| Hexavalent chromium (Cr(VI)) is a widespread highly toxic heavy metal pollutant which poses a great threat to human health, so it is in great need of efficient removal of Cr(VI) from wastewater. Iron oxyhydroxides (FeOOH) are commonly found in soil and sediments of water bodies. They naturally occur in several crystal forms, including goethite (a-FeOOH), akaganeite (P-FeOOH), lepidocrocite (y-FeOOH), and feroxyhyte (δ-FeOOH). Because of their high specific surface area and stable chemistry properties, FeOOH polymorphs have fine adsorption capacity for heavy metals ions. They can be used as the environmentally friendly water treatment agent for Cr(VI) ions directly. Therefore, the objective of this paper was to synthesize different polymorphs of FeOOH by various methods. Then the FeOOH polymorphs were characterized and applied to remove Cr(VI) in aqueous solutions. In the present work, it was especially important to study the dependence of the Cr(VI) removal efficiency on the pH value. The other objective of this paper was to immobilize the FeOOH on the surface of the ion-exchange resin and investigate the application of the synthesized R-FeOOH in removing Cr(VI). The optimal experiment conditions were determined, and the removal mechanism of Cr(VI) was also explored. In this work, the results are presented as follows.(1) The four polymorphs of FeOOH, including α-FeOOH, (3-FeOOH, y-FeOOH, and δ-FeOOH were prepared successfully. The BET analysis showed that the four polymorphs of FeOOH were mesoporous materials, and their specific surface area followed the order of γ-FeOOH>δ-FeOOH> a-FeOOH> β-FeOOH. The pHpzc values of a-FeOOH, y-FeOOH. and δ-FeOOH were about 3.6.7.1. and 8.0, respectively. However, the surface charges of β-FeOOH were always negative and the pHpzc could not be obtained.According to the comparison study of Cr(VI) removal by the four polymorphs of FeOOH. after reaction for 120 min, the highest Cr(VI) removal efficiencies by a-FeOOH, y-FeOOH, and 8-FeOOH were 94.5%,100.0%, and 84.6%, respectively. However, the β-FeOOH with low specific surface area can only achieve the Cr(VI) removal efficiency of 52.0%. In addition, in a fairly broad pH range (pH 3.0-9.0), the Cr(Ⅵ) removal efficiencies by a-FeOOH were all over 85.0%.δ-FeOOH was easily recovered due to its feature of ferromagnetic.The XPS analysis revealed that the valence state of Cr(VI) did not change during the Cr(VI) removal process. The Cr(VI) removal by the four polymorphs of FeOOH was a process of adsorption, which was simultaneously controlled by electrostatic attraction (physical adsorption) and ligand exchange (chemical adsorption). In this process, electrostatic attraction between the positively charged surface functional groups-OH2+ of FeOOH and the Cr(VI) oxyanions facilitated the ligand exchange between hydroxyl ions and Cr(VI) ions.(2) The supported R-FeOOH was prepared using the Amberlite IR120 resin as support through the hydrolysis of Fe3+ions which were exchanged on the surface of the resin. On the whole, the synthesized R-FeOOH demonstrated a strong ability in removing Cr(VI) from an aqueous solution, and the R-FeOOH was easily to be separated and collected after loading pollutants.The influence factors of preparation and Cr(VI) removal, including Fe3+concentration, H2O2 concentration, R-FeOOH dosage, initial pH value and initial Cr(VI) concentration were investigated. Results showed that the Cr(VI) removal could be achieved within a broad pH range. After reaction for 120 min, the removal efficiencies of Cr(VI) were all over 98% with a R-FeOOH concentration of 15.0 g/L for initial Cr(VI) concentration of 20.0 mg/L at initial pH values of 2.0,3.0,5.0, and 7.0. Under the optimal conditions, after reaction for 60 min, the Cr(VI) efficiency achieved 100%. Among the process of Cr(VI) removal, the reaction mechanisms included the reduction of Cr(VI) by some Fe2+ions besides the electrostatic attraction and ligand exchange. |
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