Lead Speciation and Association with Organic Matter in Various

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Lead Speciation and Association with Organic Matter in Various Particle-Size Fractions of Contaminated Soils Gautier Landrot, and SAENGDAO KHAOKAEW Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00004 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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Environmental Science & Technology

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Lead Speciation and Association with Organic Matter in Various Particle-Size Fractions of

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Contaminated Soils

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Gautier Landrot1* & Saengdao Khaokaew2

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* Corresponding author, [email protected]

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1 -Synchrotron SOLEIL, L’Ormes des Merisiers, 91190, Saint Aubin, France

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2 -Department of Soil Sciences, Kasetsart University, 50 Ngam Wong Wan Rd, Lat Yao Chatuchak, Bangkok

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10900, Thailand

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9 10 11 12 13

2nd revised manuscript word count:

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Abstract: 197 words (limit: 200 words)

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Text: 5455 words + 5 x 300 eq. w. (4 figures, 1 table) = 6955 eq. words (limit: 7000 eq. w.)

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Abstract Lead (Pb) stabilization in polluted soils treated by a Pb immobilization technique may be dependent on the

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speciation of Pb present in specific particle-size fractions of the soil. However, the scale-dependency of Pb

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speciation in contaminated soils is still not clearly understood. In this study, the natures and amounts of Pb

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chemical forms were determined in five Pb-polluted soil samples from Klity Village, Thailand, and their

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particle-size fractions. This was achieved using multiple analytical tools, including bulk Extended X-ray

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Absorption Fine Structure (EXAFS) spectroscopy at the Pb LIII edge. Results suggested that cerussite, Pb sorbed

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to goethite, and Pb-humate were present in specific amounts in all bulk samples and their particle-size fractions.

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The highest amounts of Pb-humate were found in the smallest particles of the soil samples. This Pb form was

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present in the fine particles of a soil sample, but remained undetected when analyzing the bulk sample. Since

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Pb-SOM association may impede the formation of pyromorphite in soils, the results implied that the extent of Pb

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immobilization in a polluted soil treated by P may be less than predicted if Pb speciation is only characterized at

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the macroscopic scale from the bulk soil sample prior remediation.

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1.

Introduction

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Soils polluted by lead (Pb) are ubiquitously found in the world. They represent a primary source of Pb

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exposure to human, especially young children.1 To remediate them, the phosphorus-based stabilization method

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represents one of the most cost-effective approaches.1 It consists in adding to the soil a phosphorus-bearing

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material, such as phosphoric acid,2 phosphate rocks,2, 3 or fishbones.4, 5 Phosphorus (P) then reacts with Pb to

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form pyromorphite (Pb5(PO4)3Cl), a very stable mineral.6 However, Pb speciation in a polluted soil may be an

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important factor controlling the extent of pyromorphite formation. For example, if a fraction of Pb present in a

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contaminated soil is associated to Soil Organic Matter (SOM)7 or Low Molecular Weight Organic acids

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(LMWOAc),8, 9 less pyromorphite forms when phosphorus is added to the soil. This is due to the strong capacity

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of SOM or LMWOAc to retain Pb, which could limit the amount of the metal available to react with phosphorus

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to form pyromorphite.7-9 Therefore, accurately determining the speciation of Pb present in contaminated soils

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before cleaning them up using the phosphorus-based remediation method may help estimate whether Pb can be

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efficiently stabilized.

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Metal speciation in soil samples can be determined in-situ by X-ray Absorption Fine Structure (XAFS)

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spectroscopy.10 This represents a more accurate approach than wet sequential extraction methods since the use of

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reagent solutions may modify the speciation of the target element present in the extracted soil fractions.11 Lead

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speciation in polluted soils has been determined at the macroscopic scale or scales of a few microns using bulk

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XAFS6, 7, 9, 12-15 or micro-XAFS,16 respectively. It has never been studied, using XAFS, at scales below 2 μm,

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which correspond to the size range of soil fine particles. These are small particles in soils and thus represent the

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soil solid components that have the highest potential to be transported through soils, notably via water advection.

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Therefore, if Pb present in a contaminated soil was partly associated to these water-dispersible particles, it could

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represent a reactive and mobile Pb fraction of the soil. This particle fraction of a Pb-contaminated soil thus

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particularly poses a potential environmental threat, especially if the form of Pb is bioavailable. 1 It would be then

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crucial to stabilize this fraction if the phosphorus-based method was applied to a contaminated soil to minimize

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the mobility of Pb. Microscopic clusters of pyromorphite can form after adding a P-bearing material to a Pb

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solution.3, 17, 18 Therefore, these clusters may have a lower mobility than aqueous Pb in the soil pore network.

