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Humic acids have stable radicals that are indigenous to their structure. Hydroxybenzoic acid derivatives such as gallic acid (GA) and protocatechuic acid are appropriate models for the radical properties of humic acids. Here we show that the adsorption or intercalation of gallic acid in Laponite clay results in a significant thermodynamic stabilization of gallic acid radicals. Moreover, the formed organoclay shows enhanced stability against acid dissolution. The structural details of the association of gallic acid with Laponite depend on the GA/Laponite loading. At low GA/Laponite ratios (approximately 10(-6) M of gallic acid per gram of clay), gallic acid is adsorbed at the variable charge sites of Laponite. This adsorption can be adequately described by surface complexation modeling. At higher GA/Laponite ratios (approximately 10(-3) M of gallic acid per gram of clay), X-ray diffraction data show that gallic acid is intercalated at the interlamellar sites of Laponite. In the presence of Pb2+ ions, the formed GA/Pb complex is associated with Laponite in an analogous structural manner, that is, adsorption at variable charge sites or intercalation at the interlamellar sites of Laponite, depending on the loading. Laponite stabilizes the GA/Pb radicals. At prolonged exposure to ambient O2, Laponite promotes the formation of stable oligomeric GA/Pb radical species, which are intercalated into interlamellar sites.

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The adsorption of thiram and disulfiram onto alpha-Al2O3 and montmorillonite clay has been studied in the presence of small carboxylate anions, bicarbonate, formate, and oxalate. At natural concentrations, HCO3- enhances dramatically the adsorption of both pesticides on alpha-Al2O3 and clay. An analogous significant enhancement of pesticide adsorption is also observed in the presence of formate and oxalate. Density functional theory calculations demonstrate that in solution a stable molecular complex between one molecule of thiram and one molecule of HCO3- is formed with interaction energy -35.6 kcal/mol. In addition, two H20 molecules further stabilize it by an interaction energy of -3.6 kcal/mol. This clustering [thiram- HCO3- -2H2O] leads to a change of the electronic structure and the ultraviolet-visible spectrum of thiram that is observed experimentally. Surface complexation modeling shows that the molecular cluster [thiram-HCO3- -2H2O], which bears a total net charge of -1, is responsible for the observed enhanced adsorption on the charged surface of alumina and clay at pH below their points of zero surface charge. The results reveal a novel pervasive role of carboxylate anions and particularly HCO3- on the adsorption of dithiocarbamate pesticides in natural waters.

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The interfacial chemistry of the impregnation step involved in the synthesis of cobalt catalysts supported on titania was investigated with regard to the mode of interfacial deposition of the aqua complex [Co(H(2)O)(6)](2+) on the "titania/electrolyte solution" interface, the structure of the inner-sphere complexes formed, and their relative interfacial concentrations. Several methodologies based on the application of deposition experiments and electrochemical techniques were used in conjunction with diffuse-reflectance spectroscopy and EPR spectroscopy. These suggested the formation of mononuclear/oligonuclear inner-sphere complexes on deposition of the [Co(H(2)O)(6)](2+) ions at the "titania/electrolyte solution" interface. The joint application of semiempirical quantum-mechanical calculations, stereochemical considerations, and modeling of the deposition data revealed the exact structure of these complexes and allowed their relative concentrations at various Co(II) surface concentrations to be determined. It was found that the interface speciation depends on the Co(II) surface concentration. Mononuclear complexes are formed at the compact layer of the "titania/electrolyte solution" interface for low and medium Co(II) surface concentrations. Formation of mono-hydrolyzed Ti(2)O-TiO and the dihydrolyzed TiO-TiO disubstituted configurations is very probable. In the first configuration one water ligand of the [Co(H(2)O)(6)](2+) ion is substituted by a bridging surface oxygen atom and another by a terminal surface oxygen atom. In the second configuration two water ligands of the [Co(H(2)O)(6)](2+) ion are substituted by two terminal surface oxygen atoms. Binuclear and trinuclear inner-sphere complexes are formed, in addition to the mononuclear ones, at relatively high Co(II) surface concentrations.

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