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The adsorption of hydrogen cyanide by impregnated activated carbon cloth. Part II: Reactivity of impregnated metal carboxylates towards hydrogen cyanide
OWR-6223/X8 $3 00 + .W Copyright 0 1988Pergamon Press plc
Carbon Vol. 26. No. 5. pp, 713-721, 1988 Printed in Great Britain.
THE ADSORPTION OF HYDROGEN CYANIDE BY IMPREGNATED ACTIVATED CARBON CLOTH. PART II: REACTIVITY OF IMPREGNATED METAL CARBOXYLATES TOWARDS HYDROGEN CYANIDE Department
J. F. ALDER, P. R. FIELDEN, and S. J. ShirrH* of rnstrumentation and Analytical Science, University of Manchester Institute of Science and Technology, P. 0. Box 88, Manchester M60 LQD, U.K. (Received 11 July 1987; accepred in revised form 31 March 1988)
Abstract-Activated carbon cloth was impregnated with the formates, acetates, and propanoates of manganese, cobalt, nickel, copper, and zinc. Impregnated cloth samples were challenged with 1 L min-’ 2 mg L-’ HCN in air in a test rig. The effluent gas was sampled every 2 min, the sample separated on a gas chromatograph and the cyanogen and hydrogen cyanide concentrations determined. Breakthrough curves were obtained that gave an indication of the usefulness of the jmpregnation for HCN removal from air. The behavior of all the carboxylates was consistent with a model based on reaction of HCN with saturated aqueous solution of the metal carboxylate on the charcoal surface. Storage at 60°C and 80% RH for 14 days resulted in hydrolysis of the metal carboxylate with precipitation of what were probably hydroxide species of the metals. These were much less reactive towards HCN, and breakthrough times diminished accordingly. Cobalt and nickel acetates proved to be the most efficient impregnants for the removal of HCN from air.
Key Words-Carbon panoate.
cloth, adsorption,
hydrogen cyanide, metal formate, metal acetate. metal pro-
1. fNTRODUCTION
A previous study[l] investigated the reaction of flowing streams of hydrogen cyanide (HCN) in air with cobalt and nickel acetate that were impregnated onto activated carbon cloth (ACC). In this work the formates, acetates, and propanoates of the divalent cations of manganese, cobalt, nickel, copper, and zinc were investigated in a similar way.
2. EXPERIMENTAL The activated carbon cloth used was a microporous type (produced at the Chemical Defence Establishment, Ministry of Defence, U.K.) and has been described previously[l]. Strips were washed in distilled water and dried prior to impregnation. Of the divalent metal carboxylates used, the following were laboratory reagent grade: manganese acetate tetrahydrate (John Ross Chemicals), cobalt acetate tetrahydrate (BDH), nickel formate dihydrate (BDH), nickel acetate tetrahydrate (Aldrich), copper acetate hydrate (BDH), and zinc acetate dihydrate (Aldrich). Others were synthesized by slow addition of the appropriate acid to an aqueous slurry of the divalent metal carbonate at 70 to 80°C under
was insoluble in common organic solvents, so was not used further. Copper propanoate also hydrolyzed rapidly but was impregnated into ACC from methanolic solution. The other solutions were water stable for several months-slow hydrolysis led to deposition of hydroxides. For some impregnants, a 5% (w/v) aqueous solution was prepared, and for most, a 10% (w/v) or saturated solution, if saturation occurred below 10%. The prewash and impregnation procedure used for the Co-Ni acetate study was followed in this work, using cloth strips of sufficient area for two breakthrough tests. Preequilibration was achieved by storing strips in a 0.5-L jar containing a vial of saturated potassium chloride solution. Samples were stored in this atmosphere equilibrated to 80% RH for at least four days before testing, and water uptakes determined by weighing. The HCN test assembly and filter bed preparation from strips has been described previously[ 1,2]. Standard beds were 20 layers of 2 cm dia. cloth discs. After insertion into the test rig, air at 80% RH was passed through for full equilibration, and then the flow through the bed changed to 1 L min-i 2 mg L-l HCN in 80% RH air. The effluent from the bed was
reflux. Products deposited on cooling were recrystallized from water where possible. Zinc propanoate hydrolyzed rapidly in dilute aqueous solution and
sampled every 2 min, analyzed by gas chromatography, and the components measured. A test was continued until the effluent concentration had almost reached the influent HCN concentration, which
-*Present address: Royal Military College of Canada, Kingston, Ontario, K7K .5LO, Canada.
was determined by absolute and gas chromatography methods before and after each test. The limit of detection of the gas chromatography method was (1
--
713
J. F. ALDER et al.
714
ppm (v/v) HCN or (CN), in air. All concentrations were normalized equivalent to a 2-mg L-’ HCN in air challenge. The unused portion of cloth was trimmed, weighed after drying at 35 or lOO”C, and stored in a jar at 60°C for 14 days, so that the effects of hydrolysis or carbon surface-impregnant interactions at elevated temperature could be assessed by a second HCN test. The jars were opened each day to permit ingress of fresh air, and equilibrated at room temperature, 80% RH for four days prior to repeat HCN challenge.
