High concentration of fluorine in drinking water and its harmful health
effects increase the importance of defluoridation research. There are
various methods of water defluoridation: adding chemicals, deposition,
adsorption, ion exchange, dialysis, reverse osmosis. Each of these
methods has its own advantages and disadvantages. So, chemicals
precipitation leads to the formation of large amounts of sludge, activated
carbon adsorption of fluorine occurs quite successful but it’s a costly
operation accompanied by regeneration; ion exchange, dialysis, osmosis
are costly processes. Among the ways of fluorine removal the most
attractive in terms of cost and amount of waste seems to be the adsorption
method which uses various types of inexpensive materials as adsorbents.
In the paper the adsorption properties of local diatomite modified with
aluminosilicate compounds (DMA) were investigated in removing of fluorine
from aqueous solution simulating the composition of natural water. The
natural diatomite untreated sample (D1) was used as an etalon. DMA was
synthesized by the mean of surface modification method, including
processing with NaOH and precipitating of aluminosilicate compounds into
diatomite macro-and mesopores. To characterize the D1 and DMA powder
diffractometry, low-temperature nitrogen adsorption, IR spectroscopy, DTA
analysis were used.
Fluorine adsorption has been investigated as a function of pH solution (4.5-
8.5), initial fluoride - ions concentration (2 - 20mg / l), adsorbent dose (0.2 -
2.0g / l). To determine the effect of the solution composition on the
adsorption value the experiments were carried out from the aqueous
solution of NaF, to which macro components normally present in a natural
underground water: nitrate, sulfate, chloride, and carbonate ions, mg / l:
HCO3 -- 450-1050, SO4 2--- 220-290, NO3 -- 50-350, Cl--50-250, were added.

Surface modification leads to significant changes in the structure and
properties of the obtained sorbent: the specific surface area increased from
37.5 to 81.7m2 / g, sorption pore volume increased from 0.044 to 0.106 cm3
/ g, pHpzc shifted toward a large value of 8 12 to 8.55. The DMA adsorption
capacity with respect to fluorine increased 2.5 times compared with the
initial sample of D1 in total selected range of concentrations. Maximum pH
removal of fluorine by DMA is within 4.5 - 5.5. The adsorption fluorine
isotherms by D1 and DMA were analyzed using Henry, Freundlich and BET
adsorption models. Freundlich model showed the best fitting with
experimental data for both samples (R2 = 0.9871 and 0.9901 for D1 and
DMA, respectively). To describe the time- depending adsorption results the
kinetic equations of pseudo - first and pseudo - second order were used.
Rate constants of adsorption were calculated. Adsorption kinetics is quite
well described with pseudo - second order kinetic model (R2 = 0.9641 and
1.0000 for D1 and DMA respectively). To determine the optimum conditions
of using of DMA as a sorbent for fluoride removal from water, a model
solution was prepared, corresponding to the average composition of natural
water selected from 30 different sources in the Republic of Moldova:
fluorine and macro components in the tested solution in mg/l: F- - 5, HCO3 - -
500, SO4 2—250, NO3 - —100, Cl- - 200, pH=7.5. The dosage of sorbent
required for fluorine removal to limit values (1.5 mg F-/g) under optimal
conditions (pH=4.85, τ = 120 min, Tsolution=20oC) was 3.5 g / l at an initial
concentration of fluoride in water equal 19 mgF/l. The increasing of the
anions concentration in the solution reduces the amount of adsorbed
fluoride in the following order: HCO3 -> Cl- > SO4 2- > NO3 -.
The results showed that modified diatomite - DMA can purify water of
fluorine and its concentration becomes lower then standards for potable
water require.