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DIVISION S-2-SOIL CHEMISTRY

Adsorption of Free Lead (Pb²⁺) by Pedogenic Oxides, Ferrihydrite, and Leaf Compost

^a QSAR Risk Assessment Service Inc., 360 St-Jacques W., Suite 800, Montréal, QC, Canada H1Y 2P5
^b Dep. of Soil, Crop, and Atmospheric Sci,, Bradfield Hall, Cornell Univ., Ithaca, NY 14853 USA
^c Dep. of Natural Resources Sci., McGill Univ., Macdonald Campus, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9

sebastien.sauve{at}inrs-sante.uquebec.ca

	ABSTRACT

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Iron oxides and organic matter have a high capacity to adsorb Pb and concomitantly maintain a low free Pb²⁺ activity in solution. It is, therefore, important to assess the adsorption capacity of naturally occurring materials and evaluate their potential to reduce toxicity. The free Pb²⁺ activity was measured in the solution in equilibrium with ferrihydrite (a synthetic Fe oxide), two field-collected pedogenic amorphous oxides, and leaf compost. The experiment used a factorial design varying total Pb loading and solution pH. The results show that ferrihydrite was more efficient in lowering Pb aqueous concentration than the two pedogenic oxides. Furthermore, of the two pedogenic oxides examined, a higher Pb²⁺ activity was maintained in solution at equilibrium with the most crystalline phase, which also has a lower surface area relative to the other adsorbents. Leaf compost maintained a significantly higher free Pb²⁺ activity, relative to the various oxides. The experimental data could be fitted to a semi-mechanistic model predicting free Pb²⁺ activity as a function of total Pb loadings and pH, with R² varying from 0.77 to 0.92.

Abbreviations: DPASV, differential pulse anodic stripping voltammetry • EC, electrolytic conductivity • FTIR, Fourier transform infrared spectra • XRD, x-ray diffraction

	INTRODUCTION

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LEAD CONTAMINATION OF THE ENVIRONMENT is ubiquitous: A legacy of atmospheric Pb pollution has ensured that most urban environments are contaminated by Pb to various degrees (Mielke and Reagan, 1998). In soils, Pb typically shows very low aqueous phase concentrations, with only 1 to <0.01% of the total soil Pb burden dissolved in the soil solution (Sauvé et al., 1997a, 1998c). Furthermore, only a fraction of the dissolved Pb is actually present as the free divalent Pb²⁺ species in solution (ranging from 60 to <1%, depending on pH and the concentration of dissolved Pb-binding ligands [Sauvé et al., 1997a, 1998c]). Most of the Pb in soils is precipitated as sparingly soluble mineral phases or bound by the organic matter, the clay fraction, or the Fe oxides.

Iron oxides occur in soils in varying concentrations, depending on the pedogenetic processes. The high affinity of Fe oxides for certain trace elements, Pb in particular, makes them a likely long-term sink for cationic polyvalent metals. Since a large part of trace elements, measured as soil total by acid digestion of the solid phase, is not bioavailable, it has been argued that elevated concentrations of sparingly soluble metals such as Pb do not pose a threat to the environment (Cook and Hendershot, 1996). From a bioavailability perspective, it is critical to improve our understanding of natural systems to be able to predict under which circumstances metals will be dissolved and possibly bioavailable, relative to situations where the metals are strongly bound to the solid phase and represent only a limited potential for toxicity.

Most studies on the metal adsorption properties of Fe oxides use laboratory-synthesized, well characterized materials. The actual adsorption properties of pedogenic oxides may differ from those of pure materials. Soil oxides are heterogeneous, albeit predominantly made of Fe. They are of mixed composition and often amorphous, as the presence of silica and organic matter hinders the crystallization process (Schwertmann and Taylor, 1989). The adsorption properties of naturally occurring pedogenic soil oxides must be compared with laboratory-synthesized Fe oxides before assuming that the results of adsorption studies using pure minerals can be used to predict the environmental fate of metals.

Soil organic matter has a pronounced impact on the solubility and soil adsorption of metals. Prior research suggests that soil organic matter can either enhance or inhibit adsorption, depending on other soil properties (pH, cation-exchange capacity, competing ions, etc.) (Gerritse and van Driel, 1984; Basta et al., 1993; Harter and Naidu, 1995; Römkens and Dolfing, 1998). Soil humic acids can increase adsorption, reducing both metal concentration and free metal activity. On the other hand, soil organic matter can increase dissolved organic matter and fulvic acid concentrations, which increase the total dissolved metals via complexation reactions in the soil solution, resulting in higher metal mobility (Bruemmer et al., 1986; McBride et al., 1997a; Sauvé et al., 1998b). Therefore, it is important to quantify the metal-adsorption capacity of soil organic matter relative to Fe oxides; it is also necessary to assess the extent that soil organic matter affects the adsorption properties of pedogenic Fe oxides.

