Ethylenediaminetetraacetic acid disodium salts dihydrate used in the present study was purchased from Nacalai Tesque Inc., Kyoto, Japan (Grade > 99.5%). Three types of photocatalysts were obtained from Ishiraha Sangyo Kaisha, Ltd. 1) Bare TiO₂ (ST-01); particle size 7 nm, specific surface area 300 m²/g, 2) Pt-doped TiO₂ (MPT- 623); particle size 18 nm, specific surface area 60 m²/g and 3) Fe-doped TiO₂ (MPT-625); particle size 15 nm, specific surface area 70 m²/g. Laboratory pure water was obtained from an ultrapure water system (Advantec MFS Inc., Tokyo, Japan) resulting in a resistivity >18 MΩ∙cm.

EDTA aqueous solutions were prepared with ultrapure water. Then, a 10 mL aqueous solution containing 0.1 mg/mL (0.268 μM) EDTA was put into a Pyrex glass reaction vessel (30 mL capacity). The photocatalyst powder was added into the solution to produce a given concentration of suspension. The experimental conditions for the optimization of treatment were shown in Table 1. The A 15 W black light (Toshiba Lighting & Technology Corp) with a maximum emission of 352 nm was applied as light source, which was positioned on the side of photoreactor. The light intensity was measured by a UV radiometer with a sensor of 320 - 410 nm wavelengths (UVR-400, Iuchi Co., Osaka, Japan), and the value was 1.0 mW/cm². The photocatalysts were continuously dispersed in the aqueous solution by a magnetic stirrer during the irradiation.

After the illumination, the photocatalyst was separated through the 0.45 μm Advantec membrane filter. The photocatalyst powders could be almost removed by the filtration. The amount of EDTA in the aqueous solution

was determined titrimetrically against standard Mg²⁺ using Erichrome Black T as indicator. The removal efficiency was calculated by applying the following equation:

where C₀ is the original EDTA concentration and C_t the EDTA concentration after the treatment.

In order to study the effect of doping metals on the photocatalytic degradation of EDTA in aqueous solution with TiO₂, Pt and Fe-deposited photocatalysts were evaluated for the improvement of degradation efficiency. The results are shown in Table 2. The illumination time was 60 min. The degradation efficiency with iron- doped TiO₂ was better compared with those obtained with bare and Pt-doped TiO₂. Other researchers [9] [13] have described the improvement of photocatalytic activity of 0.5 wt% Fe-doped TiO₂, which showed drastically the increases of charge-carrier lifetime to minutes and even hours beside undoped TiO₂ (average lifetime an electron/hole pair is approximately 30 ns). According to the estimation, Fe-doping may improve the photocatalytic degradation of EDTA in aqueous solution with TiO₂ semiconductors. Therefore, all subsequent experiments were performed with Fe-doped TiO₂.

To optimize the Fe/TiO₂ suspension concentration, the effect of photocatalyst dosages on the EDTA degradation in aqueous solution was investigated. The results are illustrated in Figure 1. With increasing the amounts up to 10 mg (1 mg/mL), the degradation efficiency increased. After the value, the efficiency became nearly flat. The increase in the efficiency appears to be owing to the increase in the total surface area, namely the number of active sites, available for the photocatalytic reaction as the dosage of photocatalyst increased. However, the Fe/TiO₂ photocatalyst was overdosed, the number of active sites on the Fe/TiO₂ surface may become nearly

constant due to the decreased light penetration, the increased light scattering and the loss in surface area occasioned by agglomeration (particle-particles interactions) at high solid concentration [14] . Therefore, 1.0 mg/mL (10 mg) of TiO₂ suspension concentration was adopted as the optimal amounts of photocatalyst for the sequential experiment.