some of the salts by a pH electrode. The origin of the sigmoid HCN concentration-time curve, and the gradual approach to the influent concentration exhibited by all of the impregnants, has been discussed in terms of the impregnant solution model for cobalt and nickel acetates[l] and applies here. Coating of impregnant particles with insoluble cyanides would occur for all of the metals except manganese, which does not form a stable dicyanide. Reactions between HCN and some of the impregnants, or the cloth surface itself produced cyanogen, which was frequently observed, but with breakthrough profiles of various forms (Fig. 1). [CN], generation arises from the overall reaction[9]:
3. RESULTS AND DISCUSSION
3.1 Impregnation results The carboxylates investigated as impregnants exist in solution largely as hydrated undissociated complexes[3]. When cloth is dipped in solution, the amount of sorption of the bulky hydrate complexes into micropores is likely to be low. They would more probably accumulate at polar active centers on the surface of the cloth. Sorption studies on weak electrolytes have been made, but the nature of the attracting sites is debated[4]. Any adsorbed complexes would act as centers for further adsorption, so that groups of complexes and eventually accretions form on the surface. During the drying period, these would remain as solid particles, and may further act as crystallization centres for impregnant from solution not removed by blotting. Loadings from preparations at the same temperature always increased with dip solution concentration. The practice of drying prepared cloth at 35 or 100°C was to influence the size of impregnant surface crystallites. Although there would be initial dehydration of some of the salts used, storage at 80% RH prior to test would substantially rehydrate them. Dehydration of some salts accounts for the lower apparent loadings at the higher drying temperature. Data on the state of hydration at 100°C were measured or checked[5-81 for some salts, and are indicated in tables where appropriate. There was no significant difference in behaviour on testing with HCN between salts dried at the two different temperatures for most of the materials examined. It is assumed that any structural differences due to the state of hydration do not affect the impregnantHCN reaction. 3.2 HCN challenge results The model described previously[l] involving impregnant particles in equilibrium with their saturated aqueous solution is taken to apply to the results here. The important consequence is that principal reactivity between HCN and impregnant occurs in a surface solution rather than at a gas-solid interface. As the pH of the saturated salt solution presumed to be coating the surface is significant in determining the dissociation, and hence further reaction of HCN, the pH values were measured for saturated solutions of
HCN -
t[CN],
+ e- + H+
E” = -0.37
V
which requires energy input, the origin of which may be enthalpies of other reactions, solubility of impregnants or surface-water-impregnant interactions. Cyanogen generation by unimpregnated cloth was observed previously[l], and levels were always
3.3 Manganese carboxylates All were pink crystalline solids, soluble in water up to about 10% w/v. There was a trend of increased loading with dip concentration. Drying at 100°C dehydrates the acetate[5], but the degree of dehydration of the formate and propanoate at 100°C is unknown. All showed similar behavior on HCN testing, with close approach to the influent HCN concentration after a rapid rise. Cyanogen peak concentrations of 450 to 650 ppm from formate and acetate, and 160 to 330 ppm from propanoate impregnated cloths indicate that the formation of the gas was due principally to reaction between the impregnant and HCN. After the peak, cyanogen concentrations decayed rapidly at first, and then more gradually. HCN dissociates and reacts as ionic cyanide, combining with manganese carboxylates (Table 1) eventually to give [Mn[CN],]- and [Mn[CN]$, as
Adsorption of hydrogen cyanide II
( a ) unimpregnated
cloth
715
( b ) manganese carboxylate impregnated cloth
T/min
30
T/min
( c ) cobalt, nickel or zinc carboxylate impregnated cloth
60
( d ) copper carboxylate impregnated cloth
time I I
T/min
60
T/min
60
Fig. 1. Typical HCN breakthrough profiles for metal carboxylate impregnated activated carbon cloth. T = elapsed time after challenge begins/min. L = cyanogen concentration (peak values given in Tables l-5).
Mn[CN],
is unstable[lO,ll].
However,
[Mn[CN],14-
oxidizes to [Mn[CN]J(En = 0.24 V), and it is thought that cyanogen formation would accompany reduction back to [Mn[ CN]J- , although energy input is required for both reactions under standard conditions. 3.4 Cobalt and nickel carboxylates Both of the formates gave saturated solutions at low concentrations, 5% for Co and 3% for Ni, with resultant low loadings on impregnation (see Tables 2 and 3). HCN retention times were much lower than the times observed for the acetates, the difference being attributable to the lower solubilities and pH values of the saturated solutions of the formates. The long breakthrough times of the Co-Ni acetate samples are considered due to a combination of good solubility and relatively high solution pH. The cobalt and nickel acetate complex has a structure in which acetate is quite labile permitting its easy displacement by CN- . The wide variety of complexes that can be formed[ 10,111, particularly for Co, and the
generation of the electronically stable ions Ni’+ or Co3+ by air oxidation also favour the consumption of CN-. The reactions lead finally to formation of the insoluble dicyanides, so effects due to changes in impregnant coating are apparent in the breakthrough profiles. The propanoates gave slightly lower breakthrough times-probably due to lower lability of the larger anion, although structures for these salts have not been reported. Cyanogen formation was minimal for all of these samples, and it is believed that the cloth rather than impregnant was responsible for its appearance. The changes on hot humid storage are likely to arise from the effects of hydrolysis. 3.5 Copper carboxylates The interaction of HCN with impregnated Cu2+ salts leads to the formation of cyanogen, and in all cases the generation was in near stoichiometric quantities (see Table 4). Cyanogen penetration was as immediate as measurable in all cases, at 400 to 500 ppm, and rising gradually thereafter to peak at 600
*Dehydrated at 100°C.
