- Email: [email protected]
Bromate removal in the ion-exchange process
Desalination 261 (2010) 197–201
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Bromate removal in the ion-exchange process Jacek A. Wiśniewski ⁎, Małgorzata Kabsch-Korbutowicz Institute of Environment Protection Engineering, Wroclaw University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 27 October 2009 Received in revised form 11 March 2010 Accepted 12 March 2010 Available online 12 June 2010 Keywords: Donnan dialysis Ion-exchange membranes Disinfection by-product
a b s t r a c t To remove bromates from water, use was made of anion exchange in the process of Donnan dialysis conducted with an anion-exchange membrane. Under such conditions, the removal of bromates from a onecomponent solution was found to be very high (more than 90%). It was observed that the presence of − accompanying anions (NO− 3 and HCO3 ), whose concentrations were by several orders of magnitude higher than the concentrations of bromates, brought about a decrease in the rate and efficiency of bromate removal from the feed. In the process with the Selemion AMV membrane the removal of bromates amounted to approximately 84% at a salt concentration of 300 mM NaCl in the receiver. Anion exchange in Donnan dialysis with Neosepta AFN (a membrane characterized by a high ion-exchange capacity and a high water content) proceeded at a faster rate and was found to be more efficient only with respect to the ions that dominated in the feed, i.e. nitrates and bicarbonates. In this process the removal of BrO− 3 ions was less efficient than in the process conducted with the Selemion AMV membrane, and failed to exceed 70%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Ozone is a major oxidizer and disinfectant applied in water treatment for drinking purposes. Compared to chlorine, ozone limits the formation of some halogen disinfection by-products and exerts a deactivating effect on some of the microorganisms that are resistant to chlorine (e.g. Cryptosporidium parvum) [1]. In some instances, however, the source of potable water contains bromides, which enter the groundwater as a result of sea water intrusion, or due to the penetration of industrial and agricultural wastewaters containing bromides [2,3]. If the water to be ozonated contains bromides (even of relatively low concentrations approaching 100 μg/L), this will lead to the formation of bromates, ions with potential carcinogenic implications to human organisms [1–3]. The process of bromate formation involves the following approximate reactions [3]: −
O3 þ Br →O2 þ OBr
−
−
O3 þ OBr →2O2 þ Br
−
−
−
2O3 þ OBr →2O2 þ BrO3
As can be seen, ozone oxidizes bromide to the hypobromite ion (OBr−), which is further oxidized to bromate or to an unidentified species (probably BrO− 2 ), that regenerates bromide ion [3]. ⁎ Corresponding author. Tel.: + 48 71 3203225; fax: + 48 71 2382980. E-mail address: [email protected] (J.A. Wiśniewski). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.03.029
American researchers have reported that the risk of developing a cancer disease is 10−4 when the concentration of bromates in drinking water totals 5 μg/L, and 10−5 when it amounts to 0.5 μg/L [4]. For these reasons the permissible concentration of bromates in potable water established for the European Union and the United States is now 10 μg/L [4]. The same value has recently been defined as permissible also by the WHO [5]. The concentration of bromates in the water being ozonated is influenced by a variety of factors, such as bromide ion concentration, ozone dose, reaction time, pH, and natural organic matter (NOM) concentration [2,3]. It has been found that at an initial concentration of Br− ions in the water higher than 100 μg/L, the concentration of bromates in the water upon ozonation can exceed 50 μg/L, if the ozone dose and contact time applied have provided an efficient disinfection (minimum 99% deactivation of Cryptosporidium oocysts) [1,2]. Several methods are in use for reducing the concentration of bromates in the water after ozonation. Amongst the most widely used is adsorption on granular activated carbon (GAC) [6–9]. In this process − the BrO− ion on the GAC surface [9]. The 3 ion is reduced to the Br efficiency of reduction, which initially exceeds 60%, deteriorates over time, owing to the gradual transformation of the GAC bed into a biological activated bed (BAC) [6]. It has been observed that on the BAC bed bromates are not reduced to bromides. The literature also includes references to the efficiency of other processes in removing bromates from water. And so, when preozonated water was made subject to coagulation with aluminium sulphate, the removal of bromates totaled 26% (at a relatively high coagulant dose, 100 mg/L) [10]. When potable water was treated by UV radiation, only 19% removal of bromates was attained [11]. The highest removal of bromates was achieved when use was made of
198
J.A. Wiśniewski, M.ł Kabsch-Korbutowicz / Desalination 261 (2010) 197–201
Table 1 Characteristics of the ion-exchange membranes used in Donnan dialysis. Parameter
Ion-exchange capacity, mmol/g Water content, % Thickness, mm
Membrane AMV
AFN
1.85 19.9 0.11
3.15 64.8 0.12
