A Modified Approach for in Situ Chemical Oxidation Coupled to Biodegradation Enhances Light Nonaqueous Phase Liquid Source-Zone Remediation

Long-term response mechanism of bacterial communities to chemical oxidation remediation in petroleum hydrocarbon contaminated groundwater

The limited understanding of microbial response mechanism remains as a bottleneck to evaluate the long-term remediation effectiveness of in situ chemical oxidation in contaminated groundwater. In this study, we investigated long-term response of bacterial communities throughout five remediation stages of pre-oxidation, early-oxidation, late-oxidation, early-recovery and late-recovery. By analyzing bacterial biomass, taxa, diversity and metabolic functions, this work identified the consistently suppressed glyceraldehyde-3-phosphate dehydrogenase pathway and the enrichment of naphthalene degradation pathways for secondary products, suggesting persistent oxidation stress and enhanced microbial utilization of lower-molecular weight carbon sources at the oxidation and early-recovery stages. The dominant microbial clusters shifted from r-strategists to K-strategists and then back to r-strategists, indicating their higher degradation efficiency of petroleum hydrocarbons throughout the oxidation process. The changes in stability and stochastic assembly of bacterial communities during in situ chemical oxidation suggested that oxidative stress, carbon source addition and carbon source limitation as the main influential factors of bacterial community succession at the oxidation, early-recovery and late-recovery stage, respectively. Our findings highlighted the complex recovery and underlying mechanisms of groundwater bacterial communities during in situ chemical oxidation process, and provided valuable insights for effective and long-term site management after in situ chemical oxidation practices.

Effect of Fe2+-activated persulfate combined with biodegradation in removing gasoline BTX from karst groundwater: A box-column experimental study

In-situ chemical oxidation with persulfate (PS-ISCO) is a preferred approach for the remediation of fuel-contaminated groundwater. Persulfate (PS) can be activated by various methods to produce stronger sulfate radicals for more efficient ISCO. Despite karst aquifers being widespread, there are few reports on PS-ISCO combined with Fe2+-activated PS. To better understand the effects of Fe2+-activated PS for the remediation of gasoline-contaminated aquifers in karst areas, a box-column experiment was conducted under flow conditions, using karst groundwater and limestone particles to simulate an aquifer. Gasoline was used as the source of hydrocarbon contaminants. Dissolved oxygen and nitrate were added to enhance bioremediation (EBR) and ferrous sulfate was used to activate PS. The effect of Fe2+-activated PS combined with biodegradation was compared during the periods of EBR + ISCO and ISCO alone, using the mass flow method for data analysis. The results showed that the initial dissolution of benzene, toluene, and xylene (BTX) from gasoline injection was rapid and variable, with a decaying trend at an average pseudo-first-order degradation rate constant of 0.032 d-1. Enhanced aerobic biodegradation and denitrification played a significant role in limestone-filled environments, with dissolved oxygen and nitrate utilization ratios of 59 ~ 72% and 12-70%, respectively. The efficiency of EBR + ISCO was the best method for BTX removal, compared with EBR or ISCO alone. The pseudo-first-order degradation rate constants of BTX reached 0.022-0.039, 0.034-0.070, and 0.027-0.036 d-1, during the periods of EBR alone, EBR + ISCO, and ISCO alone, respectively. The EBR + ISCO had a higher BTX removal ratio range of 71.0 ~ 84.3% than the ISCO alone with 30.1 ~ 45.1%. The presence of Fe2+-activated PS could increase the degradation rate of BTX with a range of 0.060 ~ 0.070 d-1, otherwise, with a range of 0.034-0.052 d-1. However, Fe2+-activated PS also consumed about 3 times the mass of PS, caused a further decrease in pH with a range of 6.8-7.6, increased 3-4 times the Ca2+ and 1.6-1.8 times the HCO3- levels, and decreased the BTX removal ratio of ISCO + EBR, compared to the case without Fe2+ activation. In addition, the accumulation of ferric hydroxides within a short distance indicated that the range of PS activated by Fe2+ may be limited. Based on this study, it is suggested that the effect of Fe2+-activated PS should be evaluated in the remediation of non-carbonate rock aquifers.

Controllable chemical redox reactions to couple microbial degradation for organic contaminated sites remediation: A review

Global environmental concern over organic contaminated sites has been progressively conspicuous during the process of urbanization and industrial restructuring. While traditional physical or chemical remediation technologies may significantly destroy the soil structure and function, coupling moderate chemical degradation with microbial remediation becomes a potential way for the green, economic, and efficient remediation of contaminated sites. Hence, this work systematically elucidates why and how to couple chemical technology with microbial remediation, mainly focused on the controllable redox reactions of organic contaminants. The rational design of materials structure, selective generation of reactive oxygen species, and estimation of degradation pathway are described for chemical oxidation. Meanwhile, current progress on efficient and selective reductions of organic contaminants (i.e., dechlorination, defluorination, -NO2 reduction) is introduced. Combined with the microbial remediation of contaminated sites, several consideration factors of how to couple chemical and microbial remediation are proposed based on both fundamental and practical points of view. This review will advance the understanding and development of chemical-microbial coupled remediation for organic contaminated sites.

