Indian Journal of Chemical Technology Vol.3,May 1996,pp. 131-135 Modified cellulosic nanofiltration membrane with improved characteristics for desalination and concentration of reactive dyest G Venkidachalam & S K Verma Membrane Division, Research and Development Centre, Ion Exchange (India) Ltd, Ambernath 421 501, India The concentrated stableaqueous solutionof water solublereactivedyeswithsubstantialdesaltingcould be achieved by cellulosic based asymmetric nanofiltration membranes. In this paper, the modification and performance of an in-house developed nanofiltration membrane for its efficiencyin simultaneous desalination as wellas concentration of reactivedyesisevaluated both in flat-sheetand spiral wound configuration. It has been observedthat the chemicallymodifiednanofiltration membranesare far more efficientthan unmodified membranes in their performance with respect to their dye rejection, salt permeation and solution flux under identical operating conditions. OfJate, aqueous concentrated dyes are finding better market potential compared with its powder form because of its improved quality, long shelf-life, easy applicability and less health hazardsl. Desalination of dye is essential to attain maximum concentration of dye, otherwise salts present in the dye, up to 40% concentration, may cause precipitation of dye at higher concentrations of reactive dyes. Conventional methods adopted for removal of salts from the dyes are very time consuming, cumbersome, energy intensive and hence uneco nomic. Nanofiltration is one of the best techniques developed in ever growing membrane technology to solve these kinds of highly complicated and specific industrial problems. The term nanofiltrationis drawn from the observation that the size selectivity of membrane towards no-charged solutes is approximately 1rom cut off. Commercially available nanofiltration membranes have more doi)nan ion repulsion towards sulphate ions than chloride ions3.4, which leads to the possibility of applying nanofiltration membranes in removal of hardness from waters, color removal from bleach effluents in wood and pulp industries6 and desalination of whey and dyes. Various kinds of reactive dyes and dye intermediates having 10-40% saltconcentration have been desalinated and concentrated by cellulose acetate based nanofiltration membranes 7.8. In this paper, it is attempted to optimize desalination and concentration of reactive dyes by employing modified cellulosic membrane both in the form of flattthis paper was presented in the Indo-French International Meet and XIII National Conference of Indian Membrane Society on "Recent Advances in Membrane-Based Sepa.ration Science and Technology". Kama.taka University. Dharwad, 27-28 February 1995. -sheets and spiral wound module to attain maximum salt separation and minimum dye loss without compromising flux data for different concentration of dye solutions. The dye concentration in the permeate during desalination and concentration-could be reduced by recycling the permeate. Thus, the results of desalination with simultaneous concentration of reactive dyes by using modified nanofiltration membrane system is very promising and will escalate the quality and market potential of the reactive dyes. Experimental Procedure Membrane modu/e- The optimized membrane casting conditions gave cellulosic asymmetric membrane with pore size of 10-15 A and molecular weight cut off200-500 D. The spiral wound element was fabricated. at Hydranautics Membranes India Ltd (HMIL, Baroda) by using the cellulosic nanofiltration membrane having effective membrane area of 5 sq. ft. and length of I ft. Fig. I-Flow diagram of desalination with concentration ofthe dyes by cellulosic nanofiltration membrane system [I. Feed tank, 2. pump, 3. by-pass valve, 4. pressure gauge (feed), 5. spiral wound membrane module, 6. permeate"line, 7. pressure gauge (reject), 8. reject valve, 9. flow meter, 10. thermometer and II. permeate tank]
