Adsorption of Natural Organic Polyelectrolytes by Activated Carbon: A

Environ. Sci. Technol. 1996, 30, 1336-1343

Adsorption of Natural Organic Polyelectrolytes by Activated Carbon: A Size-Exclusion Chromatography Study JAMES E. KILDUFF,† TANJU KARANFIL,† YU-PING CHIN,‡ AND W A L T E R J . W E B E R , J R . * ,† Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125, and Department of Geological Sciences, The Ohio State University, Columbus, Ohio 43210-1002

The adsorption of several different organic polyelectrolytes from aqueous solution by activated carbon was characterized. Polyelectrolytes included humic acids extracted from peat and soil, polymaleic acid, a synthetic polymer identified as a fulvic acid surrogate, and natural organic matter in Huron River (Ann Arbor, MI) water. Isotherms of individual ultrafiltration size fractions confirmed that smaller molecular size components adsorb to a greater extent on an adsorbent mass basis. The molecular weight distributions of organic polyelectrolytes remaining in solution after equilibration with various amounts of activated carbon were measured with highperformance size-exclusion chromatography (HPSEC). A comparison of molecular weight distributions demonstrated conclusively that small molecular size components are adsorbed preferentially; i.e., adsorptive fractionation on the basis of molecular size occurs. This behavior was observed for each of the wide variety of samples studied, suggesting that it may be a rather general feature of the adsorption of polyelectrolyte mixtures from solution by activated carbon.

Introduction Humic and fulvic acids are natural organic polyelectrolytes that comprise the greatest proportion of naturally-occurring dissolved organic matter in aqueous systems (1-3). Humic materials may be specifically targeted for removal from potable water supplies because they can adversely affect appearance and taste, and they can react with chlorine to form potentially carcinogenic chlorinated organic compounds. Further, the presence of macromolecular dissolved organic matter may reduce the effectiveness of water treatment processes that employ membranes or micro* Corresponding author telephone: (313)763-1464; fax: (313)7632275; e-mail address: [email protected]. † The University of Michigan. ‡ The Ohio State University.

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porous adsorbents (4-6). Even when not specifically targeted for removal, macromolecular dissolved organic matter has been shown to compete with low molecular weight synthetic organic chemicals (SOCs), reducing their adsorption rates and equilibrium capacities (4-9). Therefore, understanding the adsorption of humic substances is central to optimizing their removal from solution by activated carbon and to minimizing their impacts on the adsorption of other compounds specifically targeted for removal. Natural dissolved organic matter and humic material extracted from aqueous and terrestrial sources are polydisperse mixtures of components having different sizes and adsorption characteristics. The behavior and reactivity of natural and synthetic organic polyelectrolytes may depend, in part, on their molecular weight (MW) or size in solution. The impact of molecular weight on polyelectrolyte adsorption has been shown to depend on (i) the importance of sorbate/solute interactions (10, 11); (ii) the rate of adsorption (11-14); (iii) the polydispersity of the mixture (11, 12, 15, 16); and (iv) the ability of the adsorbate to access adsorbent surface area (17-20). This latter effect has been demonstrated in studies of polyelectrolyte adsorption by a variety of porous adsorbents including activated carbon (18-21), crystalline CaCO3 (22), and ion exchange resins (23), suggesting that the ability of an adsorbate to access adsorbent surfaces has a significant impact on the extent of adsorption. Studies using monodisperse solutes or fractions of whole solutions provide insight into the effects of molecular size, but they do not provide information about how different size components may compete when adsorbed from a mixture. Several studies have revealed that isotherms for the adsorption of humic substances on activated carbon exhibit characteristics of multicomponent competitive adsorption from mixtures. Few studies, however, have been directed toward identifying or characterizing the nature of this competition. El-Rehaili and Weber (24), using Sephadex gel-permeation chromatography calibrated with proteins, observed that the removal of total organic carbon (TOC) in smaller size fractions increased with increasing activated carbon concentration or dose, Do. However, the molecular size distributions measured were useful only for qualitative comparisons. Further, the system required sample concentration by a factor of 60, which resulted in a loss of TOC ranging from 5 to 52%. Other limitations of Sephadex as a chromatography packing material are discussed by Hine and Bursill (25). Summers and Roberts (20) used ultrafiltration fractionation to conduct a similar study using the same commercial humic material (Aldrich). They observed a shift in the molecular size distribution to larger sizes with increasing adsorbent dose, but did not observe increased removal of the smallest size fraction. Limitations of using ultrafiltration for this type of study are (i) the resolution of the technique is limited by the number of membranes available and the range of pore sizes within a particular membrane; and (ii) the sample size required is relatively large. A limitation to both of the above studies is the use of commercial humic acids, which may not be appropriate as analogues of true soil or aqueous humic substances (26).