membrane processes. Reversal electrodialysis and reverse osmosis (RO) brought about efficiencies of removal amounting up to 64% and 96%, respectively [12]. It is essential to note, however, that in these processes dissolved salts are removed from the water with a high efficiency. In this context, when the salinity of natural water is low, such a high extent of desalination is not advisable. The aim of the study reported on this paper was to remove bromates from water using the method of anion exchange in the Donnan dialysis process. In the method proposed the anion-exchange membrane separates two solutions: the feed (containing anions that are to be removed) and the receiver (an electrolyte characterized by a comparatively high concentration of the driving anion). The chemical potential gradient on both sides of the membrane induces the flow of counterions from the receiver to the feed. As the flow of co-ions in the same direction is impossible, an equivalent amount of counterions is transported from the feed to the receiver in order to maintain the electroneutrality of the two solutions [13]. In consequence, the anions in the feed (including bromates) are replaced by neutral ions, e.g. chlorides, from the feed. In this paper consideration is given to the problem of how the type of the anion-exchange membrane and the concentrations of the components (bromates, accompanying anions and driving salt) influence the rate and efficiency of anion exchange in the Donnan dialysis process. 2. Experimental The Donnan dialysis process was conducted using a laboratory dialytic set-up equipped with 20 cell pairs with anion-exchange membranes, Selemion AMV or Neosepta AFN. The working area of the membrane was 0.140 m2. The characteristics of the membranes tested are compiled in Table 1. The process was performed with recirculation of the feed and the receiver until the equilibrium concentration of bromate ions in the feed was attained. The volume ratio of the feed to the receiver was 4:1 (10 dm3:2.5 dm3). In our studies a one-component feeding solution of NaBrO3 was used (with a bromate concentration amounting to 50, 100 or 200 μg/ dm3). The multi-component feed contained NaNO3, NaHCO3, NaCl 3 (3 mM of each component) and NaBrO3 (50 μg BrO− 3 /dm ). The receiver used in the study was NaCl solution of varying concentrations: 50, 100, 200 or 300 mM.
Anion concentrations in the feed were measured in the course of the process. The concentration of nitrate ions was determined using a DREL 2000 spectrophotometer. The concentration of bicarbonates and chlorides was determined by titration (with HCl and AgNO3, respectively). Bromate concentration was measured photometrically with 3,3′-dimethylnaftidin and iodine [14] using a UV mini 1240 spectrophotometer (Shimadzu) and 50 mm glass cuvettes. Sample absorbance was measured at the wavelength of 550 nm. 3. Results The data plotted in Figs. 1 and 2 substantiate the efficient removal of bromates from the one-component solution by the anion-exchange method. As can be seen from these plots, the removal efficiencies obtained exceed 90%, regardless of whether the initial concentration of bromates was low (50 μg/dm3) or high (200 μg/dm3). It was found that when the concentration of NaCl in the receiver was relatively low and it equalled 50 mM, the extent of anion exchange exceeded 90%. The increase in NaCl concentration to 200 mM only slightly enhanced the efficiency of the process, but this was concomitant with an increase in the BrO− 3 ion flux across the membrane: from 0.038 · 10−3 mol/m2 h to 0.132 · 10−3 mol/m2 h (for bromate concentration of 200 μg/dm3). In consequence, the time required for attaining the state of equilibrium decreased from 2 h to 0.75 h (Fig. 2). − Owing to the presence of other anions in the solution (NO− 3 , HCO3 ), − the exchange of BrO− ions to Cl ions became less efficient. Figs. 3 and 4 3 visualize the decrease in the concentration of the anions and the efficiency of their removal at various salt concentrations in the receiver (for Selemion AMV). As can be seen from these data, nitrates are the ions that are removed from the water with the highest efficiency. The average flux of this anion for the attainment of its equilibrium approaches 0.110 mol/m2 h, and the efficiency of the anion removal ranges from 72 to 89%. Bicarbonates are transported across the membrane at a slower rate; the average flux of this ion for attaining the state of equilibrium varies between 0.047 and 0.051 mol/m2 h, at a removal efficiency of 67 to 83%. The average flux of the bromates transported from the multi-component solution falls between 0.0078 · 10−3 and 0.0091 · 10−3 mol/m2 h only. It is worth noting that these values were about 3 times lower than the average flux of bromates transported from the one-component solution with the same initial concentration (0.030 · 10−3 mol/m2 h). Moreover, an extent of bromate removal from the multi-component solution (varying from 66 to 85%) was much lower than it was in case of the one-component bromate solution (98–100%). It is essential to note, however, that the slow transport of bromates and relatively low efficiency of their removal from the solution containing accompanying anions is not an effect of the ionic size. The radius of the hydrated BrO− 3 ion amounts to 0.351 nm and is comparable with the radius of the NO− 3 ion (0.335 nm) [15]. Nevertheless, because of the very low concentration
Fig. 1. Decrease in concentration and extent of bromate removal from the one-component solution at various NaCl concentrations in the receiver; membrane: Selemion AMV, CBrO3 = 50 μg/dm3.
