Adsorption of Chromium (VI) by Cu (I)-MOF in Water: Optimization, Kinetics, and Thermodynamics

To investigate the adsorption behavior of Cu (I)-MOF material for chromium (VI) in water, the parameters of influencing adsorption were optimized and found as follows: the optimal pH was 6 for the adsorption of Cr (VI) by the Cu (I)-MOF, the optimal amount of adsorbent was 0.45 g·L, and the adsorption saturation time was within 180min. Subsequently, the kinetics results were fitted by four models such as pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models. Among them, the adsorption of chromium (VI) was more inclined to the pseudo-first-order model (Radj � 0.9230). +en, the isotherm data were fitted by Langmuir and Freundlich models. +e results indicated that Langmuir isotherm was the excellent match model (Radj � 0.9827). It belongs to a monolayer adsorption, and the maximum adsorption capacity was 95.92mg·g. Subsequently, the thermodynamic parameters of the adsorption were calculated as follows: enthalpy change (ΔH) was −8.583 kJ·mol, entropy change (ΔS) was −8.243 J·mol K, and the Gibbs function change (ΔG) was less than zero in the temperature range of 288–328K, indicating that the reaction was spontaneous. Finally, both the spectra of infrared and XPS supported the adsorption mechanism that belonged the ion exchange. +e spectra of XRD and SEM images shown that the structure of Cu (I)-MOF remained stable for at least 3 cycles. In conclusion, Cu (I)-MOF material has a high adsorption capacity, good water stability, low cost, and easy to prepare in large quantities in practical application. It will be a promising adsorbent for the removal of Cr (VI) from water.


Introduction
Heavy metal chromium pollution has become a serious environmental problem worldwide. Mineral mining, electroplating, leather tanning, and other industrial processes have resulted in a large amount of wastewater containing chromium [1]. It has been listed as a carcinogen by the International Agency for Research on Cancer of the World Health Organization [2]. e hexavalent chromium (Cr (VI)) and trivalent chromium (Cr (III)) ions were the main stable forms in the water, the toxicity of Cr (VI) was about 100 times higher than that of Cr (III), and high content of Cr (VI) will be highly toxic, carcinogenic, and mutagenic properties [3]. Excessive levels of Cr (VI) in drinking water will harm human health and cause the digestive system and liver damage [4]. In addition, wastewater containing chromium compounds can reduce the efficiency of the biochemical treatment system. erefore, Cr (VI) is the optimal control of pollutants in wastewater [5] and must be treated before discharge into the environment to reduce the harm to environment [6].
Treatment methods for Cr (VI) ions in water include chemical reduction, electrolysis, ion exchange, phytoremediation, and adsorption [7]. Among them, the adsorption method is widely used for the elimination of heavy metals from polluted water because of its simple operation, low cost, and less required equipment. e choice of material is very crucial for adsorption; in addition to the activated carbon, silica gel, and other traditional materials, the modified materials or new materials should be developed to improve the adsorption performance, such as chitosan [8] and its magnetic materials [9], graphene and its composites [10], nano zero-valent iron [11], and metal-organic framework (MOF) materials [12].
Compared with other materials, MOF is a new class of porous structures composed of different varieties transition metal ions (or clusters). To date, MOFs have exhibited numerous properties, such as gas adsorption, separations, sensors, drug delivery, and catalytic activity [13][14][15]. Numerous studies have been conducted on MOF for the removal of heavy metals from water bodies [16]. Particularly, the MOF can be used for the high efficiency removal of Cr (VI) in aqueous solution [17][18][19]. But some MOFs are not stable in water or difficult to produce in large quantities in practical application. A Cu (I)-MOF, (Cu[2,4,6-tris(4-pyridyl)pyridine]·NO 3 ·CH 3 OH) ∞ , previously synthesized with a 4-fold helical channels ( Figure 1) and different inorganic anions can be recognized in water via color change [20]. Cu (I)-MOF is very suitable for application in water treatment because it has very good water stability and is easy to produce in large quantities in practical application. However, the identification of heavy metals has not been studied. In this study, the adsorption behavior for Cr (VI) was investigated in aqueous solution related to the optimization, kinetics, thermodynamics, and mechanism studies.

Preparation of Cu (I)-MOF.
e Cu (I)-MOF was synthesized using a hydrothermal method as reported by the previous work [20]. Briefly, 0.031 g of ligand (2,4,6-tris (4pyridyl) pyridine) and 0.072 g of copper nitrate trihydrate were added into 100 mL reaction kettles, and then 10 mL of mixed solvent (N, N-dimethylacetamide and methanol, volume ratio is 1 : 1) was added. e reaction kettle was shaken well, screwed down, and put in the oven at 120°C for 72 h. Allow cooling to room temperature, cleaned with methanol, and orange crystals obtained. en, it was dried at room temperature and stored to use.

