Use of metal-organic framework to remove chromium (VI) from aqueous solutions

Abstract

Chromium is one of the heavy metals found in industrial wastewaters, which have highly toxic to human beings and the environment. Exposure with it may cause some hazard diseases including stomach ulcers, liver, vomiting, kidney and nerve tissue damage, cancer in the lungs, and eventually death. The main objective of this study was to evaluate the efficiency of Uio-66 and ZIF-8 in removing chromium from aqueous solutions. For the synthesis of Uio-66 and ZIF-8, hydrothermal and sol-gel methods were used, respectively. The prepared Uio-66 and ZIF-8 were identified by FTIR, XRD, FE-SEM, EDX, and BET. All experiments were done in batch conditions. Uio-66 and ZIF-8 efficiency for chromium adsorption from aqueous solutions were investigated by variables like initial concentration (10–200 mg/l), pH (3 to 11), Uio-66 and ZIF-8 dosage (0.2 to 1 g/l) and contact time (45 min). The FE-SEM image showed that the sizes of Uio-66 crystals were between 140 and 280 nm. The specific surface area and total pore volume of the prepared Uio-66 and ZIF-8 were 800 m²/g, 0.45 m³/g, 1050 m²/g, and 0.57 m³/g, respectively. The results show chromium adsorption has increased in acid conditions. Equilibrium dosage for Uio-66 and ZIF-8 was 0.4 g/l and 0.6 g/l, respectively. Adsorption equilibrium was performed after 60 min and after this time, chromium adsorption did not significantly change. The study results showed that the experimental data obtained fitted with kinetic model pseudo-order- reaction and isotherm model of Langmuir.

Introduction

Heavy metals are the environmental pollutants that human exposure to them through water and food can cause severe chronic and severe poisoning. Drinking water pollution with heavy metals is a serious problem in developing countries. Heavy metals such as lead, copper, cadmium, zinc and nickel are the most common pollutants found in industrial wastewater [1]. Chromium is one of the heavy metals found in industrial wastewaters, which have highly toxic to human beings and the environment [2]. In the environment, chromium ion can exist in hexavalent Cr and trivalent Cr forms with different toxicity. Chromium ion 6 toxicity is greater than 3 (almost 100 times) [3, 4]. The World Health Organization (WHO) has confirmed the carcinogenicity of chromium in humans [5]. The US Environmental Protection Agency (USEPA) and National Iranian Standard (NIS) have set the maximum allowed concentration for chromium in drinking waters 0.1 mg/l (or 100 ppb) and 0.05 mg/l, respectively [6, 7]. Exposure with Cr(VI) may cause some hazard diseases including stomach ulcers, liver, vomiting, kidney and nerve tissue damage, cancer in the lungs, and eventually death [4, 6]. Chromium accumulation in animal and plant tissues can cause serious problems. Cr(VI) is usually present in the effluent in the form of chromite anions (CrO₄⁻²) and dichromate (Cr₂O₇⁻²), and it does not easily precipitate by conventional methods [8]. So far, different methods have been used to remove chromium such as chemical precipitation, coagulation and flocculation processes, electrocoagulation, ion exchange, adsorption, bio-sorption, and reverse osmosis [9]. Also, these treatment methods have not been highly effective in eliminating heavy metals and also the costs in these methods are very high. Of these, the adsorption process has been considered more by many researchers. Some of its benefits include high efficiency, environmentally friendly, low cost, flexibility, simple design and operation, and cost-effective [6]. In general, adsorption is to keep a substance on the surface of the absorbent or the penetration into the internal absorbent pores [10]. So far, various metal-organic frameworks have been used to remove chromium. Some of these adsorbents are ZIF-67 [11], Uio-66 and Uio-66-NH₂ [12], U-H₄btec MOF [13], a silver-triazolato framework [14], and Cu-BTC [1]. Metal-organic frameworks, MOFs, are a new class of crystalline materials with high porosity and high surface area constructed by metal-containing nodes connected by different organic bridges, which bear multiple complexing functions [15]. MOFs are in two main parts, including organic and inorganic [16]. Compared with the conventional adsorbents, MOF adsorbents present fascinating merits because if their different compositions and structures, such as higher surface area and porosity, greater pore volume, modifiable surface, and tunable pore size [17]. The innovation of this study was the use of a new class of adsorbents to remove chromium from aqueous solutions. According to the literature review, so far, no articles have been published on the use of Uio-66 and ZIF-8 to remove chromium. It should be noted that in 2013, Shen et al. used adsorbents of Uio-66 and Uio-66-NH₂ as an efficient visible-light-driven photocatalyst for selective oxidation of chromium [12]. The new adsorbents have shown a high surface area that makes them suitable to remove different heavy metals. In this study, we report the first adsorptive of Cr (VI) from aqueous solutions with a Uio-66 and ZIF-8 to investigate the possibility of using MOFs as suitable adsorbents for the removal of Cr (VI). Among different adsorbents, University of Oslo 66 (Uio-66) and Zeolitic Imidazole Frameworks (ZIF-8) were selected. UiO-66 contains hexanuclear zirconium clusters linked by terephthalates. This MOF might have favourable interactions, such as π-τ interactions, with Cr (VI) for adsorption [18]. Therefore, the main objective of this study was to evaluate the efficiency of Uio-66 and ZIF-8 in removing chromium from aqueous solutions.

