Humic Acid Fractionation upon Sequential Adsorption onto Goethite

Abstract

Mineral−humic complexes are commonly distributed in natural environments and are important in regulating the\ntransport and retention of hydrophobic organic contaminants in soils and sediments. This study investigated the\nstructural and conformational changes of humic acid (HA) and mineral−HA complexes after sequential HA adsorption\nby goethite, using UV−visible spectroscopy, high performance size exclusion chromatography (HPSEC), Fourier\ntransform infrared (FT-IR) spectroscopy, and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The\nHA remaining in the solution after adsorption showed low polarity index values ((N + O)/C), which indicates that\npolar functional moieties are likely to adsorb on the goethite surface. In addition, we observed decreased E₄/E₆ and\nE₂/E₃ ratios of unbound HA with increasing number of coatings, implying that aliphatic rich HA fractions with polar\nfunctional moieties readily adsorb to the goethite surface. According to IR spectra, carbohydrate carbon would be\nthe important fractions associated with goethite. NMR spectra provided evidence for HA fractionation during adsorption\nonto the mineral surface; that is, aliphatic fractions were preferentially adsorbed by goethite while aromatic fractions\nwere left in solution. Relatively small molecular weight (MW) HA fractions had a greater affinity for the goethite\nsurface based on analyses of the HPSEC chromatograms, which differs from the results reported in the literature.\nFinally, our results suggest that the polar aliphatic fractions of HA were mainly adsorbed to goethite via electrostatic\nattraction and/or ligand exchange reactions.