Influence of sorption processes by iron oxides and algae fixation on arsenic and phosphate cycle in an acidic estuary (Tinto river, Spain)

Inorganic arsenic and phosphate distributions have been studied in the acidic mixing zone of the Tinto river in November 1996, June 1997, and April 1998. This mixing zone receives high inputs of As, PO3−4 and Fe in relation with acid mine drainage and run-off from phosphogypsum waste. In the early stage of mixing the dissolution of detritic Fe phases (pyrite and oxides) releases Fe and As to water. This process is counterbalanced by removal due to precipitation of Fe-oxides and Fe-organic complexes and algae fixation. In autumn, the amount of algae is negligible and a release of As, Fe and PO3−4 to the dissolved phase was observed. As a consequence, high As and PO3−4 concentrations are registered in the water (up to 43 nM for As and 330 μM for PO3−4). In spring, the removal dominates in relation with high algae productivity. As a result As and PO3−4 are depleted in the dissolved phase in spring compared to autumn and high concentrations of As (up to 1530 μg g−1 and 700 μg g−1 in June and in April, respectively) were observed in the suspended particulate matter. In autumn dissolved As is correlated with Fe whilst in spring As behaves in a similar way as PO3−4.