IV. Chapter 4. Investigation of degradation by sulfuric acid and hydrogen halides on full-
4.1 Introduction
Water is essential for humans, and at many locations on the Earth, fresh water shortages need to be solved. In consequence, membrane technology to supply alternative fresh water has been widely researched because of its lower operation and maintenance cost, as well as lesser land space requirement [4]. Nanofiltration (NF) membrane, which has ~0.5 to ~2.0 nm of pore diameter, has been rapidly attracted during the last decades due to its low operating pressure and high rejection for divalent salts or organic molecules with low molecular weight in the range from 200 to 1000 g mol-1 [5]. The high rejection of NF can be explained by the combination of two different separation mechanisms which are the solution diffusion mechanism and steric/electrostatic sieving mechanism [6]. Current commercial NF thin-film composite (TFC) polyamide (PA) membranes have generally been fabricated by interfacial polymerization (IP) technique using both m-phenylenediamine (MPD, aromatic amine monomer) and trimesoyl chloride (TMC, acyl chloride monomer) for the active layer [7, 8]. This IP technique is a good way to fabricate TFC NF membranes, because the thin and dense full aromatic PA active layer results in high membrane performance despite low operating pressure, and the membrane performance can be controlled by various fabrication factors such as the concentration of monomers/additives, reaction/curing time, and temperature for post-treatment [9].
The NF technique has been widely applied to the reutilization/removal of target compounds under acidic conditions: 1) purification of nitric acids in picture tube production [13], 2) treatment of acidic effluents in the pulp and paper industry [15], 3) regeneration of acidic streams in dairy cleaning- in-place processes (CIP) [14], 4) removal of heavy metals [18] and sulfate ions [19] in the mining and metal industry, 5) recovery of phosphorus in sewage sludge [16, 17], and 6) separation or recycling of abundant acids such as HBF4, HCl, HNO3, H2SO4, and H3BO3 in effluents from rinsing, fermentation, and extraction processes [11]. Furthermore, NF technique can be applied to wastewater containing HCl, HBr and HI from etching process for semiconductors [20-22]. Acid-resistant NF membranes are needed to use the above processes which operate at low pH condition. However, high-performance commercial NF full aromatic PA membranes, which are fabricated by MPD with TMC, are limited in the range of pH 2 to 11 [24]. Therefore, additional research on the degradation of full aromatic PA membrane by acid is needed to apply the above processes which have less than pH 2 acidic condition. Liu et al. [25]
studied the effect of 0.5 mol/l HCl on full aromatic reverse osmosis PA membrane, and PA was hydrolyzed after 30 Days filtration, following more permeations of water and salt. However, to the best of our knowledge, the effects of degradation by acid on full aromatic membrane using different pH
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range of acid and various kinds of acid containing sulfuric acid and hydrogen halides have not previously been researched. Therefore, the degradation of full aromatic membrane in various acidic conditions, with pH < 2 condition of sulfuric acid and hydrogen halides, requires systematic investigation.
Plentiful research has been conducted on the hydrolysis of amide by the biological importance of proteolytic reactions [32]. An amide bond’s stability originates from the delocalization of electrons between lone pairs of the nitrogen atom in amide and the carbonyl πCO bond [34, 37, 39]. Acid-catalyzed hydrolysis reactions firstly undergo protonation on oxygen (O) or nitrogen (N) in amide group. The O/N-protonation twists the amide’s C-N bond to lose the resonance between carbonyl πCO bonds and nitrogen lone-pair electrons, and accordingly accelerates the amide’s hydrolysis [38, 39, 81]. Ma et al.
