Influence of dissolved organic matter and Fe(II) on the abiotic reduction of pentachloronitrobenzene

Nitroaromatic pesticides (NAPs) 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 NAPs in natural systems. We studied the reduction of pentachloronitrobenzene (PCNB) to pentachloroaniline (PCA) in controlled studies using Fe(II) and surface water DOM isolates from Pony Lake, Antarctica, and Suwannee River, GA, in unfiltered and 0.45 microm filtered solutions. We observed rapid reduction of PCNB to PCA in the presence of Fe(II) and DOM (t(1/2) approximately = 30 min to 4 h) and very limited reduction in DOM-only systems. DOM in unfiltered systems inhibited iron colloid formation and potentially limited the formation of reactive Fe(ll)-iron colloid surface complexes, causing reductive transformation in Fe(II)-DOM media to be slower in some cases relative to Fe(ll)-only controls. Conversely, in 0.45 microm filtered solutions, PCNB reduction in Fe(III)-DOM media was faster than the Fe(II)-only controls, suggesting that DOM enhances the reductive capacity of Fe(ll) in the absence of iron colloids. This work shows that DOM may significantly affect the reactivity of Fe(ll) toward NAPs under suboxic and anoxic conditions in natural wetland sediments.