Impact of halogen based disinfectants in seawater on polyamide RO membranes
Introduction
Biofouling of reverse osmosis (RO) membranes is a major cause of operational problems in RO plants worldwide. Intakes for seawater RO desalination systems are typically subjected to intermittent chlorination to control biofouling in the intake structure, equipment and on the membranes. Reverse osmosis desalination systems employing conventional pre-treatment (i.e., coagulation and sand filtration) use intermittent or continuous chlorination with target residual chlorine concentrations of 0.2–4.0 mg/L as Cl2 and typical contact times between 15 and 30 min [1]. Nevertheless, RO membrane suppliers do not recommend the continuous use of oxidant (i.e. chlorine) due to poor resistance of polyamide membrane to it. Often, sodium bisulfite is added to remove excess chlorine or alternative disinfectants to chlorine are used such as ozone, chlorine dioxide, hydrogen peroxide, and chloramines. However, these oxidants also damage polyamide membranes. da Silva et al. [2] concluded, based on their work and the work of Tessaro et al. [3], that similar degradation mechanism of polyamide membranes occurs by exposure to chlorine, monochloramines, and chlorine dioxide. One of the most important considerations in assessing disinfectants is balancing biocidal effectiveness with by-product formation. Yet, the effect on the membrane must also be considered. Oxidation of membranes is a complex phenomenon due to the great variety of substances present in the feed water which may interfere to some extent in the oxidative process [2]. Generally, polyamide membrane degradation depends on the solution pH and concentration of the oxidant. For example, at lower pH and higher concentration of hypochlorite solution greater deterioration of membrane performance is expected [4].
Most polyamide RO membrane manufacturers recommend that the chlorine concentration of feed water should be lower than 0.1 mg/L. Performance change of polyamide membrane, due to chlorination, has been identified as related to specific structural changes within the polyamide [5]. The mechanism at which chlorine damages the polyamide RO membrane includes N-chlorination by substituting the hydrogen on the amide nitrogen (N–H) followed by ring-chlorination via Orton Rearrangement [6]. The chlorine substitution was linked to physical deformation or chemical cleavage of the polymer chain [5], resulting in flux changes [7], [8], [9]. Soice et al. [10] suggested that the major cause of performance decline following chlorination was by physical separation of the polyamide skin layer from the polysulfone support layer. Kwon and Leckie [5] showed that chlorine chemically changed the surface properties of polyamide membrane by bounding to its surface. The extent to which it bounded appears to be determined by the combination of the membrane surface charge and the speciation of chlorine.
Efforts are made to develop chlorine-tolerate polymer membranes for seawater desalination which will enable to extend the membrane lifetime (up to 3–5 years for commercial membranes), simplify maintenance and operation of RO systems by eliminating the need for dechlorination, and consequently reduce costs. Research groups concentrate on examining new polymer material, surface modification such as hydrophilicity and crosslinking, and structural design of monomers for polyamide.
Bromide and ammonia are the most important species that determine the fate of chlorine applied to seawater. Bromide is oxidized by chlorine to produce hypobromous acid and hypobromite ions (Eqs. (1), (2)). In seawater HOBr is probably the most reactive species, although a significant fraction is made up of molecular bromine because of the relatively high concentration of bromide tending to displace the equilibrium of reaction (3) toward the left [11].Br− + HOCl → HOBr + Cl−HOBr → OBr− + H+Br2 + H2O → HOBr + H+ + Br−Bromide and chlorine compete with ammonia to form chloramines (Eqs. (4), (5), (6)). Monochloramine is the main specie form when ammonia is in excess to chlorine (on a molar basis). At high chlorine concentration compared with ammonia, dichloramine is formed.NH3 + HOCl → NH2Cl + H2ONH2Cl + HOCl → NHCl2 + H2ONHCl2 + HOCl → NCl3 + H2OAt low ammonia concentration bromamines may be formed from the hypobromous acid produced from bromine (Eqs. (7), (8), (9)).NH3 + HOBr → NH2Br + H2ONH2Br + HOBr → NHBr2 + H2ONHBr2 + HOBr → NBr3 + H2OThe degree of halogen substitution on the nitrogen is determined by the pH and the halogen to ammonia ratio [11]. Monobromamine and dibromamine are unstable and decay rapidly under the conditions encountered in water treatment and distribution systems [1]. Since the rate of reaction (4) is about 1000 times faster than the rate of reaction (1), chloramine can be formed in seawater containing 65–120 mg/L Br− even if the ammonia concentration is low. The reaction products depend on the pH, the relative concentrations of hypochlorous acid and ammonia, the reaction time, and the temperature [11].
In this study formation and stability of halogen based disinfectants, including monochloramines, free chlorine, and free oxidants (a mixture of free chlorine and free bromine) were studied. The effect of these oxidants on polyamide RO membranes was investigated in synthetic Ocean seawater. Membrane damage was evaluated by comparing the baseline permeability and salt rejection of virgin membrane to its performance following exposure to the halogen based oxidants.
Section snippets
Membranes
Hydranautics ESPA2 composite polyamide and GE Septa™ CF polyamide RO AD flat sheet membrane were used. The main characteristics of the membranes, as listed by the manufacturers, are presented in Table 1.
Materials
Synthetic Ocean seawater was prepared according to the ASTM International standard for specification for substitute Ocean water (method D1141, 2003; [12]).
Sodium hypochlorite solution was prepared by diluting commercial bleach with de-ionized (DI) water. Its concentration was determined,
Speciation of chlorine/bromine in Ocean seawater
Fig. 2 represents the ratio between free chlorine and free bromine formed during chlorination of synthetic Ocean seawater at a constant concentration of 65 mg/L bromide and varying concentrations of chlorine, ranging from 4 to 120 mg/L as Cl2. Chlorination at molar ratio < 1.0 chlorine to bromide, at pH 8.2, resulted in formation of mostly free bromine > 95% (HOBr, OBr−). Further increase in the chlorine concentration resulted in a decrease in the free bromine species and an increase in free chlorine
Conclusions
The ratio between free chlorine and free bromine formed during chlorination of synthetic Ocean seawater was found to depend on the ratio between the bromides, naturally occurring in seawater, and the added chlorine. Chlorination at molar ratio < 1.0 Cl2 to Br2, at pH 8.2, resulted in formation of mostly free bromine > 95% (HOBr, OBr−).
Free bromine species were found to be more stable than free chlorine. While free chlorine stability was not affected by the presence of bromide, monochloramines
Acknowledgment
The authors gratefully acknowledge the financial support of Veolia Environmental Services.
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