In situ synthesis of ferrate based on calcium hypochlorite for organic matter removal

Abstract

Ferrate is a promising environment-friendly water-treatment agent due to its multiple function. 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 for synthesising ferrate by oxidising ferric with calcium hypochlorite (Ca(ClO)2). The effects of reagent dosage and temperature on ferrate (VI) were investigated. The optimum conditions for ferrate (VI) synthesis was obtained (1 g of calcium hypochlorite, 5 g of sodium hydroxide (NaOH) and 4 g of iron (III) chloride (FeCl3) in 25 ml of water at 20°C and stirring for 30 min). Under the optimum conditions, the available chlorine and sodium hydroxide amount was about one-fifth and one-half of that of traditional sodium hypochlorite oxidation, respectively. The product was characterised by ultraviolet–visible spectroscopy and X-ray photoelectron spectroscopy. It was concluded that ferrate was present in the generated ferrate solution. Low-temperature, airtight and shading conditions and the addition of sodium silicate are conducive to the preservation of fresh ferrate. 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 was about 55% higher than that by pure ferrate.