adsorption of pb and cd onto metal oxides

8/8/2019 Adsorption of Pb and CD Onto Metal Oxides

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ADSORPTION OF Pb AND Cd ONTO METAL OXIDESAND ORGANIC MATERIAL IN NATURAL SURFACE

COATINGS AS DETERMINED BY SELECTIVEEXTRACTIONS: NEW EVIDENCE FOR THE IMPORTANCE

OF Mn AND Fe OXIDES

DEMING DONG 1 , YARROW M. NELSON 2 , LEONARD W. LION 2 *, MICHAELL. SHULER 3 and WILLIAM C. GHIORSE 4

1 Department of Environmental Science, Jilin University, Changchun 130023, People's Republic of China; 2 School of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA;

3 School of Chemical Engineering, Cornell University, Ithaca, NY 14853, USA and 4 Section of Microbiology, Cornell University, Ithaca, NY 14853, USA

(First received 1 November 1998; accepted in revised form 1 April 1999)

Abstract Surface coatings (biolms and associated minerals) were collected on glass slides in the oxicsurface waters of Cayuga Lake (New York State, U.S.A.) and were used to evaluate the relativecontributions of Fe, Mn and Al oxides and organic material to total observed Pb and Cd adsorptionby the surface coating materials. Several alternative selective extraction techniques were evaluated withrespect to both selectivity and alteration of the residual unextracted material. Pb and Cd adsorptionwas measured under controlled laboratory conditions (mineral salts solution with dened metalspeciation, ionic strength 0.05 M, 25 8C and pH 6.0) before and after extractions to determine bydi erence the adsorptive properties of the extracted component(s). Hydroxylamine hydrochloride(0.01 M NH 2 OH HCl+0.01 M HNO 3 ) was used to selectively remove Mn oxides, sodium dithionite(0.3 M Na 2 S2 O 4 ) was used to remove Mn and Fe oxides, and 10% oxalic acid was used to removemetal oxides and organic materials. Several other extractants were evaluated, but preliminaryexperiments indicated that they were not suitable for these experiments because of undesirablealterations of the residual, unextracted material. The selected extraction methods removed targetcomponents with e ciencies between 71 and 83%, but signicant amounts of metal oxides and organicmaterials other than the target components were also removed by the extractants (up to 39%).Nonlinear regression analysis of the observed Pb and Cd adsorption based on the assumption of additive Langmuir adsorption isotherms was used to estimate the relative contributions of each surfacecoating constituent to total Pb and Cd binding of the biolms. Adsorption of Cd to the lake biolmswas dominated by Fe oxides, with lesser roles attributed to adsorption by Mn and Al oxides andorganic material. Adsorption of Pb was dominated by Mn oxides, with lesser roles indicated foradsorption to Fe oxides and organic material, and the estimated contribution of Al oxides to Pbadsorption was insignicant. The tted Pb adsorption isotherm for Fe oxides was in excellentagreement with those obtained through direct experiments and reported in independent investigations.The estimated Pb distribution between surface coating components also agreed well with thatpreviously predicted by an additive adsorption model based on Pb adsorption isotherms for laboratorysurrogates for Mn, Fe and Al oxides and dened biological components. # 1999 Elsevier Science Ltd.All rights reserved

Key words selective extraction, adsorption, lead, cadmium, iron oxide, manganese oxide

INTRODUCTION

The toxicity and bioaccumulation potential of heavy metals has prompted great interest in devel-oping models to describe their transport and fate inaquatic environments. Development of meaningfulmodels for trace metal phase distribution requires

an understanding of trace metal adsorption ontosolid phases and associated biolms, which is a keyfactor inuencing the residence time, bioavailabilityand e ects of toxic metals on organisms in aquaticecosystems (Krauskopf, 1956; Jenne, 1968;Turekian, 1977; Vuceta and Morgan, 1978; Murray,1987; Santschi et al. , 1997). In addition to the wellestablished e ects of solution chemistry (e.g. pH,ionic strength, metal speciation), trace metaladsorption is expected to be governed by the com-

