Micro-organisms such as bacteria are likely to be present in the weathered municipal solid waste incinerator (MSWI) bottom ash that will be used as aggregates in road construction. Microbial activity may impact the weathering process of such material and thus the rate of leaching of potentially toxic elements. Indeed, the deterioration of rocks, like that of materials of anthropogenic origin (stained glasses, cements, blast-furnace slags), depends partly on organic compounds and micro-organisms. However, the exact role of micro-organisms remains poorly understood. Some authors suggest that they play a considerable part in the degradation of stained glass in cathedrals but they do not specify neither the mechanisms, nor even the type of micro-organism (bacteria, fungi, lichens) involved. Other authors are of the opinion that bacteria accelerate the deterioration of marine basaltic glasses due to the observation of “hair channels” that could represent bacterial activity. In our opinion, the exact role of micro-organisms remains to be demonstrated apart for some well-defined materials: degradation of books by fungi and metal drains by bacteria like Thiobacillus. There is no existing experimental validation of the role of bacterial action in the degradation of complex silicates such as the glass contained in MSWI bottom ash. The major difficulty is to measure in such complex media the rate of deterioration with reliable tracers.Although studies have been conducted on the role of bacterial cells, there is however, a lack of studies that compare the degradation due to bacterial biofilms (cells-exopolymers-solution) and the degradation that occurs in abiotic systems. As a consequence, our aim was to investigate the effect of Pseudomonas aeruginosa (a common bacteria found in the environment) on the weathering of MSWI bottom slag under well-defined conditions, similar to the natural environment (neutral pH and 25 °C).Experiments were carried out at 25 °C for 19 weeks. Two conditions were performed in parallel; the first one in a sterile medium and the second one in the presence of P. aeruginosa. The culture medium used (PS medium AOUAD et al., 2005), buffered at pH 6.7, has been developed in order to detect the greatest number of elements dissolved during deterioration. The bottom slag grains were placed in containers filled with 50 ml of medium which was renewed each week to ensure the production of a measurable quantity of bacterial cells and exopolysaccharides (biofilm). The renewed solutions were analyzed using ICP/AES (Jobin-Yvon JY 124 spectrometer) and ICP/MS (Fisons VG PQ2 + spectrometer). From the measured concentrations for Si, Mg, Ca and Sr (potential tracers), the rate of dissolution expressed in masses standardized to the reactive surface area (g/m2.j) was calculated. Rates obtained in sterile condition are higher than those of the biotic one, but they converge by the end of the experiments. For Mg and Si the rates are lower than those obtained for Ca and Sr. This may be a consequence of the precipitation of a magnesium silicate hydrate.The first remarkable observation is that the grains leached in biotic medium are agglomerated and form a compact cluster whereas the grains resulting from deterioration in the abiotic medium remain free. SEM observations indicate that by the end of the experiments, the surface of the grains leached in abiotic medium do not appear to be different to those initially present. Conversely, grains deteriorated in biotic experiments are systematically covered with exopolyssacharides which explains their agglomeration; the biofilm cements the bottom ash grains together.The rate of corrosion of bottom ash decreases quickly in biotic medium despite of the renewal of the growth medium. One explanation could be that the biofilm acts as a protective barrier, thus preventing dissolution. The rate also decreases although less rapidly in the absence of bacteria. This is probably due to the formation of a layer of precipitated minerals at the surface which has also protective properties. At the end of our experiments (19 weeks) both rates are equal.
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Oct 9, 2015
Prannoy Suraneni + 2
Open Access