The scanning electron microscopic images of wood chips (WCs) and biochar (BC) without bacterial culture and with addition of HRJ4 culture is shown in Fig. 3. It was observed that BC showed additional porous nature with many crevices, huge surface area and special structure. This property was helpful in accommodating more bacterial cells with biochar (B-BC). A monolayer bacterial cell adherence pattern was observed both in B-BC and B-WC but more compact aggregation of cells forming a biofilm in B-BC due to its special structure and surface properties. In B-WC, wood chips and bacterium were poorly combined and less bacteria was embedded because that wood chips had less crevices and smaller surface area. Soil aggregate pore size played an important role on biodegradation of petroleum hydrocarbons that are biodegraded primarily at the oil–water interface (Akbari and Ghoshal, 2015). The addition of biochar in bacterium culture permitted excellent mass transportation of oxygen, nutrients, and hydrocarbons and formed a suitable micro-environment inside the system to colonize HRJ4. The SEM analysis results showed that BC was an excellent amendment than the WC for immobilization of HRJ4 in petroleum hydrocarbon degradation studies.