**3.5. Effect of initial phenol concentration**

**Figure 3a** indicates that, in the range studied, the variation of the effluent flow does not affect the profile of phenol degradation and total degradation of the same is almost completely achieved in 180 min of operation, reaching values of 99.5 and 97.4%, respectively, at effluent flow of 100 and 170 L h−1. **Figure 3b** indicates that the increase of the effluent flow of 100 to 170 L h−1 allows a higher speed of phenol mineralization, due the acceleration of the lowest value of the pH, but not interfering in the maximum value TOC conversion, around 28% with

**Figure 3.** (a) Evolution of phenol degradation as a function of the operating time. (b) Evolution of TOC conversion as a

h−1, *C*Ph0 = 500 mg L−1, *R*P/H = 50% and *Q*RG = 50%.

**Figure 4a** and **b** indicates, respectively, the time profiles of the hydroquinone and catechol formed by thermochemical phenol oxidation, for the two flows of liquid effluent, 100 and 170 L h−1. Analysing the same figures, it can be seen that the rate of formation of these species becomes appreciable after the induction period, approximately 110 min, previously observed

The evolution of the hydroquinone and catechol concentrations happened quickly because of the thermochemical oxidation reaction of phenol with high speed, regardless of the flows of liquid effluent studied. It has been observed that hydroquinone and catechol concentrations are reached when phenol consumption rate is maximum, which is identified in the process time between 140 and 150 min. After reaching the maximum hydroquinone and catechol formation, an immediate reduction in the concentration of these two species is observed, indicating that to achieve the maximum consumption of phenol, the oxidation rate of these two organic compounds becomes greater than its rate of formation, enabling to be degraded, thus favouring the formation of other organic compounds that are not acids, because the pH remained almost constant at 2.5−2.8, after

by the curves of the evolution of pH, phenol degradation and TOC conversion.

an operating time of 210 min.

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

334 Phenolic Compounds - Natural Sources, Importance and Applications

In order to evaluate the effect of initial concentration of the organic pollutant on the efficiency of the process DiCTT on thermochemical phenol oxidation, three initial concentrations of the aromatic contaminant (*C*Ph0): 500, 2000 and 3000 mg L−1, were employed keeping all other variables constant. The operating conditions used in this study are presented in **Table 4**.

**Figure 5a** and **b** presents, respectively, the evolution of the temperature and pH of the liquid effluent in the perfect mixing tank (Tank 2) during the process, varying only the initial concentration of phenol. From **Figure 5a** it can be seen that the concentration of phenol does


**Table 4.** Operational parameters for the study of the influence of the initial concentration of phenol.

not influence the heating curve of the liquid phase, reaching a temperature of approximately 350 K (77°C). An expected behaviour since the variation of concentration, in different experiments, it is not enough to significantly change the chemical and thermophysical properties of the effluent, since the natural gas flow in the process is the same for all cases, as well as the excess air and effluent flow which remained constant during these essays.

**Figure 5b** show the curves of the evolution of pH for concentrations of phenol 2000 and 3000 mg L−1. It can be seen that the initial pH value already show low values, 4 and 3, respectively, while for a *C*Ph0 equal 500 mg L−1, pH presents an initial value of 8. This can be explained due to the amount of hydrogen peroxide added. In the procedure adopted for the preparation of the synthetic effluent, the peroxide is mixed with the phenol solution in the preparation tank, causing uncontrolled reactions. As the molar stoichiometric ratio of the mix is kept constant, to higher concentrations of phenol oxidant availability in the reaction medium is greater, increasing the effect and decreasing the initial pH due to a possible premature oxidation of phenol to form organic acids.

**Figure 5.** (a) Evolution of temperature of the liquid effluent as a function of the operating time. (b) Evolution of pH as a function of the operating time. *E* = 40%, *Q*GN = 4 m3 h−1, *QL* = 170 L h−1, *R*P/H = 50% and *Q*RG = 50%.

**Figure 6a** and **b** presents, respectively, the profile of phenol degradation and TOC conversion as a function of time, for different initial concentrations of phenol studied.

**Figure 6a** show that the increase of the initial concentration of phenol from 500 to 3000 mg L−1 does not affect the duration of the first step of the reaction, called induction period, and does not have a significant effect on the phenol degradation after a time of approximately 130 min. After the induction period, around 110 min, the speed of the reaction becomes more pronounced, as was expected, reaching values of XF practically the same after an operating time of around 130 min, regardless of the initial concentration of phenol. Phenol degradation around 99% is obtained after an operating time of 180 min.

not influence the heating curve of the liquid phase, reaching a temperature of approximately 350 K (77°C). An expected behaviour since the variation of concentration, in different experiments, it is not enough to significantly change the chemical and thermophysical properties of the effluent, since the natural gas flow in the process is the same for all cases, as well as the

E3 500 4 40 170 50 50 E4 2000 4 40 170 50 50 E5 3000 4 40 170 50 50

**Table 4.** Operational parameters for the study of the influence of the initial concentration of phenol.

 **h−1) E(%) QL(Lh−1) QRG(%) RP/H(%)**

**Figure 5b** show the curves of the evolution of pH for concentrations of phenol 2000 and 3000 mg L−1. It can be seen that the initial pH value already show low values, 4 and 3, respectively, while for a *C*Ph0 equal 500 mg L−1, pH presents an initial value of 8. This can be explained due to the amount of hydrogen peroxide added. In the procedure adopted for the preparation of the synthetic effluent, the peroxide is mixed with the phenol solution in the preparation tank, causing uncontrolled reactions. As the molar stoichiometric ratio of the mix is kept constant, to higher concentrations of phenol oxidant availability in the reaction medium is greater, increasing the effect and decreasing the initial pH due to a possible premature oxidation of

**Figure 5.** (a) Evolution of temperature of the liquid effluent as a function of the operating time. (b) Evolution of pH as a

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

h−1, *QL*

excess air and effluent flow which remained constant during these essays.

phenol to form organic acids.

**Tests CPH0(mgL−1) QGN(m<sup>3</sup>**

336 Phenolic Compounds - Natural Sources, Importance and Applications

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

**Figure 6b** show the evolution of TOC conversion, identifying a time of induction period also approximately 110 min, and show a slight increase in the TOC conversion with increasing of the initial phenol concentration, reaching XT values of 27.5; 31.5 and 33.5% to initial concentration of phenol of 500, 2000 and 3000 mg L−1, respectively, after an operating time of 210 min. Regardless of the value of the initial phenol concentration, the process presents maximum rates of phenol degradation almost 100% after 170 min of operation, in addition to providing a TOC conversion, between a range of 27.5–33.5%, after 210 min, within the range of the initial concentration of phenol, being the air excess used of 40% and a combustion gases recycling rate of 50%.

**Figure 6.** (a) Evolution of phenol degradation as a function of the operating time. (b) Evolution of TOC conversion as a function of the operating time. *E* = 40%, *Q*GN = 4 m3 h−1, *QL* = 170 L h−1, *R*P/H = 50% and *Q*RG = 50%.

**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 concentration.

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 case of hydroquinone).

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

**Figure 7.** (a) Evolution of hydroquinone formation as a function of the operating time. (b) Evolution of catechol formation as a function of the operating time. *E* = 40%, *Q*GN = 4 m3 h−1, *QL* = 170 L h−1, *R*P/H = 50% and *Q*RG = 50%.
