Abstract
The increase in world population and human activities are leading to an increase in water stress in many regions of the planet, coupled with a decrease in the quality of water bodies. Advanced oxidation processes have demonstrated great potential for the reduction of almost any organic pollutant; however, it is necessary to intensify this type of treatment in order to reduce contact times and to reach a greater number of pollutants. The generation of sulfate radicals by activation of peroxymonosulfate (PMS) by divalent iron (Fe2+) and/or titanium dioxide (TiO2) were statistically studied to understand the role of these compounds as activators, using methylene blue as target pollutant because of its ease of handling and analysis. A factorial experimental design was used to study the influence of different variables (PMS, Fe2+, and TiO2) in the presence of UV-A or UV-C. There were relevant differences in the discoloration of methylene blue when analyzing the size of the effects and significance of the experiments, when UV-A or UV-C was used, being faster with UV-C. For instance, total discoloration of methylene blue was reached after 60 min with the system PMS/UV-C, while after 90 min only the 59% of methylene blue disappeared in presence of PMS/UV-A. Both Fe2+ and TiO2 in combination with PMS and UV increased the discoloration effect. So, in the presence of Fe2+, total discoloration of methylene blue was observed after 30 min in presence of UV-A, while this yield was reached in 7.5 min under UV-C. In the case of PMS/TiO2, it required 60 min under UV-A radiation to totally remove methylene blue, and around 15 min with UV-C. Statistically, the three variables were observed to have the main effect in combination with UV. Furthermore, the PMS/Fe2+ system has a significant interaction with UV-A and UV-C radiation, while the interaction of PMS/TiO2 was significant under UV-A, but with a negative effect under UV-C, or in other words the high elimination rates observed are achieved by the oxidation potential of UV-C, and the effect of PMS and TiO2 by itself.
Highlights
Advanced oxidation processes (AOP) have been shown to be efficient treatments for the removal of organic and biological pollutants in water [1,2,3,4,5,6,7], and are especially useful for the degradation of recalcitrant organic pollutants that cannot be removed by conventional methods, such as dyes or other persistent contaminants [8,9,10,11]
From a statistical point of view, TiO2 photocatalysis increases as the wavelength of UV radiation decreases, but PMS/Fe2+ contributes more to the elimination rates when subjected to UV-A radiation
Regarding Analysis of variance (ANOVA), experiments using only PMS, Fe2+, or TiO2 were all significant when subjected to both UV-A and UV-C, meaning that activation of PMS by UV radiation contributes in a relevant way to dye degradation, and that the other variables (Fe2+ and TiO2) have an important contribution to elimination rates
Summary
Advanced oxidation processes (AOP) have been shown to be efficient treatments for the removal of organic and biological pollutants in water [1,2,3,4,5,6,7], and are especially useful for the degradation of recalcitrant organic pollutants that cannot be removed by conventional methods, such as dyes or other persistent contaminants [8,9,10,11]. Sulfate radicalbased advanced oxidation processes (SR-AOPs) are emerging alternatives to hydroxyl radical-based AOPs (HR-AOPs). Both AOPs are based on the production of free radical species, with a high redox potential of 2.5–3.1 V (25 ◦C) and 2.80 V (25 ◦C) for sulfate and hydroxyl radicals, respectively [12,13,14]. SR-AOPs have some advantages, such as the lack of toxicity of persulfate salts, the ease of handling and storage, and a very low pH-dependent activity [12]. The generation of sulfate radicals requires the activation of persulfate salts such as potassium peroxymonosulfate (HSO5−; PMS). The main activation mechanisms reported are thermal, UV-assisted, or catalyzed by a metal or a metal oxide [12,13,14,15,16,17,18]
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