Nagoya University, Japan

Since 2018, Dr. Takuya Kasai is an assistant professor of Institute of Materials and Systems for Sustainability, and Department of Civil and Environmental Engineering, Nagoya University in Japan ...

... He received PhD in life sciences from Tokyo University of Pharmacy and Life Sciences in 2018. He has been studying the mechanisms of external electron transfer in microorganisms using molecular microbiology. Currently, he focuses on degradation of toxic substances in soil and/or water using anaerobic microorganisms and has collaborated with Dr. Arata Katayama who is a professor of Institute of Materials and Systems for Sustainability, and Department of Civil and Environmental Engineering, Nagoya University, Japan. Their team has isolated anaerobic microorganisms dehalogenating recalcitrant chemicals, such as dioxins and polychlorinated biphenyls, and found the external electron transfer of humic substances enhance the microbial dehalogenation. Based on the findings, their team is developing the bioelectrochemical system for anaerobic bioremediation.

 

Abstract


Characterization of solid-phase humin as external electron mediator for anaerobic reducing reactions of microorganisms

Takuya Kasai1,*, Duyen Minh Pham2, Yasushi Miyata3, Shozo Ohta1, Arata Katayama1,2

1Institute of Materials and Systems for Sustainability, Nagoya University, Chikusa, Nagoya 464-8603 Japan

2Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan

3Nagoya Municipal Industrial Research Institute, Atsuta, Nagoya 456-0058, Japan

There are anaerobic reducing reactions of microorganisms that can detoxify pollutants, such as dehalogenation and denitirification. In these reducing reactions, it has been shown that the microorganisms can utilize the external electrons directly received from the electron mediators, such as humic substances, present in soils and sediments. Humic substances are classified into three categories, based on the solubility into aqueous solutions at different pH: fulvic acids (soluble at any pH), humic acids (soluble at alkaline pH), and humin (insoluble at any pH). We found that anaerobic dehaloganating reactions of microorganisms were supported by solid-phase humin, but not by humic acids. Thus, in this study, we have characterized solid-phase humin polyphasically as the external electron mediator for microbial reduction.

Humin was extracted from various soils and sediments as reported previously (Zhang and Katayama, 2012). The freeze dried humin powder were subjected to the physical, chemical, and microbial characterizations (Zhang et al 2015), as well as electrical analyses.

All humins examined facilitated microbial reductive dechlorination of pentachlorophenol as external electron mediators (EEMs) using formate as C source, with different dechlorination rates ranging from 0.99 (µmol Cl-)∙L-1∙d-1 to 7.63 (µmol Cl-)∙L-1∙d-1. The denitrification of Pseudomonas stutzeri was also enhanced by all the humins as EEMs. The larger enhancing effects in the dechlorination and denitrification were observed in humins with higher carbon contents. Yields of the humins and the elemental compositions varied among sources. Particle size distribution analysis showed that humin consisted of mainly particles ranged as silt. Fourier transform infrared analysis showed that all the humins exhibited similar spectra with functional groups of hydroxyl, carboxyl, carbonyl and aromatic double bonds. The electron spin resonance spectra of humins prepared at different pH showed typical changes for the semiquinone-type radicals, suggestive of quinone moieties for the redox activity of the humins. The 13C-CP/MAS NMR analysis confirmed the presence of carbonyl and aromatic carbon as well as aliphatic carbon. Cyclic voltammetry analysis showed the redox-active functional groups in all the humins, with the estimated isoelectric points ranging from –0.30 to –0.13 V (versus a standard hydrogen electrode). Synchrotron-XAFS analysis suggested iron in humin was not affected by the redox conditions of humin. The electrochemical impedance spectrum and the electric capacity analysis showed the electric characteristics of humin. The polyphasic characterization suggested that the function of humin as external electron mediator was emerged from the organic fraction of humin with quinone structure.


Zhang CF and Katayama A. ES&T 46, 6575-6583, 2012

Zhang CF et al, Chemosphere, 131, 110-116, 2015