3.1 Characterization of PHAC
3.2.1. Elemental composition and functional group of PHAC
The elemental compositions of raw PH and PHAC are summarized in Table 1.
Table 1. Elemental composition of PH and PHAC
Element
|
Material type
|
Raw PH
|
PHAC
|
C (%)
|
46.52
|
62.05
|
H (%)
|
2.73
|
0.83
|
N (%)
|
0.72
|
1.32
|
S (%)
|
0.15
|
0.12
|
As can be seen, the carbon content of raw PH is lower than the PHAC. The raw PH contained 46.52% of carbon, 0.72% of nitrogen, 2.73% of hydrogen, and 0.15% of sulfur. Nevertheless, the PHAC fabrication with dual consecutive chemical activation leads to increasing the PHAC carbon content to 62.05%. In contrast, the hydrogen content dropped to 0.83%, following the same trend that was described by Umpierres et al. (Umpierres et al. 2018) who studied the preparation of ACs from tucumã (TUC) by microwave heating and demonstrated that the TUC carbon contents improved from 48.8% to 81.5% when converted to AC.
Furthermore, the FTIR test was employed to determine the functional groups of PHAC. Fig. 1 illustrates the obtained spectrum of FTIR for PHAC.
The band at 458 and 561 cm-1 may be allocated to Zn-O bands (Babu et al. 2013). The bands at 757, 798, and 877 cm-1 are related to aromatic out of the plan –CH bending (Ribas et al. 2014). The intense broadband at 1099 cm-1 is assignable to the asymmetric C–O–C ether or O–C–C of an aromatic ester. The shoulder at 1178 cm-1 could be appointed to C–O of phenol. The vibrations at detected at 1465, 1396, and 1347 cm-1 are established to the aromatic ring modes (Gupta and Nayak 2012). The band at 1570 cm-1 is assignable to the asymmetric carboxylate band (Lima et al. 2019). The bond of 2926 and 2856 cm-1 is related to asymmetric stretching of C–H groups. The broadband at 3429 cm-1 is assigned to OH stretching (Sajjadi et al. 2019a).
3.2.2. X-ray diffraction and BET of PHAC
To determine the structural characteristics of PHAC, the XRD was employed (Fig. 2)
It can be observed that PHAC have sharp distinguishing diffraction peaks at 2θ = 31.93°, 34.53 °, 36.33 °, 47.84 °, 56.77 °, 63.14 °, 66.51 °, 68.12 °, 69.29 °, 72.79 °, and 77.17 °, respectively, corresponding to zinc oxide (JCPDS file no. 01-079-0205) and related to the application of activating agents during PHAC production.
The BET analysis according to the adsorption and desorption isotherms of N2 gas is the practical method for evaluating the total pore volume (Vtotal), specific surface area (SBET), pore size, and monolayer volume of adsorbent. The adsorption and desorption isotherms of N2 gas and distribution of pore size curves based on the DFT method are illustrated in Fig. 3.
Therefore, the main properties of PHAC, including Vtotal, SBET, average pore diameter, were 0.404 cm3/g, 811.12 m2/g, and 1.21 nm, respectively. The better characteristics of AC stated by Sajjadi et al. (Sajjadi et al. 2019b). They reported that the Vtotal, SBET, and pore diameter were of NH4NO3, and NaOH activated carbon from pistachio wood were 0.994 cm3/g, 1884 m2/g, and 2.11 nm, respectively. However, in contrast, Ribas et al. (Ribas et al. 2014) synthesized acidified AC from a cocoa shell (ACC-1) and reported that the Vtotal, SBET, and pore diameter of ACC-1 were respectively 0.459 cm3/g, 522 m2/g, and 5.23 nm.
3.2.3. EDS map and PHAC morphology
The morphology of PHAC was exhibited in Fig. 4 (a). The carbon material presents some rugosity at the magnification of 5,000-fold. The results of the mapping analysis illustrated in Fig. 4 (b) also confirms that the distribution of elements at the PHAC surface is performed uniformly. According to EDS analysis (Fig. 4 (c)) of the PHAC, the peaks of C, N, O, Ca, and Zn could be seen which indicated the chemical composition of PHAC includes carbon (57.74%), nitrogen (5.98%), oxygen (16.72%), calcium (0.29%), and zinc (19.27%) which once again suggesting the synthesis of PHAC.
