Synthesis and Application of Cobalt-based Metal-organic Framework for Adsorption of Humic Acid from Water

In this study, the adsorption of humic acid on cobalt based metal-organic framework (Co-MOF) was investigated. Co-MOF was synthesized via solvothermal technique and further characterized using X-ray diffraction (XRD), Fourier transform infrared spectros-copy (FTIR), and scanning electron microscopy (SEM). The characterization results of material confirm the formation of MOF structure. The adsorption kinetics, isotherms, thermodynamics, as well as isosteric heat of adsorption were also investigated by obtaining experimental adsorption data through batch experimentation. Optimum adsorption uptake of ~91 mg g –1 was attained at pH 6 and 305 K. Regression analysis of experimental results revealed that adsorption kinetics follows a pseudo-second-order kinetic model, and adsorption can reach equilibrium at ~20 min. Adsorption isotherm data can be well fitted with Koble Corrigan isotherm. Thermodynamic parameters demonstrated that the adsorption of humic acid is a spontaneous, endothermic, and physical process, while isosteric heat evaluations revealed the heterogeneous nature of the adsorbent. Overall, the Co-MOF was a promising choice to adsorb humic acid from water.


Introduction
A natural organic matter (NOM) is a complex mixture that comprises humic, fulvic, amino acids, and humin-like substance. NOMs may exist in surface and ground water resources, and their presence might be carcinogenic because of reaction of NOM and disinfectants. At high concentration in water, the chlorination of NOM-laden water can result in disinfection by products like trihalomethanes during the chlorination process. Other than the formation of disinfection byproducts, it augments microbial growth and ultimately increases the use of disinfectants and coagulants during water treatment processes 1 . A primary component of NOM is a macro-molecular humic acid, which is typically identified through its brownish appearance in water. It mainly contains hydroxyl, carboxyl, phenolic, and carbonyl functional groups in its aliphatic and aromatic chains. Conventional treatment of a wastewa-ter bearing humic acid may lead to the formation of harmful compounds like haloacetonitriles, haloketones, trichloroacetaldehyde, halo acetic acids (HAAs), and trihalomethanes (THMs) as disinfection byproducts through reaction with halogens. Hence, elimination of HA from water is of great importance from an environmental and health point of view [2][3][4] .
Several techniques like chemical coagulation 5 , membrane separation 6 , advanced oxidation 7 , adsorption 8 , and biochemical degradation 9 have been practiced to eradicate humic acid from polluted water. Among these, adsorption could be considered as the best method because of its simplicity, lack of byproducts, and most importantly, low sensitivity to toxins. Thus, different classes of materials have been reported that can effectively treat wastewater polluted with humic acid. However, most conventional materials such as zeolitic tuff 10 , fly ash 11 , chitosan 12 , activated carbon 13 , rice husk ash 14 , and clay 15 show limited adsorption capacities (i.e., less than 50 mg g -1 ). Therefore, it is required to develop an efficient adsorbent with high affinity for humic substances. Metal-organic frameworks (MOFs) is an innovative class of crystalline microporous materials that are available in one-, two-, and threedimensional networks. In the development of MOFs, the choice of metal ion, ligand, synthesis route, and solvent has resulted in a variety of MOFs such as MIL, HKUST, IRMOF, UiO, Co-MOF, and ZIF [16][17][18] .
Cobalt-based MOFs are of great importance in many areas because of their size, redox ability of cobalt metal, and nature of metal-ligand bond in the structure. Murinzi et al. synthesized cobalt 2-6-pyridine dicarboxylate MOF and demonstrated its successful application in electrochemical sensing 19 . Weidong Fan et al. described the synthesis of 2D cobalt-based metal-organic framework (UCP-32) for removal of C 3 light hydrocarbons from methane, and fuel gas purification. Results exhibit that UCP-32 has high adsorption uptake for H 2 and higher affinity for CO 2 . Its narrow pore size allowed high separation selectivity for C 3 against methane 20 . The adsorptive removal of Cr(IV) (69.4 mg g -1 ) and As(V) (71.4 mg g -1 ) from river and wastewater samples using Co-MOF has been reported by Azile Nqombolo et. al. Further, they found that the prepared MOF showed good recyclability 21 . Different MOFs have been examined as propitious adsorbents for eradicating hazardous pollutants from wastewater like heavy metal cations and anions, dyes, etc., by electrostatic/hydrophobic interactions, π-π stacking, hydrogen bonding, and acid-base interactions [22][23][24][25][26][27] . However, the removal of humic acid (HA) onto MOFs has been rarely reported to date. Humic acid is a long chain molecule that contains various functional groups including -OH, -COOH, and these are present in anionic (i.e., HA-COO-) form in aqueous solution because of ionization 3,28 . The tendency of ionization of functional groups in HA suggests that the hydrogen bonding and electrostatic interactions may be proper approaches to attain highest removal efficiency of humic acid from aqueous solution.
Thus, the focus of this study was the use of cationic transition (redox active Co) metal and carboxylate linker in the synthesis of MOF that might exhibit strong positive charge surrounding the fra mework besides active sites formed by functional groups 29 . To ascertain the research idea, cobalt-based MOF was produced through a surfactant-assisted solvothermal method using triethylamine (TEA) as a surfactant media. The synthesized MOF material was characterized through SEM, XRD, and FTIR, and its efficacy for treatment of humic acid-bearing water was tested by batch adsorption. Isothermal, dynamic, and thermodynamic analyses were also carried out to evaluate the adsorption performance of prepared cobalt-based MOF.

