Removal of selected phthalates from aqueous solution by mesoporous-ordered carbon adsorbent

Abstract

In the present work adsorption of selected phthalates from the water samples on mesoporous carbon material was investigated. Carbon material was obtained by soft-templating method in the presence of citric acid. Adsorption process of phthalates was carried out from water solution using static method with sonication for process acceleration. Phthalates extraction from water phase, after adsorption on carbon material, was carried out using solid phase microextraction method. Extracted samples were analyzed using gas chromatography coupled with mass spectrometry method. Considering obtained experimental results it can be indicated that almost 100% of phthalates were adsorbed from water solutions. The efficiency of adsorption decreases with increasing molecular weight of phthalates.

Introduction

Phthalic acid esters (PAEs) are widely used as additives in the production of plastics to make them flexible. For example, bis(2-ethylhexyl) phthalate is one of the main phthalates used as plasticizer in production of polyvinylchloride and other polymers, described by Cao (2008). Worldwide production of PAEs and their common application in different products of everyday use has resulted in their widespread presence in environment and foods (Riosa et al., 2010).

The aim of present work was the investigation of adsorption process of six phthalates: dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-butyl phthalate (DBP), benzyl butyl phthalate (BBP), bis(2-ethylhexyl) phthalate (DEHP), and di-n-octyl phthalate (DOP) on mesoporous-ordered carbon (MOC) adsorbent.

Experimental

MOC was obtained by soft-templating method in the presence of citric acid (C-KC) applying synthesis procedure described by Choma et al. (2013). Basic, standard parameters characterizing the mesoporous structure of studied carbon were determined on the basis of experimental, low-temperature (−196℃) nitrogen adsorption–desorption isotherms. Obtained adsorbent C-KC has the following parameters: specific surface area S_BET = 780 m²/g, single-point pore volume V_t = 0.60 cm³/g, volume of micropores obtained by the alfa-s method V_mi = 0.19 cm³/g, volume of mesopores V_me = 0.41 cm³/g obtained by subtraction of the micropore volume V_mi from single-point pore volume V_t, mesopore diameter at the second maximum of the pore size distribution w_me = 5.99 nm obtained by the Kruk–Jaroniec–Sayari method, and external surface area S_ext = 0.9 m²/g obtained by the alfa-s method. For obtained carbon material images, using scanning electron microscopy (SEM), were taken. C-KC materials have ordered, homogenous porous structure with mesopores dimension about 6 nm (Figure 1).

Figure 1.

Scanning electron microscopy (SEM) image of mesoporous-ordered carbon.

Phthalates standard from Supelco in the form of 1 cm³ ampule containing six phthalates diluted in methanol was used. Concentration of each phthalate in standard was 2000 µg/cm³.

Carbon adsorbent was crushed mechanically and fraction of 0.40–0.63 mm was sieved. Then suitable amounts of carbon material were transferred into 15 cm³ glass vials with phthalate solutions and placed in the incubator set to 25℃ for 60 min. The platform with vials in the incubator was shaking, performing 150 round moves per minute. Adsorption process, on C-KC material, was carried out using 50, 100 and 150 mg of carbon in phthalates water solutions. Concentrations of each phthalate were 0.03/0.25 mg/dm³ and they were at low level concerning phthalates solubility, so the solutions of phthalates were not saturated.

Phthalates extraction from aqueous phase, before and after adsorption on carbon material, was carried out using solid phase microextraction method (SPME), described by Wideł et al. (2015). The 65 µm film of stationary phase polydimethylsiloxane/divinylbenzene (PDMS/DVB) was used for microextraction. Adsorption and thermal desorption, in microextraction procedure, were performed for 45 min and 2 min, respectively.

Adsorption process of selected phthalates was controlled by gas chromatography-mass spectrometry (GC-MS) method (Clarus 600 from PerkinElmer). The compounds were separated on Elite-5MS 30 m × 0.25 mm × 0.25 µm capillary column, using following temperature program: 60℃ held for 1 min, then increased at rate of 15℃/min up to 250℃ and held for 5 min. Total analysis time was approximately 19 min. Injection port temperature was 250℃, helium carrier gas ( ≥ 99.9999% purity) was maintained at a constant flow rate of 1 cm³/min. The ion source and transfer line temperature was set at 250℃. The electron ionization was 70 eV. Analysis was carried out in selected ion recording (SIR) mode (m/z = 149).

Results

The efficiency of adsorption decreases with increasing molecular weight of phthalate (Figure 2). The exception is decreasing adsorption between DEHP and DOP despite the same molecular weight of both phthalates. It can be explained by different structure of DEHP and DOP molecules. The DOP molecule has longer chain substituent groups and this can be the reason of lower adsorption on carbon material.

Figure 2.

Efficiency of phthalates adsorption in percentage. Time of adsorption is 60 min.

The optimum adsorption time of all studied phthalates on MOC is greater than 60 min. The exemplary adsorption kinetic curve for DBP is shown in Figure 3.

Figure 3.

Adsorption kinetics of di-n-butyl phthalate (DBP) on mesoporous-ordered carbon. Efficiency of adsorption (%) vs. adsorption time (min).

Filling the pores with DBP occurs through micropores followed by mesopores (Figure 4).

Figure 4.

The isotherm of DBP adsorption on mesoporous-ordered carbon. Temperature 298 K. Adsorption (mg/g) vs. concentration at equilibrium state (mg/dm³).

