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Published online 5 January 2006
Published in J Environ Qual 35:342-351 (2006)
DOI: 10.2134/jeq2004.0259
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Heavy Metals in the Environment

µ-XANES and µ-XRF Investigations of Metal Binding Mechanisms in Biosolids

G. M. Hettiarachchia,*, K. G. Scheckelb, J. A. Ryanb, S. R. Suttonc and M. Newvillec

a Dep. of Soil and Water, School of Earth and Environmental Science, Univ. of Adelaide, Glen Osmond, SA 5064, Australia
b National Risk Management Res. Lab., USEPA, 5995 Center Hill Ave., Cincinnati, OH 45224
c GSECARS, Univ. of Chicago, Chicago, IL 60637

* Corresponding author (ganga.hettiarachchi{at}adelaide.edu.au)

Received for publication July 7, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Micro-X-ray fluorescence (µ-XRF) microprobe analysis and micro-X-ray absorption near-edge structure (µ-XANES) spectroscopy were employed to identify Fe and Mn phases and their association with selected metals in two biosolids (limed composted [LC] and Nu-Earth) before and after treatment to remove organic carbon (OC). Spatial correlations derived from elemental mapping of XRF images showed strong correlations between Fe and Cd, Cr, Pb, or Zn (r2 = 0.65–0.92) before and after removal of most of the OC. The strong correlation between Fe and Cu that was present in intact samples disappeared after OC removal, suggesting that Cu was associated with OC coatings that may have been present on Fe compounds. Except for Fe and Cr, the spatial correlations of metals with Mn were improved after treatment to remove OC, indicating that the treatment may have altered more than the OC in the system. The Fe µ-XANES spectra of the intact biosolids sample showed that every point had varying mixtures of Fe(II and III) species and no two points were identical. The lack of uniformity in Fe species in the biosolids sample illustrates the complexity of the materials and the difficulty of studying biosolids using conventional analytical tools or chemical extraction techniques. Still, these microscopic observations provide independent information supporting the previous laboratory and field hypothesis that Fe compounds play a major role in retention of environmentally important trace elements in biosolids. This could be due to co-precipitation of the metals with Fe, adsorption of metals by Fe compounds, or a combination of both mechanisms.

Abbreviations: LC, limed composted • LCF, linear combination fitting • OC, organic carbon • µ-XANES, µ-X-ray absorption near edge struc-ture • µ-XRF, µ-X-ray fluorescence


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
BIOSOLIDS are an inevitable end product of modern wastewater treatment and have been defined as "primarily organic solid product, produced by wastewater treatment processes, that can be beneficially recycled" (USEPA, 1993a). The environmental impact and potential hazards of metals in biosolids to plants, animals, and the human food chain have been studied for decades. From an extensive body of work in the literature, it has been concluded that, by addition of biosolids to soil, the overall chemical reactivity in the soil system is altered beyond that of the simple addition of trace elements due to the addition of chemically active surfaces (e.g., OC, Fe, and Mn oxides) from biosolids processing. Further, this alteration in soil chemistry does not require large additions of biosolids (Cunningham et al., 1975a, 1975b, 1975c; Gaynor and Halstead, 1976; Street et al., 1977; Singh, 1981; Mahler et al., 1987; Bell et al., 1991; Hooda and Alloway, 1993; Brown et al., 1998). The phase(s) responsible for this alteration and for the ultimate fate of metals in biosolids has been and continues to be in dispute. In its development of regulations designed to protect human health and the environment from reasonably anticipated adverse effects of land application of biosolids, the USEPA's reliance on the difference in phytoavailability of metals in soil systems amended with biosolids (USEPA, 1993b) intensified the debate.

In studies to examine the impact of biosolids on metal–soil reactivity, researchers observed that metals added to soil as constituents of biosolids are less phytoavailable than metal salts added to the same soil. Further, studies found that metal salts added to soils with biosolids are less phytoavailable than metal salts added to like soils without biosolids (Cunningham et al., 1975a, 1975b, 1975c; Gaynor and Halstead, 1976; Mahler et al., 1987; Bell et al., 1991; Hooda and Alloway, 1993; Brown et al., 1998). These observations indicate that some phase(s) in biosolids inhibits metal availability for plant uptake. Long-term experimental studies in which the organic matter added by biosolids has decomposed and in some cases reached levels equivalent to the unamended control soil illustrate the change in soil chemistry and phytoavailability of metals caused by biosolids application are still apparent and effective, indicating that inorganic and/or very recalcitrant organic phases are responsible for the biosolids-induced reductions in metal phytoavailability (Mahler et al., 1987; Brown et al., 1998).

