Activated carbon adsorption of humic substances

Activated Carbon Adsorption of Humic Substances II. Size Exclusion and Electrostatic Interactions

R. S C O T T S U M M E R S l AND P A U L V. ROBERTS 2 Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, California 94305-4020 Received June 17, 1986; accepted May 27, 1987 The activated carbon adsorption isotherms for a wide range of h u m i c substance molecular size (MS) fractions display an inverse dependence on MS, when expressed on an adsorbent mass basis. However,

normalizing the amount adsorbed on the basis of available adsorbent surface area accounts for the size exclusionbehaviordisplayedby the MS fractions,resultingin convergenceof their isotherms.The available surface area was calculated by relating the hydrodynamic size of the macromoleculesto the adsorbent pore size. The effectsof adsorbent charge and solution ionic strength conform to adsorption electrostatic principleswhen the isothermsare expressedon an availablesurfacearea basis.Adsorbentswith progressively more positively charged surfaces adsorb more of the negatively charged humic substances. Increasing solution ionic strengthsuppressesadsorptionby a positivelychargedadsorbentand enhances the adsorption by a negativelycharged adsorbent. The characteristic, two-segmentshape of the MS fraction isotherms is interpreted based upon changes in adsorbed macromoleculeorientation. © 1988AcademicPress.Inc. INTRODUCTION

Activated carbon is a heterogeneous and microporous adsorbent having the capacity o f removing compounds of health concern from drinking water supplies. The composition o f organic compounds in most water supply sources is dominated by humic substances, which are also adsorbed. The physical nature of activated carbon is typified by a distribution of pore sizes which extends downward to values similar to or smaller than the molecular size (MS) of humic substances. The chemical nature o f activated carbon is affected by its precursor material and activation process, with both positively and negatively charged adsorbents occurring. Both the magnitude and the sign of the charged surface affect the adsorption of charged adsorbates such as humic substances.

An understanding of both the physical and the chemical interactions of activated carbon and humic substances is an essential part of the effective design of this adsorption process for contaminant removal when humic substances are present. The objectives of this study are (i) to assess the relationship of adsorbate macromolecular size to the adsorbent pore structure in terms of available surface for adsorption, and (ii) to characterize the electrostatic interactions between humic substances and activated carbon. The effect of size exclusion is evaluated by investigating the adsorption of several MS fractions by one adsorbent, whereas the effect of adsorbent surface chemistry is demonstrated through the adsorption of one humic substance by several adsorbents after accounting for the available surface area. BACKGROUND

~Present address: Engler-Bunte-Institut, Universit~t Karlsruhe, Bereich Wasserchemie, Richard-Willst~tterAllee 5, D7500 Karlsruhe, West Germany. 2 To w h o m correspondence should be addressed.

Activated Carbon The ability of activated carbon to adsorb large quantities of material is directly related 382

0021-9797/88 $3.00 Copyright© 1988by AcademicPress,Inc. All fightsof reproductionin any formreserved.

Journalof Colloidand InterfaceScience,VoL 122,No. 2, April 1988

ADSORPTION

OF HUMIC

to its porous nature. In describing the porosity, several characteristics are important: pore size, pore volume, surface area, and their spatial distribution within the carbon particle. The most commonly cited, and seemingly misunderstood, adsorbent characteristic is the specific surface area, normally measured and reported as NE-BET surface area. This surface area is most often not proportional to the adsorptive capacities of activated carbons for other substances, which can be attributed to the wide range of pore sizes present in activated carbon. However, good agreement has been found when the accessible surface area, determined by the surface area distributions, is compared to the adsorption capacities of other substances (1, 2). Attempts to relate the total pore volume or porosity to the adsorption capacity for organic compounds are also often unsuccessful; however, several studies (3-8) have reported good agreement when the pore volume distribution is used. In one study, both surface chemistry and pore structure were investigated and found to be important (9). Generally, 5 to 20% by weight of activated carbon consists ofnoncarbon atoms, with surface oxides and metals as the two major components (10). Mattson and Mark (1 l) discuss in detail both the nature and the origin of surface oxides. Acidic surface oxides are most often suggested as carboxyl, lactone, phenol, and quinone-type carbonyl oxygen functional groups. Chromene-like, pyrone-like, and isoviolanthrene-like structures have been proposed to account for the basic nature. At charged interfaces, such as the surface of activated carbon, an electrical double layer is formed, with ions of opposite charge attracted to the surface and ions of like charge repelled. The pH value at which the densities of the charge-determining ions at the surface are equal is referred to as the point of zero charge (PZC). At pH values below this value, the surface is positively charged and at pH values above this value, the surface is negatively charged. Adsorption models which incorporate electrostatic principles to account for the

SUBSTANCES,

II

383

effects of pH and ionic strength on the activated carbon adsorption of dissociating organic solutes have been proposed (12, 13).

