Characterization of a Sodium Persulfate Sustained Release Rod for in Situ Chemical Oxidation Groundwater Remediation

From Theory to Practice: Leveraging Chemical Principles To Improve the Performance of Peroxydisulfate-Based In Situ Chemical Oxidation of Organic Contaminants

In situ chemical oxidation (ISCO) using peroxydisulfate has become more popular in the remediation of soils and shallow groundwater contaminated with organic chemicals. Researchers have studied the chemistry of peroxydisulfate and the oxidative species produced upon its decomposition (i.e., sulfate radical and hydroxyl radical) for over five decades, describing reaction kinetics, mechanisms, and product formation in great detail. However, if this information is to be useful to practitioners seeking to optimize the use of peroxydisulfate in the remediation of hazardous waste sites, the relevant conditions of high oxidant concentrations and the presence of minerals and solutes that affect radical chain reactions must be considered. The objectives of this Review are to provide insights into the chemistry of peroxydisulfate-based ISCO that can enable more efficient operation of these systems and to identify research needed to improve understanding of system performance. By gaining a deeper understanding of the underlying chemistry of these complex systems, it may be possible to improve the design and operation of peroxydisulfate-based ISCO remediation systems.

A biodegradable chitosan-based polymer for sustained nutrient release to stimulate groundwater hydrocarbon-degrading microflora

Petroleum hydrocarbon-contaminated groundwater often has a low indigenous microorganism population and lacks the necessary nutrient substrates for biodegradation reaction, resulting in a weak natural remediation ability within the groundwater ecosystem. In this paper, we utilized the principle of petroleum hydrocarbon degradation by microorganisms to identify effective nutrients (NaH2PO4, K2HPO4, NH4NO3, CaCl2, MgSO4·7H2O, FeSO4·7H2O, and VB12) and optimize nutrient substrate allocation through a combination of actual surveys of petroleum hydrocarbon-contaminated sites and microcosm experiments. Building on this, combining biostimulation and controlled-release technology, we developed a biodegradable chitosan-based encapsulated targeted biostimulant (i.e., YZ-1) characterized by easy uptake, good stability, controllable slow-release migration, and longevity to stimulate indigenous microflora in groundwater to efficiently degrade petroleum hydrocarbon. Results showed that YZ-1 extended the active duration of nutrient components by 5-6 times, with a sustainable release time exceeding 2 months. Under YZ-1 stimulation, microorganisms grew rapidly, increasing the degradation rate of petroleum hydrocarbon (10 mg L-1) by indigenous microorganisms from 43.03% to 79.80% within 7 d. YZ-1 can easily adapt to varying concentrations of petroleum hydrocarbon-contaminated groundwater. Specifically, in the range of 2-20 mg L-1 of petroleum hydrocarbon, the indigenous microflora was able to degrade 71.73-80.54% of the petroleum hydrocarbon within a mere 7 d. YZ-1 injection facilitated the delivery of nutrient components into the underground environment, improved the conversion ability of inorganic electron donors/receptors in the indigenous microbial community system, and strengthened the co-metabolism mechanism among microorganisms, achieving the goal of efficient petroleum hydrocarbon degradation.

Release efficiencies of potassium permanganate controlled-release biodegradable polymer (CRBP) pellets embedded in polyvinyl acetate (CRBP-PVAc) and polyethylene oxide (CRBP- PEO) for groundwater treatment

In-situ chemical oxidation (ISCO) is a commonly used method for the remediation of environmental contaminants in groundwater systems. However, traditional ISCO methods are associated with several limitations, including safety and handling concerns, rebound of groundwater contaminants, and difficulty in reaching all areas of contamination. To overcome these limitations, novel Controlled-Release Biodegradable Polymer (CRBP) pellets containing the oxidant KMnO₄ were designed and tested. The CRBP pellets were encapsulated in Polyvinyl Acetate (CRBP-PVAc) and Polyethylene Oxide (CRBP-PEO) at different weight percentages, baking temperatures, and time. Their release efficiency was tested in water, soil, and water and soil mixture media. Results showed that CRBP-PVAc pellets with 60 % KMnO₄ and baked at 120 °C for 2 min had the highest release percentage and rate across different conditions tested. Natural organic matter was also found to be an important factor to consider for in-field applications due to its potential reducing effect with Mn O 4 - . Overall, the use of CRBP pellets offers an innovative and sustainable solution to remediate contaminated groundwater systems, with the potential to overcome traditional ISCO limitations. These findings suggest that CRBP pellets could provide sustained and controlled release of the oxidant, reducing the need for multiple injections and minimizing safety and handling concerns. This study represents an important step towards developing a new and effective approach for ISCO remediation.

