Contaminated Soil Sites Research Articles

A transformation and auxiliary extraction of Cr during electrokinetic removal of Cr-contaminated multilayer composite soil chamber.

Multilayer composite soil chamber was proposed to extract the Cr of contaminated site soil and insight into transformation of Cr fractionation associated with valence states. The variations of current, soil pH and moisture content were explored, as well as the migration of Cr fractionation and redistribution of Cr. Results indicated that duration of half peak current could be used to adjust treatment time and it varied among different composite ways. Moreover, extraction efficiency of Cr in soil near cathode was relatively higher and reached 60% when citric acid was used. Citric acid could promote the transformation between different Cr fractionations or different valence states. It could also improve the desorption of Cr, and could prevent excessive fluctuations of moisture content at the same time. Cr redistributed acrossed the soil chamber after extraction. When deionized water was used, Cr(VI) significantly migrated toward anode mainly in the form of exchangeable fractionation (EXC) while Fe-Mn oxides fractionation (Fe-Mn) which may be in the form of cationic Cr(III) hydroxides migrated toward cathode. When using citric acid, fractionations that were difficult to migrate of Cr, especially for Fe-Mn in site soils could be activated and became EXC and carbonate fractionation (CAR), then migrated to the anode or cathode. The migration of exchangeable Cr(III) was dramatically enhanced. But the use of citric acid could cause Cr(VI) transformation to Cr(III) near anode. In addition, during the migration process, EXC could go back to Fe-Mn again or transform to residue fractionation (RES).

Survival and reproduction tests using springtails reveal weathered petroleum hydrocarbon soil toxicity in boreal ecozone.

Survival and reproduction tests were conducted using two native springtail (subclass: Collembola) species to determine the toxicity of a fine-grained (< 0.005 - 0.425 mm) soil from an industrial site located in the Canadian boreal ecozone. Accidental petroleum hydrocarbon (PHC) release continuously occurred at this site until 1998, resulting in a total hydrocarbon concentration of 12,800mg/kg (soil dry weight). Subfractions of the PHC-contaminated soil were characterized using Canadian Council of Ministers of the Environment Fractions, which are based on effective carbon numbers (nC). Fraction 2 (> nC10 to nC16) was measured at 8400mg/kg and Fraction 3 (> nC16 to nC34) at 4250mg/kg in the contaminated soil. Age-synchronized colonies of Folsomia candida and Proisotoma minuta were subject to 0%, 25%, 50%, 75%, and 100% relative contamination mixtures of the PHC-contaminated and background site soil (< 100mg/kg total PHCs) for 28 and 21days, respectively. Survival and reproduction decreased significantly (Kruskal-Wallis Tests: p < 0.05, df = 4.0) in treatments of the contaminated site soil compared to the background soil. In both species, the most significant decline in survival and reproduction occurred between the 0% and 25% contaminated soil. Toxicity responses in the two springtails were ascribed to the standardized test design, short lifespans, and high fecundity in both species. This study showed that 25 + years of soil weathering has not eliminated the toxicity of fine-grained PHC-contaminated soil on two native terrestrial springtail species. Adverse effects to springtail health were attributed to exposure to soils dominated by genotoxic PHC Fraction 2 compounds and slow weathering processes due to the cold climate at the site.

The simultaneous passivation of arsenic and cadmium in contaminated soil through combined use of schwertmannite and calcium carbonate

