Self-stabilization of Zn, Pb, Cd, and As in smelter-impacted organic-rich soil: The effect of hydrous Fe oxides and ZnCd sulfide coprecipitation

Abstract

Soil constitutes a direct sink for elements mobilized due to mining and smelting activities. One of the desired pathways for reducing the bioavailability and toxicity of the contaminants is their transformation into sparingly soluble solid phases. Here, we report the formation of secondary mineral phases in extremely contaminated (up to 210 g Zn kg−1,102 g Pb kg−1, 5.68 g Cd kg−1, and 6.42 g As kg−1, respectively) organic-rich soil. Combining mineralogical techniques (SEM, XRD), a nonspecific sequential extraction (CISED) with Mössbauer and X-ray photoelectron (XPS) spectroscopies evidenced two poorly crystalline goethite components differing in crystallite size and As, Zn, Pb, Ca, Al, P substitutions and minor magnetite associated with plant roots (mainly Deschampsia caespitosa, Equisetum palustre, and Carex rostrate) directly below a layer of smelter-derived particles deposited into the soil. SEM was the only method that unambiguously documented the occurrence of ZnCd sulfide microsize aggregates incrusting plant roots and located in between the (hydrous) Fe oxides. Sequential extraction confirmed a complete As immobilization by goethite, while Cd forms a solid solution with ZnS and is lacking in the Fe hydroxides. The partitioning of Zn and Pb between the goethite and sulfide phases depends on soil water saturation. It is proposed that the coexistence of hydrous Fe oxides and nonferrous metal sulfides in the soil subsurface is possible because of redox heterogeneity of the rhizosphere and the decoupling of sulfur and iron cycles. Low mobility of biogenic sulfide ions and the protecting role of organic matter limits goethite sulfidation. The system remains active, adapting to the seasonally changeable plant roots ecology and fluctuations in water saturation. The obtained results are of value in remediation and managing strategies for contaminated soils and in reconstructing processes related to the formation and/or transformation of low-temperature sulfide deposits.