Dynamic Release Characteristics and Kinetics of a Persulfate Sustained-Release Material

Sustained-release materials are increasingly being used in the delivery of oxidants for in situ chemical oxidation (ISCO) for groundwater remediation. Successful implementation of sustained-release materials depends on a clear understanding of the mechanism and kinetics of sustained release. In this research, a columnar sustained-release material (PS@PW) was prepared with paraffin wax and sodium persulfate (PS), and column experiments were performed to investigate the impacts of the PS@PW diameter and PS/PW mass ratio on PS release. The results demonstrated that a reduction in diameter led to an increase in both the rate and proportion of PS release, as well as a diminished lifespan of release. The release process followed the second-order kinetics, and the release rate constant was positively correlated with the PS@PW diameter. A matrix boundary diffusion model was utilized to determine the PS@PW diffusion coefficient of the PS release process, and the release lifespan of a material with a length of 500 mm and a diameter of 80 mm was predicted to be more than 280 days. In general, this research provided a better understanding of the release characteristics and kinetics of persulfate from a sustained-release system and could lead to the development of columnar PS@PW as a practical oxidant for in situ chemical oxidation of contaminated aquifers.

Modeling the spreading and remediation efficiency of slow-release oxidants in a fractured and contaminated low-permeability stratum

Traditional remediation technologies cannot well remediate the low permeability contaminated stratums due to the limitation in the transport capacity of solute. The technology that integrates the fracturing and/or slow-released oxidants can be a new alternative, and its remediation efficiency remains unknown. In this study, an explicit dissolution-diffusion solution for the oxidants in control release beads (CRBs) was developed to describe the time-varying release of oxidants. Together with advection, diffusion, dispersion and the reactions with oxidants and natural oxidants, a two-dimensional axisymmetric model of solute transport in a fracture-soil matrix system was established to compare the removal efficiencies of CRB oxidants and liquid oxidants and to identify the main factors that can significantly affect the remediation of fractured low-permeability matrix. The results show that CRB oxidants can achieve a more effective remediation than liquid oxidants under the same condition due to the more uniform distribution of oxidants in the fracture and hence a higher utilization rate. Increasing the dose of the embedded oxidants can benefit the remediation to some extent, while at small doses the release time over 20 d has little impact. For extremely low-permeability contaminated stratums, the remediation effect can be significantly improved if the average permeability of the fractured soil can be enhanced to more than 10^-7 m/s. Increasing the injection pressure at a single fracture during the treatment can enlarge the influence distance of the slow-released oxidants above the fracture (e.g., 0.3-0.9 m in this study) rather than below the fracture (e.g., 0.3 m in this study). In general, this work is expected to provide some meaningful guidance for the design of fracturing and remediating low permeability contaminated stratums.

Developing a Slow-Release Permanganate Composite for Degrading Aquaculture Antibiotics

