New and conventional pore size tests in virus-removing membranes Aviv Duek a , Elizabeth Arkhangelsky a , Ronit Krush a , Asher Brenner a , Vitaly Gitis a,b, * a Unit of Environmental Engineering, Ben-Gurion University of the Negev, PO Box 653, Beer-Sheva 84105, Israel b van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands article info Article history: Received 2 August 2011 Received in revised form 20 December 2011 Accepted 27 December 2011 Available online 8 January 2012 Keywords: Porous materials Monodispersed nanoparticles Aquasols Molecular separation Pathogens abstract Microorganisms are retained by ultrafiltration (UF) membranes mainly due to size exclu- sion. The sizes of viruses and membrane pores are close to each other and retention of viruses can be guaranteed only if the precise pore diameter is known. Unfortunately and rather surprisingly, there is no direct method to determine the membrane pore size. As a result, the UF membranes are not trusted to remove the viruses, and the treatment plants are required to enhance viral disinfection. Here we propose a new, simple and effective method for UF pore size determination using aquasols of gold and silver nanoparticles. We synthesized highly monodispersed suspensions ranging in diameter from 3 to 50 nm, which were later transferred through polymer and ceramic UF membranes. The retention percentage was plotted against the particle diameter to determine the pore size for which a membrane has a retention capability of 50, 90 and 100%. The d 50 , d 90 and d 100 values were compared with data obtained from conventional transmembrane flux, polyethylene glycol, and dextran tests, and with the retention of phi X 174 and MS2 bacteriophages. The absolute pore size, d 100 , for the majority of tested UF membranes is within 40e50 nm, and can only be detected with the new tests. The average 1.2 log retention of hydrophilic phi X 174 was predicted accurately by models based on the virus hydrodynamic radii and d 100 pore size. The 2.5 log MS2 retention suggests hydrophobic interactions in addition to simple ball-through-cylinder geometry. ª 2012 Elsevier Ltd. All rights reserved. 1. Introduction Many water and wastewater treatment facilities use ultrafil- tration (UF) membranes for the safe retention of protozoa, bacteria, and viruses. The retention is mainly governed by size exclusion where the microorganisms smaller than pore size are passing through and bigger than the pore size are retained. With a microorganism to pore ratio higher than a 100, the UF membranes are trusted to provide 6 log removal values (LRV) of protozoa and bacteria (EPA, 2001; Lovins et al., 2002). Here LRV is determined as the logarithm to base 10 of the ratio of microorganism concentrations in the feed to those in the permeate. The virus size is much closer to the pore diameter, such that the UF membranes are nearly denied any virus log removal credits (EPA, 2003). The discrimination is mainly due to the absence of reliable methods for detection of virus-sized pores in the membrane active layer (Gitis et al., 2006). The detection based on the molecular weight cut-off (MWCO) (Cleveland et al., 2002) results in contradictive reports of 6-log virus retention with 100 kDa membranes (Jacangelo et al., * Corresponding author. Unit of Environmental Engineering, Ben-Gurion University of the Negev, PO Box 653, Beer-Sheva 84105, Israel. E-mail address: gitis@bgu.ac.il (V. Gitis). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 46 (2012) 2505 e2514 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.12.058