CC BY
21
DOI: 10.17277/amt.2019.04.pp.003-012
The Uptake of Pb(II) Metal Ion in Water Using Polyhydroquinone/Graphene Nanocomposite Material: Kinetics, Thermodynamics and Mechanism Studies
I. Ali1*, A.E. Burakov2, A.V. Melezhik2, A.V. Babkin2, I.V. Burakova2, E.A. Neskoromnaya2, E.V. Galunin2, A.G. Tkachev2
lDepartment of Chemistry, Jamia Millia Islamia (Central University), Jamia Nagar, New Delhi, 110025, India; 2 Department of Technology and Methods ofNanoproduct Manufacturing, Tambov State Technical University, 1, Lenindradskaya Str., Tambov, 392000, Russian Federation
* Corresponding author: Tel.: +91 921 145 82 26. E-mails: drimran_ali@yahoo.com, drimran.chiral@gmail.com
Abstract
A novel nanocomposite of polyhydroquinone / graphene was synthesized and analyzed by XRD, Raman spectroscopy, TG, SEM and TEM. It was used to remove lead metal ion in aqueous solution. The maximum removal capacity was 156.5 ^g/g at 60 min time, 6.0 pH, 3.0 g/L dose and 25 °C. The removal of lead metal ion followed Langmuir, Temkin and Dubinin-Radushkevich models. The adsorption in water was clarified by applying pseudo-first-order, Elovich and internal diffusion models. Briefly, the adsorption method used is rapid, eco-friendly and cost-effective as may be applied under natural water conditions. Thus, the nanocomposite employed can be utilized for the withdrawal of lead metal ion in any water reserve.
Keywords
Pb(II) metal ions; removal in water; nanocomposite graphene/polyhydroquinone; kinetics and thermodynamics; metal ions removal mechanism.
1. Introduction
Water is the most important commodity on the planet earth but unfortunately it is contaminated by different kinds of pollutants. This is due to rapid growth in population, industrialization and high standard of living [1-4]. Among various pollutants, metal ions contamination is very serious as these accumulate in living tissues and are non-biodegradable and carcinogenic in nature [5-9]. Water contamination caused by lead is very serious because this metal ion forms complexes with oxo-groups in several enzymes to affect virtually all steps in the process of hemoglobin synthesis and porphyrin metabolism. Other problems associated with toxic levels of lead exposure are encephalopathy, seizures and mental retardation, anemia and nephropathy. Hence, lead must be removed as much as possible from industrial effluents to prevent environmental hazard from its discharge. Water contamination due to lead is very serious because this
© I. Ali, A.E. Burakov, A.V. Melezhik, A.V. Babkin, I.V. Burakova, E.A. Neskoromnaya, E.V. Galunin, A.G. Tkachev, 2019
metal ion forms complexes with oxo-groups in several enzymes to touch effectively all steps in the procedure of hemoglobin synthesis and porphyrin metabolism. Other difficulties related to the lead contact are seizures encephalopathy, and mental retardation, nephropathy and anemia [10].
The most common industrial sources of lead metal ion are paints, plating, oil refining, batteries, pigments, television tube, printing, gasoline additives, photographic materials, matches and explosives [11, 12]. Besides, lead contamination is also due to anthropogenic activities, making it the most ubiquitous toxic metal in the environment [13]. Additionally, about 100 countries use leaded petrol; causing the environmental and water resources contamination [14]. Therefore, lead should be eliminated from water before supplying to the communities.
Various methods, including precipitation, coagulation, ion exchange, filtration, membrane
treatment, adsorption, etc. have been used to effectively remove the heavy metal ions from wastewater [15-24]. Some methods of lead removal by adsorption [25-29] are available but these are not effective as most of them have longer time. Some of them do not work with the natural pH of water. Therefore, there is a great need to develop the effective adsorption method for the removal of lead metal ion. Among various adsorbents, nanomaterials are gaining importance in water treatment as new generation adsorbents due to their remarkable properties. In view of all these facts, the efforts are made to synthesize and characterize polyhydroquinone / graphene nanocomposite material. This material was used as new generation adsorbent for lead metal ion removal in water. The kinetics, thermodynamics and mechanism studies were also carried out and discussed in this paper.
