were conducted with non-radioactive iodine (1271) compounds, the same would apply to compounds of iodine-129 and iodine131.

Claims (9)

1. A method of scavenging iodine from a gas phase containing one or more organic iodides and optionally elemental iodine, comprising: (a) irradiating the gas phase with radiation capable of photodecomposing the organic iodides present to elemental iodine, (b) providing sufficient ozone in the gas phase to react with all elemental iodine present to form solid iodine oxides, allowing this reaction to proceed, and (c) separating the solid iodine oxides and recovering the gas phase free of iodine.

2. A method according to claim 1, wherein the gas phase is air.

3. A method according to claim 1 or 2 wherein ozone is generated at a separate location and fed to the gas phase for step (b).

4. A method according to claim 2, wherein the ozone is generated photochemically in situ in said gas phase.

5. A method according to claim 4, wherein the ozone generation is effected by radiation of the gas phase with ultraviolet light of less than about 220 rim wavelength with a component near 185 nm.

6. A method according to claim 4 or 5 wherein the air is irradiated simultaneously with radiation capable both of photodecomposing the iodides present and of photochemically generating ozone, and steps (a) and (b) proceed concurrently.

7. A method according to any one of the preceding claims wherein said photodecomposition is effected by radiation of the gas phase with ultraviolet light of about 220-300 nm wavelength with intense component near 260 nm.

8. A method according to any one of the preceding claims wherein the said iodine is or comprises a radioactive iodine isotope.

9. A method according to any one of the preceding claims wherein the solid iodine oxides are removed in step (c) by heating or by washing with water.