Materials and methods

Materials (Uio-66 and ZIF-8, chromium)

Zirconium chloride, terephthalic acid (TPA) were obtained from Merk Company. Potassium permanganate (KMnO₄), N, N-dimethylformamide, Zinc nitrate hexahydrate, 2-methylimidazole, and methanol were prepared by Sigma-Aldrich. All reagents and solvents were used as received from commercial suppliers without further purification.

Synthesis of Uio-66 and ZIF-8

UiO-66 and ZIF-8 were prepared was synthesized based on the previous method [19–21]. For the synthesis of Uio-66, ZrCl₄ [22] (1 mmol or 0.2332 g) and TPA (1 mmol or 0.161 g) were dissolved in 50 ml of DMF solution. The mixed solutions were transferred to a 100 ml an autoclave. The autoclave was sealed and heated in an oven at 120 °C for 48 h. After cooling, the final white powder was washed several times with methanol. After washing with methanol, the white powder was dried at 100 °C for 12 h in vacuum conditions. Ultimately, the end powder was stored. For the synthesis of ZIF-8, 2-methylimidazole (11.350 g) and of Zinc nitrate hexahydrate (0.863 g) were dissolved in 8 ml and 80 ml deionized water, respectively. The two solutions were combined under constant stirring at room temperature for 8 h. Then, a white powder formed and that was isolated by centrifuge. Ultimately, the end product was dried 24 h. Before use for activating, ZIF-8 was dried for an overnight at 100 °C in a vacuum oven.

General characterization

The prepared Uio-66 and ZIF-8 were identified by Fourier Transform Infrared spectroscopy spectra (Spectrum two model, PerkinElmer Company), X-Ray Diffraction (X’ Pert Pro model, Panalytical Company), Energy Dispersive X-ray Spectroscopy, Field Emission Scanning Electron Microscopy (SIGMA VP-500 model, ZEISS Company), and BET surface area. The total pore volume of the adsorbents was also characterized by using nitrogen adsorption isotherms at 77 K (BElSORP Mini model, Microtrac Bel Corp).

Adsorption studies

Chromium adsorption was studied by Uio-66 and ZIF-8 zeolite imidazole framework. All experiments were done in batch conditions. Uio-66 and ZIF-8 efficiency for chromium adsorption from aqueous solutions were investigated by variables like initial concentration (10–200 mg/l), pH (3 to 11), Uio-66 and ZIF-8 dosage (0.2 to 1 g/l) and contact time (45 min). Method of one point at the time was used to determine the number of samples in this study. In this way, only one variable is considered in each step, and the rest of the variables are assumed to be constant. Finally, to determine the optimum conditions and also to determine the isotherms and kinetics adsorption, 210 samples were examined for both adsorbents (Uio-66 and ZIF-8). Also, each experiment was repeated three times. So, Therefore, the effect of variables in constant time (in this study was 45 min). In the step of determining the adsorption kinetics, the effect of the contact time was more accurately investigated. At first, a stock solution of chromium was made and stored under standard conditions. Potassium dichromate, K₂Cr₂O₇, was used to prepare the standard solution (1000 mg/l). An absorbent dosage was added to 100 ml of chromium solution. The solution pH was adjusted using HCl and NaOH (1 N). At the beginning and the end, the adsorbent was isolated from solution by centrifuge (3000 rpm, 5 min). Then, the residual chromium was measured by a spectrophotometer (UV-UVIS) [23]. The residual chromium concentration at 540 nm was determined. In the first step, the calibration curve for chromium was plotted in the range of 0.1 to 0.7 mg/L (R2 = 0.9987, Y = 1.4836 + 0.0699). All experiments were performed at laboratory temperature [24]. Eventually, adsorbed chromium (q_e, mg/g) on the Uio-66 and ZIF-8 was estimated according to Eq. 1 [25–27]:

