Adsorption and photocatalytic degradation of 2,4-dichlorophenol in TiO2 suspensions. Effect of hydrogen peroxide, sodium peroxodisulphate and ozone

2,4-dichlorophenol (DCP) was decomposed in the presence of TiO2 suspensions by illumination. The operating variables that were studied included initial DCP concentration, photocatalyst doses and initial pH. Mineralisation was slow at the optimum pH (12) for DCP degradation, with the optimum pH for both degradation and mineralisation being pHo 5. The kinetic patterns were fitted to a first order kinetic. Adsorption and rate constants were obtained by using the Langmuir–Hinshelwood model. The adsorption isotherm was fitted to the Freundlich model and the Langmuir model with high coefficients of correlation. The adsorption constant in darkness was lower than that obtained with the kinetic model. According to the FTIR results, DCP–TiO2 interaction occurs as a result of the formation of a phenolate. Of the three oxidants tested – hydrogen peroxide, ozone and sodium peroxodisulphate – only H2O2 resulted in enhancement of the photocatalytic process.

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► DCP degradation is fastest at pH 12 and mineralisation at pH 5.
► At pH 3, toxicity remains high and recalcitrant chlorinated intermediates are formed.
► The DCP adsorption isotherm fits the Freundlich model and the Langmuir model well.
► DCP interaction with the TiO2 surface gives rise to the formation of a phenolate.
► H2O2 enhances the photocatalytic process while S2O82− has the opposite effect.