Geochemical and Hydrological Reactivity of Heavy Metals in Soils - Chapter 8 ppsx

However, several studies have shown that the form the metal takes in soils is of much greater importance than the total concentration of the metal with regards to the bioavailability to

8 Zinc Speciation in

Contaminated Soils Combining Direct and Indirect Characterization Methods

Darryl Roberts, Andreas C Scheinost, and Donald L Sparks

CONTENTS

8.1 Introduction 8.2 Approaches to Determining Metal Speciation in Soils 8.2.1 Single Extraction Methods

8.2.2 Selective Sequential Extraction Methods 8.2.3 Analytical Techniques

8.2.3.1 Synchrotron-Based Methods 8.2.3.2 Microspectroscopic Approaches 8.2.4 Advantage of Combining Techniques 8.3 Case Study: Zn-Contaminated Soil in the Vicinity of a Smelter8.3.1 Site Description, Sampling, and Soil Characteristics8.3.2 XRD and EMPA Analysis

8.3.3 Sequential Extractions 8.3.4 Bulk EXAFS Spectroscopy 8.3.4.1 EXAFS Data Analysis 8.3.5 EXAFS of Soil Samples 8.3.5.1 Surface Soil8.3.5.2 Subsurface Soils 8.3.6 EXAFS Combined with Sequential Extractions 8.3.7 Synchotron-µ-XRF

8.3.8 µ-EXAFS \8.3.8.1 Surface Soil\

8.3.8.2 Subsurface Soil \8.3.9 Desorption Studies\

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introduced to soils naturally as reaction products via the dissolution of metal-bearingminerals that are found in concentrated deposits Of a thousand Superfund sitesnamed in the U.S Environmental Protection Agency’s National Priority List of 1986,40% were reported to have elevated levels of heavy metals relative to backgroundlevels.2 The fate and mobility of these metals in soils and sediments are of concernbecause of potential bioaccumulation, food chain magnification, degradation of

The effective toxicity of heavy metals to soil ecosystems depends not only ontotal metal concentrations, but also, and perhaps more importantly, on the chemicalnature of the most mobile species The long-term bioavailability to humans andother organisms is determined by the resupply of the metal to the mobile poolfrom more stable phases Thus, quantitative speciation of metal species as well astheir variation with time is a prerequisite for long-term risk assessments Thecomplex and heterogeneous array of mineral sorption sites, organic materials,metal oxides, macro- and micro-pores, and microorganisms in soils provide amatrix that may strongly sequester metal ions Noncrystalline aluminosilicates(allophanes), oxides, and hydroxides of Fe, Al, and Mn, and even the edges oflayer silicate clays, to a lesser extent, provide surface sites for the specific adsorp-

strat-egy is attempted, it is wise to determine and understand the nature of the tions of metal ions with these reactive sites These interactions can be consideredone portion of the overall concept of metal speciation in soils However, thedetermination of metal speciation in complex and heterogeneous systems such assoils and sediments is far from a trivial task

interac-Speciation encompasses both the chemical and physical form an element takes

in a geochemical setting A detailed definition of speciation includes the followingcomponents: (1) the identity of the contaminant of concern or interest; (2) theoxidation state of the contaminant; (3) associations and complexes to solids anddissolved species (surface complexes, metal-ligand bonds, surface precipitates); and

of these parameters that can be identified the better one can predict the potentialrisk of toxicity to organisms by heavy metal contaminants Prior to the applicationL1623_FrameBook.book Page 188 Thursday, February 20, 2003 9:36 AM

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of sequential extraction techniques and analytical tools, researchers often relied ontotal metal concentration as an indication of the degree of bioavailability of a heavymetal However, several studies have shown that the form the metal takes in soils

is of much greater importance than the total concentration of the metal with regards

to the bioavailability to the organism.6,7 Metal speciation in soil and aquatic systemscontinues to be a dynamic topic and of interest to soil scientists, engineers, toxicol-ogists, and geochemists alike, as there remains no sufficient method to characterizemetal contaminants in all natural settings

The lack of a universal method of determining heavy metal speciation in naturalsettings comes as a result of the complexity of soil, sediment, and aquatic envi-ronments The multiple solid phases in soils include primary minerals, phyllosil-icates, hydrous metal oxides, and organic debris Metals can potentially bind tothese sorbents by a number of sorption processes, including both chemical andphysical mechanisms The mechanism(s) of metal binding strongly influences thefate and bioavailability of metals in the environment In addition to solid phases,the soil solution is also heterogeneous in nature, containing dissolved organicmatter and other metal-binding ligands over a range of concentrations This leads

to metal-ligand complexes in the soil solution and ternary complexes at thesolid–solution interface The presence of ligands in an ion-sorbent complex hasbeen shown to influence the atomic coordination environment of the ion and,