2HZ0
Mn[WG&L
Propanoate
4H,O
Mn[02CCH312
Acetate*
Formate Mn[ 0,CH 12. 2H,O
Salt
35 104l
10
c
35 100
35 100
35 100
35 100
Drying temp. (“Cl
5
10
5
5
DIP solution concn. (% w/v)
17.3 16.6
11.9 12.4
13.1 9.2
:::
5.6 2.5
Loading (% w/w)
16.8 22.4
23.3 29.5
25.4 35.2
4 4
9 9
4 5
4 4
2 6
35.5 34.0 23.7 41.7
HCN
<2 <2
2 2
c2 2
<2 <2
[CN],
Breakthrough (times/min)
Water uptake (% w/w)
210 260
160 330
490 640
90 550
470 470
Estimated ]CNh peak level (ppm)
16.6 25.8
27.3 33.6
27.8 39.1
24.3 41.9
43.3 38.7
Water uptake (% w/w)
[CN], profiles in Fig. lb
Test before storage
Table 1. Results for manganese carboxylates
2 <2
<2 <2
<2 <2
HCN
2 <2
<2 <2
<2 <2
<2 <2
[CN]z
Breakthrough (times/min)
160 70
190 510
520 550
160 420
530 510
Estimated ICNI, peak Ievel (ppm)
Test after storage
*Dehydrated at 100°C.
Propanoate
4&O (6.17)
W02CCH312
Acetate*
Formate Co[ OzCH J22H20 (4.56)
Salt (pH of salt solution)
10
1
5 35 100
35 100
35 100 1 35 100
35 100
Drying temp. (“C)
5 10
5
DIP solution concn. (% w/v)
20.4 17.5
14.1 12.2
9.2 7.2 21.3 13.4
3.2 1.3
Loading (% w/w)
11.9 21.6
23
22.4 29.3 18 27
15
15 21 25 28
11 10
HCN
-
-
-
-
44 37
[CNIz
Breakthrough (times/min)
28.2 39.1 IS.1 22.9
32.7 41.1
Water uptake (70 wlw)
-
-
-
-
2 2
Estimated ICNL peak level (ppm)
Test before storage
Table 2. Results for cobalt carboxylate [CN], profiles in Fig. lc
16.7 29.7
23.2 31.9
40.7 23.8 35.7
29.3 38.7
Water uptake (% w/w)
-
-
12 <2 16 18
-
12
16 <2
[CN]z
7 814
-
<2 2
HCN
Breakthrough (times/min)
-
-
-
4 5
Estimated [CM, peak level (ppmf
Test after storage
*Dehydrated at 100°C.
Propanoate
Acetate* Ni[02CCHz Jz . 4Hz0 (6.27)
Formate Ni[ O&X I2 .2H,O (6.50)
Salt (pH of salt solution)
13.3 13.4
35 100
35 100
5
10
22.0 20.9
18.9 12.4
2.9 0.1
Loading (a w/w)
35 100
35 100
35 100
Drying temp. (“C)
10
5
3
DIP solution concn. (% w/v)
12.3 25.3
19.7 24.1
12.5 31.7
25.9 37.1
24.2 40.8
Water uptake (% w/w)
23 23
2a 18
29 24
11 12
97
HCN
2
16
20 _
-
2
[CN],
Breakthrough (times/min)
7
15.5 29.5
22.4 32.6
4
28.4 35.3
379
32.7 43.2
Water uptake (% w/w)
‘2
15
-
Estimated [CNL peak level (ppm)
Test before storage
Table 3. Results for nickel carboxylates [CN& profiles in Fig. lc
14 16
2 10
<2 18
-8
<2 4
HCN
2
<2 8
<2 4
-
-6
2 <2
ICN],
Breakthrough (times/min)
7 5
2
<1 1
-8 -
Estimated ICNI, peak level (ppm)
Test after storage
Q !--
n
Propanoate Cu[ O&XLH& Hz0
Acetate Cu[O,CCH,]2 2H,O
Formate Cu[ 0,CH ],HzO
Salt (pH of salt solution)
10
7
10
DIP solution concn. (% w/v)
I
35 too
35 100
35 100
Drying temp. (“C)
19.5 16.0
23.9 26.4
20.5 9.0
Loading (% w/w)
8.6 31.4
14.6 26.4
23.0 32.9
Water uptake (% w/w)
22 28
33 36
32 36
HCN
<2 <2
c2 <2
<2 2
[CN]?
