Novel kinetics model for adsorption of pollutant from wastewaters onto zeolites. Kinetics of phenol adsorption on zeolite-type silicalite

Abstract

The kinetics of isothermal adsorption of phenol from an aqueous solution onto the zeolite-type silicalite was investigated. Zeolite-type silicalite was synthesized and its basic physico-chemical properties were determined. Isothermal adsorption kinetics curves of phenol on zeolite-type silicalite were measured at temperature range from 283 to 313 K. By applying Friedman’s differential isoconversional method it was found that the adsorption of phenol on silicalite has one rate determining step. By using the ‘model-fitting’ method it was established that the kinetic of adsorption can be described with theoretical kinetic model of the two-dimensional phase-boundary controlled reaction (model R2). The kinetic parameters, activation energy ( $E_{a} = 45 {kJ mol}^{- 1}$ ) and preexponetial factor (lnA = 14.1 min⁻¹) of phenol adsorption were calculated. The thermodynamic parameters, standard enthalpy (ΔH*), standard entropy (ΔS*) and standard free Gibbs energy of adsorption (ΔG*) were calculated and discussed. A novel model for the kinetics of pollutant adsorption from wastewaters onto zeolites based on the following: zeolite pores have cylindrical shape with average radius r₀, pores in zeolite are filled simultaneously by the model ‘layer by layer’, the rate of phenol adsorption is higher than the rate of the growth of the thickness of the adsorption layer was suggested. It has been found that the adsorption kinetics can be completely described by this kinetic model.

Keywords

Kinetics adsorption phenol silicalite kinetic model

Introduction

Phenol is considered as one of the most common pollutant in the wastewater because of its presence in the majority of industrial processing and refining plants (Busca et al., 2008). Phenol is harmful even at low concentrations due to its toxicity and carcinogenic properties (Akbal and Onar, 2003). For that reasons, the US Environmental Protection Agency regulations demand lowering phenol content in the wastewater to less than 1 mg l⁻¹ (Banat et al., 2000). Various methods are used for phenol removal from wastewaters, such as biodegradation (El-Naas et al., 2010; Kurzbaum et al., 2010; Stehlickova et al., 2009), photocatalytic degradation (Ahmed et al., 2010; Deshpande and Madras, 2010; Mu et al., 2010), chemical coagulation (Perdigon-Melon et al., 2010), chemical oxidation (Bansal et al., 2010; Zazo et al., 2009), solvent extraction (Messikh et al., 2007; Vatai et al., 2009), membrane separation (Bodalo et al., 2009; Ng et al., 2010), adsorption with porous inorganic materials (Lin et al., 2009; Liu et al., 2010; Senturk et al., 2009; Sprynskyy et al., 2009; Su et al., 2011) and synthetic polymeric materials (An et al., 2009; Zeng et al., 2009, 2010).

Adsorption is one of the most frequently used methods for phenol removal due to its simplicity in handling and relatively low exploitation price. The most widely used adsorbents are activated carbon (Dabrowski et al., 2005) alumino-silicates such as clays (Ahmaruzzaman, 2008), surfactant-modiﬁed zeolites and mesoporous aluminosilicates (Bowman, 2003; Shu et al., 1997).

In spite of the huge practical importance, there are relatively low numbers of articles considering kinetics of phenol adsorption on synthetic zeolites. Adsorption isotherm for phenol from water solution onto zeolite-type silicalite 1 and 2 is shown in the work of Narita et al. (1985). The authors found that adsorption isotherm can be modelled with the Freundlich’s type isotherm and that the maximal adsorption capacity of phenol is dependent on both temperature and pH (Narita et al., 1985). Adsorption of benzene and phenol onto zeolite-type silicalites, HAlZSM-5 and NaAlZSM-5, was mathematically simulated by Klemm et al. (1998). The differences in the adsorption behaviour between benzene and phenol are attributed to the difference in Coulomb interactions between them and zeolite. Roostaei and Tezel (2004) investigated kinetic of liquid-phase phenol adsorption on different adsorbents: activated alumina (basic and acidic base), silica gel, activated carbon, zeolite Y, zeolite ZSM-5 and Filtrasorb-400. Their results showed that phenol is not adsorbed on activated alumina and silica gel, whereas kinetic of phenol adsorption on the other used adsorbents can be best described with Lagergren equation.

For the design and projecting adsorption plant for wastewater purification from phenol, the knowledge of kinetics of phenol adsorption on particular adsorbent is of crucial importance. In this regard, the adsorption of phenol from aqueous solutions on zeolite-type silicalite, which included evaluation of degree of complexity, kinetics model and the values of kinetics parameters of phenol adsorption, was thoroughly investigated in this work.

