Environmental implications of a phosphogypsum disposal area (Huelva, SW Spain)Weathering processes and mobility of contaminants

Laburpena

ABSTRACT TESEO Phosphogypsum (mainly gypsum) is a highly acidic waste by-product produced by the phosphate fertiliser industry through the wet chemical digestion of phosphate ore (mainly apatite). It is usually stack-piled near coastal areas worldwide or even thrown directly to the sea, threatening the adjacent environment (Tayibi et al., 2009), as it is considered a major potential hazard with high concentrations of contaminants and radionuclides containing many dangerous chemical reagents (Rutherford et al., 1994; Lottermoser, 2010). A huge waste facility of phosphogypsum (100 Mt; 1200 ha), stacked from 1968 until 2010, is located near the Atlantic coast of SW Spain, in an estuary formed by the confluence of the Odiel and Tinto Rivers (Huelva province). Phosphogypsum is stored in piles on the salt marshes of the Tinto River without any type of isolation, very close to the city of Huelva. The piles contain highly-contaminated groundwater that flow laterally and reach the edge of the stack forming the ''edge outflows'', which are acidic leakages that continuously pollute the estuary until nowadays (Pérez-López et al., 2015; 2016). Another source of contamination is the water stored on the surface of the piles, known as process water, which was used to slurry the phosphogypsum and to transport it from the industry to the stack. Preliminary restorations have taken place in some parts of the phosphogypsum disposal area and similar ones are planned for the future in the unrestored areas. Those restorations are based on a technical report that recognises process water as the main washing agent of phosphogypsum (Junta de Andalucía, 2009). According to this previous weathering model, the process water ponded on the surface of the stack was thought to be the main leaching agent through its infiltration and subsequently the main component of the leakages emerging as the edge outflows. However, this weathering model is questionable considering that some supposedly restored zones, i.e. without ponded process water, still discharge highly-polluted edge outflows to the estuary (Pérez-López et al., 2015; 2016). In this context, the present Ph.D. Thesis focused on three lines of investigation including (i) the weathering processes occurring in the phosphogypsum stack, and on the metal mobilisation (ii) under seawater mixing with the phosphogypsum leachates and (iii) under redox oscillations in the phosphogypsum and the salt marshes. 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 the main outcomes of the last investigation line of the thesis, Eh-pH conditions and Fe (and less S) precipitation or release during redox conditions control the geochemical processes and the mobility of the contaminants; although important metal immobilisation was not observed under the studied conditions. The formation of Fe3+ oxyhydroxides was favoured during oxic conditions following Fe oxidation, mostly at the phosphogypsum and less at the marsh soil due to the lower pH. The expected subsequent precipitation process of metal sulphides during anoxic conditions -after the release of Fe and other metals by reductive dissolution processes- was masked by the dominant precipitation of Fe phosphates that mainly controlled the behaviour of the metals. Nevertheless, the microbial activity of sulphate-reducing bacteria at the end of the experiment appeared to enhance and be consistent with the rare cases of sulphide precipitation under anoxic conditions, and it should be considered for similar future studies and potential treatment plans for the contaminants related to the phosphogypsum waste.