Breakthrough (timesimin)
570 590
720 680
850 750
Estimated ICI% peak level (ppm)
Test before storage
Table 4. Results for copper carboxylates [CNIZ profiles in Fig. Id
0.6 31.3
10.5 28.1
25.6 35.0
Water uptake (410wlw)
20 23
29 34
35 33
HCN
<2 <2
<2 <2
<2 <2
[CN],
Breakthrough (timesfmin)
530 610
610 700
630 700
Estimated [ CN]* peak level (ppm)
Test after storage
I
(5.46) \
Z@WCH31Z
Acetate
Formate Zn[ O&H], . 2Hz0 (5.75)
Salt (pH of salt solution)
3.5 100 35 100
18.8 14.6 8.8 18.6
13.4 11.9
35 100
5
10
8.0 5.1
Loading (% w/w)
35 100
Drying temp. (“C)
5
DIP solution concn. (% w/v)
20.2 26.3 10.4 13.6
26.7 32.4
27.9 32.9
Water uptake (% w/w)
11 14 1s 22
8 10
911
HCN
-
4 7
-
[CN]*
Breakthrough (times/min)
1
-
S 3
-
(ppm)
Estimated ]CNlzrk
Test before storage
Table 5. Results for zinc carboxylates [CN], profiles in Fig. lc
21.1 32.0 -
6 10 -
14 -
-
-
[CN], <2 2
-
HCN
Breakthrough (times/mm)
36.4 32.0
Water uptake (% w/w)
<2 -
-
2s 11
(ppm)
Estimated [CNlzdqeak
Test after storage
Adsorption
of hydrogen cyanide II
to 850 ppm. After this peak, which coincided with HCN breakthrough, the level dropped sharply to a low and gently decreasing level, 5 to 10% of peak concentration. Reaction with CU’+ salts is a common method for cyanogcn preparation[ll,l2]. The lack of observed HCN while the cyanogen output is high suggests that all of the influent HCN is being converted until the available supply of impregnant becomes exhausted. Solution studies show that ft~rmation of insoluble ~u~CU~~N~~~~, along with several other copper cyanide complexes accompanies the reaction of HCN with copper acetate, so the i~~pregnant would not only be consumed on reaction, but also coated with the insotuble product. The cyanogen level after the peak may be due to continued conversion by fresh impregnant slowly becoming available, and the decay of other cyano-complexes. The various carboxylates all gave similar cyanogen breakthrough profiles and times, except for the copper formate sample dried at lOO”C, which gave a much more gradual initial rise to the peak, the reason for which is uncertain.
721
with hydrogen cyanide in a manner that can be predicted from their known reactivity with the gas in solution, supporting a mechanism of reaction in an adsorbed water film on the carbon surface. There was little difference in results arising from use of different temperatures in the preparation of the impregnated cloth. The impregnants can be categorized as those which generate cyanogen from HCN by some redox reaction (Mn and Cu salts), and the remainder that remove HCN without its oxidation to cyanogen, but have no apparent capacity themselves to consume cyanogen when this is formed by the cloth surface. The solubility of the impregnants, pH of the solution formed, lability of the dissolved complexes towards CN and stability of products govern the behavior on HCN testing of cloth impregnated with salts of this latter group (Co, Ni., Zn). Of the carboxylates examined, none has been found to better the cyanogen-free HCN removal capacity of cobalt or nickel acetates.
REFERENCES
3.6 Zinc carboxylates Zinc formate samples gave quite low breakthr~~ugh times and no cyanogen before the hot and humid storage period (see Table 5). Low levels of cyanogen. consistent in amount and concentrationtime profile with those generated by the cloth alone were observed afterwards. This is attributed to reduced availability of impregnant because of hydrolysis or physical redistribution. The zinc acetate tests pave lower HCN breakthrough times than Co or Ni acetates, and no cyanogen, according with the proposed reaction mechanism. The expected reaction product is insoluble zinc dicyanide with probable formation of [ Zn[CN]f ’ Reduction in breakthrough times after the storage period are attributed to hydrolysis of the zinc acetate. Cigarette filters have been impregnated with zinc acetate for HCN removal from tobacco cmoke[ 131. 4.
CONCLUSIONS
This study has shown that metal carboxylate impregnants on humidified activated carbon cloth react
1. P.‘R. Fielden. S. J. Smith, and J. F. Alder, submitted to Carbon 26, in press. 2. P. R. Fielden, S. J. Smith. and J. F. Alder. A~7~~~,~f 111,@5 (19%). 3. A. Marteli and L. G. Sillen, Stability Constants uf Metal-Ion Complexes, Special Publication No. 17.The Chemical Society, London (1964). 4. J. S. Mattson and H. B. Mark, Acriwted Carbon: Surface Chemistrv and Adsorption from Solution. Marcel “Dekker Inc.,‘New York (1971): 5. R. C. Mehrota and R. Bohra. Metul Carhoxvlatt~s. Academic Press, London (1983). 6. K. Tenuichir~~ and M. lbzua. Kngvo Kqaku Zusshi 48, 1629 (1965). 7. J. L. Doremieux and R. Poihlane, c‘. R. Acad. Sci. Pur& Ser. C 246. 1278 (1967). 8. R. L. Martin and H. Waterman. J. Qtem. Sec. 1359 (1959). 9. W. M. Latimer, The Oxidation .Stares of the Elements and Their Potentials in Aqueous Solution, p. 137, 2nd ed. Prentice-Hall. Englewood Cliffs, NJ (1952). 10. W. P. Griffith. Quart. Rev. 16, IX8 (1962). 11. A. G. Sharpe, The Chemistry of Cyano C#rn~~exe~ of the Tr~n~s~t~~n Metals. Academic Press, London ( f976). 12 S. Nakamura. ind. Eng. Chem. Prod. Res. Develop. 7, 159 (1968). I 13. H. G. Horsewell and T. W. C. Tolman, U. S. Patents 3.403.690 (1968)and 3.5SO.MlO(196X).
