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Stoichiometric analysis of chemisorption of hydrogen-cyanide onto activated carbon cloth
Carbon Vol. 33, No. 10, pp. 1433-1441, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008.6223/95 $9.50 + 0.00
Pergamon 0008-6223(95)00093-3
STOICHIOMETRIC ANALYSIS OF CHEMISORPTION OF HYDROGEN-CYANIDE ONTO ACTIVATED CARBON CLOTH LJ. V. RAJAKOVI~,’ M. R. ILIC,~ P. B. JOVANI~~ and P. B. RADOSEVIC~ ‘Faculty of Technology and Metallurgy, Karnegijeva 4, 11 000 Belgrade, Yugoslavia ‘Institute for Technical and Medical Protection, Kataniceva 15, 11 000 Belgrade, Yugoslavia (Received 29 September
1994; accepted
in revised form 11 May 1995)
Abstract-Based on the experimental results, and on the analysis of the chemistry of cyanide ion bonding, possible mechanisms of bonding of this pollutant were assumed and confirmed. Activated carbon cloth (ACC) was impregnated with metalorganic compounds consisting of the following cations: copper, silver, iron, magnesium and aluminum, and the following anions: acetate, oxalate, tartrate, stearate and citrate. Cation and anion content of impregnants on the cloth were determined by atomic absorption spectrometry and ion and gas chromatography. The analysis of stoichiometric utilization of chemical agent was done by calculation and the results of calculating the degree of stoichiometric utilization are presented. The image of the surface morphology of the materials were made by scanning electronic microscopy (SEM). Sorption properties of ACC were examined for hydrogen-cyanide and benzene in the gas phase, by standard gravimetric procedure in static conditions, with the use of Cahn RG electrobalance. Carbon materials impregnated with copper(H)-tartrate, silver(I)-citrate, and iron(III)-citrate, which demonstrated the best properties, and the best stoichiometric utilization, are completely characterized together with determination of basic parameters of the sorption system, which enables their application as new materials for chemisorption of cyanide. The salts of magnesium and aluminium that were used have no affinity for HCN bonding. Key Words-Activated
carbon
cloth, metalorganic
I. INTRODUCTION The
contribution
sorption
of this
is in developing
paper
in the field
impregnated
carbon
of chemimateri-
l-41. The testings concerned a large number of samples of carbon materials impregnated with metalorganic compounds, including the analysis of interactions with hydrogen-cyanide. Processes that take place on the boundary surface of carbon material-by means of their influence on the strength of the bond between chemical agent (impregnant) and pollutant-determine adsorptive capacity of carbon materials [ 5-S]. This is analyzed in detail for individual cases and represents an important contribution of this paper. The purpose of this work was to find a convenient separation technique and a convenient carbon material for removing cyanide from the air on the basis of a chemisorption process[9,10]. To that purpose, ACC was additionally activated by chemical impregnation. In this way, it could be possible to separate cyanides efficiently by its bonding to an active chemical agent inside the cloth[ 11-141. Cyanogen, which is also formed as the product of reaction of metalorganic compounds with hydrogen-cyanide in the following step changes into oxamide and one HCN molecule[15]. The quantities of cyanogen that are measured during sorption are negligible and were not taken into account in the course of these testings. An especially important contribution of this paper is in determining the participation of separate components in chemisorption and total adsorption. As for the experimental field, this paper contains for the als
for
hydrogen-cyanide
sorption
from
air[
salts, hydrogen-cyanide,
stoichiometry.
first time a comprehensive research and analysis of impregnated carbon cloth before and after exposure to the impact of hydrogen-cyanide, as well as the effect of applying metalorganic compounds for expanding sorptive capacity of carbon materials. The activated carbon material impregnated in such a way can be used for producing filters for purification of air or industrial gases, both in peacetime and in war conditions, as well as for manufacturing protective clothes and associated masks. Chemical impregnation was done using metalorganic compounds. The role of metallic ions is to introduce a catalytic function, while the role of organic components is to increase the sorptive capacity of the cloth. Motivation for this work was an idea to investigate sorption on carbon “atoms”, which was experimentally achieved by using organic acids that have one, two or more carboxyl groups. Benzene was used as standard adsorbate for physical adsorption control. In this work, complete results of ACC impregnation, its characterization, and sorption capacity of the cloth are given. Directions for future research in the field of application of carbon materials impregnated with metalorganic compounds should comprise testings in dynamic conditions aimed at determining the time of protection until the penetration point. This would help to complete the testings on capacity of developed carbon materials. Moreover, it would also be very important to test sorption of pollutants such as phosgene and chlorine-cyan onto these materials, because it is assumed that these pollutants-which 1433
1434
LJ.V.
can be found in air following can be successfully sorbed as well.
2.
RAJAKOVIC et al.
accidental situationsby the same material
EXPERIMENTAL
Basic material used in this research was microporous ACC with specific surface of 1491.12 m*/g, made by carbonization and activation of viscous rayon, a standard production type, produced by “Toyobo”-Osaka (Japan). Acetates, oxalates, tartrates, stearates and citrates of copper, silver, iron, magnesium and aluminium as impregnation reagents were used. Concentrations of solutions of these salts were 0.1 mol/dm3, and they were made from MERCK and ZORKA chemicals. The experimental procedure for impregnation and characterisation of carbon cloth have been described elsewhere [ 161. Metallic ions were determined using atomic absorption flame spectrometry. The contents of acetates, oxalates, tartrates and citrates were determined by ion chromatography. The stearate content was determined by gas chromatography. Sorptive properties of the samples were examined on a gravimetric adsorption apparatus, by standard procedure in static conditions, as described previously [ 161. A Cahn RG electrobalance was used. Measurements were taken in the interval of relative pressures ranging from 0.0 to 0.3. With the Cahn electrobalance, the increase in mass of adsorbates during the introduction of portions of gas into the system was measured. The gas portion was observed by the pressure change. Adsorption was performed at an initial pressure of 13.2 Pa. The sample was previously evacuated for 120 minutes to clean the surface from impurities. Results were processed using Langmuir adsorption isotherms.
3. RESULTS AND DISCUSSION Quantitative analysis of impregnants indicated that cations and anions show various affinities towards ACC. Affinity of anions decreases in the following order: acetates, tartrates, citrates, oxalates and stearates, which corresponds to the increase of the ion size. Good adherence of tartrates and citrates to the surface could be explained by electrostatic attraction, and by the geometry and dimensions of the molecules. Tartrates and citrates attach to electropositive locations on the surface of ACC by their electronegative hydroxyl OH- and carboxyl COOgroups, by Van der Waals forces. Cations, active for reaction with cyanide ions, remain on the other side. Stearate molecules, being too large, can attach to the fiber surface only with its “tail”. As such, it is not active in reaction with cyanide ions, and, most likely, it blocks some of the micropores because of its size. The aim of impregnating the ACC with such salts was to find out whether the carbon atoms from the impregnants will be free and active for adsorbing the cyanide. However, it seems that the number of carbon
atoms is not decisive; rather, decisive are the optimally bonded copper and silver cations, if the salts are properly chosen. The content of cations, determined by the AAS method, is given in Table 1. It is noticeable that there is a difference in the percentage of impregnant held within the samples that were washed after impregnation and those that were not. Generally, the silver salts (silver(I)-acetate 12.26 mass % of silver) and copper salts (copper(II)acetate 6.72 mass % of copper) attached strongly to the surface of the fibres. This is not puzzling, bearing in mind that all silver salts are very slightly soluble in water. For the same reason, the smallest percentage of the impregnants was removed by washing when silver salts were already present on the ACC surface. The anion content for samples with washing after impregnation is given in Table 2. All tartrates and citrates were adsorbed excellently by the cloth. A very high percentage of adhering was achieved only by copper( II)-acetate, even 19.34 mass % with samples washed after impregnation. All oxalates were adsorbed very poorly: from 0.027 mass %, with cloth impregnated with copper( II)-oxalate, to 0.25 mass % with cloth impregnated with silver(I)oxalate. From Table 2 it is noticeable that, in the sample impregnated with copper(stearate, there was 2.77 mass % of stearate, while in the sample impregnated with magnesium(II)-stearate there was 2.18 mass %. However, stearate was deposited on cloth mainly in the form of powder that falls off when the cloth is shaken, so that it does not achieve the desired function. Also it visibly covered the surface of the cloth and the micropores. This was confirmed by the adsorption of hydrogen-cyanide. Micrographs obtained by SEM helped in finding out the morphological structure of ACC. Micrographs in Fig, 1 also show that silver salts were adsorbed on the cloth better than other salts. Being the least soluble, they
