Recovery of microbial diversity and activity during bioremediation following chemical oxidation of diesel contaminated soils

Treatment of petroleum hydrocarbon contaminated soil by combination of electro-Fenton and biosurfactant-assisted bioslurry process

Removing petroleum hydrocarbons (PHCs) from polluted soil is challenging due to their low bioavailability and degradability. In this study, an experiment was carried out to treat soil polluted with petroleum hydrocarbon using a hybrid electro-Fenton (with BDD anode electrode) and biological processes stimulated with long-chain rhamnolipids (biosurfactants). Electro-Fenton treatment was applied as a pretreatment before the biological process to enhance PHC biodegradability, which would benefit the subsequent biological process. The effects of initial pH, hydroxide concentration, soil organic matter composition, PHCs intermediates during the electro-Fenton process, and total numbers of bacteria in the biological process were analyzed to determine the optimum conditions. The results showed that the optimized electrolysis time for the electro-Fenton was 12 h. The change induced during pretreatment at a specified time was found suitable for the biological process stage and led to 93.6% PHC degradation in combination with the electro-Fenton-and-biological process after 72 h. The combined system's performance was almost 40% higher than individual electro-Fenton and biological treatments. GC-MS analysis confirms the formation of 9-octadecen-1-ol (Z), 2-heptadecene, 1-nonadecene, 1-heneicosene, and pentacosane as fragmentation during the PHCs degradation process. Thus, the electro-Fenton process as pretreatment combined with a biological process stimulated with rhamnolipids (biosurfactants) could be effectively applied to remediate soil polluted with PHCs. However, the system needs further research and investigation to optimize electrolysis time and biosurfactant dose to advance this approach in the soil remediation process.

Hydrocarbons removal and microbial community succession in petroleum-contaminated soil under hydrogen peroxide treatment

Chemical oxidation as a pretreatment step coupled with bioremediation for petroleum-contaminated soil may pose serious impacts on indigenous microorganisms and the available nutrients. Petroleum-contaminated soil were treated by hydrogen peroxide (H₂O₂) at initial concentrations of 105 mM (HH), 21 mM (HL), and 105 mM in three equal amounts (HT) without adding any external catalyst. The contents of total petroleum hydrocarbons (TPH) and dissolved nutrients (total organic compounds, nitrogen, and phosphate), and the indigenous bacteria community succession (analyzed by high-throughput sequencing of 16S rDNA) were investigated over 50 days. Compared to the control treatment without H₂O₂ addition, H₂O₂ treatments for the petroleum-contaminated soil significantly promoted the TPH removal especially in the first 4 days and impacted the contents of dissolved nutrients. Both of chemical oxidation and nutrients contributed to microbial community structure changes in alpha diversity. Although the soil microbial community structure had undergone significant changes after different chemical oxidation pretreatments, Firmicutes, Proteobacteria, Gemmatimonadetes, and Actinobacteria were the main bacterial phyla. Compared with adding H₂O₂ at one time, H₂O₂ added in stepwise was beneficial to indigenous bacterial diversity recovery and TPH removal. H₂O₂ oxidation treatments showed a great influence on the microbial community structures in the start-up stage, while recovery time rather than the oxidation treatments presented greater effects on the composition of the microbial community structure with the incubation time extended. Therefore, adding H₂O₂ as pretreatment for petroleum-contaminated soil showed little effect on the structure of soil indigenous microbial community from a long-term scale, and was conducive to the continuous removal of TPH by indigenous microorganisms.

