Adsorption of unsubstituted phenols by polyvinyl acetate in aqueous systems.

INTRODUCTION

Adsorption is a phenomenon in which solid or liquid surfaces have a tendency to satisfy the residual forces in it by attraction of onto and retaining on their surfaces, molecules of gases or other substances which they come in contact. This gives rise to a higher concentration of any particular component at the surface of a solid or liquid phase than is present in the bulk of the medium. The adsorbing material (usually solid or liquid) is known as adsorbent (sorbent) while the adsorbed material is the adsorbate (sorbate). The rate of adsorption and the amount of materials adsorbed depends on the nature of adsorbent, temperature, concentration, pH of the medium (Rengaraj et al, 2002; Mohan and Karthikeyan, 1997,Motoyuki, 1990). Molecules can stick to a surface in two ways leading to two types of adsorption (physisorption) and chemical adsorption (chemisorption). In physisorption, vander waals forces exist between adsorbent and adsorbate molecules which implies low amount of energy, hence the adsorbed molecules can still retain its identity. In chemisorption, the molecules of adsorbate stick to the surfaces of the adsorbent by chemical means (covalent bonds), hence molecules undergoing chemisorption may loss their original identity (Aktins, 1983).

Adsorption of small molecules by polymers from solution has been widely studied (Jaycock and Parfitt, 1981; Lu et al, 1994). For the materials to be adsorbed from solution it must be soluble in the medium, a condition which can be fulfilled in both aqueous and non-aqueous systems. When non-polymeric molecules are being adsorbed onto large (polymeric) molecules there are few groups of which are capable of attachment to the adsorbent (polymer) surfaces. As a result of the random coiled structure of polymers, loops may be formed between the different segments of its molecules, which may extend into the solution (Rosoff, 1969). It is to be expected that the equilibrium attainment of the adsorption involving polymers is rather slow because of the large number of solid-solution interfaces which polymeric molecules present (Silbeberg, 1973). It has been reported that adsorption of molecules from solution is affected by factors such as nature of adsorbent, temperature, time, concentration and nature of adsorbate (Igwe and Abia, 2006). Microporous materials such as polyvinyl acetate (PVAC) and finely divided substances such as activated carbon, silica and alumina posses' large area of active surface per unit area for adsorption and are strong adsorbents. These adsorbents are very useful in various purification processes such as treatment of industrial effluents and wastes from textile, dye, petrochemicals, tannery, pulp and paper industries (Amir et al, 2005,Ahmed and Ram, 1992; Karthikeyan and Chaudhuri, 1986). The phenomenon of adsorption could be used in the removal of undesired colours, odours and water vapours from contaminated products (Karthikejan, 1988; Mckay et al, 1981,Chiou et al, 2003).

Adsorption of phenols on polyvinyl acetate may be used for the purification of waste water, sewage waters contaminated by phenols or the removal of chlorinated pesticide e.g. TNT (2,4,6-Trinitrotoluene) and RDX (Hexahydro-1, 3,5-trinitro-1,3,5-triazine) residues and other noxious compounds from waste effluents and water supplies (Hundal et al. 1997; Jekins and Walsh, 1992). This paper reports on the rate of adsorption (kinetics) and effect of temperature (adsorption isotherms) for unsubstituted phenols by polyvinyl acetate.

MATERIALS AND METHODS

Materials

Phenol and polyvinyl acetate (PVAC) were obtained from BDH Laboratory supplies, England and were used as purchased without further purification.

Determination of maximum wavelength ([[lambda].sub.max]):

Aqueous solution of phenol ([10.sup.-3] mold[m.sup.-3]) was diluted into different concentrations ranging from ([10.sup.-3] - [10.sup.-4] mol [m.sup.-3]). After some test runs the concentrations of phenol 5 x [10.sup.-4] mold[m.sup.-3] was introduced into a plastic cell and the absorbance was determined to be 267 [cm.sup.-1] using the UV spectrophotometer (Pye Unicam SP 800). This is the maximum wavelength ([[lambda].sub.max]) of absorbance for phenol. All other absorbances reading were taken with reference to this ([[lambda].sub.max]) of 267[cm.sup.-1].

