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Theoretical Investigations and Experimental Studies of the Adsorption of Diethyl Phthalate and Di(2-ethylhexyl) Phthalate on Reduced
Graphene Oxide
Umar Yunusa
a*, Sulaiman Adamu Idris
b, Gambo Nurudeen Bello
c, Abdullahi Shehu
d, Abdulrahman Ibrahim Kubo
e, Umaru Umar
aa Department of Pure and Industrial Chemistry, Faculty of Physical Sciences, Bayero University, Kano, Nigeria.
b Department of Science Laboratory Technology, Federal Polytechnic Offa, Kwara State, Nigeria.
c Chemical Division, Industrial Development Department, FMITI, PMB 88, Garki 1, Abuja, Nigeria.
d Department of General Studies, Federal College of Agricultural Produce Technology, Kano, Nigeria.
e Department of Pure and Applied Chemistry, Adamawa State University, Mubi, Nigeria.
Corresponding author. E-mail : umaryunusa93@gmail.com
Received 12 Apr 2021, Revised 17 May 2021, Accepted 13 Jun 2021
Abstract
Through an eco-friendly solvothermal approach, a reduced graphene oxide (rGO) was successfully fabricated for the convenient removal of diethyl phthalate (DEP) and di(2-ethylhexyl) phthalate (DEHP) from water streams. The morphological and spectroscopical characterization of the prepared rGO was accomplished using SEM, FTIR, EDX and UV-visible spectroscopy. The adsorption properties of rGO was explored using static adsorption experiments. Kinetics, isotherms, thermodynamics and computational studies were performed to evaluate the adsorption process. The results disclosed that the experimental data were compatible with pseudo-second-order kinetics and the Freundlich adsorption isotherm. The kinetic diffusion model revealed that intraparticle diffusion is present but may not be the sole rate-limiting factor. Thermodynamic analysis indicated that the test adsorbent spontaneously adsorbed DEP and DEHP, driven mainly by entropy change. In the light of theoretical quantum studies, the frontier molecular orbitals, electrostatic surface potential and molecular reactivity parameters for the studied compounds were reported and discussed.
Keywords : Adsorption ; Phthalates ; Reduced graphene oxide ; DFT calculation ; Chemical reactivity.
1. Introduction
Phthalates are non-halogenated esters of 1,2-dibenzenedicarboxylic acid, prepared by the esterification of phthalic anhydride and an alcohol. Phthalates are extensively employed as plasticizers to strengthen and enhance the flexibility of plastic materials, majorly polyvinyl chloride (PVC). As common phthalate compounds, diethyl phthalate (DEP) and di(2-ethylhexyl) phthalate (DEHP) are used in wide application context including polymer-based materials, building materials, cleaning materials,
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cosmetics, household products, and medical devices [1-2]. Since they are not chemically bound in a polymeric matrix, large amounts of phthalates can leach from PVC and other products into the environment, and then migrate into water, food and air [3]. Thus, phthalates have emerged as one of the most widespread pollutants found in water streams. These chemicals have potential health risks to human, regarding to their carcinogenic, mutagenic, xenoestrogenic and endocrine disrupting effects.
Consequently, some phthalate compounds including DEP and DEHP have been categorized as priority toxic pollutants by the World Health Organization (WHO), U.S. Environmental Protection Agency (EPA) and European Environment Agency [4-6]. Therefore, the removal of phthalates from diverse environmental matrices is highly imperative.
So far, numerous remediation technologies have been used to eliminate phthalates from water systems including biodegradation, bioconversion, coagulation and advanced oxidation processes [7].
Nevertheless, these techniques are limited in their application due to technical, operational and economic constraints. In this respect, adsorption appears to be one of the most effective choices thanks to its convenience, lower energy consumption, insensitivity to toxic pollutants and simplicity of design [8]. A number of conventional adsorbents, including activated carbon, chitosan, microbial biomasses, carbon nanotubes etc., have been used to remove phthalates from aquatic medium [9]. However, some of the materials have a relatively weak affinity to phthalates resulting in low adsorption capacity.
