Effect of sodium hypochlorite exposure on polysulfone recycled UF membranes and their surface characterization
Introduction
The population growth and the water demand exceed conventional available water resources. For this reason, the production of potable water has become a worldwide concern [1,2]. Reverse Osmosis (RO) is the most widely used desalination technology and it has been objective of a huge development in the last 40 years. RO has reached to a 44% share in world desalination production capacity and an 80% share in the total number of desalination plants installed worldwide [3].
The major drawback of membrane processes is the fouling phenomena that leads to progressive loss of permeate flux [[3], [4], [5]]. Normally, this effect is compensated by increasing the process pressure, which translates into an increase of energy consumption. In order to restore the permeate flux routine cleaning protocols are commonly carried out in desalination plants (with acids, bases and complexing agents), but usually these cleaning processes will be dependent on the type of fouling [6]. Polyamide thin film composite (PA-TFC) membranes currently account for over 95% of existing RO desalination plants [7]. PA-TFC is formed by three layers: a non-woven polyester support, an asymmetric porous polysulfone (PSF) interlayer and a polyamide (PA) ultra-thin layer. One of the cleaning techniques used is to employ a basic solution of sodium hypochlorite (NaOCl). However, it is known that this cleaning agent reacts relatively fast with a wide variety of organic substrates such as PA. In this sense, when the exposure level of hypochlorite is high, PA layer could be damaged and membrane performance could be profited to change membrane morphology and impaired performance. For this reason, several studies have been conducted in order to understand membrane aging and how the PA layer can be affected by cleaning compounds such as NaOCl. Kwon et al. [8,9] conducted a systematic study of crossed-linked PA membranes after exposing them to hypochlorite. These studies were reinforced with an extended membrane surface characterization, which allowed understanding in deep how chlorine exposure affects the chemical and morphological properties of the PA layer. Ettori et al. [10] studied the effect of chlorination (ageing) in seawater (SW) RO membranes at pilot scale.
In the same direction, there are some research groups which have studied the membrane ageing of ultrafiltration (UF) membranes (PSF layer). For example, Regula et al. [11] simulated the industrial cleaning in static conditions and studied membrane ageing of hollow fibre polysulfone/polyvinylpyrrolidone membranes at different concentrations, temperatures and soaking times after several oxidant agents. They completed the study with permeability measurement and mechanical strength tests, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM). They concluded that crackles appear on all the samples more aged at higher temperature (40 °C).
On the other hand, Huisman et al. [12] analysed the causes of membrane damage for UF membranes used to treat wastewater. Their results reveal that the membranes were not damaged by chlorine, other halogens or cleaning agents, but merely by mechanical forces e.g. pressure shocks. Nevertheless, Zondervan et al. [13] found that mechanical properties do not seem to change much after intense exposure to NaOCl but they were changes if the fibre were fouled.
The widely reported sensitivity of PA layer to oxidant compounds such as NaOCl has also been taken advantage of to recycle end-of-life RO membranes. In this way, in order to find a second life to these end-of-life membranes, diverse researchers began to investigate the possibility of transforming RO membranes into UF membranes by elimination of active PA layer [[14], [15], [16], [17]]. However, our recent work reported that it is possible to eliminate totally or partially the PA layer from RO end-of-life membrane in a controlled way to obtain UF or nanofiltration (NF) membranes at laboratory scale [18]. Until now, the transformation studies have been mainly focused on the evaluation of membrane performance by means of permeability and rejection coefficients. Nevertheless, studies where a complete surface characterization of PSF layer into transformed UF membranes is conducted are scarce. As example, Lawler et al. [19] just shows ATR-FTIR of virgin PA membranes and converted (300,000 ppm h chlorine exposure). Raval et al. [20] reported ATR-FTIR spectrum and SEM images. However, this characterization was focused on end-of-life RO membranes and regenerated RO membranes that were exposed to 1000 ppm h chlorine with the objective of reusing them as low-pressure RO processes.
In our previous work [21], a surface characterization of recycled membranes (NF and UF) and their fouling layers were analysed for the first time by different spectroscopic techniques (ATR-FTIR, SEM and contact angle among others). However, the NaOCl exposure dose studied was limited up to 50,000 ppm h, which requires unusable exposure time (410 h) for the industry. Therefore, in order to make the recycling process more attractive at industrial scale, the exposure time should be reduced. The present study goes one further step and it is focused on reducing the exposure time to 48 h and quantifying and comparing the effect of chlorine on the PSF layer at higher exposure dose (300,000 ppm h). In addition, the objective of this work is to study if the PSF layer of transformed UF membranes is affected in the transformation process and to obtain a deep understanding of their recycling process. For this purpose, membrane performance and surface characterization of PSF layer were extensively investigated evaluating transformed PSF UF membrane morphology and their properties.
Section snippets
Membranes and chemical reagents
Experiments were performed on 6 transformed UF membranes that were obtained from 6 end-of-life PA-TFC RO membranes. For this purpose, membrane coupons (216 cm2) were taken out from spiral wound modules with diameter of 8”. The end-of-life membranes had originally been used for water desalination for more than three years and present fouling of different nature. On one hand, TM 720-400 (Toray) and BW30 (Dow Filmtec) modules had treated brackish water (BW). On the other hand, TM 820C-400 (Toray)
Results and discussion
With the aim of verifying that the transformation process is effective regardless of the fouling nature of each membrane, a characterization of fouling of each starting membrane has been presented by TGA analysis, ICP-MS spectroscopy and bacteria detection enumeration, before membrane surface characterization.
Conclusions
In this work, the transformation of 6 end-of-life RO membranes has been carried out employing an oxidizing agent. Two exposure doses of free chlorine have been selected to obtain transformed PSF UF membranes. 50,000 ppm h dosage was obtained by using low concentrated solution free chlorine (124 ppm) and long exposure time (410 h), while 300,000 ppm h was reach using high concentration of free chlorine (6200 ppm) and low exposure time (48 h). Membrane performance and surface morphology of these
Acknowledgements
The authors acknowledge LIFE 13 ENV/ES/000751 TRANSFOMEM European project, CTM2015-65348-C2-1-R (MINECO/FEDER, UE) INREMEM National project, the Marie Curie Amarout II Europe program and the Regional Government of Madrid through program S2013/MAE-2716- REMTAVARES-CM for the financial support of this research. Collaborative companies like SADYT, VALORIZA AGUA and GENESYS INTERNATIONAL are also gratefully acknowledged to generously donate end-of-life membranes.
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