Colloidal iron species driven enhanced H2O2 decomposition into hydroxyl radicals for efficient removal of methylene blue from water

Abstract

Colloids are wide-spread in natural waters and colloid-facilitated transport via adsorption was established as the most important mechanism for the mobilization of aqueous contaminants. This study reports another possible, but reasonable, role of colloids for the contaminants driven by redox reactions. Under the same conditions (pH 6.0, 0.3 ml 30% H2O2, and 25 °C), the degradation efficiencies of methylene blue (MB) at 240 min over Fe colloid, Fe ion, Fe oxide and Fe(OH)3 were 95.38%, 42.66%, 4.42% and 9.40%. We suggested that, Fe colloid can promote the H2O2 based in-situ chemical oxidation process (ISCO) compared with other iron species such as Fe(Ⅲ) ion, Fe oxide and Fe(OH)3 in natural water. Furthermore, the MB removal via adsorption by Fe colloid was only 1.74% at 240 min. Hence, the occurrence, behavior and fate of MB in Fe colloid containing natural water system mainly depends on the reduction-oxidation rather than adsorption-desorption process. Based on the mass balance of colloidal iron species and characterization of iron configurations distribution, Fe oligomers were the active and dominant components for Fe colloid-driven enhanced H2O2 activation among three types of Fe species. The quick and steady conversion of Fe(III) to Fe(II) was proven to be reason why Fe colloid can efficiently react with H2O2 to produce hydroxyl radicals.