Tracer test with As(V) under variable redox conditions controlling arsenic transport in the presence of elevated ferrous iron concentrations.

To study transport and reactions of arsenic under field conditions, a small-scale tracer test was performed in an anoxic, iron-reducing zone of a sandy aquifer at the USGS research site on Cape Cod, Massachusetts, USA. For four weeks, a stream of groundwater with added As(V) (6.7 muM) and bromide (1.6 mM), was injected in order to observe the reduction of As(V) to As(III). Breakthrough of bromide (Br(-)), As(V), and As(III) as well as additional parameters characterizing the geochemical conditions was observed at various locations downstream of the injection well over a period of 104 days. After a short lag period, nitrate and dissolved oxygen from the injectate oxidized ferrous iron and As(V) became bound to the freshly formed hydrous iron oxides. Approximately one week after terminating the injection, anoxic conditions had been reestablished and increases in As(III) concentrations were observed within 1 m of the injection. During the observation period, As(III) and As(V) were transported to a distance of 4.5 m downgradient indicating significant retardation by sorption processes for both species. Sediment assays as well as elevated concentrations of hydrogen reflected the presence of As(V) reducing microorganisms. Thus, microbial As(V) reduction was thought to be one major process driving the release of As(III) during the tracer test in the Cape Cod aquifer.

Abiotic Fe(III) induced mineralization of phenolic substances.

The redox process between iron(III) (in dissolved form and as the mineral phase ferrihydrite) and phenolic substances has been examined. We investigated the relationship between the structure and reactivity for the dihydrobenzene reductants catechol, hydroquinone and resorcine, and for the 2-methoxyphenol guaiacol with iron(III), by determining the rate of the Fe(III) reduction as well as the production of CO2. This work demonstrates that catechol and guaiacol will be effectively oxidized to CO2 by reducing iron(III). Hydroquinone shows a reduction of iron(III), but no accompanying mineralization could be determined. In contrast, resorcine showed no reaction with Fe(II). The deciding factor on whether or not mineralization occurs were controlled by the position of the hydroxy groups. It is shown that phenolic substances with two hydroxy groups in the orthoposition or at least one hydroxy group and a methoxy group can be oxidized to CO2 while iron(III) is reduced.