Effects of total organic carbon content and leaching water volume on migration behavior of polycyclic aromatic hydrocarbons in soils by column leaching tests

Fugacity-based multimedia transport modeling and risk assessment of PAHs in Urumqi

Currently, there is a lack of a comprehensive understanding of the behavior of polycyclic aromatic hydrocarbons (PAHs) in complex multimedia urban environmental systems. Taking Urumqi City as a case study, we developed an integrated multimedia urban environmental model to simulate the inter-media transport processes of PAHs across air, water, soil, sediment, vegetation, and impervious surfaces. The predictive results of this model were in good agreement with the actual monitoring data from 2021, confirming its accuracy. Notably, the simulated data for 2021 indicate that the total amount of PAHs in the soil reached 1.06 × 106 kg, accounting for 97.44% of the total PAHs in Urumqi City, highlighting soil as the primary sink for PAHs. Further analysis of transport fluxes revealed that atmospheric transfer pathways to soil and vegetation are the main mechanisms driving the distribution of PAHs in urban environments. Additionally, sensitivity analysis identified temperature, soil, and vegetation-related parameters as the primary factors influencing PAHs. Based on the simulated concentration, the risk assessment results showed that soil PAHs had a higher risk of carcinogenesis to human body. This study deepens our understanding of the behavior of PAHs in urban environments and provides insights into how human activities affect the fate and transformation of these contaminants in multimedia urban systems.

Spatial variations and vertical migration potentials of petroleum hydrocarbons with varying chain lengths in soils of different depths: Roles of solid and dissolved organic matters and soil texture

Petroleum hydrocarbons (PHs) in contaminated sites may threaten human health and ecological safety, while the environmental behaviors of PHs with varying carbon chains and critical influencing factors need to be elucidated, thus facilitating efficient risk management. This study explored the occurrence characteristics and spatial variations of different PHs at the depths of 0-10 m in an abandoned industrial site, as well as evaluated the effects of solid organic matter (SOM), dissolved organic matter (DOM), and soil texture on the migration potentials of PHs with varying carbon chains. Furthermore, the leaching potentials of different PHs were integrated into their risk assessment. The total concentrations of PHs in contaminated soils ranged from 22.7 to 397 mg/kg in contaminated soils, and the long-chained PHs (C22-C40) represented the major components with an average contribution of 46.9 %, followed by short-chained PHs (C10-C12, 32.0 % average) and medium-chained PHs (C13-C21, 21.1 % average). As soil depth increased, a declining trend was observed in the proportions of long-chain PHs, with an augmentation in the relative abundance of short-chain PHs. The random forest model coupling redundancy analysis showed that SOM was the major contributor to the occurrence and vertical attenuation of PHs with longer carbon chains in underground soils, with clay component exerting a greater additional effect than silt and sand components. While DOM in soils exhibited a relatively high contribution to the retention of PHs with short carbon chains. These results demonstrated the significant influence of SOM and clay on the behavior of long-chained PHs and DOM on that of short-chained ones. Besides direct human exposure, the leaching potentials of PHs, particularly short-chained ones, in soils should be considered for a more comprehensive risk assessment. The findings of this study may assist with the behavior modelling and prediction of different PHs as well as the corresponding risk control.

Characterization of a Soft Ionization by Chemical Reaction in Transfer Ion Source Hyphenated With Supercritical Fluid Chromatography-Tandem Mass Spectrometry

A commercially available dielectric barrier discharge ionization (DBDI) source was tested with supercritical fluid chromatography-mass spectrometry (SFC-MS). The compound mixture investigated comprised caffeine, theobromine, theophylline, uracil, testosterone, and pyrene, diluted in methanol. Dynamic response ranges were evaluated with multiple injections at different concentrations. Precision studies demonstrated the robustness and sensitivity of the ionization source across a concentration range of 10-1000 ng/mL. Results from this experiment showed linear regression of 0.99 or greater for all analytes tested over the range with a relative standard deviation (RSD) of less than 10% down to 10 ng/mL for all analytes except theobromine, which had an RSD of less than 10% down to 25 ng/mL. Notably, this study marks the first investigation of sensitivity for coupling a commercial DBDI source with SFC; a limit of detection less than 1 ng/mL was achieved for all compounds. This study demonstrates chromatographic separation by SFC and MS analysis for compounds that ionize poorly using traditional atmospheric pressure ionization, such as polycyclic aromatic hydrocarbons. Combining SFC with the DBDI source opens promising avenues for analyzing compounds that were previously challenging to characterize with standard atmospheric pressure ionization techniques.

