3.1 Characterisations of adsorbent
FTIR spectra of the C4, HMDI, and the C4PU polymer adsorbents are displayed in Fig. 1. According to the results, significant differences can be observed between C4, HMDI, and synthesised adsorbents, with the dotted lines indicating the main bands. Figure 1(a) showed the main characteristics vibration of p-tert-butyl calix[4]arene by the appearance of hydroxyl terminated compound at 3120 cm− 1, CH3 tert-butyl group at 2955 cm− 1, and C = C aromatic vibrations at 1604 and 1476 cm− 1. Figure 1(b) shows the main peak of HMDI which is the isocyanate peak at 2250 cm− 1. Figure 1(c-f) represents all C4PU adsorbents with the disappearance of the isocyanate peak which showed that the polymerisation of C4PU has been successfully taken place [24, 29]. The small broad IR band around 3500 − 3300 cm− 1 indicate the presence of N-H stretching vibrations from the amine group. The bands at 1736 cm− 1 correspond to the stretching vibrations of C = O group. Upon completion of the reaction, NC = O stretch of the amide group and ester group in the urethane link shows the appearance of peaks around 1566 cm− 1 and 1249 cm− 1, respectively. The amine peak, C-N appeared around 1182 cm− 1. The presence of vibrations of CH3 tert-butyl and C = C aromatic, which are identical to the calix[4]arene spectra, was found in all C4PU adsorbents.
Figure 2 shows the comparative TGA of the polymer to evaluate the stability of synthesised polymer adsorbent. C4 and PU started to have decomposition at 163 ˚C and 198 ˚C, respectively. The C4PU adsorbents undergoes gradual degradation in the temperature range of 198 ˚C to 298 ˚C. According to TGA studies, segmented PUs usually experience a two-stage degradation process with an increase in temperature. There are two phases of degradation in the C4PU polymer adsorbents, first phase is the degradation of p-tert-butyl and the hard segment of urethane link. The first and second steps of decomposition for all polymers occurred at more or less similar temperatures. The first degradation step occurred between 270 ˚C and 330°C which can be attributed to the thermal cleavage of the urethane bonds. Second phase is the degradation of aromatic ring and soft segment polyol around 400 ˚C to 500 ˚C.
From the graph, C4PU 4 seems to appear as the most stable adsorbent compared to others. This might be due to the increasing in the amount of cross-linking polymers and few more stable hydrogen bond.
Based on IUPAC classification, a BET analysis provides precise specific surface area evaluation of materials by nitrogen multilayer adsorption measured as a function of relative pressure using a fully automated analyser. Figure 3 depicted the N2 adsorption/desorption isotherm. The decline in the vertical axis is attributed to the fact that when the pressure was reduced, the amount of N2 adsorbed was reduced. The result appears to be in good agreement with the type III adsorption isotherm, which indicates that the adsorbate-adsorbent interactions are very weak. Table 2 showed the BET surface area and total pore volume for all adsorbents. Changing in molar ratio does not affect the surface area and porosity of the adsorbent as the total pore volume in all adsorbents appeared to be quite similar to one another.
Table 2
BET summary for C4PU adsorbents.
Adsorbents | BET Surface area / | Total Pore volume / |
m².g− 1 | cm³.g− 1 |
C4PU-1 | 3.0920 | 0.00911 |
C4PU-2 | 0.1021 | 0.00576 |
C4PU-3 | 0.0324 | 0.00669 |
C4PU-4 | 0.1413 | 0.00837 |
3.4 The effect of experimental conditions on adsorption studies
3.4.1 pH
The pH of the dye solution plays an important role in the whole adsorption process. Figure 5(a) shows that as the pH increases, the adsorption percentage for MB as well as MG increases from 10.2 to 56.9%, and from 15.2 to 71.5%, respectively. The graph shows that the adsorption undergoes a significant increasing in the removal of dyes after pH 5.2, which is the value for pHPZC. The maximum removal for MB is observed at pH 10, while for MG it is at pH 7. At lower pH, the presence of excess H+ ions from the solutions occupies the adsorbent surface, which causes an electrostatic repulsion with cationic dyes molecules, resulting in lower adsorption of dyes. At higher pH, negatively charged of the adsorbent surface facilitates the adsorption of cationic contaminants. Since the competition for the active site decreases, the adsorption of dyes increases [30–32].
