Biodegradation of high-concentration isopropanol by a solvent-tolerant thermophile, Bacillus pallidus

Abstract

The aerobic biodegradation of high-concentration, to 24 g l^–1, 2-propanol (IPA) by a thermophilic isolate ST3, identified as Bacillus pallidus, was successfully carried out for the first time. This solvent-tolerant B. pallidus utilized IPA as the sole carbon source within a minimal salts medium. Cultivation was carried out in 100-ml shake flasks at 60°C and compared with cultivation within a 1-l stirred tank reactor (STR). Specific growth rate (µ) was about 0.2 h^–1 for both systems, with a maximum cell density of 2.4×10⁸ cells ml^–1 obtained with STR cultivation. During exponential growth and stationary phase, IPA biodegradation rates were found to be 0.14 and 0.02 g l^–1 h^–1, respectively, in shake-flask experiments, whereas corresponding values of 0.09 and 0.018 g l^–1 h^–1 were achievable in the STR. Generation of acetone, the major intermediate in aerobic IPA biodegradation, was also monitored as an indicator of microbial IPA utilization. Acetone levels reached a maximum of 2.2–2.3 g l^–1 after 72 and 58 h for 100-ml and 1-l systems, respectively. Both IPA and acetone were completely removed from the medium following 160 and 175 h, respectively, during STR growth, although this was not demonstrated within shake-flask reactions. Growth of B. pallidus on acetone or IPA alone demonstrated that the maximum growth rate (µ) obtainable was 0.247 h^–1 at 4 g l^–1 acetone and 0.202 h^–1 at 8 g l^–1 IPA within shake-flask cultivation. These results indicate the potential of the solvent-tolerant thermophile B. pallidus ST3 in the bioremediation of hot solvent-containing industrial waste streams.