Mineralogical Hosts of Reactive Arsenic within Reduced Deltaic Soils and Sediments of Southeast Asia.

Millions of people in the deltaic regions of S/SE Asia regularly consume arsenic contaminated groundwater. Within the Mekong Delta of Cambodia, for example, arsenic persists within the groundwater despite being flushed by several pore volumes of recharge. The identity and reactivity of the minerals contributing to the persistence of arsenic in the deltaic aquifers remain elusive. Here we seek to define the molecular form of the arsenic and its host phases along two defined flow paths: i) in a seasonally saturated wetland (Seasonal Site) and ii) in a continuously saturated wetland (Flooded Site).  Sediment cores were retrieved from depths of 0.2 to 6 m at the Seasonal Site and from 0.2 to 4 m at the Flooded Site. Sediments were dried under 95%N₂/5%H₂ atmosphere. The clay size fraction (< 2 μm) was isolated by sonication for 10 h followed by gravity separation in anoxic deionized water. Synchrotron XRD and Fe EXAFS spectroscopy identified montmorillonite and magnetite as dominant Fe-bearing minerals in the clay size fraction of nearly all sediments analyzed. Citrate-bicarbonate-dithionite (CBD) extractions and 1 M HCl extractions each solubilized ~ 0.1% of the total iron in the sediments, suggesting the Fe mineralogy is dominated by minimally reactive montmorillonite. Moreover, CBD and 1 M HCl each extracted ~ 12% (on average) of the total arsenic, suggesting the small As pool is hosted by a relatively reactive, reducible form of iron. Arsenic XANES spectroscopy showed As(V) from 0.2 to 2 m, a mixture of As(V) and As(III) from 2 to 6 m, and arsenic sulfide in peat at 6 m in the Seasonal Site.  The Flooded Site contained predominantly As(III) down to 4 m. Importantly, there remains a reactive fraction of arsenic within a reducible iron fraction of the soils/sediments.