Data on efficient removal of acid orange 7 by zeolitic imidazolate framework-8.

A water stable and hybrid nonporous adsorbent, cubic zeolitic imidazolate framework-8 (ZIF-8), was synthesized for Acid orange 7 (AO7) removal from aqueous solutions in batch mode. Central composite design was utilized to explore the individual and interaction effects of pH, AO7 concentration, ZIF-8 dosage and contact time on dye adsorption. A second order polynomial equation (R2 = 0.9852, LOF = 0.1419) developed for prediction of the AO7 removal. Sorption model revealed that the adsorbent dosage and the dye concentration are major factors that controlled the AO7 removal efficiency. AO7 removal increased from 55 to 80% by increasing ZIF-8 dosage from 0.2 to 1 g/L. The dye removal, on the other hand, decreased from 84 to 70% with increasing AO7 concentration from 10 to 100 mg/L and increased from 60% to 80% by decreasing pH from 12 to 4. The dye removal followed the pseudo second order kinetic and the Langmuir isotherm model. The maximum monolayer adsorption capacity of 80.47 mg dye/g of ZIF-8 was obtained according to the Langmuir model.

Data

The XRD pattern of the synthesized ZIF-8 is shown in Fig. 1(a). Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) of as-synthesized ZIF-8 are shown in Fig. 1(b) and (c), respectively.

Fig. 1

Characteristics of the as-synthesized ZIF-8: (a) XRD pattern and (b) FTIR pattern and (c) SEM image of ZIF-8 crystals.

The experimental range and levels of four independent variables are listed in Table 1. Table 2 shows a total of 30 experimental runs that performed based on the experimental design matrix. The removal efficiencies then analyzed using the analysis of variance (ANOVA) to set a prediction model.

Table 1

Experimental range and levels of independent variables.

Factor	Coded variables	Range and levels
		−2	−1	0	1	2
pH	X1	4	6	8	10	12
AO7 Concentration (mg/L)	X2	10	32.5	55	77.5	100
ZIF-8 dosage (g/L)	X3	0.2	0.4	0.6	0.8	1
Time (min)	X4	10	30	50	70	90

Table 2

Experimental design and results for AO7 removal.

Run Order	X1	X2	X3	X4	Efficiency %	Run Order	X1	X2	X3	X4	Efficiency %
1	12	55	0.6	50	68	16	10	32.5	0.8	30	85
2	8	55	0.6	50	70	17	8	100	0.6	50	75
3	6	77.5	0.8	70	78	18	8	55	0.6	50	67
4	6	77.5	0.4	30	62	19	6	32.5	0.4	30	73
5	4	55	0.6	50	80	20	6	32.5	0.8	30	89
6	10	77.5	0.8	70	79	21	10	77.5	0.4	70	57
7	8	55	1	50	78	22	6	77.5	0.4	70	61
8	8	55	0.6	50	69	23	8	55	0.6	90	72
9	6	32.5	0.8	70	89	24	8	55	0.6	10	68
10	8	55	0.2	50	32	25	10	32.5	0.4	30	60
11	6	32.5	0.4	70	74	26	10	77.5	0.4	30	56
12	10	32.5	0.4	70	61	27	8	55	0.6	50	68
13	8	10	0.6	50	99	28	6	77.5	0.8	30	81
14	10	77.5	0.8	30	78	29	8	55	0.6	50	71
15	10	32.5	0.8	70	87	30	8	55	0.6	50	70

To evaluate the efficacy of linear, 2Fl, quadratic and cubic models, the obtained scores from the sequential modeling were compared (Table 3). The relationship between input variables and response was modeled using ANOVA which presented in Table 4. Fig. 2 depict the effect of pH and initial dye concentration on the removal of AO7. Fig. 3 shows the counter plot and response surface plot of pH and adsorbent dose on dye removal.

Table 3

Adequacy of the model tested.

Source	Sum of Squares	df	Mean Square	F-Value	p-value
Mean vs Total	1.551E+005	1	1.551E+005
Linear vs Mean	3426.5	4	856.63	17.16	<0.0001
2FI vs Linear	80.75	6	13.46	0.22	0.9659
Quadratic vs 2FI	1098.03	4	274.51	59.32	<0.0001	Suggested
Cubic vs Quadratic	44.5	8	5.56	1.56	0.2849	Aliased
Residual	24.92	7	3.56
Total	1.598E+005	30	5325.43

Table 4

Analysis of variance (ANOVA) for AO7 removal by ZIF-8.

