One-Step Fabrication of Methylthiazole-Functionalized Anion Exchange Membranes for Diffusion Dialysis

Anion exchange membranesusing brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) as starting material were prepared from one-step functionalization by 4-methylthiazole (MTz). The obtain membranes with high thermal stability and mechanical strength showed satisfied diffusion dialysis performance for acid recovery. Specifically, when the optimal membrane was evaluated to recover acid from the simulated iron polishing waste solution (1.0 mol·L-1 FeCl2 + 0.2 mol·L-1 HCl), its acid diffusion coefficient (UH+) was 0.019 m h-1 and separation factor was 40.1 at 25°C, both of these two parameters are much higher than the corresponding values of the commercial DF-120 membrane, suggesting the great potential in the practical application for acid recovery.


Introduction
Diffusion dialysis (DD) employing anion exchange membrane (AEM) as core component is recognized as the most economically promising technology for acid recovery from the industrial waste water, such as metal etching, titanium white production, stripping processes and so on [1] [2] [3] [4] [5]. It is well known that AEM has a positively charged matrix, such chemical structure may allow anions in the acidic waste solution (e.g. Cl − , (always water) because of the electrostatic attraction, and then cations (e.g. H + and metal ions) should then permeate into the side with lower concentration to fulfill the requirement of the electric neutrality [6]. Notably, such DD process is spontaneous and the driving force is only from the concentration gradient between the feed and permeate side. Therefore, DD technic reveals many advantages, especially the low economic cost and great environmental benefits, as compared with other separation processes [7] [8] [9]. Obviously, the permeate rate of proton will directly decide the acid recovery rate and the ratio of permeate rate between proton and metal ions will decide the purity of the recovered acid. The two above-mentioned parameters are designed as acid diffusion coefficient (U) and separation factor (S).
Obviously, AEMs are the critical component for the DD module. They are required to possess high ion permeability, good thermal and chemical stabilities, proper water uptake and low swelling ratio. It was found that the recent research mainly focused on the polymer chains of AEMs [10]- [15], and the investigation on the functional group for ion exchange is rare. Moreover, among the AEMs used in DD, they are always bearing quaternary ammonium groups as ion exchange groups. It is noted that there are some drawbacks of quaternary ammonium functional groups. Such as unsatisfied thermal stability and low ion permeability compared with other ion exchange groups [16]. In order to enhance the thermal stability and the ion permeability, as well as to extent the variety of the ion exchange groups, other functional agent being able to offer the newly ion exchange groups should be investigated. 4-methylthiazole (MTz) is a nucleophilic agent that has a high reaction activity, also its alkalinity (pKa 30.3) is much higher than trimethylamine (pKa 10.8) as the procedure of quaternary ammonium groups. Therefore, compared with quaternary ammonium groups, methylthiazolium from MTz should endow the obtained membrane with much higher ion permeability [16]. To our best knowledge, methylthiazole as he functional reagent for the preparation of AEMs for DD has not been investigated. Therefore, in this paper, 4-methylthiazole was selected for the functionalization of AEMs. Moreover, bromiated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) with excellent mechanical properties and good membrane forming ability was selected as the polymer backbone. Because of the high reactivity between MTz and −CH 2 Br groups of BPPO, the reaction between them can be easily carried out to get the MTz-PPO AEMs via one-step reaction.
Specifically, the main purpose of this work is to prepare AEMs with high diffusion dialysis performance based on BPPO. The fabrication process will be investigated in detail and the chemical structure of the obtained AEMs will be conducted. The diffusion dialysis-related properties like ion exchange capacity (IEC), water uptake (WU) and swelling ratio will also be investigated. More important, the DD performance of the obtained AEMs will be evaluated using the

Synthesis of MTz-PPO-x
MTz-PPO AEMs were fabricated by one-step reaction between BPPO and MTz.
Firstly, 2 g BPPO was dissolved in 18 g NMP to form a homogenous solution, a certain amount of MTz was added dropwise into the BPPO/NMP solution. The solution was firstly stirred for 12 h at 60˚C and then stirred at 90˚C for 6 h. Finally, the solution was poured into ethanol to get the brown precipitate, which was collected and washed with water, followed by drying in an oven for 12 h at 50˚C. Then the precipitate was re-dissolved in NMP to form a 10 wt% solution and then casted onto a pre-cleaned glasses, which would be dried in an oven at

