Multi-synchrotron techniques to constrain mobility and speciation of Zn associated with historical mine tailings

Abstract

Zinc speciation, chemical forms, and phytoavailability were determined within mine tailings and soil samples from a historical mining site in the Xichú district, central Mexico. For this purpose, species determination and phase mapping were conducted using XAS, μ-XRF, and μ-XRD, and chemical extraction was performed to assess chemical forms and phytoavailability. High concentrations of Zn were recorded in the mine tailings (4681 mg·kg−1) and relatively high concentrations were observed in the soils (319 mg·kg−1). Soil samples either exceeded the threshold value for exploratory assessment or yielded concentrations that may cause ecological risk. Igeo values indicate that the mine tailings were extremely polluted and soils were unpolluted to moderately polluted. Phytoavailable Zn positively correlated with total Zn content in the mine tailings (34%; 1719 mg·kg−1) and soils (36%; 69.5 mg·kg−1) and exceeded the threshold value for the development of ecotoxicological effects. The average percentages of most mobile fractions (water-soluble, exchangeable, and carbonate-bound) in surficial and composite mine tailings were ~31% and ~34%, respectively, whereas that in the surrounding soils was ~36%. According to XAS and linear combination fitting, the main phases of Zn in mine tailings were residual species, whereas in soils, the main phase was zinc carbonate (hydrozincite). Minor components of Zn were found as sulfate and sorbed onto goethite, which was confirmed by μ-XRD phase mapping. Synchrotron techniques suggest that Zn sulfate and Zn sorbed onto goethite are the main species controlling the mobility and availability of Zn. The potential high mobilization and dispersion of Zn represent potential harm to surrounding ecosystems in the historical mine tailings of central Mexico.