A study of layered-unlayered extraction of benzene in soil by SVE

Immobilized enzyme with sustainable chestnut biochar to remediate polycyclic aromatic hydrocarbons contaminated soils

Polycyclic aromatic hydrocarbons (PAHs) contaminated soil severely and are difficult to remediate. In this study, acid-modified chestnut inner shell biochar with abundant pore channels was used as the main raw materials for the immobilization of white-rot fungal crude enzyme. The maximum immobilization rate of crude enzymes (97.25%±6.20%) could be achieved under the optimal conditions of 24 h immobilization of 10 U/mL crude enzymes by 1 g biochar at 25℃ and pH = 5. Meanwhile, immobilization improved the stability of the crude enzyme. The relative activity of the immobilized crude enzyme increased by 59.32% and 49.73% (compared to the free crude enzymes) after 5 weeks of storage at 4°C and 25°C, respectively. It has been verified that chestnut-based immobilized crude enzyme can degrade 37% of benzo[a]pyrene in 10 days for PAHs-contaminated soils. An efficient, feasible, and low-cost remediation method for PAHs-contaminated soils was explored, which provides technical support for the application of crude enzymes in organic contaminated soils.

New insights in the biodegradation of high-cyclic polycyclic aromatic hydrocarbons with crude enzymes of Trametes versicolor

High-cyclic polycyclic aromatic hydrocarbons (PAHs), with complex fused aromatic structures, are widespread, refractory and harmful in soil, but the current remediation technologies for high-cyclic PAHs are often inefficient and costly. This study focused on the biodegradation process of high-cyclic benzo[a]pyrene by Trametes versicolor crude enzymes. The crude enzymes exhibited high laccase activity (22112 U/L) and benzo[a]pyrene degradation efficiency (42.21%) within a short reaction time. Through the actual degradation and degradation kinetics, the degradation efficiency of PAHs decreased with the increase of aromatic rings. And the degradation conditions (temperature, pH, Cu2+ concentration, mediator) were systematically optimised. The optimum degradation conditions (1.5 mM Cu2+, 28℃ and pH 6) showed significant degradation efficiency for the low and medium concentrations of benzo[a]pyrene. In addition, complete degradation of benzo[a]pyrene could be achieved using only 0.2 mM of HBT mediator compared with crude enzymes alone. Collectively, these results showed the high-cyclic PAHs degradation potential of Trametes versicolor crude enzymes, and provided references to evaluate applicable prospects of white rot fungus crude enzymes in PAHs-contaminated soils.

Predicting soil concentrations and remediation target values of BTEX by an off-gas based mass transfer model

Soil vapor extraction (SVE) is a widely used technology for the remediation of volatile organic compounds (VOCs) contaminated soils. Residual concentrations of VOCs are crucial for assessing the SVE process and planning when to stop this process, however, the measurement of their residual concentrations in the soil is complicated. Herein, a pseudo-first-order sequential reaction model was established to predict the mass transfer of the BTEX (benzene, toluene, ethylbenzene, xylene) between the soil and off-gas during the SVE process. Based on this mass transfer model, the residual concentrations of BTEX in the soil during the trailing stage could be accurately estimated (R2 > 0.89) by their off-gas concentrations that were directly monitored in real time. Considering the removal efficiency and operating costs, a concept of the remediation target values (RTV) was proposed for the SVE technology, and its relevant model (R2 > 0.92, NRMSE = 6.4-16.8 %) was established based on the experimental data. The remediation endpoint can be further estimated based on the RTV with an overall accuracy of 84-100 %. These findings provide a simple and fast way to predict VOC concentrations in soil with easy-to-know factors and online monitoring of off-gas concentrations and will guide and optimize the SVE process toward more economical and efficient techniques for soil remediation.

An Analysis of the Spatial Characteristics and Transport Fluxes of BTEX in Soil and Atmospheric Phases at a Decommissioned Steel Mill Site in China with a Long History

