Study On The Technic Of Arsenic Removal By Lime Precipitation And The Mechanism Of The Solution Chemistry

The Problem of arsenic contamination especially water pollution caused widespread concern of environmental scientists around the world, Since the arsenic compounds had serious toxicity and were widely used in industrial and agricultural production. So, It's meaningful to research the technology of arsenic removal from the arsenic wastewater.In this studied, the effect of pH, the Ca/As molar rations , the precipitate time and the temperature on arsenic removal by lime precipitation was investigated, with the Ca(OH)₂ solution for precipitation agent and the sodium arsenate solution for simulation arsenic water. To study the mechanism of arsenic removal through the way of the lime precipitation, the form of arsenate calcium precipitate was analyed from the solution chemistry.The results showed that, the removal rate of arsenic rise to 99.05% while controlled the pH to 12, the mole ratio of Ca/As to 6, the time of precipitate to 48 hours and the temperature to 25℃. Besides, the effluent concentration of arsenic wastewater could be significantly reduced when added the ferric to high concentration of arsenic wastewater which have been treated with Ca(OH)₂ solution, then met the requirements of Integrated Wastewater Discharge Standard 0.5mg/L. However, the removal rate of arsenic didn't changed while added the aluminum to the arsenic wastewater which have been treated with Ca(OH)₂ solution. The Ca₃(AsO₄)₂,Ca5(AsO₄)3(OH) was the stable form of the arsenate calcium precipitate. There were another forms comparable unstable such as calcium hydrogen arsenate (CaHAsO₄), calcium hydrogen arsenite (Ca(HAsO₃)₂), calcium arsenic trioxide (Ca(AsO₂)₂), they would converted to Ca₃(AsO₄)₂ in certain condition. The main factor affected the equilibrium system of precipitation– dissolution for the arsenic removal through the way of the lime precipitation was the common ion effect and the acid effect. The effect of adding in ferric on the removal rate of arsenic depended on the pH value in the solution. The calcium arsenite can convert to the Ferric arsenate in certain condition. The main factor for aluminum on the removal rate of arsenic was adsorption. Ca₃(AsO₄)₂ was the main form of arsenate calcium precipitate when the aluminum was added to the solution of arsenic removal by lime precipitation.