2.1 Chemicals

The industrial grade of para-xylene was prepared from Dr. Mojallali industrial chemical complex company. In addition, anhydrous iron (III) chloride, potassium permanganate (KMnO₄), 1,2-dichloro ethane (DCE), formaldehyde dimethyl acetal (FDA), and ethanol were prepared from Merck company. During the xylene based HCP preparation and post modification, deionized water and distilled water were used for washing purposes. Also lead nitrate ( ${\mathrm{P}\mathrm{b}({\mathrm{N}\mathrm{O}}_3)}_2$ ), cadmium nitrate ( ${\mathrm{C}\mathrm{d}({\mathrm{N}\mathrm{O}}_3)}_2.4{\mathrm{H}}_2\mathrm{O}$ ), nickel nitrate $({\mathrm{N}\mathrm{i}\left({\mathrm{N}\mathrm{O}}_3\right)}_2.6{\mathrm{H}}_2\mathrm{O})$ , and copper nitrate ( ${\mathrm{C}\mathrm{u}({\mathrm{N}\mathrm{O}}_3)}_2.3{\mathrm{H}}_2\mathrm{O}$ ) were purchased from Merck company. The aforementioned raw materials were applied without further purification.

2.2 Para-xylene based HCP synthesis

In general, the hyper cross linked polymeric adsorbent (HCP) is synthesized through Friedel-Crafts alkylation reaction by using an external cross linker agent to form covalent bonds between aromatic ring of the monomers. In this work, similar to the technique reported by Li et al. (Li, 2011), the HCP adsorbent was synthesized based on para-xylene as monomer. To obtain the highest specific surface area and maximum heavy metal ions adsorption capability, the synthesis parameters such as synthesis time and monomer to cross linker ratio were adjusted in an optimal condition that was presented by Kim et al (Kim, 2020). Typically, para-xylene (0.02 mol), formaldehyde dimethyl acetal (0.04 mol), and dichloro ethane (40 ml) were poured into a three-Neck round bottom flask, then the flask contents were agitated at 25 °C for 1 h in the argon atmosphere. Then the anhydrous iron (III) chloride powder (0.04 mol) was charged to the flask and the mixture was blended at 45 °C for 4 h. Next, the mixture temperature was increased to 85 °C, followed by mixing in the reflux condition for 24 h. Next, the obtained porous polymeric adsorbent was filtrated and aimed to remove the solvent and excess reactants, the resulting polymer was washed several times with distilled water and ethanol for 20 h by soxhlet extractor apparatus. Finally, the purified HCP adsorbent dried at 160 °C for 15 h. The para-xylene based HCP synthesis method is shown in Fig. 1.

Close

Fig. 1

Para xylene based HCP sample synthesis procedure.

Export to PPT

2.3 Para-xylene based HCP’s surface modification

The presence of electron-withdrawing groups such as amine or amide group, phenol group, or carboxylic acid group in the HCP adsorbent’s skeleton enhances the HCPs adsorbent heavy metal ions uptake capability by improving the adsorbent’s surface heterogeneity. In addition, the presence of lone pair electrons on electron donor atoms such as oxygen, nitrogen, or sulfur act as ligand active sites and improve the selectivity of the HCP in the selective adsorption of cations. In this work, surface modification of xylene based HCP sample was done by incorporation of carboxylic acid group (–COOH) to the HCP adsorbent network. Carboxylic acid modification of the HCP sample was carried out through the oxidation of the xylene’s methyl group (–CH₃) by utilizing a strong oxidizing agent namely potassium permanganate (KMnO₄). In a typical procedure, the HCP adsorbent (4 g) and distilled water (250 ml) were charged to a three-neck flask, and the contents were heated at 100 °C in the reflux conditions until the mixture reached to the boiling point. Then, potassium permanganate (KMnO₄) was added to the mixture in three stages. First, KMnO₄ (8 g) was charged to the flask followed by stirring under reflux condition and solution’s boiling point for 2 h. Next, KMnO₄ (4 g) was added to the mixture and the reaction continued for 2 h. Finally, KMnO₄ (4 g) was added to the mixture in the third stage and the mixture was kept for 3 h at the boiling point of the mixture. Next, the flask contents were cooled down and the flask contents were washed with distilled water several times. Then, aimed to remove the remained manganese ions and the carboxylic acid groups recovery, the resulting modified HCP was washed with 500 ml of 0.01 M hydrochloric acid (Vogel and Furniss, 1978). Finally, the obtained modified HCP was dried at 160 °C for 15 h in a vacuum oven. The xylene based HCP carboxylic acid functionalization procedure is shown in Fig. 2.

