Zinc speciation and desorption kinetics in a mining waste impacted tropical soil amended with phosphate

Mining is an important component of the Brazilian economy. However, it may also contribute to environmental prob-lems such as the pollution of soils with zinc and other potentially toxic metals. Our objective was to evaluate changes in the chemical speciation and mobility of Zn in a soil amended with phosphate. Soil samples were collected from a deactivated mining area in the state of Minas Gerais, Brazil, and amended with NH4H2PO4 saturated with deionized water to 70 % of maximum water retention and incubated at 25 +/- 2 degrees C in open containers for 60 days. The soil was chemically and mineralogically characterized, and sequential extraction, desorption kinetics, and speciation were car-ried out using synchrotron bulk-sample and micro-X-ray Absorption Near-Edge Structure (XANES/mu-XANES) spectros-copy at the Zn K-edge, and X-ray fluorescence microprobe analysis (mu-XRF). The combination of mu-XRF and mu-XANES techniques made it possible to identify Zn hotspots in the main species formed after phosphate remediation. The best fit combination for bulk XANES and mu-XANES was observed in Zn-montmorillonite, Zn-kerolite, Zn-ferrihydrite, and gahnite. In the course of phosphate treatment, gahnite, Zn layered double hydroxides (Zn-LDH), Zn3(PO4), and ZnO were identified by bulk XANES, while Zn-ferrihydrite, Zn-montmorillonite, and scholzite were identified by mu-XANES. Zinc in the phosphate-amended soil had the strongest partial correlations (r ' > 0.05) with Ni, Co, Fe, Cr, Mn, Si, P, Cd, Pb, and Cd, while the unamended soil showed the strongest correlation with Cu, Pb, Fe, and Si. The ap-plication of NH4H2PO4 altered Zn speciation and favored an increase in Zn desorption. The most available Zn contents after phosphate amendment were correlated with the release of exchangeable Zn fractions, associated with carbonate and organic matter.

Automated summary

Mining is important to the Brazilian economy but can cause environmental problems such as soil pollution with toxic metals like zinc. This study examined changes in the chemical speciation and mobility of zinc in soil amended with phosphate. The results showed the formation of specific zinc species and an increase in zinc desorption after phosphate amendment.