Mechanisms of Pb(II) Sorption on a Biogenic Manganese Oxide

1 mM Mn(II) was added, and which was buffered to yield a final pH of 7 after oxidation (25). ..... alignment between the center of the cavity and the ...
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Environ. Sci. Technol. 2005, 39, 569-576

Mechanisms of Pb(II) Sorption on a Biogenic Manganese Oxide M A R I O V I L L A L O B O S , * ,† JOHN BARGAR,‡ AND GARRISON SPOSITO§ Environmental Bio-Geochemistry Group, LAFQA, Instituto de Geografı´a, National Autonomous University of Mexico, Circuito Exterior, Ciudad Universitaria, Me´xico, Coyoaca´n, 04510, D.F., Me´xico, Stanford Synchrotron Radiation Laboratory, SLAC, P.O. Box 4349, Stanford, California 94309, and Division of Ecosystem Sciences, University of California, Berkeley, California 94720-3110

Macroscopic Pb(II) uptake experiments and Pb L3-edge extended X-ray absorption fine structure (EXAFS) spectroscopy were combined to examine the mechanisms of Pb(II) sequestration by a biogenic manganese oxide and its synthetic analogues, all of which are layer-type manganese oxides (phyllomanganates). Relatively fast Pb(II) sorption was observed, as well as extremely high sorption capacities, suggesting Pb incorporation into the structure of the oxides. EXAFS analysis revealed similar uptake mechanisms regardless of the specific nature of the phyllomanganate, electrolyte background, total Pb(II) loading, or equilibration time. One Pb-O and two Pb-Mn shells at distances of 2.30, 3.53, and 3.74 Å, respectively, were found, as well as a linear relationship between BrunauerEmmett-Teller (BET; i.e., external) specific surface area and maximum Pb(II) sorption that also encompassed data from previous work. Both observations support the existence of two bonding mechanisms in Pb(II) sorption: a triple-corner-sharing complex in the interlayers above/ below cationic sheet vacancies (N theoretical ) 6), and a double-corner-sharing complex on particle edges at exposed singly coordinated -O(H) bonds (N theoretical ) 2). General prevalence of external over internal sorption is predicted, but the two simultaneous sorption mechanisms can account for the widely noted high affinity of manganese oxides for Pb(II) in natural environments.

Introduction Lead(II) is a nonessential element of environmental concern for its toxic effects on plants and animals and especially for its neurological effects in humans (1, 2). It is a widespread contaminant, notably in soil environments, via atmospheric dusts and solid wastes from a variety of active and inactive sources, such as lead smelting and mining, lead alkyl compounds in gasoline, acid battery recycling, pesticides, incinerators, power plants, etc. (2). Lead immobilization processes in contaminated soils and sediments have been typically attributed to formation of highly insoluble phos* Corresponding author phone: +11-52-55-5622-4336; fax: +1152-55-5516-2145; e-mail: [email protected]. † National Autonomous University of Mexico. ‡ Stanford Synchrotron Radiation Laboratory. § University of California, Berkeley. 10.1021/es049434a CCC: $30.25 Published on Web 12/08/2004

 2005 American Chemical Society

phates [e.g., pyromorphite (3, 4)] or sulfates, to association with organic functional groups or silica (5), or to sorption by iron oxides (6, 7). However, a number of studies have identified preferential associations of Pb with manganese oxides in contaminated environments (8-10). These observations are corroborated by other studies in natural environments, which have shown that uptake of Pb by different plant species is inhibited by manganese oxides (11-13). Additionally, studies with natural and artificial biofilms indicate predominance of manganese oxides over iron oxides with respect to Pb binding capacity at pH < 8.5 (7, 14). These findings are not surprising, considering the well-known fact from laboratory experiments that manganese oxides have a high affinity for aqueous Pb(II) (15-18). Nelson et al. (19) found an extremely high Pb sorption capacity on a biologically produced manganese oxide, considerably higher than for synthetic manganese oxides. However, O’Reilly and Hochella (18) found much higher Pb sorption efficiency for synthetic manganese oxides in 5 h flowthrough column experiments as compared to natural manganese oxide varieties. But regardless of their identity, all manganese oxides studied (18) showed higher Pb sorption efficiencies than a set of environmentally relevant iron oxides studied, even those with considerably higher specific surface area, suggesting that different sorption mechanisms must take place in manganese oxides as compared to iron oxides. Similar findings, in fact, were reported long ago for equilibrium batch experiments with synthetic iron and manganese oxides (16). Lead(II) forms strong inner-sphere surface complexes with iron and aluminum oxides (20, 21). Macroscopic evidence for Pb incorporation into the interlayers of synthetic layertype manganese oxides was presented early on by McKenzie (16), and recent spectroscopic evidence suggests formation of corner-sharing complexes above/below cationic vacancies at interlayer sites in manganese(III/IV) oxides (22, 23) but not in γ-MnOOH (manganite), where Pb remains on external surfaces (22). Despite the fact that biological catalysis is the original source of most natural manganese oxides (24), no molecular structural work has been reported for Pb sorbed to biogenic manganese oxides, either those obtained from laboratory cultures or those harvested from natural environments, for example, hyporheic zones. In the present paper, we study Pb sorption by a layer type manganese oxide (phyllomanganate) produced by a common freshwater and soil bacterium, Pseudomonas putida strain MnB1. We investigated the details of the sorbed Pb(II) bonding environment by performing macroscopic measurements of loading capacities and, via extended X-ray absorption fine structure (EXAFS) spectroscopy, by molecular structural analysis of the sorption complexes formed. These experiments were carried out varying the electrolyte background, total Pb(II) loading, and equilibration time, as well as the type of phyllomanganate, by comparing the behavior of the biogenic manganese oxide with two close synthetic structural analogues identified and characterized in previous work: δ-MnO2 and acid birnessite (25).