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*Author to whom all correspondence should be addressed.Tel.: +1-607-255-7571; fax: +1-607-255-9004; e-mail:[email protected]


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position of the solid phase, particularly the contentof metal oxides and organic materials. Studies havebeen undertaken to quantify the relative roles of these components in controlling the adsorption of transition metals to surfaces in natural lake waters(Sigg, 1985), lake sediments (Tessier and Campbell,1987) and biolms (Nelson et al. , 1995). However,there remains some uncertainty about the roles of metal oxides vs organic materials in controlling theadsorption of trace metals to natural heterogeneousmaterials. Indeed, some researchers report thatmetal oxides are the single most important determi-nant of trace metal adsorption (Krauskopf, 1956;Jenne, 1968), while others report that organic ma-terials exert a stronger e ect (Balistrieri andMurray, 1983; Salim, 1983; Sigg, 1985). Addi-tionally, interactions between constituents couldalter the metal adsorption properties of these con-stituents in a heterogeneous matrix (Davis andLeckie, 1978; Balistrieri and Murray, 1982; Tippingand Cooke, 1982; Honeyman and Santschi, 1988).

The purpose of the research presented here is to usea new selective extraction approach to carefully elu-cidate the relative roles of metal oxides and organicmaterials. The resulting information is expected tofacilitate the development of trace metal adsorptionand transport models.

The use of selective extractants is a usefulapproach for determining the relative signicance of the mineral and organic components in controllingtrace metal adsorption. Extractants have previouslybeen used to dissolve metal oxide or organic com-ponents in sediments and soils along with the tracemetals associated with these components (Lindsayand Norvell, 1978; Tessier et al. , 1979; Lion et al. ,1982; Bauer and Kheboian, 1986; Martin et al. ,1987; Tessier and Campbell, 1987; Luoma, 1989;Campbell and Tessier, 1991; Young et al. , 1992;Young and Harvey, 1992). While useful for estimat-ing trace metal bioavailability, selective extractionmethods are di cult to use for accurately quantify-ing trace metals associated with specic biogeo-chemical phases because the extracted phases areoperationally dened and are subject to experimen-tal limitations such as removal of additional ma-terials besides the target component duringextraction. For example, when hydroxylamine hy-drochloride (NH 2 OH HCL+HNO 3 ) is used to

extract Mn oxides, the extraction reagent is likelyto also extract some fraction of other components,such as other metal oxides and organic materials.Another limitation is that extractants can poten-tially desorb trace metals from other componentsthat were not extracted, which would lead to anoverestimation of trace metal associated withthe target component. For example, whenNH 2 OH HCL+HNO 3 is used to extract Mn oxidesand associated trace metals, the extraction reagentmay also desorb trace metals from other surfacecomponents, such as Fe oxides. Yet another possi-

bility is that metals extracted from one solid phasemay readsorb to unextracted materials, whichwould lead to an underestimation of the importanceof the extracted component.

In the present work, trace metal adsorption wasmeasured for residues before and after selectiveextraction to avoid problems associated with de-

sorption and/or readsorption of metals from othercomponents. By determining metal adsorption iso-therms for composite surface coatings before andafter extraction, the adsorptive role of the removedcomponent(s) was revealed by di erence. The selec-tivities of the extractants were determined bymeasuring Fe, Mn and Al concentrations andchemical oxygen demand (COD) before and aftereach extraction. Since standard extractants werefound to remove signicant quantities of non-targetcomponents, non-linear regression analysis of theadsorption isotherm data was used to determine theadsorptive contribution of each surface phase. This

approach does not identify phase associations of contaminant metals already present on natural ma-terials collected from the eld because it relies onmeasuring adsorption of trace metals from denedsolutions before and after extraction. Instead, thismethod provides an alternative means for estimat-ing the reactive roles of metal oxide and organicphases in controlling trace metal adsorption infreshwater environments. In this way, this workcontributes to the mechanistic understanding of trace metal associations with adsorptive com-ponents of the heterogeneous surfaces in naturalaquatic environments.

