Abiotic Reduction of 1,3-Dinitrobenzene by Aqueous Dissolved Extracellular Polymeric Substances Produced by Microorganisms

Abstract

Extracellular polymeric substances (EPS) secreted by microorganisms make up an important constituent of labile and semilabile dissolved organic matter in surface water. There are no literature reports on the capability of EPS to reduce organic pollutants. This study demonstrated that EPS could effectively reduce a probe nitroaromatic compound, 1,3-dinitrobenzene. In aqueous dissolved EPS (60.3 mg L, produced by ), 1,3-dinitrobenzene (2.1 mg L) was completely reduced to 3-hydroxylaminonitrobenzene and 3-nitroaniline within 45 h (30°C; pH 5.6). The reduction of 1,3-dinitrobenzene followed pseudo-first-order kinetics (rate constant [], 4.3 × 10 h). Fourier transform infrared and C nuclear magnetic resonance spectroscopy analyses combined with Tollen's test suggest that hemiacetal of rhamnose structures and reduced phenolic groups in EPS acted as the reducing agents. The abiotic nature of the reaction was further verified by the electrochemical cell experiments where the measured quantity of electron transfer through the external circuit was in principle equal to the stoichiometric demand to reduce 1,3-dinitrobenzene. The reduction of 1,3-dinitrobenzene by EPS was markedly facilitated by the addition of model juglone (1 mg L), confirming the "electron shuttle" role of quinoid-like structures in EPS. Moreover, the reaction was dependent on the pH and the concentration of metal ions (Na, Zn, or Cu). Additional tests confirmed the universality of 1,3-dinitrobenzene reduction by other sources of EPS (, , , and natural freshwater biofilm). The results indicate that EPS are reductively labile and can contribute to the natural attenuation and remediation of environmental organic contaminants.