Efficient adsorption of Europhtal onto activated carbon modified with ligands (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine (M) and (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine (SCH-4); response surface methodology

Abstract

Porous activated carbon was modified with (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine (M) and (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine (SCH-4). Characterization was performed using different techniques such as SEM, FTIR and ¹H NMR analysis. Then, these new adsorbents were used for the efficient removal of Europhtal (sulfonated cobalt phthalocyanine). Response surface methodology (RSM) was used to achieve operational conditions for the efficient removal of Europhtal using an activated carbon modified with novel ligands. To investigate the individual effect of variables involved and the effect of their possible interactions on the adsorption process, several systematic experiments were designed by varying the variables such as pH, sonication time, adsorbent dosage and dye concentration in the range of 2.0–11, 2–8 min, 0.005–0.035 g and 4–30 mg L⁻¹, respectively. The optimum values of pH, sonication time, adsorbent dosage and Europhtal concentration were found to be 7, 2 min, 30 mg L⁻¹ and 0.03 g for the sample modified with M and 7, 5 min, 30 mg L⁻¹ and 0.03 g for the one modified with SCH-4, at which the removal percentages were more than 98%. Adsorption equilibrium data were well fitted to the Freundlich model with maximum adsorption capacities of 56.69 and 17.84 mg g⁻¹ for M and SCH-4, respectively. The study of adsorption kinetics indicated that the dye uptake process follows the pseudo second-order and saturation type rate expressions for each ligand.

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1 Introduction

The effluents composed of synthetic dyes are liberated from different sources to the environment. Dyes are widely utilized in various industrial activities such as coloring, paper, textiles, cosmetics and food.^1–3 Their complex (aromatic) molecular structures cause an increase in their resistance to heat, oxidizing agents and biodegradation. Most dyes are toxic and harmful to some microorganisms.⁴ The toxicity of dyes on plant, animal and human health is not considerably known.^5,6 Several versatile approaches, including ion exchange, photo-oxidation processes, flocculation, precipitation, microbiological decomposition and ozonation, have been applied for wastewater treatment and water quality enhancement. Low cost and easy-to-prepare adsorbents are the best candidates for wastewater treatment and removal of pollutants via the adsorption process.^7–14 The adsorption process provides an attractive alternative for water treatment because it is easy to handle, requires less maintenance and produces smaller amounts of sludge.^5,15–17 Most adsorbents have a high abundance and are cheap; however, they have drawbacks such as poor mechanical strength and heat resistance and relatively limited adsorption capacity for dyes.^18–20 Thus, the modification of adsorbents and improvement of their properties, viz reactive centers and surface area, is in great demand. A catalyst solution comprised of 30% of sulphonated cobalt phthalocyanine (Europhtal under the name 802) is generally used to remove mercaptan from hydrocarbon based on either its solution or liquid catalyst loaded on an activated carbon. This catalyst is extensively applied in numerous refineries worldwide and it is necessary to develop novel materials and methods for its removal from various media. In this regard, development and search for novel adsorbent, especially nanoscale adsorbents, is a good option to improve the characteristic performance of the removal procedure. Abundant reactive centers and sites on the surface of nanomaterials make them suitable for trapping other species. Activated carbon-based adsorbents with unique porous reactive sites in the presence of versatile reaction centers, such as hydroxide, carboxylic and amide groups, are good materials for subsequent modification through hydrogen bonding, π–π bonding and electrostatic reactions. Various organic reagents and chelating agents can be used for the modification of the surface of such adsorbents, leading to more improvement in the performance of the removal process.

The present study focuses on the synthesis of (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine (M) and (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine (SCH-4) and their subsequent loading on AC followed by characterization by SEM, FTIR and ¹H NMR. Then, the potential feasibility of M- and SCH-4-loaded AC for Europhtal removal (as a response) was investigated and the influences of various variables on the response were investigated and optimized. Statistical analysis was applied on the experimental data and results to make a predictive model and test the validity. The kinetic and equilibrium data were analyzed to understand the adsorption mechanism by applying different models to fit the experimental data.

