Removal of Selected Organic Compounds in Aqueous Solutions by Activated Carbon

MATERIALS AND METHODS

Reagent grade phenol, m-nitrophenol, p-nitrophenol and o-nitrophnenol (all from Merck) were of the highest purity available and used without further purification. Safranin-O was obtained from Fluka and its purity is higher than 99% were provided by Sigma Activated carbon was purchased from VWR PROLABO BDH. Chemical and physical characteristics of activated carbon were done by Turkish Scientific and Technical Research Center. The specific surface area, cross sectional area, bulk density and the porosity of the adsorbent were 1014.44 m² g^-1, 16.2 Ų, 0.63 g cm^-3 and 0.4, respectively with the elemental analysis of the adsorbent quoted at C: 86.5%, H: 1.09% and N < 1% (w/w).

The effects of important parameters such as initial concentrations of Safranin-O and phenols and temperature on the adsorptive removal of Safranin-O and phenols were studied by batch experiments. In each kinetic experiment, a known quantity of adsorbent contacted with an adsorbate solution in a 250 mL flask at desired temperature was shaken in a thermostat rotary shaker at constant agitation speed (250 rpm) for a given time intervals. At various time intervals the flasks were successfully removed, the liquid was separated from the solid by centrifugation and the remaining concentration of dye and phenols in solution were measured spectrophotometrically on a UV-1700 Shimadzu at a wavelengths of 520 nm for SO and 270, 317, 274, 279 nm for phenol, p-nitrophenol, m-nitrophenol and o-nitrophenol, respectively. Equilibrium adsorption isotherms for Safranin-O and phenols were undertaken at 25±1°C. The adsorption behaviors of the Safranin-O and phenols on the activated carbon were studied at four temperatures (298, 303, 313 and 323 K). The adsorption equilibrium isotherm experiments were repeated in duplicate and the average values are reported.

The concentration retained in the adsorbent phase (q_e, mg g^-1) was calculated by using the following equation;

(1)

where, C₀ is the initial dye concentration and C_t is concentration of the dye and phenols (mg L^-1) at any time (t). V is the volume of solution (L) and W_s is the mass of activated carbon (g).

RESULTS AND DISCUSSION

It is seen that in all the concentrations studied of Safranin-O and Phenol while the concentration of Safranin-O decreased with time up to 150min the concentration of Phenol decreased with time up to 100 min and then the curves become flattened (Fig. 1a, b). It is also seen that the equilibrium time is independent of solution concentration. The same behaviours were also seen for p-nitrophenol, m-nitrophenol and o-nitrophenol. Based on these findings, the shaking time was used in all further experiments. The amount adsorbed, q_e (mgg^-1) is plotted against the equilibrium concentration, C_s (mg L^-1) to obtain the adsorption isotherms. Adsorption isotherm data of Safranin-O and phenols are shown in Fig. 2a and b. The shape of the isotherms indicates L behavior according to Giles and

Fig. 1a:	Variations in the initial concentrations of Safranin-O with time

Fig. 1b:	Variations in the initial concentrations of phenol with time

Fig. 2a:	The adsorption isotherms of Safranin-O onto activated carbon at 298 K

Smith classification (Giles et al., 1974). Type L suggests that the aromatic rings adsorbs parallel to the surface and no strong competition exists between the adsorbate and the solvent to occupy the adsorption sites.

The correlation of the experimental adsorption data with a number of adsorption models was undertaken to gain an understanding of the adsorption behavior and the heterogeneity of the adsorbent surface. The linearized forms of Langmuir (1918), Freundlich (1906) and Temkin (1941) isotherm equations can be represented Eq. 2-4 as follows:

Fig. 2b:	The adsorption isotherms of Phenol and Nitrophenols onto activated carbon at 298 K

(2)

(3)

(4)

where, q_e is the amount of adsorbate adsorbed per unit mass of adsorbent at equilibrium in mg g^-1; C_e the final concentration at equilibrium in mg L^-1; q_max the maximum adsorption at monolayer coverage in mg g^-1; b the adsorption equilibrium constant related to the energy of adsorption in L mg^-1; K_F a Freundlich constant representing the adsorption capacity in (mg g^-1) (L mg^-1)^1/n; n a constant depicting the adsorption intensity; k₁ the Temkin isotherm energy constant in L mg^-1 and k₂ the Temkin isotherm constant. The main difference between these three isotherm models is in the variation of heat of adsorption with the surface coverage. Langmuir model assumes uniformity, Freundlich model assumes logarithmic decrease and Temkin model assumes linear decrease in heat of adsorption its surface coverage.

