Cadmium Adsorption on Aluminum Oxide in the Presence of

by a Hydro Service reverse osmosis/ion exchange system (Model LPRO-20). ... After filtering, samples were analyzed for aqueous Cd2+ with an Orion ...
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Environ. Sci. Technol. 2001, 35, 348-353

Cadmium Adsorption on Aluminum Oxide in the Presence of Polyacrylic Acid RUXANDRA M. FLOROIU, ALLEN P. DAVIS, AND ALBA TORRENTS* Environmental Engineering Program, Department of Civil and Environmental Engineering, University of Maryland, College Park, Maryland 20742

Adsorption of metals from aqueous solution onto oxide and other surfaces is known to affect trace metal transport in many natural and engineered systems. It is therefore important to understand whether dissolved metal inputs will be easily bound to particles or will be strongly complexed in solution and transported with the water phase. The effect of poly(acrylic acid) (PAA), representing a model compound for natural organic matter, on the adsorption of Cd(II) onto γ-Al2O3 was determined using batch adsorption experiments over a pH range from 4 to 10. Initially, interactions among the individual components were evaluated. Cadmium adsorption onto alumina showed a typical S-shaped metal adsorption curve. PAA adsorption onto γ-Al2O3 decreased with increase in pH. The affinity of PAA for Cd2+ increased strongly with pH. In ternary systems, the presence of PAA resulted in an enhancement of Cd(II) adsorption below pH 6, apparently due to ternary surface complex formation. Above pH 6, a decrease in cadmium adsorption onto γ-Al2O3 was observed resulting from an increase in the concentration of soluble Cd-PAA complexes. Overall, results indicate that the presence of natural organic matter could have a significant impact on the distribution and mobility of cadmium in the environment. Simple surface complexation modeling was insufficient to describe behavior in the ternary systems due to the complexity of the PAA polymer.

being structurally simpler, can be used as a model compound for the study of natural biogeopolymers such as humic substances (15). The adsorption of PAA onto oxides decreases with an increase in pH, similar to other carboxylic acids, following the change in the surface charge of the oxide (1618). Cd(II) can form bidentate and monodentate ligand complexes with PAA, depending on relative concentrations of PAA and Cd(II) and the degree of dissociation of the PAA (19). However, the number of carboxylate groups bound to each metal ion and the stability constants of the complexes of mono- and multivalent metal ions with PAA, determined by different techniques, were not always in agreement (20, 21). The examination of sorption behavior in ternary metalligand-mineral surface systems has lately been given high priority by researchers. Geochemical models that utilize single-component or binary system databases have been used to predict trends of metal ion adsorption at the oxide/aqueous interface. However, successful prediction of complex system behavior such as Cd(II)-fulvic acid-kaolinite and Cd(II)-humic acid-hematite could not be achieved by just using a combination of results from simpler systems (22, 23). Recently, the diffuse layer model was used to interpret the adsorption of Cd(II), Zn(II), Co(II), and Mn(II) onto alumina, silica, and kaolinite, in the presence of salicylic acid (24). The model employed several ternary surface complexes and described adsorption data well in the acidic pH range but failed to simulate the observed decrease in adsorption in the alkaline pH range. The goal of this study is to improve the understanding of adsorption properties within a multicomponent system which contains a polymeric complexing agent, used as a model compound for NOM (poly(acrylic acid)), a solid phase (aluminum oxide), and a metal (cadmium). PAAs are among the most popular models chosen to study the complexation of Cu(II), Ni(II), Pb(II), and Zn(II) by natural organic macromolecules (15, 25, 26). The specific tasks are to evaluate through batch adsorption experiments binary complexation reactions in the systems Cd(II)/alumina, PAA/alumina, and Cd(II)/PAA and to observe the effects of poly(acrylic acid) on cadmium adsorption onto aluminum oxide as a function of pH. Discussion is provided on the capability of a simple surface complexation model to accurately predict adsorption in ternary Cd/PAA/Al2O3 systems.

