Eliminación de contaminantes por fotocatálisis heterogénea, M.A. Blesa y B. Sánchez (Editores), Editorial CIEMAT, Madrid , ISBN: Get this from a library! Eliminación de contaminantes por fotocatálisis heterogénea. [Miguel A Blesa; CYTED (Organización);]. Get this from a library! Eliminación de contaminantes por fotocatálisis heterogénea. [Miguel Blesa;].
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Reactive Black 5 RB5 is an azo dye widely used in fotocaatlisis textile industry because of its high chemical stability. Since it does not entirely fix on the fabrics, it pollutes water sources. The reaction was carried out in an aluminum photoreactor equipped with five lamps.
The effect of TiO 2 0. Complete decolorization of RB5 was obtained at 14 h, employing 0. The highest decolorization percentage of RB5 Cytotoxicity tests on the HepG2 cell line indicated that photocatalytic degradation of RB5 did not generate cytotoxic byproducts.
Preserving and protecting natural resources, such as water, is one of the nowadays main environmental concerns. The water effluents are polluted with high concentrations of heavy metals, pesticides, herbicides, synthetic dyes, soaps, salts, pharmaceuticals, and personal care products 1.
Synthetic dyes are aromatic compounds with structural diversity, high chemical stability, and resistance to microbial attack.
Revista Colombiana de Química
Thus, these industries produce large amounts of wastewater with high concentrations of synthetic dyes 2. Large quantities of drinking water L kg -1 of textile material are consumed in the textile industry, due to inefficiencies in the dyeing process. Contamination of water sources with dyes interferes with the photosynthetic processes of aquatic organisms and impacting the health of the population that receive contaminated water 4.
Also, there are several studies related to synthetic dyes with cases of toxicity, carcinogenesis, mutagenesis, and teratogenesis in humans 5. However, these characteristics make it a toxic compound, recalcitrant to natural aquifers 6.
In this context, it is necessary to develop studies that examine methods for transforming synthetic dyes, such as RB5, into less toxic compounds. The chemical nature of synthetic dyes confers them resistance to degradation by conventional methods, including sedimentation, adsorption, oxidation with sodium hypochlorite, filtration, coagulation, flocculation, and biodegradation 7.
Therefore, advanced oxidation processes AOPs have emerged as an alternative for the treatment of waters polluted with synthetic dyes and other substances.
Additionally, it provides the possibility of using solar radiation as a primary source of energy. This character of sustainability gives significant environmental value 4. In photocatalytic processes, others semiconductors such as zinc oxide ZnOstannic oxide SnO 2zinc sulfide ZnSand cadmium sulfide CdS can be used as catalyst for the degradation of contaminants Heterogeneous photocatalytic methodologies are based on the band model.
The photo-generated electrons and positive holes cause the reduction and oxidation of the organic matter 912 Although reaction rate depends mainly on the light intensity, this parameter does not affect the reaction mechanism. At higher intensity, the absorption of radiation by the catalyst increases, which implies a more significant production of electrons in the conduction band.
This implies an increase the generation of oxidizing species with capacity to degrade the dye. It has been reported that 1 kg of dye is enough to pollute a volume of water of L or higher, that is a concentration of mg L -1 However, high substrate concentrations decrease the photonic efficiency and saturate the TiO 2 surface, which leads to catalyst deactivation and a decrease of the degradation efficiency 15 Another possible cause for such results is the UV-screening effect of the dye itself.
Fotocatálisis heterogénea para el abatimiento de tensoactivos aniónicos en aguas residuales
In order to assess the effect of the dye concentration, several of the previous works that have been executed with AOPs report dyes concentrations between 10 mg L -1 and mg L -1 17 The objective of the present work was to develop a photocatalytic methodology with TiO forocatalisisfor the remediation of aqueous solutions contaminated with RB5 concentrations of 50 mg L-1, 75 mg L-1, and mg L This presents a contribution to the ongoing discussions on dye decolorization.
An exhaustive statistical analysis to obtain suitable operating conditions to carry out the photocatalytic degradation of the dye, on a laboratory scale, was done. The objective of our research was not to identify the byproducts of the reaction, but the evaluation of its cytotoxicity; this parameter indicates the degree of toxicity of the by-products.
Besides, the cytotoxicity analysis has not carried out in most of the studies on this topic, but this fotocatalisus of analysis is important to determine the environmental impact of the treated water.
Titanium dioxide TiO 2 TiO 2 concentration 0. Fifteen tests were performed, with three central points 0.
Also, two control tests were performed, using two center point conditions: When fitting the model, residuals plot was checked, in searching of potential outliers. The RB5 photocatalytic decolorization was performed in an aluminum photoreactor R. According to the experimental design Table 1, Figure 1mL beakers were used, with mL of the dye solution and TiO 2. The reaction was monitored every 2 h. The RB5 decolorization percentage was measured as the response variable.
Degradation kinetics of the dye was determined by plotting the decolorization percentage of RB5 against the time of the photocatalytic reaction. As a part of the analysis of the results, the calculation of the area under the curve AUC was also taken into account, as it is a useful methodology when the progress of a process over time is tested.
