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Solar Photocatalytic Degradation of Humic Acids Using Copper-Doped TiO2
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Photodegradation of humic substances causes drastic changes in the UV–vis absorption and fluorescence properties of humic acids. In this study it is intended to fulfill the lack of knowledge about the spectral changes of humic acids during photocatalytic oxidation processes and elucidate the effects observed on the molecular size distribution of humic acid focusing on their analysis by UV–vis and fluorescence spectroscopy.As confirmed by the spectroscopic evaluation of the molecular size distribution data, photocatalytic degradation of humic acid leads to the formation of lower molecular size (small fractions) and higher UV absorbing compounds. For fractions less than 10 kDa, UV254 absorbing moieties in treated humic acid samples become higher than that of raw humic acid designating the generation of new species during photocatalysis. UV–vis spectroscopic changes were also evaluated by the parameters relating to the concomitant removal of the total organic carbon as well as by the ratios using absorption values at discrete wavelengths. Moreover, the fluorescence spectra of treated humic acid samples show decreasing intensity profiles with increasing photocatalytic irradiation time.
The aim of this study is to develop a shortcut method to predict the intermediates and the mechanism of decolorization reactions of azo dyes. To this purpose, Reactive Red 195 (RR195) was chosen as the representative member of azo dyes and photocatalytic decolorization reaction of RR195 in the presence of TiO2 under UV-A light irradiation was investigated. TiO2 was synthesized by an acid-catalyzed sol-gel method from an alkoxide precursor and characterized by XRD, XPS, ESEM-EDX and BET measurements. The decolorization reaction was monitored by UV–vis, FTIR, GC–MS and ESEM-EDX techniques. Conceptual Density Functional Theory was applied to the degradation reaction of the target molecule and reactivity descriptors were calculated by means of DFT/B3LYP/6-31G* level of theory. Eventually, the reactive sites of the molecule for OH radical attack were determined and the reaction mechanism was predicted by combining the results of the DFT calculations with the experimental FTIR and GC–MS analyses. The predicted mechanism was confirmed by comparison with the experimental results on simple structures reported in the literature. The results of the study suggest that TiO2/UV photocatalysis may be used as a method for treatment of diluted wastewaters in textile industries, adsorption on TiO2 surface occurs through sulfo and carbonyl groups of the dye molecule, while decolorization by the breaking of the NN double bond of the chromophore group.
Excitation emission matrix (EEM) fluorescence spectral features were evaluated for the elucidation of the photocatalytic degradation of sole humic acid (HA) and HA in the presence of clay minerals. Montmorillonite (Mt) or kaolinite (Kt) was selected to represent two different types of clay minerals. EEM fluorescence signatures displayed irradiation time dependent transformation of humic-like fluorophores to fulvic-like fluorophores in accordance with the photocatalytic mineralization of HA. The role of Ca2+ ions on the photocatalytic degradation of HA could also be visualized through EEM under the specified conditions. Upon photocatalytic treatment, the role of Mt was more pronounced with respect to the effects observed in the presence of Kt in comparison to the results achieved for sole HA. Under prolonged irradiation conditions (tirr = 180 min) almost complete removal of fluorophoric groups was detected in relation to a substantial degree of mineralization (>75% removal of dissolved organic carbon, DOC). As a correlative approach, fluorescence-derived index defined as Fluorescence Intensity (FI) represented by the ratio of the emission intensity at λemis = 450 nm to that at λemis = 500 nm, following excitation at λexc = 370 nm was successfully employed. FI values were correlated to specified UV parameters (UV254 and UV280) revealing an inverse relationship. Moreover, through normalization of the specified UV parameters to DOC contents, the derived specific UV parameters (SUVA254 and SUVA280) could also be successfully correlated to FI. The obtained results indicated that besides photocatalytic mineralization efficiencies, UV-vis and fluorescence spectroscopic properties could also be employed to gain further information on the humic structural changes attained through non-selective oxidation mechanism in comparison to the data obtained under non-oxidative conditions.
In this review, special interest was devoted to provide information on the surrogate parameters expressing both quality and quantity of organic matter for the understanding of the photocatalytic oxidation of humic substances. Detailed investigation was directed to the application of photocatalysis with reference to source, origin and modeling of organic matter. Evaluation of the literature findings emphasizes that organic matter taken from natural waters are site specific and should be characterized in detail to be comparable to other studies. Taking into account the photocatalytic degradation studies of natural organic matter, humic substances, humic acids and fulvic acids in slurry systems, a procedure could be deduced that depends on the selection of a standard model sample with a representative concentration, selection of a standard photocatalyst and dose (e.g., TiO2 Degussa P-25, 0.25 mg mL(-1)), application of standardized reaction conditions such as light intensity, pH, and temperature. Furthermore, standardized filtration step avoiding organic leaching and selection of the most suitable analytical parameter are the crucial points to be considered. The use of such a protocol could form a basis for the determination of "relative degradation efficiency" of any sample containing natural organic matter, humic substances, humic acids and fulvic acids regardless of dependency on source and origin.