Beckett et al. (1979) postulated the "time-bomb" hypothesis, in which it is proposed that the phase responsible for reduced phytoavailability of metals is organic. As the organic material added in biosolids decomposes, its complexing nature will diminish, with a subsequent release of metals to the soil system where they will behave as additions of inorganic salts to the soil. In contrast, Corey et al. (1987) predicted that biosolids adsorption chemistry is related to inorganic surfaces added during biosolids processing. As the soil metal-binding becomes saturated, the biosolids control the metal activity in the soil solution. Based on these understandings, researchers began to characterize the chemical aspects of biosolids that made biosolids-associated metals much less phytoavailable and bioavailable than metal salts.

Previous research from our laboratory with biosolids and biosolids-amended soils has demonstrated that rate of Cd sorption in biosolids was significantly greater than the rate of Cd sorption in biosolids-amended soils which, in turn, was significantly greater than in the non–biosolids-amended control soil. Further, Cd sorption increased with increasing biosolids application rate (Li et al., 2001; Hettiarachchi et al., 2003). Removal of organic carbon (OC) or Mn and/or Fe oxides reduced metal sorption but did not account for the observed differences in metal sorption between biosolids-amended soils and controls. Removal of both OC and Mn and/or Fe oxides eliminated the observed differences in sorption characteristics of the biosolids-amended and the control soils. Further, desorption experiments showed that a substantial proportion of Cd sorbed by amended and control soils could not be readily desorbed. This "apparent hysteresis" or "apparent irreversibility" was greater for biosolids-amended soils than for the control soil. As observed for sorption, removal of both OC and Fe/Mn fractions was more effective in removing the observed differences between the biosolids-amended soils and the control than was removal of either fraction alone, indicating the importance of both fractions for Cd sorption and desorption (Hettiarachchi et al., 2003).

Concentrations of freshly precipitated, X-ray amorphous metal oxides (such as Fe, Mn, and Al oxides) can be higher in biosolids than they are in soils. Surfaces of freshly precipitated metal oxides, especially those of Fe and Mn, are known to be highly reactive sites for sorption of most dissolved metal ion species. Metal sorption/desorption data illustrate the importance of the inorganic fraction, especially Fe and Mn oxides fractions in retention of toxic trace metals in biosolids-amended soils (Li et al., 2001; Hettiarachchi et al., 2003). Extraction procedures used to separate OC and Fe and Mn fractions in these experiments are not completely phase-specific or phase-selective and do not provide definitive information on retention mechanisms. Rather, they imply the relative importance of the various solid phases. Ideally, one would employ spectroscopic speciation methods to identify the forms of metals, which—combined with an understanding of their persistence—would provide information on potential environmental problems associated with metal bioavailability and mobility.

The complexity of minerals in biosolids (their chemical composition [Lake et al., 1984], lack of crystallinity, and low concentration [Essington and Mattigod, 1991]) make direct observation of mineral-associated metals in biosolids difficult. Synchrotron techniques may be able to overcome some of these problems and to provide information about reaction products in these systems and relevant risk issues. The objectives of this study were to elucidate the relationship and/or mechanistic interactions of metals in biosolids with Fe and Mn phases and to identify and speciate Fe phases in biosolids samples.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Samples
Two biosolids samples, limed-composted (LC) and Nu-Earth, used in this study were from Washington, DC, and Chicago, IL, respectively. The LC biosolids sample was derived from undigested biosolids that was treated with lime during dewatering and composted with wood chips at the USDA-ARS, Beltsville, MD, research center. The Nu-Earth biosolids were digested in an Imhoff tank and dried on a sand bed. Selected chemical properties of biosolids samples are given in Table 1. These properties were determined by microwave digestion, followed by inductively coupled plasma atomic emission spectroscopy (USEPA Method 3051, USEPA, 2001). The LC biosolids contained 41 000 mg kg–1 Fe, 719 mg kg–1 Mn, and 7.2 mg kg–1 Cd, whereas the Nu-Earth contained 25 000 mg kg–1 Fe, 302 mg kg–1 Mn, and 210 mg kg–1 Cd. As a result of effective pretreatment and diligence by municipalities, the concentrations of metals in biosolids have decreased in recent years. Thus, these two biosolids have metal contents greater than those of the 95th percentile of biosolids reported by Stehouwer et al. (2000) and are therefore high-metal biosolids. However, in the published literature (Chaney et al., 1982; Brown et al., 1998) these bisolids were categorized as low Cd biosolids (the LC sample) and very high Cd biosolids (the Nu-Earth sample) based on their classification at the time of their production. These biosolids were chosen for this study mainly because of their elevated Cd concentration. Further, Cd sorption studies done in our laboratory with soil amended with LC and Nu-Earth biosolids showed higher sorption capacity for added Cd compared with the unamended control soil, despite the high Cd concentrations in the biosolids added to the amended soil. Iron levels in these two biosolids were within the normal range for biosolids, that is, 1000 to 150 000 mg kg–1, with a median of 17 000 mg kg–1 (Tchobanoglous and Burton, 1991). Iron (in the form of a steel industry by-product) was added at the Blue Plains wastewater facility, Washington, DC (LC biosolids), to precipitate P. Iron was also added at the Chicago wastewater treatment facility to aid in dewatering of the Imhoff-digested biosolids (Nu-Earth).


