Microbial degradation of isosaccharinic acid at high pH

Abstract

Intermediate-level radioactive waste (ILW), which dominates the radioactive waste inventory in the United Kingdom on a volumetric basis, is proposed to be disposed of via a multibarrier deep geological disposal facility (GDF). ILW is a heterogeneous wasteform that contains substantial amounts of cellulosic material encased in concrete. Upon resaturation of the facility with groundwater, alkali conditions will dominate and will lead to the chemical degradation of cellulose, producing a substantial amount of organic co-contaminants, particularly isosaccharinic acid (ISA). ISA can form soluble complexes with radionuclides, thereby mobilising them and posing a potential threat to the surrounding environment or ‘far field'. Alkaliphilic microorganisms sampled from a legacy lime working site, which is an analogue for an ILW-GDF, were able to degrade ISA and couple this degradation to the reduction of electron acceptors that will dominate as the GDF progresses from an aerobic ‘open phase' through nitrate- and Fe(III)-reducing conditions post closure. Furthermore, pyrosequencing analyses showed that bacterial diversity declined as the reduction potential of the electron acceptor decreased and that more specialised organisms dominated under anaerobic conditions. These results imply that the microbial attenuation of ISA and comparable organic complexants, initially present or formed in situ, may play a role in reducing the mobility of radionuclides from an ILW-GDF, facilitating the reduction of undue pessimism in the long-term performance assessment of such facilities.

Microbial Community Evolution Is Significantly Impacted by the Use of Calcium Isosaccharinic Acid as an Analogue for the Products of Alkaline Cellulose Degradation

“…Irrespective of the source of ISA (single/mixed diastereomers), the degradation rates observed here were similar. This group of findings suggest that the metabolism of both ISA diastereomers occurs through the same metabolic pathway, however they also conflict with previous studies which suggest that α-ISA is not degradable without the presence of a terminal electron acceptor [13]. …”

“…This has resulted in a number of studies attempting to identify the impact of microbial intrusion and metabolism of ISA’s. The studies of Rout et al [8–10] and Charles et al [11] adopted the use of a synthetic CDP leachate [12] containing a mixture of both diastereomers of ISA, whilst the work of Bassil et al [13] and Kuippers et al [14] have chosen α-ISA as carbon source. A number of these studies have used the sediment from the same geographic location [15].…”

Floc Formation Reduces the pH Stress Experienced by Microorganisms Living in Alkaline Environments

“…These conditions will also result in alkaline hydrolysis of the cellulose component of the ILW, producing a range of low-molecular-weight organic compounds collectively known as cellulose degradation products (CDP) (5). The biodegradation of CDP has received considerable attention in recent years with a number of authors reporting alkaliphilic degradation under a wide range of growth conditions (6,7), including alkaliphilic methanogenic consortia operating at a pH of 11.0 (8). In some cases, these alkaliphilic communities have been shown to form flocs where the bacteria are encased in a matrix of extracellular polymeric substances (EPS) (9).…”

Herbert's Quarry, South Wales – an analogue for host-rock alteration at a cementitious radioactive waste repository?

“…The presence of Ca-rich, high-pH streamwaters at Herbert's Quarry is balanced by the the widespread CaCO 3 precipitation through interaction with atmospheric CO 2 , paucity of silicate rock, pervasive oxic conditions and low water temperature, which limits the site as an analogue for a cement leachate plume in the CDZ around a cementitious GDF. However, the site may hold interest for microbial studies at high pH as alkaliphilic bacteria may be present which are starting to be recognized as important components for deep disposal (Rizoulis et al, 2012;Bassil et al, 2014;Williamson et al, 2013) and further, targeted studies may be warranted.…”