CA1172828A - Iodine removal from a gas phase - Google Patents

Abstract

TITLE
IODINE REMOVAL FROM A GAS PHASE
INVENTOR
Andreas Ch. Vikis ABSTRACT OF THE DISCLOSURE
Iodine, usually at least partially in the form of organic iodides, can be removed from a gas phase by irradiating the gas with radiation selected to photodecompose the iodides present until organic iodides have photodecomposed to elemental iodine, providing that ozone is present in the gas phase and reacting all iodine with ozone to form solid iodine oxides which separate from the gas phase. If no organic iodides are present, the initial photodecomposition can be omitted, Where the gas phase contains oxygen the ozone can be generated photochemically in situ. Alternatively, the ozone can be generated separately and fed to the gas phase reaction zone. The method has application to removing radioactive iodine from air and other gases.

Description

:
'7~ 2~
This invention is di.rec~ed to ~he removal of iodine, usually present at least partially as organic iodides, from various gas phases including air. The remov~l of radioactlve iodine and iodides is of particular in~erest, but any isotope of iodine can be processed.
: Background and Prior Art In some industrial operations, the removal of the halogen i~dine in various forms from gaseous mi~tures, is desirable. I~ the nuclear industry especially, this removal has become a necessa~ry . procedure.

Radioactive iodines (129I and I~ are produced in nuclear ; fuel by fission and by the decay of other fission products. These radionuclides are released, primarily as elemental iodine (I2) and organic iodides (CH3I, C2H5I, C6H5I, 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 environ-. mental release limits are satisPled, the rad:l.o:Lodines must be remove~l from the air, reta:Lned in stable chemlcal forms and dlsposed in saPe envlronment un~il they become harmless by naturnl decay.
Varlous processes have been developed Eor the removal of gaseous radioiodines from airO 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 HN03 solutions (IODOX) - see U.S.
.` Patent 3,752,~76, ~ugust 14, 1973, Cathers et al, and (v) adsorp~ion 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 anlon exchaage resins ha-ve been used also see U.S. Patent 3,943,229, March 9, 1976, Keener et al. Although ~ost of the above methods appear to meet the required decontaminatlon actors, there are a number of disadvantages associated with each one o the~.
For instance, ~i) and (ii) are ineffective in removing the organic lorms, while (iii) and (v) can re~ain aliphatic iodides but not aromatic ones. The IODOY. process requires highly concentrated HN03 which is difficult to handleO The efficiency of silver-loaded - sorbents (v) is affected by impurlties present in gas or air streams.
In addition, silver-loaded sorbents are expensive. Finally, a general disadv~ntage of each of the above methods is the requirement of large volumes of solid sorbents, resins, or liquids with which to remove the radioiodlnes. 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 o~

benefit to the nuclear industry~

The iodine-o~one reaction, elther ln the vapour phase or in i~ carbon tetr~lchlorid¢, i8 known ~ a means for the prepara~lon of 1~09 and I205 ~G. Brauer, editor, "Handbook oE Preparative Inorganlc Chemistry" Vol. 1, 2nd edlS Academic Press, New York, 1963; ~I.J.

Emeleus 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 H20- I205", Acta Chem~ Scand. 22, 3309, 1968). Solid iodine oxides - have also been observed in reactions of oxygen atoms with iodine (D.I.

Walton and ~.F Phillips, "The Reaction of Oxygen Atoms with Iodine", J. Phys. Chem. 70, 1317, 1966). I,.C. Glasgow and J~E. Willard 5"Reactions of Iodine Rxc-ited with 185-nm Radiation. III. Reactions with Hydrogen, Methane, Trifluoromethane, Chloromethane, and Oxygen~