0013-936X/96/0930-1336$12.00/0

 1996 American Chemical Society

Size-exclusion chromatography is a technique particularly well suited for quantifying different size components in a mixture. Gloor et al. (27) showed that size-exclusion chromatography could be used to measure changes in molecular size distributions of lake dissolved organic matter remaining in solution after adsorption on colloidal alumina, γ-Al2O3. Bain et al. (12) used gel-permeation chromatography to measure changes in size distributions of sodium polyacrylate and sodium carboxy methyl cellulose remaining in solution after adsorption on BaSO4 crystals. Ramachandran and Somasundaran (11) used size-exclusion chromatography to study the adsorption of PSS on hematite. They showed that size-exclusion chromatography can be used to monitor the effect of molecular weight polydispersity on the adsorption of polyelectrolytes. Recent advances in HPSEC allow samples of natural humic materials and dissolved organic matter to be analyzed rapidly with little or no pretreatment, with high resolution, and at environmentally relevant concentrations (28).

Objectives The focus of the research is to better understand the behavior of humic substances in activated carbon adsorption systems. Our objectives here were to (i) investigate the impacts of molecular size and molecular size distribution on the adsorption of natural and synthetic polyelectrolytes by activated carbon; (ii) demonstrate the efficacy of high-performance size-exclusion chromatography (HPSEC) as a means for studying the adsorption of humic materials on activated carbon; and (iii) further characterize the nature of competitive interactions among components comprising natural humic substances.

Materials and Methods Municipal water that was de-ionized, distilled, and processed through a Milli-Q system (MQ-water; Millipore Inc., Bedford, MA) was used in all experiments. Macromolecules. Polystyrene sulfonate (PSS), used to calibrate the HPSEC column, was obtained in narrow molecular weight fractions from Polysciences, Inc. Weight fractions used for adsorption and ultrafiltration studies included 1.8K, 5.4K, 8K, and 18K. (Molecular weights will be referred to using K to symbolize 1000 Da). The chemicals were used as received and stored in a desiccator. Stock solutions were made up in 1 × 10-3 M phosphate buffer. The ionic strength of PSS solutions was adjusted with NaCl, and the pH was adjusted to between 6.9 and 7.0 using concentrated HCl or NaOH as required. Conductivity measurements confirmed that the PSS did not measurably contribute to the solution ionic strength at the PSS concentrations used in this study. A humic acid extracted from Laurentian soil (LaHA) was obtained from Fredrik’s Research Products, Amsterdam, The Netherlands; and a humic acid extracted from peat (PHA) was obtained from the International Humic Substance Society (IHSS). Unless otherwise noted, humic solutions were buffered with phosphate, and NaCl was used to adjust ionic strength. Polymaleic acid (PMA) was synthesized in our laboratory by the method of Spiteller and Schnitzer (29) and is the same material studied by Carter et al. (6). River water (HRW) was obtained from the Huron River in Ann Arbor, MI. HRW was filtered through a 1-µm glass fiber filter and microbially stabilized by the addition of 100 mg/L sodium azide prior to refrigerated storage. Before use in adsorption experiments, all macromolecule

TABLE 1

Physical Characteristics of F400 Carbon % of total surface area in particle surface av pore stated pore size (Å) range diameter, area, radius, pore vol, Å cm3/g 100 µm m2/g 165

