## 2 Cr(VI) Behavior in Solution and Proposed Sorption Mechanisms: Effect of pH  
Sorption is a general term encompassing a diversity of processes such as adsorption, absorption, ion exchange and surface precipitation, as well as a diversity of mechanisms depending on the physicochemical properties of sorbate and sorbent. Being a surface phenomenon, sorption usually increases with an increase of the surface area of the sorbent [17; 18; 19]. Other factors, notably the pH and the surface charge of the sorbent, may play a leading role in the process [8; 20]. Cr(VI) is present in water in different oxyanionic forms (e.g., HCrO\({}_{4}\)\({}^{-}\), CrO\({}_{4}\)\({}^{2-}\) and Cr\({}_{2}\)O\(\gamma\)\({}^{2-}\)) or as undissociated chronic acid, H\({}_{2}\)CrO\({}_{4}\), the relative abundance of each species being strongly related to the pH. The equilibria that contribute most significantly to Cr(VI) speciation in water are reported below [21; 22; 23]:  
\[\mathrm{H_{2}CrO_{4}}\rightleftarrows\mathrm{H^{+}}+\mathrm{HCrO_{4}^{-}} \qquad\mathrm{K_{1}}=0.37 \tag{1}\]  
\[\mathrm{HCrO_{4}^{-}}\rightleftarrows\mathrm{H^{+}}+\mathrm{CrO_{4}^{2-}} \qquad\mathrm{K_{2}}=3.2\times 10^{-7} \tag{2}\]  
\[2\mathrm{HCrO_{4}^{-}}\rightleftarrows\mathrm{Cr_{2}O_{7}^{2-}}+\mathrm{H_{2 }O}\qquad\mathrm{K_{3}}=35.5 \tag{3}\]  
where K\({}_{1}\), K\({}_{2}\) and K\({}_{3}\) are the equilibrium constants of the three reactions. The first two equilibria are pH-dependent whereas the third only depends on total Cr(VI) concentration. The effect of pH and total Cr(VI) concentration (based on Equations (1)-(3)) on Cr(VI) species distribution, is shown in Figure 1 in a range of concentrations commonly found in contaminated water systems [24]. As can be seen from Figure 1, HCrO\({}_{4}\)\({}^{-}\) and CrO\({}_{4}\)\({}^{2-}\) are the most abundant species at acid and basic pH, respectively. Due to the negative net charge of Cr(VI) oxyanions, Cr(VI) sorption is expected to involve electrostatic interactions or anion exchange with positively-charged sorbent sites [6; 24; 25]. The uptake of Cr(VI) should be favored by a pH below the point of zero charge, which would impart the sorbent a net positive charge [20]. Chromium sorption should be less efficient under alkaline conditions than in acidic medium, not only because of electrostatic repulsion between chromium oxyanions and negatively-charged sites on sorbent surface but also because of competition with OH\({}^{-}\) for sorbent sites. In line with the analysis above, most papers on Cr(VI) report a decreasing sorption capacity with increasing pH. A typical trend of sorption capacity vs. pH is shown in Figure 2[20; 25; 26]. [PERSON] et al. [20], for example, investigated the sorption performance of the anion-exchange resins D301, D314 and D354 for Cr(VI). The authors ascribed the observed sorption behavior with varying pH (similar to that shown in Figure 2), to an anion exchange process between Cr species and OH\({}^{-}\) anions associated to -N(CH\({}_{3}\))\({}_{2}\) functional groups of the resins, according to the following reactions:  
\[\mathrm{R-OH+HCrO_{4}^{-}}\rightleftharpoons\mathrm{R-HCrO_{4}+OH^{-}} \tag{4}\]  
\[\mathrm{2R-OH+CrO_{4}^{2-}}\rightleftharpoons\mathrm{R_{2}-CrO_{4}+2 OH^{-}} \tag{5}\]  
Figure 1: Cr(VI) species distribution (%) as a function of pH and total Cr(VI) concentration: (**A**) Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\); (**B**) CrO\({}_{4}\)\({}^{2-}\); (**C**) H\({}_{2}\)CrO\({}_{4}\); (**D**) H\({}_{\mathrm{CrO_{4}}}\). Figure 2: Typical decreasing trend of Cr(VI) sorption capacity vs. pH. \[2\text{R}-\text{OH}+2\text{HC}\text{O}_{4}^{-}\rightleftharpoons\text{R}_{2}- \text{Cr}_{2}\text{O}_{7}+2\text{OH}^{-}+\text{H}_{2}\text{O} \tag{6}\]  
\[2\text{R}-\text{OH}+\text{Cr}_{2}\text{O}_{7}^{-}\rightleftharpoons\text{R}_{2}- \text{Cr}_{2}\text{O}_{7}+2\text{OH}^{-} \tag{7}\]  
where R is the resins' matrix. The authors reported that the amount of Cr(VI) sorbed in acidic condition was double than in alkali condition.