For the experiments described here, natural bio-lms that developed on glass slides in oxic lakewaters were used to represent typical lake surfacecoating materials. It is expected that these surfacecoatings may also be representative of the materialscontained in suspended particulate material (SPM)given their morphological and compositional simi-larities. Indeed, the biolms in this study were likelyformed in part by the deposition of SPM onto theglass slides. Pb and Cd adsorption to the collectedbiolms was measured before and after selectiveextractions under conditions of controlled tempera-ture, pH and solution chemistry. Extraction e -ciency and selectivity were evaluated by analyzingfor Fe, Mn, Al and COD concentrations before andafter extractions with conventional extractants. Inaddition, several modications of conventionalextractants were tested as well as a novel extractantbased on the use of Ti(III) as a reductant. Pb andCd adsorption to each surface component was esti-mated through a non-linear regression analysis, andthe results for Pb were compared to independentpredictions based on representative laboratory sur-rogate materials for the oxide and organic phases.

Deming Dong et al.428


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MATERIALS AND METHODS

Development and characterization of natural biolms

Cayuga Lake in central New York State (U.S.A.) waschosen as the eld site for collection of biolms because of prior biolm characterization by the researchers at thissite (Nelson, 1997; Nelson et al. , 1999b). Biolms devel-oped on glass microscope slides (5.1 7.6 cm) held inpolypropylene racks (Fluoroware, Chaska, MN, U.S.A.)

that were submerged in the lake at a depth of approxi-mately 30 cm for a period of 4 weeks. Several sets of bio-lms were collected between January and March 1998,while the lake water temperature was approximately 4 8C.A similar collection method was reported by Tessier forcollection of sediments on Teon 1 sheets (Tessier et al. ,1996). Prior to placement in the lake, glass slides andracks were precleaned with detergent, soaked for 24 h insoap solution, acid washed for 24 h in 6:1 (v/v) H 2 O:HNO 3 (trace metal grade, Fisher Scientic, Pittsburgh,PA), and then rinsed in distilleddeionized water (ddH 2 O),followed by a second 24-h acid wash and a nal rinse inddH 2 O.

After retrieval from the lake, glass slides with attachedbiolms were transported within 1 h to the laboratory(submerged in lake water) for microscopic examination,chemical characterization and measurement of Pb and Cdbinding. Biolms were consistent from slide to slide (Fe,Mn and Al concentrations varied by less than 5%), allow-ing the use of di erent slides for characterizations and formeasurement of Pb and Cd binding.

Organic material in the biolms was quantied bymeasuring chemical oxygen demand (COD) using a modi-cation of Standard Method # 5220 B (APHA, 1995). TheCOD, reported here in units of mg O 2 /L, is approximatelyequivalent to 2.7 times the organic carbon content in mgC/L (assuming an oxidation state of zero for all organiccarbon in the biolm and 100% e ciency of oxidation toCO 2 ). For the COD analysis, the slides with attached bio-lms were broken into small pieces and placed in 250-mLErlenmeyer asks. To each ask was added 50 mLddH 2 O, 0.3 g HgSO 4 , 5 mL sulfuric acid reagent (w/Ag 2 SO 4 ), 25 mL of 0.00417 M K 2 Cr 2 O 7 and an additional

70 mL of sulfuric acid reagent. These solutions werere uxed for 2 h, cooled and the remaining Cr 2 O 72 was

titrated with standardized 0.025 M ferrous ammonium sul-fate.

Total extractable metal concentrations (Fe, Mn, Al) inthe biolms were determined by extracting with 50 mL of 10% HNO 3 (trace metal grade, Fisher Scientic,Pittsburgh, PA, U.S.A.) for 24 h. Acid extracts were ana-lyzed by graphite furnace atomic absorption spectrometry(GFAAS) using a Perkin Elmer (Norwalk, CT, U.S.A.)AAnalyst 100 equipped with a HGA 800 graphite furnaceand an AS-72 autosampler.

