Role of hydrophobic and electrostatic interactions for initial enteric virus retention by MF membranes

Abstract

Membranes are of increasing interest for the removal of human enteric viruses from wastewater, especially when the goal of treatment is reuse. Limited work has been undertaken on fundamental issues such as aggregation and the role of electrostatic and hydrophobic interactions, as opposed to the sieving of viruses by membranes. One apparently critical factor would be the iso-electric point (pI) of a virus. As an example of a worst-case model virus, the retention of bacteriophage MS-2 was investigated using hydrophobic (GVHP) and hydrophilic (GVWP) 0.22 μm MF membranes at different pH levels and with different salts. High retention levels were measured at the iso-electric point of MS-2, pH 3.9 (5 log retention) and pH 7 (4.3 log retention) in the presence of salts and with a hydrophobic membrane. When retention was compared on a hydrophilic membrane, it was clear that hydrophobic interactions dominated virus retention, and this was improved by salt, presumably causing reduction of the Gouy double-layer when MS-2 was charged (pH 7). This paper shows that knowledge of the adsorption characteristics of viruses and the suspending conditions are important to predict removal of viruses by hydrophobic MF membranes, and discusses some of the practical implications of these important hydrophobic interactions.

Introduction

In recent years, the reuse of wastewater for non-potable use has gained much attention. The presence of human enteric viruses is a major risk associated with wastewater reuse [1], [2], [3], [4]. Therefore, enteric virus removal requires specific attention, given their low infective dose, long-term survival in the environment and low removal efficiency in conventional wastewater treatment [5]. Detection and enumeration of human enteric viruses is expensive and time consuming, hence, bacteriophages such as the F-RNA coliphages have been suggested as useful models because of there similar size and survival in waters [6]. MS-2 is the most studied F-RNA coliphage [6], and given its low iso-electric point (pI=3.9) [7] and relative hydrophobicity [8] it makes a good worst-case strain for membrane interaction studies. The main characteristics of some human enteric viruses and bacteriophages are summarized in Table 1.

Retention of the F-RNA coliphage, Qβ, using a new MF membrane with a pore size of 0.1 μm has been reported at about 90% [5]. Removal was increased, however, to 99.5% in the presence of particulates (from pond water and activated sludge). Similarly, Otaki et al. [9] reported 40–90% removal of F-RNA coliphages to Escherichia coli K12, using a MF membrane with a pore size of 0.2 μm. Removal was increased to 88–99% with a 0.1 μm pore-size MF membrane, despite the significantly smaller diameter of the coliphages (∼24 nm). The coliphage findings appear similar to reported poliovirus removal, where up to 99% removal was observed with 0.2 μm MF membrane [10]. In contrast to the retentions on standard hydrophobic MF membranes, Herath et al. [5] reported only 35–80% removal of Qβ and 30–85% of MS-2 coliphages using 50 nm hydrophilic nucleopore membranes at various pH levels. At the pH’s around the pI of MS-2 and Qβ, the highest retention was reported and it was suggested that aggregation enhanced retention, even below their pI values [5].

Besides the use of MF membranes, the smaller pore size ultra-filtration (UF) membranes have been used for the removal of enteric viruses and not surprisingly high removal efficiencies have been achieved [10]. Nonetheless, despite the relatively large pore sizes of MF membranes (0.1–0.45 μm) for the removal of small viruses like Hepatitis A (27 nm) and Norwalk-like viruses (35–39 nm), MF membranes appear to be capable of high removal efficiencies [11], [12], [13]. In addition, there is evidence that pore size alone does not adequately describe the ability of a filter to remove particulates from solutions [14]. For example, the important factors for adsorption of viruses to membranes are the chemical composition of the membrane, the ratio of membrane pore diameter to virus diameter and hydrophobic and electrostatic interactions [10]. In previous research, the ratio of membrane pore diameter (0.22 μm) and virus diameter (MS-2, 25 nm) was one of the major areas of interest regarding the retention of viruses by MF and UF [15], [16].

Very little work has been undertaken on the importance of electrostatic and hydrophobic interactions in virus removal by membranes. One apparently critical factor would be the iso-electric point (pI) of a virus and this knowledge makes it possible to predict the likelihood of its attachment to a charged surface [17]. The charge of most viruses will be negative under the conditions present in most wastewater effluents (i.e. pH 6–7). The net neutral charged at a virus’s pI leads to maximum virus–virus coagulation [5]. Aggregation may, therefore, further promote virus retention by membranes. It has also been reported that the presence of particular ions promotes virus aggregation compared with buffers at low pH alone [18].

For electrostatic interactions of viruses, the thickness of the double layer as described by Gerba [8] (see Fig. 1) plays the most important role. In Fig. 1, the solid could represent the membrane surface and it should be noted that the concentration in the boundary layer of the membrane could differ to the concentration present in the bulk solution. Both the pH and the presence of salts (in the bulk solution) influence the thickness of the double layer. Increasing the salt concentration or valency reduces the thickness of this layer and facilitates virus adsorption to surfaces [8].

Hydrophobic interactions between viruses and surfaces may also contribute significantly to adsorption [17]. Due to the increased electrostatic repulsion at higher pH levels, hydrophobic interaction could play the major role in maintaining virus-filter adsorption [8]. Gerba goes on to conclude that some salts will have a positive effect on the adsorption of viruses to surfaces, by increasing the ordering of water molecules and promote the sequestering of hydrophobic entities [8]. Nonetheless, these findings are mostly based on adsorption of viruses to soil or sand, little is known about adsorption of viruses to membranes. Therefore, the aim of this paper was to describe the adsorption interactions, electrostatic or hydrophobic of MS-2 to MF membranes during the first stage of filtration. The adsorption of MS-2 to the membrane was investigated at different pH levels, with different salts and with hydrophobic and hydrophilic membranes using different test volumes. Before starting these experiments the effect of using stirring in the dead-end membrane experimental unit and that of MS-2 aggregation at different pH levels were determined. A final experiment was performed to see the influence of time and permeate volume on the retention of MS-2 by MF membranes (Fig. 2).