Transport characterizations of natural organic matter in ion-exchange membrane for water treatment

Transcription

1 Transport characterizations of natural organic matter in ion-exchange membrane for water treatment D.H. Kim, S.-H. Moon and J. Cho Department of Environmental Science and Technology, Kwangju Institute of Science and Technology (K- JIST), 1 Oryong-dong, Buk-gu, Gwangju , Korea ( choj@kjist.ac.kr) Abstract A series of adsorption experiments were performed to investigate the factors affecting the transport of natural organic matter (NOM) in an ion-exchange (IX) membrane. In this study, the structure of the NOM was hypothesized to be an important factor in terms of the organic fouling of IX membrane. It was found that the adsorbed mass of hydrophobic NOM constituent on the membrane surface was higher than that of either the hydrophilic or transphilic NOM constituent. NOM adsorption was seriously affected by the apparent charge of the NOM. As the apparent charge increased, NOM adsorption also significantly increased. Moreover, the molecular mass of the hydrophobic NOM acids was too high to enable them to pass through the IX membrane, and this caused an accumulated adsorption of solutes on the membrane surface, i.e. NOM fouling. In addition, both ph and ionic strength affected NOM adsorption on the surface of the IX membrane. Lower NOM adsorption resulted from a lower ph and a higher ionic strength. Keywords Adsorption; apparent charge; IX membrane; natural organic matter (NOM) Introduction Ion-exchange (IX) membranes are widely used to remove ionic materials from source water, to enable it to be used as drinking water. Such source waters contain high concentration of ions, such as nitrate and sulfate and other conductivity contributing species, i.e. total dissolved solids (Lee, 1996; Kim et al., 2000). Recent studies have showed that IX membranes or resin can successfully remove ionic trace materials, such as arsenic (Vagliasindi and Benjamin, 1998). However, NOM has been found to be a problematic solute in terms of electrodialysis (ED) processes using IX membranes. NOM with a larger relative molecular mass (RMM) cannot pass through the IX membrane pores, because the IX membrane is almost non-porous. The accumulated NOM, i.e. concentration polarization, near the IX membrane surface then increases the electrical resistance during the ED process and significantly reduces the ion transport through the IX membrane. Moreover, the relation between NOM and the IX membrane is complicated, involving several different interactions. The objectives of this study were: to evaluate the effect of the apparent NOM charge on NOM adsorption; to demonstrate the effects of NOM structure on IX membrane fouling; to characterize NOM in terms of its transport in the ED process. Materials and methods Membranes and NOM examined Three different NOM were used during this work: soil organic matter (SOM), which was purchased from Aldrich; NOM concentrated from the Nakdong River (surface water) in Korea (RNOM); and effluent organic matter (EfOM) from a wastewater treatment plant. All organic matters were analyzed in terms of molecular size, hydrophobic/hydrophilic structure, and charge density, i.e. ionizable functionality. Molecular size was measured by the high performance liquid chromatography size exclusion chromatography (HP-SEC) using a SEC column (Protein-pak 125, Waters, Milford, USA) (Aiken et al., 1992; Cho et Water Science and Technology: Water Supply Vol 2 No 5 6 pp IWA Publishing

