The abiotic transformation of nitroaromatic pesticides by iron(II) and dissolved organic matter

Nitroaromatic pesticides are hydrophobic contaminants that can accumulate in sediments by the deposition of suspended solids from surface waters. Fe(II) and dissolved organic matter (DOM), present in suboxic and anoxic zones of freshwater sediments, can transform nitroaromatic pesticides in natural systems. In this study, the abiotic chemical transformation of pentachloronitrobenzene, trifluralin and pendimethalin was studied in controlled laboratory systems containing Fe(II) and fulvic acid DOM isolates, and in natural pore waters collected from a freshwater wetland. Geochemical parameters affecting reactivity in the natural wetland were also monitored.; Rapid reduction of pentachloronitrobenzene to pentachloroaniline was observed in controlled systems in the presence of Fe(II) and DOM fulvic acid isolates from surface waters (pseudo-first-order half-life, t1/2 ≈ 30 min to 4 h). DOM in unfiltered systems inhibited iron colloid formation and possibly limited the formation of reactive Fe(II)-iron colloid surface complexes, causing reductive transformation in Fe(II)-DOM media to be slower in some cases relative to Fe(II)-only controls. Conversely, in 0.45-microm-filtered solutions pentachloronitrobenzene reduction in Fe(II)-DOM media was faster than the Fe(II)-only controls, suggesting that DOM enhances the reductive capacity of Fe(II) in the absence of iron colloids.; Differential pulse polarography (DPP) scans of natural wetland pore waters collected from Old Woman Creek (OWC; located in northern Ohio) revealed that a variety of redox-active metals exist naturally in OWC pore waters. Fe(III)-organic and Fe(II) species increased to a depth of ∼30 cm from the sediment-water interface, and a maximum for dissolved Mn(II) was observed at ∼6 cm depth. Dissolved Fe(II) was necessary for rapid pentachloronitrobenzene reduction in natural pore water (< 24 hr), and faster reduction was observed with increased pore water pH. Pentachloronitrobenzene reduction in "pH-adjusted" pore waters (acidified to pH 2.5 after pore water extraction and raised to the native pH (between 6.7 to 7.6) prior to reaction) was similar to that observed in a model system containing Fe(II) and an aquatic fulvic acid isolate. Conversely, pentachloronitrobenzene reduction in fresh, unaltered pore water was slower than that observed in "pH-adjusted" pore water. This indicated that the Fe(II) speciation and reductive capacity differs between unaltered and "pH-adjusted" samples due to a rearrangement of the naturally-occurring Fe complexes with pH-adjustment.; Trifluralin and pendimethalin reduction occurred in controlled systems containing Fe(II) and DOM surface water isolates, and in natural pore waters collected from OWC. Dissolved Fe(II) was necessary for trifluralin and pendimethalin reduction to occur, and pendimethalin reduction was faster in solutions containing Fe(II) and Suwannee River, Georgia, fulvic acid isolate relative to reactions containing Fe(II) and Pony Lake, Antarctica, fulvic acid isolate. DOM source material did not affect reactivity for trifluralin in similar systems. The reduction rate increased with increased pH for both compounds. Natural pore waters reduced both trifluralin and pendimethalin, and trifluralin degraded to multiple byproducts while pendimethalin only degraded to one major byproduct. Comparison of pseudo-first-order rate constants between controlled systems containing Fe(II) and OWC fulvic acid isolate, and natural sediment pore waters collected from OWC, showed that trifluralin and pendimethalin reduction in controlled systems was an order of magnitude faster relative to natural pore waters.; This study is the first to investigate nitroaromatic pesticide reduction in the presence of Fe(II) and DOM surface water isolates, and in natural benthic pore waters that contain high concentrations of dissolved Fe(II) and DOM. Although pentachloronitrobenzene, trifluralin and pendimethalin were reduced both in controlled s...