Experimental

Materials and methods

Phenol p.a. was purchased from Lachner, Czech R., tetra-propylammonium hydroxide (TPAOH), 40% solution in water, was purchased from Fluka, Germany. SiO₂ was synthesized according to the procedure described by Salvestrini et al. (2016). Deionized water was used in all the experiments.

Synthesis and characterization of zeolite-type silicalite

Zeolite-type silicalite was synthesized from a reaction mixture, with the molar content (TPA)₂O × 13.3SiO₂ × 184H₂O. Reaction mixture was prepared as follows. The measured weight of 21 g of fine powder of SiO₂ was suspended in 66 g water. To this suspension was added 25.06 g of 40% solution of TPAOH. The reaction mixture was placed in a tetrafluoroethylene-lined pressure vessel and heated at 200°C and autogenous pressure and kept for 24 h. The solid reaction product was recovered by filtration, washed with water and dried at 110°C. Dried sample of silicalite was calcined in air at about 600°C for 1 h. Table 1 presents basic physico-chemical properties of the zeolite.

Table 1.

Basic physico-chemical properties of synthesized zeolite-type silicalite.

Physico-chemical properties	Value	Method	Ref.
Degree of crystallinity (%)	99	XRD	Adnadjević et al. (1997)
Si/Al ratio	1200	Chemical analysis	Salvestrini et al. (2016)
Degree of hydrophobicity (%)	99.5	Toluene adsorption from aqueous solution	Vakili et al. (2005), Weitkamp et al. (1997)
Diameter granule (µm)	200	Granulometric analysis	Adnadjević et al. (1997)
Crystal size SEM (µm)	15	SEM	Adnadjević et al. (1997)
Surface area BET (m²g⁻¹)	380	Low temperature nitrogen adsorption	Adnadjević et al. (1997)

BET: brunauer–emmett–teller; SEM: scanning electron microscopy; XRD: X-Ray diffraction.

Kinetics of phenol adsorption on silicalite

Investigation of kinetics of phenol adsorption on zeolite-type silicalite was carried out by batch method. An accurately measured weight of zeolite (m) was added in the 100 g of 1% aqueous phenol solution, preheated to a predetermined temperature. The obtained suspension was homogenized by mixing at 400 r min⁻¹. In predetermined time intervals from the reaction suspension, sample aliquots were withdrawn for analysis. The samples were centrifuged at 2000 r min⁻¹ in duration of 5 min for the purpose of separation of zeolites from the sample. The speciﬁc adsorption capacity of silicalite at given time (t) was calculated from the equation

xt = \frac{(C_{0} - C_{t})}{m} m_{t} (mg \cdot g^{- 1})

(1)

where C₀ is the initial concentration of the phenol aqueous solution (wt%), C_t is the phenol concentration at given time (t) (wt%), m_t is the aqueous solution weight (mg) and m is the weight of zeolite-type silicalite (g).

Concentration of phenol in water solution after adsorption was determined by the ASTM method (International standard, ASTM D1783-01, 2012). The VIS spectra of water solution of phenol were recorded on UV-VIS spectrophotometer Shimadzu UV mini 1240, Japan. The absorbance of phenol solution was measured at 510 nm.

Degree of adsorption of phenol

The degree of adsorption of phenol was calculated from equation

α = \frac{x_{t}}{x_{\max}}

(2)

where x_t is the specific adsorption capacity of silicalite and x_max is the maximum speciﬁc adsorption capacity of silicalite for phenol which was obtained from kinetic curves.

Model-fitting method

Bearing in mind that the adsorption of phenol on zeolite is a process which is carried out at adsorbent–solution interface, the kinetic model of phenol adsorption was determined by the ‘model-fitting’ method (Brown et al., 1980; Vyazovkin and Wight, 1999). In brief, the method consists of the following. The experimentally determined conversion curve α_exp = f(t) _T has to be transformed into the so-called normalized conversion curve α_exp = f(t _R ) _T , where t_R is the so-called reduced time. The reduced time, t_R, introduced to normalize the time interval of the monitored process in accordance with Brown et al. (1980) is defined by the following equation

t_{R} = \frac{t}{t_{0.9}}

(3)

where t_0.9 is the adsorption time at which α = 0.9. By applying the reduced time, it was possible to calculate the reduced conversional curves for different kinetics models (Fu et al., 2009). The kinetics model of the investigated process was determined by analytically comparing the reduced experimental curves with the model’s normalized curves.

