Jacek Wiśniewski, Małgorzata Kabsch-Korbutowicz, Sylwia Łakomska
Bromide ions are present both in surface and ground water and their concentration ranges from several to 800 µg/L. Bromides are not reported to be detrimental to human health but their presence in the water being disinfected is a contributing factor in the formation of harmful disinfection by-products (DBP). During water disinfection with chlorine, bromides are oxidized to hypobromous acid (HOBr), which reacts with natural organic matter (NOM) to form carcinogenic brominated trihalomethanes (BrTHM). When ozone is used in water disinfection, bromides are oxidized to hypobromite ions (OBr−) and thereafter to bromates (BrO3−). Bromates are ions exerting a carcinogenic effect on human organisms. According to the data published by the US Environmental Protection Agency, the lifetime risk of cancer disease amounts to 10-4, for a man consuming daily 2 L of water containing 5 μg BrO3−/L.The above data prove that bromides or bromates should be removed from drinking water. Among the methods used for this purpose, coagulation (for bromide removal) and granular activated carbon adsorption or reverse osmosis (for bromate removal) provide the highest removal efficiency. We proposed Donnan dialysis with anion-exchange membrane for removal of bromides or bromates from water. In this process, an anion-exchange membrane separates two solutions: the feeding solution (with harmful anions) and the receiver (with a simple salt of a relatively high concentration). Transport of the driving anions (e.g. chlorides) from the receiver to the feeding solution induces an equivalent, oppositely directed anion flow to the receiver. In this way the harmful anions that occur in the solution being treated (bromides or bromates) are replaced with neutral ions from the receiver (i.e. chlorides). Donnan dialysis was performed in a laboratory dialytic set-up containing 20 cell pairs with anion-exchange membranes, Selemion AMV (Asahi Glass) or Neosepta ACS (Tokuyama Corp.). The working area of the membranes amounted to 0.140 m2. The feed was natural water enriched with bromide salt (500 µg Br−/L) or with bromate salt (50 µg BrO3−/L). The receiver was NaCl solution with concentration ranging from 50 to 300 mM. It was found that Donnan dialysis with the anion-exchange membrane Selemion AMV enables high removal efficiency of bromides from natural water containing 500 µg Br-/L. The efficiency of bromide removal amounts to 86% at a relatively low NaCl concentration in the receiver (100 mM). The exchange of bromide ions for chloride ions is paralleled by the exchange of associated anions: sulphates (with 76% efficiency) and bicarbonates (with 70% efficiency). Compared to the anion-exchange process with Selemion AMV, the process involving Neosepta ACS (an anion-exchange membrane of a compact surface structure) provides a higher efficiency of bromide removal that amounts to 90%. In this process, retention of the associated anions is relatively high: sulphates are exchanged for chlorides with the efficiency of 3% and bicarbonates – with the efficiency of 43%. The anion-exchange process with the membrane Selemion AMV offers complete removal of bromates from natural water (containing 50 µg BrO3−/L), when salt concentration in the receiver is low (100 mM NaCl). There is a concomitant exchange of other anions for chloride ions: sulphates are exchanged for chlorides with the efficiency of 93% and bicarbonates – with the efficiency of 73%. The anion-exchange process also provides complete removal of bromates from natural water, when use is made of the Neosepta ACS membrane. However, the exchange of sulphate ions and bicarbonate ions for chloride ions is poor (3% efficiency and 47% efficiency, respectively). Such treatment approach may be recommended for implementation, when the concentration of anions (especially that of bicarbonates) in the water to be treated is low.
usuwanie bromków; wymiana anionów
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