Adsorption of Charged Macromolecules Adsorption of polyelectrolytes is heavily influenced by the solution ionic strength through the shielding action of the electrolytes on the polyion and adsorbent charge. In the adsorption ofpolyelectrolytes to uncharged surfaces, higher-ionic-strength solutions result in the shielding of polymer charge and a subsequent increase in adsorption. The same principle applies in the adsorption of a polyelectrolyte to a surface of the same charge, with the added mechanism of charge shielding of the surface. In both of these cases, adsorption at low ionic strength may not occur if the nonionic adsorption energy is less than the charge repulsion energy. When the polymer and surface have charges of opposite sign, electrostatic forces promote adsorption. Any increase in the ionic strength will again result in the shielding of the polymer and surface charge, but to the detriment of adsorption. Similar effects on adsorption are observed with varying pH (14). The macromolecular size and charge are two humic substance characteristics which play a major role in the activated carbon adsorption of these macromolecules. The electrical charge ofhumic substances results from the ionization of functional groups and under most ambient conditions the humic macromolecule is negatively charged (15). This polyanionic character leads to the mutual repulsion of the like-charged groups, which controls the macromolecular size. A model of the macromolecular configuration of fulvic acid (FA) and humic acid (HA) was proposed by Ghosh and Schnitzer (16), in which humic substances behave like flexible linear macromolecules at low ionic strength (<0.01 M), pH > 3.5, and low solution concentration (<4 kg/m3), while at high ionic strength, low pH, or high concentration, a rigid spherocolloid model is thought to best represent the humic substances. Interpretation of HA diffusion rate Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988

384

SUMMERS

AND ROBERTS

experiments (17) using the Stokes-Einstein (SE) equation is consistent with such a model; increasing pH and temperature and decreasing ionic strength were found to increase the hydrodynamic size of the same HA as that used in this study. Several recent investigations of humic substance removal by activated carbon indicate that adsorption is influenced by adsorbent and humic substance properties, as well as by solution composition and experimental methodology. In most studies (4, 6, 18-20), an increased adsorption capacity with decreasing pH was found and has been attributed to the charge neutralization of the weak acid functional groups with decreasing pH (18, 20). Increased adsorption with increasing ionic strength has been observed in a number of investigations (18-21) and has been attributed to the increase in available surface area due to coiling of the macromolecule with increasing ionic strength and to an increase in hydrophobicity due to dehydration. Studies of the adsorption of different MS fractions ofhumic substances by several activated carbons have shown that smaller MS fractions were adsorbed more readily on an adsorbent mass basis (4, 6, 18, 22). Lee et al. (4) found a significant relationship between adsorption capacity and pore volume within certain ranges of pore radii of several carbons for fractionated and unfractionated humic substances. Although the effect of adsorbent surface chemistry on the adsorption of humic substances has not been investigated with activated carbon, in investigations using relatively nonporous mineral surfaces, the importance of adsorbent charge has been amply demonstrated. Increasing ionic strength has been reported to enhance humic substance adsorption with negatively charged adsorbents (23, 24) and to decrease the adsorption with positively charged adsorbents (25). MATERIALS AND METHODS

An unfractionated, purified commercial humic acid (CHA), and six MS fractions Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988

thereof, were used (26). The six MS fractions were prepared using the XM300, YM100, XM50, YMI0, YM5, YM2, and YC05 membranes (Amicon Corp.), which have nominal MW cutoffs of 300K, 100K, 50K, 10K, 5K, IK, and 0.5K (K = 103), respectively. A concentration-diafiltration method was utilized (17, 27), in which the retained solution concentration was limited to three times that of the working solution concentration to minimize size-shift effects of increasing concentration. The retained fraction was then diafiltered (dialyzed) with organic-free aqueous solution of identical inorganic species and concentration at a volume three times greater than the retained volume. Ultrafiltration conditions were normally 0.1 M NaC1, 1.0 mM Na2HPO4, and pH 7, except that one MS fraction was also ultrafiltered with no added NaC1. The five activated carbons (AC) utilized were the same as those used in Part I of this study (26): F300, F400, CET, CFT, and WVB. The 200/400-mesh particle size fraction was used and all AC were physically and chemically characterized. The surface area distribution was determined in the range below rp = 10 nm with nitrogen gas adsorption (Model 2100E, Micromeritics Inst. Corp.) (28) and in the range above rp = 10 nm with mercury intrusion porosimetry (Pore Sizer 9300, Micromeritics Inst. Corp.) (29). The extent of surface oxidation was determined by acidimetric and alkalimetric titration techniques (30). The adsorbent aliquot, 0.1 g, was contacted with 0.05 liter of 0.05 N acid (HC1) or base (NaOH) for 24 h. The AC suspension was then filtered out and the excess acid or base was titrated with 0.1 N NaOH or HCI, respectively. The difference between the excess base or acid in solution and a control sample without AC is the amount consumed. To quantify the PZC, pH drift experiments were used in which 0.1 g of AC was added to 0.02 liter of a 0.1 N NaC1 solution, whose initial pH had been adjusted with NaOH or HC1. The containers were sealed and placed on a shaker table for 24 h, after which the pH was

385

ADSORPTION OF HUMIC SUBSTANCES, II

measured. Longer times were investigated, but no further change in pH occurred. The PZC occurs when there is no change in the pH after contact with the carbon (12, 27). The adsorption isotherm techniques and conditions were the same as those used in Part I (26). RESULTS

Adsorption isotherm results for six MS fractions of CHA onto F300 AC using the constant-initial-concentration (Co) method are presented in Fig. 1. For reference, the isotherm results from the unfractionated CHA under the same experimental conditions are shown. Size exclusion is evident, as the solid-phase concentration, q, increases with decreasing molecular size for all six MS fractions. The larger MS fractions, 100K-300K through 5K80

I

(.b

I

I

I CHA-F

0,11dNoCl

y.

