Natural organic matter removal from water by complexation-ultrafiltration

Natural organic matter removal from water by complexation-ultrafiltration

Desalination 223 (2008) 91–96 Natural organic matter removal from water by complexation-ultrafiltration V. Siyanytsya, V. Kochkodan*, V. Goncharuk A...

160KB Sizes 2 Downloads 143 Views

Desalination 223 (2008) 91–96

Natural organic matter removal from water by complexation-ultrafiltration V. Siyanytsya, V. Kochkodan*, V. Goncharuk A.V. Dumansky Institute of Colloid Chemistry and Water Chemistry of National Academy of Sciences of Ukraine, Vernadsky Str. 42, 03680, Kyiv, Ukraine Tel. +38 044 4247521; Fax +38 044 4238224; email: [email protected] Received 22 January 2007; accepted 30 January 2007

Abstract An efficiency of humic acids (HA) removal from aqueous solutions by complexation-ultrafiltration (COUF) process was investigated. The rejection of commercial HA (Aldrich) in the presence of two weakly basic, cationic water-soluble polymers poly(diallyl dimethylammonium chloride) (PDADMAC) and chitosan were studied. The effect of different experimental parameters such as a mass ratio of HA to polymers, operating pressure, pH of solution on the HA removal was evaluated. It was shown that the HA rejection on ultrafiltration UPM-67 membrane (Vladipor) varied from 98.1 to 99.2% or from 90.5 to 92.5% with addition of PDADMAC or chitosan, respectively, when a HA/polymer mass ratio was changed from 1:1 to 1:7. The solute rejection to some extent improved with an increase in the concentration of polymeric complexing agents due to a higher completeness of the HA binding. It was found that a pH value of the feed solution has an important effect on HA removal in COUF with chitosan: the HA rejection increases with an increase in a degree of protonation of chitosan molecules. On the other hand, the HA removal did not essentially change with pH when PDADMAC was used as polymeric complexant owing to the lack of protonation of the quaternary amino groups of this polymer. Keywords: Ultrafiltration; Complexation; Humic acids; Poly(diallyl dimethylammonium chloride); Chitosan

1. Introduction The presence of natural organic matter (NOM) in water is associated with a number of problems including the undesirable taste, odor, and color of drinking water and the formation of harmful *Corresponding author.

by-products (especially trihalomethanes) on chlorination. [1]. During the chlorination of NOM containing surface water approximately 50–300 μg of highly toxic chlororganic compounds might be formed from 1 mg of their naturally-occurring precursors [2]. Humic acids (HA) constitute the major part of NOM therefore these substances should be extracted from water before it

Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Society and Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007. 0011-9164/06/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.01.220

92

V. Siyanytsya et al. / Desalination 223 (2008) 91–96

chlorination. Currently, the widespread methods for HA removing such as adsorption [3,4], ozonization [5], flocculation [6,7] do not always satisfy the continuing tightening of drinking water standards. In particular, it was shown [3] that the adsorption of HA on activated coals depends essentially on composition and admixtures of surface water, thus a choice of an optimal sorbent is needed for treatment of the each specific type of surface water. Ozonation of aqueous solutions of HA leads to formation and accumulation of low molecular weight organic compounds such as alcohols, aldehydes and acids that are highly stable to molecular ozone [5]. In a turn thin porous nanofiltration or reverse osmosis membranes are needed for an effective HA removal by pressure-driven membrane methods [8]. That is associated with high-energy requirements involved in these processes. Therefore of particular interest is the use of complexation-ultrafiltration (COUF) approach as an alternative water treatment method for HA removal. The basic concept of COUF is following: the target ions or molecules are rejected on wide-porous ultrafiltration membranes after binding with high molecular weight water-soluble polymers. The advantages of this process are the low-energy requirements and the high binding capacity of the polymeric complexing agents. Previously it was shown that COUF is attractive for removal of trace metallic species from dilute aqueous streams [9,10]. Dissociation of HA functional groups in aqueous solutions results in the net negative charge of their molecules and determines high affinity of humic substances towards complex formation. In this work an efficiency of HA removal from aqueous solutions by COUF using cationic watersoluble polymers such as poly(diallyl dimethylammonium chloride) (PDADMAC) and chitosan was studied.

