adsorption of pb(ii), nta, and pb(ii)–nta onto tio2

9
JOURNAL OF COLLOID AND INTERFACE SCIENCE 194, 59–67 (1997) ARTICLE NO. CS975071 Adsorption of Pb(II), NTA, and Pb(II) –NTA onto TiO 2 Muhammad S. Vohra and Allen P. Davis 1 Environmental Engineering Program, Department of Civil Engineering, University of Maryland, College Park, Maryland 20742 Received January 21, 1997; accepted July 9, 1997 sites (2). Toxic metal species extensively found at these Nitrilotriacetic acid ( NTA ) is extensively used in different in- waste sites can be complexed by organic chelates. This com- dustries because of its excellent chelating properties. Introduction plex formation can result in increased subsurface movement of NTA into the natural environment is a concern because of of heavy metal and radionuclide species, initially to the adja- mobilization of heavy metal species that may be otherwise bound cent and later to distant soil and ground waters. to natural particulate matter. The present study investigates the Additionally, NTA ( along with zeolite A and polycarbox- adsorption behavior of Pb ( II ) and NTA, both as individual species ylates) has been used as a substitute detergent for tripoly- and as complex species onto titanium dioxide. This adsorption phosphate because of eutrophication concerns in rivers and information is important in considering the TiO 2 -assisted photo- catalytic treatment of these metal – organic complexes. Pb ( II ) lakes (3). Other sources of metal–NTA contaminants in- shows a typical cationic type of adsorption behavior, whereas NTA clude water hardness control systems (4). Use of NTA on demonstrates an anionic type of adsorption trend. Results from a large scale is expected to result in mobilization and conse- stoichiometric ternary systems show a gradual increase in Pb ( II ) quently increased human uptake of toxic metal species that adsorption and a decrease in NTA removal with an increase in may normally be attached to the natural biota and particulate pH. However, for the cases of Pb(II) ú NTA, increased NTA material in the environment. Furthermore, attachment of adsorption as compared to pure NTA systems was noted even at NTA to the biological sludge during wastewater treatment higher pH. Model predictions employing MINTEQA2 software has also been reported (5). Land application of such a followed the experimental trends. Experimental and model results sludge, which may result in groundwater contamination and from ternary systems suggest adsorption of free Pb(II) and NTA, heavy metal mobilization, is an environmental concern. Nev- as well as ternary Ti–NTA–Pb(II) and Ti–O–Pb(II) –NTA 20 species. The cationic-type complexation, i.e., Ti–O–Pb(II)– ertheless, biodegradation of NTA in subsurface sandy soils NTA 20 , was essential for the successful NTA adsorption model- and in biological treatment plants, which may result in re- ing, especially at higher pH and for Pb ú NTA systems, where duced NTA concentrations, has also been described (6, 7). significant NTA adsorption was noted even at very high pH values. Lead has been extensively found both in soils and in Most of the previous metal – ligand adsorption studies did not con- groundwater at several DOE waste sites (2). Also, Pb(II) sider such a surface complexation. However, the present results is a common pollutant and is found in wastes from numerous indicate that any groundwater transport modeling of such pollut- different sources, including the plating and battery manufac- ants will require the inclusion of cationic-type surface complex- turing industries. Pb(II) that is present in the form of ation, in addition to other surface species. q 1997 Academic Press Pb(II) –NTA is not trivially eliminated from contaminated Key Words: adsorption; TiO 2 ; metal complexes; lead; NTA. waters. Generally, the complexed Pb ( II ) cannot be removed through traditional lead precipitation reactions. Recently, TiO 2 -mediated photocatalytic treatment of metal – chelate INTRODUCTION wastes has been reported (8–11). However, an understand- Nitrilotriacetic acid ( NTA ) has been widely used industri- ing of the interaction mechanisms between the metal–NTA ally because of its strong metal complexing properties. In complex and the TiO 2 surface is necessary to optimize the the nuclear industry, NTA and other chelating / complexing photocatalytic treatment process. Nevertheless, little infor- agents are used for decontamination ( cleaning process in- mation is available on NTA and metal–NTA complex ad- struments containing toxic metals and radionuclide species ) sorption onto hydrous oxide surfaces. and isotope extraction (1). Recently, concern has been ex- Chang et al. (12) studied the adsorption of several differ- pressed regarding the presence of such complexing com- ent chelating agents onto a-Fe 2 O 3 . NTA showed a typical pounds at several U.S. Department of Energy (DOE) waste anionic type of adsorption behavior, i.e., decreasing adsorp- tion with an increase in pH. Similar adsorption characteris- tics were noted by Elliott and Huang (13) for Cu(II) – 1 To whom correspondence should be addressed. Telephone: (301) 405- 1958; Fax: ( 301 ) 405-2585, E-mail: [email protected]. NTA– g-Al 2 O 3 . For an equimolar Cu–NTA system, de- 59 0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.

