Long-term impacts of chemical cleaning agents on reverse osmosis (RO) membrane properties

Abstract

Membrane fouling by organic, inorganic, colloidal, and particulate matters results in a decrease of the water flux and an increase of the operational cost in membrane processes. Thus, chemical cleaning using bases, acids, chelates, and surfactants are employed to eliminate fouling layers on the membrane surface. However, it has been acknowledged that frequent chemical cleaning resulted in a shortened membrane life and changed properties of reverse osmosis (RO) membrane. Several studies have described the impact of various cleaning chemicals on the properties and performance of polyamide RO membrane, however, the detailed deterioration mechanism of RO membrane performance by chemical cleaning agents is still unclear. In the current work, we examined the impact of long-term chemical exposure on the properties of polyamide layer of RO membrane. In Chapter 1, a research background was introduced. Then, objective and scope of this research were proposed. In Chapter 2, type and mechanism of cleaning chemicals on RO membrane were reported. After that, the impact of cleaning chemicals on the polyamide membrane and a limitation of previous studies were shown. In Chapter 3, RO membrane performances including pure water flux, NaCl passage, and boron rejection were measured to evaluate the impact of long-term exposure to four cleaning chemicals (HCl, citric acid, NaOH, and NaOH+EDTA) during 20 days. In Chapter 4, sophisticated analysis was carried out including contact angle measurement, electrokinetic analysis, and X-ray photoelectron spectroscopy (XPS) analysis to correlate the type of chemical cleaning agents with the changes of physico-chemical properties of polyamide layer on RO membrane surfaces. In Chapter 5, to evaluate the impact of surface changes during the rejection of organic matters such as contaminants of emerging concern (CECs), seven CECs were selected and tested. In Chapter 6, the conclusion of this research was presented.

In Chapter 3, the pure water flux and salt passage of the membrane after the chemical exposure were significantly changed depending on types of cleaning chemicals. The pure water flux after exposures to the base chemicals (NaOH and NaOH+EDTA) for 20 days was significantly increased from 41.2±1.2 LMH to 64.4±2.0 LMH and 53.1±2.2 LMH, respectively. As a similar trend, NaOH and NaOH+EDTA increased salt passage of RO membrane to 1.95 times and 1.59 times compared to that of the virgin membrane. In addition, after chemical exposure to NaOH for 20 days, boron rejection was reduced by 30.4% compared to the virgin membrane. In contrast, the membrane exposed to acidic chemicals (HCl and citric acid) indicated a minimal change of the pure water flux and boron rejection. The membrane exposure to HCl for 20 days, however, exhibited a decrease in salt passage to 0.59 times compared to the virgin membrane. This result provided that the membrane performances were changed depending on the type of cleaning chemicals, and NaOH induced the polyamide structure more opened by swelling effect, while HCl caused the effect called as shrinkage of polyamide membrane.

After the chemical exposure of RO membrane for 20 days, the surface properties such as wettability, surface zeta potential, and polyamide structure on RO membrane were significantly altered depending on the cleaning chemicals in Chapter 4. After the exposure to acid chemicals, the surface of RO membrane turned more hydrophilic, while the membrane exposed to base chemicals showed the decrease of hydrophilicity compared to the virgin membrane. Furthermore, the ratio of oxygen to nitrogen (O/N), which denoted the structure of amide bonding, was increased after the chemical exposure from 1.46 (virgin membrane) to 1.57 and 1.51 for HCl and NaOH, respectively. Therefore, it can be concluded that the membrane exposed to HCl and NaOH experienced a decrease in the degree of cross-linking by hydrolysis.

In Chapter 5, changes in contaminants of emerging concern (CECs) rejection by RO membrane exposed to cleaning chemicals were determined. The rejection of charged CECs was not changed significantly compared to the virgin membrane (R>96.5%). However, the rejection of acetaminophen, low molecular and neutral CECs, was significantly decreased from 89.5% to 71.9% after exposures to NaOH, but slightly increased to 93.8% after exposure to HCl for the membrane exposed 20 days. This result is in agreement with the change in the salt passage and boron rejection of RO membranes exposed to HCl and NaOH. Consequently, the results showed that NaOH causes a significant decrease in rejection of neutral and low molecular (M.W<200) CECs, possibly due to the enlarged free-volume hole radius of polyamide structure.

Therefore, this study suggested that chemical exposure test of RO membrane should be conducted in addition to cleaning efficiency tests when selecting cleaning chemicals to minimize the potential deterioration of polyamide structures of RO membrane.