## 3 Results and Discussion
### Effect of Contact Time  
The effect of contact time on the adsorption efficiency was examined, and the results are shown in Figure 2. At a pH lower than pH\({}_{\text{max}}\), a distinct difference was observed for adsorption envelopes in relation to the sorbent dose, or sorbent/sorbent ratio (surface coverage). At a sorbent dose of 0.5 and 1.0 g/L, the pH adsorption edges followed a similar trend for \(t\geq\) 15 min. At a pH of 50% of adsorption (pH\({}_{\text{50}}\)), a negligible variation occurred at pH 3.9-4.1. When the sorbent dose was1.5 g/L, the adsorption process was dependent on the hydration time. Therefore, the adsorption envelopes  
Figure 1: Log(D) versus pH for different amounts of OCP and different contact times.  
shifted to higher pH values, resulting in an increase in pH\({}_{50}\) from 2.9 to 3.9, as the time increased from 0 to 150 min. The mechanism for the oxyanion adsorption was dependent on the surface coverage and hydration of the surface sites, involving the formation of distinct surface complexes. This was in agreement with the spectroscopic study results, which indicated that chromium (VI) resulted in the formation of both monodentate and bidentate surface complexes on iron oxides [49; 50]. The proportion of these species was dependent on the metallic ion concentration. Recently, through the use of a spectroscopic study, it has been shown that the monodentate chromate complexes on ferrihydrite were predominant at a low surface coverage and a pH \(\geq\) 6.5. In contrast, bidentate surface complexes were formed at a high surface coverage and pH \(\leq\) 6 [51].  
Taking into account the previous results, it could be deduced that, at a low adsorbent dose, monodentate surface complexes prevailed for all of the examined experimental conditions. In contrast, at a higher adsorbent dose, the bidentate surface complexes became predominant at a low hydration level of sorbent materials at pH \(\leq\) pH\({}_{\max}\), and were subsequently converted to monodentate species, when hydration equilibrium was reached.  
At a pH higher than pH\({}_{\max}\), adsorption envelopes become more alkaline. The shift to this alkalinity increased by increasing the hydration time. Due to electrostatic repulsion with the negative surface charges, the chromium uptake initially involved the adsorption of a HCrO\({}_{4}\)\({}^{-}\) anion, which was followed by the subsequent slower, and less important, uptake of CrO\({}_{4}\)\({}^{2-}\) in the alkaline region. These electrostatic factors could influence both the kinetics and equilibrium of chromate ions, as observed for the adsorption of a similar oxyanion on ferrihydrite [52]. When the contact time increased, the adsorption of the chromate anion also increased, and the Log(D) = f(pH) curves show a  
Figure 2: Variation of n = f(pH) obtained at various contact time and at different OCP sorbent amounts of m = 0.5, 1.0 and 1.5 g/L: 5 min (\(\bullet\)); 30 min (\(\blacktriangle\)); 90 min (\(\bullet\)); and 150 min (\(\blacksquare\)).  
pronounced difference in the slopes at \(t=150\) min. As previously discussed, this phenomenon could be due to a change in the adsorbed fraction of the Cr (VI)-predominant surface complexes.