PAH-contaminated Soil Research Articles

The Application of Carotenoid-Coated Chitosan Nanoparticles to Reduce the PAHs Stress on Spinach Growth

Polycyclic aromatic hydrocarbons (PAHs) pose risks to human and animal health, and their accumulation in crops is a concern for the food chain in the environment. Nanoparticles (NPs) have shown potential for chemical delivery and can be used to enhance plant resistance to PAHs. In this study, carotenoid-coated chitosan nanoparticles (CCNPs) loaded with β-carotene were prepared and applied to spinach grown in PAH-contaminated soil. The size of the CCNPs varied based on reaction conditions with temperature, TPP, and pH, with sizes ranging from 260 to 682 nm. After four weeks of treatment, the spinach showed varying growth responses depending on the specific CCNP treatment. The treatment with CCNPs prepared at 20 °C, pH 6, and 10 mg/mL TPP resulted in the best spinach growth, while the treatment at 40 °C, pH 6, and a TPP concentration of 20 mg/mL hindered growth; and the growth ration increased by over 47.4% compared to the normal growing spinach, the final biomass reached 2.53 g per plant. In addition, phenanthrene (PHE) and pyrene (PYR) predominantly accumulated more in the spinach roots, with variations depending on the specific CCNP treatment. The exogenous application of CCNPs can reduce the PAH transfer to the shoots. The bioconcentration factors and transfer factors of PYR and PHE reduced differential movement within the spinach plants, and the spinach prefers PYR to PHE in biological accumulation. This study offers a new understanding of the mechanisms underlying NPs and PAHs interactions and NP’s implications for crop protection and food safety.

Propylene glycol-mixed steam enhanced extraction for an efficient and sustainable remediation of PAHs-contaminated soil

An innovative thermal desorption method, propylene glycol (PG)-mixed steam enhanced extraction, is proposed for a highly efficient remediation of polycyclic aromatic hydrocarbons (PAHs)-contaminated soil. It is found that injecting PG-mixed steam into soil column could obtain > 99% removal efficiencies of PAHs either for the pyrene-spiked soil, or for the contaminated field soil with high-molecular-weight PAHs. PG is a safe and low-cost dihydric alcohol with a boiling point higher than water. When the PG-mixed steam penetrated the contaminated soil, the PG vapor preferentially condensed to form a hot liquid with concentrated PG (e.g., from 30 wt% PG in gas phase to 90 wt% PG in the liquid phase), which would significantly solubilize the PAHs and enhance their desorption from soils. The results also revealed that the effluents derived from the PG-mixed steam could be purified by removing the desorbed PAHs using a simple coagulation treatment, and the recovered PG solution could be reused. The plant assay using wheat seeds showed that the remediated soil had a good regreening potential. Our results demonstrate that PG-mixed steam injection is a promising thermal desorption method for an efficient and sustainable remediation of PAHs-contaminated soil.

Degradation of benzo [a] pyrene in the soil enhanced by soapwort: The role of soapwort and functional microbial community

The limited bioavailability of polycyclic aromatic hydrocarbons (PAHs) in soils poses a challenge for their biodegradation. We hypotheses soapwort (Saponaria officinalis L.) as a factory in-situ providing biosurfactant, which could effectively promote the BaP removal by exogenous or native functional microbes. Rhizo-box and microcosm experiments were conducted to analyze the phyto-microbial remediation mechanism of soapwort, a plant that excretes biosurfactants known as saponins, and combined with two exogenous strains (P. chrysosporium and/or B. subtilis) for benzo[a]pyrene (BaP)-contaminated soils. The results revealed that the natural attenuation treatment (CK) BaP achieved only a 15.90% BaP removal rate after 100 days. In contrast, soapwort (SP), soapwort-bacteria (SPB), soapwort-fungus (SPF), soapwort- bacteria - fungus (SPM) mediated rhizosphere soils treatments yielded removal rates of 40.48%, 42.42%, 52.37%, and 62.57%, respectively. The analysis of the microbial community structure suggested that soapwort stimulated the introduction and native functional microorganisms, such as Rhizobiales, Micrococcales, and Clostridiales, which contributed to BaP removal via metabolic pathways. Furthermore, the efficient BaP removal was attributed to saponins, amino acids, and carbohydrates, which facilitated mobilization, solubilization of BaP, and microbial activity. In conclusion, our study highlights the potential of soapwort and specific microbial strains to effectively remediate PAH-contaminated soils.