Nowadays, graphene-based nanomaterials such as graphene oxide (GO), reduced graphene oxide (rGO) and graphene nanocomposites (GNCs) have attracted huge attention in water and wastewater treatment, particularly due to their large specific surface area, superior mechanical properties and presence of negatively charged oxygen functionalities on the surface [10-11]. Although GO and rGO exhibit similar adsorption performance, GO has a stronger hydrophilic character that could interfere with the elimination of organic pollutants from aqueous environment. Moreover, GO is highly dispersible in water which usually result in low adsorption performance [12]. Conversely, the presence of residual oxygen functionalities, defects sites, large porosity, wrinkles and π-electron domains in the planner structure of rGO, can be good characteristics to facilitate the elimination of organic pollutants from water [13].
In this sense, the present study is focused on the removal of two phthalates, DEP and DEHP by adsorption onto reduced graphene oxide. The effect of operating variables such as initial concentration, pH of the aqueous solutions, temperature and contact time on the adsorption performance has been assessed. Equilibrium, kinetic and thermodynamic characteristics of the adsorption of DEP and DEHP onto rGO are also discussed in detail. Crucially, the molecular properties of the examined phthalates including optimized geometry, HOMO-LUMO energies, charge distribution, global and local reactivity descriptors have been explored using density functional theory (DFT) calculations.
153 2. Materials and Methods
2.1. Chemicals
All the chemicals and reagents used in this work were of analytical grade. DEP (≥99.5%) and DEHP (≥98%) were procured from R&M Chemicals. Selected physicochemical properties of both phthalates are disclosed in Table 1. Graphite powder (<150 µm, 99.99%), sodium hydroxide (40.00 g/mol), sodium nitrate (84.99 g/mol), and ethanol (≥99.9%) were acquired from Sigma Aldrich. Hydrogen peroxide (≥30%), hydrochloric acid (≥ 37%), sulfuric acid (95-98%) and potassium permanganate (≥99%) were purchased from Merck. Deionized water was employed in all experiments.
2.2. Adsorbate preparation and analytical measurement
The stock solutions were prepared by dissolving the appropriate amounts of DEP and DEHP in deionized water. Then, the stock solutions were serially diluted to get experimental solutions of required phthalate concentrations. The residual concentration of DEP and DEHP in the supernatant was measured by UV-visible spectrophotometer (Labda 35; Perkin Elmer) at the wavelength of 230 and 274 nm, respectively. The analytical detection limit was about 0.1 mg L-1. The phthalates calibration curve was developed by determining the absorbance with a series of standard solutions (2−10 mg/L).
2.3. Synthesis of graphene oxide (GO) and reduced graphene oxide (rGO)
Graphite powder was oxidized to GO using a modified Hummer’s protocol [14]. In a typical procedure, 1 g of graphite powder and 0.5 g of sodium nitrate were mixed together followed by the addition of H2SO4 (23 mL) under vigorous stirring in a water bath. Then KMnO4 (3 g) was added slowly to the mixture while maintaining the temperature at 20 °C. The resulting mixture was agitated at 40 °C for 10 h and the resulting solution was diluted by adding 500 mL of water under vigorous stirring. This was followed by addition of 30% H2O2 solution (15 mL) to terminate the reaction. After centrifugation (9000 rpm, 5 min), the GO precipitate obtained was washed with 10% (v/v) HCl solution and deionized H2O in subsequent order until a final pH of 6 was attained. Finally, GO was dried and kept for subsequent use.
For synthesis of rGO, 300 mg of GO powder was mixed with C2H5OH (30 mL) and deionized water (15 mL). A stable GO dispersion was obtained after 40 min of rigorous sonication of the mixture.
