Predictive models and airflow distribution associated with the zone of influence (ZOI) during air sparging remediation

Enhanced air sparging for groundwater remediation using alginate gel-based removable hydraulic barriers

The objective of this study was to investigate the effect of a removable physical barrier on the air sparging performance using a lab-scale aquifer model was investigated. The barrier was installed in water-saturated porous media, prior to the air sparging, by injecting calcium chloride aqueous solution into the aquifer with pre-applied alginate solution. Changes in the air flow direction and air flux at the media surface during air sparging were evaluated. With a hydrogel barrier set at the center of the media, the airflow detoured the barrier resulting in a bimodal air flux distribution at the media surface. While employing two gel-formed barriers positioned away from the media's center, the airflow concentrated specifically on the gap between the barriers. The hydrogel was successfully removed using a sodium bicarbonate solution (1.0 mol/L). Using the hydrogel barrier, the performance of air sparging was significantly enhanced for removing contaminants [tetrachloroethene (PCE) and n-hexane mixture] due to increased air flux; 9.8% of PCE applied (7.8 g) was removed during 120 min air sprging for the gel barrier system whereas no PCE was removed for the control. Alginate gel did not show significant sorption capacity for PCE. It was stable in the contaminant up to 68 days with reasonable loss of its mass. Findings of this study present a promising option for air sparging process specifically targeting the contaminant source zone in the aquifer.

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.

Zone of flow: A new finding on the characteristics of airflow within the zone of influence during air sparging in aquifers

Air sparging (AS) is a popular technology for the in-situ remediation of groundwater contaminated by volatile organic compounds. The scope of the zone within which injected air exists, i.e., zone of influence (ZOI) and the airflow characteristics within ZOI are of great interest. However, few studies have shed light on the scope of the zone within which air flows, namely, the zone of flow (ZOF) and its relation with the scope of ZOI. This study focuses on the ZOF characteristics and its relation with ZOI based on quantitative observations of ZOF and ZOI using a quasi-2D transparent flow chamber. The relative transmission intensity obtained by the light transmission method presents a rapid and continuous increasing near the ZOI boundary, providing a criterion for the quantitative determination of ZOI. An integral airflow flux approach is proposed to determine the scope of ZOF based on the airflow flux distributions through aquifers. The ZOF radius decreases with the growth of particle sizes of aquifers; while it increases first and then keeps constant with the increase of sparging pressure. The ZOF radius is around 0.55- 0.82 times of the ZOI radius, which depends on air flow patterns related to particle diameters d_p, that is, 0.55- 0.62 for channel flow (d_p < 1- 2 mm), while 0.75- 0.82 for bubble flow (d_p > 2- 3 mm). The experimental results show that the sparged air is entrapped with little flowing inside ZOI regions that are outside the ZOF, which should be considered carefully in the design of AS.

Mass transfer enhancement of air sparging on VOCs contaminated low-permeability soil by establishing pressure gradient

As one of the most effective methods for remediating VOCs contaminated site, air sparging technology is not suitable to low-permeability soil due to the poor remediation efficiency. To solve this problem, an improved approach aiming for mass transfer enhancement by establishing pressure gradient in soil is proposed in this study, and the remediation efficiency, removal mechanism, as well as the mass transfer characteristic are comprehensively investigated. Test results showed that, using the proposed approach significantly reduced the time for exhaust air contaminants reaching concentration equilibrium, and improved the contaminant removal zone and extent in soil, which were especially strengthened at sparging pressures higher than 40 kPa. The total contaminant removal rate was improved by introducing the proposed approach, with a maximum improved removal rate of 23.7% at 100 kPa sparging pressure. In mechanism analysis, the recorded changes in total pore pressure and average liquid saturation illustrated the pressure drop and discrete drainage phenomena, confirming the pressure gradient and air sub-channels formed in low-permeability soil. Finally, contaminant mass transfer characteristic was quantitatively analyzed using the lumped parameter model, in which the mass transfer coefficient and the air channel influencing fraction were enhanced almost fourfold and fivefold respectively by introducing the proposed approach. Compared to the conventional approach, the improved remediation efficiency using the proposed approach tackled the in-situ remediation challenge on low-permeability soil, and further expanded the application scope of air sparging technology on VOC contaminated site.

