PAK was synthesized in a high yield by polymerizing terephthaldhyde with 2,5-hexanedione in a basic ethanolic solution using an aldol condensation reaction(34–38) (Scheme 1). FT-IR, UV and fluorescence behavior, solubility, X-ray diffraction analysis, SEM morphology, thermal behavior, surface area, and GC-mass spectroscopy were utilized to characterize the polymer structure. The FT-IR spectrum of PAK (Fig. 1A) displays the characteristic bands for the C = O stretch vibration at 1607 cm− 1 with reduced intensity, conjugated C = C bond at 1558 cm− 1, C-H aliphatic at 2856 cm− 1, and C-H Aromatic at 2929 cm− 1. Due to a large amount of the CO groups, conjugation, and alternating distribution, the wavenumber of the carbonyl group has changed to a lower value, and the peak intensity has dropped. More stacking of molecules in semicrystalline structure and greater intermolecular force resulted from this uniformity in allocation.(39 ) These findings point to polymer formation, structure elucidation, also physical and chemical behavior.
The crystallization behavior of polyketone was studied using the X-RD method. As demonstrated in Fig. 1B, PAK has a semi-wide peak at 2θ = 13.14°, indicating some degree of crystallinity. The alternating arylidene moieties between the aliphatic chain and ketonic groups provide the polymer structure some regularity and result in higher crystallinity (56.7%) than amorphously.(40, 41) This unique structure affects the chemical and physical properties of the resultant polymer, making it more resistant to solvent penetration and heat breakdown.
The resulting PAK's solubility was examined, and it was shown that the polymer is not soluble in polar protic solvents but partially soluble in a polar aprotic solvent like DMSO and DMF. This can be attributed to its structure and the high polarity content of the carbonyl group.
Thermogravimetric analysis TGA and differential scanning calorimetry DSC curves evaluated PAK's thermal stability (Fig. 2A). The DSC results show that the polymer has a high melting temperature of Tm at 580°C and a glass transition temperature of Tg at 74°C, which is beneficial because thermal stability is mainly dependent on crystalline structure increases with crystallinity. The vaporization of trapped solvent and moisture in the polymer causes a weight loss of around 1.4% at temperatures below 100°C. In contrast, the gradual weight loss in the second stage of decomposition is fast, about 6% from 110°C to 560°C due to the decays of PAK chains. Furthermore, at 700°C, the char yield is 93.9%. These findings support the existence of PAK polymorphisms with excellent thermal stability and crystalline structure.(42)
The photophysical features of PAK's UV-visible absorption and emission spectra are discussed in Fig. 2B. An absorption band at λmax 269 nm may be seen in the absorption spectrum of PAK solution (1 mg in 10 ml DMF). This peak could be attributed to the π-π* transition of arylidene conjugation with the carbonyl group of the polymer backbone, as aliphatic polyketone solutions are colorless and do not absorb UV light due to the lack of conjugation in their chains and the presence of the ketonic group.(43) While the emission spectrum of solid PAK with λexc. 280 nm exhibits a peak at λmax 511 nm, a higher intensity in the red region, the emission color is determined by the Commission International de l'Elcairage CIE chromaticity diagram with (x,y) coordination (0.691, 0.296) indicating red color emission. This emission can be linked to PAK's crystal structure and improved interchain stacking in the solid-state, resulting in this emission.(44)
Gas sorption analysis with N2 was used to study the porous structure of this PAK. Figure 3A depicts the resultant polymer's N2 adsorption and desorption data. The BET surface area appears to be 76.73 m²/g. The pore volume of the polymer is 0.0687 cc/g. The pore diameter of the polymer is 1.7911 nm, indicating that it is extremely close to being microporous. Polymer density (or concentration) affects accessible surface area but not chain length.