7O

o°£b

i

0.001M Na2HPOa pH 70 T 20"C 200/400 mesh gOOC Co 6.7 ± 0 . 2 ( ~ i - )

10K, exhibit a well-distinguished two-segment isotherm, with the first segment reaching a pseudo-plateau, followed by a substantial increase in the isotherm slope at high concentration. The two smallest MS fractions exhibit similar behavior, except that the second segment is distinguished by only one data point. As shown in Fig. 2, the four MS fractions and the unfractionated CHA adsorb on F400 AC in a manner similar to that of F300 AC (Fig. 1). The adsorptive capacity of the F400 AC is less than that of the F300 AC for all MS fractions and the unfractionated CHA. Isotherms with four CHA MS fractions were conducted at I°C ___0.5 and 41°C __+0.5 with the F300 AC. The results are shown in Fig. 3, where the solid lines without symbols represent the results at 20°C shown in Fig. 1. As with the unfractionated CHA (26), adsorption ap-

I

Ms

I

FRACTION

1

500

O.5K- I K IK-5K 5K-IOK IOK- 50K 5 O K - lOOK IOOK-5OOK

/

60 i

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1

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0

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2

3

4

LIQUID-PHASE

CONCENTRATION,

5

I 6

1 7

C ( g D O C / m ~)

FIG. 1. MS fraction adsorption equilibrium, mass basis. Adsorption isotherms for F300 AC with the unfractionated CHA and six MS fractions of CHA: 0.5K-1K, IK-5K, 5K-10K, 10K-50K, 50K-100K, and 100K-300K. Journal of Colloid and Interface Science,

Vol, 122.No. 2, April1988

386

SUMMERS AND ROBERTS

35

I

~E~ (.~ 0 C3 ~n

30

O-

25-

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UNFRAC + _

20-

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0

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03 M NaCI O.OOlM NazHPO4 pH 7.0 T 20*C 200/400 mesh ,9DOC, Co 6.7 + 0.2 [ - - ~ -- )

~

(

0

I

2

3

4

5

MS FRACTION IK-5K IOK- 50K 5 O K - lOOK IOOK-3OOK

0 V [] 0

6

7

LIQUID-PHASE CONCENTRATION, C (gDOC/m 3) FIG. 2. MS adsorption equilibrium, mass basis. Adsorption isotherms for F400 AC with the unfractionated CHA and four MS fractions of CHA: 1K-5K, 10K-50K, 50K-100K, and 100K-300K.

pears to be enhanced by increasing temperature for all four MS fractions tested. The effects of ionic strength on the adsorption of a 50K-100K MS fraction by a positively charged adsorbent, F300 (Fig. 4a), and by a negatively charged adsorbent, WV-B (Fig. 4b), were investigated. This 50K-100K MS fraction was prepared by ultrafiltration of a CHA solution with 1 m M Na2HPO4 and no NaCI. Increasing the NaC1 concentration from 0 to 0.1 M increases the adsorption capacity on an adsorbent mass basis with both activated carbons, with a significantly larger increase for the negatively charged WV-B AC. Similar results for the F300 adsorbent were found with the unfractionated CHA (26). DISCUSSION

Size Exclusion Comparison of the CHA adsorptive behavior at varying ionic strengths, Fig. 4, shows a Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988

3- to 10-fold increase in adsorptive capacity on an adsorbent mass basis, when the concentration of NaC1 is increased from 0 to 0.1 M. Similar behavior was found with the unfractionated CHA (26) and has been reported by others (18-21) with the addition of neutral salts. It is likely that this behavior is in part due to the molecular sieve effect of the adsorbent, which is influenced by the changes in the size and shape of the macromolecule. At low ionic strength the humic macromolecule expands due to the mutual repulsion of the like charges of the molecule. With increasingly higher ionic strength, the counterions in solution compress the double layer, allowing the molecule to coil and reduce its effective size. Thus, at higher ionic strength, the smaller (i.e., more tightly coiled) humic substance has access to more surface area for adsorption, as is reflected in the increase in adsorption capacity on an AC mass basis.

ADSORPTION OF HUMIC SUBSTANCES, II I00

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MS FRACTION _0.SK-IK 90 IK-5K ()
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LIQUID-PHASE CONCENTRATION, C (gDOC/m 3) FIG. 3. Effect of temperature on adsorption isotherms of CHA MS fractions with F300 AC.

To account for this size exclusion effect, adsorptive behavior should be compared on an available surface area basis rather than relative to the adsorbent mass. To quantify this approach, a relationship between the size of the molecule and the available AC surface area needs to be established. Through the use of the measured mass transfer coefficients, correlations of mass transfer and bulk diffusion coefficients (DI), and the Stokes-Einstein (SE) relationship,

rSE =

kT 6~r#Dt"

[1]

Cornel et al. (17) have estimated effective diffusion sizes, rSE, for the unfractionated and MS fractions of CHA used in this study. The relationship of AC pore radius, rp, and available surface area was quantified through the analysis of nitrogen-gas adsorption (28) and mercury intrusion experiments (29), as shown in Fig. 5. A simplified conceptualization relating the AC pore size, rp, to the size of the diffusing macromolecule, rSE, is shown in Fig. 6. A oneto-one relationship (rp:rsE) is thought to underestimate the rp cutoff, as it would not account for any decrease in the pore cross section Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988

388

SUMMERS AND ROBERTS 8

t

I

NGCl(M_~2_)

'

r

I

CHA [ 5OK-lOOK i

40

V 0.0 0 0.01 ix 0 . 0

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l

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2

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LIQUID-PHASE

CONCENTRATION,

C

(gDOC/m

7

5)

