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.
유기, 무기, 콜로이드 및 미립자 물질에 의한 막오염은 수투과도 감소와 멤브레인 공정의 운영 비용 증가를 초래합니다. 따라서, 염기, 산, 킬레이트 및 계면 활성제를 사용한 화학적 세정은 멤브레인 표면상의 오염 층을 제거하기 위해 사용됩니다. 그러나, 빈번한 화학 세정으로 막 수명이 단축되고 역삼투막의 표면특성이 변화한다는 것이 알려져있습니다. 몇몇 연구들은 역삼투막의 특성과 투과성능에 다양한 세척 화학 물질의 영향을 기술하고 있지만, 화학 세정제 장기노출에 의한 역삼투막의 저하 메커니즘은 여전히 불분명합니다. 이번 연구에서는 역삼투막 특성에 대한 장기간의 화학 물질 노출의 영향을 조사했습니다.1 장에서는 연구 배경을 소개하고, 연구의 목적과 범위를 나타내었습니다.2 장에서는 역삼투막의 세정물질의 종류와 메커니즘을 조사하고. 역삼투막에 대한 화학세정 약품의 영향, 이전 연구의 한계점에 대해 나타냈습니다.3 장에서는 20 일 동안 화학세정제(HCl, 구연산, NaOH, NaOH + EDTA)에 장기간 노출된 역삼투막의 특성 변화를 평가하기 위해 수투과도, 염투과도 및 붕소 제거율을 포함한 역삼투막 성능 변화를 확인하였습니다.4 장에서는 접촉각 측정, 표면전위 분석, X- 선 광전자 분광법 (XPS) 분석을 포함한 정교한 분석을 통해 화학 세정제에 따른 역삼투막 표면의 물리 화학적 성질의 변화를 확인하였습니다.5 장에서는 7개의 CECs (contaminants of emerging concern) 물질을 선정하여 장기간 화학세정 노출에 의한 역삼투막의 미량오염물질 제거율 변화를 평가하였습니다.6 장에서는이 연구의 결론을 제시하였습니다.
