| Three different methods were used to synthesize FeOOH. The identities of FeOOH were verified by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD peaks of the three crystallites matching well with standardβ-FeOOH. The TEM images showed that they were all nano-scaled, and the morphologies were rectangle, shuttle and spindle, respectively. The three kinds ofβ-FeOOH were used to conduct adsorption experiments to investigate their adsorption capacity of Cr(Ⅵ). The result of the adsorption experiments showed thatβ-FeOOH synthesized with organic macromolecules had high removal efficiency for Cr(Ⅵ) in comparison with other samples. So, it was employed as a modifier to prepare akaganeite coated sand (ACS). ACS was characterized through XRD, SEM/EDS and surface area analyzer. It was found that the phase of FeOOH on quartz sand was akaganeite (β-FeOOH), and the morphology was spindle. The surface area of ACS was 1.83 mg/g, which was about five times larger than that of quartz sand. Iron content and binding ability ofβ-FeOOH layer on quartz sand were investigated, the results showed that the iron content of ACS was high, and the adsorbent had high coating strength and high physical-chemical endurance.Adsorption of Cr(Ⅵ) on ACS was conducted by batch experiments and column experiments under different conditions. The key influential factors, such as adsorbent dose, pH values, coexisting ions, temperature, and contact time were studied. The thermodynamics and kinetics of adsorptive process as well as the reuse of the adsorbent were also investigated. The adsorptive removal of Cr(Ⅵ) on ACS was significantly improved compared with that on quartz sand. The removal efficiency of Cr(Ⅵ) on ACS was highly pH dependent. As pH increased, the removal efficiency increased in acidic solutions, but it decreased in alkaline environments. The optimum pH value for the adsorption process was determined to be 7.0. Competitive adsorption occurred when PO43-and SO42- were coexisted with Cr(Ⅵ) in the adsorption process. While, Cl- and NO3- had little effects on the adsorption of Cr(Ⅵ) on ACS. Higher adsorption efficiency for Cr(Ⅵ) was observed with the increasing of temperature. The adsorptive capacities of Cr(Ⅵ) under three different test temperatures (293 K,303 K, 313 K) were 0.060 mg/g,0.070 mg/g and 0.076mg/g, respectively. The adsorption behavior of Cr(Ⅵ) on ACS can be described with the Langmuir and Freundlich isotherm models. The pseudo-second-order rate equation successfully described the adsorption kinetics. The negative value of△G0 and the small positive value of△H0 showed that the adsorption of Cr(Ⅵ) in aqueous solutions by ACS was spontaneous and endothermic. Of all the tested desorbents, NaOH had the highest regenerate efficiency, and the adsorbent can be reused after regeneration. The breakthrough curve for Cr(Ⅵ) adsorbing on ACS was obtained by column experiment.In the EDS spectrum of ACS after adsorption, Cr was identified, which showed that Cr was adsorbed on ACS. The XPS result suggested that after adsorption, a part of Cr(Ⅵ) changed to Cr(Ⅲ). The electron density of oxygen was depleted, and the surface charge of oxygen was reduced. In the oxidation-reduction reaction, the oxygen atom provides electrons, iron and chromium accept electrons. |
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