Nano-goethite-mediated transformation of anthracene derivatives under low moisture conditions

Nano-goethite is widely present in soils and is active in mediating the transformation of soil-borne organic contaminants. However, little is known about the transformation efficacy and mechanisms of polycyclic aromatic hydrocarbons (PAHs) and their derivatives in the presence of nano-goethite under low moisture conditions. This study investigated the transformation of anthracene (ANT) and its amino-, methyl-, chloro- and nitro-derivatives mediated by nano-goethite at gravimetric water content of 2.3-14.5%, in combination with spectroscopic characterization and molecular computation. Nano-goethite exhibited a superior reactivity in the transformation of ANT and ANT derivatives, as a result of the single electron transfer reaction between Fe(iii) surface sites and organic reactants. The C9 and C10 atoms of ANT and ANT derivatives are the reactive sites owing to their higher Fukui function and spin density values, which was confirmed by the identification of transformation products (9,10-anthraquinone and its derivatives). The reactivity was positively correlated with the highest occupied molecular orbital (HOMO) energy. Due to their lower HOMO energies, the ANT derivatives with electron withdrawing groups (e.g., -Cl and -NO2) had slower transformation than ANT and the derivatives with electron donating groups (e.g., -NH2 and -CH3). Intriguingly, the transformation was suppressed with increasing water content, probably due to decreased sorption of hydrophobic ANT and ANT derivatives onto the increasingly moisturized nano-goethite surfaces. This study provides insights into the transformation of PAHs mediated by nano-goethite, and thus improves our understanding of the environmental fate of PAHs in soils.