Close

Fig. 2

Carboxylic acid modification procedure of the HCP sample.

Export to PPT

2.4 Adsorbent characterization

The p-xylene based HCP and the carboxylic acid modified HCP’s elemental composition were characterized by energy dispersive X-ray spectroscopy (EDS) analysis using Philips-×130 instrument, also X-ray photoelectron spectroscopy (XPS) technique was applied by an Al Kα source (XPS spectrometer Kratos AXIs Supra) device. The adsorbents morphological properties were characterized by nitrogen adsorption–desorption method at 77.3 K using ASAP 2020 M analyzer. To characterize the adsorbents structural properties and functional groups, FTIR analysis was performed by using PerkinElmer FTIR spectrometer instrument. Thermal stability of the samples were evaluated using thermogravimetric analysis (TGA) under an N₂ atmosphere (TGA, model 2960, Universal V 2.4F TA instrument). To measure the concentration of the metallic cations in liquid samples, the inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis was carried out by (ICP-OES, series 5800) instrument.

2.5 Adsorption experiments and selectivity

To perform the heavy metal adsorption experiments, the heavy metal ions contain solutions with different initial concentration including 20, 40, 60, 80, and 100 mg/l were prepared from stock solution with a concentration of 200 mg/l. Next, the resulting solution (150 ml) was charged to a flask, then the sample’s pH value was adjusted to a desired value by using 0.05 M NaOH solution or 0.05 M HCL solution, also the oil bath temperature was adjusted. The adsorption process was started by the addition of 0.1 gr adsorbent powder and stirring the flask contents at 1000 rpm. The adsorption process continued for 4 h to reach an equilibrium state, after 4 h the flask contents were filtrated and the quantity of the heavy metal ions in the residual solution was measured using ICP-OES analysis. The differences between the initial concentration and final concentration of the metallic ions refer to the quantity of adsorbed heavy metal ions. The metal ions equilibrium adsorption capacity (q_e), removal efficiency (RE%), distribution factor value (D), and selectivity value (S) in a multi component adsorption system can be calculated by using equations (1–4), respectively (Wang and Liu, 2014).

2.6 Design of experiment

Design of experiment (DOE) is a useful statistical tool, which can correlate the independent input parameters to system output (response). Applying DOE has some advantages including identifying the most effective parameters, elimination of the excessive cost of experiment, and optimization of process regarding achieving the highest yield (Bashir et al., 2010). Response Surface Methodology (RSM) is one of the applicable approaches of the DOE, which can make a relationship between inputs and output by correlating the empirical data on a quadratic function that is represented in equation (5). The resulting model’s accuracy and significance assessment can be investigated by analysis of variance (ANOVA) and other statistical criterions such as correlation coefficient (R²) and Average Absolute Relative Deviation (AARD %) values which can be calculated using equation (6) and equation (7), respectively (Zaferani et al., 2019).

2.7 Isotherm modeling

To study the heavy metal adsorption behavior of the unmodified/modified HCP samples with respect to competitive adsorption of the metallic ions in a multi component system, isotherm modeling was conducted aim to investigate the adsorbate-adsorbent interactions. To correlate the equilibrium uptake capacity to the equilibrium concentration of metallic ions in the solution, five isotherm models including Langmuir, Freundlich, Hill, Dubinin-Radushkevich (D-R), and Temkin models which are represented by equations (8–12), were utilized (Masoumi et al., 2021).

2.8 Adsorption process kinetics

The adsorption mechanism and the adsorbent's efficacy in removing metal ions have been investigated using kinetic models. Experimental adsorption data were fitted using two kinetic models: pseudo-first-order and pseudo-second-order, represented by equation (13)and equation (14), respectively (Masoumi et al., 2021).

