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PUBLICATION NUMBER
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AAT 9101050
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TITLE
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KINETICS AND MECHANISMS OF ANION ADSORPTION AND DESORPTION AT THE GOETHITE-WATER INTERFACE USING PRESSURE-JUMP RELAXATION (CHEMICAL KINETICS)
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AUTHOR
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ZHANG, PENGCHU
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DEGREE
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PHD
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SCHOOL
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UNIVERSITY OF DELAWARE
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DATE
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1990
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PAGES
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112
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ADVISER
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SPARKS, DONALD L.
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SOURCE
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DAI-B 51/08, p. 3625, Feb 1991
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SUBJECT
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AGRICULTURE, AGRONOMY (0285); GEOCHEMISTRY (0996); ENGINEERING, CIVIL (0543)
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The mechanism(s) and chemical kinetics of adsorption and desorption of inorganic ions on soils and soil constituents cannot be determined by the traditional kinetic methods such as batch, miscible displacement, and stirred-flow techniques. A pressure-jump (p-jump) relaxation apparatus with conductivity detection was employed to investigate the mechanism(s) and chemical kinetics of MoO$/sb4$, SO$/sb4$ and SeO$/sb3$ adsorption/desorption on goethite in aqueous suspensions. Single relaxations were observed in the SO$/sb4$-goethite and SeO$/sb4$-goethite systems. The mechanism for adsorption of SO$/sb4$ and SeO$/sb4$ on goethite involved simultaneous protonation of surface functional OH group and adsorption of the anion on this surface. The intrinsic rate constants showed that the adsorption process, $(logk/sbsp[ads./rm SO/sb4][int]$ = 8.3, $logk/sbsp[ads./rm SeO/sb4][int]$ = 8.55), was much faster than the desorption process, $(logk/sbsp[des. SO/sb4][int]$ = -0.84, $(logk/sbsp[des./rm SeO/sb4][int]$ = 0.52).
Double relaxations were observed for the MoO$/sb4$- and SeO$/sb3$-goethite systems. The overall reaction for both systems consisted of two steps. An outer-sphere surface complex forms during the first step and occurs at the $/beta$ layer, in which the intrinsic forward rate constants, $(logk/sbsp[/rm 1,MoO/sb4][int]$ = 3.6, $logk/sbsp[1,SeO/sb3][int]$ = 10.6), are much larger than those for the second step $(logk/sbsp[2,MoO/sb4][int]$ = 0.28, $logk/sbsp[2,SeO/sb3][int]$ = -4.0). A ligand exchange process occurred in the second step resulting in the formation of inner-sphere surface complexes.
The modified triple layer model was successfully used to describe the adsorption of MoO$/sb4$, SO$/sb4$, SeO$/sb4$, and SeO$/sb3$ on goethite with the assumption that adsorbed ions may be located at either the $/alpha$ or $/beta$ layer. Electrostatic parameters, such as surface potentials computed from the model were employed in the relaxation equations to derive the intrinsic rate constants. Excellent agreement was found between the intrinsic equilibrium constants determined from TLM model calculations $(logK/sbsp[SO/sb4 ,model][int]$ = 9.6, $logK/sbsp[SeO/sb4 ,model][int]$ = 8.64, $logk/sbsp[MoO/sb4 ,model][int]$ = -0.47, $logk/sbsp[SeO/sb3 ,model][int]$ = 20.42 for HSeO$/sb3$ and 15.48 for SeO$/sb3$) and between measurements from p-jump $(logK/sbsp[SO/sb4 ,kinetic][int]$ = 9.14, $logK/sbsp[SeO/sb4 ,kinetic][int]$ = 8.02, $logK/sbsp[MoO/sb4 ,kinetic][int]$ = -0.33 $logK/sbsp[SeO/sb3 ,kinetic][int]$ = 19.99 for HSeO$/sb3$ and 16.24 for SeO$/sb3$). These results indicate that a combination of equilibrium and kinetic studies can and must be used to ascertain mechanisms for anion adsorption/desorption on soil constituents.
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