Introduction

Methods and Materials

Iron and aluminum (hydr)oxides and natural organic matter (NOM) are the most important reactive surfaces in soils, aquifer materials, and sediments with respect to adsorption of metals (1-4). Organic matter, particularly humic substances, plays an important role in controlling the fate of metals through several mechanisms. Organic matter can adsorb onto oxides, allowing the formation of ternary surface complexes, enhancing metal adsorption (5), or blocking sorption sites. Dissolved organic matter can compete with the surface for the metal. Metals such as iron, copper, lead, and cadmium form complexes of high stability with humic substances (6-10). Research with surrogates for NOM such as weak polyelectrolytes and simple organic acids has provided valuable insights into the behavior of NOM in systems containing mineral surfaces (11-14). Poly(acrylic acid) (PAA) is a polymeric substance containing carboxylic groups and linear CH2-CH2 chains. PAA,

Aluminum Oxide C from Degussa, predominantly in the γ mineral form, was used as the adsorbent. This material has a specific surface area of 92.8 m2/g, surface site density of 2.3 OH/nm2, and point of zero charge of 8.9 (27). The PAA ([CH2CH(COONa)]n, Aldrich Chemical) employed had an average molecular weight of 5100 g/mol. Other properties include a sodium content of 18.9%, a gravity density of 0.550, and a water content of 12.1% (all from manufacturer). All solutions were prepared using reagent-grade chemicals and 18 MΩ‚cm deionized water provided by a Hydro Service reverse osmosis/ion exchange system (Model LPRO-20). Commercial buffer solutions at pH 4, 7, and 9 were used to calibrate an Accumet Model 25 Fisher Scientific pH meter before each pH measurement. Adsorption. Cadmium/γ-Al2O3. Aqueous solutions of 10-6, 10-5, and 10-4 M Cd(II) were prepared using Cd(ClO4)2‚6H2O (Aldrich Chemical), with an ionic strength of 5 × 10-3 M NaClO4 (Fisher Scientific). One hundred milliliter volumes of Cd(II) solution were added to 125 mL Nalgene plastic bottles containing 0.2 g of Al2O3 (2 g/L). A separate bottle

* Corresponding author phone: (301)405-1979; fax: (301)405-2585; e-mail: [email protected]. 348

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10.1021/es9913479 CCC: $20.00