Table 1. Content of cations on ACC determined by AAS Content of cations (%) Adsorbant ACC + Cu(II)-acetate ACC + Cu(lI)-oxalate ACC + Cu(Il)-tartrate ACC + Cu(II)-stearate ACC + Ag(I)-acetate ACC + Ag(I)-oxalate ACC + Ag(I)-tartrate ACC + Ag(I)-citrate ACC + Fe(B)-acetate ACC + Fe(II)-oxalate ACC + Fe(II)-tartrate ACC + Fe(III)-citrate ACC + Mg(II)-acetate ACC + Mg(II)-oxalate ACC + Mg(II)-stearate ACC + Al(III)-acetate ACC + AI(III)-oxalate
Washed samples 6.72 0.53 2.19 0.39 12.26 6.78 5.62 3.50 0.68 0.41 0.49 1.08 0.24 0.23 0.09 0.17 0.03
Unwashed samples 10.84 3.33 7.30 8.77 14.55 8.92 7.02 3.53 0.76 1.28 0.96 1.34 0.25 0.55 0.33 0.35 0.21
Chemisorption of hydrogen-cyanide onto activated carbon cloth Table 2. Content
of anions on ACC determined chromatography
by ion
Content of anions (%) Adsorbant ACC + Cu(II)-acetate ACC + Cu(II)-oxalate ACC + Cu(II)-tartrate ACC + Cu(II)-stearate ACC + Ag(I)-acetate ACC + Ag(I)-oxalate ACC + Ag(I)-tartrate ACC + Ag(I)-citrate ACC + Fe(II)-acetate ACC + Fe(II)-oxalate ACC f Fe(II)-tartrate ACC + Fe(III)-citrate ACC f Mg(II)-acetate ACC f Mg(II)-oxalate ACC + Mg(II)-stearate ACC + Al(III)-acetate ACC + A&III)-oxalate
Washed samples 19.34 0.03 5.60 2.77 1.79 0.25 8.42 2.58 2.07 0.04 10.32 2.82 0.22 0.04 2.18 1.50 0.07
Unwashed samples 19.99 0.15 6.68 4.11 2.70 0.28 14.11 3.55 2.27 0.05 10.87 3.38 0.24 0.04 5.46 3.18 0.07
coated the surface of each fiber with particles about 1 pm in diameter. Carbon fiber is about 15 pm thick[ 171. With samples impregnated by iron salts, there were no conspicuous traces of salts. Only with some difficulties, traces of these salts may be seen on the surface of fibres; particles are smaller than 1 pm. The fibres were smooth and shiny. Fibres impregnated with aluminium salts looked similar. Salt particles were not visible in inter-fibrous space, but traces of salt adsorbed on fiber itself were visible. With samples impregnated with magnesium salts, particles of salts, both those adsorbed on the fiber and those in interfibrous space, are highly visible. These values are close to the fiber diameter. The results of determining specific surface by benzene isotherms are given in Table 3. Specific surfaces differ from sample to sample, because of a differing percentage of impregnant added to ACC. Namely, as Kloubek also shows in his work[ 181, impregnants may partly close the pores. Chiou and Reucroft [19] reached the same conclusion. If more impregnant is used, more space is blocked, and, therefore, specific surface is further reduced. It is logical that the highest percentage of salts was imposed on ACC when copper(acetate was used: 6.72 mass % of copper (Table 1) and 19.34 mass % of acetate (Table 2). This led to a reduction of specific surface from 1491.12 to 853.75 m’/g, which is about 40%. All salts caused a greater or smaller reduction of specific surface, which was expected. Drastic reduction was observed after impregnation with copper(stearate and magnesium(II)-stearate, when the specific surface was reduced to 270.76 m’/g and 473.13 m’/g, respectively, which is unsatisfactory. For samples that were not washed after impregnation, these values are even lower, namely, 205.06 m2/g for copper( II)-stearate and 405.08 m”/g for magnesium(II)-stearate. This means that between 70 and
1435
80% of the micropores were covered. Table 2 also shows that oxalates attach very poorly to ACC. The consequence of this was that specific surfaces remained great in samples impregnated by oxalates, as compared to samples impregnated by other salts. Reduction of specific surface was from 1 to 20%. By comparing the samples that were washed after impregnation and those that were not, it was found that the sorptive capacity of benzene for all types of salts used was lower by about 15% in unwashed samples, where the concentration of impregnants was greater and, therefore, more of the pores were blocked. Chiou and Reucroft[ 191 show in their work that chemisorption is important in the region of lower pressures, while under higher pressures physical adsorption dominates. Comparing adsorption isotherms, it is seen that adsorptive capacity is lower in the region of higher pressures. This suggests that impregnant blocked or occupied a part of the adsorption area of the pores, which resulted in a smaller volume of micropores. Such effects will reduce adsorptive abilities in the region of higher relative pressures. In the region of lower relative pressures, adsorptive capacity is greater than in non-impregnated carbon cloth. Comparing the types of cupric salts, the greatest adsorptive capacity of benzene was found in material impregnated by copper(oxalate (5.3 mmo1c6n6/ g,,, at relative pressure of 0.3), copper(tartrate (3.6 mmolcuJg,&, copper( II)-acetate (3.5 mmo1c.d gAcc) and copper(stearate (2.2 rnmolcsnJgAcc, all at the same relative pressure (Fig. 2(a)). When sorption isotherms of hydrogen-cyanide were analyzed, it was noticeable that adsorption capacity did not follow the above given order. The greatest sorptive capacity for hydrogen-cyanide was found in the with copper(tartrate sample impregnated (11.9 mmol,cN/g,c, at relative pressure of 0.3 with the tendency of further increase, which is seen from the shape of the isotherm). Sorptive capacity decreases in the following order: copper( II)-oxalate ( 10.0 mmol,,/g,), copper( II)-acetate (6.3 mmol,,/ g,,,), and copper( II)-stearate (3.3 mmol,,,/g,,c), all at the same relative pressure (Fig. 2(b)). If these results are compared with sorptive capacity of samples that were not washed after impregnation, it will be obvious that there are certain discrepancies. The sorptive capacity for HCN with the sample impregnated with copper( II)-acetate increased by about 50%, while with other types of salts this capacity reduced by about 40%, which means that a large number of pores were blocked for adsorption. A characteristic example is impregnation with copper( II)-stearate, which has been discussed above. Sorptive capacities are very high with all types of silver salts used for impregnation, and they are located in a very narrow range (4.6-5.5 mmolc~,J gAc-) at relative pressure of 0.3. Sorptive capacity decreases in the following order: silver(I)-citrate, silver( I)-tartrate, silver(I)-oxalate and silver(I)-
LJ.V. RAJAKOVI~.rt
1436
Fig. 1. MIcrographs of samples (a) ACC + Ag( I&acetate,
impregnated with (h) ACC + Ag( I)-oxalate
acetate (Fig. 3(a)). It is interesting that the same order holds for the sorption of HCN. Sorptive capacity of the sample impregnated with silver( I)-acetate, washed
d.
silver salts, magnification and (c) ACC + Ag( [)-citrate.
after impregnation, was 4.8 mmol,,,/g,,, and 9.6 mmol,,,/g,,, for unwashed, relative pressure of 0.3. This suggests
549x:
(Fig. 3(b)) at the same that, in this
Chemisorption of hydrogen-cyanide onto activated carbon cloth
1437
Table 3. Specific surface and specific volume of micropore Specific surface of micropores
Specific volume of micropores
(m2/g) Washed
Adsorbant ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC
+ + + + + + + + + + + + + + + + +
of micropores
(cm3/g)
of micropores (cm3/g)
Unwashed
0.3130 0.4739 0.3225 0.0924 0.4218 0.4480 0.48 18 0.4929 0.5050 0.5396 0.3088 0.4211 0.4584 0.5436 0.1680 0.5212 0.4933 0.5482
0.2911 0.4385 0.3100 0.0692 0.3721 0.4162 0.4567 0.463 1 0.4648 0.5126 0.2954 0.3974 0.4367 0.4854 0.1407 0.4907 0.4640 0.5482
I I I I I I I I I I I I I 0.02 0.06 0.10 0.14 0.18 0.22 0.26
PIP,
PIP,
(b)
16
r
samples
802.91 1183.69 848.10 205.06 1025.21 1133.03 1225.32 1242.52 1244.43 1372.88 797.78 1068.89 1177.48 1298.74 405.08 1320.69 1252.13 1491.12
0.02 0.06 0.10 0.14 0.18 0.22 0.26 16
Specific volume
(m’ig)
samples
853.75 1299.80 896.62 270.76 1147.29 1223.61 1307.37 1341.60 1370.70 1471.30 834.98 1143.85 1250.85 1480.95 473.13 1413.85 1340.05 1491.12
Cu(II)-acetate Cu(II)-oxalate Cu(II)-tartrate Cu(II)-stearate Ag(I)-acetate Ag(I)-oxalate Ag(I)-tartrate Ag(I)-citrate Fe(II)-acetate Fe(II)-oxalate Fe(H)-tartrate Fe(III)-citrate Mg(II)-acetate Mg(II)-oxalate Mg(II)-stearate Al(III)-acetate AI(III)-oxalate
Specific surface
& X8 za
(b)
12
on*
EE 04 0.02
0.06
0.10
0.14 0.18 0.22 0.26 0.02 0.06 0.10 0.14 0.18 0.22 0.26
PIP,
PIP,
Fig. 2. (a) Adsorption isotherms of benzene for samples impregnated with copper salts: (+) ACC +Cu(II)-acetate, (A) ACC + Cu(II)-oxalate, (0) ACC +Cu(II)-tartrate and (0) ACC + Cu(lI)-stearate. (b) Adsorption isotherms of hydrogen-cyanide for samples impregnated with copper salts: (+) ACC + Cu(II)-acetate, (A) ACC+ Cu(II)-oxalate, (0) ACC+Cu(II)-tartrate and (0) ACC+Cu(II)-stearate.
