The effect of microstructural changes caused by structural-chemical
modification of diatomite with aluminosilicate nanoparticles on its adsorption
properties with respect to fluorine has been studied. The results show that
fluorine adsorption capacity of modified diatomite is considerably increased
from 10 to 58 mmol/g, the adsorption kinetics controlling mechanism has
changed: intraparticle diffusion for initial diatomite has replaced by external
diffusion; the some kind of synergism takes place after modification: the
adsorption capacity of aluminosilicate species grafted on the diatomite surface is
higher than of that of bulk aluminosilicate.
The surface modification of diatomite has been carried out by heating it in
NaOH solution with subsequent treatment with solution of aluminum salt and
ammonia. The amorphous surface silica partially is dissolved during the
treatment with NaOH with formation of aluminosilicate compound at the
addition of aluminum salt. The obtained aluminosilicate which is found to be
NaAlSiO4 with the individual particle range of about 10 nm deposited both on
the surface of the diatomite and on the inner surface of the macro- and larger
mesopores (10 nm), partially block them, which leads to the development of the
specific surface area of 81.8 m2/g, which is 2.5 times larger than the
corresponding value in the initial diatomite (37.5 m2/g), the pore blockage and
additional micropores introduced by microporous aluminosilicate increase the
volume of micropores, thereby increasing the surface area of modified
diatomite.
However despite the fact that the specific surface of the synthetic
aluminosilicate is much greater (183 m2/g), its adsorption capacity is 1.5 times
lower than that of the aluminosilicate grafted on the surface of diatomite. This
apparently is due to aluminum atom coordination change in the proximity of the
acidic silanol groups in the case of diatomite. Partial substitution of Si atoms in
silicates by Al atoms takes place as a result of their different valences an excess
negative charge, and the sorbent surface in solution adsorbs a greater number of
protons and its pHPZC is shifted to the higher values compared to initial diatomite
sample. In the process of fluorine adsorption at pH of natural water (7-9) the
surface charge of raw diatomite is negative, while that of modified diatomite is
positive. As a result, a positively charged surface of modified diatomite attracts
more fluoride ions.