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However, the extent of pyromorphite formation may be limited if Pb is associated with the residual phase,

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included in silica lattice. It can be also limited by Pb sorption to SOM present in soil.7 It is then important to

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study how much Pb, present in the soil particles smaller than 2 μm, is associated to SOM and understand the

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sorption mechanisms between Pb and SOM. The latter have been so far studied mainly based on analyses at the

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macroscopic scale of polluted soil samples or humic acids used as representative of SOM.13, 19, 20 It was found

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that Pb preferentially associate with carboxylic and alcoholic functional groups of SOM. Results obtained with

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bulk EXAFS indicated that Pb present in a contaminated soil was mainly divalent and complexed to aromatic

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salicylate and catechol-type functional groups of soil organic matter.13 A theoretical approach using the NICA-

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Donnan model, which was fitted to results obtained from macroscopic batch experiments, was employed to

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demonstrate that Pb may sorb to carboxylic-type groups via a monodentate mechanism and phenolic-type groups

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via bidendate mechanisms in humic acids present in soil samples.20 Results from Fourier Transform Infrared

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spectroscopy (FTIR) analyses suggested that Pb was bound through carboxyl- and alcoholic moieties of

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estuarine humic acids.19 It is unclear whether these retention mechanisms are similar to those occurring in the

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smallest soil particles of a Pb-contaminated soil. The scale dependency of Pb speciation in soils is also not well

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understood. It is unknown whether the Pb amounts associated to SOM in contaminated soil samples, determined

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at the macroscopic scale by bulk XAFS such as those reported in previous studies,12, 13 are similar to those

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specifically present in the soil particles smaller than 2 μm. The goal of this investigation was to study, using

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several analytical techniques including XAFS, the chemical forms of Pb in bulk soil samples collected at a

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polluted site. Lead speciation and retention mechanisms between SOM and Pb were also specifically

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characterized in the silt (2 - 50 μm particles) and fine (< 2 μm particles) fractions of the soil samples. The



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ultimate goal was to characterize the scale-dependency of Pb speciation in soils by comparing the nature and

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amount of Pb chemical forms, including Pb sorbed to SOM, constrained in bulk soil samples or their specific

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particle-size fractions. This enabled to determine for the first time whether Pb speciation in the mobile, water-

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dispersible soil particles is similar to the one constrained at the macroscopic scale from the bulk soil sample.

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This must be known to accurately predict the extent of Pb immobilization in a polluted site before treating the

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soil with a stabilization approach.

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2.

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Materials and Methods 2.1 Contaminated soil samples and particle-size extractions The soil samples were excavated in the Upper Klity Village of Kanchanaburi, Thailand. This site was

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chosen to carry out this investigation as it represents the worst case of Pb pollution in Thailand.21 In 1998,

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torrential rain flooded this area, including the site of an ore processing factory owned by a Pb mine company and

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located in the Eastern part of the village. The Pb crude ore mined by the company consisted of cerrusite

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(PbCO3), galena (PbS), and some anglesite (PbSO4) in limestone, clay, and quartz matrices.21 The flooding

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transferred the content of the factory’s waste dump to the nearby Klity river via surface runoff and contaminated

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the entire creek downstream from the factory site, including the Lower Klity village. The latter is located along

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the Klity river, a few kilometers downstream from the Upper Klity village. Over the years following this event,

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many Klity villagers are believed to have suffered severe health issues due to Pb poisoning.21

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Five soil samples were collected using a composite technique at five locations in or around the

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abandoned mining factory site of the Upper Klity Village (Figure S1). At least 15 surface soil samples (0-30 cm)

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were randomly collected per location and mixed together. Additional information on the soil samples, such as

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their geographic coordinates, is provided in SI (Figure S1). The five soil samples were referred in this study as

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“Dump Soil”, excavated next to the factory dump, which is now covered by a cement slab; “Soil Near Dump”

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excavated a few dozens of meters away from the dump; “Soccer Field” excavated in a community playground of

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the village; “Soil Near House” excavated next to an abandoned house whose owner has been relocated elsewhere

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due to the Pb pollution; and “Garden Soil” excavated in the garden of the abandoned house.

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Ultra-pure DI water used in this study was made by a Synergy Water Purification System (Millipore)

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with an 18.2 µΩ/cm resistivity at 25°C. The silt and fine fractions were extracted following an extraction method

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that did not require the use of chemical reagents.22 The sand, silt, and fine particles were successively extracted


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by sonication and wet sieving steps. The fine particles were collected via centrifugation. Details on the

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fractionation method are provided in SI.

