Formation of biogenic sulphides in the water column of an acidic pit lake: biogeochemical controls and effects on trace metal dynamics

Production of H2S by bacterial sulphate reduction in the water column of Cueva de la Mora pit lake (SW Spain) may favour an important self-mitigating capacity with respect to metal pollution. The interaction of bacterially produced H2S with dissolved metal(loid)s provokes the precipitation of various sulphides, which in turn form a 10 m-thick turbidity layer below the redoxcline. The main goal of this study was to identify the main factors driving the formation of these sulphide precipitates and their impact on the pit lake water quality. Given the severe geochemical conditions found in this lake (low pH, high metal concentrations), our findings may be relevant for a wide spectrum of environmental systems where BSR-based biotechnologies are applied. The water column has been studied by spatially resolved samplings and detailed chemical analyses, physicochemical profiling and geochemical modelling. The suspended particulate matter forming the turbidity has been chemically and mineralogically characterized. Our results suggest that the low pH (3.0-4.0) and the high metal concentrations are not inhibitory for the sulphate-reducing bacteria (SRB). The intensity of sulphide precipitation seems to be closely related with the activity of SRB, which in turn appear to depend on the availability of organic carbon produced in the photic zone. The relative location of oxygen and iron gradients and the redoxcline thickness are influencing the development of the sulphate reducing zone. The most common sulphides are CuS, As2S3 and ZnS, though formation of some other minor phases (PbS, CdS, NiS) has been also detected and/or deduced by geochemical calculations. Metal sulphide precipitation has provoked a complete removal of Cu from the monimolimnion (though it is still present at concentrations of 5.5 mg/L in the mixolimnion) and a net decrease in highly toxic trace metals (Cd, Pb, U, Th) and metalloids (As, Sb).