Carbon Vol. 20. No. 2, pp. 113-116, 1982 Printed in Great Britain.
llW-6223/82/020113-04$03.00/0 0 1982 Pergamon Press Lid.
MOISTURE SORPTION BY MODIFIED-ACTIVATED CARBONS A.M. YOUSSEF ChemistryDepartment,Faculty of Science,MansouraUniversity,Mansoura,Egypt and T. M. GHAZYand TH. EL-NABAIUWY SurfaceChemistryLaboratory,NationalResearchCentre, Dokki,Cairo,Egypt (Received10December 1980) .&&act-Modified activated carbons were prepared by the partial oxidation of activated maghara coal with concentrated nitric acid and potassium permanganate. This treatment fixes more oxygen on the carbon surface leading thus to the formation of carbon-oxygen complexes which act as primary centers for water adsorption. Not only the number of oxygen complexes that can determine the water adsorption properties but the pore size may be considered as another factor. The oxygen complexes outgassed as carbon dioxide can be oualitativelv determined bv neutralization with sodium hydroxide. These oxygen complexes are responsible for the iireversibly adsorbed water. 1. -NTAL
Carbonaceous materials contain appreciable amounts of combined oxygen and hydrogen which give rise to stable surface complexes. Several hypothetical structures have been assigned to these complexes[l, 21, and in recent years, attempts have been made to carry out more direct analysis of the surface oxide layer by studying specific chemical reaction[2-51. However the various methods of estimation have not yielded comparable results. In a previous investigation, the author was able to find a relation between the existence of the carbon-oxygen complexes and the moisture sorption of carbons. Probably some of these complexes evolve carbon dioxide on heating the carbon under reduced pressure and similarly there are distinct surface entities which evolve carbon monoxide and water vapour by this treatment. The existence of an oxide layer on the carbon surface is responsible for some important properties of carbons, e.g. their wettability, adsorptive power and dispersibility. Moreover, surface oxides of carbon play an important part as oxygen transferring agent in atmospheric oxygen electrodes. The acidity and redox behaviour of surface oxides can be used for the removal of salts from sea water. Surface oxides of carbon are considered as primary sites  for water adsorption, and therefore, it is required to fix more oxygen complexes on the carbon surface if carbon with high adsorptive capacities towards water vapour are required. It is well documented that the adsorptive capacities of vapor by non-activated (low surface area carbons) and by activated (low surface area carbons) and by activated (high surface area carbons), are very low. In this investigation trials were made to increase the adsorptive capacities of carbon, obtained from Egyptian maghara coal, towards water vapour by treatment with oxidizing solutions. The factors affecting water vapour adsorption are studied and the relation between these factors with the surface properties is discussed.
A representative sample of lower bed maghara coal, Egypt, was used, the sample is highly volatile coking coal of sub-situminous type softening at 349”.A carbonization product was obtained by destructive distillation at WC in a limited amount of air, this product is designated “M”. An activated maghara coal with 50% zinc chloride at 600°C in limited air; this active sample is designated “ZM”. Air-activated maghara coal sample “AM” was obtained by gasification with air at 500°C to 30% burnout. Carbon dioxide activated maghara coal “CM” was obtained by gasification with carbon dioxide at 800°C to 30% burn-out while steam-activated maghara coal “SW was prepared by gasification with steam at 950°C to 30% burn-out.
Oxidation of the activated carbons
Modified activated products NZM, NAM, NCM and NSM were prepared by treating ZM, AM, CM and SM respectively with concentrated nitric acid (lOmUg). The mixture was heated at 80°C till free from nitrate group. The carbon was then dried at 120°C till constant weight and stored in stoppered bottles. Another series of modified activated carbons KZM, KAM, KCM and KSM were prepared as follows: 1 g of Zm, AM, CM and SM was mixed with 50ml of aqueous solution of 0.1 B potassium permanganate at room temperature for 48 hr. The treated carbons were thoroughly washed with distilled water, dried at 120°C and stored in stoppered bottles.
Surface area determination
The surface areas were calculated from the adsorption of carbon dioxide at 25°C using a volumetric apparatus of the conventional type. 113
A. M. YOUSSEF et al.
Base adsorption capacity Adsorption of sodium hydroxide, sodium carbonate and sodium bicarbonate was studied by shaking 0.5 g carbon with 50 ml of 0.2 N base solution in Pyrex glass bottles for 48 hr and titrating an aliquot of the clear supernatant liquid against 0.1 N HCI. Estimation of confined oxygen This was made by an apparatus consisting of 50cm’ high purity silica reactor leading into (8.51) reservoir connected to a calibrated McLeod gauge to measure the pressure of the outgassed CO and CO,. CO, was first solidified in a liquid nitrogen trap and the pressure exerted by CO was followed at ditTerent temperatures. At the end of the run, the liquid nitrogen trap was replaced by carbon dioxide-acetone freezing mixture to determine the pressure exerted by the outgassed COz. Water vapour adsorption The adsorption of water vapour at 30°C was carried out with the aid of spring balances of the McBain-Bakr type[lO]. An arbitrary adsorption time of 5 hr was allowed for each point on the isotherm.