3.1 Adsorbents characterization

The FTIR spectrum of the adsorbents in the wave number range of 500–4000 cm⁻¹ are illustrated in Fig. 3.a. Based on the FTIR results, the peak at 2912 cm⁻¹ is related to aliphatic C—H stretch of methylene group which was formed through cross linking of xylene molecules, the peak at 1495 cm⁻¹ corresponds to aromatic C⚌C stretches of xylene ring molecules. In the modified HCP FTIR spectra, the peak at 1710 cm⁻¹ refer to C⚌O stretches of the carboxylic acid group, this peak proves the successfully incorporation of carboxylic acid group to HCP sample structure. The wide peak in the range of 3200 cm⁻¹ to 3600 cm⁻¹ is related to the O—H bond stretch of the carboxylic acid group, also the O—H peak in the HCP sample spectra is related to the formation of hydroxyl group through hydrolysis of the methoxy group of remained formaldehyde dimethyl acetal molecules (Kim, 2020). Thermogravimetric analysis results for both polymeric samples are presented in Fig. 3.b. According to this figure, at temperatures ranging between 80 and 100 °C, a slight reduction (about 5 %) in the content of both adsorbents was observed, which could be related to the solvent or water evaporation. Rapidly decomposition of the samples between 370 and 470 °C may be attributed to the destruction of C—C bonds of the crosslinking bridges which were yielded from the Friedel-Crafts reaction, also the modified HCP’s more weight loss can be related to the decomposition of C—O bonds of the carboxyl group (Sel, 2015). The polymeric residues of the xylene based HCP and the modified HCP adsorbents were 39.82 wt% and 33.71 wt%, respectively, at 600 °C (Maleki et al., 2022). The porous properties of the xylene based HCP sample and the carboxylic acid functionalized HCP sample were evaluated by N₂ adsorption–desorption measuring at 77.3 K, which are illustrated in Fig. 4.a. Based on Fig. 4.a, for both type of samples the type II isotherm curve were observed based on IUPAC isotherm classification. This type of isotherm curve can be attributed to the existence of micropores and meso pores in samples structure (Huang, 2009). According to the figure, sharply Nitrogen adsorption by both samples at a relative pressure range between 0 and 0.05 can be related to the presence of micropores in the samples skeleton. Additionally, the hysteresis loop presence in the adsorption–desorption isotherm at the relative pressure between 0.2 and 0.8 refers to the presence of the mesopores in the samples structure, also the hysteresis loop in the relative pressure more than 0.8 demonstrate the existence of the macropores and interparticle voids in both type of the adsorbents network (Kong et al., 2019). The porous properties characteristics of both porous polymers were reported in Table 2. According to the table findings, the BET surface area of the xylene based HCP and the carboxylic acid modified HCP are 237.73 m²/g and 133.43 m²/g, respectively. Based on the result, the carboxylic acid functionalization of the HCP sample reduced the specific surface area of the HCP sample dramatically, it can be attributed to the partial space filing of the pores by carboxylic acid functional group (He, 2017). Fig. 4.b depicts the pore size distribution of the samples, according to this figure the pore size peak of the HCP sample is obtained around 4.25 nm and for the carboxylic acid modified HCP sample, two pore size peaks are obtained around 2.96 nm and 5.03 nm. The findings of the pore size distribution plot have good conformity with the hysteresis loop existence in N₂ adsorption–desorption isotherms, which refer to the presence of the macro pores in the structure of the samples. Based on the Fig. 4.b, the carboxylic acid group incorporation to the HCP sample skeleton increased the micropore volume of the modified HCP, it can be concluded that incorporation of the carboxylic acid to polymer network can separate the mesopores volume to multiple micropores (Li, 2017). The elemental composition of the polymeric samples were investigated by using EDS analysis and the findings of the analysis are depicted in Fig. 5. According to the EDS analysis results, the xylene based HCP adsorbent consist of 94.62 % carbon, 4.12 % oxygen, and 1.26 % chlorine element that can be related to the Lewis acid catalyst in the Friedel-Craft alkylation reaction, also the modified HCP sample contains 78.58 % carbon, 3.48 % manganese, 16.73 % oxygen, and 1.21 % chlorine. Increasing the oxygen content of the modified HCP adsorbent corresponds to the incorporation of carboxylic acid into the HCP sample structure. The XPS spectra of the unmodified/modified HCP samples in the range of 100 eV to 800 eV are illustrated in Fig. 6-a. based on this figure, the resulting peaks at 198.6 eV, 285.9 eV, and 533.1 eV are attributed to Cl 1s (1.09 %), C 1s (80.64 %), and O 1s (18.27 %), respectively. The ionic form of the chlorine element ( ${\mathrm{C}\mathrm{l}}^{-}$ ) may be left over from the Friedel-Crafts reaction (Sang et al., 2020). By considering the high-resolution spectrum of the C 1s element that is illustrated in Fig. 6-b, it can be derived that the mentioned peak dissociated to three peaks correspond to C—C/C⚌C bonds (284.6 eV), C—OH bond (286.5 eV), and COOH bond (289.4 eV). Fig. 6-c demonstrates that the O 1s spectrum has two unique peaks that are related to the carboxylic acid's C⚌O bond (532.7 eV) and the C—OH bond (533.6 eV) (Manakhov, 2023; Zhang, 2013; Classen, 2007; Beamson and Briggs, 1992; Song, 2021).

Close

Fig. 3

a) FT-IR spectra of the HCP and carboxylated HCP sample, b) TGA analysis results of the both sample.

Export to PPT

Close

Fig. 4

a) Nitrogen adsorption–desorption isotherms of the samples, b) Pore size distribution plots of the HCP samples.