Source	Sum of squares	df	Mean square	F-value	p-value
Intercepts	4605.28	14	328.95	71.08	<0.0001
X1	192.67	1	192.67	41.63	<0.0001
X2	541.5	1	541.5	117.01	<0.0001
X3	2688.17	1	2688.17	580.88	<0.0001
X4	4.17	1	4.17	0.9	0.3577
X1X2	25	1	25	5.40	0.0346
X1X3	49	1	49	10.59	0.0053
X1X4	4	1	4	0.86	0.3672
X2X3	0.25	1	0.25	0.054	0.8193
X2X4	2.25	1	2.25	0.49	0.4963
X3X4	0.25	1	0.25	0.054	0.8193
X12	60.01	1	60.01	12.97	0.0026
X22	613.44	1	613.44	132.56	<0.0001
X32	293.44	1	293.44	63.41	<0.0001
X42	6.3	1	6.3	1.36	0.2616
Residual	69.42	15	4.63	–	–
Lack of Fit	58.58	10	5.86	2.7	0.1419
Pure Error	10.83	5	2.17	–	–
Cor Total	4674.7	29	–	–	–

R2: 0.9852, Adj R2: 0.9713, Pred R2: 0.9245, Adeq Precision: 41.197

Fig. 2

Counter plot and response surface plots of pH and initial dye concentration on dye adsorption (ZIF-8: 0.6 g/L, time: 50 min).

Fig. 3

Counter plot and response surface plots of pH and adsorbent dose on dye adsorption (AO7: 55 mg/L, time: 50 min).

Fig. 4 compare the removal percentages obtained experimentally for AO7 adsorption with those predicted by the model.

Fig. 4

Normal probability plot of the predicted values of AO7 uptake efficiency versus actual values (a), predicted response versus actual response (b).

The three well-known isotherm models including Langmuir, Freundlich, Temkin and Dubinin-Radushkevich [1] were fitted with equilibrium data and related constants are listed in Table 5. Fig. 5 shows the fitted experimental data with isotherm models.

Table 5

Adsorption isotherms and related constants for AO7 adsorption.

Isotherm	Formula	Plot	Parameter
Langmuir	qe = q_mbc_e1+bc_e	1q_evs. 1c_e	qmax (mg/g)	80.47
			KL (L/mg)	1
			R2	0.96
Freundlich	qe = k_Fc_e^1n.	logq_evs. logC	KF(mg/g (L/mg)1/n)	29.99
			n	2.71
			R2	0.86
Temkin	qe = RTbln(k_Tc_e)	qe vs.lnC_e	kt (L/mg)	7.94
			B1	18.39
			R2	0.69
Dubinin-Radushkevich	qe = qmexp (-βε^2)	q_evs. ε^2	qmax (mg/g)	76.74
			β	1.01E-07
			R2	0.76

Fig. 5

Fitting the experimental data with (a) Langmuir, (b) Freundlich, (c) Temkin and (d) Dubinin-Radushkevich models (ZIF-8: 0.6 g/L, pH: 6, time 12 h.).

Table 6. Adsorption kinetics and their formula for AO7 removal by ZIF-8. Three adsorption kinetic models including pseudo first order, pseudo first order and intra-particle diffusion models [2] were used to describe the kinetic data. The kinetic models and their constants are presented in Table 6 and Table 7 and the plots of these models are shown in Fig. 6.

Table 6

The kinetic models used in this work.

Kinetic Model	Formula	Plot
Pseudo first order kinetic model	Log(q_e-q_t)=logq_e-k_12.303.t	log (qe− qt) vs. t
Pseudo second order kinetic model	tq_t=1k_2qe^2+1qe.t	tq_tvs. t
Intra-particle diffusion kinetic model	q_t=k_p.t^0.5+c	qt vs. t0.5

Table 7

Constants obtained from kinetic models for AO7 adsorption.

C0 (mg/l)	qe, exp (mg/g)	Pseudo- first order			Pseudo- second order			Intraparticle diffusion
		qe, cal (mg/g)	K1 (min−1)	R2	qe, cal (mg/g)	K2 (min−1)	R2	Kp (mg/g.min−0.5)	R2
10	16	3.95	0.052	0.64	16.4	0.028	0.99	0.92	0.62
32.5	43.88	14.54	0.064	0.66	46.16	0.007	0.99	2.9	0.64
50	51.67	17.7	0.05	0.65	54.38	0.004	0.99	3.86	0.64

Fig. 6

Fitting the experimental data with (a) pseudo-first-order, (b) pseudo-second-order and (c) intraparticle diffusion kinetic models (pH: 6, ZIF-8: 0.6 g/L).

