Effects of redox oscillations on the phosphogypsum waste in an estuarine salt-marsh system

Salt marshes are natural deposits of heavy metals in estuarine systems, where sulphide precipitation associated with redox changes often results in a natural attenuation of contamination. In the present study, we focus on the effects of variable redox conditions imposed to a highly-polluted phosphogypsum stack that is directly piled over the salt marsh soil in the Tinto River estuary (Huelva, Spain). The behaviour of contaminants is evaluated in the phosphogypsum waste and in the marsh basement, separately, in controlled, experimentally-induced oscillating redox conditions. The results revealed that Fe, and to a lesser extent S, control most precipitation/dissolution processes. Ferric iron precipitates in the form of phosphates and oxyhydroxides, while metal sulphide precipitation is insignificant and appears to be prevented by the abundant formation of Fe phosphates. An antagonistic evolution with changing redox conditions was observed for the remaining contaminants such as Zn, As, Cd and U, which remained mobile in solution during most of experimental run. Therefore, these findings revealed that high concentrations of phosphates inhibit the typical processes of immobilisation of pollutants in salt-marshes which highlights the elevated contaminant potential of phosphogypsum wastes on coastal environments.This work was supported by the Regional Government of Andalusia through the ''Junta de Andalucía'' research projects and RNM-131 and by the Spanish Ministry of Economy and Competitiveness through the research project CAPOTE (CGL2017-86050-R). The authors are very grateful to the funding support for the Committee of Experts on “The environmental diagnosis and the proposal of measures of restoration of the phosphogypsum stacks of Huelva”, appointed by the City Hall of Huelva. Dr. E. M. is grateful to the funding support of International Mobility launched by the University of Granada and CEI-BIOTIC Granada 2015/2016. Dr. A. Parviainen acknowledges the ‘Juan de la Cierva – Incorporación’ (IJCI-2016-27412) Fellowship and Dr. C. Marchesi the “Ramón y Cajal” (RYC-2012-11314) Fellowship financed by the Spanish Ministry of Economy and Competitiveness (MINECO)