Environmental implications of a Phosphogypsum disposal area (Huelva, SW Spain): weathering processes and mobiility of contaminants

Abstract

The Ph.D. Thesis examines the possible pollution pathways originating the phosphogypsum leachates using stable isotopes (δ18O, δ2H, and δ34S) as geochemical tracers in order to evaluate the relationship between leachates and weathering agents of the stack. Quantification of the contribution of all possible end-members to the phosphogypsum leachates was also conducted using ternary mixing of the isotopic tracers. Accordingly, most of the outflows proved to be connected with the Tinto River and seawater end-members rather than the process water, denoting an estuarine influence and as such, presenting a different weathering model of the phosphogypsum stack, which is subjected to an open system (Papaslioti et al., 2018a). Hence, the access of intertidal water inside the phosphogypsum stack, for instance through secondary tidal channels, is the main responsible for the weathering of the waste (Papaslioti et al., 2018a). Therefore, these findings point out the ineffectiveness of the current restorations of the waste in depth and the need for a different remediation approach, because otherwise phosphogypsum leachates will continue to discharge and contaminate the estuarine environment. Phosphogypsum stacks lie within the tidal prism of the estuary resulting to the interaction of acid wastewaters and seawater. Therefore, the effects of pH increase on contaminant mobility in phosphogypsum leachates by seawater mixing were also elucidated in the current thesis. Different types of acid leachates from the phosphogypsum stack were mixed with seawater to gradually achieve pH 7. Concentrations of Al, Fe, Cr, Pb and U in mixed solutions significantly decreased with increasing pH by sorption and/or precipitation processes. Nevertheless, new insights were provided on the high contribution of the phosphogypsum stack to the release of other toxic elements (Co, Ni, Cu, Zn, As, Cd and Sb) to the coastal areas, as 80-100% of their initial concentrations behaved conservatively in mixing solutions with no participation in sorption processes (Papaslioti et al., 2018b). These toxic elements remain mobile even after mixing with the alkaline seawater and finally end up to the Atlantic Ocean contributing significantly to the total metal loads and threatening the environmental conditions of the littoral. So, these findings highlighted the urgent need to adopt new effective restoration measures to minimize the impact of the phosphogypsum leachates on the estuarine environment and subsequently on the Atlantic Ocean. The studied phosphogypsum stack, along with its basement formed by marsh soil, is a redox sensitive system (Lottermoser, 2007) and thus, the mobility of the contaminants and the related geochemical processes were studied under experimentally-controlled redox oscillations; phosphogypsum and marsh soil suspensions were subjected to six 7-day cycles of reduction and oxidation alternately, using a bioreactor system as described by Parsons et al., 2013. According to th...