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"-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" is fixed.In solutions containing Fe" and tiron,a model catechol,observed pseudo-first-order rate constants (k_(ohs)) for RDX reduction are linearly correlated with the concentration of the 1:2 Fe~(II)-tiron complex (FeL_2~(6-)),and kinetic trends are well described by -dRDX/dt = k_(FeL2~6-)-FeL_2~(6-)RDX,where k_(FeL2~(6-)) = 7.31(+-2.52) x 102 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.
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