An Examination of Co-Composting Aged Polycyclic Aromatic Hydrocarbon (PAH) Contaminated Manufactured Gas Plant Soils

Abstract

<p>Polycyclic aromatic hydrocarbons (PAHs) are predominantly generated by anthropogenic sources, are environmentally ubiquitous and a number of the compounds are known to be carcinogenic, teratogenic, mutagenic and toxic to biota. As contaminants, PAHs make up a major proportion of the pollution found in soils at manufactured gas plant (MGP) industrial sites. Depending on the concentration and the risk of the contaminated soil to the total environmental system, there may be a requirement for remediation. The remediation of this soil can pose significant clean-up costs. One biological remediation strategy, co-composting, could offer a potential cost effective and efficient clean-up strategy generating a useable end product.</p><p> This research investigates the application of a co-composting remediation strategy on two long term aged PAH contaminated soils from a former MGP site in Armidale, NSW, Australia. The aim of the project was to develop and assess an optimised co-composting system for successful removal of the 16 US EPA priority listed PAHs in the two soils and to understand the constraints on the process. This was achieved by a sequence of experiments that progressed from an analytical method development, to a series of assessments aimed to optimise the process. This included the application of different matured animal manures to determine the most suitable amendment source, different soil:amendment mixture ratios, synthetic and biological surfactants to increase PAH bioavailability, and adding a spike of phenanthrene (PHE) to understand microbial degradation capacity in the composting matrices maintained at a mesophilic (38 °C) temperature. Following this, investigation into the microbial consortium including the use of Illumina sequencing was undertaken to determine the bacterial consortia present during composting. Finally, remediation endpoint prediction modelling using hydroxypropyl-β-cyclodextrin (HPCD) was applied and a number of pre-treatment strategies trialled to enable the determination of bioaccessibility constraints for the soils being treated.</p><p> An analytical method for the extraction and clean-up of PAHs from high organic content matrices was first optimised. An exhaustive extraction technique, using ultrasonication combined with column chromatography clean-up provided between 70 -120 % recovery (%) of the 16 US EPA PAHs with < 20 % RSD. The method was then used to quantify the PAH concentrations in all experimental matrices.</p><p> A mesocosm experiment was used to evaluate and compare the effect of amending five matured animal manures: sheep, cattle, chicken, pig, horse, and the addition of nitrogen (N) and phosphorous (P), to a long term aged PAH contaminated (175 mg kg^-1) MGP soil in a cocomposting remediation strategy. The study was conducted over 31 days during which no significant reduction of PAHs or composting occurred. This was in contrast with other studies which demonstrated active degradation when matured compost was incorporated into PAH contaminated matrices. The study results indicated that a higher C:N ratio of the amendment and more frequent aeration was required (aeration) was required to trigger effective composting. Therefore, a freshly composted cattle manure was obtained from a feedlot and blended with activated wheat straw into the MGP soil at two different ratios (4:1 and 2:1) and then incubated at 38 °C, aerated and watered up to 60 % water holding capacity over a period of 56 days. Active composting occurred with significant increase and decrease of pH, EC, and reduction in bulk mass. Regardless, no significant difference in the concentration of the PAHs was observed in any treatment.</p><p> The same MGP soil was subsequently co-composted at the 4:1 soil:amendment ratio with the addition of Triton X-100, a synthetic non-ionic surfactant and a biosurfactant produced from crushing the leaves of the Red Ash Tree (<i>Alphitonia Excelsa</i>) to enhance desorption of the PAHs and increase bioavailability. A 100 mg kg<i>-1</i> spike of PHE was also used in one of the treatments to assess the capacity of the microbial consortium for PAH degradation during the composting process. Over a period of 35 days, active composting occurred for all 4:1 amendment treatments, however, no loss of PAHs was observed for any of the treatments except for the spiked PHE treatment. The spiked PHE showed an 87 % reduction in concentration. These findings strongly indicated that the limitation for PAH concentrations from the aged MGP soil was PAH bioaccessibility.