The total area was obtained by summing the area of the trapezoids under the curve, which is formed between pairs of decolorization percentage readings over time Photocatalysis in aqueous solutions of RB5. The RB5 decolorization was followed spectrophotometrically Shimadzu UV spectrophotometer at nm.
Where A 0 corresponds to the initial absorbance and A t to the absorbance after the photocatalytic reaction. Because the concentration of TiO 2 is a parameter that significantly affects the decolorization rate of the dye, it is essential to optimize it. TiO 2 particles are likely to aggregate at high concentrations, which reduces the transmission of UV light, and hence the speed of reaction 8. It was determined that the decolorization process could be improved using 0.
However, to determine the influence of the TiO 2 concentration on the dye decolorization, two concentration values of TiO 2 were applied: Thereby, six additional trials independent to the experimental design were proposed, being stricter concerning to TiO 2 concentration see tests 16—21, in Table 2. TiO2 optimization tests for RB5 decolorization. The MTT reduction method was employed to establish the cytotoxicity of the reaction mixtures corresponding to the tests with the highest percentage of RB5 decolorization, utilizing the HepG2 cell line After this period, the MTT was added to the culture dishes, and after additional 3 h of incubation at the same temperature, DMSO was added.
Finally, the absorbance at nm was determined and the minimum lethal concentration LC 50 was found.
The assays were run twice, in independent experiments, with two fotocatalisls per dilution. Doxorubicin was used as positive cytotoxicity control, as it is a cytotoxic agent intercalating the DNA; untreated cells were used as negative cytotoxicity control Under the conditions of the 10 th 0. For best interpretation, see Foocatalisis 3. RB5 Decolorization in aqueous solution for 14 h. Results obtained after applying the Box—Behnken experimental design Table 1.
Decolorization percentage of RB5 for 14 h.
Results obtained after applying the Box—Behnken experimental design. Particularly, for the heterovenea th test, the degradation was almost complete First-order kinetics for RB5 decolorization, under the conditions of test 15 0.
Additionally, other authors have reported that the degradation of azoic dyes and other substances by heterogeneous photocatalysis, using TiO 2 and UV, light can be followed by a first-order kinetics process 19 Since the kinetics were evaluated only by the RB5 concentration, it is necessary to take into account that the evaluation of other involved species may be necessary, if the hetrrogenea kinetic parameters are required In general, RB5 degradation was favored fotocatalsiis low contaminant loads 50 mg L -1see tests 2, 10, 14, and Furthermore, it was observed that the degradation of 50 mg L -1 of RB5 using 0.
This characteristic of TiO heterogensa would allow treating several loads of RB5 using the same catalyst, and therefore a system of continuous degradation could be implemented. Due to the fact that pH can affect the adsorption of dye on the catalyst surface, the point of heteroogenea charge pzc of the TiO 2 must be taken into account; the pzc corresponds to a pH value of 6.
Besides, the RB5 in aqueous solution is negatively charged because it is a sulphonated dye In this study, ffotocatalisis degradation is favored at pH 3. Under this condition RB5 degradation is faster because TiO 2 particles are positively charged, so RB5 is adsorbed easier on TiO 2 as a result of the electrostatic attraction of the positively charged photocatalyst with the dye This behavior agrees with what has been reported by other authors who claim that the degradation of azo dyes increases with decreasing pH 13 At low pH, reduction by electrons in CB may play an important role in the degradation of dyes due to the reductive cleavage of azo bonds Promising removal of the dye was also found at pH fotpcatalisis 15 th testwhich is good considering that actual polluted effluents e.
Although, as reported Threrujirapapong et al.
Using a numerical method implemented in Statgraphics, a maximum expected AUC of This is consistent with the results of the 10th and the 15th test AUC: For the two test controls with TiO 2 and without UV light, and with UV light but TiO 2 freethere were no changes, as the dye oxidation is mainly mediated by heterogennea action of the TiO 2 semiconductor in the presence of UV light, which activates the catalyst.
This result could be associated with the adsorption of the dye on the solid TiO 2 surface, as reported by Chong et al. According to the optimization tests, under the conditions of the 21 st test 0.
From the above, it is possible to conclude that the catalyst load used in the optimization tests is not a determining factor in the improvement of the process productivity Figures 5 and 6.
RB5 decolorization after 10 h, under the conditions of assays 16 to RB5 decolorization percentage in aqueous solution vs. Results obtained for six assays: This behavior was also observed with the 10 th and 15 th tests Figure 2. The decolorization percentages reached in the optimization tests did not differ significantly from each other.
However, to reduce costs and to facilitate the catalyst separation, low concentrations heterrogenea TiO 2 0. Conversely, Chong et al. In contrast, in the present work, under the conditions of the 16th test 0.
This result is due to the higher proportions of dye and catalyst that were used in this study, compared to those used by Chong fotocatwlisis al.
Besides, hegerogenea have been previously reported lower times compared to those obtained for the RB5 degradation. Though, as mentioned above it is important to note that the concentrations of dye used in the present study 50 mg L -1 are higher than those reported by other authors; therefore, it would fotocatalisid more time to remove dyes 14 h.