Removal of Phenol and o-Cresol by Adsorption onto Activated Carbon

A commercial activated carbon was utilized for the adsorptive removal of phenol and o-cresol from dilute aqueous solutions. Batch mode adsorption studies were performed by varying parameters such as concentration of phenol solution, time, pH and temperature. The well known Freundlich, Langmuir and Redlich-Peterson isotherm equations were applied for the equilibrium adsorption data and the various isotherm parameters were evaluated. The Langmuir monolayer adsorption capacities were found to be 0.7877 and 0.5936 mmole/g, respectively, for phenol and o-cresol. Kinetic studies performed indicate that the sorption processes can be better represented by the pseudo-second order kinetics. The processes were found to be endothermic and the thermodynamic parameters were evaluated. Desorption studies performed indicate that the sorbed phenol molecules can be desorbed with dil. HCl.


Introduction
Phenols and their derivatives are invariably present in the effluents from industries engaged in the manufacture of a variety of chemicals such as plastics, dyes, and in plants used for thermal processing of coal.Many of these phenols are carcinogens even when present in low concentrations.The presence of phenols in water also produces foul smelling chlorophenols during chlorination treatment of water for domestic supply.
Exposures to phenol can occur in workplace, from environmental media, from contaminated drinking water or foodstuff or from use of consumer products containing phenol 1 .All routes readily absorb phenol: oral, inhalation, and dermal 2 .Phenol is a strong eye and respiratory irritant.It is corrosive to the eyes and skin upon direct contact.Human deaths have occurred following internal ingestion of large areas of skin (>25% of total) 1 .The minimum reported lethal dose in humans 3 was 4.8g or approximately 70mg/Kg.The worldwide production of o-cresol is approximately 37000-38000 tones/annum.It is mainly used as an intermediate for the production of pesticides, herbicides, rubber and plastics, antioxidants, deodourizing and fragrances, epoxy resins, dyes, and pharmaceuticals and also as a component of disinfectants and cleaning agents.o-Cresol is toxic to fish, aquatic plants, microorganisms, soil dwelling organisms, terrestrial plants and also to humans.
The Indian Standard specifications 4 for drinking water has set the maximum allowable limit of 0.001 ppm for phenols.It is therefore necessary to reduce or eliminate phenols from water and wastewater.Though bioaccumulation and biosorption are mainly preferred for this task as the phenols are easily biodegradable, adsorption onto activated carbon is also effective.Many works have been reported in the adsorptive removal of phenolics from aqueous solutions [5][6][7][8][9][10][11] .The removal efficiency of a commercial activated carbon (M/s.Loba Company, Mumbai) towards phenol and o-cresol was attempted in the present study.The effects of adsorbate dose, pH, time and temperature were studied.

Adsorbent
The granular activated carbon supplied by M/s.Loba Company, Mumbai was used in this study.The carbon was ground and the portion retained between 150 and 250µm sieves was used for study.The adsorbent was named as CC.The characteristics of the carbon were reported elsewhere 12 .

Analysis of phenol and o-cresol
Both phenol and o-cresol were analyzed spectrophotometrically by monitoring their absorption at 270 nm.

Equilibrium adsorption experiments
All the equilibrium adsorption experiments comprised three replicate 100mL glassstoppered bottles containing selected amount of adsorbent and 50mL of adsorbate solutions of selected concentrations.Control flasks without the adsorbents are also prepared simultaneously.Mixtures were maintained in a rotary shaker (Orbitek, Chennai, India) at constant temperature (30, 45 or 60°C).After the attainment of equilibrium the contents of each flask were filtered through Whatmann No.41 filter paper, with the first 10mL discarded.The filtered samples were then analyzed for unadsorbed phenols.

pH variation studies
In order to find out the optimum pH for maximum removal of phenols, experiments were carried out with solutions of same phenol concentration but adjusted to different initial pH values (with HNO 3 or NaOH).

Desorption Studies
After equilibrium adsorption, the phenols loaded carbons were separated and washed gently with several portions of distilled water to remove any unadsorbed species.The samples were then air-dried and agitated with 0.1M solutions of HCl, AcOH or water for a period of 10 hours and the amounts of desorbed species were determined in the usual way.