$$q_e=\frac{V\left(C_o-C_e\right)}{m}$$ 1

Where, C_o and C_e are an initial and final concentration of chromium (m/l), respectively, V is the volume of chromium solution (ml), and m is the adsorbent weight (g). Also, the removal efficiency (R, %) of chromium was estimated according to Eq. 2 [28]:

$$R,\%=\frac{\left(C_0-C_t\right)}{C_0}$$ 2

Where, C_o and C_t are the initial and final concentration of chromium (mg/L), respectively. To determine the adsorption isotherms of chromium on Uio-66 and ZIF-8, the models of Langmuir (plotting C_e/q_e against C_e) and Freundlich (plotting log q_e against log C_e) were investigated. To determine the adsorption rate of chromium, the kinetic models of pseudo-first-order (plotting ln푞_푒−푞_푡) versus time) and the pseudo-second-order (plotting t/푞_푡 versus time) were used [4]. the experiments of adsorption equilibrium were investigated at optimal conditions of chromium adsorption [29]. Equilibrium studies were performed at laboratory temperature using 50 ml of chromium with varied initial concentrations (50 mg/l and 60 mg/l).

Data availability

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

Results and discussion

Characterization of prepared Uio-66 and ZIF-8

The crystallographic structure of prepared Uio-66 and ZIF-8 were studied by X-Ray Diffraction. The XRD patterns and FESEM image of the prepared Uio-66 and ZIF-8 are shown in Fig. 1. As shown in Fig. 1-A, the XRD pattern of the Uio-66 has three clear at 7°, 8.45°, 25.69° [20, 26]. The results of this study were consistent with the findings of the other researchers [16, 21, 30, 31]. Figure 1-B shows the morphology of prepared Uio-66. The FE-SEM image showed that the sizes of Uio-66 crystals were between 140 and 280 nm. In Lin study, produced Uio-66 size was between 200 to 500 nm [32]. Some properties of prepared Uio-66 are shown in Table 1. The specific surface area and total pore volume of the prepared Uio-66 was 800 m²/g and 0.45 m³/g. In various studies, the specific surface area is slightly different, which can be attributed to the synthesis method and the quality of the materials used. The XRD patterns and FESEM image of the prepared ZIF-8 are shown in Fig. 1-B and 1-C, respectively. As shown in Fig. 1-B, the XRD pattern of the ZIF-8 has peaks at 7°, 10°, 12°, 14°, 16°, and 18°. The spectrum obtained was consistent with other studies. In both the Uio-66 and ZIF-8 in the XRD spectrum, the presence of clear and distinct peaks indicated that the crystallization of MOFs has been well done [33, 34]. Unlike Uio-66, which is almost amorphous, ZIF-8 have different morphologies like cubic, leaf-shaped, and dodecahedral [19]. The results of previous studies showed which the surface area of MOFs can vary depending on morphology, the quality of used materials for synthesis, and finally synthesis conditions. As can be understood from Fig. 1-D, ZIF-8 was synthesized into a cubic shape. As shown in Table 1, the specific surface area and total pore volume of the prepared ZIF-8 was 1050 m²/g and 0.57 m³/g. The cube dimensions of prepared cubic ZIF-8 was in the range of between 455 to 760 nm. The perpend ZIF-8 was white powder and its density was 0.45 g/m³. In the studies Khan et al. and Liu, the reported surface area was 1501 m²/g and 958.4 m²/g, respectively [19, 35]. In the study of Liu et al., the surface area of dodecahedral-ZIF-8, cubic-ZIF-8, and leaf-shaped-ZIF-8 also were 1151.2 m²/g, 958.4 m²/g, and 12.7 m²/g, respectively [19]. The results of EDX showed that the percentage of C, N, Zn, and O in the ZIF-8 was 49%, 22%, 27.7%, and 1.54%, respectively.

Fig. 1.

XRD spectra and FE-SEM image of the prepared Uio-66 (a, b) and ZIF-8 (c, d)

Table 1.