FIG. 4. Effect of ionic strength on the adsorption isotherm for 50K-100K MS fraction of CHA with (a) positively charged adsorbent (F300 AC) and (b) negatively charged adsorbent (WV-B AC).

owing to the space occupied by adsorbed macromolecules. A two-to-one relationship is thought to overestimate the rp cutoff, as it would not account for any decrease in size of the macromolecules after adsorption owing to conformational changes. An intermediate value of 1.5:1 for the rp to rSE ratio is chosen as an arbitrary compromise. Using this relationship does not imply that the macromolecules adsorb in a spherical conformation identical to that assumed in the Stokes-Einstein relationship; it assumes only that adsorption is restricted to the surface area in pores larger than 1.5 times rSE. Certainly this analysis is an oversimplification of the comJournal of Colloid and Interface Science, Vol. 122,No. 2, April1988

plex situation of diffusion and adsorption of macromolecules in an AC pore, but it is employed here as an expedient quantification method. Using this approach, the adsorption isotherms of the unfractionated CHA and five adsorbents (shown in Fig. 5 of Part I (26)) were analyzed, with the results shown in Table I. The C/D isotherm parameters on an adsorbent mass basis show a factor of 4 difference in the KF values, but relatively similar 1/n values, for the five AC types. Using a measured Dl value of 1.19 × 10 -l° mE/s (17), an apparent hydrodynamic radius of 1.80 nm can be estimated by rearranging the Stokes-Einstein relation-

389

ADSORPTION OF HUMIC SUBSTANCES, II

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FIG. 4--Continued.

ship (Eq. [1]). Using the 1.5:1 ratio for ra to FSE, the available SA values at ra = 2.70 nm for the five adsorbents can be estimated from Fig. 5 and are listed in Table I. The C/D (g DOC/kg AC) and q (g DOC/kg AC) values can now be expressed in terms of available SA instead of mass AC, i.e., C/D (g D O C / m 2 AC) and q (g D O C / m 2 AC). The resulting data can be regressed using the Freundlich model or, since both ordinate and abscissa are divided by the same factor, the l/ n values will not change and the K~ on a SA basis can be calculated from the KF mass basis values using Eq. [2],

K'v = KF(SA) O/n-l),

[2]

where SA is the accessible surface area (m2/g

adsorbent) for the macromolecule size under consideration. Comparison of the relative K~ values calculated in this manner (Table I) reveals a change in the rank order of the adsorption capacities of the adsorbents. Whereas the F300 and F400 AC exhibit the lowest adsorption capacities when compared on a mass basis, i.e., the KF values in Table I, the F300 AC has the highest adsorption capacity based on accessible surface area, followed closely by F400 AC, while the WV-B AC shows the lowest capacity by a twofold margin.

Electrostatic Interactions The CHA adsorption behavior, after normalization to accessible surface area, seems to Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988

390

S U M M E R S A N D ROBERTS 800

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700

600

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PORE RADIUS, rp (nm) FIG. 5. Surface area distribution of five activated carbons ground to 200/400 mesh.

be governed by the surface electrostatic characteristics as measured by the PZC and the HC1 consumption (Table I). Since all adsorption isotherms were conducted at pH 7, four of the adsorbents (F300, F400, CET, and CFT) have a positive surface charge, as their PZC values are all greater than 9.0, while the WVB AC (PZC = 5.0) has a negative surface charge. At a system pH of 7, the humic substances are negatively charged, as they are thought to have pKa in the range of pH 3 to 6 (15). As shown in Table I, a strong correlation is found between the K~ values and the HC1 consumption and the PZC values for the five adsorbents examined. The F300 AC has the highest HC1 consumption and PZC, indicating that it is the most positively charged adsorbent; as anticipated, it has the highest adsorption capacity of the negatively charged CHA. The WV-B AC has a PZC value below the solution pH of the experiment (i.e., pH 7) and a very low HC1 consumption; thus, as expected for a negatively charged adsorbent, it has the lowJournal of Colloid and InterfaceScience, Vol. 122, No. 2, April 1988

est adsorption capacity for the negatively charged CHA due to like-charge repulsion. Variation in ionic strength has been shown to have a significant impact on the adsorption of polyelectrolytes onto charged surfaces. Through a shielding action, increasing ionic strength by adding a neutral electrolyte will cause decreased adsorption with an oppositely charged system and increased adsorption with a like-charged system. Application of the Freundlich model to the C/D isotherms of the 50K-100K MS fraction on the positively charged F300 AC and negatively charged WVB AC (Fig. 4) yields KF and l/n values shown in Table II. Through inspection of Fig. 4 and the KF values, it seems as though increasing ionic strength enhances adsorption for both negatively and positively charged adsorbents, with the latter case in apparent contradiction to electrostatic theory. Using the rSE results of Cornel et al. (17), the KF values were normalized based on available surface area (Fig. 5) and are shown in Table II as Ki:. Accounting for the surface area in this manner reverses the order in the adsorption capacity values for the oppositely charged system (F300 AC) as a function of ionic strength, reflecting the accepted influence of ionic strength on charged adsorption systems. On a surface area basis, increasing NaCI concentration from 0 to 0.1 M decreases the adsorption capacity of the positively charged adsorbent by 22% and increases that of the negatively charged adsorbent by 33%. Assuming that, apart from electrostatic interactions, the mechanisms of adsorption of humic substances are the same for

rp : tsa

I:l

1.5:1

2:1

FIG. 6. Relationship of macromolecule size, rsE, to adsorbent pore size, re, which defines the available surface area.

