Characterization of zinc in contaminated soils: complementary insights from isotopic exchange, batch extractions and XAFS spectroscopy

Isotopic exchange (IE) of trace metals is an established method for characterizing metal reactivity in soils, but it is still unclear which metal species are isotopically exchangeable. In this study, we used IE to quantify 'labile' zinc (Zn) in 51 contaminated soils that were previously studied by Zn K-edge X-ray absorption fine structure (XAFS) spectroscopy and sequential extraction (SE). All soils had been contaminated by runoff water from 17- to 74-year-old galvanized power-line towers. They covered a wide range in pH (4.0-7.7), organic carbon (0.9-10.2%), clay (3.8-45.1%) and Zn concentrations (251-30 090 mg kg-1). Isotopic exchange was also performed on selected Zn minerals used as references for linear combination fitting of XAFS spectra. The isotopically exchangeable fraction (%E) of Zn generally decreased with increasing pH, but small %E values were also observed for acidic soils with a large fraction of Zn in hydroxy-interlayered minerals (Zn-HIM). The fraction of Zn identified by XAFS spectroscopy as (tetrahedrally and octahedrally coordinated) 'sorbed Zn' agreed reasonably well with the isotopically exchangeable fraction but was in many cases larger than the %E, indicating that some 'sorbed Zn' may be isotopically non-exchangeable, such as Zn sorbed in micropores of Fe oxyhydroxides. Zinc identified by XAFS spectroscopy as Zn precipitates (Zn phyllosilicates, Zn-layered double hydroxide (Zn-LDH) or hydrozincite) or as Zn-HIM was largely isotopically non-exchangeable ('non-labile'). Comparison between IE and extraction results suggested that the isotopically exchangeable Zn was mainly extracted in the first two fractions of the SE. However, non-labile Zn was also extracted in these first two fractions for some soils, including a hydrozincite-containing soil. Despite the presence of Zn-LDH and/or Zn phyllosilicates in almost all soils, the Zn concentrations in solution and labile Zn increased with increasing soil total Zn at a given pH, which contradicts the concept of precipitation control by a single phase. Solution Zn was well predicted from the labile Zn following a sorption model.