Journal of Photochemistry and Photobiology A: Chemistry
Bulk phase degradation of Acid Red 14 by nanophotocatalysis using immobilized titanium(IV) oxide nanoparticles
Introduction
Azo dyes, which contain one or more azo bonds (NN), are among the most widely used synthetic dyes and usually become major pollutants in textile wastewaters. About 50% among annually world produced dyes (700,000 t) are azo dyes. About 15% of the total world production of dyes is lost during textile dyeing which is released in textile effluents. The discharge of dye-bearing wastewater from textile industries into natural stream and rivers poses severe problems, because of toxicity of some dyes to the aquatic life and damaging to the aesthetic nature of the environment [1], [2], [3]. Thus, there is an urgent need for textile wastewater to develop effective methods of treatment.
Heterogeneous nanophotocatalysis constitute one of the emerging technologies for the degradation of organic pollutants. Several advantages of this process over competing processes are: (1) complete mineralization, (2) no waste-solids disposal problem and (3) only mild temperature and pressure conditions are necessary [1], [3], [4], [5], [6].
At heterogeneous nanophotocatalysis, semiconductor can act as sensitizer for light-reduced redox processes due to their electronic structure, which is characterized by filled valence band and an empty conduction band. When a photon with energy of hν matches or exceeds the band gap energy, Eg, of the semiconductor, an electron, ecb−, is promoted from the valence band, VB, into the conduction band, CB, leaving a hole, hvb+ behind. Excited state conduction band electrons and valence band holes can recombine and dissipate the input energy as heat, get trapped in meta-stable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface. The hvb+ is a strong oxidant, which can either oxidize a compound directly, or react with electron donors like water or hydroxide ions to form hydroxyl radicals, which react with pollutants such as dyes. Hydroxyl radicals react with organic pollutants leading to the total mineralization of most of them [1], [3], [6], [7], [8].
Adsorption is a key factor in slurry photocatalytic system due to the large surface area of catalyst available for reaction. Thus, in a suspension system, the photocatalytic reactions are surface processes. The immobilization of TiO2 nanoparticle decreases the effective surface of catalyst. There is still an ongoing debate whether photocatalytic oxidation reactions are surface or solution processes in an immobilized system. In this study, after adsorption experiment, the absorbance of both dyes does not decrease, indicating the negligible effect of the adsorption on the dye concentration. On the other hand, it was assumed that the reactive hydroxyl radicals and other oxidizing species can diffuse into the solution bulk to react with organic pollutants.
However, in the large scale applications, the use of suspensions requires the separation and recycling of the catalyst particles from the treated wastewater prior to the discharge and can be a time-consuming expensive process. In addition, the depth of penetration of UV light is limited because of strong absorptions by both catalyst particles and dissolved dyes [9]. Above problems can be avoided by immobilization of photocatalyst over suitable supports.
The aim of the present study is to investigate the pilot scale heterogeneous photocatalytic degradation of aqueous solution of Acid Red 14 (AR 14) using an immobilized TiO2 nanoparticle photocatalytic reactor. The effects of operational parameters such as H2O2, dye concentration, anions (NO3−, Cl−, SO42−, HCO3− and CO32−) and pH were investigated. These are the major variables governing the efficiency of the process. The produced dominant aliphatic intermediates and mineralization of AR 14 were studied. Furthermore, the kinetics of the photocatalytic decolorization of AR 14 was investigated. It has been found that adsorption has a negligible effect on the aqueous dye concentration and the photocatalytic process occurred at solution bulk.
Section snippets
Reagents
AR 14 (95% ≤ purity) was obtained from BASF (Germany). The descriptions (name, color and molecular weight) of AR 14 and its chemical structure are shown in Table 1 and Fig. 1. HCO2Na, H3CCO2Na, Na2C2O4, Na2SO4 and NaNO3, NaHCO3, Na2CO3 and H2O2 were purchased from Merck. Titanium dioxide nanoparticle was utilized as a photocatalyst. Its main physical data are as follows: average primary particle size around 30 nm, purity above 97% and with 80:20 anatase to rutile.
Photocatalytic reactor
Experiments were carried out in a
Adsorption of AR 14 onto immobilized TiO2 nanoparticles
Adsorption (dark) experiments were carried out for AR 14 dye under gentle air agitation in immobilized TiO2 nanoparticles at the same experimental conditions (dye: 0.1 mM and H2O2: 6 mM and 60 min in the dark).
After adsorption experiment, the absorbance of AR 14 does not decrease, indicating the negligible effect of the adsorption on the dye concentration. On the other hand, other studies have concluded from their analyses that the photocatalytic process does not need not to occur at the catalyst
Conclusions
AR 14 could be successfully decolorized and mineralized by nanophotocatalysis in an immobilized TiO2 nanoparticle photocatalytic reactor. It has been found that adsorption has a negligible effect on the dye concentration and the photocatalytic process occurred at solution bulk. The effects of operational parameters such as H2O2, dye concentration, anions (NO3−, Cl−, SO42−, HCO3− and CO32−) and pH were investigated. Na2CO3 exhibited the strongest inhibition effect followed by NaHCO3. The
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