Biogeochemical processes controlling the speciation and transport of arsenic within iron coated sands

Iron (hydr)oxides exert a domineering control on the dissolved concentration and transport of arsenic within surface and subsurface environments. Here we examine desorption and transport of As(V) (arsenate) and As(III) (arsenite) within ferrihydrite-coated sand under hydrodynamic conditions during biotic and abiotic induced transformations. For sands containing active cultures of the Fe(III) and As(V) respiring bacterium Sulfospirillium barnesii (an iron and arsenic respiring organism), an initial pulse (within 2 days) of arsenic desorption was followed by an exponential decrease (tailing) in eluted arsenic concentration for both As(V) and As(III)-in both columns, As(III) was the only species eluted after 2 days of reaction. The pulse in arsenic release was coincident with changes in iron mineralogy brought about by Fe(II) production. For As(V) loaded columns containing a culture of As(V) respiring Bacillus benzoevorans (an organism capable of As(V) respiration but incapable of dissimilatory Fe(III) reduction), the major pulse of arsenic release was delayed but remained elevated, with As(III) being the predominant species eluted after 1 day of reaction. Arsenic desorption in sterile columns, however, was even greater than the biologically active systems for As(V) and particularly As(III). Furthermore, despite As(III) having an adsorption maximum on ferrihydrite nearly twice that of As(V) at pH 7, As(III) desorption was more rapid and extensive than for As(V). Batch desorption experiments illustrate that Fe(II) and cellular organic materials aided the retention of arsenic (i.e., they depressed the extent of desorption), particularly for As(V). Our findings therefore suggest that arsenic desorption from ferrihydrite (or other labile iron oxides) and transport is greatest at the onset of an oxidizing to reducing transition and are promoted by mildly reducing conditions where arsenic reduction occurs but limited aqueous Fe(II) concentrations result.