The partitioning of metal contaminants between solid and solution phases is adynamic process and an accurate description of this process is important in con-structing models capable of predicting heavy metal behavior in surface and sub-surface environments

A metal that has received a fair amount of attention due to its ubiquitous nature

in soils and sediments and role as a plant essential nutrient, is Zn Zinc is mined in

threat to biota and vegetation, while in areas that have elevated levels of Zn as aresult of smelting, land application of biosolids, or other anthropogenic processes,

it is often a detriment to the environment.10 At acidic pH values, Zn toxicity to plants

conditions, Zn is one of the most soluble and mobile of the trace metal cations Itdoes not complex tightly with organic matter at low pH; therefore, acid-leached soilsoften have Zn deficiencies because of depletion of this element in the surface layer.The degree of Zn bioavailability and, therefore Zn toxicity, is by and large determined

by the nature of its complexation to surfaces found in soils, such as phyllosilicates,metal oxides, and organic matter Research investigating Zn sorption using labora-tory-based macroscopic sorption experiments using oxide and clay minerals assorbents suggests Zn has variable reactivity and speciation in soils Sorption studieshave shown that Zn can adsorb onto Mn oxides, Fe (hydr)oxides and Al (hydr)oxides,and aluminosilicates.11 − 18 At alkaline pH values and at high initial Zn concentrations,the precipitation of Zn(OH)2, Zn(CO)3, and ZnFe2O4 may control Zn solubility.19,20

In these studies, however, direct determination of Zn sorption mechanisms andspeciation using spectroscopic and/or microscopic approaches was not employed,allowing room for further interpretation of the results

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With the advent of more sophisticated analytical techniques and their cation to soils and sediments, further information on the nature of Zn sorptioncomplexes in clay mineral and metal oxide systems has been gleaned Waychunas

(XAFS) spectroscopy and found that Zn forms inner-sphere adsorption plexes at low Zn sorption densities, changing to the formation of Zn hydroxidepolymers with increasing Zn sorption densities, and finally transforming to abrucite-like solid phase at the highest sorption densities in the study In a study

com-of Zn sorption on goethite, inner-sphere surface complexes were observed using

neutral to basic pH values, researchers have demonstrated that Zn can form bothinner-sphere surface complexes and Zn hydrotalcite-like phases upon sorption

find-ing in many of these studies is the fact that Zn-bearfind-ing precipitate phases oftenformed under reaction conditions well below the solubility limit of known Znsolid phases, suggesting that their formation in soils and sediments may havebeen overlooked using conventional approaches For example, the sorption kinet-ics of Zn on hydroxyapatite surfaces had an initial rapid sorption step followed

x-ray diffraction (XRD) and scanning electron microscopy (SEM) were not sitive enough to determine if precipitation was a major mechanism at high pHvalues (>7.0)

sen-With a substantial amount of Zn sorption studies performed using a tion of sophisticated analytical tools such as XAFS in mineral and metal oxidesystems, there is a natural progression to investigate Zn speciation in actual soilsand sediments By applying XAFS and electron microscopy to Zn-contaminatedsoils and sediments, Zn has been demonstrated to occur as ZnS in reduced envi-ronments, often followed by repartitioning into Zn hydroxide and/or ZnFe hydrox-ide phases, adsorption to Fe(oxyhydr)oxides, or incorporation into phyllosilicatesupon oxidation.27 − 30 Manceau et al.31 employed a variety of techniques, includingXRD, XAFS, and micro-focused XAFS to demonstrate that upon weathering ofZn-mineral phases in soils, Zn was taken up by the formation of Zn-containingphyllosilicates and, to a lesser extent, by adsorption to Fe and Mn (oxyhydr)oxides

combina-In addition to adsorption and precipitation as the primary mechanisms for Znremoval from solution, Zn may be effectively removed from solution via diffusion

of Zn ions into the micropores of Fe oxides.32,33 These studies demonstrate that inany given system, Zn may be present in one of several forms making directidentification of each species difficult using traditional approaches The majority

of studies employed to characterize the reactivity in Zn has dealt with relativelysimplistic systems, with one or two sorbent phases in question Clearly, naturalenvironments are much more complex and only after extensive studies in the abovesystems can one focus on natural samples To better illustrate this point, we nowturn our attention to the various approaches that have been used to identify metalspecies in soils and sediments, followed by a specific scenario of applying thesetechniques to Zn-contaminated soils