Development and microbial characterization of Bio-RD-PAOP for effective remediation of polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) as typical halogenated persistent organic pollutants are widely distributed in natural environments, and can be enriched and magnified in organisms via food webs. It is consequently urgent and necessary to develop techniques to completely remove these persistent organohalides. In this study, we developed a process (Bio-RD-PAOP) by integrating microbial reductive dechlorination (Bio-RD) with subsequent persulfate activation and oxidation process (PAOP) for effective remediation of PCBs. Results showed the synergistic combination of advantages of Bio-RD and PAOP in dechlorination of higher-chlorinated PCBs and of PAOP in degradation/mineralization of lower-chlorinated PCBs, respectively. For the PAOP, both experimental evidences and theoretical calculations suggested that degradation rate and efficiency decreased with the increased PCB chlorine numbers. Relative to the Bio-RD and PAOP, Bio-RD-PAOP had significantly higher PCB removal efficiencies, of which values were PCB congener-specific. For example, removal efficiency of Bio-RD-PAOP in removing PCB88 is 2.50 and 1.86 times of that of Bio-RD and PAOP, respectively. In contrast, the efficiency is 1.66 and 3.35 times of Bio-RD and PAOP, respectively, for PCB180 removal. The PAOP-derived oxidizing species (mainly sulfate free radical) significantly decreased microbial abundance, particularly of the organohalide-respiring Dehalococcoides. Notably, co-existence of other microorganisms alleviated the inhibitive effect of oxidizing species on the Dehalococcoides, possibly due to formation of microbial flocs or biofilm. This study provided a promising strategy for extensive remediation of organohalide-contaminated sites, as well as new insight into impact of PAOP-derived oxidizing species on the organohalide-respiring community.

Fenton oxidation for soil remediation: A critical review of observations in historically contaminated soils

Fenton-based treatments have received tremendous attention in recent decades as viable strategies for soil decontamination. Historically contaminated soils are characterized by particular contamination types, pollution composition patterns, soil constituents, and complex soil-pollutant interactions arising due to long-term pollutant aging. These major pitfalls dictate the remediation efficiency in a significantly different way in soils with a history of contamination than that in a spiked soil. It becomes, therefore, highly challenging to treat historically contaminated soils. Despite the immense amount of collected research data in these soils, to our knowledge, no comprehensive review of this topic has been published. This article is intended to provide a critical review of the applications, limitations, and implications of various Fenton-based processes exclusively in these soils. These processes are differentiated on the basis of experimental conditions, reaction chemistry, efficiency, and impacts on soil biota. These processes are critically evaluated to illustrate the promising techniques with a brief description of related challenges and their potential solutions. Moreover, coupling Fenton oxidation with other remediation techniques such as bioremediation, chemical reduction, and soil washing has also been discussed. The last part of this review describes the effects of these processes onto soil quality and native biota, and how they can be addressed. It is also highly demanding to identify the processes which are not likely to evolve in practice either due to their poor efficiency, treatment cost, or environmental impacts. Future critical research directions have been identified to promote research for the upscaling of this technique for real field application.

Persulfate Oxidation Coupled with Biodegradation by Pseudomonas fluorescens Enhances Naphthenic Acid Remediation and Toxicity Reduction

The extraction of bitumen from the Albertan oilsands produces large amounts of oil sands process-affected water (OSPW) that requires remediation. Classical naphthenic acids (NAs), a complex mixture of organic compounds containing O₂^- species, are present in the acid extractable organic fraction of OSPW and are a primary cause of acute toxicity. A potential remediation strategy is combining chemical oxidation and biodegradation. Persulfate as an oxidant is advantageous, as it is powerful, economical, and less harmful towards microorganisms. This is the first study to examine persulfate oxidation coupled to biodegradation for NA remediation. Merichem NAs were reacted with 100, 250, 500, and 1000 mg/L of unactivated persulfate at 21 °C and 500 and 1000 mg/L of activated persulfate at 30 °C, then inoculated with Pseudomonas fluorescens LP6a after 2 months. At 21 °C, the coupled treatment removed 52.8-98.9% of Merichem NAs, while 30 °C saw increased removals of 99.4-99.7%. Coupling persulfate oxidation with biodegradation improved removal of Merichem NAs and chemical oxidation demand by up to 1.8× and 6.7×, respectively, and microbial viability was enhanced up to 4.6×. Acute toxicity towards Vibrio fischeri was negatively impacted by synergistic interactions between the persulfate and Merichem NAs; however, it was ultimately reduced by 74.5-100%. This study supports that persulfate oxidation coupled to biodegradation is an effective and feasible treatment to remove NAs and reduce toxicity.