[81] explains the O- or N-protonation pathways, which compete with each other (e.g., undistorted amides are normally protonated at the oxygen [38]). When acid-catalyzed hydrolysis occurs on the PA membranes, permeations of water and salt increased as shown in previous researches [6, 25]. However, the occurrence of mainly O/N-protonation without hydrolysis would also affect membrane performance because of the change of hydrogen bonding and deformation of polymer structure. Firstly, when the O- protonation occurs in amide group, it changes hydrogen bonding between a hydrogen-accepting carbonyl group and a hydrogen-donating H atom (e.g., connecting N atom in amide), due to the protonated carbonyl group generated by proton bridge [112, 113]. Witt et al. [113] studied several diamides which generate a rigid and strong proton bridge between the amide groups after the O- protonation, because it can be further stabilized by changing conformations including those with axial substituents. Furthermore, Addario et al. [112] reported N-acetylated amino acids made proton bridges after the O-protonation for not only the stabilizing effect between two atoms by proton bridge, but also better stabilization for amide resonance of the N-acetyl group itself. Likewise, spatial rearrangement of protonated amide can occur by proton bridge, because of the more stabilized state. In terms of the N- protonation, on the other hand, rotation of N-C(O) bond occurs, due to the formation of a tetrahedral structure at the nitrogen in amide. That is, when a progressive pyramidalization of the N from sp2 (planar) to sp3 (tetrahedral) geometry occurs, a rotation about the N–C(O) bond of 30° takes place [34]. Because the spectral interpretation of polymeric materials is quite difficult, benzanilide can be used for the model compound of full aromatic PA membrane [114]. If the benzanilide is assumed for the model compound of full aromatic PA membrane, the amount of N-protonation in pH 0 and 1 is 76 and 24%, respectively, because the pKa value for N-protonation of benzanilide is 0.5 [115]. These spatially distorted structures caused by O/N-protonation in acidic condition can possibly affect the performance of the full aromatic PA membrane, because the application of high pressure could collapse or locally compact or mainly
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distort the polymer chain [116]. Hydroiodic acid (HI) is known as an unstable chemical, so it oxidizes to molecular iodine (I2) in air [117, 118]. Table 4.1, which shows the chemical reaction for generating halogen gases by oxidation of hydrogen halides, shows that these phenomena can be explained by spontaneous reaction. Subsequently, molecular chlorine (Cl2) and molecular bromine (Br2) can be generated from hydrochloric acid (HCl) and hydrobromic acid (HBr) in the same manner. Henry’s constants (HCP) of these halogens (Cl2, Br2, and I2) in water are 9.2 × 10-4, 7.2 × 10-3, and 2.8 × 10-
2, respectively (Fig. 4.1) [119]. These generated halogens are an another possible source to affect the surface characterization/performance of the full aromatic polyamide membrane, given that research has been widely conducted on the effect and mechanism of halogenation on the polyamide membrane [8, 120]. For example, chlorination mechanisms on polyamide membrane are mainly divided into two kinds of mechanisms: polymer depolymerization and polymer deformation (e.g., N-chlorination) [8]. Firstly, mechanisms of polymer depolymerization due to chlorination were suggested by Koo et al. [121] and Avlonitis et al. [122]. They suggested polyamide’s oxidative chain cleavage and Hoffmann degradation in alkaline chlorine solutions, respectively. Secondly, in the case of polymer deformation, nitrogen atom in a polyamide can react with chlorine to generate N-chloroamide, because the nitrogen is electron- donating group by lone pair electrons which can share with the electron-withdrawal chlorine (Clδ+) created by a temporary dipole moment [123, 124]. Figure 4.1 shows a schematic description of the possible pathway of halogenated polyamide due to halogenation, which in this work explains the formation of N-halogenation. Membrane degradation due to chlorination affects membrane flux and rejection of inorganic salts as well as pharmaceutically active compounds [125]; however, systematic studies of membrane halogenation in the presence of HCl, HBr, and HI have not yet been reported.
In this work, systematic physico-chemical characterization was conducted to explain the effect of degradation by sulfuric acid in the range of pH 0 to 2 as well as hydrogen halides at pH 0 on the full aromatic membrane. Degraded membrane samples were characterized by various analytical tools such as (1) scanning electron microscopy (SEM) for observation of surface morphology, (2) attentuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) for measurement of chemical bonds on surface, (3) X-ray photoelectron spectroscopy (XPS) for confirmation of the surface’s atomic percentages, (4) goniometer for contact angle of membrane surface, (5) zeta potential analyzer by electrophoresis for surface charge of the membrane surface. Finally, filtration experiments were investigated to explain the effects of degradation by acid on membrane performance. As far as we know, this is the first study to systematically investigate the degradation of commercial NF full aromatic membrane by sulfuric acid as well as by hydrogen halides.
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Table 4.1 Chemical reaction for generating halogen gases by oxidation of hydrogen halides and gibbs energy in standard state (for one mole at 298K and 1 bar).
Chemical reaction formula for generating halogen gases from oxidation of hydrogen halides
Gibbs energy for reaction (kJ·mol-1)
4 HCl (g) + O2 (g) → 2Cl2 (g) + 2H2O (g) -76.0
4 HBr (g) + O2 (g) → 2Br2 (g) + 2H2O (g) -236.9
4 HI (g) + O2 (g) → 2I2 (g) + 2H2O (g) -423.6
Compound Gibbs energy (kJ·mol-1)
HCl (g) -95.3
HBr (g) -53.5
HI (g) 1.3
O2 (g) 0
Cl2 (g) 0
Br2 (g) 3.13
I2 (g) 19.38
H2O (g) -228.6
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Figure 4.1 Schematic description of possible pathway of halogenated polyamide due to halogenation generated by hydrogen halides and oxygen.
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