2 Materials and methods

2.1 Materials and instruments

The Europhtal (Table 1) was purchased from Sigma Aldrich and its stock solution (100 mg L⁻¹) was prepared in double distilled water. The concentration of Europhtal was determined by obtaining and analyzing the absorbance spectra before and after each experiment by a 2800-DR (UV-Vis) spectrophotometer at 664 nm. An ultrasonic bath with a heating system (Bandelin sonorex) at a frequency of 40 kHz with a power of 130 W was used for the ultrasound-assisted adsorption procedure. For pH measurements, a Metrohm pH meter model-780 was used. The morphology and size distribution of the M- and SCH-4-loaded AC were observed by scanning electron microscopy (SEM; Hitachi S-4160) at an acceleration voltage of 15 kV. The FTIR spectra of all the compounds were recorded using a JASCO-FTIR-480 instrument in the range of 4000–400 cm⁻¹ using samples prepared in a KBr disc.

Table 1 Properties of the Europhtal dye

Product Identification
Trade name	Europhtal Additive 802
Appearance	Blue liquid solution
Chemical name or generic name	Sulfonated cobalt phthalocyanine (sodium salt)
Molecular structure
Chemical formula	C32H16−iN8Co(SO3Na)i
CAS number	90294-83-0
EINECS number	290-971-9
Shelf life	Not limited when stored in normal conditions: (temperature: 5 to 40 °C; relative humidity: less than 90%)
Maximum wavelength (λmax), nm	664 nm
USES	Desulphuration catalyst for oils
Physical state	Liquid
Colour	Blue
pH	7.0 to 9.0
Decomposition temperature	>300 °C
Relative density (water)	1.15
Solubility	Soluble in water
Acute toxicity	LD 50 > 2000 mg kg^−1 (oral rat)
Hazardous reaction with	None under normal conditions of use

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2.2 Ultrasound-assisted adsorption method

A batch process was used to evaluate the performance of Europhtal adsorption onto M- and SCH-4-loaded AC in the presence of ultrasound. Adsorption experiments were performed in a cylindrical glass vessel by adding 0.02 g of either of the adsorbents into 50 mL of Europhtal solution at known concentrations (4 and 30 mg L⁻¹) at pH 7, which was optimized by CCD. The vessel was immersed in an ultrasonic bath for 2.8 min at room temperature. Subsequently, the solutions were analyzed for determining the final Europhtal concentration using a UV-Vis spectrophotometer set at a wavelength of 664 nm. The experiments were also performed in the initial dye concentration range of 5–40 mg L⁻¹ to obtain adsorption isotherms. The dye removal percentage (R%) was calculated using the following equation:where C₀ and C_e (mg L⁻¹) are the initial and equilibrium dye concentrations in an aqueous solution, respectively. The amount of adsorbed dye (q_e (mg g⁻¹)) was calculated by the following mass balance relationship:where V (L) is the volume of the solution and W (g) is the mass of the adsorbent.

2.3 Preparation of AC from acorn

The seed shells correspond to approximately 25% of the whole acorn. To separate shell from seeds, about 2 kg of acorn was cooked in a 1.0 L glass beaker for 2 h. Then, the acorn seeds were immersed in 2 L of deionized water and boiled for two more hours to remove the water soluble phenolic compounds and to avoid their release during the adsorption experiments. Subsequently, the acorn wastes were washed with distilled water and dried at 70 °C in an air-supplied oven for 8 h. These were then ground in a disk-mill and sieved (70–100 mesh). The sieved mass was then subjected to a carbonization process in an argon atmosphere at a temperature increase rate of 5 °C min⁻¹ to the final temperature of 500 °C and maintained at that temperature for 1 h. The mass was then cooled and washed thoroughly several times by distilled water and dried.²¹

2.4 Synthesis of (1E,2E)-1,2-bis(pyridin-4-ylmethylene)-hydrazine (M)

1 mL (11 mmol) of hydrazine was added dropwise to a solution of isonicotinaldehyde (2.35 mL, 22 mmol) dissolved in ethanol (15 mL). Two drops of acetic acid were added and the mixture was stirred at room temperature for 24 h. The yellow solid that formed (yield = 73%) was filtered and washed several times with ethanol/ether (1 : 1) (see Fig. 1(a) for the synthesis procedure).

Fig. 1 Procedure for the synthesis of (a) (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine and (b) (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine.

2.5 Synthesis of (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine (SCH-4)

1 mL (11 mmol) of hydrazine was added dropwise to a solution of nicotinaldehyde (2.4 mL, 20 mmol) dissolved in ethanol (15 mL). Two drops of formic acid were added and the mixture was stirred at room temperature for 24 h. The product (yield = 73%) was filtered and washed in the same way as that for ligand M (see Fig. 1(b) for the synthesis procedure).