Adsorption Experiments.
e experiments were conducted to assess the effects of various parameters on Cr (VI) adsorption onto Cu (I)-MOF. Briefly, (1) pH was adjusted to 4, 5, 6, 7, and 8 using a certain amount of dilute HCl or NaOH solution; (2) adsorbent dosage: 0.15, 0.30, 0.45, 0.60, and 0.75g·L −1 ; (3) contact time: 5, 10, 20, 30, 60, 120, and 720 min; (4) initial Cr (VI) concentration: 25, 50, 100, 200, and 250 mg·L −1 ; and (5) temperature: 288, 298, 308, 318, and 328 K. Each adsorption experiment was carried out by mixing a weighted amount of Cu (I)-MOF with 100 mL of Cr (VI) solution in a 100 mL beaker. e mixture was shaken by magnetic stirring (120 rpm) at a definite temperature for a proper time. e supernatant fluid was obtained through the centrifuge, and the concentrations of Cr (VI) were analyzed using the flame atomic absorption spectrometer. e adsorption capacity was calculated according to the following equation: where q e is the adsorption capacity at equilibrium (mg·g −1 ), c 0 is the initial concentration of Cr (VI) ionic solution, c t is the concentration after adsorption time t (mg·L −1 ), m is the mass of adsorbent (g), and V is the volume of Cr (VI) solution (L).

Kinetics Models
where q e is the same as in equation (1), q t is the adsorption capacity of the adsorbent on chromium (VI) ion at any time t (mg·g −1 ), and k 1 is the rate constant of the pseudo-first-order equation (min −1 ) [21].

Pseudo-Second-Order Equation.
q where t, q t , and q e are the same as in equation (2), and k 2 is the rate constant of the pseudo-second-order equation (g·mg −1 ·min) [22].

Elovich Model.
where t and q t are the same as in equation (2), α is the initial rate constant (mg·g −1 ·min), and β is the desorption coefficient (mg·g −1 ) [23].

Intraparticle Diffusion Model.
where t and q t are the same as in equation (2), k p is the rate constant of the intraparticle diffusion model (mg·g −1 ·min), and C is a constant related to the thickness of the boundary layer (mg·g −1 ) [24].

Isotherm Models
where q e is the same as in equation (1), c e is the concentration of adsorbate in equilibrium solution (mg·L −1 ), q m is the maximum adsorption capacity of the adsorbent (mg·g −1 ), and K L is the Langmuir adsorption constant (L·mg −1 ), which is related to the affinity of binding sites [25].
where q e and c e are the same as in equation (6), K F is Freundlich adsorption constant ((mg·g −1 ) (L·mg −1 ) n), and the adsorption index 1/n can reflect the extent of the adsorption reaction. If the value of 1/n is between 0.1 and 0.5, the adsorption is favorable; if 1/n is greater than 1, it is unfavorable; if 1/n is zero, it is irreversible [26].

Adsorption ermodynamics.
where K θ e is the thermodynamic equilibrium constant (dimensionless) [27], ΔS θ is the reaction entropy change (J·mol −1 ·K −1 ), ΔH θ is the reaction enthalpy change (kJ·mol −1 ), R and T are the same as in equation (5), K L is the Langmuir equilibrium constant (L·mg −1 ), C θ Adsorbate is the unitary standard concentration of the adsorbate (1 mol·L −1 ), c adsorbate is the coefficient of activity (dimensionless), and ΔG θ is the Gibbs function change (kJ·mol −1 ). e values of reaction enthalpy change (ΔH θ ) and entropy change (ΔS θ ) were calculated from the slope and intercept obtained by 1/T versus lnK θ e plot.

2.9.
Characterization. e infrared spectra of surface functional groups of samples were investigated using Fourier transform infrared spectroscopy (WQF-510A, Beijing Rayleigh Analytical Instrument Co., Ltd.) with wave numbers from 4000 to 500 cm −1 . e spectra of XPS were measured with the ermos Scientific K-Alpha XPS spectrometer. e powder XRD spectra of the samples were performed by X-ray diffraction (XRD-6100, Shimadzu) using Cu Kα radiation (λ �1.5406Å), employing a scan step of 0.02°in the 2θ ranged from 5°to 40°. e morphology of samples was observed by using a scanning electron microscope (SEM, JSM-7001F).