Synthesis and characterization of cobalt-based MOF
Co-MOF was synthesized through solvothermal synthesis using the following steps. Co(NO 3 ) 2 •6H 2 O (0.70 g, 2.4 mmol), oxalic acid (0.21 g, 2.4 mmol) were dissolved in 30 mL of DMF solvent with mild stirring. TEA (0.24 g, 2.4 mmol) was added to the solution and stirred for proper mixing. Homogeneous solution was placed in a preheated oven at 120 °C for 4 h. After cooling to room temperature, pink precipitates of MOF were separated through filtration, washed with methanol to remove the unreacted substances, and dried at 40 °C for 4 h. The material was named Co-MOF. The dried Co-MOF material was examined for its morphology, crystal phases, and functional groups. Surface morphology was studied using scanning electron microscope (SU1510, Hitachi, Japan) at an accelerated voltage of 20 kV. To analyze the functional groups, Fourier transform infrared spectrometer (FTIR -4100 type A, JASCO Inc.) was used to record the spectrum in the range of 450 to 4000 cm -1 using KBr pellets. X-ray powder diffraction patterns were recorded using Bruker (Germany) D8 Advance powder diffractometer to investigate the amorphous and crystalline structure of material. The analysis conditions were 20 kV voltage, 5 mA current, X-ray source of Cu Kα (λ = 1.54021 Å) operated in a 2θ range of 5° to 40° at a scan speed of 1° min -1 . To study the surface charge characteristics of Co-MOF, point of zero charge (PZC) was determined. For this purpose, distilled water (50 mL each) with different initial pH ranging from 2 to 10 was placed in Erlenmeyer flasks. An amount of 25 mg Co-MOF was added in each flask and left overnight on stirrer to reach equilibrium at room temperature. The equilibrium solution pH was then measured and plotted against the initial solution pH to get PZC.

Adsorption experimentation
Batch adsorption experiments were performed to study the effect of different parameters such as initial pH, contact time, concentration, and temperature on the removal of humic acid from water. Stock solution of humic acid (HA) was prepared by dissolving 1 g of humic acid in 62.5 mL NaOH solution (0.1 M) (added to promote the complete dissolution of HA), and further adding distilled water to obtain 1 L solution. Stock solution was diluted for further experimentation. All the adsorption experiments were performed in a thermostatic shaker (150 rpm) by placing Erlenmeyer flask of 250 mL containing 50 mL HA solution and 25 mg Co-MOF. The initial solution pH (recorded using HI 8424, Hana Instruments, Italy) was adjusted using HCl or NaOH solution (0.1 M).
The effect of pH on the adsorption of HA was investigated by changing the initial pH of the HA solution from 2-10 at constant temperature of 305 K, contact time of 100 min, and HA concentration of 50 ppm. To evaluate kinetics and the equilibrium time for adsorption of humic acid, contact time was varied from 0-100 min while keeping constant HA concentration (50 ppm), temperature (305 K), and pH (6). For isotherms and thermodynamics studies, adsorption experiments were carried out by changing the HA concentration from 5 to 200 ppm at different temperatures (325 K, 305 K, 295 K, and 285 K), constant pH 6, and contact time of 100 min. After completion of each adsorption experiment, solution was filtered using Whatman filter paper No. 41. Initial and residual HA concentration was analyzed using UV-Vis spectrophotometer (Optima, SP-3000, Japan) by measuring the absorption at λ max = 321 nm, and then the absorbance was compared with calibration curve (Fig. 1) to reveal the respective concentration of solution.
Further, the uptake capacity 'q' (amount of HA adsorbed per unit mass of Co-MOF, mg g -1 ) was evaluated using equation (1): where C 0 (mg g -1 ) and C f (mg g -1 ) are the HA initial and final concentrations, respectively, V (L) represents the volume of humic acid solution, and m (g) is the adsorbent dosage of Co-MOF. To study the effect of various adsortpion parameters and further obtain insight into the adsorption process, adsorption mechanism and thermodynamics parameters, experimental adsoprtion data was fitted with different kinetics and isotherm models [30][31][32] . The representative equations of different models are provided in Table 1.