 2001 American Chemical Society Published on Web 12/09/2000

without the adsorbent was used as a blank control. The pH of the suspensions was adjusted to a range of 4-10 with 0.1 N HCl (36.5-38%, J. T. Baker) or 0.1 N NaOH (VWR). The bottles were placed on a horizontal shaker for 18 h; examining adsorption at shorter and longer contact times confirmed that this time was more than adequate for the suspension to equilibrate. After the equilibration time, the pH was recorded. Fourteen milliliters of each sample was transferred to polystyrene tubes and centrifuged at 1800 rpm for 25 min with a Beckman GPR Centrifuge. The supernatant was then filtered using a 0.2 µm membrane filter (Gelman Sciences), discarding the first 2 mL of the filtrate as a rinse. After filtering, samples were analyzed for aqueous Cd2+ with an Orion free cadmium ion activity electrode (Model 94-98A) while being continuously stirred. The corresponding Cd2+ standards were prepared using aqueous cadmium standard solution (1000 ppm, VWR) adjusted with 5 × 10-3 M NaClO4. The potential (mV) for each sample was correlated to a respective Cd2+ (M) concentration. The detection limit of the electrode was 1.0 × 10-6 M, and its response was linear up to 1.0 × 10-2 M Cd2+. The filtered supernatant was acidified with HNO3 and analyzed for Cd(II) using a Perkin-Elmer Model 5100 ZL atomic absorption spectrophotometer (AAS). The detection limit for the flame module was 8.9 × 10-7 M with a linear range up to 1.8 × 10-5 M Cd(II). For the graphite furnace, the detection limit was 10-8 M, and detection was linear to 2 × 10-7 M. Aqueous cadmium standard solution (1000 ppm, VWR) was used to obtain standards for AAS. The concentration of Cd(II) adsorbed onto the Al2O3 was calculated by the difference between the blank Cd(II) concentration and the filtered Cd(II) sample concentration at each pH value. No Cd(II) adsorption on the walls of plastic bottles or centrifuge tubes was detected. PAA/γ-Al2O3. A stock solution of 400 mg/L PAA was employed in order to obtain initial PAA concentrations of 50-285 mg/L at a fixed 5 × 10-3 M NaClO4 ionic strength. The adsorption experiments were performed in polystyrene tubes with a sample volume of 14 mL at 2 g/L Al2O3; the pH was adjusted to a range from 4 to 10. The samples were equilibrated for 6 h on a rotameter at 35 rpm. Studies with various equilibration times concluded that negligible change in adsorption characteristics occurred after 6 h. The equilibrated samples were centrifuged and filtered as with Cd adsorption. The PAA adsorption was expressed by measuring the total carbon (TC) of each filtrate sample using a Shimadzu total organic carbon (TOC) analyzer (Model 5000). Calibration was done with potassium biphthalate, and a detection limit of 0.5 mg/L C was obtained. Prior to the TC analysis, CO2 was eliminated from each sample by purging with nitrogen gas for about 15 min. Condensation/precipitation of PAA prevented the use of acidification in the TC analyses. Cadmium/PAA. These studies employed cadmium at 10-5 M; pH 4 and 7 were examined with 5 × 10-3 M NaClO4 as supporting electrolyte. PAA concentrations ranged from 12 to 163 mg/L (0.23 × 10-5-3.2 × 10-5 M, using a PAA average molecular weight of 5100 g/mol, as given by the manufacturer). The Cd(II)-PAA experiments were performed in 14 mL polystyrene tubes that were rotated on a horizontal shaker for 8 h. The pH and Cd2+ concentration were measured with the appropriate electrode. PAA complexed with cadmium, [Cd-PAA], was calculated as [Cd(II)] - [Cd2+]; Cd(OH)x complexes are negligible at these two pH values. Ternary Systems. Ternary systems involving all three components (Cd(II), PAA, Al2O3) were studied using welldefined component addition sequences. Three sequences were performed: two sequential adsorptions (Cd(II) first adsorbed onto Al2O3 and PAA first adsorbed onto Al2O3) and

TABLE 1. Reactions Used in Surface Complexation Modeling of the Cd(II)/Al2O3 System no. 1 2 3 a

reactions

stability constants H+

Al-OH2+ S Al-OH + Al-OH S Al-O- + H+ Al-OH + Cd2+ S Al-OCd+ + H+

log K1S ) -7.7a log K2S ) -10.2a log KAl-OCdS ) -1.5

From ref 27.

one simultaneous adsorption (Cd(II) and PAA adsorbed together onto Al2O3). First Sequential Adsorption: Cd(II)-(PAA-Al2O3). In this sequential adsorption, PAA was first adsorbed onto Al2O3. Duplicate samples containing 49, 123, and 230 mg/L PAA at a range of pH values were equilibrated with the solid as above, being shaken on a rotameter for 18 h. One of the duplicate samples was sacrificed to establish the amount of PAA adsorbed in the binary system. To the other, 200 µL of 7 × 10-4 M Cd(II) was spiked in order to obtain the desired total concentration of 10-5 M Cd(II). The samples were equilibrated again on the rotameter at room temperature for 18 h, centrifuged, and filtered, and Cd(II) and PAA levels were measured. Second Sequential Adsorption: (Cd(II)-Al2O3)-PAA. In the second sequential adsorption, 10-5 M Cd(II), 2 g/L of Al2O3, and 5 × 10-3 M NaClO4 were equilibrated for 18 h. Afterward, 200 µL of PAA was spiked into this system to produce 49, 123, and 230 mg PAA/L. These samples equilibrated for 18 h on the rotameter and were then centrifuged, filtered, and analyzed as above. Simultaneous Adsorption: (Cd(II)-PAA)-Al2O3. In the simultaneous adsorption, 49, 123, or 230 mg PAA/L was mixed with 10-5 M cadmium for 6 h and then added with 2 g/L of Al2O3 (18 h for equilibration). Batch experiments were performed over a pH range of 4-10. Modeling. The geochemical speciation program MINTEQA2/PRODEFA2 (28) was used to calculate conditional stability constants for solution-surface equilibria using the diffuse layer model (DLM). Surface acidity reactions were modeled using eqs 1 and 2 (Table 1), in which the values for the intrinsic surface acidity constants for Al2O3 were taken from (27). A surface site concentration of 7.1 × 10-4 M was calculated for 2 g/L of Al2O3.