Materials and Methods Preparation and Characterization of the Manganese Oxide Suspensions. The detailed procedures have been described previously (25). In brief, δ-MnO2 was prepared by an oxidation method combining an alkaline Mn(II) solution with KMnO4, the pH being adjusted to 8 in the final suspension; acid VOL. 39, NO. 2, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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birnessite was prepared by reduction of KMnO4 with concentrated HCl at boiling temperature, the equilibrium pH after dialysis being 5.8; and the biogenic oxide was prepared from a culture of P. putida MnB1 in a highly nutrient-enriched medium at 27-30 °C to which ca. 1 mM Mn(II) was added, and which was buffered to yield a final pH of 7 after oxidation (25). The careful, laborious separation of the biogenic oxide from bacterial organic matter, and the subsequent cleaning procedure to remove remaining organic matter and cell material based on gradual oxidation with NaOCl, has been described in detail previously (25). All three oxides were characterized by wet chemical methods to determine average Mn oxidation number, BrunauerEmmett-Teller (BET) specific surface area, and structural Na+/K+ content (cf. Table 1 in Supporting Information) and by X-ray diffraction (XRD) and spectroscopic methods to elucidate their crystal structures (25, 26). They are all layertype manganese(IV) oxides, with a 7 Å layer spacing and hexagonal sheet symmetry, whose charge arises primarily from the presence of cationic sheet vacancies. The main differences among them are in particle size and cation sorption capacity. Simulations of their XRD patterns (26) indicate coherent scattering domains in the c-direction comprising close to 3 sheets/particle, except for acid birnessite, which comprise about 6 sheets. Lateral size domains are 60, 70, and 85 Å for δ-MnO2, acid birnessite, and biogenic oxide, respectively, although BET-specific surface area follows the order acid birnessite < biogenic oxide < δ-MnO2. Lead(II) Sorption Experiments. Two different kinds of sorption experiment were performed: kinetics experiments, with reaction times from 1 to 15 days under the condition of Pb uptake saturation, and isotherm experiments at 23 °C to establish maximum Pb sorption after 5 days of equilibration, in which [Pb(II)] at equilibrium ranged from 1 to 5 µM. All concentration adjustments and measurements were performed on a gravimetric basis. For both types of experiment, aliquots of stock suspension (14.9 mg/g δ-MnO2, and 1.6 mg/g biogenic oxide) were accurately weighed into narrow-neck 60-mL Teflon bottles to yield a final solids concentration of 12 mg/L. Final volumes were adjusted to 60 mL with Milli-Q water after appropriate aliquots of 5 M NaClO4 were added to yield 0.01 M ionic strength. For acid birnessite, the freeze-dried solid was weighed into 250-mL Teflon bottles to yield 140 mg/L in the final suspension (250 mL) and adjusted to 0.01 M ionic strength with saturated (0.1339 M) KClO4 solution, because the counterion used in its synthesis was K+ (cf. Table 1 in Supporting Information). All suspensions were adjusted to pH 5.8 ( 0.1 with 0.1 or 0.01 M NaOH (KOH for acid birnessite) immediately after addition of adequate gravimetric aliquots of 6 × 10-4 M (6 × 10-3 M for the acid birnessite suspension) Pb(ClO4)2 at pH 2 in HClO4 prepared from analytical-grade Baker solid reagent. Manual readjustment of pH with NaOH was done frequently once equilibration was begun to maintain the desired pH: initially every 2 h, and later once or twice a day. Before every sample withdrawal, pH was measured and confirmed to be 5.8 ( 0.1; otherwise, it was readjusted and the sample was left to equilibrate for at least 8 h more. The concentrations of the Pb(II) standards were determined by inductively coupled plasma atomic emission spectroscopic (ICP-AES) analysis and referenced to appropriate dilutions of a commercial 1000 mg/L (4.8265 × 10-3 M) ICP standard in HNO3 solution. Initial aqueous Pb(II) concentrations in the suspensions were varied between 0 and 50 µM (0 and 100 µM for acid birnessite suspensions). Some experiments with δ-MnO2 were performed with the pH varied from 4.3 to 6.4. Carbon dioxide was not excluded, but experimental conditions were substantially undersaturated with respect to the formation of lead carbonate or lead hydroxy-carbonate solids. After equilibration for the desired time interval on a table 570