Fast-stimulating bioremediation of macro crude oil in soils using matching Fenton pre-oxidation

This study aims at exploring the mechanism of fast-stimulating bioremediation of macro crude oil using matching Fenton pre-oxidation. The 80-day biodegradation experiment for soil S1 and S2, containing macro crude oil: C₁₉-C₂₉ and C₁₇-C₂₉ respectively, was conducted after Fenton pre-oxidation with three concentrations of H₂O₂ (225 mM, 450 mM, and 900 mM). Experimental results indicated that the bioremediation efficiency of macro crude oil was up to 57.1% (8853 mg/kg, S1) and 64.4% (11,719 mg/kg, S2) for 80-day fast-stimulating bioremediation using matching Fenton pre-oxidation (450 mM H₂O₂), which was 1.8-2.6 times that (S1: 22.2-37.1%; S2: 36.1-39.6%) for slow-stimulating bioremediation using un-matching Fenton pre-oxidation. Furthermore, the high-throughput analysis revealed that genera Sedimentibacter, Caenispirillum, and Brevundimonas became the dominant bacteria after matching Fenton pre-oxidation. Meanwhile, the highest logarithmic growth rate of indigenous hydrocarbon degraders (IHD) was obtained (S1: 64% and S2: 60%) for fast-stimulating bioremediation. And the consumption of NH₄⁺-N was up to 90% and 94% in S1 and S2 within 60 days for fast-stimulating bioremediation, approximately 1.4 and 2.2 times that (S1: 65% and 62%; S2: 47% and 41%) for slow-stimulating remediation. The results showed that the macro crude oil became the main carbon source for IHD for the fast-stimulating bioremediation, resulting in the rapid growth of IHD. Thus, this study provides a fast and efficient remediation technology for bioremediation of macro crude oil-contaminated soils.

Effect of various chemical oxidation reagents on soil indigenous microbial diversity in remediation of soil contaminated by PAHs

Chemical oxidation is a promising pretreatment step coupled with bioremediation for removal of polycyclic aromatic hydrocarbons (PAHs). The effectiveness of Fenton, modified Fenton, potassium permanganate and activated persulfate oxidation treatments on the real contaminated soils collected from a coal gas plant (263.6 ± 73.3 mg kg^-1 of the Σ16 PAHs) and a coking plant (385.2 ± 39.6 mg kg^-1 of the Σ16 PAHs) were evaluated. Microbial analyses showed only a slight impact on indigenous microbial diversity by Fenton treatment, but showed the inhibition of microbial diversity and delayed population recovery by potassium permanganate reagent. After potassium permanganate treatment, the microorganism mainly existed in the soil was Pseudomonas or Pseudomonadaceae. The results showed that total organic carbon (TOC) content in soil was significantly increased by adding modified Fenton reagent (1.4%-2.3%), while decreased by adding potassium permanganate (0.2%-1%), owing to the nonspecific and different oxidative properties of chemical oxidant. The results also demonstrated that the removal efficiency of total PAHs was ordered: permanganate (90.0%-92.4%) > activated persulfate (81.5%-86.54%) > modified Fenton (81.5%-85.4%) > Fenton (54.1%-60.0%). Furthermore, the PAHs removal efficiency was slightly increased on the 7th day after Fenton and modified Fenton treatments, about 14.6%, and 14.4% respectively, and the PAHs removal efficiency only enhanced 4.1% and 1.3% respectively from 1st to 15th day after potassium permanganate and activated persulfate treatments. The oxidants greatly affect the growth of soil indigenous microbes, which cause further influence for PAHs degradation by bioremediation.