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  		Table 1. Selected chemical properties of the biosolids samples (adapted from Chaney et al., 1982, except mentioned otherwise).

 
Representative subsamples of air-dried biosolids samples stored in plastic containers at the USDA-ARS storage facility in Beltsville, MD, were shipped in November 2001 to Cincinnati, OH, in plastic bottles. These representative biosolids samples were crushed and sieved through a stainless steel sieve with a mesh size of 250 µm. For the removal of OC, approximately 1 g of biosolids sample was treated four times with 20 mL of 0.7 M reagent-grade NaOCl (adjusted to pH 8.5 with HCl) for 2 h at 90°C. The sodium hypochlorite (NaOCl) method was chosen for removal of OC in these materials because the NaOCl procedure permits the removal of OC with minimal destruction to Mn oxides and other inorganic phases (Anderson, 1963). For additional information on the OC removal method, see Li et al. (2001) and Hettiarachchi et al. (2003). Four-fold repetition of the extraction procedure outlined above removed approximately 70 to 75% of the OC from these biosolids samples as confirmed by TOC analysis. The intact (unaltered) and reduced OC biosolids samples were used for µ-XRF and µ-XANES analysis.

µ-XANES and µ-XRF Analysis
The distribution and speciation of elements in the biosolids samples were analyzed by µ-XRF mapping and µ-XANES. Experiments to collect µ-XANES spectra and µ-XRF maps were conducted at beamline Sector 13-ID-C (GeoSoilEnviro Consortium of Advanced Radiation Sources) at the Advanced Photon Source at Argonne National Laboratory, Argonne, IL. The electron storage ring operated at 7 GeV with a top-up fill status. Mapping data (µ-XRF) and µ-XANES spectra were collected at ambient temperature in fluorescence mode with a solid-state 13-element detector. The µ-XRF microprobe at APS beamline 13-ID-C is capable of collecting fluorescence data with a 1 to 10 µm beam spot size range (<10 µm resolution) and 1 to 10 mg kg–1 sensitivity, allowing study of elements at very low concentration in complex environmental samples. Biosolids samples (intact and OC removed) were situated in a 5 by 5 cm cardboard photographic slide mount by sandwiching several milligrams of the sample between two pieces of Kapton film that spanned the hole in the slide mount. Elemental distributions and associations for Fe, Mn, Pb, Zn, Cu, and Cr were determined using µ-XRF for the LC biosolids. Data analysis included background subtraction to obtain net fluorescence peak intensities. In addition, calibration was accomplished by internal standards with known concentrations. Because the absorption length limits the intensity of X-ray fluorescence, the thickness of the sample should be small (<1 mm) (Sutton and Rivers, 1999). The thickness of samples used in this study was ~250 µm or less.

