The adsorption of chromate ions H2-yA (y = 1, 2, and A = CrO42–) on iron(III) hydroxide was conducted as a function of adsorbent mass, solution pH, and hydration time. The surface complexation technique, based on the examination of the chromate distribution between the solid and liquid phases, was adopted to predict the adsorption mechanism. To specify stoichiometry of the chromate surface complexes, the proton (n > 0), and hydroxyl (n < 0) ion-exchange was evaluated at a pH range of 2–12. The obtained “n” values are ranging between -1 and 1. As a result, the sorption process involved specific chemical interaction with surface sites, resulting in 1H+ and 1OH– release of the adsorbate molecule. The surface species identified were ; ; ; ; ; ; ;  and . The logarithmic values of their complexing constants were: log K00 = 1.81 ± 0.04; log K11 = -3.53 ± 0.07; log K21 = -1.03 ± 0.23, log K1-1 = 7.15 ± 0.14 and log K2-1 = 9.62 ± 0.53. The results showed that the chromate adsorption on Fe(III) hydroxide was of electrostatic and chemical nature at pH lower than 5.5, and only of chemical nature at pH superior to 5.5. Taking into account these considerations, Fe(III) hydroxide could be considered an excellent sorbent for the removal of Cr(VI) from wastewater solutions.

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The sorption mechanism of hexavalent chromium sorption on eucalyptus barks was evaluated as a function of solution pH for different adsorbent dosages, surface coverage, and the amount of adsorbent. The chromium retention was evaluated based on the distribution coefficient (D), and this retention is attributed to  species, which is predominant between pH 1 and 6.5. The biosorption of Cr(VI) ions onto barks achieved at pH 2.0 in the highest sorbet conditions corresponding to [Cr(VI)] = 10–5 mol (V = 100 mL) is examined for various surface coverage. The surface complexes formed between chromate and eucalyptus barks were found to be > S (HCrO4) and > S (CrO4). Logarithmic stability for log K1–1 and the log K10 values of the complexes were measured and found to be -5.93 in acidic medium and -0.76 in alkaline medium, respectively. Pointed out that the adsorption of Cr(VI) on eucalyptus bark was greater than 90% in all cases, Cr(VI) recovery is strongly acidic dependent and shows maximum retention, for various sorbent amounts, at pH around 2, and this retention is attributed to  species, which is predominant between pH 1 and 6.5, the morphological surface of eucalyptus barks were examined by Scanning Electron Microscope (SEM) connected to a micro analyzer EDS.