Raman spectroscopic study of aluminum silicate complexation in acidic solutions from 25 to 150 degrees C

Raman spectroscopic measurements were performed on aqueous acid to neutral silica-bearing solutions (0.005 less than or equal to m(Si) less than or eq ual to 0.02, 0 less than or equal to pH less than or equal to 8) and Al-sil ica solutions at temperature from 20 to 150 degrees C. At 20 degrees C, the spectrum of silica-bearing solutions exhibits only the bands of water and a completely polarized band at 785 cm(-1). This band is attributed to the v (1) band of the tetrahedral Si(OH)(4) molecule. In Si(OH)(4)-AlCl3-HCl solu tions, the intensity of this band decreases with increasing Al concentratio n, temperature, and pH. This decrease can be explained by the formation of an inner sphere complex between Al3+ and Si(OH)(4) according to the reactio n: Al3+ + H4SiO40(aq) double left right arrow AlH3SiO42+ + HThe fraction of complexed silica deduced from raman spectroscopic measureme nts is in good agreement with that calculated for the similar solution comp ositions and temperatures using the complexation constant generated by Pokr ovski et al.((23)) from potentiometric measurements. At ambient temperature , the formation of aluminum silicate complex is weak and does not account f or more than ca. 5 % of the total Al in most natural waters. As temperature increases, this complex becomes more significant and can dominate Al speci ation in acid (pH less than or equal to 2) hydrothermal solutions.