Then, the mixture was transferred into a stainless-steel autoclave, and heated at reduction temperature of 150 °C for 1.5 h. Finally, the as-prepared rGO sample was washed with deionized water by centrifugation for 0.5 h at 2500 rpm, and subsequently dried in an oven at 90 °C to obtain it in the powder form [15].
2.4. Characterization studies
To elucidate the chemical and physical characteristics of the synthesized adsorbent, various analyses were conducted. The surface features were taken using a scanning electron microscope (Phenomn Pro X). The elemental composition of the sample was analyzed via energy dispersive spectroscopy (FEI Quanta FEG 650 EDX Unit). UV-visible spectroscopy was used to record the absorbance spectra
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(200–800 nm) and to assess the optical properties of the samples. FTIR spectra were obtained using Cary 630 spectrometer (Agilent Technologies). The point of zero charge (pH pzc)was determined using a reported protocol [16]. Briefly, 0.002 g of rGO was added into a set of centrifuge tubes, each containing 5 mL of an electrolyte (0.1 M NaNO3 solution). The pH of the solutions was adjusted to a value between 2 and 11 using dilute HCl or NaOH solutions. All the solutions were then stirred continuously for 24 h, after which the final pH was measured. A graph between the changes in pH (∆pH) against the initial pH was plotted and the point of intersection was taken as pHpzc.
2.5. Adsorption test and evaluation of linear models
Appropriate amount of rGO (20 mg) was added to centrifuge tubes containing 20 mL solutions of phthalates of known concentrations. The natural solution pH (without any adjustment) was maintained and the mixture was stirred at 200 rpm for 3 h at 25°C. Subsequently, the aliquots were centrifuged (2500 rpm, 5 min) and phthalates concentration in the supernatant were analyzed by UV-visible measurements. The experimental adsorption capacity, Qe (mg/g) of rGO for DEP and DEHP was calculated according to eq. (1):
(1)
where Co (mg/L) represent the initial concentration of phthalates in solution, Ce (mg/L) denotes the equilibrium concentration in the supernatant solution, V (mL) reflects the volume of the experimental solution, and m denotes the dry weight (g) of the rGO that was added to the solution. All the experiments were run in triplicate to ensure the accuracy and reproducibility of the results, and only the average values were used for the analysis. Standard deviations were found to be within ±1.5 %.
To explore the impact of solution pH, adsorption experiments were carried out by adding 20 mg of rGO into centrifuge tubes containing 20 mL of 20 mg/L phthalates solution within a pH range of 3−10.
The solutions’s pH was adjusted with 0.1 M HCl or 0.1 M NaOH solution and measured by pH meter (Crison GLP 21). The removal phthalates as a function of temperature was evaluated at variable temperatures (298, 308, 318, and 328 K). To examine the adsorption kinetics (effect of contact time), batch experiments were conducted by adding 20 mg of rGO within each tube containing 20 mL of adsorbate solution (10-40 mg/L). The tubes were shaken at 200 rpm at 30 °C for 10−150 min. Then, the pseudo-first- order model (eq. 2), pseudo-second-order model (eq. 3) and intraparticle diffusion model (eq. 4), were engaged to elucidate the dynamic binding processes of rGO.
(2)
(3)
+ C (4)
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where t is the adsorption time (min); Qe and Qt (mg/g) represent the adsorption capacity at equilibrium and t time; and k1 (min−1), k2 (g/mg min) and Kid (g mg -1min−0.5) reflect the rate constant of the pseudo-first-order, pseudo-second-order and intraparticle diffusion models, respectively. Also, C is indicative of the boundary layer thickness.
To investigate the adsorption isotherms, adsorption experiments were performed by adding 20 mg of rGO within the tubes containing 20 mL of pthalate solution with varying concentrations from 10 to 40 mg/L. The tubes were agitated at 200 rpm under room temperature (298 K) for 180 min. Langmuir (eq. 5) and Freundlich (eq. 6) isotherm models were used to fit the equilibrium data.
