Natural attenuation processes during in situ capping

Growth of Dehalococcoides spp. and increased abundance of reductive dehalogenase genes in anaerobic PCB-contaminated sediment microcosms

Polychlorinated biphenyls (PCBs) contaminate 19% of US Superfund sites and represent a serious risk to human and environmental health. One promising strategy to remediate PCB-contaminated sediments utilizes organohalide-respiring bacteria (OHRB) that dechlorinate PCBs.However, functional genes that act as biomarkers for PCB dechlorination processes (i.e., reductive dehalogenase genes) are poorly understood. Here, we developed anaerobic sediment microcosms that harbor an OHRB community dominated by the genus Dehalococcoides. During the 430-day microcosm incubation, Dehalococcoides 16S rRNA sequences increased two orders of magnitude to 10⁷ copies/g of sediment, and at the same time, PCB118 decreased by as much as 70%. In addition, the OHRB community dechlorinated a range of penta- and tetra-chlorinated PCB congeners including PCBs 66, 70 + 74 + 76, 95, 90 + 101, and PCB110 without exogenous electron donor. We quantified candidate reductive dehalogenase (RDase) genes over a 430-day incubation period and found rd14, a reductive dehalogenase that belongs to Dehalococcoides mccartyi strain CG5, was enriched to 10⁷ copies/g of sediment. At the same time, pcbA5 was enriched to only 10⁵ copies/g of sediment. A survey for additional RDase genes revealed sequences similar to strain CG5's rd4 and rd8. In addition to demonstrating the PCB dechlorination potential of native microbial communities in contaminated freshwater sediments, our results suggest candidate functional genes with previously unexplored potential could serve as biomarkers of PCB dechlorination processes.

Response of microbial communities to different organochlorine pesticides (OCPs) contamination levels in contaminated soils

Understanding microbial community structure and diversity in contaminated soils helps optimize the bioremediation strategies and performance. This study investigated the roles of environmental variables and contamination levels of organochlorine pesticides (OCPs) in shaping microbial community structure at an abandoned aged insecticide plant site. In total, 28 bacterial phyla were identified across soils with different physiochemical properties and OCPs levels. Proteobacteria, Bacterioidetes and Firmicutes represented the dominant lineages, and accounted for 60.2%-69.2%, 5.6%-9.7% and 6.7%-9.4% of the total population, respectively. The overall microbial diversities, in terms of phylogenetic diversity and phylotype richness, were correlated with the contents of hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs) in soils, as well as other soil properties including total nitrogen, dissolved organic carbon, pH and vegetation. The multivariate regression tree (MRT) analysis revealed that the soil microbial diversity was significantly impacted by vegetation, which explained 31.8% of the total variation, followed by OCPs level (28.3%), total nitrogen (12.4%), dissolved organic carbon (6.3%) and pH (2.4%). Our findings provide new insights and implications into the impacts on soil microbial community by OCPs contamination and other environmental variables, and offer potential strategic bioremediation for the management of OCPs contaminated sites.

Using biochar capping to reduce nitrogen release from sediments in eutrophic lakes

The effects of reduced nitrogen release from sediments were studied using biochar (BC) capping in simulated water-sediment systems. Dried solid waste of Phyllostachys pubescens was used to produce BC, which was then pyrolyzed at 500 °C. Subsequently, 14 sediment cores were collected, including the sediment-water interface and some overlying water, from two sites in Baiyangdian Lake (China). The sediment cores were split into two batches (A and B), and then two each were capped with soil, BC or a BC/soil mixture, and incubated for 30 days. In the BC capped cores, the ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N) and total nitrogen (TN) concentrations decreased from 0.90 mg·L^-1 to 0.05 mg·L^-1, 0.88 mg·L^-1 to 0.18 mg·L^-1, 6.93 mg·L^-1 to 2.81 mg·L^-1, respectively, in batch A and 3.51 mg·L^-1 to 0.11 mg·L^-1, 0.92 mg·L^-1 to 0.61 mg·L^-1, 8.88 mg·L^-1 to 3.32 mg·L^-1, respectively, in batch B. The sediments to water fluxes of NH₄⁺-N, NO₃^--N and TN were greatly reduced or reversed. Compared with other cappings, the BC layer was shown to absorb more NH₄⁺-N from the pore water, thereby breaking the diffusion gradient of NH₄⁺-N at the sediment-water interface, and has a good inhibitory effect on the endogenous release of NH₄⁺-N from the sediments. Additionally, in the BC capped cores, the redox potential remarkably increased and dissolved oxygen was comparatively high. This study suggests that BC capping can reduce the amount of nitrogen released from polluted sediments because the diffusion of nitrogen to the overlying water is chemically blocked by the cap.

