A. A. Abuhabib

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/60212

#### **Abstract**

The study focuses on NF membranes modification and performance improvement while desalinating brackish water. The study provides valuable information about flux and rejection changes and relationship with pressure changing before and after modification. Experimental works included in the study investigate modified and unmodified NF membranes performance while filtering synthesized single salt and mixture salt solution at various concentrations (ranged from 1000 ppm to 4000 ppm) and various pressure magnitudes (pressure ranged from 2 to 10 bars). The rejection rates witnessed an increase after membrane modification took place with about 11– 30% for magnesium sulfate and sodium sulfate, and 50–60% for sodium chloride and potassium chloride.

**Keywords:** Nanofiltration, Membrane, Desalination, Modification, UV graft

## **1. Introduction**

Nanofiltration membranes have made noticeable establishment and found a way into many industries since their first introduction in the early 1990s. The major industries in which these membranes are variously applied and served are water and wastewater industries. The characteristics of these membranes determined by high flux, high rejection of salts, and low energy consumption associated with low pressure requirements enabled these membranes to apply significantly and perfectly [1-5]. However, obtaining an improved flux and rejection as well as fouling resistance of NF membranes for various applications are of major interest for

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researchers [6]. Technically, surface modification is considerably applied to improve mem‐ brane properties in terms of flux, salt rejection, and fouling resistance [6, 7]. In addition, micropollutant removal could also be improved by surface modification [8]. Various modifi‐ cation techniques have been applied for NF membranes by researchers including radical polymerization [9], low temperature plasma [10], pre-oxidation [11], layer-by-layer alternating polyelectrolyte deposition (APD) [12], ionizing radiation [13], and photochemical techniques [14]. However, photochemical grafting techniques (mainly UV-initiated grafting) have been widely used due to their low cost of operation, mild reaction conditions, selectivity to absorb UV light without affecting the bulk polymer, and the possibility of easy incorporation into the end stages of a membrane manufacturing process [15].

Several hydrophilic monomers are commonly used including N-vinyl-2-pyrrolidinone (NVP), 2-hydroxylethyl methacrylate (HEMA), acrylic acid (AA), acrylamide (AAM), and 2-acryla‐ midoglycolic acid (AAG) for membrane surface modification of which such monomer would be grafted on the membrane surface by UV-initiated graft polymerization [7, 16-19]. Generally, modification process sufficiency is measured by measuring membrane properties and performance after modification. Two common methods are considered and followed for the UV-initiated grafting of membranes: the dip method and the immersion method. For the same support, with the same the monomer concentration and irradiation time, the degree of grafting achieved using the dip method is two to three times higher than with the immersion method. However, in certain cases, membranes modified by the dip method showed lower rejection factors compared to the membrane support and membranes modified by the immersion method [19].

Normally, UV-initiated grafting of polyethersulfone membranes involves two parallel competitive processes, crosslinking and chain scission, that determine the final membrane transport properties [20]. Both mechanisms are very important for any modified membrane where crosslinking and chain scission may affect hydrodynamic resistance and membrane selectivity, respectively. Kaeselev et al. [21] illustrated that the membrane hydrodynamic resistance is increased relatively with crosslinking while the membrane selectivity loss is affected by chain scission in a direct manner. This study focuses on NF membrane performance improvement in terms of flux and rejection for desalination applications via surface modifi‐ cation.

### **2. Materials and methods**

#### **2.1. Materials**

NF membrane used manufacturing properties, salts (NaCl, MgSO4, Na2SO4, and KCl) used for filtration experiments, chemicals used for modification (acrylic acid and ethylenediamine dihydrochloride), and experiment set-up. One commercial NF membrane denoted as NF-1 was purchased from Amfor Inc. China. A summary of membrane information is given in Table 1. The monomers used in this study were acrylic acid (99% purity, purchased from Sigma-Aldrich, USA) and ethylenediamine dihydrochloride (99% purity, purchased from Frinde‐ mann Schmidt chemicals, Germany). In addition, high purity salts including MgSO4 and NaCl, 99% purity, were purchased from John Kollin, UK.


