Adsorption of Toluene on Film Membranes of Chitosan/Nа-CMC

The sorption properties of films on bases of chitosan and sodium carboxyl methylcellulose (Na-CMC) with toluene have been checked. The sorption rate, sorption thermodynamics and isothermal properties of toluene molecules in chitosan/Na-CMC films are analyzed by adsorption-calorimetric method. Thus, it was found that toluene is adsorbed on the chitosan/Na-CMC film by the same patterns of sorption of aromatic compounds, due to the process of penetration of toluene molecules into the network of chitosan macromolecules and this is preceded by the absorption process by the surface of the chitosan/Na-CMC film.


Introduction
Nowadays, the development of the chemical industry results in a negative impact on the environment. Toxic substances in the wastewater of the oil refining industry and petrochemical factories alone are leading to a deterioration of the ecological situation. One of the environmental problems that worry the whole world is the pollution of drinking and sewerage networks. Therefore, to solve this problem, a number of studies and practical work are being carried out.
Chitosan and its modifications are widely used for the sorption of various organic compounds and metal ions. For example, the work [1] involves bunch experiments to investigate the effect of contact time, pH, and adsorbent dose on concentration, pH and adsorbent dose. The chitosan and fibers (banana and areca) were then cross-linked with glutaraldehyde to remove chromium [Cr (VI)] from water via static adsorption.
In particular, Singaporean scientists Nai Naing and Sam Fong [2] conducted studies on the use of the sorption properties of the polysaccharide chitosan and complexes based on glutaraldehyde for the purification of organic substances (benzene, toluene, ethylbenzene, xylene and styrene) from wastewater. Giffin and Davis conducted research on the use of polysaccharides for the purification of toluene from water in oil refining [3].
In this study, interpolymer complexes were formed with the presence of Na salt of chitosan and carboxymethylcellulose [4]. The sorption properties of the obtained films were tested on the basis of experiments conducted in a high-vacuum adsorption device.
The main purpose of our research is to study the effects of aromatic compounds containing radical groups on the surfaces of polymer sorbents and the laws of sorption processes, as well as the analysis of the sorption-structural state.
Used for investigations the chitosan obtained from silkworm Bombyx Mori. Na-CMC was obtained from cotton cellulose which was manufactured in "Carbonam" LTD.
The adsorption experiments are carried out in a universal high-vacuum system with a U-monometer and a completed to this system Tiana-Kalve differential microcalorimeter for measuring differential temperatures and isotherm of adsorption.

Results and Discussion
The adsorption isotherms of toluene on the chitosan/Na-CMC film membrane are shown in Figure 1 in logarithmic coordinates. The adsorption isothermal line of toluene on the chitosan/Na-CMC film membrane shows that the relative pressure of the initial adsorbate molecules is Р/Р 0 = 0.0004, the atmospheric pressure is 0.0485, the isotherm logarithm is ln (P/P 0 ) = −6.63 and the adsorbents. The amount is 0.008 mmol/g.
When calculating the adsorption isotherm of toluene on the chitosan/Na-CMC film membrane, the values of pressure at equilibrium are obtained. Equilibrium pressure is determined using a U-shaped monometer of a high-vacuum adsorption device. Experiments on this device are carried out mainly using absolutely pure adsorbents. In experimental work, a U-shaped manometer is continuously measured until the pressure reaches a constant value. Undetermined values on the U-monometer are measured on the McLeod part of the device. Measurements are made on the macro until the U-monometer reaches 0.1 mm of mercury. The value obtained is divided by the saturated vapor pressure of toluene at 303 K (36.7 mmHg) and the natural logarithmic value ln (P/P 0 ) is obtained. The main difference in the adsorption of toluene from benzene is that the initial bonding in toluene is mainly due to methyl groups. Due to the presence of a methyl group, its molecular size is larger than that of benzene.  It is known from the graph of the adsorption of toluene molecules on the membrane of chitosan/Na-CMC that the adsorption isotherm increases gradually.
In the processes that take place on the surface, mainly adsorption isotherms form an arc shape. Toluene molecules combine by hydrogen bonding with amino, hydroxide, and carboxyl groups in the sorbent. When the adsorption reaches 0.1 mmol/g, the isotherm is −1.36. After 0.1 mmol/g, the changes in isothermal values gradually decrease. The isothermal lines rise vertically and slowly approach the axis of adsorption (ordinate). When the adsorption reaches 0.5 mmol/g, the isotherm value is −0.02.
The differential heat of toluene adsorption on the chitosan/Na-CMC membrane film is shown in Figure 2.  In the seventh stage, the adsorption heat decreases from 35.21 kJ/mol to 33.72 kJ/mol and 0.048 mmol/g toluene is absorbed. Adsorption ranged from 0.424 mmol/g to 0.472 mmol/g. After 0.472 mmol/g, 0.02 mmol/g toluene is adsorbed at the end of the process and the heat goes very close to the condensation line.
Chitosan/Na-CMC are sorbed into the membrane film by 0.8 mmol/g toluene. According to the Gibbs equation, the differential molar entropy of toluene adsorption on the chitosan/Na-CMC membrane film was determined ∆S d , where the entropy of liquid toluene was assumed to be zero.
According to the Gibbs-Helmholtz equation, the differential molar entropy ∆S d of toluene adsorption on the chitosan/Na-СMС membrane film was calculated (assuming that the entropy of liquid toluene was zero). Figure 3 shows the differential entropy of adsorption of chitosan/Na-СMС film membrane toluene molecules at 303 K. Initially, the adsorption entropy of toluene molecules is 32.11 J/mol•K. The initial toluene molecules interact with the functional groups on the surface of the resulting interpolymer film membrane and are higher than the standard state entropy value, which is explained by the sorption of chitosan/Na-CMC on the film surfaces. This adsorption process continues until it reaches 0.06 mmol/g. After 0.06 mmol/g, the movement of toluene molecules is partially limited as a result of the interaction of chitosan/Na-CMC with functional groups inside the chemical bond chains that form the film membrane. Therefore, the standard case is below the entropy value.
We obtained the standard entropy of the liquid state of toluene as 0 J/mol•K. When the toluene molecules are then adsorbed, the entropy values decrease to the standard state entropy.  Entropy values indicate that the adsorption of toluene molecules on chitosan/CMC is stronger than that of chitosan.

Conclusion
Thus, it was found that toluene is adsorbed on the chitosan/Na-CMC film by the same patterns of sorption of aromatic compounds, due to the process of penetration of toluene molecules into the network of chitosan macromolecules and this is preceded by the absorption process by the surface of the chitosan/Na-CMC film.