## 3 Results and Discussion
### Chromium (VI) Adsorption Reactions and Stability Constants
The \(\mathrm{C_{10.5}}\) surface complex can also be formulated as the \(>\overline{>\mathrm{S(H_{2}Cr_{2}O_{7})}}\) species. However, the bichromatic surface complexes were not found at 0.5 and 1.0 g/L sorbent doses and were neglected, as previously reported [57]. The results show that, as the sorbent amount increased, the number of surface sites also increased. At high-hydration equilibrium, the hydrogen chromate anions (HCrO\({}_{4}\)-) and water molecules were competing for the active surface sites. At a low-hydration level of the sorbent material's surface site, the \(\mathrm{HCrO_{4}}\)- ions displaced the \(\mathrm{H_{2}O}\) molecules, in the hydration sphere of the \(\mathrm{C_{11}}\) complex. Bidentate surface species were then formed at a high sorbent dose and short contact time. These results were in accordance with previous studies, suggesting that high-chemisorbed water molecules prevented the bidentate complexes from forming, as noted for the complexation of chromium with ferrihydrite [48]. A similar substitution of water molecules by \(\mathrm{HCrO_{4}}\)- anions could take place at a higher chromium concentration, even at hydration equilibrium, explaining the formation of bidentate surface species when there is a high surface coverage, observed for the adsorption of Cr (VI) or a similar anion on ferrihydrite [29]. It should be noted that the \(\mathrm{C_{10}}\equiv\overline{>\mathrm{S(HCrO_{4})H_{0}}}\) complex could also be expressed as an outer-sphere \(\overline{>\mathrm{SOH_{2}^{+}}-\mathrm{HCrO_{4}^{-}}}\) species, since we could not distinguish between the different complexes' formulations or structures, based on one \(\mathrm{H_{2}O}\) molecule. Generally, when at a low pH value, the formation of such outer-sphere Cr (VI) surface complexes is favored, similar to that previously obtained on amorphous aluminum oxides [58], and supported by the formation of \(\mathrm{SOH_{2}}\)+ in acidic media, as was indicated in a titration experiment [59]. At a pH higher than 4.5, n varied between \(-1\) and \(0\), in all investigated conditions. The predominant surface species formed in these conditions, were \(\mathrm{C_{10}}\) and \(\mathrm{C_{1-1}}\). Thus, Cr (VI) was adsorbed via the formation of \(\overline{>\mathrm{S(OH)(HCrO_{4})H_{0}}}\) (\(\mathrm{C_{10}}\)) and \(\overline{>\mathrm{S(OH)(HCrO_{4})H_{1}}}\) (\(\mathrm{C_{1-1}}\)) complexes, that could be expressed as \(\overline{>\mathrm{SCrO_{4}}}\) and \(\overline{>\mathrm{SHCrO_{4}}}\), respectively. As the contact time increased, the optimal pH formation of protonated \(\mathrm{C_{1-1}}\equiv\overline{>\mathrm{SHCrO_{4}}}\equiv\overline{>\mathrm{ S(OH_{2}^{+})(HCrO_{4}^{-}}}\) and the un-protonated \(\mathrm{C_{10}}\equiv\overline{>\mathrm{SCrO_{4}}}\equiv\overline{>\mathrm{ S(OH)(HCrO_{4})}}\) species, shifted from 5 to 6 and 7 to 8, respectively. The adsorption of Cr (VI) increased with t, and at a higher hydration time, the adsorption reaction was likely to comply to OH- surface exchange. Thus, for a pH lower than PZC, the negative surface charge of the phosphate material reacts in acidic media to form \(>\)S(OH\({}_{2}\))+, which adsorbs chromium as \(\mathrm{HCrO_{4}}\)-. Maximal OH- exchange was observed at a pH range of 5.2-6.0, which approximately coincided with the pH range of zero charge and the iso-electric points of unloaded material. Whilst higher than PZC, the repulsion between chromium anions and negatively sorbent surface charge, increased with pH. The contribution of the columbic effect to the overall uptake of Cr (VI) could be a more important process. Nevertheless, at a pH value around the iso-electric point, the electrostatic repulsion reached a minimum value, and then the intrinsic process (with n = 0) became the major adsorption mechanism. As shown from the obtained results, the pH value became higher with contact time, reaching 8.2 at \(t\approx 150\) min.