## 3 Results and Discussion  
### Effect of pH on Chromium Adsorption  
The logarithmic variation of D with pH at different contact times, for 0.5, 1.0, and 1.5 g/L of calcium phosphate solutions, is plotted in Figure 1.  
As shown in Figure 1, all cases exhibit similar behavior; log D increased by increasing the pH, to reach the maximum at pH\({}_{\text{max}}\) of 4.0 to 5.0, before decreasing as the pH continued to increase. The maximum adsorption efficiencies were found to increase with the amount of adsorbent (m). The results also show that the pH\({}_{\text{max}}\) is dependent on the contact time; when the contact time increased from 5 to 150 min, the pH\({}_{\text{max}}\) rose from about 4 to 5.  
The variation in the extraction efficiency with the solution's pH could be related to the protonation/deprotonating of both surface groups, and to the acidity of H\({}_{2}\)CrO\({}_{4}\) (pK\({}_{\text{a1}}\) = 0.2 and pK\({}_{\text{a2}}\) = 6.5) [27; 28; 29]. According to the chromium speciation pH-diagram, the chromium (VI) was adsorbed as hydrogen chromate (HCrO\({}_{4}\)\({}^{-}\)) at pH \(\leq\) 5.0 (\(\geq\)95%), as chromate (CrO\({}_{4}\)\({}^{2-}\)) at pH \(\geq\) 7.6 (\(\geq\)95%), and as a mixture of these species between pH 5.0 and 7.6. In this case, both electrostatic and chemical sorption mechanisms could occur, and generally, it is not possible to distinguish between these two mechanisms [30]. It was assumed that the maximum adsorption occurred when the combination of a high positive surface charge and a high concentration of anionic chromium species, are achieved. Thus, the uptake of weak acid was maximal at a pH value around its dissociation constant, or near the PZC of surface sorbent materials [31]. In general, anion adsorption is strongly dependent on the pH of the medium, exhibiting the greatest removal in acidic to neutral solution. As has been demonstrated in previous studies, optimal Cr (VI) adsorption occurs at a pH lower than 4 for various sorbents, such as some metallic (oxy)hydroxides [32; 33; 34] and natural bio-sorbents, for example, larch bark [35], cooked tea dust [36], papaya seeds [37], raw Bagasse [38], and activated carbon [39]. In the case where iron and aluminum oxides are used as adsorbents, an adsorption efficiency of higher than 80% was reached at a pH of 4 to 6. The adsorption was seen to be highly dependent on the pH of the medium; the Cr (VI) uptake increased by increasing the pH values from 1.0 to 7.0, after which the uptake decreased.  
Similar results were reported for the variation of hexavalent chromium adsorption with pH solution, with other adsorbents such as clay minerals [40], oxide-coated sand [40], aluminosilicate [41], zeolite [42], chitosan [43], and hydroxyapatite [44], which all exhibited a maximum uptake at a pH of 5 to 7. It has been suggested that, at a low pH, the adsorption is also low, due to the competition between the metal ions and protons for the adsorption sites. In these cases, the uptake of Cr (VI) followed the ion exchange mechanism. From these results, it could be concluded that, at a pH higher than pH\({}_{\text{max}}\), Cr (VI) exhibited typical anionic sorption behavior, with adsorption decreasing when the pH was increased. Similar results were reported for the retention of similar anions on oxide surfaces [45; 46]. This adsorption pattern is the result of the protonation of surface hydroxyl sites and of Cr (VI) hydrolysis [47]. Thus, at a pH higher than pH\({}_{\text{max}}\), the retention of Cr (VI) was due to the interaction of HCrO\({}_{4}\)\({}^{-}\) and/or CrO\({}_{4}\)\({}^{2-}\) with OH surface groups, rather than with OH\({}_{2}\)\({}^{+}\) groups, which were predominant at a pH lower than pH\({}_{\text{max}}\)[44]. The significant influence on the adsorption of CrO\({}_{4}\)\({}^{2-}\) was found at the slope of the pH adsorption edges. Distinct pH regions with different slopes characterized the various adsorbed species. This result reflected the change in the sorption mechanism [48].