3.1. Physicochemical characterization
The surface area and also poresize distribution of adsorbents were examined by the BET way (Fig. 2-a). The three samples displayed the isotherm of type-III. The calculated SBET of PANI, CS and PANI@CS are 46 m2.g−1, 988 m2.g−1 and 1008 m2.g−1, respectively. The SBET, mesoporous volume and total pore of materials are given in Table 2. The SBET value is more important for hybrid adsorbent versus to CS and PANI which confirms the existence of more activated surface sites for PANI@CS [25].
Table 2
Textural characterization and Surface composition (at%) from XPS of adsorbents prepared before and after adsorption.
Adsorbents | Before adsorption | After adsorption |
PANI | CS | PANI@CS | PANI | CS | PANI@CS |
SBET / m2.g-1 | 46 | 988 | 1008 | 25 | 51 | 64 |
VDR (N2) / cm3.g-1 | 1.82 | 1197 | 2.10 | 1.02 | 82 | 1.54 |
Vmes / cm3.g-1 | 0.02 | 0.31 | 0.30 | 0.01 | 0.19 | 0.28 |
Vmic / cm3.g-1 | 0.01 | 0.43 | 0.44 | 0.01 | 0.08 | 0.13 |
Vtot pore | 0.03 | 0.74 | 0.74 | 0.02 | 0.27 | 0.41 |
To confirm the molecular structure of adsorbents, XPS analysis was carried out for three samples prepared. In Fig. 2-b, for the wide scan of the samples the peaks at 284 eV, 398 eV and 530 eV for C, N and O. Table 3 illustrates measured chemical composition of specimens. Moreover, the C1S peak of CS adsorbent is divided into three parts, at 284.59 eV, 286.06 eV and 288.30 eV (Fig. 3-a). The peaks indicate the C–C, C–OH and O–C–O bonds, respectively; and these obtained values are exactly matched with available literature [26, 27]. In addition, we can notice these peaks exist in both CS and PANI@CS. Moreover, Fig. 3-c displays the deconvoluted high resolution C1s spectrum of PANI@CS. The XPS spectrum can be deconvoluted into four components, at 284.56 eV, 285.66 eV, 286.94 eV and 290.96 eV, respectively. The main peak at 284.56 eV is assigned to the C–C/C–H groups present in polymers. The peaks at 285.66 eV and 286.94 eV are due to C–N and C–OH groups due to the hydroxyl groups which are incorporated between CS and PANI. There is shake-up satellite at 290.96 eV, due to the existence of C–N groups.
Table 3
XPS data of Binding Energy (BE) for adsorbents prepared.
Species | Adsorbents and BE (eV) | Observation |
CS | PANI | PANI@CS |
O1s | 531.74 | // | 531.35 | C=O, O=C–OH |
// | 532.33 | 532.50 | C–OH |
533.41 | 533.57 | 533.74 | H2O |
C1s | 284.59 | 284.60 | 284.56 | C=C, C–H, C–C |
// | 285.91 | 285.66 | C–O, C–N |
286.06 | 287.73 | 286.94 | C-OX, N–C=N |
288.30 | // | 290.96 | O-C-O |
N1s | // | 399.74 | 399.44 | =N– |
400.95 | 401.11 | 400.63 | –NH– |
402.51 | // | 401.84 | –NHx |
The N1s XPS spectrum of CS is shown in Fig. 3-d. The results indicated the peaks at 400.95 and 402.51 eV assigning to the assignments of -NH- and -NHx groups, respectively. Further, based on the N1s values in PANI@CS (Fig. 3-f), characterized by XPS it was concluded that there peaks observed at 401.84 eV, 400.63 eV and 399.44 eV which can be attributed to –NHx, –NH– and =N– groups, respectively. This result suggests that blending PANI with CS produced a hybrid absorbent with more amino functional groups on surface [28].
To further investigate of the different adsorbents prepared, the overall nitrogen content was determined. The surface elemental compositions from the XPS analysis are listed in Table 4. The PANI@CS, shows a nitrogen content of 10.80% in the surface layer. Moreover, the XPS analysis of CS adsorbent reveals a significantly less total nitrogen content compared with other adsorbents prepared. The content difference is obviously due to the new structure. Further, this nitrogen content obtains indicated that the matrix PANI were well-dispersed in the CS during the polymerization.