Understanding the role of graphene oxide in affecting PAHs biodegradation by microorganisms: An integrated analysis using 16SrRNA, metatranscriptomic, and metabolomic approaches

Graphene oxide (GO)-promoted microbial degradation technology is considered an important strategy to eliminate polycyclic aromatic hydrocarbons (PAHs) in the environment; however, the mechanism by which GO affects microbial degradation of PAHs has not been fully studied. Thus, this study aimed to analyze the effect of GO-microbial interaction on PAHs degradation at the microbial community structure, community gene expression, and metabolic levels using multi-omics combined technology. We treated PAHs-contaminated soil samples with different concentrations of GO and analyzed the soil samples for microbial diversity after 14 and 28 days. After a short exposure, GO reduced the diversity of soil microbial community but increased potential degrading microbial abundance, promoting PAHs biodegradation. This promotion effect was further influenced by the GO concentration. In a short period of time, GO upregulated the expression of genes involved in microbial movement (flagellar assembly), bacterial chemotaxis, two-component system, and phosphotransferase system in the soil microbial community and increased the probability of microbial contact with PAHs. Biosynthesis of amino acids and carbon metabolism of microorganisms were accelerated, thereby increasing the degradation of PAHs. With the extension of time, the degradation of PAHs stagnated, which may be due to the weakened stimulation of GO on microorganisms. The results showed that screening specific degrading microorganisms, increasing the contact area between microorganisms and PAHs, and prolonging the stimulation of GO on microorganisms were important means to improve the biodegradation efficiency of PAHs in soil. This study elucidates how GO affects microbial PAHs degradation and provides important insights for the application of GO-assisted microbial degradation technology.

Application of the PDMS Passive Sampling Method to Assess Bioavailability and Health Risks Associated with PAH-Contaminated Soil

Integrating bioavailability into risk assessments is an effective way to objectively assess human health risks. In this study, the bioavailability of 16 polycyclic aromatic hydrocarbons (PAHs) in contaminated soil from a coking plant was evaluated using the polydimethylsiloxane (PDMS) passive sampling method. The results showed that the bioavailability factor (BAF) of each PAH, predicted using PDMS fibers, ranged from 0.46% to 9.74%. The PDMS passive sampling method was more stable in testing the bioavailability of PAHs with more than 4 benzene rings; a preliminary relationship was established between the BAF and the log value of the octanol–water partition coefficient (logKow). After considering their bioavailability, the carcinogenic risks (CRs) and non-carcinogenic hazard quotients (HQs) associated with the 16 PAHs were reduced by 1 to 2 orders of magnitude. Only the health risks associated with benzo(a)pyrene and dibenzo(a,h)anthracene exceeded the acceptable level. The PDMS passive sampling method provides a useful tool for estimating oral bioavailability, and incorporating its results into human exposure testing can help to refine the health risk assessment of contaminants through oral ingestion.

Surfactant enhanced thermally activated persulfate remediating PAHs-contaminated soil: Insight into compatibility, degradation processes and mechanisms