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Page 1: Adsorption of Pb(II), NTA, and Pb(II)–NTA onto TiO2

JOURNAL OF COLLOID AND INTERFACE SCIENCE 194, 59–67 (1997)ARTICLE NO. CS975071

Adsorption of Pb(II ) , NTA, and Pb(II ) –NTA onto TiO2

Muhammad S. Vohra and Allen P. Davis1

Environmental Engineering Program, Department of Civil Engineering, University of Maryland, College Park, Maryland 20742

Received January 21, 1997; accepted July 9, 1997

sites (2) . Toxic metal species extensively found at theseNitrilotriacetic acid (NTA) is extensively used in different in- waste sites can be complexed by organic chelates. This com-

dustries because of its excellent chelating properties. Introduction plex formation can result in increased subsurface movementof NTA into the natural environment is a concern because of of heavy metal and radionuclide species, initially to the adja-mobilization of heavy metal species that may be otherwise bound

cent and later to distant soil and ground waters.to natural particulate matter. The present study investigates theAdditionally, NTA (along with zeolite A and polycarbox-adsorption behavior of Pb(II) and NTA, both as individual species

ylates) has been used as a substitute detergent for tripoly-and as complex species onto titanium dioxide. This adsorptionphosphate because of eutrophication concerns in rivers andinformation is important in considering the TiO2-assisted photo-

catalytic treatment of these metal–organic complexes. Pb(II) lakes (3) . Other sources of metal–NTA contaminants in-shows a typical cationic type of adsorption behavior, whereas NTA clude water hardness control systems (4). Use of NTA ondemonstrates an anionic type of adsorption trend. Results from a large scale is expected to result in mobilization and conse-stoichiometric ternary systems show a gradual increase in Pb(II) quently increased human uptake of toxic metal species thatadsorption and a decrease in NTA removal with an increase in may normally be attached to the natural biota and particulatepH. However, for the cases of Pb(II) ú NTA, increased NTA

material in the environment. Furthermore, attachment ofadsorption as compared to pure NTA systems was noted even at

NTA to the biological sludge during wastewater treatmenthigher pH. Model predictions employing MINTEQA2 softwarehas also been reported (5) . Land application of such afollowed the experimental trends. Experimental and model resultssludge, which may result in groundwater contamination andfrom ternary systems suggest adsorption of free Pb(II) and NTA,heavy metal mobilization, is an environmental concern. Nev-as well as ternary Ti–NTA–Pb(II) and Ti–O–Pb(II) –NTA20

species. The cationic-type complexation, i.e., Ti–O–Pb(II) – ertheless, biodegradation of NTA in subsurface sandy soilsNTA20 , was essential for the successful NTA adsorption model- and in biological treatment plants, which may result in re-ing, especially at higher pH and for Pb ú NTA systems, where duced NTA concentrations, has also been described (6, 7) .significant NTA adsorption was noted even at very high pH values. Lead has been extensively found both in soils and inMost of the previous metal–ligand adsorption studies did not con- groundwater at several DOE waste sites (2) . Also, Pb(II)sider such a surface complexation. However, the present results

is a common pollutant and is found in wastes from numerousindicate that any groundwater transport modeling of such pollut-different sources, including the plating and battery manufac-ants will require the inclusion of cationic-type surface complex-turing industries. Pb(II) that is present in the form ofation, in addition to other surface species. q 1997 Academic Press

Pb(II) –NTA is not trivially eliminated from contaminatedKey Words: adsorption; TiO2; metal complexes; lead; NTA.waters. Generally, the complexed Pb(II) cannot be removedthrough traditional lead precipitation reactions. Recently,TiO2-mediated photocatalytic treatment of metal–chelateINTRODUCTIONwastes has been reported (8–11). However, an understand-

Nitrilotriacetic acid (NTA) has been widely used industri- ing of the interaction mechanisms between the metal–NTAally because of its strong metal complexing properties. In complex and the TiO2 surface is necessary to optimize thethe nuclear industry, NTA and other chelating/complexing photocatalytic treatment process. Nevertheless, little infor-agents are used for decontamination (cleaning process in- mation is available on NTA and metal–NTA complex ad-struments containing toxic metals and radionuclide species) sorption onto hydrous oxide surfaces.and isotope extraction (1) . Recently, concern has been ex- Chang et al. (12) studied the adsorption of several differ-pressed regarding the presence of such complexing com- ent chelating agents onto a-Fe2O3. NTA showed a typicalpounds at several U.S. Department of Energy (DOE) waste anionic type of adsorption behavior, i.e., decreasing adsorp-

tion with an increase in pH. Similar adsorption characteris-tics were noted by Elliott and Huang (13) for Cu(II) –1 To whom correspondence should be addressed. Telephone: (301) 405-

1958; Fax: (301) 405-2585, E-mail: [email protected]. NTA– g-Al2O3. For an equimolar Cu–NTA system, de-

59 0021-9797/97 $25.00Copyright q 1997 by Academic Press

All rights of reproduction in any form reserved.

