Phosphate Adsorption and the Displacement of Structural Silicon in an Allophane Clay

Abstract

Phosphate was adsorbed at a constant pH, on allophane clay samples which had been pre-treated with selenite to remove adsorbed silicate. T h c concomitant release of selenium, silicon, and hydroxyl ion on phosphate adsorption was also measured. T h e results are compared with those obtained from a similar experiment conducted using clay which had not been treated and they substantiate the earlier conclusion that phosphate displaces structural silicon of clays. T h e amount displaced increases parabolically with increasing levels of phosphate adsorbed over the concentration range studied. I hlanuscript submitted to Soil Sci. SOC. Amer. Proc. 2 T h e main observations which led to the conclusion that the selenite does not displace the Si,t are as follows. T h e selenite adsorption isotherm levelled off at a relatively low concentration and the isotherm could be completely described using a two term Langmuir equation. T h e silicon released plotted against selenite adsorbed also reached a plateau and the curve did not show any 'discontinuity'.

The Electrophoretic Mobility of Imogolite and Allophane in the Presence of Inorganic Anions and Citrate

“…Ligand exchange is considered to be the process involved in phosphate reaction with allophane based on the phenomenon of an increase in bathing solution pH and silicic acid concentration (Rajan, 1975a(Rajan, , 1975b. This was attributed to a release of surface hydroxyls during the exchange; however, the increase in solution pH may not be unique to a ligand exchange reaction.…”

“…The phosphate reaction with allophane has been described either as discrete-or surface-precipitation (Veith and Sposito, 1977;Nanzyo, 1987) or as specific adsorption (Rajan, 1975a(Rajan, , 1975bRajan and Perrott, 1975;Yuan, 1980;Imai et al, 1981). More phosphate is retained by allophane than imogolite (Theng et aL, 1982;Clark and McBride, 1984).…”

Área superficial específica, porosidade da fração argila e adsorção de fósforo em dois latossolos vermelhos

“…Segundo Rajan (1975), Bárron et al (1988) e Celi et al (2003, a baixa adsorção de P sobre caulinita é devida ao menor número de sítios ativos, no caso OH. Isso fez com que a caulinita, embora tenha sido o filossilicato de maior representatividade na fração argila do LVDf e do LVD, tenha menor importância sobre o fenômeno de adsorção de P.…”

Phosphorus: Chemism and Interactions

“…It can affect the structure of clay minerals where fractions of P can replace Al ions from the tetrahedral structure thus destructing the structure and pulverizing clay minerals (Rajan, 1975). Long-term fertilization with MAP distninguished that phenomenon since the changes in soil texture were due to the amount of added P, i.e.…”