3.2.4. Point of zero charge
The isoelectric point (IEP) or pHpzc was used to demonstrate the electrical surface state of adsorbent, especially AC. The series of solutions using NaCl as background electrolyte was prepared with initial pH value (pHi) from pH 2.0 ± 0.2 to 11 ± 0.2 to find the pHZPC. With adding desire PHAC mass into each solution, the containers were immediately sealed, and the suspensions were shaken for 48 h at 150 rpm. After that, the final pH value (pHf) of each solution was documented, and the ΔpH was plotted against pHi. Fig. 5 illustrates the effect of influencing parameters on pHpzc of PHAC, including the concentration of background electrolyte and PHAC dose.
From Fig. 5, the pHpzc was determined from the point of intersection of the ΔpH (pHfinal-pHinitial) with pHinitial. As can be seen, the pHpzc of PHAC ranged from 6.15 to 6.41 in a medium ranging from deionized water to 1 M NaCl (Fig 5A). When the PHAC dose varied from 0.1 to 0.4 g.L-1, the pHpzc ranged from 5.96 to 6.38.
3.3. Effect of critical parameters on 4CP adsorption
3.3.1. Effect of solution pH
The 4CP adsorption by PHAC under different solution pH (3-9) was examined at 20 mg/L of 4CP concentration and shown in Fig. 6.
As illustrated in Fig. 6, the removal efficiency of 4CP improved from 55.9 ± 3.2% to 58.5 ± 2.1% with ascending solution pH from 3 to 6. After that, the 4CP removal efficiency reduced as the hydrogen ion concentration increased, and reached 46.8 ± 3.1% at a solution pH of 9.
It was realized that the 4CP removal by PHAC was extremely dependent on the solution pH, which affected the PHAC surface charge and the 4CP ionization degree. This behavior agrees with the pKa value of the 4CP (pKa 8.96). At pH 6.0, 99.89% of 4CP is presented in the unprotonated specie (Fig. 7), and 0.11% is presented in the anionic specie. Considering that pHpzc of the adsorbent is 5.96-6.41, at pH values higher than this interval, the surface of the adsorbent becomes with a superficial negative charge. At pH 9.0, 47.91% of 4CP is presented in the undissociated form, and 52.09% as dissociate form. Therefore, the dissociate form that is an anion is repulsed from the adsorbent surface (at pH 9), decreasing the percentage of removal considerably.
Besides the charge attraction (pH £ 6) and repulsion (pH ³ 6.5) mechanism, hydrogen bonding also plays a role in the mechanism of adsorption. The interactions of OH groups of 4CP with the oxygen present on the carbon surface is also quite probable (Srivastava et al. 1997). The p-p interactions also play a role in the adsorption of 4CP (p bonds of 4CP and p bonds of activated carbon).
3.3.2. Effect of ionic strength
Both natural water and industrial wastewater usually contain dissolved salts, which may interfere with the pollutant’s uptake of the adsorbents. The adsorption experiments were performed by the addition of different concentrations of NaCl to evaluate the influence of ionic strength on the 4CP adsorption (Fig. 8).
As illustrated in Fig. 8, with increasing the NaCl concentration from distilled water to 0.3 mol/L, the significant decrease in the removal of 4CP was observed. The inner and outer sphere are two surface complexes that can form along with the adsorption process. The covalent bonds are the dominant chemical link between adsorbed molecules or ions and the adsorbent surface functional groups in the inner sphere surface complexes, and during the outer-sphere surface complexes, no covalent bonds form. The ionic strength in sensitivity and adsorption decreasing with ionic strength increasing has been taken as an indication of an inner and outer-sphere surface complex, respectively (Wang et al. 2010). Based on the mentioned above, the 4CP adsorption by PHAC in the present work was followed by the formation of outer-sphere complexes.
Furthermore, other interactions, including hydrogen bonding, electrostatic attraction, or hydrophobic attraction, are involved in the adsorption process (Chen and Wang 2007). The inorganic salts (eg., NaCl, KCl, CaCl2, MgCl2) in the solution can change the strength of electrostatic interactions of the adsorbent-adsorbate (Kuśmierek et al. 2020). Theoretically, the solution salinity increasing resulted in adsorption capacity declining as the adsorbent surface and adsorbate ions has attractive electrostatic forces. Conversely, the ionic strength increasing leads to improving the adsorption efficiency when the electrostatic attraction is repulsive (Al-Degs et al. 2008). Chen and Wang (Chen and Wang 2007), found that when adsorption efficiency depends on the ionic strength due to the formation of outer-sphere complexes, the interactions of negative charge of adsorbent surface and metal ions lead to cation exchange at sorption sites. Therefore, the increasing concentration of Na+ and Clˉ in solution presumably attributed to enhancing the competition with 4CP for the PHAC sorption sites, thus reducing the 4CP adsorption efficiency.