Table 4
Surface composition of the adsorbents obtained by TPD and XPS.
Adsorbents | TPD / µmol.g−1 | XPS / at.% |
CO | CO2 | Otot | O | C | N |
CS | 1975 | 513 | 3000 | 28.75 | 67.66 | 3.59 |
PANI@CS | 1485 | 355 | 2190 | 16.09 | 73.11 | 10.80 |
The surface chemistry of those adsorbents has been also investigated by thermal programmed desorption (TPD), which presents more detailed description about the quantity and nature of surface oxygen (O) groups [29]. The quantity of O groups that transferred as CO, as CO2 and the resulting quantity of O in adsorbents have been compiled in Table 4. The CS adsorbent possesses a large amount of oxygen functionalities that decompose mainly as CO and in lower extent as CO2 (Fig. 2-c and Fig. 2-d). The PANI matrix produces the attachment of nitrogen to the surface of the CS by consumption of oxygen functional groups [30], as confirmed by XPS (Table 4). Thus, PANI@CS has lower oxygen content than the pristine CS adsorbent.
The FT-IR spectra of PANI@CS, CS and PANI before AAP retention are used as a reference for interpreting any possible structural changes (Fig. 4). FT-IR spectrum of CS presents adsorption bands at 1021, 1084 and 1153 cm−1 are due to C−O stretching vibration of primary alcoholic groups, C−N stretching vibration and saccharide unit of CS. The vibration bands at 1021 and 1153 cm−1 disappear after AAP adsorption. It can be seen also the stretching vibration at 1387 and 1456 cm−1 indicating amide III (C−N stretching) and aromatic C−C stretch, respectively. These latter bands turn to a single band at 1393 cm−1 after AAP adsorbed. On the other hand, a vibration bands at 1625 and 2872 cm−1 characteristic of C=O stretching on the bond (−NHCO−CH3) and C−H stretching of CH2 groups, respectively. The −OH and −NH stretching is between 3100 and 3400 cm−1 [32]. Moreover, in hybrid adsorbent typical bands of PANI were found at 806 cm−1 due to C-C stretching of quinoid rings and deformation of the benzenoid rings respectively. The band at 1207 cm−1 for PANI appeared due to C=N stretching vibration. The transmittance peak at 1326 cm−1 owing C-N bond, 1516 cm−1 due to C=C stretching vibration of benzenoid rings, 1573 cm−1 due to stretching vibration of quinoid rings. The bands between the ranges 3000−3500 cm−1 ascribed to secondary amines stretching (N−H) vibrations [21]. The FT−IR spectra of PANI@CS adsorbent displays the next bands at 3426, 2878, 1657, 1578, 1317 and 1078 cm−1. Compared to CS, the functional groups moved to a higher frequency band. On the contrary, the peaks at 1516 and 1208 cm−1 were disappeared. These modifications confirm that significant amounts of O−H and N−H at CS were grafted by PANI [21]; while most of the bands changed their positions after absorption.
The XRD patterns of all adsorbents were recorded in Fig. 5-a. The diffraction pattern of CS displays two peaks at 2θ = 10.36º and 20.78º indicating the ordered crystalline structure of CS. Moreover, PANI is semi-crystalline in nature as the patterns show three peaks centered at 2θ = 7.41º (011), 20.21° (020), and 25.91° (200) because of the presence of B with Q group in the polymer chain [31]. Moreover, the XRD pattern of PANI@CS displays different crystalline structure with small moves associating together to PANI and CS, which further confirmed the succeeded grafting of PANI backbone with CS chain [16, 18, 22], in the peaks positions. It is distinctly seen from XRD study that the hybrid material offer an amorphous in nature with a little crystalline part from 2θ = 10º to 30°. The CS crystallinity has been reduced after surface grafted by PANI backbone and this can be because the intermolecular forces into structure of composite formed, which is predicted for the elimination of AAP [21].
