The Release of Aluminum from Aluminosilicate Minerals. I. Kinetics

-The rates of release of AI by M NH~NO3 (pH 3) from minerals saturated with AI ions at pH 3 suggest that AI ions migrated from the surface layers and the matrix cores of kaolinite, montmorillonite, illite, and biotite, but only from the matrix core of muscovite mica. From the 0.25-0.5 p.m kaolinite and montmorillonite, part of the 'surface' A1 is released 'instantaneously' and the rest by first order kinetics, but the coarse 1.5-2.5 ttm kaolinite has only the former component. From illite and biotite, 'surface' A1 is released by 'bulk diffusion' kinetics suggesting the existence of disturbed peripheral layers of finite thickness. The diffusion coefficients, D,., for the matrix core follow the trend: mica = biotite > illite > montmorillonite > kaolinite. Based on models proposed in previous work, the ionic composition of the "surface' AI is calculated. This shows that (1) this composition varies according to the mineral from 3 to 100% AI 3§ the remainder being in the hydrolyzed form, and (2) the apparent hydrolysis constants are different for each mineral and significantly different from those of AI ions in solution. Key Words---Aluminum, Biotite, Illite, Kaolinite, Montmorillonite. I N T R O D U C T I O N The determinat ion of exchangeable A1 in acid soils and its differentiation from slowly released nonexchangeable AI has occupied the at tention of many research workers. A method commonly used is successive extract ion with an unbuffered salt solution (e.g., KCI, NH4CI, NH4NO3) adjusted to the soil pH (Skeen and Sumner , 1967; S ivasubramaniam and Tal ibudeen, 1972; Prakash and Bhasker , 1974; Cabrera and Talibudeen, 1977). Bache and Sharp (1976a) at tr ibuted this slowly released AI to disordered aluminosil icates, A1 interlayers in vermicul i te clays and organic AI complexes in soils. Not much attention has been paid however to similar measurements of the kinetics of A1 extraction from pure minerals such that rate parameters can be evaluated, especial ly for the ' s low re lease ' reaction, and related to such measurements with acid soils. The total exchangeable cations, including A1, displaced by unbuffered salt solutions f rom acid soils has been shown to be larger than the C E C measured by one of the standard methods (Lin and Coleman, 1960; Bache, 1970; Lim and Talibudeen, 1975). This suggests that displaced AI ions have a charge < 3 + per AI atom, i .e. , they are adsorbed as partially hydrolyzed AI ions on mineral surfaces. F rom studies on the composi t ion of the solution of A1 extracted f rom acid soils and clays, the exis tence o f polymeric A1 species on mineral surfaces has been suggested (Chakravart i and Tal ibudeen, 1961; Bache, 1974; Dalai, 1975; Smith and Emerson , * Permanent address: Centro de Edafologla y Biologia Aplicada del Cuarto (C.S.I.C.), Sevilla, Spain. 1976). Also Brown and N e w m a n (1973) demonst ra ted the adsorpt ion by bentoni te of a basic cat ion with OH/ AI ratio of 2.5. (For a comprehens ive rev iew see Coulter, 1969). More recent ly Bache and Sharp (1976b) were able to distinguish analytically be tween mononuclear and polynuclear AI ions in soil solutions. The work descr ibed hitherto (except Lin and Coleman, 1960; Smith and Emerson , 1976) has been on natural materials whose surface propert ies concerning adso rbed A1 are a f f ec t ed by the i r h i s to ry . In our measurements we pretreated the various minerals so that they had a common background before kinetic extractions of AI commenced . M A T E R I A L S A N D M E T H O D S