Furthermore, the available concentration of smaller molecules in thick, brittle bendable polymers can be as high as 10− 3. When chains have a low degree of flexibility and cannot compress and versatile chain integration, the total area available may be directly proportional to the twisted chain portions that hinder the chains from collecting and producing permeable pieces. Due to the packing, adjacent chains would come into contact, preventing the probe from touching the contact surface and lowering the surface area that could be accessed. Because the arylidene moieties are present, the surface area of this PAK is reduced. In these conditions, the arylidene effectively fills in a convex region on the molecule's surface.(45, 46)
Enlarging the size of an object and allowing individuals to observe smaller sections within the sample is a useful approach to inspecting the surface of the polymer. Figure 3B shows the morphology of PAK as seen using a scanning electron microscope (SEM) at various magnifications. This information has been utilized to track bulk polymers' crystal shape, structure, and some of their mechanical and optical properties. The spherical form with coalescence is founded in the SEM analysis of PAK. The crystallinity of PAK, the ordering between chains, the interchange distance, and the increased porosity are all due to the compact packing of globules, which has several uses. The presence of the flexible aliphatic chain allows for accumulation and packing between chains.(47, 48)
Pyrolysis GC/mass was used to determine the composition of a PAK sample, which was pyrolyzed at 600°C to ascertain its design. The pyrolysis temperature and the pyrograms (total ion chromatograms) are displayed in Figure S1. The breakdown products indicate the presence of a monomeric unit of the polymer at m/z 343.2. At 650°C, the breakdown product suggests a dimer unit with m/z 646.5. These findings point to one of the most widely used domains in gas chromatography: constructing a polymer chain and examining its greater thermal stability in the face of heat.
Analysis of the Factors Affecting Adsorption
Adsorption kinetics were investigated at various time intervals to measure the amount of MB dye adsorbed. We used 200 ml of dye solution with concentrations extending from 15 to 100 ppm, continuous stirring at 700 rpm; 2 ml from the stirred sample was collected every 2 minutes between each patch within a 10-minute time interval and then filtered with filter paper before analyzing with a quartz cuvette using UV/visible spectrophotometry at λmax = 660 nm. Several parameters (polymer dosage, pH, and beginning dye concentration) are tested to discover the most effective circumstances for increasing the quantity of MB dye adsorbed.
The equation below calculates the adsorption efficiency percent (AE %) of MB on polymer:(49)
$$\text{A}\text{E} \text{\%}=\frac{\left(C0-Ct\right)}{C0}*100$$
1
C 0 and Ct are the beginning MB dye and dye concentration at period t in (mg/l). The Beer-Lambert law determines the remaining concentration of MB dye. The qe and qt of the dye adsorbed is given by the Eq. (2,3):
$$\text{q}\text{e}=\frac{\left(C0-Ce\right)\text{V}}{m}$$
2
$$\text{q}\text{t}=\frac{n\left(C0-Ct\right)\text{V}}{m}$$
3
Where qe is the adsorbed quantity at equilibrium for MB dye, qt is during time t, the amount of MB dye adsorbed at equilibrium, and both qe and qt have the unit (mg g− 1).
(a) Effect of adsorbent mass (quantity)
This study aimed to see how the adsorbent dose affected the results at a constant temperature of 298 K, and a fixed concentration of MB of 20 ppm, the polymer dose in 200 ml was increased from 10 to 50 mg. Adsorption behavior was studied by obtaining a sample every 2 minutes for 10 minutes. The data revealed that when utilizing 40 and 50 mg as polymer doses, the polymer's adsorption effectiveness (AE %) was approximately 96%. When using 5 mg of polymer, 66% AE was also obtained (Fig. 4). The increased number of accessible active pore sites for adsorption and higher specific surface area are the reasons for boosting the AE of MB from 66–96% by increasing the polymer dosage.(50)
(b) Effect of initial dye concentration
All other variables and parameters, such as pH, temperature, and polymer dose, were constant, except for the MB beginning dye concentration, modified to emphasize the relationship between initial dye concentration and AE % (Fig. 5). When the first dye concentration was elevated from 15 to 30 ppm at pH 10, the AE % of the 40 mg polymer was altered considerably. When the first dye concentration was raised from 40 to 100 ppm, the AE % decreased from 92–83%. Because we employed a certain dose of polymer in our research, the number of accessible sites on the polymer’s surface is fixed. Consequently, the number of places is constant. As a result, the ratio of active and accessible sites on the polymer surface to the MB dye molecule is always larger than one at lower concentrations. However, the available active site on the polymer's surface was occupied at higher dye concentrations, reducing the AE %. As a result, the active site to MB dye molecule ratio is below one.(51)
(c) Effect of pH
The pH of the MB solution is a significant component that affects adsorption capacity directly. The adsorption efficiency of the adsorbent is evaluated using the pH of the MB solution. In this study, we changed the pH of the solution while keeping a constant temperature of 298 K and an amount of adsorbent that is constant (40 mg) in 200 ml of dye solution with a concentration of 20 ppm (Fig. 6a-c). The researchers discovered that raising the pH from 2.5 to 10 increased the AE %. This increase in the AE % can be attributed to the competition of both H+ and dye cations for an accessible site at a low pH. Furthermore, the AE % is practically constant at pH 7–8, associated with the zeta potential threshold where H+ and dye cations have identical values, showing that pH has little impact on AE percent. The previous variables are summarized in Fig. 6d-f to indicate their effects on adsorption efficiency.