Remediation characteristics of surfactant-enhanced air sparging (SEAS) technology on volatile organic compounds contaminated soil with low permeability

Surfactant-enhanced air sparging (SEAS) is an effective technology for the remediation of volatile organic compounds contamination of medium and high-permeability soil, though applying SEAS to low-permeability soil contamination has rarely been explored. In this study, a series of two-dimensional physical model tests were designed to explore the feasibility and remediation characteristics of SEAS on low-permeability soil. In the test results, the incorporation and increase in surfactant concentration promoted air channel formation in the low-permeability soil, finally reduced the capillary breakthrough pressure and improved the airflow rate. The majority of the exhausted gaseous contaminants were distributed along the horizontal direction, differing from the results observed in medium and high-permeability soils. The exhausted gaseous contaminant concentration changed slightly when the sparging pressure and surfactant concentration increased at relatively low levels and increased as the sparging pressure and surfactant concentration increased further. Increasing the air sparging pressure without surfactant incorporation or with a low surfactant concentration cannot effectively remove the contaminant, while the removal efficiency can be enhanced with further increases in surfactant concentration. The discrete remediation characteristics had been confirmed during SEAS application on low-permeability soil, then the relationships between the ratios of remediation area and remediation extent under different surfactant concentrations and sparging pressures were established for remediation efficiency evaluation. Using this method, the discrete remediation characteristics can be recreated once the surfactant concentration and the sparging pressure were chosen. On the other side, targeted improvements in the remediation area or extent can be achieved by controlling the surfactant concentration and sparging pressure. Through this study, SEAS technology and the proposed evaluation method were successfully implemented in soil with hydraulic conductivity around 9E-7 m/s, which expanded the application scope of SEAS technology for contaminant removal.

Effects of airflow rate distribution and nitrobenzene removal in an aquifer with a low-permeability lens during surfactant-enhanced air sparging

The application of surfactant-enhanced air sparging (SEAS) in heterogeneous aquifers has received increasing attention. In this study, a two-dimensional laboratory visualization device was used to study the migration and distribution mechanism of airflow and the nitrobenzene removal effect in an aquifer with a low-permeability lens during AS and SEAS. Experimental results showed that the surfactant significantly reduced the blocking effect of the geological interface on airflow, and the ΔP_e (the air entry pressure difference between the background media and the lens) value of the geological interface decreased from 1.1 kPa to 0.3 kPa when the surfactant concentration was 800 mg/L. When the surfactant injection location was at the center of the lens and the injection volume was 1 PV (pore volume of the lens), part of the airflow entered the lens through its below interface, which clearly improved the nitrobenzene removal inside and above the lens compared with AS remediation. However, when SEAS remediation was 24 h, the surfactant redistribution caused by air sparging resulted in the airflow entering the lens to bypass the lens again, which changed the spatial distribution of airflow rate and was not conducive to the continuous removal of nitrobenzene inside the lens.

Effects of different permeable lenses on nitrobenzene transport during air sparging remediation in heterogeneous porous media

Air sparging (AS) is considered an effective remediation technology for groundwater contaminated by volatile organic compounds. However, the effects of AS remediation of heterogeneous aquifers with lenses of different permeability are still unclear, which limits the application of AS technology. In this study, the effects of different permeable lenses on nitrobenzene (NB) transport were quantitatively analysed by tracking the temporal and spatial evolutions of the NB concentration and using light transmission visualisation technology to observe airflow. Experimental results showed that the NB outside the airflow zone of the heterogeneous aquifer containing a gravel lens was rapidly removed, which is a special phenomenon. Through moisture content monitoring and colour tracer technology, the bubble-induced water circulation zone in a gravel lens was discovered during AS. At this time, the zone of influence (ZOI) included air flow zone and water circulation zone, while previous studies believed that the ZOI only contained the air flow zone. The presence of water circulation zone in the heterogeneous aquifer with a gravel lens increased the ZOI area and average contaminant removal flux by 5 and 2.3 times, respectively, compared with those in homogeneous aquifer. These findings have modified the conventional cognition about the ZOI and are conducive to an in-depth understanding of the remediation mechanisms and a better design of AS technology in heterogeneous aquifers with different permeable lenses.

