Concentration of soluble and nonsoluble zinc-based impurities by unidirectional freezing: Basis for a method of sludges treatment

Heavy metals are known to be significant pollutants in sludges produced by wastewater treatment. A new technique to make the metallic contaminants migrate is to submit such sludges to a slow freezing in order to purify the frozen region and to concentrate impurities in the non-frozen zone. In this paper, we apply this method to model contaminated sludges consisting of clayey suspensions charged with a heavy metal (zinc). The effect of some physicochemical parameters (zinc and clay concentrations, pH) on the effectiveness of the method is experimentally investigated. The pH is the most relevant parameter and the highest purification rate (similar to 80%) is obtained for pH similar to 9.