Damage to the soil environment caused by petroleum hydrocarbon pollution has become increasingly evident. However, the chemical adsorption reaction between petroleum hydrocarbon pollutants and medium particles in unsaturated soil at actual sites remains unclear. To explore the reaction process and mechanism, a series of unsaturated experiments under sterile conditions were performed to simulate the soil state of site with n-hexadecane as a characteristic pollutant. Soil samples were collected regularly, and environmental factors, including oxidation-reduction potential (ORP) and pH, were monitored online. Through microscopic characterization and qualitative and semi-quantitative analysis methods, the morphological changes in medium particles before and after reaction were systematically analyzed. The characteristics of reaction products were determined using gas chromatography-mass spectrometry. The results showed that the consumption of oxygen (O2) and production of hexadecanoic acid [CH3(CH2)14COOH] were fundamental reasons for the decrease of ORP and pH. A hydroxyl radical (& sdot;OH) in medium particles might extract a methyl hydrogen (H) atom from n-hexadecane and turn it into an alkyl radical. The alkyl radical might eventually be oxidized by O2 in soil to 1-hexadecanol and CH3(CH2)14COOH. This study systematically explains the chemical adsorption reactions and mechanisms of petroleum hydrocarbon pollutants in unsaturated soils. In addition, it provides a theoretical basis for revealing the migration and transformation mechanisms of petroleum hydrocarbon pollutants in soil under hydrological conditions such as rainfall and water level fluctuations.

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 onedimensional 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 ( theta) 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 straightchain 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 (C 16 H 34 ). The oxygen consumed, hexadecanoic acid (C 16 H 32 O 2 ) 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.