## 2 Cr(VI) Behavior in Solution and Proposed Sorption Mechanisms: Effect of pH
An obvious explanation is that in acidic condition the exchanged OH- reacts with H\({}^{+}\), thus promoting the forward reactions in Equations (4)-(7). At alkaline pH values, the sorption of Cr(VI) is hampered by competition between CrO\({}_{4}\)[2]- and OH\({}^{-}\) for binding sites. At high pH (i.e., in excess of OH\({}^{-}\)), this competitive effect leads to a drastic reduction of Cr(VI) uptake (Figure 2). [PERSON] et al. [25] investigated the sorption of Cr(VI) on chitosan derivatives cross-linked and grafted with amino and carboxyl groups. The authors observed a sorption trend with increasing pH similar to that observed by [PERSON] et al. [20]. The researchers ascribed the higher uptake of Cr(VI) in acidic conditions to the protonation of amino groups in the chitosan sorbents, which would induce an electrostatic attraction to oxyanion Cr(VI) species. The same study [25] also reported a slightly lower Cr(VI) removal efficiency at pH 2 relatively to pH 4, which was attributed to chitosan instability at this pH. An alternative mechanism for water purification from Cr(VI) involves reduction to Cr(III), a process in which pH also plays a central role [6; 27; 28]. [PERSON] et al. [6] studied the reduction and immobilization of Cr(VI) with oxyhumolite and iron humate. The authors found that the amount of Cr(VI) removed from the solution in the presence of oxyhumolite decreased with increasing pH (Figure 3A). Figure 3: Residual chromium concentration in solution as a function of pH. Sorbent used: (**A**) oxyhumolite; (**B**) iron humate. Adapted from [PERSON] et al. [6]. Two mechanisms were proposed to account for that, involving direct and indirect reduction of Cr(VI). In the former mechanism, Cr(VI) is directly reduced to Cr(III) in the aqueous phase by contact with electron-donor groups of the sorbent; Cr(III) thus formed may be sorbed by cation-exchange or remain in solution. The indirect reduction pathway proposed postulates that Cr(VI) in the oxyanionic form initially binds via electrostatic interaction to the positively charged sorbent surface, then undergoes reduction by adsorbent electron-donor groups and is finally released in solution or remains bound to the sorbent. The study by [PERSON] et al. [6] clearly identified two processes: Cr(VI) reduction to Cr(III) favored at lower pH, and Cr(III) retention by the sorbent favored at higher pH. Interestingly, a somewhat different trend was observed when using iron humate as sorbent (Figure 3B). The authors related this trend to the presence of iron compounds in the sorbent material that might be able to reduce Cr(VI) over a wide pH range. The resulting Cr\({}^{3+}\) cations might be retained on the sorbent by several mechanisms, such as ion-exchange, covalent binding or surface precipitation and co-precipitation of Fe\({}^{3+}\)/Cr\({}^{3+}\) hydroxides. It is worth mention that the pH used by [PERSON] et al. [6] is below 5, hence their results are not strictly comparable with those shown in Figures 2 and 4. [PERSON] [28] examined the performance of powder carbon steel for Cr(VI) removal from water. This author found a higher Cr removal efficiency with decreasing pH, probably due to Cr(VI) reduction to Cr(III) coupled with oxidation of Fe to Fe\({}^{3+}\). Fe\({}^{3+}\) and Cr\({}^{3+}\) thus produced could react forming mixed oxide with low solubility and consuming H\({}^{+}\). On the other hand, chromate and Fe\({}^{2+}\) ions may react and form mixed Cr and Fe oxides generating H\({}^{+}\) release. The following Equations (8) and (9) illustrate some of the reactions discussed above:  
\[\mathrm{HCrO_{4}^{-}}\rightleftarrows\mathrm{H^{+}}+\mathrm{CrO_{4}^{2-}} \qquad\mathrm{K_{2}}=3.2\times 10^{-7} \tag{2}\]0  
\[\mathrm{HCrO_{4}^{-}}\rightleftarrows\mathrm{H^{+}}+\mathrm{CrO_{4}^{2-}} \qquad\mathrm{K_{2}}=3.2\times 10^{-7} \tag{2}\]1  
A similar reduction in the sorption capacity with the increasing pH was observed by [PERSON] et al. [22], who investigated the sorption and reduction of Cr(VI) to Cr(III) using pyrite (FeS\({}_{2}\)).