## 2 Cr(VI) Behavior in Solution and Proposed Sorption Mechanisms: Effect of pH
The mechanism proposed by the authors was quite complex, involving a number of simultaneous or sequential steps, including pyrite dissolution, precipitation of oxidation products, protonation/deprotonation of chromium and of oxidized surfaces. A decrease in Cr(VI) removal efficiency associated with increasing pH was ascribed, at least in part, to pyrite surface passivation due to the accumulation of oxidation products. Under acidic conditions, pyrite releases Fe\({}^{2+}\) which may  
\[\mathrm{HCrO_{4}^{-}}\rightleftarrows\mathrm{H^{+}}+\mathrm{CrO_{4}^{2-}} \qquad\mathrm{K_{2}}=3.2\times 10^{-7} \tag{2}\]2 Typical bell-shaped trend of sorption capacity as a function of pH. reduce Cr(VI) to Cr(III) in the solution phase. Interestingly, the authors also suggested that Fe\({}^{2+}\) ions could also be sorbed onto pyrite, thus activating a cyclic redox mechanism that would further favors Cr(VI) reduction. The overall reaction pathway proposed is:  
\[\mathrm{HCrO_{4}^{-}}\rightleftarrows\mathrm{H^{+}}+\mathrm{CrO_{4}^{2-}} \qquad\mathrm{K_{2}}=3.2\times 10^{-7} \tag{2}\]3  
[PERSON] et al. [29] used an additive-competitive Langmuir model to describe the effects of both pH and temperature on the chromium adsorption capacity of a commercial granular activated carbon, accounting for the chromium ion speciation. They highlighted that the optimal pH conditions were derived from the compensation between higher Cr(VI) ionization, favored at higher pH, and lower competition with OH- ions, favored at lower pH. [PERSON] et al. [26] investigated the sorption of Cr(VI) by free and immobilized biomass (_Pediastrum boranum_) and observed the typical decreasing sorption trend with increasing pH (Figure 2). This result was related by the authors to deprotonation of amino groups at the sorbent surface, which would hamper the electrostatic binding of chromium oxyanions. A distinctive effect of pH on the sorption of Cr(VI) was observed by [PERSON] and co-workers [30] using Tulsion A-27(MP) resin (Figure 4). The sorption capacity vs. pH followed a bell-shaped curve with a maximum in the pH range 5-6 (Figure 4). The authors ascribed the fall of sorption capacity at high pH to sorbent surface passivation of the adsorbent surface by precipitation of hydroxides. In contrast, the decrease at low pH was ascribed to competition of H\({}^{+}\) with Cr(VI) for sorption sites. In the study carried out by [PERSON] et al. [31], husk of _Lathyrus sativus_ was used as a sorbent for Cr(VI). It was found that the presence of functional groups such as \"NH\({}_{2}\), \"OH and PO\({}_{4}\)\({}^{3-}\) on the biomass surface markedly affected Cr(VI) uptake. More in detail, the authors suggested that the binding of Cr(VI) involved hydrogen bonds with amino and hydroxyl groups, or electrostatic interactions with protonated phosphate groups. A pH effect similar to that displayed in Figure 4 was also found by [PERSON] and co-workers [32] using chitosan-xylan-TiO\({}_{2}\) adsorbent. The authors attributed the low sorption of Cr(VI) in acidic solution (pH 4) to electrostatic repulsion between the protonated functional groups of the sorbent and positively charged chromium ions. This explanation is not convincing because, as discussed above, at pH 4 Cr(VI) is prevalently present as oxyanion. An alternative explanation might be the structural alteration of the sorbent surface in acidic environment and consequent loss of sorption capacity [33; 34; 35]. A third mechanism possibly contributing to reduce Cr(VI) sorption at low pH (<2) might be the formation of oligomers of chromium species such as Cr\({}_{3}\)O\({}_{10}\)\({}^{2-}\) and Cr\({}_{4}\)O\({}_{13}\)\({}^{2-}\)[36; 37]. [PERSON] and coworkers studied the sorption of Cr(VI) onto MnO\({}_{2}\)[24]. A pH rise from 5.9 to 8.1 remarkably reduced Cr(VI) removal.