Carbon Vol. 26. No. 5. pp, 713-721, 1988 Printed in Great Britain.
THE ADSORPTION OF HYDROGEN CYANIDE BY IMPREGNATED ACTIVATED CARBON CLOTH. PART II: REACTIVITY OF IMPREGNATED METAL CARBOXYLATES TOWARDS HYDROGEN CYANIDE Department
J. F. ALDER, P. R. FIELDEN, and S. J. ShirrH* of rnstrumentation and Analytical Science, University of Manchester Institute of Science and Technology, P. 0. Box 88, Manchester M60 LQD, U.K. (Received 11 July 1987; accepred in revised form 31 March 1988)
Abstract-Activated carbon cloth was impregnated with the formates, acetates, and propanoates of manganese, cobalt, nickel, copper, and zinc. Impregnated cloth samples were challenged with 1 L min-’ 2 mg L-’ HCN in air in a test rig. The effluent gas was sampled every 2 min, the sample separated on a gas chromatograph and the cyanogen and hydrogen cyanide concentrations determined. Breakthrough curves were obtained that gave an indication of the usefulness of the jmpregnation for HCN removal from air. The behavior of all the carboxylates was consistent with a model based on reaction of HCN with saturated aqueous solution of the metal carboxylate on the charcoal surface. Storage at 60°C and 80% RH for 14 days resulted in hydrolysis of the metal carboxylate with precipitation of what were probably hydroxide species of the metals. These were much less reactive towards HCN, and breakthrough times diminished accordingly. Cobalt and nickel acetates proved to be the most efficient impregnants for the removal of HCN from air.
Key Words-Carbon panoate.
cloth, adsorption,
hydrogen cyanide, metal formate, metal acetate. metal pro-
1. fNTRODUCTION
A previous study[l] investigated the reaction of flowing streams of hydrogen cyanide (HCN) in air with cobalt and nickel acetate that were impregnated onto activated carbon cloth (ACC). In this work the formates, acetates, and propanoates of the divalent cations of manganese, cobalt, nickel, copper, and zinc were investigated in a similar way.
2. EXPERIMENTAL The activated carbon cloth used was a microporous type (produced at the Chemical Defence Establishment, Ministry of Defence, U.K.) and has been described previously[l]. Strips were washed in distilled water and dried prior to impregnation. Of the divalent metal carboxylates used, the following were laboratory reagent grade: manganese acetate tetrahydrate (John Ross Chemicals), cobalt acetate tetrahydrate (BDH), nickel formate dihydrate (BDH), nickel acetate tetrahydrate (Aldrich), copper acetate hydrate (BDH), and zinc acetate dihydrate (Aldrich). Others were synthesized by slow addition of the appropriate acid to an aqueous slurry of the divalent metal carbonate at 70 to 80°C under
was insoluble in common organic solvents, so was not used further. Copper propanoate also hydrolyzed rapidly but was impregnated into ACC from methanolic solution. The other solutions were water stable for several months-slow hydrolysis led to deposition of hydroxides. For some impregnants, a 5% (w/v) aqueous solution was prepared, and for most, a 10% (w/v) or saturated solution, if saturation occurred below 10%. The prewash and impregnation procedure used for the Co-Ni acetate study was followed in this work, using cloth strips of sufficient area for two breakthrough tests. Preequilibration was achieved by storing strips in a 0.5-L jar containing a vial of saturated potassium chloride solution. Samples were stored in this atmosphere equilibrated to 80% RH for at least four days before testing, and water uptakes determined by weighing. The HCN test assembly and filter bed preparation from strips has been described previously[ 1,2]. Standard beds were 20 layers of 2 cm dia. cloth discs. After insertion into the test rig, air at 80% RH was passed through for full equilibration, and then the flow through the bed changed to 1 L min-i 2 mg L-l HCN in 80% RH air. The effluent from the bed was
reflux. Products deposited on cooling were recrystallized from water where possible. Zinc propanoate hydrolyzed rapidly in dilute aqueous solution and
sampled every 2 min, analyzed by gas chromatography, and the components measured. A test was continued until the effluent concentration had almost reached the influent HCN concentration, which
-*Present address: Royal Military College of Canada, Kingston, Ontario, K7K .5LO, Canada.
was determined by absolute and gas chromatography methods before and after each test. The limit of detection of the gas chromatography method was (1
--
713
J. F. ALDER et al.
714
ppm (v/v) HCN or (CN), in air. All concentrations were normalized equivalent to a 2-mg L-’ HCN in air challenge. The unused portion of cloth was trimmed, weighed after drying at 35 or lOO”C, and stored in a jar at 60°C for 14 days, so that the effects of hydrolysis or carbon surface-impregnant interactions at elevated temperature could be assessed by a second HCN test. The jars were opened each day to permit ingress of fresh air, and equilibrated at room temperature, 80% RH for four days prior to repeat HCN challenge.
