Petroleum hydrocarbon contamination is widespread and has easily accumulated in various ecosystems due to hydrophobicity and limited bioavailability. Until now, the primary bioremediation techniques have used free cell (FC) cultures to remove these recalcitrant compounds from contaminated areas. The use of immobilized microbial cell systems is a promising technique, which has been recently applied in various environmental studies. The primary objective of this review is to give an overview of the key scientific aspects related to bioremediation of hydrocarbons using immobilized cell (IC), with a particular focus on the techniques to entrap the cells. The most effective cell immobilization technique was cell entrapping inside a porous matrix. This review also examines and compares the performance of ICs in biodegradation of petroleum hydrocarbons with those of FC systems. It has been observed that the biodegradation rate can be enhanced using immobilization technique, especially in harsh environments. Moreover, several advantages of using ICs compared to FCs are also discussed. For example, due to the easier separation, possible reutilization of microorganisms, and more tolerance to pH and temperature changes, ICs are generally preferred to FC systems. Finally, examples of some limitations of IC systems and new viewpoints in this field are presented.

Oil pollution is a serious international concern due to its harmful effect on human health and the environment. This study aims to investigate the effective factors on the biodegradation of Iranian heavy crude oil by Bacillus licheniformis. For this purpose, oil removal from the artificial seawater was studied by response surface methodology (RSM). After the screening experiments, pH (4-10), NaCl concentration (0-10 g/L), and oil concentration (500-4500 ppm) were selected as influential factors. Moreover, to evaluate the bacterial capability in bioremediation of an actual polluted site, crude oil spill with a salinity of 35 g/L was experimentally simulated. The proposed model in this study clearly shows that both selected individual factors and their interactions are significantly effective on the crude oil biodegradation capacity. The results showed that Bacillus licheniformis was able to degrade crude oil at different concentrations of oil, especially at low concentrations, which are challenging in actual polluted sites. 15%-66% removal was achieved for 500-4500 ppm of crude oil after 14 days. Furthermore, according to the obtained results, this bacterium can tolerate the salinity up to 3.5%. At this salinity level, crude oil removal was 23.43 and 25.64% in neutral and alkaline conditions, respectively. Process factors were optimized, and 54.8% of crude oil was removed at optimum conditions i.e., 3500 ppm crude oil concentration, 2.5 g/L of NaCl and pH equal to 8.5. Finally, it can be concluded that the selected bacterium of this study can be more effective in harsh environments such as hypersaline and alkaline conditions.