posted on 2024-01-31, 16:38authored byXiaopeng Huang, Duo Song, Qian Zhao, Robert P. Young, Ying Chen, Eric D. Walter, Nabajit Lahiri, Sandra D. Taylor, Zheming Wang, Kirsten S. Hofmockel, Fernando Rosario-Ortiz, Gregory V. Lowry, Kevin M. Rosso
Solar photoexcitation
of chromophoric groups in dissolved organic
matter (DOM), when coupled to photoreduction of ubiquitous Fe(III)-oxide
nanoparticles, can significantly accelerate DOM degradation in near-surface
terrestrial systems, but the mechanisms of these reactions remain
elusive. We examined the photolysis of chromophoric soil DOM coated
onto hematite nanoplatelets featuring (001) exposed facets using a
combination of molecular spectroscopies and density functional theory
(DFT) computations. Reactive oxygen species (ROS) probed by electron
paramagnetic resonance (EPR) spectroscopy revealed that both singlet
oxygen and superoxide are the predominant ROS responsible for DOM
degradation. DFT calculations confirmed that Fe(II) on the hematite
(001) surface, created by interfacial electron transfer from photoexcited
chromophores in DOM, can reduce dioxygen molecules to superoxide radicals
(•O2–) through a one-electron
transfer process. 1H nuclear magnetic resonance (NMR) and
electrospray ionization Fourier-transform ion cyclotron resonance
mass spectrometry (ESI-FTICR-MS) spectroscopies show that the association
of DOM with hematite enhances the cleavage of aromatic groups during
photodegradation. The findings point to a pivotal role for organic
matter at the interface that guides specific ROS generation and the
subsequent photodegradation process, as well as the prospect of using
ROS signatures as a forensic tool to help interpret more complicated
field-relevant systems.