Results and discussion Characterization
The SEM image of synthesized Co-MOF shows well-groomed orthorhombic crystals as depicted from Fig. 2. The orthorhombic crystals of Co-MOF were further examined by XRD. Sharp diffraction peaks in the X-ray diffractograms at 7.2°, 12.4°, and 16.2° reflect the crystalline nature of MOF and respective planes of Co such as (011), (112), and (013). The intense peak at 7.2° confirms the synthesis of Co-MOF ( Fig. 3) 33,34 . Furthermore, FTIR analysis was performed to study surface functional groups. Fig. 4 shows the FTIR spectra of both oxalic acid di-hydrate and Co-MOF. In spectrum of oxalic acid dihydrate, broad peak at 3466 cm -1 represents O-H stretching vibrations, while sharp peaks at 1638 cm -1 and 1475 cm -1 correspond to asymmetrical and symmetrical stretching vibrations of C=O bond. The adsorption peaks between 3300 cm -1 to 2500 cm -1 actually represent the O-H stretching of carboxylic acid dimers in both spectrums. Broad peak located at 3467 cm -1 in the Co-MOF represents O-H stretching vibrations. Absorption peaks at 1634 cm -1 and 1475 cm -1 are assigned to asymmetric and symmetric stretching vibration of C=O in the oxalic acid. In both the spectrums, peaks at 1322 cm -1 and 1375 cm -1 correspond to the bending vibrations of O-H, while the peaks at 1166 and 1156 cm -1 and the two bands at 1065 and 1030 cm -1 are assigned to C-O stretching of dicarboxylic acid, respectively. In the FTIR spectrum of oxalic acid, intensive peak at 790 cm -1 and intensive peak at 808 cm -1 appeared in the spectrum of Co-MOF, which may be assigned to out-of-plane bending motion of O-C=O functional group. Peaks at 1322 cm -1 and 1372 cm -1 correspond to the bending vibrations of C-O and O-H, respectively. Compared to FTIR spectrum of oxalic Isotherm Two-parameter model ∑ q e,exp (mg g -1 ), experimental adsorption uptake q e,cal (mg g -1 ), calculated adsorption uptake n, number of experimental data points acid, an intensive peak at 808 cm -1 appeared in the spectrum of Co-MOF, which can be assigned to out-of-plane bending motion of O-C=O functional group and the presence of metal-oxygen bond. Small peak at low wavenumber range, i.e., 503 cm -1 also confirmed the bending vibration of Co-O in Co-MOF 35 .

Effect of initial solution pH
The pH of pollutant solution is an important parameter, which not only affects the degree of dissociation of adsorbate but also changes the surface properties of the adsorbents. The surface charge characteristics of Co-MOF are presented in Fig. 5 as an inset graph. PZC is equivalent to ~ 7.4 corresponding to which surface is considered as neutral. Co-MOF surface has net positive charge when the pH of aqueous environment is below 7.4; however, the sorbent surface becomes net negative charged above 7.4. Based on the PZC analysis, it is evident that the adsorption of negatively charged humic acid will be favored in acidic medium over Co-MOF.
To establish the conclusion from PZC analysis, the effect of initial solution pH on the uptake of Co-MOF is presented in Fig. 5. Uptake capacity of adsorbent was different under different pH conditions. Despite negative surface of adsorbent under extreme acidic conditions, i.e., between 2 to 3 pH, low uptake capacity can be observed from Fig. 5. It can be associated to formation of oxonium ions (H 3 O + ) in the solution, which decreases the dissociation of surface acidic functional groups (carboxylic and phenolic groups) present in HA. Therefore, electrostatic interactions between positively charged ad-  36 . With the rise in initial solution pH, uptake of HA increased and attained a maximum value corresponding to pH 5, after which the uptake was not changing as rapidly as before until pH 7. HA contains weekly acidic, carboxylic, and phenolic functional groups, which have the tendency to ionize at mildly acidic or neutral pH. As a result, electrostatic interaction between HA and positively charged surface of Co-MOF increases the uptake. Further increase in initial solution pH from neutral to basic (above PZC 7.4) turned the adsorbent surface to negative, which led to a decline in HA adsorption onto Co-MOF due to repulsive forces between sorbent and adsorbate. Additionally, in basic environment, HA is reported to exist in a fully dissociated form and arranged in a torus or ring-like structure, which makes it more hydrophilic and therefore less likely to adsorb on adsorbent surface 37 .