Adsorption kinetics
Langmuir and Freundlich models.
We employ the Langmuir and Freundlich models to validate our findings and better comprehend the research results. According to the Langmuir isotherm kinetic model, adsorption happens on a uniform surface of a monolayer material, and all adsorption points are equal and equivalent.(52) The Langmuir model has the following formula.
$$\frac{Ce}{qe}=\frac{\text{C}\text{e}}{qm}+\frac{1}{\left(Kl*qm\right)} \left(4\right)$$
Where Ce denotes the MB equilibrium state (mg l − 1), the equilibrium quantity adsorbed qe (mg g − 1), the high adsorption capacity qm (mg g − 1), and KL is the adsorption equilibrium constant (l mg − 1).
We use the second assumption in this analysis, the Freundlich isotherm model, which expects adsorption on diverse multilayer adsorbent material surfaces. The adsorbent material and the adsorbate have significant interactions, unlike the Langmuir isotherm type. The Freundlich isotherm type is expressed as:
$$\text{ln}qe=\text{ln}Kf+\frac{\text{ln}Ce}{n} \left(5\right)$$
C e and qe are identical to the Ce and qe in the Langmuir model. Kf is the Freundlich characteristic factor, closely linked with adsorption ability, and n is the adsorption strength.
The Langmuir and Freundlich parameters were obtained by graphing Ce/qe on the x-axis versus Ce on the y-axis and charting ln qe on the x-axis versus ln Ce on the y-axis for both models (Fig. 7a, 7b). The MB dye adsorption process on the surface of the polymer indicated in the data was fit using both models (Table 1).
Table 1
Isotherm parameters for MB adsorption onto PAK at 298 K.
Langmuir model
|
Freundlich model
|
Ce/qe= Ce/qm + 1/kLqm
|
ln(qe) = lnKF+(1/n) ln(Ce)
|
qm (mg g− 1)
|
15.652
|
n
|
4.995005
|
KL (mg g− 1)
|
0.426
|
KF (mg g− 1)
|
56.182201
|
R2
|
0.88
|
R2
|
0.9582
|
y = 0.0642x + 0.1497
|
|
y = 0.2002x + 4.0286
|
|
The results suggest that the Freundlich pattern is the best approximation for the polymer's MB dye adsorption process, with correlation coefficients of 0.88 and 0.9582, respectively. The Freundlich constant Kf was likewise found to have a high value, corresponding to a sorption capacity of 56.2.
Furthermore, the adsorption intensity was indicated by n, which was applied to evaluate the feasibility of the adsorption action. If n is 2–10, effective adsorption is displayed; if n is 1–2, moderate to difficult adsorption is indicated; if n is < 1, poor adsorption is indicated.(53) The result of the n polymer were 4.99, showing that MB was easily adsorbed onto the produced polymer, as previously reported.
Pseudo First Order (PPO) and Pseudo-second Order (PSO).
The amount of MB adsorbed on the polymer (qt) for 10 minutes is investigated. After 2 minutes, the MB dye was rapidly eliminated, with roughly 90% of the dye gone. This result was due to a free and active adsorption site in the initial stage of the reaction that MB dye molecules could occupy. The removal effectiveness falls as the available active sites on the polymer surface diminish with time until all reactive points on the polymer surface are blocked and saturated with MB dye.
We used pseudo-first- and pseudo-second-order reaction kinetic techniques and both models to test our findings and further understand the peculiarities of the adsorption mechanism. The liner formula is expressed in equations (6) and (7) for pseudo-first-order and pseudo-second-order kinetic systems, respectively (Fig. 7c, 7d).
$$\text{ln}\left(\text{q}\text{e}-\text{q}\text{t}\right)=\text{ln}qe+Kl*t \left(6\right)$$
$$\frac{t}{qt}=\frac{t}{qt}+\frac{1}{K2 q{e}^{2}} \left(7\right)$$
Where qe is the adsorbed amount at balance for MB dye, qt is the adsorbed quantity of MB dye at equilibrium at time t (mg g− 1), and Kl is the constant of pseudo-first-order rate (min− 1), and k2 is the constant of pseudo-second-order rate (g mg− 1 min− 1).