Selective extraction techniques

Each biolm-coated slide was extracted in 50 mL of extraction reagent in 150 mm plastic petri dishes using sev-eral extraction techniques. Our initial experiments with apreviously reported hydroxylamine extraction method forselective removal of Mn and Fe oxides (0.04 MNH 2 OH HCl, 25% acetic acid, 6 h at 95 8C) (Tessier et al. ,1979; Young et al. , 1992; Young and Harvey, 1992)suggested that the high temperature altered adsorptioncharacteristics of the remaining organic material. Therewas also evidence that the acetic acid increased the organiccontent (COD) of the extracted biolms. The added CODwas likely the result of acetate binding to the biolm andcould be expected to alter trace metal adsorption. Thus,we modied this extraction procedure by reducing thetemperature to 25 8C, reducing the NH 2 OH HCl concen-tration to 0.01 M, eliminating the acetic acid and reducingthe extraction time to 30 min.

Preliminary experiments with a previously reportedsodium dithionite reagent to extract Fe oxides [0.3 MNa 2 S2 O 4 with a citrate bu er (0.175 M Na-citrate+0.025citric acid)] (Anderson and Jenne, 1970; Tessier et al. ,1979) indicated that this reagent caused the organic con-tent of the biolms to increase by a factor of two asmeasured by COD. Similar to the di culty with aceticacid extraction noted above, this increase was presumablycaused by citrate binding to the biolm which would inter-

fere with accurate subsequent measurement of Pb and Cdadsorption. Thus, we eliminated the citrate bu er from thereagent, and pH was controlled at 6.0 by manual additionof dilute HNO 3 or NaOH solutions. The nal modiedextraction procedure used 50 mL of 0.3 M Na 2 S2 O 4 for40 min at pH 6.0. This extractant was prepared just beforeuse to avoid any reduction of S 2 O 4

2 .Extraction with 10% oxalic acid for 60 h (Ramsay et

al. , 1988) was employed to remove organic materials frombiolms, but also removed most of the metal oxides (seethe Results Section).

Several extraction reagents based on Ti(III) as a reduc-tant were evaluated for use in selectively removing Fe ox-ides. Hudson and Morel (1989) employed a Ti(III) reagentcontaining 0.05 M Ti, 0.05 M EDTA and 0.05 M citrate,and the extraction was carried out for 15 min at roomtemperature. The Ti(III) solutions used in this procedureare unstable without EDTA and citrate bu er. Since re-sidual EDTA or citrate could inuence Pb and Cd adsorp-tion, the use of this reagent was discontinued insubsequent experiments.

Fig. 1. Test for adsorption interference between Cd andPb. A. Pb adsorption to biolms in the presence andabsence of Cd. B. Cd adsorption in the presence andabsence of Pb. For adsorption from mixtures, the initial

levels ( mM) of Cd and Pb were equal.

Pd and Cd adsorption to surface coating components 429


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Measurement of Pb and Cd adsorption to natural biolms

Pb and Cd adsorption isotherms were obtained forextracted and unextracted biolms by measuring Pb andCd adsorption from solutions with dened metal specia-tion and initial Pb and Cd concentrations ranging from0.2 to 2.0 mM. The equilibration solutions were preparedby dilution of 1000 mg/L PbNO 3 and CdNO 3 referencesolutions (Fisher Scientic, Pittsburgh, PA, U.S.A.) usinga minimal mineral salts (MMS) solution with ionicstrength adjusted to 0.05 M with NaNO 3 (Table 1). Pband Cd speciation in the dened solutions was calculatedusing MINEQL (Westall et al ., 1976). The calculationsshowed that because of low inorganic ligand concen-trations, free Pb 2+ or Cd 2+ ions would comprise 89% of the total dissolved metal (Table 1). Three slides from eachtreatment were placed in polypropylene racks and sub-merged into each of ve 800-mL solutions with ve di er-ent Pb and Cd concentrations. These solutions werecontained in 2-L water-jacketed beakers to maintain aconstant temperature of 25 2 18C. The solutions were stir-red continuously with magnetic stirrers for 24 h whilemaintaining the pH at 6.0 2 0.1 using pH controllers (ColeParmer, Vernon Hills, IL) to regulate the addition of 0.01 M HNO 3 and NaOH. After equilibration, slides withbiolms were removed from the Pb and Cd solutions,rinsed for 1 s in metal-free MMS solution, and extractedinto 50 mL of 10% HNO 3 (trace metal grade) for 24 h in150 mm plastic petri dishes. Pb and Cd in extracts weremeasured using GFAAS as described above. The coe -cient of variation for the GFAAS analyses was less than5%.

