Potential pathways for lithium and Cr-isotopes during biogeochemical redox processes: Insights from karst-type laterites of Greece

Abstract

The present study is focused on the distribution of unexplored critical and strategic element lithium (Li) in ophiolitic peridotites and weathered products (laterites, altered ultramafic rocks), affected soil and groundwater from Central Greece. The provided Li and other trace elements data are combined with available Cr-isotope compositions as tracers for redox reactions and their geochemical cycling from the magmatic source to these weathered products, aiming to define favorable conditions and potential pathways. All samples contain organic matter, but it is higher in bauxite laterites. The activity of the associated reducing microorganisms, catalyzing redox reactions, on Al-Fe-oxides may result in the reduction of the Fe(III) to soluble Fe(II), providing pathways for the iron leaching and the increase of Al and Li, as is exemplified by a good positive correlation between Al and Li (r = 0.85). However, partly re-deposition of iron is obvious by the occurrence of nanoparticulate spheroidal and/or bacterio-morphic pyrite. Given that the Cr occurs mainly as Cr(III) in chromite, olivine, serpentine, it may be released and oxidized to Cr(VI), under oxidizing conditions. The variations in the δ⁵³Cr values in chromitites from Greece are small but detectable, ranging from the most negative slightly negative to positive, due probably to differences in the mantle processes or metasomatism, during post-magmatic stages. The variations of the δ⁵³Cr values recorded in Fe-Ni-laterites, weathered rocks and groundwater from C. Greece are much wider compared to a very small variation in chromitites and show a negative correlation versus Cr(VI), supporting the post-mobilization reduction of Cr(VI) back to Cr(III) by Fe(II), organic matter, sulfides or microorganisms. However, δ⁵³Cr values in the leachates are lower compared to these in groundwater, due probably to the reduction of Cr(VI) and its precipitation as Cr(OH)₃, and/or the adsorption and bio-accumulation processes. Elevated Li concentrations in groundwater from coastal areas contaminated by Cr(VI) may reflect the intrusion of seawater, suggesting that the reduction of Cr(VI) to Cr(III) is inhibited, due to the potential stability of Cr(VI). This may be applied to the soil-groundwater-plant/crops system, using appropriate techniques to facilitate the reduction of Cr(VI) and protect food chain.