948

12.00

0.566

86.0

12.8

0.79

0.41

solutions were sequentially filtered through a 10-µm polyethylene filter, a 1-µm glass fiber filter, and a 0.45-µm polysulfone (Gelman Sciences) filter in a 122 mm diameter stainless steel filter holder. Stock solutions were stored refrigerated in the dark. The solution pH was checked prior to adsorption experiments and adjusted to 7.0 ( 0.1 with HCl or NaOH as necessary. Activated Carbon. Calgon F400 activated carbon, a bituminous coal-based adsorbent, was chosen for the experimental work because it is widely used in water treatment applications and because it has been studied extensively by researchers in our laboratory and elsewhere. The physical properties of F400 carbon are reported in Table 1. Activated carbon obtained from the manufacturer was crushed and mechanically sieved to yield uniform particle sizes having a mean diameter of 165 µm. Carbon was washed with Milli-Q water, sonicated for 30 s in Milli-Q water to reduce the amount of fines produced during the adsorption experiment (30), oven-dried at 105 °C to constant weight, and stored in a desiccator until use. Macromolecule Adsorption Isotherms and Rate Studies. Isotherms and rate studies were conducted using the bottle-point method in serum bottles sealed with Teflonlined rubber septa and aluminum crimp seals. Rate studies indicated that statistically significant changes in solutionphase concentration did not occur after an equilibration period of 30 days. Based on the results of control vials, there was no measurable loss of macromolecular substances from the reactors, and all changes in concentration in the aqueous phase of reactors containing carbon was attributed to adsorption. After the equilibration period, samples from each reactor were filtered through a prewashed 0.45-µm polysulfone filter (Supor, Gelman Sciences) and analyzed by total organic carbon (TOC) analysis (Shimadzu TOC-500) and UV spectrophotometry (Varian Optical). The TOC measurement error as estimated by propagating the error through each step of the determination was 0.10 mg/L. The extent of adsorption (qe) was measured with a precision of 3% or better. Ultrafiltration Fractionation. Ultrafiltration fractionation of LaHA was carried out according to the protocols described by Kilduff and Weber (31). We used a semibatch ultrafiltration system (Amicon CH2PRS) consisting of a reservoir, a peristaltic pump, and hydrophilic, neutral, cellulosic-type spiral wound membrane cartridges having nominal molecular weight cutoffs of 3K, 10K, 30K, and 100K. In this research, the pressure drop (20 psig) and recirculation rate (0.833 L/min) were maintained constant for all experiments. Size-Exclusion Chromatography. Size-exclusion chromatography was carried out as described by Chin et al. (28). Ideally, HPSEC systems should be calibrated with compounds having identical structure as the samples being analyzed. When the samples are polyelectrolytes, their

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structure and potential interactions with the stationary phase may depend, in part, on solution pH and ionic strength. Indeed, a number of researchers have noted that molecular weight distributions determined using sizeexclusion chromatography may depend on eluent composition (32, 33). The use of globular proteins as molecular weight standards may result in significant overestimates of the molecular weights of humic substances and other polymers (31, 34-36). To overcome this limitation, several investigators have calibrated their systems with polymers, including polysaccharides (37) and polystyrene sulfonates (28, 31, 35). Chin and Gschwend (34) found that the coiled configuration of polystyrene sulfonate standards and Suwannee fulvic acid (an IHSS standard) were nearly identical when a mobile-phase ionic strength equivalent to 0.1 M NaCl and pH of 6.8 was used. Saito and Hayano (38) used the same mobile-phase composition and found that molecular weight distributions of humic and fulvic acids were independent of flow rate and mass injected. In this research, therefore, we used PSS standards and a mobilephase ionic strength of 0.1 M buffered to pH 6.8. The salt composition of all samples was adjusted to yield an ionic strength of 0.1 M prior to chromatographic analysis, to eliminate artifacts arising from dynamic coiling phenomena during sample transport through the chromatographic system (34).

Results and Discussion Size-Exclusion Chromatography System Calibration and Data Analysis. The HPSEC system was calibrated with four different monodisperse PSS fractions, and the column void volume was determined with acetone. Standard curves were prepared daily, and an excellent linear correlation (R2 > 0.998) was found between the log10 of the molecular weight and the elution time, t. Chromatography data was processed by first establishing a baseline. A horizontal line was constructed through the early time data prior to sample elution, and the point where the chromatogram deviated from this line was taken as the beginning of sample elution. The baseline was drawn from the point just prior to sample elution to the chromatogram at 13.33 min, which represented the lower limit of the standard curve. The baseline was subtracted from the detector response to yield a corrected chromatogram. In all cases, baseline drift, and therefore the baseline correction applied, was small. The detector response at 13.33 min was less than 2% in all chromatograms. Where necessary, chromatograms were corrected for the presence of sodium azide by subtracting the azide spectrum. The first moments of the chromatograms were determined by numerically integrating the detector response as a function of elution time from the first point of sample elution to 13.33 min. Approximately 500 data points were integrated in each chromatogram, corresponding to a time interval of 0.0167 min. The weight-averaged molecular weight, MW, was determined by N

∑ MW (t)M (t) i

MW )

i)1

MT

i

(1)

where MWi(t) is the molecular weight as a function of elution time (determined from the calibration curve); Mi(t) is the