Enhanced degradation of polycyclic aromatic hydrocarbons by indigenous microbes combined with chemical oxidation

In this study, the removal efficiency PAHs by chemical oxidation combined with microbe remediation was evaluated in two contaminated soils. The number of indigenous soil microbes decreased after the addition of chemical oxidants and then increased by nutrients addition. The total removal efficiencies of PAHs by chemical oxidation and nutrient addition followed the order: activated persulfate > potassium permanganate > modified Fenton reagent > Fenton reagent. There are 24.29-27.97%, 22.00-23.67%, 10.24-13.74% and 1.9-2.5% contributions separately due to nutrient treatment in Fenton, modified Fenton, activated persulfate and potassium permanganate treatment, which show significantly difference. The different chemical oxidants exhibited 78-90% removal efficiency for 5-6 rings PAHs, while 52-85% removal efficiency for 2-4 rings PAHs. With the addition of nutrients, the growth of indigenous microbes was enhanced significantly, and the contents of 2-4 rings PAHs in the soil were further decreased. Furthermore, the removal efficiencies of NAP and ANY were increased by more than 45%, while the removal efficiencies of ANE, FLE and PHE were about 30% at Fenton system. There was a complementary enhancing effect of microbial remediation for PAHs degradation after chemical oxidation.

Remediation of a soil chronically contaminated with hydrocarbons through persulfate oxidation and bioremediation

The impact of remediation combining chemical oxidation followed by biological treatment on soil matrix and microbial community was studied, of a chronically hydrocarbon contaminated soil sourced from a landfarming treatment. Oxidation by ammonium persulfate produced a significant elimination of polycyclic aromatic hydrocarbons (PAHs) and an increase in PAH bioavailability. Organic-matter oxidation mobilized nutrients from the soil matrix. The bacterial populations were affected negatively, with a marked diminution in the diversity indices. In this combined treatment with oxidation and bioremediation working in tandem, the aliphatic-hydrocarbon fractions were largely eliminated along with additional PAHs. The chemical and spectroscopic analyses indicated a change in soil nutrients. In spite of the high residual-sulfate concentration, a rapid recovery of the cultivable bacterial population and the establishment of a diverse and equitable microbial community were obtained. Pyrosequencing analysis demonstrated a marked succession throughout this twofold intervention in accordance with the chemical and biologic shifts observed. These remediation steps produced different effects on the soil physiology. Spectroscopic analysis became a useful tool for following and comparing those treatments, which involved acute changes in a matrix of such chronically hydrocarbon-contaminated soil. The combined treatment increased the elimination efficiency of both the aliphatic hydrocarbons and the PAHs at the expense of the mobilized organic matter, thus sustaining the recovery of the resilient populations throughout the treatment. The high-throughput-DNA-sequencing techniques enabled the identification of the predominant populations that were associated with the changes observed during the treatments.

Impact of the Fenton-like treatment on the microbial community of a diesel-contaminated soil

Fenton-like treatment (FLT) is an ISCO technique relying on the iron-induced H₂O₂ activation in the presence of additives aimed at increasing the oxidant lifetime and maximizing iron solubility under natural soil pH conditions. The efficacy of FLT in the clean-up of hydrocarbon-contaminated soils is well established at the field-scale. However, a better assessment of the impact of the FLT on density, diversity and activity of the indigenous soil microbiota, might provide further insights into an optimal combination between FLT and in-situ bioremediation (ISB). The aim of this work was to assess the impacts of FLT on the microbial community of a diesel-contaminated soil collected nearby a gasoline station. Different FLT conditions were tested by varying either the H₂O₂ concentrations (2 and 6%) or the oxidant application mode (single or double dosage). The impact of these treatments on the indigenous microbial community was assessed immediately after the Fenton-like treatment and after 30, 60 and 90 d and compared with enhanced natural attenuation (ENA). After FLT, a dramatic decrease in bacterial density, diversity and functionality was evident. Although in microcosms with double dosing at 2% H₂O₂ a delayed recovery of the indigenous microbiota was observed as compared to those subjected to single oxidant dose, after 60 d incubation the respiration rate increased from 0.036 to 0.256 μg CCO₂ g^-1soil h^-1. Irrespective of the oxidant dose, best degradation results after 90 d incubation (around 80%) were observed with combined FLT, relying on double oxidant addition, and bioremediation.

