Spatial distribution characteristics and degradation mechanism of microorganisms in n-hexadecane contaminated vadose zone.

The damage caused by petroleum hydrocarbon pollution to soil and groundwater environment is becoming increasingly significant. The vadose zone is the only way for petroleum hydrocarbon pollutants to leak from surface into groundwater. The spatial distribution characteristics of indigenous microorganisms in vadose zone, considering presence of capillary zones, have rarely been reported. To explore the spatial distribution characteristics of indigenous microorganisms in vadose zone contaminated by petroleum hydrocarbons, a one-dimensional column migration experiment was conducted using n-hexadecane as characteristic pollutant. Soil samples were collected periodically from different heights during experiment. Corresponding environmental factors were monitored online. The microbial community structure and spatial distribution characteristics of the cumulative relative abundance were systematically analyzed using 16S rRNA sequencing. In addition, the microbial degradation mechanism of n-hexadecane was analyzed using metabolomics. The results showed that presence of capillary zone had a strong retarding effect on n-hexadecane infiltration. Leaked pollutants were mainly concentrated in areas with strong capillary action. Infiltration and displacement of NAPL-phase pollutants were major driving force for change in moisture content (θ) and electric conductivity (EC) in vadose zone. The degradation by microorganisms results in a downward trend in potential of hydrogen (pH) and oxidation-reduction potential (ORP). Five petroleum hydrocarbon-degrading bacterial phyla and 11 degradable straight-chain alkane bacterial genera were detected. Microbial degradation was strong in the area near edge of capillary zone and locations of pollutant accumulation. Mainly Sphingomonas and Nocardioides bacteria were involved in microbial degradation of n-hexadecane. Single-end oxidation involved microbial degradation of n-hexadecane (C16H34). The oxygen consumed, hexadecanoic acid (C16H32O2) produced during this process, and release of hydrogen ions (H+) were the driving factors for reduction of ORP and pH. The vadose zone in this study considered presence of capillary zone, which was more in line with actual contaminated site conditions compared with previous studies. This study systematically elucidated vertical distribution characteristics of petroleum hydrocarbon pollutants and spatiotemporal variation characteristics of indigenous microorganisms in vadose zone considered presence of capillary zone. In addition, the n-hexadecane degradation mechanism was elucidated using metabolomics. This study provides theoretical support for development of natural attenuation remediation measures for petroleum-hydrocarbon-contaminated soil and groundwater.

The collaborative monitored natural attenuation (CMNA) of soil and groundwater pollution in large petrochemical enterprises: A case study.

A large in-service petrochemical enterprises in Northeast China was taken as the research object, and the Collaborative Monitored Natural Attenuation (CMNA) for soil and groundwater pollution was carried out to remedy combined pollution and reduce environmental risks. The pollutants distributions were obtained based on detailed regional investigation (Mar. 2019), and feature pollutants in soil and groundwater were then screened. The spatiotemporal variations of feature pollutants and relative microbial responses were explored during the CMNA process. Furthermore, the CMNA efficiency of the contaminated site at initial stage was evaluated by calculation of natural attenuation rate constant. The results showed that the feature pollutants in soil were 2,2',5,5'-tetrachlorobiphenyl (2,2',5,5'-TCB) and petroleum hydrocarbons (C10∼C40), and the feature pollutant in groundwater was 1,2-dichloroethane (1,2-DCA). The concentrations of all feature pollutants decreased continuously during four years of monitoring. Feature pollutants played a dominant role in the variability of microbial species both in soil and groundwater, increasing the relative abundance of petroleum tolerant/biodegradation bacteria, such as Actinobacteria, Proteobacteria and Acidobacteriota. The average natural attenuation rate constant of 2,2',5,5'-TCB and C10∼C40 in soil was 0.0012 d-1 and 0.0010 d-1, respectively, meeting the screening value after four years' attenuation. The average natural attenuation rate constant of 1,2-DCA was 0.0004 d-1, which need strengthening measures to improve the attenuation efficiency.

Microwave-induced steam distillation (MISD) remediation in petroleum hydrocarbon-contaminated sites: From process improvement to pilot application.

The process improvement, a pilot remediation test and the decontamination mechanism of microwave-induced steam distillation (MISD) for petroleum hydrocarbons (PHs) removal were conducted. Processes of multistage steam distillation and carbon reinforcement were compared to determine the best remediation process. Pilot project was then carried out to explore the applicability of MISD in site-scale remediation. The remediation efficiency, procedures and influencing factors of site-scale MISD project were studied by monitoring variations of soil moisture, temperature and PHs concentrations. Furthermore, the decontamination mechanisms of PHs were clarified based on kinetic analysis. The results showed that the multistage steam distillation could improve 10%∼15% remediation efficiency, and the carbon reinforcement could shorten remediation duration of each steam distillation stage by 50%. Pilot MISD project adopted multistage steam distillation process and went through four (initial, rapid heating-up, gentle heating-up and quasi-equilibrium) remediation stages (overall temperature ≤100 °C). The final PHs removal rate was about 60%, which would get better with greater proportion of low boiling points components and stronger vapor extraction. Kinetic studies showed that PHs was removed by steam stripping and limited by intraparticle diffusion in the "steam distillation zone", while local high temperature (＞100 °C) greatly improved PHs volatilization and provided activation energy for PHs desorbed and degraded in the "selective heating zone".

Construction, application and validation of a new algorithm for determining light nonaqueous-phase liquid fluxes in unsaturated zones.