Export to PPT

Close

Fig. 5

EDS pattern of the a) xylene based HCP, b) modified HCP sample.

Export to PPT

Close

Fig. 6

The carboxylic acid modified HCP’s XPS results, a) survey scan spectra, b) High resolution C 1s spectra, and c) High resolution O 1s spectra.

Export to PPT

3.2 RSM results

3.2.2

3.2.2 Adsorption models

The semi-empirical quadratic models for prediction of the adsorption capacity of the metallic ions in terms of operational condition were obtained based on the RSM-CCD technique. The adsorption models for prediction of adsorption capacity of the lead, cadmium, nickel, and copper ions by utilizing xylene based HCP are presented in equations (7–10), respectively. In addition, the adsorption models for the uptake of mentioned metallic ions by carboxylic acid modified HCP adsorbent are reported in equations (15–22), respectively. Furthermore, Fig. 7 (a-h) illustrate the contrast between the predicted heavy metal uptake capacities using the RSM-based model and the actual values. The figures indicate that by taking into account the adequate concentration of the predicted and the actual values adsorption capacity across the diagonal line, the superior accuracy of the presented semi-empirical models can be demonstrated.

(15)

$${\mathrm{q}}_{\mathrm{P}\mathrm{b}}=-40.27727+0.652332\ast \mathrm{A}+6.49363\ast \mathrm{B}+1.48448\ast \mathrm{C}+0.039000\ast \mathrm{A}\mathrm{B}+0.006656\ast \mathrm{A}\mathrm{C}+0.025875\ast \mathrm{B}\mathrm{C}-0.014517\ast {\mathrm{A}}^2-0.545567\ast {\mathrm{B}}^2-0.005298\ast {\mathrm{C}}^2$$

(16)

$${\mathrm{q}}_{\mathrm{C}\mathrm{d}}=-524.06385+12.00843\ast \mathrm{A}+64.87823\ast \mathrm{B}+2.50028\ast \mathrm{C}-0.287125\ast \mathrm{A}\mathrm{B}-0.031859\ast \mathrm{A}\mathrm{C}+0.061104\ast \mathrm{B}\mathrm{C}-0.099209\ast {\mathrm{A}}^2-3.64901\ast {\mathrm{B}}^2-0.007088\ast {\mathrm{C}}^2$$

(17)

$${\mathrm{q}}_{\mathrm{N}\mathrm{i}}=-485.35051+10.28783\ast \mathrm{A}+73.30781\ast \mathrm{B}+1.39564\ast \mathrm{C}-0.23208\ast \mathrm{A}\mathrm{B}-0.003153\ast \mathrm{A}\mathrm{C}+0.113104\ast \mathrm{B}\mathrm{C}-0.114150\ast {\mathrm{A}}^2-5.37443\ast {\mathrm{B}}^2-0.011220\ast {\mathrm{C}}^2$$

(18)

$${\mathrm{q}}_{\mathrm{C}\mathrm{u}}=-23.74753+3.10602\ast \mathrm{A}+34.67724\ast \mathrm{B}+2.13802\ast \mathrm{C}-0.036875\ast \mathrm{A}\mathrm{B}-0.008259\ast \mathrm{A}\mathrm{C}+0.032813\ast \mathrm{B}\mathrm{C}-0.035682\ast {\mathrm{A}}^2-1.73748\ast {\mathrm{B}}^2-0.008003\ast {\mathrm{C}}^2$$

(19)

$${\mathrm{q}}_{\mathrm{p}\mathrm{b}}=-68.33282+0.362097\ast \mathrm{A}+16.01616\ast \mathrm{B}+1.66074\ast \mathrm{C}+0.014750\ast \mathrm{A}\mathrm{B}-0.000450\ast \mathrm{A}\mathrm{C}+0.013000\ast \mathrm{B}\mathrm{C}-0.005754\ast {\mathrm{A}}^2-1.16871\ast {\mathrm{B}}^2-0.002746\ast {\mathrm{C}}^2$$

(20)

$${{(\mathrm{q}}_{\mathrm{C}\mathrm{d}})}^{0.15}=-1.09120+0.034829\ast \mathrm{A}+0.433140\ast \mathrm{B}+0.011813\ast \mathrm{C}+0.000291\ast \mathrm{A}\mathrm{B}-0.000035\ast \mathrm{A}\mathrm{C}+0.000423\ast \mathrm{B}\mathrm{C}-0.000413\ast {\mathrm{A}}^2-0.28186\ast {\mathrm{B}}^2-0.000079\ast {\mathrm{C}}^2$$

