Couture and two co-authors reported in mid-2010 that adsorption of As onto diagenetic Fe oxyhydroxides controls As mobility at the vicinity of the sediment–water interface of the two lakes with an oxygenated hypolimnion. Recycling of As due to its release to the porewaters by the reductive dissolution of Fe oxyhydroxides upon sediment burial, followed by its upward migration and its adsorption onto the freshly formed Fe oxyhydroxides is responsible for
the As enrichment observed in the surface sediments of these lakes. Peaks in [As] porewater profiles and inverse diagenetic modeling suggest that As adsorption onto the authigenic Fe oxyhydroxides is relatively slow, in contrast with laboratory adsorption experiments carried out with pure As solutions.
The authors inferred, based on the report that NOM greatly decreases the rate of As adsorption onto hematite (Redman et al., 2002), that the presence of natural organic matter in the studied lake sediments is responsible for the slow adsorption of As onto authigenic Fe oxyhydroxides. In line with the work of Redman et al. (2002), further experiments should be performed to evaluate the importance of natural organic matter in slowing down As adsorption onto Fe oxyhydroxides representative of the authigenic forms present in lake sediments (e.g., ferrihydrite, lepidocrocite).
The authors suggested that kinetic treatment of As adsorption onto Fe oxyhydroxides be considered in the development of coupled reaction-transport models to describe As behavior in sediments. In contrast to the other previously published models, the thermodynamic model proposed by Helz and Tossell
(2008), identified as Model 2 in this study, allows for the formation of AsV thioanions in sulfidic waters.
The authors' field-derived observations are that As removal in sulfidic waters occurred only when AsV oxythioanions were (according to Model 2) an important fraction of total dissolved As and when saturation with respect to FeSm(s) was reached. Based on these findings, the authors concluded that adsorption of AsV onto poorly crystalline Fe sulfide stands as a plausible mechanism for As removal from the sulfidic waters of all the lakes studied. This inference has potential implications for As transport in anoxic waters and should be tested in a wider variety of natural environments. It should also be supported by measurements of dissolved As speciation and by characterization of the nature of the sorbent and of the redox state of solid As in sulfidic environments.
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