Roles of molecular weight -fractionated extracellular polymeric substance in transformation of Au(III) to Au nanoparticles in aqueous environments

Extracellular polymeric substance (EPS) is widely distributed in natural environments and plays important roles in the biogeochemical cycling of heavy metal. Earlier works reported that EPS could reduce metal ions such as Au(III) and Ag(I) to corresponding metal nanoparticles (NPs). EPS is a complex mixture of microbiogenic polymers with wide molecular weight (MW) distribution, and the specific components of EPS responsible for Au(III) reduction and AuNPs stabilization are still not well understood. In this study, the EPS of Shewanella oneidensis MR-1 was divided into six fractions with MW of < 3, 3-10, 10-30, 30-50, 50-100, and > 100 kDa, respectively through the ultrafiltration method and the roles of MW-fractionated EPS in the reduction of Au(III) to AuNPs were investigated. It was found that the low MW (< 3 kDa) EPS was the major reducing agent in EPS but the fraction itself could not convert high concentration (> 25 mg/L) of Au(III) to stable AuNPs due to its inferior AuNPs-stabilizing capacity. The high MW (> 50 kDa) EPS could act as coating reagents to increase the stability of the formed AuNPs with sizes of 20-50 nm, but had low Au(III)-reducing activity. The carboxyl-containing substances in EPS may play crucial roles in stabilizing AuNPs. This finding is important for a better understanding of the differential roles of MW-fractionated EPS in the transformation and fate of Au(III) and AuNPs, as well as other metal ions and metal NPs in natural environments.