J.A. Wiśniewski, M.ł Kabsch-Korbutowicz / Desalination 261 (2010) 197–201
199
Fig. 2. Decrease in concentration and extent of bromate removal from the one-component solution at various NaCl concentrations in the receiver; membrane: Selemion AMV, CBrO3 = 200 μg/dm3.
Fig. 3. Decrease in concentration and extent of anion removal from the multi-component solution; membrane: Selemion AMV, CNaCl = 100 mM.
Fig. 4. Decrease in concentration and extent of anion removal from the multi-component solution; membrane: Selemion AMV, CNaCl = 300 mM.
Fig. 5. Decrease in concentration and extent of anion removal from the multi-component solution; membrane: Neosepta AFN, CNaCl = 100 mM.
200
J.A. Wiśniewski, M.ł Kabsch-Korbutowicz / Desalination 261 (2010) 197–201
Fig. 6. Decrease in concentration and extent of anion removal from the multi-component solution; membrane: Neosepta AFN, CNaCl = 300 mM.
of bromates (which is by three orders of magnitude lower than that of nitrates and bicarbonates), their share in the flux of the counterions transported from the feed to the receiver is very low. Figs. 5 and 6 illustrate the removal efficiencies obtained for bromates and accompanying anions in the Donnan dialysis process which involved the Neosepta AFN membrane. The comparison of the experimental results for the two membranes − indicates that the exchange of the NO− 3 and HCO3 anions proceeds at a faster rate when use is made of Neosepta AFN. The average flux of nitrates (for the state of equilibrium of this ion) ranges from 0.143 to 0.184 mol/m2 h, and the extent of removal varies from 73 to 86%. As for
2 the HCO− 3 ions, the average flux is between 0.071 and 0.099 mol/m h, and removal efficiency falls in the range of 70 to 77%. The higher rate of − NO− 3 and HCO3 anion transport across the Neosepta AFN membrane (approx. 1.5 or twice as high as compared to Selemion AMV) is attributable to the high ion-exchange capacity and water content of the former (Table 1). A high ion-exchange capacity enhances counterion flux across the membrane, and a high water content (which is due to the low crosslinking of the membrane matrix) facilitates ion transport, particularly when the ions are large in size, e.g. bicarbonates. These findings substantiate the results of our previous study on the removal of troublesome anions by Donnan dialysis [16,17]. Compared to Selemion AMV, the transport of bromates across the Neosepta AFN membrane proceeds at a faster rate, as was the case with the accompanying ions. The average bromate flux (for the state of equilibrium of this ion) varies from 0.019 · 10−3 to 0.035 · 10−3 mol/ m2 h. However, the extent of bromate removal from the feed is lower (63 to 70%) than in the process involving the Selemion AMV membrane (66 to 85%). In Fig. 7 the removal of bromates and accompanying anions in the Donnan dialysis process obtained with the Neosepta AFN membrane is compared with the relevant removal achieved with the Selemion AMV membrane (the efficiencies of ion removal relate to the state of equilibrium for bromates, i.e. to the lowest concentration of bromates in the ion-exchange process). Upon analysis of the foregoing the following generalizations can be made. The membrane of a high ion-exchange capacity and a loose structure, i.e. Neosepta AFN, accelerates and facilitates the transport of counterions, specifically when they are large in size (like bicarbonates). In consequence, the final concentrations of these ions in the feed (at equilibrium concentration of bromates) are lower than those attained with Selemion AMV, which corresponds with the higher efficiency of their removal. Such effect, however, causes the equilibrium concentration of bromates to stabilize at a higher level than the one observed during anion exchange with the application of Selemion AMV. Taking into account, furthermore, that the primary objective of anion exchange is to remove bromates with the highest possible efficiency, it can be concluded that the use of Selemion AMV is preferable because in that case the advantageous properties of this membrane outweigh those of Neosepta AFN.
