Application of enhanced electrokinetic remediation by coupling surfactants for kerosene-contaminated soils: Effect of ionic and nonionic surfactants

Hybrid and enhanced electrokinetic system for soil remediation from heavy metals and organic matter

The electrokinetic (EK) process has been proposed for soil decontamination from heavy metals and organic matter. The advantages of the EK process include the low operating energy, suitability for fine-grained soil decontamination, and no need for excavation. During the last three decades, enhanced and hybrid EK systems were developed and tested for improving the efficiency of contaminants removal from soils. Chemically enhanced-EK processes exhibited excellent efficiency in removing contaminants by controlling the soil pH or the chemical reaction of contaminants. EK hybrid systems were tested to overcome environmental hurdles or technical drawbacks of decontamination technologies. Hybridization of the EK process with phytoremediation, bioremediation, or reactive filter media (RFM) improved the remediation process performance by capturing contaminants or facilitating biological agents' movement in the soil. Also, EK process coupling with solar energy was proposed to treat off-grid contaminated soils or reduce the EK energy requirements. This study reviews recent advancements in the enhancement and hybrid EK systems for soil remediation and the type of contaminants targeted by the process. The study also covered the impact of operating parameters, imperfect pollution separation, and differences in the physicochemical characteristics and microstructure of soil/sediment on the EK performance. Finally, a comparison between various remediation processes was presented to highlight the pros and cons of these technologies.

Highly alkaline electrokinetic extraction: Characteristics of chromium mobilization, conversion and transport in high alkalinity soil

In site chromium (Cr) contaminated soil characterized by high alkalinity and carbonate content, protons are not effectively targeted for Cr(III) mobilization but rather accelerate the reduction of easily transportable Cr(VI) within the acidification electrokinetic (EK) system. As an alternative, the highly alkaline extraction conditions (HAECs) maintained by anolyte regulation are explored owing to the ability to desorb strong binding Cr(VI) and form anionic Cr(III)-hydroxides (Cr(OH)4-, Cr(OH)52-). The results demonstrate that HAECs were more efficient in mobilizing ions in severe alkalinity and electrical conductivity soil compared to organic acid acidifying extraction conditions (OAECs). Simultaneously, a limited amount of soluble Cr(III) was produced; however, its transportation was hindered and more noticeable in the case of Cr(VI), displaying a distinct retention phase within the intermediate soil chamber. The antagonistic interplay between electromigration and electroosmotic flow was considered the main responsible factor. The conversion intensity of Cr(VI) to Cr(III) was inhibited at HAECs. The promising mobilization and low conversion intensity contributed to total Cr removal. At HAECs, enhanced electromigration and electroosmotic flow combined with a favorable oxidation environment may facilitate in situ delivery of oxidants, offering practical implications for the EK detoxification of high alkalinity site soil contaminated with Cr. The practicability of HAECs is likely to be enhanced when the cost-benefit balance of providing a simultaneous energy supply during site treatment is resolved.

An overview of electrokinetically enhanced chemistry technologies for organochlorine compounds (OCs) remediation from soil

In recent years, electrokinetic (EK) remediation technology has gained significant attention among researchers. This technology has proven effective in the remediation of low-permeability polluted soil. Organochlorines (OCs) are highly toxic, persistent, bioaccumulative, and capable of long-distance migration. They can also accumulate through the food chain, posing significant environmental risks. This paper provides a review of the reaction mechanism of combining chemical technology with EK remediation for the removal of several typical OCs. Furthermore, the factors influencing the efficiency of EK remediation, such as pH and ζ potential, voltage gradients, electrode materials, electrolytes, electrode arrangements, and soil types, are summarized. The paper also presents an overview of recent advancements in the methods of combining chemical technology with EK remediation for the treatment of OCs contaminated soil. Specifically, the research progress in surfactants-combined EK technology, chemical oxidation-combined EK technology, chemical reduction-combined EK technology, and chemical adsorption-combined EK technology is summarized. These findings serve as a foundation for ongoing and future research endeavors in the field. Further exploration and investigation in this area are essential for advancing the field and improving environmental remediation strategies.