Copious use of antibiotics in aquaculture farming systems has resulted in surface water contamination in some countries. Our objective was to develop a slow-release oxidant that could be used in situ to reduce antibiotic concentrations in discharges from aquaculture lagoons. We accomplished this by generating a slow-release permanganate (SR-MnO₄^-) that was composed of a biodegradable wax and a phosphate-based dispersing agent. Sulfadimethoxine (SDM) and its synergistic antibiotics were used as representative surrogates. Kinetic experiments verified that the antibiotic-MnO₄^- reactions were first-order with respect to MnO₄^- and initial antibiotic concentration (second-order rates: 0.056-0.128 s^-1 M^-1). A series of batch experiments showed that solution pH, water matrices, and humic acids impacted SDM degradation efficiency. Degradation plateaus were observed in the presence of humic acids (>20 mgL^-1), which caused greater MnO₂ production. A mixture of KMnO₄/beeswax/paraffin (SRB) at a ratio of 11.5:4:1 (w/w) was better for biodegradability and the continual release of MnO₄^-, but MnO₂ formation altered release patterns. Adding tetrapotassium pyrophosphate (TKPP) into the composite resulted in delaying MnO₂ aggregation and increased SDM removal efficiency to 90% due to the increased oxidative sites on the MnO₂ particle surface. The MnO₄^- release data fit the Siepmann-Peppas model over the long term (t < 48 d) while a Higuchi model provided a better fit for shorter timeframes (t < 8 d). Our flow-through discharge tank system using SRB with TKPP continually reduced the SDM concentration in both DI water and lagoon wastewater. These results support SRB with TKPP as an effective composite for treating antibiotic residues in aquaculture discharge water.

Remediation of groundwater contaminated with trichloroethylene (TCE) using a long-lasting persulfate/biochar barrier

Groundwater contamination by chlorinated solvents causes potential threats to water resources and human health. Therefore, it is important to develop effective technologies to remediate contaminated groundwater. This study uses biodegradable hydrophilic polymers, hydroxypropyl methylcellulose (HPMC), hydroxyethyl cellulose (HEC) and polyvinyl pyrrolidone (PVP) as binders to manufacture persulfate (PS) tablets for the sustained release of persulfate to treat trichloroethylene (TCE) in groundwater. The release time for different tablets decreases in the order: HPMC (8-15 days) > HEC (7-8 days) > PVP (2-5 days). The efficiency with which persulfate is released is: HPMC (73-79%) > HEC (60-72%) > PVP (12-31%). HPMC is the optimal binder for the manufacture of persulfate tablets and persulfate is released from a tablet of HPMC/PS ratio (wt/wt) of 4/3 for 15 days at a release rate of 1127 mg/day. HPMC/PS/biochar (BC) ratios (wt/wt/wt) between 1/1/0.02 and 1/1/0.0333 are suitable for PS/BC tablets. PS/BC tablets release persulfate for 9-11 days at release rates of 1243 to 1073 mg/day. The addition of too much biochar weakens the structure of the tablets, which results in a rapid release of persulfate. TCE is oxidized by a PS tablet with an efficiency of 85% and a PS/BC tablet eliminates more TCE, with a removal efficiency of 100%, due to oxidation and adsorption during the 15 days of reaction. Oxidation is the predominant mechanism for TCE elimination by a PS/BC tablet. The adsorption of TCE by BC fits well with the pseudo-second-order kinetics and the pseudo-first-order kinetics, which describes the removal of TCE by PS and PS/BC tablets. The results of this study show that a PS/BC tablet can be used in a permeable reactive barrier for long-term passive remediation of groundwater.

Evaluation of alkaline activated sodium persulfate sustained release rod for the removal of dissolved trichloroethylene

The presence of trichloroethylene (TCE) dense non-aqueous phase liquid (DNAPL) in the subsurface can generate a dissolved phase plume in groundwater. This study developed an alkaline activated sodium persulfate (SPS) sustained release oxidation rod (alkaline SPS SR-Rod) for long-term in situ chemical oxidation accelerated treatment of TCE dissolved from TCE DNAPL, by creating a greater concentration gradient at the TCE DNPL boundary. The dissolution of TCE DNAPL (1 mL) in water (280 mL) generated ~700 mg L^-1, with a volumetric mass transfer coefficient (k_La) of 0.0187 d^-1. The alkaline SPS SR-Rod system had a k_La of 0.013 d^-1 for TCE dissolution at early stage, and thereafter aqueous TCE concentration remained below ~10 mg L^-1 over 60 d of reaction. An SPS SR-Rod life-span of 186 d, for 90% of SPS released from the rod, was estimated. In the soil-water system, aqueous TCE was maintained < 3 mg L^- 1 throughout the reaction and the soil oxidant demand was determined to be ~4 g-SPS/kg-soil in the alkaline SPS SR-Rod system. These results revealed that the use of the alkaline SPS SR-Rod can be effective as a method of treating dissolved TCE released from DNAPL contamination, and thereby accelerating TCE DNAPL removal.