Schwertmannite was proved to be an efficient material for the remediation of arsenic-contaminated soil. Given that arsenic (As) and cadmium (Cd) coexist in soils and have opposite chemical behaviors, stabilizing them simultaneously is challenging. Here, we reported a compound passivating agent (AMSC) which simultaneously passivated As and Cd in contaminated site soil containing 1659 mg/kg As and 32.26 mg/kg Cd. It consisted of alkali-modified schwertmannite (denoted as AMS) capable of preferentially adsorbing As and fine calcium carbonate (denoted as C) for mainly stabilizing Cd. It was found that the addition of AMSC into the contaminated soil at 5 % for 60 days could drastically reduce water-extractable As and TCLP-As concentrations by over 84 %, and water-extractable Cd and TCLP-Cd concentrations by over 94 %, respectively, due to the conversion of the labile fraction of As or Cd into more stable fractions. Additionally, pH in the AMSC-amended soil was increased to 6.74 from initial 4.50. As was mainly adsorbed into schwertmannite of AMSC by ligand exchange of SO42- with As, while Cd was jointly passivated mainly by calcium carbonate and fewly by schwertmannite. To identify the mineral phase changes of AMSC, quartz sands of different grain sizes were selected for Mössbauer analysis. The percentage of schwertmannite in the sediment amended with AMSC decreased slightly from 51.1 % to 48.9 % within 30 days, while Fe(III) oxyhydroxide (48.9 %) was converted to ferrihydrite (30.3 %) and lepidocrocite (20.8 %), respectively. Therefore, it was expected that AMSC could act as a high-performance and environmentally-friendly amendment to remediate As and Cd-contaminated soils.

Synergy of carboxymethyl cellulose stabilized nanoscale zero-valent iron and Penicillium oxalicum SL2 to remediate Cr(VI) contaminated site soil

Nano zero-valent iron (nZVI) acting as a high-cost disposable material in soil Cr(VI) remediation faces significant challenges due to its easily oxidizable nature and biological toxicity. In addressing this issue, the present study undertook the synthesis of a series of modified nZVI and combined the selected material with Cr(VI)-resistant filamentous fungus Penicillium oxalicum SL2 for real-site chromium pollution remediation. Adsorption experiments demonstrated that the inclusion of carboxymethyl cellulose (CMC) significantly enhanced the adsorption capacity of nZVI for Cr(VI) by 19.3% (from 73.25 to 87.4 mg/L), surpassing both biochar (37.42 mg/L) and bentonite modified nZVI (48.03 mg/L). Characterization results validated the successful synthesis of the nano composite material. Besides, oxidative stress analysis explained the unique detoxification effects of CMC on SL2, acting as a free radical scavenger and isolating layer. In real-sites soil remediation experiments, a low dosage (0.4% w/w) of nZVI/CMC@SL2 (CMC modified nZVI combined with SL2) exhibited an impressive reduction of over 99.5% in TCLP-Cr(VI) and completely transformed 18% of unstable Cr to stable forms. Notably, nZVI/CMC demonstrated its capability to facilitate SL2 colonization in highly contaminated soil and modulate the microbial community structure, enriching chromium-removing microorganisms. In summary, the synergistic system of nZVI/CMC@SL2 merges as a cost-effective and efficient approach for Cr(VI) reduction, providing meaningful insights for its application in the remediating contaminated site soils.

Simulation of microbial transport and petroleum hydrocarbon compound biodegradation in a fracture–matrix coupled system

In-situ bioremediation technology is considered an ideal and environment-friendly approach for the largescale treatment of petroleum-contaminated soil. Most earlier studies on bioremediation have focused on the biostimulation techniques, which can enhance the biodegradation capabilities of indigenous bacteria, rather than on bioaugmentation involving the introduction of exogenous microorganisms. Additionally, contaminated soil sites typically contain both natural and artificial fractures. These fractures serve as preferential transport pathways, complicating the transport of bacteria and related solutes. This study proposes a mathematical model to simulate the transport of exogenous bacteria and electron acceptors during the degradation of petroleum hydrocarbon pollutants in a fractured aquifer system. The model incorporates the processes such as microbial convection, diffusion, adsorption, chemotaxis, growth, death, and microbial metabolic reactions. Simulation results demonstrate that considering microbial transport significantly affects the prediction of contaminant degradation, particularly with regard to the transport of electron acceptors. The growth rate of bacteria has an evident effect on the efficiency of contaminant remediation, and microbial chemotaxis can significantly enhance the degradation efficiency. The proposed mathematical model serves as a valuable tool for assessing the effectiveness of bioaugmentation strategies and can aid in the design and optimization of remediation efforts for contaminated and fractured aquifers.