Increasing pH reduces the solubility of most Pb-bearing minerals, while increasing the adsorption affinity of iron oxides, organic matter, and other adsorptive surfaces. However, increasing pH also increases Pb hydrolysis, inorganic ion-pair formation, and organic matter solubility, promoting higher dissolved concentrations of Pb (Bruemmer et al., 1986; Sauvé et al., 1998b, 1998c). It is important to identify which soil components are responsible for controlling Pb concentrations in the aqueous phase and the conditions under which solubility and chemical speciation are optimum to minimize Pb bioavailability. Furthermore, although the total dissolved concentrations of Pb are important for certain aspects of environmental soil modeling (Sauvé et al., 1998c; Bruemmer et al., 1986), bioavailability and chemical reactivity may be more directly related to the free metal pool (Sauvé et al., 1998a; Knight et al., 1998; Knight and McGrath, 1995).

The objective of this study was to determine the pH-dependent Pb-adsorption properties of two pedogenic oxides compared with synthesized ferrihydrite and a leaf compost, thus exploring the relative contributions of soil oxides in relation to leaf compost in controlling free Pb²⁺ activity in contaminated soils.

	Materials and methods

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Adsorbents
We used four materials: two pedogenic oxides, a leaf compost, and a synthesized ferrihydrite. The pedogenic oxides were collected in June 1997 from acidic forest soils (humicryods) in St-Michel-des-Saints (200 km northwest of Montréal, QC, Canada). The iron oxides were accumulating on the soil surface where Fe²⁺-rich soil water was moving laterally. As the Fe²⁺ reached the soil surface it oxidized, generating iron oxides naturally. The first sample (SMS-1) was collected over grass and gravel, and the second (SMS-2) was collected over leaf litter. Both samples were wet-sieved in the following week to obtain the size fraction <250 µm, and this fraction was stored at 4°C for 6 mo before use (as slurries of 7.9 and 19% solids). A sugar maple (Acer saccharum L.) leaf compost was collected from a rural location near Ithaca, NY, air-dried, ground in a blender, and sieved to obtain the size fraction <1 mm (used in prior research, Sauvé et al., 1998c). We preferred leaf compost to reagent-purified standardized humic materials because we were interested in characterizing natural materials and contrasting their properties to laboratory-synthesized products. Synthetic ferrihydrite was prepared by slow (0.03 mL min^-1) precipitation to a pH 6 of a 4.95 mM Fe(NO₃)₃ solution with 1 mM KOH (further details in Martínez and McBride, 1998).

Characterization of Solid Phases
Basic chemical properties of the materials used can be found in Table 1 . Elemental composition of the samples was determined by digestion in concentrated HNO₃ (Mench et al., 1994) and analysis by inductively coupled plasma emission spectrometry. The impurities in the synthesized ferrihydrite come at least partially from the reagents used. The organic C and N content was measured using a combustion method. Specific surface was determined by three-point N₂ adsorption measurements after a 1-h outgas procedure with dry N₂ at 100°C. Fourier transform infrared spectra (FTIR) were obtained using a pellet containing 2 mg of sample mixed with 170 mg of KBr. Measurements were made using a Fourier-transform Perkin-Elmer 1720-X spectrometer (Perkin-Elmer, Buckinghamshire, UK) at 2 cm^-1 resolution and 100 co-added scans. X-ray diffraction (XRD) analyses were made using a theta-theta diffractometer (Scintag, Cuttertino, CA) with a solid-state intrinsic germanium detector. Specific surface, FTIR, and XRD measurements showed that the adsorbents vary in degree of crystallinity (Martínez et al., 1999). The synthetic ferrihydrite had a specific surface that was typical of a non-crystalline oxide material (170 m² g^-1); SMS-1 had an intermediate specific surface (97.9 m² g^-1); and SMS-2 had a specific surface that was consistent with that of a crystalline material (26.4 m² g^-1). These results agree with the XRD patterns for each adsorbent. The x-ray diffraction patterns showed that a two-line ferrihydrite was synthesized in the laboratory. The XRD pattern of SMS-1 also shows two broad lines, consistent with the presence of ferrihydrite, and there was no evidence of the presence of well-crystallized iron oxides. For SMS-2, the XRD pattern showed some quartz and feldspar; however, there was no indication of the presence of non-crystalline (ferrihydrite) or crystalline (hematite, goethite, or lepidocrocite) Fe oxides. Infrared analysis indicated that a combination of various Fe oxides was present in the samples. Hematite-like (in ferrihydrite) and lepidocrocite-like (in SMS-1 and SMS-2) microcrystalline structures were present, but they were not ordered well enough to produce a sharp XRD pattern. The FTIR spectra of the leaf compost was dominated by two strong absorption bands (typical of organic matter): (i) the OH stretching band, and (ii) a very intense band at 1640 nm, indicating a high COOH content and thus a high metal-binding capacity (Stevenson, 1994). The XRD and FTIR spectra are presented and discussed in more detail in Martínez et al. (1999).