107 108

2.2 Soil Characterization The bulk soil samples as well as coarse sand, fine sand, silt, and large fine fractions were digested using

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aqua regia and a BossTech digestion block (Scientific Instruments). The concentrations of Pb, phosphorus (P),

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calcium (Ca), iron (Fe), and manganese (Mn), were measured using an ICP-AES (Fisher Brand) from the

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particle-size fractions or bulk samples, respectively. The concentration of Pb in the < 0.8 µm fraction of each soil

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sample was inferred from mass balance using the bulk Pb concentration, the weight of each extracted particle-

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size fraction and its Pb concentration. Soil texture and organic carbon content were determined using the

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Hydrometer23 and Walkley-Black titration24 methods, respectively. Details of these two methods are provided in

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SI. Soil pH was measured by a pH meter (FE20, Mettler Toledo) in a soil suspension containing 50 g of soil

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sample and 100 ml of DI water. The Cation Exchange Capacity (CEC) was measured using the ammonium

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acetate method.25 The CaCl2 extraction method was employed to extract Pb from the soil samples, using a 1:10

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(w:v) soil suspension prepared with a 0.01 mol/l CaCl2 solution.26 This suspension was shaken for 3 hours.

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Additionally, the diethylenetriaminepentaacetic acid-triethanolamine (DTPA-TEA) extraction method was

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employed to extract Pb from the soil samples, using a 1:10 (w:v) soil suspension prepared with a solution of

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0.005 mol/l DTPA, 0.01 mol/l CaCl2, and 0.01 mol/l TEA, adjusted to pH 7.3. This suspension was shaken for 2

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hours.27, 28 At the end of both extraction experiments, the soil suspension was filtrated using 2.5 μm-pore size

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filtration paper (n°42, Whatman). The filtrate was analyzed by ICP-AES for Pb concentration.

124 125

The silt and fine fractions of the soil samples were analyzed by X-ray diffraction spectroscopy (XRD) using

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a D2 Phaser (Brucker), 0.05 step size, and 0.5 tour/minute rotation speed. Thin soil sample sections were

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prepared by Spectrum Petrographics, WA, USA. Briefly, each soil sample was embedded in an epoxy resin and

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cut to a 30-50 µm slab. The latter was fixed onto a quartz slide using a suprasil glue. The distribution of Pb in the

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thin soil sections and its possible spatial co-distributions with other elements were determined using Scanning

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Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM-EDS) or μ-XRF and μ-XRD at

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DiffAbs microprobe beamline, Synchrotron SOLEIL. The latter analyses were done using a 18 keV

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monochromatic beam from a Si(111) monochromator, a 4-element silicon drift fluorescence detector, and a 2D



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XRD camera (XPad). The synchrotron storage ring was operated at 2.75 GeV with a 500 mA beam current and a

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multi-bunch mode.

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2.3 Pb speciation determined by XAFS

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Most inorganic or organic Pb reference compounds analyzed by XAFS were synthesized or purchased.

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Details on the Pb standards used in this study are provided in SI. All XAFS analyses were performed at SAMBA

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beamline, synchrotron SOLEIL. The storage ring was operated at 2.75 GeV with a 100 mA beam current and an

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8-bunch mode. A stack of 1.25 mm-thick aluminum foils was placed in front of the first ionization chamber (I 0)

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to reduce the brightness of the incoming X-ray beam. The beamline was equipped with a Si(220)

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monochromator and a 35-pixel Ge fluorescence detector. The samples were analyzed at 20 K using a He

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cryostat. All scans were collected at the Pb LIII edge from 12800 to 14500 eV using a continuous scan

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acquisition mode, 10 eV/s monochromator velocity, and 0.08 s/point integration time. Each scan was then

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obtained in 170 seconds and featured 2125 data points with a 0.8 eV step size. Multiple scans were collected for

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each sample until observing no significant improvement in the signal-to-noise ratio of their merge spectrum and

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its corresponding chi EXAFS function. The latter was obtained from the raw spectrum using Demeter and Ifeffit

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software package.29 The merged XAFS spectrum corresponding to each soil sample or Pb reference compound

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was normalized using linear and quadratic function to fit the pre-edge and post-edge region of the spectrum,

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respectively. The chi EXAFS function was extracted using the Rbkg algorithm featured in Ifeffit, and k2-

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weighted. Artemis from Demeter and Ifeffit Software Package was employed to perform shell-by-shell fitting of

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the Fourier Tranform of the chi EXAFS function corresponding to the Pb-humate reference. All k2-weighted chi

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EXAFS functions, from 2 to 10 A-1, corresponding to the bulk samples, silt, or fine particle-size fraction, were

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grouped into a data matrix. A Principal Component Analysis (PCA) followed by a Target Transformation

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Analysis was performed on each data matrix, using Sixpack.30 The PCA was employed to determine the number

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of reference spectra required to describe the data matrix within experimental error. The Target Transformation

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approach31 was used to determine whether each Pb reference considered in this study could represent one of the

157

principal components of the data matrix. Lastly, a Multivariate Curve Resolution – Alternating Least-Squares

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(MCR-ALS) method was employed as an alternative approach to identify the nature of principal components,

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using the Matlab toolbox of Jaumot et al. (2015).32 This approach enabled to directly extract the spectra

160

corresponding to the principal components of the data matrix. Additional information on PCA, Target

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Transformation, and MCR-ALS is provided in SI. To identify the nature of the pure phases whose corresponding


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EXAFS spectra were extracted by MCR-ALS, an R factor was calculated for each possible combination of pure

163

phase and Pb reference (Equation 1).