Synergistic binding mechanisms of co-contaminants in soil profiles: Influence of iron-bearing minerals and microbial communities

In contaminated soil sites, the coexistence of inorganic and organic contaminants poses a significant threat to both the surrounding ecosystem and public health. However, the migration characteristics of these co-contaminants within the soil and their interactions with key components, including Fe-bearing minerals, organic matter, and microorganisms, remain unclear. This study involved the collection of a 4.3-m-depth co-contaminated soil profile to investigate the vertical distribution patterns of co-contaminants (namely, arsenic, cadmium, and polychlorinated biphenyls (PCBs)) and their binding mechanisms with environmental factors. The results indicated a notable downward accumulation of inorganic contaminants with increasing soil depth, whereas PCBs were predominantly concentrated in the uppermost layer. Chemical extraction and synchrotron radiation analysis highlighted a positive correlation between the abundance of reactive iron (FeCBD) and both co-contaminants and microbial communities in the contaminated site. Furthermore, Mantel tests and structural equation modeling (SEM) demonstrated the direct impacts of FeCBD and microbial communities on co-contaminants within the soil profile. Overall, these results provided valuable insights into the migration and transformation characteristics of co-contaminants and their binding mechanisms mediated by minerals, organic matter, and microorganisms.

The Effect of Soluble Sugar Degradation on the Evaporation of Compacted Clay

In arid climates, evaporation and water loss in surface soil can lead to the development of shrinkage cracks in the soil. The crack network in contaminated soil sites can become a rapid pathway for the infiltration and transport of contaminations, thereby increasing the range of soil contamination. Dense contaminated clay samples were prepared by using glucose as a representative soluble sugar of domestic source contaminations. Through indoor evaporation simulation tests, the effect of soluble sugar anaerobic degradation on the water loss, deformation, and crack growth of compacted clay was analyzed, and the mechanism of this effect was revealed. The results showed that glucose increased the water-holding capacity of clay, while the anaerobic degradation of glucose decreased the water-holding capacity of clay. Although glucose anaerobic degradation reduced the overall deformation of dense clay, it promoted the development of evaporative cracks on the surface of dense clay. Soluble sugar anaerobic degradation mainly affected the evaporative cracking of clay by “forming hydrogen bonds to reduce the rate of evaporative water loss in clay” and “generating CO2 to alter the structure of the clay”.

A comparative study on the removal and recovery of hexavalent chromium from tannery wastewater using an isolated strain Aspergillus proliferans LA and a known strain Aspergillus terreus

Isolation of a potent microorganism from the soil near tanneries and a comparative analysis of the removal and recovery of different concentrations of hexavalent chromium using known strain Aspergillus terreus and isolated strain Aspergillis proliferansLA are done. The isolated fungal species from the chromium contaminated soil sites located in the industrial area of Kanpur, U.P. were tested for its potential for the removal of hexavalent chromium from wastewater streams. The experiments were conducted comparing the biosorption efficiency of the isolated species Aspergillus proliferansLA and Aspergillus terreus by varying the initial Cr (VI) concentration and media constituents. The highest removal efficiency of 99.19% was shown by the isolated species Aspergillus proliferansLA for an initial Cr (VI) concentration of 10ppm followed by increasing the concentration to 50ppm whereas Aspergillus terreus showed 78.43% removal of hexavalent chromium. This was then followed by desorption using 0.8M NaOH and 75.12% hexavalent chromium was desorbed from the cells. The isolated strain Aspergillus proliferansLA was seen to show better removal and recovery results than the known strain Aspergillus terreus.

Phytotoxicity of weathered petroleum hydrocarbons in soil to boreal plant species