2. Materials and methods HA were obtained from Aldrich. Poly(diallyl dimethylammonium chloride) (PDADMAC) with molecular weight of 250,000–300,000 Dalton (Aldrich) and chitosan were used as polymeric complexing agents for HA binding. Chitosan was produced from biomass of Aspergillus niger fungi [11] and was provided by the Chercassy Technological University (Ukraine). The molecular weight of chitosan was found to be 1.9 × 105 from the viscosity measurements. Chitosan flakes were dissolved in 0.5% acetic acid. Vladipor (Vladimir, Russia) commercial polysulfonamide ultrafiltration membranes UPM-67 (MWCO of 65,000) and UPM-20 (MWCO of 17,000) were used throughout the experiments. The aqueous solution of the polymer was added to HA solution with concentration of 20 mg/L at a different HA/polymer mass ratio (0.2–1). After stirring for 30 min the feed solution was filtered through the membrane at operating pressure of 0.05–0.4 MPa created by compressed nitrogen. Ultrafiltration experiments were performed in a 200 mL dead-end stirred cylindrical cell (FM-200) with an effective membrane area of 24.6 cm2. A stirring speed in the cell was maintained at about 300 rpm. The pH of solutions was adjusted by 0.1 hydrochloric acid or sodium hydroxide. HA concentration in permeate was determined by a spectrophotometer (KFK-2M) at 254 nm. The rejection coefficient of HA was calculated as

R=

Co − C p Co

⋅ 100%,

where Co, Cp are the HA concentrations in the feed and permeate, respectively. Membrane flux was evaluated following [12]:

J=

V St

V. Siyanytsya et al. / Desalination 223 (2008) 91–96

where J is the membrane flux, V is a permeate volume passed through the membrane with an area of S within time t at operating pressure of ΔP. 3. Results and discussion As can be seen in Fig. 1(a), the HA rejection is almost independent on operating pressure with membrane UPM-20, whereas the rejection decreases with the UPM-67 one. This difference might be explained by a concentration polarization effect. The water flux through the wide porous UPM-67 membrane sharply grows with an enhance in operating pressure. As a result the HA concentration in the boundary membrane (a) 85

R, %

80 UPM-67 UPM-20

75

70 0

0.1

0.3

layer essentially exceeds those in the bulk of solution that leads to decline in the observed HA rejection. It should be note that the HA rejection did not exceed 84% even with a relatively thinporous UPM-20 membrane (MWCO of 17,000). This is obviously due to a broad molecular weight distribution of the HA [13]. Fig. 1(b) shows that the volume flux of UPM-67 membrane is practically three times higher in comparison with UPM-20, therefore the former membrane was used in further COUF studies on HA removal. The effect of HA to polymer compexants ratio on the HA rejection and volume fluxes is shown in Fig. 2. It is seen that the efficiency of HA removal slightly increases with a HA/polymer ratio, obviously due to an increase in completeness of HA binding at higher polymer concentration. From the other hand the membrane fluxes somewhat decrease with enhancing of a HA/polymer mass ratio apparently owing to an increase in a viscosity of the solutions. The volume fluxes at filtration of HA/PDADMAC feed solutions are lower than those with chitosan (Fig. 2). This is because of higher molecular weight of PDADMAC comparing with chitosan that leads to a higher viscosity of HA/PDADMAC solutions.

0.4 100 100

300

90

90

2

UPM-67 UPM-20

250

60

100

50

50

40

0.1

0.2 ΔP, MPa

0.3

0.4

Fig. 1. The rejection of HA (a) and membrane flux (b) versus operating pressure at filtration of HA solution of concentration of 20 mg/L through membranes UPM-67 and UPM-20.

