Removal of molybdenum from wastewater using modified amino‐functional framework: A study of the adsorption properties

Herein, we have employed a convenient method to study kinetic and thermodynamic parameters of the adsorption of molybdenum from wastewater by using modified Zr‐based metal–organic frameworks, UIO‐66‐NH2‐X, as adsorbents. The frameworks were fabricated with a simple and fast method from terephthalic acids and Zirconium metal ions and modified via introduction of aldehyde groups using imine condensation. This is the first report in which MOFs have been utilized as adsorbents in molybdenum adsorption. The prepared adsorbents show high adsorption efficiency, which is representative of an improvement compared to other literature. The adsorption capacity and molybdenum removal for the modified framework are 667 mg/g and 93%, respectively which is a significant and remarkable value among other previously reported articles. The experimental adsorption data could be well interpreted with the pseudo‐second‐order kinetics and the Langmuir isotherm model. Thermodynamic studies illustrated a spontaneous exothermic mechanism for the adsorption process.


| INTRODUCTION
Molybdenum is one of the active transition elements with the biological application as well as used in industry. 1 It is a vital nutrient for both humans and plants. In plants, it is indispensable to fix atmospheric nitrogen and convert nitrate into nitrite, while in humans it is needed for removing toxins and activating enzymes. 2,3 On the other hand, from industrial viewpoint, molybdenum has miscellaneous functions such as vacuum tubes, alloying agents, rubber, catalysts, and solid lubricants. 4,5 However, the excess effluent concentration of molybdenum in water caused crucial environmental problems. 6,7 Therefore the removal of molybdenum from wastewater is essential in order to eliminate its hazardous impact on the environment. 8 Different oxidation states of Molybdenum vary from Mo(II) to Mo(VI) exist in wastewater which among them molybdate ion (MoO 4 2À ) is the most soluble species. One of the most significant radionuclides is Molybdenum-99 ( 99 Mo) which is used in nuclear medicine. 99 Mo is one of the toxic compounds in radioactive waste that need to be removed in order to the protection of people and the environment. 9,10 Various methods have been investigated for molybdenum removal from wastewater such as chemical precipitation, adsorption process, ion exchange, electrochemical techniques, and biological treatment. [11][12][13][14] So far, among diverse available wastewater treatments, adsorption process is significant due to its effectiveness, easy separation, and suitable economical condition. [15][16][17] In general, developing environmentally friendly adsorbents with high adsorption capacity toward molybdenum adsorption from wastewater is still a major challenge among scientists. Many different adsorbents have been developed and applied in order to eliminate Mo(VI) from wastewater such as carbon nanotubes, pyrite, ferric oxide gel, chitosan, ion exchange resin, and so forth. [18][19][20] Metal-organic frameworks (MOFs) are a new class of porous materials that have attracted significant attention of many researchers in recent years due to their unique and special characteristics including high surface area and porosity, tunable pore structure, high thermal stability, and different accessible sites. 21  Therefore, the adsorption studies on molybdenum pollutant from wastewater was investigated over modified MOFs. With this regard, UIO-66-NH 2 has been successfully synthesized and modified with aldehyde functional groups using imine condensation (Scheme S1).
The adsorption capacity, kinetics, and thermodynamic have been evaluated in detail. Additionally, the equilibrium data were analyzed by using Langmuir isotherms. The details of the adsorption process and its effective factors will be discussed.

| Materials and instrumentation
Materials and instrumentation are explained in supporting information.

| Synthesis of adsorbents
Synthesis of adsorbents are demonstrated in supporting information.

| RESULTS AND DISCUSSION
In the present work, UIO-66-NH 2 was synthesized and successfully modified with aldehyde groups using imine condensation reaction (Scheme S1) in order to make a superior adsorbent for removal of molybdenum from wastewater.

| Characterization of adsorbents
The FT-IR spectra of the prepared compounds are depicted in Figure S1. The FT-IR spectrum of UIO-66-NH 2 ( Figure S1a To specify the morphology and particle size of synthesized compounds, scanning electron microscopy (SEM) was conducted and the micrographs are presented in Figure S3. The SEM images indicate uniform particle size of about 100 nm for UIO-66-NH 2 ( Figure S3a,b). In addition, there is no substantial changes in the morphology of the structures after modification of the framework ( Figure S3c,d).
Thermal and structural stability of the synthesized frameworks were investigated by using of thermogravimetric analysis (TGA) and the results are illustrated in Figure S4. The TGA curves show a twostep weight loss for the prepared compounds. As can be seen in Figure S4, the first weight loss for the as-synthesized UIO-66-NH 2 occurred at about 60-120 C which was related to the removal of guest molecules from the framework. The second weight loss is related to the thermal decomposition of the framework which occurred at about 400 C.
In the case of UIO-66-NH 2 -SA and UIO-66-NH 2 -OH after molybdenum adsorption, their residual mass is in accordance with the amount of Molybdenum adsorption by ICP ( Figure S4).
To determine the specific surface area, porosity, and the pore volume of the prepared frameworks, N 2 adsorption-desorption analysis at low temperatures was assessed and the results are summarized in Figure 1 and Table S1. The N 2 adsorption-desorption isotherms exhibited type I isotherms for both pristine MOF and modified frameworks. According to the values of textural parameters in Table S1, the surface area and pore volume for UIO-66-NH 2 were 886 m 2 /g and 0.45 cm 3 /g, respectively, whereas a substantial decrease for the corresponding values was observed owing to the modification of the framework. Therefore, the surface area and pore volume for UIO-66-NH 2 -SA and UIO-66-NH 2 -OH decreased to 574 m 2 /g and 0.31 cm 3 /g, and 436 m 2 /g and 0.25 cm 3 /g, respectively which affirmed the successful modification of the framework.

