Estudio de asociación con muestras comórbidas

Depending on the MWCO of the membrane, the MW is an important first indicator whether a solute will be rejected or permeate through that membrane. Generally, rejec- tion increases with increasing MW (Chen et al. 2014, Dang et al. 2014a, Fujioka et al. 2014c, Kong et al. 2016), however, exceptions exist. The MW is most important for neutral so- lutes, particularly hydrophilic ones (Fujioka et al. 2015a). Similar findings were reported by Simon et al. (2013a) who showed that rejection of neutral hydrophilic and moderately

hydrophilic neutral TrOCs (logD > 3) by the NF270 and NF90 membranes is predominantly

governed by the membrane porosity and the MW of the TrOCs. Chen et al. (2014) ob- served a large influence of MW on rejection, particularly for loose and low desalting LPRO membranes. However, the authors state rejection may be better explained by considera- tion of molecular size (height/width, see section 1.3.3.2 below) in addition to molecular weight.

1.3.3.2

Molecular size (length and width)/molecular volume

As discussed in the previous section MW is an important parameter for the evaluation of the rejection potential of a membrane for a given solute. However, MW does not always correlate with the actual size, i.e. molecular length and width of a molecule. Therefore it is essential to know the molecular size parameters in addition to MW. This was reported by Chen et al. (2014), who observed a good correlation of MW with molecular length (ML) for PFCs and non PFCs. Hence, there was a large influence of molecular length/width on the rejection of PFCs and non PFCs, particularly for the loose low desalting LPRO mem- brane. An increase of rejection of hydrophilic neutral and hydrophilic negatively charged compounds with increasing molecular width was shown by Dang et al. (2014a). Another

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study found that of the various physicochemical parameters studied, molecular volume shows the best correlation with permeate flux and rejection of pesticides and derivatives for the NF90 membrane tested (Hidalgo et al. 2016).

1.3.3.3

Minimum projection area/Equivalent width

The minimum projection area (MPA) is a relatively new parameter and it was first used to correlate compound size and rejection (e.g. Fujioka, et al. 2015a). These authors carried out a study with RO membranes of different membrane materials, namely polyamide, the most widely used material for NF/RO, and cellulose triacetate. They observed a good cor- relation of the rejection of n-nitrosamines with MPA for the polyamide RO membrane. For the cellulose triacetate RO membrane correlation of rejection of n-nitrosamines with MPA showed to be worse than for the polyamide RO membrane since the largest compounds, N-nitrosodipropylamine (NDPA) and N-nitrosodi-n-butylamine (NDBA), did not fit the cor- relation curve. The authors assumed these compounds might adsorb more progressively leading to an increased diffusion across the membrane and consequently a decreased rejection.

Simon et al. (2013a) correlated the rejection of hydrophilic and moderately hydrophilic neutral TrOCs by NF270 and NF90 membranes to their molecular dimensions, namely equivalent width, which is ‘defined as √S/2, where S is the minimum area of a rectangle enclosing the projection of the molecule on the plane perpendicular to the length-axis’. The rejection of the studied TrOCs increased with their equivalent width due to steric hin- drance.

1.3.3.4

Charge

Charge is a quantitative measure of the strength of an acid in solution i.e. the larger pKa

the smaller the extent of dissociation at any given pH. Therefore, the pKa value of a com-

pound is a particularly important parameter in relation to the feed pH. Solutes that disso- ciate in the range of feed pH become ionic. As a consequence, electrostatic interactions between solutes and a (charged) membrane surface come into effect. These electrostatic interactions, often referred to as charge repulsion, is an important rejection mechanism that is entirely ineffective to neutral compounds (Dang et al. 2014a).

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Chen et al. (2014) showed a greatly enhanced rejection of PFCs in dissociated state, PFCs are ionic at pH > 3, by a significantly greater efficacy of electrostatic exclusion. The authors also observed a greater influence of the electrostatic charge effect for loose membranes (Chen et al., 2014). In pH experiments with the NF270 and ESPA2 membranes Dang et al. (2014a) observed higher rejections for most charged TrOCs compared to neutral species, which was attributed to electrostatic repulsion.

1.3.3.5

Hydrophobicity/hydrophilicity

Some basic information on hydrophobicity/hydrophilicity of membranes has already been given in section 1.3.1.3. These are generally applicable to solutes as well.

Hydrophobicity and hydrophilicity of a solute are used to describe their attraction or ab- sence thereof to adsorb to a membrane surface. Hydrophobicity and hydrophilicity strongly depend on pH and are closely associated with hydrophobicity/hydrophilicity of the membrane surface or even the surface of a covering fouling layer.

Chen et al (2014) showed the impact of hydrophobic adsorption on the rejection of PFCs particularly for loose membranes for which the size exclusion effect is less pronounced. A high hydrophobic adsorption of the solutes to the membrane surface was proposed as an initial rejection mechanism. However, the authors suggested a possible slippage of PFCs through the membrane, partially due to the slender molecular structure of these compounds. Governed by steric forces and/or diffusion a lower overall rejection for hy- drophobic compounds was observed. In contrast, hydrophilic NFCs were better retained since they did not adsorb to the membrane. A similar observation, i.e. an increase of re- jection with increasing hydrophilicity was reported by Dang et al. (2014a). On the other hand, hydrophobic TrOCs adsorbed to the membrane surface which led to increased par- titioning of these compounds into the membrane. Consequently, a rise in transport through the membrane occurred resulting in a lower rejection (Dang et al. 2014a). How- ever, these authors did not see a strong correlation between adsorption and the hydro-

phobicity of TrOCs. Additionally, there was no correlation between adsorption and logD

(both for hydrophilic and hydrophobic compounds). This was attributed to the existence of other impact factors (physicochemical characteristics of TrOCs and the membrane ma- terial), such as molecular size and charge of the compounds as well as pore size, charge, and surface roughness properties of the membranes. These parameters exerted a con- siderable influence on the adsorption of TrOCs to the membranes. (Dang et al. 2014a)

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