164 R factor =

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∑n i |ChiPb standard (i)−Chipure phase (i)| |ChiPb standard (i)|

Equation 1

166 167

The n and i parameters in Equation 1 refer to the total number of data points and the ith data point in the chi

168

EXAFS spectra, respectively. Prior to each R calculation, the chi spectrum corresponding to the pure phase

169

extracted by MCR-ALS was rescaled using an amplitude factor, which was obtained using a non-linear least-

170

square regression fitting approach. This was used to minimize the difference between the amplitude of the pure

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phase spectrum rescaled by the amplitude factor, whose value was floated in the fit, and the amplitude of the Pb

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reference spectrum. Details on the methodology employed to quantify lead chemical forms in each soil fraction

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are provided in SI.

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3. Results and Discussion

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3.1 Preliminary characterization of the soil samples

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The highest Pb concentration allowed in Thai soils is set at 400 mg/kg.33 Lead concentrations in all

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Klity soil samples (Table S2) were much higher than this limit. They were nearly 10,000 mg/kg in three soil

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samples and almost reached 100,000 mg/kg in two soil samples, which were excavated next to the dump of the

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old factory site or a few dozens of meters away from it. Therefore, these two soil samples may contain

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significant amount of Pb originating from the ores processed at the old mine factory since the [Pb] was around

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650,000 and 35,000 mg/kg in the concentrate and tailing parts of the ore, respectively.21 The total organic carbon

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content varied from ~ 1.1 to 3.5 % between the five soil samples. A fraction of the organic carbon present in the

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soil samples, especially Dump Soil, might originate from the organic additives used at the old factory in the ore

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flotation step, including potassium-amyl-xanthate (KAX), dispersant, and pine oil.21

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The amounts of sand, silt, and fine particles extracted for each soil samples by the sonication/wet

186

sieving method were similar to those extracted with the hydrometer method (Table S3). Lead was present in all

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extracted particle-size fractions of each soil sample (Figure S3). It was mainly (~ 80 % or higher) present in the

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particles larger than 2 µm in the two soil samples featuring a ~ 10 % w/w Pb total concentration. These fractions

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could correspond to large lead ore particles, since the two samples were collected at, or near the factory dump. In

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contrast, a large amount of Pb (~ 40 % or higher) was present in the fine particles in the three other soil samples,



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which featured a ~ 1 % Pb total concentration. These Pb-bearing fine particles may correspond to Pb ore

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particles originating from the dump and weathered or size-fractionated by transport since the three soil samples

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were collected a few hundred meters away from the dump (Figure S1). The sizes of the particles in the extracted

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nano fine fractions, including the average sizes or maximum sizes, were specific to the soil samples (Figure S4).

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The largest particles in all extracted nano fine fractions were found for the Soil Near House and Soccer Field

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samples, and had sizes of about 800 nm.

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Results from SEM-EDS analyses of the soil samples suggested that dolomite (CaMg(CO₃)₂) was

198

present in all soil samples since large microscopic grains (> 10 µm) containing both Ca and Mg were observed at

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the same locations in most maps collected (Figure S5). Similarly, results from XRD analyses indicated the

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presence of dolomite in the silt and fine fractions of some soil samples, and also cerussite (PbCO3) (Figure S6).

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This is consistent with results reported in a previous study, which demonstrated based on sequential extractions

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that Pb was significantly distributed with the carbonate fractions in soil samples from Klity village.21 No study,

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however, has so far employed an in-situ technique, such as XAFS, to accurately determine Pb speciation in the

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polluted soils of Klity, whose Pb contamination is regarded as one of the worst environmental disasters in

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Thailand.

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3.2 Lead speciation

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Results from PCA revealed that three principal components were present in the bulk, silt, or fine sample

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mixture (Figure S7). Among all Pb references considered in this study (Figure 1), cerussite, Pb-humate, and Pb-

209

sorbed-to-goethite references always gave acceptable SPOIL values when these three standards were

210

individually target-transformed from each data matrix (Table 1). These references could then represent the three

211

principal components present in the bulk, silt, and fine sample mixtures. They were, however, not the only Pb

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standards that gave acceptable SPOIL values for each sample mixture (Table 1). A second approach was then

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employed to identify the nature of the three principal components present in each sample mixture. An MCR-ALS

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method was used to extract, from each sample mixture, the EXAFS spectra corresponding to the three principal