</p><p> To further assess potential effectiveness of the co-composting process and limitations of co-composting for aged PAH compounds, the co-composting treatment was trialled on a second MGP soil type, with PAH contamination (489 mg kg^-1), and dissimilar physico-chemical characteristics but composting period was reduced to 14 days. Again, no significant degradation of PAHs was observed for any of the treatments except the spiked PHE treatment was reduced by 79 % between day 0 and day 14. This supported previous findings that co-composting was ineffective due to limited bioaccessibility of the aged PAHs in the MGP soils. In light of the significant degradation of the spiked PHE, an assessment of bacterial and fungi present and their capacity for PAH degradation during co-composting was undertaken. Using PCR and solid hydrocarbon spray plates it was revealed that there were few bacteria in the MGP soil alone, with very little 16S rRNA product observed (indicative gel assessment only) and 8.0 x 10⁵ colony forming units (CFU). The number of bacterial CFUs which established in MGP soil following a spike of PHE, 3.75 x 10⁵ indicated that bacteria capable of tolerating PHE were present, however PHE degrading bacteria or fungi could not be enumerated. It was revealed by qPCR amplication that bacteria had significantly increased with the incorporation of the amendment source and that during each incremental time periods of co-composting, there was a change associated with the bacteria population present. Notable, a significant increase in the number of gene copies occurred with the addition of the amendment (19134 gene copies g^-1 soil) at day 0 in comparison to the MGP soil (404 gene copies g^-1), and then at day 3 a spike in the relative abundance of gene copies in the amendments masses occurred with (218856 gene copies g^-1 soil) and without PHE (40235 gene copies g^-1 soil). This showed that bacterial present in the matrix were suppressed following the addition of a readily available PAH. Following day 3, at day 7 and day 35, the gene copy numbers then became similar for both treatments, with declining trends typical for a composting mass. Illumina sequencing was used to determine which bacteria were responsible for the PHE degradation and assess the consortium change over the compositing period. The Illumina sequencing showed 16 genera of known PAH degrading bacteria, belonging to the Actinobacteria, Bacteriodetes, Firmicutes, Proteobacteria and Verrunmicrobia phyla. In particular, bacteria from the phylum Proteobacteria, the order Burkholderiales, were proportionally greater in the 4:1 amended treatment spiked with PHE than the treatment without PHE. This suggested that bacteria from the order Burkholderiales may have played an active role in the degradation of the spiked PHE. Overall, the findings indicate that there was potential bacterial capacity for PAH degradation from the MGP soils when the amendment was incorporated. Consequently, and because no degradation of the aged PAHs during the co-composting experiments was observed, the limitation on remediation was therefore determined to be a lack of PAH bioaccessibility.</p><p> The final components of the project therefore focused on understanding the bioaccessibility of the PAH in the long-term aged MGP soils. Hydroxypropyl-β-cyclodextrin (HPCD) extractions combined with linear regression modelling was used to determine the 16 US EPA PAH bioaccessibility. The modelling consistently showed that the predicted total PAH remediated endpoint concentrations at approximately 1:1 with the actual processed soils. The modelling results support strongly that limitations for co-composting bioremediation on aged MGP soils was PAH bioaccessibility. The application of pre-treatment strategies, pulverisation, a Fenton-like reaction (hydrogen peroxide) and thermal (165 °C) desorption showed little increase in the bioaccessibility of the PAHs. With the pulverisation pre-treatment, an improvement in bioaccessible was observed, however, the predicted endpoint concentration was greater than the MGP control soil. The thermal pre-treatment also showed some potential in application, however, the temperature would have to be reduced to prevent volatilisation of PAHs. Only the thermal pre-treatment showed some potential to increase bioaccessibility of individual PAHs, with three PAHs (fluoranthene, benz(a)anthracene and phenanthrene), being improved by 62 - 99 %.</p><p> The use of the co-composting remediation strategy on long term aged PAH contaminated MGP soils was not suitable for the aged MGP soils assessed in this study. This is the first time an in-depth co-composting study has been conducted on an aged MGP soil and it has revealed a number of important findings. The trails suggest that PAH bioaccessibility was the limiting factor but that optimisation of a thermal desorption pre-treatment coupled with co-compositng may yield some potential for successful remediation. Overall, the key findings of this study support the need for detailed piloting of remediation strategies. The understanding of bioaccessibility for contaminated matrices prior to implementation of remedial processes may save significant time and cost. The use of a simple bioaccessibility assay tool and modelling may be appropriate for this purpose.</p>