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sample mass as a function of elution time; and MT is the total mass of the sample. The number-averaged molecular weight, MN, was determined by N

∑ M (t) i

i)1

MN )

(2)

N

∑ M (t)/MW (t) i

i

i)1

The mass of the sample is given by

Mi(t) ) Q[hi(t)](∆t)[1/a(t)]

(3)

where Q is the flow rate, hi(t) is the UV detector response, ∆t is the time interval, and a(t) is the sample absorptivity as a function of elution time. MT was determined by integrating Mi(t) over the entire chromatogram. For a constant flow rate, eqs 1 and 2 are written N

∑MW (t)[h (t)](∆t)[1/a(t)] i

MW )

i

i)1

(4)

N

∑ h (t)(∆t)[1/a(t)] i

i)1

And the number-averaged molecular weight was determined by N

∑ h (t)(∆t)[1/a(t)] i

MN )

i)1

(5)

N

∑ h (t)(∆t)[1/a(t)]1/MW (t) i

i

i)1

Sample polydispersity was determined from the ratio of the weight to number-averaged molecular weights. If the time interval is constant, and the sample absorptivity does not vary with molecular weight, eqs 4 and 5 reduce to the expressions given by Yau et al. (39) and used by Chin et al. (28). To apply eqs 4 and 5, an estimate of 1/a(t) was needed. The function 1/a(t) can be calculated from the standard curve if 1/a as a function of molecular weight is known: 1/a(t) ) 1/a(MW)MW(t). Our approach was to first determine 1/a (TOC/UV, mg cm-1 L-1) as a function of molecular size by measuring the slope of correlations between TOC and UV absorbance for several ultrafiltration size fractions. TOC/UV correlations, shown in Figure 1, were linear over the range of concentrations investigated, with R2 values >0.99. This finding is consistent with the results of other research (19, 20, 40). Because the molecular weight of different size fractions were not known a priori, the determination of 1/a(MW) and hence 1/a(t) was an iterative process: molecular weight information was needed to calculate 1/a(MW), but 1/a(MW) was needed to calculate molecular weights. The iterative procedure was started by estimating the molecular weight of the size fractions assuming that 1/a(t) was constant. Then, 1/a was regressed as a function of log10(MW) using an empirical second-order polynomial function. This regression equation and the standard curve were used in eqs 4 and 5 to determine new estimates of the molecular weight of the size fractions. This process was repeated until the molecular weight estimates converged,

TABLE 2

Effect of 1/a(t) on Computed Mw

FIGURE 1. TOC/UV correlations for Laurentian HA size fractions prepared by ultrafiltration. The lines represent linear regression fits to the experimental data.

FIGURE 2. Effect of molecular weight on the TOC/UV correlation (1/a) of Laurentian HA size fractions prepared by ultrafiltration. The solid line represents an empirical second-order polynomial regression fit to the experimental data. 95% confidence limits on the linear TOC/UV correlation slope are smaller than the symbols used to represent the data.

which took only four iterations. The final relationship between 1/a and log10(MW) is shown in Figure 2. The trend of decreasing absorptivity (increasing 1/a) with increasing molecular weight observed in this study is consistent with the findings of several researchers (41-45). However, this cannot be considered a general trend because other research has found either no trend or an increase in absorptivity with an increase in molecular weight (28, 46). The molecular weight determination error was estimated by propagating the error of each step through eqs 4 and 5. This procedure requires estimates of the standard errors of each variable: (i) the standard error in log10(MWi) was estimated as the standard error of the standard curve prediction; (ii) the standard error of the absorbance response was assumed to be 0.0005 absorbance unit; and (iii) the standard error for the TOC/UV was taken as 0.33 mg cm-1 L-1, the largest value exhibited by any fraction. In all cases, the standard error of MW was less than 10% of the calculated parameter value. The error associated with assuming a constant sample absorptivity for the LaHA can be estimated by comparing the molecular weight estimates determined assuming a constant 1/a(t) and those made using an empirically determined function. The results of this analysis are tabulated in Table 2. The assumption of a constant 1/a(t)

nominal UF size fraction

constant 1/a(t)

empirical 1/a(t)

