Speciation of Aqueous Iron and Arsenic for Assessing Mechanisms of Arsenic Attenuation in Groundwater

Groundwater pollution by natural arsenic can be induced by the reduction of subsurface iron oxides that host arsenic. Characterization of the chemical species of arsenic and iron in groundwater is critical to diagnosing the cause of arsenic mobilization and assessing the potential for downgradient attenuation. A streamlined plan is reported to preserve and analyze aqueous Fe(II), Fe(III), As(III), and As(V). In succinic acid, both Fe(II) and As(III) are stable for 9 days, allowing adequate time for the analyses. The revised Fe speciation is based on the o-phenanthroline (o-phen) method, with limits of quantitation (LOQ) of 0.2 and 0.07 μM for Fe(II) and Fe(III), respectively. This method can tolerate 100× of Ca²⁺, 100× of Mg²⁺, 100× of Mn²⁺, and 500× of F^–. The revised As speciation is based on the analysis of As(V) with mixed mode LC-ESI-MS. The LOQ for As(V) is 0.13 μM. A generic protocol has been developed to prepare synthetic groundwater with the desired compositions of 11 major ions. These methods have been applied to characterize the oxidation and sequestration of Fe(II) and As(III) in synthetic groundwater when exposed to air over 16 h.