Controllable Preparation Of ?-Fe₂O₃ And FeOOH And Its Adsorption On Cr???

At present,the severity of Cr???and its hardness of removal are one of the biggest problems concerning heavy metal pollution control.Among many control methods of heavy metal pollution,the adsorption method,is suitable for large-scale industrial wastewater treatment due to its simple operation and low equipment requirements.But the cost and adsorption effect are largely related to the selection and preparation of adsorbents.The iron oxide is widely found in nature.In particular,?hydroxyl?iron oxide possesses a great potential of application in water treatment and catalysis because of its special physical and chemical properties,stable structure and rich hydroxyl.Moreover,hydroxyl iron oxide is also an important precursor for the preparation of other iron-based compounds.Iron oxide is the most common product obtained by using hydroxy iron oxide as precursors,and it shows prominent effect in many fields.Therefore,the study of iron oxide and ferric oxide is a hot spot in recent years in respect to the study of water pollution control.In view of the advantages of iron oxide and ferric oxide in water pollution control,in this paper it intends to prepare hydroxy iron oxide and iron oxide continuously by hydrothermal method at low temperature,and to study its adsorption performance to Cr???.In view of the advantages of ?-Fe₂O₃ and FeOOH in water pollution control.In this paper,hydroxy iron oxide and iron oxide with different morphology,different particle size and different adsorption properties of Cr???were prepared controlledly by changing the type of iron salt and adjusting the pH value of the corresponding solution at low temperature.The phases and morphology were analyzed by XRD and SEM respectively.The adsorption properties and the surface state of the adsorbent were analyzed by ICP-OES,SEM/EDS and XPS respectively.The effects of initial concentration of pollutants,pH value,ionic strength and coexisting ions on the adsorption effect were studied.The experimental data were fitted by isothermal adsorption equation and kinetic equation.The effects of ?-Fe₂O₃,?-FeOOH and ?-FeOOH on the adsorption characteristics of Cr???and the difference of adsorption mechanism are studied.The results are as follows:?1?The optimization of the preparation process was based on FeCl₃.The effects of synthesis temperature,synthesis time and Fe³⁺ concentration on the phase,crystallization and Cr???of the composites were studied.The optimum experimental conditions for the synthesis of adsorbents were determined.The temperature is 70?,the reaction time is 72 h and the concentration of Fe³⁺ is 0.2mol/L.?2?The anion species and pH value of the solution had great influence on the phase of the composites.Cl-and NO-3 are favorable for the formation of ?-FeOOH and ?-FeOOH,respectively.?-Fe₂O₃ is suitable for synthesis in solution pH 3 to 10,When the solution pH is greater than or equal to 11,FeCl₃,Fe?NO₃?₃,Fe₂?SO₄?₃ solution can be synthesized with good crystallinity of ?-FeOOH.And with the increase of the pH value of the solution,the phase showed a tendency of ?-Fe₂O₃ to ?-FeOOH.?3?The pH value of iron salt solution is the key factor to synthesize ?-Fe₂O₃.The anion species in iron salt can obviously affect the morphology of ?-Fe₂O₃.?-Fe₂O₃ synthesized in FeCl₃ solution is a kind of spherical particles with clear edge edge.Fe?NO₃?₃ solution is an irregular square particle with edge-like grains,and the agglomeration of particles is serious.The ?-Fe₂O₃ synthesized in the Fe₂?SO₄?₃ solution at pH 9 is a triangular double cone shape,with a small spherical particle at pH 10.The For the three kinds of iron salt,the crystallinity of ?-Fe₂O₃ synthesized under acidic conditions is weaker than that of alkaline condition,the particle size is larger,and the removal efficiency of Cr???is better.The crystallinity of the synthesized iron oxide in the acidic environment is worse than that synthesized in the alkaline environment,but the removal efficiency of Cr???is better.?4?The anion species and the pH value of the solution are all important factors for the synthesis of ?-FeOOH.?-FeOOH is a spherical particle in the Fe?NO₃?₃ solution at pH?2.In Fe₂?SO₄?₃ solution,in the range of acid,the shape of ?-FeOOH is short rod,and with the increase in pH,will?001?direction of growth,and ultimately grow into needle or long plate.?5?The removal rate of Cr???in ?-Fe₂O₃,?-FeOOH and ?-FeOOH decreases with the increase of initial concentration of Cr???in the range of 10mg/L²10mg/L.The unit adsorption capacity showed an increasing trend.The results of isothermal adsorption equation show that the adsorption of ?-Fe₂O₃ on the Cr???is more consistent with the Freundlich isothermal adsorption equation,which is multi-molecular layer adsorption The adsorption of ?-FeOOH and ?-FeOOH on the Cr???is in accordance with the Langmuir isothermal adsorption equation,which is monomolecular adsorption.?6??-Fe₂O₃,?-FeOOH and ?-FeOOH have a small amount of dissolution at the pH value of 2.The amount of dissolution is from large to small is ?-FeOOH> ?-FeOOH>?-Fe₂O₃.But in the pH range of 4 to 10,can be stable.The adsorption effect of the three adsorbents in the acidic environment is obviously better than that in the alkaline environment.?7?The kinetic study shows that the adsorbent process of Cr???is very fast,and the adsorption process can be simulated with quasi-second order kinetic model.Both the initial adsorption rate V0 and the reaction rate constant K2 increased with the increase of temperature.The activation energies of ?-Fe₂O₃,?-FeOOH and ?-FeOOH were increased in the range of chemical adsorption.Both physical adsorption and chemical adsorption were observed in the process,with mainly physical adsorption in early stage,and chemical adsorption in the latter stage.