In Situ Chemical Oxidation Research Articles

Oxalic acid enhanced ferrous/persulfate process for the degradation of triclosan in soil: Efficiency, mechanism and a column study

Triclosan (TCS) is a compound in pharmaceutical and personal care products (PPCPs) that preferentially accumulates in soil and may threaten human health. Iron-based persulfate activation processes (Fe-PAPs) are a promising technique in soil remediation but are limited by their narrow pH application range and resultant iron precipitation. In this study, oxalic acid (OA) was specifically introduced to improve the efficiency of Fe2+-activated persulfate (PS) processes for TCS degradation in soil. The complexation of OA and Fe2+ improved the total iron concentration in the Fe2+/OA/PS system and prevented iron precipitation. Fe-minerals in soil were also dissolved by OA, which participated in PS activation. The most appropriate proportion of Fe2+/OA/PS is 1:1:1, and approximately 85% of the TCS was degraded within 240 min compared with 45% in the Fe2+/PS system. SO4•- and 1O2 played important roles on TCS oxidation, while O2•- and CO2•- mainly participated in the iron cycle. The weak stretch of Fe-O in OA containing treatments in ATR-FTIR measurements provided evidence that OA dissolved the Fe-minerals. The introduction of OA in the Fe2+/PS system in four different soils reduced the negative effect of soil organic matter on TCS degradation. Three types of intermediates including hydroxylation, cleavage of ether bonds and cyclization products, were identified by LC-Q-TOF-MS. Column experiments were used to study the feasibility of in situ chemical oxidation (ISCO) remediation of TCS in the Fe2+/OA/PS system. Our findings indicated the ability of OA to be used as a cheating agent in Fe-PAPs for TCS soil remediation.

Effect of phosphate on peroxymonosulfate activation: Kinetics, mechanism and implication to in-situ chemical oxidation for water decontamination

Peroxymonosulfate (PMS, HSO5-) is an increasingly popular oxidant for groundwater remediation via in situ chemical oxidation (ISCO). However, it is desired to evaluate the effect of phosphate on the PMS oxidation of organics in detail due to the saline characteristics of groundwater. In this study, ofloxacin (OFX) as a typical antibiotic was chosen as a model pollutant. The results indicated that 64% of OFX was degraded by PMS in the presence of HPO42- (PMS/HPO42- system), and the efficiency increased from 64% to 80% when the HPO42- concentration was increased from 0.1 mol/L to 0.3 mol/L. The corresponding pseudo-first-order reaction constants were 0.0086 and 0.02 min-1, respectively. In comparison, PO43- and H2PO4- showed almost no reactivity. Electron spin response (ESR) analysis and quenching tests proved that SO4·- and 1O2 were the main oxidizing species for OFX degradation in PMS/HPO42- system. Hence, HPO42- was the most effective form in PMS activation and its performance was highly pH and ionic strength sensitive. Given the ambient pH and phosphate concentration in saline groundwater, a considerable amount of HPO42- can be produced, which then serves as a homogeneous PMS activator in ISCO process. This finding provides new insights into ISCO process using PMS.

Revisiting the Role of Dissolved Silicate in Catalyzing the Hydrogen Peroxide Process with Iron-Bearing Minerals.

Injecting H2O2 into aquifers is a widely used in-situ chemical oxidation (ISCO) technology for groundwater remediation. Dissolved silicate was reported to decrease the reactivity of iron (Ⅲ)-bearing minerals toward H2O2. In this study, the effect of naturally occurring levels of dissolved silicate (≤1 mM) on the catalyzing hydrogen peroxide (CHP) with Fe(II) minerals is revaluated, and new observations that contradict with previous studies are reported. Specifically, dissolved silicate enhanced the CHP process by Fe(II) minerals. In the presence of Fe(II) minerals, siderite and ferrous oxide (FeO), which had a stronger dissolution tendency than Fe(III) minerals, dissolved silicate could prevent the dissolved iron species from precipitation through a coordinating effect, therefore reinforcing the homogeneous CHP process and the degradation of 2,4-dichlorophenol. The solution pH decreased due to the generation of degradation intermediates, and the solution acidification in turn promoted further dissolution of Fe(II) minerals. FeO particles exhibited the strongest silicate adsorption among the minerals, therefore a higher initial silicate concentration of 1 mM was needed to observe the enhancing effect. This study redefines the role of dissolved silicate on CHP process and provides clues to the design of efficient H2O2-based ISCO system for the remediation of groundwater.