132 INDIAN J. CHEM. TECHNOL., MAY 1996 eed Table tivity. 24.84.5 Feed Permeate 2200 1-1, 91.3 1400 22.84 62.5 23.20 54.5 mho 1000 gsfd I---Characteristic 122 1-1. Permeate rejection conductivity mho % conduc- Saltflux Temperature 37 of cellulosic nanofiltration membrane at 100 psi transmembrane pressure Chemical modification of the nanofiltration membrane- The chemical modification of nanofiltration membranes was carried out by treating with different concentrations of alkali followed by treatment with polyhydroxy or polycarboxylic acids (A & B) having different hydrophobic and hydrophilic nature. Treatments were done by varying time and concentration of the coat materials. Module modification was carried out with the help of positive flow pump at 1 atm. Nanofiltration membrane module characteriza-. tion- The membrane module was fixed in the experimental set-up as shown in the Fig. 1, and characterized by studying its pure water permeability (PWP), % salt rejection with 1000 ppm NaCl and dye rejection with various concentrations of reactive red dye at 100 psi trans-membrane pressure (TMP) with a feed flow rate of 5 L/min. Dye desalination and concentration-dye desalination and concentration were carried out with varying concentration of reactive red dye (obtained from Narmada Chemicals, Bombay) having variation in chemical composition of dye and salts. Observations were made considering dye rejection, salt rejection and flux. The concentration of the dye was estimated with colorimetric analysis against standard calibration graph of salt free pure dye. Composition ofthe salt and dye in the feed during the experiment was calculilted based on the weight composition against pure dye. The percentage salt rejection was quantified based on the conductivity data. Results and Discussion c Suitability of cellulosic nanofiltration membrane- A new type of very low pressure asymmetric cellulosic nanofiltration membrane spiral wound module has been developed by Ion Exchange (India) Ltd, that will fractionate the monovalent salts from divalent salts which was reported in our earlier paper9 This nanofiltration membrane allowed NaCl to pass through more freely than MgS04 This spiral wound element exhibited PWP of24.84 gsfd and flux of23.20 gsfd with 1000ppmNaCland22.84 gsfd with 1000 ppm MgS04, at aforementioned conditions (Table 1). This membrane also showed a rejection of 54.5% for NaCI and 91.37% for MgS04 at 1000 ppm Table 2-The effect of treatment time on characteristic of polyhydroxyl polymer treated nanofiltration membrane, tested with feed dye -conc;entrat1on of 2% TDS, at 100 psi TMP treatment, 0.5 1.0 I.5 2.0 2.5 3.0 3.5 h PWP gsfd 15.97 18.75 20.14 17.63 17.85 17.45 17.86 % Salt rejection 55.55 53.12 83.87 83.33 84.61 73.39 78.46 % Dye rejection 99.85 99.84 99.85 99.79 99.76 99.69 99.81 concentrations. These results impliedthat the behavior of cellulosic nanofiltration membrane is similar to commercially available surface charged thin film composite (TFC) membranes which are recommended for their applications in softening of water5 The dramatic improvement was achieved by modifying nanofiltration membrane for its suitability in desalination and concentration of reactive dyes. The unmodified nanofiltration membrane is only suitable for desalination of monovalent salt based dyes. The modification of this nanofiltration membrane extends its applications in the desalination of divalent salts based reactive dyes as well, which was confirmed by its MgS04 rejection and was observed to be 22.22%. Modification of nanofiltration membrane with polymer having polyhydroxyl groups- Thlt nanofiltration membrai1e was surface coated with more hydrophilic polyrp.erhaving polyhydroxyl group in order to increase the surface hydrophilicity, thus, enhancing water flux and reducing dye permeation through the membrane. It was observed that variation in the time of treatment varies the characteristics of the membrane. The optimum time of exposure was found to be 90 min, the increase in time of treatment increases the dye a~ well as salt rejection with decrease in PWP as shown in Table 2. These results conclude that the coating of this polymer brings the membrane more towards reverse osmosis rather than increasing its perfor" mance in desalination of dyes.