Decontamination of water co-polluted by copper, toluene and tetrahydrofuran using lauric acid

Co-contamination by organic solvents (e.g., toluene and tetrahydrofuran) and metal ions (e.g., Cu²⁺) is common in industrial wastewater and in industrial sites. This manuscript describes the separation of THF from water in the absence of copper ions, as well as the treatment of water co-polluted with either THF and copper, or toluene and copper. Tetrahydrofuran (THF) and water are freely miscible in the absence of lauric acid. Lauric acid separates the two solvents, as demonstrated by proton nuclear magnetic resonance (¹H NMR) and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). The purity of the water phase separated from 3:7 (v/v) THF:water mixtures using 1 M lauric acid is ≈87%v/v. Synchrotron small angle X-Ray scattering (SAXS) indicates that lauric acid forms reverse micelles in THF, which swell in the presence of water (to host water in their interior) and ultimately lead to two free phases: 1) THF-rich and 2) water-rich. Deprotonated lauric acid (laurate ions) also induces the migration of Cu²⁺ ions in either THF (following separation from water) or in toluene (immiscible in water), enabling their removal from water. Laurate ions and copper ions likely interact through physical interactions (e.g., electrostatic interactions) rather than chemical bonds, as shown by ATR-FTIR. Inductively coupled plasma—optical emission spectrometry (ICP-OES) demonstrates up to 60% removal of Cu²⁺ ions from water co-polluted by CuSO₄ or CuCl₂ and toluene. While lauric acid emulsifies water and toluene in the absence of copper ions, copper salts destabilize emulsions. This is beneficial, to avoid that copper ions are re-entrained in the water phase alongside with toluene, following their migration in the toluene phase. The effect of copper ions on emulsion stability is explained based on the decreased interfacial activity and compressional rigidity of interfacial films, probed using a Langmuir trough. In wastewater treatment, lauric acid (a powder) can be mixed directly in the polluted water. In the context of groundwater remediation, lauric acid can be solubilized in canola oil to enable its injection to treat aquifers co-polluted by organic solvents and Cu²⁺. In this application, injectable filters obtained by injecting cationic hydroxyethylcellulose (HEC +) would impede the flow of toluene and copper ions partitioned in it, protecting downstream receptors. Co-contaminants can be subsequently extracted upstream of the filters (using pumping wells), to enable their simultaneous removal from aquifers.

Integration of in-situ chemical oxidation and permeable reactive barrier for the removal of chlorophenols by copper oxide activated peroxydisulfate

In-situ chemical oxidation (ISCO) and permeable reactive barrier (PRB) have been used in field practices for contaminated groundwater remediation. In this lab-scale study, a novel system integrating ISCO and PRB using peroxydisulfate (PDS) as the oxidant and copper oxide (CuO) as the reactive barrier material was developed for the removal of 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP). The influences of chlorophenol concentration and flow rate on the system performance were first evaluated using synthetic solutions. The removal efficiencies of target chlorophenols were greater than 90% when sufficient PDS was supplied ([PDS]/[chlorophenol]>1). It was also found that the removal efficiencies decreased with the increasing chlorophenol concentrations (10-150 μM) and flow rates (1.8-14.4 mL/min). When three real groundwaters were employed, the removal efficiencies of 2,4-DCP and 2,4,6-TCP slightly reduced to 90% and 85%, respectively. For PCP, the removal efficiency dropped to 20% in two groundwaters with relatively high levels of alkalinity. The influences of pH and TOC were found to be insignificant for the range investigated (pH 6.5-8.7 and TOC = 0.4-1.5 mgC/L). The reduced removal efficiency could be due to the formation of weaker radicals and the stronger competition between bicarbonate ions and PDS for the activation sites on the CuO surfaces.