BTEX (benzene, toluene, ethylbenzene, and xylene), as characteristic pollutants in chemical plant sites, are widely present in the environment and pose a serious threat to the health and safety of nearby residents. Studying the spatial distribution characteristics and transport fluxes of BTEX in soil and air at contaminated sites and the health risks they pose to humans is of great significance for fine pollution control and environmental management. This study took a typical decommissioned steel plant as a case study. A total of 23 soil and air samples were collected from different locations to investigate the spatial distribution characteristics of BTEX in soil and air. The transport and fate of BTEX in soil and air were evaluated using the fugacity model, and finally, a human health risk assessment was conducted. The results indicate a relatively severe level of benzene pollution in both soil and air. The maximum exceedance factor of benzene in soil samples is 31.5, with the concentration exceedance depth at 1.5 m. The maximum concentration of benzene in air samples is 4.98 μg·m-3. Benzene, at 5.9% of the site, shows a low flux with negative values, while other components at various locations all exhibit a trend of transport from the soil phase to the atmospheric phase. Benzene is the pollutant that contributes the most to the transport flux from soil to air within the site. The coking area and sewage treatment area are key areas within the steel mill where BTEX accumulate easily in the soil. The non-carcinogenic risk values of the individual components of BTEX in the soil are below the acceptable risk level. However, the carcinogenic risk value of benzene in the children's exposure scenario exceeds the carcinogenic risk level of 10-6. The carcinogenic risk range of various components of BTEX in the air is 2.63 × 10-6~3.88 × 10-5, with 28.6% of the locations exceeding the threshold of 10-6. The range of the total HI (hazard index) is 2.08 × 10-4~1.81 × 10-1, all of which is below the safety threshold of 1. The results of this study will provide scientific support for the fine pollution control and environmental management of industrial contaminated sites with BTEX as their typical pollutants.

A comparative study of DNAPL migration and transformation in confined and unconfined groundwater systems

To explore the migration and transformation process of dense non-aqueous liquid (DNAPL) pollutants' multiphase flow, specifically nitrobenzene (NB), in confined groundwater (CG) versus unconfined groundwater (UG), a two-dimensional sandbox experimental device was designed and constructed. This involved constructing a vadose zone-UG- aquitard-CG structure, which was then subjected to different scenarios. Real-time analysis and numerical simulation methods were established and employed, with a particular focus on the detailed investigation results of actual contaminated site. The study found that when the same amount of NB was injected, the special structure of the CG layer resulted in a more pronounced reverse diffusion of NB in both the dissolved and NAPL phases. This was especially true for the dissolved phase, which was more likely to diffuse reversely. Meanwhile, CG did not directly interact with the vadose zone, and there was no loss of gas phase NB after the leakage in CG. As a result, higher concentrations of dissolved phase NB were generated, leading to the emergence of a larger area of NB contaminant plumes with CG flow. Importantly, the simulation study of the actual site and the laboratory experimental results were found to be validated, further validating the conclusion that direct leakage of NB into CG results in a higher concentration and larger area of dissolved phase contaminant plume, causing more serious pollution to the groundwater environment.

A numerical simulation of thermally-enhanced soil vapor extraction with a validation of an actual contaminated site

Thermally-enhanced soil vapor extraction (T-SVE) remediation technology is widely used in organic-contaminated sites due to its high efficiency, short remediation period and controllable secondary contamination. However, the remediation efficiency is affected by the complex site factors, which leads to the uncertainty of the remediation process and energy waste. Thus, it is necessary to optimize T-SVE systems to accurately remediate the sites. In this work, a pilot site of reagent factory in Tianjin was taken as the research object to validate the model, and the T-SVE process parameters of a VOCs-contaminated sites were predicted by this simulation method. The simulation results showed that the Nash efficiency coefficient E of the measured and simulated temperature rise data in the study area was 0.885, and the linear correlation coefficient R of the measured and simulated concentrations of cis-1,2-dichloroethylene after remediation was 0.877, indicating that this simulation method is highly reliable. Based on this numerical simulation method, some parameters of the T-SVE process at the VOCs-contaminated site of an insulation plant in Harbin were simulated and optimized. Included a heating well spacing of 3.0 m, extraction pressure of 40 Kpa, extraction well influence radius of 4.35 m, extraction flow rate of 2.97 × 10-4 m3/s, and a theoretical number of 25 extraction wells (adjusted to 29 wells in practice), and the corresponding extraction well layout has been designed. The results can provide a technical reference for the future application of T-SVE in the remediation of organic-contaminated sites.

