ODOR A B A T E M E N T WITH POTASSIUM
P E R M A N G A N A T E SOLUTIONS HANS S. POSSELT AND ARNO H. R E l D l E S Research Department, Carus Chemical Co., Inc., LaSalle, Ill.
With the objective of evaluating potassium permanganate for use in air pollution abatement, we investigated its odor-destroying properties on a number of malodorous organic and inorganic compounds. Reaction conditions similar tQ those existing in gas-scrubbing devices were employed. Threshold odor determinations before and after permanganate treatment were used for measuring the deodorizing actian. Although permanganate will not destroy all odors, its spectrum of effectiveness appears to warrant its consideration as an air pollution abatement chemical, particularly since odoriferous samples from process streams of asphalt and rendering plants were successfully treated by the permanganate method.
HE ODOR-DESTROYING properties of potassium permanganate T h a v e been recognized for many decades. In fact, many of its current industrial uses involving purification applications are based on its ability to alter or destroy bad-tasting and/or odoriferous inorganic and organic contaminants by oxidation. The major use of permanganate in this capacity is for the control of taste and odor in water treatment. I n other applications, it is used in the purification of industrial gases such as carbon dioxide. Permanganate has been proposed for the purification of air in air-conditioning systems ( 3 ) . The latest development in this area involves the so-called “purple pellets” ( Z ) , manufactured by Borg-Warner Corp. under the trade name Purafil, which represent a combination of KMnO4 with activated alumina and have been proved a n effective means of destroying a wide variety of odors. As a newcomer into the field of air pollution abatement, potassium permanganate has recently been applied successfully to the control of odors in cattle feed yards, as reported by Faith (7). I t has also been used for scrubbing off-gases from rendering and asphalt plants. Results of these field trials were so encouraging that a more systematic investigation of the odorreducing qualities of K M n 0 4 in aqueous solution was called for. The study was directed toward a variety of odorous chemical compounds, both organic and inorganic. Malodorous air and liquid samples collected a t rendering and asphalt plants were also investigated. Both qualitative and quantitative experiments were made. T h e qualitative series gave an indication as to whether a given compound would be deodorized by KMn04, whereas in the quantitative series threshold odor levels were measured before and after permanganate treatment. The prime objective of the study was to see whether or not permanganate scrubbing would eliminate or lessen objectionable odors from specific sources under conditions similar to those in air-scrubbing towers-namely, with a very short retention time (in the order of seconds) and a t a practical p H range.
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I h E C PRODUCT RESEARCH A N D DEVELOPMENT
Chemistry of Permanganate Action
KMnO4 is a strong oxidizing agent. I t will attack oxidizable matter under a wide variety of conditions; however, pH, concentrations, contact time, and temperature must be considered important variables. In air scrubbing, the most practical p H range is from 8 to 10, because most permanganate oxidations of organic compounds are catalyzed by hydroxyl ions. At a slightly alkaline p H range, permanganate will react with an oxidizable compound as shown in the following equation : 2KMn04
+ HzO
reducing
2 K O H ? 2MnO2 f 3 0
The oxygen formed in this reaction does not appear in molecular form, but immediately combines with the reducing agent. Inasmuch as the p H is on the alkaline side, a secondary benefit accrues in some deodorization reactions. Any malodorous lower fatty acids formed by oxidation of the corresponding alcohols or aldehydes will be retained as odorless alkali metal salts in the scrubbing solutions. In alkaline and neutral solutions, K M n 0 4 is noncorrosive to most commonly used scrubber materials. Adjusting the p H to far beyond 10 will result in excessive consumption of KMnO4, which is due to needless secondary oxidation of the primary oxidation products. In cases of extreme alkalinity, oxygen is lost by spontaneous deoxidation of the K M n 0 4 under the influence of OH- ions. Allowing the p H to fall below 7 into the slightly acidic range will generally stop most of the permanganate action. Satisfactory results are again obtained in a strongly acidic medium, but the free permanganic acid is very corrosive, thus requiring expensive materials of construction. In scrubbing air which contains CO, or other acidic or acidforming substances, it is most important to add a suitable buffer to the permanganate. Alkali and alkaline earth metal bicarbonates have been proposed for this purpose by Kuehner ( 3 ) . Alkali metal borates work equally well. Concentration of the KMnO4 is also important, as this variable-in conjunction with pH-will determine the redox
compared with the smell coming out of the blank reference. In the qualitative series only large odor differences were noted; in the quantitative measurements. ASTM Method D 1391-57 was used. In this method, a sample of the gas is diluted with odor-free air until a dilution is achieved in which the odor can barely be perceived. The ratio of the total volume of this diluted sample to the volume of original sample is a measure of the concentration of odor in the original sample. I n this context, an odorless air sample has a n odor concentration of 1.