some of the salts by a pH electrode. The origin of the sigmoid HCN concentration-time curve, and the gradual approach to the influent concentration exhibited by all of the impregnants, has been discussed in terms of the impregnant solution model for cobalt and nickel acetates[l] and applies here. Coating of impregnant particles with insoluble cyanides would occur for all of the metals except manganese, which does not form a stable dicyanide. Reactions between HCN and some of the impregnants, or the cloth surface itself produced cyanogen, which was frequently observed, but with breakthrough profiles of various forms (Fig. 1). [CN], generation arises from the overall reaction[9]:
3. RESULTS AND DISCUSSION
3.1 Impregnation results The carboxylates investigated as impregnants exist in solution largely as hydrated undissociated complexes[3]. When cloth is dipped in solution, the amount of sorption of the bulky hydrate complexes into micropores is likely to be low. They would more probably accumulate at polar active centers on the surface of the cloth. Sorption studies on weak electrolytes have been made, but the nature of the attracting sites is debated[4]. Any adsorbed complexes would act as centers for further adsorption, so that groups of complexes and eventually accretions form on the surface. During the drying period, these would remain as solid particles, and may further act as crystallization centres for impregnant from solution not removed by blotting. Loadings from preparations at the same temperature always increased with dip solution concentration. The practice of drying prepared cloth at 35 or 100°C was to influence the size of impregnant surface crystallites. Although there would be initial dehydration of some of the salts used, storage at 80% RH prior to test would substantially rehydrate them. Dehydration of some salts accounts for the lower apparent loadings at the higher drying temperature. Data on the state of hydration at 100°C were measured or checked[5-81 for some salts, and are indicated in tables where appropriate. There was no significant difference in behaviour on testing with HCN between salts dried at the two different temperatures for most of the materials examined. It is assumed that any structural differences due to the state of hydration do not affect the impregnantHCN reaction. 3.2 HCN challenge results The model described previously[l] involving impregnant particles in equilibrium with their saturated aqueous solution is taken to apply to the results here. The important consequence is that principal reactivity between HCN and impregnant occurs in a surface solution rather than at a gas-solid interface. As the pH of the saturated salt solution presumed to be coating the surface is significant in determining the dissociation, and hence further reaction of HCN, the pH values were measured for saturated solutions of
HCN -
t[CN],
+ e- + H+
E” = -0.37
V
which requires energy input, the origin of which may be enthalpies of other reactions, solubility of impregnants or surface-water-impregnant interactions. Cyanogen generation by unimpregnated cloth was observed previously[l], and levels were always
3.3 Manganese carboxylates All were pink crystalline solids, soluble in water up to about 10% w/v. There was a trend of increased loading with dip concentration. Drying at 100°C dehydrates the acetate[5], but the degree of dehydration of the formate and propanoate at 100°C is unknown. All showed similar behavior on HCN testing, with close approach to the influent HCN concentration after a rapid rise. Cyanogen peak concentrations of 450 to 650 ppm from formate and acetate, and 160 to 330 ppm from propanoate impregnated cloths indicate that the formation of the gas was due principally to reaction between the impregnant and HCN. After the peak, cyanogen concentrations decayed rapidly at first, and then more gradually. HCN dissociates and reacts as ionic cyanide, combining with manganese carboxylates (Table 1) eventually to give [Mn[CN],]- and [Mn[CN]$, as
Adsorption of hydrogen cyanide II
( a ) unimpregnated
cloth
715
( b ) manganese carboxylate impregnated cloth
T/min
30
T/min
( c ) cobalt, nickel or zinc carboxylate impregnated cloth
60
( d ) copper carboxylate impregnated cloth
time I I
T/min
60
T/min
60
Fig. 1. Typical HCN breakthrough profiles for metal carboxylate impregnated activated carbon cloth. T = elapsed time after challenge begins/min. L = cyanogen concentration (peak values given in Tables l-5).
Mn[CN],
is unstable[lO,ll].
However,
[Mn[CN],14-
oxidizes to [Mn[CN]J(En = 0.24 V), and it is thought that cyanogen formation would accompany reduction back to [Mn[ CN]J- , although energy input is required for both reactions under standard conditions. 3.4 Cobalt and nickel carboxylates Both of the formates gave saturated solutions at low concentrations, 5% for Co and 3% for Ni, with resultant low loadings on impregnation (see Tables 2 and 3). HCN retention times were much lower than the times observed for the acetates, the difference being attributable to the lower solubilities and pH values of the saturated solutions of the formates. The long breakthrough times of the Co-Ni acetate samples are considered due to a combination of good solubility and relatively high solution pH. The cobalt and nickel acetate complex has a structure in which acetate is quite labile permitting its easy displacement by CN- . The wide variety of complexes that can be formed[ 10,111, particularly for Co, and the
generation of the electronically stable ions Ni’+ or Co3+ by air oxidation also favour the consumption of CN-. The reactions lead finally to formation of the insoluble dicyanides, so effects due to changes in impregnant coating are apparent in the breakthrough profiles. The propanoates gave slightly lower breakthrough times-probably due to lower lability of the larger anion, although structures for these salts have not been reported. Cyanogen formation was minimal for all of these samples, and it is believed that the cloth rather than impregnant was responsible for its appearance. The changes on hot humid storage are likely to arise from the effects of hydrolysis. 3.5 Copper carboxylates The interaction of HCN with impregnated Cu2+ salts leads to the formation of cyanogen, and in all cases the generation was in near stoichiometric quantities (see Table 4). Cyanogen penetration was as immediate as measurable in all cases, at 400 to 500 ppm, and rising gradually thereafter to peak at 600
*Dehydrated at 100°C.