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shaker (with minimal suspension contact with caps and lids), Pb sorption was quantified indirectly by measuring [Pb(II)] by ICP-AES analysis in the supernatant solution obtained after filtration of 10-20 mL of suspension (previously centrifuged) through a e0.05 µm mixed (nitrate and acetate) cellulose membrane by means of a syringe and then immediately acidification of the filtrate with 60 µL of concentrated ultrapure HCl/10 mL of filtrate. Filter membranes were confirmed not to sorb Pb(II) at any of the aqueous concentrations investigated at pH values from 4 to 7. Aqueous Mn(II) and Pb(II) were simultaneously monitored and the former was confirmed not to appear under any of the experimental conditions investigated, which suggests no Mn(IV) reduction and, therefore, no Pb(II) oxidation had occurred. For the kinetics experiments, the remaining suspension was left to continue shaking until the next withdrawal, then the cycle was repeated, correcting appropriately for the previous withdrawals at each step. Sample Preparation for Pb(II) EXAFS Spectroscopy. Solids concentrations of 20-60 mg of the manganese oxides/L were prepared in total volumes ranging from 1 to 4 L in Teflon or polyethylene bottles. Ionic strength was adjusted to 0.01 M with NaClO4 or NaCl (KClO4 or KCl for acid birnessite) as described above. Total Pb(II) added was either 8 µM, for low sorption loadings, or 100 µM, for high loadings, and pH was adjusted to 5.8 (or 7.0 for selected δ-MnO2 suspensions) with NaOH (KOH for acid birnessite). Finally, equilibrations were performed on a horizontal table shaker, usually lasting for 7 days, but for just 1 day for the biogenic oxide and for acid birnessite suspension replicates. Suspensions were filtered through 0.2 µm membranes with vacuum suction by use of 15 mL glass towers. This process could require 8-9 h but was necessary to concentrate enough solid paste to load the small-volume sample holders for synchrotron EXAFS spectroscopy. The high volume-low solids concentrations employed were required to ensure Pb(II) sorption near saturation values while keeping aqueous Pb(II) low enough to maintain the system undersaturated with repect to any possible precipitated Pb solids. Pb(II) EXAFS Spectroscopy. Wet pastes obtained from filtering as described above were loaded into 1-mm-thick Teflon holders with 0.003-in.-thick Kapton windows. An additional 0.005-in. polycarbonate film was used to eliminate adhesive-sample contact to avoid potential chemical reactions with the redox-reactive acrylate adhesive. Fluorescentyield lead L3-edge EXAFS spectra were collected at room temperature on wiggler beamline 4-3 [Si(111) monochromator crystals] at the Stanford Synchrotron Radiation Laboratory by use of a Lytle detector filled with Xe gas, and 3Al + Ge6x filters. Four to five scans were collected for each sample up to a k value of 12 Å-1. Spectra were backgroundsubtracted, splined, and fit in k-space by use of an EXAFSPAK program (27) and the SIXPack interface (28), which makes use of the IFEFFIT engine (29). Amplitude and phase functions were derived from fitting spectral data for a PbO reference by use of the FEFF7 code with single-scattering paths. The parameter ∆E0 was allowed to float during optimization, but was linked to a common value for all shells considered for a given sample. The amplitude reduction factor (S02) was fixed at 0.843 for all shells (20). The interatomic distance (R), coordination number (N) for first-shell oxygens, and Debye-Waller parameter for thermal and static disorder (σ2) were allowed to float freely during optimization. Amplitude reduction for Pb-Mn shells was assumed to be contributed by the respective fractions occupying external [Mn(1)] and internal [Mn(2) and Mn(3)] sites, fext and fint, respectively. Thus, the theoretical N values were considered and fixed for a triple-corner-sharing (TCS) interlayer complex [N(2) and N(3) ) 6] and for a double-corner-sharing (DCS)