Preliminary experiments with Pb and Cd adsorptionmeasured together and separately showed that Cd did notinterfere with Pb adsorption to the biolms and vice versaunder the conditions of these experiments (Fig. 1). Thispermitted the simultaneous measurement of Pb and Cdadsorption in subsequent experiments.

Statistical analyses

As described above, none of the selective extractionsremoved only one component from the biolms withoutalso partially removing at least one of the other com-ponents as well. Accurate determination of Pb and Cd as-sociated with each individual component (by di erencebefore and after extraction) thus required consideration of contributions from the partial fractions of the other com-ponents removed from the slides. Tessier et al . (Tessier etal. , 1996) recently addressed this problem by using simul-taneous solution of two equations for Fe and Mn contri-butions to trace metal binding. Because our work includedadditional variables (i.e. Fe and Mn, as well as Al oxides

and organic materials) Pb and Cd adsorption to each com-ponent was estimated with non-linear regression analysesof all of the isotherm data including unaltered biolmsand biolms after each of the three extractions. The model

used for the regression analysis considered total adsorp-tion by the biolm at a given Pb or Cd concentration(Gtotal , mmol Pb or Cd/m

2 ) to be the sum of contributionsfrom four constituents (Fe, Mn and Al oxides and COD):

Gtotal C Fe GFe C Mn GMn C Al GAl C COD

GCOD , 1

where C Fe , C Mn , C Al and C COD are the surface concen-trations of each component ( mmol Fe, Mn or Al/m 2 andmg COD/m 2 ) and the G terms are adsorption on a perquantity of material basis (e.g. mmol Pb/ mmol Fe). G foreach component was expressed as a Langmuir adsorptionisotherm:

Gi Gmaxi K i M

2

1 K i M 2, 2

where Gi is the adsorption of M 2+ by component i perunit surface area, Gi max is the maximum adsorption of M 2+ by component i , K i is the Langmuir equilibrium coef-cient and [M 2+ ] is the concentration of free Pb or Cdmetal ions. The predicted metal adsorption to bare glassslides at each metal concentration was subtracted from theobserved metal adsorption. Adsorption to each componentis expressed per unit nominal surface area of the glassslides containing the biolm, not the total surface area of the adsorbing phase.

The nonlinear regression was performed using SAS soft-ware (SAS Version 6.12, SAS Institute, Cary, NC). The re-gression minimized the error associated with a total of eight variables (four values of Gi

max and four values of K i ).The data set consisted of adsorption data for the unex-tracted biolms plus biolms extracted with each of thethree extractants, with triplicate samples at ve metal con-centrations, for a total of 60 observations. The regressionwas initialized with estimates for each Gi

max based on theassumption that all components adsorbed equal surfaceconcentrations of metal. If the algorithm did not initiallyconverge when all eight variables were regressed, the re-gression was performed iteratively for the four values of Gi

max and the four values of K i until convergence on bothG i

max and K i was obtained.

Table 1. Composition and Pb/Cd speciation of MMS solution used in metal adsorption experiments

Component or species Concentration ( mM) Concentration (mg/l)

MMS medium a CaCl 2 2H 2 O 200 30MgSO 4 7H 2 O 140 35

(NH 4 )2 SO 4 910 120KNO 3 150 15

NaHCO 3 10 0.84KH 2 PO 4 5 0.70

Pb speciation b Pb 2+ 89%PbSO 4 9%

PbOH + 1%

Cd speciation b Cd 2+ 89%Cd Cl + 1.5%Cd SO 4 4.6%

Cd NO 3+ 4.9%

a Ionic strength adjusted to 0.05 M w/NaNO 3 ; pH adjusted to 6.0 before autoclaving.b Pb and Cd speciation calculated by MINEQL for a total Pb/Cd concentration of 1.0 mM.



