Interactions of Goethite and Natural Organic Matter: Fractionation and Impact on Contaminant Reduction

Elucidating how organic matter and iron oxide particles interact will advance the understanding of geochemical processes and facilitate the development and implementation of methods to address groundwater pollution. The sorption and fractionation of three well-characterized organic matter isolates, Suwannee River natural organic matter (SRNOM), Suwannee River fulvic acid (SRFA), and Eliot Soil humic acid (ESHA), onto goethite nanoparticles in batch reactors and goethite-coated sand in column reactors were studied. Total organic carbon, molecular size, and optical properties were measured before and after exposure to solids in batch and column reactors, as well as in solution after the resuspension of equilibrated solids in a fresh buffer. Fluorescent, aromatic, high-molecular-weight material more strongly sorbed and stayed sorbed upon exposure to fresh solution. The effects of the different organic matter fractions on reduction of 4-chloronitrobenzene (4-ClNB) were also quantified. In both batch and column reactors, the reactivity of NOM-treated particles toward 4-ClNB in the presence of Fe(II) was inhibited by all isolates as compared to that in NOM-free reactors, with SRFA inhibiting reaction rate constants the most followed by ESHA and SRNOM. There are clear spectral and size differences in the organic matter that remains in the liquid medium compared to the NOM that binds to the mineral surfaces, but both of these materials inhibit Fe(II)-mediated reactions on the surface. Materials high in aliphatic and carboxylate contents, rather than the more strongly sorbing fluorescent aromatic material, appear to inhibit reactivity.