The criterion for comparing was the sum of squares of the deviation from the models’ normalized curves. As a kinetics model, the model for which the sum of squares of the deviation from the experimental curve gives a minimal value is chosen. A set of the reaction models is used to determine the model which best describes the kinetic of phenol adsorption on zeolite, where f(α) is the analytical expression describing the kinetic model and g(α) is the integral form of the kinetics model (Lazarevic et al., 2011).

Differential isoconversion method

The Friedman’s (1964) differential isoconversion method is based on the isoconversional principle. In accordance with the isoconversional principle, the rate of reaction (dα/dt) on fixed value of conversion is dependent solely on temperature and can be shown by the equation

{(\frac{d α}{dt})}_{α_{i}} = A_{a_{i}} \exp (- \frac{E_{a, α_{i}}}{RT}) f (α_{i})

(4)

where

E_{a, α_{i}}

is the apparent activation energy and

A_{α_{i}}

is the preexponential factor for particular degree of adsorption, R is the gas constant, T is the temperature and f(α _i ) is the reaction model associated with a certain theoretical reaction mechanism. From the slope and intercept of the dependence of ln(dα/dt) on 1/T, the values of the kinetics parameters

E_{a, α}

and A_α are calculated.

Fitting of experimental data

The kinetics data for the isoconversional method (kinetic model R2, Arrhenius and Eyring equations) are fitted with a linear regression method. The linear correlation coefficient was used to minimize deviation of experimental data from the used equations. The parameters and the coefficients of correlations were calculated by using commercial software Origin Lab Pro 8.

Results and discussion

The isothermal dependence of speciﬁc adsorption capacity of the silicalite for phenol on time (i.e. kinetic curves) is shown in Figure 1.

Figure 1.

Dependence of the isothermal specific adsorption capacity of silicalite on time for phenol adsorption at 283 K (), 293 K (), 298 K (), 303 K () and 313 K ().

From Figure 1, it can be seen that the isothermal dependences of speciﬁc adsorption capacity of silicalite versus time at all of the investigated temperatures are similar by shape. At the beginning of the adsorption process, the specific adsorption capacity increases linearly with time. After that the increase with time slowed and becomes convex by shape. At the end of the adsorption process, the value of specific adsorption capacity of silicalite becomes independent of time-plateau region. With an increase in temperature, the slope of the linear part of the kinetic curve increases as well as the maximum speciﬁc adsorption capacity x_max. That increase of the slope points on the increase in the adsorption rate, whereas the increase in x_max with temperature indicates on adsorption type-activated adsorption.

In order to examine rate-determining step of adsorption (mass transfer, phase boundary and chemical reaction), several kinetics models are used to test the experimental data.

In the work of Roostaei and Tezel (2004), it was established that the adsorption kinetics of phenol on zeolite-type Y and ZSM-5 can be described with the model of first-order chemical reaction.

When the adsorption kinetics of phenol can be described with the model of first-order chemical reaction, then the dependence of −ln(1−α) on time gives a straight line. Thus, herein obtained isothermal dependences of −ln(1−α) on time for different temperatures are shown in Figure 2.

Figure 2.

Dependence of −ln(1−α) on time at 283 K (), 293 K (), 298 K (), 303 K () and 313 K ().

From the results shown in Figure 2, it is obvious that the dependence of −ln(1−α) versus time is considerably deviating from linearity through the whole range of α. Therefore, the adsorption kinetics of phenol on silicalite is not possible to be described with the model of first-order chemical reaction.

When the adsorption kinetics of phenol can be described with the model of second-order chemical reaction, then the dependence of (1−α)⁻¹−1 on time gives a straight line. The obtained isothermal dependences, (1−α)⁻¹−1= f(t), for different temperatures are shown in Figure 3.

Figure 3.

Dependence of (1−α)⁻¹−1 on time at 283 K (), 293 K (■), 298 K (), 303 K () and 313 K ().

Based on the results shown in Figure 3, similarly to the ones in Figure 2, it is obvious that the dependence of (1−α)⁻¹−1 versus time is considerably deviating from linearity through the whole range of α and the adsorption kinetics is not possible to be described with the model of second-order chemical reaction.

In the case when the rate-determining step of phenol adsorption is the intraparticle diffusion of phenol, the dependence α² = f(t) should be a straight line (Weber and Morris, 1963; Yousef et al., 2011). The isothermal dependences, α²= f(t), for phenol adsorption at different temperatures are shown in Figure 4.

Figure 4.

Dependence of α² on time at 283 K (), 293 K (), 298 K (), 303 K () and 313 K ().