4. Conclusions
Fig. 7. Comparison of anion removal efficiencies in the Donnan dialysis process attained with Selemion AMV and Neosepta AFN, at various NaCl concentrations.
1. Donnan dialysis enables efficient removal of bromates from a onecomponent solution. At a relatively low salt concentration in the receiver (50 mM NaCl) the extent of removal for this anion exceeds 90%. 2. Bromate removal from the multi-component solution proceeds with lower rate and lower efficiency comparing with onecomponent solution. In the Donnan dialysis process using the
J.A. Wiśniewski, M.ł Kabsch-Korbutowicz / Desalination 261 (2010) 197–201
Selemion AMV membrane the removal of bromates does not exceed 85% at a relatively high NaCl concentration in the receiver (300 mM). 3. The application of Neosepta AFN (a membrane with a high ionexchange capacity and a high water content) to the Donnan dialysis process facilitates and accelerates the transport of the counterions that dominate in the feed (nitrates and bicarbonates), thus producing a higher removal efficiency of those ions from the solution as compared to the use of Selemion AMV. This, however, reduces the efficiency of bromate removal, which approaches 70% in the process involving Neosepta AFN. Acknowledgement
[5] [6]
[7]
[8] [9] [10]
[11]
The work was supported by grant N523 4489 93 (2008–2010) from the Polish Ministry of Science and Higher Education.
[12]
References
[13]
[1] U. von Ganten, Ozonation of drinking water: part II. disinfection and by-product formation in presence of bromide iodide or chlorine, Water Res. 37 (2003) 1469–1487. [2] K. Tyrola, E. Diamadopoulos, Bromate formation during ozonation of groundwater in coastal areas in Greece, Desalination 176 (2005) 201–209. [3] T.P. Bonacquisti, A drinking water utility's perspective on bromide, bromate and ozonation, Toxicology 221 (2006) 145–148. [4] B.M. De Borba, J.S. Rohrer, Ch.A. Pohl, Ch. Saini, Determination of trace concentrations of bromate in municipal and bottled drinking waters using a
[14] [15] [16] [17]
201
hydroxide-selective column with ion chromatography, J. Chromatogr. A 1085 (2005) 23–32. Draft Guideline for Drinking Water Qualitythird ed, World Health Organization, Geneva, Switzerland, 2003. M. Asami, T. Aizawa, T. Morioka, W. Nishijima, A. Tabata, Y. Magara, Bromate removal during transition from new granular activated carbon (GAC) to biological activated carbon (BAC), Water Res. 33 (1999) 2797–2804. M.L. Bao, O. Griffini, D. Santianni, K. Barbieri, D. Burrini, F. Pantani, Removal of bromate ion from water using granular activated carbon, Water Res. 33 (1999) 2959–2970. M.J. Kirists, V.L. Snoeyink, J.C. Kruithof, The reduction of bromate by granular activated carbon, Water Res. 34 (2000) 4250–4260. W.J. Huang, Y.L. Cheng, Effect of characteristics of activated carbon on removal of bromate, Sep. Purif. Technol. 59 (2008) 101–107. H. Selcuk, Y. Vitosoglu, S. Ozaydin, M. Bekbolet, Optimization of ozone an coagulation processes for bromate control in Istanbul drinking waters, Desalination 176 (2005) 211–217. S. Peldszus, S.A. Andrews, R. Souza, F. Smith, I. Douglas, J. Bolton, P.M. Huck, Effect of medium-pressure UV irradiation on bromate concentrations in drinking water, a pilot-scale study, Water Res. 38 (2004) 211–217. J.P. van der Hoek, D.O. Rijnbende, C.J.A. Lokin, P.A.C. Bonne, M.T. Loonen, J.A.M.H. Hofman, Electrodialysis as an alternative for reverse osmosis in an integrated membrane system, Desalination 117 (1998) 159–172. H. Strathmann, Ion-exchange Membrane Separation Processes, Elsevier, Amsterdam, 2004. Merck applications. Bromate in water and drinking water. Photometric determination with 3,3′-Dimethylnaftidin and iodine. E.R. Nighitingale, Phenomenological theory of ion solvation. Effective radii of hydrated ions, J. Phys. Chem. 63 (1959) 1381–1387. J. Wiśniewski, A. Różańska, T. Winnicki, Removal of troublesome anions from water by means of Donnan dialysis, Desalination 182 (2005) 339–346. J. Wiśniewski, A. Różańska, Donnan dialysis with anion-exchange membranes as a pretreatment step before electrodialytic desalination, Desalination 191 (2006) 210–218.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Bromate removal in the ion-exchange process Jacek A. Wiśniewski ⁎, Małgorzata Kabsch-Korbutowicz Institute of Environment Protection Engineering, Wroclaw University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 27 October 2009 Received in revised form 11 March 2010 Accepted 12 March 2010 Available online 12 June 2010 Keywords: Donnan dialysis Ion-exchange membranes Disinfection by-product
