Abstract
Biodegradation is a key process for the remediation of sites contaminated by petroleum hydrocarbons (PHCs), but this process is not well known for the (semi)-arid coastal environments where saline conditions and continuous water level fluctuations are common. This study differs from the limited previous studies on the biodegradation of PHCs in Qatari coastal soils mainly by its findings on the biodegradation kinetics of the selected PHCs of benzene and naphthalene by indigenous bacteria. Soil samples were collected above, across, and below the groundwater table at the eastern coast of Qatar within a depth of 0 to -40 cm. Environmental conditions combining low oxygen and high sulfate concentrations were considered in this study which could favor either or both aerobic and anaerobic bacteria including sulfate-reducing bacteria (SRB). The consideration of SRB was motivated by previously reported high sulfate concentrations in Qatari soil and groundwater. Low- and high-salinity conditions were applied in the experiments, and the results showed the sorption of the two PHCs on the soil samples. Sorption was dominant for naphthalene whereas the biodegradation process contributed the most for the removal of benzene from water. Losses of nitrate observed in the biodegradation experiments were attributed to the activity of nitrate-reducing bacteria (NRB). The results suggested that aerobic, NRB, and most likely SRB biodegraded the two PHCs, where the combined contribution of sorption and biodegradation in biotic microcosms led to considerable concentration losses of the two PHCs in the aqueous phase (31 to 58% after 21 to 35 days). Although benzene was degraded faster than naphthalene, the biodegradation of these two PHCs was in general very slow with rate coefficients in the order of 10-3 to 10-2 day-1 and the applied kinetic models fitted the experimental results very well. It is relevant to mention that these rate coefficients are the contribution from all the microbial groups in the soil and not from just one.
Highlights
The process of biodegradation has been given considerable interest globally for the purpose of cleaning up the sites contaminated by petroleum hydrocarbons (PHCs) [1,2,3,4,5,6]
The main objectives of this study were (1) to quantify the biodegradation kinetics of two PHCs in a-arid saline coastal soil, which was influenced by groundwater table fluctuations and seawater intrusion, by indigenous bacteria at a low oxygen concentration; (2) to investigate the potential difference in biodegradation behavior with depth by considering the vadose and saturated zones of shallow coastal soil environment of Qatar, and the effect of salinity on biodegradation due to seawater intrusion, and (3) to propose a model which takes into account the biodegradation kinetics of the PHCs used in this study to predict the fate of PHCs in-arid saline coastal soil environments
The findings of this study indicated that the dominant process in the batch experiments was sorption for naphthalene and biodegradation for benzene
Summary
The process of biodegradation has been given considerable interest globally for the purpose of cleaning up the sites contaminated by petroleum hydrocarbons (PHCs) [1,2,3,4,5,6]. Some studies have shown that tide-induced seawater circulations can cause frequent groundwater table fluctuations in coastal aquifers which in turn can enhance the biodegradation of PHCs after an oil spill (e.g., [15,16,17,18]) This enhancement can occur as a result of the redistribution or diffusion of electron acceptors (e.g., oxygen) and donors (e.g., nonaqueous phase liquids or LNAPLs) across the groundwater table during water table rise and fall, helping local bacteria for their metabolisms. Kostka et al [20] characterized the microbial community involved in the biodegradation of PHCs (C8 to C40) in Gulf of Mexico beach sands which were exposed to heavy oil contamination from the well-known Deepwater Horizon oil spill in 2010 They identified up to 24 bacterial strains capable of degrading these PHCs. The biodegradation of naphthalene in seawater-impacted coastal sediments was studied by Jin et al [21] with enriched cultures established using seawater and modified minimal media containing naphthalene. Their results showed that naphthalene and other polycyclic aromatic hydrocarbons (PAHs) such as phenanthrene and anthracene could be degraded in the sediments over time
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