The samples were mounted on the rotation axis of an x–y–{theta} stepping-motor stage. Fluorescence data were collected at ambient temperature for a 400 by 400 µm area (for the LC sample) or a 220 by 120 µm area (for the Nu-Earth sample) with a step size of 10 µm using a solid-state energy-dispersive X-ray detector that allows simultaneous detection of fluorescence signals from multiple elements. Aluminum foil was used to diminish the background fluorescence from Fe. At each position, the fluorescence signal from a given element is proportional to the integrated number of atoms of that element along the transect of the synchrotron beam. Because the biosolids samples had higher levels of Fe than the other metals, especially Cr and Mn, elemental mapping was performed twice, at 14 000 and 7102 eV. The elemental mapping collected at 7102 eV provided maps for elements with an absorption edge less than that of Fe (that is, Mn and Cr) without interference from background Fe fluorescence. From the LC biosolids sample, three to four µ-XANES spectra of Fe and Mn were collected over the range of –200 to +600 eV above the K-edge of Fe (~7111 eV) and the K-edge of Mn (~6550 eV) in fluorescence mode from 7 to 8 randomly chosen hotspots of Fe and Mn chosen from the elemental maps (Fig. 1 ). In addition, µ-XANES spectra were collected for Cr at several Fe and Mn hot spots in the LC biosolids sample.



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  		Fig. 1. X-ray fluorescence maps of selected metals in the intact LC biosolids sample. Area of a single map is 400 by 400 µm. The color scheme employed ranges from white–yellow for high fluorescence signal to blue–black for low fluorescence signal. Shading is relative across the single map. The eight markers noted as 1 to 8 represent locations for which µ-XANES analyses were conducted. The XANES spectra are presented in Fig. 6 and 7, and the XANES fitting is presented in Table 2.

 


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  		Fig. 6. The XANES spectra of selected Fe-containing standard minerals and several different Fe hotspots in the LC biosolids sample. Dotted lines indicate the linear combination XANES fits using all the standard compounds that are listed in Table 2. Vertical lines represent white line peaks for Fe2+ and Fe3+, respectively.

 


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  		Fig. 7. The first derivative of the XANES spectra of selected Fe-containing standard minerals and several different Fe hotspots in the LC biosolids sample. The numbers correspond with locations shown on Fig. 1. Vertical dotted lines represent white line peaks for Fe2+ and Fe3+, respectively.

 

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  		Table 2. Percentages of Fe species in selected "Fe hotspots" in the limed composted biosolids sample determined by linear combination fitting of XANES spectra.

 
The XANES spectra of seven standard Fe compounds were also collected to compare with µ-XANES Fe spectra collected from Fe hotspots in the intact and OC-removed biosolids samples. Model Fe compounds included siderite (FeCO3), vivianite (Fe3(PO4)2·8H2O), magnetite (Fe3O4), ferrihydrite (Fe2O3·nH2O), goethite ({alpha}-FeOOH), green rust with chloride (Fe6–xIIFexIII(OH)12[(Cl)x·3H2O]), and green rust with sulfate (Fe6–xIIFexIII(OH)12[(SO4)x/2·3H2O]). Further description of those samples can be found in Hansel et al. (2001). In complex environments rich in OC such as biosolids, some Fe phases could exist as organic Fe compounds. However, in this study we only used inorganic compounds as our standards. It should be noted that, for the purpose of employing standards for comparison, the use of uncomplexed inorganic model compounds is a widely accepted practice; however, using inorganic salts as contaminant sorbents in complex environmental matrixes should be avoided.

The Fe µ-XANES spectra for a particular hotspot were averaged. Then the edge energy was calibrated, pre-edge was subtracted (by a linear function), and the spectrum was normalized to the second-order polynomial to be equal to one. The reduced spectra for the biosolids samples were analyzed by linear combination fitting (LCF) using IFEFFIT software (Newville, 2001). Spectra for the model compounds were reduced and normalized as for the spectra of the biosolids. The linear combination XANES fitting procedure attempted to reconstruct the biosolids spectra using all combinations of the seven model spectra (siderite, vivianite, magnetite, ferrihydrite, goethite, green rust with chloride, and green rust with sulfate). For each biosolids sample, the combination with the lowest reduced {chi}2 was chosen as the most likely set of components, with reduced {chi}2 defined as the sum of squares of the fit residual divided by the estimated uncertainty in the data, and normalized by the number of degrees of freedom in the fit (i.e., the number of data points minus the number of components used). For all normalized XANES spectra for the biosolids, the uncertainty in the data was estimated to be 0.01 by statistical rendering within IFEFFIT software (Newville, 2001). Standards that had partial contributions <5% were removed, though this criterion was only used once and did not result in a significantly larger reduced {chi}2. The results of these analyses are given in Table 2. The accuracy of this fitting procedure depends on how well the reference standards represent the samples and the data quality (Roberts et al., 2002). A reduced {chi}2 near 1 indicates a reliable fit. Because a limited number of model spectra were used in the fits, the best-fit compositions may not give the true composition, but these results can be used to describe the variations among samples.