2HZ0
Mn[WG&L
Propanoate
4H,O
Mn[02CCH312
Acetate*
Formate Mn[ 0,CH 12. 2H,O
Salt
35 104l
10
c
35 100
35 100
35 100
35 100
Drying temp. (“Cl
5
10
5
5
DIP solution concn. (% w/v)
17.3 16.6
11.9 12.4
13.1 9.2
:::
5.6 2.5
Loading (% w/w)
16.8 22.4
23.3 29.5
25.4 35.2
4 4
9 9
4 5
4 4
2 6
35.5 34.0 23.7 41.7
HCN
<2 <2
2 2
c2 2
<2 <2
[CN],
Breakthrough (times/min)
Water uptake (% w/w)
210 260
160 330
490 640
90 550
470 470
Estimated ]CNh peak level (ppm)
16.6 25.8
27.3 33.6
27.8 39.1
24.3 41.9
43.3 38.7
Water uptake (% w/w)
[CN], profiles in Fig. lb
Test before storage
Table 1. Results for manganese carboxylates
2 <2
<2 <2
<2 <2
HCN
2 <2
<2 <2
<2 <2
<2 <2
[CN]z
Breakthrough (times/min)
160 70
190 510
520 550
160 420
530 510
Estimated ICNI, peak Ievel (ppm)
Test after storage
*Dehydrated at 100°C.
Propanoate
4&O (6.17)
W02CCH312
Acetate*
Formate Co[ OzCH J22H20 (4.56)
Salt (pH of salt solution)
10
1
5 35 100
35 100
35 100 1 35 100
35 100
Drying temp. (“C)
5 10
5
DIP solution concn. (% w/v)
20.4 17.5
14.1 12.2
9.2 7.2 21.3 13.4
3.2 1.3
Loading (% w/w)
11.9 21.6
23
22.4 29.3 18 27
15
15 21 25 28
11 10
HCN
-
-
-
-
44 37
[CNIz
Breakthrough (times/min)
28.2 39.1 IS.1 22.9
32.7 41.1
Water uptake (70 wlw)
-
-
-
-
2 2
Estimated ICNL peak level (ppm)
Test before storage
Table 2. Results for cobalt carboxylate [CN], profiles in Fig. lc
16.7 29.7
23.2 31.9
40.7 23.8 35.7
29.3 38.7
Water uptake (% w/w)
-
-
12 <2 16 18
-
12
16 <2
[CN]z
7 814
-
<2 2
HCN
Breakthrough (times/min)
-
-
-
4 5
Estimated [CM, peak level (ppmf
Test after storage
*Dehydrated at 100°C.
Propanoate
Acetate* Ni[02CCHz Jz . 4Hz0 (6.27)
Formate Ni[ O&X I2 .2H,O (6.50)
Salt (pH of salt solution)
13.3 13.4
35 100
35 100
5
10
22.0 20.9
18.9 12.4
2.9 0.1
Loading (a w/w)
35 100
35 100
35 100
Drying temp. (“C)
10
5
3
DIP solution concn. (% w/v)
12.3 25.3
19.7 24.1
12.5 31.7
25.9 37.1
24.2 40.8
Water uptake (% w/w)
23 23
2a 18
29 24
11 12
97
HCN
2
16
20 _
-
2
[CN],
Breakthrough (times/min)
7
15.5 29.5
22.4 32.6
4
28.4 35.3
379
32.7 43.2
Water uptake (% w/w)
‘2
15
-
Estimated [CNL peak level (ppm)
Test before storage
Table 3. Results for nickel carboxylates [CN& profiles in Fig. lc
14 16
2 10
<2 18
-8
<2 4
HCN
2
<2 8
<2 4
-
-6
2 <2
ICN],
Breakthrough (times/min)
7 5
2
<1 1
-8 -
Estimated ICNI, peak level (ppm)
Test after storage
Q !--
n
Propanoate Cu[ O&XLH& Hz0
Acetate Cu[O,CCH,]2 2H,O
Formate Cu[ 0,CH ],HzO
Salt (pH of salt solution)
10
7
10
DIP solution concn. (% w/v)
I
35 too
35 100
35 100
Drying temp. (“C)
19.5 16.0
23.9 26.4
20.5 9.0
Loading (% w/w)
8.6 31.4
14.6 26.4
23.0 32.9
Water uptake (% w/w)
22 28
33 36
32 36
HCN
<2 <2
c2 <2
<2 2
[CN]?