Some properties of Uio-66 and ZIF-8 based on BET

Adsorbent	SABET (m2 g−1)	Total pore volume (m3 g−1)	Mean pore diameter (nm)
Uio-66	800	0.45	2.33
ZIF-8	1050	0.57	1.54

In the adsorption processes, the solution pH has an important effect on the reaction progression and adsorption capacity. The solution pH affects the distribution of charge on the adsorbent surface and dissociation the target pollutant [36]. Chromium adsorption at different pH was investigated. The effect of solution pH on adsorption of chromium on the Uio-66 and ZIF-8 was investigated over a pH range of 3 to 11 with a contact time of 45 min. Figure 2 shows the adsorption efficiency of chromium with adsorbents Uio-66 and ZIF-8. The selective concentration of chromium for these experiments was 50 mg/l. As shown in Fig. 2, chromium adsorption has increased in acid conditions. The relatively lower pH was favourable for the chromium adsorption. When the solution pH decreased from 11 to 3, the capacity of chromium adsorption on the Uio-66 and ZIF-8 increased from 15.6 to 85.7 mg/g and from 35.4 to 150 mg/g, respectively. According to studies on the distribution of different species of chromium at different pH, its various forms are H₂CrO₄ (chromic acid), H₂CrO₄⁻ (hydrogen chromate ions), and CrO₄⁻² (chromate ions). Also, chromic acid and hydrogen chromate ions are the dominant forms when the solution pH is lower than 6.8, while only chromate ions are stable when the pH is above 6.8 [11, 37]. For accurate examination of the chromium adsorption process on adsorbents, pH_ZPC was first measured [38]. pH_ZPC for Uio-66 and ZIF-8 was obtained 7 and 9, respectively. Therefore, at lower pH than these values, the surface charges of the adsorbents are positive. Consequently, in these conditions, the strong electrostatic interaction of among adsorbents and chromium anions occurs when the solution pH is lower than pH_ZPC. Li et al. (2015) reported that ZIF-67 adsorbent has a significant effect on chromium adsorption at acid pH [11]. In this study, when the pH value decreases from 11 to 5, the adsorption capacity of Cr (VI) on the ZIF-67 increases from 3.27 to 13.2 mg/g. The results of this study are consistent with other researchers’ findings [11, 39–41]. Maleki et al. (2015) reported that they used Cu-BTC adsorbent to chromium adsorption [1]. Khosravi et al. (2018) reported that the high capacity of chromium adsorption at acidic pH is due to the presence of different forms of Cr(VI) in the solution, including H₂CrO₄, HCrO₄⁻, CrO₄²⁻, and Cr₂O₇²⁻ [6]. Mobarak et al. (2018) used CTAB/H₂O₂-clay to remove chromium. The results showed that the highest adsorption capacity was observed at pH ≈ 2. [4]. Based on Khosravi et al. report (2018), Chromium adsorption reactions are as follows [37]:

$$C{r}_2{O_7}^{-2}+H_{2}O\Rightarrow 2\mathit{HCr}{O_4}^{-}$$ 3

$$\mathit{HCr}{O_4}^{-}\Rightarrow \mathit{Cr}{O_4}^{-2}+H^{+}$$ 4

$$\mathit{HCr}{O_4}^{-}+7H^{+}+3e^{-}\Rightarrow 4H_{2}O$$ 5

$$\mathit{Cr}{O_4}^{-2}+14H^{+}+6e^{-}\Rightarrow 2C{r}^{3+}+7H_{2}O$$ 6

$$C{r}_2{O_7}^{-2}+14H^{+}+6e^{-}\Rightarrow 2C{r}^{3+}+7H_{2}O$$ 7

Fig. 2.

The pH effect onto chromium (VI) adsorption by Uio-66 and ZIF-8

In alkaline conditions, the concentration of hydroxyl ions (OH⁻) ions increases in solutions and these ions have a higher attraction to the Uio-66 and ZIF-8 surface than CrO₄⁻². Because of the changing of chromium species (from HCrO₄⁻ and Cr₂O₇²⁻ to CrO₄⁻²) and the presence of competitive ions (such as OH⁻) under basic conditions, the chromium adsorption capacity was significantly reduced [4]. Of course, this decrease in the adsorption capacity in Uio-66 was higher because the pH_ZPC (≅7) is lower than that of ZIF-8 (≅9). At pH above the pH_ZPC, the charge on the surface of the adsorbents is negative. In this study, the best pH for removing chromium 7 was reported. Maximum adsorption of chromium has been achieved at the pH ≅ 3. As a result, the pH of 3 was selected for further experiments.

