- Email: [email protected]
Effect of cationic polymer additives on the adsorption of humic acid onto iron oxide particles
Colloids and Surfaces A: Physicochemical and Engineering Aspects 194 (2001) 123– 131 www.elsevier.com/locate/colsurfa
Effect of cationic polymer additives on the adsorption of humic acid onto iron oxide particles Eun Kyoung Kim, Harold W. Walker * Department of Ci6il and En6ironmental Engineering and Geodetic Science, The Ohio State Uni6ersity, 470 Hitchcock Hall, 2070 Neil A6enue, Columbus, OH 43210, USA Received 1 May 2000; accepted 9 May 2001
Abstract Improving the removal of natural organic matter (e.g. humic acid) during drinking water treatment is important in order to minimize the formation of disinfection by-products (DBPs). Although polymeric flocculants are often used to improve turbidity removal during the coagulation process, little information is available regarding the influence of these polymers on NOM removal. In this study, the adsorption of humic acid onto polymer-coated iron oxide particles was investigated as a way to improve humic acid removal during the coagulation process. Monodisperse iron oxide particles and well-characterized humic acid were used as a model system. The adsorption of humic acid onto bare iron oxide particles decreased as solution pH increased. When iron oxide particles were coated with a cationic polymer, adsorption doubled at high pH ( 9.5) and low salt concentration (0.001 M NaCl). The greater adsorption of humic acid on polymer-coated surfaces at high pH was largely due to changes in the electrostatic interactions between humic acid and the particle surface. Coating the iron oxide particles with cationic polymer resulted in the reversal of the negative particle surface charge, thus providing more favorable conditions for humic acid adsorption. Little change in humic acid adsorption was observed, however, for polymer-coated particles at near neutral pH values ( 6.8) or at high salt concentration. These data suggest that under certain conditions polymers may enhance humic acid adsorption onto iron oxide surfaces, a process which may improve DBP precursor removal during drinking water treatment. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Cationic polymer; Humic acid; Adsorption
1. Introduction Natural organic matter (NOM) reacts with chlorine during drinking water treatment processes to form disinfection by-products (DBPs), * Corresponding author. Tel.: + 1-614-292-8263; fax: +1614-292-3780. E-mail address: [email protected] (H.W. Walker).
such as trihalomethanes (THMs) and haloacetic acids (HAAs) [1]. DBPs are potential carcinogens [2] and are also linked to several health problems associated with the central nervous system [3]. Based on the health effects of DBPs, a maximum contaminant level (MCL) for THMs was set at 0.1 mg l − 1 by United States Environmental Protection Agency (USEPA) in 1979. New limits, as part of Stage I of the Disinfection/Disinfectant
0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 7 9 1 - 9
124
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
By-Product (D/DBP) Rule, will be set in 2001 at 0.080 mg l − 1 for THMs and 0.060 mg l − 1 for HAAs. Stage II of D/DBP Rule will set limits at 0.040 mg l − 1 for THMs and 0.030 mg l − 1 for HAAs in the year 2002 [4]. In order to minimize the formation of DBPs, many drinking water utilities have focused on improving the removal of disinfection by-product precursors, such as humic and fulvic acids. For example, the USEPA has established guidelines for carrying out so-called ‘enhanced coagulation’, in which the metal salt coagulation process is optimized for NOM removal [5]. The implementation of enhanced coagulation generally requires higher metal salt usage, thereby increasing cost and the amount of waste sludge produced [6]. Therefore, new approaches for improving NOM removal during coagulation will reduce the formation of DBPs and significantly improve the economics of water treatment. The removal of NOM by enhanced coagulation occurs largely through adsorption to metal hydroxide floc particles, and depends on source water pH, alkalinity, coagulant type and dosage, and the type and concentration of NOM [7]. A number of studies have shown that the adsorption of NOM onto iron oxide particles increases with decreasing pH and increasing ionic strength. Cronzes et al. [6] and Davis et al. [8] have shown that at low pH organic protonation increases, thus reducing the negative charge density of organic matter, and subsequently, coagulant demand. As a result, pH modification has become one of the approaches used by water treatment plants to improve NOM removal. The characteristics of organic matter also affect treatability, with NOM removal generally increasing with increasing molecular weight and hydrophobicity [9– 12]. The major mechanisms controlling NOM adsorption on metal oxides have been summarized by Gu et al. [13] and include (1) electrostatic interactions [14– 16], (2) ligand exchange–surface complexation [13,17,18], (3) hydrophobic interactions [19], and (4) cation bridging [20]. For example, Gu et al. [13] determined using microcalorimetry, FTIR, and 13C NMR that specific functional groups on NOM, such as carboxyl
and hydroxyl functional groups, are amenable to adsorption onto iron oxide surfaces by a surface complexation–ligand exchange mechanism at low pH conditions. Steeg et al. [21] and Vermeer et al. [22] found that increases in salt result in higher amounts of adsorbed NOM. They proposed that the increase in NOM adsorption was due to the screening of electrostatic interactions between surface-adsorbed NOM molecules. Murphy et al. [19] found that simple inorganic ions (Na+, Ca2 + , Cl−) are readily hydrated [23], and this facilitates the formation of hydrophobic mineral surfaces, which may also enhance NOM adsorption. Many water treatment plants utilize synthetic polymer additives to improve turbidity removal during coagulation. Although a number of studies have examined the influence of polymers on particle removal [24–27], little is known about the interaction of polymeric additives with NOM. In one of the few studies in this area, Lurie et al. [25] examined the influence of polymer on NOM removal in the presence of iron and aluminum salts. They found that NOM removal increased with increasing polymer dosage during metal salt coagulation, possibly due to electrostatic interactions between polymer and NOM in solution. While earlier studies have provided insight into the factors influencing NOM removal during coagulation, little information is available regarding specific interactions between NOM, polymeric additives, and metal hydroxide surfaces during water treatment processes. The purpose of this study was to investigate the effect of cationic polyelectrolytes on NOM removal during metal salt coagulation. In particular, this paper examines how the coating of floc particles with polymeric additives influences the adsorption of NOM to particle surfaces. Experiments were carried out with model iron oxide particles, polymeric flocculants, and well-characterized humic materials at a number of pH values and ionic strengths. The results from this study provide important new insight into the process of sequential adsorption of charged polymers on surfaces. At a more practical level, these data provide new information about the role of polymeric flocculants in controlling NOM removal during water treatment operations.
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
2. Materials and methods
2.1. Materials Humic acid was supplied by Aldrich Chemical Company (Milwaukee, WI). Stock solutions of humic acid were prepared in Milli-Q water (18.2 MV cm), filtered with 0.45 mm glass fiber filters (GF/C, Millipore Corporation, Bedford, MA), and then stored at 4 °C prior to use. Iron oxide particles (hematite) were obtained from Atlantic Equipment Engineers (Bergenfield, NJ) and had a purity of 99.9% as determined by the manufacturer. The particles had a mean diameter of 3079 18.5 nm, which was determined at the Ohio State University using a Photon Correlation Spectrometer (90 Plus particle size analyzer, Brookhaven Instruments Corporation, NY). Particle suspensions were sonicated for 10 min prior to size determination in order to break up any aggregates present in the stock suspensions. To prepare particle solutions for adsorption experiments, a specific amount of iron oxide was mixed in carbonate buffer at a desired pH and ionic strength for 1 h, or until the pH of the solution stabilized. Polydiallyl dimethyl, a high molecular weight, cationic polymer (‘C-3299’) was supplied by Polydyne Inc. (Riceboro, GA).