Although advanced oxidation processes (AOPs) based on persulfate (PS) is an attractive approach for repairing polycyclic aromatic hydrocarbons (PAHs) contaminated soils, limited oxidizability of PAHs and efficient in-situ activation of PS hinder its practical applications. In this study, we comprehensively examined the contributions of five representative surfactants on the oxidative remediation of PAHs-contaminated soil in terms of degradation kinetics of the pollutants, and further proposed an innovative coupling strategy of surfactant-enhanced thermally activated PS remediating PAHs-contaminated soil. The results showed that the degradation process of PAHs in soil was significantly facilitated only via adding sodium dodecyl benzenesulfonate (SDBS) and fitted the pseudo-first-order kinetic pattern. The removal of phenanthrene (PHE) reached 98.56% at 50 mM PS, 50 °C, 5 g L−1 SDBS and 48 h reaction time, accompanying an increase of 25% in reaction rate constant from 0.0572 h−1 (without SDBS) to 0.0715 h−1. More importantly, SDBS-enhanced thermally activated PS degrading PAHs with higher benzene rings were more effective as the reaction rate constants of pyrene (PYR) and benzo(a)anthracene (BaA) were significantly increased by 49.40% and 56.86%. Additionally, only appropriate dosages (5–10 g L−1) of SDBS facilitated the oxidative degradation of PHE, as well as the aging time of contaminant-soil contact slowed down the enhancement of oxidative degradation of PHE by SDBS. Scavenger experiments demonstrated that SO4·- and 1O2 were the dominant reactive oxygen species. Finally, a possible oxidative degradation pathway of PHE was proposed, and the toxicity of derived intermediates got alleviation by the assessment using the Toxicity Estimation Software Tool. This investigation was promising for in situ scale-up remediation of PAHs-contaminated soil.

Soil physiochemical properties and bacterial community changes under long-term polycyclic aromatic hydrocarbon stress insitu steel plant soils

In situ soils were collected at two depths in Jinan and Hangzhou steel plants, which both have a long history of operation and polycyclic aromatic hydrocarbons (PAHs) contamination. The richness of 16 S rRNA gene and bacterial community of the soil were determined by real-time PCR and high-throughput sequencing. Soil physicochemical properties, PAHs contamination characteristics, and their interrelationships were also analyzed. In general, the PAHs contamination decreased with increasing soil depths. The physicochemical properties and PAH concentration of soil had synergistic impacts on the composition of the bacterial community. The long-term higher PAHs stress in Hangzhou contaminated soil (982 mg kg−1) increased the bacterial abundance and diversity, while that of Jinan contaminated soil (63 mg kg−1) decreased bacterial abundance and diversity. The pH value, sand content of the soil were positively correlated (P < 0.05) with the bacterial diversity including Simpson, Shannon, Observed_species and Chao1 indexes., and the other soil properties exhibited negative correlations with different strengths. The abundances of Curvibacter, Pseudomonas, Thiobacillus, Lysobacter, and Limnobacter were positively correlated with the PAHs concentration (P < 0.01). Additionally, the network structure of the PAHs-contaminated soils was more complex compared to that of uncontaminated soils, with stronger linkages and correlations between the different bacteria. These findings provide a theoretical basis for microbial remediation of PAHs-polluted soil.

The use of an algal bioindicator in the assessment of different chemical remediation strategies for PAH-contaminated soils and sediments

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous petroleum-derived pollutants that tend to accumulate in soils and sediments. Among the in situ chemical oxidation technologies applied at PAH-contaminated sites are Fenton reagent (FN), persulfate ion (PS) and permanganate ion (PM). The natural phytoplankton of an aquatic environment is useful as a biological indicator of the treatment efficacy of pollutant degradation. The objective of this work was to assess the use of an algal species, native from Los Barreales Reservoir (Argentina), as an efficacy indicator of the chemical remediation of PAH-contaminated soils and sediments. Oxidations with FN, PS or PM were carried out on soils and sediments previously contaminated with two types of PAHs (phenanthrene (Ph) or Benzo(a)anthracene (Ba)), analysing pH, residual oxidant, PAH degradation and heterotrophic bacteria abundance in soils or sediments after 7 days. Then, a bioassay was carried out by incubating the obtained soils or sediments, and the native alga Scenedesmus quadricauda (Turpin, Brébisson), analysing its abundance after 7 days. S. quadricauda abundance was related to PAH degradation efficacy, residual oxidants and changes in system pH. In this sense, the application of PS produced the lowest growth of the indicator in assays both with soils and sediments. Fenton reaction results varied depending on the contaminating PAH, with the best percentages of PAH degradation and growth of S. quadricauda for Ba. PM oxidation resulted in the best development of the indicator. This study constitutes a first approach to the use of S. quadricauda as an efficiency indicator of soil/sediment chemical remediation.