Effects of soil colloids on adsorption and migration of benzo(a)pyrene on contaminated sites under runoff infiltration processes

In the environment, soil colloids are widespread and possess a significant adsorption capacity. This makes them capable of transporting different pollutants, presenting a potential risk to human and ecological well-being. This study aimed to examine the adsorption and co-migration characteristics of benzo(a)pyrene (BaP) and soil colloids in areas contaminated with organic substances, utilizing both static and dynamic batch experiments. In the static adsorption experiments, it was observed that the adsorption of BaP onto soil colloids followed the pseudo-second-order kinetic model (R2 = 0.966), and the adsorption isotherm conformed to the Langmuir model (R2 = 0.995). The BaP and soil colloids primarily formed bonds through π-π interactions and hydrogen bonds. The dynamic experimental outcomes revealed that elevating colloids concentration contributed to increased BaP mobility. Specifically, when the concentration of soil colloids in influent was 500 mg L-1, the mobility of BaP was 23.2 % compared to that without colloids of 13.4 %. Meanwhile, the lowering influent pH value contributed to increased BaP mobility. Specifically, when the influent pH value was 4.0, the mobility of BaP was 30.1 %. The BaP's mobility gradually declined as the initial concentration of BaP in polluted soil increased. Specifically, when the initial concentration of BaP in polluted soil was 5.27 mg kg-1, the mobility of BaP was 39.1 %. This study provides a support for controlling BaP pollution in soil and groundwater.

Features of the polycyclic aromatic hydrocarbon's spatial distribution in the soils of the Don River delta

PAHs are one of the most toxic organic compounds classes which is obligatory controlled all over the world. There is a luck of studies devoted to the PAHs levels and sources identification in the south of Russia. The features of the PAHs accumulation and spatial distribution in hydromorphic soils (Fluvisol) were studied on the example of the soils of the Don River delta floodplain landscapes. It has been shown that changes in the PAHs content in soils depended on the type and intensity of the emission source. A factor analysis and multivariate linear regression analysis were carried out to determine the features of the spatial distribution for individual PAH compounds, considering the properties of soils and typical differences in the emission source. The most polluted areas in the studied area located along the transit line of the long-distance tankers, where the content of the most toxic high molecular PAHs compounds reached 8862 ng g-1. As a result of regression analysis, a relationship was established between the PAHs accumulation rate with the content of silt (particles less than 0.001 mm in size) and Ca2+ and Mg2+ exchangeable cations in the soil (at p-level < 0.0001). Differences in individual PAH content for medium and heavy loamy Fluvisol and depend on the influence of different types of pollution sources.

Transport and retention of n-hexadecane in cadmium-/naphthalene-contaminated calcareous soil sampled in a karst area

Studying the transport of petroleum hydrocarbons in cadmium-/naphthalene-contaminated calcareous soils is crucial to comprehensive assessment of environmental risks and developing appropriate strategies to remediate petroleum hydrocarbons pollution in karst areas. In this study, n-hexadecane was selected as a model petroleum hydrocarbon. Batch experiments were conducted to explore the adsorption behavior of n-hexadecane on cadmium-/naphthalene-contaminated calcareous soils at various pH, and column experiments were performed to investigate the transport and retention of n-hexadecane under various flow velocity. The results showed that Freundlich model better described the adsorption behavior of n-hexadecane in all cases (R2 > 0.9). Under the condition of pH = 5, it was advantageous for soil samples to adsorb more n-hexadecane, and the maximum adsorption content followed the order of: cadmium/naphthalene-contaminated > uncontaminated soils. The transport of n-hexadecane in cadmium/naphthalene-contaminated soils at various flow velocity was well described by two kinetic sites model of Hydrus-1D with R2 > 0.9. Due to the increased electrostatic repulsion between n-hexadecane and soil particles, n-hexadecane was more easily able to breakthrough cadmium/naphthalene-contaminated soils. Compared to low flow velocity (1 mL/min), a higher concentration of n-hexadecane was determined at high flow velocity, with 67, 63, and 45% n-hexadecane in effluent from cadmium-contaminated soils, naphthalene-contaminated soils, and uncontaminated soils, respectively. These findings have important implications for the government of groundwater in calcareous soils from karst areas.