Calcium sulfide-organosilicon complex for sustained release of H2S in strongly acidic wastewater: Synthesis, mechanism and efficiency

Sulfide precipitation is an efficient method to remove Cu(II) and As(III) from strongly acidic wastewater, but the instantaneous release of H₂S from traditional sulfuration reagents causes serious H₂S pollution. Moreover, the obtained precipitates are mixtures of CuS and As₂S₃, leading to difficulties in resource recovery. In this study, a calcium sulfide-organosilicon complex (CaS-OSCS), in which CaS was coated into a matrix of {[O_1.5Si(CH₂)₃NH]CS}_n (OSCS) via the coordination bonding, was developed. OSCS, as a matrix of CaS-OSCS, can ensure the sustained and stable release of H₂S under strongly acidic conditions owing to its low swelling (1.75% swelling ratio) and excellent acid resistance. The release longevity of H₂S from CaS-OSCS extended from 5 min up to 50 min compared with that from CaS because the hydrophobic OSCS prevented solution diffusing to the pores of CaS-OSCS and thus slowed down the hydrolysis of CaS in pores. 99% of Cu(II)/As(III) was precipitated without H₂S escape, and the dosage of sulfuration reagents was reduced by 30%. In addition, CaS-OSCS improved the selective separation of copper from wastewater, and a separation factor between Cu(II) and As(III) reached 2376. This study provides a potential approach for the elimination of H₂S pollution and selective recovery of copper.

In-situ generation of reactive oxygen species using combination of electrochemical oxidation and metal sulfide

In-situ chemical oxidation (ISCO) is commonly practiced to degrade organic pollutants in various fields. However, ISCO is deteriorated the oxidation efficiency due to the non-selective and self-decomposition of reagents. Therefore, in-situ generation of oxidants is being proposed to compensate for the demerits of conventional ISCO. In this study, the aim is to suggest a novel in-situ generation system using the combination of electrochemical oxidation (EO) and pyrite oxidation. It is hypothesized that EO system can generate the oxygen species, which can activate the pyrite surface to produce more oxidants. We evaluated three systems (1) EO system (2) pyrite oxidation system (3) combined system using sulfanilamide as a common antibiotic. The EO system degraded completely sulfanilamide and generated 150 μM of H₂O₂ and 8 mg/L of DO even at 10 mA. In other words, EO system can directly oxidize the sulfanilamide and produce oxygen species. The pyrite system produced 204 and 24 μM of hydroxyl radicals at pH 3 under oxic and anoxic conditions, respectively, and 118 and 20 μM at pH 7. Pyrite oxidation can generate more reactive species in the presence of oxygen. The combined system enhanced the oxidation-rate constant to 1.5 times (from 0.2561 to 0.3502 h^-1). The additional supply of oxygen showed a higher oxidation rate to 1.5 and 1.3 times higher than single EO or pyrite oxidation, respectively. As a result, the co-presence of pyrite and oxygen shows a synergistic effect on the oxidation of the organic pollutant. Our results suggest that electrochemical generation of the oxygen species in the presence of pyrite is a promising technique to oxidize organic pollutants in groundwater.

Modeling the release and spreading of permanganate from aerated slow-release oxidants in a laboratory flow tank

Aerated, slow-release oxidants are a relatively new technology for treating contaminated aquifers. A critical need for advancing this technology is developing a reliable method for predicting the radius of influence (ROI) around each drive point. In this work, we report a series of laboratory flow tank experiments and numerical modeling efforts designed to predict the release and spreading of permanganate from aerated oxidant candles (oxidant-wax composites). To mimic the design of the oxidant delivery system used in the field, a double screen was used in a series of flow tank experiments where the oxidant was placed inside the inner screen and air was bubbled upward in the gap between the screens. This airflow pattern creates an airlift pump that causes water and oxidant to be dispersed from the top of the outer screen and drawn in at the bottom. Using this design, we observed that permanganate spreading and ROI increased with aeration and decreased with advection. A coupled bubble flow and transport model was able to successfully reproduce observed results by mimicking the upward shape and spreading of permanganate under various aeration and advection rates.

