Fotodestruição de compostos potencialmente toxicos utilizando TiO2 e luz solar




The destruction of potentially toxic compounds by photocatalysis was studied in this work in two different systems: a) the system using colloidal TiO2, where the influence of silver deposits was evaluated for the photodegradation of phenol. b) the system using TiO2 P 25 supported on a glass plate as part of a plug flow reactor using solar light as source of radiation. The colloids were synthesized through the hydrolysis of titanium tetraisopropoxide in acidic medium. The photoreduction of Ag íons onto TiO2 surface was achieved by the UV illumination of the colloid in the presence of methanol as electron donor. The characterization of the colloids was made by UV/Vis absorption, what showed a threshold at 375 nm for the naked TiO2. An absorption band at 424 nrn was observed after silver photo- deposition, corresponding to the metallic silver deposits. The colloids prepared showed high photocatalytic activity for the degradation of phenol when compared to TiO2 P 25 (Degussa). The phenol mineralization yielded 76% at pH 3, quantified by total organic carbon analysis (TOC). The presence of photodeposited silver (1 and 2% w/w) did not result in an increase of reaction rate. This is related to the reoxidation of silver by the photogenerated holes during irradiation. This was verified by the decrease of characteristic colloidal silver band absorption. This reoxidation was inhibited by the presence of a good electron donor such as methanol. The solar reactor consisted in a glass plate with supported TiO2 where the solution falls over as it is exposed to the solar light. The rector is operated with a single pass or recirculating the sample. PhenoI and dichoroacetic acid were used as model compounds in the photodegradation studies. The single pass of 1 mmol/L phenol solution at pH 8 and flow rate of 2.7 L/h resulted in 55% of phenol and 30% of total organic carbon degradation at the average light intensity of 31.2 W/m. The percentage of phenol degradation can be increased by the addition of Fe ions and the presence of silver photodeposited on TiO2. The photodegradation of industrial phenolic effuents (30 mg/L of phenol and 150 mg/L TOC) resulted in 68.6 % of phenol and 65.7% of TOC degradation at pH 8 and a flow rate of 2.7 L/h. A synergistic effect was observed when the Fenton reagent (5 mg/L Fe + H2O2 200 mg/L, at pH 3.5) was added to TiO2/solar light system, resulting in 96.7% of phenol degradation. It was observed a linear dependence of percentage of degradation with the increase of solar light intensity for the range of 20 to 30 W/ m. The percentage of degradation dacays exponentially with the increase of molar flow rate (mmol/min.). Experiments with single as well as recirculation of the sample indicated that there is no mass transfer limitation in this system.


catalise fotoquimica fisico-quimica coloides

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