Laura Valdés Sánchez

Aniline mineralization by AOP's: anodic oxidation, photocatalysis, electro-Fenton and photoelectro-Fenton processes

Abstract The aniline degradation in acidic medium of pH≈3 under photocatalytic and electrochemical conditions has been investigated. The efficiency for substrate mineralization in each process has been comparatively analysed by the decrease in TOC of aniline solutions. Particular emphasis has been made on the role of Fe(II) ions and H 2 O 2 . The electrochemical experiments performed in the presence of both species (electro-Fenton conditions) leads to a fast aniline mineralization, which is notably increased by UVA irradiation (photoelectro-Fenton process). In photocatalysis with TiO 2 suspensions, the presence of H 2 O 2 and Fe(II) ions in solution notably increases the aniline degradation rate at the initial stages of the process, whereas the opposite effect occurs at long irradiation times. Benzoquinone, hydroquinone, nitrobenzene, phenol and 1,2,4-benzenetriol were detected as intermediates by HPLC in both, electrochemical and photocatalytic experiments. Short chain aliphatic acids, such as maleic and fumaric acids, were only found in the electrochemical experiments. Ammonium ions (75–80% of initial nitrogen) were generated in all solutions tested. A general reaction pathway that accounts for aniline mineralization to CO 2 involving those products is proposed.

Aniline degradation by combined photocatalysis and ozonation

Abstract The combination of TiO 2 -assisted photocatalysis and ozonation in the degradation of aniline in aqueous solution is investigated. From the experimental results obtained it is observed that the ozonation pretreatment followed by photocatalysis strongly increases the yield of TOC removal in comparison to either ozonation or photocatalysis carried out separately. The opposite sequence (photocatalysis pretreatment followed by ozonation) does not enhance the efficiency of aniline degradation. Nevertheless, the highest TOC removal was achieved by simultaneous ozonation and photocatalysis. A mechanism involving the formation of an ozonide anion radical previous to the generation of OH radicals is suggested to explain the synergic effect between ozone and TiO 2 under illumination.

Photocatalyzed destruction of aniline in UV-illuminated aqueous TiO2 suspensions

The photocatalytic oxidation of aniline in aqueous suspensions of TiO2 under UV-illumination is studied. The process follows a Langmuir-Hinshelwood kinetic model with an adsorption constant of 1.1 × 103 dm−3 mol−1 and a reaction rate constant of 8.3 × 10−7 mol dm−3 s−1. The yield of photodegradation strongly depends on pH; higher yields are reported at very acid and basic media and also at pHs near the point of zero charge of TiO2. On the other hand, the yield of the photodegradation increases with the addition of small amounts of Fe2+ in solution. Hydrohydroquinone is the main intermediate product detected, phenol, paraquinone and nitrobenzene being other minor products formed during irradiation. Under illumination, the organic nitrogen is transformed to the inorganic forms: predominantly NH4+ at acid pH, while NO2− and NO3− appear at higher pHs. Percentages of TOC reduction of about 85% are attained after 8 h of irradiation of aniline solutions at pH 3 and 6 in the presence of TiO2.

Degradation of 2,4-dichlorophenoxyacetic acid by in situ photogenerated fenton reagent

The oxidation of 2,4-dichlorophenoxyacetic acid using ZnO as photocatalyst is investigated. This photodegradation process depends on the mass of semiconductor and on the light intensity, being almost independent of the pH of the solution, except in alkaline media and near to the point of zero charge of the semiconductor. The experimental data show that the photodegradation process follows Langmuir-Hinshelwood kinetics. The role of Fe(II) ions in the yield of the 2,4-D photodegradation is analyzed. It is shown that the addition of a small amount of Fe(II) increases the rate of photodegradation. Also, an increase in the rate of 2,4-D dechlorination is observed, although a lower degree of mineralization is detected in presence of Fe(II) ions than in their absence.

Solar activated ozonation of phenol and malic acid

The effect that sunlight has on the degradation rate of two model organic compounds, phenol and malic acid, by ozone is studied. The effect seems to be due to both direct light absorption (300-320 nm photons) by ozone, which produces the pollutant degradation, and light absorption by reaction intermediates. The presence of such a light notably improves the reactivity of ozone toward the organic species, leading to a faster and complete mineralization even at large initial total organic carbon values. The use of artificial sunlight (Xe lamp) is also explored. Finally, the simultaneous presence of sunlight and other ozone degradation catalyst like transition metal ions is studied, showing the beneficial effect of such a combination.