Extracellular Polymeric Substances Govern the Surface Charge of Biogenic Elemental Selenium Nanoparticles Rohan Jain,* ,†,⊥ Norbert Jordan, ‡ Stephan Weiss, ‡ Harald Foerstendorf, ‡ Karsten Heim, ‡ Rohit Kacker, § Rene ́ Hü bner, ∥ Herman Kramer, § Eric D. van Hullebusch, ⊥ Franc ̧ ois Farges, # and Piet N. L. Lens † † UNESCO-IHE, Institute for Water Education, Westvest 7, 2611AX Delft, The Netherlands ‡ Helmholtz-Zentrum Dresden − Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany § Process & Energy Laboratory, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands ∥ Helmholtz-Zentrum Dresden − Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstraße 400, 01328 Dresden, Germany ⊥ Universite ́ Paris-Est, Laboratoire Ge ́ omate ́ riaux et Environnement (EA 4508), UPEM, 77454 Marne la Valle ́ e, France # Institut de Mineralogie, de Physique des Materiaux et de Cosmochimie (IMPMC), Musé um National d’Histoire Naturelle, Universite ́ Pierre-et-Marie Curie and CNRS, UMR 7590 Paris, France * S Supporting Information ABSTRACT: The origin of the organic layer covering colloidal biogenic elemental selenium nanoparticles (Bio- SeNPs) is not known, particularly in the case when they are synthesized by complex microbial communities. This study investigated the presence of extracellular polymeric substances (EPS) on BioSeNPs. The role of EPS in capping the extracellularly available BioSeNPs was also examined. Fourier transform infrared (FT-IR) spectroscopy and colorimetric measurements confirmed the presence of functional groups characteristic of proteins and carbohydrates on the BioSeNPs, suggesting the presence of EPS. Chemical synthesis of elemental selenium nanoparticles in the presence of EPS, extracted from selenite fed anaerobic granular sludge, yielded stable colloidal spherical selenium nanoparticles. Furthermore, extracted EPS, BioSeNPs, and chemically synthesized EPS-capped selenium nanoparticles had similar surface properties, as shown by ζ-potential versus pH profiles and isoelectric point measurements. This study shows that the EPS of anaerobic granular sludge form the organic layer present on the BioSeNPs synthesized by these granules. The EPS also govern the surface charge of these BioSeNPs, thereby contributing to their colloidal properties, hence affecting their fate in the environment and the efficiency of bioremediation technologies. ■ INTRODUCTION Selenium is an essential nutrient in the human diet. 1 However, higher concentrations of selenium, especially those of the selenium oxyanions selenate and selenite, are toxic to humans, animals, and aquatic life. 2−4 Therefore, regulatory agencies have set limits on total selenium discharges, e.g., the Environmental Protection Agency of the United States has recommended a discharge limit of 5 μgL −1 total selenium in freshwater. 5 Anaerobic bioreduction of dissolved selenium oxyanions to elemental selenium is considered a promising technology for the remediation of wastewaters containing selenium oxy- anions. 6,7 However, the produced biogenic elemental selenium is in the form of colloidal spherical nanoparticles with a diameter of 50−500 nm. 8,9 Such colloidal biogenic elemental selenium nanoparticles (BioSeNPs) are present in high concentrations in the effluent of upflow anaerobic sludge blanket reactors (UASBs), in which anaerobic granules treat selenium-rich wastewaters. 7 Buchs et al. 10 showed that the colloidal properties of these BioSeNPs determine their transport and fate in the environment as well as the bioremediation efficiency. Thus, it is important to understand the factors governing the colloidal properties of BioSeNPs. Capping agents are known to affect the surface properties of chemically produced metal(loid) nanoparticles, including surface charge and colloidal stability (Figure S1 in Supporting Information). 11 For instance, sterically stabilized silver nano- particles capped by polyvinylpyrrolidone (PVP) do not agglomerate while electrostatically stabilized silver nanoparticles by citrate do agglomerate at low pH or high ionic strength. 12 Electrosterically stabilized silver nanoparticles by branched polyethyleneimine capping are more resistant to agglomeration at low pH or high ionic strength as compared to citrate- stabilized silver nanoparticles. 12 Proteins such as bovine serum albumin (BSA), which stabilize silica nanoparticles through electrosteric mechanisms, 13 are also known to stabilize Received: September 2, 2014 Revised: December 22, 2014 Accepted: December 23, 2014 Published: December 23, 2014 Article pubs.acs.org/est © 2014 American Chemical Society 1713 DOI: 10.1021/es5043063 Environ. Sci. Technol. 2015, 49, 1713−1720