Facet-mediated degradation of 6PPD on hematite: From direct electron transfer to reactive oxygen species-driven accelerated oxidation

As a widely used tire antioxidant, N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine (6PPD) is released into soil ecosystems through tire wear. However, knowledge gaps exist regarding the transformation mechanism and ecological implications resulting from the interaction between 6PPD and soil minerals. This study revealed that the degradation rates of 6PPD varied among different hematite facets, reaching 97 % for the hematite nano-rhombohedra (HNR) with {104} facets and 55 % for the hematite nanoplate (HNP) with {001} facets after a one-hour reaction. DFT calculations demonstrated that the higher reactivity of HNR was closely related to the stronger affinity between its surface Fe(III) and amino groups of 6PPD. This process led to more vigorous electron transfer, as verified by more environmentally persistent free radicals (EPFRs) formation. Moreover, more reactive oxygen species (ROS) induced by HNR were involved in 6PPD degradation, which significantly drove the oxidation of EPFRs and other intermediate products. The toxicity prediction suggested the reduced toxicity of the degradation products of 6PPD. Our results shed light on the catalytic potential of hematite in 6PPD degradation, offering new insights into alleviating the environmental pressures associated with 6PPD in soil.