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.

Retention effect and mode of capillary zone on the migration process of LNAPL pollutants based on experimental exploration.

Two-dimensional sand tank experiments were designed to investigate the retention process of the capillary zone during the migration of light non-aqueous phase liquid (LNAPL) pollutants. The fine sand and silt media experiments simulated the LNAPL migration process given a shallow point source leakage scenario. The results indicate that the LNAPL was retained in the capillary zone. A retention factor, based on the ratio of the change in the vertical migration velocity of the LNAPL front with time, was proposed to quantitatively characterize the retention effect. The retention factor and time satisfied the function of σ=A×exp(-kt). And the retention factor increased gradually with time, indicating the enhanced retention effect of capillary zone on the vertical migration of LNAPL. The concentration change rate was then used to investigate the LANPL redistribution process, which had a relationship with time of νc=B×ln(t)+C. The capillary zone could be divided from top to bottom into a weak retention zone (B > 0, vc < 0), a strong retention zone (B < 0), and a barrier zone (B > 0, vc > 0). The retention effect of capillary zone on LNAPL migration gradually strengthened during the vertical migration of LNAPL. In addition, the coefficient B had a relationship with the environmental factors (i.e., EC, pH, and ORP) of B=a×sin(b×α×ß×γ)c and the fitting coefficient R2 of the function was above 0.913 for both media, indicating a strong correlation between the LNAPL redistribution process and the key environmental factors.

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.

Sorption of Sr in granite under typical colloidal action.

The sorption behaviour of Sr into granite was studied with the presence or absence of typical colloids (goethite, bentonite and humic acid). A batch technique was used to analyse the influencing process of colloids and key factors. Moreover, scanning electron microscopy and Fourier transform infrared spectroscopy were used to characterize granite samples before and after the batch experiments. The experiments showed that the presence of colloids promotes the sorption of Sr in the broken granite system; when goethite, bentonite or humic acid (HA) was present, the sorption capacity percentages were 1.8, 2.13 and 1.93 times higher, respectively, than those in the granite only system. As the initial Sr2+ concentration increased, the sorption of Sr increased, but the sorption percentage decreased; the sorption percentage reached a maximum at pH = 7 and decreased as the acidity or alkalinity of the solution increased. The sorption of granite may be related to the interstitial water of the hydroxyl, quartz, and feldspar, the intergranular water of granite groups and the water molecules attached to the granite. Moreover, the surface of the granite sample was rougher after the sorption experiment.