Influence of temperature on selenium mobility under contrasting redox conditions: a sediment flow-through reactor experiment

We studied selenium (Se) sequestration in minimally disturbed lacustrine sediments using flow-through reactors (FTR) in response to organic matter lability, selenium (Se) speciation and temperature (4 and 23°C). Initial sediment was composed of either fresh or aged organic matter (OM), and was fed with environmentally relevant, low Se concentrations and filtered lake water. We monitored Se concentration as well as speciation along with pH and the concentrations of dissolved OM, NO3-, NO2-, Fe(II), SO42- and HS- in the outflow of FTRs during 8 experimental phases along increasing Se concentrations. All experiments sequestered a large proportion of Se. Fresh, labile OM removed 50% more Se than aged, more recalcitrant OM. Along with a highest proportion of reduced redox-sensitive species in the reactors with fresh OM, this result is consistent with reducing conditions promoting Se sequestration. Inflowing selenite was sequestered to a larger extent than inflowing selenate. Lastly, only selenate reduction responded strongly to temperature. At 100 nM inflow, selenate was sequestered at a rate of 92 pmol cm-3 d-1 at 23°C, which lowered to 80 pmol cm-3 d-1 at 4°C. Outflow Se speciation for selenate reduction experiments comprised mostly of organic Se species at 23°C and, in contrast, solely of selenate at 4°C. We hypothesize that selenate reduction proceeded via microbial processes, in line with reactions catalyzed by enzymes being temperature dependent. Overall, our findings suggest that the mobilisation and warming of the boreal and permafrost carbon pools may increase the capacity of aquatic environments to sequester Se, lowering its bioavailability.