The conditions for photocatalytic degradation of ethylenediaminetetraacetic acid (EDTA) in aqueous solution with Fe-doped titanium dioxide (TiO2) were optimized. The degradation efficiencies with Fe-doped TiO2 were better, compared with those obtained with bare TiO2 and Pt-doped TiO2. The effect of various experimental factors, such as photocatalytic dosage, temperature, solution pH and light intensity on the photocatalytic degradation of EDTA by Fe-doped TiO2 was investigated. The photocatalytic degradation treatment for the wastewater containing EDTA is simple, easy handling and low cost.
The treatment of radioactive liquid waste from radiochemical plants and nuclear power plants become one of urgent problems for the environmental safety. Ethylenediaminetetraacetic acid (EDTA; C10H16N2O8, CAS #60- 00-4) has been widely used as a decontaminating agent in radiochemical and nuclear power plants. The addition of EDTA into the radioactive liquid waste can give the complexation of some of the precipitant cations, which results in the interference in their removal by the conventional treatment process such as chemical precipitation and ion exchange [
Since EDTA is stable, has low biodegradability, is rarely degradable by chlorine, is hardly retained by activated carbon fibers and is resistant to ozone treatment, it is a crucial step to perform a pretreatment step for the removal of EDTA for a better treatment of the liquid waste [
Recently, advanced oxidation method based on the photocatalysis has been employed successfully for the degradation of organic pollutants. For instance, the liquid waste containing bisphenol [
Thus far, several studies have been reported for the photocatalytic degradation of EDTA with titanium dioxide (TiO2) semiconductors [
In the present work, the photocatalytic degradation of EDTA in the aqueous solution with Fe-doped TiO2 has been investigated. Moreover, the treatment conditions such as the photocatalytic dosage, temperature, solution pH and light intensity have been optimized.
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
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
. Degradation conditions.
EDTA | 0.1 mg/mL, 10 mL |
---|---|
Photocatalyst | TiO2, Pt/TiO2, Fe/TiO2 |
Photocatalyst dosage | 0 - 20 mg/10 mL |
Temperature | 10˚C - 50˚C |
Solution pH | 3 - 8 |
Light intensity | 0 - 4.5 mW/cm2 |
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 Ct 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
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
. Effect of doping metal on the photocatalytic degradation of EDTA in aqueous solution.
Photocatalyst | Degradation efficiency (%) |
---|---|
TiO2 | 68 |
Pt/TiO2 | 73 |
Fe/TiO2 | 91 |
Effect of Fe/TiO2 dosage on the photocatalytic degradation of EDTA in water. EDTA; 0.1 mg/mL, irradiation time; 60 min, temperature; 25˚C, light; black light 2.0 mW/cm2
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 [
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
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
Effect of temperature on the photocatalytic degradation of EDTA in water with Fe/TiO2. Fe/TiO2; 1 mg/mL, EDTA; 0.1 mg/mL, irradiation time; 60 min, light; black light 2.0 mW/cm2
Effect of pH on the photocatalytic degradation of EDTA in water with Fe/TiO2. Fe/TiO2; 1 mg/mL, EDTA; 0.1 mg/mL, irradiation time; 60 min, temperature; 25˚C, light; black light 2.0 mW/cm2
the efficiency was roughly constant (approximately 80%). Above pH 6, the efficiency gradually decreased with pH. The pH value of zero point charge (zpc) pHzpc of TiO2 particles is equal to around six as TiIV-OH [
The effect of light intensity on the photocatalytic destruction of EDTA in water with Fe/TiO2 was investigated. The results are illustrated in
In the semiconductor Fe/TiO2 material with a band gap (Eg), upon the illumination with radiation having an energy greater relative or equal to Eg, 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
The optimization of photocatalytic degradation conditions of EDTA in water using Fe-doped TiO2 was investi-
Effect of light intensity on the photocatalytic degradation of EDTA in water with Fe/TiO2. Fe/TiO2; 1 mg/mL, EDTA; 0.1 mg/mL, irradiation time; 60 min, temperature; 25˚C, light; black light
Schematic representation of the mechanism proposed for the photocatalytic degradation of EDTA over Fe/TiO2
gated. Fe-doped TiO2 was very effective for the photocatalytic degradation of EDTA in aqueous solution, compared with bare TiO2 and Pt-doped TiO2. Since iron is one of cheap and convenient metals, the photocatalyic degradation technology developed may be applied into the treatment of radioactive liquid waste containing EDTA complexing agent.
The present research was partly supported by Grant-in-Aid for Scientific Research (C) 24510096 from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. All experiments were conducted at Mie University. Any opinions, findings, conclusions, or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of the supporting organizations.