Efficiency and mechanism on photocatalytic degradation of fluoranthene in soil by Z-scheme g-C3N4/α-Fe2O3 photocatalyst under simulated sunlight.

Polycyclic aromatic hydrocarbons (PAHs) in soil have potential harm on human health. However, remediation of PAH-contaminated soils through photocatalytic technology remains a challenge. Therefore, the photocatalyst g-C3N4/α-Fe2O3 was synthesized and applied to photocatalytic degradation of fluoranthene in soil. The physicochemical properties of g-C3N4/α-Fe2O3 and various degradation parameters, such as catalyst dosage, the ratio of water/soil, and initial pH, were investigated in detail. In soil slurry reaction system (water/soil=10:1, w/w), the optimal degradation efficiency on fluoranthene was 88.7% after simulated sunlight irradiation for 12 h (contaminated soil=2 g, initial fluoranthene concentration=36 mg/kg, catalyst dosage=5%, and pH=6.8), and the photocatalytic degradation followed pseudo-first-order kinetics. The degradation efficiency of g-C3N4/α-Fe2O3 was higher compared with P25. Degradation mechanism analysis showed that •O2- and h+ are the main active species in photocatalytic degradation process of fluoranthene by g-C3N4/α-Fe2O3. Coupling g-C3N4 and α-Fe2O3 enhances the interfacial charge transport capacity via Z-scheme charge transfer route and inhibits the recombination of photogenerated electrons and holes of g-C3N4 and α-Fe2O3, then significantly improves the production of active species and photocatalytic activity. Results showed that photocatalytic treatment of soil by g-C3N4/α-Fe2O3 is an effective strategy for remediation of soils contaminated by PAHs.

Deep relationships between bacterial community and polycyclic aromatic hydrocarbons in soil profiles near typical coking plants.

Bacterial communities play an important role in maintaining the normal functioning of ecosystems; therefore, it is important to understand the effects of polycyclic aromatic hydrocarbons (PAHs) on the bacterial community. In addition, understanding the metabolic potential of bacterial communities for PAHs is important for the remediation of PAH-contaminated soils. However, the deep relationship between PAHs and bacterial community in coking plants is not clear. In this study, we determined the bacterial community and the concentration of PAHs in three soil profiles contaminated by coke plants in Xiaoyi Coking Park, Shanxi, China, using 16S rRNA and gas chromatography coupled with mass spectrometry, respectively. The results show that 2 ~ 3 rings PAHs are the main PAHs and Acidobacteria (23.76%) was the dominant bacterial community in three soil profiles. Statistical analysis showed that there were significant differences in the composition of bacterial communities at different depths and different sites. Redundancy analysis (RDA) and variance partitioning analysis (VPA) illustrate the influence of environmental factors (including PAHs, soil organic matter (SOM), and pH) on the vertical distribution of soil bacterial community, and PAHs were the main factors affecting the bacterial community in this study. The co-occurrence networks further indicated correlations between bacterial community and PAHs and found that Nap has the greatest effect on bacterial community compared with other PAHs. In addition, some operational taxonomic units (OTUs, OTU2, and OTU37) have the potential to degrade PAHs. PICRUSt2 (Phylogenetic Investigation of Communities by Reconstruction of Unobserved States) was used for further study on the potential of microbial PAHs degradation from a genetic perspective, which showed that different PAH metabolism genes were present in the genomes of bacterial communities in the three soil profiles, and a total of 12 PAH degradation-related genes were isolated, mainly dioxygenase and dehydrogenase genes.

Bacterial benz(a)anthracene catabolic networks in contaminated soils and their modulation by other co-occurring HMW-PAHs