3.3.3. Effect of temperature
The 4CP batch adsorption experiments were carried out at different adsorption temperatures ranging from 283 to 313 K at different initial 4CP concentrations and constant PHAC dose (0.4 g/L) and pH of 6, and obtained results are shown in Fig. 9.
It can be seen from Fig. 9, with increasing adsorption temperature, at a particular concentration of 4CP, the 4CP removal percent increased too. The 4CP adsorption increased from 57.36% to 68.81% (20 mg/L of 4CP), 51.91% to 61.52% (30 mg/L of 4CP), 47.82% to 56.76% (40 mg/L of 4CP), 41.37% to 51.80% (50 mg/L of 4CP) and 37.20% to 49.79% (60 mg/L of 4CP) with the climbing the adsorption temperature from 283 to 313 K. The main effects of temperature on the adsorption process are including (i) the solution viscosity decreasing and enhancing the diffusion rate of sorbate within adsorbate pores (ii) internal bonds breaking of sorbent active surface sites and generating the additional sorption sites (Boudrahem et al. 2009). The results indicated that the adsorption of 4CP adsorbed by PHAC is favored with the increase of the temperature. Shaarani et al. (Shaarani and Hameed 2011) reported similar results and described that the 2,4-dichlorophenol (2,4DCF) adsorption by ammonia-modified AC is gradually improved with adsorption temperature ascending from 30 to 50°C.
3.3.2. Influence of AC dose
The influence of the dose of PHAC on adsorption efficiency of 4CP was investigated by changing the PHAC dose ranging from 0.5 to 2.5 g/L. Fig. 10 shows the 4CP removal efficiency variations as a function of PHAC dose at various initial 4CP concentrations (50-150 mg/L).
The adsorption efficiency of 4CP by PHAC was observed to improve gradually when the PHAC dose was increased from 0.5 to 2.5 g/L for all initially studied concentrations (50-150 mg/L). With rising dose of PHAC from 0.5 to 2.5 g/L, the 4CP removal efficiency enhanced from 78.6 ± 3.9% to 100.0 ± 0.0%, from 73.4 ± 3.2% to 100.0 ± 0.0%, and from 66.9 ± 3.3% to 100.0 ± 0.0% at 50, 100, and 150 mg/L of 4CP concentration of, respectively. When a higher dose of PHAC is used, the higher amount of surface and pore volume of the sorbent is available and providing greater active sites of sorption and more functional groups, and as a result, the higher 4CP adsorption efficiency is occurred (Li et al. 2003). Latip et al. (Badu Latip et al. 2020) evaluated the 2,4DCF adsorption from wastewater by Fe3O4@AC and showed that the removal efficiency of 2,4DCF enhanced upon the sorbent dose ascending to 20 mg/L, but reached constant removal efficiency on further increasing the sorbent dose.
3.3.3. Effect of contact time
In order to realize the equilibrium time for maximum 4CP adsorption, the experiments of 4CP adsorption by PHAC were conducted as a function of contact time. Fig. 11 presents the variation of 4CP removal under different 4CP concentrations (50-150 mg/L) by changing the contact time ranging from 2 to 120 min.
As shown in Fig. 11, the progress in contact time leads to improve the removal efficiency of 4CP. The instant 4CP removal was observed in the first part of the adsorption process (contact time lower than 20 min), which dropped gradually and attained equilibrium at around 60 min. More vacant sites on the sorbent surface area available in the initial stage of the adsorption process and lead to an increasing gradient of concentration between adsorbate in bulk solution and sorbent surface, and as a result, the fast-initial adsorption occurred. This behavior may be related to the strong, attractive forces between the active sorbent site and 4CP molecules and quick 4CP diffusion into the interparticle matrix to reach the rapid equilibrium (Sathishkumar et al. 2007). Further contact time increasing leads to declining the availability of active sites and uncovered surface area and decreasing the driving force. Consequently, the extended contact time is required to attain equilibrium for slowly diffusing of 4CP molecules into the sorbent intraparticle pores.(Li et al. 2010). Besides, the carve of 4CP removal by PHAC as a function of contact time is single and continuous to achieve the saturation and indicating the monolayer coverage possibility of 4CP on the PHAC outer surface (Bilgili et al. 2012). Hameed et al. (Hameed et al. 2008) described that the higher initial 4CP concentration leads to a superior driving force and enable to overcome 4CP mass transfer boundary between bulk solution and sorbent surface.