The spiked materials were then shaken every other day for 15 d. The bottles were kept loosely covered (to prevent dust deposition) but not closed, to allow gas exchange and keep the solutions aerobic and in equilibrium with ambient CO₂. Then, the tubes were centrifuged at 15000 x g to separate the supernatants. The pH and electrolytic conductivity were determined in the supernatants before filtration. The pH was measured using a Fisher 805MP meter (Fisher Scientific, Pittsburgh, PA) and a combination glass electrode (Orion model 91-55, Orion Research, Boston, MA). Electrolytic conductivity (EC) was measured using a YSI model 31 meter (YSI, Yellow Springs, OH). Electrolytic conductivity was also used to estimate the ionic strength (IS), assuming (where IS in unitless and EC is in mS cm^-1) (Griffin and Jurinak, 1973). The solutions were then passed through 0.22-µm cellulosic membranes and analyzed for labile Pb using differential pulse anodic stripping voltammetry (DPASV) (Florence, 1986; Opydo, 1989; Sauvé et al., 1998b, 1998c). For the DPASV analysis, a hanging mercury drop electrode was used. The samples were prepurged for 8 min with N₂ (gas) to remove dissolved oxygen. We used a -0.8 V reducing potential deposition step of 1 min (with stirring) followed by a 30-s homogenization period (without stirring). We then carried out metal stripping using current ranges (sensitivity) between 0.1 µA and 0.5 mA, as necessary. The DPASV determinations allow the discrimination of organically bound Pb from the inorganic labile Pb (Florence, 1986). A chemical equilibrium model (MINEQL⁺, Schecher and McAvoy, 1994), combined with the constants compiled in Sauvé et al. (1998b), can then be used to partition the measured DPASV-labile Pb into those inorganic ion-pairs calculated to be most prevalent in the solution (namely PbOH⁺, Pb⁰₂, Pb⁺₃, PbHCO^-₃, PbCO⁰₃, Pb^2-₂, PbNO⁺₃, PbCl^-, and PbSO⁰₄. An estimate of free Pb²⁺ is obtained by taking the difference between DPASV and the sum of inorganic chemical species (Sauvé et al., 1998b). Actual free Pb²⁺ may be lower than this estimate if significant levels of ASV-labile organo-Pb complexes exist in solution (Florence, 1986). Thus in organic-rich systems, this method might overestimate free Pb²⁺ activity, although a comparison with other free Pb²⁺ speciation methods showed that ASV gave the lowest, most restrictive free metal activity results (Sauvé et al., 1997a; Sauvé, 1999).

	Results and discussion

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Lead Adsorption
Figure 1 illustrates the Pb adsorption isotherms as a function of both pH and total Pb content (log₁₀). Although total Pb is not equivalent to adsorbed Pb, with solid/liquid partitioning ratios of 10⁶, the difference is negligible. Most of the variability in the solution free Pb²⁺ activity is explained by pH and total Pb content. Thus, both adsorption and free Pb²⁺ activity are related directly to pH and metal loading. The data were fit to a semi-mechanistic model of the following form:

(1)

where a, b, and the constant were determined using linear statistical regressions (Wilkinson, 1992). Although the regression is simplistic, it was derived from a mass-action driven model of metal adsorption–complexation, assuming competitive binding of H⁺ and Pb²⁺ ions (McBride et al., 1997b; Sauvé, 1999). A relationship of this form has also been applied with success to soil solution speciation of Cu²⁺ (Sauvé et al., 1997b; Sauvé, 1999), Pb²⁺ (Sauvé et al., 1997a), and Cd²⁺ (Sauvé et al., 2000) using field-collected soils of various origins. This suggests that a relatively simple model can be used to evaluate the fate of metals in contaminated soils.

