shows efficiency values of degradation and relative concentration (C/C0
) alongside the respective treatments suggested in the experimental design, using Fe2
) as iron source and MnSO4
) as manganese source. In this table, run—corresponds to the experiment number; pH, [H2
], e t (reaction time)—are the values that were used for each experiment for each variable as explained in Table 1
; Efficiency—is the value in percentage of the degradation efficiency for each experiment; C/C0
—is the relative concentration after degradation of each experiment. The best experimental run is highlighted.
3.1. Degradation of TMP by the Fenton-Like Process
To facilitate the visualization of the variables which had a significant effect on the TMP degradation, the Pareto chart may be visualized in Figure 1
, where the investigated effects are shown. Those which exceeded the red line are significant, while the ones that did not exceed the line are insignificant and do not influence the TMP degradation by the Fenton-like process.
According to the observation of the Pareto Graph (Figure 1
), regarding the experimental design (Box–Behnken) used on this experiment to analyze the TMP degradation, it was found that the variables which had higher efficiency (significant effect) were the t (reaction time) quadratic and linear effect, [Fe2+
] linear effect, [Mn2+
] linear effect, and the interaction pH vs. [Mn2+
The reaction time showed a linear effect: the longer the reaction time, the lower the efficiency. However, there is also a negative quadratic effect, which indicates that when the reaction time is increased to a certain point (from 30 to 60 min), the efficiency is decreased, and from this point (60 to 90 min) the efficiency is increased. It is believed that this fact occurred because at the very beginning of the reaction, the process degrades the original micropollutant. however, as the time passes, the formation of intermediate compounds may be occurring, which interfere with the reading of the micropollutant concentration. Nevertheless, at longer reaction times, the Fenton process continues to degrade these intermediate products with an increase of the system’s efficiency. Hasan et al. [43
] verified that the maximum efficiency is 88.4% on phenol degradation by the Fenton process occurring in 20 min, the efficiency being decrescent with the increase of the reaction time. This achieved 66% in 100 min and 41.8% in 180 min. A similar fact was also reported by Marinho [44
], when using the Fenton process for the degradation of estrogen. The author obtained ≅40% of degradation efficiency at 10 min, however, at 15 min of reaction, the efficiency decreased to ≅20%, returning to increase after 30 min of reaction, reaching ≅60% of efficiency at 60 min.
The graph of the contours of the TMP relative concentration (C/C0
) under the effect of the independent variables which had significant effects is displayed in Figure 2
] had a significant effect on the TMP degradation, according to Figure 1
. On increasing the [Fe2+
], the degradation efficiency of the Fenton process also increases, however the [Fe2+
] tended to have a positive quadratic effect. It may be stated that, if the [Fe2+
] keeps increasing from a certain concentration, there will be a decrease on the process efficiency. The excess of Fe2+
ions may become harmful to the process efficiency, because they may generate competition with hydroxyl radicals and the organic compound degrading, decreasing, therefore, the degradation efficiency, according to that also reported by Wu et al. [45
]. Huang et al. [27
] observed a similar effect (quadratic) when they evaluated other transition metals as Mn2+
The pH 3, 4, and 5 were tested in this experiment, obtaining good efficiencies for all of them. However, it is not possible to make further inferences regarding these results considering only the effect of pH, since this variable alone did not present a significant effect (Figure 1
). There was significant interaction between the independent variables pH and [Mn2+
] (Figure 1
and Figure 2
), interaction of inverse relation, considering that the higher efficiencies expected for this interaction are obtained in the absence of Mn2+
and pH 5. An interesting observation is that if the substrate is more acidic (pH 3), to increase the efficiency of TMP degradation it is estimated (Figure 2
) that the addition of Mn2+
would be beneficial. In other words, the lower the pH, the higher the concentration of Mn2+
must be to obtain better efficiencies of degradation. This observation is according to the reports from Nidheesh and Gandhimathi [20
]. They tested, in the degradation of Rhodamine B, several transition metals (Fe0
) on the electro-Fenton process and observed that both Cu2+
provided better degradation efficiencies if the initial pH of the reaction was more acidic—the highest efficiency (73.59%) was obtained with pH 2.5.
Balci et al. [46
], with their experiments of glyphosate degradation by the modified electro-Fenton process, on testing Ag+
as catalysts, observed that the Mn2+
was clearly a better catalyst than the other ones. In this paper, on TMP degradation, according to that observed in Figure 1
, the utilization of a transition metal as Mn2+
as catalyst for the Fenton-like process (in the form of MnSO4
O), was not a good alternative. This was verified by the significant linear effect that it had on the TMP degradation. To corroborate this affirmation, in Figure 3
the global average of the relative concentration in all runs tested is displayed, for the three levels of the factor [Mn2+
It has been observed, in similar papers (modified Fenton and modified photo-Fenton), that better efficiencies are obtained with the utilization of Mn2+
. Huang et al. [27
], for instance, compared the degradation of 2,4-D on the photo-Fenton and modified photo-Fenton process and reported that up to a time of 90 min of reaction, the conventional photo-Fenton process provided higher efficiencies (60%). However, after 90 min, it maintained this range of efficiency and the modified photo-Fenton process in approximately 180 min reached efficiencies of around 100%.
3.3. Optimization of the Fenton-Like Process
Based on the results obtained in this experiment, it was verified that the addition of Mn2+ on the Fenton process, transforming it into denominated Fenton-like, did not provide an increase of the degradation efficiency of the antibiotics analyzed. This fact may be verified by the linear effect, the greater the [Mn2+], the lower the efficiency of the process degradation.
The regression model of TMP degradation was considered significant and demonstrated a non-significant lack of fit (Table 3
). Thus, it enabled the analysis of the optimal conditions for the tested variables. Through the utilization of the desirability function, it is possible to verify the optimal conditions for each studied variable, as shown in Figure 5
In Figure 5
, it may be seen that for [Fe2+
] and time, there is more than one value for the maximum condition of desirability. Therefore, the best efficiencies on TMP degradation are obtained below pH 5, [H2
] between 4.41 and 6.17 mmol L−1
] between 0.81 and 1.61 mmol L−1
] at 0.00 mmol L−1
and t of 30 and, or, 90 min.
Ben et al. [47
], studying the conventional Fenton process for the degradation of sulfonamides in effluents with pH of ≅8.5, testing pH of 3, 5 and 8.8, concluded that it was not be necessary to acidify the pH to 3, as they obtained high efficiencies even at pH 5.
The most economical condition would be the pH 5, condition which would not require the necessity to acidify the effluent, giving lower expenses without acidification; [H2
] and [Mn2+
], respectively at 4.41, 0.81, and 0.00 mmol L−1
, for lower expense of inputs; and t of 30 min, for the shorter time of process. Despite the visualization of a t of 30 min in this experiment being optimum for the desirability, it would be more prudent to use t of 90 min (Figure 5
), bearing in mind that, on pollutant degradation, as time passes, the formation of intermediate products may occur that demand longer reaction time.