Fig. 3. (a) Adsorption isotherms of benzene for samples impregnated with silver salts: (+) ACC +Ag(I)-acetate, (A) ACC + Ag( I)-oxalate, (0) ACC + Ag( I)-tartrate and (q) ACC + Ag( I)-citrate. (b) Adsorption isotherms of hydrogen-cyanide for samples impregnated with silver salts: (+) ACC + Ag(I)-acetate, (A) ACC + Ag(I)-oxalate,
case, it is possible to achieve better chemisorptive ability by increasing the quantity of impregnant. Samples impregnated with silver(I)-citrate did not show significant changes in sorptive capacities for HCN whether they were washed or not washed. Sorptive capacity at a relative pressure of 0.3 was 11.1 mmol,,.,/g,,, and 11.5 mmol,,,/g,,,, respectively. This means that adding further quantities of impregnant, beyond a certain percentage, does not
produce any effect. It could be concluded that all chosen silver salts may be efficient as impregnants, although the material with impregnated silver( I)-citrate has somewhat better characteristics than the others. Adsorptive isotherms of benzene in the samples impregnated with iron salts showed that the highest capacity was obtained with samples impregnated with
(0) ACC + Ag( I)-tartrate
and
(q) ACC + Ag(I)-citrate.
1438
LJ.V. RAJAKOVI~'etal.
iron(oxalate and subsequently washed (6.0 mmol CsHs/gAcc at relative pressure of 0.3). The capacity of iron(acetate was (5.8 mmol,-,H,/gA,c), iron(III)citrate (4.6 mmolc6,Jg,,c), and iron(tartrate (3.6 mmolc,,,/g,c,), all at the same relative pressure (Fig. 4(a)). Sorptive capacity for HCN increases from samples impregnated with iron(acetate (7.4 mmolHcN/gAc-), iron( II)-oxalate (8.5 mmol,,,/ iron(tartrate (10.7 mmol,c,/g,cc), to gACC)> iron(III)-citrate (14.1 mmol,,,/gA,), the last showing excellent sorptive capacity (Fig. 4(b)). Taking into consideration both the sorption of benzene and that of hydrogen-cyanide, material impregnated with iron(III)-citrate possesses the best sorption characteristics. As could be expected, sorptive capacity for benzene of samples impregnated with magnesium(II)-acetate and magnesium(II)oxalate can be very high, but this is not the result of the impregnant used. Material impregnated with magnesium( II)-stearate confirms the theory discussed magnesium( II)-stearate blocked above. Namely, pores completely, so that sorptive capacity for benzene was very low (1.9 mmol,6,JgA,,). Sorptive capacity for HCN is very low for all samples impregnated with magnesium salts; it was not higher than with any of the samples. As in the 7.4 mmol&g,cc case of chemisorption of hydrogen-cyanide, the abilities of impregnant as chemisorbent were to be examined it is obvious that magnesium salts are not adequate for this purpose, because they do not show
l-
1 I I I I I I I I I I I I 0.02 0.06 0.10 0.14 0.18 0.22 0.26 P/P, Y--
(b)
affinity for HCN. Therefore, they will not be considered any further. Materials impregnated with aluminium salts showed very high sorptive capacity for benzene, above 5.1 mmolc,a,/g,cc. Isotherms for HCN show that this case is similar to the previous one. Sorptive capacity of these materials for HCN is very low, barely above 7.4 mmol,,,/gAcc. Therefore, they will not be considered any further, either. Adsorption isotherms of hydrogen-cyanide for non-impregnated ACC are given in Fig. 5. When cyanide ions chemically bond on ACC, precipitate and complex compounds are formed. These processes are carried out in two stages, with the formation of characteristic compounds in both cases. By favoring the conditions for producing complex compounds, it is possible to achieve higher stoichiometric utilization of chemical impregnant[20,21], which, in turn, gives ACC of higher sorptive capacity. ACC is a convenient medium for bonding complex compounds, due to adsorptive and chemical driving forces inside the structure of the cloth [ 22-241. Based on the experimental results, and on the analysis of the chemistry of cyanide ion bonding, possible mechanisms of bonding of this pollutant can be assumed and confirmed. Structural activity of ACC is twofold: the ACC itself without any chemical impregnant maintains its adsorptive functions, but the impregnant, added to ACC, acts chemically towards formation of precipitate and complex compounds. Besides the adsorptive and chemical forces, which affect the active bonding of the pollutant, other important factors are: the geometry of the system, flow of the air through the pores of the system, diffusion rate, and the kinetics of surface reactions. The system is obviously complex. For non-specific pollutants, the mechanism of bonding to a chemical agent is very important. Movement of the chemical agent and cyanides inside ACC, and the mechanism of bonding, may be presented as follows: in the first stage, cyanide ions and copper form the precipitate of copper(I)-cyanide, Cu(CN). In the second stage, new quantities of cyanide ions decompose the precipitate, and a tetracyanocuprate( I) complex ion [Cu(CN).Jis formed. Further quantities of cyanide ions could only be adsorbed and this process contin-
l6r 0.02 0.06 0.10 0.14 0.18 0.22 0.26 PIP, Fig. 4. (a) Adsorption isotherms of benzene for samples impregnated with iron salts: (+) ACC+ Fe(H)-acetate, (A) ACC + Fe(U)-oxalate, (0) ACC + Fe( II)-tartrate and (0) ACC + Fe( III)-citrate. (b) Adsorption isotherms of hydrogen-cyanide for samples impregnated with iron salts: (+) ACC + Fe( II)-acetate, (A) ACC + Fe(II)-oxalate, (0) ACC+ Fe(II)-tartrate and (0) ACCf Fe(III)-citrate.
0.02 0.06 0.10 0.14 0.18 0.22 0.26 PIP, Fig. 5. Adsorption isotherms for non-impregnated (0) adsorption isotherm of hydrogen-cyanide.
