The era of low-permeability sites remediation and corresponding technologies: A review

Microbial adaptation and genetic modifications for enhanced remediation in low-permeability soils

Low-permeability soils, characterized by fine texture and high clay content, pose significant challenges to traditional soil remediation techniques due to limited hydraulic conductivity, restricted nutrient flow, and reduced oxygen availability. These unique properties enable low-permeability soils to function as natural barriers in environmental protection; however, they also trap contaminants, making traditional remediation efforts challenging. This review synthesizes current knowledge on microbial adaptation and genetic engineering approaches that enhance the effectiveness of bioremediation in such environments. Key microbial adaptations, including anaerobic metabolism, extracellular enzyme production, and stress response mechanisms, allow individual microbes to adapt in low-permeability soils. Additionally, community-level strategies like microhabitat creation, biofilm formation, and functional redundancy further support microbial resilience. Advancements in genetic engineering now enable the modification of microbial traits-such as soil adhesion, nutrient utilization, and stress tolerance-to enhance bioremediation efficacy. Synthetic biology techniques further allow for the design of tailored microbial consortia that work cooperatively to degrade contaminants in complex soil matrices. This review highlights the integration of microbial and genetic engineering strategies, offering a comprehensive overview that informs current practices and guides future research in low-permeability soil remediation.

Insights into the Relationship between Temperature Variation and NAPL Removal during In Situ Thermal Remediation of Soil in the Presence of NAPL-Water Co-boiling: A Two-Dimensional Visualized Sandbox Study

Thermal remediation effectively treats sites contaminated with nonaqueous phase liquids (NAPL) by heating soil. A key process is the co-boiling at the water-NAPL interface, which lowers the boiling point due to combined vapor pressures, potentially reducing energy needs. However, determining the optimal end time for heating is challenging due to the invisible nature of underground NAPL, often resulting in excessive energy use. The initial NAPL pool size and distance from the heat source influence the spatiotemporal evolution of the NAPL-water interface, defining three zones: the co-boiling equilibrium zone, a nonequilibrium zone, and an unaffected zone. The temperature data collected by fixed temperature sensors can reflect the spatiotemporal evolution of these zones, offering valuable insights into NAPL removal. This study tackles these challenges using a two-dimensional visualized sandbox integrated with real-time image processing and an array of temperature sensors to monitor the NAPL removal and temperature variation. The results reveal semiquantitatively the impact of different initial NAPL amounts and spatial distributions on temperature variations. An optimized strategy is proposed for temperature sensor positioning, and a qualitative relationship is established between the temperature increase and NAPL removal. These findings can enhance our understanding of subsurface temperature dynamics, supporting more efficient, decarbonized remediation practices.

Evaluation of electrokinetic-assisted phytoremediation efficiency of dibutyl phthalate contaminated soil by maize (Zea mays L.) under different electric field intensities

The excessive accumulation of dibutyl phthalate (DBP) in soil poses a serious threat to soil ecosystems and crop safety production. Electrokinetic-assisted phytoremediation (EKPR) has been considered as a potential technology for remediating organic contaminated soils. In order to investigate the effect of different electric fields on removal efficiency of DBP, three kinds of electric fields were set up in this study (1 V·cm-1, 2 V·cm-1 and 3 V·cm-1). The results showed that 59 % of DBP in soil was removed by maize (Zea mays L.) within 20 d in low-intensity electric field (1 V·cm-1), and the accumulation of DBP in maize tissues decreased significantly compared to the non-electrified treatment group. Interestingly, it could be observed that the low-intensity electric field could maintain ion homeostasis and improve the photosynthetic efficiency of the plant, thereby relieving the inhibition of DBP on plant growth and increasing the chlorophyll content (94.1 %) of maize. However, the removal efficiency of DBP by maize decreased significantly under the medium-intensity (2 V·cm-1) and high-intensity electric field (3 V·cm-1). Moreover, the important roles of soil enzyme and rhizosphere bacterial community in low-electric field were also investigated and discussed. This study provided a new perspective for exploring the mechanism of removing DBP through EKPR.

Experimental Study on the Temperature-Dependent Static, Dynamic, and Post-Dynamic Mechanical Characteristics of Municipal Solid Waste

Municipal solid waste (MSW) has huge potential to be recycled as construction material, which would have significant benefits for environmental conservation. However, the cornerstone of this undertaking is a solid comprehension of the mechanical response of MSW in real-world engineering locations, taking into account the effects of stress levels and temperature. In this paper, well-mixed MSW samples were sieved and crushed to produce standardized specimens in cylindrical molds. A series of static, dynamic, and post-cyclic shear tests were conducted on the MSW at temperatures ranging from 5 °C to 80 °C with normal stresses of 50 kPa, 100 kPa, and 150 kPa. The experimental findings demonstrate that the static, dynamic, and post-cyclic mechanical response of MSW presents temperature range-dependency; temperature variation between 5 °C and 20 °C affects MSW's mechanical reaction more than variation in temperature between 40 °C and 80 °C under various stress settings; at 5 °C~80 °C, the static peak shear strength of MSW is the highest, being followed by the post-cyclic peak shear strength, while the dynamic peak shear strength is the lowest; the sensitivity of the dynamic shear strength of MSW to temperature variation is the largest, being followed by the post-cyclic peak shear strength, and the static peak shear strength is the lowest.