Little information dealing with the temperature effect on the photocatalytic degradation of pollutants in water by Fe/TiO₂ has been presented. Therefore, the effect of temperature on the photocatalytic degradation of EDTA in aqueous solution using TiO₂ was investigated in the range of 10˚C - 50˚C. The results are shown in Figure 2. The degradation efficiency of EDTA increased as the temperature increased up to 25˚C. Above the temperature, the efficiency curve was almost flat. In the photocatalytic degradation of imazaquin in the TiO₂ suspension [15] , the effect of temperature was studied in the range 20˚C - 40˚C, and the rate constants increased with increasing temperature. Ishiki et al. [16] have evaluated the photocatalytic degradation of imazethapyr herbicide at TiO₂/H₂O interface. The temperature effect was investigated using a suspension between 20˚C and 40˚C, and the herbicide was more easily degraded at lower temperatures in the TiO₂ suspension, due to the decrease in the physisorption between the TiO₂ surface and the imazethapyr molecules. Therefore, various tendencies of the temperature effect seem to be observed for different target pollutants. All subsequent irradiations were performed at 25˚C because of the operating cost for the photodegradation system.

The amphoteric behavior of most semiconductor oxides affects the surface charge of the photocatalyst. Therefore, the role of initial pH on the degradation efficiency for EDTA was investigated in the pH range 3 - 8, as illustrated in Figure 3. Although the degradation efficiency tended to increase with increase in pH up to around 6,

the efficiency was roughly constant (approximately 80%). Above pH 6, the efficiency gradually decreased with pH. The pH value of zero point charge (zpc) pH_zpc of TiO₂ particles is equal to around six as Ti_IV-OH [17] . This means that the TiO₂ surface becomes positively charged, Ti_IV-, when the pH is lower than this value and becomes predominatly negative, Ti_IV-O⁻, for a pH value above about six. Generally, the pH changes can have a non-insignificant result not only on the mode of adsorption of the EDTA substrate on TiO₂ surface, but also on the selectivity of the photodegradative reaction occurring on the particle surface since redox reactions are very sensitive to changes in the surface potential. In the pH range of 3 to 8, the EDTA chemical species and their metal complex species are anion in the solution. Therefore, the photocatalytic degradation of EDTA may be accelerated in an acid medium. Similar reaction has been suggested by a number of researchers. Consequently, 5.7 was selected for the optimal experimental conditions because of the unnecessary of chemical remediation including neutralization process.

The effect of light intensity on the photocatalytic destruction of EDTA in water with Fe/TiO₂ was investigated. The results are illustrated in Figure 4. The degradation efficiency increased rapidly with increase in the light intensity up to 0.5 mW/cm², and then the efficiency increased gradually. Since the catalyst Fe/TiO₂ powders are suspended in a stirred solution, the light intensity will affect the degree of absorption of light by the catalyst surface. Ollis [18] reviewed the effect of light intensity on the kinetics of photocatalysis and stated that 1) at low light intensities, the rate would increase linearly with increasing light intensity; 2) at intermediate light intensities, the rate would depend on the square root of the light intensity; and 3) at high light intensities, the rate is independent of light intensity. Therefore, the results obtained in the photocatalytic degradation of EDTA in aqueous TiO₂ suspension were reasonable.

In the semiconductor Fe/TiO₂ material with a band gap (E_g), upon the illumination with radiation having an energy greater relative or equal to E_g, the promotion of an electron (e^‒) from the valance band (VB) to the conduction band (CB) takes place. Concomitantly, the formation of a positive hole (h⁺) in the VB occurs. Photogenerated electrons and holes can either undergo undesired recombination or migrate to the surface of the system, where they can initiate reactions with adsorbed species. Whereas holes in the VB are powerful oxidizing species that can produce hydroxyl radicals (•OH) from the reaction with H₂O, photogenerated electrons in the CB are involved in the formation of •OOH. These oxidizing species may attack the pollutant EDTA in the aqueous solution. The presence of the Schottky barrier can decrease the recombination of photogenerated electron-hole pairs consequently prolong their lifetime, and greatly enhance the photocatalytic activity of TiO₂. Iron loading can result in stronger Schottky barrier effect, and therefore shows better photocatalytic activity of TiO₂, as illustrated in Figure 5 [19] .