2.6 Central composite design (CCD)

To reduce the number of experiments required, a standard CCD (with 31 experiments) under RSM was applied. It helped us to determine regression model equations and operating conditions at which the maximum R% (as response) is achieved.^22–24 Moreover, the advantage of this procedure is the possibility of finding the effects of interaction between each pair of the variables involved, such as pH (x₁), sonication time (x₂), adsorbent dosage (x₃) and Europhtal concentration (x₄), on the response (R%) in addition to their individual effects. As mentioned above, two materials were used as an adsorbent, namely, M- and SCH-4-loaded activated carbon. The ranges of variation were set to 2–11 for pH, 2–8 min for sonication time, 0.005–0.035 g for adsorbent amount, and 4–30 mg L⁻¹ for Europhtal concentration (see Tables 2 and 3). CCD has been discussed in detail in the literature.^13,19,25 In general, the following equation may be used to plot the experimentally obtained response versus the variables involved.

Table 2 Factors in CCD and their levels

Factors	Levels
	Low (−1)	Central (0)	High (+1)	−α	+α
x 1: pH	4.25	6.5	8.75	2	11
x 2: sonication time (min)	3.5	5	6.5	2	8
x 3: adsorbent (g)	0.0125	0.02	0.0275	0.005	0.035
x 4: dye concentration (mg L^−1)	10.5	17	23.5	4	30

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Table 3 Experimental runs and responses

Run	x 1	x 2	x 3	x 4	M	SCH-4
					Europhtal R%	Europhtal R%
1	8.75	3.5	0.0125	10.5	70.059	73.548
2	8.75	3.5	0.0125	23.5	56.111	62.209
3	4.25	6.5	0.0275	23.5	87.209	95.312
4	4.25	3.5	0.0275	10.5	94.230	94.820
5	8.75	6.5	0.0125	10.5	76.219	64.400
6	6.50	8.0	0.0200	17.0	92.277	90.079
7	11.0	5.0	0.0200	17.0	64.070	70.370
8	4.25	3.5	0.0275	23.5	90.285	86.140
9(C)	6.50	5.0	0.0200	17.0	90.079	91.320
10	2.00	5.0	0.0200	17.0	78.930	84.830
11	6.50	2.0	0.0200	17.0	88.537	76.404
12	8.75	3.5	0.0275	23.5	93.181	88.251
13	8.75	3.5	0.0275	10.5	95.977	92.215
14	6.50	5.0	0.0050	17.0	48.863	65.640
15(C)	6.50	5.0	0.0200	17.0	89.030	90.706
16	6.50	5.0	0.0350	17.0	95.454	93.632
17	4.25	6.5	0.0125	23.5	65.934	72.012
18	8.75	6.5	0.0125	23.5	52.189	82.954
19	6.50	5.0	0.0200	4.00	97.159	95.238
20(C)	6.50	5.0	0.0200	17.0	90.545	84.980
21	6.50	5.0	0.0200	30.0	84.120	94.495
22	4.25	3.5	0.0125	10.5	81.976	90.384
23(C)	6.50	5.0	0.0200	17.0	90.545	90.944
24	8.75	6.5	0.0275	23.5	94.525	97.413
25	4.25	6.5	0.0275	10.5	95.348	90.909
26	4.25	3.5	0.0125	23.5	64.102	60.240
27	4.25	6.5	0.0125	10.5	79.6511	79.084
28(C)	6.50	5.0	0.0200	17.0	85.144	85.433
29(C)	6.50	5.0	0.0200	17.0	85.144	88.432
30(C)	6.50	5.0	0.0200	17.0	85.144	88.076
31	8.75	6.5	0.0275	10.5	94.827	90.797

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In this equation, β₀, β_i, and β_ii are constant coefficients corresponding to zero-, first- and second-order terms, respectively. β_ij is the coefficient of interaction term. Depending on the extent to which each term plays a role this model may be varied.

2.7 Model fitting and statistical analysis

The CCD-based experimental data were analyzed using Minitab software version 17.0 to be fitted to a second-order polynomial equation, and then the regression coefficients were obtained. The analysis of variance (ANOVA) was performed to justify the significance and adequacy of the developed regression model. The adequacy of the model was also evaluated by calculating the determination coefficient (R²) in addition to testing it for the lack of fit.