2. Experimental
2.1. Preparation of PHQ/Graphene
nanocomposite
The nanocomposite material under study contained the following components: GO - 0.3-1.0 wt.%, /-benzoquinone (C6H602; Laverna Lab. Ltd., Moscow, Russia) - 3-4 wt.%, and deionized water. They were diversified, and the three-constituent mixture was kept into an ultrasonic instrument for 40-70 min (oscillation frequency (22 ± 0.4) kHz). The laboratory setup for nanocomposite preparation was purged with an inert gas for 30 min and closed to permit the procedure to continue in a non-oxygen atmosphere. The subsequent mixture was heated to 95 °C, following aging for 6 h in nonstop thrilling at 160 rpm in an inert gas environment. The prepared products were chilled to room temperature and cleaned to separate the marked product. The resultant material was eroded with deionized water heated to 50-70 °C, and emptied on filter for 5-10 min to eliminate wetness from the surface. Thus, the ultimate product characterized a water paste (dry residue 4-6 wt.%) having 70-80 wt.% polyhydroquinone (PHQ) and 20-30 wt.% GO. It was dehydrated and utilized for adsorption studies. A diagram showing the preparation of nanocomposite is revealed in Fig. 1.
2.2. Adsorption study
The batch mode was used to remove lead metalions in water using a thermostatic water bath shaker. The nano-adsorbent was removed from the solution by centrifugation, and the equilibrium metal ion concentration was determined by atomic absorption spectrometer (MGA-915MD, Lumex, St. Petersburg,
Fig. 1. A schematic representation of polyhydroquinone / graphene nanocomposite material synthesis
Russia). The sorption conditions varied were as follows: contact time (2-120 min), concentrations (50-400 ^g/L), pH (2.0-9.0), and dose (1.0-7.0 g/L) at room temperature. The lead concentration at equilibrium ( Ce, ^g/L) was measured by the following equation:
C = (C -Ct),
(1)
where Ci is the initial concentration, and Ct is the
concentration at particular time t (all - ^g/L). The lead percentage removal (Rp, %) was evaluated according to the following equation:
Rp =[(C -C)/ct]ioo.
2.3. Kinetic study
(2)
The lead stock solution (1.0 mg/L) was prepared by dissolving PbNO3 of AR grade into double deionized water. The consequence concentrations were obtained by diluting the stock solution. The experiments were carried out for three times (n = 3) at constant room temperature (25 °C). The stirring times were 2, 5, 10, 15, 30, 60, and 120 min. The glass tubes (50 mL) having the adsorbent were shaken at 120 rpm on a Multi Bio RS-24 programmable rotator (Biosan Ltd., Riga, Latvia). These were centrifuged on a 5810R device (Eppendorf AG, Hamburg, Germany) at 10,000 rpm for 10 min. The remaining lead concentration was determined with atomic absorption spectrometer.
2.4. Adsorbent characterization
The material was characterized using a number of techniques like Raman spectroscopy using the DXR™ Raman microscope (Thermo Fisher Scientific Inc., Waltham, MA USA), thermogravimetric analysis using an STA 449 F3 Jupiter thermal analyzer (NETZSCH-Feinmahltechnik GmbH, Selb, Germany), that permits concurrent thermogravimetry (TG) and differential scanning calorimetry (DSC) dimensions. The outcomes of this sort of studies gave information on the material thermal stability and temperature transitions rising during destructuring. The temperature program had heating from 800 to 900 °C at a rate of 10 °C per min. To determine the quantitative and qualitative phase compositions of the samples in examination, a Difrey 401 desktop X-ray diffractometer (Scientific Instruments CJSC, St. Petersburg, Russia) was utilized. The scanning electron microscopy (SEM) was utilized for surface structure characterization using a Neo 40 instrument (Zeiss AG, Oberkochen, Germany), and transmission electron microscopy (TEM) using a JEM-2010 instrument (JEOL Ltd., Tokyo, Japan).
