Environmental Transformations of Metals in a Hypersaline Mediterranean Agricultural Wetland (Laguna Honda, South Spain)

This study investigated the biogeochemical processes governing heavy metal sequestration in Laguna Honda, a hypersaline Mediterranean wetland affected by intensive olive cultivation located in the Guadalquivir Basin River (S Spain). A network of sediment samples was analyzed from the wetland using X-ray fluorescence, ICP-MS, and high-resolution electron microscopy (FESEM/HRTEM). Geochemical data were treated with statistical factor analysis. Sediments show significant enrichment of Cu, Hg, and Au due to anthropogenic agrochemical treatments. Mineralogical characterization revealed authigenic pyrite framboids and metallic sulfide nanoparticles (CuS and HgS) within organic-rich sediments, where sequestration is primarily probably driven by the metabolic activity of sulfate-reducing microorganisms (SRMs) under reducing conditions. Additionally, Au enrichment is attributed to mechanical transport and coagulation of pesticide-derived nanoparticles, exhibiting a distinct geochemical correlation with rare earth elements (REEs). We concluded that hypersaline wetlands act as critical environmental sinks, where microbial activity, high organic matter content, and detrital transportation interact to facilitate the immobilization and stabilization of toxic trace elements through the precipitation of low-solubility sulfide phases (Cu and Hg) and the mechanical accumulation of nanoparticles (Au).