3 Results and discussion

3.1 Characterization of adsorbent

The FTIR and ¹H NMR spectra of the ligand M were acquired and are shown in Fig. 2. The FTIR absorption band at 1628 cm⁻¹ for the imine factor, ν_C=N, confirms the formation of the Schiff-base ligand. Some important absorption bands of the ligands are given in Table 4 and are in good agreement with the reported ones.^26,27

Fig. 2 FT-IR and ¹H NMR spectra of ligand (1E,2E)-1,2-bis(pyridin-4ylmethylene)hydrazine (M).

Table 4 Wavenumbers (cm⁻¹) of the vibrational modes of ligand

Ligand	ν C=N imine	ν C=N imine	ν C–N, νC–N aromatic
M	2941.17	1628.18	1595.39, 1552.48, 1417.97
SCH-4	2925.95	1625.28	1586.23, 1482.09, 1417.00

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Fig. 3 shows the FTIR and ¹H NMR spectra of the ligand SCH-4. The absorption band appears around the frequency of 1625 cm⁻¹ in the FTIR spectrum of the ligand SCH-4, which corresponds to ν_C=N, confirming the formation of the Schiff-base ligand. Detailed absorption bands of this ligand are given in Table 4, which are consistent to those reported elsewhere.²⁸

Fig. 3 FT-IR and ¹H NMR spectra of ligand (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine (SCH-4).

The morphology of the non-modified AC (Fig. 4(a)) and that of the modified one with either ligand M (Fig. 4(b)) or SCH-4 (Fig. 4(c)) were characterized by SEM, which showed that the approximate particle size was in the range of 40–80 nm. The surface modification is clearly seen after the deposition of ligands onto AC (Fig. 4(b) and (c)).

Fig. 4 SEM of AC (a) non-modified and (b) modified by (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine (M) and (c) modified by (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine (SCH-4).

3.2 Central composite design (CCD)

A total of 31 experiments (runs) were designed by Minitab software version 17.0. The tests for the significance of the regression model for both responses were evaluated and the results of ANOVA tests are presented in Table 5. From ANOVA, the factors with p-values less than 0.05 were distinguished to be significant. The p-values (0.343 and 0.0554 for M and SCH-4 data, respectively) that correspond to the lack of fit for the models applied indicated a non-significant lack of fit. However, the model for predicting R% of Europhtal, corresponding to either M- or SCH-4-based adsorbent, was reconstructed to filter out non-significant factors. Thus, the predictive models were applied as follows:

R%_Europhtal = 64.9 + 5.49x₁ + 2228x₃ − 2.416x₄ − 0.827x₁² − 71468x₃² + 179.7x₁x₃ + 69.7x₃x₄, for M-loaded AC (4)

R%_Europhtal = 114.3 + 0.25x₁ − 1.77x₂ + 1761x₃ − 5.92x₄ − 0.584x₁² − 0.686x₂² − 43475x₃² + 0.0322x₄² + 0.2195x₁x₄ + 0.491x₂x₄ + 36.4x₃x₄, for SCH-4-loaded AC (5)

Table 5 Analysis of variance (ANOVA)

Source of variation	M					SCH-4
	Sum of square	Df^a	Mean square	F-value	P-value	Sum of square	Df^a	Mean square	F-value	P-value
a Degree of freedom.
x 1	127.729	1	127.729	17.9149	0.005484	88.295	1	88.295	12.9980	0.011295
x 1 ^2	500.791	1	500.791	70.2394	0.000157	249.621	1	249.621	36.7469	0.000914
x 2	2.319	1	2.319	0.3253	0.589131	114.514	1	114.514	16.8577	0.006316
x 2 ^2	8.404	1	8.404	1.1787	0.319299	68.180	1	68.180	10.0368	0.019366
x 3	3565.419	1	3565.419	500.0748	0.000001	1785.555	1	1785.555	262.8525	0.000004
x 3 ^2	462.131	1	462.131	64.8171	0.000196	171.012	1	171.012	25.1747	0.002410
x 4	511.799	1	511.799	71.7833	0.000148	45.679	1	45.679	6.7245	0.041037
x 4 ^2	10.297	1	10.297	1.4443	0.274728	53.054	1	53.054	7.8101	0.031384
x 1 x 2	1.491	1	1.491	0.2092	0.663497	11.573	1	11.573	1.7037	0.239624
x 1 x 3	147.147	1	147.147	20.6383	0.003921	25.270	1	25.270	3.7200	0.102025
x 1 x 4	0.423	1	0.423	0.0593	0.815772	164.866	1	164.866	24.2701	0.002640
x 2 x 3	0.769	1	0.769	0.1079	0.753718	0.055	1	0.055	0.0081	0.931297
x 2 x 4	3.634	1	3.634	0.5097	0.502087	366.998	1	366.998	54.0260	0.000325
x 3 x 4	184.878	1	184.878	25.9304	0.002239	50.325	1	50.325	7.4084	0.034559
Lack-of-fit	101.970	10	10.197	1.4302	0.342826	263.462	10	26.346	3.8784	0.055384
Pure error	42.779	6	7.130			40.758	6	6.793
Total	5660.113	30				3483.314	30