Preparation of Calibration Curve:

From a stock solution of [10.sup.-3] mold[m.sup.-3] of the phenol other concentrations were obtained by dilution to appropriate concentration ranging from [10.sup.-3] - [10.sup.-4] mold[m.sup.-3]. A plot of absorbance against concentration of phenol was obtained (Fig.1). It was from this plot that all concentrations with their absorbances were obtained from the subsequent experiments.

Kinetic Study of Adsorption of Phenol by PVAC at Different Temperatures:

Polyvinylacetate (0.25g) was weighed into each of eight (8) conical flasks and distilled water (25.0ml) added to each flask. After ten minutes the flask was immersed into a thermostated water bath (Gallenkamp, London), which has been preset to the temperature of the experiment of 30[degrees]C. Thereafter, a known concentration of phenol solution ([10.sup.-3] - [10.sup.-4]mold[m.sup.-3]) was added to each of the eight flasks in turn. A stop clock was started immediately and after an interval of five minutes the eighth (8th) flask, which was, the last in the series was removed and the absorbance obtained using the UV spectrophotometer. The same procedure was repeated for the rest of the flask with time intervals ranging from 5 - 40 minutes for the temperature under investigation. In each case, the solution was immediately decanted and the absorbance determined. The amount of phenol adsorbed per gramme of 0.25g of polyvinyl acetate and the time adsorption reaches equilibrium were determined and are as presented in Table 1.

Adsorption Isotherm for Phenol-PVAC System at Different Temperatures:

Solutions of phenol of concentrations of range 1-16 x [10.sup.-4] mold[m.sup.-3] were used. Polyvinyl acetate (0.25g) was weighed into conical flasks with distilled water (25ml), after ten minutes interval, aqueous phenol solution of known concentration was added in each case to the PVAC solution at the temperature under study. The adsorption reaction was allowed to proceed for the time upon which it was assumed that equilibrium has been obtained. The absorbance of the phenol solution before and after the adsorption reaction was deduced from the calibration curve. The change in absorbance ([DELTA]A) divided by the weight of PVAC (0.25g) was obtained as the effect of temperature in each case.

RESULTS AND DISCUSSION

Table 1 shows the variation of the amount of phenol adsorbed by polyvinyl acetate (0.25g) at different temperatures with time. The rate of adsorption of phenol rises rapidly until it attained equilibrium at about 20 mins, which was found to be the equilibrium time ([t.sub.eq] = 20mins) for the four temperatures under investigation. Also the rate of adsorption steadily decreases with rise in temperature even for the same time of adsorption. The rate of adsorption at a given time expressed in terms of change of the concentration of phenol with respect to the fixed mass of PVAC was determined using the expression below:

([DELTA]C)/0.25g mol/[dm.sup.3] x[10.sup.-4] (1)

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The values of rate of adsorption obtained at the temperatures studied were 10.30, 7.30,4.30 and 0.00 for 30,35,40 and 45[degrees]C respectively. High rates of adsorption at lower temperatures suggest that phenol is probably physically adsorbed on the PVAC matrix. This indicates the formation of a monolayer of phenol over the homogenous adsorbent (PVAC), which increases with time until description sets in (Jekins and Walsh, 1992,Katsoyiannis and Zouboulis, 2000). Desorption indicates the irreversible nature of adsorption process, at equilibrium, the rate of both adsorption and desorption approaches unity. Gupta et al (1986) and Kadirvelu et al (2000) reported similar observation for adsorption of azo dyes onto secondary cellulose acetate and chrome dyes onto activated solutes from solution leads to formation of monolayer of molecules on the surface of adsorbent similar to Langmuir adsorption (Atkins, 1983,Horsfall et al, 2004). The steady rise in the adsorption of phenol may be affected by the spongy, porous nature of PVAC and also due to the participation of specific functional groups in both the adsorbent and adsorbate. Macro and micro pores present active surface sites in the adsorption of the phenol. The penetration and attachment of phenol molecules into the binding sites of the polymer matrix (PVAC) may also be enhanced by hydrogen bonding and Van der Waals forces between the carbonyl oxygen of the PVAC and the hydroxyl group of the phenol molecules as the suggested structure shown:

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Fontana and Thomas (1981) reported that adsorption of poly (alkyl-methacrylate) molecules on silicone occurred through hydrogen bonding. The segment between the adsorbed groups can form loops, which extend into solution, the adsorption being affected by the number of polymer adsorbate contacts and dimension of adsorbed layers. It is not uncommon in aqueous system that phenol molecules may be hydrogen bonded with the hydroxyl group in the aqueous medium.

