| Chromium, existing in two major oxidation states such as Cr(Ⅲ) and Cr(Ⅵ), mainly arising from the discharge wastewater of various industries including mining operation, metal plating, leather tanning, and pigment manufacturing, is among the common and persistent surface and ground water contamination. And Cr(Ⅵ) is most toxic and carcinogenic to organism.Presently, it is hot topic noticed by researchers that the biology and mineral materials such as alumino-silicates minerals, carbonate minerals and iron oxyhydroxides, are utilized to remove the heavy metals. Iron oxyhydroxides (FeOOH), as a group ofα,β,γ-FeOOH polymorphs, are commonly found in some soils, sedmiments of water bodies, and acid mine drainage natural environments. They can availably remove the heavy metals from the contaminated environments by the approachs of coprecipitation, ions exchange and adsorption. Due to holding the stable chemistry properties, more large specific surface areas and fine particle structures, iron oxyhydroxides, as the adsorbent materials of heavy metals and other contaminants in environment media, are doubly noticed. However, little information is available on the FeOOH prepared under various conditions, existed differences in their phases and structural properties, inducing some differences in their environmental functions.Therefore, the objective of the paper firstly is to systematically investigate the synthesis of iron oxyhydroxides by the various methods, for example, ferric hydrolysis and neutralization , ferrous oxidation by air, ferrous biooxidation and gel-network precipitation methods, under the different conditions such as the kind of iron salt, pH, temperature, exterior ion, biological oxidation, and fixation by organic biological molecule. The resulting products are characterized by spectral methods to examine the phases and structural properties of iron oxyhydroxides. The other objective is that some of the characterized FeOOH are applied to remove Cr(Ⅵ). In the present works, it is especially important work to formation of the bio-akaganeite and its application in removal of Cr(Ⅵ), which provides a scientific proof for searching the potential adsorbent materials. The results of all works are presented as following.The iron oxyhydroxides were prepared by hydrolysis and neutralization of ferric ion from FeCl3, Fe(NO3)3 and Fe2(SO4)3 salts, under the conditions of the various pH values and aging for about 6 days at 60℃.Results showed that ferrihydrite formed in the ferric solutions containing Cl-, NO3- and SO42- at pH values of 8 and 10, except that the poor crystalline akaganeite phase generated in the FeCl3 solution at pH10.It testified that the presence of Cl- was favorable for the formation of akaganéite. Meanwhile, the poor crystalline goethite phase was observed to be formed in FeCl3 or Fe(NO3)3 solution, but not be formed in Fe2(SO4)3 solution at pH12.It indicated that the presence of SO42- obviously inhibited the formation of goethite. However, the goethite phase formed in Fe2(SO4)3 solution when addition of ferrous ion, indicating ferrous ion could promote the formation of goethite in SO42--rich solution. In addition, it was usually easy to the crystalline goethite be transformed from the above generated ferrihydrite precipitates by aging at 60℃. Furthermore, the phase of akaganeite also was obtained in the Cl--rich acid (pH<5) solution by aging at 60℃.Tthe resulting akaganeite (A1 and A2) and goethite (G1 and G4) comprised of fine particle, with large specific surface area, and interface property, were valid and potential adsorbents for removal of Cr(Ⅵ), with a maximal sorption capacity of 24-27mg g-1.Iron oxyhydroxides were also prepared from ferrous chemical oxidation by air at room temperature, with various reaction pH values, the H2PO4- ion or Acidithiobacillus ferrooxidans. Results showed that lepidocrocite for sample Lep(6.7), the mixture of lepidocrocite and goethite for samples of Gth+Lep(6.7) and Gth+Lep(7.0), and goethite for samples of Gth(8.5), Gth(11) and Gth(12), were obtained from FeSO4 solution with a constant pH of 6.7, a final pH of 6.7 or 7.0, and a final pH of 8.5,11 or 12, respectively. And all the above FeOOH were composed of small particles with the short-rod shaped morphologis. At one time, in the (NH4)2Fe(SO4)2 solution with a reaction pH range of 6.7-4.0, lepidocrocite