Used lubricant oils (ULOs) strongly bind to soil particles and cause persistent pollution.
In this study, soil microcosm experiments were conducted to model the ex situ bioremediation
of a long term ULO-polluted area. Biostimulation and various inoculation levels of
bioaugmentation were applied to determine the efficacy of total petrol hydrocarbon
(TPH) removal. ULO-contaminated soil microcosms were monitored for microbial respiration,
colony-forming units (CFUs) and TPH bioconversion. Biostimulation with inorganic nutrients
was responsible for 22% of ULO removal after 40 days. Bioaugmentation using two hydrocarbon-degrader
strains: Rhodococcus quingshengii KAG C and Rhodococcus erythropolis PR4 at a small
inoculum size (107 CFUs g−1 soil), reduced initial TPH concentration by 24% and 29%,
respectively; the application of a higher inoculum size (109 CFUs g−1 soil) led to
41% and 32% bioconversion, respectively. After 20 days, all augmented CFUs decreased
to the same level as measured in the biostimulated cases, substantiating the challenge
for the newly introduced hydrocarbon-degrading strains to cope with environmental
stressors. Our results not only highlight that an increased number of degrader cells
does not always correlate with enhanced TPH bioconversion, but they also indicate
that biostimulation might be an economical solution to promote ULO biodegradation
in long term contaminated soils.