Application of novel nanobubble-contained electrolyzed catalytic water to cleanup petroleum-hydrocarbon contaminated soils and groundwater: A pilot-scale and performance evaluation study

Soil and groundwater contamination caused by petroleum hydrocarbons is a severe environmental problem. In this study, a novel electrolyzed catalytic system (ECS) was developed to produce nanobubble-contained electrolyzed catalytic (NEC) water for the remediation of petroleum-hydrocarbon-contaminated soils and groundwater. The developed ECS applied high voltage (220 V) with direct current, and titanium electrodes coated with iridium dioxide were used in the system. The developed ECS prototype contained 21 electrode pairs (with a current density of 20 mA/cm2), which were connected in series to significantly enhance the hydroxyl radical production rate. Iron-copper hybrid oxide catalysts were laid between each electrode pair to improve the radical generation efficiency. The electron paramagnetic resonance (EPR) and Rhodamine B (RhB) methods were applied for the generated radical species and concentration determination. During the operation of the ECS, high concentrations of nanobubbles (nanobubble density = 3.7 × 109 particles/mL) were produced due to the occurrence of the cavitation mechanism. Because of the negative zeta potential and nano-scale characteristics of nanobubbles (mean diameter = 28 nm), the repelling force would prevent the occurrence of bubble aggregations and extend their lifetime in NEC water. The radicals produced after the bursting of the nanobubbles would be beneficial for the increase of the radical concentration and subsequent petroleum hydrocarbon oxidation. The highly oxidized NEC water (oxidation-reduction potential = 887 mV) could be produced with a radical concentration of 9.5 × 10-9 M. In the pilot-scale study, the prototype system was applied to clean up petroleum-hydrocarbon polluted soils at a diesel-oil spill site via an on-site slurry-phase soil washing process. The total petroleum hydrocarbon (TPH)-contaminated soils were excavated and treated with the NEC water in a slurry-phase reactor. Results show that up to 74.4% of TPH (initial concentration = 2846 mg/kg) could be removed from soils after four rounds of NEC water treatment (soil and NEC water ratio for each batch = 10 kg: 40 L and reaction time = 10 min). Within the petroleum-hydrocarbon plume, one remediation well (RW) and two monitor wells (located 1 m and 3 m downgradient of the RW) were installed along the groundwater flow direction. The produced NEC water was injected into the RW and the TPH concentrations in groundwater (initial concentrations = 12.3-15.2 mg/L) were assessed in these three wells. Compared to the control well, TPH concentrations in RW and MW1 dropped to below 0.4 and 2.1 mg/L after 6 m3 of NEC water injection in RW, respectively. Results from the pilot-scale study indicate that the NEC water could effectively remediate TPH-contaminated soils and groundwater without secondary pollution production. The main treatment mechanisms included (1) in situ chemical oxidation via produced radicals, (2) desorption of petroleum hydrocarbons from soil particles due to the dispersion of nanobubbles into soil pores, and (3) enhanced TPH oxidation due to produced radicals and energy after nanobubble bursting.

Review of the application of surfactants in microemulsion systems for remediation of petroleum contaminated soil and sediments

Microemulsions are important for soil and sediment remediation technology. The characteristics of the surfactants that make up these microemulsions include low sorption into soil or sediments, low surface and interfacial tension, the ability to penetrate tiny pores, and good solubilization of contaminants. This review revealed that microemulsions formulated with nonionic and anionic surfactants have higher recovery efficiencies for hydrophobic contaminants than cationic ones, as evidenced by the surveyed studies reporting effective remediation of soils and sediments using on microemulsions. These microemulsified systems have been found to remove petroleum and its derivatives from soil or sediments at percentages ranging from 40 to 100%. As such, this review can aid with the choice of surfactants used in microemulsions for remediation, such as those with plant-based components, which are promising solutions for the remediation of contaminated soils due to their contaminant extraction efficiency and biodegradability.