Distribution and biodegradation potential of polycyclic aromatic hydrocarbons (PAHs) accumulated in media of a stormwater bioretention

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds that can be captured and accumulate in the bioretention cell media, which may lead to secondary pollution and ecological risks. This research aimed to understand the spatial distribution of 16 priority PAHs in bioretention media, identify their sources, evaluate their ecological impact, and assess the potential for their aerobic biodegradation. The highest total PAH concentration (25.5 ± 1.7 μg/g) was observed 1.83 m from the inlet and 10-15 cm deep. The individual PAHs with the highest concentrations were benzo [g,h,i]perylene in February (1.8 ± 0.8 μg/g) and pyrene in June (1.8 ± 0.8 μg/g). Data indicated that primary sources of PAHs were fossil fuel combustion and petroleum. The ecological impact and toxicity of the media were assessed by probable effect concentrations (PECs) and benzo [a]pyrene total toxicity equivalent (BaP-TEQ). The results showed that the concentrations of pyrene and chrysene exceeded the PECs, and the average BaP-TEQ was 1.64 μg/g, primarily caused by benzo [a]pyrene. The functional gene (C12O) of PAH-ring cleaving dioxygenases (PAH-RCD) was present in the surface media, which indicated that aerobic biodegradation of PAHs was possible. Overall, this study revealed the PAHs accumulated most at medium distance and depth, where biodegradation may be limited. Thus, the accumulation of PAHs below the surface of the bioretention cell may need to be considered during long-term operation and maintenance.

Seasonal assessment of the distribution, source apportionment, and risk of water-contaminated polycyclic aromatic hydrocarbons (PAHs)

The research aims to evaluate the seasonal differences in the distribution, source, and risks of water-contaminated PAHs. The PAHs were extracted by the liquid-liquid method and analyzed with GC-MS, and a total of eight PAHs were detected. There was a percentage increase in the average concentration of the PAHs from the wet to the dry season in the range of 20 (Anthracene)-350 (Pyrene)%. Total PAHs (∑PAHs) range from 0.31 to 1.23 mg/l in the wet period and from 0.42 to 1.96 mg/l in the dry period. The distribution of the average PAHs in mg/l showed that Fluoranthene ≤ Pyrene < Acenaphthene < Fluorene < Phenanthrene < Acenaphthylene < Anthracene < Naphthalene in wet period and while Fluoranthene < Acenaphthene < Pyrene < Fluorene < Phenanthrene < Acenaphthylene < Anthracene < Naphthalene in the dry period. The children were exposed to non-carcinogenic risk through non-dietary ingestion due to the accumulative effect (HI) of the PAHs in the dry period. Furthermore, the naphthalene was responsible for ecological and carcinogenic risk in the wet period, while the fluorene, phenanthrene, and anthracene were responsible for ecological and carcinogenic risk in the dry period. However, while adults and children are both susceptible to carcinogenic risk through the oral channel during the dry period, only children are susceptible to non-carcinogenic risk through this pathway. The multivariate statistical analysis revealed the influence of physicochemical parameters on the detected PAHs and also showed the PAHs' sources to be mainly combustion, pyrolysis, and vehicular emission.

Influence of temperature and precipitation on the fate of polycyclic aromatic hydrocarbons: simulation experiments on peat cores from a typical alpine peatland in Central China