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the regression analysis indicates that the greatestcontribution to total Pb adsorption was from Mnoxides, followed by lesser contributions from Fe ox-ides and organic material (Fig. 4). The estimatedcontribution to Pb adsorption by Al oxides wasnegligible (Table 3, GAl

max =0.00 2 0.0035 mol Pb/

mol Al). For Cd, the regression analysis indicatedthat Fe oxides exerted the greatest inuence on Cdbinding, followed by lesser contributions from Aloxides, Mn oxides and organic material (Fig. 5).However, at low Cd concentrations (


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than that of organic material and Al oxides and

similar to that of Fe (Fig. 5). Errors associated withestimated adsorption to each phase are depicted inFig. 6, which shows Pb and Cd adsorption at asingle adsorbate concentration (0.5 mM). The stan-dard errors depicted in Fig. 6 were determined byconsidering the propagated errors from both Gmaxand K estimations. These calculations indicate thatthe higher adsorption of Pb by Mn is statisticallysignicant. The estimated Pb adsorption by Fe wasnot signicantly di erent from that of organic ma-terial.

The distribution of Pb between biolm com-ponents estimated by the non-linear regressionanalysis is similar to that estimated for CayugaLake biolms using an adsorption additivity model(Nelson et al. , 1999b). In the additivity model, totaladsorption was predicted from the sum of contri-butions of individual components that were deter-mined using Pb adsorption isotherms for purelaboratory surrogate materials selected to represent

natural Fe, Mn, and Al oxides and organic ma-

terial. When the adsorption capacity of laboratory-derived biogenic Mn oxides was used as the surro-gate for natural Mn oxides (Nelson et al. , 1999a),the additivity model predicted a strong role of Mnoxides (Nelson et al . , 1999b) similar to thatobserved in the present work. The additivity modelused Pb adsorption to pure cultures of microorgan-isms to estimate Pb adsorption to the organic phaseof the biolms, and resulted in a lower estimationof the role of organic material than in the presentwork. The low concentration of Pb associated withAl oxides predicted by the selective extractions alsoagrees with predictions based on laboratory adsorp-

tion isotherms. Based on previously measured Pbadsorption to gAl 2 O 3 (Nelson, 1997; Nelson et al. ,1999b), the expected Gmax for Al oxide in the unex-tracted Cayuga Lake biolms would be 1.2 mmolPb/m 2 , which is much lower than Pb adsorptionmeasured for the other oxide and organic phases.

The regression analysis provides Langmuiradsorption isotherms for each of the biolm com-ponents investigated, and these can be compared toPb adsorption isotherms for representative labora-tory materials determined under the same con-ditions (MMS solution matrix at 25 8C, pH 6.0,ionic strength 0.05 M). The regression-derived Pbadsorption isotherm for the Fe oxide component of the biolms was very similar to Pb adsorption toamorphous Fe oxyhydroxide previously measuredin our lab (Nelson et al. , 1995) as well as to thatestimated using a model described by Benjamin andLeckie (1981) (Fig. 7). The excellent agreement of the Pb adsorptive behavior of Fe oxides obtainedfrom these distinctly di erent approaches suggeststhat the isotherm parameters have a predictive uti-lity. The agreement of the regression results for thePb isotherm to those independently attained byother methods also suggests the extraction approach

Fig. 5. Estimated Cd adsorption to metal oxide and organic components of unextracted Cayuga Lakebiolms based on non-linear regression analysis of Cd adsorption isotherm data for extracted and

unextracted biolms. Error bars indicate 2 one standard deviation.

