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Redox cycling of iron and manganese - the role of biosorption

Final Activity Report Summary - FEMN_BIOSORP (Redox cycling of iron and manganese - the role of biosorption)

Prokaryotes are an essential component of the Earth's biota and can play a pivotal role in the functioning of natural environments. Microorganisms have high surface to volume ratios and, in particular, bacterial cell walls can behave as a solid phase sorbent for dissolved ions because of the presence of surface reactive functional groups such as carboxyl, phosphoryl, hydroxyl and amino groups. This implies that microorganisms can not only exert significant influence on the mobility of ions in natural environments, but also that the use of microorganisms as sorbent material offers interesting perspectives for developing environmentally friendly and cost-effective water treatment technologies.

Here, we investigated metal and ion sorption to the gram-negative organism shewanella putrefaciens as a representative subsurface bacterium and a potentially suitable organism for bioremediation and water treatment applications. Emphasis was put on the active response of bacteria upon exposure to different solutes or changes in pH and on its consequences for the bacterial ion sorption characteristics. Comparing the acid-base properties and sorption behaviour of viable and heat-killed cells of s. putrefaciens demonstrated that viable bacteria could be conceived as passive sorbents and that their active response to environmental perturbations could have a significant effect on their capability and the underlying mechanisms of metal and anion immobilisation.

More specifically, viable s. putrefaciens showed different base buffering capacity compared to inactivated cells. This difference in buffering capacity was also reflected in the pH dependency on Cu2+ uptake by s. putrefaciens, which was about two times higher for vital cells at elevated pH. However, the difference between viable and heat-killed bacteria was less pronounced regarding the sorption of Zn2+, F- and phosphate. Moreover, the difference between viable and heat-killed cells manifested in the kinetics of Mn2+ sorption to s. putrefaciens when exposing the bacteria over a time period of about one month to Mn2+ containing solution. Dissolved Mn2+ concentrations decreased within a period of several minutes after the addition of heat-killed cells and reached equilibrium values which remained stable over at least one month. In contrast, Mn2+ removal by vital cells continued over the entire incubation period.

Electron microscopy, infrared spectroscopy and synchrotron radiation-based X-ray absorption spectroscopy were used in order to unravel the underlying sorption mechanism. Results from microscopic and spectroscopic analyses indicated that changes in the microbial biomass, bioprecipitation of Mn(II) salts and oxidation of Mn(II) leading to the formation of Mn oxides potentially contributed to the long-term Mn2+ removal in the presence. The contribution of the different processes depended on temperature and initial Mn2+ concentration: at relatively high Mn2+ concentrations, more than 200 mg L-1, formation of Mn phosphates was dominant, while at the relatively lower Mn2+, 100 to 125 mg L-1, concentration indications for the formation of Mn oxides and other Mn(II) precipitates were obtained. The formation of Mn precipitates was temperature depended and most intensive above 20 degrees Celsius. In addition, the production of biomolecules was influenced by temperature and below 10 degrees Celsius the formation of polymeric sugars was dominant. Release of polysaccharides and protein-like compounds was encountered in the incubation experiments with bacteria, however no indications for strong complexation of the investigated metals by the exudates were found.

This study demonstrated that the characteristics of bacteria regarding metal uptake and immobilisation could change significantly when they were exposed to metal over longer time periods and heat-killed cells might not represent the sorption behaviour of live cells. The large variation in possible Mn removal mechanisms which was observed in this study highlighted the possible complexity of the influence that bacteria could exert on metal mobility in natural environments and might open new perspectives in using bacteria for the removal of metals from waste water streams.