Nanoscale Zero-Valent Iron Coated with Magnesium Hydroxide for Effective Removal of Cyanobacteria from Water

The effects of nanoscale zero-valent iron (NZVI, as a benchmark) and a novel core–shell structured nanoparticle, magnesium hydroxide coated NZVI (NZVI@Mg­(OH)₂), on the removal and viability of a cyanobacterium, Microcystis aeruginosa, in water were investigated. Dosing of these two nanoparticles at concentrations between 20 and 100 mg/L led to various degrees of sedimentation of cyanobacterial cells from the water column. NZVI@Mg­(OH)₂ exhibited much better ability to remove cyanobacteria than NZVI. A cell removal efficiency of 86.6% was achieved after 0.5 h with the lowest dosage (20 mg/L) of NZVI@Mg­(OH)₂, whereas the highest dosing concentration of NZVI (100 mg/L) resulted in the removal of only 57.5% of cells. This improved effectiveness can be attributed to the reduction in electrostatic repulsion between the cyanobacterial cells and NZVI@Mg­(OH)₂. The membrane integrity of the settled cyanobacterial cells in the sediment was significantly disrupted after the addition of a 100 mg/L dose of NZVI, which may lead to the release of unwanted intracellular metabolites. In contrast, the viability of the settled cyanobacteria was nearly unaffected after treatment with NZVI@Mg­(OH)₂. It can be concluded that NZVI@Mg­(OH)₂ is highly effective in the removal of cyanobacteria from the water column. The Mg­(OH)₂ coating not only protects the NZVI cores from corrosion in water but also reduces the toxicity of NZVI toward cyanobacterial cells when NZVI technology is applied for cyanobacteria control and removal in water supply and treatment.