Mechanistic Tests", J. Phys. Chem. 77, 1585, 1973) observed that7 in the gas phase, approximately four ozone molecules were corlsumed per I2 molecule reacted and a solid yellow product was deposited on the walls of the reactor. Also, W.F~ ~amilton et al ("Atmospheric Iodlne Abates Smog Ozone"9 Science 140, 190, 1963), interested in the application of the reaction in removing traces of ozone from aircraft cabins and other enclosed atmospheres 9 observed that low concentra-; ~ .
tions of iodine (~ 3 x 10 9 mol/L) were effective in reducing about ~2 x 10 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.I.S.) me~hod has been developed to remove radioactive iodine from air (~.F. Torgerson and ` I.M. Smith, "AECL Iodine Scrubbing Project", Proc. of 15th DOE Nuclear Air Cleaning Conference, August 1978, CONF-780819). In this CIS
method the entire bulk of the air effluents containlng radioactive .~r iodines is subjected to a high voltage discharge and I409 is formed along wlth other reactlon products.
Summary of the InventLon The present invention provides a method of removing iodine (I2) from gaseous mixtures including same, comprising reacting I2 with ozone 03 to form solid iodine oxides, and separating the solid - oxides, sufficient o~one 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 capahle o-f photodecom-poslng the iodides present to elemental iodine;

~ ~'7 ~J~ ~ ~

(b) providing suffLcient ozone in the gas phase to react with all elemental iodlne present to form solld lodlne oxldes~ allowlng this reaction to proceed, and (c) separating the solid iodlne 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 ~he gas phase contains oxygen, the ozone can be generated in situ photochemically. Alternatively5 the ozone can be generated elsewhere and fed;to the gas phase Eor the step (b) reaction. The elemental iodine released in step ~aj, and any . .
elemental iodine already present reacts with o~one to yleld solid iodine oxides whlch quickly separate from the gas phase.
, Advantages of this method are:
(i) It is appllcable to the removal o all organlc rad-loiodines as well as to ~he removal of elemental iodine.
~ ii) A simple scrubber (no lnternal components), which is easily adaptable for remote operatlon in radloactive environments, can be - designed.
, (iii) It avoids the need o~ soli,d so~bents, llqulcls or other suh-strates whlch result in complex ~andling procedures "~ecome poisoned by the accumulatlon ~f air lmpurlties (N0x, ~120, C0~, ~1) and must be disposed~ after a few regenerations, as contaminated nuclear waste.
Description of Drawing The single drawing illustrates schematically the photochemi-cal method. Ultraviolet light is used to selectively decompose the organlc radioiodines (RI). The released elemental iodine ~I2) and the I2 already present in the air are reacted with 03 to form solid I205 which deposits on the walls of the scrubber, leaving the air lodine-free. Scrubber #1 can be valved ln to replace #2 when the latter is saturated with I205) or #1 could be operated in paral]el to increase throughput.

. - ~
'7~

Detailed ~escription The gas phase to be treated can be any gas produced in industrial processes or present in nuclear reactor or nuclear ~uel reprocessing environments. Usllally the gas phase will comprise air .
but other gases such as NO , H20, C02, 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 129I and 1 lI
The gas phase containing organic iodides (RI) is first ~'r exposed to radiation capable of photodecomposing the iodides present to elemental iodine (I2) Ultraviolet radiation wavelengths less ....
than 300 nm is sufflcient. Preferably the radiation should be in the - range of 220-300 nm, ~here air does not absorb, and lt should be most :
- intense near 260 nm where the organLc iodlde absorptLon i6 maxLmum.
Any suitable source oE such radiatlon may be used, e.g. mercury-vnpour lamps or la-~ers. The extent of clecompositlon of the organic iodides is a f~nction o~ the intenslty of the radiation and of the resldence time of the stream in the illuminated zone. In the presence o oY.y-gen, as in air, the organic group (~) will be oxidized to the corre-sponding alcohols and aldehydes. The lodine atoms will form elemental iodine (I2). The tempera~ure for this reaction is nat critical:
room temperature normally i5 preferred.
The lodine from the photodecomposition, plus any elemental iodine present inltially, is reacted with ozone in step (b). The ozone can he generated in a separate generator and fed to the gas phase. PreferablyJ ozone can be fed continuously to a gas stream moving through a reactLon zone. Where the gas phase contains o~ygen, (as in air)~ the ozone can be generated in situ by lrradiatlllg with ~,;' ", ~ ~ 7 ~ J~
. ' .
ultraviolet radiatLon less than 220 nm in wavelength The iodine reacts with ozone to yield solid iodine oxides which ~eposit out of the gas phase. The initial oxide formed is believed to be I40~) but if thi~ oxide is heated to, or the reaction carried out at, about 120-140C then I205 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 I409 is formed, or in the temperature range of 100~200~C, in which case the I205 is Eormed. Fixation of the iodine in the more stable I205 form is preferred.

