Defining the Controlling Factors of Organic Carbon and Iron (Hydr)Oxide Reactivity On Arsenic Release In Mekong Delta Sediments.

Drinking of arsenic-contaminated groundwater has lead to the chronic poisoning of tens of millions of people throughout the large deltas of S/SE Asia. Under the anaerobic conditions of these deltaic sediments, microbially driven oxidation of organic carbon coupled to the dissimilatory reductive dissolution of arsenic-bearing iron (hydr)oxides causes the transfer of arsenic from the solid to the aqueous phase. The reactivity and quantity of organic carbon and arsenic-bearing iron (hydr)oxides dictate the rate of arsenic release from soil/sediment to pore-water. Methods exist for quantifying the reactivity of iron (hydr)oxides, although they have been applied sparingly for predicting arsenic release; however, no widely-accepted method exists for quantifying organic carbon reactivity. Moreover, methods are lacking for measuring the reactivity of organic carbon and arsenic in the field. Here we developed proxies for the reactivity of organic carbon and arsenic in Mekong Delta sediments in Cambodia. The reactivity of organic carbon and As-bearing iron (hydr)oxides were examined along two contrasting sediment profiles: 1) a (near) permanently flooded wetland having sustained anaerobic conditions with co-burial of organic carbon and arsenic bearing sediment and 2) a seasonally flooded wetland experiencing aerobic-anaerobic cycles and less carbon input. Samples were retrieved through excavation from the surface to 6 m depth. Incubations of fresh sediment with sterile, anoxic groundwater medium (pH 7.1) were initiated in the field immediately upon sediment collection under nitrogen atmosphere. Prior to sediment addition, eleven treatment additions to the groundwater medium were performed in triplicate: 0.1, 1, and 10 mM glucose, 6.67 mM lactate + 10 mM acetate + 10 mM ethanol, 0.035 and 0.35% Fe³⁺ (w/w) (as As-loaded goethite), 3.5% Fe³⁺ (both as As-loaded goethite and as As-loaded Al-substituted ferrihydrite), 10 mM glucose + As-loaded goethite (3.5% Fe³⁺), 10mM glucose + As-loaded Al-substituted ferrihydrite (3.5% Fe³⁺), and no amendment. Each of the eleven treatments had an abiotic counterpart achieved by antibiotic addition. Iron (hydr)oxide addition alone did not stimulate arsenic or iron(II) release at both sites. In contrast, high rates of organic carbon (e.g., glucose) alone increased aqueous arsenic and iron(II) concentrations, suggesting organic matter reactivity governs the arsenic release rate in these sediments. The addition of organic carbon and iron hydr(oxides) together released iron(II), but not arsenic, suggesting the added iron (hydr)oxides sequestered any liberated arsenic.  Organic matter reactivity is the rate limiting factor controlling arsenic release. We then couple the incubations with detailed chemical (spectroscopic and microscopic) analysis of the As-bearing iron (hydr)oxides and organic matter.  Collectively, these measurements allow parameterization of key sediment reactivity essential for spatial and temporal prediction of arsenic fate.