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8.2 APPROACHES TO DETERMINING METAL

SPECIATION IN SOILS

For most contaminated sites where practical considerations of limited money andresources are operational, the most efficient and cost-effective method of determiningheavy metal speciation is often desired One of the most commonly used approacheshas been to measure total metal concentration and correlate this to the amount ofmetal that may be bioavailable, based on thermodynamic considerations However,total concentration approaches overlook the fact that not all of the metal may belabile or available for uptake.7 Slightly more discriminating in the amount of metalextracted is the approach of single extractions using chemicals such as EDTA andDTPA This approach has been successfully applied to soils for both fertility assess-ments and for estimating the degree of contamination for heavy-metal impactedsites.34,35 These approaches generally cannot estimate the amount of slowly availablemetal that is released over time since extractions are carried out over a period ofseveral hours Moreover, the exact speciation of the metal is not gleaned using thesetypes of approaches However, these approaches continue to be developed and are

of great benefit given their relatively low cost and availability

8.2.2 S ELECTIVE S EQUENTIAL E XTRACTION M ETHODS

A more rigorous and complete alternative to determining metal speciation via totalmetal concentration and one-step extractions is the use of sequential extractions.Sequential extraction methods for heavy metals in soils and sediments have beendeveloped and employed in an effort to provide detailed information on metal origin,biological and physicochemical availability, mobilization, and transport.36,37 Aftermany studies and refinements, the chemical extractions steps are designed to selec-tively extract physically and chemically sorbed metal ions, as well as metals occluded

in carbonates, Mn (hydr)oxides, crystalline and amorphous Fe (hydr)oxides, andmetal sulfides The resulting extract is operationally defined based on the proposedchemical association between the extracted species and solid phases in which it isassociated Given that the extraction is operationally defined, the extracted metalmay or may not truly represent the defined chemical species, so care must be taken

to report the step in which it was removed rather than the phases it is associatedwith Many studies investigating the impact of mining and metallurgic activities onsoils have utilized various sequential extraction techniques in an effort to speciateheavy metals.38 − 40

The use of sequential extractions for metal speciation has other limitations andpitfalls as well These include (1) the incomplete dissolution of a target phases; (2)the removal of a nontarget species; (3) the incomplete removal of a dissolved speciesdue to re-adsorption on remaining soil components or due to re-precipitation with

These limitations are becoming more evident with the progress in research couplingsequential extractions with analytical techniques capable of directly determiningmetal speciation in soils and sediments.39,41,43,44,46 These studies, and future studies,L1623_FrameBook.book Page 191 Thursday, February 20, 2003 9:36 AM

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will certainly aid in explaining why selectively extracted metal fractions are often

sequential extractions will become more complete and universal, significantlyimproving our understanding of metal partitioning and mobility in soils Despite thelimitations of these approaches, however, sequential extractions continue to be valu-able for relative comparisons between contaminated sites, and due to their wide-spread availability and relative ease

in complex heterogeneous materials such as soils and sediments, a selective and

tech-niques is XRD For characterization of crystalline phases and minerals, XRD isextremely useful However, metal-contaminated soils and sediments often containthe metal in a form such that it is a minority phase below the detection limit ofthe instrument, or the important reactive phase is amorphous and only produces

a large background in the diffractogram Other ray–based techniques include ray fluorescence (XRF) spectroscopy and x-ray photoelectron spectroscopy (XPS).XRF has been used for decades to determine the concentration of trace metals insoils and sediments, with lower detection limits becoming more common with

con-centrations with no insight into metal speciation XPS, however, is a sensitive analytical technique that provides elemental chemical state and semi-quantitative information.49 The pitfall to this technique is that it isex situ, andrequires samples be dried and placed under ultra high vacuum that may lead to