(5)
(6)
For the Langmuir isotherm, the separation factor, RL was represented by eq. (7):
(7)
The terms in the Langmuir and Freundlich model, Qe (mg/g) and Ce (mg/L) are the uptake and concentration of phthalates at equilibrium; Qm (mg/g) is the maximum adsorption capacity and KL
(L/mg) is the Langmuir constant linked to the free energy of adsorption; KF (mg/g)(L/mg) is the Freundlich constant which is reflective of the adsorption capacity. The constant n in the Freundlich model indicates the deviation of adsorption from the linearity [17].
2.6. Quantum chemical calculations
All computations were performed using Materials Studio 8.0 software package (BIOVIA, Accelrys).
The molecular structures of DEP and DEHP were modeled using ChemDraw Ultra 7.0 software.
Density Functional Theory (DFT) method was engaged for the geometry optimizations by using B3LYP functional together with DND basis set. Selected quantum chemical parameters including the energy of the highest occupied molecular orbital (EHOMO), energy of the lowest unoccupied molecular orbital (ELUMO), the energy gap, ∆E (∆E = EHOMO-ELUMO), potential of ionization, I (─EHOMO), electron affinity, A (─ELUMO), electronegativity, χ (eq. 8), global hardness, η (eq. 9), chemical softness, σ (eq.
10), chemical potential, μ (eq. 11), and electrophilicity power, ω (eq. 12), were obtained [18].
(8)
(9)
(10)
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(11)
\
(12)
Table 1. Physicochemical data of DEP and DEHP [6]
Phthalate Structure Molecular weight Log Ko/w
Diethyl phthalate (DEP)
O O
O
O
222.24 2.65
Di(2-ethylhexyl) phthalate (DEHP)
O
O O
O
390.56 8.39
3. Results and Discussion
3.1. Morphological and chemical characterization of GO and rGO
The morphological and anatomical features of GO and rGO were elucidated by capturing SEM images (Figure 1). The images which provide visual context revealed the layered and folded features on the textural surface of the GO sheets (Figure 1a), which may be linked to the presence of sp3 sites of oxygen functionalities. The wrinkle and clumped features were explicitly observed in the rGO image (Figure 1b), which may be ascribed to some structural defects in the graphitic domains. The EDX elemental analysis (Table 2) showed that only C and O were present in the GO and rGO. It was observed that the GO exhibited the lowest C/O atomic ratio of 1.79, which signify the higher oxygen composition. Results from FTIR analysis are displayed in Figure 2. The FTIR spectrum of GO displays a broad peak (3000-3600 cm-1) centered at 3301 cm−1, which is often associated with O-H stretching vibrations of alcohols/phenol, or may be due water molecules adsorbed in GO layers [19].
This peak was clearly absent in rGO spectrum implying that the GO is more hydrophilic. The sharp peak at around 1623 cm-1 is assigned to the stretching vibration of C=O of carboxylic acid and carbonyl groups present at the edges of GO plane [20]. Then a peak at the 1405 cm-1 was attributed to absorption of O-H bending vibrations. The point of zero charge analysis (Figure 3) revealed that the pHpzc of rGO was observed at pH 4.1. Thus, it is anticipated that the adsorbent surface will bear a net negative charge above pH 4.1, and a net positive charge below pH 4.1. The UV-visible absorbance spectra of GO and rGO are presented in Figure 4. GO exhibits a prominent absorption peak at 229 nm, which is ascribed to the π-π* transitions of C-C bonds [21]. When GO was reduced, the absorbance
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peak observed at 230 nm redshifts to 265 nm, indicating the partial restoration of the sp2 conjugation of aromatic structure [22].