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60 VOHRA AND DAVIS

TABLE 1 AdsorptionPhysicochemical Characteristics of Degussa (P25) TiO2 Adsorption at concentrations of 5 1 1006 to 1003 M for

Pb(II) , NTA, and Pb(II) –NTA onto 2 g/L TiO2 was deter-Characteristic Description Referencemined. The ionic strength was fixed at 5 1 1003 M using

Crystal structure 80% anatase (29) NaClO4. The solutions were purged with N2 for at least one20% rutile half hour before TiO2 addition to minimize dissolved CO2Isoelectric point 5.6 (30)

and O2. Samples of 100 mL of TiO2 suspension were trans-6.5 (16)ferred to 125 mL HDPE bottles which were covered withSpecific surface area 55 m2/ga (16)

56 m2/g (31) aluminum foil to minimize light exposure. The pH of theseSurface site density 2.2 1 1004 mol/gb (31) TiO2 suspension samples was adjusted from 2 to 10 with

4.2 1 1004 mol/g (31) dilute HClO4 or NaOH employing an Orion digital pH ana-2.74 1 1004 mol/ga (16)

lyzer/502. After shaking overnight at normal room tempera-ture, the final suspension pH was noted. The samples werea Used in the present study.

b Strong acid sites neutralized at NaOH activities below 1002 N. then filtered using 0.2 mm microfilters (Gelman); a smallinitial amount of the filtrate was rejected as a rinse. Thefiltered samples were acidified using HNO3.

creasing adsorption of both Cu and NTA species with in- A Perkin–Elmer atomic absorption spectrophotometercreasing pH and near equal adsorption of Cu and NTA spe- (AAS Model 5100ZL) was employed for lead analyses. Bothcies at pH ú 6 suggested the adsorption of the Cu–NTA flame and Zeeman graphite furnace methods were used. Forcomplex onto the Al2O3. Greater adsorption of divalent cop- the flame method, the optimum working range was 2.2 to 96per–aspartate species as compared to monovalent CuNTA0

mM. The graphite furnace was employed for lead detectionions suggested an electrostatic interaction between these between 0.05 and 0.5 mM. Indirect analysis of dissolvedcomplexes and the hydrous oxide surface. Swanson (14) NTA was completed employing a Shimadzu total organicalso noticed a transformation in Ni(II) adsorption behavior carbon (TOC) analyzer (Model 5000). A 2% standard devi-from cationic to anionic with addition of NTA, again indicat- ation was noted for detection of 0.5 ppm TOC samples: 0.5ing adsorption of the Ni(II) –NTA complex onto ppm constitutes 7% TOC for 1004 M NTA experiments andFe2O3rnH2O. Decreased Ni(II) adsorption with an increase 0.7% for 1003 M NTA studies. In the present work, onlyin the NTA concentration was also found. two data points were lower than 0.5 ppm (the last points

The present study investigates Pb(II) , NTA, and Pb(II) – both in Figs. 2 and 3b). At all pH values, the amount ofNTA complex adsorption onto TiO2. Both experimental and Pb(II) and NTA adsorbed onto TiO2 was calculated by sub-adsorption modeling results are discussed with Pb(II) and tracting suspension concentrations from the original concen-NTA present both at stoichiometric and nonstoichiometric trations. This adsorbed amount is presented as a percentageconcentrations. Few studies have attempted to describe and of the blank concentration (without TiO2) at specific pHmodel ternary metal–ligand–surface interactions at nonstoi- values.chiometric metal–ligand levels. Competition between sev-eral surface and solution complexation reactions which con- Modeling Approachtrol the fate of metal and ligand species have been described.

MINTEQA2/PRODEFA2, a geochemical speciationSpecific adsorption of these species onto TiO2 surface sitesmodel provided by the U.S. Environmental Protectionlikely affects the photocatalytic degradation process. Find-Agency, was used for adsorption modeling (15). The diffuseings from the present study will help in optimizing the photo-layer model (DLM) incorporated in MINTEQA2 was em-catalysis of such contaminants.ployed to model the adsorption of Pb(II) , NTA, andPb(II) –NTA onto the TiO2. Surface complexation reactions

MATERIALS AND METHODS used for modeling are given in Table 2. Intrinsic surfaceacidity constants for TiO2 surface speciation (using theDLM) are taken from Stone et al. (16). Important solutionChemical reagents used in this study include: NTA

(Na2HNTA, Aldrich) , Pb(ClO4)2r3H2O (EM Science), complexation reactions employed in the modeling are pre-sented in Table 3. FITEQL, a nonlinear least squares optimi-NaClO4rH2O (Fisher Scientific) , HClO4 (69–72%, Fisher

Scientific) , HNO3 (Fisher) , and NaOH (Baker) . Degussa zation software, was employed to estimate lead-only andNTA-only adsorption constants.P25 TiO2, widely used in photocatalytic studies, was used

as the adsorbent. Several physical and chemical properties With respect to reaction 13, the lead hydroxide precipita-tion reaction, higher Pb(II) solubility constants for freshof P25 are given in Table 1. Deionized water from a Hydro-

Service reverse osmosis/ ion exchange system (Model ( log KsoÅ 014.93) as compared to aged (log Kso Å019.85)solution precipitates have been proposed (17). For the pres-2PRO-20) was used in all experiments.