Selective solvent filters for non-aqueous phase liquid separation from water

Injectable filters permeable to water but impermeable to non-polar solvents were developed to contain non-aqueous phase liquids (NAPL) in contaminated aquifers, hence protecting downstream receptors during NAPL remediation. Filters were produced by injecting aqueous solutions of 0.01% chitosan, hydroxyethylcellulose and quaternized hydroxyethylcellulose into sand columns, followed by rinsing with water. Polymer sorption onto silica was verified using a quartz-crystal microbalance with dissipation monitoring. Fluorescence and gas chromatography mass spectroscopy showed low ppm range concentrations of non-polar solvents (e.g., hexane and toluene) in water eluted from the filters (in the absence of emulsifiers). The contact angles between polymer-coated surfaces and hexane or toluene were > 90°, indicating surface oleophobicity. Organic, polar solvents (e.g. tetrahydrofuran and tetrachloroethylene, TCE) were not separated from water. The contact angles between polymer-coated surfaces and TCE was also > 90°. However, the contact area with polymer coated surfaces was greater for TCE than non-polar solvents, suggesting higher affinity between TCE and the surfaces. Emulsifiers can be used to facilitate NAPL extraction from aquifers. Emulsion separation efficiency depended on the emulsifier used. Emulsions were not separated with classical surfactants (e.g. Tween 20 and oleic acid) or alkaline zein solutions. Partial emulsion separation was achieved with humic acids and zein particles.

Role of coke-bounded environmentally persistent free radicals in phenanthrene degradation by hydrogen peroxide

Emission of polycyclic aromatic hydrocarbons (PAHs) is accompanied with the discharge of carbonaceous particles during the coke production. To degrade the adsorbed PAHs, hydrogen peroxide (H₂O₂) was applied as an oxidising agent, which might be activated by the inherent environmentally persistent free radicals (EPFRs) on coke particles. The transformation of phenanthrene (PHE), selected as model molecule, was achieved in H₂O₂/coke particle system without the addition of additional activating agent. This process consumed the particle-bounded EPFRs, inducing the decreasing of spin density from 1.92 × 10¹⁸ to 4.4 × 10¹⁷ spins g^-1 in 30 min of reaction time. Electron paramagnetic resonance (EPR) technique coupled with spin-trapping agent 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to probe the potential formation of reactive oxygen species. A higher capture [[Formula: see text]] concentration was observed with larger decreases in EPFRs concentration, indicating that EPFRs were the main contributor to the formation of [Formula: see text]. The obtained results suggested that the activation of H₂O₂ by EPFRs on coke particles resulted in the generation of hydroxyl radical ([Formula: see text]), which then back-reacted with adsorbed PHE. The finding of this study shed light on a new remediation technology for toxic carbonaceous byproducts discharged during the coke production.

'Emulsion locks' for the containment of hydrocarbons during surfactant flushing

Reversible double water in oil in water (W/O/W) emulsions were developed to contain subsurface hydrocarbon spills during their remediation using surfactant flushing. Double emulsions were prepared by emulsifying CaCl₂ solutions in canola oil, and subsequently by emulsifying the W/O emulsions in aqueous sodium alginate solutions. The formation of double emulsions was confirmed with confocal and optical microscopy. The double emulsions reversed and gelled when mixed with the surfactants sodium dodecyl sulfate (SDS) and cocamidopropyl betaine (CPB). Gels can act as 'emulsion locks' to prevent spreading of the hydrocarbon plume from the areas treated with surfactant flushing, as shown in sand column tests. Shear rheology was used to quantify the viscoelastic moduli increase (gelation) upon mixing the double emulsion with SDS and CPB. SDS was more effective than CPB in gelling the double emulsions. CPB and SDS could adsorb at the interface between water and model hydrocarbons (toluene and motor oil), lowering the interfacial tension and rigidifying the interface (as shown with a Langmuir trough). Bottle tests and optical microscopy showed that SDS and CPB produced W/O and O/W emulsions, with either toluene or motor oil and water. The emulsification of motor oil and toluene in water with SDS and CPB facilitated their flow through sand columns and their recovery. Toluene recovery from sand columns was quantitated using Gas-Chromatography Mass-Spectroscopy (GC-MS). The data show that SDS and CPB can be used both for surfactant flushing and to trigger the gelation of 'emulsion locks'. Ethanol also gelled the emulsions at 100 mL/L.