To study the effect of initial concentration, the chromium concentration of 10 to 200 mg/l was used. Figure 3 the effect of initial chromium concentration by Uio-66 and ZIF-8 adsorbents. At a constant pH, by increasing metal concentration, at first, the adsorption removal and capacity increased. In this study, the optimal concentration of chromium was found 10 mg/l. The different adsorption behaviour can be explained that with the increasing initial concentrations, the affinity of chromium molecules to adsorb on Uio-66 and ZIF-8 were increased (compared with lower concentrations) [42]. As a result, with increasing the concentrations of chromium, the adsorption capacity increased. After some time, the amount of chromium adsorption decreased. At low concentration, enough adsorption site onto Uio-66 and ZIF-8 surface are exist for chromium, and conversely [42]. As a result with an increase of the chromium concentration, adsorption was decreased. This fact can be explained by the mass transfer of Cr⁶⁺ and pressure onto the inner adsorption sites of the adsorbent [13]. In lower concentrations, also, chromium molecules are adsorbed on Uio-66 and ZIF-8 adsorbent surface, rapidly, but with the increasing initial chromium concentration, gradually adsorbents surfaces become saturated. Finally, the adsorption decreased because of the repulsion between chromium molecules [43, 44]. The results of this study are consistent with the findings of other researchers [1].

Fig. 3.

The effect of initial chromium concentration by Uio-66 and ZIF-8 (pH: 3, contact time: 45 min)

Figure 4 shows the effect of Uio-66 and ZIF-8 dose and removal efficiency of chromium. It was observed that chromium adsorption increased with the increasing adsorbent dose until reaching an equilibrium dose. Equilibrium dosage for Uio-66 and ZIF-8 was 0.4 g/l and 0.6 g/l, respectively. In the following by increasing the adsorbent dose, chromium adsorption was not significant. Based on the results of another study, the increase of adsorption with adsorbent dose can be caused by high adsorbent surface and the availability of more adsorption sites [1]. At a higher dose than optimum (0.4 g/l and 0.6 g/l) by increasing than the dosage, the adsorption capacity of adsorbent was almost constant. The cause of that can be because of overlapping or aggregation of adsorption sites of adsorbents which finally lead to a decrease in the total surface area [1, 45]. The results of this study are consistent with the findings of other researchers. The findings of Mobarak et al. (2018) showed that dose increases from 0.05 to 2 mg, adsorption efficiency increases because of added sorption sites. However, increasing the dose does not have any effect on the adsorption efficiency [4].

Fig. 4.

The effect of adsorbent dose of Uio-66 and ZIF-8 onto chromium adsorption (pH: 3, contact time: 45 min, and initial concentration: 50 mg/l)

Contact time is one of the leading factors determining the adsorption behaviour of a process [36]. To understanding the amount of chromium adsorbed onto adsorbents at various time, contact time was used. Figure 5 shows the effect of contact time on chromium adsorption onto Uio-66 and ZIF-8. Initially, chromium was adsorbed quickly onto Uio-66 and ZIF-8. Adsorption equilibrium was performed after 60 min and after this time, chromium adsorption did not significantly change. After this time (60 min), qe were nearly constant because of the saturation of active sites and the attainment of dynamic equilibrium step [4]. Table 2 shows calculated parameters of kinetic models for the Cr (VI) adsorption onto Uio-66 and ZIF-8. In order to choose the best kinetic model, the coefficient of determination (R²) is checked. Actually, the agreement between the model predicted values and experimental data was determined by R² [41]. As illustrated in Table 2, the pseudo-second-order model for Uio-66 has the highest square R. As a result, this model was a fit model to describe the adsorption kinetics of chromium on Uio-66. Also for ZIF-8, pseudo-second-order model has the highest coefficient of determination (R²). As a result, this model was a fit model to describe the adsorption kinetics of chromium on ZIF-8. So, it can be concluded that in the adsorption the rate-limiting step is mainly chemisorption which involves valence forces, occurred possibly because of exchange of electrons among chromium and adsorbents [41]. The present results showed which chromium adsorption onto Uio-66 and ZIF-8 was very fast and more than 95% of the equilibrium adsorption capacity was achieved in the first 60 min. The equilibrium time of Uio-66 and ZIF-8 for chromium adsorption was approximately 350 min (5.83 h), respectively. Table 3 shows the calculated parameters of isotherm models for the chromium adsorption onto Uio-66 and ZIF-8. As shown in Table 3, the Langmuir isotherm model has the most coefficient of determination, so, chromium adsorption onto both Uio-66 and ZIF-8 conform to the model. The Langmuir model suggests monolayer adsorption process and the homogeneous surface of adsorbent which indicates that active sites of chromium adsorption have been distributed throughout the surface of adsorbents [41]. According to this model, the maximum of chromium adsorption capacity for Uio-66 and ZIF-8 was 85.7 mg/g and 150 mg/g, respectively. As is clear, the two parameters surface area and pore volume have a significant impact on the amount of pollutant adsorption. The adsorption capacity varies and depends mainly on the initial chromium concentration and characteristics of the individual adsorbent [41]. Maleki et al. (2015) reported that the maximum adsorption capacity of chromium on Cu-BTC is on activated carbon adsorbent 48 mg/g [1]. Table 4 shows a comparison of the adsorption chromium capacity onto Uio-66 and ZIF-8 with other adsorbents. According to Table 4, the adsorption chromium capacity ZIF-8 is significantly more than that of other adsorbents.