Effect of Fenton pre-oxidation on mobilization of nutrients and efficient subsequent bioremediation of crude oil-contaminated soil

Fenton pre-oxidation and a subsequent bioremediation phase of 80 days were used to investigate the importance of matching concentration of residual indigenous bacteria and nutrient levels on subsequent bioremediation of crude oil. Experiments were performed using either high (>10^7.7 ± 0.2 CFU/g soil) or low (<10^5.9 ± 0.1 CFU/g soil) concentrations of bacteria and three different nutrient levels: enough (C/N > 9.8), moderate (C/N:5-9.8), and lacking nutrient level (C/N < 5) conditions. Weak Fenton pre-oxidation (225 mM H₂O₂ and 2.9 mM Fe²⁺) resulted in highly matching between nutrient level and the population of residual indigenous bacteria. Up to 53% of total petroleum hydrocarbon (TPH) and 58% of main hydrocarbon (C₁₅C₂₅, during the first 10 days) were removed from the soil. Under matching conditions, the activity of indigenous bacteria and nutrient mobilization were enhanced, promoting the bioremediation of crude oil. In addition, the biodegradation of long chain molecules (C₂₆C₃₀) required a high level of NH₄⁺-N.

Combination of Fenton processes and biotreatment for wastewater treatment and soil remediation

There is a continuously increasing worldwide concern for the development of wastewater and contaminated soil treatment technologies. Fenton processes and biological treatments have long been used as common technologies for treating wastewater and polluted soil but they still need to be modified because of some defects (high costs of Fenton process and long remediation time of biotreatments). This work first briefly introduced the Fenton technology and biotreatment, and then discussed the main considerations in the construction of a combined system. This review shows a critical overview of recent researches combining Fenton processes (as pre-treatment or post-treatment) with bioremediation for treatment of wastewater or polluted soil. We concluded that the combined treatment can be regarded as a novel and competitive technology. Furthermore, the outlook for potential applications of this combination in different polluted soil and wastewater, as well as the mechanism of combination was also discussed.

Impact of chemical oxidation on indigenous bacteria and mobilization of nutrients and subsequent bioremediation of crude oil-contaminated soil

Fenton pre-oxidation provides nutrients to promote bioremediation. However, the effects of the indigenous bacteria that remain following Fenton oxidation on nutrient mobilization and subsequent bioremediation remain unclear. Experiments were performed with inoculation with native bacteria and foreign bacteria or without inoculation after four regimens of stepwise pre-oxidations. The effects of the indigenous bacteria remaining after stepwise oxidation on nutrient mobilization and subsequent bioremediation over 80 days were investigated. After stepwise Fenton pre-oxidation at a low H₂O₂ concentration (225×4), the remaining indigenous bacterial populations reached their peak (4.8±0.17×10⁶CFU/g), the nutrients were mobilized rapidly, and the subsequent bioremediation of crude oil was improved (biodegradation efficiency of 35%). However, after stepwise Fenton pre-oxidation at a high H₂O₂ concentration (450×4), only 3.6±0.16×10³CFU/g of indigenous bacteria remained, and the indigenous bacteria that degrade C₁₅-C₃₀ alkanes were inhibited. The nutrient mobilization was then highly limited, and only 19% of total petroleum hydrocarbon was degraded. Furthermore, the recovery period after the low H₂O₂ concentration stepwise Fenton pre-oxidation (225×4) was less than 20 days, which was 20-30 days shorter than with the other pre-oxidation treatments. Therefore, stepwise Fenton pre-oxidation at a low H₂O₂ concentration protects indigenous bacterial populations and improves the nutrient mobilization and subsequent bioremediation.

