Membranes chlorine-tolerant membrane

The dimensionally stable characteristic of the metal anode made the development of the membrane chlorine ceU possible. These cells are typically arranged in an electroly2er assembly which does not allow for anode-to-cathode gap adjustment after assembly. Also, very close tolerances are required. 

Chemical attack is often a result either of fouling prevention or cleaning in response to fouling. Chlorine and hypochlorite damage most RO and NF membranes, as do oxidants generally (see discussion of chlorine tolerance below). 

Chlorine Tolerance Most of the best RO membranes are attacked by oxidants, and they are particularly susceptible to chlorine. A particularly sensitive locus for attack is the amidic hydrogen. Cellu-losic membranes are generally less sensitive, and pass the chlorine into the permeate giving downstream biocidal activity, veiy useful for under-the-sink RO. These factors are largely responsible for CA s survival in RO membranes. Chlorine, whatever its vices, has the virtue of being a known, effective, residual bactericide and a good inhibitor of... 

Membranes are commonly rated for their chlorine tolerance in ppm-hours, simply the product of the concentration and the contact time. Tolerance is temperature dependent. 

Thin film composite (TFC) is an ultrathin barrier membrane on polysulfone support layer, of good chemical stability. It has a wide operating pH range of 2.0 to 12.0 at 0 to 40 °C, but cannot tolerate chlorine. TFC membranes are better at rejecting silica than CA membranes. 

Cellulose membranes can generally tolerate a pH of 3 to 6 and 0.3 to 1.0 ppm of chlorine while TFC membranes can generally tolerate a pH of 3 to 11 and < 0.05 ppm of chlorine. Membrane tolerance is also described as the permittee cumulative ppm-hours of membrane exposure to chlorine. TFC membranes can range from 1000 to 12,000 ppm h. Always check specific filter specifications. RO membranes... 

Cellulose acetate was the first high-performance reverse osmosis membrane material discovered. The flux and rejection of cellulose acetate membranes have now been surpassed by interfacial composite membranes. However, cellulose acetate membranes still maintain a small fraction of the market because they are easy to make, mechanically tough, and resistant to degradation by chlorine and other oxidants, a problem with interfacial composite membranes. Cellulose acetate membranes can tolerate up to 1 ppm chlorine, so chlorination can be used to sterilize the feed water, a major advantage with feed streams having significant bacterial loading. 

For a few years after the development of the first interfacial composite membranes, it was believed that the amine portion of the reaction chemistry had to be polymeric to obtain good membranes. This is not the case, and the monomeric amines, piperazine and phenylenediamine, have been used to form membranes with very good properties. Interfacial composite membranes based on urea or amide bonds are subject to degradation by chlorine attack, but the rate of degradation of the membrane is slowed significantly if tertiary aromatic amines are used and the membranes are highly crosslinked. Chemistries based on all-aromatic or piperazine structures are moderately chlorine tolerant and can withstand very low level exposure to chlorine for prolonged periods or exposure to ppm levels... 

Since the late 1970 s, researchers in the US, Japan, Korea, and other locations have been making an effort to develop chlorine-tolerant RO membranes that exhibit high flux and high rejection. Most work, such as that by Riley and Ridgway et.al., focuses on modifications in the preparation of polyamide composite membranes (see Chapter 4.2.2).11 Other work by Freeman (University of Texas at Austin) and others involves the development of chlorine-tolerant membrane materials other than polyamide. To date, no chlorine-resistant polyamide composite membranes are commercially available for large-scale application. 

Unlike CA membranes, polyamide membranes cannot tolerate free chlorine or any other oxidizers. Some manufacturers quote 200 - 1,000 ppm-hrs of exposure until the membrane rejection is lost.21 This means after 200 - 1,000 hours of exposure to 1 ppm free chlorine, the membrane rejection will be unacceptably low. Chlorine attack is faster at alkaline pH than at neutral or acidic pH. 

Polyamide, composite membranes are very sensitive to free chlorine (recall from Chapter 4.2.1 that cellulose acetate membranes can tolerate up to 1 ppm free chlorine continuously). Degradation of the polyamide composite membrane occurs almost immediately upon exposure and can result in significant reduction in rejection after 200 and 1,000-ppm hours of exposure to free chlorine (in other words after 200-1,000 hours exposure to 1 ppm free chlorine). The rate of degradation depends on two important factors 1) degradation is more rapid at high pH than at neutral or low pH, and 2) the presence of transition metals such as iron, will catalyze the oxidation of the membrane. 

Membrane material. During the early days, membranes were usually made of cellulose acetate. At present, membranes can also be made from aromatic polyamide and thin-filmed polymer composites. Different membrane materials have their own distinctive characteristics, such as hydraulic resistance, pH range, temperature range, chlorine tolerance, and biodegradation tolerance. 

Incidentally, all of the cellulose acetate membranes will tolerate a limited amount of residual chlorine which allows chlorine to be used for feedwater disinfection. 

As noted above, the polyamide and polyurea membranes cannot tolerate an oxidizing agent, such as residual chlorine in the feed. Consequently, if these membranes are used and the feed has a residual chlorine content, then it is necessary to remove it. This is usually done by adding a stoichiometric excess of sodium bisulfite or sodium thiosulfate in accordance with recommendations from the membrane manufacturer. 

This section intends to provide a review on the advanced materials used in the recent development of TFC-NF membranes and the effects of the advanced materials on the improvement of composite membrane properties with respect to permeability/selectivity, chlorine tolerance, solvent stability, fouling resistance, etc. The advanced materials that are used in composite membrane fabrication to improve either the top active layer properties or substrate properties can be generally categorized into (a) active monomer, (b) surfactant/ additive, (c) nanoflller, and (d) polymeric substrate. 

Development of Membranes for Boron Removal, Chlorine Tolerance, and Antibiofouling.38... 

High product water recovery was achieved with Toyobo s advanced modules design and with high permeability CTA membranes. Further, CTA membranes are tolerant to residual chlorine in feed water up to 5 ppm compared to 2 ppm for cellulose acetate (CA) and <0.1 ppm for PA membranes [59]. This prevents biofouHng, which is a major advantage. 

High performance thin-film composite membranes for reverse osmosis applications were fabricated by coating solutions of a highly chlorine-tolerant disulfonated PAES [92,93]. As base monomers, 4,4 -dichlorodiphenyl sulfone and 4,4 -biphenol are used. 4,4 -dichlorodiphenyl sulfone is then directly sulfonated to get a disulfonated monomer, 3,3 -disutfonate-4,4 -dichlorodiphenyl sulfone. These monomers can be directly copolymerized on a commercially available porous polysulfone support. 

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