Results Binary Systems. Cd(II) Adsorption. Adsorption of cadmium onto γ-Al2O3 as a function of pH at three different initial Cd(II) concentrations is shown in Figure 1. Increasing adsorption with increasing pH is found, as expected for adsorption of metals onto oxides. The adsorption of Cd(II) begins at a pH value less than 5.5; essentially 100% adsorption is observed beyond pH 8. The percent adsorption decreased as the concentration of Cd(II) increased, and thus the ratio of adsorbate to adsorbent increased. PAA Adsorption. Adsorption of poly(acrylic acid) onto γ-Al2O3 as a function of pH is presented in Figure 2. PAA is strongly adsorbed over the entire pH range studied. At a fixed PAA concentration, adsorption decreases almost linearly with increasing pH, similar to that noted by other investigators (19, 21). Based on PAA adsorption isotherms performed at pH 5, 7, and 9, the maximum adsorption density found at the low pH (1.2 mg/m2) is about three times greater than at the higher pH (0.4 mg/m2). Similar observations were found by Kunjappu et al. (29) in their study of the equilibria of PAA (MW ) 40 000) with alumina. The higher adsorption density at the low pH can be attributed to a highly “coiled” conformation of the polymer due to the low charge development at this pH (the average pKa is 4.5), with the result that VOL. 35, NO. 2, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Percent Cd(II) adsorption on 2 g/L Al2O3 in Cd-Al2O3 system, I ) 5 × 10-3 M NaClO4. Lines represent model fits (log KAlOCd+ ) -1.9, -1.5, and -0.75 at 10-4, 10-5, and 10-6 M Cd(II)).

FIGURE 2. Percent PAA adsorption on 2 g/L Al2O3 in PAA-Al2O3 system, I ) 5 × 10-3 M NaClO4.

FIGURE 3. Free cadmium ion concentration ([Cd2+aq]) data at pH 4 (Cd(II) ) 10-5 M, I ) 5 × 10-3 M NaClO4). more of the polymer is needed for complete surface coverage. At the higher pH values, however, since the polymer is more “stretched” due to electrostatic repulsion, the polymer occupies more area and therefore a lower maximum adsorption density is obtained. Cadmium-PAA Interactions. Experimental data obtained with 10-5 M Cd(II) at pH 4 using different PAA concentrations showed a decrease in the equilibrium Cd2+ concentrations with increasing PAA concentrations (Figure 3). At the lowest PAA level, little complexation occurred. However, Cd-PAA complexes were formed as higher concentrations of PAA were added, providing more available PAA sites. At pH 7, all Cd2+ concentrations were less than the detection limit, indicating that the total cadmium in solution is more than 90% 350