Self-propelled micromotors based on natural pyrolusite for dynamic remediation of organic contaminated soil

For in situ chemical oxidation (ISCO) technologies in soil remediation, one of the key challenges is the limited diffusion range of chemical oxidants in soil without external agitation. Herein, self-propelled micromotors based on natural pyrolusite with abundant micro/nanobubble generation were developed as active heterogeneous catalysts to improve the mass transport of remediation reagents. The micromotors composed of natural pyrolusite and Fe2O3 were employed as activators of peroxymonosulfate (PMS) and hydrogen peroxide (H2O2) for dynamic remediation of soil contaminated by polycyclic aromatic hydrocarbons (PAHs). The prepared micromotors exhibited speeds of 185 ± 33.7 µm/s, 280 ± 53.9 µm/s and 312 ± 66.2 µm/s in 1 %, 3 % and 5 % H2O2, respectively. The dual effect of NPF micromotors in catalytic degradation and mass transport led to a pyrene degradation efficiency of 100 % in 5 h and a total organic content removal rate of 93.36 % in the reaction condition of 1.0 g/L of NPF, 20 mM of PMS, 20 mM of H2O2, 84.10 mg/kg of pyrene and natural pH. Meanwhile, the main reactive oxidant species (ROS) and degradation pathway of pyrene were investigated. This research demonstrates the high potential of natural-pyrolusite-based micromotors for in situ remediation of organic contaminated sites.

Quantitative study of in situ chemical oxidation remediation with coupled thermal desorption

In situ chemical oxidation (ISCO) is widely used as an efficient remediation technology for groundwater pollution. However, quantitative studies of its reactive remediation process under coupled thermal desorption technology are scarce. Based on laboratory experiments and site remediation, the chemical oxidation remediation reaction process was quantified, and the apparent reaction equation of the ISCO process was constructed. And then, a numerical model coupled with Hydraulic-Thermal-Chemical (HTC) fields was built to quantitatively describe the remediation process of an actual contaminated site. The simulation results fit well with the site monitoring data, and the results indicated that thermal desorption strengthens the ISCO remediation effect. In addition, the HTC model is expanded to build a conceptual and numerical model of a coupled remediation system, including heating and remediation wells. The results showed that high-temperature conditions enhance the activity of remediation chemicals and increase the rate of remediation reaction to obtain a better remediation effect. The heating wells increase the regional temperature, accelerating the diffusion of pollutants and remediation chemicals, and promoting adequate contact and reaction. Based on this crucial mechanism, thermal desorption coupled with ISCO technology can significantly improve remediation efficiency, shorten the remediation cycle, and precisely control agent delivery with the help of numerical simulation to avoid secondary contamination.

Monitoring of in-situ chemical oxidation for remediation of diesel-contaminated soil with electrical resistivity tomography

In-situ chemical oxidation (ISCO) with persulfate, an electrically conductive oxidant, provides a powerful signal for noninvasive geophysical techniques to characterize the remediation process of hydrocarbon contaminants. In this study, remediation with ISCO is conducted in laboratory sandboxes to evaluate the ability of electrical resistivity tomography (ERT) for monitoring the base-activated persulfate remediation process of diesel-contaminated soil. It was found that the resistivity of contaminated sand significantly decreased from 846 Ω·m to below 10 Ω·m after persulfate injection, and all measured chemical parameters showed a noticeable increase. Natural degradation and contamination plume migration were not evident in a reference sandbox without treatment. The area with a resistivity ratio < 0.95 based on imaging before and after injection indicated downward migration of the oxidation plume due to density-driven flow. A comparison between remediation and reference sandboxes showed that the observed resistivity decrease can be due to both contaminant degradation as well as the oxidation plume itself in the contaminated source zone. In contrast, the resistivity decrease in the area with low contamination concentration is attributed to the oxidation plume alone. The derived relationships between resistivity and contaminant indicators further emphasize that the contribution of contaminant consumption to resistivity change in the source area is 25.6%, while it is <16% in the low or non-contaminated area. Although this study showed that resistivity is not solely affected by the chemical transformation of diesel components, it can be combined with sampling data to allow an assessment of the effectiveness of ISCO treatment and to identify target areas for subsequent treatment.

