Deep separation of arsenic and alkali from alkaline arsenic containing solution using hydrothermal lime precipitation method

Abstract

A new strategy of lime precipitation of arsenic (As) under hydrothermal conditions was proposed for the treatment of alkaline arsenic-containing wastewater (AAW). On the basis of theoretical analysis, a study was conducted on the reaction law and mechanism of lime precipitation of arsenic in hydrothermal systems, as well as the environmental stability of precipitation products. Results of theoretical calculations showed that increasing the reaction temperature was beneficial the formation of insoluble Ca5(AsO4)3OH, which promoted the removal of arsenic. It is shown that the arsenic concentration could be reduced from 35.38 mg/L to 9.16 mg/L with increasing the reaction temperature from 90 °C to 210 °C. The experimental results further confirmed the above conclusion. When the reaction temperature was increased from 90 °C to 210 °C, the arsenic concentration in the final liquid decreased from 706.21 mg/L to 15.21 mg/L. The leaching toxicity evaluation results of the products showed that the stability of the arsenic precipitates was improved significantly with the hydrothermal conditions. The arsenic leaching concentration for the products obtained at 170 °C was only 2.1 mg/L, which was lower than the U.S. Environmental Protection Agency (USEPA) limit of 5 mg/L for solid waste containing hazardous arsenic. It is found that the improved stability of the arsenic precipitates could be attributed to the increased crystallinity and crystal size of the product obtained under hydrothermal conditions.