Introduction
Accompanied by industrial production, the issue of hexavalent Cr6+ contamination in water environments is becoming increasingly severe. The adsorption method for removing Cr6+ from water has attracted attention due to its operational simplicity and low cost. MOF as adsorbents possess structural advantages such as large surface area and high porosity. Hybridization of MOF can increase the adsorption sites, enhance reaction activity, and further improve the adsorption performance of MOF. Research on synthesizing MOFs using KHP as an organic ligand is scarce. In this study, KHP was utilized as the organic ligand to synthesize urea-doped N/MOF(Fe) via a solvothermal method. The optimal urea doping concentration of N/MOF(Fe) was characterized using SEM, TEM, BET, XRD, XPS, and other techniques. The effects of factors such as the initial concentration of Cr6+, the dosage of N/MOF(Fe), the initial pH of the solution, the presence of coexisting ions on the adsorption reaction, and the corresponding mechannisms, were analyzed. The adsorption kinetic equation was established, the adsorption isotherm model, and adsorption thermodynamic parameters, were calculated.
Methods
After determining the optimal preparation conditions for MOF(Fe) through orthogonal experiments, a solution was prepared by adding 0.306 g of KHP 1.212 g of Fe(NO3)3·9H2O, 0.6 mL of acetic acid, and 0.028 mmol/L of urea into 24 mL of DMF. The solution was stirred until completely dissolved and then transferred to a reaction vessel lined with PTFE. The reaction was carried out at 180 ℃ for 20 h. Afterward, a reddish-brown precipitate was obtained by centrifugation. The precipitate was washed several times with deionized water and ethanol, followed by drying overnight at 80 ℃. The resulting material was named as N/MOF(Fe). A certain amount of the adsorbent was added to a 140 mL solution of Cr6+. At specific intervals, aliquots of the solution were taken out and the absorbance was measured to study the N/MOF(Fe) adsorption behavior.
Results and discussion
Orthogonal experiments indicate that controlling the pH value is crucial for the adsorption of Cr6+ by MOF(Fe). The optimal preparation conditions for MOF(Fe) are a reaction temperature of 180 ℃, a reaction time of 20 hours, a metal-to-ligand molar ratio of 1.0:0.7, and a pH value of 2.3. After urea doping, the material's adsorption capacity and adsorption rate were significantly enhanced. When the urea doping concentration was 0.028 mol/L, the adsorption rate of N/MOF(Fe) reached its maximum. N/MOF(Fe) exhibited a spherical structure with a lattice spacing of 0.12 nm. The diffraction peaks were broad, indicating low crystallinity and diffraction intensity. BET analysis showed that the adsorption of N/MOF(Fe) followed a type II isotherm with a hysteresis loop, indicating primarily mesoporous characteristics with pore size distribution mainly around 5 nm. With the increase in urea doping concentration, the pore volume and pore size of N/MOF(Fe) first increased and then decreased. The adsorption effect followed the same trend. Characterization by XRD and XPS revealed that the Fe3+ present in N/MOF(Fe) did not undergo a change in oxidation state during the adsorption process. There was a certain amount of Cr3+ in the solution.The optimal experimental conditions were found to be an initial Cr6+ concentration of 5 mg/L, an adsorbent dosage of 0.8 g/L, and no adjustment of the initial pH of the solution. Electrostatic interactions enabled N/MOF(Fe) to exhibit superior adsorption performance in acidic environments. The SO42– being divalent, exerted stronger electrostatic attraction compared to chromate ions in the solution, leading to a more significant competitive adsorption effect. Apart from SO42–, other coexisting ions have little effect on the adsorption of N/MOF(Fe). BET analysis indicated that the material regenerated twice exhibited similar N2 adsorption-desorption isotherms to the original material. The regeneration process had a slight effect on the pore size, pore volume, and specific surface area of N/MOF(Fe), but it did not significantly affect its adsorption performance. N/MOF(Fe) demonstrated good repeatability in reuse tests, with the adsorption rate remaining at around 70% even after multiple regenerations. The n in the Freundlich model is 8.13, indicating that the adsorption process is the main chemical adsorption and is easy to occur. In the intra-particle diffusion model, the intercepts of all three stages were non-zero, reflecting that surface diffusion is not the only limiting step. The a in the fitting result of Temkin model is 0.671, which proves that the adsorption of Cr6+ by N/MOF (Fe) is an exothermic process. By BQ, the mechanism of Cr6+ adsorption by N/MOF(Fe) was investigated, revealing the involvement of ·O2– in the reduction of Cr6+ during the adsorption process.
Conclusions
Urea was employed as a structure-directing agent to modify MOF(Fe), resulting in the preparation of N/MOF(Fe) via a solvothermal method. The adsorption reaction followed a second-order kinetic equation, and the adsorption model conformed to the Langmuir model. The adsorption process was characterized as a spontaneous exothermic reaction. N/MOF(Fe) demonstrated excellent regeneration performance, with the adsorption process involving physicochemical reactions. This process facilitated the in-situ adsorption and reduction of Cr6+ by N/MOF(Fe), rendering it more environmentally friendly compared to traditional adsorbents.