Kinetics study
The results of the effect of contact time for the adsorption of HA over Co-MOF are presented in Fig. 6. It can be noticed that 90 % (~ 85 mg g -1 ) of HA is adsorbed in less than 10 minutes and becomes slower with time. However, there is no significant change after an hour, which implies equilibrium under imposed experimental conditions. Rapid adsorption of HA in a few minutes indicates abundantly available vacant sites and high affinity between HA moieties and MOF surface. The equilibrium uptake of Co-MOF was observed to be 89 mg g -1 . The kinetic data was analyzed through adsorption controlled kinetic models, i.e., pseudo-first and second order kinetic models. These models consider that the formation of surface complex would be the rate-determining step in the adsorption process. Table 2 summarizes the results of linear regression of kinetic data. Best fit model can be decided by comparing the values of some statistical parameters such as regression coefficients (R 2 ), and residual sum of squares (RSS). From Table 2, results show that the pseudo-second-order model is superior to first order kinetic model with high R 2 and comparatively low RSS values. Additionally, there is an insignificant difference between calculated equilibrium adsorption capacity (q e,cal ) and experimental (q e,exp ) uptake capacity for pseudo-second-or- der model. A graphical plot based on a pseudo-second-order model can also be viewed in Fig. 6, which shows the goodness of fit kinetic data. It can be concluded that liquid film diffusion only plays its role in the adsorption of HA onto Co-MOF. The equilibrated uptake after few minutes may also be linked to depleting bulk concentration of pollutant during batch adsorption.

Adsorption isotherms
In batch adsorption, equilibrium is established between adsorbate in the solution phase with adsorbate onto Co-MOF surface at a constant temperature. Corresponding to each mentioned temperature in Fig. 7, the batch experiments were performed with different initial concentrations.
For a fixed isotherm, each data point corresponds to equilibrium concentration that was achieved for different initial concentration of the pollutant. The isotherm results follow the type III isotherm, which suggest that adsorbate-adsorbate reaction supersedes the adsorbate-adsorbent interactions. The HA uptake is low at low initial concentrations until surface coverage is sufficient, then the interactions of free and adsorbed species start to dominate the process 38,39 . Type III isotherm can be the result of the presence of large adsorbate molecules, which are too large to access the micropores, and adsorption takes place on the surface and in extra-crystalline pores. The steepness of isotherms at higher temperature indicates that the probability of collisions among adsorbate moieties raises the adsorption capacity. Furthermore, it can be concluded that HA uptake over Co-MOF is endothermic in nature. Although the mechanism of adsorption of pollutant from aqueous phase is complicated in nature, the correlation of experimental data with theoretical isotherm models provides a clue to key mechanism steps involved in the adsorption of HA. Two-parameter isotherm models, namely Freundlich and Dubinin-Radushkevich, and three-parameter model, namely Hill and Koble-Corrigan, were used to fit the experimental data of adsorption (equations provided in Table 1).
Nonlinear regression of experimental data produced the isotherm results shown in Table 3. Freundlich isotherm equation produces fairly high correlation coefficient for all temperatures, indicating better agreement of this model equation with experimental equilibrium adsorption data of HA onto Co-MOF. K F varies from 3.44 to 0.37 when the experimental temperature decreases from 325 K to 285 K. The parameter K F links with adsorption capacity, and its increase with temperature reveals endothermic nature of adsorption. At all temperatures, n F is less than one, indicating that bond energy decreases with surface density, i.e., heterogeneous nature of Co-MOF. R 2 value for DR isotherm is too low to be considered for the description of mechanism of adsorption of HA onto Co-MOF 40 . A three-parameter Hill's isotherm model produces high correlation coefficient (R 2 ) for all temperatures, but gives very high values of q max . Apart from obtaining higher values of q max from Hill isotherm, the change in up-   Table 3 shows that the Koble Corrigan isotherm is in good agreement with adsorption equilibrium experimental data of humic acid for all temperatures. n KC > 1 confirms the applicability of KC isotherm model, and increase in parameter A KC with temperature reveals that the adsorption of humic acid onto Co-MOF is favorable at a higher temperature. Comparison of isotherm models shows that Koble Corrigan model is the best-fitted model among all isotherms, and reveals that the surface of Co-MOF is heterogeneous. For greater visibility, the experimental data was also compared with KC model results through Fig. 7b, where it can be seen that both are in good agreement with each other.

