*3.8. Degradation of DB78 by an H2O2*/*Pulsed Light AOP*

The degradation of DB78 by the H2O2/pulsed light AOP can be observed in Figure 9. The process was able to degrade the dye by more than 95%, which can be considered a significant reduction in the contamination potential of this dye if released to the environment. The respective pseudo-first order degradation rate was 0.012 cm2/J (*R*<sup>2</sup> = 0.9967). Prolonging the treatment would not be efficient as data extrapolation allows predicting 99% degradation after supplying an energy of 383 J/cm2, which means expending 49% more energy to increase the degradation by only 4%.

**Figure 9.** Degradation of DB78 by the H2O2/pulsed light advanced oxidation process in model wastewater.

#### **4. Conclusions**

The present study showed that eggshell can be successfully utilized as a biosorbent for the removal of DB78 dye from water solutions. It was found that the adsorption of DB78 onto eggshell followed pseudo-second-order kinetics. The Freundlich isotherm was the best model to describe adsorption. The maximum adsorption of DB78 onto eggshell was obtained at pH 5 and 0.5 g of adsorbent dosage. According to the Langmuir isotherm, the maximum adsorption capacity of DB78 onto eggshell was 13 mg/g. Taking into account the results obtained for the *nF* and *RL* parameters, the adsorption is considered as favorable. The analysis of thermodynamic parameters revealed that the adsorption process is endothermic and spontaneous at the three temperatures analyzed. Desorption studies were conducted and the results showed that the eggshell was reusable in different adsorption/desorption cycles. Complementing the adsorption process by an H2O2/pulsed light advanced oxidation process allows further decreasing the release of pollutant dyes to the environment, proving that the combination of both techniques can be used successfully in the removal of dyes from wastewater at higher concentrations of dye.

**Author Contributions:** The authors who sign the following manuscript have made significant contributions, allowing achieving the set objectives: A.M.-S. carried out the adsorption experiments; J.A.P. wrote the manuscript and interpreted the kinetic and isotherm results obtained; M.I.R.-L. graphed the results; V.M.G.-L. carried out the photolysis of remaining dye by using pulsed light; E.N.-D., and J.A.G. were responsible for the conception, design, assessment, and revision of the work. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
