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

Selenium is an essential micronutrient, whose mobility in cold region underpins both crop quality and the aquatic food web. Our objective is to test for the influence of temperature (4 and 23°C) on Se sequestration in undisturbed sediments using flow-through reactors (FTR) containing fresh or aged organic matter (OM), under environmental relevant low Se conditions. Se speciation along with concentration of ancillary parameters (pH, DOC, NO3-, NO2-, Fe(II), SO42-, HS-) were measured in the outflow during 8 experimental phases in which Se concentrations were sequentially increased. FTR sequestered most of the Se, with fresh OM removing 50% more than aged OM. Temperature response was lower for selenite than for selenate. At 100 nM inflow selenite, in the presence of fresh OM, Se effluxes, comprising of undefined organic Se, were of 0.94 nM d-1 at 23°C and of 0.85 nM d-1 at 4°C. In contrast, at 100 nM inflow selenate, effluxes were of 0.9 nM d-1 at 23°C and an order of magnitude higher at 8.4 nM d-1 at 4°C. Outflow Se was composed of selenite and organic Se species at 23°C and of selenate at 4°C. We hypothesize that selenate reduction was controlled by microbial processes, influenced by temperature, while selenite transformation proceeded via abiotic reactions.