Thermodynamic studies
Thermodynamic parameters provide an idea about the spontaneity and nature of adsorption process. These parameters include Gibbs free energy ΔG° (kJ mol -1 ), change in enthalpy ΔH° (kJ mol -1 ), and entropy change ΔS° (kJ mol -1 K -1 ). The pertinent equations relating to all thermodynamic pa-rameters are summarized in Table 1. The thermodynamic equilibrium constant (K D = C ad /C e ), ratio of equilibrium concentration of adsorbed HA onto Co-MOF to the equilibrium concentration of HA in filtered solution) was evaluated from experimental equilibrium data. The Van't Hoff plot, i.e., ln K D vs 1/T, in Fig. 8, produced the values of ΔS° and ΔH°; however, ΔG° was obtained from the equation provided in Table 1. The estimated values of thermodynamic parameters for adsorption of HA onto Co-MOF are listed in Table 4. The Gibbs free energy (ΔG°) is negative at all temperatures, indicating that the HA adsorption process is globally spontaneous and feasible. The negativity of ΔG° increases as with temperature, implying that the adsorption of humic acid is thermodynamically favorable at a higher temperature. Positive value of ΔH° at all concentrations confirms the endothermic character of HA adsorption.
The adsorption process is usually viewed as a combination of two steps: firstly, the desorption of pre-adsorbed water molecules on the surface of Co-MOF, and secondly, the adsorption of HA species onto Co-MOF surface. In the adsorption process, HA may have to displace more than one water molecule, hence, resulting in positive values of change in enthalpy. ΔH° also helps to conclude whether the adsorption mechanism is physical or chemical. If ΔH° is less than 80 kJ mol -1 then the adsorption can be considered as physisorption where the Van der Waal's forces and electrostatic interactions play their role in the adsorption. However, the adsorption process follows the chemisorption mechanism if ΔH° is greater than 80 kJ mol -1 41 . For HA adsorption over Co-MOF, ΔH° varied in the range of 24 to 31 kJ mol -1 , suggesting a physisorption process. Moreover, positive value of ΔS° revealed the affinity of HA for Co-MOF, and suggested the randomness at solid/liquid interface, thereby indicating the structural variations in Co-MOF 42,43 . Overall thermodynamics parameters indicated the spontaneity, feasibility, as well as endothermicity of HA adsorption onto Co-MOF. In addition, isosteric heat of adsorption (DH x ) at a fixed surface coverage was calculated using Clausius-Clapeyron equation mentioned in Table 1. The isosteres related to different equilibrium adsorption capacities of HA are presented in Fig. 9. The correlation coefficients obtained through linear regression for each isostere and enthalpies of isosteres are given in Table 5. Isosteric heat of adsorption was found to be smaller than 80 kJ mol -1 confirming that the adsorption of humic acid onto MOF surface involves physical forces between adsorbate and MOF. Variations in (DH x ) corresponding to uptake capacity of HA indicated that Co-MOF has energetically heterogeneous surface. Results revealed that (DH x ) increased with increase in surface coverage, probably due to the dominance of intermolecular interaction between adsorbed HA molecules. This behavior suggests the presence of possible strong lateral interaction of humic acid moieties when adsorbed on the surface of Co-MOF that already has adsorption sites of different activities. Performance of the Co-MOF to adsorb HA from aqueous phase was compared to other materials reported in the literature ( Table 6). The comparison indicated reasonably good uptake of prepared Co-MOF.