Canada has extensive petroleum hydrocarbon (PHC) contamination in northern areas and the boreal forest region from historical oil and gas activities. Since the 2013 standardization of boreal forest species for plant toxicity testing in Canada, there has been a need to build the primary literature of the toxicity of weathered PHCs to these species. A series of toxicity experiments were carried out using fine-grained (<0.005–0.425 mm) background (100 total mg/kg total PHCs) and weathered contaminated soil (11,900 mg/kg total PHCs) collected from a contaminated site in northern Ontario, Canada. The PHC mixture in the contaminated site soil was characterized through Canadian Council of Ministers of the Environment Fractions, as indicated by the number equivalent normal straight-chain hydrocarbons (nC). The soil was highly contaminated with Fraction 2 (>nC10 to nC16) at 4790 mg/kg and Fraction 3 (>nC16 to nC34) at 4960 mg/kg. Five plant species (Elymus trachycaulus, Achillea millefolium, Picea mariana, Salix bebbiana, and Alnus viridis) were grown from seed in 0%, 25%, 50%, 75%, and 100% relative contamination mixtures of the PHC-contaminated and background soil from the site over 2–6 weeks. All five species showed significant inhibition in shoot length, shoot weight, root length, and/or root weight (Kruskal-Wallis Tests: p < 0.05, df = 4.0). Measurements of 25% inhibitory concentrations (IC25) following PHC toxicity experiments revealed that S. bebbiana was most significantly impaired by the PHC-contaminated soil (410–990 mg/kg total PHCs), where it showed <35% germination. This study indicates that natural weathering of Fraction 2- and Fraction 3-concentrated soil did not eliminate phytotoxicity to boreal plant species. Furthermore, it builds on the limited existing literature for toxicity of PHCs on boreal plants and supports site remediation to existing Canadian provincial PHC guidelines.

Extended application of diffusive gradients in thin films (DGT) in assessing arsenic bioavailability and human health risks in brownfield soils

The environmental and human health risks of arsenic (As) mainly depend on its bioavailability in the soil. However, precise determination of bioavailable As in situ is still challenging due to the complexity of soil types and hydrogeological conditions in polluted sites. Diffusive gradients in thin films (DGT) is one of the reliable analysis technology in the in situ analysis of bioavailable chemicals in waters, sediments and farmland soils. However, few studies have investigated the application of DGT in evaluating the bioavailability of heavy metals in brownfield soils. In this study, DGT was extended applied to evaluate the availability of As in 16 contaminated soil samples from seven brownfield sites. The correlation between the bioavailable As measured by DGT and the bioaccessible As determined by the Unified BRAGE method (UBM) assay was also analyzed. In addition, the human health risks were evaluated, and the DGT-based prediction model for bioavailable As was also developed (R2=0.852, P< 0.001). The results indicated that considering bioavailability in risk assessments can improve the accuracy of calculations for site-specific health risks and provide guidance for effective remediation. The study will demonstrate the effectiveness of DGT in characterizing bioavailable heavy metals in contaminated site soils and provide decision-making support for the soil remediation.

Efficient remediation of PAHs contaminated site soil using the novel slow-release oxidant material

The eco-friendly slow-release material for persulfate (PS) using chitosan (CS) and urea (U) composite as frame structure (CS-U@PS) was firstly used to remediate the site soil contaminated by polycyclic aromatic hydrocarbons (PAHs) from Hefei city, China. The influences of glutaraldehyde dosage and urea dosage on the slow released properties of CS-U@PS was investigated and a longer release time of 190.0 d to 591.0 d was achieved, indicating the promising tunability according to the contents of pollutants in site soil. The effects of various factors (cross-linking agent dosage, initial PAHs concentration, CS-U@PS dosage, hydrogen peroxide dosage, etc.) on the degradation of soil spiked with PAHs (80 and 8 mg·kg−1) by CS-U@PS were studied, and the degradation pathway of PAHs was speculated. According to the EPR results, urea played an important role in promoting the 1O2 generation, which was the main reactive oxidative species. Finally, the long-term remediation of the industrial site soil with total PAHs of 553.2 mg·kg−1 was performed using 50 mg·g−1soil CS-U@PS and 90.22% removal rate was obtained after nine months. Moreover, the monitoring results of soil respiration gas show that, in addition to CO2, there is no additional greenhouse gas generated in the process of soil remediation, suggesting that the remediation of site soil using CS-U@PS has an excellent application prospect.

