Zinc mobility and speciation in soil covered by contaminated dredged sediment using micrometer-scale and bulk-averaging X-ray fluorescence, absorption and diffraction techniques

Abstract

The mobility and solid-state speciation of zinc in a pseudogley soil (pH ϭ 8.2-8.3) before and after contamination by land-disposition of a dredged sediment ([Zn] ϭ 6600 mg kg Ϫ1 ) affected by smelter operations were studied in a 50 m 2 pilot-scale test site and the laboratory using state-of-the-art synchrotronbased techniques. Sediment disposition on land caused the migration of micrometer-sized, smelter-related, sphalerite (ZnS) and franklinite (ZnFe 2 O 4 ) grains and dissolved Zn from the sediment downwards to a soil depth of 20 cm over a period of 18 months. Gravitational movement of fine-grained metal contaminants probably occurred continuously, while peaks of Zn leaching were observed in the summer when the oxidative dissolution of ZnS was favored by non-flooding conditions. The Zn concentration in the Ͻ50 m soil fraction increased from ϳ61 ppm to ϳ94 ppm in the first 12 months at 0 -10 cm depth, and to ϳ269 ppm in the first 15 months following the sediment deposition. Higher Zn concentrations and enrichments were observed in the fine (Ͻ2 m) and very fine (Ͻ0.2 m) fractions after 15 months (480 mg kg Ϫ1 and 1000 mg kg Ϫ1 , respectively), compared to 200 mg kg Ϫ1 in the Ͻ2 m fraction of the initial soil. In total, 1.2% of the Zn initially present in the sediment was released to the environment after 15 months, representing an integrated quantity of ϳ4 kg Zn over an area of 50 m 2 . Microfocused X-ray fluorescence (XRF), diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy techniques were used to image chemical associations of Zn with Fe and Mn, and to identify mineral and Zn species in selected points-of-interest in the uncontaminated and contaminated soil. Bulk average powder EXAFS spectroscopy was used to quantify the proportion of each Zn species in the soil. In the uncontaminated soil, Zn is largely speciated as Zn-containing phyllosilicate, and to a minor extent as zincochromite (ZnCr 2 O 4 ), IV Zn-sorbed turbostratic birnessite (␦-MnO 2 ), and Zn-substituted goethite. In the upper 0 -10 cm of the contaminated soil, ϳ60 Ϯ 10% of total Zn is present as ZnS inherited from the overlying sediment. Poorly-crystalline Zn-sorbed Fe (oxyhydr)oxides and zinciferous phyllosilicate amount to ϳ20 -30 Ϯ 10% each and, therefore, make up most of the remaining Zn. Smaller amounts of franklinite (ZnFe 2 O 4 ), Zn-birnessite and Zn-goethite were also detected. Further solubilization of the Zn inventory in the sediment, and also remobilization of Zn from the poorly-crystalline neoformed Fe (oxyhydr)oxide precipitates, are expected over time. This study shows that land deposition of contaminated dredged sediments is a source of Zn for the covered soil and, consequently, presents environmental hazards. Remediation technologies should be devised to either sequester Zn into sparingly soluble crystalline phases, or remove Zn by collecting leachates beneath the sediment.

Zinc Redistribution in a Soil Developed from Limestone During Pedogenesis

“…5). Such a strong affinity of Zn for Fe oxides was already observed in polluted soils (Manceau et al, 2000;Isaure et al, 2005).…”

“…The contribution of apatite to the quantities of Zn extracted during step 3 is therefore limited, although not totally excluded, and Zn extracted during step 3 was thus mainly released by Mn oxides. According to Manceau et al (1997) who studied the Fe-Mn concretions of these soils, Mn oxides occur as birnessite that sorbs Zn, as also shown in polluted soils (Isaure et al, 2005).…”

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

“…This spectrum, and therefore its radial structure function (RSF), are almost identical to the Zn-rich phyllosilicate reference Si 4 (Zn 2.1 Mg 0.9 )O 10 ·nH 2 O (ZnKer210) ( Zn-phyllosilicate is the dominant Zn species for the type II spectra. This species has been detected by EXAFS spectroscopy in the clay fraction of soils Scheinost et al, 2002;Manceau et al, 2004;Isaure et al, 2005), and also in the Scarpe sediment (Isaure et al, 2002). In all these studies, it was always mixed with other Zn species in the analyzed sample volume and the EXAFS spectrum of this component was never obtained separately.…”

“…Alternatively, the size of the X-ray probe can be adapted to a particular scale dependent on the heterogeneity of the sample. For soils and sediments, the micrometer scale of resolution is appropriate because fine particles of similar composition and structure formed by biotic and abiotic natural processes tend to aggregate Isaure et al, 2002Isaure et al, , 2005Manceau et al, 2002b;Kirpichtchikova et al, 2003;Manceau et al, 2004;Sarret et al, 2004). This second approach is preferable because it is less invasive than chemical extraction (Ostergren et al, 1999;Calmano et al, 2001).…”

Zinc surface complexes on birnessite: A density functional theory study

“…However, other birnessite samples shown to contain the Zn IV -TCS species comprise sheets of octahedra with only Mn(IV) present [sediment birnessite, Isaure et al (2005); bacteriogenic birnessite and δ-MnO 2 , Toner et al (2006); quartz-coating birnessite, Manceau et al (2007); plant-root Zn-Mn coprecipitate, Lanson et al (2008)]. Therefore, it does not seem necessary to connect the existence of Zn IV -TCS uniquely to the presence of Mn(III) substituted in the octahedral sheet and the fundamental basis for the occurrence of the four-coordinate surface species remains unclear.…”