2.2. Adsorption measurements 2.2.1. Humic acid adsorption onto iron oxide The adsorption of humic acid on iron oxide was determined using a UV-VIS 2401 PC double-beam spectrophotometer (Shimadzu Corporation, MD) at 254 nm. To perform a humic acid adsorption experiment, humic acid solutions were prepared with 0.001 or 0.1 M NaCl in carbonate buffer. A fraction of a well-mixed iron oxide suspension (10 g l − 1) was then pipetted into series of humic acid solutions at varying concentration (5– 40 mg l − 1). The final iron oxide concentration for all adsorption experiments was 50 mg l − 1. The final solution was adjusted to the desired pH with 0.1 M HCl or 0.1 M NaOH. The solutions were mixed for 18 h to equilibrate. Preliminary experiments verified that after 18 h no measurable change occurred
125
in the adsorbed amount. After 18 h, samples were centrifuged and then filtered with 0.22 mm filters (HA, Millipore Corporation, Bedford, MA) to separate the supernatant from particles. Control experiments were carried out to verify no adsorption onto the walls of the reaction vessels or filter materials and that filtration effectively removed all iron oxide particles from the solution. The pH was monitored at both the beginning and end of each adsorption experiment. Little variation in pH was observed during the course of an adsorption experiment. The amount of humic acid adsorbed was determined based on the measured depletion in solution.
2.2.2. Cationic polymer adsorption onto iron oxide The adsorption of polymer on iron oxide particles was determined by a similar approach using a total organic carbon analyzer (TOC 5000A, Shimadzu Corporation, MD). For the polymer adsorption tests, the cationic polymer solution was prepared with Milli-Q water at various concentrations (5–50 mg l − 1) in carbonate buffer. The pH was adjusted to approximately 7 or 9 with 0.1 M HCl or 0.1 M NaOH and the salt concentration fixed at either 0.001 or 0.1 M NaCl. Next, a 10 g l − 1 well-mixed iron oxide solution was pipetted into a series of polymer solutions for a final hematite concentration of 50 mg l − 1 and the mixture was mixed for 24 h. Following equilibration, samples were centrifuged to separate the supernatant and the particles, and the TOC of the supernatant was measured using the TOC analyzer. In these experiments, it was not possible to use filtration to separate the polymer in solution from the adsorbed polymer fraction due to a loss of polymer during the filtration step. However, control experiments were carried out and showed that centrifugation effectively removed all particles from the suspension and that no polymer or humic acid was lost during this process. 2.2.3. Humic acid adsorption on polymer-coated iron oxide To coat iron oxide particles, cationic polymer at 50 mg l − 1 was prepared in carbonate buffer
126
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
solution and the pH adjusted to pH 7 or 9 and the salt concentration fixed at either 0.001 or 0.1 M. Next, a well-mixed iron oxide solution was pipetted into the cationic polymer solution for a final iron oxide concentration of 50 g l − 1, and the mixture was equilibrated for 24 h. After 24 h, samples were centrifuged to separate the supernatant and the particles. Following separation, a series of humic acid solutions at varying concentration (5– 40 mg l − 1) were added to the polymer-coated particles and the particles resuspended by gentle agitation. The pH and salt concentration of humic acid solutions matched that of the supernatant removed during the coating process. The solutions were then mixed for 18 h. After 18 h, samples were centrifuged and then filtered with 0.22 mm filters (HA, Millipore Corporation, Bedford, MA) to separate the supernatant from the particles. The concentration of humic acid remaining in solution was determined using the UV-Vis spectrophotometer and the amount adsorbed calculated based on humic acid depletion in solution. Control experiments indicated that no measurable desorption of polymer occurred following resuspension of coated particles in polymer-free buffer.
2.3. Electrophoretic mobility measurements The electrophoretic mobility of iron oxide particles, both in the presence and absence of cationic polymer, was determined using an electrophoretic light scattering instrument (ZetaPlus, Brookhaven Instruments Corp., NY). All mobility measurements were carried out at 25 °C and a salt concentration of 0.001 M NaCl. The pH of solution was adjusted by adding varying amounts of either 0.1 M NaOH or 0.1 M HCl. For mobility measurements with cationic polymer, iron oxide particles were suspended in a 50 mg l − 1 polymer solution for 24 h to allow for saturation of the surface. Then, the particles were separated from the remaining polymer in solution by centrifugation, resuspended in new buffer, and the mobility measured using the ZetaPlus. Each calculated mobility value represents the average of at least five independent measurements.
3. Results and discussion
3.1. Adsorption of humic acid on bare iron oxide The adsorption of humic acid on iron oxide particles was investigated to determine the affinity of humic acid for the bare iron oxide surface as function of pH (7.4 and 9.6) and salt concentration (0.001 and 0.1 M). These results are shown in Fig. 1. As can be seen, the isotherms are of the ‘high affinity’ type with the maximum amount adsorbed rapidly reaching saturation. This behavior has been seen by a number of other researchers examining the adsorption of humic substances onto iron oxide surfaces [28–30]. The well-defined plateau in adsorption is indicative of monolayer coverage of humic acid on the particle surface, and a decrease in the amount of adsorbing surface remaining as the surface excess of humic acid increases. The possible exception to these observations is the data at pH 9.6 and 0.001 M NaCl (filled circles). Under these conditions, adsorption is low and no distinct plateau region was observed. The adsorbed amounts observed in Fig. 1 are within the range of those seen by other researchers [16], considering differences in the specific surface area of iron oxides and variations in the properties of humic substances in these different studies.
Fig. 1. Adsorption of humic acid on iron oxide particles at two salt concentrations and two pH values. (The experiments were carried out at pH 7.4 and 0.1 M NaCl ( ), pH 9.6 and 0.1 M NaCl ( ), pH 7.4 and 0.001 M NaCl () and pH 9.6 and 0.001 M NaCl ( )).
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
The adsorption of humic acid onto bare iron oxide increased with decreasing pH, at least at low NaCl concentration, consistent with earlier studies [13]. For example, at pH 7.4 and 0.001 M NaCl (), the adsorption maximum of humic acid occurred at approximately 10 mg g − 1 whereas the maximum amount of humic acid adsorption observed at pH 9.6 and 0.001 M NaCl ( ) was less than 8 mg g − 1. The dependence of the adsorbed amount on pH is consistent with a ligand exchange adsorption mechanism [16,18]. As pH decreases from 9.6 to 7.4, the increase in hydroxyl groups on the iron oxide surface provides more sites for ligand exchange reactions, and hence, greater humic acid adsorption. At high salt concentration (0.1 M NaCl), however, no measurable difference in adsorption as a function of pH was observed. For both pH 7.4 and 9.6 at 0.1 M NaCl, the maximum amount adsorbed was 15 mg g − 1. In general, an increase in salt concentration resulted in a significant increase in humic acid adsorption on iron oxide for both pH 7.4 and 9.6. At pH 7.4 and a salt concentration of 0.001 M (open circles), the maximum amount adsorbed was approximately 10 mg g − 1. At the same pH but a salt concentration of 0.1 M (open squares), the maximum amount adsorbed increased to 15 mg g − 1. A similar effect was observed at high pH. At pH 9.6, the maximum amount adsorbed was 8 mg g − 1 at 0.001 M NaCl (filled circles) and increased to 15 mg g − 1 at 0.1 M NaCl (filled squares). The increase in amount adsorbed with increasing ionic strength was likely a result of changes in repulsive electrostatic interactions between humic acid segments. The screening of repulsive electrostatic interactions leads to a more compact humic acid conformation and closer packing of humic acid molecules on the iron oxide surface. This effect has been shown earlier in a number of other studies examining humic acid and polyelectrolyte adsorption [31– 33].