FIGURE 1. Pb(II) uptake curves on the biogenic manganese oxide and synthetic analogues. (a) Rate of saturation values at 23 °C, pH 5.8. Total Pb(II) added initially, in micromoles per gram of oxide: black squares, 1565; red circles, 2571; green triangles, 3450; green diamonds, 3497 (pH 6.4). (b) Isotherms at 23 °C, pH 5.8, 5-day equilibrations (lines are meant only as visual guides). (c) pH dependence of saturation values on δ-MnO2, 5-day equilibrations. (d) Experimental relationship between (external) specific surface area and maximum Pb(II) uptake. Diamonds denote our experimental data. (*) Not used for regression due to equilibration at different pH than 6. (+) Calculated (circle) assuming equal moles of Mn per gram of manganese oxide as in our biogenic oxide (diamond), with same Sa. The value was used only once to calculate the regression. external complex [N(1) ) 2], and the reduction factors S02 were defined as 0.843Nf, where fext ) 1 - fint.

Results and Discussion Lead(II) Sorption Experiments. Saturation kinetics data for Pb(II) sorption at pH 5.8 by the three manganese oxides are shown in Figure 1a, with sorbed concentrations reported as the molar ratio Pb/Mn. The sorption rate for saturating concentrations was high, constant values of Pb/Mn being reached after about 1 day of equilibration (Figure 1a). Sorption isotherms at 23 °C were H-type [high affinity (30)] for all oxides (Figure 1b), with saturation reached at [Pb(II)] < 1 µM, the maximum Pb/Mn then increasing in the order acid birnessite (0.17) < biogenic manganese oxide (0.27) < δ-MnO2 (0.31). Nelson et al. (19) reported a maximum Pb/Mn ≈ 0.55 at pH 6 and 25 °C for the biogenic manganese oxide produced by Leptothrix discophora but found Pb/Mn ≈ 0.22 for a synthetic manganese oxide they assumed to be δ-MnO2. Significant dependence of Pb(II) sorption capacity on pH was observed for δ-MnO2 (Figure 1c), and this should be true for the other two manganese oxides as well. These high Pb(II) sorption capacities (equivalent to 26, 26, and 37 µmol/m2 external surface for the biogenic oxide, δ-MnO2, and acid birnessite, respectively) suggestsassuming no polymeric sorbates or surface precipitates are formeds that Pb(II) must be entering the layer structure of the oxides

by intraparticle diffusion. The three manganese oxides studied comprise almost exclusively Mn(IV) (25, 26) and, in the absence of edge-surface charge, would be electrically neutral if the cationic sites in the sheet structure were fully occupied. The presence of cation vacancy sites (hereafter referred to as “sheet vacancies”) in the manganese oxide structure has been invoked (22, 23) to explain the presence of excess negative structural charge and the interlayer retention of sorbed Pb(II). Recent simulations of X-ray diffraction and EXAFS spectroscopic data for the three manganese oxides have provided evidence for and quantitative estimates of these cation vacancies (26). Influence of Specific Surface Area on Lead(II) Sorption. The maximum value of Pb/Mn was found to depend on BET (external) specific surface area in a very linear fashion (Figure 1d), including the two saturation Pb/Mn values reported by Nelson et al. (19). Regression analysis of the data in Figure 1d yields

Pb(II)sorbed/Mn ) 0.0020Sa + 0.0891R2 ) 0.99

(1)