Polycyclic aromatic hydrocarbons (PAHs) are major environmental pollutants in a number of point source contaminated sites, where they are found embedded in complex mixtures containing different polyaromatic compounds. The application of bioremediation technologies is often constrained by unpredictable end-point concentrations enriched in recalcitrant high molecular weight (HMW)-PAHs. The aim of this study was to elucidate the microbial populations and potential interactions involved in the biodegradation of benz(a)anthracene (BaA) in PAH-contaminated soils. The combination of DNA stable isotope probing (DNA-SIP) and shotgun metagenomics of 13C-labeled DNA identified a member of the recently described genus Immundisolibacter as the key BaA-degrading population. Analysis of the corresponding metagenome assembled genome (MAG) revealed a highly conserved and unique genetic organization in this genus, including novel aromatic ring-hydroxylating dioxygenases (RHD). The influence of other HMW-PAHs on BaA degradation was ascertained in soil microcosms spiked with BaA and fluoranthene (FT), pyrene (PY) or chrysene (CHY) in binary mixtures. The co-occurrence of PAHs resulted in a significant delay in the removal of PAHs that were more resistant to biodegradation, and this delay was associated with relevant microbial interactions. Members of Immundisolibacter, associated with the biodegradation of BaA and CHY, were outcompeted by Sphingobium and Mycobacterium, triggered by the presence of FT and PY, respectively. Our findings highlight that interacting microbial populations modulate the fate of PAHs during the biodegradation of contaminant mixtures in soils.

Effect of eucalyptus saponin and sophorolipid amendment on soil and sediment microbial communities and seed germination: potential application for PAH bioremediation

PurposeThe potential toxic effects due to the application of sophorolipid and the novel saponin biosurfactant extracted from the leaves of Eucalyptus camaldulensis for the purpose of bioremediating PAH-contaminated soils and sediments were studied.MethodsThe phytotoxic effects of sophorolipid and eucalyptus saponin were determined based on the seed germination assays carried out using the seeds of Lactuca sativa L and compared with commonly used rhamnolipid and Tween 20 surfactants. Further, biosurfactant-induced changes in soil/sediment bacterial structure and diversity were investigated by conducting Miseq amplicon sequencing of the bacterial genes.ResultsGermination indices (GI) demonstrated the non-phytotoxic effects (GI > 80%) of saponin and sophorolipid biosurfactants (100–500 mg/L), while rhamnolipid demonstrated greater phytotoxicity than Tween 20 at high concentrations (500 mg/L). Saponin-amended soil resulted in greater bacterial diversity and richness compared to controls, while sophorolipid produced the opposite effect. These significant variations were not observed in sediment samples. Incubation of biosurfactants for 20 and 40 days did not result in significant changes in bacterial diversity and structure in any of the samples. Increased abundance of some of the PAH-degrading bacteria was noted at OTU level, in the presence of saponin and sophorolipid. Saponin had less impact on native soil/sediment bacteria relative to sophorolipid based on the prevalence of the significantly shifted OTUs.ConclusionAs saponin and sophorolipid were shown to have no adverse impacts on the microbiome, and non-phytotoxic effects, their sustainable applications to remediate PAH-contaminated soils and sediments can be recommended.

Influencing factors of bioaugmentation treatment of PAH-contaminated soils in slurry bioreactors

Polycyclic aromatic hydrocarbons (PAHs) are a group of contaminants with low bioavailability and biodegradability. As a result, removing PAHs from contaminated soils is often inefficient. In this study, soils spiked with fluoranthene and pyrene were inoculated with Aeromonas sp. BCP-3, a newly isolated strain from coke-plant sludge, and treated in slurry bioreactors. The removal of PAHs was significantly affected by aeration rate, slurry water-soil mass ratio, stirring rate, temperature, and the concentration of Tween-80. Such effects were attributed to changes in BCP-3 growth, PAHs solid-liquid mass transfer, and slurry particle size. Notably, under optimal conditions (i.e., aeration rate of 1.5 L/min, temperature of 30 °C, stirring rate of 200 rpm, water-soil mass ratio of 1.5:1, and Tween-80 concentration of 1.3 %), 96.3 % and 97.2 % of initially spiked fluoranthene and pyrene were removed within 12 days, respectively. A PAH-contaminated soil was collected from a local coking plant and treated under the same conditions, except for the Tween-80 concentration. For the real soil, the Tween-80 concentration was 4 % to ensure a significant proportion of PAHs released into the liquid phase. Twenty-one days after inoculation of BCP-3, the removal efficiencies of low molecular weight PAHs and high molecular weight PAHs were 82.2 % and 51.8 %, respectively. The removal efficiencies of PAHs were significantly higher than the slurry without BCP-3 or HgCl2-sterilized slurry. BCP-3 improved the bioavailability and biodegradability of PAHs, resulting in effective PAH removal in slurry bioreactors. Overall, successful bioaugmentation was achieved with the addition of Tween-80 and the optimization of operating conditions.