New insights into persulfate decomposition by soil minerals: radical and non-radical pathways

Persulfate (PS)-based in situ chemical oxidation (ISCO) has been widely used for pollutant remediation in soil and groundwater. However, the underlying mechanism of interactions between mineral and PS was not fully explored. In this study, several soil model minerals including goethite, hematite, magnetite, pyrolusite, kaolin, montmorillonite, and nontronite were selected to investigate their potential effects on PS decomposition and free radical evolution. It was found the decomposition efficiency of PS by these minerals varied significantly, and both the radical and non-radical decomposition processes were included. Pyrolusite has the highest reactivity for PS decomposition. However, PS decomposition is prone to form SO42- through non-radical pathway, and thus, the amounts of free radicals (e.g., •OH and SO4•-) produced are relatively limited. However, PS mainly decomposed to produce free radicals in the presence of goethite and hematite. In the presence of magnetite, kaolin, montmorillonite, and nontronite, PS both decomposed to produce SO42- and free radicals. Furthermore, the radical process exhibited the high degradation performance for model pollutant such as phenol with relatively high utilization efficiency of PS, while non-radical decomposition has limited contribution to phenol degradation with extremely low utilization efficiency of PS. This study deepened the understanding of interactions between PS and minerals during the PS-based ISCO in soil remediation.

Impact of aqueous environments on hydrogen peroxide activation by manganese oxides: Kinetics and the critical role of bicarbonate

MnO2 activating H2O2 is a promising way in the field of advanced oxidation processes (AOPs) and in situ chemical oxidation (ISCO) to remove contaminants. However, few studies have focused on the influence of various environmental conditions on the performance of MnO2–H2O2 process, which restricts the application in real world. In this study, the effect of essential environmental factors (ionic strength, pH, specific anions and cations, dissolved organic matter (DOM), SiO2) on the decomposition of H2O2 by MnO2 (ε-MnO2 and β-MnO2) were investigated. The results suggested that H2O2 degradation was negatively correlated with ionic strength and strongly inhibited under low pH conditions and with phosphate existence. DOM had a slight inhibitory effect while Br−, Ca2+, Mn2+ and SiO2 placed negligible impact on this process. Interestingly, HCO3− inhibited the reaction at low concentrations but promoted H2O2 decomposition at high concentrations, possibly due to the formation of peroxymonocarbonate. This study may provide a more comprehensive reference for potential application of H2O2 activation by MnO2 in different water systems.

Persulfate-based ISCO for field-scale remediation of NAPL-contaminated soil: Column experiments and modeling

An experimental and computational investigation of in situ chemical oxidation (ISCO) of weathered diesel fuel in soil columns was undertaken to validate a reactive-transport model capable of predicting contaminant mass reduction from a residual source zone. Reactivity tests with contaminated groundwater in batch reactors were used to estimate a priori the kinetic parameters of a phenomenological model of the oxidation of petroleum hydrocarbon (PHC) mixture fractions. The transport model, which incorporated groundwater flow, dissolution of main PHC fractions, and homogeneous reaction in the aqueous phase, was subsequently validated against experimental data of ISCO in soil columns using repetitive treatments with unactivated and alkaline-activated persulfate. No significant effect of the initial concentration of persulfate on the remediation performance was observed in the batch system, but alkaline activation significantly improved performance. The alkaline-activated persulfate treatment achieved ∼80% removal of the initial NAPL mass in soil columns. The combination of models and experiments described herein should enable the rational design of field-scale advanced oxidation strategies for the removal of weathered petroleum hydrocarbons. This expectation was supported by a comprehensive demonstration study at a historical site contaminated by weathered diesel fuel present as a residual source within the soil and dissolved within groundwater.