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components, labelled Pure Phase A, B, and C. The lowest R factor values associated to Pure Phase A, B, and C

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in each sample mixture were obtained with cerussite, Pb-humate, and Pb-sorbed-to-goethite standards,

217

respectively (Table S4). The shape of the EXAFS spectrum of cerussite matched well (R factor < 0.2) with the

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one of Pure Phase B extracted by MCR-ALS from each sample mixture (Figure 2). Shape differences were

219

observed between the EXAFS spectra of Pb-humate and Pure Phase A or Pb-sorbed-to-goethite and


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Figure 1

EXAFS spectra collected at the Pb LIII edge of a) Pb(aqueous), Pb sorbed species, inorganic Pb

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references, and b) Pb organic complexes

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Table 1

SPOIL values obtained after target-transforming each Pb reference compound using

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the bulk, silt, or fine data matrix. All acceptable SPOIL values (i.e. ≤ 4.5) per data matrix are highlighted with a

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specific color.

Data matrix Bulk

Silt

Fine

Pb-humate

3.0

2.5

2.0

Pb sulfate

6.4

4.5

3.6

Pb sorbed to birnessite

3.6

4.5

8.3

Hydroxylpyromorphite

12.3

5.5

3.3

Galena

16.0

21

16.4

Pb dioxide

6.8

3.1

4.4

Pb hydrogen phosphate

5.2

10

3.9



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Hydrated Pb oxides

5.6

2.6

1.8

Alamosite

7.5

4.4

3.8

Pb sorbed to goethite

3.9

3.3

4.3

Synthetic chloropyromorphite

11.3

5.7

3.3

Cerussite

2.3

3.0

1.1

Pb aqueous

4.2

5.5

11.3

Natural chloropyromorphite

9.1

9.1

10.4

Hydrocerussite

17.5

16

5.9

Natural plumbogummite

11.2

6.2

6.1

Pb oxide

10.1

9.4

7.6

Pb(IV) acetate

5.0

2.4

2.6

Pb malonate

4.4

3.8

11.3

Pb malate

5.0

5.1

11.1

Pb formate

12.8

4

1.9

Pb benzoate

4.4

4.4

10.7

Pb succinate

5.6

3.2

2.5

Pb oxalate

12.6

6.7

5.0

Pb citrate

6.4

3.1

2.0

Pb catechol

7.9

9

13.4

Pb salicylate

6.1

5.5

3.2

Pb(II) acetate

13.9

5

2.2

227 228

Pure Phase C (Figure 2). This could indicate differences in Pb speciation between the Pb references and

229

extracted pure phases. This could also indicate the presence of a mixture of Pb chemical forms existing in the

230

samples and appearing by PCA as one principle component. 34 Therefore, the nature of the pure phases present in

231

each sample mixture could not be unambiguously identified from the XAFS data processed by the Target

232

Transformation or MCR-ALS approach. However, results obtained from both approaches suggested that

233

cerussite, Pb-humate, and Pb-sorbed-to-goethite were, among all references considered, the species the most

234

likely present in each sample mixture. Additional evidences on the possible presence of cerussite and Pb-sorbed-

235

to-goethite in the sample mixtures were obtained using analytical techniques other than bulk XAFS. The

236

presence of cerussite in the soil samples was detected by bulk (Figure S6) and micro (Figure 3 c) X-ray

237

diffraction. The presence of cerussite in the soil samples is likely since this mineral phase is known to be the

238

main Pb form found in the region of the study site.35 Cerussite may represent a stable Pb form as each soil

239

sample was slightly or moderately alkaline based on soil pH (Table S2). Results from µ-XRD indicated the


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240

241 242

Figure 2

Chi EXAFS spectrum of Pure Phase A, B, and C extracted by MCR-ALS from the

243

bulk, silt, or fine data matrix, and Chi EXAFS spectrum of Pb reference that provided the lowest R factor value

244

for each extracted pure phase

245 246

presence of goethite in the Klity soil samples (Figure 3 c). Lead sorbed to iron oxides can represent one of the

247

main Pb forms in soils.36, 37 A study aiming to determine Pb speciation in a contaminated soil, using XAFS,

248

reported that Pb was significantly associated with iron and manganese oxides. 12 These types of associations

249

could occur in the soil samples since a spatial co-distribution between Pb and Fe as well as Pb and Mn were

250

observed in some µ-XRF maps (Figure 3 a, Figure S8). They were particularly obvious in the µ-XRF maps at

251

specific locations, which appeared as Fe or Mn hot spots. These hot spots could be > 100 µm2 in size, such as

252

Spot 1 & 2 shown in Figure 3 a. It was, however, likely that more Pb was associated to iron oxides than

253

manganese oxides in the soil samples. The iron contents in the five soil samples were between 5 and 60 times

254

higher than the manganese contents (Table S2), suggesting a higher content of iron oxides than manganese

255

oxides. Also, the soil samples were excavated in Kanchanaburi province whose soils are essentially oxisols,38

256

which are known to be rich in iron oxides. Lastly, iron oxides and soil organic matter are considered as principal



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257 258

Figure 3

a) µ-XRF co-distribution map of Pb, Fe, and Mn, and b) the corresponding Pb/Fe and

259

Pb/Mn pixel intensity scatter plots, for the soil sample “Soil Near House”. Spot 1 and Spot 2 encircled in the

260

map are examples of macroscopic hot spots where Pb/Mn and Pb/Fe seemed to be present at the same locations.

