The effect of pH in the adsorption of Alizarin and Eriochrome Blue Black R onto iron oxides.

The adsorption process of two dyes as a function of pH on three different adsorbents (goethite, Co-goethite, and magnetite) has been analyzed. Typical anionic adsorption behavior was observed for both dyes onto goethite and Co-goethite. The adsorption level was practically constant in the range of pH studied when the adsorbent was magnetite. The constant capacitance model (CCM) was employed to fit the experimental results. The surface complexes proposed from the adsorption data were in agreement with the patterns obtained from FTIR spectroscopy and a molecular mechanics calculation. Goethite has very good performance as adsorbent of Alizarin and Eriochrome Blue Black R. The presence of a foreign cation in Co-goethite does not improve the adsorption abilities of goethite. At low pH, the amounts of Alizarin and Eriochrome Blue Black R adsorbed on goethite and Co-goethite are similar. However, a higher dependence with the increase of pH is observed by Eriochrome Blue Black R. On magnetite, the dye adsorption shows less affinity for both dyes. Electronic and steric considerations can explain the trends found in the adsorption of the two dyes on the three iron oxides studied in this work.

Removal of Fluorescein using different iron oxides as adsorbents: effect of pH.

In this work the adsorption process of Fluorescein (dye with aril-methane group) as a function of pH on three different adsorbents: goethite, Co-goethite, and magnetite has been studied experimentally and theoretically. FTIR and Raman spectroscopy have been performed in an attempt to confirm the structure of surface complexes formed by sorption of the Fluorescein to different iron oxides. Typical anionic adsorption behaviour was observed for this dye onto goethite and Co-goethite whereas the adsorption level was practically constant in the range of pH studied when the adsorbent was magnetite. The diffuse layer model was employed to fit the experimental results. The surface complexes proposed from the adsorption data were in agreement with the patterns obtained from FTIR and Raman spectroscopy. The surface structure of the oxides affects the adsorption process and the final adsorbed amount at the equilibrium. Our model of diffuse double layer with the addendum of the effect of hydrophobic forces fits well the adsorption data of Fluorescein on iron oxides at different pH in the studied range. At lower pH electrostatic forces by ligand-exchange are predominant. In the range of pH 9-11 hydrophobic forces are managing the Fluorescein adsorption on the iron oxides, with the formation of outer-sphere complexes through van der Waals/hydrophobic forces. It is interesting that in the three iron oxides studied, the adsorbed amount in this range is similar.