The corresponding relationships between temperature, precipitation, and polycyclic aromatic hydrocarbon (PAH) concentration in a typical ombrotrophic peatland in Dajiuhu, Shennongjia, were quantitatively characterized by field sampling tests validated with simulation experiments. The PAH concentrations of peat cores in Dajiuhu peatland ranged from 262 to 977 ng·g^-1, with a mean value of 536 ± 284 ng·g^-1. PAHs were mainly composed of 2-3 ring PAHs, accounting for 31.7% ± 2.00% and 31.7% ± 5.00%, respectively. The concentration of PAHs in peat cores showed a significant decrease with increasing temperature, while the low molecular weight PAHs (LMW-PAHs) were more sensitive to temperature changes compared to the high molecular weight PAHs (HMW-PAHs). Besides, with the increase of quantity and velocity of leaching liquid, PAHs in peat were first transferred in the form of attached large-size particles and then gradually entered the aqueous phase. According to the IPCC projections of global warming, Dajiuhu peatland will release 956 ± 26.3 kg·°C^-1 PAHs into gas phase during 2030-2052, and a conservative projection based on local temperature trends showed that 459 ± 12.6 kg·°C^-1 PAHs will be released into gas phase by 2047 in Dajiuhu peatland. The projected release fluxes of PAHs in Dajiuhu peatland with precipitation volume and precipitation velocity are 381 ± 201 kg·100 mm^-1 and 1052 ± 167 kg·min·mL^-1, respectively, which are primarily from peat into particulate and aqueous phase.

Assessment of seasonal variation in distribution, source identification, and risk of polycyclic aromatic hydrocarbon (PAH)-contaminated sediment of Ikpoba River, South-South Nigeria

The study aims to assess the seasonal variation in distribution, source identification, and risk of 20 polycyclic aromatic hydrocarbons (20 PAHs) in the sediment of the Ikpoba River, south-south Nigeria. The PAHs were extracted in an ultrasonic bath with a mixture of n-hexane and dichloromethane (1:1 v/v). The extract was cleaned by silica-alumina gel mixed with anhydrous Na₂SO₄ in a chromatography column, eluted by n-hexane, and analysed by gas chromatography-mass spectrometry. The range of the average PAHs in mg.kg-dw was 0.15 (Nap)-0.54 (Acy) and 0.13 (D.al.P)-0.99 (Acy) in wet and dry periods correspondingly, indicating an increase in concentration from wet to dry period. However, the rings of the average concentration of the PAHs show 6 and 3 rings to be the highest values during the wet and dry seasons, respectively. Based on the human health risk analysis, the hazard quotient (HQ) and hazard index (HI), and carcinogenic risk indices showed low non-carcinogenic and carcinogenic risk for both seasons. The ecological risk analysis showed the mean effect range median quotient (mERMQ) recorded a medium-low effect on the biota of the locations, except in AS3 during the wet season and also in WS8 and WS9 during the dry season. The minimum value of the toxic equivalent quotient (TEQ) was > 0.2 mg/kg, which indicated a recommendation for the clean-up of the Ikpoba River. The isomer ratio and the principal component analysis (PCA) revealed the sources of the PAHs to be majorly combustion, followed by pyrolytic and petrogenic sources for both seasons.

Migration Behavior and Influencing Factors of Petroleum Hydrocarbon Phenanthrene in Soil around Typical Oilfields of China

Petroleum spills and land contamination are becoming increasingly common around the world. Polycyclic aromatic hydrocarbons (PAHs) and other pollutants found in petroleum are constantly migrating underground, making their migration in soil a hot research topic. Therefore, it is of great significance to evaluate the migratory process of petroleum hydrocarbons in petroleum-polluted soil to clarify its ecological and environmental risks. In this study, Phenanthrene (PHE) was used as a typical pollutant of PAHs. The soil was gathered from three typical oilfields in China, and a soil column apparatus was built to simulate the vertical migration of PHE in the soil. The migration law and penetration effect of PHE in various environmental conditions of soil were investigated by varying the ionic strength (IS), pH, particle size, and type of soil. According to the literature, pH has no discernible effect on the migration of PHE. The migration of PHE was adversely and positively linked with changes in IS and soil particle size, respectively. The influence of soil type was mainly manifested in the difference of organic matter and clay content. In the Yanchang Oilfield (YC) soil with the largest soil particle size and the least clay content, the mobility of PHE was the highest. This study may reveal the migration law of PAHs in soils around typical oilfields, establish a new foundation for PAH migration in the soil, and also provide new ideas for the management and control of petroleum pollution in the soil and groundwater.