The µ-XANES spectra of the biosolids' Mn spots were compared with µ-XANES spectra of four different Mn oxide standards. This helped to determine the oxidation state of Mn in the Mn hotspots of the biosolids samples. Manganese standards included: MnSO4·H2O and MnCO3 as Mn2+ standards, Mn2O3 as an Mn3+ standard, and Na-birnessite (NaMn7O14·2.8H2O) as an Mn4+standard.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Distributions and Correlations of Elements in Biosolids
Figure 1 shows the X-ray fluorescence maps (400 by 400 µm) for selected metals at one location in the intact LC biosolids sample. The color scheme employed utilized white–yellow for high concentrations and blue–black for low concentrations. There are several areas of high Fe concentration hotspots scattered in the 400 by 400 µm image. Though the Mn concentration was considerably less than that of Fe, the spatial distribution of Mn was similar to that of Fe. Further, zones enriched with Zn, Cu, and Cr also corresponded with the zones of Fe and Mn enrichment in the intact LC biosolids sample (Fig. 1). The spatial distribution of Pb in the area chosen for mapping did not correlate with that of Fe. In comparison, all the elements mentioned above (Mn, Zn, Cu, Cr, and Pb) as well as Cd exhibited a distribution similar to that of Fe in the Nu-Earth sample (data not shown). The X-ray fluorescence map for Cd in LC sample was not collected. The spatial relationships that exist among these metals suggest that their ultimate fates are intertwined.

Correlations between fluorescent X-ray photons of Fe and Mn in both biosolids samples were strong and positive, with an r2 value ~0.8; only the data for the LC sample are shown here (Fig. 2 ). There are several possibilities that could explain these results, including co-precipitation of Fe and Mn, sorption of Mn onto Fe compounds or a combination of these mechanisms. We cannot rule out the possibility of co-precipitation of Fe and Mn compounds in the LC biosolids sample during treatments at the Blue Plains treatment plant and subsequently during the composting procedure at Beltsville. Nelson et al. (2002) reported that Fe(II) and Mn(III, IV) (hydr)oxide mixtures could form via a number of different mechanisms in the transition from reducing to oxidizing conditions. Abiotic oxidation of Fe(II) is faster than oxidation of Mn(II) (Stumm and Morgan, 1995). The oxidation of Mn(II) is kinetically inhibited and generally requires biological catalysis (Ghiorse, 1984; and Nealson et al., 1988). Depending on the relative concentrations of Fe(II) and Mn(II) and the presence of Mn-oxidizing organisms, mixtures of Fe oxides and biogenic Mn might have formed simultaneously or sequentially, both during treatments at the Blue Plains treatment plant and during composting at Beltsville.



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  		Fig. 2. The correlation between the fluorescence signals of Fe collected at 14 000 eV and Mn at 7102 eV in (a) the intact and (b) OC removed LC biosolids sample. Each point on the graph represents a pixel in Fig. 1.

 
Strong, positive correlations were observed between the X-ray fluorescence of Cu, Zn, and Cr and that of Fe in the intact LC biosolids sample (Fig. 3 ). The association of these elements with Fe compounds suggests that either these elements co-precipitated with Fe phases and/or that they were sorbed onto the functional groups of Fe phases. If this relationship were due to sorption, this elemental accumulation with Fe compounds would indicate that there was selectivity between those elements and the functional groups of Fe phases compared with any other discrete inorganic and organic components. There was little correlation between Pb and Fe in the intact LC biosolids sample. In contrast, the Nu-Earth biosolids sample showed a strong correlation between Pb and Fe with r2 = 0.9 (data not shown), which is in agreement with previous research (Hansel et al., 2001). The lack of correlation between Pb and Fe in the LC biosolids sample may have been due to the poor spatial distribution of Pb in the area chosen for mapping.



