Occurrence of Zn/Al hydrotalcite in smelter-impacted soils from northern France: Evidence from EXAFS spectroscopy and chemical extractions

Abstract

Zinc speciation was studied by EXAFS spectroscopy, m-SXRF elemental mapping, XRD, and chemical extraction methods in two smelter-impacted soils sampled near one of the largest Pb and Zn processing plants in Europe, which is located in northern France about 50 km south of Lille. The tilled and wooded soils chosen for study differ in Zn concentration (ª600 and 1400 mg/kg, respectively), soil pH (7.5 and 5.5, respectively), and organic matter content (1.5 and 6.4 wt% TOC, respectively). In both soils, the occurrence of Fe-and Zn-rich (up to 10 wt% Zn) slag particles ranging in size from a few micrometers to a few millimeters, was shown by m-SXRF elemental mapping of soil thin sections as well as by SEM and chemical analysis of different soil size fractions. For both soils, XRD analysis of the dense coarse fraction, which contains up to 10 wt% Zn, revealed the presence of a minor amount (1-1.5 wt%) of crystalline ZnS (sphalerite and wurtzite). In this fraction, EXAFS data show that Zn is mainly incorporated in the tetrahedral sites of a magnetite-franklinite solid solution.The clay fraction (<2 mm) represents the largest pool of Zn in both soils, with 77 and 62% of the total Zn in the tilled and wooded soils, respectively. However, XRD was not able to detect any Znbearing phases in this fraction. Comparison of Zn K-EXAFS data of untreated and chemically treated samples from the bulk (<2 mm) and the clay (<2 mm) soil fractions with Zn K-EXAFS data from more than 30 model compounds suggests that Zn is present in the following chemical forms: (1) Zn outer-sphere complexes, (2) Zn-organic matter inner-sphere complexes, (3) Zn/Al-hydrotalcite (Zn/ Al-HTLC), (4) phyllosilicates in which Zn is present in the dioctahedral layer at dilute levels, and (5) magnetite-franklinite solid solutions inherited from the smelting process. The presence of exchangeable Zn outer-sphere complexes and of Zn inner-sphere complexes on organic matter is indicated by the relative increase of second-neighbor contributions in the EXAFS RDFs after chemical treatments with 0.01 M CaCl 2 and 0.1 M Na 4 P 2 O 7 . The occurrence of Zn/Al-HTLC is demonstrated by the persistence of a Zn-Zn pair correlation at 3.10 ± 0.04 Å (i.e., edge sharing ZnO 6 octahedra in the trioctahedral layer structure) in EXAFS data of Na 4 P 2 O 7 treated soil samples and its disappearance after treatment with 0.45 M HNO 3 . This latter treatment also revealed the occurrence of Znbearing phyllosilicate minerals, as shown by two Zn-Mg/Al/Si pair correlations at 3.05 ± 0.04 Å and 3.26 ± 0.04 Å, and of magnetite-franklinite solid solutions, as indicated by a Zn-Mn/Fe/Zn pair correlation at 3.50 ± 0.04 Å. Significant changes in the relative proportions of the different forms of Zn between the two soils explain their different responses to chemical treatments and emphasizes the relationships between solid state speciation and mobility of Zn in soils. et al. 1999). Heavy metals and metalloids can be sequestered in soils via abiotic and/or biotic reaction pathways through the forma...

Zn Speciation in the Organic Horizon of a Contaminated Soil by Micro-X-ray Fluorescence, Micro- and Powder-EXAFS Spectroscopy, and Isotopic Dilution

“…Juillot et al (19) found that 12% of Zn was CaCl 2 -exchangeable, and concluded from EXAFS and chemical extraction data that outer-sphere Zn was present in this soil. In this study, the comparison of Zn contained in the bulk soil before fractionation and the weighted sum of Zn in the size fractions shows that ~ 20% of Zn was leached during size fractionation (Table 1).…”

“…Outer-sphere and organic Zn species have been suggested to amount ~ 50-60% in an organic acidic soil (pH 5.5) (19), and otherwise are present in subordinate amount in soils dominated by mineral constituents. In these soils, a varieties of Zn-sorbed and Zn-substituted secondary species were identified, including phyllosilicates, iron oxyhydroxides, manganese oxides, lithiophorite, hydrotalcite and phosphate (8,16,18,19,22). On the basis of these results, Zn-organic complexes seem to prevail in highly organic soils at moderately acidic pH.…”

“…In natural systems, all these species may be present altogether, and the EXAFS signal from organic and outer-sphere complexes is then masked by the more intense signal from other species. To obtain the EXAFS spectra of pure species, one can combine powder EXAFS on bulk sample with chemical treatments (8,17,19) or use micro-focused EXAFS (µEXAFS) spectroscopy (16,18,(20)(21)(22). In practice, however, most natural systems are still heterogeneous at the micrometer-scale, and µEXAFS spectra may be a weighted sum of component spectra from several individual species.…”

The effect of phytostabilization on Zn speciation in a dredged contaminated sediment using scanning electron microscopy, X-ray fluorescence, EXAFS spectroscopy, and principal components analysis

“…If the element of interest is present in several forms, their identification can be challenging because the EXAFS signal is the weighted sum of the individual spectra from the component species. To reduce the number of components or modify their proportions, physical and chemical treatments can be used to remove or concentrate certain species Isaure et al, 2002;Scheinost et al, 2002;Juillot et al, 2003). Alternatively, the size of the X-ray probe can be adapted to a particular scale dependent on the heterogeneity of the sample.…”

Colloids and suspended particulate matters influence on Ni availability in surface waters of impacted ultramafic systems in Brazil

“…readily be transported and suspended in water, while the biggest and heaviest ones settle in the basin. These spherical particles are typical of ash produced during metallurgic processes [54,55]. They result from the combustion, melting and volatilization of ore minerals and commonly consist of slag and matte droplets, graphite, and condensed particles, which are collected by filters.…”