(21)

$${{(\mathrm{q}}_{\mathrm{N}\mathrm{i}})}^{0.45}=-24.45907+0.605728\ast \mathrm{A}+3.76580\ast \mathrm{B}+0.084171\ast \mathrm{C}+0.002274\ast \mathrm{A}\mathrm{B}-0.000190\ast \mathrm{A}\mathrm{C}+0.006087\ast \mathrm{B}\mathrm{C}-0.007139\ast {\mathrm{A}}^2-0.275649\ast {\mathrm{B}}^2-0.000714\ast {\mathrm{C}}^2$$

(22)

$${\mathrm{q}}_{\mathrm{C}\mathrm{u}}=-82.40925+2.33099\ast \mathrm{A}+12.46638\ast \mathrm{B}+0.811348\ast \mathrm{C}-0.188542\ast \mathrm{A}\mathrm{B}+0.012222\ast \mathrm{A}\mathrm{C}+0.075146\ast \mathrm{B}\mathrm{C}-0.016481\ast {\mathrm{A}}^2-0.465378\ast {\mathrm{B}}^2-0.005058\ast {\mathrm{C}}^2$$

Close

Fig. 7

Predicted versus actual heavy metal adsorption data utilizing HCP sample for a) lead, b) cadmium, c) nickel, d) copper, and utilizing carboxylic acid modified for e) lead, f) cadmium, g) nickel, h) copper.

Export to PPT

3.2.3

3.2.3 Effect of operational factors on adsorption process

3.2.3.1

3.2.3.1 Adsorption by xylene based HCP

To peruse the impact of operational factors including temperature, pH, and initial concentration of the solution on the metallic ions uptake capacity of the xylene based HCP adsorbent, perturbation plots are provided and illustrated in Fig. 8. In these plots, the A, B, and C curves indicate the impact of variation of the temperature, pH, and initial concentration on adsorption capacity. By default, the reference point of the perturbation plot was considered at the center point of the design space with a coded value equal to 0, the actual value of the A, B, and C terms at the reference point are 45 °C, 7, and 60 mg/l, respectively. According to Fig. 8, heavy metal ions adsorption by HCP adsorbent is strongly affected by increasing the initial concentration of metallic ions in the solution, but the effect of the other factors on uptake capacity is dependent on the adsorbed metal ion type. As can be seen in Fig. 8.a, the lead ion adsorption by HCP adsorbent is not sensitive to the temperature and pH alteration due to the A and B curves status which are near to the horizontally state. According to Fig. 8.b and Fig. 8.c, the cadmium and nickel uptake are deeply influenced by changing temperature and pH value, and the maximum adsorption capacity of these ions are obtained at a temperature of 45 °C and pH of 7. Based on Fig. 8.d, the copper adsorption capacity can increase by gaining pH value up to 9 at the reference point of temperature and initial concentration. To better understand of the mentioned factors effect on HCP adsorbent’s uptake capacity and investigation of the factors interaction, three dimensional response surfaces are provided based on RSM model that are illustrated in Fig. 9 and Fig. 10. The Fig. 9 depicts the HCP adsorbent’s uptake capacity dependency to temperature and pH at a constant initial concentration of 60 mg/l. Based on the figure, the maximum uptake capacity of lead ion can be obtained at about 87.5 mg/g at T = 50 °C and pH = 9, also for cadmium and nickel adsorption by HCP adsorbent, a similar condition is observed in which maximum adsorption capacity of these ions at T = 45 °C and pH = 7 are obtained 58.67 mg/g and 64.81 mg/g, respectively. According to Fig. 9.d, the highest adsorption capacity of copper ion at the initial concentration of 60 mg/g, can be reached at T = 45 °C and pH = 9. Based on the figures findings, the solution pH has a key role in the heavy metals adsorption capability of HCP adsorbent. In general, decreasing in solution’s pH causes a reduction in the uptake capacity of the metallic ions, this happens due to gaining in hydronium ions (H₃O⁺) concentration which result in improving the repulsion force between hydronium ions and metallic ions, also increasing the pH of the solution (pH greater than 9) causes the formation of metal hydroxide and decreases the adsorption capacity of HCP adsorbent (Qin, 2019). Fig. 10 depicts effect of the pH and initial concentration of metallic ions on the HCP uptake capability at a constant temperature of 45 °C. According to Fig. 10, at a constant pH value, increasing the initial concentration of the ions causes improving the uptake capacity of the solid sorbent, it can be attributed to the improving mass transfer driving force and diminished mass transfer resistance of the metallic ions diffusion into the HCP adsorbent’s cavities (Gonte and Balasubramanian, 2016). Based on the findings, the uptake capacity of the metallic ions by xylene based HCP sample at a similar condition are in descending order Pb²⁺> Cu²⁺> Ni²⁺> Cd²⁺.