The amount of adsorbate adsorbed at equilibrium per unit mass of mesoporous carbon material ‘a’, was calculated by the following equation

$$a=\frac{V(c_0-c_{\mathrm{eq}})}{m}$$

(1) where V is volume of the adsorbate solution in dm³; c₀ and c_eq are the initial and equilibrium concentrations in mg/dm³, respectively, and m is mass of adsorbent in g.

Adsorbed amount of selected phthalates on mesoporous carbon adsorbent were shown in Figure 2. The efficiency of adsorption decreases with increasing molecular weight of phthalate.

The kinetics of adsorption of DBP on mesoporous carbon adsorbent was shown in Figure 3.

According to Wang (2015), the kinetic data were further analyzed using the pseudo-second-order equation which can be expressed as

$$\frac{\mathrm{da}}{\mathrm{dt}}=k_2(a_{\mathrm{eq}}-a{)}^2$$

(2)

Integrating equation (2) for the boundary conditions t = 0 to t > 0 and a = 0 to a > 0 and rearranging to obtain the linearized form which is shown as follows

$$\frac{t}{a}=\frac{1}{k_2{a}_{\mathrm{eq}}^2}+\frac{t}{a_{\mathrm{eq}}}$$

(3) where k₂ is the pseudo-second-order rate constant (g/(mg·min)). The plot of t/q versus t of equation (3) should give a linear relationship, from which a_eq and k₂ can be determined from the slope and intercept from the plot. After the calculations, the following results were obtained: k₂ = 6.14 g/(mg·min), a_eq = 0.2428 mg/g for monolayer coverage. The correlation coefficient R² is 0.9997.

Isotherm adsorption of DBP on mesoporous carbon adsorbent is shown in Figure 4. The shape of the adsorption isotherms shown in Figure 4 can provide multilayer adsorption of DBP above 0.07 mg/dm³ of equilibrium concentration.

According to Wang (2015), the linearized form of Freundlich (F) equation is given in the following equation

$$\text{ln}a=\text{ln}{K}_F+(1/n)\text{ln}{c}_{\mathrm{eq}}$$

(4) where a is the amount of adsorbate adsorbed per unit mass of adsorbent at equilibrium in mg/g, c_eq is the final concentration at equilibrium in mg/dm³, K_F is the composite empirical constant in dm³/mg and 1/n is an empirical parameter. The Freundlich parameter 1/n relates to the surface heterogeneity. In general, 0 < 1/n < 1 indicated that the adsorption behavior of adsorbate on adsorbent was carried out favorable and heterogeneous sorption.

The linearized Langmuir [L] equation is

$$\frac{1}{a}=\frac{1}{c_{\mathrm{eq}}{K}_L{a}_m}+\frac{1}{a_m}$$

(5) where a is the amount of adsorbate adsorbed per unit mass of adsorbent at equilibrium in mg/g, c_eq is the final concentration at equilibrium in mg/dm³, a_m is the maximum adsorption at monolayer coverage in mg/g, and K_L is the adsorption equilibrium constant related to the energy of adsorption. The dimensionless constant separation factor for equilibrium parameter is R_L

$$R_L=\frac{1}{1+K_L{c}_0}$$

(6)

The value of R_L suggest whether system’s conditions is favorable (0 < R_L < 1).

The following equation indicates the Dubinin–Radushkevich [D–R] isotherm

$$\text{ln}(a)=\text{ln}{a}_m-B{\varepsilon }^2$$

(7)

$$\varepsilon =\mathrm{RT}\text{ln}(c_0/c_{\mathrm{eq}})$$

(8) where c₀ and c_eq are the initial and equilibrium concentrations in mg/dm³, a_m is the maximum adsorption at monolayer coverage in mg/g, B is a constant related to the sorption energy, ɛ is the Polanyi potential, R is the universal gas constant, and T is absolute temperature in K.

The mean free energy of adsorption (E), defined as the free energy change when 1 mol of molecule is transferred from infinity in solution to the surface of the sorbent and was calculated from the B value using the following relation

$$E=\frac{1}{\sqrt{2B}}$$

(9)

The magnitude of E is useful for estimating the type of adsorption process. According to the literature, the value E (6.35 kJ/mol) indicated a physi-sorption process of adsorption of DBP on mesoporous carbon adsorbent.

On the basis of equations (4) to (9) the calculation of parameters for selected adsorption models of monolayer coverage were performed on heterogeneity character of adsorbent surface. Results of calculation these parameters were shown in Table 1.

Table 1.

The adsorption isotherm parameters of DBP on mesoporous carbon.

L	am = 0.209 mg/g	KL = 88.43 dm3/mg	RL = 0.043	R2 = 0.9032
F		KF = 1.85 dm3/mg	1/n = 0.371	R2 = 0.9292
D–R	am = 0.212 mg/g	B = 1.2· × 10−2 mol2/kJ2	E = 6.35 kJ/mol	R2 = 0.8914

Conclusions

The efficiency of adsorption decreases with increasing molecular weight of phthalate.

The kinetic data were described by the pseudo-second-order equation.

The equilibrium data fitted with Freundlich isotherm are better than Langmuir and D–R isotherms.

The D–R isotherm indicated a physi-sorption process of DBP adsorption through micropores filling of carbon adsorbent. However data shown in Table 1 consider the calculations for equilibrium concentrations of DBP up to 0.058 mg/dm³ and these data are related to the process of filling the adsorbent’s monolayer.

Filling the pores with DBP occurs through micropores can be followed by mesopores. Mesoporous carbon adsorbent (approx. 68% of mesoporosity) was applied because it can accelerate the diffusion processes during the adsorption.