Effects of physical aging processes on the bioavailability of heavy metals in contaminated site soil amended with chicken manure and wheat straw biochars

The physicochemical properties of biochars undergo slow changes in soils due to the natural aging processes, which influences their interaction with heavy metals. The effects of aging on immobilization of co-existing heavy metals in contaminated soils amended with fecal and plant biochars possessing contrasting properties remain unclear. This study investigated the effects of wet–dry and freeze–thaw aging on the bioavailability (extractable by 0.01 M CaCl2) and chemical fractionation of Cd and Pb in a contaminated site soil amended with 2.5% (w/w) chicken manure (CM) biochar and wheat straw (WS) biochar. Compared to that in the unamended soil, the contents of bioavailable Cd and Pb in CM biochar-amended soil decreased by 18.0% and 30.8%, respectively, after 60 wet–dry cycles, and by 16.9% and 52.5%, respectively, after 60 freeze–thaw cycles. CM biochar, which contained significant levels of phosphates and carbonates, effectively reduced the bioavailability of Cd and Pb and transformed them from the labile chemical fractions to the more stable ones in the soil during the accelerated aging processes, mainly through precipitation and complexation. In contrast, WS biochar failed to immobilize Cd in the co-contaminated soil in both aging regimes, and was only effective at immobilizing Pb under freeze–thaw aging. The changes in the immobilization of co-existing Cd and Pb in the contaminated soil resulted from aging-induced increase in oxygenated functional groups on biochar surface, destruction of the biochar's porous structure, and release of dissolved organic carbon from the aged biochar and soil. These findings could help guide the selection of suitable biochars for simultaneous immobilization of multiple heavy metals in co-contaminated soil under changing environmental conditions (e.g., rainfall, and freezing and thawing of soils).

Novel insights into the predominant factors affecting the bioavailability of polycyclic aromatic hydrocarbons in industrial contaminated areas using PLS-developed model

Bioavailability is recognized as a useful technical standard for risk assessment and pollution rehabilitation. However, knowledge on the bioavailability of polycyclic aromatic hydrocarbons (PAHs) in contaminated site soils is still limited, especially concerning the influential mechanism. With an abundance of soil collections from nine industrial areas in China, the bioavailabilities, as conceptually defined as bioconcentration factors (BCFs) of PAHs were analyzed using biomimetic extraction of hydroxypropyl-β-cyclodextrin (HPCD). Apart from the total content of PAHs varying with the different pyrogenic sources, the BCFs were greatly dependent on the soil physicochemical properties from the spatial scale and inversely proportional to the number of rings. Pearson correlation analysis indicated a weak relationship between bioavailability and the soil dissolved organic matter (DOM), pH and particle size. To incorporate the soil physicochemical properties and structural characteristics of PAHs determined by density functional theory (DFT), the optimum model for bioavailability was developed for BCFs by partial least square (PLS) analysis. The PLS-derived model was shown to be predictive within the applicability domain (AD). The structural characteristics, e.g., molecular polarizability and frontier orbital energy level that favor the soil adsorption of PAH isomers via dispersion interactions, and electron exchanges were indicated to be more impactful on bioavailability than soil environmental factors. However, soil factors should not be neglected, because the pH, DOM, etc. were significantly influential. It makes sense that the higher DOM causes greater bioavailability via increasing the free-dissolved fractions of PAHs. Interestingly, the effect of pH on bioavailability was spectrally validated by excitation-emission matrix (EEM) fluorescence, showing that the interaction between DOM and pyrene strengthened the fluorescence quenching of chromophores with the decline in pH.

Isolation, genetic identification and optimization of hydrocarbon degrading bacteria from petroleum contaminated soil in Raigad region