The TGA thermograms of CS, PANI and PANI@CS are present in Fig. 5-b. The TGA curve of CS displays two stages of weight loss, the first occurring in the range of 25 to 160°C attributed to loss of H2O molecules with the weight loss approximately 3.49%. The primary degradation of CS started at 230°C and it was completely degraded at near 900°C with a weight loss of almost 13.27% [34]. TGA of PANI@CS presented three different steps of weight loss. The first step starting from 25 to 160°C, can attributed to the loss of adsorbed H2O with a weight loss of almost 7.59%. The second decomposition step occurs between 220ºC and 450°C, correspond to degradation (approximately 23.96%). The third step of the TGA curve is after 500ºC to 900ºC with a weight loss of almost 13.78%. These values prove the loss of the thermal stability for PANI@CS compared to the CS, confirming the formation of the hybrid adsorbent.
The SEM image of CS presents full amorphous regions as illustrated in (Fig. 6-a). The image clearly depicts the surface of CS is coarse and consisting of holes and uneven lumps that make it suitable for adsorption [35]. The SEM micrographs of PANI@CS as shown in (Fig. 6-b), reveal that the inclusion of PANI chain has great influence on morphology of the adsorbents that result in an interlocking arrangement of material. A close investigation of the surface reveals that PANI matrix is uniformly distributed on CS forming a network interconnecting each other and distributed on the entire surface during the polymerization process.
3.2. Adsorption assessments
3.2.1. Effect of pH
pH is one of the most significant parameters for adsorption process since the surface charge of molecules changes according the pH of the environment [18]. Thus, the influence of pH on the AAP adsorption by the three adsorbents was tested. The plot showing the adsorption of AAP on CS, PANI and PANI@CS is given in Fig. 7-a. The results of CS showed that adsorption efficiency increased as pH increased from 2.0 to 6.0. After increasing pH to 7.0, the adsorption percent slightly increased, and then it is quickly decreased to pH 12.0 values. However, for PANI, the adsorption percent decreased as the pH is increased. Obviously, PANI@CS has a higher affinity for AAP adsorption rather than both CS and PANI overall investigated pH range. Thereby, the deposition of PANI matrix on the CS surface is a efficacious strategy to ameliorate its AAP uptake ability. Moreover, it can be inferred that the imine and amine groups in PANI@CS hybrid adsorbent act as very helpful hosts to bind AAP from the aqueous solution. For the increase from 7.0 to 12.0, the capacity decreased for all adsorbents prepared. This behaviour is linked to the dissociation of functional groups on the adsorbent surface at pH above 6.0 (pKa = 9.5), causing adsorbate and adsorbent to be negatively charged. In this condition, electrostatic repulsion happens between the adsorbent and adsorbate molecules, causing the elimination to be disadvantaged [36]. This behaviour is in accord with other studies reporting the adsorption of AAP [36, 37]. Thereby, pH 7.0 was suggested as an optimum pH value for experiments of sorption of the studied AAP from aqueous solutions.
3.2.2. Effect of contact time
The shaking time between adsorbed and adsorbent is an essential role, as it enables us to know more in detail the characteristics, the design of the adsorption process, the mechanism as well as the adsorbate adsorption ratio and equilibrium time [38]. The influence of the time on the adsorption was studied and the equilibrium time was determined in the experiments carried out with the most suitable adsorbents with maximum adsorption capacity at pH 7.0 at 25 ºC (Fig. 7-b). As can be seen from the figure, at the end of 480 min, AAP adsorption to CS and PANI@CS reached the maximum level with values 359.84 mg.g−1 and 383.48 mg.g−1, respectively. In the adsorption processes, it is expected that equilibrium will be established between the adsorbent and the substance to be removed after a certain period of time. Because the functional groups on the adsorbent material reach fullness, the adsorption reaches equilibrium after a certain period of time [18, 19]. On the other hand, it is observed that the maximum adsorption (242.84 mg.g−1) on PANI was achieved within only 360 min.
3.2.3. Adsorption kinetics
Figure 7-b. illustrates the impact of contact time on AAP adsorption by CS, PANI and PANI@CS with 15000 mg.L−1 initial concentration. The AAP was noted to rapidly achieve equilibrium (300 min) by PANI. However, onto CS and PANI@CS, AAP need longer contact time (480 min) to arrive equilibrium. The augmentation in equilibrium time with hybrid adsorbent is because to the increased contest for new active sites of the PANI@CS.
Adsorption kinetics of AAP on materials obtained was studied by using pseudo-1st -order (PFO) and pseudo-2nd -order (PSO) laws. The adsorption kinetics have been investigated in the initial concentration of 15000mg.L−1.
