In situ synthesis of ferrate based on calcium hypochlorite and its application to oxidative degradation of methylene blue and chlorpyrifos in water

Ferrate is a promising environment-friendly water treatment agent due to its multiple functions in water treatment. However, its large-scale application in sewage treatment is limited because of its instability in aqueous solution and the high cost of pure ferrate. This study demonstrated an improved wet oxidation process to synthesize ferrate by oxidizing ferric with calcium hypochlorite (Ca(ClO)2) under alkaline conditions. The effects of reagent dosage and temperature on ferrate (VI) were investigated. The optimum condition for ferrate(VI) synthesis was obtained(1 g of Ca(ClO)2, 5 g of NaOH, and 4 g of FeCl3 in 25 mL of water at 20 °C, and stirring for 30 min). Under the optimum condition, the available chlorine and sodium hydroxide amount was about 1/5 and 1/2 of that of traditional sodium hypochlorite oxidation, respectively. The product was characterized by ultraviolet-visible spectroscopy and X-ray photoelectron spectroscopy. It was concluded that ferrate was present in the generated ferrate solution. Fresh ferrate had four absorption peaks in the wavelength range of 500-600 nm (508, 525, 545, and 570 nm), which were two more characteristic absorption peaks than pure ferrate (VI) (508 and 570 mm). Low temperature, airtight and shading conditions are conducive to the preservation of fresh ferrate, and the addition of sodium silicate as a stabilizer can increase its stability. At pH 5.0, fresh ferrate could effectively remove methylene blue (MB) and chlorpyrifos in water, and the average removal rate of MB or chlorpyrifos by fresh ferrate was about 55% higher than that by pure ferrate.