3.4.2 Adsorbent dosage
A series of adsorption tests with varied dosages at an optimum environment, which is 10 mg.L− 1 solution at pH 10 for MB and 10 mg.L− 1 solution at pH 7 for MG, were conducted to investigate the effect of adsorbent dose on the adsorption of MB and MG dyes. Figure 5(b) shows the effect of the adsorbent dosage on MB and MG adsorption. The adsorption percentage of MB increases from 57.8–93.1% when the adsorbent dosage is increased from 0.4 g to 2.4 g, whereas the adsorption percentage of MG increases from 45.2–92.2% when the adsorbent dosage is increased from 0.4 g to 1.6 g. MG dye was adsorbed more effectively in this study, despite a modest difference in dye adsorption percentages.
From the graph, the optimum dosage for MB is 1.8 g / 150 ml, while the optimum dosage for MG is 1.0 g / 150 ml, as seen in the graph. It is found that as the dosage was increased, the number of adsorbed dyes decreased. This is attributed to an increase in adsorbent dosage that results in unsaturated adsorption sites. In short, the lower adsorption capacity in higher dosage is caused by the partial aggregation of the copolymer and restricted residual sites of dyes adsorption [14, 33].
3.4.3 Initial dye concentration
The effect of initial dye concentrations (10–30 mg.L− 1) on the adsorption of both dyes was investigated at different contact time onto C4PU adsorbents. For both MB and MG dyes solutions, experiment was carried out in the presence of optimum dosage of adsorbents. This experiment was conducted over a period of time ranging from 30 to 180 min. As shown in Fig. 5(c), the adsorption percentage decreases with increasing initial dyes concentrations. The adsorption percentage of MB and MG drops as the initial dye concentration increases, from 84.0–46.3% and 74.9–53.9% respectively. As concentration of each dye increases, more active site of the C4PU adsorbent become engaged in the process, resulting in dye adsorption decreases due to saturation. At high concentration, the ratio of initial dye molecules to the accessible surface area is high, thus the available adsorption sites become fewer, resulting in a lower adsorption percentage [34, 35].
3.4.4 Contact time
Figure 5(d) shows that the dye adsorption percentage by C4PU adsorbent increases as contact time increases until equilibrium is reached. The contact time curve is continuous and leads to saturation, demonstrating the possibility of MB and MG monolayer coverage on the adsorbent surface. There are two phases involved in this process. The quick adsorption phase took 10–45 min to attain relative adsorption equilibrium, whereas the second, less steep phase represents the slow sorption of dyes within the pores of adsorbent. The graph shows that as contact time increases, both dyes’ adsorption increases, and equilibrium is attained in 60 min for MB, and 90 min for MG. It is mostly due to the saturation of the active site, which prevents further adsorption [35].
3.4.5 Adsorption isotherm
Two adsorption isotherm models, Langmuir and Freundlich, were employed to analyse the adsorption behaviour of C4PU adsorbent on MB and MG dyes. The Langmuir model assumes that adsorption is localized on a monolayer and all adsorption sites on the adsorbent are homogeneous and have the same adsorption capacity [36]. The Langmuir isotherm equation is:
$$\frac{{C}_{e}}{{q}_{e}}= \frac{1}{{q}_{max}} {C}_{e}+ \frac{1}{{q}_{max}.{K}_{L}}$$
3
where Ce is the equilibrium concentration (mg.L− 1), qe is the amount of adsorbed dye at equilibrium (mg.g− 1), qmax (mg.g− 1) is the Langmuir constants that are related to the adsorption capacity, and KL (L.mg− 1) is the adsorption rate. The value of R2 obtained from the linear graph of Ce/qe versus Ce is shown in Fig. 6.
