Measurement of Phenols Dearomatization Via Electrolysis: The UV-Vis Solid Phase Extraction Method

Dearomatization levels during electrochemical oxidation of p-methoxyphenol (PMP) and p-nitrophenol (PNP) have been determined through UV-Vis spectroscopy using solid-phase extraction (UV-Vis/SPE). The results show that the method is satisfactory to determine the ratio between aromatic and aliphatic compounds and reaction kinetics parameters during treatment of wastewater, in agreement with results obtained from numerical deconvolution of UV-Vis spectra. Analysis of solutions obtained from electrolysis of substituted phenols on antimony-doped tin oxide (SnO2-Sb) showed that an electron acceptor substituting group favored the aromatic ring opening reaction, preventing formation of intermediate quinone during oxidation.

Trabajo publicado:
Water Research, Volume 44, 2010, Pages 911-917
Ronald Vargas, Carlos Borrás, Jorge Mostany, Benjamin R. Scharifker

Hydrogen bond interactions at the TiO2 surface: Their contribution to the pH dependent photo-catalytic degradation of p-nitrophenol

We have obtained pKa values of p-nitrophenol–TiO2 by measuring the adsorption equilibrium constants of p-nitrophenol (PNP) on the TiO2 surface at different pH values. These values have been obtained from Langmuir isotherms and from a plot of 1/rate vs. 1/[PNP]o obtained during TiO2 catalyzed solar light photodegradation of PNP. Two limit equilibrium constants are readily obtained depending on the solution pH: at pH 5 at which the TiO2 surface is mainly positively charged and at pH 8 when it is negatively charged. With these and other adsorption equilibrium constants and the PNP pKa value in solution, thermodynamic cycles are established in order to obtain the PNP pKa when it is adsorbed on positively charged, neutral and negatively charged TiO2 surfaces. From these pKa values useful information on the PNP–TiO2 interaction is readily obtained. For instance, the PNP nitro group interacts with the TiO2 surface via a hydrogen bond, arising from the complex of water molecules with the Ti4+ ions on its surface. The weaker the hydrogen bond donor, the stronger the oxygen nitro group basicity. Therefore, pKa changes on the phenolic hydroxyl group result from these interactions. Linear free energy correlations, maximum PNP adsorption capacity values (QL) and FTIR-ATR, spectrum support this proposal. A kobs vs.pH degradation profile of p-nitrophenol is also provided.

Trabajo publicado:
Journal of Molecular Catalysis A: Chemical, Volume 300, Issues 1-2, 2 March 2009, Pages 65-71
Ronald Vargas, Oswaldo Núñez

The photocatalytic oxidation of dibenzothiophene (DBT)

TiO2/UV solar light degradation of dibenzothiophene (DBT) aqueous solutions readily occurs in neutral and acid media. For instance, at neutral pH, t1/2 = 30 min are found for DBT degradation and mineralization. In neutral and acid media the rate limiting step for mineralization is the same as for degradation and corresponds to the DBT sulfone formation. Benzaldehyde is the major reaction product. In addition, it also acts as an intermediate in the mineralization to CO2 +water + sulfate. The results support the Langmuir–Hinshelwood degradation mechanism. Therefore, instead of pseudo-first-order rate constants, apparent rate constants that depend on concentration are obtained. Only at basic pH these apparent rate constant are independent of the [DBT]0. The solubility of DBT in water is increased by one-order of magnitude when solutions of the surfactant Triton X-100 is used at [Triton X-100] >CMC (critical micelle concentration). Under these conditions DBT is readily degraded without degrading Triton X-100 at t1/2 ca. 120 min. DBT is solubilized into Triton X-100 micelles from where it exchanges with water and becomes available for degradation. Triton X-100 monomers slightly compete with DBT for the TiO2 catalytic site. The degradation method established in this work might be used, for instance, to decontaminate soil contaminated with sulphur-rich heavy oil following a pre-treatment consisting of washing out the solid with Triton X-100 solutions, prior to photolysis.

Trabajo publicado:
Journal of Molecular Catalysis A: Chemical, Volume 294, Issues 1-2, 15 October 2008, Pages 74-81
Ronald Vargas, Oswaldo Núñez

Photocatalytic TiO2 – assisted decomposition of Triton X-100: inhibition of p-nitrophenol degradation

A decrease in the apparent pseudo first-order rate constant is observed in the photocatalyzed (TiO2) degradation of surfactant Triton X-100 (Triton) when its concentration is increased. The measured rate versus the concentration profile is consistent with a hyperbolic form (rate increases with concentration) as described by the Langmuir–Hinshelwood (LH) model. The rate is then given by the expression: r=kK[Triton]/(1+K[Triton]) but the apparent rate constant by kapp=kK/(1+K[Triton]o), where k=0.66 mg/(L min) and K=0.037 L/mg. Therefore, at low [Triton]o, kapp=kK but at high [Triton]o, kapp=k/[Triton]o, that is, an inverse function of the reactant concentration. Although, in the latter case the reaction does not follow first-order kinetics, its pseudo first-order deviation is not easily noticeable. Therefore, this decrease in kapp with reactant concentration may limit its use when rate constants are compared to evaluate degradation efficiency or when it is used to show reaction inhibition. However, we have detected p-nitrophenol inhibition induced by Triton using kapp values. Inhibition is observed at [Triton]o > CMC. These inhibitions are consistent with the LH model given by the expression: r=k’K’[phenol]/(1+K’[phenol]o+K([Triton]o), where [phenol] is equal during all kinetic runs.

Trabajo publicado:
Journal of Physical Organic Chemistry, Volume 21, Issue 12 (December 2008), pp 1072-1078
Gloria Pardo, Ronald Vargas, Oswaldo Núñez