Nanoparticles for Treatment of Arsenic - Nanotechnologies for Water Environment Applications - page 119

Adsorptive filters are suitable for point-of-entry (POE), point-of-use (POU), and
small water treatment systems because they are simple to use and the spent media are
easier to handle than backwash suspension. Although the reverse osmosis technique can
also be used for these applications, the cost and residuals-handling requirements for this
technology make it less attractive. Common media used for arsenic treatment include
activated alumina, anion exchange resin, iron-modified activated alumina, and granular
ferric hydroxide and oxide. Anion exchange resins and activated alumina have a
relatively low removal capacity for As(V) and don't remove As(III). Iron-based
adsorbents (i.e., GFH and GFO) have higher adsorption capacities than activated
alumina. However, the costs of these adsorbents are much higher than activated
alumina, and they are not as strong as activated alumina. Nanotechnology-based TiO 2
adsorbent has been invented recently for drinking water treatment (Meng et al., 2005).
The advantages of the TiO 2 adsorbent over the existing media are: high adsorption
capacity, effective As(V) removal up to a pH of 8, good removal of As(III) in a pH
range of 6.5 to 8, less effect of common anions, such as phosphate, on arsenic removal,
relatively high strength.

5.3 Treatment of Arsenic Using Nanocrystalline TiO 2

5.3.1 Synthesis and Characterization of Nanocrystalline TiO 2

The nanocrystalline TiO 2 can be prepared by hydrolysis of a titanium sulfate
solution which has been detailed in patent description (Meng et al., 2005). Briefly, a
titanium dioxide in anatase form was prepared by hydrolysis of a titanium sulfate
solution. The pH of the slurry was adjusted to a range between 4 and 9 with sodium
hydroxide, and the slurry was filtered to collect the titanium oxide solids. The titanium
oxide solids were washed with water to remove salts, then dried at 105°C for 2 hours.
Samples of the dried titanium oxide product were sieved to obtain a 100-standard U.S.
mesh fraction.

The X-ray powder diffraction analysis determined the TiO 2 produced is in
anatase form with an average crystalline particle size of about 7 nm. The Brunauer-
Emmett-Teller (BET) specific surface area and total pore volume of the sample are
determined to be 329 m 2 /g and 0.415 cm 3 /g, respectively, by a static volumetric gas
adsorption technique (Meng et al., 2005). The available surface hydroxyl content, i.e.,
the number of hydroxyl groups available for chemical reaction, is determined to be 6.0
mmol/g with an acid-base titration method in accordance with Sigg and Stumm (1980).

As shown in Figure 10.2, the TiO 2 surface charge curves obtained using a
titration procedure for ionic strength at 0.01 and 0.001 M KNO 3 solution intersect at pH
5.8, which is determined to be the point of zero charge (PZC) of TiO 2 . This is in