Interaction of metallic cations between the inter face of SiO2(s)/solution

The transport and fate of metallic
cations are issues of concern with regard to
the natural environment, such as rivers, lakes,
groundwater, and soil in waste disposal sites.
When cations exist in an oxide suspension
system, it has been proposed that a transition
takes place from mononuclear adsorption to
hydrolysis to multinuclear adsorption and,
finally, to precipitation on the oxide surface
or in bulk solution with increasing system pH
or cation concentration. SiO2 is the most
abundant oxide in the earth crust and can
significantly influence a variety of nutrients
and pollutants in aquatic and soil
environments. Moreover, SiO2 is widely
applied to many industrial processes
including use as filtration medium in water
treatment plants and base substrate in
catalysts. Thus, the objectives of this study
are to explore the interaction of various
metallic cations (Al(III) and Fe(III)) between
the interface of SiO2 and aqueous solution.
The TEM images show that pure silica
particles are aggregate and that the diameter
of a primary particle is around 15-25 nm. It
is obviously shown from the results that
discrete Fe(OH)3 precipitate forms both at
low pH (2.40) and at high pH (7.00) around
the unchanged size of base SiO2 particle.
However, no discrete Al(OH)3 phase is
evident in the Al(III)/SiO2 systems. The
invisibility of discrete Al(OH)3 precipitate
suggests a strong association between Al-
(hyro)oxide and the SiO2 surface. With an
increasing system pH, the Al(OH)3
precipitate gradually forms on the surface ofthe base SiO2 particle and the particle size
becomes larger.
The charge reversals (CR) observed in the
Al(III)/SiO2 system represent, in order of
increasing
pH, the point-of-zero charge (PZC) on the
SiO2 substrate (CR1), the pH of surface
nucleation of Al(OH)3 (CR2), and, at high pH,
the PZC of the Al(OH)3 coating (CR3). The
observed electrophoresis mobility of
Fe(III)/SiO2 is the overall result contributed
by negatively charged SiO2 and positively
charged Fe(OH)3.