391

A D S O R P T I O N OF H U M I C SUBSTANCES, II TABLE I Normalization of C H A Adsorption Isotherms Based on Accessible Surface Area

Adsorbent

Kv"

l/n

SA~

Ki:¢

PZC pH

HC1a consumption

F300 F400 CET CFT WV-B

7.96 6.74 26.7 15.0 12.5

0.334 0.302 0.340 0.350 0.314

27.5 21.5 290 124 212

0.876 0.792 0.633 0.654 0.317

10.2 10.0 9.6 9.3 5.0

0.49 0.35 0.15 0.28 0.06

a (g DOC/kg AC)/(g DOC/kg AC) TM. b m2/g AC. c (mg D O C / m 2 AC)/(mg D O C / m 2 AC) l/'. d meq/1000 m 2.

these two oppositely charged adsorbents, then a continued increase in ionic strength theoretically would yield identical isotherms for these adsorbents on a surface area basis (31). Molecular Size Fractions

As shown in Figs. 1 to 3, the MS fractions of CHA exhibit increasing adsorption on an adsorbent mass basis with decreasing MS. Cornel et al. (17) have established an inverse relationship between MS and rSE based upon the diffusion coefficients of the MS fractions of CHA. Using these rSE values at three different temperatures and the 1.5:1 ratio for rv to rSE, the available SA can be estimated for each of the different MS fractions, as shown in Fig. 7 for T = 20°C. The rSE values and the

available SA values for F300 AC at three temperatures (1, 20, and 41°C) and for F400 AC at 20°C are shown in Table III. For the F300 AC at 20°C, a 6-fold difference in available SA is found between the largest (100K-300K) and the smallest (0.5K-IK) MS fractions. For the 0.5K-1K MS fraction, a change in temperature from 41 to 1°C yields a 40% decrease in rSE, resulting in a 2.4-fold increase in the available SA. Incorporating the size exclusion into the MS fraction adsorption isotherms of Fig. 1 (F300 AC, T = 20°C) by normalizing the solid-phase concentration values based on available surface area yields the isotherms shown in Fig. 8. The adsorptive capacity normalized to available SA results in remarkably similar behavior for the wide range of MS fractions tested for

TABLE II Normalization of Adsorption Capacity Based on Accessible Surface A r e a - - C H A 5 0 K - 1 0 0 K MS Fraction NaCI (M)

Kra

1/n

r2

rsE (nm)

SA (m2/gAC)

K["~

F300

0 0.01 0.1

3.11 5.13 10.7

0.339 0.333 0.251

0.994 0.994 0.984

5.93 2.62 1.41

8.0 18.2 45.0

0.787 0.741 0.618

WV-B

0 0.01 0.1

1.72 5.80 19.3

0.418 0.354 0.265

0.984 0.999 0.997

5.93 2.62 1.41

36.7 123 313

0.211 0.259 0.283

Adsorbent

Note. Ultrafiltration at 0.0 M NaCI and 1 m M Na2HPO4.

° (g DOC/kg AC)/(g DOC/kg AC) TM. b (mg D O C / m 2 AC)/(mg D O C / m 2 AC) TM. Journal of Colloid and Interface Science, Vol. 122,No. 2, April 1988

392

SUMMERS

AND

ROBERTS

125 TOTAL SURFACE AREA (mZ/gAC)

< X FSO0 • F400

E too

820 890 2 0 0 / 4 0 0 mesh

-IK-5K

75 < o c~

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r IOK-50K

>_ ~

5OK- lOOK f lOOK-300K

25

5 2~

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0 [ I

2

3

4

5

6

7

8 9 I0

PORE RADIUS, rp (nm) FIG. 7. S u r f a c e a r e a d i s t r i b u t i o n o f t h e F 3 0 0 a n d F 4 0 0 a c t i v a t e d c a r b o n s , s h o w i n g a v a i l a b l e s u r f a c e a r e a for different C H A M S fractions.

the F300 AC. In the concentration range below 4 g DOC/m 3, only the smallest MS fraction displays any distinguishable difference, which may be due to an underestimation of the surface area in the pore size range below 1.5 nm

(28, 29). In the concentration region above 5 g DOC/m 3, the slopes of the isotherms increase significantly and a dependence of the amount adsorbed, q', on the MS begins to establish itself, with the largest macromolecules having

T A B L E III Stokes-Einstein

Radius and Available Surface Area for MS Fractions of CHA

Availablesurface area (m2/g AC) Radius r~ (nm)

Temperature (*C): CHA MS fraction 0.5K-IK 1K-5K 5K-10K 10K-50K 50K-100K 100K-300K

F400 A C b

F300 AC*

1

20

41

1

20

41

20

0.660 0.952 -1.49 1.84 --

0.889 1.19 1.40 1.77 2.10 2.50

1.11 1.38 -1.93 2.23 --

209 105 -40.0 26.5 --

116 77.4 46.0 28.0 22.3 19.0

87.5 48.0 -25.0 21.0 --

-47.0 -21.5 19.5 18.0

a T o t a l specific surface a r e a (N2-BET) = 8 2 0 m2/g. b T o t a l specific surface a r e a (N2-BET) = 8 9 0 m2/g. Journal of Colloid and Interface Science, Vol. t22, No. 2, April 1988

ADSORPTION

OF HUMIC

SUBSTANCES,

393

II


E (J oc'~ v

FRACTION 1.2 - -

E 1.0

0.SK- I K I K- 5K 5K-IOK IOK- 50K 5 O K - lOOK IOOK-3OOK

-e-

@

z

o I-,< (2/

bZ

0.8

W

0.6

(.3 Z 0 (.3

0.4

W < -1(3_

0

.