Optimization of Adsorption Conditions
e pH value of the solution will affect the types, binding sites, valence state, and surface charge of metal ions, even the arrangement of molecular structure [28]. Cr (VI) adsorption capacity at various pH values was performed after equilibration with Cu (I)-MOF. As shown in Figure 2, the adsorption capacities decreased with the increase of the pH value. A similar tendency was reported on Cr (VI) adsorption [29]. Although the maximum adsorption amount has been achieved at pH 4.0 experimentally, the pH of 6.0 was selected for next experiments. Because, the pH value is required between 6 and 9 in the actual water treatment process according to the standard of wastewater discharge.

Effect of Adsorbent Dosage.
e amount of Cu (I)-MOF material directly affects the adsorption efficiency of Cr (VI) ions, and the dose selection will directly guide for the removal of Cr (VI) in the process of water treatment. As shown in Figure 3, at the early stage, the equilibrium adsorption amount was proportional to the amount of adsorbent. However, they were inversely proportional at the late stage. e former maybe explained by the sufficient surface area was available for the adsorption of Cr (VI), while the latter might be explained by the comparatively less available specific surface area with increase in adsorption of Cr (VI) ions. erefore, 0.45 g·L −1 of Cu (I)-MOF material was selected as the optimal dosage for Cr (VI) ions adsorption for the next experiments. Figure 4, the rapid adsorption rate occurred within 120 minutes, and the adsorption capacity of Cr (VI) ions gradually reached equilibrium within 180 min. Because, Cu (I)-MOF material surface has the sufficient unsaturated sites at the initial stage of the adsorption. With the increase of time (>180 min), some adsorption sites bonded with Cr (VI), and the adsorption rate increased slowly and reached completely equilibrium at 720 min. erefore, 180 min was selected as the optimal time for Cr (VI) ions adsorption in the next studies.

Adsorption Kinetics.
e adsorption kinetics can provide the information on adsorption efficiency and reactor scale, which can provide the reference for the design parameters of water treatment equipment. To illustrate the adsorption kinetics, the adsorption experiments were performed at different times. e kinetic results fitted by four models included pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion model. e pseudo-firstorder model can provide a simple monophasic description mathematically [21]. e pseudo-second-order model assumed that the adsorption rate is controlled by the chemisorption [22]. e Elovich model assumed that the active sites on solid surface are heterogeneous [23], and it can indicate the different activated energies for chemisorption. For the intraparticle diffusion model, it was usually used to estimate whether intraparticle diffusion, bulk diffusion, or film diffusion is the rate limiting step [24]. e parameters of the four models were calculated by nonlinear fitting approach to avoid expressive errors [30]. Based on the comparison of the values of the adjusted determination coefficients (R 2 adj ), as given in Table 1, the pseudo-first-order model (R 2 � 0.9230) was the highest value among the four models, which indicated that the pseudofirst-order model was more suitable for the adsorption of Cr (VI) onto Cu (I)-MOF in aqueous solution.

Adsorption Isotherm.
To elucidate the isotherm mechanism, the adsorption experiments were performed at different initial concentrations of Cr (VI). Cr (VI) adsorption by Cu (I)-MOF was investigated by two models included Langmuir and Freundlich. Briefly, the Langmuir adsorption isotherm model often describes as monolayer adsorption [25]; it assumes a uniform surface bond and no interaction with other adsorbate molecules. On the contrary, the Freundlich isotherm assumes that adsorbate molecules bond to the adsorbent on a multilayer surface and they are influenced by other adsorbates [26].
As given in Table 2, the highest degree of R 2 adj (0.9827) was related to the Langmuir model, which indicated that the adsorption of Cr (VI) was more inclined to the Langmuir model. Accordingly, adsorption behavior belonged to monolayer adsorption (homogeneous surface), and the Langmuir isotherm model predicted the maximum adsorption capacity of 95.92 mg·g −1 .
MOFs are hybrid materials composed of metal (oxide) cation clusters and multiple ligands. ey have many advantages, such as large specific surface area, special metal   sites, easy aperture adjustment, and easy surface modification. Compared with other materials (Table 3), Cu (I)-MOF material as adsorbent has a larger adsorption capacity, which was much higher than those of most adsorption materials. But the most striking feature is very good water stability and easy to prepare in large quantities in practical application. It has a broad application prospect for pollutant removal in water environment.

Adsorption ermodynamics.
To investigate the adsorption thermodynamics, the adsorption experiments were performed at different temperatures. e parameters of thermodynamics are given in Table 4; the negative value of ΔH θ suggested exothermic reaction in the process of adsorption, while negative ΔS θ suggested that there was a decrease in randomness at the solid/solution interface during the adsorption of Cr (VI) onto the Cu (I)-MOF. Importantly, the spontaneous degree of reaction was estimated by the Gibbs energy (ΔG): ΔG < 0, spontaneous; ΔG � 0, equilibrium state; ΔG > 0, not spontaneous. e values of ΔG were less than zero in the temperature range of 288-328 K. eir absolute values decreased with an increase of temperature, which implied that the reaction was spontaneous, and the increasing temperatures were unfavorable to adsorption.

