Ceramic capillary membranes with tailored pore sizes and functionalizations for virus retention

Abstract

The aim of this work is the design of a versatile and effective virus filtration system based on ceramic which offers both high membrane throughput rates and high retention capacities for a wide variety of viruses. For this purpose, an extrusion process based on yttria stabilized zirconia (YSZ) powders with particle sizes of 30 nm, 40 nm or 90 nm is implemented to shape tubular membranes. After sintering at 1050 °C for 2 h a defect-free homogenous structure with open porosities of around 50 % is achieved. By increasing the initial YSZ particle size, increased average membrane pore sizes ranging from 24 nm to 146 nm are obtained. A high membrane pore size leads to reduced virus retention capacities in combination with increased water permeate fluxes. Capillaries made of YSZ-40nm are promising candidates as they fulfill the virus filter criterion of 4 log reduction values (LRVs) required by the World Health Organization in combination with a membrane flux of ~30 L/(m2hbar). Capillaries made of YSZ-90nm do not fulfill the virus filter criterion due to their average pore size of ~150 nm. However, they show a membrane flux of ~150 L/(m2hbar) and can therefore be used for high flux filtration applications, if an adequate adsorption capacity for viruses by a membrane surface functionalization is provided. Therefore, a straightforward chemical functionalization strategy using aminosilanes with one, two or three amino groups per silane molecule, namely 3-aminopropyltriethoxysilane (APTES), N-(2-aminoethyl)-3-aminopropyltriethoxysilane (AE-APTES) and n-(3-trimethoxysilylpropyl)diethylenetriamine (TPDA), is used. The zeta-potential of the membrane surface is converted from negative to positive. Therefore, the virus retention capacity for the model virus MS2 (diameter = 25 nm, isoelectric point (IEP) = 3.9) is significantly increased for neutral feed solutions based on monovalent and divalent salts (NaCl, MgCl2) from a LRV of <0.3 for non-functionalized membranes to LRVs of up to 9.6 ± 0.3 for the TPDA functionalized capillaries due to electrostatic interactions. A lower LRV of 6.4 is observed for feed solutions which are based on only monovalent ions (NaCl). Therefore, the TPDA functionalized surface is simulated at the atomistic scale using explicit-solvent molecular dynamics in the presence of either Na+ or Mg2+ ions. This shows that the binding free energy reveals that the Mg2+ ions remain adsorbed to the membrane surface, whereas Na+ ions form only a weakly bound with the surface. Due to the adsorbed Mg2+ ions an upright orientation of the TPDA molecules and opposed to that a more tilted orientation in the presence of Na+ ions is found. Therefore, a better accessibility of the TPDA molecules for the viruses and thus a better virus retention capacity is found when using feed solutions based on Mg2+ ions. The retention capacity for the model virus PhiX174 (diameter = 26 nm, IEP = 6.2) viruses cannot be increased by TPDA functionalized membranes due to the repulsion between the positively charged bacteriophages and the positively charged pore wall surfaces at a neutral pH. Therefore, a hydrophobic functionalization of the ceramic membranes with silanes offering carbon chains is performed. Virus retention increases most strongly for capillaries functionalized with C8-chains (n-octyltriethoxysilane, OTS), showing LRVs of ~9 for both viruses (MS2 and PhiX174) with throughput rates of up to ~400 L/(m²h) even under varying feed conditions. Accordingly, such hydrophobic ceramic membranes are a versatile alternative to conventional polymeric membranes for virus retention applications.