In recent years, accumulation of petroleum hydrocarbons in soil and water is a global concern due to its genotoxicity and carcinogenic nature. Bioremediation of petroleum contaminates sites with natural isolates having ability to degrade hydrocarbons, are safe alternative for effective biodegradation. Present study is focused on isolation, characterization and optimization of novel isolates from petroleum contaminated soil with hydrocarbon degrading potential. Thirteen different petroleum contaminated soil sites were sampled from Raigad region to isolate indigenous hydrocarbon degrading bacteria. DCPIP screening assay was performed to select best isolates which utilize hydrocarbon as carbon source. The two best isolates S9D2 and S13D1 were identified as Rhodococcus ruber and Azospirillum zeae respectively by 16S rRNA gene-based sequencing and Phylogenetic analysis. One factor at a time (OFAT) with a spectrophotometer approach was used to optimize growth parameter. Optimum growth of Rhodococcus ruber was observed with 2 % diesel concentration at 37°C temperature and pH 7.0, while Azospirillum zeae showed better growth at 1% substrate concentration, pH 6.0. and temperature 30°C. The current study revealed that both the isolates have ability to degrade hydrocarbons present in diesel, and can be used for effective bioremediation tools

An experimental investigation on the effect of soluble sugar degradation on the of domestic-source-contaminated soil sites formation and evolution

The percolation-degradation process of soluble domestic pollution is very important for the evolution of soil properties and the formation of contaminated sites. The main objective of this study is to investigate the influence of glucose seepage-degradation on the permeability of clay through an indoor percolation test in combination with thermogravimetric measurement with glucose as a representative domestic contaminant soluble sugar. We can conclude that the permeability of clay was significantly impacted by the seepage-degradation of soluble sugar. With a focus on the role of soluble sugars in domestic source pollutants on clay, the formation and evolution of the domestic source contaminated soil site went through three main stages: “generation of domestic source contaminated liquid & formation of S–C zone”, “contraction of S–C zone & formation of E-C zone and C zone”, and “disappearance of S–C zone & contraction of E-C zone and C zone”. The clay permeability decreased, the migration range shrinked, and the pollution level of the clay near the source of the contaminants increased with increasing soluble sugar solution concentration.

Triaxial test and microstructure analysis of soil from alkali contaminated sites

The protection of site cultural relics is facing great challenges of site soil instability and contamination. Targeting at the special geological environment of Kaifeng region, the affecting laws of different concentrations of alkali on mechanical properties of site soil in this region are studied in this paper. The triaxial test is conducted to study the mechanical properties of site soil, including its failure pattern, stress-strain relationship, stress path and strength index, etc. The X-ray diffraction test and scanning electron microscope test are used to analyze the composition, particle state, and pore characteristics of the soil before and after contamination, and to evaluate the stability of the site soil. At the site combining with microstructure changes and macro mechanical properties. The results show that, from the macroscopic surface, with the increase of alkali concentration, the failure shear plane usually appears along the 45° direction. From the perspective of mechanical properties, the stress-strain relationship of the site soil is caused by the strengthening of soil deformation, and the alkali concentration increases, and the curve gradually develops from hardening type to softening type. The stress path is the line connecting the points in the sequence of the stress change process, which is different, and the corresponding soil deformation and strength characteristics will also appear very different. The stress path of soil with different alkali concentration is different. Under the same σ3 condition, the stress σ1 is different. The stress change is expressed in macroscopic terms as different forms of soil damage. Under 0 % concentration, the strength index of soil is 82.14 kPa, 21.67°, and the alkali concentration is 4–16 %, the internal friction Angle is gradually increased, and the cohesion is slightly reduced. From the micro perspective, the main components of the contaminated site soil are SiO2, Al2O3, CaO and Fe2O3. In the process of alkali-soil reaction, alkali reacts with oxides in soil to form Na2AlO2, which has a certain cementation effect. SiO2 reacts with NaOH solution to generate Na2SiO3, which has strong cohesiveness and strength. The newly generated cementing material in the soil increases, and the bonding force between particles increases. In addition, some cements fill the inner pores and improve the shear strength of soil. Under the same pressure, the stress-strain curve of soil gradually developed from hardening type to softening type with the increase of alkali concentration. In the stress-strain curve of soil, with the increase of alkali concentration, the soil reaches the strength at failure under small deformation. The increase of alkali concentration reduces the plasticity and increases the brittleness of soil. At this time, if the stress is localized, shear bands will appear and the stability of soil will decrease. The study of soil properties changes caused by different alkali pollution concentrations can provide a scientific basis for the reinforcement and protection of soil sites or ancient cultural foundations in Kaifeng area.

