Role of organically complexed iron(II) species in the reductive transformation of RDX in anoxic environments

Organically complexed iron species can play a significant role in many subsurface redox processes, including reactions that contribute to the transformation and degradation of soil and aquatic contaminants. Experimental results demonstrate that complexation of Fe-II by catechol- and thiol-containing organic ligands leads to formation of highly reactive species that reduce RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and related N-heterocyclic nitramine explosive compounds to formaldehyde and inorganic nitrogen byproducts. Under comparable conditions, relative reaction rates follow HMX << RDX << MNX < DNX < TNX. Observed rates of RDX reduction are heavily dependent on the identity of the Fe-II-complexing ligands and the prevailing solution conditions (e.g., pH, Fe-II and ligand concentrations). In general, reaction rates increase with increasing pH and organic ligand concentration when the concentration of Fe-II is fixed. In solutions containing Fe-II and tiron, a model catechol, observed pseudo-first-order rate constants (k(obs)) for RDX reduction are linearly correlated with the concentration of the 1:2 Fe-II-tiron complex (FeL26-), and kinetic trends are well described by -d[RDX]/dt = k(FeL)2 > 6-[FeL26-][RDX], where k(FeL)2 > 6- = 7.31(+/- 2.52) x 10(2) M-1 s(-1). The reaction products and net stoichiometry (1 mol of RDX reduced for every 2 mol of Fe-II oxidized) support a mechanism where RDX ring cleavage and decomposition is initiated by sequential 1-electron transfers from two Fe-II-organic complexes.