3. Results and Discussion
3.1. Characteristics of PHQ/Graphene nanocomposite
The polymerization of p-benzoquinone in water solution during heating in inert environment was studied in detail [30]. Concisely, the polymerization phenomenon contains the formation of insoluble PHQ [-C6H2(OH)2-]„ and monomeric p-benzohydroquinone. During the sequence of nanocomposite synthesis, p-benzohydroquinone incompletely reduced GO to graphene, so that the product acquired showed PHQ molecules bonded to some graphene layered sheets. The structure of the prepared PHQ / graphene
nanocomposite is given in Fig. 1. The SEM images in different magnifications are given in Fig. 2a - x100.00, and Fig. 2b - x30.00. A perusal of these figures indicates graphene like structure i.e. carbon layer flakes - graphene nanoplatelets. The TEM images are given in Fig. 3a, b and again a perusal of these figures clearly indicates some layered graphene structures with less than 10 layers on the material surface. These are chiefly flawed with distinct fragments of well-defined graphene structures are existing. Consequently, it can be confirmed that reduction and polymerization of ^-benzoquinone of GO to graphene was occurred in parallel when preparing reported materials. The energy-dispersive X-ray (EDX) spectra is given in Fig. 4 and a perusal of this indicates the availability of 91-95 % (atomic) of carbon and 5-9 % of oxygen, that is usual allowing for the attendance of PHQ. Raman spectra were obtained for the final material and intermediate materials (GO and p-benzoquinone) and it is given in Fig. 5. A perusal of this figure clearly indicates the presence of peaks at ~ 1,580-1,590 cm-1, corresponding to G band connected to sp2 hybridized carbon atoms showing higher energy. Another peak was at ~ 1,332 cm-1; corresponding to D band associated with sp hybridized of carbon with low energy. In contrast with the Raman spectra obtained for GO, the peaks of p-benzoquinone are accompanied with characteristic bands at ~ 1,271 and ~ 1,188.5 cm-1 usually of even twist vibrations of C — H bonds in benzene molecules. The highest bands intensity of the existing spectra can be seen at ~1,424.5 cm-1, that is usually of double C = C bonds vibrations in the benzene ring.
The spectra of the PHQ/graphene nanocomposite indicated the typical peaks at ~ 1,580-1,590 cm-1; corresponding to G band and ~ 1,332 cm-1; corresponding to D band; similar to the GO spectra.
a) b)
Fig. 2. SEM images of the PHQ/graphene nanocomposite at magnification x 10000 (a) and x3000 (b)
a) b)
Fig. 3. TEM images of the PHQ/graphene nanocomposite atmagnification:
a - 10 nm; b - 20 nm
Fig. 4. X-ray diffraction pattern for the GO and PHQ/graphene nanocomposite
Fig. 5. Raman spectra recorded for the GO, ^-benzoquinoneand PHQ/graphene nanocomposite
Yet, a decrease was noted in the strength of D band connected to stable double C = C bonds formation, that affected the presence of the matching intensity band at ~ 1,420 cm-1. The composite material prepared was also categorized by deficiency of the typical peaks of
deformation vibrations of C—H bonds in benzene molecules; perhaps relating to oxidation of polymerized particles of /-benzoquinone through modification of the GO surface.
The interpretation of TG/DSC curves of /-benzoquinone, GO and PHQ/graphene nanocomposite are given in Fig. 6, that indorse thermal stability of PHQ/graphene nanocomposite in programmable heating to 600 °C. At ~ 200 °C, the sample weight starts to decline, that is escorted by the discharge of thermal energy (exothermic reaction of organic component combustion). The monotonic practice carries on to ~ 500 °C temperature, thus varying sample weight by 72 %. Additional heating (~ 500-650 °C) was attended by thermal energy absorption and a reduction in the sample weight related to demolition of graphene layers. The decrease in the sample weight was owing to exothermic decomposition of GO (DSC curve peak at ~ 220 °C) in 75-250 °C temperature range. Additional heating up to ~ 200-500 °C was occurred with weight loss (-13.31 %) with a secured accent of the DSC curve in exothermic caused the direction that may be obviously detected. The type of weight loss within this temperature area may be clarified by combustion of sample organic constituent. The exothermic band at ~ 500 °C is a usually of destruction of ordered carbon layers in the sample. The remaining weight of the material was 15.28 %. The organic behavior of /-benzoquinone showed on its thermal instability in programmable heating. The boiling point of pure /-benzoquinone is ~ 116 °C, being linked to safe thermal energy absorption and a >50 % sample weight loss. Additionally, heating to 150 °C was attended by a peak in the DSC curve describing the endothermic behavior of the phenomenon. The residual mass of sample was ~ 5 % at ~ 200 °C heating temperature.