The deposits of the iodine oxides form on any suraces in the reaction zone. The deposits can be removed by physJc~l or chemlcal means, such as bg heating to 300C or more, Ln whlch casP the iodine is rbcovered as elemen~aL Lodine (I2), or by washing wlth water, in which case the iodine will be recovered ~8 aqueous I03.

Where the iodine oxides are radioactive, the concentrated forms recovered above can be disposed of by a number of methods which are be~7ond the scope of this invention.
The following examples are illustrative.
Example I

It is shown in this example that elemental iodine (I2) r ac~s with oæone (03) to yield solid I~09. This reaction can be used ~ se to remove elemental iodine from air or other gas, but - lt is also considered to be a key reaction ln the abatement of organic iodine from air by the photochemical method dlsclosed herein.
....~,_ ~7~

The I2-03 reaction was studied in a flow sy~tem uslng ~ oxygen a~ the carrier gas at room temperature (20-25C~, a pressure of ,; .
,, 100 kPa and flow rates in the range of l to 17 cm (NTP)~sec . The reaction vessel was mad~ out of glass in tlle form of a cylinder 50 cm , long and 2.5 cm I.D. The concentration of iodine in the reaction ves-sel was controlled in the range of 10 to 10 mol/L by varylng the temperature of a fine iodine saturator, and also~ by varying the oxygen flow ràte thro~lgh the system~ O~one was generated separately ' in o~ygen by a corona discharge, and entered the reactor through a ,, 10 l-mm diameter nozzle at concentrations of 10 to 10 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 lodine removal by reaction wit'h o~one was cletermined by condensing the unreacted iodine ir- a trap located past the ozone detector. The trap was packed with 2-mm diameter glass beads and was cooled to -73C with a dry~ice and ~lcetone bath. The accumulated iodlne was ~ubseq~ently dlssolved in CC14 and anfllysed spectrophotometrLcally in the vislble absorp~ion bancl.
The faint violet colour of gaseous iodine was observed to disappear on reaction with ozone i~ediately past the ozone nozzle and a visible lemon-yellow powder was observed to form in the gas phase and settle on the walls of ~he reaction vessel. When heated to between 100C and 200C with a heat gun, the solid deposit released , iodine and ~urned white. The remaining white solid on the reaction ; vessel walls decompos2d en~irely to its constituents (I2 and 2) ;~ when heated in the range of 400 to 500C with an oxygen-na~ural gas '~ flame. The latter procedure was routinely employed for the removal of the iodine oxide deposlts. Data relating to the efficiency of iodine removal and to the reaction rate, are given in Table 1.

:' .

1~ '7~B~

:
,. .
~ .
.

TABLE l _ TION R~TE DATA*

.' . .
.
_. . . ~ _ .. _.
03 Concentra~ion I~ Concentration Reaction .(~IL) ~LfL) Time Initlal/Final . Initial/Final (s~
__ .. ,.. ~ ___ . .23.2/8.~10,3J2.6 21 21.5/9.310.7/3.~ 21 3~l.0/20,5l~.~tl.6 21 : 60.~ 1.310~8/0.04 21 .
82.4/G7,27~5/0~15 24 : 60,~ 0,07.~/0.15 ~3 .O/Z3.27.2/0119 2l.
~01/78.516/~ 0,03 66 156/ -- **~jc 0,0~ ~.00 ___~ ~_ __ _ * These data were obtained In a flow system uslng oxygen ~he carrier gas, at room temperature (20-25C~, a total pressure of l00 kPa and flow rates ln ~he range of 3-17 cm3(NTP~s 1 ** Not n)easured~
;-`
:

