# Optimization of Organic Content Removal from Aqueous Solutions by Fenton-Ozonation
## 2 Materials and Methods
### Analysis Methods  
Kinetic monitoring of the concentration of the solution to be treated was carried out using HPLC (DAD detection), along with monitoring of changes in total organic carbon (TOC) over time. The HPLC method was validated in terms of linearity, sensitivity, and precision. Calibration was performed using at least five concentration levels  
Figure 1: Ozonation pilot experimental set-up.  
of 2,4-dichlorophenoxyoxyacetic acid (2,4-D) (Sigma-Aldrich, \(\geq\)97%). The limit of detection (LOD) and limit of quantification (LOQ) were determined following ICH guidelines, using the standard deviation of the response and the slope of the calibration curve [27]. The resulting values were 0.24 mg\(\cdot\)L\({}^{-1}\) (LOD) and 0.72 mg\(\cdot\)L\({}^{-1}\) (LOQ). TOC measurements were validated in a similar manner. Calibration was performed using standard TOC solutions from Sigma Aldrich. The LOD and LOQ for TOC were calculated analogously to the HPLC method and were determined to be 0.63 mg\(\cdot\)L\({}^{-1}\) and 1.90 mg\(\cdot\)L\({}^{-1}\), respectively.  
HPLC analysis was used to evaluate the degradation (abatement) of 2,4-D during the oxidation, while TOC measurements were used to monitor the mineralization of the molecule as the treatment progressed. Operating conditions for the HPLC analyses are provided in Table 1.  
The efficiency of the processes to eliminate 2,4-D was determined from the degradation rate (\(Y_{HPLC}\)), which represents the removal efficiency of the concentration of the pollutant in the solution, and the mineralization rate (\(Y_{TOC}\)), which gives the mineralization efficiency (transformation into CO\({}_{2}\) and H\({}_{2}\)O) of 2,4-D. These parameters are given by the following equations, in which \(TOC_{0}\) et \(TOC_{t}\) are the concentrations of total organic carbon (mg L\({}^{-1}\)) at time \(t\) and at \(t=0\), and C\({}_{0}\) and C\({}_{t}\) are the concentrations of the solutions (mg L\({}^{-1}\)) at time \(t\) and at \(t=0\) (determined by HPLC).  
\[Y_{HPLC}\ (\%)= \frac{C_{0}-C_{t}}{C_{0}}\times 100, \tag{1}\]  
\[Y_{TOC}\ (\%)= \frac{TOC_{0}-TOC_{t}}{TOC_{0}}\times 100, \tag{2}\]  
The modeling of the degradation kinetics of 2,4-D was carried out according to the pseudo-first-order model. For the case of ozonation, the concentration of ozone and hydroxyl radicals was considered constant. In this case, the degradation was dependent on the concentration of 2,4-D in the solution [28; 29] and is given by the following equation:  
\[-\frac{dC}{dt}=k_{app}\times C, \tag{3}\]  
In this equation, \(C\) represents the pollutant concentration in mg L\({}^{-1}\) and \(K_{app}\) is the apparent degradation rate constant, expressed in min\({}^{-1}\). Integration of the rate law between \(t=0\) and any time \(t\), where the pollutant concentrations are \(C_{0}\) and \(C_{t}\), respectively, leads to the following expression, in which \(k_{app}\) corresponds to the slope of the variation linear plot of \(ht(C_{t}/C_{0})\) versus time.  
\[\ln\left(\frac{C_{t}}{C_{0}}\right)=-K_{app}\times t, \tag{4}\]  
\begin{table}
\begin{tabular}{l l} \hline
**Column** & **Restek Raptor C18 (100 mm \(\times\) 3 mm \(\times\) 2.7 \(\mu\)m)** \\ \hline Temperature (\({}^{\circ}\)C) & 40 \\ Mobile phase & A/B (65/35): A: water + acetic acid (90/10) \\  & B: acetonitrile \\ Analysis mode & Isocratic \\ Flow rate (mL min\({}^{-1}\)) & 0.8 \\ Pressure (bar) & 290 \\ Injection volume (\(\mu\)L) & 10 \\ Wavelength (nm) & 280 \(\pm\) 4 \\ Retention time (min) & 1.6 \\ \hline \end{tabular}
\end{table}
Table 1: HPLC operating conditions.