**Table 1.** NF membranes manufacturer characteristics

#### **2.2. Methods**

researchers [6]. Technically, surface modification is considerably applied to improve mem‐ brane properties in terms of flux, salt rejection, and fouling resistance [6, 7]. In addition, micropollutant removal could also be improved by surface modification [8]. Various modifi‐ cation techniques have been applied for NF membranes by researchers including radical polymerization [9], low temperature plasma [10], pre-oxidation [11], layer-by-layer alternating polyelectrolyte deposition (APD) [12], ionizing radiation [13], and photochemical techniques [14]. However, photochemical grafting techniques (mainly UV-initiated grafting) have been widely used due to their low cost of operation, mild reaction conditions, selectivity to absorb UV light without affecting the bulk polymer, and the possibility of easy incorporation into the

Several hydrophilic monomers are commonly used including N-vinyl-2-pyrrolidinone (NVP), 2-hydroxylethyl methacrylate (HEMA), acrylic acid (AA), acrylamide (AAM), and 2-acryla‐ midoglycolic acid (AAG) for membrane surface modification of which such monomer would be grafted on the membrane surface by UV-initiated graft polymerization [7, 16-19]. Generally, modification process sufficiency is measured by measuring membrane properties and performance after modification. Two common methods are considered and followed for the UV-initiated grafting of membranes: the dip method and the immersion method. For the same support, with the same the monomer concentration and irradiation time, the degree of grafting achieved using the dip method is two to three times higher than with the immersion method. However, in certain cases, membranes modified by the dip method showed lower rejection factors compared to the membrane support and membranes modified by the immersion

Normally, UV-initiated grafting of polyethersulfone membranes involves two parallel competitive processes, crosslinking and chain scission, that determine the final membrane transport properties [20]. Both mechanisms are very important for any modified membrane where crosslinking and chain scission may affect hydrodynamic resistance and membrane selectivity, respectively. Kaeselev et al. [21] illustrated that the membrane hydrodynamic resistance is increased relatively with crosslinking while the membrane selectivity loss is affected by chain scission in a direct manner. This study focuses on NF membrane performance improvement in terms of flux and rejection for desalination applications via surface modifi‐

NF membrane used manufacturing properties, salts (NaCl, MgSO4, Na2SO4, and KCl) used for filtration experiments, chemicals used for modification (acrylic acid and ethylenediamine dihydrochloride), and experiment set-up. One commercial NF membrane denoted as NF-1 was purchased from Amfor Inc. China. A summary of membrane information is given in Table 1. The monomers used in this study were acrylic acid (99% purity, purchased from Sigma-Aldrich, USA) and ethylenediamine dihydrochloride (99% purity, purchased from Frinde‐

end stages of a membrane manufacturing process [15].

method [19].

24 Desalination Updates

cation.

**2.1. Materials**

**2. Materials and methods**

#### *2.2.1. Membrane modification*

The membrane was modified using a monomer solution of 4% acrylic acid and 1% ethylene‐ diamine dihydrochloride (w/v) following the immersion method [7, 22]. The monomer was grafted on the membrane surface using a commercial UV device (LC5) supplied by Hama‐ matsu, Japan. The membrane was exposed to UV radiation for 5 minutes. The unmodified membrane was marked with a UV time of 0 min. The modification procedure is detailed as followed:


#### *2.2.2. Membrane permeation and rejection*

Filtration experiments were performed in a stainless steel cylindrical batch cell (HP 4750), served as a dead-end filtration system (cell volume: 300 cm³), supplied by Sterlitech (UK). The working pressure in the cell was applied by a nitrogen gas cylinder in the range of 2 to 10 bar for unmodified and modified membranes; the experiments were conducted at room temper‐ ature. The membrane active area was 14.6 cm2 . Membranes were washed with ultrapure water and compacted at 10 bar of pressure for 20 minutes prior to use. No further pretreatment was performed on the membranes, bearing in mind that the manufacturer did not provide certain instructions for preparing the commercial membrane prior to use. A new filter was used for each experiment. Ultrapure water was used with conductivity below 1 μs/cm. The system was flushed with ultrapure water before and after use. Solutions of MgSO4, NaCl, Na2SO4, and KCl at four various concentrations each (1000, 2000, 3000, and 4000 ppm) were used as the feed for unmodified and modified membranes to measure their rejection and determine the best performing membranes in terms of rejection. The concentrations of the feed and permeate were measured depending on solution conductivity measurements using a commercial conductiv‐ ity meter supplied by Martini instruments (Romania).