nonactivated maghara coal. The carbon dioxide areas decrease with treatment with either nitric acid or potassium permanganate. Treatment with these oxidizing solutions fix oxygen complexes on the surface which occupy a fraction of the surface. Table 1 indicates that the decrease in carbon dioxide areas goes parallel with the uptake from NaOH. Sodium hydroxide is adsorbed by the oxygen complexes and therefore the amounts of this base adsorbed by the carbon may be taken as a measure for the fraction of the surface occupied by the oxygen complexes. It is seen from Table 1 that in all cases, titration with NaHCO$ gives the lowest values of base neutralization capacities while titration with NaOH gives the highest values. Titration with Na2C03 measures intermediate values. These results indicate that the strength of the base plays an important role in determining the base adsorption capacity. Probably weak acid sites are only neutralized by strong bases. It is evident also from Table 1 that the amount of carbon dioxide evolved could be taken as a quantitative measure of the total neutralization capacity, i.e. to the values absorbed from NaOH. The results obtained by Puri et a/.[121 indicated a very good agreement between the outgassed carbon dioxide and the uptake by carbon
3. REWLTS AND DISCUSSION
adsorption of carbon dioxide at 25°C was interpreted using the Dubinin-Polanyi equation (abbreviated as D-P equation) The
log v = log vo - B(RT log P,PP.
where V(cd/g) is the amount of carbon dioxide adsorbed under a relative pressure P/P0 at an absolute temperature 7” and R is the gas constant. Figure 1 shows the linear D-P plots of carbon dioxide adsorbed on maghara coal and some of its activation products. The micropore volume VOcould therefore be calculated in each case and apparent areas were accordingly estimated using the value of 25.3 AZas the molecular area of carbon dioxide [ll]. The D-P areas of activated maghara coal as measured by carbon dioxide, are considerably higher than the carbon dioxide area of
Fig. 1. Linear D-P plots of carbon dioxide adsorbed on nonactivatedand activatedmagharacoals.
Table 1. Surface area, base neutralizationcapacity,CO2evolved and irreversiblyadsorbedwater for the various investigatedcarbons
Base capacity(mequilg) CO+volved m equivlg
Irrev. water (m&t)
ZM 6: :!M
295 425 345 268 478
0.05 0 :0.32
0.08 0 0.45 :
0.10 If is*
: 0 106 0
NSM KZM KAM KCM KSM
0.80 0.45 0.30 0.45 0.50
I.56 0.82 0.60 0.88 1.00
1.50 0.75 0.63 0.92 1.03
;! 90 7 12 15
Moisture sorption by modified-activated carbons
of NaOH or BaOH or Ba(OH),, expressed in the same units. The moisture sorption isotherms of M, AM, NZM, NAM,NCM and NSM are given in Fig. 2. It is seen that the isotherms of oxidized-activated carbons are type II with open hysteresis in which desorption isotherms do not meet the adsorption ones even at zero relative pressure. Further, a certain amount of water was left even on outgassing the system again at the isotherm temperature to constant weight. For the NZM sample the isotherm is very steep in its initial part exhibiting also open hysteresis. The only exceptional cases are those of nonoxidized carbon (M and ZM in Fig. I), where the sorption-desorption isotherms meet at some intermediate relative pressures. The irreversibly adsorbed water is given in Table 1 expressed as (mg/g). It has been proved in a previous investigation that the surface areas of carbons as measured by water vapour adsorption do not have any scientific significance. The uptake of water vapour has been proved to be a measure of the adsorption sites on the coal surface. i.e. a measure of the oxygen functional groups. Figure 3 shows a relation between irreversibly adsorbed water (mgfg) and the amount of carbon dioxide outgassed in (m equivalent/g), for oxidized-activated samples. It is seen that the relation is a straight line indicating that the amount of irreversibly adsorbed water depends on the number of the oxygen complexes. Exceptionally
NZM and KZM do not lie on the straight line and the irreversible adsorbed water is by far very high to be compared with the number of oxygen complexes as determined by the outgassed carbon dioxide. In the case of zinc-chloride-activated maghara coal, treatment with either nitric acid or potassium permanganate fix oxygen on carbon atoms located in micropores. Therefore, in spite of the existence of a relatively small number of complexes, the removal of water even by outgassing this microporous carbon seems difficult. Accordingly, not only is the number of the oxygen complexes a factor in determining the irreversibly adsorbed water, but the pore size of carbon is another factor. The shape of the water adsorption isotherm of “NZM" is another evidence for this explanation. This isotherm exhibits a very steep
Fig. 3. Relative between outgassed carbon dioxide and irreversibly adsorbed water.
initial portion characteristic of microporous sorbents such as molecular-sieve zeolites. Evidently the straight line in Fig. 3 represents oxidized-activated carbons in which the activation process involves the gasification with oxidizing gases which cause erosion of the micropores producing thus relatively wider pore structures [ 131. From the previously presented results, the surface oxides on carbons represent water adsorption sites. In the initial pressure region adsorption is chiefly due to the formation of hydrogen bonds between water molecules and primary adsorption centers. Adsorbed water molecules may act as secondary adsorption centers, which can retain other molecules by means of hydrogen bonds. As the pressure rises the probabilities of adsorption will increase, owing to the increase in the number of secondary adsorption centers, that is, adsorbed water molecules themselves. The steep rise in the adsorption isotherm in the region of mean equilibrium relative pressure 0.345 is connected with the appearance and growth of these complexes from the associated water molecules. Although water adsorption capacities here found by modified activated carbon are still low compared to the capacity of other dehydrating agents, the results seem promising and stimulate further investigation. The sorption of water vapours by active carbons differs drastically, both in its nature and mechanisms, from the sorption of vapours of many other substances encountered in present day technology. A number of essential problems still remain unsolved in this field and researches should give it serious attention. REFERENCES
Fig. 2(a). Adsorption desorption isotherms of water vapour on samples, M, Zm and NZM. Fig. 2(b). Adsorption-desorption isotherms of water vapour on samples NAM,NCM and NSM.
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