Effects of fluid composition in fluid injection experiments in porous media

Experiments on fluid flow in porous media, using fluids loaded with solids of various grain sizes, have been conducted in a modified Hele-Shaw setup. This setup utilised weakly cemented porous media with specific hydraulic and mechanical properties. Fluid injection in coarse granular media with clean or low-concentration fine particles, results in infiltration only, with pressure close to the material tensile strength, while injection in finer granular material causes damage alongside infiltration, with the fluid pressure still close to the material tensile strength. When larger particle sizes or higher particle concentrations are used in the mixture, the fluid travels further within the porous medium, primarily influenced by the grain size of the granular medium. In the latter case, the Darcy flow equation with an effective permeability term can be employed to determine the pressure differential. For the largest particle sizes included in the fluid, the equation is still applicable, but the effective permeability requires adjustment for particle size within the fluid rather than the granular medium. This is crucial when the injection point is locally clogged. The experiments show that fracturing conditions are controlled by different mechanisms. Dimensional and statistical analysis was used to classify the injection pressures to regimes predicted by fracturing theory or by Darcy law with modified effective permeabilities. The findings show that both the material properties and fluid composition are important designing parameters.

Spatiotemporal dynamic temperature variation dominated by ion behaviors during groundwater remediation using direct current

Direct current (DC) electric field has shown promising performance in contaminated site remediation, in which the Joule heating effect plays an important role but has been previously underappreciated. This study focuses on the spatiotemporal characteristics and mechanism of temperature change in heterogeneous porous media with applied DC. The heating process can be divided into four phases: preferential heating of the low permeability zone (LPZ), rapid heating in the middle region, temperature drop and hot zone shift, and reheating. The dynamic ion behaviors with complex interplays among reactions, electrokinetic-driven migration, and mixed convection induced an uneven redistribution of ions and dominated the heating rate and temperature distribution. The concentration of major ions near the pH jump decreased to 1% of the initial value, even though ions were continuously pumped into the heating zone. This ion depletion caused a drop in current, heating rate, and temperature. Here ions cannot be delivered rapidly into the ion-depleted zone by electromigration due to the potential flattening in the surrounding region. The presence of LPZ intensified the nonuniformity of ion redistribution, where a regional focusing of water-soluble ions was observed, and weakened the temperature rebound compared with that using homogeneous sand. These results provide a new perspective on the regulation of DC heating in site remediation.

Mechanisms and extended kinetic model of thermal desorption in organic-contaminated soil

Thermal desorption is a preferred technology for site remediation due to its various advantages. To ensure the effective removal of different pollutants in practical applications, it is necessary to understand the kinetic behaviors and removal mechanisms of pollutants in thermal desorption process. This paper explored the thermal desorption processes of five organic pollutants (nitrobenzene, naphthalene, n-dodecane, 1-nitronaphthalene, and phenanthrene) at 50-350 °C in two different subsoils with 6-18% moisture content. The results suggested that the thermal desorption process was well-fitted by the exponential decay model (R2 = 0.972-0.999) and could be divided into two distinct stages. The first stage was relatively fast and highly influenced by soil moisture, while the second stage showed a slower desorption rate due to the constraints imposed by the soil texture and structure. The influence of soil moisture on thermal desorption depended on the octanol/water partition coefficient (KOW) of pollutants. Pollutants with log KOW values lower than the critical value exhibited enhanced thermal desorption, while those with log KOW values higher than the critical value were inhibited. The critical value of log KOW might be between 3.33 and 4.46. Changes in soil texture and structure caused by heating promoted thermal desorption, especially for naphthalene, 1-nitronaphthalene and phenanthrene. The differences in texture and structure between the two soils diminished as the temperature increased. Finally, an extended kinetic model under changing temperature conditions was derived, and the simulation results for the two subsoils were very close to the actual thermogravimetric results, with the differences ranging from -1.28% to 0.94% and from -0.67% to 1.35%, respectively. These findings propose new insights into the influencing mechanisms of soil moisture and structure on the thermal desorption of organic pollutants. The extended kinetic model can provide reference for future kinetic research and guide practical site remediation.

