Assessment of metals mobility during the alkaline treatment of highly acid phosphogypsum leachates.

This research evaluates the feasibility of an alkaline treatment system for highly acid leachates from a phosphogypsum stack located in an estuarine environment degraded by such pollution. The presented methodology consists of the addition of a Ca(OH) solution to the different types of phosphogypsum-related acidic leachates with the aim to increase their pH and subsequently, to provoke the precipitation and immobilization of the dissolved contaminants. In fact, phosphates and fluorides reached removal of 100% and 90%, respectively. As regards metals, removal values close to 100% were reached for Fe, Al, Cr, Cd, U and Zn, whereas it did not seem to be totally effective for other elements such as As (removal of 57-82%) and Sb (4-36%). The decrease of contaminant concentrations was caused probably by co-precipitation and/or adsorption to phosphate phases, together with by fluoride precipitation. The solid phases formed during the treatment were subjected to two standard leaching tests (EN 12457-2 from the EU and TCLP from the US) in order to conduct a risk and management assessment. In this context, some of the precipitates formed during the treatment would be classified as hazardous wastes, due to the high concentration of As leached. Moreover, the potential economic costs of a convectional active treatment system were also explored. This study sets the basis for a new research line with the aim to minimise the impact of the phosphogypsum stacks worldwide to their adjacent environment.