phase generated, while ferrihydrite formed in the solution when the exterior H2PO4- ion was added. It indicated that H2PO4- ion inhibited the formation of lepidocrocite phase. In addition, in the FeSO4 solution with a constant pH of 4.0, goethite phase could form in the solution with/without Acidithiobacillus ferrooxidans cells, but the microbe obviously accelerated the oxidation of ferrous ion. Furthermore, it was noticed that the similar morphologies of sphere shaped for the resulting particles with the diameter of 20nm, were obtained from the solutions, though the pH of the reaction solutions are 6.7-4.0 and 4.0, respectively.Furthermore, during testing the stability of sample Lep(-6.7) withγ-FeOOH phase, it was observed not to be transformed to goethite in aqueous solution with a solid/liquid weight ratio of 1/1000 and a pH of 5.5 or 7.0,when the resulting suspensions were placed undisturbedly at 24℃for 70 days, but better crystallineγ-FeOOH be indentified for solid sample at the 70th day than at the 30th day. Moreover, in the above similar experimental conditions, the capacities of keeping Cr(Ⅵ) for sample Lep(-6.7) withγ-FeOOH phase and sample of Gth(8.5) withα-FeOOH were determined by the released percentages of Cr(Ⅵ) from the saturated Cr(Ⅵ)-loading samples. Results showed that the quantities of Cr(Ⅵ) released fromγ-FeOOH(pH7.0),γ-FeOOH(pH5.5),α-FeOOH(pH7.0) andα-FeOOH(pH5.5) orderly were 0.72, 0.82, 0.87 and 0.96 mg g-1, resulting a increasing order. In addition, at the 70th day, the corresponding released percentages of Cr(Ⅵ) for the above samples were 17.9, 15.7, 31.1 and 15.3%, respectively. Obviously, the released percentage of Cr(Ⅵ) forα-FeOOH(pH7.0) is about two times of those of other samples. It proved that Gth(8.5) withα-FeOOH had a lower ability of keeping Cr(Ⅵ) in aqueous solution with a pH of 7.0, following higher risk to the ambient.Akaganeite biosynthesis from FeCl2 solution oxidized by chrolide-acclimated Acidithiobacillus ferrooxidans LX5 cells at 28℃,with spindle-shape approximately 200 nm in length with an axial ratio of about 5 and the spindles having a rough surface, its chemical formula of the crystalloid akaganeite could be expressed as Fe8O8(OH)7.1(Cl)0.9 with Fe/Cl molar ratio of 8.93. The biogenic akaganeite had a specific surface area of about 100 m2 g-1by BET method. In addition, the results of effect of Cl-/SO42- mole ratio showed that sulfate inhibited drastically the formation of akaganeite, resulting in only schwertmannite occurrence in the ferrous solution containing both sulfate and chloride. The presence of chloride in ferrous solution containing sulfate would enable the obtained schwertmannite possess different morphologies and characteristics depending on Cl-/SO42- mole ratio in initial reaction system. It implied that iron(Ⅲ) oxyhydroxysulfate might be the only iron mineral phase in acidic ferrous and Acidithiobacillus ferrooxidans -rich solutions as long as the presences of trace sulfate. Finally, in the reaction time of 0-10 days, the resulting precipitates were always the phase ofβ-FeOOH,except that it had a better crystallinity after reaction for 3 days than ahead. The above obtained bio-akaganeite had a notable advantage in removal of Cr(Ⅵ), comparing the chemo-akaganeite. Firstly, the bio-akaganeite didn't dissolve in the aqueous solutions with the pH ranges of 3-12. Furthermore, it was testified that the bio-akaganeite had a similar removal efficiency of Cr(Ⅵ) in a wide pH rangs of 3-8. In addition, under the pH of 5.5 and 7.0, the bio-akaganeite had a maximal load capacity of 58.5 and 42.2 mg g-1, respectively, correspondly being about 2 and 1.5 times of the load capacities of chemo-akaganeite (A1) and chemo-goethite (G4). It concluded that the adsorption of Cr(Ⅵ) on bio-akaganeite by the major adsorption mechanism of surface complexiation models and the kinetic model of Lagergren secord order rate equation for Cr(Ⅵ) in aqueous solution with a pH of 7.0.In the last work, a novel gel-network precipitation method was developed to synthesize goethite in the presence of the optimum concentrations of glutin (12%) and FeCl3 solution (0.6 M). Resulting particle had better monodispersity, and had a short rod-type shape approximately 110 run in length with an average diameter of about 35 run. |
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