% error

100K

1320 2522 3398 5820 19563

1383 2696 3747 6792 22033

4.56 6.45 9.31 14.3 11.2

results in an underestimation of the molecular weight, and the relative error generally increases with molecular weight. Errors will be lowest for less polydisperse samples having lower molecular weights. All data reported for the LaHA is corrected for differences in absorptivity among size fractions; however, no attempt was made to make a similar correction for other humic substances studied. It can be seen from the data in Table 2 that the HPSECdetermined molecular weights of humic size fractions prepared by ultrafiltration are significantly smaller than the nominal membrane molecular weight cutoffs. This is because the manufacturer determined nominal cutoff values using globular proteins, which have different structures than humic and fulvic acids. This finding is consistent with previous results reported in the literature (31, 34-36). Adsorption from Mixtures. As demonstrated by Weber et al. (40), humic acids are mixtures of components having different adsorption properties. Components having different adsorption affinities for the surface or different abilities to access adsorbent surface area may compete for adsorption sites. Typically, components of humic substances cannot be uniquely identified or quantified; therefore, a lumped concentration parameter such as total organic carbon (TOC) must be used. A salient feature of adsorption from mixtures quantified by a lumped parameter is that the isotherm depends on the experimental conditions employed. The components of the mixture removed from solution depend on the relative magnitude of the initial adsorbate (TOC) concentration and the adsorbent dose, Do. When the adsorbent dose is low relative to the initial adsorbate concentration, adsorption sites are limited, and only the most adsorbable components of the mixture are removed from solution. When the adsorbent dose is high, more adsorption sites are available, and a greater proportion of less adsorbable components are removed from solution. As a result, the extent of adsorption (for a given equilibrium TOC concentration) increases with decreasing adsorbent dose when isotherms are measured using a constant adsorbent dosage and variable initial adsorbate concentration. When isotherms are measured using a constant initial adsorbate concentration and a variable adsorbent dose, the extent of adsorption increases with decreasing initial TOC concentrationsadsorption capacity increases upon dilution. Variable-dose isotherms were measured at two different initial TOC concentrations to demonstrate that the Laurentian humic acid exhibited multicomponent competitive adsorption. The isotherms, shown by the open symbols in Figure 3, illustrate that the isotherm relationship is not only a unique function of the equilibrium concentration (Ce) but also depends on the initial TOC concentration. Furthermore, the extent of adsorption increases upon dilution; therefore, at any given value of Ce, lower initial TOC concentrations result in higher qe values.


9

1339

FIGURE 3. Adsorption of Laurentian HA on F400 carbon at two initial concentrations using the variable-dose methodology (open symbols). Isotherms were normalized by the modified Freundlich isotherm model (solid line) expressed in terms of nonadsorbed DOM per adsorbent mass, Ce/Do (closed symbols).

Preferential Adsorption. Preferential adsorption is a special case of adsorption from mixtures. When preferential adsorption occurs and the adsorbent dose is low, the most adsorbable component in a mixture is removed from solution exclusively. The next most adsorbable component is removed only when the adsorbent mass is increased sufficiently to completely remove the most adsorbable component. This progression continues until the adsorbent mass is increased sufficiently to remove all but the least adsorbable component. Therefore, for a given distribution of component adsorbabilities, the components removed from solution and the extent of adsorption depend only on the fractional reduction in the initial solution concentration (15, 16, 20). Thus, the composition of the mixture at equilibrium, not the solution concentration, determines the extent of adsorption. A unique isotherm may be obtained when the amount adsorbed is expressed in terms of a parameter that is correlated with the mixture composition. One such parameter is the amount of nonadsorbed solute per unit mass of adsorbent, computed by normalizing the equilibrium concentration by the adsorbent dose (15, 20). As shown by Koopal (15) normalization of isotherms in this way takes account of the polydispersity of the mixture. Therefore, unlike non-normalized isotherms, parameters obtained from normalized isotherms may be compared directly regardless of the experimental conditions employed to measure isotherm data. It was found that this technique of normalizing isotherms could be applied to commercial and natural humic materials (20) and to natural humics after coagulation and ozonation (47). The normalized isotherm data was described by a modified Freundlich isotherm model:

qe ) KF(Ce/Do)n

(6)

where qe is the amount adsorbed per unit mass of adsorbent (mg of TOC/g); Ce is the equilibrium liquid-phase concentration (mg/L); Do is the adsorbent dose (mg/L); KF is an empirical constant that represents the adsorption capacity at a value of Ce/Do equal to unity, and n is an empirical constant. Using this technique, we were able to normalize the variable-dose isotherms of Laurentian humic acid shown by the open symbols in Figure 3. The results, shown by the solid symbols in Figure 3, imply that (i)