Given the myriad of reactive phases in soils and their complex distribution inthe soil matrix, a technique capable of providing spatial and morphological infor-mation on heavy metal speciation is desired Microscopic techniques may resolvethe different reactive sites in soil at the micron level, thus allowing for a moreselective approach to speciation Examples of these techniques include SEM, elec-tron microprobe analysis (EMPA), and transmission electron microscopy (TEM) Inorder to glean elemental information and ratios, all the above techniques are oftencoupled with an energy dispersive spectrometer (EDS) While the above techniqueshave given insight into elemental associations and metal distributions in contami-nated soils and sediments, they do have a few drawbacks The most notable are thatEDS is only sensitive to greater than 0.1% elemental concentration, it is insensitive

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data.29,41,43 A study investigating Zn speciation in contaminated sediments found thatSEM coupled with x-ray EDS only provided elemental concentrations, but discerning

to locate Hg grains within a Hg-contaminated sample and was unable to distinguish

other studies have pointed out similar shortcomings of these techniques in speciatingmetal phases in soils and sediments.41,43 In all of these studies, the authors mentionand/or use XAFS as a more robust technique to characterize the metal phases andcomplexes found in their samples Indeed, given its sensitivity to amorphous species,

capable of discerning between the myriad of possible surface species occurring onthe submicrometer scale in soils and sediments We now turn our attention to theuse of this technique in determining metal speciation in natural environments

8.2.3.1 Synchrotron-Based Methods

The application of synchrotron light sources to address environmental issues hasprovided insight into the reaction mechanisms of heavy metals at interfacesbetween sorbent phases found in soils and the soil solution The most widely usedtechnique for this has been XAFS The term XAFS is a general term encompassingseveral energies around an absorption edge for a specific element, namely the pre-edge, near-edge (XANES), and extended portion (EXAFS) Each region providesspecific information on an element depending on the selected energy range, makingXAFS an element-specific technique Several articles provide excellent overviews

region, electron transitions lead to an absorption edge from which chemical mation of the target element, such as oxidation state, can be deduced EXAFS canprovide the identity of the ligands surrounding the target element, specific bond

information is extremely useful in speciation of metals in soils and sediments as

it provides quantitative information on the geometry, composition, and mode ofattachment of a metal ion at a sorbent interface.5 Given the intensity of synchrotronfacilities, this technique has a detection limit down to 50 ppm and can target aspecific element, potentially with little interference from other elements in the

not possible with any other technique Features that have dramatically increasedthe use of XAFS in environmental studies include more available synchrotronfacilities, more routine data analysis due to computer-based packages, and word

of mouth via professional meetings and journal articles Many studies can be found

in the literature detailing the use of this technique in order to speciate metals insoils and sediments.3,31,41,43,45,47,51,54 − 57 Nonetheless, XAFS does have limitationsand is by no means the only technique one should use for speciation of heavymetals in environmental samples

In soils and other natural samples, metal ions may partition to more than onereactive site, with each sorbent–sorbate complex providing a unique spectroscopicsignal In addition, the x-ray beam hitting the sample will inevitably bombard theL1623_FrameBook.book Page 193 Thursday, February 20, 2003 9:36 AM

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sorbent phase or other minerals in the matrix which may cause fluorescence, resulting

in an interference with the spectrum of the central element of interest (e.g., for Co

spectra obtained in doing these types of measurements represents the sum of all the

Therefore, the determination of all metal species is only as good as the ability toanalyze the data successfully In order to discriminate between species and quantifythem in a multispecies system, the target species must have different oxidation states,

or vary in atomic distances by ≥0.1 Å and/or coordination numbers by ≥1.59 Using

a nonlinear least-squares fit of the raw data or a shell-fitting approach of transformed data, typically only two species may be detected within a given sampleand there is a tendency to overlook soluble species with weak or missing second-shell backscattering in the presence of minerals with strong second-shell backscat-tering.31 This latter point often leads to an inability to successfully detect minormetal-bearing phases, even though they may be the most reactive or significant inthe metal speciation Discrimination among species has also been achieved usingthe linear combination fit (LCF) technique, where spectra of known reference speciesare fitted to the spectrum of the unknown sample LCF has been successfullyemployed to identify and quantify up to three major species, including minerals andsorption complexes.43,55 The success of the speciation depends critically on a spectraldatabase containing all the major species coexisting in the unknown sample, under-scoring the need to have a thorough database of reference spectra One way todetermine single species in a multispecies system separated by space is to use micro-

