Metal mobility and mineral transformations driven by bacterial activity in acidic pit lake sediments: evidence from column experiments and sequential extraction

Purpose: Biogeochemical processes occurring in lake sediments are crucial for element cycling. In acidic pit lakes, they play a major role in acidity neutralization and toxic metal immobilization. However, the relationship between bacterial activity (e.g., iron and sulfate reduction), mineral neoformation, and metal mobility is often difficult to establish and has been rarely reported. The aim of this study was to identify mineral products formed by bacterial activity and their influence on trace metal dynamics during early diagenesis. 

Materials and methods: The study focuses on several acidic pit lakes in SW Spain. We combined chemical analyses of intact sediment cores taken from shallow and deep zones with experimental sediment core incubations aimed at visualizing geochemical and mineralogical transformations occurring in the sediments during early diagenesis. The sediment incubations commenced with a total resuspension and homogenization and lasted for periods of several months. During this time, pore water and overlying water was extracted and analyzed to record geochemical changes. The sediments were also sectioned and chemically analyzed after the experiments to investigate carbon, nutrient, sulfur, iron, and trace metal mobility. Intact sediment cores were analyzed by sequential extraction and differential X-ray diffraction to examine metal speciation and the presence of secondary minerals not detectable by conventional techniques. 

Results and discussion: Shallow sediments are more biogeochemically reactive due to their higher organic carbon content. In columns prepared with these littoral sediments, a sharp pH gradient appeared in the upper 4 cm soon after the beginning of the incubations, suggesting alkalinity generation and concentration of microbial metabolisms (iron and sulfate reduction). This pH gradient provokes the precipitation of Al and leads to the formation of a milky, gelatinous layer in the water/sediment interface. This layer quickly dehydrates and recrystallizes to more stable phases. At the same time, bacterially produced H ₂ S reacts with different metals (Cu, Zn) which are thus immobilized in the sediments as secondary sulfides. However, these two metals are also removed from solution by purely abiotic mechanisms like sorption or coprecipitation. 

Conclusions: Carbon seems to be the major factor limiting bacterially mediated metal and acidity attenuation in sediments of these acidic pit lakes. Metal immobilization is not a simple process and takes place through a combination of direct precipitation (Al oxyhydroxysulfates, Cu and Zn sulfides) and indirect mechanisms (sorption, coprecipitation, cation exchange). The precipitation of Al is of prime importance for pH buffering and immobilization of other metals (Cu, Zn). This process is not evident in routine chemical analyses of sediment cores, but it can be inferred by a combination of experimental and laboratory techniques such as the ones used here. P and N fertilization of these pit lakes would greatly increase the natural rates of acidity neutralization.