Breakthrough (timesimin)
570 590
720 680
850 750
Estimated ICI% peak level (ppm)
Test before storage
Table 4. Results for copper carboxylates [CNIZ profiles in Fig. Id
0.6 31.3
10.5 28.1
25.6 35.0
Water uptake (410wlw)
20 23
29 34
35 33
HCN
<2 <2
<2 <2
<2 <2
[CN],
Breakthrough (timesfmin)
530 610
610 700
630 700
Estimated [ CN]* peak level (ppm)
Test after storage
I
(5.46) \
Z@WCH31Z
Acetate
Formate Zn[ O&H], . 2Hz0 (5.75)
Salt (pH of salt solution)
3.5 100 35 100
18.8 14.6 8.8 18.6
13.4 11.9
35 100
5
10
8.0 5.1
Loading (% w/w)
35 100
Drying temp. (“C)
5
DIP solution concn. (% w/v)
20.2 26.3 10.4 13.6
26.7 32.4
27.9 32.9
Water uptake (% w/w)
11 14 1s 22
8 10
911
HCN
-
4 7
-
[CN]*
Breakthrough (times/min)
1
-
S 3
-
(ppm)
Estimated ]CNlzrk
Test before storage
Table 5. Results for zinc carboxylates [CN], profiles in Fig. lc
21.1 32.0 -
6 10 -
14 -
-
-
[CN], <2 2
-
HCN
Breakthrough (times/mm)
36.4 32.0
Water uptake (% w/w)
<2 -
-
2s 11
(ppm)
Estimated [CNlzdqeak
Test after storage
Adsorption
of hydrogen cyanide II
to 850 ppm. After this peak, which coincided with HCN breakthrough, the level dropped sharply to a low and gently decreasing level, 5 to 10% of peak concentration. Reaction with CU’+ salts is a common method for cyanogcn preparation[ll,l2]. The lack of observed HCN while the cyanogen output is high suggests that all of the influent HCN is being converted until the available supply of impregnant becomes exhausted. Solution studies show that ft~rmation of insoluble ~u~CU~~N~~~~, along with several other copper cyanide complexes accompanies the reaction of HCN with copper acetate, so the i~~pregnant would not only be consumed on reaction, but also coated with the insotuble product. The cyanogen level after the peak may be due to continued conversion by fresh impregnant slowly becoming available, and the decay of other cyano-complexes. The various carboxylates all gave similar cyanogen breakthrough profiles and times, except for the copper formate sample dried at lOO”C, which gave a much more gradual initial rise to the peak, the reason for which is uncertain.
721
with hydrogen cyanide in a manner that can be predicted from their known reactivity with the gas in solution, supporting a mechanism of reaction in an adsorbed water film on the carbon surface. There was little difference in results arising from use of different temperatures in the preparation of the impregnated cloth. The impregnants can be categorized as those which generate cyanogen from HCN by some redox reaction (Mn and Cu salts), and the remainder that remove HCN without its oxidation to cyanogen, but have no apparent capacity themselves to consume cyanogen when this is formed by the cloth surface. The solubility of the impregnants, pH of the solution formed, lability of the dissolved complexes towards CN and stability of products govern the behavior on HCN testing of cloth impregnated with salts of this latter group (Co, Ni., Zn). Of the carboxylates examined, none has been found to better the cyanogen-free HCN removal capacity of cobalt or nickel acetates.
REFERENCES
3.6 Zinc carboxylates Zinc formate samples gave quite low breakthr~~ugh times and no cyanogen before the hot and humid storage period (see Table 5). Low levels of cyanogen. consistent in amount and concentrationtime profile with those generated by the cloth alone were observed afterwards. This is attributed to reduced availability of impregnant because of hydrolysis or physical redistribution. The zinc acetate tests pave lower HCN breakthrough times than Co or Ni acetates, and no cyanogen, according with the proposed reaction mechanism. The expected reaction product is insoluble zinc dicyanide with probable formation of [ Zn[CN]f ’ Reduction in breakthrough times after the storage period are attributed to hydrolysis of the zinc acetate. Cigarette filters have been impregnated with zinc acetate for HCN removal from tobacco cmoke[ 131. 4.
CONCLUSIONS
This study has shown that metal carboxylate impregnants on humidified activated carbon cloth react
1. P.‘R. Fielden. S. J. Smith, and J. F. Alder, submitted to Carbon 26, in press. 2. P. R. Fielden, S. J. Smith. and J. F. Alder. A~7~~~,~f 111,@5 (19%). 3. A. Marteli and L. G. Sillen, Stability Constants uf Metal-Ion Complexes, Special Publication No. 17.The Chemical Society, London (1964). 4. J. S. Mattson and H. B. Mark, Acriwted Carbon: Surface Chemistrv and Adsorption from Solution. Marcel “Dekker Inc.,‘New York (1971): 5. R. C. Mehrota and R. Bohra. Metul Carhoxvlatt~s. Academic Press, London (1983). 6. K. Tenuichir~~ and M. lbzua. Kngvo Kqaku Zusshi 48, 1629 (1965). 7. J. L. Doremieux and R. Poihlane, c‘. R. Acad. Sci. Pur& Ser. C 246. 1278 (1967). 8. R. L. Martin and H. Waterman. J. Qtem. Sec. 1359 (1959). 9. W. M. Latimer, The Oxidation .Stares of the Elements and Their Potentials in Aqueous Solution, p. 137, 2nd ed. Prentice-Hall. Englewood Cliffs, NJ (1952). 10. W. P. Griffith. Quart. Rev. 16, IX8 (1962). 11. A. G. Sharpe, The Chemistry of Cyano C#rn~~exe~ of the Tr~n~s~t~~n Metals. Academic Press, London ( f976). 12 S. Nakamura. ind. Eng. Chem. Prod. Res. Develop. 7, 159 (1968). I 13. H. G. Horsewell and T. W. C. Tolman, U. S. Patents 3.403.690 (1968)and 3.5SO.MlO(196X).