3.2. Polymer adsorption on iron oxide Adsorption of cationic polymer on iron oxide particles at pH values of 6.7 and 9.6 and two salt concentrations (0.1 and 0.001 M NaCl) is shown
127
Fig. 2. Effect of salt concentration and pH on the adsorption of cationic polymer C-3299 on iron oxide. (The experiments were carried out at pH 6.7 and 0.1 M NaCl ( ), pH 9.6 and 0.1 M NaCl ( ), pH 6.7 and 0.001 M NaCl () and pH 9.5 and 0.001 M NaCl ( )).
in Fig. 2. For the cationic polymer used in these studies, the amount adsorbed increased with increasing pH for both salt concentrations. At low ionic strength, the amount adsorbed was roughly 0.5 mg g − 1 at pH 6.7 (open circles) and increased to over 1.0 mg g − 1 at pH 9.5 (filled circles). At high ionic strength, the amount adsorbed was 1.2 mg g − 1 at pH 6.7 (open squares) and increased to around 2 mg g − 1 at pH 9.6 (filled squares). The amount of cationic polymer adsorbed at both pH values and ionic strengths was lower than that observed for humic acid. The highest amount of polymer adsorbed was approximately 2 mg g − 1, which is significantly less than the range of humic acid adsorbed amounts (8–15 mg g − 1). Electrostatic interactions likely play an important role in controlling adsorption of this cationic polymer on the iron oxide surface. To gain better insight into the role of electrostatic interactions in controlling polymer adsorption, the electrophoretic mobility of the particles was determined as a function of pH. These data are shown in Fig. 3 for a salt concentration of 0.001 M NaCl. For the bare iron oxide particles, the isoelectric point (IEP) was between pH values 8.3 and 9.2. This is consistent with earlier studies, which have found the point of zero charge (PZC) of hematite to be approximately 8.5 [23,34]. At low pH values (pHB7), the bare particles were positively charged with an electrophoretic mobil-
128
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
ity of around 4 mm s − 1 V − 1 cm − 1. At pH 7, the mobility of the bare particles decreased sharply and became negative at pH values of 9 or higher. At pH values above 9, the electrophoretic mobility approached −4 mm s − 1 V − 1 cm − 1. The presence of cationic polymer resulted in a positive electrophoretic mobility at all measured pH values. At low pH (pHB 7), the mobility of the polymer-coated particles was approximately 5 mm s − 1 V − 1 cm − 1, slightly higher than the mobility of the bare iron oxide particles. In this range of pH values, both the polymer and iron oxide particles were positively charged, and therefore, the electrostatic interactions between polymer and particle were repulsive and adsorption was low. The fact that measurable amounts of adsorption took place at these low pH values, and that the mobility increased slightly upon adsorption of polymer, suggests that there may also have been a non-electrostatic component to the adsorption process. At high pH values (pH\9), the surface charge of the bare iron oxide particles was negative, which resulted in attractive electrostatic polymer –particle interactions and higher adsorption. The mobility of the polymer-coated particles remained positive at high pH values but decreased slightly with increasing pH. At pH 10, the polymer-coated particles had a mobility of 2 mm s − 1 V − 1 cm − 1, while the bare hematite particles had a mobility of − 3 mm s − 1 V − 1 cm − 1.
Fig. 3. Electrophoretic mobility of hematite particles as a function of pH in the presence () and absence ( ) of cationic polymer. All data were collected at a salt concentration of 0.001 M NaCl.
Fig. 4. Adsorption of humic acid on iron oxide at pH 9.6 and 0.001 M NaCl in the absence () and presence ( ) of a cationic polymer C-3299 and at pH 9.6 and 0.1 M NaCl in the absence ( ) of a cationic polymer C-3299.
3.3. Humic acid adsorption on polymer-coated iron oxide The adsorption of humic acid onto iron oxide particles in the presence and absence of adsorbed cationic polymer at pH 9.6 and 0.001 M NaCl is presented in Fig. 4. As can be seen, the adsorption of humic acid on polymer-coated iron oxide particles at pH 9.6 (filled circles) was significantly greater than adsorption on the bare particles (open circles). The maximum adsorption density nearly doubled, increasing from roughly 8 mg g − 1 on bare iron oxide to 15 mg g − 1 on polymercoated iron oxide. The amount of humic acid adsorbed on the polymer-coated iron oxide surface at low salt concentration approached the levels observed at 0.1 M NaCl (filled squares), at least at high equilibrium concentrations. The coating of the iron oxide surface with cationic polymer also influenced the shape of the humic acid adsorption isotherm. In the presence of cationic polymer, the initial slope of the isotherm was lower compared with humic acid isotherms observed at high salt. A number of mechanisms may play a role in controlling the adsorption of humic acid onto polymer-coated iron oxide. Coating the iron oxide particles with a cationic polymer will influence both specific chemical and electrostatic interactions between humic acid and the iron oxide
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
surface. At high pH, adsorption of a cationic polymer resulted in the reversal of the negative electrophoretic mobility of the particles. The subsequent decrease in repulsive electrostatic forces between humic acid and iron oxide particles facilitated the attachment of humic acid to positively charged sites on the polymer or to available functional groups on the iron oxide surface. It should be noted that interactions between cationic polymer and humic acid in solution could also play a role in controlling adsorption. However, in these experiments, unadsorbed polymer was removed from solution prior to exposure to humic acid, and therefore, polymer– humic interactions in solution were not important. It is interesting to note in Fig. 4 that the high affinity character of humic acid adsorption at high salt (filled squares) (i.e. steep increase in adsorbed amount at low equilibrium concentration) was not observed on polymer-coated particles (filled circles). The shape of an adsorption isotherm is influenced by the molecular weight of the polymer, the monodispersity of the polymer suspension, as well as the affinity of individual polymer segments for the surface. Typically, high affinity isotherms are obtained for monodisperse suspensions of high molecular weight polymers with high segmental adsorption energies. For the data in Fig. 4, the decrease in affinity at low equilibrium concentrations may be a result of changes in the affinity of specific humic acid moieties for the iron oxide surface in the presence of polymer. The steady increase in adsorbed amount may also reflect some re-configuration of humic acid on the iron oxide surface. Perhaps at low equilibrium concentrations humic acid molecules adsorb via attachment to positively charged polymer segments on the particle surface. As the surface coverage of humic acid increases, humic acid molecules adsorb to less favorable sites on the bare iron oxide surface. Desorption experiments were carried out and showed that no measurable amount of polymer was released to solution during the course of an experiment. Therefore, the low affinity character of the isotherm cannot be explained by desorption of polymer into solution.
129
The results of humic acid adsorption at pH 7.4 and low salt concentration (0.001 M NaCl) in the presence (filled circles) and absence (open circles) of adsorbed polymer are shown in Fig. 5. For these conditions, coating the particles with polymer had little effect on humic acid adsorption. In the absence of polymer, the maximum adsorption density observed was 12 mg g − 1. Upon coating the particles with polymer, the maximum amount of humic acid adsorbed slightly decreased to approximately 10 mg g − 1. At this pH and salt concentration polymer adsorption was low (B1 mg g − 1), and therefore, polymer molecules had little effect on humic acid adsorption. The slight decrease in adsorption upon coating the particles may have been a result of competition between humic acid and polymer for surface sites at these solution conditions. Fig. 6 shows humic acid adsorption isotherms at pH 9.6 and high salt concentration (0.1 M NaCl) in the presence (filled circles) and absence (open circles) of adsorbed polymer. At high salt concentration, the polymer had no measurable effect on humic acid adsorption. In both the presence and absence of polymer, the maximum amount adsorbed was approximately 17 mg g − 1. In this case, adsorption appears to be controlled largely by non-electrostatic interactions between humic acid and the hydrous oxide surface. It is reasonably given that even at high ionic strength polymer adsorption was an order of magnitude less than humic acid adsorption.
