Treatment of dye wastewater nanofiltration concentrates containing high anion levels by a pH-sensitive nano-sized Fe(iii)@silica microgel

Abstract

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)@mSiO₂). The total color and COD degradation efficiencies of Cys-Fe(III)@mSiO₂ 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)@mSiO₂ 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)@mSiO₂ is accelerated with the addition of L-cysteine; this increases the Fe(II) concentration and enhances the generation rate of hydroxyl radicals (˙OH). Additionally, the apparent rate constant of Cys-Fe(III)@mSiO₂ is almost 5 times higher than that of Fe(III) Fenton-like degradation systems.