Characterization and Cr(VI) removal optimization by a bacterial strain with high heavy metals tolerance isolated from the Atoyac River

Abstract

Chromium pollution of water bodies is an important environmental problem that has risen as a result of more industrialized societies. As a residual of the textile, construction, cellulose transformation, plastics, pharmaceutical, food, automobile, ceramics, paint and alloy manufacturing industries, hexavalent chromium is discarded into the environment as a highly soluble and toxic element leading to carcinogenic, respiratory, skin, renal, hepatic, gastrointestinal, cardiovascular, hematological and reproductive effects in humans. To address this global pollution problem, numerous studies in bioremediation technologies have been researched in the past years, focusing on the development of environmentally friendly, low cost and adaptable tools for Cr(VI) removal. Among these technologies, bacterial reduction of Cr(VI) is of great interest as it facilitates its removal by precipitation into a less soluble form, Cr(III). This present master's thesis focused on the research and characterisation of a bacterial strain isolated from the Atoyac River, one of the most polluted rivers in Mexico. The bacteria denominated as 2.15a was taxonomically identified and the effects of pH, temperature and chromium concentration were evaluated in both live cells and dry biomass for their potential to remove chromium. The bacterial strain was identified as Klebsiella pneumoniae by biochemical tests and growth in selective media. With a MIC of 800 ppm of Cr(VI) the bacterial strain showed great potential as a bioremediation tool. The optimal conditions for Cr(VI) removal by the live strain were identified as pH 8 and temperature of 37.5°C; this bacteria showed excellent removal values when the metal concentration was between 50 and 150 ppm, reaching removal percentages after 24 hours of 96.5% to 47.5% as the concentration increases. The bacteria was able to reduce Cr(VI) in presence or absence of oxygen, being the aerobic condition the optimal for chromium removal. Bacterial dry biomass also showed the ability to remove Cr(VI) at optimal conditions of pH 5 and a temperature of 25°C, showing removal percentages of 73.7% to 31.1% in concentration ranges of 50 to 150 ppm after 24 hours. Taken together, the results reveal the great potential of the strain 2.15a as a bioremediation tool, with optimal chromium removal conditions close to those of the environment. The live bacteria could be implemented into many bioremediation strategies, while its biomass has the capacity of acting as an efficient renewable material for Cr(VI) removal.