View larger version (73K): [in this window] [in a new window]   Fig. 1 The free Pb2+ activity estimated from the anodic stripping voltammetry–labile measurements is represented as a function of solution pH and total metal loading (log10) for the different adsorbents. Ferrihydrite represents the synthesized ferrihydrite, SMS-1 represents the most amorphous pedogenic oxide, SMS-2 represents the most crystalline pedogenic oxide, and Leaf represents the leaf compost. The graph surfaces are obtained by distance-weighted least square smoothing, noting that the regions outside the data are extrapolated (Wilkinson, 1992)

The adsorption properties of the various materials can be compared more directly using the data in Table 2 to calculate the expected Pb²⁺ activities over the same range of pH and fixed total Pb loads (Fig. 2) . Figure 2 is simply an illustration of the numerical consequence of regressions given in Table 2 (experimental data shown in Fig. 1). Figure 2 suggests that Pb-binding by ferrihydrite, as a surrogate for laboratory-synthesized amorphous Fe oxides, overestimates Pb binding and consequently underestimates equilibrium Pb²⁺ activity by one to six orders of magnitude relative to the sorption properties of soils (see Fig. 2). Even when comparing ferrihydrite with the pedogenic oxides, it is clear that these naturally occurring materials have a lower affinity for Pb. The discrepancy between adsorption by synthetic and pedogenic oxides is lowest at low pH and increases with pH. Similarly, the discrepancy is most marked at low total Pb loadings and decreases with higher loadings (remembering that the lowest Pb levels are more realistic). At the highest loading illustrated, the amorphous pedogenic oxide (SMS-1) is similar to ferrihydrite, whereas the more crystalline pedogenic Fe oxide (SMS-2) shows adsorption properties similar to soils (Fig. 2). The leaf compost supports a free Pb²⁺ activity in solution similar to that in soils, and much higher than that resulting from equilibrium with the pedogenic and synthetic oxides.

View larger version (19K): [in this window] [in a new window]   Fig. 2 The calculated pH-dependent free Pb2+ activity [shown as negative M log10(Pb2+)] is represented for three arbitrary metal loadings (20, 200, and 2000 mg Pb kg-1). The lines are obtained from the regressions reported in Table 2. They do not represent actual measurements; they simply reflect the shape of the regressions. The triangles () represents ferrihydrite; the circles (•) represents the most amorphous pedogenic oxide (SMS-1); the asterisks (*) represents the most crystalline pedogenic oxide (SMS-2); the squares () represents the leaf compost; and the lozenges () represent data obtained from an independent study of field-collected Pb-contaminated soils (Sauvé et al., 1997a). The inserts use the same x–y scales to compare the pH-dependent free Pb2+ relationship calculated using a model similar to Eq. [1] and Table 2 but based on different arbitrary surface coverage levels of 0.5, 5, and 50 µg Pb m-2 (regression data not presented)

Data in Fig. 2 suggest that increasing organic matter content and the heterogeneity of the pedogenic oxide tend to reduce the Pb adsorption affinity of the material studied (in increasing order of adsorption affinity: ferrihydrite > pedogenic oxides > soils > organic matter). This is somewhat surprising considering that organic matter has a high affinity for metals, including Pb (Stevenson, 1976; Logan et al., 1997). We do not believe this is because Pb has a low affinity for organic matter, but rather because Pb has a much higher affinity for certain mineral surfaces (Fe oxides and phosphates in particular) relative to organic matter. Therefore, organic matter may block the reactive sites on Fe-oxides and indirectly increase Pb solubility. Furthermore, the formation of dissolved organo-Pb complexes will increase the concentration of Pb in solution and prevent its adsorption.

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
A synthetic Fe oxide had a higher Pb adsorption affinity than two naturally occurring pedogenic oxides. This suggests that environmental fate models cannot reliably use the data obtained from adsorption isotherms of laboratory-synthesized Fe oxide materials to predict the behavior of heterogeneous pedogenic oxides. This study shows that organic matter (in the form of leaf compost) had the weakest tendency to remove Pb from solution compared with the oxide materials tested. It also suggests that under certain situations, organic matter may maintain soil Pb in a more soluble and labile form than would be the case in its absence.

	ACKNOWLEDGMENTS

 
This project was made possible through grants from an NRI competitive Grants Program/USDA award number 95-37107-1620 to M. McBride, an operating grant to W. Hendershot, and a post-doctoral fellowship to S. Sauvé, both from the Natural Sciences and Engineering Research Council of Canada. Comments on the manuscript were provided by Leonard Lion.

Received for publication November 9, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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