Straight-line charts illustrate the data for both models and the kinetic parameters and correlation coefficient (R2) (Table 2). The pseudo-second-order model had a higher linearity of correlation coefficient (0.9469) than the pseudo-first-order model Vs. (0.811), suggesting that the pseudo-second-order model provides the most accurate estimations of reaction kinetics in our manufactured polymer.
Table 2
Kinetic parameters for the adsorption of MB dye on PAK.
Pseudo-first order model
|
Pseudo-second order model
|
ln(qe − qt) = lnqe – kl*t
|
t/qt=(1/k2qe^2) + t/qe
|
qe.c (mg g− 1)
|
57.67
|
qe.c (mg g− 1)
|
44.25
|
k1 (min− 1)
|
0.0678
|
k2 (mg g− 1 min− 1)
|
0.033
|
R2
|
0.811
|
R2
|
0.9469
|
y = -0.0678x − 4.0548
|
|
y = 0.0226x + 0.0153
|
|
Heavy metal removal studies
Many types of heavy metals have great toxicity and are non-biodegradable, which leads to a highly bad impact on the health of humans, the environment, and animals. Therefore, removing heavy metals from sewer water is considered a big problem and a station for everyone's attention worldwide. Many researchers and many technologies work to develop the removal process like membrane filtration, reverse osmosis, electrochemical treatment, extraction, and irradiation.
In this article, we investigate the capacity of a produced polymer to remove heavy metals based on the presence of polyketones as functional groups that can generate more chemical than physical adsorption interactions. At pH 6, we created our study on the reduction of metal (Zn2+, Cu2+, Ni2+, Co2+, Cd 2+ and Fe2+) from wastewater as one of the effective methods used recently by the polymer in wastewater treatment.
Adsorption removes heavy metals from a solid surface, creating equilibrium by keeping adsorbed heavy metal concentrations in water constant. The conventional interaction is chemical adsorption to attain higher adsorption capacity for heavy metal removal. The projected chemical adsorption pathway is depicted in Fig. 8.
The polymer's structure shape, surface area, and strong disposal capacity make it a better adsorbent for heavy metal removal. Because we have ketone groups in the polymer main chain, removing metal ions by chemical adsorption involved chelation ion exchange, which resulted in electrostatic contact between metal ions and polymer (Fig. 9).(54, 55) The maximum adsorption capacities (Cd2+ < Ni2+ < Co2+< Zn2+< Cu2+ <Fe3+) were within the equilibrium time approaching 1 hour, with AE 93.83%, 92.68%, 92.26%, 90.43%, 84.52%, and 77% for Fe, Cu, Zn, Co, Ni, and Cd, respectively. The maximal adsorption capabilities of pka and other adsorbents for MB are listed in Table 3. As can be shown, the adsorption capacity attained in this investigation was significantly higher than that previously reported in the literature. This finding indicates that the pka polymer has a high potential for MB removal.(56–67)
Table 3
The maximum adsorption capacities of pka and other adsorbents for MB.
adsorbents
|
Pollutant
|
adsorption capacity (mg/g)
|
Referances
|
PPy/SD
|
MB
|
34.36
|
49
|
G an GO
|
MB, MV, RB, OG, Pb(II), Cu(II), Cd(II), Zn
(II),
|
17.3
|
50
|
PANINTs/Silica
|
MB
|
25
|
51
|
Fe3O4@PAmA-BAmPD-TCATS
|
MB
|
31.64
|
52
|
SW-ZnO-PANI
|
MB
|
20.6
|
53
|
PProDOT/MnO2
|
MB
|
13.94
|
54
|
potato (Solanum tuberosum) plant
|
MB and MG (malachite green)
|
52.6
|
55
|
AC
|
MB
|
47.62
|
56
|
SNCM
|
MB
|
20.0
|
57
|
Fe2O3-ZrO2/Black cumin
|
arsenic and
|
38.1
|
58
|
PAK
|
MB, Cu(II), Cd(II), Zn
(II),
|
57.67
|
This study
|