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This layer hydrogen bonding may be competing with the phenol-PVAC hydrogen bonding. It is likely that the rate of these two hydrogen bondings may be different at different times and temperatures. Table 1 also shows that the time adsorption reaches equilibrium ([t.sub.eq]) was approximately the same (20 mins) for all the temperatures under investigation. However, the amount of phenol adsorbed by PVAC at [t.sub.eq] varied with the different temperatures, increasing with rise in temperatures. Therefore considering economics and practical aspects, an equilibrium contact time of 20 mins is optimum time for the adsorption of phenol by PVAC under the conditions investigated.

Adsorption Isotherms of Phenol Adsorption onto PVAC Matrix.

Adsorption isotherms are useful in understanding the sorption interaction. Figure 2 shows the effect of temperatures on amount of phenol molecules adsorbed in terms of adsorption isotherms. The concentration of phenol adsorbed was found to decrease with increase in temperature at a particular concentration. However, deviation from this pattern is likely to exist since the adsorption goes Langmuirian and deviation have been reported (Aktins, 1983, Gladstone and Lewish, 1990).

Generally at higher temperature, desorption may set in since adsorption involves the release of heat. Moreso,there is increase solubility of phenol at higher temperature and more of it may be retained in the bulk of the solution than adsorbed. It has been reported that the amount of adsorbate (phenol) adsorbed decrease with increase in concentration or pressures (Barrow, 1976). Hence, at higher temperatures and concentrations of adsorbate, molecules from the surface of the adsorbent leave leading to decrease adsorption. (Jaycock and Parfitt, 1981, Silva and Brunner, 2004).

CONCLUSION

Adsorption of unsubstituted phenol molecules onto PVAC matrix increases with a decrease in temperature. Adsorption is a surface phenomenon, which involves release of heat, which is not favoured at high temperatures. The conformation of the data from adsorption studies to Langmuir isotherm indicates the formation of monolayer of adsorbate on the adsorbent surface. Hence, phenol molecules were physisorbed on the PVAC matrix at equilibrium time and temperature discussed. PVAC could be used as an adsorbent for the purification of aqueous system contaminated by phenols.

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A.U. Israel, S.A. Umoren and U. D.Akpabio

Department of Chemistry, Faculty of Science, University of Uyo, P. M.B 1017, Uyo, Nigeria.

For correspondence/reprints (A. U Israel), E-mail: [e-mail]

Table1: VARIATION OF AMOUNT OF PHENOL ADSORBED BY PVAC AT
DIFFERENT TEMPERATURES WITH TIME AND ADSORPTION.

Temperature     Absorbance       Con of phenol     Con of
([degrees]C)    (267[lambda]     adsorbed mol/     unadsorbed
                [cm.sup.-1])     [dm.sup.3]        phenol (mol/
                                 x[10.sup.-4]      [dm.sup.3]x
                                                   [10.sup.-4])

30              0.48             2.20              2.52
                0.45             2.20              2.72
                0.45             2.25              2.73
                0.46             2.25              2.75
                0.42             2.20              2.80

35              0.66             2.18              1.83
                0.64             3.05              1.95
                0.62             3.00              2.00
                0.60             2.95              2.00
                0.62             3.00              2.00

40              0.82             3.87              1.13
                0.84             3.90              1.10
                0.80             3.81              1.20
                0.72             3.73              1.28
                0.80             3.70              1.30

45              1.10             5.00              0.00
                0.90             4.80              0.20
                0.91             4.74              0.26
                0.90             4.72              0.28
                0.96             4.70              0.30

Temperature       [DELTA]C       Time
([degrees]C)    0.25g (mol/     (mins)
                [dm.sup.3]x
                [10.sup.-4])

30              10.3              5
                10.9             10
                10.9             20
                11.6             30
                11.2             40

35              7.30              5
                7.80             10
                6.00             20
                8.20             30
                8.00             40

40              4.50              5
                4.40             10
                4.75             20
                5.10             30
                5.20             40

45              0.00              5
                0.80             10
                1.04             20
                1.10             30
                1.20             40