Nitroaromatic Reduction Kinetics as a Function of Dominant Terminal Electron Acceptor Processes in Natural Sediments

Abstract

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.

Assessment of the geochemical reactivity of Fe-DOM complexes in wetland sediment pore waters using a nitroaromatic probe compound

“…We attribute these differences to changes in the speciation of the metal complexes resulting from alterations in solution pH and carbonate concentration as a consequence of Ar-sparging. Although iron sulfide complexes and other dissolved sulfur species have been measured by others within this half-cell potential range (À0.1 to À0.7 V) in natural pore waters (Rozan et al, 2000), the neutral pH of unpurged pore waters (range 6.7-7.6), high dissolved Fe(II) content, lack of characteristic sulfidic odor and lack of black precipitate (indicative of the presence of iron sulfides, per Hoferkamp and Weber (2006)) indicated that the contribution of inorganic sulfur species towards OWC pore water chemistry is likely to be minimal.…”

“…Mn(II) reaches a maximum around 6 cm depth, and either remains constant or decreases slightly at depths >6 cm. Although Mn(II) is present in all samples, previous work with NAC probe compounds in natural systems have emphasized the importance of dissolved Fe(II) on NAC reactivity under iron-reducing conditions (Heijman et al, 1995;Rü gge et al, 1998;Hoferkamp and Weber, 2006). Thus, we expect dissolved Fe(II) to act as an important redox-active metal capable of participating in electron-transfer reactions in OWC sediment pore waters.…”

“…Nitroaromatic compounds (NACs) have been used to probe Fe redox reactions in environmentally relevant media such as soil columns, contaminated aquifer material, batch sediment slurries and synthetic laboratory systems (Klausen et al, 1995;Heijman et al, 1995;Rü gge et al, 1998;Klupinski et al, 2004;Hoferkamp and Weber, 2006;Hakala et al, 2007;Coló n et al, 2008). NACs are ideal redox probes because (in the case of mononitroaromatic compounds) the nitro moiety is completely reduced to its aniline derivative thus simplifying mass balance calculations during the course of a reaction.…”

“…Abiotic NAC reduction has been observed in the presence of aqueous landfill leachate material containing high concentrations of dissolved Fe(II) and DOM (Rü gge et al, 1998) and in homogeneous solutions of Fe(II) and fulvic acids (Hakala et al, 2007). Work by Hoferkamp and Weber (2006) suggested that NAC (pcyanonitrobenzene) reduction rates in batch sediment slurries are in part affected by Fe(II) speciation.…”

Reduction and conversion of 2,4,6-trinitrotoluene (TNT) by sulfide under simulated anaerobic conditions

“…Studies of model and natural sediment systems have shown that the reductive transformation of nitroaromatic compounds (NACs) under anaerobic conditions occurs primarily through abiotic processes rather than biotransformation [16]. Elovitz and Weber [15] found that over 95% of 25 M TNT was lost from the aqueous phase of an aerobic sediment-water system within the first 2 h, with 50% being removed from the solution by sorption.…”