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Abstract
Known bacteria capable of degrading carbon nanotubes (CNTs) were very limited, and the only known degradation mechanism was the Fenton reaction driven by Labrys sp. WJW with siderophores only under iron-deficient conditions. There was no useful information about the degradation rates or long-term stability and continuity of the degradation reaction. CNT degradation was considered to require a long period of time for several months or more. In this study, we challenged long-term continuous degradation of oxidized (carboxylated) single-walled CNTs (O-SWCNTs) using bacteria of the genus Shewanella, which are widely present in the environment and can drive the Fenton reaction through alternating anaerobic-aerobic growth conditions under more general environmental conditions. We first examined the effect of O-SWCNTs on the growth of S. oneidensis MR-1 and it was revealed that O-SWCNTs promote the growth up to 30 μg/mL but inhibit at 40 μg/mL and above. Then, S. oneidensis MR-1 was subjected to incubation cycles consisting of 21-h anaerobic and 3-h aerobic periods in the presence of 30 μg/mL O-SWCNTs and 10 mM Fe(III) citrate. We found key factors to continue the bacterial Fenton reaction, and finally, achieved long-term continuous degradation of O-SWCNTs over 90 d by maintaining pH near neutral and replenishment of Fe(III) citrate at 60 d, reaching a degraded fraction of 56.3%. S. oneidensis MR-1 produces Fe(II) from Fe(III) citrate, a final electron acceptor for anaerobic respiration during the anaerobic period, and ·OH was generated through the Fenton reaction by Fe(II) and H2O2 produced by MR-1 during the aerobic period. The ·OH was responsible for O-SWCNT degradation, which was inhibited by scavengers of H2O2 and ·OH. Raman spectroscopy and X-ray photoelectron spectroscopy showed that the graphitic structure in O-SWCNTs was oxidized, and electron microscopy showed that long CNT fibers initially aggregated became short and isolated during the degradation. Since Shewanella spp. and iron are ubiquitous in the environment, this study suggests the applicability of a bacteria-driven Fenton reaction to the degradation of CNTs under a wide range of conditions using this genus and contributes to developing novel methods for waste treatment and environmental bioremediation against CNTs.
