The removal of OMPs is governed by different parameters based on the membrane characteristics, aqueous media/solute characteristics, operating conditions, membrane fouling as well as OMP characteristics, but generally membranes are designed to work as a physical barrier (semi- permeable) that catches or rejects constituents greater than its pore size while allowing water to permeate through it. However, studies have shown that other significant physicochemical phenomenal activities occur during membrane processes. Combinations of certain properties of OMPs, solute parameters as well properties of membranes are reported to orchestrate observed removal mechanisms. Properties of OMPs such as molecular weight and size (MW, length and width), hydrophobicity or hydrophilicity, Charge characteristics and chemical structure (i.e occurrence of electron withdrawing or donating functional group) are reported to have significant effects on their rejection by membrane filtration (Chon et al., 2012; Nghiem and Hawkes, 2007a; Tadkaew et al., 2011). On the other hand, properties of membranes also play a major role in facilitating the rejection of contaminants. They may include membrane’s molecular weight cut- off (MWCO), pore size, surface charge- measured as zeta potential, hydrophobicity or hydrophilicity - measured as contact angle, and surface morphology - measured as roughness (Bellona et al., 2004; Schäfer et al., 2011). While these parameters are being understood there is still need for more investigations on the removal mechanisms in order to achieve a more realistic and predictable performance as presently reported mechanisms show varied results.
3.3.1 Size exclusion
This is the fundamental mechanism of membrane filtration where pollutants are sieved out based on their sizes. OMPs with size larger than the membrane pore size are retained because of the sieving effect (Fig 3.2). This mechanism is well understood especially in MF and UF application for removal of particulate matters and suspended solids which are large in size. As regards to
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OMPs which usually are very small in size (molecular weight), inconsistent results have been observed where OMPs with larger size than pore size of membrane are not retained as expected during filtration process (Tadkaew et al., 2011). Furthermore, reports show that size of OMPs should not be exclusively based on molecular weight but molecular length and width (shape) should be considered; and thus attributed the inconsistency in results to this (Chon et al., 2012; Tadkaew et al., 2011). Size exclusion mechanism is mostly observed with uncharged (neutral) OMPs as studies show a correlation between rejection of uncharged OMPs and their molecular weight and/or width (Kimura et al., 2004, 2003b; Ozaki and Li, 2002).
3.3.2 Adsorption
Adsorption of OMPs to polymeric membrane surfaces plays a significant role in the rejection of OMPs. Adsorption is mainly influenced by hydrophobic surface interactions and hydrogen bonding between OMPs and the membranes (Figure 3.2). Association of OMP with retained matter or introduced adsorbent material such as powdered activated carbon (PAC) can also facilitate adsorption. Adsorption could sometimes be confused with deposit formation activities which usually causes fouling. Membrane fouling and presence of humic acid from organic matter retained on surface and pores of membrane could also increase adsorption activities by changing the membrane surface characteristics and pore size, since membrane surface morphology, roughness, active layer thickness and pore size contribute significantly to adsorption effects (Nghiem and Hawkes, 2007a; Schäfer et al., 2011).
Adsorption site on membrane surfaces is relatively low, however larger pore sizes (MF, UF) tend to record higher adsorption than smaller pore sizes (NF/RO) (Tang et al., 2007). Some studies also suggest that apart from adsorption to membrane surface occurring, absorption into the membrane pore structure internally could also occur; this concept appears reasonable but very debatable because of the complexity involved and huge variety of membrane material and structure available. Studies show that adsorption rate decrease with time as a result of saturation after the initial stages of filtration, also adsorption effects measured at the initial stages of filtration show high retention but may be an over estimation of the retention because once membrane becomes saturated, retention decreases significantly (Comerton et al., 2007; Hu et al., 2007; Kimura et al., 2003b). The solution chemistry such as pH as well as solute-solute interaction also affects adsorption. In real waste water where the aqueous solution contains a mixture of OMPs and other pollutants, adsorption phenomena may become complex and theoretically unpredictable.
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Figure 3.2 OMP removal mechanism in MAP processes in Polymer based membranes
3.3.3 Hydrophobic interaction
Hydrophobic interaction between OMPs and membrane surface also affects the adsorption phenomena. Hydrophobicity of OMPs which is a function of octanol–water partition coefficient (log Kow) and of the membrane measured as contact angle (Mulder, 1996), promote interaction
and adsorption of hydrophobic OMP to hydrophobic membrane surfaces. Generally compounds with relatively high hydrophobicity (log Kow > 2.5) are expected to adsorb onto solid phases rather
than being soluble in water. Hydrophobic OMPs are therefore expected to adsorb to the surface of hydrophobic membranes surfaces by hydrophobic interactions. While hydrophobic adsorption contributes to retention, it must be noted that membrane fouling can be excavated by the
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hydrophobicity of the membrane affecting membrane operating conditions. Biofouling of membrane surfaces can make the surfaces hydrophilic and thus impact on the rejection of hydrophobic OMPs.
3.3.4 Electrostatic exclusion
Surface charge of membrane could induce electrostatic interaction between charged OMP molecules and the membrane surfaces (Figure 3.2). Electrostatic exclusion could be as result of repulsive force between negatively charged OMPs and negatively charged membrane surfaces. Several studies have shown this phenomenon and compared the retention to uncharged compounds (Kim et al., 2005; Kimura et al., 2003a). Electrostatic repulsion is not expected to change with time of filtration, however studies show that there may be changes due to the effects solution chemistry (Acero et al., 2010; Nghiem et al., 2006).