The solar-photocatalytic degradation mechanisms and kinetics of 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) using TiO2 have been investigated both individually and combined. The individual solar-photocatalytic degradation of both phenolic compounds showed that the reaction rates follow pseudo-first-order reaction. During the individual photocatalytic degradation of both 4-CP and 2,4-DCP under the same condition of TiO2 (0.5 g L(-1)) and light intensities (1000 mW cm(-2)) different intermediates were detected, three compounds associated with 4-CP (hydroquinone (HQ), phenol (Ph) and 4-chlorocatechol (4-cCat)) and two compounds associated with 2,4-DCP (4-CP and Ph). The photocatalytic degradation of the combined mixture (4-CP and 2,4-DCP) was also investigated at the same conditions and different 2,4-DCP initial concentrations. The results showed that the degradation rate of 4-CP decreases when the 2,4-DCP concentration increases. Furthermore, the intermediates detected were similar to that found in the individual degradation but with high Ph concentration. Therefore, a possible reaction mechanism for degradation of this combined mixture was proposed. Moreover, a modified Langmuir-Hinshelwood (L-H) kinetic model considering all detected intermediates was developed. A good agreement between experimental and estimated results was achieved. This model can be useful for scaling-up purposes more accurately as its considering the intermediates formed, which has a significant effect on degrading the main pollutants (4-CP and 2,4-DCP).

The influences of ferric ions (Fe3+) and hydrogen peroxide (H2O2) on the degradation of combined chlorophenols during solar/TiO2 process were investigated. 4-Chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) were used as a chlorophenols combined mixture. Fe3+ ions have shown considerable effect on the solar-photocatalytic degradation rate of the mixture and its intermediates. Different amounts of Fe3+ ions were used, and the optimum value was 10 mg/L. At these conditions, three major intermediates were detected including hydroquinone (HQ), phenol (Ph), and 4-chlorocatechol (4-cCat). However, the concentrations of these aromatic intermediates were less than that of using TiO2 alone as well as the degradation time was reduced to 150 min. H2O2 was also effectively used as a degradation enhancer at various concentrations together with Fe3+ ions in order to improve the solar-photocatalytic degradation rate and the optimum value was 3.41 mM. The degradation efficiency of the combined mixture increased sharply in the presence of H2O2. Additionally, an extremely high degradation rate of the main pollutants and their intermediates was achieved when using Fe3+ ions and H2O2 together with TiO2. Furthermore, only two intermediates HQ and Ph were observed in this case. The complete degradation of the main chlorophenolic compounds and their intermediates was achieved within 130 min of solar irradiation.

The present study reports a mechanism and kinetic model of solar photocatalytic degradation of 4-chlorophenol (4-CP) and its intermediates based on the experimental results. Three intermediate compounds hydroquinone (HQ), 4-chlorocatechol (4cCat) and phenol (Ph) were found during the solar degradation of different 4-CP concentrations using 0.5g/L TiO 2 as a photocatalyst. In order to obtain more details about the photocatalytic reaction pathway and the kinetic model, set of experiments were carried out using the major intermediates (HQ and 4cCat) as model reactants. To minimise the number of variables and give more accuracy to the kinetic model, the adsorption constants of 4-CP and its intermediates were obtained experimentally. The reaction mechanism for the photocatalytic degradation of 4-CP is proposed. The proposed model predicts well the concentrations of 4-CP and its by-products during the solar photocatalytic degradation at different initial concentrations. The model provides a very good fit of the experimental data and works for a wide range of 4-CP initial concentrations (25-100 mg/L).