Bioremediation of polycyclic aromatic hydrocarbons contaminated soil under the superimposed electric field condition

A comprehensive review of soil remediation contaminated by persistent organic pollutants using electrokinetic: Challenging enhancement techniques

The hydrophobic, hard-to-naturally-decompose compounds, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and pesticides, are categorized as persistent organic pollutants (POPs). POPs are toxic/hazardous and present serious risks to human health. Electrokinetic (EK) remediation is highly flexible and cost-effective, suitable for both in-situ and ex-situ applications. It effectively targets a wide range of contaminants, including metals and organic compounds, especially in low-permeability and low-hydraulic conductivity soils, where traditional methods are less effective. This technology is easy to install and can be combined with other strategies for enhanced remediation in complex soil environments. This paper underscores EK remediation as a promising method for addressing soil pollution caused by these organic pollutants, especially in low-permeability soil. The present review starts with the classification, toxicity effects, and source of POPs in the environment. Theoretical aspects and fundamentals of EK, including transport mechanisms and principles, are also reviewed. The theoretical underpinnings of effective factors are comprehensively explored, such as surface charge, zeta potential, pHpzc, and numerical modeling of transport fluxes. Moreover, a comprehensive examination is undertaken regarding the operation and design considerations of the EK process, encompassing factors like pH, electrode arrangement, electrolyte, and voltage. Subsequently, it is highlighted that EK has the potential to come in synergistically in contact with other remediation technologies to augment the POPs' degradation. Various enhancement techniques are also explored, including solvent extraction, chemical oxidation, bioremediation, and permeable reactive barriers to combine with EK. Each method is examined in terms of its advantages, limitations, recent developments, and ongoing research. Finally, the potential and challenges associated with enhanced EK methods combined with other techniques for the removal of POPs were reviewed.

Defense systems of soil microorganisms in response to compound contamination by arsenic and polycyclic aromatic hydrocarbons

Arsenic and PAHs impose environmental stress on soil microorganisms, yet their compound effects remain poorly understood. While soil microorganisms possess the ability to metabolize As and PAHs, the mechanisms of microbial response are not fully elucidated. In our study, we established two simulated soil systems using soil collected from Xixi Wetland Park grassland, Hangzhou, China. The As-600 Group was contaminated with 600 mg/kg sodium arsenite, while the As-600-PAHs-30 Group received both 600 mg/kg sodium arsenite and 30 mg/kg PAHs (phenanthrene:fluoranthene:benzo[a]pyrene = 1:1:1). These systems were operated continuously for 270 days, and microbial responses were assessed using high-throughput sequencing and metagenomic analysis. Our findings revealed that compound contamination significantly promoted the abundance of microbial defense-related genes, with general defense genes increasing by 11.07 % ∼ 74.23 % and specific defense genes increasing by 44.13 % ∼ 55.74 %. The dominate species Rhodococcus adopts these general and specific defense mechanisms to resist compound pollution stress and gain ecological niche advantages, making it a candidate strain for soil remediation. Our study contributes to the assessment of ecological damage caused by As and PAHs from a microbial perspective and provides valuable insights for soil remediation.

Effect of electric fields strength on soil factors and microorganisms during electro-bioremediation of benzo[a]pyrene-contaminated soil

Electro-bioremediation is a promising technology for remediating soils contaminated with polycyclic aromatic hydrocarbons (PAHs). However, the resulting electrokinetic effects and electrochemical reactions may inevitably cause changes in soil factors and microorganism, thereby reducing the remediation efficiency. To avoid negative effect of electric field on soil and microbes and maximize microbial degradability, it is necessary to select a suitable electric field. In this study, artificial benzo [a]pyrene (BaP)-contaminated soil was selected as the object of remediation. Changes in soil factors and microorganisms were investigated under the voltage of 1.0, 2.0, and 2.5 V cm-1 using chemical analysis, real-time PCR, and high-throughput sequencing. The results revealed noticeable changes in soil factors (pH, moisture, electrical conductivity [EC], and BaP concentration) and microbes (PAHs ring-hydroxylating dioxygenase [PAHs-RHDα] gene and bacterial community) after the application of electric field. The degree of change was related to the electric field strength, with a suitable strength being more conducive to BaP removal. At 70 d, the highest mean extent of BaP removal and PAHs-RHDα gene copies were observed in EK2.0 + BIO, reaching 3.37 and 109.62 times those in BIO, respectively, indicating that the voltage of 2.0 V cm-1 was the most suitable for soil microbial growth and metabolism. Changes in soil factors caused by electric fields can affect microbial activity and community composition. Redundancy analysis revealed that soil pH and moisture had the most significant effects on microbial community composition (P < 0.05). The purpose of this study was to determine the appropriate electric field that could be used for electro-bioremediation of PAH-contaminated soil by evaluating the effects of electric fields on soil factors and microbial communities. This study also provides a reference for efficiency enhancement and successful application of electro-bioremediation of soil contaminated with PAHs.