2 D.H. Kim et al. al., 1999, 2000). Standard solutions were made using various polystyrene sulfonates (PSS: 0.25 k, 1.8 k, 4.6k, and 8.0 k daltons) to produce an RMM calibration curve. XAD-8/4 resins were used to fractionate NOM into hydrophobic, transphilic, and hydrophilic constituents. Functionality was measured by micro titration. In addition, the apparent organic matter charges were measured using the method proposed by Kim et al. (2001). Apparent charge was defined as the valence per unit mole of the NOM. Cation-exchange membrane (CMX) and anion-exchange membrane (AMX) (Tokuyama Corp., Japan) were used. These were characterized in terms of their surface charges, i.e. zeta potential, and ion exchange capacity. The electrophoretic method (ELS 8000, Otzca, Japan) was used to measure zeta potential of the IX membrane. Polylatex in 10 3 M NaCl solution was used as the standard measurement particle. To measure the ion exchange capacity, a membrane was placed in a 5N NaCl solution and then soaked in a 0.1 N NaOH solution, which exchanges chloride ions with hydroxyl ions. Ion capacity was then estimated by measuring the amount of detached chloride ions as meq of Cl /g of dried membrane. Experimental protocols The experimental part of this study consisted of two phases. Phase I included the adsorption tests and phase II the ED tests. Adsorption tests and affinity tests were performed using a diffusion cell fitted with an IX membrane (with an active surface area of 13.7 cm 2 ) to determine NOM adsorption and affinity under various phs and ionic strengths. Phase I tests were conducted in three stages. The first stage involved determining the apparent charge of the NOM. Three types of organics, including SOM, RNOM and EfOM, were used. The second stage concerned investigating the effect of NOM structure on NOM adsorption. NOM fractions isolated with XAD resins were used for the demonstration. The third stage involved an investigation of the effects of solution chemistry on NOM adsorption. Both ph and ionic strength effects were evaluated on NOM adsorption. Phase II tests were related to NOM transport in ED process. During this stage of testing, long-term fouling tests were performed using a bench scale ED apparatus to determine the NOM transport behavior in the ED process. The electrodialyzer (TS-1, Tokuyama Corp., Japan) used in this work accomodates two pairs of cells, made up of CMX and AMX membranes with an active surface area of 100 cm 2 (Tokuyama Corp., Japan). NaCl solution of 0.05 M (5 L) was cirulated as a concentrated solution and 800 ml of 3.0 wt% Na 2 SO 4 was used as an electrode rinse solution. 0.1 M NaCl solution (5 L) was used as the dilution solution, and the solution also included NOM of ca. 110 mg/l as DOC to investigate transport and fouling phenomena. The flow rates of both dilute and concentrated solutions were adjusted to be in the range L/min, and a direct current of 0.6 A was used unless otherwise stated. The ph effect was also evaluated during this phase. Results and discussion Electrochemical characterization of the AMX membrane and tested NOM The ion-exchange capacity of the AMX membrane was estimated to be 6.20 mol/l (or 1.40 meq/g-dried membrane). The zeta potential of the AMX membrane was measured as a function of ph (not shown in this paper), and determined to be +7.6 mv at ph 7. The isoelectric-point (i.e.p.) was not measured, because the zeta potential of this AMX membrane was positive in the ph range between 3 and 10. The apparent charges of SOM, RNOM, and EfOM were 10.0, 5.5 and 3.7, respectively. The fundamental properties of the IX membranes used in this study are described in Table

3 Table 1 The characteristics of IX membranes Membrane code AMX CMX Type Strong base Strong acid (anion permeable) (cation permeable) Electric resistance (ohm cm 2 ) Water content (g H 2 O/g dried) Exchange capacity (meq/g dried membrane) ~1.8 Thickness (mm) Effect of the apparent charge on NOM adsorption Figure 1 shows the effect of the apparent charge of different NOM types on NOM adsorption. The adsorbed SOM mass, which had the highest apparent charge ( 10.0) was greater than that of EfOM, which has the lowest apparent charge ( 3.7). The difference between the masses of SOM and EfOM adsorbed increased significantly as the NOM concentration increased (see Figure 1). It is believed that attraction forces may increase with increasing NOM negative charge due to the positive charge of the anion exchange membrane. Throughout the results obtained, the apparent charge of NOM acids was found to be an effective fouling indicator for anion-exchange membranes D.H. Kim et al. Effect of NOM structure on adsorption Different NOMs fractionated by XAD resins were used to evaluate the effect of NOM structure on adsorption. Concentrated RNOM solution was used for these tests. To provide the same concentration conditions, ionic strength and ph were adjusted to be 10 ms/cm and ph 7, respectively. As expected, the mass of hydrophobic NOM adsorbed was much greater than that of the transphilic and hydrophilic NOM (see Figure 2). In particular, 13.1 times more hydrophobic NOM was adsorbed than hydrophilic NOM at a concentration of 20 mg/l. This can be explained by the fact that ionizable functional groups, such as carboxyl and hydroxyl, are present more in the hydrophobic NOM than in the transphilic or hydrophilic NOM. Thus, it is expected that the difference in the adsorbed mass between hydrophobic and hydrophilic NOM may increase if the solution becomes more alkaline. In conclusion, the determination of NOM structure may be used as a good indicator of membrane fouling Adsorbed mass (mg/cm 2 ) SOM RNOM EfOM Concentration (mg/l) Figure 1 Effect of apparent charge on NOM adsorption 447

