Thallium (Tl) redox state determines its speciation and
fate in
aqueous environments. Despite the high potential of natural organic
matter (NOM) providing the reactive groups to complex and reduce Tl(III),
the kinetics and mechanisms by which NOM influences the Tl redox transformation
have remained insufficiently understood. Here, we studied the reduction
kinetics of Tl(III) in acidic Suwannee River fulvic acid (SRFA) solutions
under dark and solar-irradiated conditions. Our results show that
the thermal Tl(III) reduction occurs by the reactive organic moieties
in SRFA, with the electron-donating capacities of SRFA increased with
pH and decreased with [SRFA]/[Tl(III)] ratios. Solar irradiation promoted
Tl(III) reduction in SRFA solutions as a result of ligand-to-metal
charge transfer (LMCT) within the photoactive Tl(III) species as well
as an additional reduction process mediated by a photogenerated superoxide.
We demonstrated that the formation of Tl(III)–SRFA complexes
decreased the reducibility of Tl(III), with the kinetics dependent
on the nature of the binding component and SRFA concentrations. A
“three ligand class” kinetics model has been developed
and satisfactorily describes Tl(III) reduction kinetics over a range
of experimental conditions. The insights presented here should assist
in understanding and predicting the NOM-mediated speciation and redox
cycle of Tl in a sunlit environment.