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61ADSORPTION OF Pb(II) , NTA, AND Pb(II) –NTA

TABLE 2TiO2 Surface Complexation Reactions used in AdsorptionModeling Employing the Diffuse Layer Model (I Å 0 M )

logNo. Reactions Ks

int

1 Ti–OH / H/} Ti–OH/

2 3.9a

2 Ti–OH } Ti–O0 / H/ 08.7a

3 Ti–OH / Pb2/} Ti–O–Pb/ / H/ 1.2b

4 Ti–OH / NTA30 / H/} Ti–NTA20 / H2O 14.43b

5 Ti–OH / Pb2/ / NTA30} Ti–O–Pb–NTA20 / H/ 7.7b

6 Ti–OH / Pb2/ / NTA30} Ti–NTA–Pb / OH0 19.46b

a (16).b Present study.

FIG. 1. Pb(II) adsorption onto TiO2 (2 g/L TiO2; ionic strength Å 51 1003 M NaClO4rH2O). Lines represent model fit.ent Pb(II) –NTA–TiO2 system, good Pb(II) adsorption

modeling results were obtained considering a Pb(OH)2 solu-bility constant intermediate to the fresh and aged samples

concentrations, i.e., 1.1 1 1005 and 1.1 1 1004 M, a sharp(log Kso Å 016.85).adsorption edge similar to those found at lower Pb(II) levelsin the present study, were noticed.RESULTS AND DISCUSSION

For modeling Pb(II) adsorption, a monodentate inner-sphere-type complexation between the Pb(II) species andPb(II) AdsorptionTiO2 surface sites was considered (Eq. [3] , Table 2). Spe-

Results for Pb(II) adsorption are presented in Fig. 1; cies other than monodentate complexes, as mentioned bytypical cation adsorption behavior is observed. For 1004 M Stumm (19), are also possible and may produce better modellead, 100% adsorption is achieved at pH near 5.5 with the fits at the expense of a more complex model with a greateradsorption edge occurring between pH 3.5 and 5.5. For the number of fitting parameters. However, this study presents5 1 1006 M Pb(II) system, the adsorption edge is approxi- a simple model which can predict the adsorption trends, ifmately between pH 3 and 5. However, experiments con- not exactly. Modeling results are represented by solid linesducted at 1003 M Pb(II) show a gradual increase in lead in Fig. 1 with log K s

int Å 1.2. At 1004 and 5 1 1006 Mremoval, beginning around pH 4, with complete removal Pb(II) concentrations, only specific adsorption was pre-occurring only at pH above 9.5. James and Healy (18) re- dicted to cause Pb(II) removal since adequate TiO2 surfaceported a broad adsorption edge for 1.1 1 1003 M Co(II) sites are available for the adsorption (5.5 1 1004 mol/L).adsorption onto TiO2 (rutile) . However, at lower Co(II) However, in the case of 1003 M Pb(II) , both adsorption

and precipitation are predicted to cause metal removal fromTABLE 3

Important Solution Speciation Reactions (T Å 257C, I Å 0 M )

No. Reactions log K

7 NTA30 / 4 H/} H4NTA/ 17.394a

8 NTA30 / 3 H/} H3NTA 16.394a

9 NTA30 / 2 H/} H2NTA0 13.274a

10 NTA30 / H/} HNTA20 10.334a

11 Pb2/ / NTA30} PbNTA0 12.690a

12 Pb2/ / H/ / NTA30} PbHNTA 15.259a

13 Pb(OH)2(s) } Pb2/ / 2 OH0 016.850b

14 Pb2/ / H2O } PbOH/ / H/ 07.677a

15 Pb2/ / 2 H2O } Pb(OH)2 / 2 H/ 017.120a

16 Pb2/ / 3 H2O } Pb(OH)03 / 3 H/ 028.027a

17 2 Pb2/ / H2O } Pb2(OH)3/ / H/ 06.062a

18 3 Pb2/ / 4 H2O } Pb3(OH)2/4 / 4 H/ 023.748a

19 Pb2/ / 4 H2O } Pb(OH)204 / 4 H/ 039.567a

FIG. 2. NTA adsorption onto TiO2 (2 g/L TiO2; ionic strength Å 5 1a MINTEQA2.b Modified—see text. 1003 M NaClO4rH2O). Lines represent model fit.

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62 VOHRA AND DAVIS

FIG. 3. Stoichiometric Pb(II) –NTA systems. (a) 1003 M Pb(II) –NTA adsorption onto TiO2 (2 g/L TiO2; ionic strength Å 5 1 1003 MNaClO4rH2O). Lines represent model fit. (b) 1004 M Pb(II) –NTA adsorption onto TiO2 (2 g/L TiO2; ionic strength Å 5 1 1003 M NaClO4rH2O).Lines represent model fit. (c) 1004 M Pb(II) –NTA solution speciation diagram (ionic strength Å 5 1 1003 M NaClO4rH2O).

solution at higher pH; e.g., 28% adsorption and 72% precipi- primary precipitates formed in aqueous lead systems and didnot find Pb(OH)2(s) precipitate under normal temperaturetation were predicted at pH 9.5.