70

70

150

0

80

80

200

0

1

60 3

JV, L/m2 h

350

R, %

JV, L/m2 h

(b)

0.2

ΔP, MPa

93

50 40

4 1:1

1:3 1:5 HA/ Polymer ratio

1:7

Fig. 2. The HA rejection (1, 2) and membrane fluxes (3, 4) at different HA/PDADMAC (1, 4) and HA/Chitosan (2, 3) concentration ratios. The initial HA concentration is 20 mg/L. Membrane UPM-67, ΔP = 0.3 MPa.

V. Siyanytsya et al. / Desalination 223 (2008) 91–96

hand a decrease in a number of protonated groups in chitosan molecules with an increase in pH value brings to smaller repulsive forces between macromolecular segments. Obviously this leads to more dense structure of a gel layer formed from chitosan molecules on the membrane surface that results in a decline of the volume flux when pH changes from acidic to alkali values (Fig. 2, curve 2). It may be seen in Fig. 3 that the volume flux curve has a distinct minimum at a pH value of about 8.5 that corresponds to the isoelectric point of the chitosan [15]. At this point the aggregation of chitosan molecules is the highest as a result of which the most dense macromolecular gel layer is formed on the membrane surface. At

H2C

CH2 N +

H3C

Cl− CH3

+

R

H2C

− +

COOH

CH2

n

where R−COOH is a HA molecule. It can be assumed that chitosan macromolecules bind HA anions via electrostatic interactions with protonated aminogroups. Fig. 3 shows the dependencies of the HA rejection and volume flux versus pH at filtration of HA/chitosan solution through UPM-67 membrane. As can be seen, the HA rejection increases with decrease of solution pH. It is known [14] that chitosan molecules in acidic media are protonated (log Kp = 6.3):

H 3C

N +

_ OOC

+ HCl

n CH3 R

pH > 8.5 a sharp rise in membrane flux is observed, obviously, due to formation at this conditions a large amount of insoluble aggregates 60 100 98

55 1

96

50 2

94 92

RC-NH2 + H ↔ RC-NH , +

+ 3

JV, L/m2 h

It was found (Fig. 2) that high degrees of the HA removal (98.1 and 90.5% in the case of PDADMAC and chitosan, respectively) were obtained at a HA/polymer ratio of 1:1 (20:20 mg/L). Obviously at this ratio a number of positively charged sites in polymer macromolecules is sufficient for effective binding of HA molecules and there is no need in addition of the excessive amounts of the polymers. A higher removal of HA with PDADMAC is obviously explained by chemical nature of this polymer which macromolecules due to the presence of quaternary amino groups are capable to strong interaction with negatively charged HA molecules:

R, %

94

45

90 40

where RC-NH2 is a molecule of chitosan. Due to protonation chitosan molecules are capable to bind HA anions via electrostatic interactions. With pH increasing, a number protonated chitosan molecules decreases that results in lower HA rejection (Fig. 3, curve 1). On the other

4

6

8

10

12

pH

Fig. 3. The HA rejection (1) and volume flux (2) versus pH at filtration of HA/chitosan solution at the ratio of 1:1 (20:20 mg/L) through membrane UPM-67. Operating pressure is 0.3 MPa. Degree of permeate collection is 15%.

V. Siyanytsya et al. / Desalination 223 (2008) 91–96

the HA removal should not change essentially in COUF process with pH alteration. However, an increase in a pH value of the HA/PDADMAC feed solution from 7.1 to 9.1 leads to some decrease in HA rejection (Fig. 4(a)). This is likely because in the presence of the excessive quantity of OH ions the PDADMAC macromolecules gain more rigid conformation, enhancing the passage of macromolecules across the membrane [10].

(a)

98

pH = 7.1 96 R, %

pH = 9.1

94 92

5

10 15 20 25 Degree of permeate collection, %

30

4. Conclusions

(b)

pH = 7.1

60

pH = 9.1 JV, L/m2 h

50 40 30 20 10 5

10 15 20 25 Degree of permeate collection, %

95

30

Fig. 4. The rejection of HA (a) and membrane fluxes (b) versus degree of permeate collection at filtration of HA/PDADMAC solution at the ratio of 1:1 (20:20 mg/L) at various pH values through membrane UPM-67. Operating pressure is 0.3 MPa.