An analytical algorithm coupling free-phase migration, precipitation, and natural attenuation through volatilization and biodegradation (FPVB) was developed to calculate the flux of light nonaqueous-phase liquid (LNAPL) leaking from unsaturated zone to groundwater. Sandbox and soil column experiments were performed to identify the LNAPL migration characteristics and states to provide data to establish and verify FPVB algorithm. For free-phase migration, the Kinematic Oily Pollutant Transport (KOPT) model was used to determine LNAPL movement velocity and leakage time. The correlations of water saturation, residual LNAPL saturation and the cumulative dissolution ratio of residual LNAPL were described using an empirical formula for the precipitation leaching process. Equations for diesel volatilization kinetics and first order degradation were used to describe the natural attenuation processes. Coupling the algorithms for the different stages gave the final FPVB algorithm. The FPVB algorithm was used to describe the pollution situation at a real site, and the results were consistent with the actual situation. The FPVB algorithm could be used to quickly assess the scale and degree of pollution with little information on the parameters for the actual LNAPL leakage event.

Response relationship of environmental factors caused by toluene concentration during leaching of capillary zone.

Due to the leaching of capillary water, the petroleum pollutants initially trapped in vadose zone may migrate to lower aquifer, thus increasing the risk of groundwater pollution. In order to explore the effect of capillary leaching on toluene-contaminated soil and the relationship between toluene concentration (TC) and environmental factors (EFs) during the leaching process, the sterilized and non-sterilized soil column experiments were designed. The EFs were used to estimate TC. The results showed that the difference between leaching and volatilization rates directly determined the changing trend of toluene concentration in capillary water. The toluene concentration in the medium always showed decreasing trend due to leaching. The indigenous microbial community structure of the non-sterilized soil column was analyzed by 16S rRNA sequencing. It was found that indigenous microorganisms could degrade toluene after 33.0 days of acclimatation. The microbial population was dominated by bacteria, among them the Ellin6055 strain and Pseudomonas, Pseudoxanthomonas, Cupriavidus, Bdellovibrio, Sphingobium, Phenylobacterium, Ramlibacter, Bradyrhizobium, Shinella genera. The Pseudomonas was the most crucial bacterial genus that degraded toluene. Indigenous microbial degradation was the fundamental reason for strong response relationship. Furthermore, we suggested a relationship of function between environmental factors (pH, DO, ORP) and time (t) for toluene attenuation: C0+ln(eAtαBγCß)=CToluene, (α, ß, γ represent the pH, DO, and ORP in leaching capillary water, respectively; A, B, and C represent undetermined coefficients), and the fitting coefficient R2 > 0.950. This relationship can only characterize the attenuation process of capillary zone leaching on toluene. However, it may still be utilized to give a theoretical foundation for understanding the dynamic of pollutant concentration change processes under specific environmental factors.

Response of environmental factors to attenuation of toluene in vadose zone.

Contaminated groundwater migrates in reverse direction under capillary force in vadose zone, and the attenuation process of pollutant adsorption and microbial degradation changes the environment of vadose zone. In this study, the response of toluene to environmental factors during reverse migration and attenuation of toluene from aquifer to vadose zone was studied by column experiment and experimental data analysis. The changes of environmental factors, including potential of hydrogen (pH), dissolved oxygen (DO), and oxidation-reduction potential (ORP), and toluene concentration were monitored by soil column experiment under sterilized and non-sterilized conditions. The 16S rRNA molecular biological detection technology was used to quantitatively analyze the impact of microbial degradation on the environment. Finally, the correlation between environmental factors and concentration in the attenuation process of toluene in the vadose zone was quantitatively studied by Pearson Correlation Coefficient (PCC) and multivariate statistical equation. The results showed that pH was primarily affected by microbial degradation, and DO and ORP were primarily affected by both adsorption and microbial degradation. The attenuation of toluene was divided into two stages: adsorption dominated (0~26 d) and microbial degradation dominated (26~55 d). The degradation amounts of microorganisms at each position in the non-sterilized column from bottom to top were 9.37%, 55.34%, 68.64%, 75.70%, 66.03% and 42.50%. At the same time, the article proposes for the first time that there is an obvious functional relationship between environmental factors (DO, ORP, pH), time (t) and concentration (CToluene):CToluene=C0+A100t+Bα+Cß+D100γ, (α,ß,γ are the pH, DO and ORP of capillary water, respectively; A, B, C and D are all undetermined coefficients), R2 > 0.95. The results of this study may facilitate the use of simple and easy-to-obtain environmental factors to characterize the dynamic process of pollutant concentration changes.

Analysis of the LNAPL Migration Process in the Vadose Zone under Two Different Media Conditions.

This study focused on the processes of free infiltration, precipitation displacement, and natural attenuation of the LNAPL under the condition of near-surface leakage. Sandbox experiments were performed to explore the migration characteristics of LNAPL in the vadose zone with two media structures and the influences of the soil interface on the migration of LNAPL. The results indicate that the vertical migration velocity of the LNAPL infiltration front in medium and coarse sand was 1 order of magnitude higher than that in fine sand and that the LNAPL accumulated at the coarse-fine interface, which acted as the capillary barrier. Displacement of precipitation for LNAPL had little relationship with rainfall intensity and was obviously affected by medium particle size, where coarse sand (40.78%) > medium sand (20.5%) > fine sand (10%). The natural attenuation rate of the LNAPL in the vadose zone was related to the water content of the media; the natural attenuation rate of fine sand was higher. This study simulated the process of the LNAPL leakage from the near surface into the layered heterogeneous stratum, improved the understanding of the migration of the LNAPL under different stratum conditions, and can provide support for the treatment of LNAPL leakage events in the actual site.