Fig. 5. Adsorption of humic acid on iron oxide at pH 7.4 and 0.001 M NaCl in the absence () and presence ( ) of cationic polymer C-3299.
130
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
Fig. 6. Adsorption of humic acid on iron oxide at pH 9.6 and 0.1 M NaCl in the absence () and presence ( ) of cationic polymer C-3299.
A schematic interpretation of the effect of cationic polymer on the adsorption of humic acid to iron oxide particles is shown in Fig. 7 for low
Fig. 7. Schematic interpretation of the effect of cationic polymer on the adsorption of humic acid on iron oxide particles at two pH values
salt conditions. At low pH, in the absence of polymer, the particles had a net positive charge. Under these conditions, both non-electrostatic and electrostatic interactions are favorable for adsorption and the surface concentration of humic acid on the bare particles was high. Coating the particles with polymer at low pH resulted in slightly greater positive surface charge density, but perhaps influenced specific chemical interactions between the humic acid and the hematite surface. As a result, humic acid adsorption decreased slightly at this pH value in the presence of polymer. At high pH, the bare particles had a net negative charge, and therefore, humic acid adsorption was low. Coating the particles with cationic polymer under these conditions reversed the charge density of the iron oxide and resulted in favorable electrostatic interactions and significantly higher humic acid adsorption.
3.4. Significance of cationic polymer additi6es in water treatment plants As mentioned in Section 1, many water treatment plants utilize polymeric additives to enhance turbidity removal during coagulation. Upon addition of polymer, both the electrostatic and surface chemical characteristics of floc surfaces will be altered. It is expected that the net charge of floc surfaces will be shifted to higher, positively charged values in the presence of cationic polymer. This suggests that the typical reduction of pH carried out for optimum removal of natural organic matter by coagulation using metal salts may not be required when cationic polymers are added. It should be noted, however, that humic molecules in solution could possibly interact with cationic polymer additives in solution, and therefore, exert a ‘polymer demand’. Polymer conditioning of metal hydroxide sludge with subsequent recycle may provide an effective approach for enhancing natural organic matter removal during metal salt coagulation while minimizing polymer demand. Further work is required to elucidate the relative importance of surface and solution interactions in controlling the adsorption and removal of humic substances during polymer-enhanced coagulation processes.
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
4. Conclusions Coating iron oxide flocs with cationic polymer significantly increased adsorption of humic acid at high pH, but had less effect at low pH. The adsorption of cationic polymer converted negatively charged iron oxide to positively charged particles, thereby inducing electrostatic attraction between the humic acid molecules and the coated particle surfaces. These results suggest that cationic polymers may facilitate adsorption of humic acid on floc surfaces at high pH. Little enhancement in adsorption was observed at low pH or high ionic strength. Under these conditions high adsorption of humic acid was observed in both the presence and absence of cationic polymer. Polymer conditioning of floc surfaces may be an attractive alternative for water plants treating source water with high alkalinity in which reduction of pH to optimum values for NOM removal requires significant acid or coagulant addition.
Acknowledgements Funding from the Ohio Water Development Authority (OWDA) is gratefully acknowledged.
References [1] G.L. Amy, M.R. Collins, C.J. Kuo, P.H. King, J. Am. Water Works Assoc. 79 (1987) 43. [2] D.A. Okun, J. Environ. Eng. 122 (1996) 453. [3] K. Walker, S.H. Swan, G. DeLorenze, B. Hopkins, Epidemiology 9 (1998) 134. [4] USEPA. National primary drinking water regulation; Disinfectants and disinfection by-products; Proposed Rule. Fed. Reg. 5-: 145:38668, 19 July 1994. [5] Malcolm Pirnie, Inc. Guidance Manual for Enhanced Coagulation and Enhanced Precipitative Softening, United States Environmental Protection Agency, Washington D.C., September, 1993. [6] G. Crozes, P. White, M. Marshall, J. Am. Water Works Assoc. 87 (1995) 78.
131
[7] M.C. White, J.D. Thompson, G.W. Harrington, P.C. Singer, J. Am. Water Works Assoc. 89 (1997) 64. [8] J.A. Davis, R. Gloor, Environ. Sci. Technol. 15 (1981) 1223. [9] G.L. Amy, J. Bedessem, R.A. Sierka, J. Am. Water Works Assoc. 84 (1992) 67. [10] D.M Owen, G.L. Amy, Z.K. Chowdhury, R. Paode, G. McCoy, K. Viscosil, J. Am. Water Works Assoc. 97 (1995) 46. [11] M.R. Collins, G.L. Amy, C. Steelink, Environ. Sci. Technol. 20 (1986) 1028. [12] M.J. Semmens, A.B. Staples, J. Am. Water Works Assoc. 78 (1986) 76. [13] B. Gu, J. Schmitt, Z. Chen, L. Liang, J.F. MaCarthy, Geochim. Cosmochim. Acta 59 (1995) 219. [14] A. Amirbahman, T.M. Olson, Environ. Sci. Technol. 27 (1993) 2807. [15] E. Tipping, M.A. Hurly, M.M. Reddy, Environ. Sci. Technol. 24 (1990) 1507. [16] A.W.P. Vermeer, Langmuir 15 (1998) 4210. [17] E. Tipping, Geochim. Cosmochim. Acta 45 (1981) 191. [18] R.L. Pafitt, A.R. Fraser, V.C. Farmer, J. Soil Sci. 28 (1977) 289. [19] E.M. Murphy, Environ. Sci. Technol. 28 (1994) 1291. [20] S. Chen, W.P. Inskeep, S.A. Williams, P.R. Callis, Soil Soc. Am. J. 56 (1992) 67. [21] G.M.H. van de Steeg, M.A. Cohen Stuart, A. de keizer, B.M. Bijsterbosch, Langmuir 8 (1992) 2538. [22] A.W.P. Vermeer, F.A.M. Leermakers, L.K. Koopal, Langmuir 13 (1997) 4413. [23] W. Stumm, Chemistry of the Solid – Water Interference, Wiley, New York, 1992. [24] M. Edwards, M. Benjamin, J.E. Tobiason, J. Am. Water Works Assoc. 86 (1994) 105. [25] M. Lurie, M. Rebhun, Water Sci. Technol. 36 (1997) 96. [26] X. Yu, P. Somasundaran, J. Colloid Interface Sci. 177 (1996) 283. [27] T. Matsumoto, Y. Adachi, J. Colloid Interface Sci. 204 (1998) 328. [28] C.H. Ho, N.H. Miller, J. Colloid Interface Sci. 106 (1985) 281. [29] G. Sposito, The Surface Chemistry of Soils, Oxford University Press, New York, 1984. [30] B. Gu, Environ. Sci. Technol. 28 (1994) 38. [31] M. Bjelopavlic, G. Newcombe, E. Hayes, J. Colloid Interface Sci. 201 (1999) 271. [32] C.L. Tiller, C.R. O’Melia, Colloids Surf. 73 (1993) 189. [33] K. Kormohlen, H. Lewandowski, H.-D. Narres, M.J. Schwuger, Colloids Surf. 163 (2000) 45. [34] A.W.P. Vermeer, W.H. Riemsdijk, L.K. Koopal, Langmuir 14 (1998) 2810.