Fig. 6. TG and DSC curves constructed for the GO, p-benzoquinone and PHQ/graphene nanocomposite
Associating two X-ray diffraction arrangements obtained for GO and PHQ/graphene nanocomposite, it is interesting to note that a very exhaustive peak characteristic of primary GO at 29 = 12.0° is lacking in the pattern of the PHQ/graphene nanocomposite. A perusal of Fig. 4 confirms that the material has some layered graphene established by presence of an extensive band at 29 = 38.0°. The peaks at 29 = (20.0-27.0)° showed the accessibility of the structures with an interlayer distance better than of graphite and few layered graphene, but less than of the original GO. These may characterize graphene layers stacks on the surface of which PHQ molecules were chemically attached. The unidentifiable peaks in the X-ray pattern apparently resembled to organic materials.
3.2. Adsorption studies
The adsorption of lead metal ion was carried out in batch mode utilizing composite nanomaterial as sorbent. The different variables optimized are presented herein.
3.2.1. Concentration effect
The lead concentrations used were 50, 100, 150, 200, 250, 300, 350 and 400 pg/L. The other variables were 60 min contact time, pH 6.0, and 3 g/L dose. All the experiments were carried out at
160
140
120
100
¡4 80
О 60
40
20
0
100
200 300 Conc. (mg/L)
a)
400
70 65 60
1M 55
1Й)
3 50 о 45 40 35 30
room temperature of 25 °C. The results of this set experiments are given in Fig. 7a. A perusal of this
figure clearly indicates that adsorption increased from
50 to 300 pg/L. Further increase in the concentration could not augment the adsorption. The amounts adsorbed at 50, 100, 150, 200, 250 and 300 pg/L were 32.1, 58.8, 80.3, 99.1, 120.5 and 143.5 pg/g. Furthermore, the amounts of adsorption were 144.0 and 145.0 pg/g at 350 and 400 pg/L concentrations. These data clearly indicate that 300 pg/L was the optimized lead concentration.
3.2.2. Contact time effect
The time values used were 2, 5, 10, 15, 30, 60 and 120 min. The other variables were 300 pg/L concentration, pH 6.0 and 3 g/L dose. All the experiments were carried out at the room temperature of 25 °C. The results of these experiments are given in Fig. 7b. A perusal of this figure clearly indicates that adsorption increased from 2 to 60 min time. Further increase in time could not augment the adsorption. The amounts adsorbed at 2, 5, 10, 15, 30 and 60 min were 48, 49, 49.5, 52.4, 59.3 and 63.3 pg/g. Furthermore, the amount of adsorption was 64.2 pg/g at 120 min. These data clearly indicate that 60 min was the optimized time of the lead metal ion.
70 65 '60
500
Q 55 50 45
70 60 50 40
0
30 20 10
20
40
60 80 Time (min.)
b)
100
120
140
Fig. 7.
4 6 8 0 2 4 6
Dose (g/L) pH
c) d)
Effect of various parameters on adsorption of the lead metal ion:
a - concentration.; b - contact time; c - dose; d- pH
10
0
0
0
3.2.3. Dosage effect
The different doses used were 1, 3, 5 and 7 g/L. The other variables were 300 ^g/L concentration, 60 minutes contact time and pH 6.0. All the experiments were carried out at room temperature of 25 °C. The results of these experiments are given in Fig. 7c. A perusal of this figure clearly indicates that adsorption increased from 1 to 3 g/L dose. Further increase in dose could not augment the adsorption. The amounts adsorbed at 1 and 3 g/L doses were 37.5 and 63.2 ^g/g. Furthermore, the amounts of adsorption were 63.5 and 64.0 ^g/g at 5 and 7 g/L doses. These data clearly indicate that 3 g/L dose was the optimized time of lead metal ion.
3.2.4. pH effect
The different pH values used were 2.0, 4.0, 6.0 and 9.0. The other variables were 300 ^g/L concentration, 3 g/L dose and 60 minutes contact time. All the experiments were carried out at room temperature of 25 °C. The results of these experiments are given in Fig. 7d. A perusal of this figure clearly indicates that adsorption increased from 2 to 6 pH. Further increase in pH decreased adsorption. The amounts adsorbed at pH 2.0, 4.0 and 6.0 were 12, 23 and 58 ^g/g. Furthermore, the amount of adsorption was 18 ^g/g at pH 9.0. These data clearly indicate that pH 6 was the optimized one of lead metal ion. A decrease in the adsorption at high pH may be due to the formation of lead hydroxide. It means that lead hydroxide is less adsorbed than lead metal ion species.