The experimental isotherm data were fitted to these equations by applying linear regression analysis. One way to assess the goodness of fit of experimental isotherm data to these equations is to check the regression coefficients obtained during the regression analysis. As shown in Table 1, adsorption data for all compounds obeyed mostly Langmuir isotherm. The efficiency of adsorption process can be predicted by the dimensionless equilibrium parameter R_L, which is defined by the following equation:

(5)

Table 1:	Parameters of Langmuir, Freundlich and Tempkin adsorption isotherm equations at 298 K

where, b is the Langmuir constant (L mg^-1), C₀ the initial concentration of SO and phenols in mg L^-1. Isotherm is considered to be unfavourable when R_L >1, linear when R_L = 1, favorable when 0<R_L<1 or irreversible when R_L = 0. The R_L values calculated for the adsorption of SO and phenols studied are given in Table 1 for the highest initial concentrations. Since all the R_L values are between 0 and 1, the adsorption processes in all cases are favourable.

It is known that the Langmuir and Freundlich isotherm constants do not give any idea about the adsorption mechanism. In order to understand the adsorption type, equilibrium data was tested with D-R isotherm. D-R equation can be written as:

(6)

where, ε, RT 1n (1+1/C_e), K' is the constant related to the adsorption energy, R the gas constant (8.314 J mol^-1 K^-1) and T is the temperature in Kelvin. The quantities of q, q_max and C_e have the same meaning as above. Straight lines are obtained upon plotting lnq versus ε² indicating that adsorption of both Safranin-O and phenols on activated carbon also obey the D-R isothermal equation in the entire concentration range studied. Values of q_max and K' calculated from the intercepts and slopes of the plots were given in Table 2. From the value of K' it is possible to calculate the mean free energy of adsorption (E_a), defined as the free energy change when 1 mol of ion is transferred to the surface of the solid from infinity in solution using the following Eq.:

(7)

It is known that magnitude of E_a is useful for estimating the type of adsorption and if this value is between 8 and 16 kJ mol^-1 the adsorption type can be explained by ion exchange. But the value of E_a found in this study is within the energy range of physical adsorption (E_a< 8). The calculated values of E_a are between 2 and 4 kJ mol^-1 for Safranin-O and phenols (Table 2). This indicates that adsorption of Safranin-O and phenols onto activated carbon are physical in nature.

The study of adsorption kinetics of Safranin-O, phenol, m-nitrophenol, p-nitrophenol and o-nitrophenol describe the solute uptake rate and evidently this rate controls the residence time of adsorbate uptake at the solid-solution interface. In case of m-, p- and o-nitrophenols trends of their kinetics curves are similar to those of phenol and therefore, similar reasoning can be given. Adsorption rate constants for the Safranin-O and phenol were calculated by using pseudo-first-order, second-order and intraparticle diffusion kinetic models and which were used to describe the mechanism of the adsorption. The conformity between the experimental data and the model-predicted values was expressed by the correlation coefficients (R²). A relatively high R² values indicate that the model successfully described the kinetics of the Safranin-O and Phenol adsorption.

A pseudo-first-order equation can be expressed in a linear form as:

(8)

Table 2:	Parameters of D-R adsorption isotherm at 298 K

Table 3:	Comparison of the pseudo-first and second-order kinetic models of Safranin-O and phenol adsorption for the different initial concentrations on activated carbon at 298 K

where, q_e and q are the amount of Safranin-O and phenol adsorbed (mg g^-1) on the adsorbents at the equilibrium and at time t, respectively and k₁ is the rate constant of adsorption (min^-1) (Lagergren, 1898). Values of k₁ were calculated from the plots of ln (q_e-q) versus t for different concentrations of Safranin-O and Phenol, respectively and shown in Table 3.

The pseudo-second-order adsorption kinetic rate equation is expressed as:

(9)

where, k₂ is the rate constant of pseudo-second-order adsorption (g mg min). Integrating and applying the initial conditions, we have a linear form as:

(10)

where, q_e is the amount of dye adsorbed at equilibrium (mg g^-1). Values of k₂ and q_e were calculated from intercept and the slope of the linear plots of t/q_t versus t (Ho and McKay, 1999).

It may be observed from Table 3 that the adsorption of Safranin-O and Phenol on activated carbon fits pseudo-second order kinetics better than the first order. The same observation can be said phenol adsorption. It can be seen from the better fit of q_e values by using pseudo-second order kinetics when experimental q_e values are compared.

In adsorption studies, it is necessary to determine the rate-limiting step. Therefore, the results obtained from the experiments were used to study the rate-limiting step in the adsorption process. The rate constant for intraparticle diffusion (k_id) is calculated by the following equation:

(11)

where, q is the amount dye adsorbed (mg g^-1) at time (t) and k_id (mg g^-1 min^0.5) is the rate constant for intraparticle diffusion. Values of k_id were calculated from the slope of the linear plots of q versus t^0.5 (Namasivayam and Yamuna, 1995). The k_id values were obtained from the slope of the linear portions of the curves and were found to be as 10.517 and 12.33mg g^-1min^0.5 for the initial concentrations of 125 and 200mg L^-1, for Safranin-O and 10.824 and 21.409 mg g^-1min^0.5 for the initial concentrations of 20 and 40 mg L^-1 for Phenol, respectively. It is seen from Fig. 3a and b, that all the plots have the same general features (the initial curved portion and a linear portion). The initial curved portion is attributed to the boundary layer diffusion, while the linear portion is responsible for the intraparticle