The PFO explains the adsorption happens between solid and liquid system depending to the elimination capability of adsorbent materials were expressed by the relations are presented in Table 1. Where; Qe (mg.g−1): equilibrium concentration, Qt (mg.g−1): concentration at time t, k1 (min−1) : PFO rate constant of adsorption. PFO linear regression plot of \(log({C}_{e}-{C}_{t})\) versus t presents a weak correlation coefficients (Table 5) displaying that the PFO kinetic was not succeed by adsorption.
Table 5
PFO and PSO kinetics for AAP elimination on adsorbents at pH:7.0, 298K and Co:15000mg.L−1.
Adsorbents | Qeq.Exp (mg.g−1) | PFO | PSO |
k1 min−1 | Qeq.Cal mg.g−1 | R2 | k2.ads g.mg−1.min−1 | Qeq.Cal mg.g−1 | R2 |
PANI | 242.49 | 0.0103 | 214.48 | 0.78 | 0.00015 | 196.08 | 0.94 |
CS | 359.84 | 0.0066 | 298.26 | 0.57 | 0.00026 | 208.33 | 0.97 |
PANI@CS | 385.25 | 0.0129 | 348.01 | 0.92 | 0.00088 | 344.82 | 0.98 |
The PSO kinetic shows a chemisorption phenomenon from solutions [43]. Also, the linear formula is written by the relations are presented in Table 1. Where; k2 (g.mg−1.min): PSO rate constant, Qt (mg.g−1): concentration at time t, Qe (mg.g−1): adsorption capacity, k2Qe2 (g.mg−1.min): initial adsorption rate.
The PFO and PSO kinetic data are calculated taking into account the correlation values (R2). According to Table 5, the adsorption kinetic of AAP by three adsorbents is better defined by the PSO kinetics data than the PFO models because of the high R2. This shows that AAP removal by CS and PANI are dominated by chemical adsorption [43].
The adsorption rate values measured based on PSO kinetic are 88 ˟ 10−5, 26 ˟ 10−5 and 15 ˟ 10−5 g.mg−1·min−1 for PANI@CS, CS and PANI, respectively, reflecting a faster adsorption process by hybrid adsorbent. This is possibly because the better feasibly accessible to active sites of hybrid material compared with other adsorbents used. This augments the electrostatic attraction between PANI@CS adsorbent and AAP molecules, consequent a speed adsorption rate. Several investigators illustrated the succeeded usage of the PSO equation for the represent empirical kinetics values of AAP adsorption by several materials adsorbent (Table 6) [4, 8, 12, 36–44].
Table 6
The adsorptive capacity of several adsorbents for removal of AAP
Adsorbents | Adsorption Efficiency | pH | Ref. |
AC from Oak acorn | 45.45 | 3.0 | [4] |
Chemically modified Orange peel | 28.09 | // | [8] |
SAC from Biomass waste | 356.22 | 5.0 | [12] |
AC from Butia capitata | 100.60 | 7.0 | [36] |
NH4Cl-induced AC | 233.00 | 7.1 | [37] |
ABSAC | 145.40 | 8.0 | [38] |
AC from Dende coconut | 70.62 | 2.0 | [39] |
AC from Babassu coconut | 71.39 | 2.0 | [39] |
Rice husk ash | 7.65 | 8.0 | [40] |
Chemically Modified AC | 75.00 | 6.5 | [41] |
AC Pellets | 105.00 | 7.0 | [42] |
Commercial AC | 261.04 | 7.0 | [43] |
AC from Fly ash | 270.30 | 7.0 | [44] |
PANI@CS | 385.25 | 7.0 | This work |
3.3. Adsorption isotherm modeling
The monolayer adsorption capacity, the amount adsorbed was measured by the linear equation presented in Table 1, where Qeq (mg.g−1): amount adsorbed, Ceq (mg.L−1): equilibrium concentration of the adsorbate, Qm (mg.g−1): maximum adsorption capacity of adsorbent and Kl (L.mg−1): Langmuir constant.