Ihe initial reaction product was shown b~ elemental analysis ; to have the stoichiometric composition of I409. Its behaviour as a function of temperature was studied by thermogravimetric analysis. It was shown that the I409 solid decomposed to I2 and I205 (also a , solid) when heated to more than 100C, according to equation (1).
5I409 (s) >~1~0 > 9I205 (s) ~ I2 (g) (1) , .
The I205 subsequently decomposed to I2 and 2 when heated above 300C, 2I25 (s> ~ ~ ~ 2I2 (g) ~~ 52 (g) (2) The stoichiometry of the I2-03 reaction was obtained by measuring the amount of ozone consumed per iodine molecule removed from the gas-phase. This was done 11nder 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 ; o~one were consumed per iodine molecule Eixed.
Experiments to determlne the rate O~ the I2-03 reactlon were carried out as fol:low~. A flow system ~t room temperat-lre and total pressure of 100 kPa, with nLtrogen: oxy~4en 2:1 as the carrier gas, was used. A cy1indrical reaction vessel (SS cm 1ong 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 I2 pressures of 2 to 10 Pa, and initial 03 pressures of 20 to 100 Pa.
Reaction times ranged from 32 to 120 seconds.
'- A simplified form of the rate law in integrated form, when ozone is in excess, can be written ln [I 1 = k[03]it = ln DE

where t = vol. of scrubber/flow of gas du-rillg time t, k = rate con-stant~ DF = decontamination factor, and subscrlpts 1 and t refer to initial and final (at time t) concentrations.
_9_ ~ ~t7~

; Data were obtained from measurements of the rate of I2 consumption (and also from rate of 03 consumption). The rate con-stant was calculated to be (1.5 -~ 0.1) x 10 dm3 mol 1s 1. This value for k can be used to estimate decontamination ~actors for various situations.
Example II
Abatement of both elemental and organic iodine from air by - photochemical means is demonstrated in this example. Under lllumina-tion with ultraviolet radiation (comprising 254 nm~ the CH3I present decomposed to iodine and methyl radicals. In the presence of -10 Mol/h of ozone, which was generated by the absorption of ultra-vivlet radiation (comprising 185 nm3 by the oxygen in air, the iodine liberated from the CH3I, and any other elemental iodine, reacted at , according to the discus-- a temperature of 120-140C to yield I205 ; sion in Example I. Ihe CH3 radicals were oxidized in air to form paraformaldehyde primarily.
Dry air was used as the carrier gas ~t flow rates Ln the range of 8-45 cm3(STP) 6 1, Iodine and methyl lodide concentratLolle in the range of 1-50~L/I oE air were 0l7tairle~ by saturatlon oi: ~epa-rate alr streams which m:Lxed with the maLn carrier stream before the scrubber. The total pressure in the scrubber was 100 kPa~ The CH3I
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 [trademark] Model G37T6VH 39 W mercury lamp was used as a source of ultraviolet radiation. This lamp was tubular (79 cm in length, 1.6 crn O.D.~ and emitted radlation at 254 nm which photodi6sociated the organic iodide, and also radiation at 185 nm whlch was absorbed by the oxygen in air to generate ozone.
~he scrubber was made entLrely of quartz by glass-blowing :

. I'able 2 Photochemical ~econtamin~t~on Data for CH3I and I2 . . ~ .

_, . _ .~ Before Scrubber After Scrubber Decontaminatlon Factor _ _ _ . ., . (microliters per liter of air) : O ~- 1 O ~O~QZ _ ? 2~0 025. Ll 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 . ~;85 0 0.22 < 0.05 l~4 > 100 : . 8.0 0 0.13 _ 62 .~
9.5 0 0.013-0.125 _ (76-130) 10.6 0 0~071 ~ 150 ~.
20.0 0 o.o67 0.087 _ (230-300) ;~ 23.5 0 0.10 ~ 230 ..
., . 32.0 0 0.076 ~ l~20 ,~ llO.O 0 0.053-0.077 _ (520-750)

2.427.17 0.008 < 0.03 290 > 280 5.1425.5 0.10 0.245 52 110 .~ . 10.423.6 0.21 0.1Z 49 240 (a) These data were obtained at a total flow-rate of 42 cm3(STP)s 1, a total pressure of 100 kPa and a temperature of 120-140C.

