Abstract
The study focused on assessing the influence of rhamnolipids on the phytotoxicity of diesel oil-contaminated soil samples. Tests evaluating the seed germination and growth inhibition of four terrestrial plant species (alfalfa, sorghum, mustard and cuckooflower) were carried out at different rhamnolipid concentrations (ranging from 0 to 1.200 mg/kg of wet soil). The experiments were performed in soil samples with a different diesel oil content (ranging from 0 to 25 ml/kg of wet soil). It was observed that the sole presence of rhamnolipids may be phytotoxic at various levels, which is especially notable for sorghum (the germination index decreased to 41 %). The addition of rhamnolipids to diesel oil-contaminated soil samples contributed to a significant increase of their phytotoxicity. The most toxic effect was observed after a rhamnolipid-supplemented diesel oil biodegradation, carried out with the use of a hydrocarbon-degrading bacteria consortium. The supplemention of rhamnolipids (600 mg/kg of wet soil) resulted in a decrease of seed germination of all studied plant species and an inhibition of microbial activity, which was measured by the 2,3,5-triphenyltetrazolium chloride tests. These findings indicate that the presence of rhamnolipids may considerably increase the phytotoxicity of diesel oil. Therefore, their use at high concentrations, during in situ bioremediation processes, should be avoided in a terrestrial environment.
Highlights
The use of biosurfactants has been recognised as an environmentally friendly way of enhancing the removal of petroleum products from contaminated soil (Mulligan 2009)
We investigated the influence of rhamnolipids on the seed germination and growth of four common terrestrial plant species in diesel oil-contaminated soil samples
The biosurfactant did not exhibit any phytotoxicity at lower concentrations (75 mg/kg), an increase in the concentration of rhamnolipids resulted in a decrease of the GI for alfalfa, mustard and sorghum
Summary
The use of biosurfactants has been recognised as an environmentally friendly way of enhancing the removal of petroleum products from contaminated soil (Mulligan 2009). The application of rhamnolipids contributes to an increased effectiveness of soil flushing processes (at concentrations up to 50 g per 1 kg soil), as well as a higher biodegradation efficiency (at average concentrations below 1 g/l) (Urum et al 2004; Santa Anna et al 2007; Makkar and Rockne 2003), the number of studies focused on assessing their effect on phytotoxicity in a petroleum-contaminated terrestrial environment is very limited (Millioli et al 2009) With this in mind, we investigated the influence of rhamnolipids on the seed germination and growth of four common terrestrial plant species (alfalfa, sorghum, mustard and cuckooflower) in diesel oil-contaminated soil samples. The results of this study may provide an interesting insight into the potential toxicity of rhamnolipids, applied during the bioremediation of petroleum-contaminated soil
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