Both multilayer (physisorption) and monolayer (chemisorption) can be measured using the Freundlich isotherm. This model is based on the heterogeneous equilibrium on adsorbents surface. The formula for Freundlich isotherm is illustrated in Table 1, where; n and Kf (mg1−1/n.L1/n.g−1): Freundlich isotherm constant and Ceq (mg.L−1): equilibrium concentration.
Basically, the adsorption phenomenon of porous adsorbent possesses three steps. First, the pollutant moved from solution to adsorbents surface by a liquid-boundary film. Secondly, this pollutant moved from adsorbents surface to intraparticle active site and finally, a strong interaction of pollutant molecules with the disposable sites on both the internal and external surfaces of adsorbents [45].
As a result of the adsorption studies carried out at the specified concentrations, the maximum AAP concentration to be adsorbed by different adsorbents was determined by plotting the amount of adsorbed AAP against the concentration values (Fig. 7-c). As display in Table 7, the fitting data of Langmuir equation for AAP adsorption were better than Freundlich equation, which exhibited the adsorption phenomenon, appertained to a chemisorption adsorption. And the n value (n>1) of Freundlich equation confirmed that the three adsorbents was conducive to absorb AAP. As a result, this model is unable to justify the experimental capacity for AAP where the values of Kf obtained from this model are significantly lower than the experimental values, as shown in Table 7. Also, the R2 values were smaller than those obtained from the Langmuir model. Thus, the Freundlich model does not fully explain the experimental results for these elements [46]. Combined with the kinetics and isotherm analysis data, the chemisorption was prevailing on adsorption of AAP by adsorbents prepared.
Table 7
Langmuir & Freundlich isotherms constants for AAP removal by adsorbents at pH:7.0 and 298K.
Adsorbents | Langmuir | Freundlich |
Qm (mg.g−1) | KL (L.mg−1) | RL | R2 | KF (mg1−1/nL1/ng−1) | n | R2 |
PANI | 101.01 | 0.087 | 0.101 | 0.99 | 28.64 | 5.21 | 0.71 |
CS | 243.90 | 0.539 | 0.007 | 0.99 | 45.84 | 3.27 | 0.75 |
PANI@CS | 196.08 | 0.024 | 0.172 | 0.98 | 30.95 | 4.33 | 0.76 |
3.4. Adsorption thermodynamics.
Temperature effects on the adsorption of AAP by PANI, CS and PANI@CS adsorbed from aqueous solution with pH 7.0 were investigated at different temperatures in the range from 25 to 45 ºC. In Fig. 7-d, it can be found that the AAP adsorption decreases with increasing temperature. In the adsorption process the thermodynamic parameters of enthalpy change ΔH, free energy change ΔG and entropy change ΔS can be measured by the formulas presented in Table 1, where; ΔS (kJ.mol−1): entropy, ΔH (kJ.mol−1): enthalpy, T (K): absolute temperature and R (8.314 J.mol−1.K−1): general gas constant.
The negative values ΔG displays the adsorption of AAP molecules by CS adsorbent is extremely favorable (Table 8). Furthermore, ΔG values are found to decrease with the augmentation of temperature is proved the spontaneous character of the adsorption processes at increased temperatures, this may demonstrated that the increased of adsorbents pores and more surface modification. The ΔH positive values illustrate the endothermic character of the elimination and it may also exhibit the adsorption processes physiosorption. The increasing positive values of ΔS at higher temperature may because to the increasing mobility and reducing the size of AAP which causes to increase in kinetics energy of the AAP. It favors the fast diffusion of AAP towards the adsorbents [47].The same way was followed in the case of PANI and PANI@CS adsorbents, which confirmed that the bonding between AAP and adsorbents related to formation of PANI matrix on CS surface.
Table 8
Thermodynamic data for the AAP adsorption by the adsorbents prepared.
Adsorbents | T / K | ΔG / kJ.mol−1 | ΔH / kJ.mol−1 | ΔS / kJ.mol−1 |
PANI | 298 | −10.17 | 29.05 | 0.131 |
303 | −10.81 |
313 | −12.16 |
318 | −12.79 |
CS | 298 | −9.18 | 27.86 | 0.123 |
303 | −9.46 |
313 | −10.05 |
318 | −12.01 |
PANI@CS | 298 | −9.01 | 25.25 | 0.114 |
303 | −9.34 |
313 | −10.20 |
318 | −11.46 |