Figure 1. SEM images of (a) GO and (b) rGO
Figure 2. FTIR spectra of GO and rGO
a b
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Figure 3. pHpzc of rGO
Figure 4. UV-visible absorption spectra of GO and rGO
Table 2. EDX elemental analysis of GO and rGO
Sample C O C/O
GO 63.06 36.94 1.71
rGO 69.22 30.78 2.24
3.2. Effect of pH on DEP and DEHP adsorption
One of the crucial experimental factors influencing the adsorption capacity of adsorbent is pH of solution. The solution pH could alter both the existing form of the examined adsorbate and the charges on the surface of the target adsorbent [23]. The influence of solution pH on the phthalates adsorption was displayed in Figure 5. As can be observed in Figure 5, the adsorption yield of DEP and DEHP was higher throughout the acidic range, and maximum uptake was observed at pH 4 and 5, respectively. The lower adsorption at basic medium was ascribed to the fact that at higher pH (pH>pHPzc), the rGO surface acquires a negative charge, while phthalates molecules hydrolyze to
159
produce phthalate anions. Therefore, the adsorption capacity of rGO will decline due to the increase in the electrostatic resistance between the phthalate anions and negatively charged adsorbent [24].
Figure 5. Effect of solution pH on DEP and DEHP adsorption onto rGO
3.3. Effect of contact time and initial concentration
Figure 6 displays the adsorption capacity of rGO with respect to time at different initial concentrations (10–40 mg/L). It was observed that the amount of phthalates adsorbed increased with the increase in initial adsorbate concentration. This was attributed to the increase in mass transfer driving force with increase in initial phthalate concentration. The time taken to attain equilibrium is important for practical pollution management. In the present study, the adsorption of both DEP and DEHP initially increased and then levelled off within 60 min. No significant change in adsorption capacity was observed by increasing the contact time beyond this period. Thus, the equilibrium time for the adsorption of DEP and DEHP onto rGO is established as 60 min. When an equilibrium condition was attained, the phthalate molecule in the solution is in a state of equilibrium with the molecule adsorbed by the rGO.
Figure 6. Adsorption capacity against time at different initial concentration of (a) DEP (b) DEHP
160 3.4. Adsorption kinetics
The adsorption kinetics was probed by fitting the adsorption data to various kinetic models (Figures 7 and 8). Kinetic fits were developed to determine the rate constants and theoretical adsorption capacity.
The linear fitting results revealed that the pseudo-second-order model was more compatible with both DEP and DEHP adsorption data, as reflected by the high interrelationship coefficient (R2) values. This finding suggest that the adsorption rate is principally controlled by chemisorption between phthalates and rGO. Furthermore, the theoretical value of Qe,cal predicted by the PSO model is close to the corresponding experimental value, Qe,exp (Tables 3 and 4).
The adsorption kinetic data were further processed using intraparticle diffusion model in order to determine which process (film diffusion or intraparticle diffusion) is the rate-limiting step. The fitting result (Figure 9), demonstrate that all the straight lines fitted do not pass through the origin, which shows that binary, but not single, film/intraparticle diffusions are the rate-determining steps for DEP and DEHP adsorption.