Effect of cationic polymer additives on the adsorption of humic acid onto iron oxide particles Eun Kyoung Kim, Harold W. Walker * Department of Ci6il and En6ironmental Engineering and Geodetic Science, The Ohio State Uni6ersity, 470 Hitchcock Hall, 2070 Neil A6enue, Columbus, OH 43210, USA Received 1 May 2000; accepted 9 May 2001
Abstract Improving the removal of natural organic matter (e.g. humic acid) during drinking water treatment is important in order to minimize the formation of disinfection by-products (DBPs). Although polymeric flocculants are often used to improve turbidity removal during the coagulation process, little information is available regarding the influence of these polymers on NOM removal. In this study, the adsorption of humic acid onto polymer-coated iron oxide particles was investigated as a way to improve humic acid removal during the coagulation process. Monodisperse iron oxide particles and well-characterized humic acid were used as a model system. The adsorption of humic acid onto bare iron oxide particles decreased as solution pH increased. When iron oxide particles were coated with a cationic polymer, adsorption doubled at high pH ( 9.5) and low salt concentration (0.001 M NaCl). The greater adsorption of humic acid on polymer-coated surfaces at high pH was largely due to changes in the electrostatic interactions between humic acid and the particle surface. Coating the iron oxide particles with cationic polymer resulted in the reversal of the negative particle surface charge, thus providing more favorable conditions for humic acid adsorption. Little change in humic acid adsorption was observed, however, for polymer-coated particles at near neutral pH values ( 6.8) or at high salt concentration. These data suggest that under certain conditions polymers may enhance humic acid adsorption onto iron oxide surfaces, a process which may improve DBP precursor removal during drinking water treatment. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Cationic polymer; Humic acid; Adsorption
1. Introduction Natural organic matter (NOM) reacts with chlorine during drinking water treatment processes to form disinfection by-products (DBPs), * Corresponding author. Tel.: + 1-614-292-8263; fax: +1614-292-3780. E-mail address: [email protected] (H.W. Walker).
such as trihalomethanes (THMs) and haloacetic acids (HAAs) [1]. DBPs are potential carcinogens [2] and are also linked to several health problems associated with the central nervous system [3]. Based on the health effects of DBPs, a maximum contaminant level (MCL) for THMs was set at 0.1 mg l − 1 by United States Environmental Protection Agency (USEPA) in 1979. New limits, as part of Stage I of the Disinfection/Disinfectant
0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 7 9 1 - 9
124
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
By-Product (D/DBP) Rule, will be set in 2001 at 0.080 mg l − 1 for THMs and 0.060 mg l − 1 for HAAs. Stage II of D/DBP Rule will set limits at 0.040 mg l − 1 for THMs and 0.030 mg l − 1 for HAAs in the year 2002 [4]. In order to minimize the formation of DBPs, many drinking water utilities have focused on improving the removal of disinfection by-product precursors, such as humic and fulvic acids. For example, the USEPA has established guidelines for carrying out so-called ‘enhanced coagulation’, in which the metal salt coagulation process is optimized for NOM removal [5]. The implementation of enhanced coagulation generally requires higher metal salt usage, thereby increasing cost and the amount of waste sludge produced [6]. Therefore, new approaches for improving NOM removal during coagulation will reduce the formation of DBPs and significantly improve the economics of water treatment. The removal of NOM by enhanced coagulation occurs largely through adsorption to metal hydroxide floc particles, and depends on source water pH, alkalinity, coagulant type and dosage, and the type and concentration of NOM [7]. A number of studies have shown that the adsorption of NOM onto iron oxide particles increases with decreasing pH and increasing ionic strength. Cronzes et al. [6] and Davis et al. [8] have shown that at low pH organic protonation increases, thus reducing the negative charge density of organic matter, and subsequently, coagulant demand. As a result, pH modification has become one of the approaches used by water treatment plants to improve NOM removal. The characteristics of organic matter also affect treatability, with NOM removal generally increasing with increasing molecular weight and hydrophobicity [9– 12]. The major mechanisms controlling NOM adsorption on metal oxides have been summarized by Gu et al. [13] and include (1) electrostatic interactions [14– 16], (2) ligand exchange–surface complexation [13,17,18], (3) hydrophobic interactions [19], and (4) cation bridging [20]. For example, Gu et al. [13] determined using microcalorimetry, FTIR, and 13C NMR that specific functional groups on NOM, such as carboxyl
and hydroxyl functional groups, are amenable to adsorption onto iron oxide surfaces by a surface complexation–ligand exchange mechanism at low pH conditions. Steeg et al. [21] and Vermeer et al. [22] found that increases in salt result in higher amounts of adsorbed NOM. They proposed that the increase in NOM adsorption was due to the screening of electrostatic interactions between surface-adsorbed NOM molecules. Murphy et al. [19] found that simple inorganic ions (Na+, Ca2 + , Cl−) are readily hydrated [23], and this facilitates the formation of hydrophobic mineral surfaces, which may also enhance NOM adsorption. Many water treatment plants utilize synthetic polymer additives to improve turbidity removal during coagulation. Although a number of studies have examined the influence of polymers on particle removal [24–27], little is known about the interaction of polymeric additives with NOM. In one of the few studies in this area, Lurie et al. [25] examined the influence of polymer on NOM removal in the presence of iron and aluminum salts. They found that NOM removal increased with increasing polymer dosage during metal salt coagulation, possibly due to electrostatic interactions between polymer and NOM in solution. While earlier studies have provided insight into the factors influencing NOM removal during coagulation, little information is available regarding specific interactions between NOM, polymeric additives, and metal hydroxide surfaces during water treatment processes. The purpose of this study was to investigate the effect of cationic polyelectrolytes on NOM removal during metal salt coagulation. In particular, this paper examines how the coating of floc particles with polymeric additives influences the adsorption of NOM to particle surfaces. Experiments were carried out with model iron oxide particles, polymeric flocculants, and well-characterized humic materials at a number of pH values and ionic strengths. The results from this study provide important new insight into the process of sequential adsorption of charged polymers on surfaces. At a more practical level, these data provide new information about the role of polymeric flocculants in controlling NOM removal during water treatment operations.
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
2. Materials and methods
2.1. Materials Humic acid was supplied by Aldrich Chemical Company (Milwaukee, WI). Stock solutions of humic acid were prepared in Milli-Q water (18.2 MV cm), filtered with 0.45 mm glass fiber filters (GF/C, Millipore Corporation, Bedford, MA), and then stored at 4 °C prior to use. Iron oxide particles (hematite) were obtained from Atlantic Equipment Engineers (Bergenfield, NJ) and had a purity of 99.9% as determined by the manufacturer. The particles had a mean diameter of 3079 18.5 nm, which was determined at the Ohio State University using a Photon Correlation Spectrometer (90 Plus particle size analyzer, Brookhaven Instruments Corporation, NY). Particle suspensions were sonicated for 10 min prior to size determination in order to break up any aggregates present in the stock suspensions. To prepare particle solutions for adsorption experiments, a specific amount of iron oxide was mixed in carbonate buffer at a desired pH and ionic strength for 1 h, or until the pH of the solution stabilized. Polydiallyl dimethyl, a high molecular weight, cationic polymer (‘C-3299’) was supplied by Polydyne Inc. (Riceboro, GA).