3.3. Adsorption isotherms
The Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models were used to explain the experimental results of lead metal ion on the reported adsorbent. These models were also utilized to understand the sorption phenomenon.
3.3.1. Langmuir isotherm
The Langmuir model defines the removal on a uneven surface through mono-layer adsorption. It also explains the unbroken energies of adsorption on the sorbent surface with no movement of adsorbent. This model is expressed as below [31].
1/ Qt = 1/XmbCt +1/ Xm , (3)
where Ct and Qt have normal meaning; Xm is a constant and shows maximum monolayer sorption capacity of the sorbent, ^g/g; b is another constant co-related to the binding energy of lead metal ion onto the sites of adsorbent, L/^g. The values of Xm give a knowledge of active sites whilst b is dynamic force at
equilibrium. The values of Xm and b were designed from slope and intercept of a straight line of the graph (1/Qt vs. 1/Ct ). Langmuir graph of lead is given in Fig. 8a. Langmuir isotherm fixed the sorption data of lead onto the adsorbent. The values of Xm and b and regression constants R are 185.19, 0.012 and 0.981 (Table 1). These values showed good binding capacities of lead metal ion on adsorbent surface. The value of the dimensionless constant [separation factor RL] was determined by the following equation
Rl = 1 (1 + bCe). (4)
The value was 0.35 and this lower than 1.0 vale indicates favourable sorption.
3.3.2. Freundlich isotherm
The Freundlich model is appropriate for huge numbers of sorption sites with no restraint of monolayer merely; with non-uniform spreading of sorption heat and affinities on the heterogeneous surface. The adsorbate quantity on the surface of the sorbent is the precis of all sites. The sorption energies reduce exponentially on the close of sorption occurrence. This model is expressed by following equation
1
log Qe =-loge + log kF , (5)
n
where kF, (mg/g), and n are experiential constants and relating to the comparative sorption capacity of adsorbent and sorption strength. The values of n from 1.0 to 10.0 show satisfactory sorption. The graph of logQe vs. logCe was straight lines and intercept connected to kF. Adsorption strength was calculated by slope (1/n). The Freundlich model is given in Fig. 8b, and the coefficients values - in Table 1. The kF value was 6.70, and the n value was 1.75 with 0.964 regression coefficient. These values indicated good sorption. The values of regression coefficient in case of the Langmuir was greater than the Freundlich model; indicating the validity of the former model.
3.3.3. Temkin isotherm
The Temkin model defines sorbate and sorbent relations. The sorption heat of all the molecules in layer drops linearly with reportage owed to adsorbate-adsorbent interactions. Also, sorption is categorized by an even spreading of binding energies; up to nearly maximal binding energy. Tmkin model is given by following equation
Qe = (RTjBT )ln Ce + (RTlBT )ln K
t >
(6)
0.035
a
0.03 ■ 2.4 2.2
0.025 ■ 2
0.02 ■ 0.015 ■ • £18 w: 5 1.6
0.01 ■ 1.4
0.005 ■ 1.2
160 140 120 100 a 80 60 40 20 0
0 0.01 0.02 0.03 0.04 0.05 0.06 1/Ce
1.5 2
logCe
2.5
a)
b)
•
5.5 5 4.5 4 3.5 3
1.5 2
logCe
2.5
0.05 0.1
E2
0.15
c)
d)
Fig. 8. The different models for Pb(II) metal ion sorption:
a - Langmuir; b - Freundlich; c - Tempkin; d - Dubinin-Radushkevich (D-R) isotherm
Modeling parameters for the lead adsorption on polyhydroquinone/graphene nanocomposite material
Table 1
Langmuir isotherm
Freundlich isotherm
Qm
b,
^g/g L/^g
Rl r
k, n,
vgjg ^g/L
R
Temkin isotherm
KT, Bt,
L/^g kJ/mol
Dubinin-Radushkevich
R
Qm,
VgJg
E, kJ/mol
R
185.19 0.012 0.35 0.981 6.70 1.75 0.964 2.82
0.024 0.920 141.25
0.21
0.898
1
where KT is an equilibrium binding coefficient giving maximum binding energy, L/g; BT coefficient is connected to heat of sorption; R is gas constant (0.008314 kJ/mol/K) and T is Kelvin temperature. A graph of Qt vs. logCe of lead metal ion was linear (Fig. 8c). The coefficients BT and KT were designed from slope and the intercept (Table 1). The magnitudes of BT, KT and regression constant were 2.82, 0.024 and 0.920 (Table 1). These values confirmed robust connections between lead metal ion and the reported sorbent. It means that Temkin model is applicable to the experimental results.
