## 3 Results
### Electrochemical Reduction of Cr(VI)
#### 3.4.2 Reticulated Vitreous Carbon (RVC)  
Figure 8 presents the results of application constant current electrolysis for 350 mL samples of leachate, containing 5.9 or 3.5 g Cr(VI)\(\cdot\)L\({}^{-1}\) in 0.5 M H\({}_{2}\)SO\({}_{4}\), in a small batch reactor under vigorous magnetic stirring. The applied current was 0.5 A which, given the real area of the RVC electrode (see the experimental section), corresponds to a cathode current density of ca. 2.4 mA\(\cdot\)cm\({}^{-2}\). The near-linearity of the C vs. t curves implies zero order kinetics. There are three kinetic regimes for the electro-reduction of Cr(VI), i.e., (a) the adsorption of HCrO\({}_{4}\)\({}^{-}\) (see also the aforementioned discussion of RDE experiments), (b) the electron transfer, and (c) the mass transport. The last two cases are both characterized by 1 st-order heterogeneous kinetics. The first one provided that a full monolayer of strongly adsorbed HCrO\({}_{4}\)\({}^{-}\) is readily formed over a wide concentration range; thus, leading to reactant surface saturation, potentially resulting in reaction rate independent of concentration. This is also implied by the fact that the current efficiency of the process (defined as the charge consumed for Cr(VI) reduction--based on [PERSON]'s law--divided by the total charge passed) was estimated as 104% and 108% for the two samples. This indicates that, despite the continuous decrease in bulk concentration, the steady-state reduction of a continuously replenished adsorbed layer was able to sustain the applied  
Figure 7: Current vs. potential LSV curves, recorded with GC RDE electrode at various rotation speeds (in the 200–2500 rpm range) and a 20 mV\(\cdot\)s\({}^{-1}\) potential scan rate, in a 0.05 M K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) + 0.5 M H\({}_{2}\)SO\({}_{4}\) solution. Inset: Koyecký–Levich plot for currents at \(-\)0.6 V vs. KCl-saturated calomel reference electrode (SCE). current. Noting also that Cr(VI) was converted to Cr(III) by 97.5% in 6 h (for 5.9 g Cr(VI)\(\cdot\) L\({}^{-1}\) initial concentration and by 89.8% in 3 h for 3.5 g Cr(VI)\(\cdot\) L\({}^{-1}\) initial concentration). Figure 9 presents the results of constant current electrolysis for 800 mL sample of leachate, containing 5.1 g Cr(VI)\(\cdot\) L\({}^{-1}\) in 0.5 M H\({}_{2}\)SO\({}_{4}\) in the batch-recirculation reactor arrangement, described in the experimental section. The applied current was 0.5 A which, given the real area of RVC electrode embedded in the plate-and-frame electrochemical cell, corresponds to a cathode current density of ca. 1.25 mA\(\cdot\)cm\({}^{-2}\). The shape of the concentration vs. time curves points now to more complicated kinetics (as a plot of InC/C\({}_{\rm o}\) was not found linear). At the relatively lower flow rates used (33 L\(\cdot\)min\({}^{-1}\)), the mass transport limitations are likely to prevail. This is also in line with the overall current efficiency of the process, which is now estimated as only 35%. As the concentration of Cr(VI) in the bulk solution decreases and the mass transport is not sufficient to sustain a full monolayer of adsorbed HCrO\({}_{4}\)\({}^{-}\), a competitive reduction process (based on dissolved oxygen reduction, or hydrogen evolution) becomes operative to keep the current constant. The conversion of the Cr(VI) contained in 800 mL of a 5.1 g Cr(VI)\(\cdot\) L\({}^{-1}\) wastewater was 99.8% in 36 h.  
In order to improve the current efficiency, the batch-recirculation reactor was also operated at constant potential, by selecting the applied potential to be in the range of potentials as recorded during the constant current operation; hence, the value of 3 V was selected. Figure 10 presents the results of constant potential electrolysis at 3 V of an 800 mL sample of leachate, containing 4.1 g Cr(VI)\(\cdot\) L\({}^{-1}\) in 0.5 M H\({}_{2}\)SO\({}_{4}\). The shape of the concentration vs. time curves points again to complicated kinetics (as a plot of InC/C\({}_{\rm o}\) was not found to be linear). The recorded current started from values at ca.