Thermal Remediation of Soil Contaminated with Polycyclic Aromatic Hydrocarbons: Pollutant Removal Process and Influence on Soil Functionality

Thermal remediation has been widely used for the removal of polycyclic aromatic hydrocarbon (PAH) from contaminated soil. The method has a high removal rate for semi-volatile organic pollutants; however, soil functionality is affected by the method because of the alteration of the soil properties. In this study, experimental soil was impregnated with phenanthrene (Phe), pyrene (Pyr), and benzo(a)pyrene (BaP); after natural air-dry aging, the thermal remediation experiment was carried out, using a tube-furnace and thermal gravimetry-Fourier transform infrared (TG-FTIR) equipment. More than 84% of the Phe and Pyr were lost in the aging stage, whereas the BaP was stable with 41% retention in the soil. After the thermal treatment, the desorption and decomposition of the pollutants and organic matter led to the removal of the PAHs; about 1% of the PAHs remained in the soil treated at 400 °C. The presence of the PAHs can promote the thermal reaction by slightly reducing the reaction activation energy by ~7-16%. The thermal remediation had a significant influence on the physical properties of the soil and destroyed the bioavailability by reducing the organic matter content. Therefore, a comprehensive consideration of effective PAH removal while preserving soil functionality may require a low temperature (100 °C) method for thermal remediation.

Impact of pyrene (polycyclic aromatic hydrocarbons) pollutant on metabolites and lipid induction in microalgae Chlorella sorokiniana (UUIND6) to produce renewable biodiesel

Pyrene (polycyclic aromatic hydrocarbon), an anthropogenic organic pollutant prevalent in various ecological units, receives more attention for bioremediation and energy transformation using microalgae. In this study, we have used pyrene pollutant (50-500 ppm) to evaluate the half-maximal inhibitory concentrations (IC₅₀) of Chlorella sorokiniana and the impact on metabolites as well as the induction of lipid biosynthesis to produce renewable biodiesel. Pyrene concentration at 230 ppm (IC₅₀) caused half-maximum inhibition for the 96 h incubation. The harvest in the stationary stage (day 16) for C. sorokiniana revealed a biomass generation of 449 ± 7 mg L^-1 and 444 ± 8 mg L^-1 dcw in the control medium and pyrene IC₅₀ medium, respectively. An insignificant decline in biomass generation (1.2%) was observed due to the stress effect of the pyrene IC₅₀ medium on metabolic biosynthesis. Although contrary to biomass generation, IC₅₀ of pyrene assisted to induce lipid biosynthesis in C. sorokiniana. The improvement in lipid biosynthesis was observed as ~24% higher in pyrene IC₅₀ compared to the control medium. The chemical composition of the microalgae biomass, metabolites, and lipids was examined using FTIR spectra. The extracted lipid was transesterified to produce biodiesel via methanolic-H₂SO₄ catalysis. The renewable biodiesel obtained was evaluated using FTIR and ¹H NMR spectra. The transformation efficiency of the lipid of C. sorokiniana in biodiesel was calculated as ~81%. This research offers the incentive in lipid biosynthesis in microalgae cells using pyrene for the production of renewable and sustainable ecological biofuels along with bioremediation of pyrene.

Trends in predictive biodegradation for sustainable mitigation of environmental pollutants: Recent progress and future outlook

The feasibility of in-silico techniques, together with the computational framework, has been applied to predictive bioremediation aiming to clean-up contaminants, toxicity evaluation, and possibilities for the degradation of complex recalcitrant compounds. Emerging contaminants from different industries have posed a significant hazard to the environment and public health. Given current bioremediation strategies, it is often a failure or inadequate for sustainable mitigation of hazardous pollutants. However, clear-cut vital information about biodegradation is quite incomplete from a conventional remediation techniques perspective. Lacking complete information on bio-transformed compounds leads to seeking alternative methods. Only scarce information about the transformed products and toxicity profile is available in the published literature. To fulfill this literature gap, various computational or in-silico technologies have emerged as alternating techniques, which are being recognized as in-silico approaches for bioremediation. Molecular docking, molecular dynamics simulation, and biodegradation pathways predictions are the vital part of predictive biodegradation, including the Quantitative Structure-Activity Relationship (QSAR), Quantitative structure-biodegradation relationship (QSBR) model system. Furthermore, machine learning (ML), artificial neural network (ANN), genetic algorithm (GA) based programs offer simultaneous biodegradation prediction along with toxicity and environmental fate prediction. Herein, we spotlight the feasibility of in-silico remediation approaches for various persistent, recalcitrant contaminants while traditional bioremediation fails to mitigate such pollutants. Such could be addressed by exploiting described model systems and algorithm-based programs. Furthermore, recent advances in QSAR modeling, algorithm, and dedicated biodegradation prediction system have been summarized with unique attributes.