9. Conclusions and outlook
Water-stable MOFs and MOF-based composites are green materials that exhibited the superior adsorption ability for removing target metal species. The adsorptive processes were generally fitted well with pseudo-second-order kinetic and Langmuir isotherm models. Several dominated adsorptive mechanisms were proposed, including the strong coordination between heavy metals/radionuclides and the binding sites (e.g., hydroxyl, thiol, amide substituents and other oxygen-containing groups) of organic ligands of water-stable MOFs, electrostatic interactions, anion exchange or SC-SC transformation, and combined adsorption-reduction process. Additionally, these materials also showed the excellent visible light-driven photocatalytic performance for reducing Cr(VI) by forming reactive photo-generating electrons in the systems and by decreasing the recombination of electron-hole pairs. Also, MOFs could catalyze the reduction of Cr(VI) by formic acid effectively, which appears to be a green technology in sustainable water purification. The superior selectivity, good stability, and reusability of MOFs during the treatments, indicating the promising applications of these emerging techniques for treating water and wastewater contaminated with different kinds of heavy metal ions and radionuclides. There are still great challenges before applying MOFs in practical water treatment. The primary consideration is the upscale potential of synthesis, which has not been addressed adequately in literature. Nowadays, most of the MOFs are synthesized by solvothermal methods in organic solvents such as N,N’-dimethylformamide (DMF) and methanol. The impact of used solvent to the environment should be considered thoroughly. Negative effects of organics many be minimized by recycling the solvents. A progress is being in developing environmentally benign synthetic conditions of aqueous phase (Li et al., 2018) and solvent-free synthesis (Zou et al., 2016).
