Effective flocculation of harmful algae Microcystis aeruginosa by nanoscale metal–organic framework NH2-MIL-101(Cr)

Harmful algal bloom outbreak in lakes drives water-quality deterioration worldwide. Here, we recognized metal–organic frameworks (MOFs) as a potentially powerful weapon for the elimination of cyanobacterial blooms. We demonstrated that the water-stable Cr(III)-based MOFs, structured as NH2-MIL-101, could be used for removal of cyanobacterium Microcystis aeruginosa by coagulation/flocculation. The NH2-MIL-101(Cr) MOFs exhibited remarkable algal removal performance, with > 95% flocculation efficiency against M. aeruginosa within 1.5 h at 30 mg/L dosage or 3 h at 20 mg/L dosage. NH2-MIL-101(Cr) exhibited an excellent algal flocculation capacity over a wide range of pH conditions and cell densities. Especially, the algal removal capacity of NH2-MIL-101(Cr) was superior to commercial flocculants such as ferric chloride and chitosan. Furthermore, NH2-MIL-101(Cr) exhibited algal removal efficiencies of 97% after 24 h in real creek water, confirming the excellent performance of the prepared MOFs under natural conditions. The outstanding algal removal capacity of the prepared MOFs could likely be attributed to the adhesion of aggregated MOFs to the algal surface and co-precipitation with algae from the water solution. NH2-MIL-101(Cr) showed much higher algal removal efficiency compared with MIL-101(Cr), emphasizing the contribution of amine groups in algae-MOFs attachment and co-aggregation. This study presents the first account of using a water-stable MOFs as a mean of flocculant to achieve harmful algal removal, shedding new light on the potentials of MOFs in water treatment applications.