Efficient remediation of the field soil contaminated with PAHs by amorphous porous iron material activated peroxymonosulfate

An amorphous porous iron material (FH) was firstly self-synthesized using a simple coprecipitation approach and then utilized to activate peroxymonosulfate (PMS) for the catalytic degradation of pyrene and remediation of PAHs contaminated soil on site. FH exhibited more excellent catalytic activity than traditional hydroxy ferric oxide and possessed stability at a pH range of 3.0–11.0. According to quenching studies and electron paramagnetic resonance (EPR) analyses, non-radicals (Fe(IV) = O and 1O2) were the major reactive oxygen species (ROS) in the FH/PMS system's degradation of pyrene. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) of FH before and after the catalytic reaction, as well as active site substitution experiments and electrochemical analysis all verified that PMS adsorbed on FH could produce more abundant bonded hydroxyl groups (Fe–OH) which dominated the radical and non-radical oxidation reactions. Then, a possible pathway for pyrene degradation was presented according to gas chromatography-mass spectrometry (GC–MS). Furthermore, the FH/PMS system exhibited excellent catalytic degradation in the remediation of PAH-contaminated soil at real sites. This work provides a remarkable potential remediation technology of persistent organic pollutants (POPs) in environmental and will contribute to understanding the mechanism of Fe-based hydroxides in advanced oxidation processes.

New insight into the mechanisms of autochthonous fungal bioaugmentation of phenanthrene in petroleum contaminated soil by stable isotope probing

Autochthonous fungal bioaugmentation (AFB) is considered a reliable bioremediation approach for polycyclic aromatic hydrocarbon (PAH) contamination, but little is known about its mechanisms in contaminated soils. Here, a microcosm experiment was performed to explore the AFB mechanisms associated with two highly efficient phenanthrene degrading agents of fungi (with laccase-producing Scedosporium aurantiacum GIG-3 and non-laccase-producing Aspergillus fumigatus LJD-29), using stable-isotope-probing (SIP) and high-throughput sequencing. The results showed that each fungus markedly improved phenanthrene removal, and microcosms with both fungi exhibited the best phenanthrene removal performance among all microcosms. Additionally, AFB markedly shifted the composition of the microbial community, particularly the phenanthrene-degrading bacterial taxa. Interestingly, based on SIP results, strains GIG-3 and LJD-29 did not assimilate phenanthrene directly during AFB, but instead played key roles in the preliminary decomposition of phenanthrene though secretion of different extracellular enzymes to oxidize the benzene ring (GIG-3 bioaugmentation with laccase, and LJD-29 bioaugmentation with manganese and lignin peroxidases). In addition, all functional degraders directly involved in phenanthrene assimilation were indigenous bacteria, while native fungi rarely participated in the direct phenanthrene mineralization. Our findings provide a new mechanism of AFB with multiple fungi, and support AFB as a promising strategy for the in situ bioremediation of PAH-contaminated soil.

Using soil enzyme Vmax as an indicator to evaluate the ecotoxicity of lower-ring polycyclic aromatic hydrocarbons in soil: Evidence from fluorescein diacetate hydrolase kinetics