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Positive and negative signs in each equation imply synergistic and antagonistic effect of the variables, respectively. The values of determination coefficient (R²) were found to be 0.985 and 0.968 for the Europhtal removal percentage using M- and SCH-4-loaded AC, respectively, confirming the good agreement between the experimental and predicted values.

3.3 Response surface methodology

The curvature natures of Fig. 5 and 6 show the response surface plots of removal (%) and confirm a strong interaction among the variables.

Fig. 5 Response surfaces for the Europhtal removal by M-loaded AC: (a) x₁–x₃ and (b) x₃ and x₄.

Fig. 6 Response surfaces for the Europhtal removal by SCH-4-loaded AC: (a) x₁–x₄; (b) x₂–x₄ and (c) x₃ and x₄.

Fig. 5(a) and 6(a) strongly support the fact that elevating the adsorbent amount and dye concentration leads to an enhancement in the Europhtal removal percentage. An increase in the dye concentration leads to a decrease in the removal percentage. The low and high initial dye concentrations exhibit an opposite trend. The small ratio of solute concentrations to non-occupied reactive adsorbent sites causes an acceleration of dye adsorption and a subsequent increase in removal percentage, while the saturation of adsorption sites is associated with a reduction in the removal percentage at a higher amount of target compounds. The opposite correlation is exhibited for the initial Europhtal concentration; the removal percentage and actual amount of adsorbed Europhtal clearly indicate that the adsorption of Europhtal depends on its initial concentration.

Fig. 5(b) shows the combined effect of adsorbent and pH on the adsorption of Europhtal onto adsorbent at a constant Europhtal initial concentration (30 mg L⁻¹). Increasing pH leads to a significant influence on the removal percentage, and at a constant pH, removal percentage increases with the amount of M. The Europhtal removal decreases with lower amounts of adsorbent, and an approximately constant trend was seen with higher amounts of adsorbent.

The increase in the amount of adsorbent leads to a significant decrease in sonication time (Fig. 6(b)), while the removal percentage at a fixed sonication time has a positive correlation with the amount of adsorbent.

Fig. 6(d) shows the interaction of pH with initial Europhtal concentration and their relation with removal percentage. The pH has a positive correlation with removal percentage, while a higher pH and lower adsorbent concentration lead to the achievement of a lower removal percentage. At a low initial pH, the protonation of the functional groups of adsorbent leads to the generation of a positive charge and appearance of strong attractive forces between the Europhtal molecule and adsorbent surface, thus increasing the removal percentage.

3.4 Optimization

The profile for a desirable option with predicted values in the Minitab software was used for the optimization of the process (Fig. 7). The profile for desirable responses was chosen after specifying the DF for each dependent variable (removal percentage) by assigning predicted values. The scale in the range of 0.0 (undesirable) to 1.0 (very desirable) was used to obtain a global function (D) that its maximum (100%) value related to Europhtal adsorption was achieved in this study.

Fig. 7 Profiles for predicated values and the desirability function for the removal percentage of Europhtal by ligands M and SCH-4. Dashed line indicates the current values after optimization.

The maximum removal (>98.8%) was obtained at the optimum conditions set as follows: 2 min, 0.03 g of adsorbent, initial Europhtal concentration of 30 mg L⁻¹ at pH 7 using AC-M and 5 min, 0.03 g of adsorbent, initial Europhtal concentration of 30 mg L⁻¹ at pH 7 using AC-SCH-4. The validity of duplicate assenting experiments at the optimized value of all parameters was investigated. The results were closely correlated with the data obtained from the optimization analysis using CCD.