ACC:
Chemisorption of hydrogen-cyanide onto activated carbon cloth
1439
Table 4. Degree of stoichiometric utilization of chemical impregnant
Adsorbant
Contribution of physical adsorption (%)
Contribution of chemisorption W)
88.7 90.9 89.4
11.3 9.1 10.6
ACC + Cu(II)-tartrate ACC + Ag(I)-citrate ACC + Fe(III)-citrate
ues until the sorptive capacity of the cloth is completely exhausted. The analysis of stoichiometric utilization of chemical agent started by calculating, for the sample impregnated with copper-tartrate, the number of mols of copper absorbed by a gram of carbon cloth. The obtained value is 0.00034 mol,Jg,,,. If this sample by similar computation we bonds 0.32 gnc,lg,cc, obtain 0.0119 mol,,N/g,,c. The residue chemisorptive mass after desorption under vacuum amounts to 11.3%, which means 0.00134 molnCN/gACC, as is shown in Table 4. It ensues that 0.00034 mol Cu bonds 0.00134 mol HCN. The obtained stoichiometric ratio is Cu+ : CN- = 1 : 3.9. This means that the formation of complex ions is favored in relation to precipitate compounds, especially of ion [CU(CN),]~and a part of precipitate CuCN. This is the most favorable stoichiometric ratio, because it gives the highest stoichiometric utilization of the chemical agent. Already in the sample impregnated with copper-oxalate the ratio was Cu+ :CN- = 1: 2.8, which means that the formation of precipitate compounds is favored. In other words, mostly CuCN and some [Cu(CN)J ions are formed. The sample impregnated with silver-citrate has and bonds 0.00101 molrtc,/g,cc. 0.00032 mol,,/g,,, The obtained stoichiometric ratio is Ag’:CN= 1: 3.1. This ratio would lead to the conclusion that the stoichiometric utilization is even over lOO%, which is of course impossible, but it can be concluded that the formation of complex ions [Ag(CN),]is certainly favored, while the rest belongs to good physical adsorption. The sample impregnated with iron-citrate has 0.00020 mol,/g,,, and bonds The obtained stoichiometric 0.00148 mol,,.,/g,c,. ratio is Fez+ : CN- = 1: 7.4. This is close to the theory about the formation of the [Fe(CN),14ion, while the rest is in greater contribution of physical adsorption, that is, the mass retained by physical bondings Table 5. Adsorption
Degree of stoichiometric utilization
Stoichiometric ratio 3.9 3.1 7.4
0.97 Xl Xl
on the sample. The results of calculating the degree of stoichiometric utilization are presented in Table 4. It is obvious that the best stoichiometric utilization of the chemical agent dominates among the selected samples, which is exactly what makes the corresponding materials so good. The materials impregnated with inorganic copper and silver salts, tested in a previous period, were compared with new materials impregnated with organic copper and silver salts. In that way, it was possible to isolate the contribution to adsorption by the organic component alone. Table 5 gives the values of hydrogen-cyanide sorption for the samples of materials impregnated with both organic and inorganic salts. It is evident that there is a difference in the sorptive capacity of materials impregnated with inorganic salts in relation to those impregnated with organic salts. A contribution by the organic component to the sorptive capacity exists and it amounts to even as much as 50% in relation to the contribution by the inorganic component. After determination of adsorption isotherms for materials impregnated with pure acids-tartaric and citric acids (Fig. 6kit is now possible to decompose the contributions by each separate component to the overall adsorption. Figure 7 shows histograms for selected materials impregnated with copper-tartrate, silver-citrate and iron-citrate. As was initially assumed, the metallic cation is responsible for chemisorption and its share in total adsorption is about 10%. The organic component increases physical adsorption and this can be seen by comparing the adsorption isotherms for hydrogencyanide in non-impregnated carbon cloth (Fig. 5) and in carbon cloth impregnated with organic acids (Fig. 6). Nevertheless, total adsorption is not a simple sum of adsorptions of separate components. This is obvious from the presented histograms, since sorption
capacity of hydrogen-cyanide for samples impregnated organic and inorganic salts
Adsorbant ACC + CuCO,Cu(OH), ACC + CUCO,CU(OH)~ ACC + Cu(II)-tartrate ACC + Cu(II)-oxalate ACC + Cu(II)-acetate ACC + Ag(I)-nitrate ACC + Ag(I)-citrate
with
Contents of cations (%)
Adsorption capacity (mm&,/&u,)
1.3 5.8 2.19 0.53 6.72 2.4 3.50
7.4 4.8 11.9 10.0 6.3 4.4 11.1
1440 16
r (a) 0.02
0.06
LJ.V. RAJAKOWC et crl
0.10
0.14
0.18
0.22
0.26
0.18
0.22
0.26
PIP0 @)
0.02
0.06
0.10
0.14 Q/Q0
Fig. 6. Adsorption isotherm of HCN for samples impregnated with (a) tartarlc
16 -
acid and (b) citric acid.
r
CU
Tanaric acid
Figure 8 presents adsorption isotherms for hydrogen-cyanide at various temperatures in carbon cloth impregnated with copper-tartrate. As was expected, the adsorption isotherms show reduction in physical adsorption with increase in temperature. After desorption by vacuuming until initial pressure, the residue masses were bonded by strong chemisorption effect on the samples. It can be seen that the change in adsorption temperature influences much more physical adsorption than chemisorption. Figure 9 presents the histogram of the temperature influence on the contribution by each component in adsorption. It was concluded that sorption of cyanide slightly increases in the case of the material impregnated only with copper ions. which means that, in the course of sorption, there is a chemical reaction between copper and cyanide. In the case of sorption of cyanide onto material impregnated only with tartaric acid, the sorptive capacity is lower at an increased temperature. This can be explained only by the fact that physical adsorption of cyanide is taking place on the surface. In the case of applying coppertartrate, the sorptive capacity of cyanide at an increased temperature decreases. This is understandable, since the share of physical adsorption is much higher than the share of copper as the component that is the carrier of the chemisorptive function. Sorption for the material impregnated with coppertartrate also decreases with the increase in temperature, because in this case as well there is a considerable share of physical adsorption within the total adsorption.
Cu-tanratc
16 16
12
5 X:: ‘t;q
8
M
r
1 (0) 25oc
2 -4 D
Al7
Citric acid
Ag-citrate
16 0.02
0.06
0.10
0.14
0.18
0.22
0.26
PIP, E E -4
Fig. 8. Adsorption isotherms nated with copper-tartrate
M
of HCN for samples impregat different temperatures.
u Fe
Fig. 7. Adsorption
capacity
Citric acid
of HCN
Fe-citrate
for several
components.
onto materials impregnated with organic salts is higher. This means that an additional synergetic effect exists in this case. In that way, the assumption from the Introduction of this paper has been proved, namely, that the organic component has the function of increasing the sorption capacity of carbon materials.
CU
25°C
Fig. 9. AdsorptIon
Tartaric 41°C
25°C
acid 47QC
Cu-tartrate 25°C
41nc
capacity of HCN for several components at different temperatures.
Chemisorption of hydrogen-cyanide onto activated carbon cloth 4. CONCLUSIONS
The purpose of this work was to study, investigate and develop impregnated activated carbon cloth, which unites, in its structure, adsorptive and chemisorptive properties; this would contribute to the study and application of such materials for air purification from toxic gasses, especially cyanides. On the basis of the results obtained in this experimental work, the following conclusions could be made: 1. ACC impregnation with metalorganic compounds was performed by acetates, oxalates, tartrates, citrates and stearates of copper, silver, iron, magnesium and aluminium. Samples with and without washing after impregnation were tested. 2. Characterisation of impregnated ACC was performed by modern techniques. The content of metallic cations was determined by atomic absorption spectrometry, while the content of organic acid anions was determined by ion and gas chromatography. The surface composition images and the materials morphology were made using scanning electron microscopy. isotherms for benzene and 3. Adsorption hydrogen-cyanide in impregnated ACC were obtained by gravimetric adsorption apparatus. It was found that chemisorptive activity was realised by adding an active chemical agent (impregnant) into the material structure. The effects of metallic ions and their organic salts on hydrogen-cyanide and benzene were studied. Bonding efficacy of the pollutants was analysed, and it was found that the greatest contribution was made by the following materials: ACC impregnated with copper( II)-tartrate, then silver(I)-citrate, and iron(lII)-citrate. 4. By analysis of possible chemistry in the structure of ACC it was found that in the pollutant and chemical agent (impregnant) reaction, precipitate and complex compounds were formed. The precipitate and complex compounds ratio depends directly on pollutant concentration. At higher concentrations, formation of complex compounds predominates, and at lower concentrations formation of precipitates predominates. 5. Temperature influence on sorption was tested by determining the capacity of impregnated material in the process of hydrogen-cyanide sorption at two different temperatures. The testings used carbon material impregnated only with copper ions, only with tartaric acid and finally with copper-tartrate. It was concluded that sorption of cyanide slightly increases in the case of the material impregnated only with copper ions, which means that in the course of sorption there is a chemical reaction between copper and cyanide. 6. The contribution by organic component to
1441
total adsorption was obtained by comparing sorptive capacity of materials impregnated with organic salts and materials impregnated with inorganic salts of the same cation. 7. In the case of material impregnated with copper-tartrate the contribution by each individual component to the overall adsorption was determined. Contribution of chemisorption is about lo%, while the rest belongs to physical adsorption, to which the contribution is given by elementary carbon cloth and organic component of impregnant. Total adsorption is not a simple sum of adsorptions of each component separately, which means that an additional synergetic effect exists in such a case. This proves the assumption that the organic component has the function of increasing the overall sorptive capacity. Acknowledgement-The authors wish to thank the Department of Toxicology for supporting the research presented in this paper and also to the Serbian Research Council for their support. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
M. M. Dubinin, Carbon 25(5), 593 (1987). M. M. Dubinin, Carbon 27(3), 457 (1989). P. G. Hall and P. M. Gittins, Carbon 23(4), 353 (1985). P. L. Gai, Carbon 27(l), 41 (1989). Lj. V. Rajakovic and M. M. Mitrovic, Enuiron. Pollut. 75, 279 (1992). B. McEnaney, Carbon 26(3), 267 (1988) K. S. W. Sing. Carbon 27(l). 5 (1989). H. F. Stoeckii, Carbon 28(l); 1 (1990). G. B. Freeman, P. B. Rao and P. J. Reucroft, Carbon 18(l), 21 (1980). V. R. Deitz and J. A. Rehrmann, Carbon 28(2/3), 363 (1990). M. R. Ilic, P. B. Jovanic, P. B. RadoSeviC and Li. V. Rajakovic, J. Serb. Chem. Sot. 60, 507 (1995). _ M. R. Ilic and Lj. V. Rajakovic, 6th Congress of Toxicology of Yugoslavia 2, 407 (1994). M. R. IliE, P. B. Jovanic, P. B. RadoSeviC and Lj. V. Raiakovic, I Congress of Electronic Microscopv __ of Yugoslavia, Novi s‘gd (1994). M. R. IIiC, P. B. Jovanic, P. B. RadoSevic and Lj. V. Rajakovic, II Symposium on Glasses and Ceramics, Arandjelovac (1994). V. R. Deitz and C. J. Karwacki, Carbon 32(4), 703 (1994). M. R. Ilic, P. B. Jovanic, P. B. RadoSevii: and Lj. V. Rajakovic, Separation Sci. Technol. 30( 13), 2707 (1995). J. Economy and R. Y. Lin, Appl. Pal. Symp. 29, 199 (1976). J. Kloubek and J. Medek, Carbon 24(4), 501 (1986). C. T. Chiou and P. J. Reucroft, Carbon 15, 49 (1977). Lj. V. Rajakovic, PhD Thesis, Faculty of Technology and Metallurgy, University of Belgrade, Belgrade (1986). Lj. V. Rajakovic, Separation Sci. Technol. 27( ll), 1423 (1992). L. Goetwick and L. H. How, Carbon 31(3), 46 (1993). G. Epstein et al., Carbon 31(6), 114 (1993). L. M. Taw et al., Carbon 31(4), 66 (1993).