3.3.4. Dubinin-Radushkevich isotherm
The Dubinin-Radushkevich (D-R) isotherm describes if adsorption is physical or chemical. Gaussian energy spreading on uneven surface is explained by this model. This model is expressed by the following equations:
ln Qt = ln qQm -ße2
(
e = RT ln
1
1 + —
C„
\
(7)
(8)
8 = 1/VP, (9)
lnQt vs. 82 plot with a traditional line (Fig. 8d) is a sign of this model applicability. The constants Qe and s (Table 1) were ascertained by slope and intercept of graph. The magnitudes of Qe and s constants of the lead removal were 141.25 and 0.21. The magnitude of s is lower than 8; indicating physical adsorption. The magnitude of regression constant is 0.898; indicating good applicability of this model. Finally, the adsorption was found to be physical in nature.
3.4. Kinetics studies
The adsorption mechanism of lead metal ion is determined by kinetic modeling. It was contingent on the physical and chemical types of adsorbate and adsorbent. Consequently, two models were tried using the experimental data. These are deliberated below.
3.4.1. Pseudo-first-order kinetic model
This model is expressed by the following equation dQt/dt = kx(Qe -Qt). (10)
Equation (10) was integrated with boundary conditions, t = 0 with Qt = 0 and t = t with Qt = Qt , to get the lineralized equation as below.
ln(Qe - Qt ) = lne - k1t, (11)
where Qe, Qt and k1 (min-1) are lead metal ion amount (^g/g) adsorbed at equilibrium, at any time and equilibrium rate coefficient. These values were determined and given in Table 2. The rate coefficient was determined by conniving the ln(Qe - Qt) vs. t plot (Fig. 9a) at 25 °C temperature. The pseudo-first-order rate constant was 0.049 (1/min) with 0.934 regression constant (R ); approving the suitability of this model. The theoretical and experimental values of Qe were 19.38 and 156.5 ^g/g. Thus, the experimental data agreed well with the pseudo-first order kinetic model.
3.4.2. Pseudo-second-order kinetic model
The pseudo-second-order kinetic model was also tested for the adsorption of lead metal ion. This model is expressed by the following eqaution
dQt/dt = k2(Qe -Qt)2, (12)
where Qe, Qt and t have similar connotations as clarified above. The overhead equation was integrated with boundary conditions, t = 0 with Qt = 0 and t = t with Qt = Qt . The subsequent equation is given below.
tQ = Vk2Qe2 + t/Qt , (13)
k2 is the rate constant of pseudo-second-order sorption, g/^g/min; Qe and k2 were intended from the slope and intercept of plot t/Qt vs. (Fig. 9b). The intended parameters are given in Table 2. The magnitudes of k2 was 6.45x10-3. The values of theoretical and experimental Qe were 65.35 and 156.5, respectively, while the regression constant was 0.999. Of course, the regression coefficients and closeness of theoretical and experimental values inclined towards the applicability of this model but the extreme low rate constant ruled out its applicability. Therefore, the lead adsorption was found to be pseudo first order kinetics.
3.4.3 Elovich kinetic model
The Elovich kinetic model is used to express the rate of sorption and desorption processes. This model is expressed as below.
dQt/dt = a exp(-PQt), (14)
where a is an early sorption rate, ^g/(g min); p is desorption coefficient, g/^g. The above equation was integrated utilizing boundary situations, t = 0 with Qt = 0 and t = t with Qt = Qt . Further presumptuous a, p, t >> 1, equation (12) changes to equation
Qt =pln(ap) + plnt. (15)
Table 2
Kinetic parameters for the lead metal ion
Kinetic models Kinetic parameters
Pseudo-first-order k1 (1/min) = 0.049
Experimental Qe (^g/g) = 156.5
Theoretical Qe (^g/g) = 19.38
R2= 0.934
Pseudo-second-order k2 (^g/(g min)) = 6.45 x 10-3
Experimental Qe (^g/g) = 156.5
Theoretical Qe (^g/g) = 65.35
R2 = 0.999
Elovich a (^g/(g min)) = 1,412.53
P fog/g) = 0.21
R2= 0.846
Intraparticle diffusion kipd1 (M-g/(g min05)) = 2.65
Intercept = 43.02
R2= 0.954
Liquid film diffusion kfd fog/(g min)) = 0.024
Intercept = 1.35
R2= 0.876
15 20 Time (min.)