VENKIDACHALAM & VERMA: MODIFIED CELLULOSIC NANOFILTRATION MEMBRANE 133 Modification of membrane with polycarboxylic acids'-most of the reactive dyes are having net negative charge at neutral ph. In order to exploit this characteristic of the dyes, the membrane was modified to give negative charge over the surface. Two polymers (A & B) of polycarboxylic acids having different hydrophilicity were used to investigate efficacy of their coating after alkali treatment of the membrane. It was observed that coating of polymer A, which is more hydrophobic than polymer B, effectively increases the PWP with increase in salt as well as dye rejections as shown in Table 3, but more salt permeation with higher water flux is desired in desalination of reactive dyes. Polymer B coated membrane exhibits encouraging results as listed in Table 4. Increase in time of coating with polymer B decreases the PWP within an acceptable limit but increases the salt permeation as well as dy.erejection substantially. This shows that the polycarboxylic acids having more hydrophilicity coatefficiendy with substantial negative charge over the surface, thus increasing the hydrophilicity as well as decreasing dye adsorption on the membrane surface. M odijication of nanofiltration membrane with activating agent (NaOH}- The surface modification of nanofiltration membranes with different concentrations and treatment time of activating agent gave promising results as listed in Tables 5 and 6. This activating agent deacetylates the cellulose acetate resulting in the formation of a thin layer of cellulose on the membrane surface. It has been established that cellulose possesses more partial negative charge and hydrophilicity than cellulose acetate. Therefore, this treatment increases the surface negative charge as well as hydrophilicity of the membrane. It is also expected that a small increment in the pore size may occur due to hydrolysis. However,longer treatment times lead to degradation of the membrane due to base polymer hydrolysis. Optimum time of treatment and concentration of the activating agent will depend upon the regular application of the membrane in dye industries, i.e. nature, molecular weight and types of salts contamination in the dye. All investigations were made with reactive red dye having molecular weight of 1000 D. In this investigation, the optimum time of Table 3- The effect of treatment time on characteristic of polycarboxylic acid polymer (A) treated nanofiltration membrane, tested with feed dye concentration of 2% TDS, at 100 psi TMP 21.69 % 99.95 rejection 69.44 99.94 53.75 PWP Salt Dye gsfd treatment, 30 20 15 10 5 min 2 Table 6- The effect of treatment time on characteristic of 0.4% activating agent treated nanofiltration membrane, tested with feed dye concentration of 2% TDS, at 100 psi TMP 34.29 26.91 rejectiongsfd 43.98 59.07 67.46 79.86 98.31 98.92 99.69 99.42 30.00 99.85 % 36.66 99.86 10.9 16.66 5.43 PWP 1.19 Salt Dye Table 4-- The effect oftr~atment time on characteristic of polycarboxylic acid polymer (B) treated nanofiltration membrane, tested with feed dye concentration of 2% TDS. at 100 psi TMP 21.743.33 20.045 20.83 99.87 % rejection 41.66 99.90 99.91 33.33 PWP Salt Dye gsfd 100.00 80.00 Table 5- The effect of concentration of activating agent (NaOH) 60.00 on characteristic oftreated nanofiltration membrane. tested with feed dye concentration of 2% TDS, at 100 psi TMP ~ 40.00 Concentration 0.'0 0.10 0.20 O.JO O.S) 0.60 48.84 65.32 99.26 29.88 38.62 58.72 99.56 99.60 99.79 99.84 % rejection 12.50 30.02 18.60 16.66 "10 8.24 Salt Dye SALT gsfd PWPREJECTION 0.00 CDNCEMTRATION OFlttE ACTIVATING AGENT, 0.00 20.00 0'. Fig. 2----'Effectof I concentration of the activating agent on the per~ formance of treated membrane
134 INDIAN J. CHEM. TECHNOL., MAY 1996 treatment and concentration of activating agent is observed to be 15 min and 0.4% respectively. The effect of concentration of activating agent in the treat, ment of nanofiltration membrane was studied with respect to its PWP, % salt and dye rejections and is dep'icted in Fig. 2. It is observed that increasing the concentration of the activating agent decreases the salt rejection substantially with a negligible decrease in dye rejection while the PWP changes enormously. It is experienced from these studies that the treatment of membrane with a lower concentration of activat, ing agent will be suitable for dyes having lower molecular weight. Change in the dye composition in simultaneous desalination and concentration of the reactive dyes-the spiral wound membrane element was modified with, activating agent and was used in the simultaneous desalination and concentration of the reactive dye. The water soluble reactive dye used