4 50 45 Hydrophobic 40 Transphilic D.H. Kim et al. Adsorbed mass (mg/cm 2 ) Hydrophilic Concentration (mg/l) Figure 2 Effect of NOM structure on adsorption Effect of solution chemistry on adsorption For NOM solutions with NOM concentrations of 10, 25, 50, and 100 mg/l at ph 7, without any ionic strength adjustment, the total adsorbed NOM masses were 15.4, 38.6, 113.5, and mg/cm 2, respectively (see Figure 3). For NOM solutions with DOC concentrations of 10, 25, 50, and 100 mg/l at ph 10 without ionic strength adjustment, the total adsorbed masses of NOM were 79.1, 99.3, 156.0, and mg/cm 2, respectively. This was attributed to increased NOM ionization under alkaline condition due to the presence of ionizable functional groups, such as carboxylic (ionized in the ph range of 3 8) and phenolic (ionized in the ph range of 8 12). The effect of ph on adsorption was significant at a low NOM concentration range. On the contrary, the ph effect at a high NOM concentration range was relatively small. It is believed that the ion exchange capacity of IX membrane is limited at higher ph values. However, the adsorbed mass increased under conditions which favored adsorption, i.e. at high concentration and high ph, and it is thought possible that adsorption occurred Adsorbed mass (mg/cm 2 ) I = 0 M ph 7 I = 0 M ph 10 I = 0.01 M ph 7 I = 0.01 M ph 10 I = 0.1 M ph 7 I = 0.1 M ph Concentration of RNOM (mg/l) Figure 3 Effect of ionic strength on adsorption

5 between the IX membrane and the bulk NOM, i.e. single adsorption layer, as well as between the adsorbed NOM (on the IX membrane) and the bulk NOM, i.e. multiple adsorption layer after the saturation of sorption sites. NOM adsorption decreased, as ionic strength increased, because more chloride ions accumulated near the IX membrane surface, thus competing with the negatively charged NOM acids. NOM transport during ED To evaluate NOM transport during ED, a diluted NOM solution with a DOC of ca. 110 mg/l was used as influent. The final NOM concentrations in the dilute and concentrated solutions were and 2.7 mg/ L, respectively, at ph 7, and 92.0 and 3.5 mg/l, respectively, at ph 10. Details of the concentrations and molecular mass distributions of each NOM fraction at ph 7 and 10 are summarized in Table 2. At both ph 7 and 10, the major NOM fraction transported into the concentrate compartment, i.e. after passing through the IX membrane, was the hydrophobic fraction, due to its relatively high negative charge density. However, the transport of NOM acids occurred to a lesser extent than that hypothesized probably because the NOM acids had molecular weights that were too large to allow them to pass through the IX membrane, consequently causing an accumulation (or adsorption) of hydrophobic NOM near (or on) the membrane s surface. On the other hand, the hydrophilic NOM fraction was transported relatively more easily into the concentration compartment, than the hydrophobic NOM fraction. However, the hydrophilic NOM has a relatively low negative charge density, and thus only a small portion of the hydrophilic NOM (particularly NOM with lower molecular mass) may be transported during the ED operation. From these results, it can be concluded that the NOM acids can be considered to be major foulants D.H. Kim et al. Table 2 NOM characteristics of each NOM fraction in different compartments of the ED process ph 7 NOM constituents DOC (mg/l) % fraction Molecular mass Initial solution Total ,570 Hydrophobic ,840 Transphilic ,650 Hydrophilic Diluted solution Total ,460 Hydrophobic ,840 Transphilic ,610 Hydrophilic Concentrated solution Total Hydrophobic ,260 Transphilic Hydrophilic ph 10 Initial solution Total ,590 Hydrophobic ,960 Transphilic ,620 Hydrophilic Diluted solution Total ,460 Hydrophobic ,890 Transphilic ,580 Hydrophilic Concentrated solution Total Hydrophobic ,400 Transphilic Hydrophilic