Figure 7. Pseudo-first-order kinetic fit for (a) DEP and (b) DEHP adsorption onto rGO
Figure 8. Pseudo-second-order kinetic fit for (a) DEP and (b) DEHP adsorption onto rGO
a b
a
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Figure 9. Intraparticle diffusion fit for (a) DEP and (b) DEHP adsorption onto rGO
Table 3. Kinetic parameters for adsorption of DEP onto rGO
Model Kinetic
parameters
Initial DEP concentration (mg/L)
10 20 30 40
Pseudo-first-order Qe,exp (mg/g) k1 (1/min) Qe,cal (mg/g) R2
9.99 0.05 6.71 0.9670
15.03 0.06 9.52 0.9958
20.50 0.07 11.94 0.9851
24.99 0.06 13.23 0.9385 Pseudo-second-order k2 (g/ mg.min)
Qe,cal (mg/g) R2
0.015 10.50 0.9989
0.015 15.58 0.9995
0.015 20.70 0.9996
0.009 25.83 0.9997 Intraparticle diffusion kid
C R2
0.4712 5.3291 0.7773
0.6318 8.8760 0.7136
0.6926 13.518 0.6743
0.8931 16.068 0.7840
Table 4. Kinetic parameters for adsorption of DEHP onto rGO
Model Parameters Initial DEHP concentration (mg/L)
10 20 30 40
Pseudo-first-order Qe,exp (mg/g) k1 (1/min) Qe,cal (mg/g) R2
9.7 0.05 6.87 0.9595
14.50 0.07 9.82 0.9909
19.8 0.07 11.52 0.9822
24.6 0.04 11.66 0.9831 Pseudo-second-order k2 (g/ mg.min)
Qe,cal (mg/g) R2
0.015 10.21 0.9989
0.015 15.08 0.9994
0.015 20.37 0.9997
0.009 25.45 0.9997 Intraparticle diffusion kid
C R2
0.4710 5.0472 0.7731
0.6295 8.4201 0.7015
0.6945 13.103 0.6862
0.9097 15.501 0.7862
a b
162 3.5. Adsorption isotherms
Isotherm models were employed to fit the equilibrium data in order to predict the mechanism of adsorption. The isotherm parameters were determined by linear regression of the equilibrium data and the fitting results are indicated in Table 5. It can be seen that the Freundlich model provides a better fit than the Langmuir model, as evidenced by its higher interrelationship coefficient (R2), whose value is close to unity. The good fit of the Freundlich isotherm signify that the configurations of the binding sites in rGO were heterogeneous with respect to their affinity towards DEP and DEHP molecules [25].
The parameters KF and n of the Freundlich model can serve as relative indicators of the adsorption capacity and affinity, respectively. The value of the Freundlich parameter (KF) for DEP was greater than that for DEHP, indicating that rGO exhibits a greater DEP adsorption capacity than DEHP. The values of 1/n obtained were all < 1, which signifies that the adsorption of phthalates on rGO was favorable [26].
Table 5. Isotherm parameters of phthalates adsorption on rGO
Isotherm model Parameters DEP DEHP
Langmuir qmax (mg/g)
KL (L/mg) RL
R2
5.583 0.21 0.20 0.9685
4.187 0.23 0.68 0.9790
Freundlich KF (L/mg)
1/n R2
14.50 0.44 0.9965
10.9 0.4 0.9944
3.6. Effect of temperature and thermodynamic analysis
Temperature has an appreciable impact on the adsorption process since a change in temperature could alter the uptake capacity of a given adsorbent. Therefore, the influence of solution temperature on phthalates removal was examined by varying the solution temperature from 298 to 328 K (Figure 10).
The amount of DEP and DEHP adsorbed was found to increase slightly with the increase in temperature implying that the adsorption was an endothermic process. This may be a result of increase in the rate of diffusion of the phthalate molecules with increasing temperature which ultimately results in more adsorbate molecules acquiring sufficient energy to undergo interactions with active sites at the surface of rGO [27].
The thermodynamic analysis was carried out by determining the thermodynamics parameters such as enthalpy change ΔH (J/mol), entropy change ΔS (J/mol K), and Gibbs free energy change ΔG (J/mol).
The Gibbs free energy change, ΔG can be computed using eq. (13):
(13)
where R denotes the universal gas constant with a value of 8.314 (J/molK), T is the absolute temperature (K), and Kd is the distribution coefficient that can be represented by eq. (14):
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(14)
where Cads (mg/L) denotes the equilibrium phthalate concentration adsorbed by rGO and Ce (mg/L) represent the equilibrium phthalate concentration remaining in solution. The relationship between ΔG, ΔH, and ΔS is expressed by eq. (15):
(15)
By inserting eq. (13) into eq. (15), the Van't Hoff eq. (16) is obtained:
(16)
ΔH and ΔS values were estimated from the slope and intercept of plot ln Kd vs 1/T, respectively.