where Sa is the specific surface area in square meters per gram. Equation 1 predicts the total Pb sorbed at pH 6 for these and similar layer-type manganese(IV) oxides solely from their values of Sa. This remarkable statistical relationship can be interpreted as evidence for significant Pb sorption on VOL. 39, NO. 2, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Pb L3-edge EXAFS experimental results and simulations of manganese oxides equilibrated at pH 5.8 and 0.01 M NaClO4. (I) K3-weighted χ(k) functions: (a) 1 mmol/g Pb on the biogenic oxide, 7 days of equilibration; the thin gray line is a 2-shell fit; (b) 1 mmol/g Pb on the biogenic oxide, 1 day of equilibration; (c) 0.6 mmol/g Pb on acid birnessite, 7 days of equilibration; (d) 0.6 mmol/g Pb on acid birnessite, 1 day equilibration; (e) 2 mmol/g Pb on δ-MnO2, 7 days equilibration; (f) 0.4 mmol/g Pb on δ-MnO2, 4 days equilibration; (g) 0.4 mmol/g Pb on δ-MnO2, 4 days equilibration, NaCl; (h) 0.4 mmol/g Pb on δ-MnO2, 4 days equilibration, NaCl, pH 7. (II) Radial structure functions of spectra a-e from panel I. Spectra are sequential from bottom to top in both panels. the external surfaces of the manganese oxide particles (see Supporting Information). The nonzero intercept can be assigned to an internal maximum uptake, independent of Sa, that is common to all manganese oxide analogues. The fraction of this internal uptake from the total may then be computed as 0.52 () 0.0891/0.17) for acid birnessite, 0.33 () 0.0891/0.27) for the biogenic oxide, and 0.29 () 0.0891/0.31) for δ-MnO2. These values show a general preponderance of external over internal sorption at pH 6 in these layer oxides. The value of internal sorption can also be used to estimate the mole fraction of cationic vacancies in the manganese oxide particles, 0.043 {) (0.0891/2)/[(0.0891/2) + 1]}, considering that each such vacancy holds a 4e negative charge and thus can bind up to two Pb atoms. This number is low compared to estimates of vacancy content from XRD simulations (26), which range from 0.06 to 0.17 for the three manganese oxides. The difference may be explained by competition from strong adsorption of protons at these sites (cf. Figure 1c), making internal Pb sorbed determined at pH 6 not representative of the maximum interlayer sorption capacity and thus an inadequate measure of total vacancy content. The relationship in eq 1 thus leads to a unifying structural picture of the three layer-type manganese oxides, where the main difference among them is in particle size as reflected by their external specific surface area. Evidently particle size in suspension is small enough that not only interlayer sorption but also external surface sorption plays an important roles in most cases the more important role. Lead(II) L3-Edge EXAFS Spectra. Our Pb L3-edge XANES results (cf. figure in Supporting Information) gave no evidence of Pb(II) oxidation after reaction with the manganese oxides, in agreement with previous reports (22, 23). Instead, they show a pattern and white line position consistent with Pb(II) in a distorted trigonal pyramidal coordination geometry, as is commonly observed for sorbed Pb(II) on metal oxides (20). The raw EXAFS spectra collected were fit up to maximum k 572

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values of 11-11.5 Å-1 (Figure 2, panel I). A strong beat pattern was observed near 5.6 Å-1, as also noted by Matocha et al. (22) for acid birnessite. The corresponding radial structure functions (RSFs) showed two main peaks at ∼1.7 and ∼3.3 Å R + ∆R (Figure 2, panel II), due to first Pb-O and Pb-Mn shells, respectively (22, 23). These peaks were found at the same radial distances, irrespective of Pb loading, as also reported by Manceau et al. (23) for H+-birnessite. However, note the lower amplitudes of the second peak for Pb(II)equilibrated δ-MnO2 (Figure 2, panel II, spectra e-h) as compared to the other samples. This suggests a lower proportion of Mn near neighbors per Pb(II) sorbed on this oxide. Table 1 lists FEFF simulation results for Pb equilibrated on the manganese oxide samples for all experimental conditions investigated. We were able to simulate the EXAFS spectra considering a total of four shells around sorbed Pb. The fitted interatomic distances are remarkably similar (Table 1), irrespective of the type of oxide or loading conditions, but with some variation in the number of near neighbors obtained. Fits were attempted with Pb and Mn as secondshell neighbors. In all cases, Mn was found to provide the superior fit, and no evidence was found for surface-bound oligomeric Pb(II) species. Finally, homogeneous precipitation of Pb(II) phases should not have occurred under the experimental conditions used. Structural Interpretation of the EXAFS Spectra. The first Pb-O shell was found at 2.31 ( 0.01 Å (Table 1), in close agreement with Matocha et al. (22) but generally smaller than the values found by Manceau et al. (23) for different Pb loadings on H+-birnessite. The Pb(II) coordination complexes that display a 2.30 Å Pb-O bond length typically exhibit a highly distorted trigonal pyramidal first-shell coordination geometry, with three oxygens/hydroxyls forming the triangular base and lone-pair electrons defining the pyramid apex (20). This is consistent with our XANES results (cf. figure in Supporting Information). Coordination numbers (N) for

