Microbial reduction of Fe(III) coupled to the biodegradation of isosaccharinic acid (ISA)

Abstract

Isosaccharinic acid (ISA) forms through alkaline hydrolysis of cellulose in intermediate level nuclear waste and is a strong complexant of metals with the potential to mobilize priority radionuclides in an underground geological disposal facility (GDF). In this study, microbial ISA degradation was studied under anaerobic conditions with Fe(III) oxyhydroxide as the terminal electron acceptor at pH 7 to 10, representative of conditions in the geosphere surrounding a geological disposal facility. A multidisciplinary approach was used to study the fate and limits of ISA biodegradation, including mineralogical analyses (XRD, ESEM and TEM), alongside geochemical profiling of ISA and its biodegradation products, and microbial community analysis by 16S rRNA gene sequencing. Under the conditions imposed, ISA degradation was constrained to a pH limit of ≤9 and was degraded via Fe(III) reduction and fermentation. Biominerals, resulting from ISA degradation, were analyzed. These revealed two Fe(II) minerals, siderite [FeCO3] and vivianite [(Fe3PO4)2·8H2O], not normally associated with the microbial reduction of ferrihydrite in laboratory cultures, possibly as a result of chelation of ISA with Fe(III) that made the electron acceptor more soluble. After the enrichment at pH 7, a complex microbial consortium, including Clostridia and close relatives to known Fe(III)-reducing bacteria Geobacter sp. and Rhodoferax ferrireducens, was associated with these activities. The results are discussed in the context of geological disposal safety case development, including the potential impact on priority radionuclides including U(VI), Np(V), and Tc(VII), which are highly susceptible to reductive immobilization when in contact with Fe(II)-bearing biominerals.