Influence of salts on virus adsorption to microporous filters

We investigated the direct and indirect effects of mono-, di-, and trivalen t salts (NaCl, MgCl2, and AlCl3) on the adsorption of several viruses (MS2, PRD-1, phi X174, and poliovirus 1) to microporous filters at different pH values. The filters studied included Millipore HA (nitrocellulose), Filteri te (fiberglass), Whatman (cellulose), and 1MDS (charged-modified fiber) fil ters. Each of these filters except the Whatman cellulose filters has been u sed in virus removal and recovery procedures. The direct effects of added s alts were considered to be the effects associated with the presence of the soluble salts. The indirect effects of the added salts were considered to b e (i) changes in the pH values of solutions and (ii) the formation of insol uble precipitates that could adsorb viruses and be removed by filtration. W hen direct effects alone were considered, the salts used in this study prom oted virus adsorption, interfered with virus adsorption, or had little or n o effect on virus adsorption, depending on the filter, the virus, and the s alt. Although we were able to confirm previous reports that the addition of aluminum chloride to water enhances virus adsorption to microporous filter s, we found that the enhanced adsorption was associated with indirect effec ts rather than direct effects. The increase In viral adsorption observed wh en aluminum chloride was added to water was related to the decrease in the pH of the water. Similar results could be obtained by adding HCl. The incre ased adsorption of viruses in water at pH 7 following addition of aluminum chloride was probably due to flocculation of aluminum, since removal of flo cs by filtration greatly reduced the enhancement observed. The only direct effect of aluminum chloride on virus adsorption that we observed was interf erence with adsorption to microporous filters. Under conditions under which hydrophobic interactions were minimal, aluminum chloride interfered with v irus adsorption to Millipore, Filterite, and 1MDS filters. In most cases, l ess than 10% of the viruses adsorbed to filters in the presence of a multiv alent salt and a compound that interfered with hydrophobic interactions (0. 1% Tween 80 or 4 M urea).