Nitroaromatic reduction kinetics as a function of dominant terminal electron acceptor processes in natural sediments

The reductive transformation of p-cyanonitrobenzene (pCNB) was investigated in laboratory batch slurries exhibiting dominant terminal electron accepting processes (TEAPs). Pseudo-first-order rate constants (k(obs)) were measured for the reduction of pCNB in nitrate-reducing, iron-reducing, sulfate-reducing, and methanogenic sediment slurries. Reduction was extremely slow in nitrate-reducing slurries but increased in slurries exhibiting TEAPs with significant concentrations of solution phase Fe(ll). As the reduction of pCNB progressed in the Fe(ll) rich systems, significant but nonstoichiometric decreases in aqueous Fe(ll) concentration were measured. Normalization of k(obs) to initial aqueous Fe(ll) concentrations (k(obs)/[Fe(ll)]t=0) gave values ranging from 0.0040 to 0.0052 d(-1) microM(-1) for nitrate-reducing, iron-reducing, and methanogenic sediment slurries as well as sulfate-reducing sediment slurries in which lactate served as a source of organic carbon. The k(obs)/ [Fe(ll)]t=0 ratios were 1-fold greater for sulfate-reducing batch slurries amended with acetate and iron-reducing slurries equilibrated with a 3% H2 atmosphere indicating that the electron source and system parameters such as pH play a determinant role in the reaction kinetics. Although these data demonstrate that aqueous phase Fe(ll) must be present for significant reduction to occur, a limited role for aqueous phase Fe(ll) as a quantitative indicator of reactivity is suggested.