1.8 ( 0.4 2.1 ( 0.5 1.9 ( 0.5 1.8 ( 0.3 1.8 ( 0.3 2.30 ( 0.01 2.31 ( 0.01 2.33 ( 0.02 2.31 ( 0.01 2.31 ( 0.01

a Unless otherwise noted. b f values denote estimates of the fraction of total Pb in external (edge) sites and at internal (interlayer) sites. These were calculated as part of the amplitude reduction factor (S 2) as 0 follows: S02 ) Nf(0.843), where N was fixed to 6 for Pb-Mn(2)int and (3)int and to 2 for Pb-Mn(1)ext, and fext ) 1 - fint. c Fixed value in the optimization procedure. d 0.4 mmol/g. e 2 mmol/g. f 0.6 mmol/g. g 1 mmol/g.

-20 ( 2 -18 ( 2 -16 ( 2 -13 ( 2 -15 ( 2 0.023 ( 0.01 0.026 ( 0.01 0.023 ( 0.01 0.021 ( 0.006 0.022 ( 0.005 5.51 ( 0.09 5.50 ( 0.09 5.51 ( 0.09 5.57 ( 0.05 5.56 ( 0.05 0.01 0.01 0.01 0.01 0.01 0.24 ( 0.06 0.41 ( 0.05 0.36 ( 0.08 0.50 ( 0.05 0.54 ( 0.04 3.70 ( 0.02 3.72 ( 0.02 3.73 ( 0.02 3.73 ( 0.02 3.72 ( 0.01 0.024 ( 0.008 0.012 ( 0.003 0.016 ( 0.007 0.012 ( 0.004 0.008 ( 0.002 0.76 ( 0.06 0.59 ( 0.05 0.64 ( 0.08 0.50 ( 0.05 0.46 ( 0.04 3.53 ( 0.05 3.52 ( 0.03 3.55 ( 0.05 3.51 ( 0.03 3.50 ( 0.02

2.0 ( 0.5 2.31 ( 0.02

0.011 ( 0.002 0.012 ( 0.002 0.011 ( 0.003 0.008 ( 0.002 0.009 ( 0.002

-19 ( 2 0.021 ( 0.01 5.49 ( 0.08 0.01 0.27 ( 0.07 3.70 ( 0.02 0.020 ( 0.008 0.73 ( 0.07 3.53 ( 0.06

1.9 ( 0.4 2.3 ( 0.5 ,d

δ-MnO2 4 d δ-MnO2,d 4 d, NaCl δ-MnO2,d 4 d, NaCl, pH 7 δ-MnO2,e 7 d acid birn.,f 1 d acid birn.,f 7 d biogenic,g 1 d biogenic,g 7 d

2.31 ( 0.02 2.32 ( 0.02

0.012 ( 0.003

-20 ( 2 -21 ( 2 0.019 ( 0.009 0.02 ( 0.01 5.49 ( 0.08 5.48 ( 0.07 0.01 0.01 0.25 ( 0.08 0.26 ( 0.06 3.71 ( 0.02 3.71 ( 0.02 0.022 ( 0.009 0.021 ( 0.007 0.75 ( 0.08 0.74 ( 0.06 3.55 ( 0.07 3.54 ( 0.05

σ2 (Å2)

fextb R (Å) σ2 (Å2)

N sample (loading)/ conditions

R (Å)

0.012 ( 0.003 0.014 ( 0.002

e0 σ2 (Å2)

R (Å)

fintb

(Å2)c

R (Å)

Pb-Mn (3)int σ2 Pb-Mn (2)int shells Pb-Mn (1)ext Pb-O (1)

TABLE 1. Optimized Simulation Parameters of L3-Edge Pb EXAFS for Pb-Equilibrated manganese Oxides in 0.01 M NaClO4 at pH 5.8a