Fig. 5.

Effect of contact time on chromium adsorption onto Uio-66 and ZIF-8

Table 2.

Calculated parameters of kinetic models for the chromium adsorption onto Uio-66 and ZIF-8

kinetics (Uio-66)	Parameters	Cr (VI) concentration (mg l−1)		Kinetics (ZIF-8)	Parameters	Cr (VI) concentration (mg l−1)
		50	80			50	80
pseudo first order	k1	0.0065	0.0111	pseudo first order	k1	0.0065	0.0111
	R2	0.663	0.689		R2	0.7630	0.778
	qcal	2.8730	20.9639		qcal	2.8730	20.9639
pseudo second order	k2	0.0566	0.0040	pseudo second order	k2	0.0566	0.0040
	R2	0.987	0.978		R2	0.896	0.995
	qe (cal)	27.6	54.6		qe (cal)	58.6	120.6

Table 3.

Calculated parameters of isotherm models for the chromium adsorption onto Uio-66 and ZIF-8

Isotherm (Uio-66)	Parameters	Cr (VI) concentration (mg l−1)		Isotherm (ZIF-8)	Parameters	Cr (VI) concentration (mg l−1)
		50	80			50	80
Freundlich	n	3.45	4.56	Freundlich	n	2.42	3.78
	R2	0.876	0.884		R2	0.785	0.874
	KF	20.12	21.34		KF	23.4	23.7
Langmuir	KL	1.675	1.243	Langmuir	KL	0.876	0.985
	R2	0.983	0.965		R2	0.897	0.894
	qm	35.78	85.45		qm	76.65	150.85

Table 4.

Comparison of the chromium adsorption capacity onto Uio-66 and ZIF-8 with other adsorbents

Adsorbent	qe (mg g−1)	Reference
Cu-TBC	48	[1]
ZIF-67	15.43	[11]
treated waste newspaper	59.88	[46]
Novel Diazene/Methoxy-Laced Coordination Polymer	106.13	[36]
sliver-triazolate MOF	38.6	[14]
CTAB/H2O2	67.05	[4]
Mesoporous carbon microspheres (MCMs)	156.3	[41]
Carbon nano-onions (CNOs)	23.527 (pH ≈ 7), 27.855 (pH ≈ 3)	[47]
GnZVI/PAC	53.48	[37]
Uio-66	85.7	(This study)
ZIF-8	150	(This study)

Conclusion

In this study, metal organic frameworks of Uio-66 and ZIF-8 were synthesized based on previous works. Chromium adsorption was dependent on adsorbent dose, solution pH, and initial concentration, and contact time. The residual chromium concentration at 540 nm was determined using a spectrophotometer (UV-UVIS). The rate of chromium adsorption was higher under acidic conditions. pH_ZPC for Uio-66 and ZIF-8 was obtained 7 and 9, respectively. The findings of this study showed that chromium adsorption capacity in ZIF-8 (150 mg/g) was higher than Uio-66 (85.6 mg/g) at an initial concentration of 50 mg/L. maximum adsorption of chromium has been achieved at the pH ≅ 3. One of the reasons for higher adsorption of chromium in ZIF-8 (1050 m²/g) can be a higher surface area than Uio-66 (800 m²/g). In this study, the optimal concentration of chromium was found 10 mg/l. The equilibrium time of chromium adsorption onto Uio-66 and ZIF-8 was approximately 350 min. The findings of the study showed that the experimental data obtained fitted with kinetic model pseudo-order- reaction and isotherm model of Langmuir.

Author contribution

KM Thabayneh distributed, collected and read the dosimeters, analyzed, wrote the text, reviewed and approved the final manuscript.

Compliance with ethical standards

Ethics approval and consent to participate

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Not applicable.

Competing interests

The author declares that they have no competing interests.