Remediation of crude oil contaminated soil through an integrated biological-chemical-biological strategy

A plausible approach to remediating petroleum contaminated soil is the integration of chemical and biological treatments. Using appropriate chemical oxidation, the integrated remediation can be effectively achieved to stimulate the biodegradation process, consequently bolstering the overall remediation effect. In this study, an integrated biological-chemical-biological strategy was proposed. Both conventional microbial degradation techniques and a modified Fenton method were employed, and the efficacy of this strategy on crude oil contaminated soil, as well as its impact on pollutant composition, soil environment, and soil microorganism, was assessed. The results showed that this integrated remediation realized an overall 68.3 % removal rate, a performance 1.7 times superior to bioremediation alone and 2.1 times more effective than chemical oxidation alone, elucidating that the biodegradation which had become sluggish was invigorated by the judicious application of chemical oxidation. By optimizing the positioning of chemical treatment, the oxidization was allowed to act predominantly on refractory substances like resins, thus effectively enhancing pollutant biodegradability. Concurrently, this oxidating maneuver contributed to a significant increase in concentrations of dissolvable nutrients while maintaining appropriate soil pH levels, thereby generating favorable growth conditions for microorganism. Moreover, attributed to the proliferation and accumulation of degrading bacteria during the initial bioremediation phase, the microbial growth subsequent to oxidation showed rapid resurgence and the relative abundance of typical petroleum-degrading bacteria, particularly Proteobacteria, was substantially increased, which played a significant role in enhancing overall remediation effect. Our research validated the feasibility of biological-chemical-biological strategy and elucidated its correlating mechanisms, presenting a salient reference for the further studies concerning the integrated remediation of petroleum contaminated soil.

Groundwater and soil contamination by LNAPL: State of the art and future challenges

Contamination by Light Non-Aqueous Phase Liquids (LNAPL) represents a challenge due to the difficulties encountered in its underground assessment and recovery. The major risks arising from subsoil LNAPL accumulation face human health and environment, gaining a social relevance also in the frame of a continuously changing climate. This paper reports on a literature review about the underground contamination by LNAPL, with the aims of providing a categorization of the aspects involved in this topic, analyzing the current state of the art, underlying potential lacks and future perspectives. The review was focused on papers published in the 2012-2022 time-interval, in journals indexed in Scopus and WoS databases, by querying "LNAPL" within article title, abstract and/or key words. 245 papers were collected and classified according to three "key approaches" -namely laboratory activity, field based-data studies and mathematical simulations- and subordinate "key themes", so to allow summarizing and commenting the main aspects based on the application setting, content and scope. Results show that there is a wide experience on plume dynamics and evolution, detection and monitoring through direct and indirect surveys, oil recovery and natural attenuation processes. Few cues of innovations were found regarding both the use of new materials and/or specific field configuration for remediation, and the application of new techniques for plume detection. Some limitations were found in the common oversimplification of the polluted media in laboratory or mathematical models, where the contamination is set within homogeneous porous environments, and in the low number of studies focused on rock masses, where the discontinuous hydraulic behavior complicates the address and modeling of the issue. This paper represents a reference for a quick update on the addressed topic, along with a starting point to develop new ideas and cues for the advance in one of the greatest environmental banes of the current century.

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.

Preferential removal of benzene, toluene, ethylbenzene, and xylene (BTEX) by persulfate in ethanol-containing aquifer materials

The effective approaches to eliminate impacts of ethanol on the biodegradation of benzene, toluene, ethylbenzene, and xylene (BTEX) are concerned in the bioremediation of groundwater contaminated with ethanol-blended gasoline. In situ chemical oxidation (ISCO) is a common technique widely used for the remediation of contaminated groundwater. However, the selectivity of ISCO for BTEX and ethanol removal is poorly understood. Therefore, a batch experiment was performed with different aquifer materials, including calcareous soil, basalt soil, granite soil, dolomite, and sand. Gasoline was used to provide dissolved BTEX and ethanol reagent was used as additive to improve the quality of gasoline and to reduce the possibility of air pollution caused by gasoline. Persulfate (PS) was used as a chemical oxidant to oxidize organic contaminants. The target concentrations of BTEX and ethanol were 20 mg/L and 1000 mg/L, respectively. The results showed that ethanol could be preferentially degraded in the absence of PS and inhibit BTEX biodegradation. However, BTEX could be preferentially removed prior to ethanol in all aquifer materials used at ambient temperature, when PS was added at a PS/BTEX molar ratio of 150. Over 94% BTEX in sand, dolomite, and granite soil was preferentially removed with the first-order decay rate constants of 0.890-2.703 day-1 within the first ~ 10 days, followed by calcareous and basalt soil at the constants of 0.123-0.371 day-1. Ethanol could compete with BTEX for sulfate radical at the first-order decay rate constants of 0.005-0.060 day-1 for the first 25 days, which was slower than that of BTEX. The pH quickly decreased to < 2.5 in dolomite, sand, and granite soil, but maintained > 6.2 in calcareous soil. Rich organic matter in calcareous and basalt soil had an inhibition effect on BTEX oxidation by PS. The pH buffer in calcareous soil may imply the potential of PS oxidation combined with bioremediation in carbonate rock regions.