potential of the system. Some substances which are not or are only very slowly oxidized by K M n 0 4 in very dilute solutions are attacked rapidly if the permanganate concentration is raised. In regard to by-products of the reaction, K O H is converted into potassium carbonate and bicarbonate by COz from the atmosphere and the MnOs shown in the equation appears first in colloidal, later in gel form, and finally separates as a sludge.
One disadvantage is that this method differentiates only between odor intensities but not odor characteristics. This became apparent in many experiments where permanganate changed a n evil smell into a much less objectionable odor, but still caused the threshold odor number of the residual odor to be above 1.
Experimental
The experimental apparatus used is shown in Figure 1. With a n oilless gas pump, air was passed a t a controlled rate through a U-tube containing a small quantity of the odorous substance suspended on Raschig rings. Depending on the vapor pressure of the substance involved, the C-tube was either heated or cooled by means of a controlled temperature bath in order to load the air stream with a concentration of contaminant strong enough to be easily perceptible by smell. When compounds were already in the gaseous state a t room temperature. the vaporizer was bypassed by connecting the gas source directly to the absorption flasksfor example. the samples of exhaust air from rendering and asphalt plants had been collected in pressure cylinders under 50-p.s.i. pressure. Liquid samples, such as cooker condensates from the rendering plant or scrubber liquid samples from asphalt plants, were put into the U-tube vaporizer. The odorous air was then passed through two gas-washing bottles with fritted disk dispersers which were connected in parallel. One of the wash bottles contained 900 ml. of 1% K M n 0 4 solution a t a p H of 8.5. The other contained the same volume of water buffered to a p H of 8.5. The rate of gas flow through each bottle was kept constant at 1 liter per minute by means of a rotameter-type flowmeter. The contact time of the gas with the scrubbing solution was found to be close to 1 second. Each experiment was run for 1 hour after the water scrubber became saturated with contaminant. In evaluating the effectiveness of deodorization by K M n 0 4 on a given substance, the intensities of odor emerging from the wash bottles containing the permanganate solution were
Results of Quantitative Experiments
The results of the quantitative experiments are shown in Table I . T h e first group of five compounds listed is aliphatic mercaptans. The sickening smell of these thiols is well known; actually one of the compounds listed, butanethiol, is a major constituent in the discharge of the skunk. The threshold odor numbers were reduced by several orders of magnitude by K M n 0 4 (but not to l ) , but the residual odor was much more tolerable than that of the original mercaptan. In other words, permanganate not only reduces odor intensities but changes the characteristics of the residual odor. In the second group, other sulfur compounds are given; the effect of the K M n 0 4 is apparent from the threshold odor numbers before and after treatment. T h e amines selected were all odorous. The lower aliphatic amines have a typical fishy odor, cadaverine is found in decaying meat, and skatole has been identified as the odoriferous principle in human feces. In the case of triethylamine not
!I
...
..
/I% KMnO. SOLUTION pH 8.5; 900rnl.
JI
k
ROTAMETER (connected only
GAS WASHING BOTTLE
'b=P
CONTROLLED TEMPERATURE WATER BATH
/
Roschig rings
(reference) pH 8.5
E
J f r i t t e d disc (coorse)
Figure 1.
Apparatus used in KMn04 oxidation experiments with air pollutants VOL. 4
NO. 1
MARCH 1965
49
Table 1.
