Treatment of dye wastewater nanofiltration concentrates containing high anion levels by a pH-sensitive nano-sized Fe(iii)@silica microgelElectronic supplementary information (ESI) available. See DOI: 10.1039/c7nj02575k

Herein, Fe( iii ) ions showed a pH-sensitive dual function on the surface of an l -cysteine-modified nano-sized silica microgel (Cys-Fe( iii )@mSiO 2 ). The total color and COD degradation efficiencies of Cys-Fe( iii )@mSiO 2 are higher than 95% and 83%, respectively, for high salt level nanofiltration concentrates without other post-treatment methods. In the first stage, Fe( iii ) ions act as an enhanced Fenton-like degradation catalyst on the surface of Cys-Fe( iii )@mSiO 2 for the treatment of actual nanofiltration concentrates in dye wastewater under acidic conditions (pH < 5.5). Then, the pH value of the nanofiltration concentrates increases (pH > 7.5) upon the simple addition of Ca(OH) 2 , and the same Fe( iii ) ions act as an excellent coagulant component for the nanofiltration concentrates with the aid of the silica microgel in the second stage. Characterization via transmission electron microscopy (TEM), X-ray diffraction (XRD), infrared (FTIR) spectroscopy, and related techniques demonstrates that the Fe( iii )/Fe( ii ) redox cycle in Cys-Fe( iii )@mSiO 2 is accelerated with the addition of l -cysteine; this increases the Fe( ii ) concentration and enhances the generation rate of hydroxyl radicals (&z.rad;OH). Additionally, the apparent rate constant of Cys-Fe( iii )@mSiO 2 is almost 5 times higher than that of Fe( iii ) Fenton-like degradation systems. The pH-sensitive dual effects of Fe( iii ) in heterogeneous Fenton-like degradation and enhanced coagulation with silica microgel for the treatment of refractory nanofiltration concentrates.