3.5 Adsorption equilibrium study

Common isotherm models, such as Langmuir, Freundlich, Temkin and Dubinin–Radushkevich (D–R), are generally applied to discuss the equilibrium characteristics of adsorption processes.^29–32 The constant parameters of the isotherms equations for this adsorption process and their correlation coefficient (R²) are summarized in Table 6. Based on the linear form of the Langmuir isotherm model²⁹ (according to Table 6), the values of K_L (the Langmuir adsorption constant (L mg⁻¹)) and Q_max (theoretical maximum adsorption capacity (mg g⁻¹)) were obtained, respectively, from the intercept and slope of the plot of C_e/q_e vs. C_e.

Table 6 Various isotherm constants and correlation coefficients calculated for the adsorption of Europhtal dye onto ligands

Isotherm	Equation	Plot	Parameters	M	SCH-4
				0.03 g	0.02 g
Langmuir	C e/qe = 1/KaQm + Ce/Qm	A Plot Ce/qe versus Ce should indicate a straight line of slope 1/Qm and an intercept of 1/(KaQm)	Q max (mg g^−1)	56.69	17.84
			K a (L mg^−1)	6.563	19.33
			R ^2	0.983	0.978
Freundlich	ln qe = ln KF + (1/n)ln Ce	The values of Kf and 1/n were determined from the intercept and slope of linear plot of ln qe versus ln Ce, respectively	1/n	0.283	0.339
			K F (L mg^−1)	4.539	3.217
			R ^2	0.992	0.994
Temkin	q e = B1 ln KT + B1 ln Ce	Values of B1 and KT were calculated from the plot of qe against ln Ce	B 1	7.019	3.244
			K T (L mg^−1)	184.32	87.28
			R ^2	0.918	0.971
Dubinin–Radushkevich	ln qe = ln Qs − Kε^2	Values of Qm and B were calculated from the plot of ε^2 against ln qe	Q s (mg g^−1)	38.63	14.48
			B × 10^−8	1.10	1.70
			E	6742	5423
			R ^2	0.797	0.933

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The parameters of the Freundlich isotherm model,³⁰ such as K_F and 1/n (the capacity and intensity of the adsorption), were calculated from the intercept and slope of the linear plot of ln q_e versus ln C_e, respectively (see Table 6). The values of 1/n were found to be 0.283 and 0.3385 corresponding to M-AC and SCH-4-AC, which show the high tendency of the adsorbents for the adsorption of Europhtal. Furthermore, the high R² values (0.992 and 0.994 for M-AC and SCH-4-AC, respectively) show the suitability of Freundlich isotherm model for fitting the experimental data over the whole concentration range.

The heat of adsorption and the adsorbent–adsorbate interaction were evaluated using the Temkin isotherm model.³³ B represents the Temkin constant related to the heat of the adsorption (J mol⁻¹), T is the absolute temperature (K), R is the universal gas constant (8.314 J mol⁻¹ K⁻¹) and K_T is the equilibrium binding constant (L mg⁻¹). R² values obtained from applying this model are lower than that corresponding to the Freundlich and Langmuir models. Therefore, it is concluded that the Freundlich isotherm represents a better fit to the experimental data than the Langmuir isotherm.

Another commonly used model to explain equilibrium data for the adsorption onto a surface in liquid phase is Dubinin–Radushkevich (D–R),³² which is used to estimate the porosity apparent free energy and the characteristics of adsorption. In the D–R isotherm, K (mol² (kJ²)⁻¹) is a constant related to the adsorption energy, Q_max (mg g⁻¹) is the theoretical saturation capacity, and ε is the Polanyi potential. The slope of the plot of ln q_e versus ε² gives K and the intercept yields the Q_m value. The mean free energy of the adsorption (E) is known as the free energy change when the energy required to transfer one mole of ion from the solution to the surface of the adsorbent is calculated. The value of the correlation coefficient obtained from the D–R model was considerably lower than that from the other isotherms mentioned above. This means that the D–R equation represents the poorer fit to the experimental data compared to the other isotherm equations.

