Modifying Fe3O4 microspheres with rhodamine hydrazide for selective detection and removal of Hg2+ ion in water

► Fe3O4-immobilized rhodamine hydrazide (Fe3O4-R6G) was prepared and characterized. ► The Fe3O4-R6G can be quickly separated from a suspension by an external magnet. ► The Fe3O4-R6G can be used repeatedly with low loss. ► The Fe3O4-R6G exhibits high selectivity for sensing Hg2+ in aqueous media. ► Prepared Fe3O4-R6G was efficient in treating three natural waters.