Logistical drawbacks to using XAFS include the availability of synchrotron lightsources, the increased demand for beam time at these facilities, and the difficulty inanalyzing data Clearly, the number of metal-impacted sites requiring metal speci-ation information far exceeds the amount of time available at synchrotron facilities.The combination of XAFS with more routine speciation techniques, such as sequen-tial extractions, is important, as the former technique has been able to detect artifactsand other shortcomings of the latter technique and may eventually lead to more

techniques with XAFS, the number of species may be reduced by chemical tion prior to attempting their identification by XAFS Moreover, the use of twoindependent methods for determining metal speciation in soils may provide a morereliable result than either of the methods alone

separa-8.2.3.2 Microspectroscopic Approaches

To date, standard bulk XAFS has been the most widely used synchrotron-basedtechnique used to characterize heavy metals in environmental samples However, insoils and sediments, microenvironments exist that have isolated phases in higherconcentrations relative to the average of the total matrix.53 For example, the microen-vironment of oxides, minerals, and microorganisms in the rhizosphere has been

Often these phases may be very reactive and of significance in the partitioning ofL1623_FrameBook.book Page 194 Thursday, February 20, 2003 9:36 AM

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heavy metals, but may be overlooked using other analytical techniques that measure

an area constituting the average of all phases With focusing mirrors and other

devices, the x-ray beam bombarding a sample may go down to a few square microns

in area, nearing the size of the most reactive species in soils, enabling one to

distinguish between individual species in a heterogeneous system In order to

maps to be obtained prior to analysis While EMPA is often not sensitive enough to

detect trace metals in soil, µ-SXRF offers sufficient sensitivity to investigate the

spatial distribution of trace metals and their spatial correlation with other elements

state of target elements in environmentally relevant samples since first- and

-EXAFS.31,61 − 63 With the advent of brighter, third-generation sources,µ-EXAFS has

been used to speciate metals in soils and sediments.30,31,64,65

In this brief overview of the approaches to speciation of metals in soils, sediments,

and other environmentally relevant settings, it is clear that no single technique

enables one to get an accurate and precise determination of metal speciation In fact,

several of the aforementioned studies that used a combination of chemical extraction

and analytical techniques such as XRD, microscopy, and x-ray absorption techniques

arrived at the conclusion that the most thorough results were achieved in combining

techniques.38,41,54 Since no single characterization method gives a complete

descrip-tion of surface structure or the geometric details of sorpdescrip-tion complexes, it is important

illustrate this point, the remainder of the chapter focuses on the combination of

several analytical techniques in determining and quantifying Zn speciation in a soil

contaminated as a result of smelting operations In addition, results from a leaching

experiment will serve to link metal speciation to metal bioavailability Each

tech-nique is presented in its own section, with a summary comparing and contrasting

the usefulness of each result This has been the focus of two separate papers, and

advantages of combining techniques will become clear, particularly when it comes

to determining the shortcomings of each technique

8.3 CASE STUDY: Zn-CONTAMINATED SOIL IN THE

VICINITY OF A SMELTER

8.3.1 S ITE D ESCRIPTION , S AMPLING , AND S OIL C HARACTERISTICS

Emissions from the Palmerton smelting plant in Palmerton, Pennsylvania have

con-taminated over 2000 acres of land on the north-facing slope of nearby Blue Mountain

in the Appalachians (Figure 8.1) The Zn smelting facilities (Smelters I and II) are

located in east-central Pennsylvania near the confluence of Aquashicola Creek and

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opened in 1898 by the New Jersey Zinc Company in order to process zinc sulfide

(sphalerite) from New Jersey ore In 1980, the plants stopped Zn smelting and in

1982 the U.S Environmental Protection Agency placed the facilities on its national

priorities list as a Superfund site The sphalerite ores contained approximately 55%

the facilities had an average annual output of metals measuring 47 Mg of Cd, 95

Mg of Pb, and 3,575 Mg of Zn Daily metal emissions since 1960 ranged from 6000

Sulfuric acid produced by smelting processes was also deposited in the surrounding

areas, contributing to strongly acidic soil pH values As a consequence, the dense

forest vegetation of Blue Mountain was completely lost and soils on hill slopes

almost completely eroded, exposing the underlying bedrock Several attempts have

been made to remediate the site and some revegetation has been successful, but

exposed soil surfaces and bedrock are still prevalent.69

The most heavily contaminated soil collected from a profile directly above

Smelter II was selected for detailed experiments The soil was collected from a pit

between exposed bedrocks, where a shallow soil profile <15 cm in depth persisted

The topsoil consisted of a 3- to 6-cm thick layer of dark, hydrophobic organic debris

consisting of only partially decomposed plant residues and soil organic matter The

accumulation of this amount of organic matter, which does not exist in surrounding

forest soils, is an indication of drastically reduced biodegradation The consolidated

subsoil about 20 cm in thickness is most likely the remainder of the original Dekalb

Undisturbed and bulk samples were collected from both topsoil and subsoil In

FIGURE 8.1 Location of Blue Mountain sampling site in the vicinity of the Palmerton

Smelter, Palmerton, Pennsylvania.