oxygens varied between 1.8 and 2.3 (Table 1). Typically, it is not possible to observe all three first-shell oxygens in EXAFS because the asymmetry of the first oxygen shell causes destructive interference in the oxygen backscattering, which then is not adequately described by harmonic Debye-Waller amplitude-reduction terms (20). Thus, values of N for Pb-O in the range of 1.5-2.5 are common for Pb(II) sorbed to metal oxides in a trigonal pyramidal geometry (20-22). The second RSF peak (at about 3.3 Å R + ∆R) was fitted by considering two different first Mn shells: one for externally bound Pb(II) as a double-corner-sharing (DCS) complex (Figure 3b), and another one for an internal triple-cornersharing (TCS) Pb complex at interlayers (Figure 3a). The PbMn distance due to external Pb was found at 3.53 Å with fractional contributions ranging from 0.5 to 0.75, while the distance for internal Pb was found at 3.72 Å with the complementary fractional range of 0.25-0.5 (Table 1). These simulations predict a general predominance of externally bound Pb(II) over internal sorption, and the values agree quite well with the macroscopic sorption analysis performed above. Also, this external predominance explains why previous simulations of Pb sorption on similar layer manganese oxides by Matocha et al. (22) and by Manceau et al. (23) yielded numbers (N) of near-neighbor Mn atoms considerably below 6 when fitted with a single shell at 3.7-3.8 Å, for the internal complex. For the TCS complex (Figure 3a), to reconcile the Pb-O distances with the Pb-Mn distances found in our simulations (Table 1; Figure 2, panel II), from knowledge of the interatomic Mn-O and Mn-Mn distances in the three manganese oxide structures (26), it is necessary to move the corner O atoms a small distance away from the 3-fold axis of the vacancy site (dilation of the vacancy hole) as occurs in chalcophanite, or into the sheet, along the (short) shared edges, as also suggested by Manceau et al. (23) for Zn-birnessite, and commonly observed in dioctahedral layer structures (32). In our model, moving the oxygens 0.08 Å along the edges allows a shortening of the Pb-Mn distance from 3.85 Å to the 3.73 Å obtained from the EXAFS simulations. The Pb-Mn distance of 3.53 Å for the DCS complex at particle edges (Figure 3b) is crystallographically possible (22) by placing the Pb atom in an intermediate alignment between the center of the cavity and the plane of the sheet structure formed by the two bound oxygens. The 5.50 Å Pb-Mn shell (Figure 2, panel II; Table 1) corresponds in our idealized model to a second shell of Mn atoms (N ) 6) for the TCS complex, at interlayers (Figure 3a). Site Reactivity and Particle Size. Relative contributions to the total number of reactive sites from edges and sheets by use of the calculated residual charges from simple Pauling (33) bond-valence calculations (see Supporting Information) may be analyzed as a function of sheet dimensions and vacancy content. Sites may be normalized to monovalent charges, for example, one site with a 2+ charge is considered equivalent to two 1+ sites, etc. Figure 4 shows a plot resulting from this kind of analysis for two vacancy contents, 8% and 25%, and for lateral particle size as measured along the crystallographic a direction (with b ) 0.5a). The interatomic distances considered in order to construct Figure 4 were: Mn(IV)-O ) 1.90 Å, unshared O-O edges () Mn-Mn of adjacent octahedra) ) 2.87 Å, shared O-O edges ) 2.52 Å, and sheet thickness ) 1.9 Å (26). For the typical range of radial coherent scattering domain sizes in poorly crystalline manganese oxides, that is, 6-9 nm (26), and twice those values, one may calculate a contribution of edge sites (Figure 4) that ranges from 65% to 35% of the total reactive sites for an 8% vacancy content, and from 40% to 15% of the total reactive sites for a 25% vacancy content. These illustrative calculations further support our EXAFS and macroscopic data analysis, in that although Pb is indeed incorporated into the VOL. 39, NO. 2, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Graphic model showing configurations of Pb complexes on poorly crystalline layer manganese oxides, based on EXAFS interatomic distances. (a) Triple-corner-sharing (TCS) complex above/below vacant sites showing the two 6-folded Mn distances PbMn(2) and (3) obtained from Table 1; and (b) double-corner-sharing (DCS) complex at particle edges. O/OH/OH2 groups bound to Pb but not to Mn atoms are not shown.

FIGURE 4. Relationship between particle size and relative contribution of different crystal regions in ideal phyllomanganate, for b ) 1/2a, and two different sheet vacancy contents. layer structure of the manganese oxides to occupy interlayer sites at cation vacancies, occupation of edge sites cannot be neglected and may indeed contribute half or more (if vacancy content is low) to the total Pb sorbed. Even for larger particle sizes of 30-40 nm [e.g., crystalline hexagonal birnessites (31)], so long as the vacancy content is well below 25%, the contribution of edge sites is still expected to be above 10% (Figure 4). More accurate bond valence calculations may be performed (cf. Table 2 in Supporting Information) by use of parameters and concepts from Brown and Altermatt (35) and Bargar et al. (34) (see also ref 30). The analysis shows that the singly coordinated oxygens (to Mn) at particle edges (Figure 3b) are most likely doubly protonated; doubly coordinated oxygens at both particle edges (Figure 3b) and 574