Biostimulation proved to be the most efficient method in the comparison of in situ soil remediation treatments after a simulated oil spill accident

The use of in situ techniques in soil remediation is still rare in Finland and most other European countries due to the uncertainty of the effectiveness of the techniques especially in cold regions and also due to their potential side effects on the environment. In this study, we compared the biostimulation, chemical oxidation, and natural attenuation treatments in natural conditions and pilot scale during a 16-month experiment. A real fuel spill accident was used as a model for experiment setup and soil contamination. We found that biostimulation significantly decreased the contaminant leachate into the water, including also the non-aqueous phase liquid (NAPL). The total NAPL leachate was 19 % lower in the biostimulation treatment that in the untreated soil and 34 % lower in the biostimulation than oxidation treatment. Soil bacterial growth and community changes were first observed due to the increased carbon content via oil amendment and later due to the enhanced nutrient content via biostimulation. Overall, the most effective treatment for fresh contaminated soil was biostimulation, which enhanced the biodegradation of easily available oil in the mobile phase and consequently reduced contaminant leakage through the soil. The chemical oxidation did not enhance soil cleanup and resulted in the mobilization of contaminants. Our results suggest that biostimulation can decrease or even prevent oil migration in recently contaminated areas and can thus be considered as a potentially safe in situ treatment also in groundwater areas.

Reflection on Molecular Approaches Influencing State-of-the-Art Bioremediation Design: Culturing to Microbial Community Fingerprinting to Omics

Bioremediation is generally viewed as a cost effective and sustainable technology because it relies on microbes to transform pollutants into benign compounds. Advances in molecular biological analyses allow unprecedented microbial detection and are increasingly incorporated into bioremediation. Throughout history, state-of-the-art techniques have informed bioremediation strategies. However, the insights those techniques provided were not as in depth as those provided by recently developed omics tools. Advances in next generation sequencing (NGS) have now placed metagenomics and metatranscriptomics within reach of environmental engineers. As NGS costs decrease, metagenomics and metatranscriptomics have become increasingly feasible options to rapidly scan sites for specific degradative functions and identify microorganisms important in pollutant degradation. These omic techniques are capable of revolutionizing biological treatment in environmental engineering by allowing highly sensitive characterization of previously uncultured microorganisms. Omics enables the discovery of novel microorganisms for use in bioaugmentation and supports systematic optimization of biostimulation strategies. This review describes the omics journey from roots in biology and medicine to its current status in environmental engineering including potential future directions in commercial application.

A novel three-stage treatment train for the remediation of trichloroethylene-contaminated groundwater

The proposed treatment train removed TCE and its by-products effectively and there was no problem with the connection of chemical oxidation and anaerobic bioremediation in the novel treatment train technology.

Coupling chemical oxidation and biostimulation: Effects on the natural attenuation capacity and resilience of the native microbial community in alkylbenzene-polluted soil

Coupling chemical oxidation with bioremediation could be a cost-effective system to cope with soil and groundwater pollution. However, the effects of chemical oxidation on autochthonous microbial communities are scarcely known. A detailed analysis that considers both the efficiency of the two technologies and the response of the microbial communities was performed on a linear alkylbenzene-polluted soil and groundwater samples. The impacts of a modified Fenton's reaction (MFR) at various dosages and of permanganate on the microbiota over 4 weeks were assessed. The permanganate and MFR negatively affected microbial abundance and activity. However, the resilience of certain microbial populations was observed, with a final increase in potential hydrocarbon-degrading populations as determined by both the alkB gene abundance and the predominance of well-known hydrocarbon-degrading phylotypes such as Rhodococcus, Ochrobactrum, Acinetobacter and Cupriavidus genera as determined by 16S rRNA-based DGGE fingerprinting. The assessment of the chemical oxidant impact on autochthonous microbiota should be considered for the optimization of coupled field remediation technologies.

Microbial Community Response of an Organohalide Respiring Enrichment Culture to Permanganate Oxidation