a b s t r a c t To remove bromates from water, use was made of anion exchange in the process of Donnan dialysis conducted with an anion-exchange membrane. Under such conditions, the removal of bromates from a onecomponent solution was found to be very high (more than 90%). It was observed that the presence of − accompanying anions (NO− 3 and HCO3 ), whose concentrations were by several orders of magnitude higher than the concentrations of bromates, brought about a decrease in the rate and efficiency of bromate removal from the feed. In the process with the Selemion AMV membrane the removal of bromates amounted to approximately 84% at a salt concentration of 300 mM NaCl in the receiver. Anion exchange in Donnan dialysis with Neosepta AFN (a membrane characterized by a high ion-exchange capacity and a high water content) proceeded at a faster rate and was found to be more efficient only with respect to the ions that dominated in the feed, i.e. nitrates and bicarbonates. In this process the removal of BrO− 3 ions was less efficient than in the process conducted with the Selemion AMV membrane, and failed to exceed 70%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Ozone is a major oxidizer and disinfectant applied in water treatment for drinking purposes. Compared to chlorine, ozone limits the formation of some halogen disinfection by-products and exerts a deactivating effect on some of the microorganisms that are resistant to chlorine (e.g. Cryptosporidium parvum) [1]. In some instances, however, the source of potable water contains bromides, which enter the groundwater as a result of sea water intrusion, or due to the penetration of industrial and agricultural wastewaters containing bromides [2,3]. If the water to be ozonated contains bromides (even of relatively low concentrations approaching 100 μg/L), this will lead to the formation of bromates, ions with potential carcinogenic implications to human organisms [1–3]. The process of bromate formation involves the following approximate reactions [3]: −
O3 þ Br →O2 þ OBr
−
−
O3 þ OBr →2O2 þ Br
−
−
−
2O3 þ OBr →2O2 þ BrO3
As can be seen, ozone oxidizes bromide to the hypobromite ion (OBr−), which is further oxidized to bromate or to an unidentified species (probably BrO− 2 ), that regenerates bromide ion [3]. ⁎ Corresponding author. Tel.: + 48 71 3203225; fax: + 48 71 2382980. E-mail address: [email protected] (J.A. Wiśniewski). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.03.029
American researchers have reported that the risk of developing a cancer disease is 10−4 when the concentration of bromates in drinking water totals 5 μg/L, and 10−5 when it amounts to 0.5 μg/L [4]. For these reasons the permissible concentration of bromates in potable water established for the European Union and the United States is now 10 μg/L [4]. The same value has recently been defined as permissible also by the WHO [5]. The concentration of bromates in the water being ozonated is influenced by a variety of factors, such as bromide ion concentration, ozone dose, reaction time, pH, and natural organic matter (NOM) concentration [2,3]. It has been found that at an initial concentration of Br− ions in the water higher than 100 μg/L, the concentration of bromates in the water upon ozonation can exceed 50 μg/L, if the ozone dose and contact time applied have provided an efficient disinfection (minimum 99% deactivation of Cryptosporidium oocysts) [1,2]. Several methods are in use for reducing the concentration of bromates in the water after ozonation. Amongst the most widely used is adsorption on granular activated carbon (GAC) [6–9]. In this process − the BrO− ion on the GAC surface [9]. The 3 ion is reduced to the Br efficiency of reduction, which initially exceeds 60%, deteriorates over time, owing to the gradual transformation of the GAC bed into a biological activated bed (BAC) [6]. It has been observed that on the BAC bed bromates are not reduced to bromides. The literature also includes references to the efficiency of other processes in removing bromates from water. And so, when preozonated water was made subject to coagulation with aluminium sulphate, the removal of bromates totaled 26% (at a relatively high coagulant dose, 100 mg/L) [10]. When potable water was treated by UV radiation, only 19% removal of bromates was attained [11]. The highest removal of bromates was achieved when use was made of
198
J.A. Wiśniewski, M.ł Kabsch-Korbutowicz / Desalination 261 (2010) 197–201
Table 1 Characteristics of the ion-exchange membranes used in Donnan dialysis. Parameter
Ion-exchange capacity, mmol/g Water content, % Thickness, mm
Membrane AMV
AFN
1.85 19.9 0.11
3.15 64.8 0.12
