Photocatalytic degradation of Doxycycline in aqueous solution using Fe2O3 andFe2O3-Bi2WO6 catalysts

Photocatalytic degradation of Doxycycline in heterogeneous media constituted by nanoparticles of Fe2O3 and Fe2O3-Bi2WO6 was studied. The Bi2WO6 was prepared by hydrothermal method, and the Fe2O3-Bi2WO6 was obtained by mechanical mixing of Bi2WO6 to an amount of Fe2O3 and their photocatalytic activity to degrade doxycycline (Dox) under UV irradiation supported by H2O2 were studied. The H2O2 reacts with photogenerated electrons leading to the production of hydroxyl radicals (OH). The Fe2O3 acts like a Fenton reagent, accelerating the production of OH. In the present study Bi2WO6-Fe2O3/ H2O2 system demonstrate much higher photocatalytic efficiency to degrade Dox than pure Bi2WO6.


Introduction
Increasing demand and shortage of clean water sources due to population growth and long-term droughts have become an issue worldwide. It is estimated that millions of people die of severe waterborne diseases annually.
Water is necessary for the survival of all forms of life on our planet. For humans, access to clean and safe water is of fundamental importance for achieving and maintaining acceptable living conditions. One of the greatest achievements of the past century, regarding the protection of public health and the environment, was the development and the wide application of efficient treatment methods for water and wastewaters 1,2 . However, one of the most critical environmental challenges that we have to face in the 21st century is a sustainable supply of clean and safe water because clean water has become an increasingly scarce resource. In view to suppress the worsening of clean water shortage, development of advanced with lowcost and high-efficiency water treatment technologies to treat and reuse wastewater is essential.
In recent years, a broad range of organic pollutants has been identified in the aquatic environment, in concentrations ranging from ng/L to mg/L 1,2 . Despite their low concentration, several effects of these organic pollutants on aquatic organisms have been reported, such as acute and chronic toxicity, endocrine disruption and bioaccumulation.
As Doxycycline (Dox) is from a wide-range antibiotic compound with activity against many species of bacteria is used as a pharmacological agent to treat chronic prostatitis, sinusitis, syphilis, chlamydia, pelvic inflammatory disease, acne, rosacea, and rickettsial infections. However, the doxycycline is commonly detected in the aquatic environment and Wastewater Treatment Plants effluents 2 .
Unfortunately, conventional chemical and biological oxidation treatment methods being employed for water and wastewater usually fail to remove most antibiotics 1-6 . Also, physical treatment methods of water and wastewater, such as adsorption and membrane filtration, transfer these compounds from one phase to another, rather than removing them 1,2,5 . Therefore, for their abatement, advanced treatment methods of water and wastewater are required, in particular in the case that the treated domestic wastewater effluents may be considered for reuse applications 2,7 .
In recent years, several advanced water and wastewater treatment methods have been developed 1,2 . Among them, photocatalysis has received considerable attention as a promising green technology for environmental clean-up.
The development of new photocatalysts is attracting enormous importance. Among them, the Bismuth tungstate (Bi2WO6) is a typical n-type direct bandgap semiconductor with a bandgap of 2.75 eV and has prospective applications catalysis 8- 10 . In addition, it has been found that Bi2WO6 might act as a stable photocatalyst for the photochemical decomposition of organic contaminants 11 . Additionally, the unique structure of the photocatalyst could enhance its photoreactivity the photoactivity of Bi2WO6, in which the transfer of electrons to the surface was improved along with the layered network 12,13 . Bi2WO6 exhibit a possible catalytic ability to numerous organic chemicals, and it is mostly synthesized by hydrothermal or solvothermal method.
In order to enhance catalytic efficiency, Bi2WO6 catalyst could be doped with metal or metal oxide, which might trap the photogenerated electrons and restrain the recombination of hole-electron pair 14,15 . H2O2 is a typical oxidative agent and has been often used as ineffective water treatment. As an electron capture agent, H2O2 can also react with photogenerated electrons to produce hydroxyl radicals (OH • ) as established in eq. (1) 5, 16 .
Therefore, when H2O2 is present with Bi2WO6/Fe2O3 composite, it could interact with the composite and affect the photocatalytic capacity displaying higher photodegradation efficiency.
The present work proposes to dope Bi2WO6 with Fe2O3 to prepare Bi2WO6-Fe2O3 composite and investigate its efficiency to degrade Doxycycline as an organic pollutant in the presence of H2O2. In previous work 17 , we have tested the efficiency of the synthesized composite in the degradation of Methyl Orange day and Phenol as artificial pollutants.

