## 4 Conclusions  
The adsorption of hexavalent chromium on OCP material was thoroughly investigated. One goal of this study was to develop a method for studying the surface complexation of OCP. The obtained distribution coefficient (D) was dependent on the contact time, pH, and surface coverage. The treatment of Log(D) = f(pH) experimental data were used to evaluate H\({}^{+}\)/OH\({}^{-}\) exchange stoichiometry in adsorption reactions, and the results were used to specify the predominant Cr (VI) surface species. At hydration equilibrium, protonated hexavalent chromium formed \(\overline{>{\rm SHCrO}_{4}}\) and unprotonated \(\overline{>{\rm SCrO}_{4}}\) complexes, under all explored conditions. When the hydration equilibrium was not reached at a low surface coverage, the protonated species disappeared, to the benefit of the bidentate (\(\overline{>{\rm S(HCrO}_{4})_{2}}\)) complex. The stability constants were logK\({}_{10}=0.123\), for \(\overline{>{\rm SHCrO}_{4}}\), which could be formulated as \(\overline{>{\rm S(OH}_{2}^{+})({\rm HCrO}_{4}^{-})}\) and logK\({}_{11}=-4.0\) for \(\overline{>{\rm SCrO}_{4}}\), in acidic media.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Species** & **\(\pi\)** & **Adsorption Reaction** & **logK\({}_{\rm in}\)** & **logK\({}_{\rm out}\)** \\ \hline \(\overline{>{\rm SOH}_{2}^{+}-{\rm HCrO}_{4}^{-}}\) & 0 & \((\overline{>{\rm SOH}})+{\rm H}_{\rm CrO}_{4}\leftrightarrow\overline{>{\rm SGH }_{2}^{-}-{\rm HCrO}_{4}^{-}}\) & 0.1 & \\ \(\overline{>{\rm SHCrO}_{4}}\) & 0 & \((\overline{>{\rm SOH}})+{\rm H}_{\rm CrO}_{4}\leftrightarrow\overline{>{\rm SHCrO }_{4}+{\rm H}_{\rm O}}\) & 0.1 & \\ \(\overline{>{\rm SCG}_{4}}\) & +1 & \((\overline{>{\rm SOH}})+{\rm H}_{\rm CrO}_{4}\leftrightarrow\overline{>{\rm SCrO }_{4}+{\rm H}_{\rm O}+{\rm 1H}^{+}}\) & \(-4.0\) & 4.1 \\ \(\overline{>{\rm S(HCrO}_{4})_{2}}\) & 0.5 & \((\overline{>{\rm SOH}})+2{\rm H}_{\rm CrO}_{4}\leftrightarrow\overline{>{\rm SHCrO }_{4}+{\rm H}_{\rm O}+{\rm 1H}^{+}}\) & 3.0 & 2.4 \\ \hline \multicolumn{5}{c}{**Lower acidic to alkaline medium (pH = 5)**} \\ \(\overline{>{\rm SHCrO}_{4}}\) & 0 & \((\overline{>{\rm SOH}})+{\rm HCrO}_{4}\leftrightarrow\overline{>{\rm SCrO }_{4}+{\rm H}_{\rm O}}\) & 0.2 & \\ \(\overline{>{\rm SHCrO}_{4}}\) & \(-1\) & \((\overline{>{\rm SOH}})+{\rm HCrO}_{4}\leftrightarrow\overline{>{\rm SHCrO }_{4}+{\rm 1 OH}^{-}}\) & \(-6.7\) & 6.1 \\ \hline \end{tabular}
\end{table}
Table 1: Surface complexation constants for Cr (VI) sorption onto Octacalcium phosphate.  
In alkaline media, the log K1-0 = 0.2 for \(\overline{>\text{SCrO}_{4}}\)) and log K1-1 = \(-\)6.4 for \(\overline{>\text{SHCrO}_{4}}\). Whilst, for the bidentate \(\overline{>\text{S}(\text{HCrO}_{4})_{2}}\) surface species, the logK10.5 = 2.9.The electrostatic effect was evaluated for the predominant adsorption reactions. The obtained results suggested that Cr (VI) adsorption on OCP was of an electrostatic nature in acidic solutions, and of a chemical nature in lower acidic to alkaline solutions. The results could have practical and promising applications in the fields of environmental health, for the removal of hazardous chromium from industrial wastewater before dumping it into the environment (water and soil). This could enhance the environmental risk management process and will play a major role in preventing future coastal contamination.