Assessment of bimetallic Zn/Fe0 nanoparticles stabilized Tween-80 and rhamnolipid foams for the remediation of diesel contaminated clay soil

Diesel contamination of soil due to oil spills, disposal of refinery waste, oil exploration constitutes a major environmental problem. This paper reports the remediation of diesel contaminated clay soil using Zn/Fe⁰ bimetallic nanoparticle stabilized Rhamnolipid (RMLP) and Tween-80 (TW-80) surfactant foams. Fe⁰, and Zn (x wt%)/Fe⁰ (x = 0.2, 2.0, and 10.0) bimetallic nanoparticles are synthesized by using sodium borohydride reduction method. The average particle size (from FESEM) is calculated to be 62, 57, 42 and 35 nm for the Fe⁰, Zn (0.2)/Fe⁰, Zn (2)/Fe⁰ and Zn (10)/Fe⁰ nanopowders, respectively. The highest foamability and foam stability of 109.6 and 108.5 mL, respectively are observed for the RMLP (12 mg/l) surfactant foam stabilized with 6 mg/l Zn (10)/Fe⁰ nanoparticles. The surface tension values reduce to the lowest value of 28.1 and 31.4 mN/m with the addition of 6 mg/l of Zn (10)/Fe⁰ powder in RMLP and TW-80 solutions of 12 mg/l, respectively. The maximum diesel removal efficiency of 83.8 and 59%, is achieved by RMLP (12 mg/l) foam stabilized by Zn (10)/Fe⁰ nanoparticles (6 mg/l) for the clay soil contaminated with 100 and 500 μl/g of diesel, respectively. The physicochemical properties of the nanoparticles are studied to explain the foam properties and the remediation behavior. These findings regarding the nanoparticle stabilized foams can offer a cost-effective environment friendly commercial solution for soil remediation in the future.

Roles of oxidant, activator, and surfactant on enhanced electrokinetic remediation of PAHs historically contaminated soil

Electrokinetic (EK) remediation technology can enhance the migration of reagents to soil and is especially suitable for in situ remediation of low permeability contaminated soil. Due to the long aging time and strong hydrophobicity of polycyclic aromatic hydrocarbons (PAHs) from historically polluted soil, some enhanced reagents (oxidant, activator, and surfactant) were used to increase the mobility of PAHs, and remove and degrade PAHs in soil. However, under the electrical field, there are few reports on the roles and combined effect of oxidant, activator, and surfactant for remediation of PAHs historically contaminated soil. In the present study, sodium persulfate (PS, oxidant, 100 g L-1) or/and Tween 80 (TW80, surfactant, 50 g L-1) were added to the anolyte, and citric acid chelated iron(II) (CA-Fe(II), activator, 0.10 mol L-1) was added to catholyte to explore the roles and contribution of enhanced reagents and combined effect on PAHs removal in soil. A constant voltage of 20 V was applied and the total experiment duration was 10 days. The results showed that the removal rate of PAHs in each treatment was PS + CA-Fe(II) (21.3%) > PS + TW80 + CA-Fe(II) (19.9%) > PS (17.4%) > PS + TW80 (11.4%) > TW80 (8.1%) > CK (7.5%). The combination of PS and CA-Fe(II) had the highest removal efficiency of PAHs, and CA-Fe(II) in the catholyte could be transported toward anode via electromigration. The addition of TW80 reduced the electroosmotic flow and inhibited the transport of PS from anolyte to the soil, which decreased the removal of PAHs (from 17.4 to 11.4% with PS, from 21.3 to 19.9% with PS+CA-Fe(II)). The calculation of contribution rates showed that PS was the strongest enhancer (3.3~9.9%), followed by CA-Fe(II) (3.9~8.5%) (with PS), and the contribution of TW80 was small and even negative (-1.4~0.6%). The above results indicated that the combined application of oxidant and activator was conducive to the removal of PAHs, while the addition of surfactant reduced the EOF and the migration of oxidant and further reduced the PAHs removal efficiency. The present study will help to further understand the role of enhanced reagents (especially surfactant) during enhanced EK remediation of PAHs historically contaminated soil.

