Electron Microscopy Evidence of Zn Bioauthigenic Sulfides Formation in Polluted Organic Matter-Rich Sediments from the Chicamocha River (Boyacá-Colombia)

Abstract

Electron microscopy and sediment geochemical data from a river basin (the upper Chicamocha river basin, UCRB, Boyacá province, Colombia) affected by anthropogenic activities (wastewater discharges, smelting and agricultural activities) showed the existence of heterogeneously distributed Zn particles in the sediments and sediments with Zn contents above the regional background (42 mg/kg). The objective of this study was to evidence the ZnS sedimentary neoformation in organic matter rich sediments deposited in anthropogenic reservoirs to reveal the processes involved in the sedimentary uptake of Zn from potential pollution sources. The highest Zn concentrations are found in clay minerals and organic matter-rich sediments (up to 427 mg/kg) deposited in slow-flowing reaches of the river associated to La Playa dam. Quartz-rich sediments poor in organic matter deposited in fast flowing segments of the river show very low Zn contents (1–12 mg/kg). Electron microscopy images showed ZnS nanoparticles forming cell-shaped aggregates suggesting that sulfate-reducing microorganisms acted as templates for the partial binding of Zn and for the nucleation and growth of zinc sulfide minerals. A good correlation of Zn with total organic carbon (r = 0.936) and the low potential redox of these sediments (−233 mV) suggest that organic matter was able to maintain oxygen depleted conditions appropriate to the Zn accumulation in the sediments. Our results demonstrate that potentially toxic Zn, originating from anthropic activities, was partially immobilized in organic matter-rich sediments through the precipitation of sulfides.