Impact of simulating real microplastics on toluene removal from contaminated soil using thermally enhanced air injection

This paper investigated the impacts of various real microplastics (MPs), i.e., polyethylene (PE) and polyethylene terephthalate (PET) with different sizes (1000-2000 and 100-200 μm) and different dosages (0.5 and 5% on a dry weight basis), on the toluene removal during the thermally enhanced air injection treatment. First, microscopic tests were carried out to determine the MPs' microstructure and behavior. The PE was mainly a small block, and PET appeared filamentous and sheeted with a larger slenderness ratio. Second, the interactions between MPs and toluene-contaminated soils were revealed by batch adsorption equilibrium experiments and low-field magnetic resonance. The morphological differences and dosage of the MPs impacted soils' total porosity (variation range: 39.2-42.7%) and proportion of the main pores (2-200 μm). Third, the toluene removal during the air injection consisted of compaction, rapid growth, rapid reduction, and tailing stages, and the MPs were regarded as an emerging solid state to affect these removal stages. The final cumulative toluene concentrations of soil-PET mixtures were influenced by total porosity, and those of soil-PE mixtures were controlled by total porosity (influence weight: 0.67) and adsorption capacity (influence weight: 0.33); meanwhile, a self-built comprehensive coefficient of MPs can reflect the relationship between them and cumulative concentrations (correlation coefficient: 0.783).

An innovative method for soil vapor extraction to improve extraction and tail gas treatment efficiency

This study aims to improve soil vapor extraction (SVE) to address its shortcomings in treating halogenated hydrocarbon-contaminated soil. Indoor simulation experiments based on SVE were conducted to provide technical guidance for the remediation of 1,2-DCA-contaminated soil, with the overall intention of soil repair and ecological restoration. A thermal oxidation SVE (TOSVE) system was designed on the basis of SVE technology for application in the remediation of low-permeability soil contaminated with halogenated hydrocarbons from a chemical plant in Northeast China. Laboratory simulation experiments were conducted based on TOSVE technology to study the removal of target pollutants under different organic contents, moisture and air speeds. For the first time, a new material, scoria, was added to the oxidant at different proportions, and its effect on the exhaust gas treatment efficiency was examined. Thermal extraction improved the extraction efficiency of pollutants from low-permeability soil. Moreover, the adsorption–oxidation effect of 0.1–0.25 mm scoria prepared by 20% Na₂S₂O₈ on 1,2-dichloroethane (1,2-DCA) in tail gas was higher than that of the oxidant without scoria, indicating that scoria is effective in tail gas treatment.

Evaluation of the effectiveness of VOC-contaminated soil preparation based on AHP-CRITIC-TOPSIS model

Currently, several methods have been adopted for the laboratory preparation of artificial volatile organic compound (VOC) contaminated soils (VCSs). However, it remains unclear whether the prepared contaminated soils are homogenous. In this study, two representative VOCs, toluene and perchloroethylene, were separately mixed with a kaolin-based soil using six preparation methods. Thereafter, the homogeneity and recovery of the contaminated kaolin prepared using these methods were determined and analyzed. The six procedures were quantitatively assessed according to the comprehensive evaluation mathematical model (AHP-CRITIC-TOPSIS), and the final score order of the different procedures was: A > C > E > B > F > D. Additionally, the qualitative evaluation of the procedures was performed based on the phase transformation and mass transfer during the mixing processes. Based on these discussions, method A, which was considered to be optimal, was then adopted for further investigations with various natural soils. The results showed that this optimal method could be applied to natural soils and revealed that the adsorption-related characteristics of natural soils, including total organic carbon, specific surface area, pore volume, pH, plastic limit, particle size, and mineral composition, influenced the homogeneity and recovery through mass transfer. In addition, it was also observed that the chemical properties of VOCs, including molecular structure, vapor pressure, and the octanol/water partition coefficient, could also affect the effectiveness of sample recovery. Through this study, researchers can prepare VCSs with excellent homogeneity and low loss rates to conduct standardized tests for technology development.

Statistical Evaluation of Quantities Measured in the Detection of Soil Air Pollution of the Environmental Burden

The article highlights the investigation of the relationships between measured quantities during the atmospheric geochemical survey of contaminated soil and the environmental burden of the industrial establishment in eastern Slovakia. Statistical data processing was undertaken from the measured values of pollutants. The basic statistical characteristics of the monitored indicators were defined here. With the help of regressive and correlative analysis, dependency was confirmed between examined values, further expressed by a mathematical relationship. We analysed variability of the measured variables due to the influence of changed input quantities by the non-parametric Wilcox test. The statistical data processing helps us to identify the dependency between the measured values and improves valorization of the pollution of a given environmental burden. This was due to the handling of organic pollutants and the production of basic organic and inorganic chemicals stated for other industries. Chemical analysis of soil air helps us to determine the extent and amount of soil contamination by pollutants. Individual pollutants have their own characteristic properties and their negative effects on biota, the environment and humans are different.