Effectiveness of Oxygen-Saturated Seawater Injections and Air Sparging Technologies in Remediation of Coastal Marine Sediments from Sludge

The occurrence of hypoxic muddy sediments on shallow beaches and other sheltered areas is a well-known environmental problem, which negatively affects coastal areas, tourism potential, the public use of beaches and sediment biodiversity. The usual solution is limited to dredging and removal of sludge to a landfill site. In this study, a laboratory-scale experiment was performed to determine the effectiveness of two technologies: a modification of air sparging and a new approach based on injecting oxygen-saturated seawater in hypoxic muddy sediments (oxygen-saturated seawater injections method), for remediating sludge in coastal sediments, minimizing environmental impact respect to dredging. Our results showed that both technologies significantly increased dissolved oxygen content in pore water, facilitating the oxidation of more than 90% of the organic matter, and other reduced inorganic compounds such as sulphide, with the consequent increase in sulphate concentration from 0.3 to 3.0 g·L^-1. Moreover, a rise of redox potential from - 258 mV to above 200 mV, and a dramatic drop in chemical oxygen demand were also indicators that oxic conditions had been restored. After 65 days, soft, black, muddy and hypoxic sediment with high organic matter content and a characteristic foul odour was transformed into well-oxygenated sediment, which had a low organic matter content and had lost its initial shiny black colour and odour. The main difference between both technologies was the depth influenced by sediment remediation; oxygen-saturated seawater injections affected deeper areas than clean pressurized air injections.

Effects of surfactant injection position on the airflow pattern and contaminant removal efficiency of surfactant-enhanced air sparging

Surfactant-enhanced air sparging (SEAS) is an effective remediation technique for VOCs-contaminated soil. In this study, three types of tests are performed to investigate the effects of the surfactant injection position on the airflow pattern, contaminant removal efficiency, and airflow path control. The three tests are conventional air sparging (CAS), entire SEAS (ESEAS), where the surfactant is incorporated into the entire contaminated soil, and local SEAS (LSEAS), where the surfactant is injected locally at different positions. With increasing distance between the injection position and the central axis, the LSEAS test results approach the results measured in the CAS test. When the surfactant is injected directly at the central axis, a high contaminant removal rate of 89% is obtained, which is even higher than that obtained for the ESEAS test. As the injection position moves away from the central axis, the removal rate decreases. Furthermore, when the injection position is close to the sparging point, the surfactant can successfully control the airflow path. Based on the test results, a critical distance between the surfactant injection position and sparging point exists where high remediation efficiency can be achieved. This optimal surfactant injection position is specific to each contamination site.

Effects of air flowrate distribution and benzene removal in heterogeneous porous media during air sparging remediation

The decrease of remediation effect during air sparging (AS) remediation in heterogeneous porous media has attracted increasing attention. In this study, an improved light transmission visualization method was used to investigate the air accumulation, migration and flowrate distribution in benzene-contaminated heterogeneous porous media during AS. Experimental results indicated that the benzene removal rate in the porous media was mainly controlled by air flowrate distribution which could be used as a major factor to evaluate the remediation effect. Visualization of air migration showed that air accumulation occurred below the geologic heterogeneous interface when ΔPe > 0 kPa (ΔPe: the air entry pressure difference of the media above and below the interface), and the accumulation thickness and length presented exponential decay increases with increasing ΔPe and air injection rates. Air flowrate was monitored by gas flow sensors, and the flowrate distributions were found as Gaussian distribution when ΔPe ≤ 0 kPa, trapezoidal distribution when 0 <ΔPe< 0.3 kPa and fingered distribution when ΔPe ≥ 0.3 kPa. Fingered distribution of air flowrate resulted in extremely nonuniform benzene removal above the interface and reduced the overall benzene removal rate. These findings reveal the reasons for the poor performance of AS remediation in heterogeneous porous media, leading to a better understanding of the remediation mechanisms in heterogeneous aquifer.