Fluorescein diacetate hydrolase (FDA hydrolase) is a reliable biochemical biomarker of changes in soil microbial activity and quality. However, the effect and mechanism of lower-ring polycyclic aromatic hydrocarbons (PAHs) on soil FDA hydrolase are still unclear. In this work, we investigated the effects of two typical lower-ring PAHs, naphthalene (Nap) and anthracene (Ant), on the activity and kinetic characteristics of FDA hydrolases in six soils differing in their properties. Results demonstrated that the two PAHs severely inhibited the activities of the FDA hydrolase. The values of Vmax and Km dropped by 28.72–81.24 % and 35.84–74.47 % at the highest dose of Nap, respectively, indicating an uncompetitive inhibitory mechanism. Under Ant stress, the values of Vmax decreased by 38.25–84.99 %, and the Km exhibited two forms, unchanged and decreased (74.00–91.61 %), indicating uncompetitive and noncompetitive inhibition. The inhibition constant (Ki) of the Nap and Ant ranged from 0.192 to 1.051 and 0.018 to 0.087 mM, respectively. The lower Ki of Ant compared to Nap indicated a higher affinity for enzyme-substrate complex, resulting in higher toxicity of Ant than Nap to soil FDA hydrolase. The inhibitory effect of Nap and Ant on soil FDA hydrolase was mainly affected by soil organic matter (SOM). SOM influenced the affinity of PAHs with enzyme-substrate complex, which resulted in a difference in PAHs toxicity to soil FDA hydrolase. The enzyme kinetic Vmax was a more sensitive indicator than enzyme activity to evaluate the ecological risk of PAHs. This research offers a strong theoretical foundation for quality control and risk evaluation of PAH-contaminated soils through a soil enzyme-based approach.

Effects of different carbon substrates on PAHs fractions and microbial community changes in PAHs-contaminated soils

Different types of carbon substrates were widely used in soil remediation. However, differences of their impacts and related mechanisms on degradation of polycyclic aromatic hydrocarbons (PAHs) and microbial community structures in contaminated soil still remain unclear. Here, we investigated the effects of corn straw (S), glucose (G), straw combined with glucose (SG), and sodium azide (N, as an abiotic control) on PAHs fractions and bacterial communities in soil. After 70 days’ microcosm experiments, total PAHs concentrations were significantly reduced by 30.9%, 19.5% and 44.6% under S, G and SG treatments. Water soluble, acid soluble and residual PAHs under all treatments were significantly decreased after 70 days of incubation, while organically bound PAHs were increased by 11.4%, 22.7% and 36.1% under G, S and SG treatments. Additionally, straw and glucose application increased relative abundance related PAHs-degrading bacteria and the copy numbers of gram-negative (PAHs-RHDα GN) and gram-positive genes (PAHs-RHDα GP) in the contaminated soil. Redundancy analysis (RDA) and Random Forest (RF) indicated that PAHs fractions are crucial factors for biodegradation of PAHs in PAHs-contaminated soils amended with carbon substrates. These suggested that carbon substrates contributed to PAHs conversion from residual PAHs (nonlabile fractions) to organically bound PAHs and thus increased the potential for PAHs conversion to water-soluble and organic acid-soluble PAHs, which were more easy to be utilized by soil microorganisms. This study revealed the new insights of different carbon substrates on degradation and dynamic changes of PAHs fractions and the better potential of combined application of straw and glucose in enhancing degradation of PAHs in PAHs-contaminated soils.

Remediation of PAH-contaminated soil using a combined process of soil washing and adsorption by nano iron oxide/granular activated carbon composite

This study investigated the application of magnetic granular activated carbon (MGAC) in soil washing process for the removal of polycyclic aromatic hydrocarbons (PAHs) from real contaminated soil. The magnetic composite was prepared using a facile-modified co-precipitation method. The presence of the synthesized magnetite (Fe3O4) nanoparticles on the composite surface was confirmed by the SEM and XRD characterization tests. The surface area and total pore volume of MGAC were determined to be 837.9 m2/g and 0.5 cm3/g, respectively, which were approximately 10% smaller than those for the bare GAC. The Langmuir model fitted the adsorption isotherm data the best; therefore, monolayer adsorption was predominant in the treatment process with MGAC. The maximum PAH adsorption capacity of MGAC was obtained at 153.5 µg/mg for the total PAHs, which was nearly 20% lower than that of bare GAC (190.3 µg/mg). The soil washing parameters were optimized, and the results demonstrated that the total PAH removal reached a maximum of 47.4% at an MGAC dose of 2% (w/w), washing time of 24 h, a liquid-to-soil ratio of 15:1, stirring speed of 100 rpm, pH of 8.3, and a temperature of 25 °C. For low molecular weight (LMW) PAHs, the removal from soil ranged from 34.6% to 57.7%, whereas the removal percentage of high molecular weight (HMW) PAHs was determined to be between 27.9 and 67.1 %. This indicated that the optimized soil washing with MGAC had a better efficiency for some HMW PAHs than the LMW PAHs. According to the thermodynamic studies, the adsorption of PAHs onto MGAC in the soil washing process was non-spontaneous (ΔG° > 0) and endothermic (ΔH° > 0) with increased randomness at the solid/solution interface of MGAC as temperature increases. The obtained results confirmed the efficiency of the magnetized activated carbon as adsorbent in soil washing to remove the hazardous PAH compounds from soil.