10 km

Palmerton

Walnutport

Little Gap Aquaschicola Creek

Blue Mountain

N

Jim Thorpe

Lehigh River

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addition, a sediment sample was collected from an artificial pond nearby, which was

dry at the time of sampling

Soil samples were air dried, sieved to collect the <2-mm size fraction, and then

measure-ments, the undisturbed samples, aggregates of several centimeters in diameter, were

air dried, embedded in acrylic resin (LR-White), cut, and polished into thin sections

DDI water for 24 h, sonified to break up aggregates, and wet sieved to collect the

<250-µm fraction From the subsoil sample, dark concretions 0.5 to 2 mm in diameter

were hand collected and ground in a mortar and pestle for XAFS analysis Soil pH

an X-Lab 2000 energy-dispersive x-ray fluorescence spectrometer (Spectro)

to generate polarized x-rays The lower detection limit was 0.5 mg/kg for most

metals Results of these analyses are presented in Table 8.1

determined by powder XRD using a Philips Norelco 1720 instrument equipped with

with 0.04° steps and a counting time of 5 sec per step Results indicated that quartz

is the most abundant mineral in both the topsoil and subsoil In addition to quartz,

the subsoil contained gibbsite, an Al-interlayered clay mineral (determined by ion

saturation and heating), and evidence of amorphous Fe and Mn oxides

Diffracto-grams of the topsoil showed peaks from franklinite (ZnFe2O4), a spinel-type mineral

XRD analysis of the subsoil did not reveal the presence of any Zn-bearing minerals

Similar studies done on soils with increased levels of heavy metals also had difficulty

identifying metal-bearing species using XRD, even when these species were readily

TABLE 8.1

Palmerton Soil Sample Characteristics

Source: Reprinted with permission from Scheinost, A.C et al., Environ Sci Technol 36, 5021,

copyright 2002 American Chemical Society.

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the best way to identify metal-bearing phases in soils, it does provide information

on possible sorbent phases present in the soil that may be capable of complexingmetal ions

Electron microprobe analysis was performed on the resin-embedded thin sections

microprobe equipped with wavelength-dispersive spectrometers (WDSs) Severalelements (Si, Al, S, P, K, Ca, Zn, Mn, Fe, and Pb) were mapped, and then thecompositions of selected sample areas were determined with higher precision Theimages were taken using a backscattered electron detector so that low Z elementsappear dark and high Z elements are bright Backscattered electron images (BSEs)and selected elemental distributions collected by EMPA analysis are shown for thetopsoil (Figure 8.2) and the subsoil (Figure 8.3) The main spherical entity in thetopsoil image is an organic aggregate with moieties of metal-bearing phases distrib-uted throughout, indicated by the bright white spots in the BSE Several Zn grains

most of these are associated with Fe and S Detailed quantitative WDSs of suchspots gave Fe/Zn ratios of 1 to 2 in agreement with those in franklinite, and S/Znratios of about 1 indicating either Zn sulfide or sulfate Regions of enriched Si and

K were also present, most likely representing quartz and K-feldspars, respectively.EMPA was less successful in identifying Zn-bearing phases in the subsoil, indicatingthat Zn was not found in abundant portions in any one phase Aluminum, Si, and

Fe in the maps in the subsurface soil indicated the presence of clay minerals and/ormetal oxides While not considered in this study, Pb was identified in both the surfaceand subsurface samples using EMPA In the surface soil, Pb was more evenlydistributed throughout the organic aggregates, and concentrated in two Pb-richparticles in the subsoil sample This suggests, qualitatively, that Zn and Pb behavedifferently in these soils

Sequential extractions (SSEs) were performed on the surface and subsurface soils,

remained after step 6, was digested with a two-step microwave procedure For the

and 1 ml of 48% HF, and then heated for 5 min at 150 W and for 15 min at 350

added and heated for 25 min at 250 W until a pressure of 15 bars was reached Novisible residues were left after this procedure Extracted Zn was measured by atomicabsorption spectrometry (Varian Spectra 220 Fast Sequential) The percentage of

sum of all SSE steps was in good agreement with the total amount determined by

In the topsoil, 86% of Zn was extracted in steps 6 and 7, indicating that Zn was

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FIGURE 8.2 Backscattered electron image of surface soil and corresponding x-ray elemental dot maps White colors indicate highest concentration

of target elements, and dark spots indicate low concentration.