Close

Fig. 8

Perturbation plots of the heavy metal uptake by xylene based HCP a) lead, b) cadmium, c) nickel, d) copper.

Export to PPT

Close

Fig. 9

3D response surfaces representing the HCP adsorbent’s uptake capacity dependency to the pH and temperature at initial concentration of 60 mg/l for a) Lead, b) Cadmium, c) Nickel, d) Copper.

Export to PPT

Close

Fig. 10

3D response surfaces representing the HCP adsorbent’s uptake capacity dependency to the pH and initial concentration at pH of 7, for a) Lead, b) Cadmium, c) Nickel, d) Copper.

Export to PPT

3.2.3.2

3.2.3.2 Adsorption by carboxylic acid modified HCP

Similar to xylene based HCP adsorbent, the heavy metal uptake capacity of the carboxylic acid functionalized HCP adsorbent is studied in this section. To investigate the effect of temperature, pH, and solution initial concentration on the uptake capacity of the functionalized HCP, perturbation plots are prepared and illustrated in Fig. 11. According to the Fig. 11.a, slightly gaining in lead ion uptake capacity by carboxylic acid modified HCP sample is observed. Based on Fig. 11.b and Fig. 11.c, the adsorption capacity of the cadmium and nickel ions are deeply affected by changing the temperature and pH value of the solution. Compare to xylene based HCP sample, the carboxylic acid modified HCP sample shows more adsorption capacity for cadmium and nickel ions at pH value more than 7, it can be related to the presence of the hydroxide ions (OH^–) in the solution which causes deprotonating the carboxylic acid groups and making this group as an active site for chelate formation or exchanging heavy metal ions (Ehgartner, 2021). According to these figures, increasing the pH value more than 8.5 will be decreased the adsorption capacity of the cadmium and nickel ions, it can be attributed to the formation of the metal hydroxide in the solution.

Close

Fig. 11

Perturbation plots of the heavy metal uptake by carboxylic acid modified HCP a) Lead, b) Cadmium, c) Nickel, d) Copper.

Export to PPT

Aim to show the modified HCP sample uptake capability dependency to the mentioned parameters and investigation of the factors interaction, the three dimensional response surfaces are illustrated in Fig. 12 and Fig. 13. The Fig. 12 indicates the effect of the temperature and pH on the uptake capacity at a constant initial concentration equal to 60 mg/l. According to Fig. 12.a, the highest uptake capacity of the lead ion (q = 88.6 mg/g) can be obtained at T = 35 °C and pH = 7.5, also similar is observed for copper ion adsorption capacity based on Fig. 12.d. Based on this figure, the maximum value of adsorption capacity of the copper ion (q = 87.9 mg/g) at the initial concentration of 60 mg/l can be obtained at T = 50 °C and pH = 8.5. Fig. 12.b and Fig. 12.c show that the highest adsorption capacity of the cadmium and nickel ions can be obtained at T = 45 °C and pH = 8.5 and T = 45 °C and pH = 8, respectively. Fig. 13 depicts the heavy metal uptake capacity dependency to the metal ions initial concentration and the solution’s pH value at a constant temperature of 45 °C. According to Fig. 13, by increasing the initial concentration of the solution, the uptake capacity of the metallic ions are increased dramatically due to improving the mass transfer driving force. Based on Fig. 13 results, it can be concluded that increasing the initial concentration of the solution to more than 80 mg/l slightly improves the adsorption capacity of the cadmium and the nickel ions, however, the lead and the copper ions uptake capacities will be increased sharply even at higher initial concentration, it can be related to the adsorbent saturation from the cadmium and the nickel ions at the concentration more than 80 mg/l (Vakili, 2019). Based on the results, the heavy metal ions uptake capacity of the carboxylic acid functionalized HCP sample at the reference point are in descending order Pb²⁺> Cu²⁺> Cd²⁺ > Ni²⁺.

Close

Fig. 12

3D response surfaces representing the carboxylic acid modified HCP adsorbent’s uptake capacity dependency to the pH and temperature at initial concentration of 60 mg/l for a) Lead, b) Cadmium, c) Nickel, d) Copper.

Export to PPT

Close

Fig. 13

3D response surfaces representing the carboxylic acid modified HCP adsorbent’s uptake capacity dependency to the pH and initial concentration at pH of 7, for a) Lead, b) Cadmium, c) Nickel, d) Copper.

