No abstract

Understanding the nature of chemical bonding between natural organic matter (NOM) and clays is an important step toward understanding the chemistry controlling the fate and bioavailability of organic contaminants in soils and sediments. Many organic contaminants preferentially associate with NOM, and adsorption of NOM to mineral surfaces affects this association. To examine possible bonding mechanisms of NOM to clays, adsorption mechanisms of salicylic acid onto illite clay were investigated through the use of attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy and molecular orbital (MO) calculations. Batch adsorption experiments under acidic and neutral conditions were performed to sorb salicylic acid onto the illite surface. ATR-FTIR spectra were collected to determine the vibrational frequencies of organic surface complexes. MO calculations on salicylate-aluminate and salicylate-silicate clusters were used to model possible surface complexes. Strong correlations were found between theoretical and observed infrared frequencies. Based on these correlations, we suggest that Al-O-C linkages form between salicylate anions and Al 3+ octahedra at the illite clay edges. A monodentate complex involving one carboxylate oxygen is most probable under neutral conditions; a bidentate complex involving both carboxylate oxygen atoms and two vicinal Al 3+ octahedra is likely to dominate at lower pH. The predicted monodentate surface complex is similar to that suggested by Murphy et al. (Sci. Total Environ. 1992, 117/118, 413) for humic acid-clay bonding. This similarity between salicylic and humic acids suggests that simple organic ligands are good analogues for more complex humic acid-clay interactions.

Herein we present preliminary results of a study of the distribution and chemical composition of clay minerals in rocks recovered from the TransAtlantic Geotraverse (TAG) hydrothermal mound. This study is part of Leg 158 of the Ocean Drilling Program, an effort to evaluate the subsurface secondary mineral distribution and nature of alteration at the active TAG mound, located at 26°N latitude on the Mid-Atlantic Ridge. X-ray diffraction analyses and petrographic and scanning electron microscopy indicate that the clay minerals include chlorite, smectite, and a mica-like clay. Chemical analyses by flame atomic absorption spectrophotometry of clay-mineral separates and inductively coupled plasma-atomic emission spectrophotometry analyses of basalt alteration rims indicate that the chlorite and smectite have similar major-element compositions and that both clay minerals have lower SiO 2 and Al 2 O 3 and higher Fe 2 O 3 and MgO than bulk basalt. Cu and Zn are elevated above unaltered basalt concentrations in the smectites and chlorites and in bulk-rock alteration rims. Spatial distributions through the mound of SiO 2 / Fe 2 O 3 , MgO, and Zn in clay minerals and alteration rims suggest that fluids circulating through the TAG mound originate from two sources: (1) a shallowly circulating fluid that is less evolved and nearer to unaltered seawater chemical composition, and (2) a deeply circulating fluid that is more evolved fluid and changed from unaltered seawater by more extensive chemical reaction with underlying basalt. Clay-mineral compositions suggest that the southeasterly and shallow central portions of the mound are influenced by a greater proportion of the shallowly circulating, less evolved fluid, and the northwesterly and deeper central portions of the mound are influenced by a greater proportion of the deeply circulating, more evolved hydrothermal fluid.