While in situ chemical oxidation is often used to remediate tetrachloroethene (PCE) contaminated locations, very little is known about its influence on microbial composition and organohalide respiration (OHR) activity. Here, we investigate the impact of oxidation with permanganate on OHR rates, the abundance of organohalide respiring bacteria (OHRB) and reductive dehalogenase (rdh) genes using quantitative PCR, and microbial community composition through sequencing of 16S rRNA genes. A PCE degrading enrichment was repeatedly treated with low (25 μmol), medium (50 μmol), or high (100 μmol) permanganate doses, or no oxidant treatment (biotic control). Low and medium treatments led to higher OHR rates and enrichment of several OHRB and rdh genes, as compared to the biotic control. Improved degradation rates can be attributed to enrichment of (1) OHRB able to also utilize Mn oxides as a terminal electron acceptor and (2) non-dechlorinating community members of the Clostridiales and Deltaproteobacteria possibly supporting OHRB by providing essential co-factors. In contrast, high permanganate treatment disrupted dechlorination beyond cis-dichloroethene and caused at least a 2-4 orders of magnitude reduction in the abundance of all measured OHRB and rdh genes, as compared to the biotic control. High permanganate treatments resulted in a notably divergent microbial community, with increased abundances of organisms affiliated with Campylobacterales and Oceanospirillales capable of dissimilatory Mn reduction, and decreased abundance of presumed supporters of OHRB. Although OTUs classified within the OHR-supportive order Clostridiales and OHRB increased in abundance over the course of 213 days following the final 100 μmol permanganate treatment, only limited regeneration of PCE dechlorination was observed in one of three microcosms, suggesting strong chemical oxidation treatments can irreversibly disrupt OHR. Overall, this detailed investigation into dose-dependent changes of microbial composition and activity due to permanganate treatment provides insight into the mechanisms of OHR stimulation or disruption upon chemical oxidation.

Microbial dynamics during and after in situ chemical oxidation of chlorinated solvents

In situ chemical oxidation (ISCO) followed by a bioremediation step is increasingly being considered as an effective biphasic technology. Information on the impact of chemical oxidants on organohalide respiring bacteria (OHRB), however, is largely lacking. Therefore, we used quantitative PCR (qPCR) to monitor the abundance of OHRB (Dehalococcoides mccartyi, Dehalobacter, Geobacter, and Desulfitobacterium) and reductive dehalogenase genes (rdh; tceA, vcrA, and bvcA) at a field location contaminated with chlorinated solvents prior to and following treatment with sodium persulfate. Natural attenuation of the contaminants tetrachloroethene (PCE) and trichloroethene (TCE) observed prior to ISCO was confirmed by the distribution of OHRB and rdh genes. In wells impacted by persulfate treatment, a 1 to 3 order of magnitude reduction in the abundances of OHRB and complete absence of rdh genes was observed 21 days after ISCO. Groundwater acidification (pH<3) and increase in the oxidation reduction potential (>500 mV) due to persulfate treatment were significant and contributed to disruption of the microbial community. In wells only mildly impacted by persulfate, a slight stimulation of the microbial community was observed, with more than 1 order of magnitude increase in the abundance of Geobacter and Desulfitobacterium 36 days after ISCO. After six months, regeneration of the OHRB community occurred, however, neither D. mccartyi nor any rdh genes were observed, indicating extended disruption of biological natural attenuation (NA) capacity following persulfate treatment. For full restoration of biological NA activity, additional time may prove sufficient; otherwise addition electron donor amendment or bioaugmentation may be required.

Geochemical and microbiological characteristics during in situ chemical oxidation and in situ bioremediation at a diesel contaminated site

While in situ chemical oxidation with persulfate has seen wide commercial application, investigations into the impacts on groundwater characteristics, microbial communities and soil structure are limited. To better understand the interactions of persulfate with the subsurface and to determine the compatibility with further bioremediation, a pilot scale treatment at a diesel-contaminated location was performed consisting of two persulfate injection events followed by a single nutrient amendment. Groundwater parameters measured throughout the 225 day experiment showed a significant decrease in pH and an increase in dissolved diesel and organic carbon within the treatment area. Molecular analysis of the microbial community size (16S rRNA gene) and alkane degradation capacity (alkB gene) by qPCR indicated a significant, yet temporary impact; while gene copy numbers initially decreased 1-2 orders of magnitude, they returned to baseline levels within 3 months of the first injection for both targets. Analysis of soil samples with sequential extraction showed irreversible oxidation of metal sulfides, thereby changing subsurface mineralogy and potentially mobilizing Fe, Cu, Pb, and Zn. Together, these results give insight into persulfate application in terms of risks and effective coupling with bioremediation.