PNNL

Abstract

The reductive biotransformation of 6-line ferrihydrite located within porous silica (intragrain ferrihydrite) by Shewanella oneidensis MR-1 was investigated and compared to the behavior of 6-line ferrihydrite in suspension (free ferrihydrite). The effect of buffer type (PIPES and NaHCO3) and phosphate (P) on the extent of reduction and formation of Fe(II) secondary phases was investigated under anoxic conditions. Electron microscopy and micro X-ray diffraction were applied to evaluate the morphology and mineralogy of the biogenic precipitates and to study the distribution of microorganisms on the surface of porous silica after bioreduction. Kinetic reduction experiments with free and intragrain ferrihydrite revealed contrasting behaviour with respect to the buffer and presence of P. The overall amount of intragrain ferrihydrite reduction was less than that of free ferrihydrite [at 5 mmol L-1 Fe(III)T]. In the intragrain ferrihydrite suspensions, 200-300 µmol L-1 dissolved Fe(III) was released during the initial stages of incubation; no Fe(III)aq was detected in the free ferrihydrite suspensions. Reductive mineralization was not observed in the intragrain ferrihydrite incubations without P, and all biogenic Fe(II) concentrated in the aqueous phase. Distinctive surface precipitates of Fe(II) phosphates with spherical morphology were observed on porous silica when P was present. These precipitates were well colonized by microorganisms and fragments of extracellular materials at the end of incubation.