membrane processes. Reversal electrodialysis and reverse osmosis (RO) brought about efficiencies of removal amounting up to 64% and 96%, respectively [12]. It is essential to note, however, that in these processes dissolved salts are removed from the water with a high efficiency. In this context, when the salinity of natural water is low, such a high extent of desalination is not advisable. The aim of the study reported on this paper was to remove bromates from water using the method of anion exchange in the Donnan dialysis process. In the method proposed the anion-exchange membrane separates two solutions: the feed (containing anions that are to be removed) and the receiver (an electrolyte characterized by a comparatively high concentration of the driving anion). The chemical potential gradient on both sides of the membrane induces the flow of counterions from the receiver to the feed. As the flow of co-ions in the same direction is impossible, an equivalent amount of counterions is transported from the feed to the receiver in order to maintain the electroneutrality of the two solutions [13]. In consequence, the anions in the feed (including bromates) are replaced by neutral ions, e.g. chlorides, from the feed. In this paper consideration is given to the problem of how the type of the anion-exchange membrane and the concentrations of the components (bromates, accompanying anions and driving salt) influence the rate and efficiency of anion exchange in the Donnan dialysis process. 2. Experimental The Donnan dialysis process was conducted using a laboratory dialytic set-up equipped with 20 cell pairs with anion-exchange membranes, Selemion AMV or Neosepta AFN. The working area of the membrane was 0.140 m2. The characteristics of the membranes tested are compiled in Table 1. The process was performed with recirculation of the feed and the receiver until the equilibrium concentration of bromate ions in the feed was attained. The volume ratio of the feed to the receiver was 4:1 (10 dm3:2.5 dm3). In our studies a one-component feeding solution of NaBrO3 was used (with a bromate concentration amounting to 50, 100 or 200 μg/ dm3). The multi-component feed contained NaNO3, NaHCO3, NaCl 3 (3 mM of each component) and NaBrO3 (50 μg BrO− 3 /dm ). The receiver used in the study was NaCl solution of varying concentrations: 50, 100, 200 or 300 mM.
Anion concentrations in the feed were measured in the course of the process. The concentration of nitrate ions was determined using a DREL 2000 spectrophotometer. The concentration of bicarbonates and chlorides was determined by titration (with HCl and AgNO3, respectively). Bromate concentration was measured photometrically with 3,3′-dimethylnaftidin and iodine [14] using a UV mini 1240 spectrophotometer (Shimadzu) and 50 mm glass cuvettes. Sample absorbance was measured at the wavelength of 550 nm. 3. Results The data plotted in Figs. 1 and 2 substantiate the efficient removal of bromates from the one-component solution by the anion-exchange method. As can be seen from these plots, the removal efficiencies obtained exceed 90%, regardless of whether the initial concentration of bromates was low (50 μg/dm3) or high (200 μg/dm3). It was found that when the concentration of NaCl in the receiver was relatively low and it equalled 50 mM, the extent of anion exchange exceeded 90%. The increase in NaCl concentration to 200 mM only slightly enhanced the efficiency of the process, but this was concomitant with an increase in the BrO− 3 ion flux across the membrane: from 0.038 · 10−3 mol/m2 h to 0.132 · 10−3 mol/m2 h (for bromate concentration of 200 μg/dm3). In consequence, the time required for attaining the state of equilibrium decreased from 2 h to 0.75 h (Fig. 2). − Owing to the presence of other anions in the solution (NO− 3 , HCO3 ), − the exchange of BrO− ions to Cl ions became less efficient. Figs. 3 and 4 3 visualize the decrease in the concentration of the anions and the efficiency of their removal at various salt concentrations in the receiver (for Selemion AMV). As can be seen from these data, nitrates are the ions that are removed from the water with the highest efficiency. The average flux of this anion for the attainment of its equilibrium approaches 0.110 mol/m2 h, and the efficiency of the anion removal ranges from 72 to 89%. Bicarbonates are transported across the membrane at a slower rate; the average flux of this ion for attaining the state of equilibrium varies between 0.047 and 0.051 mol/m2 h, at a removal efficiency of 67 to 83%. The average flux of the bromates transported from the multi-component solution falls between 0.0078 · 10−3 and 0.0091 · 10−3 mol/m2 h only. It is worth noting that these values were about 3 times lower than the average flux of bromates transported from the one-component solution with the same initial concentration (0.030 · 10−3 mol/m2 h). Moreover, an extent of bromate removal from the multi-component solution (varying from 66 to 85%) was much lower than it was in case of the one-component bromate solution (98–100%). It is essential to note, however, that the slow transport of bromates and relatively low efficiency of their removal from the solution containing accompanying anions is not an effect of the ionic size. The radius of the hydrated BrO− 3 ion amounts to 0.351 nm and is comparable with the radius of the NO− 3 ion (0.335 nm) [15]. Nevertheless, because of the very low concentration
Fig. 1. Decrease in concentration and extent of bromate removal from the one-component solution at various NaCl concentrations in the receiver; membrane: Selemion AMV, CBrO3 = 50 μg/dm3.