3.3.7. Kinetic study
In order to realize the rate-controlling step and to determine the involved mechanisms and dynamics in 4CP adsorption, the kinetic models including pseudo-first-order, pseudo-second-order, Elovich, and Avrami fractional-order (Eqs. (3-6)) were considered as summarized in Table 2.
The obtained experimental data on 4CP adsorption by PHAC with changing contact time were fitted with the kinetic models (Table 2) by using an Origin 8.5 software. The kinetic models’ validity and their fitting quality were specified by calculating the determination coefficient (R2) and adjusted R2 (R2adj) by Eq. (7) and (8).
Fig. 12 illustrated the obtained experimental data and predicted values of kinetic models, and estimated kinetic constants are summarized in Table 3.
Table 3: Values of kinetic parameters and fitting coefficients for 4CP adsorption
Kinetic model
|
Constant parameter
|
Initial 4CP concentration (mg/L)
|
50
|
100
|
150
|
Pseudo-first order
|
qe
|
24.56 ± 0.19
|
47.88 ± 0.36
|
68.93 ± 0.74
|
kf
|
0.23 ± 0.01
|
0.217 ± 0.009
|
0.24 ± 0.01
|
R2adj
|
0.996
|
0.995
|
0.991
|
Reduced Chi-Square
|
0.28
|
0.95
|
4.21
|
Pseudo-second order
|
qe
|
26.18 ± 0.29
|
51.10 ± 0.78
|
73.42 ± 0.80
|
kS
|
0.014 ± 0.001
|
0.007 ± 0.001
|
0.0051 ± 0.0010
|
R2adj
|
0.993
|
0.988
|
0.994
|
Reduced Chi-Square
|
0.40
|
2.65
|
2.95
|
Elovich
|
α
|
100.64 ± 8.6
|
166.3 ± 14.4
|
323.67 ± 27.09
|
β
|
0.30 ± 0.04
|
0.15 ± 0.02
|
0.11 ± 0.02
|
R2adj
|
0.934
|
0.925
|
0.942
|
Reduced Chi-Square
|
4.03
|
17.69
|
27.83
|
Avrami fractionary
|
qe
|
24.78 ± 0.09
|
48.17 ± 0.28
|
69.76 ± 0.54
|
kAV
|
0.225 ± 0.005
|
0.216 ± 0.007
|
0.23 ± 0.01
|
nAV
|
0.82 ± 0.02
|
0.86 ±0.04
|
0.77 ± 0.05
|
R2adj
|
0.999
|
0.998
|
0.996
|
Reduced Chi-Square
|
0.04
|
0.49
|
1.72
|
The results revealed that, for the studied initial 4CP concentration, the R2adj value of Avrami fractional-order was higher than other kinetic models and indicating more suitability of this kinetic model for 4CP adsorption prediction by PHAC as a function on contact time.
3.3.4. Influence of initial concentration of 4CP
The obtained results related to the influence of the initial concentration of 4CP on the efficiency of 4CP adsorption at PHAC dose of 1 and 2 g/L is illustrated in Fig. 13.
As can be observed, in both doses of PHAC, with expanding initial 4CP concentration, the 4CP adsorption efficiency was declined. When 1 and 2 g/L of PHAC applied, with increasing initial concentration of 4CP ranging from 50.0 to 300.0 mg/L, the 4CP adsorption efficiencies were reduced from 93.5 ± 2.5% to 71.5 ± 1.9% and from 99.8 ± 0.2%, respectively. This can be explained by the adsorption sites saturation of the adsorbent surface, which indicated a possible formation of a monolayer of 4CP molecules at the interface with the adsorbent (Yadav et al. 2020). The obtained results are in line with the presented results with Kilic et al. (Kilic et al. 2011), who reported that the adsorption efficiency of AC diminished with an increasing initial concentration of phenol.
