Water oxide interface, speciation

Arai Y, Sparks DL (2002) Residence time effects on arsenate surface speciation at the aluminum oxide-water interface. Soil Sci 167 303-314... 

Speciation of Adsorbed Ions at the Oxide/Water Interface... 

The modeling results suggest that the physico-chemical state of the oxide/water interface has a significant effect on the speciation of ions at the surface. For example, adsorbed metal ions may be more easily hydrolyzed (. . at lower pH) than aquo metal ions in bulk solution. Consider the following surface equilibria of Cu(II) (2), i.e.. 

Apted MJ, Waychunas GA, Brown GE Jr (1985) Structure and speciation of iron complexes in aqueous solutions determined by X-ray absorption spectroscopy. Geochim Cosmochim Acta 492081-2089 Arai Y, Elzinga EJ, Sparks DL (2001) X-ray absorption spectroscopic investigation of arsenite and arsenate adsorption at the aluminum oxide-water interface. J Colloid Interface Sci 235 80-88. 

Comprehensive recent surveys of the triple layer model are to be found in J. A. Davis and J. O. Leckie, Speciation of adsorbed ions at the oxide/water interface, in Chemical Modeling in Aqueous Systems (E. A. Jenne, ed. American Chemical Society, Washington, D.C., 1979) and in R, O, James and... 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

Many of the important chemical reactions controlling arsenic partitioning between solid and liquid phases in aquifers occur at particle-water interfaces. Several spectroscopic methods exist to monitor the electronic, vibrational, and other properties of atoms or molecules localized in the interfacial region. These methods provide information on valence, local coordination, protonation, and other properties that is difficult to obtain by other means. This chapter synthesizes recent infrared, x-ray photoelectron, and x-ray absorption spectroscopic studies of arsenic speciation in natural and synthetic solid phases. The local coordination of arsenic in sulfide minerals, in arsenate and arsenite precipitates, in secondary sulfates and carbonates, adsorbed on iron, manganese, and aluminium hydrous oxides, and adsorbed on aluminosilicate clay minerals is summarized. The chapter concludes with a discussion of the implications of these studies (conducted primarily in model systems) for arsenic speciation in aquifer sediments. 

Chisholm-Brause, CJ, Morris DE, Richard RE (1992b) Speciation of uranium(VI) sorption complexes on montmorillonite. Water-Rock Interactions, Proc 7th Int Symp 1 137-140 Chisholm-Brause CJ, O Day PA, Brown GE Jr, Parks GA (1990a) Evidence for multinuclear metal-ion complexes at solid/water interfaces from X-ray absorption spectroscopy. Nature 348 528-531 Chisholm-Brause CJ, Roe AL, Hayes KF, Brown GE Jr, Parks GA, Leckie JO (1989b) XANES and EXAFS study of aqueous Pb(II) adsorbed on oxide surfaces. Physica 158,674-676 Chowdhury TR, Basu GK, Mandal BK, Biswas BK, Samanta G, Chowdhury UK, Chanda CR, Lodh D, Roy SL, Saha , Roy S, Kabir S, Quamruzzaman Q, Chakraborti D (1999) Arsenic poisoning in the Ganges delta. Nature 401 545-546... 

ATR (FTIR), DR (UV-Vis-near-IR), NMR, EPR, and RAMAN spectroscopies have been also used to examine whether inner-sphere complexes are formed upon the deposition of TMIS on the surface of an oxidic support and, in some cases, to find the interface speciation. The structure of a deposited TMIS could be changed upon drying. Thus, the apphcation of a spectroscopic technique before drying is in some cases critical. In this respect ATR is very useful because it allows the strong IR absorption of the water molecules to be overcome and interfacial species to be identified. 

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