J.A. Wiśniewski, M.ł Kabsch-Korbutowicz / Desalination 261 (2010) 197–201
199
Fig. 2. Decrease in concentration and extent of bromate removal from the one-component solution at various NaCl concentrations in the receiver; membrane: Selemion AMV, CBrO3 = 200 μg/dm3.
Fig. 3. Decrease in concentration and extent of anion removal from the multi-component solution; membrane: Selemion AMV, CNaCl = 100 mM.
Fig. 4. Decrease in concentration and extent of anion removal from the multi-component solution; membrane: Selemion AMV, CNaCl = 300 mM.
Fig. 5. Decrease in concentration and extent of anion removal from the multi-component solution; membrane: Neosepta AFN, CNaCl = 100 mM.
200
J.A. Wiśniewski, M.ł Kabsch-Korbutowicz / Desalination 261 (2010) 197–201
Fig. 6. Decrease in concentration and extent of anion removal from the multi-component solution; membrane: Neosepta AFN, CNaCl = 300 mM.
of bromates (which is by three orders of magnitude lower than that of nitrates and bicarbonates), their share in the flux of the counterions transported from the feed to the receiver is very low. Figs. 5 and 6 illustrate the removal efficiencies obtained for bromates and accompanying anions in the Donnan dialysis process which involved the Neosepta AFN membrane. The comparison of the experimental results for the two membranes − indicates that the exchange of the NO− 3 and HCO3 anions proceeds at a faster rate when use is made of Neosepta AFN. The average flux of nitrates (for the state of equilibrium of this ion) ranges from 0.143 to 0.184 mol/m2 h, and the extent of removal varies from 73 to 86%. As for
2 the HCO− 3 ions, the average flux is between 0.071 and 0.099 mol/m h, and removal efficiency falls in the range of 70 to 77%. The higher rate of − NO− 3 and HCO3 anion transport across the Neosepta AFN membrane (approx. 1.5 or twice as high as compared to Selemion AMV) is attributable to the high ion-exchange capacity and water content of the former (Table 1). A high ion-exchange capacity enhances counterion flux across the membrane, and a high water content (which is due to the low crosslinking of the membrane matrix) facilitates ion transport, particularly when the ions are large in size, e.g. bicarbonates. These findings substantiate the results of our previous study on the removal of troublesome anions by Donnan dialysis [16,17]. Compared to Selemion AMV, the transport of bromates across the Neosepta AFN membrane proceeds at a faster rate, as was the case with the accompanying ions. The average bromate flux (for the state of equilibrium of this ion) varies from 0.019 · 10−3 to 0.035 · 10−3 mol/ m2 h. However, the extent of bromate removal from the feed is lower (63 to 70%) than in the process involving the Selemion AMV membrane (66 to 85%). In Fig. 7 the removal of bromates and accompanying anions in the Donnan dialysis process obtained with the Neosepta AFN membrane is compared with the relevant removal achieved with the Selemion AMV membrane (the efficiencies of ion removal relate to the state of equilibrium for bromates, i.e. to the lowest concentration of bromates in the ion-exchange process). Upon analysis of the foregoing the following generalizations can be made. The membrane of a high ion-exchange capacity and a loose structure, i.e. Neosepta AFN, accelerates and facilitates the transport of counterions, specifically when they are large in size (like bicarbonates). In consequence, the final concentrations of these ions in the feed (at equilibrium concentration of bromates) are lower than those attained with Selemion AMV, which corresponds with the higher efficiency of their removal. Such effect, however, causes the equilibrium concentration of bromates to stabilize at a higher level than the one observed during anion exchange with the application of Selemion AMV. Taking into account, furthermore, that the primary objective of anion exchange is to remove bromates with the highest possible efficiency, it can be concluded that the use of Selemion AMV is preferable because in that case the advantageous properties of this membrane outweigh those of Neosepta AFN.