The Freundlich isotherm model assumes that multi-layer adsorption processes occur on heterogeneous surfaces. The Freundlich isotherm linear equation is shown as follows:
$$\text{ln}{q}_{e}= \frac{1}{n}\text{ln}{C}_{e}+\text{ln}{K}_{f}$$
4
where 1/n is the heterogeneity factor and Kf (L.mg− 1) is a constant related to the adsorption energy. Both are derived from plotting the ln qe versus ln Ce, as shown in Fig. 7. The Langmuir isotherm indicates that sorbate is covered in monolayers on the surface of the sorbent, whereas the Freundlich isotherm suggests that the sorbent surface is heterogeneous and multilayer formation occurs. Table 3 shows the correlation coefficient values for all of the variables.
Isotherm studies examine the interaction between an adsorbate and an adsorbent at a certain temperature under equilibrium conditions. For both MB and MG dyes, the Langmuir isotherm model provides the highest R2 value of 0.9703 and 0.9842, respectively. As a result, the Langmuir isotherm is the greatest fit for explaining the adsorption process.
In Langmuir isotherm, to determine the adsorption process is either favourable or non-favourable, it can be classified by a dimensionless constant called equilibrium parameter, RL, by following the equation below :
$${R}_{L}= \frac{1}{1+{K}_{L}{.C}_{o}}$$
5
where KL is Langmuir isotherm constant, and Co is the initial dyes concentration (mg.L− 1). The RL from the equation above can be evaluated to be either unfavourable (RL ˃ 1), linear (RL = 1), or favourable (0 ˂ RL ˂ 1) [37]. According to the RL value provided in Table 4, MB and MG experienced favourable adsorption. The Langmuir isotherm was found to be the best fit for the adsorption of MB and MG by C4PU adsorbent.
Table 4
Isotherm constants of MB and MG dyes adsorption.
Isotherm model | MB | MG |
Langmuir isotherm | qmax / mg.g− 1 | 1.619 | R² | 0.9703 | qmax / mg.g− 1 | 2.192 | R² | 0.984 |
KL / L.mg− 1 | 10.436 | RL | 0.0075 | KL / L.mg− 1 | 0.359 | RL | 0.277 |
Freundlich isotherm | KF / L.mg− 1 | 0.947 | R² | 0.430 | KF / L.mg− 1 | 0.659 | R² | 0.926 |
1/n | 0.237 | | | 1/n | 0.443 | | |
Table 5 compares the maximum adsorption capacity (qmax) of C4PU with that of other adsorbents for the removal of MB and MG dyes and few adsorbents has lower adsorption capacities than C4PU. The qmax of C4PU adsorbent is quite on the lower side, as seen in the table.
Table 5
Comparison of the maximum adsorption capacity, qmax of MB and MG dyes with various adsorbents.
Adsorbents | qmax / mg.g− 1 | References |
MB | MG |
Zeolite | 25.0 | - | [38] |
C4ABS | 23.0 | - | [39] |
PANI C-dot initiated polymerization | 19.2 | | [40] |
PProDOT/MnO2 (1:2) | 13.9 | - | [41] |
PANI nanoparticles | 6.1 | - | [42] |
Calix[6]Arene-Modified Lead Sulphide (Pbs) | 5.5 | - | [43] |
Fly ash | 1.3 | - | [38] |
Chitosan/PANI | - | 29.1 | [13] |
Chlorella-based biomass | - | 18.4 | [44] |
Saccharomyces cerevisiae | - | 17.0 | [45] |
PU / chitosan composite foam | - | 16.7 | [37] |
Arundo donax root carbon (ADRC) | - | 8.7 | [46] |
Tamarind Fruit Shell | - | 2.0 | [47] |
Unsaturated polyester Ce(IV) phosphate | - | 1.0 | [48] |
C4PU | 1.6 | 2.2 | This work |
3.4.6 Adsorption kinetics
The adsorption behaviour of MB and MG dyes onto C4PU adsorbents was studied using pseudo-first-order and pseudo-second-order models. The pseudo-first-order kinetic model states that if adsorption is controlled by diffusion steps, the reaction rate is proportional to the number of ions left in the solution. The adsorption rate is considered to be proportional to the difference between the saturated concentration and the C4PU adsorption quantity over time. Below is the integral equation for pseudo-first-order:
$$\text{ln}\left({q}_{e}-{q}_{t}\right)=\text{ln}{q}_{e}-{k}_{1}t$$
6
where k1 is the rate constant of adsorption (1.min− 1), qe is the quantity of dye adsorbed at equilibrium (mg.g− 1), and qt is the equilibrium concentration at various times t (mg.L− 1). The rate constant in this model was determined by the slope of the plot of ln (qe − qt) over time (t) as shown in Fig. 8.
