Ecotoxicity of oil sludges and residuals from their washing with surfactants: soil dehydrogenase and ryegrass germinatio
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RESEARCH ARTICLE
Ecotoxicity of oil sludges and residuals from their washing with surfactants: soil dehydrogenase and ryegrass germination tests Diego Ramirez 1
&
Liz J. Shaw 1 & Chris D. Collins 1
Received: 6 May 2020 / Accepted: 18 October 2020 # The Author(s) 2020
Abstract Oil sludge washing (OSW) with surfactants and co-solvents is used to recover the oil, and this process leaves some residuals (sediments and surfactant solution). Currently, there are no data on the ecotoxicological effects of these OSW residuals from different sludges. This study evaluated the toxicity of OSW residuals from washing four types of oil sludges with five surfactants (Triton X-100 and X-114, Tween 80, sodium dodecyl sulphate (SDS) and rhamnolipid) and a co-solvent (cyclohexane). The toxicity of the residuals was evaluated with the impact on the soil microbial dehydrogenase activity (DHA) and ryegrass (Lolium perenne) seed germination. There was a high DHA detected directly in the sludges and all OSW residual combinations, but this activity could not be attributed to the DHA itself but to some chemical interferences. The DHA was then tested in the soils amended with the OSW residuals to simulate a bioremediation scenario. There were no chemical interferences in this case. In general, the INTF concentrations were significantly higher at low concentrations, 1 and 5% (p < 0.01). There were no significant differences in the DHA at high concentrations of OSW residuals (10, 25 and 50%) which implied that the concentration of the contaminants is not directly proportional to the levels of ecotoxicity. Unexpectedly, the INTF values of the 10, 25 and 50% rhamnolipid-OSW residuals were significantly lower than the Triton X-100 residuals. The ryegrass germination rates were higher than 70% with no apparent phytotoxicity symptoms in the seedlings. Particularly, there was a highly significant negative effect of the residuals on the germination rates at high concentrations (p < 0.01). Given that the extractable petroleum hydrocarbon (EPH) concentrations in the OSW residualamended soils in both DHA and germination tests were very low (13–21 ppm), other co-contaminants could be contributing to the toxicity. These findings implied that biotreatment techniques can be applied to treat the OSW residuals if necessary. Keywords Oil sludge washing (OSW) . Surfactants . Toxicity . Dehydrogenase activity (DHA) . Germination test . Ryegrass
Introduction
Responsible Editor: Kitae Baek * Chris D. Collins [email protected] Diego Ramirez [email protected] Liz J. Shaw [email protected] 1
Department of Geography and Environmental Science, University of Reading, Reading RG6 6DW, UK
Approximately one billion tonnes of oil sludges have been accumulated worldwide (Mirghaffari 2017), and this substantially affects the functioning of the petroleum industry and impacts the environment. These wastes are principally composed of crude oil, water and sediments. Moreover, oil sludges can have some traces of heavy metals such as copper (Cu),
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