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  		Fig. 3. Correlations between the fluorescence signals of Fe and Cu, Zn, Pb, and Cr in the intact LC biosolids sample. The fluorescence signals of Fe, Cu, Zn, and Pb were collected at 14 000 eV and Cr was collected at 7102 eV. Each point on the graph represents a pixel in Fig. 1.

 
It should also be noted that part of the correlation observed (especially at low counts, close to the x, y intersection) between Fe and each of other metals could be due to the grain size being much smaller than beam size, which would lead to differences in the illuminated density of each x–y sample spot. Although this is not the main effect seen here, such effects could result in higher apparent correlations than actually occur.

The Nu-Earth sample was studied to determine if Cd was associated with Fe compounds in a biosolids sample. The area of a single map shown in Fig. 4 is 220 by 120 µm. Figure 4a shows an overview of one of the micro-spots of the Nu-Earth biosolids sample that was subjected to µ-XRF analysis. The curved feature in the optical image was an organic object coated heterogeneously with dark material (Fig. 4a). It was identified as an organic object because it did not refract light in the light microscope and because it was fibrous. Iron (Fig. 4b) and Cd (Fig. 4c) distributions of the map area are also shown. As in Fig. 1, white indicates a high concentration and black indicates a low concentration. Iron occurred in several hotspots scattered on this organic object, indicating that the dark coated material could be an Fe-rich compound(s). X-ray fluorescence intensity from Cd was considerably less than that from Fe, indicating the Cd concentration was less than that of Fe. However, the correlation between Fe and Cd in the map of X-ray fluorescence data was strong and positive with an r2 value of 0.95 (Fig. 4d). The strong associations of Cd and other elements with Fe observed in these microscopic studies support the macroscopic laboratory observations of the importance of Fe and Mn in adsorption–desorption of Cd in biosolids-amended soils (Li et al., 2001; Hettiarachchi et al., 2003).



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  		Fig. 4. (a) Optical image of 220 by 120 µm area for the intact Nu-Earth biosolids sample, (b) X-ray fluorescence map of Fe, (c) X-ray fluorescence map of Cd, and (d) correlation between Fe and Cd fluorescence signals.

 
The environmental abundance of Fe oxides with high specific surface area, the large number of electron-donating functional groups, and the affinity of certain metals for Fe oxide surfaces have resulted in trace-element adsorption to Fe oxides being extensively studied. At the Blue Plains water treatment facility, Fe was added to the LC biosolids in the form of "pickle liquor," a by-product of the steel industry that is essentially FeSO4 in H2SO4. This solution was dripped into the activated sludge process waters about 1 h before the end of that treatment. Dissolved Fe was oxidized in the aerobic water at pH > 7 and precipitated. Subsequently, the newly formed Fe oxides could sorb P and other anions, such as molybdate, arsenate, and organic matter (R.L. Chaney, personal communication, 2002). Moreover, these freshly precipitated Fe phases in the LC biosolids may be X-ray amorphous or micro-crystalline (not detectable by X-ray diffraction).

With recent advances in synchrotron-based X-ray spectroscopy techniques, many researchers have attempted to study mechanisms of elemental associations at the oxide–water interface (Charlet and Manceau, 1992; Manceau et al., 2000; Waychunas et al., 2002). Those studies showed that sorption of elements by Fe (hydr)oxides mostly involve adsorption, surface precipitation, and/or co-precipitation reactions. In addition to oxide–metal complexes, the formation of ternary complexes (oxide surface–metal–ligand) is also possible, as in the case of Cu–humic complexes on Fe oxides (Alcacio et al., 2001). Both inorganic and organic ligands can be involved in ternary complex formation. Enhanced Zn and/or Pb adsorption by Fe and Al oxides and/or soils has been shown to occur in the presence of phosphate or sulfate (Bolland et al., 1977; Shuman, 1986; Weesner and Bleam, 1998), suggesting the possibility of ternary complexation. Similarly, enhanced adsorption of phosphate by gibbsite in the presence of divalent metals such as Ca and Cd also suggests the adsorption of metal–phosphate complexes on the oxides (Helyar et al., 1976). Speculatively, sorbed anions such as phosphate, molybdate, and arsenate might have some positive effect on metal accumulation on Fe oxide surfaces.