2.2. Adsorption measurements 2.2.1. Humic acid adsorption onto iron oxide The adsorption of humic acid on iron oxide was determined using a UV-VIS 2401 PC double-beam spectrophotometer (Shimadzu Corporation, MD) at 254 nm. To perform a humic acid adsorption experiment, humic acid solutions were prepared with 0.001 or 0.1 M NaCl in carbonate buffer. A fraction of a well-mixed iron oxide suspension (10 g l − 1) was then pipetted into series of humic acid solutions at varying concentration (5– 40 mg l − 1). The final iron oxide concentration for all adsorption experiments was 50 mg l − 1. The final solution was adjusted to the desired pH with 0.1 M HCl or 0.1 M NaOH. The solutions were mixed for 18 h to equilibrate. Preliminary experiments verified that after 18 h no measurable change occurred
125
in the adsorbed amount. After 18 h, samples were centrifuged and then filtered with 0.22 mm filters (HA, Millipore Corporation, Bedford, MA) to separate the supernatant from particles. Control experiments were carried out to verify no adsorption onto the walls of the reaction vessels or filter materials and that filtration effectively removed all iron oxide particles from the solution. The pH was monitored at both the beginning and end of each adsorption experiment. Little variation in pH was observed during the course of an adsorption experiment. The amount of humic acid adsorbed was determined based on the measured depletion in solution.
2.2.2. Cationic polymer adsorption onto iron oxide The adsorption of polymer on iron oxide particles was determined by a similar approach using a total organic carbon analyzer (TOC 5000A, Shimadzu Corporation, MD). For the polymer adsorption tests, the cationic polymer solution was prepared with Milli-Q water at various concentrations (5–50 mg l − 1) in carbonate buffer. The pH was adjusted to approximately 7 or 9 with 0.1 M HCl or 0.1 M NaOH and the salt concentration fixed at either 0.001 or 0.1 M NaCl. Next, a 10 g l − 1 well-mixed iron oxide solution was pipetted into a series of polymer solutions for a final hematite concentration of 50 mg l − 1 and the mixture was mixed for 24 h. Following equilibration, samples were centrifuged to separate the supernatant and the particles, and the TOC of the supernatant was measured using the TOC analyzer. In these experiments, it was not possible to use filtration to separate the polymer in solution from the adsorbed polymer fraction due to a loss of polymer during the filtration step. However, control experiments were carried out and showed that centrifugation effectively removed all particles from the suspension and that no polymer or humic acid was lost during this process. 2.2.3. Humic acid adsorption on polymer-coated iron oxide To coat iron oxide particles, cationic polymer at 50 mg l − 1 was prepared in carbonate buffer
126
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
solution and the pH adjusted to pH 7 or 9 and the salt concentration fixed at either 0.001 or 0.1 M. Next, a well-mixed iron oxide solution was pipetted into the cationic polymer solution for a final iron oxide concentration of 50 g l − 1, and the mixture was equilibrated for 24 h. After 24 h, samples were centrifuged to separate the supernatant and the particles. Following separation, a series of humic acid solutions at varying concentration (5– 40 mg l − 1) were added to the polymer-coated particles and the particles resuspended by gentle agitation. The pH and salt concentration of humic acid solutions matched that of the supernatant removed during the coating process. The solutions were then mixed for 18 h. After 18 h, samples were centrifuged and then filtered with 0.22 mm filters (HA, Millipore Corporation, Bedford, MA) to separate the supernatant from the particles. The concentration of humic acid remaining in solution was determined using the UV-Vis spectrophotometer and the amount adsorbed calculated based on humic acid depletion in solution. Control experiments indicated that no measurable desorption of polymer occurred following resuspension of coated particles in polymer-free buffer.
2.3. Electrophoretic mobility measurements The electrophoretic mobility of iron oxide particles, both in the presence and absence of cationic polymer, was determined using an electrophoretic light scattering instrument (ZetaPlus, Brookhaven Instruments Corp., NY). All mobility measurements were carried out at 25 °C and a salt concentration of 0.001 M NaCl. The pH of solution was adjusted by adding varying amounts of either 0.1 M NaOH or 0.1 M HCl. For mobility measurements with cationic polymer, iron oxide particles were suspended in a 50 mg l − 1 polymer solution for 24 h to allow for saturation of the surface. Then, the particles were separated from the remaining polymer in solution by centrifugation, resuspended in new buffer, and the mobility measured using the ZetaPlus. Each calculated mobility value represents the average of at least five independent measurements.
3. Results and discussion
3.1. Adsorption of humic acid on bare iron oxide The adsorption of humic acid on iron oxide particles was investigated to determine the affinity of humic acid for the bare iron oxide surface as function of pH (7.4 and 9.6) and salt concentration (0.001 and 0.1 M). These results are shown in Fig. 1. As can be seen, the isotherms are of the ‘high affinity’ type with the maximum amount adsorbed rapidly reaching saturation. This behavior has been seen by a number of other researchers examining the adsorption of humic substances onto iron oxide surfaces [28–30]. The well-defined plateau in adsorption is indicative of monolayer coverage of humic acid on the particle surface, and a decrease in the amount of adsorbing surface remaining as the surface excess of humic acid increases. The possible exception to these observations is the data at pH 9.6 and 0.001 M NaCl (filled circles). Under these conditions, adsorption is low and no distinct plateau region was observed. The adsorbed amounts observed in Fig. 1 are within the range of those seen by other researchers [16], considering differences in the specific surface area of iron oxides and variations in the properties of humic substances in these different studies.
Fig. 1. Adsorption of humic acid on iron oxide particles at two salt concentrations and two pH values. (The experiments were carried out at pH 7.4 and 0.1 M NaCl ( ), pH 9.6 and 0.1 M NaCl ( ), pH 7.4 and 0.001 M NaCl () and pH 9.6 and 0.001 M NaCl ( )).
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
The adsorption of humic acid onto bare iron oxide increased with decreasing pH, at least at low NaCl concentration, consistent with earlier studies [13]. For example, at pH 7.4 and 0.001 M NaCl (), the adsorption maximum of humic acid occurred at approximately 10 mg g − 1 whereas the maximum amount of humic acid adsorption observed at pH 9.6 and 0.001 M NaCl ( ) was less than 8 mg g − 1. The dependence of the adsorbed amount on pH is consistent with a ligand exchange adsorption mechanism [16,18]. As pH decreases from 9.6 to 7.4, the increase in hydroxyl groups on the iron oxide surface provides more sites for ligand exchange reactions, and hence, greater humic acid adsorption. At high salt concentration (0.1 M NaCl), however, no measurable difference in adsorption as a function of pH was observed. For both pH 7.4 and 9.6 at 0.1 M NaCl, the maximum amount adsorbed was 15 mg g − 1. In general, an increase in salt concentration resulted in a significant increase in humic acid adsorption on iron oxide for both pH 7.4 and 9.6. At pH 7.4 and a salt concentration of 0.001 M (open circles), the maximum amount adsorbed was approximately 10 mg g − 1. At the same pH but a salt concentration of 0.1 M (open squares), the maximum amount adsorbed increased to 15 mg g − 1. A similar effect was observed at high pH. At pH 9.6, the maximum amount adsorbed was 8 mg g − 1 at 0.001 M NaCl (filled circles) and increased to 15 mg g − 1 at 0.1 M NaCl (filled squares). The increase in amount adsorbed with increasing ionic strength was likely a result of changes in repulsive electrostatic interactions between humic acid segments. The screening of repulsive electrostatic interactions leads to a more compact humic acid conformation and closer packing of humic acid molecules on the iron oxide surface. This effect has been shown earlier in a number of other studies examining humic acid and polyelectrolyte adsorption [31– 33].