Export to PPT

3.3 Heavy metal adsorption from steel industry waste water

To investigate the potential of the HCP adsorbents for the removal of the heavy metal ions from steel industry waste water, metallic ions uptake experiments were conducted at the optimum pH and temperature condition by utilizing the xylene based HCP and the carboxylic acid modified HCP samples. The waste water’s characteristics are summarized in the Table 6. Based on the table. the pH value of the waste water is higher than optimum value, so it was adjusted to the optimum pH with the help of the HCl solution (0.05 M), followed by performing adsorption test. The results of measuring the removal efficiency of the metallic ions uptake are illustrated in Fig. 14. based on this figure, the carboxylic acid modified HCP sample exhibited more affinity to adsorb metallic ions than HCP sample except in the case of nickel adsorption, which is the opposite result.

Close

Fig. 14

Removal efficiency of the metallic ions uptake from steel industry waste water.

Export to PPT

3.4 Adsorption equilibrium study and isotherm modeling

To obtain the isotherm models’ parameters, the models were fitted on the equilibrium adsorption data that obtained at the temperature of 35 °C, pH of 7, and initial concentration between 20 and 100 mg/l. The models’ fitted curves were graphically illustrated in Fig. 15 and Fig. 16 for the xylene based HCP and the carboxylic acid modified HCP sample, respectively. Furthermore, the parameters of the models and their correlation coefficients (R²) were reported in Table 7. Based on the results presented in Table 7, the Freundlich model's K_F constant, which reflects the interaction strength between the adsorbed particles and the solid sorbent, indicates improving the lead, cadmium, and copper ions tendency to be adsorbed by the carboxylic acid modified adsorbent, but a relative decrease for nickel ion uptake by the modified HCP can be observed. The range of the Freundlich constant n_f, between 1 and 10, indicates that both adsorbents are suitable for the adsorption of metallic ions (Soltani et al., 2015). Furthermore, the dominant physical adsorption of the metallic ions by both adsorbents is shown by the E value being less than 8 kJ/mol, as determined by the Dubinin-Radushkevich model. According to the Hill isotherm model’s q_m values which shows the best fitting ability for correlating experimental data, the adsorption tendency of metallic ions follows the sequence of lead > copper > cadmium > nickel. The sequence of the metal ions adsorption and lead selective removal can be explained as follows: (I) the larger electronegativity difference between lead (2.33) and nickel (1.91), cadmium (1.69), and copper (1.90), makes the lead as the best candidate to be attracted by adsorbent surface (Taha et al., 2016). (II) The ionic radius of the metallic ions are in a descending order lead (1.33 Å) > cadmium (1.09 Å) > copper (0.87 Å) > nickel (0.83 Å). Metal with a larger ionic radius show more polarizability and more tendency to be adsorbed by heterogeneous surface (Bayo, 2012). (III) The hydrated radius of the lead, nickel, copper, and cadmium ions are 4.01 Å, 4.04 Å, 4.19 Å, and 4.26 Å, respectively. Metals with lower hydrated radii reveal more affinity to be adsorbed due to their easier diffusion into adsorbent pores. (IV) The enthalpy of hydration of the mentioned cations follows the sequence of lead (−1481 kJ/mol) < cadmium (−1807 kJ/mol) < copper (−2100 kJ/mol) < nickel (−2105 kJ/mol). Metal with higher enthalpy of hydration tend to be as hydrated and show lower affinity to be adsorbed, therefor the lead ion with the lowest enthalpy of hydration among other metallic ions preferentially adsorbed by solid sorbent (Panayotova and Velikov, 2003).

Close

Fig. 15

Comparison of isotherm models and experimental values of adsorption capacity of xylene based HCP at 35 °C and pH = 7 for uptake of (a) lead (b) cadmium, (c) nickel, (d) copper.

Export to PPT

Close

Fig. 16

Comparison of isotherm models and experimental values of adsorption capacity of carboxylic acid modified HCP at 35 °C and pH = 7 for uptake of (a) lead (b) cadmium, (c) nickel, (d) copper.

Export to PPT

As a result, the above mentioned reasons are not able to determine individually the adsorption selectivity in a multi component system, so the simultaneously effect of the aforementioned explanations causes the heavy metal uptake sequence of Pb(II) > Cu(II) > Cd(II) > Ni(II). Similar result about adsorption sequence in a multi component system was reported by Bayo.J (Bayo, 2012).