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FIGURE 4. Percent Cd(II) adsorption on 2 g/L Al2O3 in the simultaneous adsorption (10-5 M Cd(II), I ) 5 × 10-3 M NaClO4). complexed with PAA. Clearly the degree of complexation increased as pH increased. Ternary Systems. Simultaneous Adsorption, (Cd-PAA)Al2O3. The effects of the presence of PAA can be seen by examining the Cd(II) adsorption (Figure 4). In comparison with the binary system, the adsorption in the presence of PAA shows vastly different trends. At acidic pH values, for all three PAA concentrations evaluated, the adsorption of Cd(II) onto Al2O3 is enhanced. At an approximate molar ratio of Cd(II):PAA of 1:1 (PAA ) 49 mg/L or 0.96 × 10-5 M), Cd(II) adsorption follows the trend of a metal-type adsorption; however, the adsorption is shifted toward a lower pH range by more than 1/2 of a pH unit. The Cd(II) adsorbed reached 100% at pH 6.5 and remained at this value. When the Cd(II)-PAA ratio was decreased to 1:2.4 (PAA ) 123 mg/L or 2.4 × 10-5 M) and 1:4.5 (PAA ) 230 mg/L or 4.5 × 10-5 M), Cd(II) adsorption also followed a metal-like adsorption trend at acidic pH values. The lower the ratio Cd(II):PAA (more PAA), the more Cd was adsorbed on the alumina in the lower pH region. Murphy and Zachara (30) observed enhanced metal uptake onto alumina at low pH with humic acid addition. Similarly, the presence of fulvic acid resulted in enhancement of Cd(II), Cu(II), and Pb(II) adsorption onto kaolinite below pH 5 (22). However, Lo et al. (31) observed either no effect or decreased adsorption of Cd(II) onto landfill soil samples in the presence of humic acid, depending on the humic acid concentration. As the humic acid level increased, the adsorption efficiency was significantly decreased from pH 4 to 8. However, at alkaline pH values (Figure 4), the higher the PAA concentration, the lower the amount of Cd(II) adsorbed and the pH at which the Cd(II) adsorption reached its maximum value decreased. At pH 9, Cd(II) adsorption decreased to 65% at 123 mg/L PAA and to 45% at 230 mg/L PAA. Comparable effects of humic substances on the sorption of Cu2+ onto alumina were observed by Davis (32), indicating similarities between humic substances and PAA with respect to their interactions in oxide-metal ion systems. The adsorption of Cu2+ onto goethite was increased at low pH in the presence of simple organic acids and decreased at pH > 5 at higher organic acid concentrations (33). However, Davis and Bhatnagar (34) noticed only the enhancement of Cd(II) adsorption onto hematite in the presence of humic acid. In a recent study (35), Th(IV) sorption to hematite was enhanced at low pH values in the presence of colloidal organic matter and the addition of HA enhanced U(VI) sorption relative to the HA-free system. Figure 5 shows the corresponding PAA adsorption in the ternary PAA-Cd-Al2O3 systems. The PAA adsorption displays the same trends as in the binary system, decreasing linearly with increasing pH. At 123 and 230 mg/L PAA adsorption was undistinguishable from that without Cd(II). However, at

FIGURE 5. Percent PAA adsorption on 2 g/L Al2O3 in the simultaneous adsorption (10-5 M Cd(II), I ) 5 × 10-3 M NaClO4). the lowest PAA concentration (49 mg/L), less PAA was adsorbed when the Cd(II) was present. Apparently, the Cd formed complexes with PAA that did not adsorb at low pH and may have blocked possible adsorption sites. Configuration changes may also be important. Sequential Adsorption, (Cd-Al2O3)-PAA; (PAA-Al2O3)-Cd. Figure 6 presents the adsorption of Cd(II) for the three different sequences of adsorption investigated. At 49 mg/L PAA the data indicate that exposure sequence had no influence on Cd adsorption. At 123 mg/L PAA there is the suggestion of a difference in the adsorption trend for the (Cd-Al2O3)-PAA adsorption sequence. However, these data always resulted from a major shift toward higher pH (>2 pH units) during the final equilibration, and points at lower pH were difficult to obtain. This same resistance to lower pH limited data collection for this sequence at 230 mg/L. In this sequence, added PAA complexes adsorbed Cd(II) and desorbs it from the surface. The surface responds by accepting H+ from solution, via the reverse of eq 1, raising the pH. Overall, it is concluded that the Cd(II) adsorption for the three sequences did not differ at low PAA levels, and (Cd-PAA)Al2O3 and (PAA-Al2O3)-Cd systems did not differ at high levels. Adsorption results independent of addition sequence were obtained in a study of the influence of fulvic acid on Cu, Cd, and Pb uptake by kaolinite (22). However, the fraction of nickel adsorbed to hydrous ferric oxide in the presence of EDTA was dependent on the component addition (36). Moreover, Davis and Bhatnagar (34) observed a general trend with respect to Cd(II) adsorption onto hematite in the presence of humic acid as being (Cd-hematite)-humics > (humics-Cd)-hematite > (humics-hematite)-Cd.