261

c) µ-XRD patterns taken at Pb and Fe hot spots chosen randomly from the µ-XRF maps of the soil samples, and

262

XRD reference patterns of cerussite and goethite.

263 264

Pb-sinks in soils.39 A study13 demonstrated that Pb was mainly associated to catechol and salicylate-types

265

functional groups of SOM in a soil polluted by alkyl-tetravalent Pb compounds. This was found using a linear

266

combination method where multiple EXAFS reference spectra were considered. Each of them corresponded to a

267

complex between Pb and an organic acid representing a specific functional group in SOM. Another

268

investigation12 employed a different approach to study the association between Pb and SOM in polluted soils.

269

Instead of considering multiple EXAFS standards to represent the association between Pb and SOM in the soil

270

samples, only a single reference was employed in the study, which consisted in a Pb-humate EXAFS spectrum.

271

These two different approaches were used in the present investigation to study the association between Pb and

272

SOM. A Pb-humate reference was employed as a global representative of Pb-SOM association. The Pb -formate,

273

-acetate, -oxalate, -malonate, -succinate, -malate, -citrate references represented aliphatic carboxylic acids of

274

SOM. The Pb -catechol, -salicylate, -benzoate references represented aromatic functional groups of SOM. The

275

Pb -glycinate represented amino acid functional group of SOM. None of these standards representative of a

276

specific type of Pb-SOM association, including the Pb-catechol and Pb-salicylate references, was identified as


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277

principal components in the bulk soil samples or their extracted particle-size fractions based on the Target

278

Transformation and MCR-ALS methods. However, results obtained from these two methods suggested that the

279

third principal components in all sample mixtures may correspond to the Pb-humate reference. This was

280

consistent with the results reported in a previous study, which indicated that more than 80 % of Pb in a

281

contaminated soil was in the form of Pb humate.12 Almost no Pb could be extracted from the soil samples by the

282

CaCl2 extraction approach, which is used to extract only water-soluble metal28 (Table S5). In contrast, the

283

amount of Pb extracted from each soil sample with the DTPA-TEA method was much higher than the amount of

284

Pb extracted with the CaCl2 extraction method (Table S5). The DTPA-TEA method is used to extract water-

285

soluble metal, exchangeable, and partially organic-bound metals.28 Hence, these results were consistent with the

286

possibility of having a Pb fraction present in the soil samples in the form of organic-bound Pb. Accordingly, the

287

linear combination fitting of each bulk sample or particle-size fraction was done using cerussite, Pb-sorbed-to-

288

goethite, and Pb-humate standard (Figure 4). Lead was entirely in the form of cerussite in the samples collected

289

in or near the dump, based on XAFS analyses of the bulk samples. This contrasted with the results reported in a

290

former study,21 which indicated that about 20 % of Pb present in a mine tailing sample collected in Klity Village

291

was in a carbonate form. This amount was quantified with a sequential extraction method. This approach is less

292

accurate than the XAFS technique to quantify metal species in soils as the use of reagent solutions may modify,

293

in the extracted soil fractions, the speciation of the target element, which can reabsorb to other soil components

294

during the extraction steps.11 This could explain the difference between the cerussite content in the dump soil

295

measured in this study and the lead carbonate amount in the sample studied in the previous investigation.

296

Alternatively, this difference could be due to the fact that the samples studied in the two investigations were not

297

collected at the same exact locations in Klity Village. The amount of Pb-humate in the < 0.5 µm fraction of

298

Garden Soil was more than twice the amount of Pb-humate in the silt or 0.5-2 µm fraction. Similarly, the amount

299

of Pb humate was much higher in < 0.4 µm fraction than 0.4-2 µm fraction of Soil Near Dump. The highest

300

amounts of Pb-humate in Soil Near House and Soccer Field were also found in the nano fine fraction of these

301

two soil samples. The amounts of Pb-humate were then the highest in the smallest solid particles in all soil

302

samples studied in this investigation except Dump Soil as Pb speciation was not determined in its < 0.6 µm

303

fraction.