10 µm

FIGURE 8.3 Backscattered electron image of subsurface soil and corresponding x-ray elemental dot maps White colors indicate highest concentration

of target elements, and dark spots indicate low concentration.

Untreated sample

1 1 M NH4NO3 Exchangeable metal ions,

water soluble metal salts

24 h, 20 °C

2 1 M NH4OAc (pH 6) Weakly complexed metals and

metals bound by carbonates

24 h, 20 °C

3 0.1 M NH3OHCl + 1 M

NH4OAc (pH 6)

Metals bound by Mn (hydr)oxides

Source: Reprinted with permission from Scheinost, A.C et al., Environ Sci Technol., 36, 5021,

copyright 2002 American Chemical Society.

FIGURE 8.4 Amount of Zn removed from surface and subsurface soils at each step of the

selective sequential extraction procedure Error bars indicate the standard deviation of

repli-cates (Reprinted with permission from Scheinost, A.C et al., Environ Sci Technol., 36, 5021,

copyright 2002 American Chemical Society.)

0 20 40 60

Extraction Step

topsoil subsoil

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the remaining 7% of Zn was released during steps 2 to 5 In addition, substantialamounts of Mn were released during all steps, with a maximum of 36% for step 6,

Fe was released during step 6, large amounts remained in the residual fraction,indicating the presence of a fairly stable Fe phase (Table 8.3) In the subsoil, whichwas less acidic (pH 3.9) and contained less Zn (900 mg/kg compared to 6200 mg/kg)than the topsoil, 58% of the total Zn was the readily exchangeable form (Figure 8.4,Table 8.3) The remaining Zn species were almost exclusively extracted during steps

6 and 7, suggesting that a similarly stable phase(s) as in the topsoil were present.Based on this first set of results, it was clear that the quantities of Zn species, andperhaps the identity of species, are different in the surface and subsurface soils

Zinc K-edge (9659 eV) EXAFS spectra of soil samples and Zn reference compoundswere collected at beam line X-11A at the National Synchrotron Light Source (NSLS),Upton, New York Details on the experimental setup and sample preparation can be

holders and sealed with Kapton tape For the surface soil, samples were dry sievedand the <2-mm fraction was collected For the subsurface soil, the <2-µm and <250-

µm fractions were collected In addition, dark nodules measuring 0.5 to 2 mm indiameter were collected from the subsurface soil and ground in an agate mortar andpestle All samples were measured at room temperature in fluorescence-yield modeusing a Stern-Heald-type (Lytle) detector filled with Kr gas Data scans were mea-sured in at least triplicate and up to ten scans, depending on Zn concentration in thesample, and then averaged to improve the signal-to-noise ratio Once raw XAFSdata were collected, they were converted to wave vector (k) units by assigning theorigin of the abscissa to the first inflection point of the edge EXAFS chi(k) functionswere derived from the spectra by modeling the post-edge region with a spline

Hanning window, resulting in radial structure functions (RSFs) The interatomicdistances shown in the RSF graphs are uncorrected for phase shift so that the truedistance is not represented

8.3.4.1 EXAFS Data Analysis

Two approaches were used in analysis of EXAFS data in order to most accuratelydetermine Zn speciation in the samples: a multishell fitting approach of the RSF

(PCA) Both methods rely on a linear least-squares fitting procedure For the tishell fitting, structural parameters of the first- and second-coordination shells weredetermined for model compounds and soil samples using theoretical paths generated

mul-by FEFF7.73 The estimated error using this approach is as follows: for bond distance

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the Debye-Waller factor (σ2), were determined for soil samples using the above

for the soil samples to the same parameters for reference materials (Figure 8.5, afterFourier transformation to R space), one is able to arrive at conclusions as to thepossible Zn species present in the sample However, soil samples are heterogeneous

in nature and the speciation represents an average of all species present In multiphasesystems such as soils, multishell fitting may lead to misjudging the number ofstructural parameters.31 Nonetheless, this method can be quite useful if care is takenand if combined with other fitting approaches