Facile one-step synthesis of 3D hierarchical flower-like magnesium peroxide for efficient and fast removal of tetracycline from aqueous solution

Hierarchically three dimensional (3D) flower-like magnesium peroxide (MgO₂) nanostructures were synthesized through a facile one-step precipitation method. The effects of magnesium salt, reaction temperature, precipitant and surfactant on the morphology and structure of MgO₂ were systematically investigated. The as-obtained samples using magnesium sulfate, ammonia and trisodium citrate were composed of 3D flowers assembled by numerous nanosheets, and SO₄^2- played a vital role in the formation of flower-like nanostructures. The 3D flower-like MgO₂ possessed high active oxygen content of 24.10 wt% and large specific surface area of 385 m²/g. Ten mg of flower-like MgO₂ could efficiently degrade 90 % of tetracycline (TC) within 60 min under stirring condition. ESR tests and radical quenching experiments suggested that hydroxyl radicals were crucial for TC degradation. Moreover, the column filled with flower-like MgO₂ could quickly and efficiently eliminate TC with the assistance of air flow, and the degradation efficiency almost had no decrease even after twenty consecutive runs. Significantly, the concentrations of magnesium and iron ions dissolved in the filtrate from the column were far below the limits of drinking water standards. Additionally, the possible degradation pathways of TC were also proposed according to the determination of generated intermediates during the degradation process.

Carbon Isotope Fractionation of Substituted Benzene Analogs during Oxidation with Ozone and Hydroxyl Radicals: How Should Experimental Data Be Interpreted?

Oxidative processes frequently contribute to organic pollutant degradation in natural and engineered systems, such as during the remediation of contaminated sites and in water treatment processes. Because a systematic characterization of abiotic reactions of organic pollutants with oxidants such as ozone or hydroxyl radicals by compound-specific stable isotope analysis (CSIA) is lacking, stable isotope-based approaches have rarely been applied for the elucidation of mechanisms of such transformations. Here, we investigated the carbon isotope fractionation associated with the oxidation of benzene and several methylated and methoxylated analogs, namely, toluene, three xylene isomers, mesitylene, and anisole, and determined their carbon isotope enrichments factors (ε_C) for reactions with ozone (ε_C = -3.6 to -4.6 ‰) and hydroxyl radicals (ε_C = 0.0 to -1.2‰). The differences in isotope fractionation can be used to elucidate the contribution of the reactions with ozone or hydroxyl radicals to overall transformation. Derivation of apparent kinetic isotope effects (AKIEs) for the reaction with ozone, however, was nontrivial due to challenges in assigning reactive positions in the probe compounds for the monodentate attack leading to an ozone adduct. We present several options for this step and compare the outcome to quantum chemical characterizations of ozone adducts. Our data show that a general assignment of reactive positions for reactions of ozone with aromatic carbons in ortho-, meta-, or para-positions is not feasible and that AKIEs of this reaction should be derived on a compound-by-compound basis.

Response of environmental variables and microbial community to sodium percarbonate addition to contaminated sediment

Sodium percarbonate (SPC) is a common reagent used for in situ remediation of contaminated soil. Current studies focus on the effects of SPC on pollutant removal; however, a knowledge gap exists for the biochemical process following SPC addition. In this study, a microcosm batch experiment was conducted to investigate the residual effect caused by different doses of SPC addition on native microbial communities, as well as on the environmental variables of contaminated sediments. The obtained results showed that the more SPC was added, the more dissolved matters were generated and the oxidation-reduction potential was lowered. Furthermore, the metabolic activities of the microbial community were enhanced and the microbial community structure responded differently to different SPC doses: the microbes that increased at high SPC dose mainly belonged to the phylum Firmicutes, the class Clostridia, and the genera Petrimonas and Proteiniclasticum. The microbes that increased at medium SPC dose mainly belonged to the class Alphaproteobacteria and the genus Brevundimonas. In contrast, vulnerable microbes mainly belonged to the phylum Acidobacteria, the class Caldisericia, Holophagae, and the genus Sulfuricurvum. Microbes capable of fermentation, ureolysis, and chemohetrotrophy increased. These results indicate that SPC addition could indirectly provide both electron acceptors and donors, thus improving the metabolic activities of the microorganisms in the contaminated sediment. Furthermore, the utilized SPC dose should be considered to achieve the optimal benefit for in situ remediation. This study forms a valuable reference for the application of SPC in ecological engineering.