:

, ....

concentrically two quartz tubes. The inrler one w~s Z.2 cm O.D and 2.0 cm I~D., and the outer one was 8.0 cm O.D. and 7.5 cm l.D. The over-al1 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 ~1as heated to a temperature of 120-140C with electrical tape woùnd around the outer surfaces of the scrubber.
Representative data on the removal of I2, ~13I and mix-tures 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 C~13I and I2, were detected itl the scrubber effluents.
Instead, solid deposits of I205 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 iodlne oxide 60lids, ~lth respect to release of I2, following deposltion ln the scrubber was also testecl.

After the. depositlon of 1.1 x 10 ~ mol t2 over a perlod oE l~l~ mln, thc I2 flow and the light source were turned oEf and the air flow through .
the scrubber-was maintained at 40 cm3(STP).s 1. Over a period of 16~7 h; a total of ~3 x 10 mol I2 (~0.2% of the deposited I2) was collected downstream of the scrubber. The srnall amount of I2 col-lected may have been due to the degradation of the deposits or to unreacted I2 which was adsorbed on the surfaces of the scrubber.
Even if the release was entirely due to the degradation of the depos-its, it is too low to aEfect the separation of I2 from air by this method.
~itrogen dioxide t NO2), WhiCh is a common impurity in the off-gas streams of nuclear fuel recycle facilities, reacts fast wlth 1~7~8 03 to form N205. It eonsumes one 03 molecule Eor every two N02 mole-cules and thus impedes the fixation o I2 with 03. It was, however~

shown in this study that as long as sufficient excess 03 is maintain in the system, the efficiency of the I2-03 fixation st~p remains unabated~
The only effect of H20 vapour predicted and also confirmed in ~his study is the conversion of the I205 to Lts hydrated forms (HIO3, HI308) 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 respons-ible 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 oæone to yield I~O9 and that I~l09 decomposed to I205 and I2 at temperatures in excess oE 100C. Thus the net reaction iII excess ozone, in the range of 12O~1IIOOC~ 1B the con-version o~ I2 to I205.
The conversion of CE~3I to elemental iocline and the oxidized form of the methyl radical ~paraformaldehyde) must be the result of the photo-dissociation of the C1-13I following absorption of 254 nm radiation which is not absorbed by any of the other components ~N2~ 2~ C02) in air. rne CH3I 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 C113I photodecomposition at the low partial pressures of CH31 employed here.
Following dissociatlon of Cl13I to C113 and I, the iodine atoms, which do not leact with 2' recombine to gLve I2 and the CF[3 radicals react with oxygen to yield paraformaldehyde as the final --~3-g~

product. In the presence of ozone generated by the 185 nm radiation the I2 is fixed as I2O5 which is quite stable, as discussed above~
It should be observed that the efficiencies of removal of CH3I and I2 from air as well as the scale of operation were intended for demonstration of the concept only and not for optimum scavenging.
~1igher 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 than one scrubber in serles. 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 (127I~ compounds, the same would apply to compounds of iodine-129 and iodine-131.

,

Claims (10)

1. A method of scavenging iodine, I2, from gaseous mixtures including same, comprising reacting I2 with ozone, O3, to form solid iodine oxides, and separating the solid oxides, sufficient ozone being provided to react with all iodine present.

2. The method of claim 1 where the gaseous mixture is air and the ozone is generated photochemically in situ.

3. A method of scavenging iodine from a gas phase which includes organic iodides, comprising:
(a) irradiating the gas phase with radiation capable of photodecom-posing 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.

4. The method of claim 3 wherein the gas phase is air and ozone is generated photochemically in situ.

5. The method of claim 3 wherein the iodine is present as radioactive isotopes thereof.

6. The method of claim 3 wherein said photodecomposition is effected by radiation comprising ultraviolet of about 220-300 nm wave-length with intense component near 260 nm.

7. The method of claim 4 wherein the ozone generation is effected by ultraviolet radiation of less than about 220 nm wavelength with a component near 185 nm.

8. The method of claim 4 wherein the air is irradiated simul-taneously with radiation capable both of photodecomposing the iodides present and of photochemically generating ozone, and steps (a) and (b) proceed concurrently.

CLAIMS (cont.)

9. The method of claim 3 wherein ozone is generated at a sepa-rate location and fed to the gas phase for step (b).

10. The method of claim 3 wherein the solid iodine oxides are removed by heating or by washing with water.