| Industrial heavy metal pollution has become a serious environmental and sanitary problem all over the world in recent years. Heavy metals can not only have toxic and harmful effects on organisms living in water, but also accumulate throughout the food chain to affect human beings. Heavy metals such as hexavalent chromium has carcinogenic, teratogenic, mutagenic effects on human body, which is listed as one of the most harmful chemicals to human health, also is one of the three internationally recognized cancer-causing metals.There is sufficient evidence available to demonstrate that the toxicity, mobility, and bioavailability of heavy metal contaminants in soils and aquatic environments is largely controlled by their interactions with these iron-aluminum oxides. Therefore, the study of the adsorption of metal ions on the interface of iron-aluminum oxides/H2O play an essential role to understand the interaction of natural iron-aluminum oxides and heavy metals contained in wastewater.In this paper, the adsorption of Cr(Ⅲ),Cr(Ⅵ), Pb(Ⅱ), Cu(Ⅱ) on hydrous iron-aluminum oxides was studied.The main research work and results are summarized as follows:The microstructure of hydrous iron-aluminum oxides was characterized by XRD, FT-IR and BET surface. The effect of the most relevant operating conditions (e.g. pH, concentration of metal ions, cations, anions, organic acids and temperature) was investigated. The adsorption capacity was found as follows:Fh> FAh-5/2> FAh-5/8, Cr(Ⅲ) sorption on hydrous iron-aluminum oxides is mainly inner sphere surface complexation. The presence of Al3+ competed with Cr3+ on the surface specific adsorption sites of sorbents, but competed only non-specific adsorption sites by Cu2+, Zn2+ and Cd2+. The trivalent chromium adsorption was enhanced by SO42-,C2O42-,F-, HCO3-. The adsorption of Cr(Ⅲ) in the presence of organic acids(citric acid and tartaric acid) increased with the increase of the initial concentration, and reached a maximum at a certain concentration, and then decreased gradually. The Langmuir model describes the adsorption behaviors of Cr(Ⅲ) ions on the three sorbents better. The mean free energy of adsorption indicated that the adsorption of Cr(Ⅲ) onto sorbents may be carried out via chemical mechanism. And the pseudo-second-order model represented better the sorption kinetics of Cr(Ⅲ) ions. The intraparticle diffusion was involved in the adsorption process, it can not be accepted as the only rate controlling step for the adsorption of Cr(Ⅲ) onto hydrous iron-aluminum oxides. Finally, the experimental results of adsorption isotherm and desorption isotherm revealed that the adsorption of Cr(Ⅲ) ions on hydrous iron-aluminum oxides was irreversible.The adsorption of Cr(Ⅵ) on the interface of hydrous iron-aluminum oxides/H2O was examined in aqueous solution with respect to the pH value, ionic strength, anions and organic acids et al. The kinetic and thermodynamic study of Cr(Ⅵ) were investigated in depth. Insensitivity to ionic strength has been taken as an indication for hydrous iron-aluminum oxides, specific bonding played an important role in the adsorption of Cr(Ⅵ). The results showed that anions and organic acids reduced the adsorption of hexavalent chromium through competition with Cr(Ⅵ) for sorption sites. The Langmuir model described the adsorption behaviors of Cr(Ⅵ) on hydrous iron-aluminum oxides better and the adsorption of Cr(Ⅵ) was irreversible. The kinetic data fitted well the pseudo-second-order kinetics. AH0 and AS0 calculated showed that the adsorption of Cr(Ⅵ) onto hydrous iron-aluminum oxides was endothermic, entropy increasing.The adsorption of Pb(Ⅱ),Cr(Ⅲ) and Cu(Ⅱ) on ferrihydrite was also investigated under the similar conditions to what was implemented in the adsorption of Cr(Ⅲ). The results showed that the Langmuir model can be used to describe the adsorption behaviors of Pb(Ⅱ),Cr(III) and Cu(Ⅱ) ions on ferrihydrite. The order of adsorption capacity was found as follows:Pb(Ⅱ)> Cu(Ⅱ)> Cr(Ⅲ). The competitive sorption of Pb(Ⅱ),Cr(Ⅲ) and Cu(Ⅱ) on ferrihydrite were studied. Lead showed a much greater capacity to inhibit the sorption of the other two metals, whereas the opposite was true for Cu. The pseudo-second-order model represented better the adsorption kinetics of Pb(Ⅱ),Cr(Ⅲ) and Cu(Ⅱ). The activation energy of Pb(Ⅱ), Cr(Ⅲ) and Cu(Ⅱ) are equal to 49.19,83.51 and 50.94 kJ/mol and the activation enthalpies are equal to 27.62,80.89 and 29.81 kJ/mol. The activation entropy are equal to-168.51,-118.24 and -160.83 J/(mol K), respectively. |
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