The immobilisation of bacteria on biochar has shown potential for enhanced remediation of petroleum hydrocarbon-contaminated soil. However, there is a lack of knowledge regarding the effect of bacterial immobilisation on biosolids-derived biochar for the remediation of diesel-contaminated soil. This current study aimed to assess the impact of the immobilisation of an autochthonous hydrocarbonoclastic bacteria, Ochrobacterium sp. (BIB) on biosolids-derived biochar for the remediation of diesel-contaminated soil. Additionally, the effect of fertiliser application on the efficacy of the BIB treatment was investigated. Biochar (BC) application alone led to significantly higher hydrocarbon removal than the control treatment at all sampling times (4887–11,589 mg/kg higher). When Ochrobacterium sp. was immobilised on biochar (BIB), the hydrocarbon removal was greater than BC by 5533 mg/kg and 1607 mg/kg at weeks 10 and 22, respectively. However, when BIB was co-applied with fertiliser (BIBF), hydrocarbon removal was lower than BIB alone by 6987–11,767 mg/kg. Quantitative PCR (q-PCR) analysis revealed that the gene related to Ochrobacterium sp. was higher in BIB than in the BC treatment, which likely contributed to higher hydrocarbon removal in the BIB treatment. The results of the q-PCR analysis for the presence of alkB genes and FTIR analysis suggest that the degradation of alkane contributed to hydrocarbon removal. The findings of this study demonstrate that bacterial immobilisation on biosolids-derived biochar is a promising technique for the remediation of diesel-contaminated soil. Future studies should focus on optimising the immobilisation process for enhanced hydrocarbon removal.

This study aimed to investigate the impact of biosolids-derived biochar on the remediation of Australian soil contaminated with diesel while investigating the role of biochar in the remediation. To achieve the latter aim, sodium azide (NaN₃) was added to a separate biochar treatment (BN) to alter the bacterial community structure. Biochar (B) reduced detectable hydrocarbons by 2353 mg/kg compared to the control (C) treatment at week 24. However, the BN treatment reduced the hydrocarbon concentration by 3827 and 6180 mg/kg, relative to B and C, respectively. Soil toxicity significantly decreased at week 24 compared to the start of the remediation in B, but not in the control. Biochar and control treatments generally showed a similar bacterial community structure throughout the incubation, while the bacterial community structure in BN differed significantly. Biodegradation was found to play a significant role in hydrocarbon removal, as the variation in the bacteria community coincided with differences in hydrocarbon removal between B and BN. The increased removal of hydrocarbons in the BN treatment relative to B coincided with increased and reduced relative abundances of Gordonia and JG30-KF-CM45 genera, respectively. This study showed that NaN₃ led to a transient and selective inhibition of bacteria. This study makes an important contribution towards understanding the use of NaN₃ in examining the role of biochar in the remediation of diesel-contaminated soil. Overall, we conclude that biochar has the potential to enhance the remediation of diesel-contaminated soil and that biodegradation is the dominant mechanism.

The immobilisation of bacteria on biochar has shown potential for enhanced remediation of petroleum hydrocarbon-contaminated soil. However, there is a lack of knowledge on the effect of bacterial immobilisation on biosolids-derived biochar for the remediation of diesel-contaminated soil. The aim of this current study was to assess the impact of the immobilisation on biosolids-derived biochar of an autochthonous hydrocarbonoclastic bacteria, Ochrobacterium sp. (BIB) on the remediation of diesel-contaminated soil using a laboratory based mesocosm study. Additionally, the effect of fertiliser application on the efficacy of the BIB treatment was investigated. Biochar (BC) application alone led to significantly higher hydrocarbon removal compared to the control treatment at all sampling times (4,887 – 11,589 mg/kg higher). When Ochrobacterium sp. was immobilised on biochar (BIB), the hydrocarbon removal was higher than BC by 5,533 mg/kg and 1,607 mg/kg at weeks 10 and 22, respectively. However, when BIB was co-applied with fertiliser (BIBF), the hydrocarbon removal was lower than BIB by 6,987 – 11,767 mg/kg. Quantitative PCR analysis revealed that the gene related to Ochrobacterium sp. was higher in BIB than in the BC treatment, which likely contributed to higher hydrocarbon removal in the BIB treatment. The findings of this study demonstrate that bacteria immobilisation on biosolids-derived biochar is a promising technique for the remediation of diesel-contaminated soil. Future studies should focus on optimising the immobilisation process for enhanced hydrocarbon removal.