Equilibrium studies
The isotherm plots obtained for the adsorption of phenol and o-cresol are shown in Figure 1.The isotherms were of L2 type under Giles 13 classification.
Where q e is the adsorption capacity in mg/g; C e is the equilibrium concentration of adsorbate (mg/L); K F and n are Freundlich constants; K L and b are Langmuir constants; q m is the Langmuir monolayer adsorption capacity and K R , b R and β are Redlich-Peterson isotherm constants.The equilibrium data obtained were fitted to the above three isotherm equations separately and the parameters evaluated are presented in Table 1 along with the correlation coefficient values for each fit.  1 that of the three isotherm models used it is the threeparameter Redlich-Peterson model that fits the experimental data with best correlation coefficient values.This is expected as the Redlich-Peterson equation has three fitting parameters, especially with β being a more direct parameter for adsorbate-adsorbent interaction strength.
The Langmuir monolayer capacity, K L was higher for phenol than o-cresol.A possible explanation can be given by invoking a 'bottle-neck' model for the pores present on the adsorbents' surface.According to this model, the pores resemble bottles in that they have very small openings like bottlenecks.After an appropriate monolayer outer surface coverage, there will be a certain population of phenol molecules at the pore entrance and as there are no substituents on the benzene ring of phenol, the pore entrance is relatively free for the movement of other unadsorbed phenol molecules into the pores.Therefore, more and more of phenol molecules can diffuse and subsequently be adsorbed in pores.But the situation for the adsorption of o-cresol is somewhat different in that the methyl groups of adsorbed o-cresol molecules may block the pore openings.Transport of unadsorbed cresol molecules from solution into the pores, therefore, is hindered which leads to lesser filling of pores.The overall amount of adsorbed cresol is thus less than that of phenol.
The Langmuir b values determined are further used to calculate the dimensionless separation factor, R L 15, 16, defined as where C i is the initial solute concentration.The RL values calculated are listed in Table 2.The magnitude of R L value gives an idea about the nature of adsorption equilibrium: the process is non-spontaneous when R L is greater than one; favorable when R L lies between 0 and 1; and irreversible when R L is zero.In all the systems studied, R L values were comprised between 0 and 1 indicating favorable on the activated carbon.

Effect of pH
The effect of pH of solution on the adsorption percentage of phenol and o-cresol is shown in Figure 2.There is no significant change in the amount of adsorption with respect to change in the solution pH.But the very slight decrease in the adsorption extent is noted with respect to increase in solution pH.This could be due to the increased solubility of phenol molecules at alkaline conditions, which results in greater affinity for the phenol molecules to remain in solution rather than to get adsorbed onto the carbon surface.

Adsorption kinetics
The kinetic behavior as shown in Figure 3 is fitted to the following three rate equations reported in the literature, namely, first order model 17 , pseudo-second order model [18][19][20] and the Elovic model [21][22] .
where, q t = amount adsorbed at time t, mg/g q e = amount adsorbed at equilibrium, mg/g q e(1) = adsorption capacity predicted by the I order model, mg/g k 1 = first order rate constant (min -1 ) k 2 = second order rate constant (gmg -1 min -1 ) = h/q e(2) 2 q e(2) = adsorption capacity predicted by the II order model, mg/g α (Elovic model) = h (II order model) = initial sorption rate (mgg -1 min -1 ) β = desorption constant (gmg -1 ) t = contact time, min The parameters evaluated for the three rate models were shown in Tables 3, 4 and 5.It is observed that both the first order and the Elovic model are quite inefficient than the pseudo-second order rate equation (inferred from the r 2 values) in predicting correctly the sorption dynamics.It is to be noted that the q e values predicted by the second order model agree very closely to the experimental values.

Effect of temperature
The adsorption of phenols on CC increased with increase in temperature (Figure 4), suggesting that these processes are endothermic.The isotherm parameters for the equilibrium at temperatures of 45 and 60°C are presented in Tables 6 and 7.It can be seen from Tables 6 and 7 that the value of q m decreases and K L increases for the adsorption of phenols on CC with increase in adsorption temperature.
Where K C is the equilibrium constant for the distribution of phenols between the liquid and solid phases; T is absolute temperature, °K and R the gas constant.Van't Hoff plots were constructed for each system and ∆H and ∆S were calculated from the slope and intercept of the plots, respectively (Table 8)

Desorption studies
The results with water, 0.1 M acetic acid and 0.1 M hydrochloric acid are presented in Table 9.We are to understand that desorption increases with increase in the acidity of the desorbing medium; desorption is higher in hydrochloric acid than in acetic acid.This does not imply that the phenol molecules are adsorbed by an ion-exchange mechanism, but rather could be due to the increased solubility of the phenol molecules in the acidic media.

Conclusions
The work presented in this work has shown that the commercial activated carbon can be successfully used for the adsorptive removal of phenolic pollutants from water.The three-parameter Redlich-Peterson isotherm equation better represented the equilibrium adsorption data.Kinetic studies point to the fact that the sorption dynamics of phenols are predicted more accurately by the pseudo-second order rate model.pH has no significant role to play in the adsorptions and are found to be endothermic in nature.The thermodynamic parameters were evaluated.

Figure 1 .
Figure 1.Isotherm plots for adsorption of phenol and o-cresol on activated carbonThe equilibrium adsorption data obtained were then fitted to Freundlich, Langmuir and Redlich-Peterson isotherm 14 equations:

Figure 2 .
Figure 2. Effect of pH on the adsorption of phenols on CC

Figure 3 .
Figure 3. Kinetic curves for adsorption of phenols on CC

Figure 4 .
Figure 4. Adsorption of phenol on CC: Effect of temperature

Table 1 .
Isotherm parameters for the adsorption of phenols on CC at 30°C

Table 2 .
R L values for the adsorption of phenols on CC at 30°C

Table 3 .
First order parameters for the adsorption of phenols on CC

Table 4 .
Pseudo-second order parameters for the adsorption of phenols on CC

Table 5 .
Elovic parameters for the adsorption of phenols on CC

Table 6 .
Isotherm parameters for adsorption of phenols on CC at 45°C

Table 7 .
Isotherm parameters for adsorption of phenols on CC at 60°C

Table 8 .
Thermodynamic parameters for the adsorption of phenols on CC

Table 9 .
Desorption of phenols