a)
60 80 Time (mins.)
b)
140
Fig. 9. The plots showing:
a - pseudo-first-order kinetic plot; b - pseudo-second order kinetic plot for the lead metal ion adsorption
The magnitudes a, p and R2are given in Table 2. The values of a, p and R2 were 1,412.53 pg/(g min), 0.21 pg/g and 0.846, respectively. These values were indicative of extremely higher adsorption rate than desorption. Furthermore, the value of regression constant was close to one, confirming the suitability of this model. These data confirmed fast adsorption rate and that is why the pseudo-second-order model was not accepted for the experimental results.
3.5. Adsorption mechanism
Usually, sorption growths are measured by intra-particle diffusion and film diffusion mechanisms. The relaxed step is rate decisive one, which governs the sorption occurrence. The experimental data was tailored to intraparticle and film diffusion models to evaluate the mechanism of lead metal ion uptake on the reported sorbent.
3.5.1. Intraparticle diffusion kinetic model
The sorption of lead metal ion on the adsorbent surface comprises dissimilar procedures like i) transport of lead metal ion from bulk solution by liquid film to the adsorbent surface, ii) uptake of adsorbate on adsorbent surface and iii) transfer of adsorbate inside the pores of sorbent. Consequently, a sorption mechanism is measured either by the surface sorption kinetics or the transportation occurrence (film and intraparticle diffusion) mechanism or by both procedures. The other step is very quick and may not be rate decisive. The first and third steps may be the rate decisive ones. Consequently, these stages were deliberated by two models. The transfer of the lead ion from solution to sorption sites might be intended by the association between quantity of adsorbed lead metal ion and square root of interaction time. For this, the equation is given below
(16)
When plot of Qt vs. t0 5 is a straight line and passed via origin then slope relates to rate constant kipd and sorption is measured by intra-particle
diffusion. This graph was drawn (not presented herein). The magnitude of rate constant was intended to be 2.65 pg/(g min0.5).The magnitudes of intercept and regression coefficient were 43.02 and 0.954 (Table 2). The graph did not go via the origin; approving no applicability of this model.
3.5.2. Liquid film diffusion kinetic model
This model was given by Boyd et al. [32] and states that the boundary plays a significant role in the sorption procedure. The liquid film diffusion model is given by the following equation
or
ln(l - QtlQe ) = -f ln(l - F) = -f,
(17)
(18)
Qt = -W
0.5
where F (Qt¡Qe) is an insignificant achievement of the
equilibrium; f is a film diffusion rate coefficient.
When plot of ln(l - F) vs. t is a straight line with zero
intercepts then sorption is thought to be a through film diffusion mechanism. The magnitudes of the film diffusion rate constant, intercept and regression constant (R2) were 0.024, 1.35 and 0.876 (Table 2). The straight line agreed via origin with a minor departure from zero intercept (1.35). This discrepancy from zero might be because of the high rate of stirring utilized in the kinetics experimental. Also, the discrepancy between the mass transfer rate in the opening and last stages of sorption might be answerable for a minor deviance from zero. The comparable results are also described in [33-35]. Accordingly, the lead metal ion sorption on the reported sorbent was regulated by the liquid film diffusion mechanism.
4. Conclusion
In this study, a novel graphene-based nanocomposite sorbent functionalized with polyhydroquinone is reported. It was analyzed by the X-ray diffractometry, TG, SEM, Raman spectroscopy and TEM techniques. The adsorption was observed in the Langmuir, Temkin and Dubinin-Radushkevich (D-R) isotherms. The uptake of the lead metal ion was 156.5 ^g/g at optimum conditions. The adsorption in water was clarified by applying the conventional pseudo-first-order, Elovich and internal diffusion models. Briefly, the method is rapid, eco-friendly and cost-effective as may be applied at pH of natural water (6.0); with low contact time (60 min) and dose (3 g/L). Minor interaction time and dose modes are the best qualities of this method. Thus, the defined method can be utilized for the withdrawal of lead metal ion in any water reserve economically.
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