Characterization
Fourier Transform Infrared Spectroscopy (FTIR) spectra were measured using a where 2 3 Na CO C and 2 3 Na CO V are the concentration and the consumed volume of Na 2 CO 3 solution, and dry W is the dry weight of the membrane.
Water uptake (WU) and swelling ratio refer to the respective weight and length change after fully hydration. Specifically, the dry membrane was weighted and its length was measured, then it was immersed in water at room tempera- where dry W and wet W are the dry and wet weights of the membrane, respectively, and dry L and wet L are the dry and wet lengths of the membrane.
Diffusion dialysis (DD) test was conducted according to the previously reported method [6]. where

Results and Discussion
As mentioned above, MTz-PPO AEMs were synthesized via facile one-step reaction. The final obtained membranes are uniform, transparent and mechanically strong (See Figure 1). What is more important, their thickness and shape can be easily controlled.
FT-IR analysis can be used to identify the presence of the functional groups as well as the chemical composition of the membranes. As shown in Figure 2, the H. Hu, W. Song Open Journal of Physical Chemistry  spectrum of BPPO membrane has a characteristic band at ca. 588 cm −1 , attributing to the C-Br stretching [17]. By contrast, the selected MTz-PPO-5 membranes displays a characteristic band at ca. 3395 cm −1 , attributing to the −OH stretching from the adsorbed water molecules [18]. It also displays two characteristic bands at 1663 cm −1 and 1578 cm −1 , corresponding to the stretching of C=N and C-N groups [19], respectively. Besides, one newly formed characteristic band at ca. AEMs and the MTz-PPO-5 AEM was selected. As can be seen in Figure 3, BPPO membrane starts to degrade at 250˚C, suggesting that BPPO is an excellent starting material because of the good thermal stability. Therefore, MTz-PPO-5 membrane with the similar polymer chain also possesses good thermal stability, and its starting degradation temperature is about 175˚C, due to the thermal degradation of the charged thiazolium rings, which is also higher than that of quaternary ammonium groups (ca. 140˚C) [21]. That is because of the presence of the resonance structure, improving the stability of the ion exchange groups.
Such results are satisfied and the thermal stability of the obtained AEMs can meet the requirement of the practical application.
The IEC values of the MTz-PPO AEMs are shown in Figure 4. From MTz-PPO-1 AEM to MTz-PPO-5 AEM, the IEC values increase from 0.48 to 2.07 mmol•g −1 . It is noted that the IEC value was determined by the amount of the ion exchange groups, which was formed by the reaction between −CH 2 Br groups of BPPO polymer and MTz, because that the amount of BPPO was fixed, the ratio of the amount of MTzto −CH 2 Br will directly decide the functional degree as well as the amount of quaternary ammonium groups [22]. From MTz-PPO-1 to MTz-PPO-5 membrane, the ratio of MTz to −CH 2 Br increases from 20% to 100%, so it is easy to understand the increasing tendency of the IEC value of MTz-PPO AEMs.   a result, the swelling ratio will also increase. In one word, the increasing hydrophilicity should play a positive role on water uptake and swelling ratio of the membranes. So the increasing trends of water uptake and swelling ratio from MTz-PPO-1 to MTz-PPO-5 are easily to be understood.

Conclusion
In summary, a facile one-step reaction for the fabrication of MTz-PPO AEMs with high diffusion dialysis has been developed. The prepared AEMs bearing methylthiazolium as ion exchange groups show high ion exchange capacity, proper water uptake, good swelling and thermal stabilities. The acid diffusion coefficient (U) and separation factor of the optimal AEM were 0.019 m•h −1 and 40.1 at 25˚C, respectively, when being employed in the diffusion dialysis test from the simulated acidic waste solution. In one word, the satisfied diffusion dialysis performance of the obtained AEMs prepared from a low cost strategy can allow the practical application in acid recovery. For instance, the optimal membrane herein can be used for the acid recovery from the industrial waste water, such as metal etching, titanium white production, and stripping processes and so on.