How important is abiotic dissipation in natural attenuation of polycyclic aromatic hydrocarbons in soil?

Photodegradation of polycyclic aromatic hydrocarbons on soil surface: Kinetics and quantitative structure-activity relationship (QSAR) model development

Polycyclic aromatic hydrocarbons (PAHs) have attracted much attention because of their widespread existence and toxicity. Photodegradation is the main natural decay process of PAHs in soil. The photodegradation kinetics of benzopyrene (BaP) on 16 kinds of soils and 10 kinds of PAHs on Hebei (HE) soil were studied. The results showed that BaP had the highest degradation rate in Shaanxi (SN) soil (kobs = 0.11 min-1), and anthracene (Ant) was almost completely degraded after 16 h of irradiation in HE soil. Two quantitative structure-activity relationship (QSAR) models were established by the multiple linear regression (MLR) method. The developed QSAR models have good stability, robustness and predictability. The model revealed that the main factors affecting the photodegradation of PAHs are soil organic matter (SOM) and the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (Egap). SOM can function as a photosensitizer to induce the production of active species for photodegradation, thus favoring the photodegradation of PAHs. In addition, compounds with lower Egap are less stable and more reactive, and thus are more prone to photodegradation. Finally, the QSAR model was optimized using machine learning approach. The results of this study provide basic information on the photodegradation of PAHs and have important significance for predicting the environmental behavior of PAHs in soil.

Study on the accelerated biodegradation of PAHs in subsurface soil via coupled low-temperature thermally treatment and electron acceptor stimulation based on metagenomic sequencing

In situ anoxic bioremediation is a sustainable technology to remediate PAHs contaminated soils. However, the limited degradation rate of PAHs under anoxic conditions has become the primary bottleneck hindering the application of this technology. In this study, coupled low-temperature thermally treatment (<50 °C) and EA biostimulation was used to enhance PAH removal. Anoxic biodegradation of PAHs in soil was explored in microcosms in the absence and presence of added EAs at 3 temperatures (15 °C, 30 °C, and 45 °C). The influence of temperature, EA, and their interaction on the removal of PAHs were identified. A PAH degradation model based on PLSR analysis identified the importance and the positive/negative role of parameters on PAH removal. Soil archaeal and bacterial communities showed similar succession patterns, the impact of temperature was greater than that of EA. Soil microbial community and function were more influenced by temperature than EAs. Close and frequent interactions were observed among soil bacteria, archaea, PAH-degrading genes and methanogenic genes. A total of 15 bacterial OTUs, 1 PAH-degrading gene and 2 methanogenic genes were identified as keystones in the network. Coupled low-temperature thermally treatment and EA stimulation resulted in higher PAH removal efficiencies than EA stimulation alone and low-temperature thermally treatment alone.

Roles of Natural Phenolic Compounds in Polycyclic Aromatic Hydrocarbons Abiotic Attenuation at Soil-Air Interfaces through Oxidative Coupling Reactions

Little information is available on the roles of natural phenolic compounds in polycyclic aromatic hydrocarbons (PAHs) attenuation at dry soil-air interfaces. The purpose of this study was to determine the roles of model phenolic constituents of soil organic matter (SOM) on the abiotic attenuation of PAHs. The phenolic compounds can significantly change the attenuation rates of PAHs, among which hydroquinone was the most effective in promoting anthracene and benzo[a]anthracene attenuation. Product identification and sequential extraction experiments revealed hydroquinone enhanced the formation of oxidative coupling products and promoted the incorporation of PAHs into humic analogues, thereby reducing potential risks to humans and ecosystems. Electron paramagnetic resonance spectroscopy analyses showed both PAHs and phenolic compounds could donate electrons to Lewis acid sites of soil minerals, resulting in the generation of persistent free radicals (PFRs). PFRs could promote the generation of ·OH to enhance PAH oxidation and could cross-couple with PAHs, resulting in high-molecular-weight oxidative coupling products. This study revealed for the first time the reaction mechanism between PAHs and phenolic components of SOM under relatively dry conditions and provided new insights into promoting PAHs detoxification in soils but also a potential strategy to increase the organic carbon sequestration.

