## 3 Results and Discussion
### Degradation of HCQ by BDD-Electrolysis in Real Water Matrices
On the one hand, the CV curves did not show significant current signals due to the interaction between the water species composition and BDD surface when the non-polluted sample was investigated, confirming that no significant matrix effect was attained. On the other hand, the typical behavior of a non-reversible system was observed when polluted water matrix was studied. The peak in the CV at +1.58 V can be understood in terms of an electroactivity of HCQ towards the diamond electrode, suggesting that the organic substrate can be directly oxidized at BDD surface [39]. Meanwhile, when the potential is increased, a smooth current signal was achieved as an extension of the HCQ peak at +1.58 V. This result indicated that other organic compounds are present in the water matrix, which could be oxidation's by-products from HCQ [19]. However, these intermediates are indirectly oxidized by free heterogeneous \({}^{\bullet}\)OH radicals [33; 37] and other oxidants [11; 40] that can be produced at higher potentials [32]. This behavior is in accordance with the HCQ and COD decays in Figure 3.  
From data in Figure 3a, kinetic studies were carried out under pseudo-order conditions [41]. Under these experimental circumstances, the concentration of \({}^{\bullet}\)OH in solution was kept in excess with respect to the HCQ in solution which ensures that the reaction of depollution was considered under pseudo-first-order conditions [42]. Then, the kinetic experiments were performed by monitoring the decay of HCQ concentration in real water matrix as a function of time. In view of the principles of chemical kinetics, the rate expression for the decay reaction of HCQ solution can be written as (9) [19],  
\[\text{-d[HCQ]/dt}=\text{k[HCQ]}^{\text{m}}\text{ [}^{\bullet}\text{OH]}^{\text{m}} \tag{9}\]  
where [HCQ] and [\({}^{\bullet}\)OH] are molar concentrations of HCQ and \({}^{\bullet}\)OH, respectively. m and n are the order of the reaction with respect to the given reactant, separately. k represents the apparent rate constant for the decay reaction. Under an excess of \({}^{\bullet}\)OH concentration in solution, Equation (11) was rewritten as (10),  
\[\text{-d[HCQ]/dt}=\text{k[HCQ]}^{\text{m}} \tag{10}\]  
By using the natural logarithm of HCQ concentration with the reaction time (min) (inset of Figure 3), linear relationships were estimated (R\({}^{2}\) = 0.99), which suggested that the decay of HCQ concentration followed pseudo-first order kinetics, and the positive of the slope of this line equals the apparent rate constants of 0.0554 min\({}^{-1}\), 0.0855 min\({}^{-1}\) and 0.118 min\({}^{-1}\) at 15, 30 and 45 mA cm\({}^{-2}\), respectively, demonstrating an improvement in the reaction rate when an increase on the \(j\) was attained [32]. Figure 2:0 Linear polarization curves of (a) non-polluted and (b) polluted river water matrices with BDD anode at 50 mV s\({}^{-1}\) as scan rate. As can be seen in Figure 3b, COD decay obtained by applying 15, 30 and 45 mA cm\({}^{-2}\) was about 18, 16 and 8, respectively, after 90 min of BDD-electrolysis. Taken into consideration that up to 68% of COD removal was obtained at 15 mA cm\({}^{-2}\) after 90 min of BDD-electrolysis, while 71% and 84% were achieved by applying 30 and 45 mA cm\({}^{-2}\), respectively. This results in evidence that the \(j\) influenced the degradation of HCQ, which is justified due to the efficiency production of oxidizing species, mainly free heterogeneous \({}^{\bullet}\)OH radicals at BDD surface [19; 43; 44]. [PERSON] and coworkers [19] have reported significant insight regarding the electrochemical treatment of HCQ in synthetic effluents, confirming that the large production of \({}^{\bullet}\)OH radicals from EO of water at the BDD surface of BDD attack immediately HCQ at the vicinity of BDD surface and decompose it into small fragments. In fact, they observed that the pH value of the solutions decreased at the beginning of the treatment due to the formation of acid intermediates during EO of HCQ [19].