Fig. 3a:	Plots of amount of Safranin-O adsorbed vs t0.5 for intraparticle diffusion of SO

Fig. 3b:	Plots of amount of phenol adsorbed vs t0.5 for intraparticle diffusion of phenol

diffusion. The linear portions of the curves do not pass the origin in Fig. 3a and b. This indicates that mechanism of Safranin-O and Phenol removal on Activated Carbon is complex and both, the surface adsorption as well as intraparticle diffusion contribute to the rate determining step. Similar trends were also observed for nitrophenols.

The adsorption of Safranin-O and phenols on activated carbon were studied at temperatures of 293, 303, 313 and 323 K. The free energy of adsorption (ΔG) was calculated from the following equation;

Fig. 4:	Plot of lnK vs. 1/T for estimation of the thermodynamic parameters for the adsorption of Safranin-O, Phenol and Nitrophenols onto activated carbon

(12)

where, K is the equilibrium constant and T is the solution temperature (K); R is the gas constant (8.314 J mol^-1 K). The apparent enthalpy of adsorption, (ΔH) and entropy of adsorption, (ΔS), were calculated from adsorption data at different temperatures using the Van`t Hoff equation (Shawabkeh and Tutunji, 2003):

(13)

Values of ΔH and ΔS were computed from the slopes and intercepts of linear variations of ln K with the reciprocal of temperature (Fig. 4). As seen in Table 4 ΔG° values were negative in the studied temperature range of 293-323K for both Safranin-O and phenols indicating that the adsorption process led to a decrease in Gibbs free energy. Negative ΔG° indicates the feasibility and spontaneity of the adsorption process. The negative values of ΔH° indicate that the process is exothermic. Physical adsorption and chemisorption can be classified, to a certain extent, by the magnitude of the enthalpy change. It is accepted that bonding strengths of <84 kJ mol^-1 are those of physical adsorption type bonds. Chemisorption bond strengths can range from 84 to 420 kJ mol^-1. Based on this the adsorption of Safranin-O, phenol and nitrophenols on activated carbon appears to be a physical adsorption process. The enthalpy of adsorption of organic molecules from aqueous solution on activated carbon is usually within the range 8-65 kJ mol^-1. The negative values of ΔS° suggest that the decreased randomness at the solid-solution interface during the adsorption of the Safranin-O and phenols in aqueous solution on the activated carbon. The less negative values of ΔS° found for phenols reveal that a more ordered arrangement of phenol molecules is shaped on the adsorbent surface.

On the basis of the data, it is concluded that active carbon can be used to remove Safranin-O and phenols from aqueous solutions. The experimental adsorption data showed good correlation with

Table 4:	Thermodynamic parameters for Safranin-O, phenol, p-nitrophenol, m-nitrophenol and o-nitrophenol adsorption on activated carbon under different temperatures

the Langmuir isotherm model. Regardless of temperature, the capacity of the activated carbon used to adsorb these compounds presented following order:

The lowest adsorption affinity of Safranin-O onto activated carbon comparing with nitrophenols including Phenol can be explained by molecular size of Safranin-O. Phenols, having the smaller molecular size, are quickly adsorbed onto activated carbon surface when comparing with Safranin-O. This was also confirmed by the equilibrium time. Solubility which is the main physicochemical features of phenols playing an important role in adsorption mechanisms. The solubility of phenol was higher than all studied nitrophenols, thus, phenol could be dissociated easily in the water. The higher adsorption affinity of m-nitrophenol onto activated carbon among the studied all nitrophenols can also be explained lower solubility in water. The difference between phenol and nitrophenols come from having -NO₂ group of nitrophenols since -NO₂ group acts as a strong electron-withdrawing group. As a conclusion the presence of nitro group on reduction of the electron density of the aromatic ring and the interaction of phenols and carbon is enhanced which leads to an increase in adsorption capacity. It can be concluded that this is also possibly the major mechanism for the adsorption of nitrophenols onto activated carbon. Thermodynamical parameters were also evaluated for the basic dye Safranin-O and phenols and revealed that the adsorption is exothermic in nature. Although activated carbons are relatively expensive and difficult to regenerate, adsorption over activated carbon is one of the most effective methods employed in the treatment of wastewaters containing both different classes of dyes and low-molecular weight organic compounds such as phenols. Our experimental results also showed that the activated carbon with a large surface area (1014.44 m² g^-1) could potentially be used in the removal of safranin-O, phenol and nitrophenols in aqueous solutions and industrial wastewater managements.

ACKNOWLEDGMENT

This research was supported by the Research Fund of Marmara University.