3.2. Polymer adsorption on iron oxide Adsorption of cationic polymer on iron oxide particles at pH values of 6.7 and 9.6 and two salt concentrations (0.1 and 0.001 M NaCl) is shown
127
Fig. 2. Effect of salt concentration and pH on the adsorption of cationic polymer C-3299 on iron oxide. (The experiments were carried out at pH 6.7 and 0.1 M NaCl ( ), pH 9.6 and 0.1 M NaCl ( ), pH 6.7 and 0.001 M NaCl () and pH 9.5 and 0.001 M NaCl ( )).
in Fig. 2. For the cationic polymer used in these studies, the amount adsorbed increased with increasing pH for both salt concentrations. At low ionic strength, the amount adsorbed was roughly 0.5 mg g − 1 at pH 6.7 (open circles) and increased to over 1.0 mg g − 1 at pH 9.5 (filled circles). At high ionic strength, the amount adsorbed was 1.2 mg g − 1 at pH 6.7 (open squares) and increased to around 2 mg g − 1 at pH 9.6 (filled squares). The amount of cationic polymer adsorbed at both pH values and ionic strengths was lower than that observed for humic acid. The highest amount of polymer adsorbed was approximately 2 mg g − 1, which is significantly less than the range of humic acid adsorbed amounts (8–15 mg g − 1). Electrostatic interactions likely play an important role in controlling adsorption of this cationic polymer on the iron oxide surface. To gain better insight into the role of electrostatic interactions in controlling polymer adsorption, the electrophoretic mobility of the particles was determined as a function of pH. These data are shown in Fig. 3 for a salt concentration of 0.001 M NaCl. For the bare iron oxide particles, the isoelectric point (IEP) was between pH values 8.3 and 9.2. This is consistent with earlier studies, which have found the point of zero charge (PZC) of hematite to be approximately 8.5 [23,34]. At low pH values (pHB7), the bare particles were positively charged with an electrophoretic mobil-
128
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
ity of around 4 mm s − 1 V − 1 cm − 1. At pH 7, the mobility of the bare particles decreased sharply and became negative at pH values of 9 or higher. At pH values above 9, the electrophoretic mobility approached −4 mm s − 1 V − 1 cm − 1. The presence of cationic polymer resulted in a positive electrophoretic mobility at all measured pH values. At low pH (pHB 7), the mobility of the polymer-coated particles was approximately 5 mm s − 1 V − 1 cm − 1, slightly higher than the mobility of the bare iron oxide particles. In this range of pH values, both the polymer and iron oxide particles were positively charged, and therefore, the electrostatic interactions between polymer and particle were repulsive and adsorption was low. The fact that measurable amounts of adsorption took place at these low pH values, and that the mobility increased slightly upon adsorption of polymer, suggests that there may also have been a non-electrostatic component to the adsorption process. At high pH values (pH\9), the surface charge of the bare iron oxide particles was negative, which resulted in attractive electrostatic polymer –particle interactions and higher adsorption. The mobility of the polymer-coated particles remained positive at high pH values but decreased slightly with increasing pH. At pH 10, the polymer-coated particles had a mobility of 2 mm s − 1 V − 1 cm − 1, while the bare hematite particles had a mobility of − 3 mm s − 1 V − 1 cm − 1.
Fig. 3. Electrophoretic mobility of hematite particles as a function of pH in the presence () and absence ( ) of cationic polymer. All data were collected at a salt concentration of 0.001 M NaCl.
Fig. 4. Adsorption of humic acid on iron oxide at pH 9.6 and 0.001 M NaCl in the absence () and presence ( ) of a cationic polymer C-3299 and at pH 9.6 and 0.1 M NaCl in the absence ( ) of a cationic polymer C-3299.
3.3. Humic acid adsorption on polymer-coated iron oxide The adsorption of humic acid onto iron oxide particles in the presence and absence of adsorbed cationic polymer at pH 9.6 and 0.001 M NaCl is presented in Fig. 4. As can be seen, the adsorption of humic acid on polymer-coated iron oxide particles at pH 9.6 (filled circles) was significantly greater than adsorption on the bare particles (open circles). The maximum adsorption density nearly doubled, increasing from roughly 8 mg g − 1 on bare iron oxide to 15 mg g − 1 on polymercoated iron oxide. The amount of humic acid adsorbed on the polymer-coated iron oxide surface at low salt concentration approached the levels observed at 0.1 M NaCl (filled squares), at least at high equilibrium concentrations. The coating of the iron oxide surface with cationic polymer also influenced the shape of the humic acid adsorption isotherm. In the presence of cationic polymer, the initial slope of the isotherm was lower compared with humic acid isotherms observed at high salt. A number of mechanisms may play a role in controlling the adsorption of humic acid onto polymer-coated iron oxide. Coating the iron oxide particles with a cationic polymer will influence both specific chemical and electrostatic interactions between humic acid and the iron oxide
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
surface. At high pH, adsorption of a cationic polymer resulted in the reversal of the negative electrophoretic mobility of the particles. The subsequent decrease in repulsive electrostatic forces between humic acid and iron oxide particles facilitated the attachment of humic acid to positively charged sites on the polymer or to available functional groups on the iron oxide surface. It should be noted that interactions between cationic polymer and humic acid in solution could also play a role in controlling adsorption. However, in these experiments, unadsorbed polymer was removed from solution prior to exposure to humic acid, and therefore, polymer– humic interactions in solution were not important. It is interesting to note in Fig. 4 that the high affinity character of humic acid adsorption at high salt (filled squares) (i.e. steep increase in adsorbed amount at low equilibrium concentration) was not observed on polymer-coated particles (filled circles). The shape of an adsorption isotherm is influenced by the molecular weight of the polymer, the monodispersity of the polymer suspension, as well as the affinity of individual polymer segments for the surface. Typically, high affinity isotherms are obtained for monodisperse suspensions of high molecular weight polymers with high segmental adsorption energies. For the data in Fig. 4, the decrease in affinity at low equilibrium concentrations may be a result of changes in the affinity of specific humic acid moieties for the iron oxide surface in the presence of polymer. The steady increase in adsorbed amount may also reflect some re-configuration of humic acid on the iron oxide surface. Perhaps at low equilibrium concentrations humic acid molecules adsorb via attachment to positively charged polymer segments on the particle surface. As the surface coverage of humic acid increases, humic acid molecules adsorb to less favorable sites on the bare iron oxide surface. Desorption experiments were carried out and showed that no measurable amount of polymer was released to solution during the course of an experiment. Therefore, the low affinity character of the isotherm cannot be explained by desorption of polymer into solution.
129
The results of humic acid adsorption at pH 7.4 and low salt concentration (0.001 M NaCl) in the presence (filled circles) and absence (open circles) of adsorbed polymer are shown in Fig. 5. For these conditions, coating the particles with polymer had little effect on humic acid adsorption. In the absence of polymer, the maximum adsorption density observed was 12 mg g − 1. Upon coating the particles with polymer, the maximum amount of humic acid adsorbed slightly decreased to approximately 10 mg g − 1. At this pH and salt concentration polymer adsorption was low (B1 mg g − 1), and therefore, polymer molecules had little effect on humic acid adsorption. The slight decrease in adsorption upon coating the particles may have been a result of competition between humic acid and polymer for surface sites at these solution conditions. Fig. 6 shows humic acid adsorption isotherms at pH 9.6 and high salt concentration (0.1 M NaCl) in the presence (filled circles) and absence (open circles) of adsorbed polymer. At high salt concentration, the polymer had no measurable effect on humic acid adsorption. In both the presence and absence of polymer, the maximum amount adsorbed was approximately 17 mg g − 1. In this case, adsorption appears to be controlled largely by non-electrostatic interactions between humic acid and the hydrous oxide surface. It is reasonably given that even at high ionic strength polymer adsorption was an order of magnitude less than humic acid adsorption.
