PNNL

Abstract

The success and long-term performance of bioremediation processes employing iron-reducing bacteria depends on several factors. A crucial factor is the availability of Fe(III) as an electron acceptor which may be dictated by both chemical (e.g., oxide mineralogy) and physical (distribution of Fe(III) in space) effects. The iron content of subsurface media usually is obtained through different extraction techniques performed in a well-mixed batch experiment. For structured media where preferential flow prevails over the matrix flow, however, the iron content determined from homogenized samples may not well represent the iron content available for microbial activity. Metal reducing bacteria may be physically excluded from a significant fraction of pores due to their sizes. In this study we performed Fe(III) oxide extraction on an intact core of saprolite where intact structure was preserved. An unsaturated flow setup was modified to allow the extraction of oxalate-extractable Fe(III) oxides under two pore tensions, 15 and 0 cm of water. The result suggested the existence of Fe(III) oxide distribution with its mass mainly contained within the finer pore domain of matrix potential larger than 15 cm. Less than 15.5% mass (an upper bound) of oxalate-extractable Fe (III) oxides were present in domains of pore tension less than 15 cm. Hence the use of extraction results from well mixed batch extraction techniques can overestimate the quantity of Fe(III) oxides accessible to bacteria in structured media. To the extent that Fe (III) oxide minerals play an important role in contaminant biogeochemistry and solute transport, the distribution of Fe(III) oxides in structured subsurface media are critical to our understanding of these processes.