6 D.H. Kim et al. during the ED process, because they can easily approach the membrane surface, due to the membrane s negative charge density, but cannot pass through the membrane because of their large molecular size. In addition, the amount of transported NOM under alkaline conditions was 1.7 times higher (based on DOC) than that transported under neutral conditions, because of NOM charge increase under alkaline condition due to its ionizable functional groups. This result is similar to that obtained from the adsorption tests. Conclusions The NOM adsorption tests showed that NOM adsorption is affected by the apparent charge and structure of NOM. In addition, the solution chemistry of the feed, including its ph and ionic strength, influenced the NOM adsorption on the membrane surface. As ph increased and ionic strength decreased, the amount of NOM adsorbed increased. Thus, lower ph and higher ionic strength reduced fouling of the IX membrane, i.e. AMX, with NOM. It was expected that the major NOM fraction transported through the membrane during the ED operation is the NOM acids, due to their high negative charge density. However, hydrophilic NOM was transported into the concentration compartment more than the NOM acids because of its small molecular size. Thus, both the charge and the molecular size of NOM significantly affected NOM transport during the ED process. It was also found that IX membrane fouling was affected by the ph of the feed solution. Acknowledgements This was supported by the National Research Laboratory (NRL) program (Cleaner Separation Laboratory) of Korea Institute of Science and Technology Evaluation and Planning (KISTEP) (Project No N-NL-01-C-185). References Aiken, G.R., McKnight, D.M., Thorn, K.A., and Thurman, E.M. (1992). Isolation of hydrophilic organic acids from water using nonionic macroporous resins. Org. Geochem., 18, 567. Cho, J., Amy, G. and Pellegrino, J. (1999). Membrane filtration of natural organic matter: initial Comparison of rejection and flux decline characteristics with ultrafiltration and nanofiltration membranes. Wat. Res., 33, Cho, J., Amy, G. and Pellegrino, J. (2000). Membrane filtration of natural organic matter: factors and mechanisms affecting rejection and flux decline with charged ultrafiltration (UF) membrane. J. Membrane Sci., 164, 89. Croude, J.P., Violleau, D., Bodaire, C. and Legube, B. (1999). Removal of hydrophilic and hydrophobic constituents by anion exchange resin. Wat. Sci. Tech., 45(9), Kim, D.H., Lee, S. and Cho, J. (2000). Selectivity and adsorptivity of various anions in ion-exchange membranes. Proceedings of WQTC 2000, November, Salt Lake City, UT, USA. Kim, D.H, Moon, S.H. and Cho, J. (2001). Investigation of the adsorption and transport of natural organic matter (NOM) in ion-exchange membrane, desalination (submitted). Lee, H.J. (1996). Pretreatment and Membrane Fouling in Electrodialysis of Fermentation Waste. MSc thesis, Kwangju Institute of Science and Technology, Korea. Miyyoshi, H. (1998). Diffusion coefficients of ions through ion exchange membrane in Donnan dialysis using ions of different valence. J. of Membrane Sci., 141, 101. Vagliasindi, F.G.A. and Benjamin, M.M. (1998). Arsenic removal in fresh and NOM-preloaded ion exchange packed bed adsorption reactors. Wat. Sci. Tech., 38(6),