Experimental Study of Artificial Ground Freezing by Natural Cold Gas Injection

The application of conventional artificial ground freezing (AGF) has two disadvantages: low freezing rate and small frozen range. In this study, a new method with natural cold gas injection was proposed, whereby the shallow soils and water can be frozen rapidly due to the effect of the heat convection. Cold gas from −15 °C to −10 °C, in the winter of northeast China, was injected into the laboratory-scale sand pipe; evolution of the induced frozen front and water migration were studied, and then, the feasibility of the new method was analyzed. According to the evolution of the induced frozen front, the freezing process was divided into an initial cooling stage, phase transition stage, and subcooled stage. The results showed that the increase of initial water content at the beginning of the experiments had little effect on the time required for completing the initial cooling stage, while the time required for the phase transition would increase in nearly the same proportion. In addition, the increase of the cold gas flow rate could not only strengthen the cooling rate of the initial cooling stage but also shorten the phase transition time; thereby, the freezing rate was increased. The freezing rate could reach 0.18–0.61 cm/min in the direction of cold gas flow, and compared to the conventional AGF (months are required for approximately 1 m), the freezing efficiency was greatly improved.

Mechanism study of the air migration and flowrate distribution in an aquifer with lenses of different permeabilities during air sparging remediation

The poor performance of air sparging (AS) remediation in heterogeneous porous media is receiving increasing attention. However, understanding of the air migration and flowrate distribution mechanisms in heterogeneous aquifers is still lacking. In this study, for experimental purposes, a heterogeneous aquifer with lenses of different permeabilities was designed in the laboratory. The effects of the double interface between a lens and the background media on the air migration were visually observed for the first time, and four types of double interfaces and their related air flowrate distributions were identified. These were bimodal distribution (∆P_e ≤ -1.1 kPa, i.e., the air entry suction difference between the background media and the lens), fingered distribution for a low-permeability lens (-1.1 <∆P_e ≤ -0.3 kPa), Gaussian distribution (-0.3 <∆P_e < 0.4 kPa), and fingered distribution for a high-permeability lens (∆P_e ≥ 0.4 kPa). The experimental results indicated that double interface characteristics and air injection rates affected air accumulation behavior. A mathematical model was established to simulate the experimental data of the air flowrate distribution, and it could well describe the air flowrate distribution patterns in heterogeneous aquifers. These findings are significant for improving our understanding of the mechanisms of air migration and flowrate distribution in heterogeneous aquifers, leading to a better design and prediction of the AS remediation required for heterogeneous aquifer pollution.

A mechanism study of airflow rate distribution within the zone of influence during air sparging remediation

In this study, an improved laboratory two-dimensional airflow visualization device was developed for the quantitative analysis of airflow distribution at different heights from the sparger (20, 30, and 40cm) within the zone of influence (ZOI). The results indicated that the measured airflow rate distribution appeared Trapezium when the height was 20cm; however, the airflow rate matched a Gaussian distribution when the heights became 30cm and 40cm. The conical shape of the ZOI was observed in the experimental processes. The experimental results verified that the airflow distribution within the ZOI conformed to turbulent jet theory. According to turbulent jet theory, the distribution of the airflow rate changes from Trapezium to Gaussian, and the jet boundary mixed layer is a linear extension in the processes of jets. Through our study, it was found that this theory could be applied to airflow distribution and predictive models for the ZOI in air sparging remediation. The shape of the ZOI should be cone-like and the boundary layer of the ZOI is a linear extension in air sparging process. All the results from this study can provide theoretical support for the design and prediction of air sparging remediation for groundwater pollution.

A comprehensive guide of remediation technologies for oil contaminated soil - Present works and future directions

Oil spills result in negative impacts on the environment, economy and society. Due to tidal and waves actions, the oil spillage affects the shorelines by adhering to the soil, making it difficult for immediate cleaning of the soil. As shoreline clean-up is the most costly component of a response operation, there is a need for effective oil remediation technologies. This paper provides a review on the remediation technologies for soil contaminated with various types of oil, including diesel, crude oil, petroleum, lubricating oil, bitumen and bunker oil. The methods discussed include solvent extraction, bioremediation, phytoremediation, chemical oxidation, electrokinetic remediation, thermal technologies, ultrasonication, flotation and integrated remediation technologies. Each of these technologies was discussed, and associated with their advantages, disadvantages, advancements and future work in detail. Nonetheless, it is important to note that no single remediation technology is considered the best solution for the remediation of oil contaminated soil.

CAPSULE

This review provides a comprehensive literature on the various remediation technologies studied in the removal of different oil types from soil.