Complexation of neptunium(V) with Bacillus subtilis endospore surfaces and their exudates

The neptunyl ion is very toxic and has the potential to be highly mobile in the environment. In an effort to understand how its interactions with biological surfaces may affect its movement in the environment, we investigated neptunyl interactions with Bacillus subtilis endospores and their exudates. The exudates were dominated by dipicolinic acid. Spectrophotometric investigations of the chemical form of neptunyl in exudate solutions are consistent with the formation of 1:1 neptunyl–dipicolinate complexes. Using neptunyl–endospore adsorption data and spectrophotometric measurements of neptunyl–dipicolinate complexes, we determined thermodynamic stability constants for both species. Neptunyl adsorption onto the endospore surface decreased with an increasing pH, which corresponds to increasing aqueous complexation of neptunyl by dipicolinate. Adsorption was also highly ionic strength dependent with adsorption increasing as ionic strength decreased. With stability constants determined in this work, we compared controls on neptunyl partitioning in a simulated system with B. subtilis endospores, vegetative cells, and generic natural organic matter. Neptunyl complexation by B. subtilis endospore exudates exerted the greatest biological control in the simulated systems.