Katharine J. Tufano

Arsenic Chemistry in Soils and Sediments

Publisher Summary This chapter explores and describes the biological and chemical processes that control the partitioning of As between the solid and aqueous phase. Strong partitioning of As on soil solids is mostly disrupted by the onset of anaerobic conditions (anaerobiosis), leading to increased aqueous concentrations of As. Variations in As chemistry, compounded by biogeochemical transformations of the soil matrix, upon the onset of anaerobic conditions transpire to produce a convolution of reactions that have varying impacts on As retention. Arsenic desorption upon anaerobiosis is ascribed to both the reduction of As, from arsenate to arsenite, and iron(III)—the latter leading to the reductive dissolution of ferric (hydr)oxides that act as principal sinks of As. The two modes of As reduction, respiratory or detoxification, both require As to be released from the solid phase. The primary reductases for respiratory reduction, ArrA and ArrB, both reside interior to the outer membrane of bacteria, and thus it is likely that As(V) must desorb from the surface, cross the outer membrane, and then undergo reduction.

Nuclear magnetic resonance relaxation measurements as a means of monitoring iron mineralization processes

[1] In this laboratory study, we assessed the measurement of nuclear magnetic resonance (NMR) relaxation times as a means of monitoring iron mineralization processes. We conducted experiments in which columns containing ferrihydrite-coated quartz sand reacted with aqueous Fe(II) solutions to form goethite, lepidocrocite and magnetite. An observed increase in the volume of water relaxing with long relaxation times in the NMR relaxation time distribution corresponds to the formation of goethite and lepidocrocite; a decrease in the average (mean log) relaxation time, and a broadening of the relaxation time distribution, corresponds to the formation of magnetite. These results indicate that NMR relaxation times are sensitive to changes in iron mineralogy and illustrate the potential use of NMR for monitoring iron mineralization processes. Citation: Keating, K., R. Knight, and K. J. Tufano (2008), Nuclear magnetic resonance relaxation measurements as a means of monitoring iron mineralization processes, Geophys. Res. Lett., 35, L19405,