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at sheet vacancies (Figure 3a) are probably singly protonated; and triply coordinated oxygens at sheet vacancies (Figure 3a) are saturated and most likely unreactive [in contrast to what occurs for iron(III), aluminum(III), and manganese(III) oxides]. All sites are predicted not to show considerable H-bonding. Lead(II) may bind only to singly or to doubly coordinated oxygens, exchanging for one proton in both. The likely configurations for Pb complexes at these sites are (Mn2-O)3Pb0 (as in the TCS complex deduced from EXAFS), or a monodentate complex Mn2-O-PbOH1/3+, at doubly coordinated oxygens; and (Mn-OH)2PbOH1/3+ (as in the DCS complex from EXAFS), or a monodentate complex Mn-OHPbOH2/3+, at singly coordinated oxygens. This analysis, however, does not exclude the possibility of a complex at the edges where Pb is bound to a doubly and to a singly

coordinated O, as in (Mn-OH)PbOH(Mn2O)0, a neutral complex that shares an edge with a Mn(IV) octahedron at particle edges. However, this latter complex would require Pb-Mn distances that are much smaller [ca. 3.3 Å (20, 21)] than those found from EXAFS, and thus we discard formation of such complexes. Competition between Pb(II) and protons for sorption sites may explain the pH-dependent behavior shown in Figure 1c, which applies to the other oxides as well, with reactions such as the following occurring at interlayer sites

(Mn2O)3Pb0 + 3H+ T 3Mn2OH1/3+ + Pb2+ or the following at external sites

(Mn-OH)2PbOH1/3+ + 3H+ T 2Mn-OH22/3+ + Pb2+ + H2O Controls on Lead(II) Sorption Capacity. The macroscopic investigation of maximum Pb(II) uptake on layer manganese oxides has allowed us to probe some structural features related to the reactivity of these oxides for metal sorption, where two different site types were identified: external and internal sites. Their corresponding contributions for Pb uptake (and likely that of other metals) is due to the interplay between their relative abundance (directly related to the surface area available) and the differences in reactivity of singly and doubly coordinated oxygens, respectively. As particle size increases, and thus Sa decreases, one may expect a decrease in the proportion of external vs internal sites. Maximum sorption values for all oxides studied, as well as for those reported in the literature, are in agreement with a common internal Pb(II) loading capacity, suggesting a similar internal structure with equal contents of vacant sites in the sheet lattice [despite differences in the number of stacked sheets per particle as deduced from XRD analysis (25)]. Thus, for example, the higher affinity for Pb(II) reported by Nelson et al. (19) for their biogenic manganese oxide as compared to ours can be attributed mainly to the higher specific surface area of the former oxide, in turn likely due to a smaller particle size and not to an intrinsically higher affinity for Pb(II) or a higher interlayer sorption capacity. Our results suggest that this conclusion may apply to all layer manganese(IV) oxides of natural origin. Additionally, despite strong Pb binding occurring at external and interlayer sites, manifest in the steep initial slopes of the sorption isotherms (cf. Figure 1b), protons still compete strongly for sorption sites at pH values as high as 6 (cf. Figure 1c); thus, site saturation by sorbed Pb(II) cannot be achieved below this pH. It has yet to be established whether this pH dependence is applicable to both types of site and to other metals. Nonetheless, it was observed by EXAFS that at pH values between 6 and 7, occupancy of both types of sites occurs concurrently (similar Pb-Mn f values) regardless of the total concentration of adsorptive Pb(II) available.

Acknowledgments M.V. thanks Sam Webb (Stanford Synchrotron Radiation Laboratory) for his guidance and training in the use of the SixPack EXAFS software. This research was funded by the National Science Foundation, Collaborative Research Activities in Environmental Molecular Science (CRAEMS) program (CHE-0089208). Stanford Synchrotron Radiation Laboratory is a national user facility operated on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.

Supporting Information Available Influence of specific surface area on lead(II) sorption; site reactivity by Pauling bond valence analysis; table showing

physicochemical properties of the manganese oxides investigated (from ref 25); table showing bond-valence analysis for reactive functional groups and Pb sorption complexes, and a figure showing Pb L3-edge XANES on manganese oxides. This information is available free of charge via the Internet at http://pubs.acs.org.

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Received for review April 15, 2004. Revised manuscript received October 19, 2004. Accepted October 25, 2004. ES049434A