Analysis of the OC reduced LC biosolids sample (from which nearly 75% of the original OC had been removed) indicated that all the elemental correlations except Fe and Cu remained strong (Fig. 5 ) and comparable to the intact sample (Fig. 1). The hypochlorite oxidation facilitated observation of elemental associations in an OC reduced sample relative to the intact sample. The disappearance of the Fe and Cu relationship after removal of OC suggests that Cu was associated with OC coatings that may have been present on Fe compounds. This observation would be in agreement with Pearson's hard and soft acid and base principle (HSAB) (Pearson, 1963). Sposito et al. (1981) showed that fulvic acid extracted from sewage sludge exhibited two distinct classes of functional groups: strongly acidic (soft) and weakly acidic (hard). The first class, the most strongly acidic functional groups, appeared to form stronger complexes with cations while the second group appeared to comprise several kinds of weakly acidic functional groups (hard), forming weak complexes with metals. Evangelou et al. (2002) observed that Cu2+ was more covalently bonded by humic acid derived from corn than was Ca2+ and Cd2+, and the nature of the covalent bond character appeared to be independent of pH. In other words, metal ion complexation ranked from most acidic metal (Cu2+, soft) to least acidic metal (Ca2+, hard). Although the study by Evangelou et al. (2002) was done using corn humic acid, it complemented the former study and shows that organic matter functional groups follow the HSAB principle. Moreover, humic acids strongly bind to Fe oxide mineral surfaces by inner sphere surface associations. Thus, in a system rich in both organic matter and mineral oxides, potential binding arrangements include metal cations complex to dissolved organic ligands, metal cations bound to mineral oxide surfaces only, metal cations bound to organic matter surfaces only, metal cations bound to organic matter that is itself bound to a mineral oxide surface, and/or metal–cation bridging between a mineral oxide surface and organic bonding sites (McBride, 1994; Alcacio et al., 2001). The present study indicates that the relatively soft metal Cu may have been associated with OC that itself was bound to Fe compounds.



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  		Fig. 5. Correlations between fluorescence signals of Fe and Cu, Zn, Pb, and Cr in the OC-removed LC biosolids sample.

 
Elemental correlations between Mn and Cr in both samples were strong before and after treatment to remove OC (r2 values were 0.74 and 0.6, before and after treatment, respectively). Further, Cr µ-XANES collected from different Mn and Fe hotspots of the intact LC biosolids sample indicated that the oxidation state of Cr was Cr(III) (data not shown). Many researchers have previously reported the importance of Mn oxides on Cr solubility/mobility in aqueous systems (Eary and Rai, 1987; Kendelewicz et al., 1999; Weaver et al., 2002). Manganese oxides are well known for their extraordinary sorption capacity and oxidizing ability (McKenzie, 1980; Fendorf et al., 1992). Manganese oxides have also repeatedly been shown to be the only naturally occurring inorganic phase capable of oxidizing aqueous Cr(III) complexes to aqueous Cr(VI) (Eary and Rai, 1987; Fendorf and Zasoski, 1992). Chromium(VI) is the most common redox-active inorganic ground water contaminant in the USA, as well as in some other industrialized countries, as it is more labile than Cr(III) (Nriagu and Nieboer, 1988; NRC, 1994). In contrast, possible reducing agents such as Fe(II) and organic matter could reduce Cr(VI) to Cr(III) (Eary and Rai, 1991). Chromium µ-XANES spectra collected in this study were from both Fe and Mn hot spots, making it difficult to verify the influence of Mn phases on the oxidation state of Cr.

The correlations between Mn and other elements (Zn, Cu, and Pb) in the intact sample were poor (r2 values were 0.18, 0.14, and 0.01 for Zn, Cu, and Pb, respectively). Some researchers have stated that Mn oxides are the solid phase most responsible for the long-term retention of heavy metals in soil and waters (Leeper, 1978; Balistrieri and Murray, 1982). It appeared that this was not the case for the biosolids samples of the present study. Manganese concentrations in the studied biosolids were minor (Table 1), and the chances for Mn to adsorb or co-precipitate with other metals alone without Fe might have been low due to the abundance of Fe compounds. The correlations between Mn and Zn, Cu, or Pb in the LC biosolids sample were improved after treatment to remove OC. Improvements in the correlations between Mn and the studied elements may also indicate that the treatment to remove OC had some effects on Mn compounds. Due to the high redox potential and high pH during the OC removal, some Mn in the sample might have been oxidized. This may have caused the redistribution of some elements in the biosolids sample, consequently improving the elemental correlations. This hypothesis can be supported by the µ-XANES analysis that showed in the intact biosolids sample the majority of Mn hotspots were dominated by the +2 oxidation state, whereas in the OC reduced sample the +4 oxidation state dominated (data not shown).