From the results presented in Figure 4, one can recognize that the plots of α² versus time considerably deviate from the linearity through the whole range of α. The established deviation from the linearity indicates that the kinetics of phenol adsorption cannot be described with the interparticle diffusion model.

By applying the differential isoconversion method (Vyazovkin and Lesnikovich, 1990), the shape of the dependence $E_{a}$ = f(α) was found with the aim to analyse the degree of kinetics complexity of this adsorption process. The dependences ln(dα/dt)α= f(1/T) for different degrees of phenol adsorbed are presented in Figure 5 and from the slope (a) and intercept (b) of these straight lines; the values of the kinetics parameters (E_a,_α and ln A_α) for each value of the degree of phenol adsorbed were obtained. The dependence of E_a versus α is given in Figure 6.

Figure 5.

Dependences of ln(dα/dt)α on inverse temperature (1/T) for different degrees of phenol adsorbed.

Figure 6.

Dependence of E_a,_α versus the degree of phenol adsorbed.

The results shown in Figure 6 reveal that the value of the activation energy does not change with the degree of phenol adsorption. Consequently, phenol adsorption on silicalite has only one well-defined rate-determining step with E_a 45 kJ mol⁻¹. In regard to this, the kinetic model of the investigated adsorption process was determined by using model-fitting method. Figure 7 shows experimentally normalized conversion adsorption curves at all of the investigated temperatures.

Figure 7.

Plot of α versus t_R at 283 K (), 293 K (■), 298 K (), 303 K () and 313 K ().

As can be seen from Figure 7, all the experimentally normalized conversion adsorption curves are identical in shape, which implies on same kinetics model independent of temperature. By a comparative analysis of the experimentally normalized conversion adsorption curves with the shapes of the normalized conversion adsorption curves for different theoretical models, one can conclude that those experimentally kinetics data best fitted with the kinetic model for phase-boundary controlled reaction, contracting area (R2). In accordance with that, one can conclude that the rate-determining step of the phenol adsorption is the rate of movement of adsorbent–solution interface. In the case when the kinetics of the phenol adsorption can be described with the R2 model, the dependence $[1 - {(1 - α)}^{1 / 2}]$ on time should give a straight line.

Figure 8 shows the isothermal dependence of $[1 - {(1 - α)}^{1 / 2}]$ on time.

Figure 8.

The dependences of [1 −(1 − α)^1/2] versus time at 283 K (), 293 K (■), 298 K (), 303 K () and 313 K ().

From Figure 8, it can be clearly seen that those dependencies are straight lines which confirm the validity of kinetic model chosen for the adsorption of phenol on zeolite-type silicalite. The values of kinetic model for the investigated temperatures are given in Table 2.

Table 2.

The rate of model constants and kinetic parameters.

T (K)	k_M (min⁻¹)	R	Kinetic parameters
283	0.0065	0.9992
293	0.0136	0.9993	E_a (kJ mol⁻¹) = 45 ± 2
298	0.0190	0.9998	ln A (min⁻¹)= 14.1 ± 0.8
303	0.0250	0.9986
313	0.0410	0.9992

The increase in the values of models’ adsorption rate constant with temperature is in accordance with the Arrhenius equation, which enables to calculate models’ kinetic parameters, E_a and ln A. The obtained value of the models’ E_a is in full agreement with the previously determined value for E_a calculated by using isoconversional method, which strongly confirms the above hypothesis about the existence of only one well-defined rate-determining step.

Based on the established kinetic model of phenol adsorption on silicalite, it is possible to assume kinetics model for pollutant adsorption on zeolites. The basic assumptions of this model are zeolite contains N pores having cylindrical shape with average radius r₀ and pores in zeolite are filled simultaneously by the model ‘layer by layer’. The rate of phenol adsorption is higher than the rate of the growth of the thickness of the adsorption layer. In that case, the radius of cylindrical pores of zeolite in momentum of time r can be expressed by the equation

r = r_{0} - kt

(5)

where k is the rate constant of the increase of the thickness of the adsorption layer.

Then, the degree of adsorption of phenol onto silicalite is given by the equation

α = \frac{N ρ π h r_{0}^{2} - N ρ π h r^{2}}{N ρ π h r_{0}^{2}}

(6)

where ρ is the density of adsorbed phenol and h is the cylinder height.

