Microbially-mediated release of As from Mekong Delta peat sediment Asta Andrés, María Pilar Frutschi, Manon Viacava, Karen Loreggian, Luca Le Pape, Pierre Le Vo, Phu Fernández, Ana María Morin, Guillaume Bernier-Latmani, Rizlan Pyrite oxidation Pyrite dissolution Ammonium This study is funded by the Swiss National Science Foundation (grant number 200021_157007), by the CODEV-EPFL Seed Money Fund, and by RESCIF-EPFL under the framework of a CARE-RESCIF initiative. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Peat layers within alluvial sediments are considered effective arsenic (As) sinks under reducing conditions due to the binding of As(III) to thiol groups in natural organic matter (NOM) and the formation of As-bearing sulfide phases. However, their possible role as sources of As for anoxic groundwaters remains unexplored. Here, we perform laboratory experiments to provide evidence for the role of a sediment peat layer in releasing As. Our results show that the peat layer, deposited about 8,000 years ago in a paleomangrove environment in the nascent Mekong Delta, could be a source of As to porewater under reducing conditions. X-ray absorption spectroscopy (XAS) analysis of the peat confirmed that As was bound to NOM thiol groups and incorporated into pyrite. Nitrate was detected in peat layer porewater, and flow-through and batch experiments evidenced the release of As from NOM and pyrite in the presence of nitrate. Based on poisoning experiments, we propose that the microbially mediated oxidation of arsenic-rich pyrite and organic matter coupled to nitrate reduction releases arsenic from this peat. Although peat layers have been proposed as As sinks in earlier studies, we show here their potential to release depositional- and/or diagenetically-accumulated As. 2026-02-18T09:46:42Z 2026-02-18T09:46:42Z 2019 journal article Published version: Asta, Maria P. et al. Microbially-mediated release of As from Mekong Delta peat sediment. Environmental Science & Technology Vol 53/Issue 17. August 7, 2019. DOI:10.1021/acs.est.9b02887 https://hdl.handle.net/10481/111164 10.1021/acs.est.9b02887 eng http://creativecommons.org/licenses/by-nc-nd/4.0/ open access Attribution-NonCommercial-NoDerivatives 4.0 Internacional American Chemical Society