Degradation of emerging aromatic micropollutants by UV-based oxidation processes Chapter uri icon

abstract

  • Ultraviolet (UV) radiation is frequently applied to disinfect water intended for human consumption and wastewater. Due to the greater chemical contamination of water, UV radiation is increasingly proposed as a technology to remove organic micropollutants, underlining its high efficacy to eliminate certain pesticides and pharmaceuticals from water. Significant advances have recently been made in our understanding of the photo-chemical processes undergone by organic contaminants and pharmaceuticals in aqueous medium. However, fewer data are available on their photochemical transformation. The objective of this chapter is to summarize the efficacy of ultraviolet (UV) radiation in the direct or indirect photodegradation of some emergent micropollutants. For this purpose, i) a kinetic study was performed, determining the quantum yield of the process; and ii) the influence of the different operational variables was analyzed [initial concentration of pollutant, pH, presence of natural organic matter compounds, radicals promoters addition (K2S2O8, H2O2, activated carbon, activated carbon/TiO2) and chemical composition of water], and iii) the time course of total organic carbon (TOC) concentration and toxicity during micropolluntant photodegradation was studied. The very low quantum yields obtained for the compounds studied are responsible for the low efficacy of the quantum process during direct photon absorption in micropollutant phototransformation. The R254 values obtained show that the dose habitually used for water dis-infection is not sufficient to remove this type of compounds; therefore, higher doses of UV irradiation or longer exposure times are required for their removal. The concentration of organic micropolluntants has a major effect on their photodegradation rate. The study of the influence of pH on the values of parameters ε (molar absorption coefficient) and k%27E (phodegradation rate constant) showed no general trend in the behavior of emergent pollutants as a function of the solution pH. The components of natural organic matter, gallic acid (GAL), tannic acid (TAN), and humic acid (HUM), may act as promoters and/or inhibitors of OH%27radicals via photoproduct ion of H2O2. It is interesting to note that the addition of radical promoters such as H2O2 or K2S2O8 markedly increased the effectiveness of UV radiation through the generation of HO· or SO4·-radicals, respectively. Regardless of the system considered, the results obtained showed that the micro-pollutant degradation rate was higher with lower concentrations. The solution pH had a major effect on micropollutant degradation with the UV/H2O2, and UV/K2S2O8 systems. The presence of activated carbon during the pharmacecutical photodegradation process markedly increases the removal rate. The results obtained indicate that activated carbon exerts the greatest synergic effect on diatrizoic acid removal by the UV/AC system, with a synergic contribution >53 %25 at one minute of treatment. Moreover, the presence of activated carbons with a high carboxyl groups content enhances 2,4-D photodegradation by the UV/TiO2 system. Carboxyl groups in the graphene planes of the activated carbon participate in the additional generation of OH radicals by interacting with the electrons produced by the UV/TiO2 system. © 2014 by Nova Science Publishers, Inc. All rights reserved.
  • Ultraviolet (UV) radiation is frequently applied to disinfect water intended for human consumption and wastewater. Due to the greater chemical contamination of water, UV radiation is increasingly proposed as a technology to remove organic micropollutants, underlining its high efficacy to eliminate certain pesticides and pharmaceuticals from water. Significant advances have recently been made in our understanding of the photo-chemical processes undergone by organic contaminants and pharmaceuticals in aqueous medium. However, fewer data are available on their photochemical transformation. The objective of this chapter is to summarize the efficacy of ultraviolet (UV) radiation in the direct or indirect photodegradation of some emergent micropollutants. For this purpose, i) a kinetic study was performed, determining the quantum yield of the process; and ii) the influence of the different operational variables was analyzed [initial concentration of pollutant, pH, presence of natural organic matter compounds, radicals promoters addition (K2S2O8, H2O2, activated carbon, activated carbon/TiO2) and chemical composition of water], and iii) the time course of total organic carbon (TOC) concentration and toxicity during micropolluntant photodegradation was studied. The very low quantum yields obtained for the compounds studied are responsible for the low efficacy of the quantum process during direct photon absorption in micropollutant phototransformation. The R254 values obtained show that the dose habitually used for water dis-infection is not sufficient to remove this type of compounds; therefore, higher doses of UV irradiation or longer exposure times are required for their removal. The concentration of organic micropolluntants has a major effect on their photodegradation rate. The study of the influence of pH on the values of parameters ε (molar absorption coefficient) and k'E (phodegradation rate constant) showed no general trend in the behavior of emergent pollutants as a function of the solution pH. The components of natural organic matter, gallic acid (GAL), tannic acid (TAN), and humic acid (HUM), may act as promoters and/or inhibitors of OH'radicals via photoproduct ion of H2O2. It is interesting to note that the addition of radical promoters such as H2O2 or K2S2O8 markedly increased the effectiveness of UV radiation through the generation of HO· or SO4·-radicals, respectively. Regardless of the system considered, the results obtained showed that the micro-pollutant degradation rate was higher with lower concentrations. The solution pH had a major effect on micropollutant degradation with the UV/H2O2, and UV/K2S2O8 systems. The presence of activated carbon during the pharmacecutical photodegradation process markedly increases the removal rate. The results obtained indicate that activated carbon exerts the greatest synergic effect on diatrizoic acid removal by the UV/AC system, with a synergic contribution >53 %25 at one minute of treatment. Moreover, the presence of activated carbons with a high carboxyl groups content enhances 2,4-D photodegradation by the UV/TiO2 system. Carboxyl groups in the graphene planes of the activated carbon participate in the additional generation of OH radicals by interacting with the electrons produced by the UV/TiO2 system. © 2014 by Nova Science Publishers, Inc. All rights reserved.

publication date

  • 2014-01-01