Fig. 6. Estimated Pb and Cd adsorption to metal oxideand organic components of Cayuga Lake biolms for dis-solved metal concentrations of 0.5 mM. Error bars indicateasymptotic standard error of the mean for the non-linear

regression analysis.



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Fig. 7. Comparison of Pb adsorption to Fe oxide predicted by the non-linear regression analysis of extracted biolm data to that measured for Fe colloids and that reported by Benjamin and Leckie

(1981). Temperature=25 8C, pH 6.0. Adsorption to Fe colloid data after Nelson et al. (1995).

Fig. 8. Comparison of Pb adsorption to Mn oxide predicted by the non-linear regression analysis of extracted biolm data to that measured for biogenic Mn oxide and a fresh abiotically precipitated Mn

oxide. Data for adsorption to biogenic Mn oxide after Nelson et al. (1999a).

Fig. 9. Comparison of Pb adsorption to organic material predicted by the non-linear regression analysisof extracted biolm data to that measured for bacterial biolms. Adsorption to L. discophora and B.

cepacia biolms after Nelson et al. (1999b).



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used in this study can yield realistic estimates of thebehavior of adsorptive phases in nature.

Regression analysis of selective extraction dataindicated greater Pb adsorption to Mn oxides(approx. 2 ) than that observed for laboratory pro-duced biogenic and abiotic Mn oxides (Fig. 8).Similarly, the selective extraction technique suggestsgreater (approx. 2 ) Pb adsorption by organic ma-terials than that observed for laboratory biolmsproduced by two di erent species of bacteria (Fig.9). The possible overestimation of Pb adsorption tothe Mn oxide and organic phases may have resultedfrom performing the regression analysis with con-sideration of only four adsorbing components (Mn,Fe, Al and organic materials). While there could beadditional adsorbing phases and/or adsorptionmechanisms inuencing Pb and Cd adsorption inthe surface coatings, the regression analysis wasforced to converge for only the four components.Thus, any adsorption to other components not con-sidered would be included with adsorption attribu-

ted to these four components. This could lead tooverestimation of metal adsorption to one or morecomponents. Alternatively, the laboratory surro-gates for Mn oxide and organic matter may adsorbless Pb than their naturally occurring counterparts.However, the excellent agreement of the results forPb isotherms on Fe oxides with estimations pre-viously made using laboratory adsorption isothermsand the reasonable (approx. 2 ) agreement withother surrogate biolm components suggests thatthe contribution of other adsorptive components islikely to have been small.

CONCLUSIONS

The selective extraction method presented here isunique because of the measurement of trace metaladsorption before and after extraction. Thisapproach avoids the possibility of desorption of trace metals from components other than the targetcomponent(s) being extracted. The selective extrac-tions removed target components with e cienciesbetween 71 and 83%, but signicant amounts of metal oxides and organic materials other than thetarget components were also removed by the extrac-tants (up to 39%). Because of this, the amount of Pb and Cd adsorption associated with each phase

could not be determined by a simple calculation,and a nonlinear regression analysis was used to esti-mate relative contributions of each surface constitu-ent. This analysis suggested a very strong role of Mn oxides in controlling Pb adsorption to the lakebiolms and lesser but signicant roles of Fe oxidesand organic material. Adsorption of Cd to the lakebiolms was dominated by Fe oxides, with lesserroles of Mn and Al oxides and organic material.The results for Pb agree with previous results of amodel based on Pb adsorption to laboratory surro-gate materials for Mn, Fe and Al oxides and

dened organic constituents. This agreementsuggests that the extraction method presented hereprovides a reliable estimate of the relative contri-butions of each component to total trace metaladsorption.

Acknowledgements This research was supported by the

National Science Foundation under Grants BES-97067715and CHE-9708093. Support for D.D. was provided by afellowship from the People's Republic of China. We aregrateful for the generous assistance of Jery Stedinger andGeorge Casella with the statistical analyses, and to LindaWestlake for the provision of a dock for sampling CayugaLake.

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adsorption of pb and cd onto metal oxides

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