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

Harald Neidhardt; Sebastian Rudischer; Elisabeth Eiche; Magnus Schneider; Emiliano Stopelli; Vu T Duyen; Pham T K Trang; Pham H Viet; Thomas Neumann; Michael Berg

doi:10.1016/j.jhazmat.2021.125128

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

J Hazard Mater. 2021 Jun 5:411:125128. doi: 10.1016/j.jhazmat.2021.125128. Epub 2021 Jan 14.

Authors

Affiliations

¹ Geoecology, Eberhard Karls University Tübingen, 72070 Tübingen, Germany. Electronic address: harald.neidhardt@uni-tuebingen.de.
² Geoecology, Eberhard Karls University Tübingen, 72070 Tübingen, Germany.
³ Institute of Applied Geosciences, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany.
⁴ Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland.
⁵ Key Laboratory of Analytical Technology for Environmental Quality and Food Safety Control (KLATEFOS), VNU University of Science, Vietnam National University, Hanoi, Vietnam.
⁶ Applied Geochemistry, Technical University of Berlin, 10623 Berlin, Germany.

PMID: 33485236
DOI: 10.1016/j.jhazmat.2021.125128

Abstract

Although phosphate (PO₄^3-) 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 PO₄^3- 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 PO₄^3- 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 PO₄^3- 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 PO₄^3- and As and dynamics of the biogeochemical Fe-cycle.

Keywords: Arsenic; Floodplain aquifer; Groundwater; Iron cycle; Phosphate; Redox transition.

Publication types

Research Support, Non-U.S. Gov't