Phosphate immobilisation dynamics and interaction with arsenic sorption at redox transition zones in floodplain aquifers: Insights from the Red River Delta, Vietnam

Although phosphate (PO43-) may play a decisive role in enriching toxic arsenic (As) in the groundwater of many Asian deltas, knowledge gaps exist regarding its interactions with As. This study investigates the simultaneous immobilisation of PO43- and As in aquifer sediments at a redox transition zone in the Red River Delta of Vietnam. The majority of PO43- and As was found to be structurally bound in layers of Fe(III)-(oxyhydr)oxide precipitates, indicating that their formation represents a dominant immobilisation mechanism. This immobilisation was also closely linked to sorption. In the surface sorbed sediment pools, the molar ratios of total P to As were one order of magnitude higher than found in groundwater, reflecting a preferential sorption of PO43- over As. However, this competitive sorption was largely dependent on the presence of Fe(III)-(oxyhydr)oxides. Ongoing contact of the aquifer sediments with iron-reducing groundwater resulted in the reductive dissolution of weakly crystalline Fe (III)-(oxyhydr)oxides, which was accompanied by decreased competition for sorption sites between PO43- and As. Our results emphasise that, to be successful in the medium and long term, remediation approaches and management strategies need to consider competitive sorption between PO43- and As and dynamics of the biogeochemical Fe-cycle.

Automated summary

This study found that the majority of PO43- and As in groundwater sediments in the Red River Delta of Vietnam are structurally bound in layers of Fe(III)-(oxyhydr)oxide precipitates, indicating their formation represents a dominant immobilisation mechanism. Competitive sorption between PO43- and As largely depends on the presence of Fe(III)-(oxyhydr)oxides, and ongoing redox processes may decrease competition for sorption sites between the two.