Investigation and Remediation of Contaminated Site Soil Environment

Investigation and Remediation of Contaminated Site Soil Environment

Ecological risk assessment of contaminated soil using improved "Weight of Evidence"approach:A case study of electroplating site in Jingjiang, Jiangsu, China

Ecological risk assessment of soil in contaminated sites provides the scientific basis for accurately developing soil quality standards, confirming remediation targets, and making safe use of contaminated soil. It thus is a critical mean to protect soil health and safety. "Weight-of-Evidence" (WoE) has been widely used in ecological risk assessment due to its systematic, integrated and scientific properties. However, most current WoE approaches are poor in objectivity and comparability because they rely too much on expertise scoring in weighing and the difficulty to collect complete evidence bodies with quantitative and comprehensive information. Focusing on those issues above, we developed an improved framework of WoE approach for ecological risk assessment of contaminated site soil based on the "Four-Step" framework of EPA coupled with the concept of hierarchy. Assessment methods and procedures for each tier were unified. Weights were weighed quantitatively through multiple criteria decision analysis. The relative independence among bodies of evidence was assured by the pre-establishment of hierarchy. The "site specific" was stressed based on matrix trails and field investigation. Finally, a case study in an electroplating site in Jingjiang was conducted to verify the approach. Results of the case study suggested that the approach was practical and that the assessment results were objective, scientific, and accurate.

Enhanced remediation of PAHs-contaminated site soil by bioaugmentation with graphene oxide immobilized bacterial pellets

Bioaugmentation is considered as a promising technology for cleanup of polycyclic aromatic hydrocarbons (PAHs) from contaminated site soil, however, available high-efficiency microbial agents remain very limited. Herein, we explored graphene oxide (GO)-immobilized bacterial pellets (JGOLB) by embedding high-efficiency degrading bacteria Paracoccus aminovorans HPD-2 in alginate-GO-Luria-Bertani medium (LB) composites. Microcosm culture experiments were performed with contaminated site soil to assess the effect of JGOLB on the removal of PAHs. The results showed that JGOLB exhibited greatly improved mechanical strength, larger specific surface area and more enriched mesopores, compared with traditional immobilized bacterial pellets. They significantly increased the removal rate of PAHs by 18.51% compared with traditional bacterial pellets, reaching the removal rate at 62.86% over 35 days of incubation. Moreover, the increase mainly focused on high-molecular-weight PAHs. JGOLB not only greatly increased the abundance of embedded degrading bacteria in soil, but also significantly enhanced the enrichment of potential indigenous degrading bacteria (Pseudarthrobacter and Arthrobacter), the functional genes involved in PAHs degradation and a number of ATP transport genes in the soil. Overall, such nanocomposite bacterial pellets provide a novel microbial immobilization option for remediating organic pollutants in harsh soil environment.

Distribution and bioaccessibility of polycyclic aromatic hydrocarbons in industrially contaminated site soils as affected by thermal treatment

Thermal treatment can not only efficiently remove volatile pollutants but also distinctly alter the speciation of organic carbon (C) and the behaviors of residual pollutants in contaminated soils. Here we examined the distribution and bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) in industrially contaminated site soils affected by thermal treatment (temperature ranging of 105–650 ℃) using synchrotron-based infrared microspectroscopy and n-butanol extraction (a mild solvent extractant). In the pristine soils, the sequestration and distribution of PAHs were simultaneously controlled by aromatic C, aliphatic C and clay minerals. Desorption efficiency of PAHs was substantially increased with increasing temperature, whereas the residual PAHs were strongly immobilized within their binding sites evidenced by their dramatically decreased bioaccessibility. Aliphatic and carboxylic C were gradually decomposed and/or carbonized with increasing temperature. In contrast, aromatic C remained relatively recalcitrant during the thermal treatment and was the key controlling factor for the desorption of residual PAHs in the soils with either thermal treatment or n-butanol extraction. This study is the first to visualize the changes in the binding sites and bioaccessibility of PAHs induced by thermal treatment, which have important implications for understanding the sequestration mechanisms of organic pollutants in soil and optimizing the remediation technique.