Discussion

FIGURE 6. Simultaneous and sequential adsorption of Cd(II) (%) onto Al2O3 in the presence of PAA at (a) 49 mg/L, (b) 123 mg/L, and (c) 230 mg/L PAA.

Cd(II) Adsorption. The uptake of Cd(II) by the alumina surface was modeled using the diffuse layer electrostatic model. Equation 3 (Table 1) was employed, where Al-OCd+ represents the Cd(II) adsorbed onto the surface, modeled as a monodentate inner-sphere surface complex. To best describe the data, the adsorption constants for the three Cd(II) concentrations were somewhat different (log KAlOCd+ ) -0.75 for 10-6 M, -1.5 for 10-5 M, and -1.9 for 10-4 M). With these constants, the single inner-sphere surface complex was adequate to represent the adsorption data. Modeling results are shown as the lines in Figure 1. The higher value of the 10-6 M Cd(II) constant indicates a greater affinity of the cadmium for the surface. This observation is consistent with a heterogeneous surface where the average site energy and, thus, the average sorption constant, decreases with increasing site coverage. Cowan et al. (37) found similar conclusions studying the adsorption of Cd(II) onto iron oxides.

PAA Adsorption. Polyanion adsorption depends strongly on electrostatic parameters such as the surface charge and the polymer charge, which can both depend on the pH and the ionic strength. However, it is known that polyanions adsorbed on an oppositely charged surface experience not only an electrostatic attraction to the surface but also an electrostatic repulsion within the polymer itself. The latter case takes place at high pH when ionization of carboxylic groups increases the repulsion among the polymer segment, leading to a stretch in length. Since PAA is a large polymer, one surface site complexes with only a fraction of the PAA molecule. It has been observed that compounds such as long-chain fatty acids may sorb onto oxides at least partially by surface complex formation (13). Moreover, studies of the adsorption of acrylic acid onto alumina have also suggested that hydrogen bonding between the protonated surface and carboxylic groups play a role in VOL. 35, NO. 2, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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the adsorption (11). This bonding changes with pH. Hydrophobic contributions to adsorption and stochastic issues of adsorption of multiple functional groups on a single molecule may also be important (38). Attempts were made to model PAA adsorption using simple surface complexation modeling. A reasonable fit to the data could be obtained considering that a fraction of the functional groups within a PAA molecule adsorbed on the alumina surface sites. However, this modeling did not consider all the other factors likely to be important to polymer adsorption. Cd-PAA Complexation. At pH 4, the percent of cadmium complexed with PAA increased from negligible to 80% as the PAA concentration increased, whereas at pH 7 the degree of cadmium complexation approached 100% over the entire range of PAA concentrations examined. Competitive binding of H+ decreases at higher pH, allowing a larger fraction of the cadmium to be complexed by PAA. Additionally, as a polymer, the PAA can form both monodentate and bidentate ligand complexes with Cd(II) (16). Ternary System. The addition of PAA, a strong complexing polymer, has a profound affect on Cd(II) adsorption onto alumina. The increase in Cd2+ adsorption in the low pH region due to the presence of PAA may result either through formation of Cd-PAA-oxide ternary surface complexes or from a reduction in positive surface charge in the low pH region due to adsorption of PAA anions, resulting in a more favorable electrostatic environment for Cd2+ adsorption. Decreased Cd2+ adsorption at high pH likely results from strong solution complexation between Cd2+ and PAA. Based on the binary data of Figure 1, at high pH values, the CdPAA complex is the only possible aqueous cadmium species in solution. Any noncomplexed Cd(II) should be adsorbed (or precipitated). As shown in Figure 5, with an increase in pH and PAA, more PAA remains soluble, corresponding to conditions where less Cd(II) is adsorbed. Similarly, the enhancement of Cd adsorption at low pH must be attributable to PAA on the alumina surface. In the ternary system, the mass balance for total cadmium at lower pH is

