Facet-induced fractionation of humic acid by hematite and the promoted-photodegradation of 17β-estradiol catalyzed by hematite-humic complex

Hematite is a ubiquitous mineral with different dominant facets in the environment, which could adsorb humic acids (HA) to form photoactive hematite-HA complex. In this study, we prepared hematite nanocubes (HNC), hematite nanoplates (HNP) and hematite nanorhombs (HNR) with dominant facets as {012}, {001} and {104}, respectively. The abilities of the three hematites to adsorb and fractionate HA were compared. Our results indicated that the components with low molecular weight, high aromaticity and more oxygenated functional groups of HA were preferentially adsorbed. The fractionation degree followed the order of HNP > HNR > HNC, attributing to the coordination ability of different hematites. After hematite formed complexes with HA, it could promote the photodegradation of 17β-estradiol under visible light. The photoactivities of the three hematite-HA complexes were also compared. Since HNP adsorbed the most photochemically active HA components, HNP-HA showed the strongest enhancement for the degradation of 17β-estradiol. During the photodegradation process, the excited triplet state of HA (3HA*) and superoxide radical (O2•−) were identified as the dominant reactive species. Our results provide new insights into the role of hematite facets towards adsorption and fractionation of HA and photodegradation of co-existing contaminants, which would improve the understanding of the fate of pollutants in the presence of hematite and HA.