Application of Biosolids-Derived Biochar for the Remediation of Australian Soil Contaminated with Diesel

Soil contamination, an aftermath of the Industrial Revolution, is a common environmental problem in many countries, including Australia, China, and the United States. In Australia alone, 160,000 sites are estimated to be contaminated, with petrogenic hydrocarbon being one of the common contaminants. Exposure to petrogenic hydrocarbons has a direct and indirect negative impact on both the soil and living things exposed to it. For example, the soil carbon to-nitrogen ratio can be altered when the soil is contaminated with petrogenic hydrocarbons, while humans can experience carcinogenic, haemotoxic, and many other toxicological health problems on exposure to this contaminant. Additionally, soil contamination threatens the actualisation of several United Nations Sustainable development goals, such as clean water and sanitation for all. There is an urgent need to remediate petroleum hydrocarbon-contaminated sites, considering the magnitude of this problem and its impact on the ecosystem. Several techniques exist to remediate petroleum-contaminated soils. Among the approaches, bioremediation or biological remediation stands out as a sustainable technique for the remediation of petroleum hydrocarbon-contaminated soil in terms of cost, environmental benefits, and social acceptance. However, bioremediation alone may be slow and thus unsuitable. Additives such as biostimulation (nutrients, or aeration), and bioaugmentation (microorganisms) can be introduced to the contaminated soil to reduce the remediation time. One additive that has found relevance in recent times is a carbon-based product called biochar, which is produced by heating biomass, including waste materials, in an oxygen-limited environment, typically at a temperature of 300 - 700 oC. Its use in remediation is an opportunity to address other serious environmental issues like waste management and climate change. Adding biochar to the soil enhances carbon sequestration through returning carbon to the soil. Also, converting waste such as biosolids and manure to biochar will reduce the volume of this waste streams and in turn lead to greenhouse gases emission reduction. The research described in this thesis aims to investigate the role of biosolids-derived biochar in the remediation of Australian soil contaminated with diesel. While the role of biochar in the remediation of petroleum hydrocarbon-contaminated soil has been investigated in other countries, the potential of biochar in the remediation of Australian soil contaminated with diesel remains unknown. Additionally, results from the literature show discrepancies in the effect of biochar on the remediation of petroleum hydrocarbon-contaminated soil, with variation in biochar production conditions likely playing a role. The first experimental chapter of this research sought to understand (i) the effect of biochar on the remediation and ecotoxicity of Australian soil contaminated with diesel at a total petroleum hydrocarbon (TPH) concentration of 16,220 mg/kg and (ii) how the biochar pyrolysis temperature (350, 500, and 900 oC), biochar application dose (2, 5, and 10% w/w), and fertiliser dose (0, 1, and 2% w/w) influences the efficacy of biochar on hydrocarbon removal. The Taguchi design of experiment (DOE) methodology was used to design the experiment. The main finding from this chapter was that the Environment Protection Authority (EPA) Victoria maximum threshold for Category D waste (5,000 mg/kg) was achieved in the best biochar treatment (900/10/1 – pyrolysis temperature/biochar application dose/fertiliser dose). In contrast, this threshold was exceeded in the control by 2,533 mg/kg in the same incubation time. Furthermore, hydrocarbon removal was affected by treatment conditions; biochar pyrolysis temperature, biochar application dose, and fertiliser. The addition of fertiliser inhibited the efficacy of biochar in hydrocarbon removal, thus indicating that co-applying biosolids biochar with fertiliser is not recommended in the remediation of diesel-contaminated soil. To further investigate the range of biochar’s effectiveness, in Chapter 4 the efficacy of biochar was examined in soil with a higher TPH concentration (54,147 mg/kg). The treatment condition used for this chapter was as follows: pyrolysis temperature – 900 oC, biochar application dose – 5%, and fertiliser dose – 0%. The result showed that the addition of biochar led to 2,353 mg/kg lower soil TPH concentration relative to the control after 24 weeks. Although the incubation period differed from Chapter 3, the findings from Chapter 4 demonstrate that biosolids biochar can still achieve similar removal even though the TPH concentration is altered to 54,147 mg/kg. Therefore, the result from this thesis suggests that biochar effectiveness was not significantly affected when the hydrocarbon concentration was altered from 1.6% to 5.4%. The second part of Chapter 4 dealt with understanding the major mechanism since this knowledge may result in optimisation of biochar application in terms of TPH removal. To achieve this, sodium azide (NaN3), a selective bacterial inhibitor, was added to one of the biochar treatments (BN) to alter the bacterial community structure. While NaN3 has been used to study biochar mechanisms, surprisingly this is the first study to incorporate quantitative PCR and 16S rRNA sequencing. This novelty provided a new approach to studying biochar mechanisms. After 24 weeks of incubation, hydrocarbon removal was significantly higher in the biochar treatment with NaN3 (BN) than in the biochar treatment (3,827 mg/kg more). While the soil bacterial community structure in the control and the biochar treatment were more similar at the phylum level, the biochar treatment differed significantly from the BN treatment in terms of the composition of the bacterial community, which coincided with significant differences in hydrocarbon removal between both treatments. These findings showed that the bacterial community significantly influences hydrocarbon removal in diesel-impacted soil amended with biosolids biochar, demonstrating that biodegradation was the major mechanism. This observation led to the final results chapter, where the effect of biochar co-application with a hydrocarbonoclastic bacterium (via immobilisation) on hydrocarbon removal on dieselcontaminated soil with a TPH concentration of 62,027 mg/kg was examined for 22 weeks. The bacterium was isolated from the BN treatment in Chapter 4 and was identified as Ochrobactrum sp. using 16S rRNA Sanger sequencing. The results revealed that bacterial immobilisation on biochar with Ochrobactrum sp. (BIB) resulted in a statistically significantly higher hydrocarbon removal from week 10 till the end of incubation, with 5,533 and 1,607 mg/kg higher removal in BIB than the biochar treatment at week 10 and 22, respectively. Overall, this suggests that the co-application of biochar (via immobilisation) with the bacteria was beneficial in enhancing the remediation of petroleum hydrocarbon-contaminated soil, especially at week 10. Another key finding from this study was that BIB was less effective in hydrocarbon removal when combined with fertiliser, demonstrating the non-beneficial role of fertiliser in the soil used in this study. As the residual total petroleum hydrocarbon (TPH) concentration does not truly give an indication of soil ecotoxicity, soil toxicity was carried out in Chapters 3 and 4, using the Microtox assay. The results revealed that biochar application generally led to reduced soil toxicity relative to the control. For example, in Chapter 4, the biochar treatment significantly reduced soil toxicity, while the control was not able to cause a significant reduction in soil toxicity after 24 weeks of incubation. This research has demonstrated the efficacy of biochar in the remediation of Australian dieselcontaminated soil and has shown that: (i) biochar has a potential for remediation and reducing the ecotoxicity of Australian soil contaminated with diesel; (ii) the efficacy of biochar in remediation is affected by the biochar pyrolysis temperature, biochar dose, and fertiliser dose; (iii) altering the hydrocarbon concentration from 1.6% to 5.4% does not have a significant influence on biochar efficacy in remediation (iv) biodegradation was the major mechanism in biochar-based remediation; (v) ecotoxicity should be integrated with the hydrocarbon concentration in assessing the success of remediation; (vi) the immobilisation of bacteria on biochar resulted in higher hydrocarbon removal than the sole biochar or bioaugmentation treatment; and (vii) fertiliser was detrimental for hydrocarbon removal when it was co-applied with bacteria immobilised biochar.