Pergamon 0008-6223(95)00093-3
STOICHIOMETRIC ANALYSIS OF CHEMISORPTION OF HYDROGEN-CYANIDE ONTO ACTIVATED CARBON CLOTH LJ. V. RAJAKOVI~,’ M. R. ILIC,~ P. B. JOVANI~~ and P. B. RADOSEVIC~ ‘Faculty of Technology and Metallurgy, Karnegijeva 4, 11 000 Belgrade, Yugoslavia ‘Institute for Technical and Medical Protection, Kataniceva 15, 11 000 Belgrade, Yugoslavia (Received 29 September
1994; accepted
in revised form 11 May 1995)
Abstract-Based on the experimental results, and on the analysis of the chemistry of cyanide ion bonding, possible mechanisms of bonding of this pollutant were assumed and confirmed. Activated carbon cloth (ACC) was impregnated with metalorganic compounds consisting of the following cations: copper, silver, iron, magnesium and aluminum, and the following anions: acetate, oxalate, tartrate, stearate and citrate. Cation and anion content of impregnants on the cloth were determined by atomic absorption spectrometry and ion and gas chromatography. The analysis of stoichiometric utilization of chemical agent was done by calculation and the results of calculating the degree of stoichiometric utilization are presented. The image of the surface morphology of the materials were made by scanning electronic microscopy (SEM). Sorption properties of ACC were examined for hydrogen-cyanide and benzene in the gas phase, by standard gravimetric procedure in static conditions, with the use of Cahn RG electrobalance. Carbon materials impregnated with copper(H)-tartrate, silver(I)-citrate, and iron(III)-citrate, which demonstrated the best properties, and the best stoichiometric utilization, are completely characterized together with determination of basic parameters of the sorption system, which enables their application as new materials for chemisorption of cyanide. The salts of magnesium and aluminium that were used have no affinity for HCN bonding. Key Words-Activated
carbon
cloth, metalorganic
I. INTRODUCTION The
contribution
sorption
of this
is in developing
paper
in the field
impregnated
carbon
of chemimateri-
l-41. The testings concerned a large number of samples of carbon materials impregnated with metalorganic compounds, including the analysis of interactions with hydrogen-cyanide. Processes that take place on the boundary surface of carbon material-by means of their influence on the strength of the bond between chemical agent (impregnant) and pollutant-determine adsorptive capacity of carbon materials [ 5-S]. This is analyzed in detail for individual cases and represents an important contribution of this paper. The purpose of this work was to find a convenient separation technique and a convenient carbon material for removing cyanide from the air on the basis of a chemisorption process[9,10]. To that purpose, ACC was additionally activated by chemical impregnation. In this way, it could be possible to separate cyanides efficiently by its bonding to an active chemical agent inside the cloth[ 11-141. Cyanogen, which is also formed as the product of reaction of metalorganic compounds with hydrogen-cyanide in the following step changes into oxamide and one HCN molecule[15]. The quantities of cyanogen that are measured during sorption are negligible and were not taken into account in the course of these testings. An especially important contribution of this paper is in determining the participation of separate components in chemisorption and total adsorption. As for the experimental field, this paper contains for the als
for
hydrogen-cyanide
sorption
from
air[
salts, hydrogen-cyanide,
stoichiometry.
first time a comprehensive research and analysis of impregnated carbon cloth before and after exposure to the impact of hydrogen-cyanide, as well as the effect of applying metalorganic compounds for expanding sorptive capacity of carbon materials. The activated carbon material impregnated in such a way can be used for producing filters for purification of air or industrial gases, both in peacetime and in war conditions, as well as for manufacturing protective clothes and associated masks. Chemical impregnation was done using metalorganic compounds. The role of metallic ions is to introduce a catalytic function, while the role of organic components is to increase the sorptive capacity of the cloth. Motivation for this work was an idea to investigate sorption on carbon “atoms”, which was experimentally achieved by using organic acids that have one, two or more carboxyl groups. Benzene was used as standard adsorbate for physical adsorption control. In this work, complete results of ACC impregnation, its characterization, and sorption capacity of the cloth are given. Directions for future research in the field of application of carbon materials impregnated with metalorganic compounds should comprise testings in dynamic conditions aimed at determining the time of protection until the penetration point. This would help to complete the testings on capacity of developed carbon materials. Moreover, it would also be very important to test sorption of pollutants such as phosgene and chlorine-cyan onto these materials, because it is assumed that these pollutants-which 1433
1434
LJ.V.
can be found in air following can be successfully sorbed as well.
2.
RAJAKOVIC et al.
accidental situationsby the same material
EXPERIMENTAL
Basic material used in this research was microporous ACC with specific surface of 1491.12 m*/g, made by carbonization and activation of viscous rayon, a standard production type, produced by “Toyobo”-Osaka (Japan). Acetates, oxalates, tartrates, stearates and citrates of copper, silver, iron, magnesium and aluminium as impregnation reagents were used. Concentrations of solutions of these salts were 0.1 mol/dm3, and they were made from MERCK and ZORKA chemicals. The experimental procedure for impregnation and characterisation of carbon cloth have been described elsewhere [ 161. Metallic ions were determined using atomic absorption flame spectrometry. The contents of acetates, oxalates, tartrates and citrates were determined by ion chromatography. The stearate content was determined by gas chromatography. Sorptive properties of the samples were examined on a gravimetric adsorption apparatus, by standard procedure in static conditions, as described previously [ 161. A Cahn RG electrobalance was used. Measurements were taken in the interval of relative pressures ranging from 0.0 to 0.3. With the Cahn electrobalance, the increase in mass of adsorbates during the introduction of portions of gas into the system was measured. The gas portion was observed by the pressure change. Adsorption was performed at an initial pressure of 13.2 Pa. The sample was previously evacuated for 120 minutes to clean the surface from impurities. Results were processed using Langmuir adsorption isotherms.