[CdT] ) [Cd(II)ads] + [Cd2+aq] + [Cd-PAAaq]

(5)

where [Cd(II)ads] is the overall cadmium adsorbed and is determined from the difference between the total cadmium concentration introduced in the system and the [Cd(II)aq] concentration in solution determined by AAS measurements:

[Cd(II)ads] ) [CdT] - [Cd(II)aq]

(6)

The [Cd-PAAaq] concentration in solution is obtained combining eqs 5 and 6:

[Cd-PAAaq] ) [Cd(II)aq] - [Cd2+aq]

(7)

Also, the overall cadmium adsorbed, [Cd(II)ads], is the sum of two components:

[Cd(II)ads] ) [Cd2+ads] + [Cd-PAAads]

(8)

[Cd2+ads] can be found based on modeling the cadmium adsorption in the binary system Cd(II)-Al2O3 using eq 3 (Table 1). Accordingly, [Cd-PAAads] can be obtained from eq 8. Figure 7 shows the Cd speciation according to eq 5 at pH < 7. For all PAA concentrations, a similar trend is apparent with increasing pH. Under the most acidic conditions, the Cd speciation is dominated by Cd2+aq. At increasing pH values, solution-phase Cd-PAAaq complexes become important, and all three species become of near equal importance. Finally, at the highest pH, both solution-phase species fall to zero as 352

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FIGURE 7. Cadmium speciation in the simultaneous adsorption (%) ([Cd(II)ads], [Cd-PAAaq], [Cd2+aq]) (a) 49 mg of PAA/L, (b) 123 mg of PAA/L, and (c) 230 mg of PAA/L. Cd adsorption onto the alumina dominates. This sequence occurs at lower pH for the higher PAA concentrations. At pH < 7, model results showed that [Cd2+ads] values were at least 2 orders of magnitude lower than [Cd(II)ads] concentrations (regardless of whether log KAlOCd ) - 0.75 or -1.5). Even from Figure 1 it is observed that Cd2+ adsorption is insignificant below pH 5.5. Therefore [Cd2+ads] concentrations are considered negligible; any enhancement in adsorption is caused by Cd-PAA ternary surface complexes. Recently, a few investigations have used surface complexation modeling (SCM) to describe interactions in ternary metal-ligand-surface environments. Scroth and Sposito (22) found that SCM failed to describe fulvic acid adsorption by kaolinite in a study of a metal-fulvic acid-kaolinite systems. Benhahya and Garnier (24) used the model and ternary surface complexes to describe the effects of salicylic acid on Cd(II) interactions with alumina, silica, and kaolinite; the

model described well the metal adsorption in the acidic pH range but failed to predict the decrease observed at alkaline pH. A simple SCM cannot adequately predict metal interactions in systems involving a complexing polymer ligand, such as PAA. Surface complexation modeling does not account for the macromolecular, multisite, and hydrophobic nature of the PAA polyelectrolyte. Modifications to include these other interactions are necessary to appropriately describe adsorption processes. Environmental Significance. The impact of PAA (as a surrogate for NOM) can significantly modify the mobility of metals in an aqueous environment. Under acidic conditions, ternary surface complexes are indicated, and an enhancement in metal adsorption is noted, lowering the metal mobility from that in the absence of polyelectrolyte. However, at higher PAA concentrations and at pH above 6, an increase in metal-PAA complexes in solution is found, suggesting greater mobilization of metal as metal-organic complexes. Cd(II) is bound to the PAA, either in solution or on the surface, over a wide pH range. These relationships should be considered in any efforts to model the interactions of cadmium in natural systems.

Acknowledgments Appreciation is extended to the Graduate School of the University of Maryland, College Park for the fellowship to RMF. We also thank the anonymous reviewers for their thoughtful comments.

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Received for review December 6, 1999. Revised manuscript received October 11, 2000. Accepted October 23, 2000. ES9913479

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