Effects of biodegradable and non-biodegradable microplastics on bacterial community and PAHs natural attenuation in agricultural soils

Anthropogenic activities such as in situ straw incineration and the widespread use of agricultural film led to the accumulation of polycyclic aromatic hydrocarbons (PAHs) and microplastics (MPs) in agricultural soils. In this study, four biodegradable MPs (BPs), including polylactic acid (PLA), polybutylene succinate (PBS), poly-β-hydroxybutyric acid (PHB) and poly (butylene adipate-co-terephthalate) (PBAT) and non-biodegradable low-density polyethylene (LDPE) were selected as representative MPs. The soil microcosm incubation experiment was conducted to analyze MPs effects on PAHs decay. MPs did not influence PAHs decay significantly on day 15 but showed different effects on day 30. BPs reduced PAHs decay rate from 82.4% to 75.0%- 80.2% with the order of PLA < PHB < PBS < PBAT while LDPE increased it to 87.2%. MPs altered beta diversity and impacted the functions to different extents, interfering in PAHs biodegradation. The abundance of most PAHs-degrading genes was increased by LDPE and decreased by BPs. Meanwhile, PAHs speciation was influenced with bioavailable fraction elevated by LDPE, PLA and PBAT. The facilitating effect of LDPE on 30-d PAHs decay can be attributed to the enhancement of PAHs-degrading genes and PAHs bioavailability, while the inhibitory effects of BPs were mainly due to the response of the soil bacterial community.

Exploring the PAHs dissipation and indigenous bacteria response in soil amended with two different microbial inoculants

This study investigated the bioremediation of PAHs in soil by two different microbial inoculants prepared with Paracoccus aminovorans HPD-2 and the carrier humic acid (HA) or montmorillonite (Mont). After incubation for 42 d, the greatest removal of PAHs, 42.8 % or 41.6 %, was observed in microcosms with 0.2 % HA inoculant or 2 % Mont inoculant. The PAH removal efficiency in these treatments was significantly greater than that in soil amended only with planktonic HPD-2. Bacterial community analysis showed that the survival of Paracoccus aminovorans was enhanced in the treatments with Mont inoculant compared with the treatments with HA inoculant or with HPD-2 alone. Moreover, the diversity of PAH-degrading bacterial genera was greater in the treatments containing Mont inoculant than in the treatments containing HA inoculant. These results indicate that the organic material HA and inorganic material Mont promote PAH removal in different ways. Specifically, HA promotes PAHs bioavailability to accelerate the degradation of PAHs in soil, whereas Mont protects PAH-degrading microorganisms to promote pollutant removal. Overall, the findings suggest that HA and Mont are promising materials for microbial immobilization for the bioremediation of PAH-contaminated soil.

Occurrence of polycyclic aromatic compounds and interdomain microbial communities in oilfield soils

Soil polycyclic aromatic compound (PAC) pollution as a result of petroleum exploitation has caused serious environmental problems. The unclear assembly and functional patterns of microorganisms in oilfield soils limits the understanding of microbial mechanisms for PAC elimination and health risk reduction. This study investigated the polycyclic aromatic hydrocarbons (PAHs) and substituted PAHs (SPAHs) occurrence, and their impact on the bacteria-archaea-fungi community diversity, co-occurrence network and functionality in the soil of an abandoned oilfield. The results showed that the PAC content in the oilfield ranged from 3429.03 μg kg^-1 to 6070.89 μg kg^-1, and risk assessment results suggested a potential cancer risk to children and adults. High molecular weight PAHs (98.9%) and SPAHs (1.0%) contributed to 99.9% of the toxic equivalent concentration. For microbial analysis, the abundantly detected degraders and unigenes indicated the microbial potential to mitigate pollutants and reduce health risks. Microbial abundance and diversity were found to be negatively correlated with health risk. The co-occurrence network analysis revealed nonrandom assembly patterns of the interdomain microbial communities, and species in the network exhibited strong positive connections (59%). The network demonstrated strong ecological linkages and was divided into five smaller coherent modules, in which the functional microbes were mainly involved in organic substance and mineral component degradation, biological electron transfer and nutrient cycle processes. The keystone species for maintaining microbial ecological functions included Marinobacter of bacteria and Neocosmospora of fungi. Additionally, benzo [g,h,i]pyrene, dibenz [a,h]anthracene, indeno [1,2,3-cd]perylene and total phosphorus were the key environmental factors driving the assembly and functional patterns of microbial communities under pollution stress. This work improves the knowledge of the functional pattern and environmental adaptation mechanisms of interdomain microbes, and provides valuable guidance for the further bioremediation of PAC-contaminated soils in oilfields.