The pseudo-second-order kinetic model can be described as the reaction rate being proportional to the concentrations of the two reactants, assuming chemical adsorption steps regulate the adsorption [49]. Below is the equation for pseudo-second order model:
$$\frac{t}{{q}_{t}}=\frac{t}{{k}_{2}{{q}_{e}}^{2}}+\frac{t}{{q}_{e}}$$
7
where k2 is the second order-rate constant (g.mg− 1.min− 1) that can be determined for different dyes concentrations per the linear plots of t/qt versus t, as shown in Fig. 9.
Pseudo-first-order and pseudo-second-order kinetic models were used to fit the kinetic data. The linear plot yields the adsorption rate constants and correlation coefficients, which are reported in Tables 6 and 7 for MB and MG, respectively. The regression coefficient (R2) is used to verify the model’s validity, and R2 of pseudo-second-order appeared to be close to unity, which fits the experimental data.
Table 6
Kinetic study of MB adsorption at 1.8 g of C4PU adsorbent, pH 10, and 60 min contact time.
Cₒ / | qe,exp / | Pseudo-first-order Model | Pseudo-second-order Model |
mg.L− 1 | mg.g− 1 | k1 / | qe,cal / | R² | k2 / | qe,cal / | R² |
| | 1.min− 1 | mg.g− 1 | | 1.min− 1 | mg.g− 1 | |
10 | 0.890 | 0.070 | 0.886 | 0.9118 | 0.197 | 0.905 | 0.9102 |
20 | 1.827 | 0.071 | 0.743 | 0.7694 | 1.182 | 1.785 | 0.9995 |
25 | 1.945 | 0.012 | 0.755 | 0.4033 | 0.600 | 1.953 | 0.9979 |
30 | 1.555 | 0.049 | 0.838 | 0.7587 | 0.754 | 1.438 | 0.9981 |
Table 7
Kinetic study of MG adsorption at 1.0 g of C4PU adsorbent, pH 7, and 90 min contact time.
Cₒ / mg.L− 1 | qe,exp / mg.g− 1 | Pseudo-first-order Model | Pseudo-second-order Model |
k1 / | qe,cal / | R² | k2 / | qe,cal / | R² |
1.min− 1 | mg.g− 1 | | 1.min− 1 | mg.g− 1 | |
10 | 0.80 | 0.046 | 0.418 | 0.834 | 0.722 | 0.776 | 0.995 |
15 | 1.46 | 0.065 | 0.559 | 0.852 | 0.847 | 1.455 | 0.999 |
20 | 1.48 | 0.037 | 0.670 | 0.731 | 0.595 | 1.384 | 0.998 |
25 | 1.69 | 0.072 | 0.930 | 0.821 | 0.369 | 1.708 | 0.997 |
30 | 1.64 | 0.064 | 0.762 | 0.585 | 0.382 | 1.653 | 0.994 |
3.4.7 Reusability studies
The reusability experiments of C4PU adsorbent in MB and MG dyes are shown in Fig. 10. HNO3 was chosen for this experiment because it provides more active acidic surface groups like carboxyl and lactone, which leads to a reduction of the basic dyes. The data shows that there is some irreversible adsorption of MB and MG on its surface after a total of four cycles [14, 50]. As a result, the adsorption percentage decreases in subsequent cycles. For MB and MG, the adsorption percentage dropped from 72.6% to 33.5% and 72.1% to 44.0%, respectively. From the graph, the adsorption percentage of both dyes becomes constant after three reusability cycles, thus, it can be concluded that this adsorbent may be reused only once to ensure for the effective removal of MB and MG dyes.