µ-XANES Speciation of Iron and Manganese Phases
The µ-XANES spectra of selected Fe standards and several different Fe hotspots are shown in Fig. 6 . The µ-XANES spectra of standard Fe compounds show that there are distinct features in each individual spectrum that can be used to identify the mineral species in the unknown biosolids sample. For example, the Fe–K edge spectra for samples containing only siderite show an absorption edge that begins at about 7115 eV and peaks at about 7125 eV. In addition to the main edge, siderite had a characteristic secondary peak at about 7138 eV. For samples containing only ferrihydrite, an edge jump begins at about 7120 eV and peaks at about 7130 eV. The LCFs of the Fe µ-XANES spectrum of an intact biosolids sample against the spectra of the standard Fe compounds showed that there were significant amounts of ferrihydrite (greater percentage) at all points, and that at least one other species was present at each sampled point (see Table 2). The XANES analysis suggested that a range of Fe minerals was present, and many points showed a mixture of oxidation states as well. First derivatives of the Fe–K edge showed that the tested points of the biosolids sample contained Fe2+ and Fe3+ mixtures in varying degrees (Fig. 7 ). The variation of Fe minerals in the biosolids samples illustrates the difficulty of trying to study the composition of biosolids Fe using conventional analytical techniques.

Examination of the µ-XANES spectra of Mn in the intact sample indicated that the majority of Mn hotspots were closely associated with Fe hotspots and contained more Mn(II) than Mn(IV) species (data not shown). The presence of some Mn(IV) in the intact biosolids sample and the strong positive correlation between the spatial distribution of Fe and Mn support the hypothesis that co-precipitation of Fe and Mn(IV) oxides exists in the system. Moreover, the Mn µ-XANES spectrum at Point 7 (Fig. 1) was very similar to that of MnCO3, indicating the possible presence of MnCO3 in the micro sites of the intact sample. The LCFs showed that there was no FeCO3 (siderite), but Point 7 consisted of about 65% of ferrihydrite and 35% of vivianite (Fig. 1 and Table 2), indicating that at that point Fe and Mn existed as discrete precipitates.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Microscopic observations made in this study support previous macroscopic observations, in both laboratory (Li et al., 2001; Hettiarachchi et al., 2003) and field (Brown et al., 1998), and suggest that Fe solid phases play an important role in metal retention in biosolids samples. This could be due to co-precipitation of these elements with Fe, adsorption of these elements onto Fe compounds, or a combination of both of those mechanisms. Since the spatial distribution of OC in these biosolids samples was unknown, our µ-XRF results could not show definitively in which phase the trace metals were specifically bound (whether directly with Fe oxides, in ternary complexes, or as organic complexes that are sorbed to Fe compounds). However, the consistency of correlations between Fe and trace metals (other than Cu) before and after the oxidation of the majority of organic matter in these samples provides indirect evidence for trace metals being associated mainly with the Fe compounds, and correlates with previous Cd adsorption and desorption studies. Furthermore, this study suggests that even if the metals were associated with organic matter, decomposition of organic matter will not result in substantial mobility of the trace metals as free ions because they will become associated with nearby Fe and Mn compounds, thus negating the "time-bomb" hypothesis. This study also shows that the employment of synchrotron-based research methods to examine complex environmental samples will greatly enhance our understanding of such systems.


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge three anonymous reviewers for their time and effort to improve this manuscript, as well as Rufus Chaney (USDA-ARS), C. Impellitteri (USEPA), and S. Al-Abed (USEPA) for their helpful discussions and suggestions. This research was supported in part by an appointment to the Postdoctoral Research Participation Program at the National Risk Management Research Laboratory administration by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USEPA. Although the research in this paper has been undertaken by the USEPA, it does not necessarily reflect the views of the Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. This work was performed at GeoSoilEnviro CARS (GSECARS), Sector 13, Advanced Photon Source at Argonne National Laboratory. The GSECARS is supported by the National Science Foundation-Earth Sciences, Department of Energy–Geosciences, W.M. Keck Foundation, and the USDA. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract no. W-31-109-Eng-38.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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