The photocatalytic characteristics of nanometer-sized TiO 2 powder prepared by a homogeneous-precipitation process (HPP) were compared with those of a commercial powder to determine which powder was better able to remove metal ions, such as lead and copper, from aqueous equimolar metalethylenediaminetetraacetic acid (EDTA) solution. In aqueous lead-EDTA solution, the TiO 2 powder fabricated by HPP had 3.5 times higher initial adsorption of lead ion and a 1.5 times faster rate for the complete elimination of lead ions than did the commercial powder. In an aqueous copper-EDTA solution, the TiO 2 powder fabricated by HPP also showed higher initial adsorption and a faster elimination rate for copper ions than did the commercial powder. Similarly, the photocatalytic properties were enhanced as the specific surface area increased, and the TiO 2 powder fabricated by HPP, which consisted of coagulated primary particles 20 nm in size, with chestnut-burr shapes, had a larger specific surface area (ϳ180 m 2 /g) than that of the commercial powder (ϳ55 m 2 /g).

ZnO nanopowder was prepared by a novel “solution-combustion method (SCM)” and it was used as a semiconductor photocatalyst to evaluate its photoreduction properties. Aqueous Pb-EDTA and Cu-EDTA solutions of heavy metal ions (Pb and Cu) were used for the photocatalysis reaction under UV illumination. The result was then compared with other semiconductor photocatalyst powder such as TiO2 powder (P25; Degussa) and TiO2 powder prepared by homogeneous precipitation process at low temperature (HPPLT). In the removal of Pb++ ions, the ZnO nanopowder showed 2.6 fold higher removal rate than P25 TiO2 powder and 1.8 fold higher than HPPLT TiO2. Also the ZnO nanopowder showed the highest removal rate of the Cu++ ions from the solution among the different photocatalyst powders compared. The superior photoreduction ability of the ZnO nanopowder appears to be due to its excellent UV absorption characteristics. This was confirmed by the photoluminescence (PL) measurement.

A kind of photocatalytic ZnO nanopowder and two kinds of photocatalytic TiO 2 nanopowders were prepared. The ZnO nanopowder was synthesized by the solution combustion method, which was invented by the present authors. One of the TiO 2 nanopowders was synthesized by HPPLT (homogeneous precipitation process at low temperature) method in this work. The other TiO 2 nanopowder (P-25 TiO 2 , a popular commercial titania photocatalyst) was purchased. These photocatalytic nanopowders were immobilized by several kinds of binders. The immobilization characteristics were examined by XRD, ultrasonication, and SEM. The photocatalytic characteristics were measured by photocatalytic Ag ion extraction rate. The Ag ions were extracted from real (not artificial) photographic waste fixing solution. The Ag ion extraction rate with the immobilized ZnO nanopowder was more than twice that reported for other immobilized photocatalysts.

ZnO nanopowders were prepared by Solution Combustion Method (SCM). The ZnO nanopowders synthesized using Zn(OH)2 and glycine as an oxidant and a fuel (with fuel/ oxidant ratio, F/O=0.8), showed excellent crystalline and photocatalytic characteristics. To confirm photocatalytic activities of the prepared ZnO powder, total organic carbon (TOC) was tried to remove from aqueous phenol solution. Surprisingly, SCM ZnO nanopowder shows 1.6 fold higher destruction rates of the organic pollutant than P-25 TiO2 nanopowder which is known as a kind of standard photocatalyst.

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