We firstly selected the Zr-MOF (Zr6O4(OH)4(OH)4(H2O)4(1,3,6,8-tetrakis(p-benzoate)pyrene)2), known as NU-1000, because it shows a mesoporous matrix with a high density of accessible Zr-OH sites to capture P-compounds. As agriculture wastewater contains phosphate ions and other hazardous agrochemicals (like organophosphorus pesticide, OP), we decided to evaluate the ability of NU-1000 to capture phosphate ions and a commercial P-pesticide (fenamiphos) as OP-model. Interestingly, we demonstrated that NU-1000 not only shows a high adsorption capacity of both P-pollutants separately, but also it is able to capture both compounds simultaneously without detriment to the individual adsorption performances (0.77 mmol P-pollutant per gram of material). In addition, NU-1000 proved to show an elevated selectivity toward both P-pollutants in presence of potential interferences typically found in wastewater. Only highly concentrated hydrogen carbonate solutions competed with phosphate ions for binding the sorption sites. Furthermore, a strategy was designed to recover selectively the captured P-pollutants, both in batch and under dynamic conditions. Thus, the phosphate ions could be selectively recovered toward their reuse as P-fertilizers while the adsorbent could be regenerated, showing an excellent recyclability during at least 3 cycles.
Encouraged by these promising results, we decided to take a step-forward by preparing a Zr-MOF through a green synthetic route toward the simultaneous capture/recovery of phosphate ions and degradation of an organophosphorus pesticide. To do it, we selected the Zr-MOF (Zr6O4(OH)4(trimesate)2(formate)6), known as MOF-808. A microwave-assisted synthetic route, using water as solvent, was designed to prepare MOF-808 materials with controlled particle size. In addition, the performance of several MOF-808 systems with different particle size toward decontamination of water containing phosphate ions and an OP-pesticide (methyl-paraoxon) was evaluated. Interestingly, all materials were able to efficiently capture phosphate ions and degrades fully the OP-pesticide into innocuous compounds, with the materials with bigger crystals showing the worse performance. In this regard, a spectroscopic analysis with several techniques (i.e. 1H- and 31P-NMR and UV-Vis spectroscopy) proved that while the phosphate ions were captured inside the cavities, the catalytic degradation of the toxic OP took place onto the particle surface. Furthermore, it was demonstrated the feasibility of recovering the captured phosphate ions toward their reuse as well as the recyclability of the adsorbent during several adsorption/degradation-regeneration cycles.
Overall, the results derived from PSust-MOF project have demonstrated the potentiality of Zr-MOFs in P-sustainability, because they not only enable the reversible capture of phosphate ions (favoring P-circular economy), but also degrade toxic OP-pesticide presented in agriculture waste water, reducing toxicity risks. Up to date, the results derived from PSust-MOF projects have been published in prestigious international scientific journals, including Materials Today Chemistry and Journal of Materials Chemistry A, and they have been presented in International and National Conferences, highlighting the oral presentations at XXVIII Reunion Bienal de la Real Sociedad Española de Química, 44th International Conference on Coordination Chemistry and the 8th International Conference on Metal-Organic Framework and Open Framework Compounds.