of chitosan molecules, which can not be deposited on the membrane surface [10]. Dependencies of HA rejection and membrane fluxes versus a degree of permeate collection at ultrafiltration of HA/PDADMAC solution at pH values of 7.1 and 9.1 are presented in Fig. 4. As can be seen, the HA rejection increases to some extent with increasing of a degree of permeate collection. This is, obviously, due to possible formation of self-rejected gel layer of PDADMAC macromolecules on the membrane surface. It can be anticipated that owing to the lack of protonation of the quaternary amino groups of PDADMAC,

The HA removal from aqueous solutions by COUF using cationic water-soluble polymers such as PDADMAC and chitosan was studied. The HA rejection enhanced from 98.1 to 99.5% (in the case of PDADMAC) and from 90.5 to 99.2% (in the case of chitosan) when a HA/ polymer ratio varied over a range of 1:1 to 1:7. It was found that at filtration of HA/chitosan feed solution the HA rejection increases at pH lowering as a result of the increasing of degree of protonation of chitosan molecules while the HA removal did not change essentially with pH when PDADMAC was used for the solute binding owing to the lack of protonation of the quaternary amino groups of this polymer. The data obtained show a possibility to obtain a high degree of HA removal from aqueous solutions via HA binding with water soluble cationic polymers such as PDADMAC and chitosan with following ultrafiltration on wide-porous membranes at low (0.1–0.4 MPa) operating pressures. References [1]

[2]

M. Watson and C.D. Hornburg, Low-energy membrane nanofiltration for removal of color, organics and hardness from drinking water supplies, Desalination, 72 (1989) 11. L. Alekseeva, The technology of preparation of drinking water to prevent halohenorganic compounds formation, Ph.D. Thesis, Municipal Academy, Moscow, 1988.

96 [3]

[4]

[5] [6]

[7]

[8]

[9]

V. Siyanytsya et al. / Desalination 223 (2008) 91–96 J.J. Mc Creary and V.L. Snoeyink, Characterization and activated carbon adsorption of several humic substances, Water Res., 14 (1980) 151. G.V. Slavinskaya, Purification of natural waters from humic acids by conjuction of anionits and activated coals, Sorp. Chromatogr. Proc., 3 (2003) 286. P. Meijers, Quality aspects of ozonation, Water Res., 11 (1976) 646. Yu I. Veitzer and D.M. Mintz, High molecular weight flocculants in water treatment, Stroiizdat, Moscow, 1975. D.B. Babcock and P.C. Singer, Chlorination and coagulation of humic and fulvic acids, J. Amer. Water Works Assoc., 71 (1979) 149. R. Bian, Y. Watanabe, N. Tambo and G. Ozawa, Removal of humic substances by UF and NF membrane systems, Water Sci. Tech., 40 (1999) 121. K.E. Geckeler and K. Volchek, Removal of hazardous substances from water using ultrafiltration in conjunction with soluble polymers, Environ. Sci. Technol., 30 (1996) 725.

[10] R.-S. Juang and C.-H. Chiou, Ultrafiltration rejection of dissolved ions using various weakly basic watersoluble polymers, J. Membr. Sci., 177 (2000) 207. [11] T. Solodovnik, V. Unrod and Y. Lega, Chitincontaining complexes from mycelium of Aspergillus for sorption of petroleum, in: R.A.A. Muzzarelli and C. Muzzarelli (Eds.), Chitosan in pharmacy and chemistry, Atec, Milan, 2002, pp. 475–478. [12] M.T. Bryk, E.A. Tsapyuk and A.A. Tverdii, Membrane technology in the industry, Technicsa, Kiev, 1990. [13] A.E. Childress and M. Elimelech, Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes, J. Membr. Sci., 119 (1996) 253. [14] R.A.A. Muzzarelli, Chitin and its derivatives: new trends of applied research, Carbohydr. Polym., 3 (1983) 53. [15] M.W. Anthonsen, K.M. Varum, A.M. Hermanssen, et al., Aggregates in acidic solutions of chitosan detected by static laser light scattering, Carbohydr. Polym., 25 (1994) 13.