Fig. 5. Adsorption of humic acid on iron oxide at pH 7.4 and 0.001 M NaCl in the absence () and presence ( ) of cationic polymer C-3299.
130
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
Fig. 6. Adsorption of humic acid on iron oxide at pH 9.6 and 0.1 M NaCl in the absence () and presence ( ) of cationic polymer C-3299.
A schematic interpretation of the effect of cationic polymer on the adsorption of humic acid to iron oxide particles is shown in Fig. 7 for low
Fig. 7. Schematic interpretation of the effect of cationic polymer on the adsorption of humic acid on iron oxide particles at two pH values
salt conditions. At low pH, in the absence of polymer, the particles had a net positive charge. Under these conditions, both non-electrostatic and electrostatic interactions are favorable for adsorption and the surface concentration of humic acid on the bare particles was high. Coating the particles with polymer at low pH resulted in slightly greater positive surface charge density, but perhaps influenced specific chemical interactions between the humic acid and the hematite surface. As a result, humic acid adsorption decreased slightly at this pH value in the presence of polymer. At high pH, the bare particles had a net negative charge, and therefore, humic acid adsorption was low. Coating the particles with cationic polymer under these conditions reversed the charge density of the iron oxide and resulted in favorable electrostatic interactions and significantly higher humic acid adsorption.
3.4. Significance of cationic polymer additi6es in water treatment plants As mentioned in Section 1, many water treatment plants utilize polymeric additives to enhance turbidity removal during coagulation. Upon addition of polymer, both the electrostatic and surface chemical characteristics of floc surfaces will be altered. It is expected that the net charge of floc surfaces will be shifted to higher, positively charged values in the presence of cationic polymer. This suggests that the typical reduction of pH carried out for optimum removal of natural organic matter by coagulation using metal salts may not be required when cationic polymers are added. It should be noted, however, that humic molecules in solution could possibly interact with cationic polymer additives in solution, and therefore, exert a ‘polymer demand’. Polymer conditioning of metal hydroxide sludge with subsequent recycle may provide an effective approach for enhancing natural organic matter removal during metal salt coagulation while minimizing polymer demand. Further work is required to elucidate the relative importance of surface and solution interactions in controlling the adsorption and removal of humic substances during polymer-enhanced coagulation processes.
E.K. Kim, H.W. Walker / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 123–131
4. Conclusions Coating iron oxide flocs with cationic polymer significantly increased adsorption of humic acid at high pH, but had less effect at low pH. The adsorption of cationic polymer converted negatively charged iron oxide to positively charged particles, thereby inducing electrostatic attraction between the humic acid molecules and the coated particle surfaces. These results suggest that cationic polymers may facilitate adsorption of humic acid on floc surfaces at high pH. Little enhancement in adsorption was observed at low pH or high ionic strength. Under these conditions high adsorption of humic acid was observed in both the presence and absence of cationic polymer. Polymer conditioning of floc surfaces may be an attractive alternative for water plants treating source water with high alkalinity in which reduction of pH to optimum values for NOM removal requires significant acid or coagulant addition.
Acknowledgements Funding from the Ohio Water Development Authority (OWDA) is gratefully acknowledged.
References [1] G.L. Amy, M.R. Collins, C.J. Kuo, P.H. King, J. Am. Water Works Assoc. 79 (1987) 43. [2] D.A. Okun, J. Environ. Eng. 122 (1996) 453. [3] K. Walker, S.H. Swan, G. DeLorenze, B. Hopkins, Epidemiology 9 (1998) 134. [4] USEPA. National primary drinking water regulation; Disinfectants and disinfection by-products; Proposed Rule. Fed. Reg. 5-: 145:38668, 19 July 1994. [5] Malcolm Pirnie, Inc. Guidance Manual for Enhanced Coagulation and Enhanced Precipitative Softening, United States Environmental Protection Agency, Washington D.C., September, 1993. [6] G. Crozes, P. White, M. Marshall, J. Am. Water Works Assoc. 87 (1995) 78.
131
[7] M.C. White, J.D. Thompson, G.W. Harrington, P.C. Singer, J. Am. Water Works Assoc. 89 (1997) 64. [8] J.A. Davis, R. Gloor, Environ. Sci. Technol. 15 (1981) 1223. [9] G.L. Amy, J. Bedessem, R.A. Sierka, J. Am. Water Works Assoc. 84 (1992) 67. [10] D.M Owen, G.L. Amy, Z.K. Chowdhury, R. Paode, G. McCoy, K. Viscosil, J. Am. Water Works Assoc. 97 (1995) 46. [11] M.R. Collins, G.L. Amy, C. Steelink, Environ. Sci. Technol. 20 (1986) 1028. [12] M.J. Semmens, A.B. Staples, J. Am. Water Works Assoc. 78 (1986) 76. [13] B. Gu, J. Schmitt, Z. Chen, L. Liang, J.F. MaCarthy, Geochim. Cosmochim. Acta 59 (1995) 219. [14] A. Amirbahman, T.M. Olson, Environ. Sci. Technol. 27 (1993) 2807. [15] E. Tipping, M.A. Hurly, M.M. Reddy, Environ. Sci. Technol. 24 (1990) 1507. [16] A.W.P. Vermeer, Langmuir 15 (1998) 4210. [17] E. Tipping, Geochim. Cosmochim. Acta 45 (1981) 191. [18] R.L. Pafitt, A.R. Fraser, V.C. Farmer, J. Soil Sci. 28 (1977) 289. [19] E.M. Murphy, Environ. Sci. Technol. 28 (1994) 1291. [20] S. Chen, W.P. Inskeep, S.A. Williams, P.R. Callis, Soil Soc. Am. J. 56 (1992) 67. [21] G.M.H. van de Steeg, M.A. Cohen Stuart, A. de keizer, B.M. Bijsterbosch, Langmuir 8 (1992) 2538. [22] A.W.P. Vermeer, F.A.M. Leermakers, L.K. Koopal, Langmuir 13 (1997) 4413. [23] W. Stumm, Chemistry of the Solid – Water Interference, Wiley, New York, 1992. [24] M. Edwards, M. Benjamin, J.E. Tobiason, J. Am. Water Works Assoc. 86 (1994) 105. [25] M. Lurie, M. Rebhun, Water Sci. Technol. 36 (1997) 96. [26] X. Yu, P. Somasundaran, J. Colloid Interface Sci. 177 (1996) 283. [27] T. Matsumoto, Y. Adachi, J. Colloid Interface Sci. 204 (1998) 328. [28] C.H. Ho, N.H. Miller, J. Colloid Interface Sci. 106 (1985) 281. [29] G. Sposito, The Surface Chemistry of Soils, Oxford University Press, New York, 1984. [30] B. Gu, Environ. Sci. Technol. 28 (1994) 38. [31] M. Bjelopavlic, G. Newcombe, E. Hayes, J. Colloid Interface Sci. 201 (1999) 271. [32] C.L. Tiller, C.R. O’Melia, Colloids Surf. 73 (1993) 189. [33] K. Kormohlen, H. Lewandowski, H.-D. Narres, M.J. Schwuger, Colloids Surf. 163 (2000) 45. [34] A.W.P. Vermeer, W.H. Riemsdijk, L.K. Koopal, Langmuir 14 (1998) 2810.