Activation of Persulfate for Groundwater Remediation: From Bench Studies to Application

Persulfate-based in situ chemical oxidation (ISCO) has been increasingly used for the remediation of contaminated groundwater and soil. In recent years, there have been numerous studies in the literature on all aspects of the activation of persulfate for contaminant removal at the laboratory scale, including the ways and mechanisms for the activation, the pathways of contaminant degradation, the factors associated with the activation performance, the methods characterizing the processes, etc. In contrast, studies in the literature on the practical use of the activated persulfate at the field scale are fewer, and at the same time have not been reviewed in an organized way. This review was initiated to summarize on the current research on the applications of activated persulfate for actual site remediation, and to extract the knowledge necessary for the formation of applicable technologies. The remediation efficiency and mechanism of activated persulfates by heat, alkaline, metal-based, and electrokinetic activated technologies are described. The major factors including pH, the persistence of persulfate, and the radius of influence and soil property during ISCO remediation applications were presented and discussed. Finally, the rebound process and impact towards microbial communities after in-situ chemical oxidation on site application were discussed.

Degradation of 1,2,3-trichloropropane by unactivated persulfate and the implications for groundwater remediation

Persulfate (PS) is widely used as an in situ chemical oxidation (ISCO) technology for groundwater and soil remediation. While conventional theory generally assumes that PS needs to be “activated” to produce reactive radicals for pollutant degradation, herein, PS without explicit activation system was discovered for the degradation of 1,2,3-TCP with the generation of reactive oxidation species (ROS). Comparison of five common ISCO oxidants (PS, peroxymonosulfate, hydrogen peroxide, potassium permanganate, and sodium percarbonate) indicated that only unactivated PS was able to degrade 1,2,3-TCP in both pure water and 12 natural water samples. 50 μM 1,2,3-TCP degradation can be continued as long as there is enough PS (50 mM). The degradation rate of 1,2,3-TCP increased 450 % when the PS concentration increased from 10 mM to 50 mM and 500 % when the temperature increased from 25 °C to 45 °C. Electron paramagnetic resonance (EPR) analyzes, hydroxyl radicals (·OH) probe reaction and radical quenching experiments confirmed the involvement of both sulfate radicals (SO4·-) and ·OH that were responsible for 1,2,3-TCP degradation and ·OH played a more important role. HCO3−, Cl− and NOM are three groundwater matrix species that are most likely to inhibit PS oxidation of 1,2,3-TCP. Compared to activated PS, unactivated PS is more promising and more practical for groundwater remediation, since it has several advantages: (1) longer lifetime and better long-term availability; (2) ability of enduring contaminant degradation; (3) applicable for low-permeability zones remediation and potential to alleviate contaminant rebound or tailing problems; (4) environmental friendly; and (5) lower cost. Overall, results of this study show that unactivated PS is a promising in situ remediation technology that may be a good candidate for the most challenging low permeable zone remediation.

Acute Toxicity Evaluation of Lindane-Waste Contaminated Soils Treated by Surfactant-Enhanced ISCO.

The discharge of lindane wastes in unlined landfills causes groundwater and soil pollution worldwide. The liquid waste generated (a mixture of 28 chlorinated organic compounds, COCs) constitutes a dense non-aqueous phase liquid (DNAPL) that is highly persistent. Although in situ chemical oxidation (ISCO) is effective for degrading organic pollutants, the low COCs solubility requires high reaction times. Simultaneous injection of surfactants and oxidants (S-ISCO) is a promising technology to solve the limitation of ISCO treatment. The current work studies the remediation of highly polluted soil (COCs = 3682 mg/kg) obtained at the Sardas landfill (Sabiñáñigo, Spain) by ISCO and S-ISCO treatments. Special attention is paid to acute soil toxicity before and after the soil treatment. Microtox®, modified Basic Solid-Phase Test (mBSPT) and adapted Organic Solvent Sample Solubilization Test (aOSSST) were used for this scope. Persulfate (PS, 210 mM) activated by alkali (NaOH, 210 mM) was used in both ISCO and S-ISCO runs. A non-ionic and biodegradable surfactant selected in previous work, Emulse®3 (E3, 5, and 10 g/L), was applied in S-ISCO experiments. Runs were performed in soil columns filled with 50 g of polluted soil, with eight pore volumes (Pvs) of the reagents injected and 96 h between successive Pv injections. The total treatment time was 32 days. The results were compared with those corresponding without surfactant (ISCO). After remediation treatments, soils were water-washed, simulating the conditions of groundwater flux in the subsoil. The treatments applied highly reduced soil toxicity (final soil toxicity equivalent to that obtained for non-contaminated soil, mBSPT) and organic extract toxicity (reduction > 95%, aOSSST). Surfactant application did not cause an increase in the toxicity of the treated soil, highlighting its suitability for full-scale applications.