Characteristics and mechanisms of controlled-release KMnO4 for groundwater remediation: Experimental and modeling investigations

Controlled release materials (CRMs) are emerging oxidant delivery techniques for in-situ chemical oxidation (ISCO) for groundwater remediation. Successful implementation of CRM relies on good understandings of the kinetics and mechanism of controlled release of reactive agents. In this study, batch experiments and model simulations were conducted to explore the impacts of CRM properties (composition and size) and environmental conditions (temperature, pH, water volume and anions) on KMnO₄ release from KMnO₄ -paraffin controlled release beads. Experimental results indicated that higher KMnO₄: paraffin mass ratio resulted in shorter release longevities and higher release rate. Larger bead resulted in lower release rate, longer release longevity, and more KMnO₄ released. Higher incubation temperature resulted in higher release rate and shorter release longevity, but did not affect the total mass of KMnO₄ released. Acidic pH decreased the total mass of KMnO₄ released while alkaline pH did not affect KMnO₄ release. The presence of SO₄^2-, CO₃^2-, Cl^- and Br^- had negligible impacts on KMnO₄ release. A dissolution-diffusion conceptual model was developed. The above experimental observation and the associated controlled release mechanisms can be qualitatively explained by the conceptual model. A more detailed two-film boundary mathematical model was developed to simulate KMnO₄ release process. Comparison of modeling results with experimental data suggest that the new mathematical model gave a good quantitatively predication. Overall, this study shows that properly designed CRM can sustain release for years, thus representing a cost-effective and low-maintenance groundwater remediation technology. Both CRM properties and environmental conditions significantly affect the release kinetics and longevity, therefore these factors should be considered in the design and maintenance of CRM-based ISCO system.

Gelation on demand using switchable double emulsions: A potential strategy for the in situ immobilization of organic contaminants

Switchable double emulsions (water in oil in water, W/O/W) are proposed for the in situ immobilization of subsurface organic contaminants such as toluene, hexane or benzene. Primary W/O emulsions were prepared by emulsifying 250 mL of 0.36 M CaCl₂ aqueous solutions in 1 L of canola oil (with 12.5 g/L of ethylcellulose, EC, and 2.5 g/L of calcium stearate). In the primary W/O emulsion the water droplets in oil were ≈8 μm, as observed using an optical and a confocal microscope. EC and calcium stearate adsorbed at the oil water interface (as demonstrated by interfacial tension measurements), forming films which stabilized the W/O emulsions (as verified with bottle tests). Experiments conducted using a Langmuir trough suggest that EC and calcium stearate films did not desorb from the oil-water interface upon compression. Crumpling tests and optical microscopy observations indicate that EC and calcium stearate films were skin-like, and buckled when deformed. To obtain double W/O/W emulsions the primary emulsions were emulsified in a 0.75 wt% solution of sodium alginate, with 2 mL/L of Tween 20 and 10 g/L of NaCl. The formation of W/O/W emulsions was verified through optical microscopy and confocal microscopy observations. In the absence of the contaminants the double emulsions were stable, as observed by resting them on the bench over three days and agitating them with a multi-action wrist shaker for 30 min. Also, they had low shear elastic (G' = 2.67 ± 0.58 Pa) and viscous (G″ = 1.69 ± 0.24 Pa) moduli, which should facilitate their transport through geological media (e.g. soil) to polluted areas. Upon mixing with toluene, hexane or benzene at concentrations ranging from 5% to 17%, the double emulsions were destabilized. Emulsion destabilization caused the release of CaCl₂, which crosslinked sodium alginate and formed gels in which the contaminants were incorporated. The gelation rate and the magnitude of the viscoelastic moduli depended on the contaminant type and concentration, and on the mixing time. Gelation occurred fastest with the highest toluene concentrations tested (9% to 17%), but the highest elastic moduli were measured with 9% toluene concentrations for the longest mixing times tested (90 s). Gelation occurred slowest with hexane, likely due to the poor solubility of EC in hexane. Because of their ability to gel exclusively in contaminant proximity, the double emulsions studied offer a potential strategy to control the migration of plumes of contaminants such as toluene, hexane or benzene.