3.3.7. Isotherm study
The experiments of 4CP adsorption by PHAC were conducted in the batch mode system to obtain 4CP concentrations in the bulk liquid phase. Several adsorption models can be used to describe experimental data of adsorption isotherms. According to Table 3, the equilibrium data were modeled with the Freundlich, Langmuir, and Liu isotherms (Eqs. (9-11)).
Fig. 14 shows the obtained fitting results of adsorption data at the studied dose of PHAC with different isotherm equations, and the obtained parameters were summarized in Table 4.
Table 4: Values of isotherm parameters and correlation coefficients for the adsorption of 4CP
Isotherm model
|
Constant
parameter
|
AC dose (g/L)
|
|
1.0
|
2.0
|
Freundlich
|
KF
|
36.15 ± 3.62
|
39.79 ± 3.85
|
nF
|
2.48 ± 0.16
|
3.66 ± 0.04
|
R2adj
|
0.992
|
0.979
|
Reduced Chi-Square
|
4.96
|
4.02
|
Langmuir
|
Qmax
|
241.94 ± 14.72
|
128.34 ± 11.09
|
KL
|
0.062 ± 0.012
|
0.16 ± 0.05
|
R2adj
|
0.983
|
0.950
|
Reduced Chi-Square
|
10.16
|
9.06
|
Liu
|
Qmax
|
448.45 ± 20.9
|
273.73 ± 21.7
|
Kg
|
0.0098 ± 0.001
|
0.0089 ± 0.001
|
nL
|
0.59 ± 0.13
|
0.39 ± 0.13
|
R2adj
|
0.993
|
0.980
|
Reduced Chi-Square
|
4.02
|
4.24
|
The results show that the higher R2adj related to Liu isotherm indicates that this model is more suitable than other isotherms for the prediction of 4CP adsorption by PHAC. As reported in Table 4, the theoretical values of Qmax of PHAC estimated by Liu isotherm at 1 and 2 g/L of PHAC dose were 448.45 ± 20.9 mg/g and 273.73 ± 21.7 mg/g, respectively. The comparison of the Qmax value of various AC are summarized in Table 5 and depicted that the obtained Qmax in present study is relatively higher than other studies.
Table 5: Qmax value of some AC for 4CP adsorption
Raw material
|
Chemical activation agent
|
Qmax (mg/g)
|
Refs.
|
Milk vetch
|
H3PO4
|
87
|
(Noorimotlagh et al. 2016)
|
Sewage sludge
|
KOH
|
192
|
(Monsalvo et al. 2012)
|
Jackfruit
|
H2SO4
|
277
|
(Jain and Jayaram 2007)
|
Sewage sludge
|
KOH
|
358
|
(Monsalvo et al. 2011)
|
Pistachio shells
|
NaOH
|
428
|
(Tseng et al. 2010)
|
Olive stones
|
H3PO4
|
436
|
(Termoul et al. 2006)
|
Corncob
|
KOH
|
446
|
(Tseng and Tseng 2005)
|
Pomegranate husk
|
ZnCl2/NaOH
|
448
|
Present study
|
3.3.8. Reusability and stability
The reusability of the PHAC was assessed through five adsorption-desorption cycles. Each of cycle was conducted at the optimum conditions (4CP concentration: 50 mg/L, PHAC dose: 2.0 g/L, solution pH: 6.0, and contact time: 60 min) and the desorption cycle was carried out using 5 mL of acetonitrile/methanol (1/1, v/v), vibrated under 180 rpm for 2 h at room temperature. The adsorbent was subsequently filtered and dried overnight for next use. The results regarding the 4CP removal efficiency for five cycles of PHAC are given in Fig. 15.
As illustrated in Fig. 15, the adsorption efficiency of 4CP was slightly declined after five consecutive cycles. The 4CP adsorption efficiency slightly reduced from 99.6% in first cycle to 80 % after fifth run and indicating that as-prepared PHAC has a satisfactory reusability potential and can be recycled for several times with adsorption efficiency higher than 80%. These findings elucidate that the operation cost in practical applications could significantly be reduced due to the high operational reusability. However, the reason of this negligible reduction can be attributed to either minimal mass loss of PHAC during operation or reduction of the adsorption capacity of PHAC via blocking of the surface pores by 4CP.