304

All XAFS analyses in this study were done using a 2 mm x 0.5 mm X-ray beam. The results indicated

305

that cerussite was the unique Pb chemical form in the bulk sample of Soil Near Dump or its extracted silt

306

particles, while Pb-humate and Pb-sorbed-to-goethite were present in its < 0.4 µm or 0.4-2 µm particles (Figure



Page 14 of 25

307

4). This implied that if an X-ray beam as small as a fine particle (i.e. < 2 microns) was employed to locally

308

determine, using XAFS, Pb speciation in the bulk sample of Soil Near Dump, it would be possible to find not

309

only cerussite but also Pb-humate and Pb-sorbed-to-goethite. Therefore, the results suggested that the nature and

310

amounts of the Pb chemical phases determined by XAFS from the bulk soil sample could be dependent on the X-

311

ray beam size, and thus scale of analysis. To our knowledge, this possible scale-dependency in Pb speciation

312

constrained by XAFS has never been reported in the literature.

313

314 315 316

Figure 4

317

corresponding amounts in mg/kg (details of calculations in SI) of Pb chemical forms in the five bulk soil samples

318

and their particle-size fractions.

Linear combination fitting results, with Pb-goethite referring to Pb-sorbed-to-goethite, and the

319 320 321 322

3.3 Mechanisms of Pb sorption onto SOM In two previous studies,12, 40 the magnitude of the fourier transform of the EXAFS spectrum corresponding to Pb-humate was fitted using a shell-by-shell fitting approach to determine at the molecular level the sorption


Page 15 of 25


323

mechanisms involved between Pb and SOM. The results from these two studies were not identical to each other

324

(Figure S9). Results from the first investigation40 indicated that Pb was surrounded by four oxygen atoms at 2.30

325

Ǻ, and two carbon atoms at 3.25 Ǻ. This implied that Pb was sorbed to humic acids as an inner-sphere complex.

326

In contrast, results reported in the second study12 indicated that Pb was only surrounded by ~3 oxygen atoms at

327

2.41 Ǻ. A possible second shell could not be fitted due to limitation in data quality. A similar approach was

328

employed in the present study to fit the magnitude of the Fourier transform of the EXAFS spectrum

329

corresponding to the Pb-humate standard (Figure S9). An R factor of 0.006, which indicated a reasonable fit

330

since the value was lower than 0.05,34 was obtained for the first model considered where Pb was surrounded by

331

four oxygen atoms at 2.35 Ǻ and 0.5 atoms of carbon at 2.88 Ǻ (Figure S9). The latter distance is similar to the

332

2.96 Ǻ distance between Pb and C in Pb succinate.41 An R factor of 0.018 was obtained for the second model

333

considered where Pb was only surrounded by four oxygen atoms at 2.32 Ǻ. Therefore, this R factor also

334

indicated a good fit despite its value was higher than the one corresponding to the R factor obtained for the first

335

model. Each model could be then validated based on the goodness of fit represented by the R factor. However,

336

the Hamilton test42 (Table S6) performed on these two models revealed that there was a 2.5 % chance that the

337

two fits were similar to each other. Since this percentage was lower than 5 % and a 95 % confidence interval was

338

considered for this test, the difference between the two fits did not occur by random variation. Therefore, the first

339

model was significantly better than the second one based on this statistical test. This implied that the presence of

340

the carbon atom at 2.88 ± 0.02 Ǻ could not be ruled out and a sorption mechanism as an inner-sphere complex

341

between Pb and humic acids did occur. Lead was then strongly associated to SOM since these two entities,

342

associated together as an inner-sphere complex, shared chemical bounds between each other.10 This explains the

343

strong capacity of SOM to retain Pb in soils, which has been mainly observed so far from macroscopic-scale

344

analyses.7, 12, 13 However, the small coordination number (0.5 ± 0.2) associated to the Pb-C shell could suggest

345

that the metal was sorbed to humic acids at more than one sorption site, possibly as outer-sphere and multi inner-

346

sphere complex mechanisms. Several sorption sites instead of one would be consistent with the fact that no Pb-

347

organic complex representative of Pb sorbed to a specific SOM functional group was identified as one of the

348

three principal components present in each soil sample.

349

In this investigation, two chemometric approaches, Target Transformation and MCR-ALS, were

350

employed to study Pb speciation. Results demonstrated that the methods can complement each other. Therefore,

351

these methods could be simultaneously applied to other heterogeneous and complex systems to help identify the

352

principal chemical forms of an element present in the sample mixture. It was found that the natures and/or



Page 16 of 25

353

amounts of Pb chemical forms varied in the particle-size soil fractions of Pb-polluted soils and could differ from

354

those determined from the bulk soil samples. Specifically, the highest amounts of Pb-humate were found in the

355

smallest particles of the soil samples. This Pb form was present in the < 0.4 µm and 0.4-2 µm particle size

356

fractions of a soil sample (Soil Near Dump), which represented 6019 and 2350 mg/kg, or 6.5 and 2.5 % of the