Metal removal higher than the available surface sites can and pressure conditions, even when specific methods wereemployed to exclude CO2. Nevertheless, consideration ofbe attributed to surface precipitation and/or surface polymer-

ization (19–22). Therefore, it can be proposed that initially, Pb(OH)2(s) precipitation for studies where lead was presentat levels greater than NTA resulted in close model fits towhen sufficient TiO2 surface sites are available, adsorption

as represented by Eq. [3] is the dominant Pb(II) removal experimental data as discussed later. Also with these experi-ments, dissolved CO2 should be very low due to N2 purging.process. However, as the number of surface sites starts be-

coming limited, adsorption and polymerization/surface Consideration of specific surface precipitation/polymeriza-tion reactions in the model, as discussed by Katz and HayesPb(OH)2(s) precipitation causes Pb(II) removal. As condi-

tions are altered to strongly favor precipitation, any further (21, 22), would likely produce model agreement at 1003 MPb(II) .lead removal occurs because of surface/bulk Pb(OH)2(s)

precipitation. Therefore in the modeling process, Pb(OH)2(s)NTA Adsorptionwas defined as the primary precipitate (Eq. [13], MIN-

TEQ2A). In contrast, experiments by Marani et al. (23) Experimental and model results for 1003 , 5 1 1004 , and1004 M NTA adsorption are presented in Fig. 2. The amounthave indicated that PbCO3(s) and Pb3(CO3)2(OH)2(s) are the

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63ADSORPTION OF Pb(II) , NTA, AND Pb(II) –NTA

of NTA adsorbed decreases with an increase in the suspen- employed to model the ternary systems. For the Ti–NTA–sion pH. NTA adsorption seems to plateau around pH 2, Pb species, log K s

int equal to 19.46 was used (Eq. [6]) .whereas at pH above 7, insignificant NTA adsorption is Additionally, a cationic type of surface complexation, i.e.,noted. At 1003 M NTA, a maximum adsorption of 14% was Ti–O–Pb–NTA20 (Eq. [5] , log K s

intÅ 7.7) was also neces-observed. Reducing the NTA concentration by half (5 1 sary. This surface complexation was added to explain NTA1004 M) results in a twofold increase in percentage adsorp- adsorption at high pH, e.g., 3.5% NTA adsorption at pH 9.3tion. For 1004 M NTA a maximum of 86% adsorption is for the 1003 M Pb(II) –NTA study (Fig. 3a) . NTA adsorp-achieved at pH 3.2 because of the comparatively high num- tion at high pH was not noticed for NTA-only studies (Fig.ber of surface sites available at the lower NTA concentration. 2); in fact, insignificant NTA adsorption was noticed aboveDecreasing NTA adsorption onto g-Al2O3 with increasing pH 7. Also, NTA adsorption modeling for nonstoichiometricpH was also noted by Elliott and Huang (13). Chang et al. Pb(II) –NTA studies (as will be presented later) required(12) also noticed a typical anionic-type adsorption behavior the introduction of Ti–O–Pb–NTA20 complexation. Re-for NTA onto a-Fe2O3. cently, Nowack et al. (25) also reported that successful mod-

For modeling NTA adsorption, a monodentate inner eling of Fe(III) –EDTA adsorption onto hydrous ferric oxidesphere complexation between NTA30 and neutral TiO2 sur- and goethite requires both anionic- and cationic-type interac-face sites was considered (Eq. [4] , Table 2). Surface com- tions.plexation reactions 3 and 4, are identical in form to those Model predictions, as given in Figs. 3a and 3b, follow theemployed by Bryce et al. (24) for Ni/EDTA adsorption experimental trends, i.e., increasing Pb(II) and decreasingonto hydrous ferric oxide. Formation of Ti–NTA20 surface NTA adsorption as pH increases. Table 4 presents modelspecies and a log K s

int value equal to 14.43 provided model estimations for the different predicted species between pHfits which follow the experimental trends (Fig. 2) . 2 and 10. Generally, Ti–O–Pb/ and Ti–NTA20 were pre-

dicted as the dominant adsorbed species. However, notice-able concentrations of Ti–NTA–Pb and Ti–O–Pb–NTA20

Pb(II) –NTA Adsorptionsurface complexes were also predicted at low and high pH,

Equimolar systems. Figures 3a and 3b show results for respectively.1003 and 1004 M Pb(II) –NTA systems, respectively. The Both experimental and modeling results present very dif-adsorption of Pb(II) increases with increasing pH, whereas ferent adsorption trends for Pb(II) –NTA–TiO2 systems asthat of NTA decreases. Both changes are gradual and occur compared to other metal–NTA adsorption studies. In bothover a wide pH range. Additionally, Fig. 3c provides aqueous systems, Pb(II) and NTA removals converge approximatelyspeciation for a 1004 Pb(II) –NTA system without TiO2. up to pH 4 and diverge with a further increase in pH. Also