Groundwater chlorinated solvent plumes remediation from the past to the future: a scientometric and visualization analysis

Contamination of groundwater with chlorinated hydrocarbons has serious adverse effects on human health. As research efforts in this area have expanded, a large body of literature has accumulated. However, traditional review writing suffers from limitations regarding efficiency, quantity, and timeliness, making it difficult to achieve a comprehensive and up-to-date understanding of developments in the field. There is a critical need for new tools to address emerging research challenges. This study evaluated 1619 publications related to this field using VOSviewer and CiteSpace visual tools. An extensive quantitative analysis and global overview of current research hotspots, as well as potential future research directions, were performed by reviewing publications from 2000 to 2022. Over the last 22 years, the USA has produced the most articles, making it the central country in the international collaboration network, with active cooperation with the other 7 most productive countries. Additionally, institutions have played a positive role in promoting the publication of science and technology research. In analyzing the distribution of institutions, it was found that the University of Waterloo conducted the majority of research in this field. This paper also identified the most productive journals, Environmental Science & Technology and Applied and Environmental Microbiology, which published 11,988 and 3253 scientific articles over the past 22 years, respectively. The main technologies are bioremediation and chemical reduction, which have garnered growing attention in academic publishing. Our findings offer a useful resource and a worldwide perspective for scientists engaged in this field, highlighting both the challenges and the possibilities associated with addressing groundwater chlorinated solvent plumes remediation.

Remediation of cadmium-contaminated soil: GLDA-assisted extraction and sequential FeCl3-CaO-based post-stabilization

Cadmium (Cd) contamination of farmland soils is a growing concern because of its highly toxic impact on ecosystems and human health. Chelator-assisted washing and chemical immobilization are effective remediation strategies for Cd-contaminated soils. Ethylenediaminetetraacetic acid (EDTA) has traditionally been used for soil washing, but its persistence in the environment and subsequent toxicity have raised significant ecological concerns. Consequently, biodegradable chelators have gained increasing attention as eco-friendly alternatives to the persistent chelator, EDTA. Therefore, this study evaluated the performance and efficacy of three biodegradable chelators: L-glutamate-N,N'-diacetic acid (GLDA), methylglycine-diacetic acid (MGDA), and 3-hydroxy-2,2'-iminodisuccinic acid (HIDS) in comparison to EDTA for remediating a real Cd-contaminated agricultural soil. The influence of treatment parameters, including chelator variants, washing time, chelator concentration, solution pH, and liquid-to-soil ratio (L/S) on Cd extraction was studied and optimized to attain the maximum removal rate. Following chelator-assisted washing, the efficacy of a stabilization preference combining FeCl3 and CaO in reducing the leaching potential of residual Cd in chelator-washed soil residues was also investigated. GLDA demonstrated comparable Cd extraction efficiency to EDTA, and the Cd extraction efficiency was found to be positively correlated with the soil washing parameters. However, under the optimized conditions (chelator concentration: 10 mmol L-1; washing time: 3 h; solution pH: 3; L/S ratio: 10:1), GLDA exhibited a higher Cd extraction rate than EDTA or the other chelators. Furthermore, a post-treatment process incorporating FeCl3 and CaO substantially diminished the water-leachable Cd content in the resultant soil residues. The proposed remediation strategy, which combines chemically assisted washing and stabilization, could be a practical option for extracting bulk Cd from soil and reducing the leaching potential of residual Cd.

Controlled-Release Materials for Remediation of Trichloroethylene Contamination in Groundwater

Groundwater contamination by trichloroethylene (TCE) presents a pressing environmental challenge with far-reaching consequences. Traditional remediation methods have shown limitations in effectively addressing TCE contamination. This study reviews the limitations of conventional remediation techniques and investigates the application of oxidant-based controlled-release materials, including encapsulated, loaded, and gel-based potassium permanganate since the year 2000. Additionally, it examines reductant controlled-release materials and electron donor-release materials such as tetrabutyl orthosilicate (TBOS) and polyhydroxybutyrate (PHB). The findings suggest that controlled-release materials offer a promising avenue for enhancing TCE degradation and promoting groundwater restoration. This study concludes by highlighting the future research directions and the potential of controlled-release materials in addressing TCE contamination challenges.

Air sparging remediation of VOCs contaminated low-permeability soil based on pressure gradient control

Air sparging (AS) is deemed unacceptable for remediating VOCs contaminated soil with low-permeability. To improve air flow and contaminant removal in sparging process, an original approach, termed as pressure gradient-enhanced air sparging (PGEAS) approach, is proposed by controlling pressure gradient in soil. Then the remediation efficiency, mass transfer characteristics, and remediation mechanism are investigated. Results showed that, the PGEAS approach accelerates gaseous contaminant exhaust, reduces residue contamination in soil, and promotes total contaminant removal, finally results in an improved remediation efficiency compared to the conventional approach. Controlled by sparging pressure and flow distance, the pressure gradient is created in soil, and a critical value needs to be exceeded to enhance the VOCs removal and mass transfer characteristics. The measured results of pore pressure and liquid saturation confirm a notable pressure gradient and drainage behavior in soil, which indicate the massive air subchannel formation during air sparging. At a two-dimensional scale, discrete distributions of contaminant concentrations in exhaust air and soil are presented, the removal extent and area are both enhanced using the PGEAS approach with a pressure gradient higher than the critical value. The reached conclusions are of great importance to contaminant removal in heterogeneous stratigraphy at sites.