The second approach to fitting EXAFS data relied on linear least-squares fitting

of reference chi(k)*k3 to the experimental chi data In order to select the number andidentity of Zn-bearing reference spectra to be used for LCF, PCA was employed.With PCA consideration is given to the statistical variance within an experimentaldataset and breaks it down into principal components The statistically meaningfulnumber of components to regenerate the original input spectra and whether thesecomponents correspond to specific species is possible with PCA.30,75 Both the number

and identity of species in a set of samples can be estimated without requiring a priori

assumptions In order to make a large database of Zn-bearing references available,

a large number of samples were collected or synthesized and analyzed using EXAFSand are outlined in the following paragraph The selection of the reference mineralsand sorption samples made was based on the mineralogy of the soil, reports of otherresearches in the literature, and common phases encountered in laboratory studies

TABLE 8.3 Percent Metals Removed from Palmerton Soil Profile by SSE

Minerals provided by the Museum of Natural History, Washington, D.C., include:franklinite (ZnFe2O4), hydrozincite (Zn5(OH)6(CO3)2), smithsonite (ZnCO3), hemi-morphite (Zn4Si2)7(OH)2⋅H2O), and chalcophanite ((Zn,Fe,Mn)Mn3O7.3H2O); andsphalerite (ZnS) (Aldrich, 99.9+% purity) The Mineral Collection of the Swiss

from Bodenmais, Bayrischer Wald, Germany A natural lithiophorite sample

Swit-zerland Aqueous Zn2+ was prepared by dissolving 10 mmol/l of Zn(NO3)2 (Zn nitrate,

double-hydroxide phase was synthesized in the laboratory following the method of

(two-line, freshly precipitated); high-surface area gibbsite (synthesized and aged 30days, 90 m2 g−1); birnessite (45 m2 g−1); hydroxy-Al interlayered vermiculite (Al-verm) (University of Missouri Source Clays Repository, Sanford vermiculite, cleaned,

90 m2 g−1); and fulvic acid (Aldrich, 99% purity).76,77 For sorption samples, in an N2atmosphere, 10 g/l of solids were titrated with a 0.1 M Zn(NO3)2 stock solution toachieve Zn loadings of ca ±0.5 µmol/m2 for ferrihydrite and 1.5 µmol/m2 for the

FIGURE 8.5 Normalized Zn-EXAFS k3 -weighted chi spectra of reference materials and sorption samples used as empirical models for linear combination fitting (Reprinted with

permission from Roberts, D.R et al., Environ Sci Technol., 36, 1742, copyright 2002.

American Chemical Society.)

2 3 4 5 6 7 8 9 10 11 12

k ( Å -1 )

Zn- ferrihydrite Zn-birnessite

Zn-fulvic acid

aqeuous Zn 2+

gahnite

Zn-Al LDH lithiophorite chalcophanite hemimorphite

© 2003 by CRC Press LLC

period Solids were separated by centrifuging at 10,000 rpm for 10 min and stored

in a refrigerator as wet pastes until analysis The raw chi(k) × k3 EXAFS data for thereference mineral and sorption samples are presented in Figure 8.5 Visually, one caneasily identify characteristic backscattering features of heavier elements that can assist

in the initial identification of mineral samples, while adsorbed samples show spectradominated by backscattering from the first O shell Fit results for these referencesamples are presented in Table 8.4

8.3.5 EXAFS OF S OIL S AMPLES

8.3.5.1 Surface Soil

spectrum in both the left and right panels show representative fits for LCF andmultishell fitting, respectively (dashed lines = fits and solid lines = raw data) Forthe topsoil sample, multishell fitting revealed that Zn was tetrahedrally coordinated

could be fit with either a Zn or Fe atom at a distance of 3.49 Å Coordination numbersand distances of the O shell and the Zn shell are in line with those of franklinite

FIGURE 8.6 Bulk Zn-EXAFS k3 -weighted chi (left panel) and corresponding radial structure functions (right panel) resulting from Fourier analysis of chi data for surface and subsurface soil samples The dotted line in the top spectrum of the left panel results from LCF fitting, and in the right panel from a shell-fitting approach (Reprinted with permission from Roberts,

D.R et al., Environ Sci Technol., 36, 1742, copyright 2002 American Chemical Society.)

TABLE 8.4

EXAFS Parameters for Zn Reference Minerals and Sorption Samples