For 1003 M Pb(II) –NTA at pH 2.1, 12 and 16% Pb(II) between pH 4 and 6 a very gradual change in Pb(II) removaland NTA adsorption, respectively, are noted. However, neg- is noted, whereas above pH 6 Pb(II) removal increases sig-ligible Pb2/ adsorption occurs at such low pH (1003 M nificantly. Elliott and Huang (13) noticed decreasing adsorp-Pb(II) , Fig. 1) , suggesting that in the presence of NTA, a tion of both Cu and NTA onto g-Al2O3 with an increase inligand-type complexation, e.g., Ti–NTA–Pb results, pro- the solution pH in equimolar Cu/NTA systems. Also, forducing lead adsorption at acidic pH. Formation of such a pH § 6, near equal adsorption of both species was noticed.ligand-type surface complexation should decrease with in- These observations suggest adsorption of the Cu–NTA0

creasing pH and consequently reduced adsorption of both complex. In the case of TiO2, the surface either breaks thelead and NTA should result. However, experimental results Pb(II) –NTA complex or selectively competes for Pb(II)show increasing lead removal with an increase in pH; for or NTA adsorption versus the aqueous Pb(II) –NTA com-1003 M Pb(II) –NTA, up to 40% lead was adsorbed at high plexation. However, the g-Al2O3 surface may not introducepH. Similar, albeit more pronounced, results are noted for such a phenomenon, thus resulting in Cu(II) –NTA adsorp-the 1004 M Pb(II) –NTA system, where 10 and 96% Pb(II) tion as a unit. Swanson (14) also noticed anionic-type Niremoval is noted at pH 2.1 and 9, respectively. Additionally adsorption behavior onto Fe2O3rnH2O in the presence offor 1004 M Pb(II) –NTA, 91% NTA removal as compared NTA. Furthermore, the present adsorption results for Pb andto 10% Pb(II) adsorption at pH 2.1 indicates that free NTA NTA also directly contrast with data from other researchersalso adsorbs. Comparing Figs. 3a and 3b, much higher free who noticed only ligand-type metal-complex adsorption forNTA adsorption is noted for 1004 M Pb(II) –NTA compared Ni(II) –EDTA onto g-Al2O3 (26) and Co(II) –EDTA ontoto the 1003 M Pb(II) –NTA study. In the case of 1003 M d-Al2O3 (27). Our work with TiO2 and equimolar Pb(II) –Pb(II) –NTA, increased competitive adsorption of NTA and EDTA also indicates ligand-type adsorption of the Pb(II) –Pb(II) –NTA species results in lower free NTA adsorption. EDTA complex; experimental and model results with non-

Adsorption modeling results are also presented in Figs. stoichiometric systems suggest the formation of an addi-3a and 3b. Surface complexation constants for Pb(II) and tional Ti–O–Pb–EDTA30 species (28). These observations

indicate that Pb(II) –NTA adsorption onto TiO2 is a muchNTA as found previously in the binary studies have been

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64 VOHRA AND DAVIS

TABLE 4Predicted Adsorbed Species (%) for 1003 and 1004 M Pb(II)–NTA Studies (MINTEQA2)

2 4 6 8 10pH

Pb(II)–NTA (M) 1003 1004 1003 1004 1003 1004 1003 1004 1003 1004

Ti–O–Pb/ 0.2 0.3 14.0 60.9 18.3 59.6 18.3 61.3 24.4 95.3Ti–NTA20 14.8 97.8 11.0 64.5 9.2 32.3 5.1 14.3 0.1 0.03Ti–NTA–Pb 4.6 0.1 2.8 1.9 0.04 0.03 õ0.01 õ0.01 õ0.01 õ0.01Ti–O–Pb–NTA20 õ0.01 õ0.01 õ0.01 õ0.01 0.07 0.04 2.7 1.0 3.1 0.02Total NTAads 19.4 97.9 13.8 66.4 9.3 32.4 7.8 15.3 3.2 0.05Total Pbads 4.8 0.4 16.8 62.8 18.4 59.7 21.0 62.3 27.5 95.3

more complex process than these other systems, apparently ering the Ti–O–Pb–NTA20 species demonstrated littleNTA adsorption at higher pH. However, consideration ofdue to the relative values of the surface and solution com-

plexation constants. Ti–O–Pb–NTA20 species resulted in improved predictionsas shown in Figs. 4 and 5. Table 5, which presents the modelPb(II)ú NTA. Adsorption results for 1003 Pb(II)/1004

predictions for the various surface species, shows that NTAM NTA and 5 1 1004 Pb(II)/1004 M NTA are given inadsorption modeling requires Ti–O–Pb–NTA20 surfaceFigs. 4 and 5, respectively. Though most of the Pb(II) iscomplexation, especially at higher pH where it representsremoved at high pH (through adsorption/precipitation), ap-all of the adsorbed NTA. The modeling results also showproximately 5 1 1005 M (5%) dissolved Pb(II) remains inthe decrease followed by the increase in NTA adsorptionthe former system and 3.5 1 1005 M (7%) in the latter.from pH 2 to 6, as the dominant adsorbed NTA speciesModel estimations using parameters determined in the previ-shifts from Ti–NTA20 to Ti–O–Pb–NTA20 . Formation ofous analysis for Pb(II) removal agree with the experimentalPb(OH)2(s) at higher pH and consequent adsorption of NTAresults. Significant Pb(OH)2(s) precipitation is predicted, i.e.,onto such precipitates are also possible. However, for the60% at pH 10.1, 1003 M Pb(II) and 23% at pH 10.6, 5 11003 M Pb(II) –NTA study (Fig. 3a) approximately 3.5%1004 M Pb(II) .ligand adsorption was noted at pH 9.3, and for 5 1 1004

