## 3 Results and Discussion
### Effect of the Initial Hexavalent Chromium Concentration  
In this part of experimental section, the examined parameter was the initial concentration of the hexavalent chromium. With the increase of the initial CR(VI) concentration, and by keeping Fe(II) concentration constant, the molar ratio [Fe(II)]/[Cr(VI)] was decreased. In particular, for initial Cr(VI) concentrations of 50, 100, 200 and 400 \(\mu\)g Cr(VI)/L, the molar ratio of Fe/Cr decreases from 9.6, to 9.3, 4.65, 2.32 and 1.15 respectively. Figure 4 shows the corresponding results. With the gradual increase of the initial chromium concentration from 50 \(\mu\)g/L to 400 \(\mu\)g/L, a decrease in chromium removal rate was noticed. In particular, for initial chromium (VI) concentration at 50 \(\mu\)g/L and 100 \(\mu\)g/L the removal rate was around 88% and 83%, respectively. For the initial concentration at 200 \(\mu\)g/L the removal rate is at around 78% and for the initial chromium concentration at 400 \(\mu\)g/L the removal rate is at around 65%. The residual chromium concentrations achieved were 6, 17, 44 and 140 \(\mu\)g/L chromium, respectively. Figure 3: Removal of hexavalent chromium in relation to linear velocity. Experimental conditions: initial Cr(VI) concentration: 50 \(\mu\)g/L, ferrous iron dose: 0.5 mg/L, Linear velocity values: 4, 8, 12, 15 m/h, at pH 7.3. As the initial Cr(VI) concentration is increasing, the Cr(VI) removal rate is decreasing, apparently because the molar ratio Fe(II)/Cr(VI) becomes lower. Higher removal values at these ratios could not be achieved at the pH value at which the experiment was conducted, because of the competition of Fe(II) oxidation with the oxygen from water, since the kinetics of Fe(II) oxidation is mainly pH dependent and at pH values above 7, Fe(II) oxidation by oxygen is very fast. This causes a competition between Cr(VI) and O\({}_{2}\) to oxidize Fe(II). In particular, [PERSON] and [PERSON], (1997) [PERSON] and [PERSON], (1997) [PERSON] and [PERSON], (1997) have reported that Cr(VI) oxidizes Fe(II) faster than O\({}_{2}\) for [Cr(VI)] = [O\({}_{2}\)] by a factor of about \(3\times 10^{4}\) at pH 4, \(6\times 10^{3}\) at pH 6 and \(1\times 10^{3}\) at pH 8. This highlights that by increasing the pH of water, increased Fe(II) concentration is need to oxidized the same concentration of Cr(VI). For higher initial Cr(VI) concentrations, the final concentrations were much higher than the current and proposed limits for Cr(VI) in water. The interesting finding of this experiment, however, is that although the percentage of Cr(VI) removal is decreasing and the residual concentrations of Cr(VI) are increasing, the amount of Cr(VI) removed in \(\mu\)g/L is increasing. For example, when the initial concentration was 50, the amount of Cr(VI) removed was about 45 \(\mu\)g/L, whereas for initial concentration of Cr(VI) of 400 \(\mu\)g/L, the amount removed was 256 \(\mu\)g/L. Both results were obtained by the same dosing quantity of Fe(II), namely 0.5 mg/L. This might indicate an autocatalytic effect of Cr(VI) concentration on Cr(VI) reduction by Fe(II), i.e., by increasing the kinetics of Fe(II) oxidation by Cr(VI). This might be critical in the removal of high concentrations of Cr(VI) and provides evidence that multiple additions of Fe(II) in the water might be more efficient that dosing all the Fe(II) quantity at once. These results are in agreement with a study by [PERSON] and [PERSON] (2001) [PERSON] and [PERSON] (2001), who examined Cr(VI) reduction by dissolved ferrous in under-stoichiometric conditions, i.e., representing a deficient amount of Fe(II). In addition, the idea of multiple dosing of the reagent could also contribute to the reduction of the produced sludge. Indeed, this technology produces some sludge, which has to be handled.