Figure 11 displays the computed thermodynamic parameters of DEP and DEHP adsorption on to rGO.
The negative value of ΔG and a positive value of ΔH for all the systems indicate that the adsorption processes are spontaneous and endothermic. In addition, the positive value of ΔS represents an increase of the random movement of phthalate molecules from bulk water to the rGO surface. The results also suggest that the adsorption processes of DEP and DEHP on to the adsorbent were driven mainly by entropy change.
Figure 10. Effect of temperature on the adsorption of DEP and DEHP onto rGO
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Figure 11. Van’t Hoff plot for estimation of thermodynamic parameters
3.7. Theoretical investigation of DEP and DEHP reactivity using DFT approach 3.7.1. Frontier molecular orbitals (FMOs) analysis
In order to assess the effect of geometrical and electronic structural factors on the adsorption yield of DEP and DEHP, quantum chemical calculations were undertaken. The optimized geometry, electron density map as well as the graphic distribution of HOMO and LUMO frontier orbitals of the studied phthalate molecules are displayed in Figure 12. As shown in Figure 12, the HOMO orbital distribution is essentially localized around the benzene ring for both DEP and DEHP. On the other hand, the LUMO extends over the benzene ring and oxygen atoms for both adsorbates. The FMOs energy parameters (EHOMO and ELUMO) play an important role in highlighting the chemical reactivity at atomic level [28]. The higher the value of EHOMO, the better is the capability of a molecule to give electrons to the suitable acceptor resulting in better adsorption efficiency [29]. The ELUMO can be regarded as the vulnerability of molecule towards attack by nucleophile and consequently accepting electrons. By comparing DEP and DEHP on the basis of EHOMO (Table 6), we can notice that DEP which exhibited higher uptake in the adsorption test, has the highest (least negative value) value of EHOMO (-5.808 eV).
It is well known fact that the lower the value of energy gap (ΔE), the more the reactivity of adsorbate molecule and consequently the capability of adsorption on a given surface is enhanced [30]. The data in Table 6 showed that DEP has lower ΔE value (3.659 eV) than that obtained by DEHP (3.687 eV), thus DEP has higher reactivity than DEHP leading to higher adsorption on rGO. The trend of the ΔE and EHOMO is in conformity with the adsorption efficiency order obtained from the experimental results (DEP>DEHP).
3.7.2. Global molecular reactivity
There are many DFT parameters, which are employed as reactivity descriptors in chemical reactions.
The values of some important reactivity parameters of the molecules under investigation are disclosed in the Table 6. The reactivity and stability of DEP and DEHP molecules can be linked to chemical hardness (ή). Generally, the electronic systems with hard molecules have the least tendency to react
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while systems with soft molecules have a higher tendency to react. In the present study, the DEP dye has a small value of the ή (1.829 eV) compared to DEHP (1.843 eV), which implies that DEP is more reactive than DEHP. This interpretation is also applicable to the softness parameter (σ) which reveals that the molecule of DEP (0.546 eV) is softer than that of DEHP (0.542 eV) [31-32]. Results recorded in Table 6 illustrate that DEP has the highest electrophilicity index value (4.325 eV) in comparison with DEHP (4.179 eV). A reactive and strong electrophile is characterized by a high value of electrophilicity power (ɷ) and chemical potential (µ) [28].The values of µ calculated for the studied phthalates are DEP (µ = 3.978 eV) and DEHP (µ = 3.926 eV). Hence, the DEP molecule presents a high value of ɷ and µ compared to DEHP, accordingly, the DEP molecule is more reactive electrophile than DEHP. These results are in good agreement with the experimental observations related to higher uptake of DEP on rGO compared to DEHP.