Chemicals and Catalysts preparation
All the chemicals used in this work were analytical grade without further purification.
The Bi2WO6 as synthesized from Bi (NO3)3.5H2O according to the published procedure 18

Characterization
The composite structure was analyzed by X-ray diffraction (XRD) using a Rich Seifert 3000 diffractometer with Cu-K.
The UV-visible diffuse reflection spectra (DRS) of the photocatalyst were measured by a Cary 100 (Varian) spectrometer at the range of 250-600 nm using BaSO4 as a reflectance standard and were converted from UV-visible absorption spectra according to the Kubelka-Munk equation.

Photocatalytic experiments
Photocatalytic activity of the photocatalysts prepared was evaluated by photocatalytic degradation of Dox under UV light irradiation. A reflective enclosure equipped with four circular fluorescent tubes type Philip 15 W with a maximum emission at 365 nm was used as the light source. The solution was placed in a central position in a quartz tube. The initial concentration of Dox in pure water was 4x10 -5 mol/L, and a concentration of 1g/L of the photocatalyst and H2O2 (5%) was stirred.
Before the experiment, the mixed solution (catalyst+Dox) was magnetically stirred in the dark for 20 min to ensure the establishment of an adsorption-desorption equilibrium between the catalyst and Dox. At given times, a definite volume of the suspension was extracted and filtered using a filter (RC 0.45 µm). The Dox in the suspension was determined by measuring the absorbance value at approximately 275 nm using a carry 300 spectrophotometer.
Photolysis of Dox under UV and in the absence of catalysts was carried out under the same conditions experimental used in this work. The photocatalytic degradation mechanism is shown in Fig.1.  The pure Bi2WO6 absorbs in the array from UV light to a visible light region less than 450 nm the band-gap was estimated to be 2,7eV which is similar to the value in the literature 19 . The Fe2O3 absorbs in the entire UV-visible range, with a band-gap of 3, 02 eV. After composited with Bi2WO6, the light absorption of the Bi2WO6-Fe2O3 composites increased in the visible range.

Photocatalytic activity
The photocatalytic activity of the catalysts prepared was evaluated by the photodegradation of doxycycline.
Preliminary experiments were carried out in order to verify that Dox was removed by the various heterogeneous photoassisted processes under UV illumination, investigating the effect of H2O2 on Dox under UV. It was observed that in the absence of the photocatalyst, no reduce in Dox concentration was found under UV illumination. Therefore photolysis did not occur. Likewise, no significant difference in the initial concentration of Dox was found, in the dark and the presence of H2O2.
UV irradiation over aqueous Dox/ Fe2O3-Bi2WO6 dispersions leads to reduce the absorption intensity. With the increase of irradiation time, the feature absorption band at 275 nm for Doxgets smaller gradually, and the absorbance of the band at 275 nm decreases from 2,582 to 0, 19.
Though the mechanisms remain ambiguous, it is known that H2O2 allow the photogenerated electron to produce OH•, which is a potent oxidative species to degrade organic chemicals [24][25][26] . As an electron capture agent, H2O2 can also react with photogenerated electrons to produce hydroxyl radicals (OH•) as established in eq. (1).
In the system with Fe2O3/ H2O2, an amount of OH• was produced through Fenton reaction as shown in eq. (2), and as results, the photodegradation of Dox was higher than pure Fe2O3.
In the presence of H2O2, the photogenerated electrons of Bi2WO6 could react with H2O2 to produce OH • as eq. (1), the photodegradation of Dox was much higher than pure Bi2WO6, meaning that OH • played an essential part in the photodegradation.
In the Bi2WO6-Fe2O3 system, the amount of OH • produced could be attributed to the heterogeneous Fenton reaction in the surface of Fe2O3 with H2O2 just as eq.(2) and the coupling effect taking place at the interfaces of Fe2O3 and Bi2WO6, leading to a higher production of OH • compared to pure Bi2WO6 in the presence of H2O2.
It was established that Bi2WO6-Fe2O3/H2O2 presented higher photocatalytic productivity to degrade Dox than Bi2WO6/H2O2. These could be explained by the additional OH • produced in the Bi2WO6-Fe2O3/H2O2 system in comparison to that in the Bi2WO6/H2O2 system during 120 min irradiation.
In previous work 17 , we found that the Bi2WO6-Fe2O3/H2O2 presented higher photocatalytic activity in the removal of Methyl Orange and Phenol used as artificial pollutatants.

Conclusion
Doping Bi2WO6 with Fe2O3 improved the photocatalytic capacity to degrade Dox in the presence of H2O2. The OH • generated by the H2O2 added is crucial to the process of degradation. The Fe2O3 operated as a Fenton-like reagent, accelerating the production of OH • , increasing the photodegradation efficiency to Dox in the presence of H2O2 under UV light irradiation.
The Fenton process requires acidic condition (pH around 3); the Bi2WO6-Fe2O3/H2O2 system could overcome this inconveniency and be applicable for broad pH conditions of wastewater.