Distribution, sources and risk assessment of PAHs in soil from the water level fluctuation zone of Xiangxi Bay, Three Gorges Reservoir

Information on PAH distribution in the water level fluctuation zone (WLFZ) of Three Gorges Reservoir is limited. In this study, we investigated PAH distribution and sources and assessed PAH risks, over one annual water level fluctuation cycle (June 2017-June 2018) at four elevations spanning the WLFZ (145 m, 155 m, 165 m and 175 m) at seven locations in the water level fluctuation zone along Xiangxi River. The mean total PAH concentration in June 2018 (953 ng g^-1) was significantly higher than in June 2017 (494 ng g^-1), and the horizontal and vertical distributions of PAHs changed significantly. The changes in distribution patterns provided evidence for the cause of increased PAH levels, which were attributed to construction of the Xiangxi River Bridge. Thus, this study of PAH dynamics in the WLFZ soils of Xiangxi Bay also provided valuable information on the impact of bridge construction on WLFZ soils. The change in PAH levels among stations implicated sediment disturbance resulting from bridge construction as the major contributor to the increased PAH levels. Source characterization, based on the ratios of certain PAHs, indicates that PAHs are mainly from the combustion of petroleum fuels, biomass and coal. These ratios indicated that the proportion of PAHs from fuel combustion increased from 2017 to 2018, implicating the heavy equipment used during bridge construction as another source of the increased PAH levels. The incremental lifetime cancer risk (ILCR) model was used to assess the health risk of the PAHs and the range among all age groups (10^-5-10^-4) indicates a potential health risk. The mean effects range-median quotient (M-ERM-Q) was used to assess the ecological risk of PAHs and the range (0.1-0.5) indicates low to medium risk. The increase in PAH levels from 2017 to 2018 increased the risk to public health and the environment. The results of this investigation provide a reference for ecological restoration of the WLFZ and support development of effective policies for environmental and public health. Further, the results provide information on the impact of bridge construction on WLFZ soils and identify research needed to more fully understand PAH dynamics in WLFZ soils.

Leaching of Polycyclic Aromatic Hydrocarbons from the Coal Tar in Sewage Wastewater, Acidic and Alkaline Mine Drainage

Polycyclic aromatic hydrocarbons (PAHs) have been a problem in the environment for an extended period. They are mostly derived from petroleum, coal tar and oil spills that travel and are immobilized in wastewater/water sources. Their presence in the environment causes a hazard to humans due to their toxicity and carcinogenic properties. In the study, coal tar was analyzed using Gas Chromatography–Mass Spectrometry (GC–MS) and a concentration of 787.97 mg/L of naphthalene, followed by 632.15 mg/L of phenanthrene were found to be in the highest concentrations in the various water sources such as sewage, alkaline and acid mine drainage. A design column was used to investigate the leaching process and assessments were conducted on 300 mL of the various water sources mentioned, with 5 g of coal tar added and with monitoring for 4 weeks. The influence of the physiochemical properties of the receiving water sources, such as sewage, and acid and alkaline mine drainage, on the release of PAHs from the coal tar was assessed. The acidic media was proven to have the highest release of PAHs, with a total concentration of 7.1 mg/L of released PAHs, followed by 1.2 mg/L for the sewage, and lastly, 0.32 mg/L for the alkaline mine drainage at room temperature.

Microplastics lag the leaching of phenanthrene in soil and reduce its bioavailability to wheat

Microplastics wildly occur in soil and they can become the carriers of persistent contaminants. However, the influence of microplastics on polycyclic aromatic hydrocarbons vertical translocation in the soil system after rainfall is limitedly understood. Here, experiments were conducted to study the influence of polyethylene (PE), polystyrene (PS) and polyvinyl chloride (PVC) microplastics on the leaching behavior and bioavailability of phenanthrene (Phe). The adsorption capacity of phenanthrene on the microplastics followed the order of PS > PE > PVC. The Phe concentrations in the top soil layer after 15 days of leaching with water were 30.25, 28.32 and 27.25 mg kg^-1 for the treatments of Phe-PS, Phe-PE and Phe-PVC respectively, which is consistent with the adsorption capacities of microplastics. The concentrations of Phe were correlated with the microplastic adsorption capacities at soil depths of 5-45 cm. Under long-term leaching, Phe could reach the deeper soil layer. Phe concentrations significantly decreased in the leachate over time. Phe concentrations in wheat had a positive correlation with that in leachate/leached top soil layer. Our findings are beneficial to accurately evaluate the ecological risk of the combined contamination of PAHs and microplastics, and improve the understanding of the environmental behaviors of different microplastics.

