| Ferrihydrite(Fh) is commonly found in natural systems. Ferrihydrite often serves as a scavenger of heavy metals because of its extremely high surface area and reactivity. 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 oxides. Therefore, the study of the adsorption of metal ions on the interface of iron oxides/H2O play an essential role to understand the interaction of natural iron oxides and heavy metals contained in wastewater. Our group has investigated the preparation and mechanism of iron oxides in solution, the applications of which in environment. In the investigation, ferrihydrite was prepared by the following three different procedures (called Fh-1, Fh-2, Fh-3). Procedure 1:ferrihydrite was prepared according to the most references. Procedure 2 and procedure 3 was developed by our group. The preliminary results showed that there are some differences in their micro-structure and the phase transformation behavior of ferrihydrite. The differences on catalytic decomposition of H2O2 and adsorption on Zn(Ⅱ) has been examined. In this paper, clearly defined three procedures were used to prepare ferrihydrite. The adsorption of Cu(Ⅱ) and Pb(Ⅱ) on the three ferrihydrites was studied. Meanwhile, the effect of transformation of ferrihydrite in the presence of trace Fe(Ⅱ) on its adsorption was investigated. The results obtained in this paper will be helpful for us to understand the dependence of the adsorption properties of ferrihydrite on its formation conditions.The main research work and results are summarized as follows:The adsorption of Cu(Ⅱ) on the ferrihydrite was studied in detail. The effect of the most relevant operating conditions (e.g. pH, concentration of metal ions, dose of adsorbent and temperature) was investigated. The Langmuir model describes the adsorption behaviors of Cu(Ⅱ) ions on the three Fhs better. And the pseudo-second-order model represented better the sorption kinetics of Cu(Ⅱ) ions. In the system of Fh-1, Fh-2 and Fh-3, the adsorption capacities are equal to 8.74,13.33 and 14.39 mg/g. The activation energy are equal to59.28,46.71 and 46.26 kJ/mol and the activation enthalpies are equal to 48.10,38.39 and 37.96 kJ/mol. The activation entropy are equal to-127.5,-151.5 and-155.8 J/mol-K, respectively. The adsorption-desorption isotherms were employed to evaluate the degree of reversible adsorption of Cu(Ⅱ) on the three ferrihydrites. The results showed that the adsorption capacity of Fh-3 was the largest under the same conditions. The degree of reversible adsorption of Cu(Ⅱ) on the Fh-1 is the highest. Fh-3 has a stronger ability to bind Cu(Ⅱ) ions. The transformation of ferrihydrite decreases its adsorption capacity for heavy metal ions. When Fe(Ⅱ) and Cu(Ⅱ) were introduced into the research system simultaneously, the ability for Fh-3 to bind Cu(Ⅱ) ions is better than others.The adsorption of Pb(Ⅱ) adsorption on the three ferrihydrites was also investigated under the similar conditions to what was implemented in the adsorption of Cu(Ⅱ). The results showed that the adsorption capacity of Fh-3 was the largest under the same conditions. The Fredunlich model describes the adsorption behaviors of Pb(Ⅱ) ions on the three Fhs better. The pseudo-second-order model represented better the sorption kinetics of Pb(Ⅱ) ions. In the system of Fh-1, Fh-2 and Fh-3, the activation energy are equal to 93.79,66.76 and 58.10 kJ/mol and the activation enthalpies are equal to 91.60,64.34 and 55.77 kJ/mol. The activation entropy are equal to 14.70, -64.31 and -95.57 J/mol·K, respectively. The data of both ageing and adsorption-desorption experiments reveal that both Fh-2 and Fh-3 have the stronger ability to bind Pb(Ⅱ) ions. The degree of reversible adsorption of Pb(Ⅱ) on the Fh-1 is higher than others. |
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