Materials and methods

Chemicals and reagents

AO7 used in the experiments was obtained from Alvan Sabet Company. All other reagents and chemicals used in the preparation of the adsorbent were purchased from Merck Company.

ZIF-8 synthesis and characterization

Zeolitic imidazolate framework-8 (ZIF-8) is a water stable and porous member of metal organic framework (MOFs). ZIF-8 was prepared at room temperature exactly according to the literature [3]. All of the diffraction patterns in XRD and FTIR tests were in accordance with those in the literature for ZIF-8, indicating that the synthesized adsorbent has pure ZIF-8 phase. The SEM image shows the uniform cubic shapes of the micrometer-sized crystals of ZIF-8. The BET surface area and total pore volume of ZIF-8 are 978 m2/g and 0.51 cm3/g, respectively [3].

Sorption experiments

Batch mode experiments were employed in this work. All experiments were carried out in duplicate and mean values were reported. The experiments were conducted in incubator shaker at 25 °C and at 200 rpm. After agitating, the residual concentration of AO7 in the solution was determined via spectrophotometer at 484 nm.

Experimental design and modeling

The AO7 removal was carried out by four chosen independent variables using central composite design (CCD). A second order polynomial equation was applied to explore the influence of variables in terms of linear, quadratic and cross product:

In the above equation, Y is the predicted response (AO7 uptake), Xi and Xj are the independent variables, β0 is a constant value, βi, βii, and βij are the regression coefficients for linear, second order, and interaction effects, respectively and is the error of the model [4], [5], [6], [7]. The higher F-value (59.32) and smaller p-value (<0.0001) in Table 3, illustrates that the quadratic model gives the best fit to experimental data. The amount of lack of fit was non-significant (0.1419), and the values of R2, adjusted R2 and predicted R2 were found to be 0.9852, 0.9713 and 0.9245 respectively, which represents the experimental data predicted well by the model [8]. The uniform distribution of predicted data near to regression line indicated the adequacy of the developed model. The empirical relationship between the independent variables and the response based on the experimental data, after modifying the model, was explained as:

The above equation explains how AO7 uptake by ZIF-8 and influenced by the individual variables. The positive sign for each term represents that AO7 adsorption increased with increasing the variable value and vice versa [9].

Effect of various process variables

As Fig. 2 illustrates, by increasing the pH from 4 to 12, the dye removal efficiency was decreased. The AO7 uptake also decreased by increasing initial AO7 concentration from 10 to 100 mg/L. The degree of AO7 uptake, on the other hand, increased with increasing adsorption dosage from 0.2 to 1 g/L.

Adsorption isotherms

In order to improve the adsorption mechanism pathways and effective design of adsorption systems, it is necessary to understand and interpret the adsorption isotherms [10], [11], [12]. Sorption experiments for isotherm modeling were done by applying 0.6 g ZIF-8/L to solutions containing 10, 25, 50, 75 and 100 mg AO7/L at pH of 6 and 25 °C for 12 h. As mentioned in Fig. 5 and Table 5, the adsorption data fitted well by the Langmuir model and the maximum monolayer adsorption capacity of ZIF-8 for AO7 was obtained 80.47 mg/g.

Adsorption kinetics

Kinetic studies evaluate the mechanism and rate of adsorption process [13], [14], [15], [16], [17]. By comparing the obtained regression coefficient (R2) values for each model, it can be stated that adsorption of AO7 by ZIF-8 obeyed pseudo second order model.

Specifications table

Subject area	Water pollution
More specific subject area	Adsorption, water and wastewater treatment
Type of data	Table, figure
How data was acquired	After sorption process, the residual dye concentrations were determined using DR-5000 spectrophotometry (Unico UV2100PC) at 484 nm.
Data format	Analyzed
Experimental factors	The adsorbent was prepared at room temperature using a precipitation method.
Experimental features	The adsorption of dye was measured by changing the contact time, dose, pH, dye concentration. Kinetic and equilibrium data also modeled.
Data source location	Gonabad, Khorasan Razavi province, Iran
Data accessibility	Data are included in this article.

Value of the Data• The characterization data in the article will be informative for researchers who work on adsorption process using ZIFs. • The mathematical model in this data article can be used to predict the dye removal and to set the optimum condition. • Kinetic and isotherm models and their coefficients presented in the data article, are informative to design real treatment units. • The data will be interesting for researchers who work on Metal Organic frameworks (MOFs).