Persistence and vertical distribution of neonicotinoids in soils under different citrus orchards chrono sequences from southern China

Continual input of neonicotinoid insecticides occurs in the citrus orchards from southern China. However, it is still unknown about the variations in the distribution and accumulation of neonicotinoids in soil profiles along a long-term chronosequence of cultivation and the driving factors contributing to these shifts. Here, changes of neonicotinoids in the 0-100 cm soil profiles with distinct orchard cultivation age (1, 10, and 20 years) were investigated, and their related factors were further determined. The results showed that the total levels of five target neonicotinoids (∑₅NEOs) in the soil profiles were in the range of 0-25.76 ng/g dw. Imidacloprid was the most dominating neonicotinoid, followed by thiamethoxam. We observed higher neonicotinoid accumulations in the soil profiles from the citrus orchards after 10 and 20 years of cultivation. Neonicotinoids migrated deeper into the soil profiles in orchards with a longer time since cultivation. Imidacloprid, thiamethoxam, and the total amount of neonicotinoid (∑₅NEOs) were mainly affected by the cultivation age of citrus orchards (accounting for 58.9% variance; P < 0.001); whereas clothianidin, acetamiprid, and thiacloprid were mainly influenced by soil depths (accounting for 66.9-85.2% variance; P < 0.05). Redundancy analyzes further indicated that the enhanced accumulation of neonicotinoids was mainly correlated with the increase of soil organic carbon (SOC) content and soil porosity, and the reduction of bulk density in the profiles of citrus orchards with increasing cultivation age. This study highlights the finding that we should give more concerns about the contamination and ecological risks of neonicotinoids in the orchards with a long cultivation age.

Leaching behavior of polycyclic aromatic hydrocarbons (PAHs) from oil-based residues of shale gas drill cuttings

Cuttings are the main solid residues which are generated from drilling operations. Due to the presence of heavy and radioactive elements, the environment risk posed by cuttings has attracted increasing attention. In this work, a short-term static immersion experiment was carried out to investigate the leaching of polycyclic aromatic hydrocarbons (PAHs) from oil-based residues of shale gas drilling cuttings. Furthermore, the effects of some relevant environmental factors controlling the leaching behavior were evaluated, including the different particle sizes, pH, extraction time, solid-to-liquid (S/L) ratio and dissolved organic matter (DOM) concentration. The results showed that (1) the concentrations of leached PAHs gradually increased with prolonged leaching time, but the cumulative amount of PAHs released during leaching was less than 3% of the total. (2) The Elovich, parabolic diffusion and power function models were found to fit the experimental data better than the first-order kinetic equation, indicating that the leaching of PAHs was controlled by the coupling of diffusion and chemical reactions at the source surface. (3) Different environmental factors had different impacts on the leaching of PAHs: the shaking time and presence of DOM increased leachability, the particle size and S/L ratio decreased leachability, and the pH did not affect the leachability of PAHs. Therefore, PAHs leaching was a complex process, and it is of scientific and environmental interest to conduct the leaching tests under the simulated environmental conditions.

Leaching behavior and transformation of total mercury and methylmercury from raw and lime-conditioned sewage sludge under simulated rain

Mercury (Hg) that leaches from municipal sewage sludge (MSS) landfill under natural rain is of increasing concern. The column leaching experiments were conducted to investigate the leaching characteristics of total mercury (THg) and methylmercury (MeHg) as well as pH, total organic carbon (TOC), and total suspended solids (TSS) in the raw sludge (RS) and lime-conditioned sludge (LCS) under simulated rain with different acidities (pH 6.5 and 2.9). Results showed the release of MeHg in the leachates presented different patterns from THg. And the final amounts of MeHg in the MSS columns were 1.49 (RS at pH = 6.5), 1.88 (RS at 2.9), 1.97 (LCS at pH = 6.5), and 2.06 times (LCS at pH = 2.9) higher than the initial amounts, suggesting methylation of inorganic Hg (IHg) occurred in the leaching process. The leaching efficiencies of THg and MeHg in RS was lower than that in LCS, indicating lime was more favorable for the release of THg and MeHg. And lower values of pH of the simulated rain promoted the release of THg and MeHg from RS while the opposite was true for LCS. This study provides a better understanding of the release and biogeochemical transformations of Hg in MSS.