**4. Conclusions and recommendations**

**Figure 7a** and **b** show, respectively, the results obtained in the quantification of the concentrations of hydroquinone and catechol formation. The evolution of the concentration profiles of hydroquinone and catechol in function of the time confirm clearly the induction time of reaction, around 110 min, identified initially by the curves of time evolution of the phenol degradation (**Figure 6a**) and TOC conversion (**Figure 6b**). It can be that maximum values of hydroquinone and catechol concentration formed in approximately 140 min operating time, reaching the maximum speed of phenol degradation and TOC conversion and that the catechol concentrations are always higher than the hydroquinone

338 Phenolic Compounds - Natural Sources, Importance and Applications

After 140 min, both hydroquinone and catechol concentrations decrease, thus allowing the formation of other organic compounds that are not acids, because the pH becomes practically constant (pH = 3) after 140 min of operation (**Figure 5b**). It can be that the products resulting from the oxidation of hydroquinone and catechol are possibly aldehydes (Glyoxal, for example, in the case of hydroquinone and catechol) and alkenes (1,4-dioxo-2-butene, for example, in the

The phenol oxidation produces catechol and hydroquinone [20]. Analyses indicated a higher catechol production then hydroquinone. This may be explained by the mesomeric effect. This signifies an electron re-distribution to the ortho position, which increases its reactivity at this

**Figure 7.** (a) Evolution of hydroquinone formation as a function of the operating time. (b) Evolution of catechol formation

= 170 L h−1, *R*P/H = 50% and *Q*RG = 50%.

h−1, *QL*

as a function of the operating time. *E* = 40%, *Q*GN = 4 m3

position of the molecule due to the proximity of opposing charges [4, 27].

concentration.

case of hydroquinone).

The Advanced Oxidation Processes (AOPs) are found to be an environmental friendly process for the degradation and mineralization of refractory compounds. The limitations of conventional processes in wastewater treatment necessitate study on the AOPs. Thus, applications of the Direct Contact Thermal Treatment (DiCTT) process is a promising technique increasingly used to remove phenolic compounds in water. The method advantages are operational and capital costs lower than other process and ability to allow total degradation and higher mineralization of target compounds when compared to conventional AOPs. On the other hand, further efforts are conducted to overcome the empirical aspect by studying the operating parameters, as well as the optimization of the process.

The complete degradation of phenol (almost 100%) was obtained independently of the flows of liquid effluent, 100 and 170 L h−1, and, of the initial phenol concentrations, 500, 2000 and 3000 mg L−1 over a 180-min period. A TOC conversion of almost 35% was observed corresponding to an operational time of approximately 210 min at a *QL* of 170 L h−1, which allows the speed of phenol mineralization is faster, but without interfering in the final value of almost 28% TOC conversion after 210 min of operation process. The flows of liquid effluent of 170 L h−1 was considered to be the best operating condition for the DiCTT process. An induction time of approximately 110 min was identified from the concentration profiles of hydroquinone and catechol. The concentrations of these intermediates tented to decrease independently of the flows of liquid effluent and of the initial phenol concentrations, indicating the formation of the other organic compounds, which were not acids (constant pH) according to data reported in the literature.
