Meromictic Lakes as On-Field Laboratories for Microbial Fuel Cells
A) Conformation of the lake basin: the glacial origin of the lake, together with the steep walls of the basin and the depth of 120 m, creates the conditions for the evolution of a meromicticcondition (Fig 1). A first layer of water (about 50 m) is continuously recirculated and oxygenated; a second layer finds the depletion of oxygen; a third layer is constituted of stationary water.
B) Water analyses: analysis of the water composition at various depths reveals stratifications of different metals, especially of manganese.
C) Physico-chemical variations: based on home-built sensors, oxygen, pH, sulphide, conductivity, and temperature profiles were obtained, demonstrating the unique correlations between these parameters (Fig. 2).
D) compositionof the rocks around the lake: carbonatic dolomite mineral, with small percentages of manganese, is susceptible to leaching and acidic dissolution.
For the first time, the presence of a well-defined zone appearing as a “white cloud” was revealed near the interfacial region of the oxygen depletion. This cloud is rich in Mn. A mechanism is herein proposed for the white cloud formation by means of analysis of Mn lacustral concentrations as well as of redox and pH variations. This phenomenon is referred to the first chemical dissolution of the rocks by the slightly acidic monimolimion zone. Mn(II) solution diffuses upwards and meets the zone of oxygen depletion, where pH increases. Mn(OH)2 (white hydroxide) starts to floculate, and buildup of the Mn salts takes place. Moving downwards, the hydroxide meets again the acidic zone, thus dissolving. Moving upwards, the white hydroxide encounters oxygen and an higher redox potential. At these pH (about 8), transformation of Mn(OH)2 to Mn3O4is quick and produces a reddish-brown powder, easy to precipitate. From time to time, hydrodynamics and convective diffusion lead to mixing of the water of the differen layers, and the brown color is visible even on the surface of the lake.
The presence of these layers, characterized by different chemistry, pH and redox potential (more than 500 mV of potential difference), might be exploited to achieve electrical current generation and Mn depollution of the waters. The role of microrganism in the cycling of manganese is now under investigation aiming to the experiemntation of bio-electrochemical systems. The electrolysis catalyzed by bacteria naturally present in the lake is able to produce large amounts of Mn2O3 from Mn(II) hydroxide. A lab-scale experiment demonstrates that this process is viable. As-produced Mn2O3precipitates on the electrode surface in a non-insulating porous layer, with the final result of collecting Mn and clear the water from the white cloud effect.
In this scenario, microbial fuel cells (MFC) might produce power by utilizing the microbial capacity of catalyzing the oxygen reduction at the cathode, and the Mn(OH)2 oxidation at the anode. Another type of MFC might develop power by reduction of Mn2O3.
Furthermore, microbial electrolysis cells (MEC) could produce valuable products.
In our opinion, the white cloud layer is a product of long-term building up of the Mn(II) concentration, and the process of water clarification may be achieved by a non-intrusive, long-term, application of the bacterial catalyzed electrochemical systems described.
Fig. 1: Scheme of a typical meromictic lake, with the three main characteristic stratifications: mixolimion, chemocline, and monimolimion and Depth-profiles of the Idro lake, by sensor probes: A) redox potential; B) sulphide pontentiometric signal; C) temperature; D) O2 content.