4. Conclusions
Fig. 7. Comparison of anion removal efficiencies in the Donnan dialysis process attained with Selemion AMV and Neosepta AFN, at various NaCl concentrations.
1. Donnan dialysis enables efficient removal of bromates from a onecomponent solution. At a relatively low salt concentration in the receiver (50 mM NaCl) the extent of removal for this anion exceeds 90%. 2. Bromate removal from the multi-component solution proceeds with lower rate and lower efficiency comparing with onecomponent solution. In the Donnan dialysis process using the
J.A. Wiśniewski, M.ł Kabsch-Korbutowicz / Desalination 261 (2010) 197–201
Selemion AMV membrane the removal of bromates does not exceed 85% at a relatively high NaCl concentration in the receiver (300 mM). 3. The application of Neosepta AFN (a membrane with a high ionexchange capacity and a high water content) to the Donnan dialysis process facilitates and accelerates the transport of the counterions that dominate in the feed (nitrates and bicarbonates), thus producing a higher removal efficiency of those ions from the solution as compared to the use of Selemion AMV. This, however, reduces the efficiency of bromate removal, which approaches 70% in the process involving Neosepta AFN. Acknowledgement
[5] [6]
[7]
[8] [9] [10]
[11]
The work was supported by grant N523 4489 93 (2008–2010) from the Polish Ministry of Science and Higher Education.
[12]
References
[13]
[1] U. von Ganten, Ozonation of drinking water: part II. disinfection and by-product formation in presence of bromide iodide or chlorine, Water Res. 37 (2003) 1469–1487. [2] K. Tyrola, E. Diamadopoulos, Bromate formation during ozonation of groundwater in coastal areas in Greece, Desalination 176 (2005) 201–209. [3] T.P. Bonacquisti, A drinking water utility's perspective on bromide, bromate and ozonation, Toxicology 221 (2006) 145–148. [4] B.M. De Borba, J.S. Rohrer, Ch.A. Pohl, Ch. Saini, Determination of trace concentrations of bromate in municipal and bottled drinking waters using a
[14] [15] [16] [17]
201
hydroxide-selective column with ion chromatography, J. Chromatogr. A 1085 (2005) 23–32. Draft Guideline for Drinking Water Qualitythird ed, World Health Organization, Geneva, Switzerland, 2003. M. Asami, T. Aizawa, T. Morioka, W. Nishijima, A. Tabata, Y. Magara, Bromate removal during transition from new granular activated carbon (GAC) to biological activated carbon (BAC), Water Res. 33 (1999) 2797–2804. M.L. Bao, O. Griffini, D. Santianni, K. Barbieri, D. Burrini, F. Pantani, Removal of bromate ion from water using granular activated carbon, Water Res. 33 (1999) 2959–2970. M.J. Kirists, V.L. Snoeyink, J.C. Kruithof, The reduction of bromate by granular activated carbon, Water Res. 34 (2000) 4250–4260. W.J. Huang, Y.L. Cheng, Effect of characteristics of activated carbon on removal of bromate, Sep. Purif. Technol. 59 (2008) 101–107. H. Selcuk, Y. Vitosoglu, S. Ozaydin, M. Bekbolet, Optimization of ozone an coagulation processes for bromate control in Istanbul drinking waters, Desalination 176 (2005) 211–217. S. Peldszus, S.A. Andrews, R. Souza, F. Smith, I. Douglas, J. Bolton, P.M. Huck, Effect of medium-pressure UV irradiation on bromate concentrations in drinking water, a pilot-scale study, Water Res. 38 (2004) 211–217. J.P. van der Hoek, D.O. Rijnbende, C.J.A. Lokin, P.A.C. Bonne, M.T. Loonen, J.A.M.H. Hofman, Electrodialysis as an alternative for reverse osmosis in an integrated membrane system, Desalination 117 (1998) 159–172. H. Strathmann, Ion-exchange Membrane Separation Processes, Elsevier, Amsterdam, 2004. Merck applications. Bromate in water and drinking water. Photometric determination with 3,3′-Dimethylnaftidin and iodine. E.R. Nighitingale, Phenomenological theory of ion solvation. Effective radii of hydrated ions, J. Phys. Chem. 63 (1959) 1381–1387. J. Wiśniewski, A. Różańska, T. Winnicki, Removal of troublesome anions from water by means of Donnan dialysis, Desalination 182 (2005) 339–346. J. Wiśniewski, A. Różańska, Donnan dialysis with anion-exchange membranes as a pretreatment step before electrodialytic desalination, Desalination 191 (2006) 210–218.