3.6 Kinetics study

To examine the mechanism of the adsorption process such as mass transfer and chemical reaction, a suitable kinetic model is needed to analyze the rate data. Many models, such as homogeneous surface diffusion model, pore diffusion model and heterogeneous diffusion model (also known as pore and diffusion model), have been extensively applied in batch reactors to describe the transport of adsorbates inside the adsorbent particles.^34,35 The kinetics of Europhtal adsorption onto M-AC and SCH-4-AC was assessed using different models such as pseudo-first-order,³⁶ pseudo-second-order,³⁷ intraparticle diffusion³⁸ and Elovich³⁹ models (see Table 7). Among these models, the criterion for their applicability was based on the respective correlation coefficient (R²) and agreement between the experimental and calculated value of q_e. The high values of R² (∼0.999) and good agreement between two q_e values obtained from applying pseudo-second-order kinetic model indicate that the adsorption of Europhtal on either of M-AC and SCH-4-AC follows this model (Table 7).

Table 7 Kinetic parameters for the adsorption of Europhtal dye onto the ligands

Model	Equation	Plot	Parameters	Value of parameters
				M	SCH-4
				5 mg L^−1	40 mg L^−1
First-order kinetic		Plot the values of log (qe − qt) versus t to give a linear relationship, from which k1 and qe can be determined from the slope and intercept, respectively	k 1 (min^−1)	0.087	0.126
			q e (calc) (mg g^−1)	1.060	1.373
			R ^2	0.914	0.874
Pseudo-second-order kinetic		Plot the values of (t/qt) versus t to give a linear relationship, from which k2 and qe can be determined from the slope and intercept, respectively	k 2 (min^−1)	0.005	0.149
			q e (calc) (mg g^−1)	6.20	33.45
			R ^2	0.999	0.999
Intraparticle diffusion	q t = Kdif t^1/2 + C	The values of Kdiff and C were calculated from the slopes of qt versus t^1/2	K diff (mg g^−1 min^−1/2)	0.226	0.715
			C (mg g^−1)	4.790	30.163
			R ^2	0.972	0.920
Elovich		Plot the values of (qt) versus ln (t) to give a linear relationship from which α and β can be determined from the slope and intercept, respectively	β (g mg^−1)	2.477	1.1756
			α (mg g^−1 min^−1)	99.89	35.842
			R ^2	0.940	0.960
			q e (exp) (mg g^−1)	5.989	33.139

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In this study, we have chosen two long, rigid bipyridyl-based ligands, (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine and (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine to react with cobalt phthalocyanine. They are natural ligands with two suitable sites for chelating with metals. The N atoms in the aromatic rings (pyridyl groups) connect to metal centers. These ligands can link two metals (or metals groups).^26,27 Schematic diagrams for the adsorption of this catalyst on both the new adsorbents are presented in Fig. 8 and 9.

Fig. 8 Reaction between (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine and cobalt phthalocyanine.

Fig. 9 Reaction between (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine and cobalt phthalocyanine.

3.7 Application to real samples

To observe the efficiency of M-AC and SCH-4-AC for the removal of Europhtal dye from real effluents, three effluent streams were collected separately and the adsorption experiments were performed. Adsorption experiments were conducted using the optimum conditions. More than 96% of dye removal was achieved for all the effluents (Table 8).

Table 8 Percentage removal of the Europhtal dye from four different effluents (N = 3)

Samples	Initial dye concentration (mg L^−1)	Removal (%)
		M-AC	SCH-4-AC
River water	30	99.50 ± 1.4	99.21 ± 0.7
Sanitary wastewater	30	98.75 ± 2.5	99.12 ± 1.21
Industrial waste	30	97.54 ± 1.9	96.63 ± 2.5
Water output from the refinery	30	97.62 ± 2.1	96.45 ± 1.7

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4 Conclusion

Activated carbon was modified with ligands (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine (M) and (1E,2E)-1,2-bis(pyridin-3-ylmethylene)hydrazine (SCH-4) and characterized using SEM, FTIR and ¹H NMR analysis. A very small amount of each adsorbent (0.03 g) was used for the efficient adsorption of Europhtal assisted by ultrasonication. The statistical modeling of the experiments resulted in the prediction of optimal conditions at which a very high percentage of Europhtal was rapidly removed within a couple of minutes. Different models were applied to study the isotherm and kinetics of the adsorption process, which showed the applicability of the Freundlich isotherm and pseudo-second-order kinetics for both the adsorbents. The adsorption capacities of M-AC and SCH-4-AC were found to be 56.69 mg g⁻¹ and 17.84 mg g⁻¹, respectively.

Acknowledgements

The authors express their appreciation to the Graduate School and Research Council of the Islamic Azad University and Fars Science and Research Branch for the financial support of this work.

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