3. RESULTS AND DISCUSSION Quantitative analysis of impregnants indicated that cations and anions show various affinities towards ACC. Affinity of anions decreases in the following order: acetates, tartrates, citrates, oxalates and stearates, which corresponds to the increase of the ion size. Good adherence of tartrates and citrates to the surface could be explained by electrostatic attraction, and by the geometry and dimensions of the molecules. Tartrates and citrates attach to electropositive locations on the surface of ACC by their electronegative hydroxyl OH- and carboxyl COOgroups, by Van der Waals forces. Cations, active for reaction with cyanide ions, remain on the other side. Stearate molecules, being too large, can attach to the fiber surface only with its “tail”. As such, it is not active in reaction with cyanide ions, and, most likely, it blocks some of the micropores because of its size. The aim of impregnating the ACC with such salts was to find out whether the carbon atoms from the impregnants will be free and active for adsorbing the cyanide. However, it seems that the number of carbon
atoms is not decisive; rather, decisive are the optimally bonded copper and silver cations, if the salts are properly chosen. The content of cations, determined by the AAS method, is given in Table 1. It is noticeable that there is a difference in the percentage of impregnant held within the samples that were washed after impregnation and those that were not. Generally, the silver salts (silver(I)-acetate 12.26 mass % of silver) and copper salts (copper(II)acetate 6.72 mass % of copper) attached strongly to the surface of the fibres. This is not puzzling, bearing in mind that all silver salts are very slightly soluble in water. For the same reason, the smallest percentage of the impregnants was removed by washing when silver salts were already present on the ACC surface. The anion content for samples with washing after impregnation is given in Table 2. All tartrates and citrates were adsorbed excellently by the cloth. A very high percentage of adhering was achieved only by copper( II)-acetate, even 19.34 mass % with samples washed after impregnation. All oxalates were adsorbed very poorly: from 0.027 mass %, with cloth impregnated with copper( II)-oxalate, to 0.25 mass % with cloth impregnated with silver(I)oxalate. From Table 2 it is noticeable that, in the sample impregnated with copper(stearate, there was 2.77 mass % of stearate, while in the sample impregnated with magnesium(II)-stearate there was 2.18 mass %. However, stearate was deposited on cloth mainly in the form of powder that falls off when the cloth is shaken, so that it does not achieve the desired function. Also it visibly covered the surface of the cloth and the micropores. This was confirmed by the adsorption of hydrogen-cyanide. Micrographs obtained by SEM helped in finding out the morphological structure of ACC. Micrographs in Fig, 1 also show that silver salts were adsorbed on the cloth better than other salts. Being the least soluble, they
Table 1. Content of cations on ACC determined by AAS Content of cations (%) Adsorbant ACC + Cu(II)-acetate ACC + Cu(lI)-oxalate ACC + Cu(Il)-tartrate ACC + Cu(II)-stearate ACC + Ag(I)-acetate ACC + Ag(I)-oxalate ACC + Ag(I)-tartrate ACC + Ag(I)-citrate ACC + Fe(B)-acetate ACC + Fe(II)-oxalate ACC + Fe(II)-tartrate ACC + Fe(III)-citrate ACC + Mg(II)-acetate ACC + Mg(II)-oxalate ACC + Mg(II)-stearate ACC + Al(III)-acetate ACC + AI(III)-oxalate
Washed samples 6.72 0.53 2.19 0.39 12.26 6.78 5.62 3.50 0.68 0.41 0.49 1.08 0.24 0.23 0.09 0.17 0.03
Unwashed samples 10.84 3.33 7.30 8.77 14.55 8.92 7.02 3.53 0.76 1.28 0.96 1.34 0.25 0.55 0.33 0.35 0.21
Chemisorption of hydrogen-cyanide onto activated carbon cloth Table 2. Content
of anions on ACC determined chromatography
by ion
Content of anions (%) Adsorbant ACC + Cu(II)-acetate ACC + Cu(II)-oxalate ACC + Cu(II)-tartrate ACC + Cu(II)-stearate ACC + Ag(I)-acetate ACC + Ag(I)-oxalate ACC + Ag(I)-tartrate ACC + Ag(I)-citrate ACC + Fe(II)-acetate ACC + Fe(II)-oxalate ACC f Fe(II)-tartrate ACC + Fe(III)-citrate ACC f Mg(II)-acetate ACC f Mg(II)-oxalate ACC + Mg(II)-stearate ACC + Al(III)-acetate ACC + A&III)-oxalate
Washed samples 19.34 0.03 5.60 2.77 1.79 0.25 8.42 2.58 2.07 0.04 10.32 2.82 0.22 0.04 2.18 1.50 0.07
Unwashed samples 19.99 0.15 6.68 4.11 2.70 0.28 14.11 3.55 2.27 0.05 10.87 3.38 0.24 0.04 5.46 3.18 0.07
coated the surface of each fiber with particles about 1 pm in diameter. Carbon fiber is about 15 pm thick[ 171. With samples impregnated by iron salts, there were no conspicuous traces of salts. Only with some difficulties, traces of these salts may be seen on the surface of fibres; particles are smaller than 1 pm. The fibres were smooth and shiny. Fibres impregnated with aluminium salts looked similar. Salt particles were not visible in inter-fibrous space, but traces of salt adsorbed on fiber itself were visible. With samples impregnated with magnesium salts, particles of salts, both those adsorbed on the fiber and those in interfibrous space, are highly visible. These values are close to the fiber diameter. The results of determining specific surface by benzene isotherms are given in Table 3. Specific surfaces differ from sample to sample, because of a differing percentage of impregnant added to ACC. Namely, as Kloubek also shows in his work[ 181, impregnants may partly close the pores. Chiou and Reucroft [19] reached the same conclusion. If more impregnant is used, more space is blocked, and, therefore, specific surface is further reduced. It is logical that the highest percentage of salts was imposed on ACC when copper(acetate was used: 6.72 mass % of copper (Table 1) and 19.34 mass % of acetate (Table 2). This led to a reduction of specific surface from 1491.12 to 853.75 m’/g, which is about 40%. All salts caused a greater or smaller reduction of specific surface, which was expected. Drastic reduction was observed after impregnation with copper(stearate and magnesium(II)-stearate, when the specific surface was reduced to 270.76 m’/g and 473.13 m’/g, respectively, which is unsatisfactory. For samples that were not washed after impregnation, these values are even lower, namely, 205.06 m2/g for copper( II)-stearate and 405.08 m”/g for magnesium(II)-stearate. This means that between 70 and
1435
80% of the micropores were covered. Table 2 also shows that oxalates attach very poorly to ACC. The consequence of this was that specific surfaces remained great in samples impregnated by oxalates, as compared to samples impregnated by other salts. Reduction of specific surface was from 1 to 20%. By comparing the samples that were washed after impregnation and those that were not, it was found that the sorptive capacity of benzene for all types of salts used was lower by about 15% in unwashed samples, where the concentration of impregnants was greater and, therefore, more of the pores were blocked. Chiou and Reucroft[ 191 show in their work that chemisorption is important in the region of lower pressures, while under higher pressures physical adsorption dominates. Comparing adsorption isotherms, it is seen that adsorptive capacity is lower in the region of higher pressures. This suggests that impregnant blocked or occupied a part of the adsorption area of the pores, which resulted in a smaller volume of micropores. Such effects will reduce adsorptive abilities in the region of higher relative pressures. In the region of lower relative pressures, adsorptive capacity is greater than in non-impregnated carbon cloth. Comparing the types of cupric salts, the greatest adsorptive capacity of benzene was found in material impregnated by copper(oxalate (5.3 mmo1c6n6/ g,,, at relative pressure of 0.3), copper(tartrate (3.6 mmolcuJg,&, copper( II)-acetate (3.5 mmo1c.d gAcc) and copper(stearate (2.2 rnmolcsnJgAcc, all at the same relative pressure (Fig. 2(a)). When sorption isotherms of hydrogen-cyanide were analyzed, it was noticeable that adsorption capacity did not follow the above given order. The greatest sorptive capacity for hydrogen-cyanide was found in the with copper(tartrate sample impregnated (11.9 mmol,cN/g,c, at relative pressure of 0.3 with the tendency of further increase, which is seen from the shape of the isotherm). Sorptive capacity decreases in the following order: copper( II)-oxalate ( 10.0 mmol,,/g,), copper( II)-acetate (6.3 mmol,,/ g,,,), and copper( II)-stearate (3.3 mmol,,,/g,,c), all at the same relative pressure (Fig. 2(b)). If these results are compared with sorptive capacity of samples that were not washed after impregnation, it will be obvious that there are certain discrepancies. The sorptive capacity for HCN with the sample impregnated with copper( II)-acetate increased by about 50%, while with other types of salts this capacity reduced by about 40%, which means that a large number of pores were blocked for adsorption. A characteristic example is impregnation with copper( II)-stearate, which has been discussed above. Sorptive capacities are very high with all types of silver salts used for impregnation, and they are located in a very narrow range (4.6-5.5 mmolc~,J gAc-) at relative pressure of 0.3. Sorptive capacity decreases in the following order: silver(I)-citrate, silver( I)-tartrate, silver(I)-oxalate and silver(I)-
LJ.V. RAJAKOVI~.rt
1436
Fig. 1. MIcrographs of samples (a) ACC + Ag( I&acetate,
impregnated with (h) ACC + Ag( I)-oxalate
acetate (Fig. 3(a)). It is interesting that the same order holds for the sorption of HCN. Sorptive capacity of the sample impregnated with silver( I)-acetate, washed
d.
silver salts, magnification and (c) ACC + Ag( [)-citrate.