357

total Pb mass in the soil sample, respectively (Figure 4). These amounts of Pb-humate present in the fine

358

particles, which potentially represent the most mobile solid fraction in a soil, remained undetected when

359

analyzing the bulk soil sample (Figure 4). This could be due to the ~ 10 % uncertainty in amounts measured by

360

the linear combination method employed.34 The association between Pb and SOM may impede the formation of

361

pyromorphite in soils.7 Therefore, the results implied that the extent of Pb immobilization in a polluted soil

362

treated by P may be less than predicted if Pb speciation is characterized at the macroscopic scale from the bulk

363

soil sample prior remediation. Determining the mobility of soil particles and the specific speciation of Pb

364

associated to them may represent a suitable approach to assess how much Pb can be effectively immobilized

365

before applying a Pb stabilization technique to a polluted soil.

366

The results of this study, which focused on tropical soils, and those of previous investigations,12, 13, 36

367

which mainly focused on non-tropical soils, suggested that Pb-SOM association commonly occurs regardless of

368

the type of soil or climate. Research is then needed to develop an effective treatment method that could

369

specifically limit Pb-SOM association to optimize Pb immobilization before or during application of a Pb

370

stabilization technique to a polluted soil. Such method currently does not exist, although the phosphorus-based

371

Pb immobilization technique has already been employed at the large scale, notably in Pb-polluted urban soils of

372

Oakland, CA, USA.43 Most of these polluted soils were essentially residential gardens, thus potentially

373

containing in them significant amounts of SOM.

374

Acknowledgments

375

The authors thank Thailand’s Pollution Control Department (PCD) for assistance in soil sampling. They

376

thank Solenn Reguer for assistance in data acquisition at DiffAbs beamline, synchrotron SOLEIL. Stephanie

377

Blanchandin and Karine Chaouchi are acknowledged for laboratory assistance, as well as Guillaume Morin for

378

donating samples of PbO2 and PbS mineral compounds.

379

Supporting Information

380 381

Additional information on soil sample locations, Pb references, parameters of Hamilton test, and all chemometric methods employed. Results related to extraction experiments, physicochemical natures of soil


Page 17 of 25


382

samples or Pb references, including bulk XRD, SEM-EDS, micro XRF, particle sizes of < 0.8 µm fractions, R-

383

factor values, and Pb-SOM sorption mechanisms constrained by EXAFS.

384

4.

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Ma, Q. Y.; Logan, T. J.; Traina, S. J., Lead Immobilization from Aqueous Solutions and Contaminated

Admassu, W.; Breese, T., Feasibility of using natural fishbone apatite as a substitute for hydroxyapatite

Giammar, D.; Xie, L.; Pasteris, J., Immobilization of Lead with Nanocrystalline Carbonated Apatite

Cotter-Howells, J. D.; Champness, P. E.; Charnock, J. M.; Pattrick, R. A. D., Identification of

Hashimoto, Y.; Takaoka, M.; Oshita, K.; Tanida, H., Incomplete transformations of Pb to pyromorphite

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Debela, F.; Arocena, J. M.; Thring, R. W.; Whitcombe, T., Organic acids inhibit the formation of

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TOC 84x47mm (200 x 200 DPI)

ACS Paragon Plus Environment


Figure 1

EXAFS spectra collected at the Pb LIII edge of a) Pb(aqueous), Pb sorbed species, inorganic Pb references, and b) Pb organic complexes 221x162mm (200 x 200 DPI)


Page 22 of 25

Page 23 of 25


Figure 2 Chi EXAFS spectrum of Pure Phase A, B, and C extracted by MCR-ALS from the bulk, silt, or fine data matrix, and Chi EXAFS spectrum of Pb reference that provided the lowest R factor value for each extracted pure phase 113x191mm (300 x 300 DPI)



Figure 3 a) µ-XRF co-distribution map of Pb, Fe, and Mn, and b) the corresponding Pb/Fe and Pb/Mn pixel intensity scatter plots, for the soil sample “Soil Near House”. Spot 1 and Spot 2 encircled in the map are examples of macroscopic hot spots where Pb/Mn and Pb/Fe seemed to be present at the same locations. c) µ-XRD patterns taken at Pb and Fe hot spots chosen randomly from the µ-XRF maps of the soil samples, and XRD reference patterns of cerussite and goethite. 264x184mm (200 x 200 DPI)


Page 24 of 25

Page 25 of 25


Figure 4 Linear combination fitting results, with Pb-goethite referring to Pb-sorbed-to-goethite, and the corresponding amounts in mg/kg (details of calculations in SI) of Pb chemical forms in the five bulk soil samples and their particle-size fractions. 306x208mm (300 x 300 DPI)


Lead Speciation and Association with Organic Matter in Various

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