Ecotoxic effect in Allium cepa due to sphalerite weathering arising in calcareous conditions

Abstract

The ecotoxic effect of Zn species arising from the weathering of the marmatite-like sphalerite ((Fe, Zn)S) in Allium cepa systems was herein evaluated in calcareous soils and connected with its sulfide oxidation mechanism to determine the chemical speciation responsible of this outcome. Mineralogical analyses (X-ray diffraction patterns, Raman spectroscopy, scanning electron microscopy and atomic force microscopy), chemical study of leachates (total Fe, Zn, Cd, oxidation-reduction potential, pH, sulfates and total alkalinity) and electrochemical assessments (chronoamperometry, chronopotentiometry, cyclic voltammetry, and electrochemical impedance spectroscopy) were carried out using (Fe, Zn)S samples to elucidate interfacial mechanisms simulating calcareous soil conditions. Results indicate the formation of polysulfides (S_n^2-), elemental sulfur (S⁰), siderite (FeCO₃)-like, hematite (Fe₂O₃)-like with sorbed CO₃^2- species, gunningite (ZnSO₄·H₂O)-like phase and smithsonite (ZnCO₃)-like compounds in altered surface under calcareous conditions. However, the generation of gunningite (ZnSO₄·H₂O)-like phase was predominant bulk-solution system. Quantification of damage rates ranges from 75 to 90% of bulb cells under non-carbonated conditions after 15-30 days, while 50-75% of damage level is determined under neutral-alkaline carbonated conditions. Damage ratios are 70.08 and 30.26 at the highest level, respectively. These findings revealed lower ecotoxic damage due to ZnCO₃-like precipitation, indicating the effect of carbonates on Zn compounds during vegetable up-taking (exposure). Other environmental suggestions of the (Fe, Zn)S weathering and ecotoxic effects under calcareous soil conditions are discussed.

Keywords: (Fe, Zn)S weathering; Allium cepa; Calcareous soil conditions; Ecotoxic effect; Gunningite (ZnSO4·H2O)-like formation; Outer-surface precipitation mechanisms.