2

0.I M Noel O.DOIM NatHPO4 pH 7.0 T ZO*C 200/400 mesh F300

~

I Q

.J 0 CO

• gDOC, 6.7 t 0.2 ~'--~-~-}

Co

O

0

~

I

2

3

4

,5

6

7

LIQUID-PHASE CONCENTRATION, C (gDOC/m 3)

FIG. 8. Adsorptionisothermsfor F300 AC with the six MS fractionsof CHA on an availableadsorbent surfacearea basis.

the highest adsorption capacity. The results for the MS fraction isotherms at T = 1 and 40°C (Fig. 3) and with F400 AC (Fig. 2) normalized in this manner behave similarly to the results in Fig. 8, both in variability and magnitude. The lack of dependence on MS in the lowconcentration region is often interpreted as reflecting the adsorption of the macromolecules in a relatively fiat configuration with very few segments extending from the surface (31, 32). The enhanced adsorption in the higherconcentration region suggests that fewer segments of the macromolecules are attached to the surface, such that more macromolecules can be adsorbed per unit surface area. The MS dependence also supports this concept of a change in adsorbed configuration in the higher-concentration region. If the same number of segments attaches to the surface for both small and large macromolecules, then

a greater mass of the large macromolecules can be adsorbed per unit surface area of the adsorbent. This hypothesized change in configuration with solution concentration is consistent with other macromolecule adsorption studies using NMR and EPR spectroscopy (33), gel permeation chromatography (34), electrophoresis (35), microcalorimetry (36), and stepwise adsorption of radiolabeled proteins (37). Those authors (33-37) have concluded that macromolecules adsorb in a fiat configuration at low solution concentrations, as there is little or no competition for adsorption space. At higher concentrations, there is more competition for space, and macromolecules adsorb with a smaller number of sites and consequently more segments extending into solution. Interpretation of stepped isotherms in terms of adsorbed configuration has also been utilized for the adsorption of proteins (37-39), Journal of Colloid and Interface Science, Vol. 122, No. 2, April t988

394

SUMMERS AND ROBERTS

nonionic (40) and ionic surfactants (41), and dyes (42). O f the above-mentioned studies, the case of ionic surfactant adsorption would most closely approximate the adsorption of humic substances because of the presence of aromatic, aliphatic, and charged groups (15). Although the transition from the fiat adsorbed-phase configuration to one in which only a few segments are attached is likely to be gradual, a pseudo-plateau is found in the intermediate-concentration range. A measure of this region of MS independence would be useful in facilitating comparisons of MS fractions under varying system conditions. An estimate was made of the adsorptive capacity at the point of intersection of the low-slope (pseudo-plateau) region and the high-slope region in the C range 4 to 5 g D O C / m 3. These transition values, qb, for the F300 AC at 1, 20, and 41 °C (Figs. 1 and 3) and F400 AC at 20°C (Fig. 2) are given in Table IV. An increase in temperature from 1 to 41°C results in a 4 0 50% increase in qb on a per mass adsorbent basis. Accounting for the different available surface areas (Table III) results in a normalized adsorption capacity at the transition point,

q'b, also shown in Table IV. The q~ values at a given temperature are in close agreement with each other, as shown by the low standard deviations (SD) and the coefficient of variation (CV) of 12-16%. F r o m the similar adsorption behavior after adjustment for size exclusion, it can be inferred that the molecular size fractions are chemically similar; this conclusion is supported by U V and IR spectroscopic characterizations, which indicate similar chemical compositions of the MS fractions (27). The values of the a m o u n t of D O C adsorbed at the transition point, q~ (Table IV), are equivalent to 0.8-1.0 mg C H A / m 2 ( T = 20°C), since carbon accounts for approximately 53% (by mass) of the C H A content. For the same C H A as that used in this study, an average value of 0.9 mg C H A / m 2 for three mineral adsorbents has been reported (43). An adsorbed a m o u n t of polymer in the range 12 m g / m 2 is generally characteristic of the first plateau in m a n y polymer adsorption isotherms (44). The similarity between this range of q~, values from previous work and the values in this study supports the hypothesized fiat ad-

TABLE IV Adsorption Capacity at the Transition Point: Mass and Surface Area Basis

Adsorbent: Temperature (*C):

CHA MS fraction 0.5K-IK IK-5K 5K-10K 10K-50K 50K-100K 100K-300K

qb

q~

(S DOC/kg AC)

( m g D O C / m 2 AC)