Adsorption Mechanism.
e kinetic results were well fitted to the pseudo-second-order model, which indicated that Cr (VI) adsorbed by Cu (I)-MOF belongs to the chemisorption instead of physical adsorption. Chemisorption includes chemical bonding, coordination bonding, acid-base interactions, and ion exchange [18]. First, the possibility of chemical bonding can be ruled out because the chemical valence states of C, N, and Cu in MOF materials did not change before and after adsorption. Second, coordination bonds are not possible because the central coordination atom Cu is already saturated [20]. ird, for acid-base interaction, it is even less likely because there are no other cations in the system except for H + , only nitrate in the MOF material, and chromate ions in water. erefore, the ion exchange was the most likely adsorption mechanism in aqueous solution.
For one thing, Cu (I)-MOF is rich in NO 3 − ion; on the other hand, the pH-dependent of increased Cr (VI) adsorption indicated that HCrO 4− ion was adsorbed into Cu (I)-MOF. Because, the Cr (VI) ions exist in the form of HCrO 4 − and CrO 4 2− in aqueous solutions as follows: e chromates mainly exist in the form of HCrO 4 − under acidic conditions (2.2 < pH < 6.49), while it exists in the form of CrO 4 2− at pH > 6.49. To sum up, Cr (VI) ions exist in the form of HCrO 4 − at pH of 6.0 in water. Both the results of infrared spectra and X-ray photoelectron spectroscopy (XPS) supported the ion exchange pathway. In detail, the peak of NO 3 − weakened at 1368 cm −1 [20] in the IR spectra, while a new peak of HCrO 4 − appeared at 918 cm −1 [19] (Figure 5) suggested that the ion exchanges were partly happened during the adsorption process. Furthermore, the new peak corresponding to Cr 2p in XPS was clearly appeared after Cr (VI) adsorption onto Cu (I)-MOF material ( Figure 6(a)). In addition, the high-resolution XPS spectrum illustrated that the peak at 576.78 and 587.28 eV, corresponding to the Cr 2p 3/2 orbital peak, was considered to be Cr (III) [37], and the peak at 578.58 and 587.68 eV, corresponding to the Cr 2p 1/2 orbital peak, was considered to be Cr (VI) (Figure 6(b)). ese results implied that a portion of Cr (VI) was reduced to Cr (III) after being adsorbed by Cu (I)-MOF under acidic condition. Above results clearly showed that ion exchange successfully occurred during the process of adsorption.

Stability.
e stability of Cu (I)-MOF was assessed through three cycles of adsorption. As shown in Figure 7, the X-ray diffraction (XRD) spectra of Cu (I)-MOF before and after Cr (VI) adsorption showed similar patterns after three cycles. It was only that some peaks were less intense, indicating that the basic framework of the Cu (I)-MOF did not change after Cr (VI) adsorption. Furthermore, as shown in SEM images (Figure 8), the framework of Cu (I)-MOF was not obvious collapse and decomposition. ese results suggested that Cu (I)-MOF as a new type of adsorbent can play an important role in removing Cr (VI) in water at least three cycles.
adj is the adjusted determination coefficient; k 1 is the first-order rate constant; q e is the equilibrium adsorption capacity; k 2 is the second-order rate constant; α is the initial rate constant; β is the desorption coefficient; k p is the rate constant; C is a constant related to the thickness of the boundary layer.

Conclusion
e Cr (VI) absorbed by the Cu (I)-MOF was investigated for the optimization, isotherm, kinetics, thermodynamics, and mechanism. First, the optimal pH was 6, the optimal amount of adsorbent was 0.45 g·L −1 , and the adsorption saturation time was 180 min, respectively. Second, the kinetics results revealed that the adsorption of chromium (VI) followed the pseudo-first-order model. ird, the isotherm study indicated that the Langmuir isotherm was the excellent match model. It belongs to a monolayer adsorption, and the maximum adsorption capacity was 95.92 mg·g −1 . Fourth, the thermodynamic parameters of the adsorption demonstrated that the reaction was spontaneous at the temperature range of 288-328 K. Finally, both infrared and XPS results supported the adsorption mechanism was the ion exchange pathway in water. In conclusion, Cu (I)-MOF material has a large of adsorption capacity, good water stability, low cost, and easy to prepare in large quantities in practical application. It will expect the removal of Cr (VI) ions from water as a new type of adsorbent. In the actual treatment, it can be used for the advanced treatment of the wastewater or the purification system of domestic supply water.

Data Availability
e data used to support the findings of this study are included within this article.