in these experiments had the composition of64% dye and 36% salts. The composition of the reactive dye and salt was quantified by its weight ratio with,respect to salt free pure dye. The dye solution having an initial concentration of,7.3% dye by weight was used as the feed. To simulate industrial operations, the dye solution having initial feed composition continuously replaces the amount of permeate to a certain period which means that this process consists a mixture of continuous as well as batcr operation. As shown in Table 7, during desalination with simultaneous concentration of dye, the dye rejection by modified nanofiltration membrane decreases at higher concentration of dye and the flux is Table 7- Thestudy of simultaneous desalination and concentration of reactive red dye using spiral wound membrane module modified by activating(naoh)havingareaof5ft2at 100psi Operation permeate time, Dye 1401.29.52 1896.0 1804.8 921.96 729.87 15.90 19.78 15.97 17.49 209.86 287.47 335.98 574.64 375.76 99.66 99.78 99.67 99.68 20.284 99.64 99.43 10.20 99.48 (%) 28.0 11.19 15.31 28.8 10.45 12.82 18.44 gsfd 238.0 31.0 39.2 37.5 % 29.2 30.0 10.27 11.79 32.2 rejection 27.5 14.83 10.0 99.39 Feed 99.11 98.97 7.57 7.32 8.58 7.52 35 7.98 6.84 5.705 7.60 4.95 6.66 11.36 11.49 9.85 6.94 5.55 5.26 3.22 'C % conc. Flux Dye Feed rejection (ppm) Salt TDS. temp. 0 20 ~0. 2 E ~ 0 15.'=.S ~ 0 10 '0 '2 'E ~ 0'05 c ou 000 000 4 00 8,00 12 00 16'00 20'00 Concentration ot dye in the te~).,. Fig. 3-Effect of feed concentration on % dye rejection during desalination and concentration of the reactive dyes Table 8-- The rate of change salt and dye composition in the feed of reactive dye during simultaneous desalination and concentration by activating agent (NaOH) modified nanofiltration membrane spiral wound nanofiltration membrane Feed TDS Composition of feed operation, min % % Dye % Salts 0.3 5 10 15 21 26 35 45 55 65 75 7.35 7.52 8.58 10.2 10.45 11.19 12.82 15.~1 15.90 18.44 20.28 65.65 70.29 75.46 76.46 75.20 82.82 87.11 87.89 88.95 89.64 92.42 34.35 29.61 24.53 23.53 24.80 17.18 12.88 12.11 11.05 10.36 7.58
VENKIDACHALAM & VERMA: MODIFIED CELLULOSIC NANOFILTRATION MEMBRANE 135 'observed to decrease correspondingly. The reason for this observation could be attributed to more dye adsorption at higher concentration of dye on the membrane surface which leads to passage of dye as well as salt solutions through the membrane. Fig. 3 shows that the decrease of dye rejection is not linear to that of increase in the concentration of dye in the feed during the concentration. This clearly indicates that the adsorptionofdyewith pressure leads tomoredyepassage during the operation. The salt rejection also increases during the concentration which can be attributed to the same reason. The dye permeation also increases with time of operation with a dye loss of 0.87%. The amount of dye adsorbedoverthe membrane surface, contributing to a major dye loss, will be negligible when it is subjected to a large volume of feed solution. The composite dye permeation is only 0.06% by weight which is well within the level of acceptance in the dye industries. The change in composition of dye and salt is listed in Table 8. Initial rate of decrease of salt level in the feed during the concentration is faster than the later part ofthe operation. This is because, the salt concentration in the feed is decreasing with time thus, increasing the % salt rejection. From this experiment, a result of synergistic coupling of desalination and simuitaneous concentration, the dye having a final compositon of92.42% dye and 7.58% salts is achieved. Conclusions The modified asymmetric nanofiltration membrane, developed by Ion Exchange (India) Ltd, can be used for dye desalination as well as concentration of water soluble reactive dyes which are having both monovalent and divalent salts. Furthermore, the performance of cellulosic nanofiltration membranes can be improved by either changing the chemical composition of the membrane or modifying the membrane surface by varying time and concentration of the activation agent. In view of achieving the most efficient nanofiltration membrane sy~tem, both aforecited approaches are being attempted. References I Lacroix R, US Pat 4523924 (June 18,1985). 2 Peterson R J, J Merribr Sci, 83 (1993) 81. 3 Tsuru T, Vrairi M, Nakao S & Kimura S, Desalination, 81 (1991) 219. 4 Feeman K D N & Stocker T P, Desalination, 62 (1987) 183. 5 Comstock D L, Desalination, 76 (1989) 61. 6 Bindolf A, Davis C J, KerrC A & Bucklay C A, Desalination, 67 (1987) 455. 7 Hugelshofer P, US Pat 4500321 (Feb. 19, 1985). 8 Lacroix R, US Pat 4955987 (Sep. 11, 1990). 9 Venkidachalam G & Verma S K, 12th IMS Conference, CSMCRI, Bhavnagar, 1994.