Characterization of Cr(vi) removal from aqueous solutions co-contaminated with Cu(ii) using an indigenous yeast

Abstract

Chromium(VI) compounds are known to be extremely toxic to living organisms due to their strong oxidizing nature. Biological Cr removal offers an alternative to traditional physic-chemical methods and is considered as a sustainable technology of lower impact on the environment. Resistant microorganisms have been studied they may exhibit Cr-bioaccumulation or biospeciation mechanisms.The endophytic Rhodotorula mucilaginosa 7Apo1 from sugar cane was tested on their Cr(VI) removal and resistant mechanism when it was incubated in YM medium plus different concentrations of Cr(VI) and Cr(VI)+Cu(II). Residual Cr(VI) (1,5-Diphenylcarbazide method) and total Cr (Atomic Absoprtion Spectroscopy) were estimated. Cell observations were made by scanning electron microscopy SEM and by EDS dispersive energy spectroscopy.7Apo1 was able to remove 100% of the 0.5 mM Cr(VI) initial concentration after 96 h of cultivation, whereas with 0.25mM Cu(II) + 0.5mM Cr(VI) the removal was reached at 48 h. Similar behavior was observed at higher concentrations [1 mM Cr(VI) and 0.25 mM Cu(II) + 1mM Cr(VI)], but not reaching the total Cr(VI) removal.Most of the total chromium was detected in the cell-free supernatants (which might be ascribed to the trivalent state) with a minimal residual proportion in the biomass digested with acid. The analysis by SEM showed changes in the general appearance and shape of cells according to the control cultures whilst EDS analysis of 7Apo1 exposed to Cr showed spectra with almost imperceptible low levels of the metal on the cell surface.The results herein presented put in evidence that the capability of the yeast 7Apo1 for Cr(VI) removal are highly influenced by the presence of low concentrations of Cu(II). Accordingly, bioreduction may be suggested as the main mechanism implicated in the removal of Cr(VI) from cultures. Furthermore, low bioaccumulation is not located on the cell surface.