Quantification of Functional Groups and Modeling of Their Ionization Behavior in the Active Layer of FT30 Reverse Osmosis Membrane

Abstract

A new experimental approach was developed to measure the concentration of charged functional groups (FGs) in the active layer of thin-film composite reverse osmosis (RO) and nanofiltration (NF) membranes as a function of solution pH. FT30 RO membrane, with a fully aromatic polyamide (PA) active layer sandwiched between a polysulfone support and a coating layer, was used. The experiments consisted of saturating charged FGs with heavy ion probes, and determining the ion probe concentration by Rutherford backscattering spectrometry (RBS). Deprotonated carboxylic groups were saturated with Ag+, and protonated amine groups with W04(2-). The ionization behavior of carboxylic and amine groups was modeled based on acid-base equilibrium theory. While the ionization behavior of amine groups was satisfactorily described by one dissociation constant (pKa = 4.74), two pKa values (5.23 and 8.97) were necessary to describe the titration curve of carboxylic groups. These results were consistent with the bimodal pore size distribution (PSD) of FT30 active layer reported in the literature. The calculated total concentrations of carboxylic and amine groups in the active layer of the FT30 RO membrane studied were 0.432 and 0.036 M, respectively, and the isoelectric point (IEP) was 4.7. The total concentration of carboxylic and amine groups revealed that the degree of cross-linking of the PA active layer of the FT30 RO membrane studied was 94%.

Reverse osmosis membrane composition, structure and performance modification by bisulphite, iron(III), bromide and chlorite exposure

“…of dissociation constants required to fit the data. As previously reported for other RO 184 membranes with fully aromatic PA active layer two dissociation constants (n = 2) resulted in 185 the most accurate representation of the experimental data (Coronell et al, 2009(Coronell et al, , 2008 HSO3_ex-Fe-ClO2, HSO3_ex-ClO2, HSO3_ex-Fe-ClO2-Br), and the second set of samples with a 201 lower concentration than the stoichiometrically required (samples HSO3_lm, HSO3_lm-Fe, 202…”

“…Figure 5 presents the 306 concentration of accessible deprotonated carboxylic groups of the active layer, being higher at 307 greater pHs. The ionisation behaviour of the active layer functional groups was modelled by an 308 acid-base equilibrium, with two dissociation constants (Coronell et al, 2008), pka1 and pka2, 309 which is consistent with a bimodal pore size distribution. A larger pore presents a lower 310 dielectric constant surrounding the functional group, and thus, the energy needed for 311 ionisation is greater, leading to a lower pka (Coronell et al, 2008).…”

“…The ionisation behaviour of the active layer functional groups was modelled by an 308 acid-base equilibrium, with two dissociation constants (Coronell et al, 2008), pka1 and pka2, 309 which is consistent with a bimodal pore size distribution. A larger pore presents a lower 310 dielectric constant surrounding the functional group, and thus, the energy needed for 311 ionisation is greater, leading to a lower pka (Coronell et al, 2008). As a result, ion probing 312 assays provide information about the membrane active layer pore size distribution, and hence, 313 by comparing the virgin and the exposed RO membranes, insights into the structural changes 314 suffered by the degraded membrane.…”

“…The later, referred as aggregate pores, have been 36 related to void space between polymer aggregates (Coronell et al, 2008) and their size has 37 been estimated in 1.0 -1.6 nm in diameter (Saenz de Jubera et al, 2012). The smaller ones, 38 named network pores, have been associated to the interstitial space between polymer 39 branches within an aggregate (Coronell et al, 2008) and their diameter has been estimated in 40 0.4 -0.8 nm (Saenz de Jubera et al, 2012). The ionization behaviour of functional groups in 41 the PA active layer, carboxylic and amine, has been modelled by an acid-base equilibrium with 42 the aqueous solution (Coronell et al, 2008).…”

“…The smaller ones, 38 named network pores, have been associated to the interstitial space between polymer 39 branches within an aggregate (Coronell et al, 2008) and their diameter has been estimated in 40 0.4 -0.8 nm (Saenz de Jubera et al, 2012). The ionization behaviour of functional groups in 41 the PA active layer, carboxylic and amine, has been modelled by an acid-base equilibrium with 42 the aqueous solution (Coronell et al, 2008). Ion probing experiments combined with 43…”