Enhanced removal of VOCs from wood by coupling extraction with surfactant and, oxidation using ultrasound-activated persulfate

Wood contains abundant extractives and volatile oils, which release strong odor and hazardous volatile organic compounds (VOCs). Free radicals generated by advanced oxidation processes (AOPs) can be used to efficiently destroy organic components in the aqueous phase. However, the limited water availability of hydrophobic extractives and non-water soluble volatile oils in wood may decrease the effectiveness of traditional in-situ chemical oxidation (ISCO). The poor permeability of wood-blocks, the limited of matter transmission, and the short half-life of radicals result in the low utilization of free radicals. To tackle this issue, surfactants can be combined with an ultrasound-activated persulfate (PS) process to construct a biphasic wood/water system to enhance the utilization of aqueous-phase radicals and the mass transfer of extractives and volatile oils. Various controlling factors were evaluated, including surfactant concentration, PS dosage and ultrasound temperature. The GC-MS analysis indicated that the removal rate of wood VOCs was ∼66.9 % without recycling treatment. The electron paramagnetic resonance (EPR) analysis further revealed that SO 4 •- and •OH as the main reactive species. The evaluation of wood characteristics shows that the process has no serious damage. The results demonstrate that the proposed method is effective for the oxidative removal of VOCs from wood. • VOCs source can be effectively oxidized in biphasic system. • C12-MADS surfactant was highly compatible with the ultrasound-activated persulfate. • Synergistic of ultrasonic irradiation and surfactant played a crucial role in the desorption of VOCs from wood. • The EPR analysis reveals that S O 4 - · and ·OH are the primary reactive species. • The key wood-VOCs was analyzed by the HS-SPME/GC-MS.

Simultaneous removal of carbamazepine and Cd(II) in groundwater by integration of peroxydisulfate oxidation and sulfidogenic process: The bridging role of SO42-

Heat-activated PDS oxidation (HAPO) has been widely used for in-situ chemical oxidation (ISCO) of micropollutants in groundwater, whereas the aesthetic demerit of additional SO42− production is largely overlooked. In this study, the sulfidogenic process is used to offset the aesthetic demerit, and the production of SO42− is then employed to recycle heavy metals. The innovative integration technology with PDS oxidation and sulfidogenic process via the bridging role of SO42− was reported to remove micropollutants and heavy metals in groundwater simultaneously. HAPO could completely degrade CBZ, producing 400 mg/L SO42− with the addition of 0.50 g/L PDS. Sulfate-reducing bacteria (SRB) utilize SO42− generated from HAPO as the electron acceptor in the sulfidogenic process, removing and recycling Cd(II) via the precipitation of CdS. The SRB tolerance experiment revealed the viability of PDS oxidation coupled with the sulfidogenic process via the bridging role of SO42−. Overall, the integration technology is a green and promising technology for simultaneous micropollutants removal and heavy metals recycling in groundwater.

Phase transformation-driven persulfate activation by coupled Fe/N–biochar for bisphenol a degradation: Pyrolysis temperature-dependent catalytic mechanisms and effect of water matrix components

Fe–N co-doped biochar is recently an emerging carbocatalyst for persulfate activation in situ chemical oxidation (ISCO). However, the involved catalytic mechanisms remain controversial and distinct effects of coexisting water components are still not very clear. Herein, we reported a novel N-doped biochar-coupled crystallized Fe phases composite (Fe@N-BC800) as efficient and low-cost peroxydisulfate (PDS) activators to degrade bisphenol A (BPA), and the underlying influencing mechanism of coexisting inorganic anions (IA) and humic constituent. Due to the formation of graphitized nanosheets with high defects (AI index>0.5, ID/IG = 1.02), Fe@N-BC800 exhibited 2.039, 5.536, 8.646, and 23.154-fold higher PDS catalytic activity than that of Fe@N-BC600, Fe@N-BC400, N-BC, BC. Unlike radical pathway driven by carbonyl group and pyrrolic N of low/mid-temperature Fe@N–BCs. The defective graphitized nanosheets and Fe-Nx acted separately as electron transfer and radical pathway active sites of Fe@N-BC800, where π-π sorption assisted with pyrrolic N and pore-filling facilitated BPA degradation. The strong inhibitory effects of PO43− and NO2− were ascribed to competitive adsorption of phosphate (61.11 mg g−1) and nitrate (23.99 mg g−1) on Fe@N-BC800 via electrostatic attraction and hydrogen bonding. In contrast, HA competed for the pyrrolic-N site and hindered electron delivery. Moreover, BPA oxidation pathways initiated by secondary free radicals were proposed. The study facilitates a thorough understanding of the intrinsic properties of designed biochar and contributes new insights into the fate of degradation byproducts formed from ISCO treatment of micropollutants.

