CONICET | Buscador de Institutos y Recursos Humanos

Recently, conductive materials started being applied as filling material of treatment wetlands, giving rise to the new technology of Bioelectrochemical Wetlands or METland filters. The conductive material enhances bacterial activity on the system boosting treatment efficiency. It allows the occurrence of a process known as direct interspecies electron transfer (DIET) in which electro-active microorganisms exchange electrons either by direct contact or through conductive materials, without relying on chemical intermediates. Major drawbacks for the application of bioelectrochemical wetlands are the cost and the availability of the conductive materials. Biochar is a conductive and biocompatible material obtained through the thermal decomposition (pyrolysis) of biomass residues and vegetable wastes and appears as a valid candidate for its use as fillingmaterial on bioelectrochemical wetlands. Its electrical conductivity, a parameter of major importance for the process of DIET, increases with pyrolysis temperature, but also does its cost. At low pyrolysis temperature chemicals such as quinones and other aromatic compounds that can be used as electron acceptors and electron donors for the growth of bacteria are produced andremain biochar. Thus, low biochar obtained at low temperatures may also enhance bacterial activity. In this work, we show the results of our first experiments aimed at finding the pyrolysis conditions that result in an enhance of the bacterial activity and wastewater treatment without compromising the cost. Biochar were obtained through pyrolysis of prunning residues atdifferent operational temperatures ranging from 400 to 1200 ºC. The composition of the materials was analyzed through infrared spectroscopy (FT-IR) and Raman spectroscopy assays, to determine the relative amount of possible bacterial electron donors or acceptors. To analyze the growth of electro-active bacteria with biochar as chemical electron acceptor, Geobacter suLfurreducens, a model electro-active bacteria was grown in batch with this material as the sole electron acceptor and its growth was followed by counting in a Neubauer chamber. Also, the electrical conductivity of the materials was measured through a 4 point probe and electrodes of the materials were prepared and used as growing substrate for Geobacter sulfurreducens in electrochemical cells, to study the capacity of the cells to directly exchange electrons with the material.Electrodes were polarized at 0.4 V vs SHE and the growth of the bacteria was followed by measuring the current through chronoamperometry. Cyclic voltammetries were performed to analyze redox processes and plausible limitations in the growth of the bacteria. Materials obtained at lower temperatures allowed a higher growth of electro-active bacteria when used as chemical electron acceptors, whereas materials obtained at higher temperatures showed higher growth of bacteria when used as electrode with current densities values comparable to those obtained with graphite, the most common electrode material used for the growth of these bacteria. Following assays, once the pyrolysis process is scaled up, will be aimed at determining which of these processes (growth as chemical electron acceptor or electron transfer through conductive material) is of greater importance for the performance of bioelectrochemical wetlands.

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