Effects of bioaugmentation by isolated Achromobacter xylosoxidans BP1 on PAHs degradation and microbial community in contaminated soil

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic pollutants ubiquitous and persistent in soil. In order to provide a viable solution for bioremediation of PAHs-contaminated soil, a strain of Achromobacter xylosoxidans BP1 with superior PAHs degradation ability was isolated from contaminated soil at a coal chemical site in northern China. The degradation of phenanthrene (PHE) and benzo[a]pyrene (BaP) by strain BP1 was investigated in three different liquid phase cultures, and the removal rates of PHE and BaP by strain BP1 were 98.47% and 29.86% after 7 days under the conditions of PHE and BaP as the only carbon source, respectively. In the medium with the coexistence of PHE and BaP, the removal rates of BP1 were 89.44% and 9.42% after 7 days, respectively. Then, strain BP1 was investigated for its feasibility in remediating PAH-contaminated soil. Among the four PAHs-contaminated soils treated differently, the treatment inoculated with BP1 exhibited higher removal rates of PHE and BaP (p < 0.05), especially the CS-BP1 treatment (inoculation of BP1 into unsterilized PAHs-contaminated soil) showed 67.72%, 13.48% removal of PHE and BaP, respectively, over 49 days of incubation. Bioaugmentation also significantly increased the activity of dehydrogenase and catalase in the soil (p＜0.05). Furthermore, the effect of bioaugmentation on the removal of PAHs was investigated by measuring the activity of dehydrogenase (DH) and catalase (CAT) during incubation. Among them, the DH and CAT activities of CS-BP1 and SCS-BP1 (inoculation of BP1 into sterilized PAHs-contaminated soil) treatments inoculated with strain BP1 were significantly higher than those of treatments without BP1 addition during incubation (p < 0.01). The structure of the microbial community varied among treatments, but the Proteobacteria phylum showed the highest relative abundance in all treatments of the bioremediation process, and most of the bacteria with higher relative abundance at the genus level also belonged to the Proteobacteria phylum. Prediction of microbial functions in soil by FAPROTAX analysis showed that bioaugmentation enhanced microbial functions associated with the degradation of PAHs. These results demonstrate the effectiveness of Achromobacter xylosoxidans BP1 as a PAH-contaminated soil degrader for the risk control of PAHs contamination.

Effects of co-modified biochar immobilized laccase on remediation and bacterial community of PAHs-contaminated soil.

Considering the stability and economy of immobilized enzymes, this study prepared co-modified biochar immobilized laccase product named Fe3O4@NaBC@GA@LC via orthogonal experimental design and explored its possibility of remediating polycyclic aromatic hydrocarbons (PAHs) contaminated soil in steel plants. Compared with the free laccase treatment, the relative activity of Fe3O4@NaBC@GA@LC remained 60% after 50 days of incubation at room temperature. The relative activity of Fe3O4@NaBC@GA@LC could still retain nearly 80% after five reuses. In the process of simulating the PAHs-contaminated site treatment experiment in Hangzhou Iron and steel plant, immobilized laccase exhibited efficient adsorption and degradation performances and even the removal rate of 5-ring PAHs reached more than 90% in 40 days, resulting in improving urease activity and dehydrogenase in the soil and promoted the growth of a PAH degrading bacteria (Massilia). Our results further explained the efficient degradation effects of Fe3O4@NaBC@GA@LC on PAHs, which make it a promising candidate for PAHs-contaminated soil remediation.