Ligand enhanced bio-oxidation of structural Fe(II) in illite coupled with nitrate reduction

Abstract

Coupled reaction of Fe(II) oxidation and nitrate reduction plays an important role in biogeochemical cycling of Fe and N, but biologically redox-inert clay minerals are seldom considered in this process. Herein, we report significant effects of ethylenediaminetetraacetic acid (EDTA) and oxalate on microbial oxidation of structural Fe(II) in illite and montmorillonite when coupled with reduction of nitrate in the presence of Pseudogulbenkiania sp. strain 2002. Without ligand, the extent of structural Fe(II) oxidation in illite was low and delayed. Because intracellular organic matter served as a more bioavailable electron donor, the molar ratio of Fe(II) oxidation relative to nitrate reduction (<0.04) was much lower than the expected stoichiometric ratio (=5). However, ligand amendment markedly shortened the lag time, enhanced Fe(II) oxidation, and resulted in a higher ratio of Fe(II) oxidation relative to nitrate reduction (=0.15). EDTA showed a greater enhancement effect than oxalate, due to its higher complexation ability. The likely reason was the formation of Fe(II)-ligand complex through ligand-induced partial dissolution of illite, which was oxidized more easily than structural Fe(II) in illite. The resulting Fe(III)-ligand complex was re-reduced by structural Fe(II) in illite via an interfacial electron transfer process. Therefore, Fe(III)-ligand/Fe(II)-ligand complexes effectively served as electron shuttle between illite and cells to enhance Fe(II) oxidation in illite. The ligand-promoting effect was less significant for montmorillonite, likely because its structural Fe(II) was already more bioavailable than that in illite. Our results reveal the significant role of natural ligands in expanding the electron pool available to support microbial oxidation of solid-state Fe(II) in soils and sediments and have important implications for coupled Fe and N cycles and other biogeochemical processes in the environment.