Threshold Odor Number Reduction in Polluted Air by KMn04 Threshold Odor N o . HzO, KMn04, 7 % Pollutant PH 8.5 concn., PH 8.5
Mercaptans Butanethiol Pentanethiol Hexanethiol Heptanethiol Octanethiol
200,000 >loo, 000
85,000 3,200 3,500
Other Sulfur Compounds Mercaptoacetic acid 65 2-Mercaptoethanol 30 .411yl isothiocyanate 2,500 Thiophenol 1,300 Thiophene 4,000 Dimethylamine Trimethylamine Triethylamine Cadaverine Indole Skatole
Amines 1,300 2,700 60-70 20 5 60-100
11 20 200 45 5
1 1 1 13a 135 20 20 50-65" 1 1 1 1 1 1 25" 1
much of a reduction in threshold odor took place; however, odor characteristics were changed from a n intense fishy to a far more agreeable smell. Most of the phenols tried were completely deodorized by permanganate. Interestingly, m-chlorophenol proved to be more oxidation-resistant than the other phenols investigated. A group of miscellaneous odorous organic compounds was also studied. Surprisingly, acetaldehyde, which is normally very rapidly oxidized by K M n 0 4 , showed a relatively high residual threshold odor number, probably the result of inadequate contact time with the permanganate solution. The last group in Table I consists of actual samples taken in rendering and asphalt plants. Although significant threshold odor values remained after the K M n 0 4 treatment, the charac-
I&EC
Qualitative
Fast reaction
PRODUCT RESEARCH A N D DEVELOPMENT
Deodorization Experiments with KMn04 HzS, SO2, COS, NO,,putrescine, mono-
ethanolamine, aniline, formaldehyde Slow reaction Very slow reaction
16" 10.5" 20" 6.ja
Miscellaneous Organic Compounds 105 Styrene 2,000 Allyl acetate 1,700 25" 1 Acrolein 140,000 1 Benzaldehyde 80 200 Acetaldehyde 1,700 40" 1-Butanol 150 Off-gas of bone elevators 20-25" (Rendering plant) 100-140 Cooker condensate. 250" (Rendering plant) 4,000 1 Off-gas of asphalt plant 15-20 Residual odor characteristics much improved.
50
II.
33"
Phenols Phenol o-Cresol o-Chlorophenol m-Chlorophenol p-Chlorophenol
Table
cs2 CO, XH3, benzene, toluene, nitrobenzene, pyridine, acetone, methyl isobutyl ketone, dipropyl ketone, and diisobutyl ketone
teristics of the rendering odor had been changed from downright nauseating to tolerable or even perfume-like. Qualitative Odor Reduction Experiments
Table I1 deals with compounds that were tested only qualitatively. Such inorganic compounds as H & SOz, COS, and K O were oxidized rapidly and completely, as were a number of organic compounds. Carbon disulfide reacted with permanganate rather slowly. A selection of compounds not attacked by K M n 0 4 is also shown in Table 11. Although permanganate oxidation undoubtedly takes place, the reaction rates are so slow that under normal conditions a permanganate solution will do little, if any, good. Role of KMn04 in Odor Abatement
Review of the experimental data indicates that potassium permanganate solutions have a pronounced deodorizing effect upon a wide variety of odorous compounds. All contaminants in a given polluted atmosphere may not be oxidized completely by scrubbing with KMn04, but the major portion of them should be eliminated, resulting in a substantial reduction in air pollution. The economics of scrubbing off-gases with K M n 0 4 will be favorable in most cases when relatively small concentrations of pollutants are to be removed. Thus, on the basis of our laboratory and field experience, air pollution abatement by means of chemical oxidation using potassium permanganate can be considered feasible from both the technical and the economic point of view. literature Cited
(1) Faith, W. L., "Odor Control in Cattle Feed Yards," 57th
Annual Meeting Air Pollution Control Association, Houston, Tex.. June 1964. (2) Gamson, B. W. et al., U. S.Patent 3,049,399 (Aug. 14,1962). (3) Kuehner, R. L.,Zbid., 2,683,074(July 6,1954). RECEIVED for review September 28,1964 ACCEPTEDDecember 21, 1964 Division of Water, Air, and Waste Chemistry, 148th Meeting, ACS, Chicago, Ill., September 1964.
