Abstract: The removal mechanisms of silicate using an aluminum hydroxide
co-precipitation process was investigated and compared with an adsorption process, in
order to establish an effective and validated method for silicate removal from wastewater.
Adsorption isotherms, XRD and FT-IR analyses showed that silicate uptake occurred by
adsorption to boehmite for initial Si/Al molar ratios smaller than two, but by precipitation
of poorly crystalline kaolinite for the ratios larger than two, in both co-precipitation and
adsorption processes. Silicate was removed by two steps: (i) an initial rapid uptake in a few
seconds; and (ii) a slow uptake over several hours in both processes. The uptake rate in the
first step was higher in the co-precipitation process than in adsorption process, presumably
due to increased silicate adsorption to boehmite and rapid precipitation of kaolinite. These
results suggest that silicate removal using aluminum salts could be effectively achieved if
the pH adjustment and aluminum concentration are strictly controlled.