Arsenic Speciation and Iron Mineralogy in Sediment from Meghna and Brahmaputra River Banks of Bangladesh

Unusually high concentrations of arsenic appear to form lenses within subsurface sediment of Meghna River bank. The formation of these As-rich strata is attributed to trapping of groundwater arsenic at a redox interface when reducing, As-containing groundwater discharges to oxic river water. The riverbank sediment was characterized for speciation and mineralogy of arsenic and iron to ascertain the possibility that freshly precipitated Fe oxyhydroxide is responsible for arsenic immobilization,.

Speciation of sedimentary As was determined by differential pulse cathodic stripping voltammetry (DPCSV) after 1M anaerobic phosphate extraction and by X-ray absorption spectroscopy (XAS). Within the Meghna River sediments, arsenic species in sediments are mixed As(III) and As(V), and on average, 23±23% and 68±20% of total As, respectively. Within sediments of the Brahmaputra River bank, which contained no sediment arsenic enrichment, As(V) predominates. Because dissolved As(III) in shallow groundwater represents > 90% of dissolved total As, oxidation of As(III) has occurred during groundwater discharge. Using XAS, ferrihydrite is shown to be the dominant Fe mineral in Meghna Riverbank sediment enriched with As, while a variety of Fe minerals including ferrihydrite, biotite, and goethite are present in Brahmaputra Riverbank sediment.

To evaluate the mobility of arsenic sorbed to Fe minerals formed at the river bank, Megna sediments with ~90 mg/kg As were incubated with artificial groundwater for 1 month with or without 1mM lactate. Without lactate, arsenic was not mobilized, whereas a significant amount of sedimentary As was mobilized with lactate through reductive dissolution of Fe oxyhydroxides and reduction of As(V) to As(III) on the surface of sediment. If irrigation pumping in the village reverses groundwater flow direction and draws DOM rich river water to the subsurface, then As may be mobilized and transported to the aquifer on a seasonal time scale.