## 3 Results
### Electrochemical Reduction of Cr(VI)  
#### 3.4.1 Exploratory Voltammetry Using a Glassy Carbon Rotating Disc Electrode  
Figure 5 presents the linear sweep voltamograms (LSVs; current vs. potential curves recorded as the potential is varied at a 10 mV\(\cdot\)s\({}^{-1}\) potential scan rate), obtained with the use of a GC RDE (at a 200 rpm rotation speed) in a solution containing 0.05 M K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) in the presence of either 0.5 M HCl or 0.25 M H\({}_{2}\)SO\({}_{4}\). Since Cr(VI) at pH \(<\)1 is known to exist mainly in its HCrO\({}_{4}\)\({}^{-}\) form [35], the negative currents correspond to the following electrochemical Reaction (5) [36]. \[\mathrm{HCrO_{4}}^{-}+7\mathrm{H}^{+}+3\mathrm{e}^{-}\rightarrow\mathrm{Cr}^{ 3+}+4\mathrm{H}_{2}\mathrm{O} \tag{5}\]  
Figure 4: Cr\({}_{\mathrm{total}}\) and Ca extraction from the thermally treated tannery sludge at 700 \({}^{\circ}\)C, using different common acids, applied in different (initial) concentrations. Figure 5: Current vs. potential linear sweep voltamogram (LSV) curves, recorded with the use of a glassy carbon (GC) rotating disc electrode (RDE) at a 200 rpm rotation speed and a 20 mV\(\cdot\)s\({}^{-1}\) potential scan rate, in 0.05 M K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) solutions, containing 0.5 M HCl or 0.25 M H\({}_{2}\)SO\({}_{4}\). Both LSV curves coincide at similar currents of ca. 4 mA at the most negative potential applied (\(-\)0.70 V vs. SCE), where electrode kinetics are expected to be fast and the current controlled by mass transfer. However, they differ significantly in terms of their current onset potential and shape (at the lower negative potentials in the mixed/kinetic control regime). In the presence of Cl\({}^{-}\) ions, the onset potential is more negative and there is also a significant current decrease at ca. \(-\)0.20 V vs. SCE. Note also that a similar difference between HCl and H\({}_{2}\)SO\({}_{4}\) has been reported for the Cr(VI) reduction by using an Au electrode [37]. The adverse effect of HCl acid on HCrO\({}_{4}\)\({}^{-}\) reduction may be due to the stronger Cl\({}^{-}\) anion adsorption on carbon [38] (i.e., competitive to HCrO\({}_{4}\)\({}^{-}\) adsorption), as well as to the low solubility of CrCl\({}_{3}\), which may precipitate on the electrode surface, produced after the Cr(III) reaction with Cl\({}^{-}\) ions. Having established that H\({}_{2}\)SO\({}_{4}\) is the best selection for the electrochemical treatment of Cr(VI) solutions, the optimal acid concentration should be subsequently identified. Figure 6 presents the LSV curves obtained with the GC RDE (at 1600 rpm) in 0.05 M K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) solutions of different sulfuric acid concentrations in the 0.05-1 M range. For acid concentrations below 0.5 M H\({}_{2}\)SO\({}_{4}\) (i.e., with 1 M of protons available for reaction), there is a strong effect of acidity on the Cr(III) reduction currents. This is to be expected since, according to Reaction (5), seven protons take part in the electro-reduction of one Cr-containing specie; hence 0.05 M K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) require a minimum of 2 \(\times\) 0.05 \(\times\) 7 = 0.7 M of protons. Finally, Figure 7 presents the effect of rotation rate (mass transfer) conditions in 0.05 M K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) + 0.5 M H\({}_{2}\)SO\({}_{4}\) solution at rotation rates (f) in the 200-2500 rpm range. It can be observed that the current at potentials more negative than ca. 0.60 V increases with the rotation speed, indicating mass transfer control (partial or complete); by contrast, the ill-defined wave at the foot of the curve (in the ca. 0.70-0.60 V range) remains largely unaffected, further confirming that it corresponds to a HCrO\({}_{4}\)\({}^{-}\) adsorption pre-wave. Considering, also, the area A = 0.07065 cm\({}^{2}\), maximum reduction currents are in the 70-190 mA\(\cdot\)cm\({}^{-2}\) range.