In both systems, approximately 85% NTA adsorption isPb(II) /1003 M NTA (Fig. 6) approximately 4% NTA ad-observed at lower pH. Experimental results also show asorption was noted at pH 9.5, whereas for both the systemsslight decrease followed by an increase in NTA adsorptionno Pb(OH)2(s) formation was predicted even at increasedbetween pH 2 to 4 and 4 to 6, respectively (Figs. 4 andpH (Tables 4 and 6). Hence, interactions between NTA and5). Additionally, an exceptionally high NTA adsorption isfresh lead/hydroxide precipitates may be possible, but willnoticed even at pH up to 10, which was not observed fornot explain NTA removal at higher pH as noted for thesethe 1004 M NTA-only studies (Fig. 2) . Model results assystems.described below followed these experimental trends.

Initial modeling results for these systems without consid-

FIG. 5. 5 1 1004 Pb(II)/1004 M NTA adsorption onto TiO2 (2 g/LTiO2; ionic strength Å 5 1 1003 M NaClO4rH2O). Lines represent modelFIG. 4. 1003 Pb(II)/1004 M NTA adsorption onto TiO2 (2 g/L TiO2;

ionic strength Å 5 1 1003 M NaClO4rH2O). Lines represent model fit. fit.

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65ADSORPTION OF Pb(II) , NTA, AND Pb(II) –NTA

TABLE 5Predicted Adsorbed and Precipitated Species (%) for 1003/5 1 1004 M Pb(II) and 1004 M NTA Studies (MINTEQA2)

2 4 6 8 10pH

Pb(II) (M) 1003 5 1 1004 1003 5 1 1004 1003 5 1 1004 1003 5 1 1004 1003 5 1 1004

Ti–O–Pb/ a 0.26 0.26 21.4 37.4 36 62.8 35.1 70.2 29.9 59.8Ti–NTA20 b 96.2 97 85.9 81.3 1.6 8.2 õ0.01 õ0.01 õ0.01 õ0.01Ti–NTA–Pbb 0.97 0.6 0.57 0.9 õ0.01 õ0.01 õ0.01 õ0.01 õ0.01 õ0.01Ti–O–Pb–NTA20 b õ0.01 õ0.01 0.77 0.3 96.7 87.9 96.7 96.7 47.8 47.8Pb(OH)2(s)

a õ0.01 õ0.01 õ0.01 õ0.01 õ0.01 õ0.01 49.7 õ0.01 59.9 19.9Total NTAads

b 97.2 97.6 87.3 82.5 98.2 96.1 96.7 96.7 47.8 47.8Total Pbads

a 0.4 0.4 21.5 37.7 45.6 80.4 44.8 89.5 34.7 69.4

a Percentage of the total Pb(II) concentration.b Percentage of the total NTA concentration.

NTA ú Pb(II) . Results for 5 1 1004 Pb(II)/1003 M Pb(II) –NTA species are also given in Figs. 6 and 7. Forboth studies, experimental and model results indicate a grad-NTA and 1004 Pb(II)/1003 M NTA are reported in Figs. 6

and 7, respectively. Similar to the stoichiometric systems, ual increase in Pb(II) removal in the intermediate pH rangedue to the formation of Ti–O–Pb/ species (Table 6). Corre-decreasing NTA and increasing Pb(II) adsorption is noted

with increasing pH. At the lowest experimental pH, maxima spondingly, the model predicts a gradual decrease in dis-solved Pb(II) –NTA over the same pH range. Model predic-of 15–16% NTA adsorption is found in both systems. The

lead adsorption increases with an increase in pH; however, tions as given in Table 6 show that above pH 4 adsorptionof free lead and free NTA are the dominant removal mecha-complete lead removal is not observed within the experimen-

tal pH range. The NTA nearly flat adsorption edge for both nisms. At pH below 4, significant individual Pb(II)/NTAadsorption and Ti–NTA–Pb complex formation results.systems covers the pH range of 2 to 9.

Figures 6 and 7 also present the modeled adsorption re- Small amounts of Ti–O–Pb–NTA20 complexation werealso predicted at higher pH.sults. For both systems, significant Pb(II) adsorption is no-

ticed even at acidic pH which does not agree with trends Comparing Figs. 3b (1004 M Pb(II) –NTA) and 7 (1004

Pb(II)/1003 M NTA) also indicates that at the constantfound from Pb(II)-only adsorption studies. This is expectedto result because of Ti–NTA–Pb complex formation. Ac- (1004 M) lead concentration, an increase in NTA concentra-

tion from 1004 to 1003 M results in significantly decreasedcordingly, Ti–NTA–Pb predictions at pH 2.2 in both sys-tems, i.e., 8–9%, matched the experimental Pb(II) removal. Pb(II) adsorption in the intermediate pH range. Similarly,

Elliott and Huang (13) found decreased Cu(II) adsorptionOverall, for 5 1 1004 Pb(II)/1003 M NTA, good Pb(II)adsorption predictions result up to pH 7. However, at higher onto g-Al2O3 at NTA concentrations higher than Cu(II) .

