Peat formation concentrates arsenic within sediment deposits of the Mekong Delta

Abstract

This is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online at Geochimica et Cosmochimica Acta, published by Elsevier. Copyright restrictions may apply. doi: 10.1016/j.gca.2014.10.021 Mekong River Delta sediment bears arsenic that is released to groundwater under anaerobic conditions over the past several thousand years. The oxidation state, speciation, and distribution of arsenic and the associated iron bearing phases are crucial determinants of As reactivity in sediments. Peat from buried mangrove swamps in particular may be an important host, source, or sink of arsenic in the Mekong Delta. The total concentration, speciation, and reactivity of arsenic and iron were examined in sediments in a Mekong Delta wetland by X-ray fluorescence spectrometry (XRF), X-ray absorption spectroscopy (XA S), and selective chemical extractions. Total solid-phase arsenic concentrations in a peat layer at a depth of 6 m below ground increased 10-fold relative to the overlying sediment. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that arsenic in the peat was predominantly in the form of arsenian pyrite. Arsenic speciation in the peat was examined further at the micron-scale using XRF and X-ray absorption near-edge structure (XANES) spectroscopy coupled with principal component analysis. The multiple energy XRF mapping and XANES routine was repeated for both iron and sulfur phase analyses. Our XRF/ XANES analyses confirm arsenic association with pyrite -a less reactive host phase than iron (hydr)oxides under anaerobic conditions. The arsenian pyrite likely formed upon deposition/formation of the peat in a past estuarine environment (~ 5.5 ka BP), a process that is not expected under current geochemical conditions. Presently, arsenian pyrite is neither a source nor a sink for aqueous arsenic in our sediment profile, and under present geochemical conditions represents a stable host of As under the reducing aquifer conditions of the Mekong Delta. AbstractThis is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online at Geochimica et Cosmochimica Acta, published by Elsevier.

Indigenous arsenic(V)‐reducing microbial communities in redox‐fluctuating near‐surface sediments of the Mekong Delta

“…The surface sediments are reduced upon wetting during the monsoon season, giving rise to biogeochemically diverse conditions, which is reflected in the diversity of arrA phylotypes. However, deeper depths (>4 m) remain reduced throughout both the dry and wet seasons, where As concentrations are no longer strongly correlated with Fe and alkalinity (Kocar et al ., ; Stuckey et al ., ). Further, because arsenic is present mostly in reduced forms [as As(III)] at these depths, As(V)‐respiration plays a minor role in As cycling.…”

Biomarker-indicated extent of oxidation of plant-derived organic carbon (OC) in relation to geomorphology in an arsenic contaminated Holocene aquifer, Cambodia

“…This is important because many studies of arsenic in groundwater at this site have used KS to interpret these sediments due to the similarities in stratigraphy across much of the Kandal Province area and at KS 20,22,23,35,45,51–56 . This means that, despite its quality, we suggest that KS is not a suitable framework for this area of Kandal Province as the site is under a different geomorphological system.…”

Reactivity and speciation of mineral-associated arsenic in seasonal and permanent wetlands of the Mekong Delta

“…Synchrotron powder XRD was performed at beam line 11-3 at SSRL with an incident beam energy of 12735 eV. Samples were prepared as described previously (Stuckey et al, 2015). Peak identification was performed using JADE XRD Pattern Processing Ver.…”

“…Similar to analyses described in Stuckey et al (2015), bulk As X-ray absorption spectroscopy (XAS) was performed at beam line 11-2 at the Stanford Synchrotron Radiation Laboratory (SSRL). Briefly, energy selection was maintained by a Si (220) monochromator calibrated to 11874.0 eV for the As K-edge of Na3AsO4.…”