F300

F400

F300

F400

1

20

41

20

1

20

41

20

55 25 -9.0 8.0 --

60 30 15 12 10 8.0

-34 -13 12 --

-22 -12 11 7.5

0.26 0.24 -0.23 0.30 --

0.52 0.39 0.33 0.43 0.45 0.42

-0.71 -0.52 0.57 --

-0.47 -0.56 0.56 0.42

)( 0.26 SD 0.03 CV 0.12

0.42 0.06 0.13

0.60 0.10 0.16

0.50 0.07 0.14

Journal of Colloid and InterfaceScience, Vol. 122, No. 2, April 1988

ADSORPTION OF HUMIC SUBSTANCES, II sorbed configuration in the concentration region below 4 g D O C / m 3. The dependence of the amount adsorbed on the MS in the high-concentration region (Fig. 8) can be seen by comparing the values of q' at a liquid-phase concentration of 6.0 g DOC/m 3, on a per unit surface area basis, q~. These values of q~ for F300 AC at 20°C are shown in Table V along with the values at 1 and 40°C and for F400 AC at 20°C. In all four systems a significant dependence on MS can be found, with increases in q~ ranging from 20 to 115% for the MS ranges investigated. An increase in adsorption capacity with increasing temperature is found for the MS fractions (Fig. 3 and Tables IV and V) and for the unfractionated CHA, as shown in Fig. 8 and Table III of Part I (26). The q~ values of the MS fractions (Table IV) increase by 0.17 mg DOC/m 2 AC per 20°C increase in temperature and the Ki: values of the unfractionated CHA increase by 15% per 20°C increase. This temperature-enhanced adsorption is indicative of conformational changes of macromolecules from the bulk state to the adsorbed state, which include dehydration of the macromolecule as well as the desorption of water from the surface (40). From comparing the accessible surface areas (Tables II-IV) with the total surface areas (N2-

TABLE V Adsorption Capacityat a Liquid-Phase Concentration of 6.0 g D O C / m 3 Adsorption capacity q~ (mg DOC/m z AC) F300

F400

Adsorbent: Teraperature (*C):

CHA MS fraction 0.5K-1K 1K-5K 5K-10K 10K-50K 50K-100K 100K-300K

1

20

41

20

0.31 0.33 -0.44 0.45 --

0.77 0.65 0.70 0.68 1.03 1.40

-1.03 -1.09 1.24 --

-0.58 -0.69 0.76 0.98

395

BET) of the adsorbents (Fig. 5), it appears that less than 25% of the total surface area is present in pores large enough to be permeated by the humic substances investigated. For large macromolecules, the accessible fraction of the surface area is only 1 to 2%, e.g., the 100K-300K MS fraction (Table III) or the 5 0 K - 1 0 0 K MS fraction at low ionic strength (Table V). In contrast, the smallest humic macromolecule studied, the 0 . 5 K - 1 K MS fraction, has access to 25% of the total surface area. It is also instructive to note that the available surface area for a macromolecule of a given size is not proportional to the total surface area for any of the five adsorbents investigated. The total surface areas of the adsorbents vary by less than a factor of 2 (Fig. 5), whereas, for example, the surface area accessible to a macromolecule with rSE = 1.80 nm (rp = 2.70 nm) varies by more than a factor of 13 (Table II). This nonproportionality and the surface chemistry differences help explain the lack of correlation between the specific adsorption capacities and the total surface area measurements reported in most studies. SUMMARY AND CONCLUSION Analysis of adsorption isotherm results normalized with respect to available adsorbent surface area, rather than to adsorbent mass, facilitates the comparison of different-sized macromolecules. By relating the hydrodynamic radius of a humic macromolecule to the pore radius of an adsorbent, the available surface area for adsorption can be calculated. For a given adsorbent and a range of MS fractions of a humic acid, normalization of the adsorptive capacity on a surface area basis accounts for the size exclusion effect. For a given humic substance and five adsorbents with a range of pore structures, normalization on a surface area basis makes it possible to evaluate the influence of surface chemistry on adsorption behavior. Adsorbents with a more positively charged surface have higher adsorption Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988

396

SUMMERS AND ROBERTS

capacities for t h e negatively charged h u m i c substance a n d t h e results o f e x p e r i m e n t s at different ionic strengths qualitatively a d h e r e to electrostatic principles for b o t h positively a n d negatively c h a r g e d adsorbents. A n e x p l a n a t i o n o f the t w o - s e g m e n t isot h e r m b e h a v i o r o f M S fractions is p r o p o s e d based u p o n a d s o r b e d m a c r o m o l e c u l e orientation. A t low adsorbent dose, c o m p e t i t i o n for a d s o r p t i o n sites is high a n d the a d s o r b e d m a c r o m o l e c u l e s are closely p a c k e d , while at high a d s o r b e n t dose, c o m p e t i t i o n is low a n d the m a c r o m o l e c u l e s adsorb in a flat configuration. ACKNOWLEDGMENT This work was supported by Grant R-809601 from the Exploratory Research Program of the U.S. Environmental Protection Agency. It has not been subjected to the Agency's internal review, and no endorsement should be inferred. REFERENCES 1. Juhola, A. J., Carbon 13, 437 (1975). 2. Hashimoto, K., Miura, K., Yoshikawa, F., and Imai, I., Ind. Eng. Chem. Process. Des. Dev. 18(1), 72 (1979). 3. Chudyk, W. A., Snoeyink, V. L., Beckmann, D., and Temperly, T. J. J. Amen Water Works Assoc. 71(9), 529 (1979). 4. Lee, M. C., Snoeyink, V. L., and Crittenden, J. C., J. Amen Water Works Assoc. 73(8), 440 (1981). 5. Suidan, M. T., Snoeyink, V. L., Thacker, W. E., and Dreher, D. W., in "Chemistry of Wastewater Treatment" (A. J. Rubin, Ed.). Ann Arbor Science, Ann Arbor, 1978. 6. Sontheimer, H., Frick, B., Fettig" J., H6rner, G., Hubele, C., and Zimmer, G., "Adsorptionsverfahren zur Wasserreinigung." DVGW-Forschungsstelle am Engler-Bunte Institut tier Universit~t Karlsruhe (TH), 1985. 7. Seewald, H., Klein, J., and Jiintgen, H., in "Fundamentals of Adsorption" (H. L. Myers and G. Belfort, Eds.). Engineering Foundation, New York, 1984. 8. Urano, K., Koichi, Y., and Yamamoto, E., J. Colloid Interface Sci. 86, 43 (1982). 9. Graham, D., J. Phys. Chem. 59, 896 (1955). 10. Smisek, M., and Cerny, S., "Active Carbon." Elsevier, Amsterdam, 1970.

Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988

11. Mattson, J. S., and Mark, H. B., "Activated Carbon: Surface Chemistry and Adsorption from Solution." Dekker, New York, 1971. 12. Miiller, G., Radke, C. J., and Prausnitz, J. M., J. Colloid Interface Sci. 103, 466 (1985). 13. Kropal, L. K., Z, WasserAbwasserForsch. 16(3), 91 (1983). 14. Hesse[ink, F. Th., in "Adsorption from Solution at the So[id/Liquid Interface" (G. D. Partitt and C. H. Rochester, Eds.). Academic Press, London, 1983. 15. Thurman, E. M., "Organic Geochemistry of Natural Waters." Martinus Nijboff/Junk Publ., Dordrecht, 1985. 16. Ghosh, K., and Schnitzer, M., Soil Sci. 129(5), 266 (1980). 17. Cornel, P. K., Summers, R. S., and Roberts, P. V., J. Colloid Interface Sci. 110, 149 (1986). 18. McCreary, J. J., and Snoeyink, V. L., Water Res. 14, 151 (1980). 19. Randtke, S. J., and Jepsen, C. P., J. Amen. Water Works Assoc. 74(2), 84 (1982). 20. Weber, W. J., Voice, T. C., and Jodellah, A., J. Amen. Water Works Assoc. 75(12), 612 (1983). 21. Weber, W. J., Pirbazari, M., Long. J. B., and Barton, D. A., in "Activated Carbon Adsorption of Organics from the Aqueous Phase" (I. H. Suffet and M. J. MeGuire, Eds.), Vol. I. Ann Arbor Science, Ann Arbor, 1980. 22. Manos, G. P., and Tsai, C., Water, Air, Soil Pollut. 14, 419 (1980). 23. Tipping. E., and Heaton, M. J., Geochim. Cosmochim. Acta 47, 1393 (1983). 24. Wong" K., and Laskowski, J. S., Colloids Surf 12, 319 (1984). 25. Davis, J., Geochim. Cosmochim. Acta 46, 2381 (1982). 26. Summers, R. S., and Roberts, P. V., J. Colloid Interface Sci. 122, 367 (1988). 27. Summers, R. S., Ph.D. dissertation, Department of Civil Engineering, Stanford University, Stanford, CA, 1986. 28. Barrett, E. P., Joyner, L. G., and Halenda, P. P., J. Amer. Chem. Soc. 73, 373 (1951). 29. Gregg, S. J,, and Sing, K. S. W., "Adsorption, Surface Area and Porosity." Academic Press, London, 1982. 30. Boehm, H. P., in "Advances in Catalysis" (D. D. Eley, H. Pines, and P. B. Weisz, Eds.), Vol. 16. Academic Press, New York, 1966. 31. Papenhuijzen, J., van der Schee, H. A., and Fleer, G. J. J. Colloid Interface Sci. 104, 540 (1985). 32. Sato, T., and Ruch, R., "Stabilization of Colloidal Dispersions by Polymer Adsorption." Dekker, New York, 1980. 33. Cohen Stuart, M. A., Fleer, G. J., and Bijsterbosch, B. H., J. Colloid Interface Sci. 90, 321 (1982).

ADSORPTION OF HUMIC SUBSTANCES, II 34. Furusawa, K., and Yamamoto, K., J. Colloid Interface Sci. 96, 268 (1983). 35. Foissy, A., E1Attar, A., and Lamarche, J. M., J. Colloid Interface Sci. 96, 275 (1983). 36. Capelle, A., in "Activated Carbon, A Fascinating Materiar' (A. Capelle and F. deVooys, Eds.). Norit N. V., Amersfoort, The Netherlands, 1983. 37. DeBaillou, N., Voegel, J. C., and Schmitt, A., Colloids Surf 16, 271 (1985). 38. Fair, B. D., and Jamieson, A. M., J. Colloidlnterface Sci. 77, 525 (1980). 39. Soderquist, M. E., and Walton, A. G., J. Colloid Interface Sci. 75, 386 (1980). 40. Clunie, J. S., and Ingram, B. T., in "Adsorption from Solution at the Solid/Liquid Interface" (G. D. Par-

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fitt and C. H. Rochester, Eds.). Academic Press, London, 1983. 41. Hough, D. B., and Rendall, H. M., in "Adsorption from Solution at the Solid/Liquid Interface" (G. D. Parfitt and C. H. Rochester, Eds.). Academic Press, London, 1983. 42. Giles, C. H., in "Adsorption from Solution at the Solid/Liquid Interface" (G. D. Partitt and C. H. Rochester, Eds.). Academic Press, London, 1983. 43. Altmann, R. S., Ph.D. dissertation, Department of Civil Engineering, Stanford University, Stanford, CA, 1984. 44. Silberberg, A., in "Encyclopedia of Polymer Science and Engineering," Vol. 1. Wiley, New York, 1985.

Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988