Coupling surfactants with ISCO for remediating of NAPLs: Recent progress and application challenges.

Non-aqueous phase liquids (NAPLs) pose a serious risk to the soil-groundwater environment. Coupling surfactants with in situ chemical oxidation (ISCO) technology is a promising strategy, which is attributed to the enhanced desorption and solubilization efficiency of NAPL contaminants. However, the complex interactions among surfactants, oxidation systems, and NAPL contaminants have not been fully revealed. This review provides a comprehensive overview on the development of surfactant-coupled ISCO technology focusing on the effects of surfactants on oxidation systems and NAPLs degradation behavior. Specifically, we discussed the compatibility between surfactants and oxidation systems, including the non-productive consumption of oxidants by surfactants, the role of surfactants in catalytic oxidation systems, and the loss of surfactants solubilization capacity during oxidation process. The effect of surfactants on the degradation behavior of NAPL contaminants is then thoroughly summarized in terms of degradation kinetics, byproducts and degradation mechanisms. This review demonstrates that it is crucial to minimize the negative effects of surfactants on NAPL contaminants oxidation process by fully understanding the interaction between surfactants and oxidation systems, which would promote the successful implementation of surfactant-coupled ISCO technology in remediation of NAPLs-contaminated sites.

Degradation of phenol using Fe(II)-activated CaO2: effect of ball-milled activated carbon (ACBM) addition

In-situ chemical oxidation (ISCO) based on peroxide activation is one of the most promising technologies for removing organic contaminants from natural groundwater (NGW). However, use of the most common form of hydrogen peroxide (H2O2) is limited owing to its significantly rapid reaction rate and heat generation. Therefore, in the present study, the activation of calcium peroxide (CaO2), a slow H2O2 releasing agent, by Fe(II) was proposed (CaO2/Fe(II)), and the phenol degradation mechanisms and feasibility of NGW remediation were investigated. The optimum molar ratio of [phenol]/[CaO2]/[Fe(II)] (phenol = 0.5 mM) was 1/10/10, resulting in 87.0–92.5% phenol removal within 120 min under a broad initial pH range of 3–9. HCO3−, PO43−, and humic acid significantly inhibited degradation, whereas the effects of Cl−, NO3−, and SO42− were negligible. Reactive oxygen species (ROS) were identified based on the results of phenol degradation in the presence of scavengers and electron spin resonance (ESR) spectroscopy, which demonstrated that 1O2 played the dominant role, supported by •OH, in CaO2/Fe(II). Phenol removal in NGW (67.81%) was less than that in distilled and deionized water (DIW, 92.5%) at a [phenol]/[CaO2]/[Fe(II)] ratio of 1/10/10. However, phenol removal was significantly improved (∼100%) by increasing the CaO2 and Fe(II) doses to 1/20/20–40. Furthermore, when 125–250 mg L−1 of ball-milled activated carbon (ACBM) was added (CaO2/Fe(II)-ACBM), phenol removal was enhanced from 67.81% to 90.94–100% in the NGW. CaO2/Fe(II)-ACBM exhibited higher total organic carbon (TOC) removal than CaO2/Fe(II). In addition, no notable by-products were detected using CaO2/Fe(II)-ACBM, whereas the polymerisation products of hydroxylated and/or ring-cleaved compounds, that is, aconitic acid, gallocatechin, and 10-hydroxyaloin, were found in the reaction with CaO2/Fe(II). These results strongly suggest that CaO2/Fe(II)-ACBM is highly promising for groundwater remediation, minimizing degradation byproducts and the adverse effects caused by the NGW components.

Pore-Scale Mechanisms of Solid Phase Emergence During DNAPL Remediation by Chemical Oxidation.

