Stability of bacterium-mercury complexes and speciation of soluble inorganic mercury species

Soluble inorganic mercury ions from aqueous solutions were retained on the external membrane of both lyophilized bacterial cells and living cells. Desorption studies were performed using both types of cells. Distribution coefficients of mercury between the aqueous and solid phases, as well as the conditional stability constants of mercury ions for the external membranes of bacteria were calculated. Loaded membranes and the amount of the unadsorbed ions were measured by electrothermal atomic absorption spectrometry (ETAAS) and/or gamma spectrometry. The 203Hg radiotracer (t12 = 46.6 days) was used to monitor recoveries using gamma spectrometry in combination with an optimized extraction procedure. Depending on the pH of the solution mercury ions can be selectively retained by the bacterial external membrane and the speciation of the mercury(II) and mercury(I) ions is possible. Best retention pHs for both mercury ions and bacteria were the following: mercury(II): 1 (E. coli) and 3 (P. putida), and mercury(I): 4 (E. coli) and 8 (P. putida). Interelement effects from other ions in the retention process of mercury were also investigated. A technique involving a solid extraction stage of mercury from aqueous samples at ng ml−1 level was developed and applied to the determination of mercury by ETAAS. The mercury-biomass pellet is separated by centrifugation and treated with a nitric acid solution determining the content of mercury in the supernatant solution. The best detection limit (3 times stand. dev. of the blank) and the best sensitivity (Hg concentration for an absorbance peak of 0.0044s) of the method for the applied concentration procedure were, respectively, of 15.0 ng ml−1 and 3.0 ng ml−1 for mercury(I) and 22.0 ng ml−1 and 5.5 ng ml−1 for mercury(II).