Membrane filtration

Historically, reverse osmosis (RO) membranes were designed to remove salt from seawater [82]. Over the next decades, a range of specialized membranes have become available for a variety of purposes, including nanofiltration (NF) membranes, which were designed for use as water softeners [83]. Today, RO and NF membranes are widely used in water treatment because they are able to remove various contaminants other than salts, including harmful trace organics, viruses, and dissolved organic matter. Sedlak found that NDMA precursors were 98% removed by RO [40,84]. However, RO and NF membranes are not always effective at removing small, neutral, and hydrophilic compounds such as NDMA. RO and NF only partially remove NDMA [3,39,40,84-87].

The more porous microfiltration (MF) and ultrafiltration (UF) membranes are unable to remove NDMA; however, they can reject its precursors with moderate efficiency (50%) [40].

To predict NDMA retention by RO and NF understanding the retention mechanisms is required. Although the actual rejection mechanisms for RO and NF are complex and not yet fully understood, key factors have been identified. These factors can be categorized as size exclusion, charge exclusion, and solute-membrane affinity [88], and they are related to the properties of the following - solute, membrane, solution (liquid matrix), and operating conditions [89,90]:

• Solute properties: molecular mass (MW) or molecular diameter, acid dissociation constant (pKa), polarity, and hydrophobicity (log Kow).

• Membrane properties: molecular weight cutoff(MWCO), surface charge (zeta potential), and roughness.

• Water chemistry: pH, ionic strength, solute-solute interactions with other substances, in particular organic matter and colloidal matter.

• Operating conditions: pressure, flow rate, and recovery.

• Fouling status of the membrane.

Organic Compound

Organic Compound

Figure 2 Predicting NDMA removal using a solute-matrix-membrane interaction scheme. Adapted from Ref. [89]. It is assumed that the membrane is negatively charged. The gray boxes highlight the case of NDMA. Left-hand side represents NF and "loose" RO;right-hand side represents "tight" RO.

Figure 2 Predicting NDMA removal using a solute-matrix-membrane interaction scheme. Adapted from Ref. [89]. It is assumed that the membrane is negatively charged. The gray boxes highlight the case of NDMA. Left-hand side represents NF and "loose" RO;right-hand side represents "tight" RO.

Fig. 2 shows a schematic indicating solute—matrix—membrane interactions and their impact on organics rejection by membranes. The gray path follows the case for a negatively charged membrane and NDMA at pH values encountered during wastewater treatment and in environmental waters. Depending on the membrane, NDMA is smaller or larger than the MWCO.

Size exclusion depends on the MW and chemical structure of the solute as it relates to the membrane MWCO and/or pore size. The MWCO is defined as the molar mass above which more than 90% of a given compound is rejected. Compounds used for MWCO determinations are typically aqueous sugar or polyethylene glycol solutions. However, today there is no generally accepted industry standard [91], and MWCO ratings are not always comparable. As a first approximation, any solute that is larger than the MWCO will be efficiently rejected. NDMA has a MW of 73 Da, which is smaller than the MWCO of NF membranes (typically 200— 500 Da). Although RO membranes are usually considered nonporous (i.e., no MWCO), some manufacturers report MWCO of approximately 100 Da for their RO membranes (Koch Membranes 2008). Because the MWCO of RO membranes (nonexistent to 100 Da) is closer to NDMA's MW (73 Da) than that of NF membranes (200—500 Da), tight RO membranes would be expected to better remove NDMA than NF membranes.

Holding other properties constant, a compound that is more charged is better rejected by NF and RO. The pKa of NDMA is less than 1 [92], which renders this amine compound uncharged at ambient pH. Therefore, membrane charges do not contribute to NDMA rejection.

Finally, NDMA is expected to sorb poorly on the membrane and instead stay in the water phase due to its hydrophilicity (log Kow — —0.57). For these reasons, NDMA is predicted to be poorly rejected by NF membranes and poorly to moderately rejected by RO membranes, depending on their "looseness," that is, MWCO (Fig. 2).

The available data show that RO and NF membranes do not achieve complete NDMA removal. Table 6 summarizes the available literature on NDMA removal by membranes in field and laboratory experiments. The data indicate a wide range of NDMA rejection values (10-70%), likely caused by different membrane—solute—matrix interactions. For example, in her laboratory studies, Steinle-Darling showed a decrease in NDMA rejection due to membrane fouling and water chemistry [87]. This is in agreement with other studies showing that fouling decreases rejection of small, uncharged contaminants [96,97].

To date, there is very little published data on NDMA rejection by NF membranes. An example is a study by Bellona et al. who determined the rejection of Filmtec NF90 [39,89] to be 42—47% NDMA. NF90 appears to be a borderline case because, based on the membrane's salt rejection and surface chemistry [98], NF90 could be classified as an RO membrane.

Advances in material science, such as membrane coatings, show promises in improving NDMA rejection by membranes. For example, a polyether polyamide block copolymer (PEBAX) coating increased NDMA rejection by LFC3 and BW-30 RO membranes by 6% and 15%, respectively, to 76% in both cases [87]. Further optimization of membranes may lead to greater removal efficiencies of small pollutants and greater reliance on membrane technology for organics removal.

Currently regulatory limits may be achieved by treating water with advanced oxidation (UV) or by blending it with water containing lower levels of NDMA.

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