3.5 Adsorption process kinetics

By fitting the kinetic models on the experimental heavy metal adsorption data, collected at the initial concentration of 100 mg/l, pH of 7, and different temperatures such as 25 °C and 35 °C, the models’ parameters were obtained that are reported in Table 8. According to the Table 8, it is evident that the R² values more than 0.99 derived from the pseudo-second-order model were higher than those of the pseudo-first-order adsorption model. Furthermore, the estimated qe values derived from the pseudo second-order model showed a strong correlation with the actual values (q_exp), indicating the reliability of the model for the adsorption system being studied. According to the proper fit of both kinetic models, it can be concluded that the adsorption process utilizing both HCP samples were occurred through chemical and physical adsorption mechanism simultaneously (Yang et al., 2018). The values of k₂ (Table 7) also suggested that the adsorption rates followed the sequence of Ni²⁺> Cd²⁺> Cu²⁺> Pb²⁺. The fitted adsorption data and the predicted values ​​based on the kinetic models are illustrated in Fig. 17-a, and Fig. 17-b for the HCP sample, and the carboxylic acid modified HCP sample, respectively.

Close

Fig. 17

Comparison of kinetic models and experimental values of adsorption capacity of lead, cadmium, nickel, copper at 35 °C and pH = 7 by (a) xylene based HCP, (b) carboxylic acid modified HCP.

Export to PPT

3.6 Comparison of the samples efficiency with similar studies

In this section, a comparison between the present work and similar studies on the heavy metal adsorption from multi component systems using porous polymeric adsorbents, was done. The results of the literature review are summarized in Table 9. As presented in the Table 7, the xylene based HCP and the modified HCP sample show high overal uptake capacities of q = 408.04 mg/g and q = 417.04 mg/g, respectively. By comparing the results of the present study with other works in a similar operating condition, it can be concluded that both types of HCP adsorbents show high performance in simultaneously heavy metal adsorption in a multi component system.

3.7 Thermodynamic modelling

In engineering processes, thermodynamic concepts such as entropy and Gibbs free energy are employed to assess the spontaneity of a process. Additionally, thermodynamic parameters can provide insight into the adsorption process's nature, particularly whether it involves an endothermic or exothermic. In the present research, the thermodynamic analysis of the heavy metal adsorption process was performed by computing the thermodynamic parameters, such as Gibbs free energy (ΔG°), entropy (ΔS°), and enthalpy (ΔH°) at a pH value of 7, an initial concentration of 60 mg/l, and the temperature between 30 °C and 60 °C, using equations (23–25).

Close

Fig. 18

Van’t Hoff’s plots for adsorption of lead, cadmium, nickel and copper ions by (a) HCP adsorbent (b) carboxylic acid modified HCP.

Export to PPT

3.8 Regeneration efficiency of adsorbents

From an economic perspective, the reusability of the adsorbent is a critical factor for industrial applications. To assess the recyclability of the adsorbents, ten cycles of adsorption were performed at a temperature of 45 °C and a pH value of 7 using both types of adsorbents. To regenerate the spent samples, the adsorbents were subsequently mixed with 0.05 M hydrochloric acid (HCl) and stirred for 2 h. The results of the recycling efficiency are presented in Fig. 19. According to this figure, the adsorption potential of carboxyl group-modified HCP for adsorbing lead, cadmium, nickel, and copper decreased by 4.5 %, 2.1 %, 1.2 %, and 3.13 %, respectively. Additionally, the adsorption potential of xylene-based HCP for adsorbing lead, cadmium, nickel, and copper reduced by 2.66 %, 1.26 %, 1.2 %, and 1.96 %, respectively, after ten cycles. Based on the results, both types of adsorbents could be considered as high-value adsorbents for industrial applications due to the slight decrease in their efficiency.

Close

Fig. 19

Regeneration efficiency of (a) xylene based HCP, and (b) carboxylic acid modified HCP sample.

Export to PPT

3.9 Adsorption mechanism

The heavy metal ions adsorption by the HCP sample is affected by several factors, such as the chemical properties of the sorbent and the physicochemical characteristics of the metal ions. The interaction between the sorbent and the metal ions occurs through various mechanisms, such as coordination, ion exchange, and electrostatic attraction (Wu, 2013; Zare-Dorabei et al., 2016). Specifically, the HCP network exhibits a chelating interaction between its carboxyl groups and the heavy metal ions, leading to forming the stable complexes. Additionally, the benzene ring in the adsorbent structure participates in a cation-π interaction with the metal ions, enhancing the adsorption efficiency. Moreover, the HCP network's porous structure, which includes micro-, meso-, and macropores, provides a large surface area for the uptake of metallic ions. The HCPs are primarily composed of microporous material due to the presence of benzene co-monomers. This allows the multiple benzene rings of HCPs to generate strong cation-π interactions with metal ions. The heavy metal adsorption mechanisms including cation-π interaction and complexation are graphically illustrated in Fig. 20.

Close

Fig. 20

Cation-π interaction and complexation mechanisms of the divalent heavy metal adsorption.

Export to PPT