EFFECT OF CHLORINE DIOXIDE ON POLYAMIDE MEMBRANES AT VARIOUS PH VALUES

Abstract

Membrane separation process is an important process that it can convert seawater into potable water. These days, polyamide membranes are replacing cellulose membranes because of their high thermal, chemical, and hydrolytic stabilities. Seawater mostly contains organics and microorganisms that make membrane desalination systems weak due to bio-fouling. Therefore, pretreatment processes designed to eliminate organics and inactivate microorganisms are essential in desalination systems. Chlorine is widely used as a microorganism disinfectant in the industry. However, residual chlorine in the water causes PA membrane performance to degrade easily. It has been known that cross-linked aromatic PA membranes are very delicate to chlorine attack. Therefore, its incompatibility with free chlorine necessitates the search for alternate disinfectants. Halogens such as iodine (I2) and bromine (Br2), ozone (O3), monochloramine (MCA) and chlorine dioxide (ClO2) have been reported to be less aggressive as disinfectants. ClO2, One of them, is a potent and useful oxidizing agent used in water treatment. Especially, it has attracted attention as a replacement for chlorine disinfectant. However, previous researches cannot suggest accurate mechanism about effect of ClO2 on membranes, especially polyamide membrane. So, the objective of this study was to investigate the performance changes of cross-linked polyamide membranes caused by chlorine dioxide solutions at various pH values. The study has performed on three commercially available membranes (NF270, NF90 and BW30 from Dow-FilmTec©. The NF90 and BW30 membranes used as representative the aromatic polyamide membranes, whereas, the NF270 membrane has the semi-aromatic piperazine based membrane. Chlorine dioxide (ClO2) stock solution was prepared by the reaction of sodium chlorite (NaClO2) solution with sulfuric acid (H2SO4). The concentration of the chlorine dioxide stock solution was measured using an Iodometric Method with a 0.1 N sodium thiosulfate standard solution. The membrane degradation experiments were conducted by soaking tests under 3 different conditions: (i) constant chlorine dioxide concentration at various pH values, (ii) diverse chlorine dioxide concentrations at fixed pH, (iii) constant chlorine dioxide concentration at fixed pH values which we selected for a long time. After finished, the surface chemical structures of the virgin and modified membranes were identified by using ATR-FTIR, SEM, XPS, Contact angle and TOF-SIMS. Also, the performances of the virgin and modified membranes were identified by using the filtration systems. In the first part of this investigation, the Dead-end system was used to measure the water flux as performance of the membrane. NF90 and BW30 membranes treated 100 ppm ClO2 solutions at various pH values are similar tendency in term of the flux increase upon the pH value. However, water flux of NF270 membranes treated ClO2 slightly changed regardless of pH values. FE-SEM and ATR-FTIR data indicate that oxidizing ability of chlorine dioxide increases according to increasing pH values. The prominent chemical structures of the NF90 and BW30 polyamide membrane was easily attacked and broken by ClO2 upon increasing pH values. However, water flux of NF270 membranes treated chlorine dioxide slightly changed regardless of pH values. This indicates that the NF270 membrane has strong resistance to ClO2. In the second and third part of this investigation, changes of polyamide NF90 chemical structure were focused in detail. At pH 4, the flux decline and salt rejection rise are likely due to polymer rearrangement in the membrane after exposure. In this study, the molecules of chlorine dioxide in aqueous acidic solution (pH 4) were supposed to react with each other and change to chlorine. As a result, the chlorine dioxide made the membrane more hydrophilic as the chlorine did. At pH 12, the flux rise and salt rejection decline are why the polyamide structure was attacked and deteriorated by chlorine dioxide. So, the polysulfone layer under polyamide revealed. Consequently, Water flux awfully increased due to large pores that polysulfone layers have. Through these results, it guesses that collapse of polymer chains occurred in the membrane structure due to chlorine dioxide at pH 12. In the final part of this investigation, mechanism of polyamide degradation due to chlorine dioxide at specific pH values was proposed. ClO2 molecules at acidic condition are changed into chlorine molecules. As a result, new generated chlorine can attack and degraded the NF90 polyamide structure. The molecules of ClO2 in aqueous basic solution (pH 12) reacts rapidly each other and occurs the disproportionation. Also, chlorine dioxide at pH 12 has a highly strong oxidizing power, which can attack easily and deteriorate the polyamide structure. Second, in accordance with the pH values of ClO2 solution, the polymer structure of polyamide membrane such as NF90 changes and is attacked differently. The amide polymers of the polyamide membrane are deprotonated in chlorine dioxide solution at pH 12 and might be attacked easily the chlorine dioxide radical due to accepting one electron. However, it seems to need further experiments to prove this hypothesis. Ultimately, this study can propose an optimum pH condition of chlorine dioxide as disinfectant instead of chlorine: Neutral pH (pH 7) is the optimal pH for ClO2 disinfectant due to little change of flux and surface. Also, this study can propose that NF270 can be used when ClO2 is used as disinfectant in desalination process from seawater to portable water.