In situ chemical oxidation (ISCO) has proven successful in the remediation of aquifers contaminated with dense nonaqueous phase liquids (DNAPLs). However, the treatment efficiency can often be hampered by the formation of solids or gas, reducing the contact between remediation agents and residual DNAPLs. To further improve the efficiency of ISCO, fundamental knowledge is needed about the complex multiphase flow and reactive transport processes as new solid and fluid phases emerge at the microscale. Here, via microfluidic experiments, we study the pore-scale dynamics of trichloroethylene degradation by permanganate. We visualize how the remediation evolves under the influence of solid phase emergence and explore the roles of injection rate, oxidant concentration, and stabilization supplement. Combining image processing, pressure analysis, and stoichiometry calculations, we provide comprehensive descriptions of the oxidant concentration-dependent growth patterns of the solid phase and their impact on the remediation efficiency. We further corroborate the stabilization mechanism provided by phosphate supplement, which is effective in inhibiting solid phase generation and thus highly beneficial for the oxidation remediation. This work elucidates the pore-scale mechanisms during remediation of chlorinated solvents with a particular context in the solid phase production and the associated effects, which is of general significance to understanding various processes in natural and engineered systems involving solid phase emergence or aggregation phenomena, such as groundwater and soil remediation.

Comparative study of the performance of controlled release materials containing mesoporous MnOx in catalytic persulfate activation for the remediation of tetracycline contaminated groundwater

Controlled release materials (CRMs) are an emerging oxidant delivery technique for in-situ chemical oxidation (ISCO) that solve the problems of contaminant rebound, backflow and wake during groundwater remediation. CRMs were fabricated using ordered mesoporous manganese oxide (O-MnOx) and sodium persulfate (Na2S2O8) as active components, for the removal of antibiotic pollutants from groundwater. In both static and dynamic groundwater environments, persulfate can first be activated by O-MnOx within CRMs to form sulfate radicals and hydroxyl radicals, with these radicals subsequently dissolving out from the CRMs and degrading tetracycline (TC). Due to their excellent persulfate activation performance and good stability, the constructed CRMs could effectively degrade TC in both static and dynamic simulated groundwater systems over a long period (>21 days). The TC removal rate reached >80 %. Changing the added content of O-MnOx and persulfate could effectively regulate the performance of the CRMs during TC degradation in groundwater. The process and products of TC degradation in the dynamic groundwater system were the same as in the static groundwater system. Due to the strong oxidizing properties of sulfate radicals and hydroxyl radicals, TC molecules were completely mineralized within the groundwater systems, resulting in only trace levels of degradation products being detectable, with low- or non-toxicity. Therefore, the CRMs constructed in this study exhibited good potential for practical application in the remediation of organic pollutants from both static and dynamic groundwater environments.

Toluene NAPL Oxidation by Ferrous Activated Persulfate in a Fractured Rock Glass Replica

AbstractNon‐aqueous phase liquids (NAPLs), such as toluene, often contaminate the subsurface. In this study, we focus on the transformation of toluene NAPL trapped in a single glass replica of a rock fracture via in situ chemical oxidation (ISCO) with ferrous activated persulfate. The trapped toluene consisted of a main trapped blob surrounded by smaller blobs. Over 53 days, successive persulfate injections into the fracture were interspersed with periods of water flushing. During persulfate injections, effluent toluene concentrations were below 0.04 mM. The rebound toluene concentrations during intervening water flushing periods decreased from 0.9 to 0.3 mM over the study. The smaller toluene blobs were removed by dissolution and partial oxidation. The main toluene blob was reduced in area by 35.5% and in volume by 37.3% due to dissolution and partial oxidation. The main blob changed in shape with reduction in size and there was also formation of an oxidation by‐product zone around the blob. The reduction in toluene effluent concentrations with time over successive persulfate injections and periods of water flushing was attributed to reductions in the NAPL‐water interfacial area of the blob and the formation of the by‐product zone surrounding the blob, which resulted in limited toluene dissolution. Analysis of by‐products of toluene oxidation by ferrous activated persulfate suggests that oligomers were part of the by‐product zone formed in the fracture. Gas bubbles were also observed in the fracture and may have formed from toluene oxidation and persulfate decomposition.