Bio-Electrochemical Systems in the Subalpine Lake Idro (Italy)

The subalpine Lake Idro in the north of Italy is here described from a physico-chemical and electrochemical point of view. The glacial origin of the lake, together with the steep walls of the basin and the maximum depth of 120 m, creates the conditions for the evolution of a stable meromictic condition during the year. The presence of a superficial layer of aerobic water (a mixolimion of about 50 m) and of a deep constant layer under anaerobiosis (monimolimion) makes it a unique environment for electrochemical studies. A peculiarity of this lake is the presence of a cloudy white layer at the chemocline, detected at 40-60 m of depth by scuba divers and never reported in literature. The hydrographic basin is mostly composed of dolomite, the typical rock of the Alps. The general chemical composition is Mg(Ca)CO₃, but it could include impurities such as Fe and Mn.
Many are the peculiarities converging to produce a unique environment for Mn cycling in the mixolimion, chemocline, and monimolimion characterizing the lake. On the basis of home-built sensors and a standard multiparametric probe, oxygen, pH, sulphide, conductivity, red-ox and temperature profiles were determined. Strict correlations were found between these parameters and the different oxidation states of Mn (Mn⁺², Mn⁺³ and Mn⁺⁴) detected in the water and in the particulate. Equilibria with Fe and other elements have been considered.

A chain of copper alloy (CuNi 70/30) and carbon steel was immersed at different depths in the water down to the bottom of the lake, in a point where the lake depth is the maximum (120 m), to study electrochemical and biological characteristics of the lake. Unique correlations between the chemical-physical parameters and electrochemical characteristics of copper alloys specimens, exposed at different depth in the lake, were demonstrated. The open circuit potential of copper alloys, measured versus the same Ag/AgCl reference electrode used for the sulphide measurements, varied as a function of the oxygen concentration in the different water layers and stabilized in time in each layer (from +200 mV down to -600 mV vs. Ag/AgCl). The results indicated that, in different ways, handmade sensors can be cost effective tools, able to give insight into the redox conditions of water.

The stratification of the lake poses a new interesting studies for MFCs. In this environment,  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.

The natural scenario of the Lake Idro suggests studies of new bio-electrochemical technologies that could be applied for environment-depollution purposes.