Remediation techniques for elimination of heavy metal pollutants from soil: A review

Contaminated soil containing toxic metals and metalloids is found everywhere globally. As a consequence of adsorption and precipitation reactions, metals are comparatively immobile in subsurface systems. Hence remediation techniques in such contaminated sites have targeted the solid phase sources of metals such as sludges, debris, contaminated soils, or wastes. Over the last three decades, the accumulation of these toxic substances inside the soil has increased dramatically, putting the ecosystem and human health at risk. Pollution of heavy metal have posed severe impacts on human, and it affects the environment in different ways, resulting in industrial anger in many countries. Various procedures, including chemical, biological, physical, and integrated approaches, have been adopted to get rid of this type of pollution. Expenditure, timekeeping, planning challenges, and state-of-the-art gadget involvement are some drawbacks that need to be properly handled. Recently in situ metal immobilization, plant restoration, and biological methods have changed the dynamics and are considered the best solution for removing metals from soil. This review paper critically evaluates and analyzes the numerous approaches for preparing heavy metal-free soil by adopting different soil remediation methods.

Efficiency of diesel-contaminated soil washing with different tween 80 surfactant concentrations, pH, and bentonite ratios

Soil contaminated with diesel fuel is a hazard to the environment and people; therefore, it needs to be remediated. Soil washing enhanced with Tween 80 (TW80), non-toxic and non-ionic surfactant, can effectively remove diesel from contaminated soils. In this study, the effects of 0.01%, 0.1%, 0.5%, 1%, and 1.5% (v/v) [TW80] concentrations; 0%, 5%, and 15% (w/w) bentonite; and variation in pH on washing efficiency were examined in a batch test. The prepared samples were physiochemically characterized on the basis of particle size, zeta potential, cation exchange capacity (CEC), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis. When the bentonite content in soil was 5% or 15%, 1.5% [TW80] solution exhibited the highest washing efficiency. The diesel removal efficiencies in soil with 0% bentonite were slightly higher than those in soils with 5% and 15% bentonite because of the increase in adsorption sites by bentonite; consequently, diesel could not be easily washed out. The extracted n-alkanes showed that the percentage of carbon number 20 was higher than that of the other even-numbered carbons in the retained washed samples analyzed by gas chromatography-mass spectrometry (GC-MS). In all the washing tests, the diesel removal efficiencies in soil with 15% bentonite and 0.1% [TW80] were lower than those in soil with 15% bentonite and water because of adsorption. The bentonite samples washed with TW80 have different morphologies, with a voluminous structure composed of the fusion of all layered structures, as supported by SEM results. Changes in the diesel content and residual TW80 content in the soil before and after washing were shown by the carbon content in the EDS results. The mechanism of the washing effect was investigated by CEC and zeta potential measurements. This study may aid in selecting appropriate conditions for improving washing efficiencies in future field applications.

Effect of additives on the remediation of arsenic and chromium co-contaminated soil by an electrokinetic-permeable reactive barrier

To enhance the remediation efficiency of arsenic (As) and chromium (Cr)co-contaminated soil, the effect of various combinations of reducing and chelating agents on the removal of As and Cr was studied in the present work by using electrokinetic technology coupled with a permeable reactive barrier (EK-PRB). In an experiment with EK-PRB, reducing agents (ascorbic acid and citric acid) and chelating agents (EDTA-2Na) were applied together with CaAl-layered double hydroxide (CaAl-LDH) to pretreat As and Cr co-contaminated soil. The chelating agents increased the removal efficiency of As and Cr, while the reducing agent only improved As removal in co-contaminated soil. The best removal efficiencies of As and Cr were 41.2% and 46.8%, respectively. The reducing agents promoted the production of As(III) and enhanced the migration of As. However, a large amount of Cr(VI) was reduced to Cr(III), which affected the migration of Cr. Although the addition of chelating agents partly increased the migration of Cr(III), the removal of total chromium (TCr) still decreased. In this remediation system, a PRB can effectively capture and fix As and Cr. The results indicated that As was mainly adsorbed on the surface of CaAl-LDH, while the surface adsorption and intercalation of CaAl-LDH were the main mechanisms for Cr.