| Adsorption study of the synthesized frameworks
To evaluate the adsorption proficiency of the prepared frameworks, the adsorption of Mo was studied using UIO-66-NH 2 -SA and UIO-66-NH 2 -OH as the adsorbents. The pristine MOF (UIO-66-NH 2 ) have no capability for Mo adsorption. Therefore, the adsorption process was only performed with modified frameworks. Additionally, the effect of contact time, temperature, pH, pollutant concentration, and the amount of adsorbents was also investigated in the adsorption process.

| Molybdenum removal ability of the synthesized compounds
The removal of molybdenum from wastewater (Mo. R.) was calculated by the following equation: where C 0 and C t are the initial and the final concentration at contact time t (mg/L), respectively. On the other hand, the results of N 2 adsorption-desorption isotherms and textural parameters for modified frameworks after the adsorption of molybdenum indicate that molybdenum was adsorbed in both modified frameworks (Table S1,  adsorbent, the ionization degree, and consequently the structure of the adsorbate molecules. 29 Thus, the solution pH was evaluated in the range of 3-11. The pH of the solution was adjusted by 0.1 M HCl or 0.1 M NaOH, and the results are depicted in Figure S6. As can be observed in Figure S6, with the enhancement of the solution pH from 3 to 7, the amount of removed Mo increased, and then decreased upon the further increment of the solution pH. Thus, the best results for the pH value for both adsorbents were 7.

| The effect of adsorbent dosage
The adsorption process was performed with various adsorbent dosages and the results are summarized in Table S2. The results indicated that Mo removal percentage was enhanced upon increasing of the amount of adsorbent from 10 to 100 mg. Accordingly, 100 mg was obtained as the optimal value for both adsorbents.

| Effect of temperature on adsorption process
In order to evaluate the effect of temperature, the adsorption process was studied at different temperatures in the presence of UIO-66-NH 2 -SA and UIO-66-NH 2 -OH as adsorbents (Table S3). According to the results, the amount of removed Mo reduced upon temperature increment from 298 to 318 K. Thus, room temperature is the favorable temperature for the adsorption process. Adsorption isotherms with respect to the temperature are discussed later.

| The effect of molybdenum concentration
To elucidate the effect of pollutant, adsorption experiments were conducted with the diverse initial concentrations of Mo using UIO-66-NH 2 -SA and UIO-66-NH 2 -OH as adsorbents and the results are presented in Table S4. Based on the results, it is obvious that Mo removal decreased upon increasing Mo concentration owing to the occupation of active sites of the adsorbents.

| Adsorption kinetics
The study of adsorption kinetics is one of the significant factors in the adsorption process as it provides noteworthy information about the diffusion mechanism and the reaction pathway as well as the effect of   Table S6. According to the results, since the Langmuir isotherm shows a better fit with the adsorption experimental data for both adsorbent, the adsorption process occurs uniformly and in a monolayer form.

| Adsorption thermodynamics
To investigate the desirability of the adsorption process, adsorption thermodynamics was studied. The calculation method for thermodynamic parameters has been explained in supporting information.
To evaluate adsorption thermodynamic parameters, adsorption experiments were carried out over UIO-66-NH 2 -SA and UIO-66-NH 2 -OH as adsorbents at 298 K and the results are depicted in Table S7. Negative values were obtained for ΔG which revealed that the adsorption process occurred spontaneously.
3.4.9 | The plausible mechanism of the adsorption process As mentioned above, aldehyde functional groups incorporated into amine groups using imine condensation to form salicylaldehyde imine and hydroxynaphthaldehyde imine (Scheme 1, pathway I).
Molybdenum metal has the ability to combinate with suitable ligands to increase its coordination number and form octahedral geometry. On the other hand, oxygen and nitrogen atoms have high affinity to interact with metal centers. Therefore, in the next step, after the addition of molybdate anions, molybdenum atoms interacts and forms a Schiff base complex with the oxygen atoms of the hydroxyl groups and the nitrogen atom C N bonds (Scheme 1, pathway II). 30 Overall, we were able to achieve excellent results compared to others in very mild condition (room temperature and neutral pH).
In order to investigate the amount of molybdenum leakage absorbed from the prepared adsorbents, the leaching process was studied for adsorbed molybdenum during 2 months. According to the results, the leaching amount of adsorbed molybdenum from adsorbents after a period of 2 months was very low and was reported to be equal to 0.7 and 1.1% for UIO-66-NH 2 -SA and UIO-66-NH 2 -OH, respectively.
To assess the proficiency of the prepared adsorbents in view of the recent progress, a comparison was made between our prepared adsorbents and other adsorbents in which diverse compounds were applied to adsorb molybdate anions and the results are summarized in  Figure S12).

| CONCLUSION
In summary, a metal-organic framework with amine functional groups was synthesized and successfully modified with aldehyde functional groups using imine condensation. The prepared materials were utilized as an efficacious adsorbent to remove molybdenum from wastewater. This is the first report in which MOFs have been applied as adsorbents in molybdenum adsorption. High adsorption capacity was observed for both adsorbents compared with other previously reported materials. The experimental adsorption data could be well interpreted with the pseudo-second-order kinetics and the Langmuir isotherm model. Thermodynamic parameters indicated an exothermic spontaneous mechanism for the molybdenum adsorption process.
Since the present molybdenum in industrial effluents ( 99 Mo) has the same structure with the molybdenum used in this work, the prepared adsorbents can be used to adsorb radioactive molybdenum from industrial wastewater.

DATA AVAILABILITY STATEMENT
Data available on request from the authors.