after impregnation, was 4.8 mmol,,,/g,,, and 9.6 mmol,,,/g,,, for unwashed, relative pressure of 0.3. This suggests
549x:
(Fig. 3(b)) at the same that, in this
Chemisorption of hydrogen-cyanide onto activated carbon cloth
1437
Table 3. Specific surface and specific volume of micropore Specific surface of micropores
Specific volume of micropores
(m2/g) Washed
Adsorbant ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC ACC
+ + + + + + + + + + + + + + + + +
of micropores
(cm3/g)
of micropores (cm3/g)
Unwashed
0.3130 0.4739 0.3225 0.0924 0.4218 0.4480 0.48 18 0.4929 0.5050 0.5396 0.3088 0.4211 0.4584 0.5436 0.1680 0.5212 0.4933 0.5482
0.2911 0.4385 0.3100 0.0692 0.3721 0.4162 0.4567 0.463 1 0.4648 0.5126 0.2954 0.3974 0.4367 0.4854 0.1407 0.4907 0.4640 0.5482
I I I I I I I I I I I I I 0.02 0.06 0.10 0.14 0.18 0.22 0.26
PIP,
PIP,
(b)
16
r
samples
802.91 1183.69 848.10 205.06 1025.21 1133.03 1225.32 1242.52 1244.43 1372.88 797.78 1068.89 1177.48 1298.74 405.08 1320.69 1252.13 1491.12
0.02 0.06 0.10 0.14 0.18 0.22 0.26 16
Specific volume
(m’ig)
samples
853.75 1299.80 896.62 270.76 1147.29 1223.61 1307.37 1341.60 1370.70 1471.30 834.98 1143.85 1250.85 1480.95 473.13 1413.85 1340.05 1491.12
Cu(II)-acetate Cu(II)-oxalate Cu(II)-tartrate Cu(II)-stearate Ag(I)-acetate Ag(I)-oxalate Ag(I)-tartrate Ag(I)-citrate Fe(II)-acetate Fe(II)-oxalate Fe(H)-tartrate Fe(III)-citrate Mg(II)-acetate Mg(II)-oxalate Mg(II)-stearate Al(III)-acetate AI(III)-oxalate
Specific surface
& X8 za
(b)
12
on*
EE 04 0.02
0.06
0.10
0.14 0.18 0.22 0.26 0.02 0.06 0.10 0.14 0.18 0.22 0.26
PIP,
PIP,
Fig. 2. (a) Adsorption isotherms of benzene for samples impregnated with copper salts: (+) ACC +Cu(II)-acetate, (A) ACC + Cu(II)-oxalate, (0) ACC +Cu(II)-tartrate and (0) ACC + Cu(lI)-stearate. (b) Adsorption isotherms of hydrogen-cyanide for samples impregnated with copper salts: (+) ACC + Cu(II)-acetate, (A) ACC+ Cu(II)-oxalate, (0) ACC+Cu(II)-tartrate and (0) ACC+Cu(II)-stearate.
Fig. 3. (a) Adsorption isotherms of benzene for samples impregnated with silver salts: (+) ACC +Ag(I)-acetate, (A) ACC + Ag( I)-oxalate, (0) ACC + Ag( I)-tartrate and (q) ACC + Ag( I)-citrate. (b) Adsorption isotherms of hydrogen-cyanide for samples impregnated with silver salts: (+) ACC + Ag(I)-acetate, (A) ACC + Ag(I)-oxalate,
case, it is possible to achieve better chemisorptive ability by increasing the quantity of impregnant. Samples impregnated with silver(I)-citrate did not show significant changes in sorptive capacities for HCN whether they were washed or not washed. Sorptive capacity at a relative pressure of 0.3 was 11.1 mmol,,.,/g,,, and 11.5 mmol,,,/g,,,, respectively. This means that adding further quantities of impregnant, beyond a certain percentage, does not
produce any effect. It could be concluded that all chosen silver salts may be efficient as impregnants, although the material with impregnated silver( I)-citrate has somewhat better characteristics than the others. Adsorptive isotherms of benzene in the samples impregnated with iron salts showed that the highest capacity was obtained with samples impregnated with
(0) ACC + Ag( I)-tartrate
and
(q) ACC + Ag(I)-citrate.
1438
LJ.V. RAJAKOVI~'etal.
iron(oxalate and subsequently washed (6.0 mmol CsHs/gAcc at relative pressure of 0.3). The capacity of iron(acetate was (5.8 mmol,-,H,/gA,c), iron(III)citrate (4.6 mmolc6,Jg,,c), and iron(tartrate (3.6 mmolc,,,/g,c,), all at the same relative pressure (Fig. 4(a)). Sorptive capacity for HCN increases from samples impregnated with iron(acetate (7.4 mmolHcN/gAc-), iron( II)-oxalate (8.5 mmol,,,/ iron(tartrate (10.7 mmol,c,/g,cc), to gACC)> iron(III)-citrate (14.1 mmol,,,/gA,), the last showing excellent sorptive capacity (Fig. 4(b)). Taking into consideration both the sorption of benzene and that of hydrogen-cyanide, material impregnated with iron(III)-citrate possesses the best sorption characteristics. As could be expected, sorptive capacity for benzene of samples impregnated with magnesium(II)-acetate and magnesium(II)oxalate can be very high, but this is not the result of the impregnant used. Material impregnated with magnesium( II)-stearate confirms the theory discussed magnesium( II)-stearate blocked above. Namely, pores completely, so that sorptive capacity for benzene was very low (1.9 mmol,6,JgA,,). Sorptive capacity for HCN is very low for all samples impregnated with magnesium salts; it was not higher than with any of the samples. As in the 7.4 mmol&g,cc case of chemisorption of hydrogen-cyanide, the abilities of impregnant as chemisorbent were to be examined it is obvious that magnesium salts are not adequate for this purpose, because they do not show
l-
1 I I I I I I I I I I I I 0.02 0.06 0.10 0.14 0.18 0.22 0.26 P/P, Y--
(b)
affinity for HCN. Therefore, they will not be considered any further. Materials impregnated with aluminium salts showed very high sorptive capacity for benzene, above 5.1 mmolc,a,/g,cc. Isotherms for HCN show that this case is similar to the previous one. Sorptive capacity of these materials for HCN is very low, barely above 7.4 mmol,,,/gAcc. Therefore, they will not be considered any further, either. Adsorption isotherms of hydrogen-cyanide for non-impregnated ACC are given in Fig. 5. When cyanide ions chemically bond on ACC, precipitate and complex compounds are formed. These processes are carried out in two stages, with the formation of characteristic compounds in both cases. By favoring the conditions for producing complex compounds, it is possible to achieve higher stoichiometric utilization of chemical impregnant[20,21], which, in turn, gives ACC of higher sorptive capacity. ACC is a convenient medium for bonding complex compounds, due to adsorptive and chemical driving forces inside the structure of the cloth [ 22-241. Based on the experimental results, and on the analysis of the chemistry of cyanide ion bonding, possible mechanisms of bonding of this pollutant can be assumed and confirmed. Structural activity of ACC is twofold: the ACC itself without any chemical impregnant maintains its adsorptive functions, but the impregnant, added to ACC, acts chemically towards formation of precipitate and complex compounds. Besides the adsorptive and chemical forces, which affect the active bonding of the pollutant, other important factors are: the geometry of the system, flow of the air through the pores of the system, diffusion rate, and the kinetics of surface reactions. The system is obviously complex. For non-specific pollutants, the mechanism of bonding to a chemical agent is very important. Movement of the chemical agent and cyanides inside ACC, and the mechanism of bonding, may be presented as follows: in the first stage, cyanide ions and copper form the precipitate of copper(I)-cyanide, Cu(CN). In the second stage, new quantities of cyanide ions decompose the precipitate, and a tetracyanocuprate( I) complex ion [Cu(CN).Jis formed. Further quantities of cyanide ions could only be adsorbed and this process contin-
l6r 0.02 0.06 0.10 0.14 0.18 0.22 0.26 PIP, Fig. 4. (a) Adsorption isotherms of benzene for samples impregnated with iron salts: (+) ACC+ Fe(H)-acetate, (A) ACC + Fe(U)-oxalate, (0) ACC + Fe( II)-tartrate and (0) ACC + Fe( III)-citrate. (b) Adsorption isotherms of hydrogen-cyanide for samples impregnated with iron salts: (+) ACC + Fe( II)-acetate, (A) ACC + Fe(II)-oxalate, (0) ACC+ Fe(II)-tartrate and (0) ACCf Fe(III)-citrate.
0.02 0.06 0.10 0.14 0.18 0.22 0.26 PIP, Fig. 5. Adsorption isotherms for non-impregnated (0) adsorption isotherm of hydrogen-cyanide.
