Preparation Of Potassium Ferrate And Its Arsenic Wastewater

Potassium ferrate(Ⅵ) has been considered as an environmental friendly oxidant in water and wastewater treatment and has a good application prospect because of its strong oxidbillity and coagulability, high sterilizing effeciency and without secondary pollution. However, current preparation method of potassium ferrate was complicated and high-cost. The large-scale industry application of potassium ferrate has not been achieved until now.In this study, the preparation method of potassium ferrate(Ⅵ) was improved and the effects of reaction time, alkalinity, chlorine dosage and reaction temperature on hypochlorite preparation were studied. Effective chlorine concentration of about 15% was obtained under optimum condition for hypochlorite preparation. Further, the effects of mole rate of iron resourse, reaction temperature, reaction time, and the dose of added agents on preparetion of potassium ferrate were investigated with iron nitrate chosen as iron resouce. The results show that the purity of potassium ferrate was more than 99% and the yield of potassium ferrate was about 67% under the optimum reaction condition. The structure and performances of prepared sample were characterized by SEM(Scanning Electron Microscope),EDS(Energy Dispersive Spectrometer),XRD(X-ray Diffraction),IR(Infrared Spectrometer), illustrating the prepared sample was in line with the properties of potassium ferrate(Ⅵ).Afterwards, the removal efficiency of arsenic in wastewater by prepared potassium ferrate was investigated. Arsenic-containing wastewater (100mg/L of arsenic) was treated by Oxidation-Flocculating process with potassium ferrate, and the effects of Fe/As mass ratio, pH value, temperature and reaction time on arsenic removal efficiency were studied. Results show that the optimum condition was Fe/As mass ratio of 3, pH value of 5-7, temperature of 25℃and reaction time of 30min.The arsenic concentration after treatment was lower than 10μg/L, reaching Standard for drinking water quality (GB5749-2006). Thermodynamical equilibrium of arsenic(Ⅲ) Oxidation by potassium ferrate and the pC-pH diagrams of the Fe3+-AsO43--H2O system at 298.15K were calculated and drawn according to the theory of thermodynamic equilibrium. As shown from the pC-pH diagrams ,two phases (solid phase of Fe(OH)3(s) and FeAsO4(s) and aqueous phase) coexisted at pH 2.53, where the solubility of FeAsO4(s) is 10-3.06mol/L With pH ranged from 5 to 7, arsenic(Ⅲ) can be completely oxidized to arsenic(Ⅴ) in the form of precipitate by using excess addition of potassium ferrate.Consequently, the preparation method of potassium ferrate by hypochlorite oxidation was improved, the removal efficiency of arsenic by potassium ferrate was investigated, and the mechanism for arsenic removal by potassium ferrate was discussed based on thermodynamic equilibrium. The results of the study could provide theoretical principle for the treatment of arsenic-containing wastewater by potassium ferrate and new thought for industry application of potassium ferrate.