Comparison of heavy metal removals from aqueous solutions by chemical precipitation and characteristics of precipitates

Abstract

Typical chemical precipitation methods using lime (Ca(OH)2), soda ash (Na2CO3) and sodium sulfide (Na2S) for removals of heavy metals (i.e. Zn (II), Cu (II) and Pb (II)) from aqueous solutions were compared by jar tests. A focus was especially given to particle size differences and chemical phase conversion of precipitates. A removal of 99.99% from aqueous solutions with three precipitants was achieved for copper and zinc at an initial concentration of 100 mg/L. And lead was efficiently removed (99.75%) by sodium sulfide. In contrast, the maximal lead removals with lime or soda ash precipitation were only 76.14% and 97.78%. The mean particle size of precipitates was in the range of 55 nm–45 μm, depending on properties of precipitants and heavy metal to precipitant ratios. The settling performance of the sludge derived from precipitation was dominated by particle size and Zeta-potential of precipitates. It was observed that ultra-fine copper sulfide particles resulted from the precipitation were around 55 nm and did not settle in 12 h due to electrostatic repulsion force between particles. The main compounds in the sludge obtained from precipitation were metal hydroxides and metal sulfides. However, spontaneous dehydration of metal hydroxide, oxidation of sulfide and atmospheric carbonation were identified by means of XRD and thermal analyses, which is invaluable to the disposal and utilization of the sludge.