ACC:
Chemisorption of hydrogen-cyanide onto activated carbon cloth
1439
Table 4. Degree of stoichiometric utilization of chemical impregnant
Adsorbant
Contribution of physical adsorption (%)
Contribution of chemisorption W)
88.7 90.9 89.4
11.3 9.1 10.6
ACC + Cu(II)-tartrate ACC + Ag(I)-citrate ACC + Fe(III)-citrate
ues until the sorptive capacity of the cloth is completely exhausted. The analysis of stoichiometric utilization of chemical agent started by calculating, for the sample impregnated with copper-tartrate, the number of mols of copper absorbed by a gram of carbon cloth. The obtained value is 0.00034 mol,Jg,,,. If this sample by similar computation we bonds 0.32 gnc,lg,cc, obtain 0.0119 mol,,N/g,,c. The residue chemisorptive mass after desorption under vacuum amounts to 11.3%, which means 0.00134 molnCN/gACC, as is shown in Table 4. It ensues that 0.00034 mol Cu bonds 0.00134 mol HCN. The obtained stoichiometric ratio is Cu+ : CN- = 1 : 3.9. This means that the formation of complex ions is favored in relation to precipitate compounds, especially of ion [CU(CN),]~and a part of precipitate CuCN. This is the most favorable stoichiometric ratio, because it gives the highest stoichiometric utilization of the chemical agent. Already in the sample impregnated with copper-oxalate the ratio was Cu+ :CN- = 1: 2.8, which means that the formation of precipitate compounds is favored. In other words, mostly CuCN and some [Cu(CN)J ions are formed. The sample impregnated with silver-citrate has and bonds 0.00101 molrtc,/g,cc. 0.00032 mol,,/g,,, The obtained stoichiometric ratio is Ag’:CN= 1: 3.1. This ratio would lead to the conclusion that the stoichiometric utilization is even over lOO%, which is of course impossible, but it can be concluded that the formation of complex ions [Ag(CN),]is certainly favored, while the rest belongs to good physical adsorption. The sample impregnated with iron-citrate has 0.00020 mol,/g,,, and bonds The obtained stoichiometric 0.00148 mol,,.,/g,c,. ratio is Fez+ : CN- = 1: 7.4. This is close to the theory about the formation of the [Fe(CN),14ion, while the rest is in greater contribution of physical adsorption, that is, the mass retained by physical bondings Table 5. Adsorption
Degree of stoichiometric utilization
Stoichiometric ratio 3.9 3.1 7.4
0.97 Xl Xl
on the sample. The results of calculating the degree of stoichiometric utilization are presented in Table 4. It is obvious that the best stoichiometric utilization of the chemical agent dominates among the selected samples, which is exactly what makes the corresponding materials so good. The materials impregnated with inorganic copper and silver salts, tested in a previous period, were compared with new materials impregnated with organic copper and silver salts. In that way, it was possible to isolate the contribution to adsorption by the organic component alone. Table 5 gives the values of hydrogen-cyanide sorption for the samples of materials impregnated with both organic and inorganic salts. It is evident that there is a difference in the sorptive capacity of materials impregnated with inorganic salts in relation to those impregnated with organic salts. A contribution by the organic component to the sorptive capacity exists and it amounts to even as much as 50% in relation to the contribution by the inorganic component. After determination of adsorption isotherms for materials impregnated with pure acids-tartaric and citric acids (Fig. 6kit is now possible to decompose the contributions by each separate component to the overall adsorption. Figure 7 shows histograms for selected materials impregnated with copper-tartrate, silver-citrate and iron-citrate. As was initially assumed, the metallic cation is responsible for chemisorption and its share in total adsorption is about 10%. The organic component increases physical adsorption and this can be seen by comparing the adsorption isotherms for hydrogencyanide in non-impregnated carbon cloth (Fig. 5) and in carbon cloth impregnated with organic acids (Fig. 6). Nevertheless, total adsorption is not a simple sum of adsorptions of separate components. This is obvious from the presented histograms, since sorption
capacity of hydrogen-cyanide for samples impregnated organic and inorganic salts
Adsorbant ACC + CuCO,Cu(OH), ACC + CUCO,CU(OH)~ ACC + Cu(II)-tartrate ACC + Cu(II)-oxalate ACC + Cu(II)-acetate ACC + Ag(I)-nitrate ACC + Ag(I)-citrate
with
Contents of cations (%)
Adsorption capacity (mm&,/&u,)
1.3 5.8 2.19 0.53 6.72 2.4 3.50
7.4 4.8 11.9 10.0 6.3 4.4 11.1
1440 16
r (a) 0.02
0.06
LJ.V. RAJAKOWC et crl
0.10
0.14
0.18
0.22
0.26
0.18
0.22
0.26
PIP0 @)
0.02
0.06
0.10
0.14 Q/Q0
Fig. 6. Adsorption isotherm of HCN for samples impregnated with (a) tartarlc
16 -
acid and (b) citric acid.
r
CU
Tanaric acid
Figure 8 presents adsorption isotherms for hydrogen-cyanide at various temperatures in carbon cloth impregnated with copper-tartrate. As was expected, the adsorption isotherms show reduction in physical adsorption with increase in temperature. After desorption by vacuuming until initial pressure, the residue masses were bonded by strong chemisorption effect on the samples. It can be seen that the change in adsorption temperature influences much more physical adsorption than chemisorption. Figure 9 presents the histogram of the temperature influence on the contribution by each component in adsorption. It was concluded that sorption of cyanide slightly increases in the case of the material impregnated only with copper ions. which means that, in the course of sorption, there is a chemical reaction between copper and cyanide. In the case of sorption of cyanide onto material impregnated only with tartaric acid, the sorptive capacity is lower at an increased temperature. This can be explained only by the fact that physical adsorption of cyanide is taking place on the surface. In the case of applying coppertartrate, the sorptive capacity of cyanide at an increased temperature decreases. This is understandable, since the share of physical adsorption is much higher than the share of copper as the component that is the carrier of the chemisorptive function. Sorption for the material impregnated with coppertartrate also decreases with the increase in temperature, because in this case as well there is a considerable share of physical adsorption within the total adsorption.
Cu-tanratc
16 16
12
5 X:: ‘t;q
8
M
r
1 (0) 25oc
2 -4 D
Al7
Citric acid
Ag-citrate
16 0.02
0.06
0.10
0.14
0.18
0.22
0.26
PIP, E E -4
Fig. 8. Adsorption isotherms nated with copper-tartrate
M
of HCN for samples impregat different temperatures.
u Fe
Fig. 7. Adsorption
capacity
Citric acid
of HCN
Fe-citrate
for several
components.
onto materials impregnated with organic salts is higher. This means that an additional synergetic effect exists in this case. In that way, the assumption from the Introduction of this paper has been proved, namely, that the organic component has the function of increasing the sorption capacity of carbon materials.
CU
25°C
Fig. 9. AdsorptIon
Tartaric 41°C
25°C
acid 47QC
Cu-tartrate 25°C
41nc
capacity of HCN for several components at different temperatures.
Chemisorption of hydrogen-cyanide onto activated carbon cloth 4. CONCLUSIONS
The purpose of this work was to study, investigate and develop impregnated activated carbon cloth, which unites, in its structure, adsorptive and chemisorptive properties; this would contribute to the study and application of such materials for air purification from toxic gasses, especially cyanides. On the basis of the results obtained in this experimental work, the following conclusions could be made: 1. ACC impregnation with metalorganic compounds was performed by acetates, oxalates, tartrates, citrates and stearates of copper, silver, iron, magnesium and aluminium. Samples with and without washing after impregnation were tested. 2. Characterisation of impregnated ACC was performed by modern techniques. The content of metallic cations was determined by atomic absorption spectrometry, while the content of organic acid anions was determined by ion and gas chromatography. The surface composition images and the materials morphology were made using scanning electron microscopy. isotherms for benzene and 3. Adsorption hydrogen-cyanide in impregnated ACC were obtained by gravimetric adsorption apparatus. It was found that chemisorptive activity was realised by adding an active chemical agent (impregnant) into the material structure. The effects of metallic ions and their organic salts on hydrogen-cyanide and benzene were studied. Bonding efficacy of the pollutants was analysed, and it was found that the greatest contribution was made by the following materials: ACC impregnated with copper( II)-tartrate, then silver(I)-citrate, and iron(lII)-citrate. 4. By analysis of possible chemistry in the structure of ACC it was found that in the pollutant and chemical agent (impregnant) reaction, precipitate and complex compounds were formed. The precipitate and complex compounds ratio depends directly on pollutant concentration. At higher concentrations, formation of complex compounds predominates, and at lower concentrations formation of precipitates predominates. 5. Temperature influence on sorption was tested by determining the capacity of impregnated material in the process of hydrogen-cyanide sorption at two different temperatures. The testings used carbon material impregnated only with copper ions, only with tartaric acid and finally with copper-tartrate. It was concluded that sorption of cyanide slightly increases in the case of the material impregnated only with copper ions, which means that in the course of sorption there is a chemical reaction between copper and cyanide. 6. The contribution by organic component to
1441
total adsorption was obtained by comparing sorptive capacity of materials impregnated with organic salts and materials impregnated with inorganic salts of the same cation. 7. In the case of material impregnated with copper-tartrate the contribution by each individual component to the overall adsorption was determined. Contribution of chemisorption is about lo%, while the rest belongs to physical adsorption, to which the contribution is given by elementary carbon cloth and organic component of impregnant. Total adsorption is not a simple sum of adsorptions of each component separately, which means that an additional synergetic effect exists in such a case. This proves the assumption that the organic component has the function of increasing the overall sorptive capacity. Acknowledgement-The authors wish to thank the Department of Toxicology for supporting the research presented in this paper and also to the Serbian Research Council for their support. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
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