These observations suggest that at higher NTA concentra-pH, significant underestimation of Pb(II) adsorption occurs.Similar disagreement was noticed for the 1004 Pb(II)/1003 tion, aqueous metal–NTA complexation competitively dom-

inates the adsorption of metal species, as shown in Fig. 7.M NTA study. Pb(II) precipitation was not predicted ineither of these situations. Model results for dissolved This does not occur at 1004 M Pb(II) –NTA because the

TABLE 6Predicted Adsorbed Species (%) for 5 1 1004/1004 M Pb(II) and 1003 M NTA Studies (MINTEQA2)

2 4 6 8 10pH

Pb(II) (M) 5 1 1004 1004 5 1 1004 1004 5 1 1004 1004 5 1 1004 1004 5 1 1004 1004

Ti–O–Pb/ a 0.3 0.3 4.5 2.8 17.5 21.9 19.1 28.9 36.1 74.1Ti–NTA20 b 15.5 16.2 12 12.9 5.4 3.1 4.1 1.7 0.1 0.02Ti–NTA–Pba 5.1 5.6 3 2.8 0.1 0.07 õ0.01 õ0.01 õ0.01 õ0.01Ti–O–Pb–NTA20 a õ0.01 õ0.01 õ0.01 õ0.01 õ0.01 õ0.01 0.54 0.12 1.2 0.08Total NTAads

b 18.1 16.8 13.5 13.2 5.4 3.1 4.4 1.7 0.7 0.02Total Pbads

a 5.4 5.9 7.5 5.6 17.6 22 19.6 29 37.3 74.2

a Percentage of the total Pb(II) concentration.b Percentage of the total NTA concentration.

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66 VOHRA AND DAVIS

NTA concentration (and driving force for solution complex-ation) is an order of magnitude less. Also, as shown in Fig.7, at higher pH Ti–O–Pb/ complex formation is favoredover solution Pb(II) –NTA species, resulting in increasedPb(II) adsorption.

CONCLUSIONS

The adsorption of Pb(II) , NTA, and the Pb(II) –NTAcomplex onto TiO2 has been investigated employing bothexperimental and modeling approaches. Pb(II) shows a typi-cal cationic type of adsorption. Model fits were obtainedusing the geochemical speciation software MINTEQA2 em-ploying a diffuse layer surface complexation model. In thecase of 1003 M Pb(II) , significant metal precipitation is FIG. 7. 1004 Pb(II)/1003 M NTA adsorption onto TiO2 (2 g/L TiO2;predicted at higher pH. NTA shows an anionic type of ad- ionic strength Å 5 1 1003 M NaClO4rH2O). Lines represent model fit.sorption behavior, i.e., decreasing adsorption with increasingpH, with no adsorption above pH 6–7. In ternary systems,

increases. Further increase in pH results in gradually reducedadsorption of free Pb(II) and NTA as well as the Pb(II) –adsorption although a significant amount of NTA remainsNTA complex, i.e., Ti–NTA–Pb, was expected and consid-adsorbed even at very high pH. Such a trend was not notedered in the modeling. NTA adsorption modeling also re-in the cases of NTA alone. These results were explained byquired a cationic ternary type of interaction, i.e., Ti–O–Pb–change in the dominance of the adsorbed NTA from Ti–NTA20 . For stoichiometric Pb(II) –NTA systems, increas-NTA20 to Ti–O–Pb–NTA20 under these experimental con-ing Pb(II) and decreasing NTA adsorption is noticed withditions. Hence results from the present study show that aque-an increasing pH, both experimentally and based on modelous transport modeling of metal–ligand complexes will re-predictions. An increase in the NTA concentration from 1004

quire consideration of cationic-type surface complexes into 1003 M, keeping total Pb(II) constant at 1004 M, resultedaddition to the conventional ligand-type surface species toin decreased Pb(II) adsorption at intermediate pH. The ther-correctly predict the movement of such pollutants in themodynamic favorableness of solution Pb(II) –NTA speciessubsurface environment.formation over the surface Ti–O–Pb/ complex at high NTA

concentration was suggested as the likely reason for thistrend. Model predictions agree with this observation. ACKNOWLEDGMENTS

In the experimental cases of excess lead, i.e., 1003 Pb(II)/This study was supported by the National Science Foundation through1004 M NTA and 5 1 1004 Pb(II)/1004 M NTA, a unique

Grant BCS-9358209. Thanks goes to the Degussa Company for providingNTA adsorption behavior was noticed. Between pH 2 andthe TiO2 sample and Shalini Jayasundera for TOC analysis.4, adsorption decreases whereas between pH 4 and 6 it again

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