Fig. 2 presents the changing concentrations of benzene, toluene,
ethylbenzene and p-xylene under different doses of immobilized
Mycobacterium sp. CHXY119 as a function of incubation
time. It was clear that benzene, toluene, ethylbenzene and
p-xylene exhibited different biodegradation efficiencies by immobilized
Mycobacterium sp. CHXY119. Ethylbenzene was completely
removed after 120 h of incubation, and the average biodegradation
rate was 0.239 mg l−1 h−1; p-xylene reached 100% biodegradation
after 210 h of contact, and the average biodegradation rate was
0.119 mg l−1 h−1. Moreover, there was no lag of biodegradation for
both ethylbenzene and p-xylene, i.e., biodegradation started as soon
as the pollutants came in contact with the cells. In contrast, toluene
exhibited a biodegradation lag of about 80 h, followed by complete
removal after 400 h of contact with the immobilized Mycobacterium
sp. CHXY119 at an average biodegradation rate of 0.087 mg l−1 h−1.
Immobilized Mycobacterium sp. CHXY119 had the worst efficiency
for removing benzene, which is the most serious threat to human
health among the BTEX compounds and has a minimum permitted
dose of 0.005 mg l−1 in drinking water. Only 5% of benzene was
removed after 400 h of incubation.