Table 6. Computed quantum chemical parameters for studied compounds using B3LYP/PWC/DND level of calculation
Phthalate EHOMO
(eV)
ELUMO
(eV)
∆E (eV)
µ (eV)
ή (eV)
σ(eV-
1)
ɷ (eV)
I (eV)
A (eV) DEP -5.808 -2.149 3.659 3.978 1.829 0.546 4.325 5.808 2.149 DEHP -5.770 -2.083 3.687 3.926 1.844 0.542 4.179 5.770 2.083
3.7.3. Local molecular reactivity
Promising local sites of reactivity in DEP and DEHP molecules were examined by calculating the Fukui indices, fk+ and fk─ as indicators to the prospective sites of nucleophilic and electrophilic attacks, respectively. Atomic sites with high values of fk+ are more prominent to nucleophilic attacks, while those with high values of fk─ are said to be highly vulnerable to electrophilic attacks [33]. The Fukui functions were computed using the finite difference (FD) approximation approach as expressed by eqs.
(17 and 18) [34]:
for electrophilic attack (17)
for nucleophilic attack (18)
where fk─, and fk+ correspond to electrophilic and nucleophilic Fukui functions, respectively; qk(N) denote the atomic population on the kth atom for the neutral molecule, while qk(N─1) and qk(N + 1) reflect the atomic population on the kth atom for its cationic and anionic species, respectively.
In addition to the information concerning nucleophilic and electrophilic capacity of a given atomic site in the molecule, a dual descriptor (Δfk) was proposed which is given by eq. (19) [35]:
(19)
166
if Δfk > 0, then the site is favored for a nucleophilic attack, whereas if Δfk < 0, then the site may be favored for an electrophilic attack.
DEP DEHP
Figure 12. Optimized geometry and electron density distributions of frontier molecular orbitals of the investigated compounds at the level B3LYP/PWC/DND
Optimized structureLUMOHOMOElectron density
167
The calculated Fukui function indices and dual descriptor are displayed in Table 7. From the values of Fukui functions fk+ and fk, it can be stated that the most active sites prone for nucleophilc attacks are C4 and C7 for DEP and DEPH, respectively. Likewise, O9 is the favorite site for the electrophilic attack in DEP, whereas O10 is the active site susceptible to electrophilic attack in DEHP. Thus, the O atoms adjoining the benzene ring of the phthalate molecules are less disposed to attacks by electron- rich species. These findings are in conformity with the distribution of frontier molecular orbitals over the benzene ring and oxygen atoms for both adsorbates.
Table 7. Fukui indices and dual descriptors of studied compounds at level B3LYP/PWC/DND Molecules Atom Mulliken Atom Hirshfield
(fk+/ fk─) fk+ fk─ ∆fk (fk+/ fk─) fk+ fk─ ∆fk
DEP C4/09 0.086 0.064 0.022 C4/O9 0.087 0.067 0.02
DEHP C7/O10 0.085 0.068 0.017 C7/O10 0.074 0.065 0.01
Conclusion
In this study, a reduced graphene oxide (rGO) was synthesized and has been tested for the adsorption of DEP and DEHP molecules from aqueous medium. The fabricated rGO material was characterized by SEM, FTIR, EDX, pHpzx and UV-visible spectroscopy. The effects of operational parameters such as initial concentration, solution pH, temperature and contact time on the adsorption process were investigated. The results revealed that the adsorption process was pH dependent and equilibrium was attained after the contact time of 60 min for both DEP and DEHP. The isotherm data is more in line with the Freundlich model. Intraparticle diffusion model suggested that the rate-determining step is jointly controlled by both film diffusion and intrparticle diffusion. From thermodynamic investigation, it was learned that, the adsorption process is spontaneous and endothermic in nature. Finally, the theoretical studies carried out on the DEP and DEHP molecules using the quantum chemical descriptors derived from density functional theory (DFT) calculations have proved that the DEP molecules are more electrophilic, reactive and have the ability to adsorb more easily on the rGO surface compared to the DEHP molecules.
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