## 1 Introduction  
The hexavalent chromium Cr (VI) generated from various industrial processes, such as metallurgy, dyes, paints, inks, and plastics, is a major global concern, due to its harmful effects on humans and nature [1]. As a result, the presence of this metal cation in nature is well controlled. The maximal concentration level of Cr (VI) allowed in drinking water, as determined by the US-Environmental Protection Agency (EPA), is 0.05 mg/L [2]. Compliance with the EPA's chromium rule requires additional industrial monitoring. Designing a treatment process which reduces the number of chromium ions in industrial effluent to an acceptable level, has become crucial. A number of methods for this purpose have already been developed, among which are reduction and precipitation [3], ion exchange [4], solvent extraction [5], adsorption [6], and electrochemical precipitation [7]. When considering these methods, adsorption is the most promising technique [8]. This process is usually performed using conventional adsorbents, such as silica, zeolites, iron(III) (hydr)oxides, and activated carbon, or nonconventional adsorbents, such as red mud, sewage sludge, and bone char [9; 10; 11]. Phosphate materials (synthetics and minerals) have also been used as effective adsorbents for heavy metals from wastewaters and polluted soils. They have an excellent stabilization efficiency for several metal ions. For this reason, they are highly efficient metal adsorbents [12; 13; 14], particularly calcium phosphate. This material has a large specific area, high thermal and geochemical stability, low solubility, high ionic exchange capacity, and high stability towards ionization by radiation. For these reasons, it has been used as a backfill material for geological repositories for nuclear waste [15], and as an adsorbent in engineered barriers for environmental restoration [16]. It was also reported that phosphates with an amorphous structure are more efficient adsorbents of lead, uranium, and plutonium [17]. Although several adsorbents have been developed, and the retention of the metals hasbeen extensively studied, the sorption mechanisms are still rather difficult to identify. This could be because several phenomena, such as iso-morphous substitutions, surface complexation, and dissolution-precipitation, can occur simultaneously during the sorption process [18; 19; 20; 21]. In addition, there is a lack of data on the adsorption of many metals on amorphous or poorly-crystallized phosphates. Published studies show that the uptake of hexavalent chromium by calcium phosphate, exhibits typical anionic (such as \(\mathrm{HCrO_{4}}^{-}\) and \(\mathrm{CrO_{4}}^{2-}\)) sorption behavior, and that the adsorption decreases by increasing the pH [22]. They also show that Cr (VI) adsorption is favored on phosphate that ispositively charged, at a low to neutral pH level (i.e., high point of zero charge (PZC)) [23]. The findings suggest that the retention of Cr (VI) by phosphates occurs through an electrostatic attraction and via binding to the surface functional groups \(\mathrm{OH_{2}}^{+}\) and \(\mathrm{OH^{-}}\). In other studies, it was found that hydroxyapatite (HAp) and tricalcium phosphate (TCP) composite are able to adsorb a significant amount of chromium (VI), at a pH level of about 5. So, the composites showed a lower PZC than HAp (6.2-8.5) [22; 23; 24]. Moreover, there is no systematic understanding of the mechanism of chromium immobilization that involves the protonation/deprotonation of surface hydroxyl groups, and their interaction with the metal oxyanion. The combined effect of both pH and contact time on the adsorption mechanism requires more investigation (Figure 1). Taking these considerations into account, the present study aims to investigate the complexation of hexavalent chromium with low-crystallized octacalcium phosphate (OCP).