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FIGURE 4. Effects of DOM macromolecular size on adsorption of Laurentian HA size fractions prepared by ultrafiltration. The lines represent regression fits of the modified Freundlich isotherm model to the experimental data. TABLE 3

Adsorption of Laurentian HA Size Fractions Prepared by Ultrafiltration fraction soil humic acid > polymaleic acid > river water organic matter. The trends observed in this research are consistent with recent measurements reported in the literature (35, 36). The trends of polydispersity observed are consistent with those of Beckett et al. (35), who found that fulvic acids are generally less disperse than humic substances regardless of source and that surface water organic matter is generally less disperse than organic matter extracted from soils, peat and coal. The magnitude of changes in molecular weight distributions resulting from preferential adsorption should depend on the initial polydispersity of the sample. Less polydisperse distributions would exhibit a smaller change in average molecular weight as a given fraction of the initial solute mass is adsorbed from solution; as the polydispersity approaches unity, the change would approach zero. Our

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data are consistent with this expectation. As shown in Figure 9, organic matter having higher polydispersity, such as the LaHA and the PHA, exhibit statistically significant changes in average molecular size after contact with activated carbon (p values equal to 0.001 and 0.006, respectively). The more narrowly distributed PMA shows an increase in molecular weight after contact with activated carbon, but of a lesser magnitude (p ) 0.074). HRW organic matter does not exhibit a statistically significant trend. If complete preference is exhibited for the low molecular size fractions, then preferential adsorption will increase both the weight- and number-averaged molecular weights and will decrease the polydispersity of the molecular size distribution. Such effects should also depend on the initial polydispersity of the sample. As shown in Figure 10, a statistically significant trend (p ) 0.047) was exhibited for the PHA, whereas the polydispersity of PMA and HRW is essentially constant. The Laurentian soil humic acid exhibits unique behavior, with polydispersity increasing with preferential adsorption. This trend was observed regardless of whether corrections were made for changes in absorptivity as a function of molecular weight. For MW, MN, and polydispersity to increase simultaneously, the weight-averaged molecular weight must have increased at a faster rate than the number-averaged molecular weight. This suggests that the small molecular size components are composed, in part, of compounds which adsorb to a lesser extent than larger components or which do not adsorb at all. The presence of a nonadsorbing fraction seems unlikely, as there was no evidence for such a fraction in the isotherms measured, in which we examined adsorbent to adsorbate mass ratios of up to 145 mg of carbon/mg of TOC. Furthermore, there is no evidence of a significant low molecular weight nonadsorbing fraction in the HPSEC chromatograms. Therefore, we hypothesize that some of the low molecular weight components in the LaHA are rather weakly adsorbing compared to other components having a similar or larger size. Thus, while preferential adsorption is observed for the LaHA, the preference is not complete. In this context, increases in molecular size distribution polydispersity may provide evidence of heterogeneity in the chemical structure of low molecular size components. The basis for preferential adsorption of smaller molecular sizes is most likely due to a combination of factors. Smaller molecular sizes have higher diffusion coefficients and are capable of reaching the adsorbent surface more quickly than larger molecules (14). Moreover, they can diffuse more easily into the adsorbent pore structure, where they can access a greater adsorbent surface area. Displacement of the small molecular sizes by larger ones may be difficult because of electrostatic repulsion between charged segments of the polyelectrolyte molecule (11) and their exclusion from a portion of the intraparticle adsorbent surface. The adsorption and HPSEC data presented in this study for organic polyelectrolytes from a variety of sources corroborates the results of El-Rehaili and Weber (24) and Summers and Roberts (20) and suggests that the preferential adsorption of low molecular weight components is a general feature of the adsorption of polyelectrolyte mixtures from solution by activated carbon.

Acknowledgments The authors thank Meredith Weiner and Jae Son, workstudy students at the University of Michigan, for their

assistance in collecting laboratory data. This publication is a result of work sponsored by the National Science Foundation (Grant CES-8702786) and by the National Institute of Environmental Health Sciences (Grant 5P42E50491102). Partial financial support for J.E.K., provided through the American Society of Civil Engineers Research Fellowship Program, and for T.K., provided through the Turkish Scientific Research Council (TUBITAK), is gratefully acknowledged.

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Received for review July 20, 1995. Revised manuscript received November 21, 1995. Accepted November 22, 1995.X ES950547R X

Abstract published in Advance ACS Abstracts, February 15, 1996.


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Adsorption of Natural Organic Polyelectrolytes by Activated Carbon: A

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