That is

α = (1 - \frac{r^{2}}{r_{0}^{2}})

(7)

In equation (6) replacing the r value with r₀ − kt, the following is given

α = 1 - {(\frac{r_{0} - kt}{r_{0}})}^{2}

(8)

which can be rearranged to

1 - α = {(1 - \frac{k}{r_{0}} t)}^{2}

(9)

that is

[1 - {(1 - α)}^{1 / 2}] = k_{M} t

(10)

where

k_{M}

is the model rate constant of the increase of the thickness of the adsorption layer. Since equation (9) is identical with the integral form of reaction model R2, that is successfully used for describing the kinetics of phenol adsorption on silicalite, with great degree of assurance it can be claimed that the kinetics of phenol adsorption on silicalite is described with the suggested kinetic model.

Since the adsorption of phenol on zeolite is a thermally activated process and has one well-defined kinetics rate-determining stage, it is possible to calculate values of thermodynamic parameters, the standard enthalpy (ΔH*) and the standard entropy (ΔS*) of adsorption by applying Eyring equations to calculate values of thermodynamic parameters (Eyring, 1935)

k = \frac{χ k_{B} T}{h} \exp (\frac{Δ S^{*}}{R}) \exp (- \frac{Δ H^{*}}{R})

(11)

where k is the reaction rate constant, R is the gas constant, k_B is Boltzmann constant, h is Planck’s constant, T is the temperature, and

Δ S^{*}

and

Δ H^{*}

are the entropy and enthalpy of activation. Division of equation (10) by T through and taking the natural logarithm leads to the following logarithmic equation

\ln \frac{k}{T} = \ln \frac{k_{B}}{h} + \frac{Δ S^{*}}{R} - \frac{Δ H^{*}}{RT}

(12)

The values for the enthalpy and entropy of activation for the process can be determined from the kinetic data obtained from ln(k/T) on 1/T plot. The dependences ln(k_M/T)= f(1/T) are presented in Figure 9.

Figure 9.

Dependences of ln(k_M/T) on inverse temperature (1/T).

Dependence is a straight line with slope (a) and intercept (b). From the slopes and intercepts, the values were obtained

Δ H^{*} = a \cdot R

(13)

Δ S^{*} = (b - \ln \frac{k_{B}}{h}) \cdot R

(14)

The values of the standard free Gibbs free energy of adsorption ( $Δ G^{*}$ ) can be described as

Δ G^{*} = Δ H^{*} - T Δ S^{*}

(15)

The values of the thermodynamic parameters ( $Δ H^{*}$ and $Δ S^{*}$ ) of adsorption and the effect of temperature on standard free Gibbs energy of adsorption ( $Δ G^{*})$ are shown in Table 3.

Table 3.

Values of thermodynamic parameters for adsorption of phenol.

Thermodynamic parameters of the activated complex
ΔG* (kJ mol ⁻ ¹)	283 K	−5.1
	293 K	−3.5
	298 K	−2.8
	303 K	−2.1
	313 K	−1.1
ΔH* (kJ mol⁻¹)		−42.7
ΔS* (J mol⁻¹ K⁻¹)		−133.9

Thermodynamic parameters $Δ H^{*}$ , $Δ S^{*}$ and $Δ G^{*}$ have negative values. The negative value of (ΔG*) indicates that the adsorption of phenol on silicalite is a spontaneous process (Fu et al., 2009) and that the silicalite has a high affinity to phenol. In addition, the value of ΔG* increases with the increase in temperature suggesting spontaneous adsorption process inversely proportional to the temperature. The negative value of enthalpy changes (ΔH*) implies that the adsorption of phenol onto silicalite has an exothermic nature, since the energy that is released during the phenol adsorption onto silicalite is higher than the energy required to break intermolecular interactions between phenol and water. The negative values of entropy (ΔS*) show the randomness at the solid–liquid interface during the adsorption process since the degree of freedom of phenol molecules decreased during the adsorption.

Conclusion

Silicalite selectively adsorbs phenol from water solutions. The adsorption of phenol onto silicalite is a spontaneous, exothermal, thermally activated process. The rate of adsorbed phenol and its maximal quantity increases with the increase in temperature. The adsorption of phenol was carried out by the model ‘layer by layer’. The rate of phenol adsorption is kinetically limited with the rate of adsorption layer growth. The kinetics of phenol adsorption can be described with the theoretical kinetics model of two-dimensional boundary phase (R2). The activation energy for phenol adsorption is independent of the degree of phenol adsorption. A novel model for the kinetics of pollutant adsorption from their water solutions onto zeolites was suggested which is successfully applied to thoroughly explain the investigated adsorption of phenol onto silicalite.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: The presented investigations were supported by The Ministry of Education, Science and Technological Development of the Republic of Serbia, through the Project 172015.

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