Evaluation of sediment capping with activated carbon and nonwoven fabric mat to interrupt nutrient release from lake sediments

The aim of this study was to assess the potential application of activated carbon (AC) and nonwoven fabric mats (NWFM) for thin-layer capping in remediation of sediments containing high amounts of carbon, nitrogen, and phosphorus. Laboratory column incubation experiments were performed to analyze the efficiencies of AC and NWFM for blocking nutrients. Under uncapped conditions, dissolved oxygen (DO) was exhausted within three days but under NWFM/AC capping conditions (with NWFM above the AC capping layer), the presence of DO was prolonged until Day 33. Chemical oxygen demand (COD) was lower under all capped conditions than under uncapped conditions, with lowest COD observed with NWFM/AC capping. NH₄-N occupied the highest percentage of total nitrogen in the overlying water and its percentage increased as the DO concentration decreased. The capping efficiencies for NH₄-N, T-N, and PO₄-P with NWFM/AC capping were (66.0, 54.2, and 73.1) %, respectively, which were higher than for other capping conditions. In the case of T-P, capping efficiencies under all capping conditions were almost 100%, indicating that both AC and NWFM effectively interrupted phosphorus release from sediments. Placing NWFM above the AC capping layer was more effective than the opposite arrangement. It can be concluded that NWFM and AC can be successfully used for remediation of lake sediments with high amounts of nitrogen and phosphorus.

Ex-Situ Remediation Technologies for Environmental Pollutants: A Critical Perspective

Pollution and the global health impacts from toxic environmental pollutants are presently of great concern. At present, more than 100 million people are at risk from exposure to a plethora of toxic organic and inorganic pollutants. This review is an exploration of the ex-situ technologies for cleaning-up the contaminated soil, groundwater and air emissions, highlighting their principles, advantages, deficiencies and the knowledge gaps. Challenges and strategies for removing different types of contaminants, mainly heavy metals and priority organic pollutants, are also described.

Reducing the recruitment of sedimented algae and nutrient release into the overlying water using modified soil/sand flocculation-capping in eutrophic lakes

The effect of simultaneously removing algal blooms from water and reducing the resuspension and nutrient release from the sediment was studied using modified local soil/sand flocculation-capping (MLS-capping) in simulated water-sediment systems. Twenty one sediment cores in situ with overlying water containing algal blooms were collected from Meiliang Bay of Lake Taihu (China) in July 2011. The algal cells in the water were flocculated and sunk to the sediment using chitosan modified local soils, and the algal flocs were capped with modified and nonmodified soil/sand and then incubated at 25 °C for 20 days. In the MLS-capping treated systems, the TP concentration was reduced from 2.56 mg P L(-1) to 0.06-0.14 mg P L(-1) and TN from 14.66 mg N L(-1) to 6.03-9.56 mg N L(-1) throughout the experiment, whereas the sediment to water fluxes of TP, TN, PO(4)-P, and NH(4)-N were greatly reduced or reversed and the redox potential remarkably increased compared to the control system. A capping layer of 1 cm chitosan-modified sand decreased the resuspension of the sediment by a factor of 5 compared to the clay/soil/sediment systems and the overlying water kept clear even under constant stirring conditions (200 rpm). The study suggested that by using MLS-capping technology it is possible to quickly reduce the nutrient and turbidity of water by flocculating and capping the algal cells into the sediment, where the resuspension of algal flocs is physically reduced and the diffusion of nutrients from sediment to the overlying water chemically blocked by the MLS capping layers.

Evaluation of a laboratory-scale bioreactive in situ sediment cap for the treatment of organic contaminants

The development of bioreactive sediment caps, in which microorganisms capable of contaminant transformation are placed within an in situ cap, provides a potential remedial design that can sustainably treat sediment and groundwater contaminants. The goal of this study was to evaluate the ability and limitations of a mixed, anaerobic dechlorinating consortium to treat chlorinated ethenes within a sand-based cap. Results of batch experiments demonstrate that a tetrachloroethene (PCE)-to-ethene mixed consortium was able to completely dechlorinate dissolved-phase PCE to ethene when supplied only with sediment porewater obtained from a sediment column. To simulate a bioreactive cap, laboratory-scale sand columns inoculated with the mixed culture were placed in series with an upflow sediment column and directly supplied sediment effluent and dissolved-phase chlorinated ethenes. The mixed consortium was not able to sustain dechlorination activity at a retention time of 0.5 days without delivery of amendments to the sediment effluent, evidenced by the loss of cis-1,2-dichloroethene (cis-DCE) dechlorination to vinyl chloride. When soluble electron donor was supplied to the sediment effluent, complete dechlorination of cis-DCE to ethene was observed at retention times of 0.5 days, suggesting that sediment effluent lacked sufficient electron donor to maintain active dechlorination within the sediment cap. Introduction of elevated contaminant concentrations also limited biotransformation performance of the dechlorinating consortium within the cap. These findings indicate that in situ bioreactive capping can be a feasible remedial approach, provided that residence times are adequate and that appropriate levels of electron donor and contaminant exist within the cap.

Kinetics and inhibition of reductive dechlorination of trichloroethene, cis-1,2-dichloroethene and vinyl chloride in a continuously fed anaerobic biofilm reactor

Anaerobic bioreactors containing Dehalococcoides spp. can be effective for the treatment of trichloroethene (TCE) contamination. However, reductive dehalogenation of TCE often results in partial conversion to harmless ethene, and significant production of undesired cis-1,2-dichloroethene (cis-DCE) and vinyl chloride (VC) is frequently observed. Here, a detailed modeling study was conducted focusing on the determination of biokinetic constants for the dechlorination of TCE and its reductive dechlorination intermediates cis-DCE and VC as well as any biokinetic inhibition that may exist between these compounds. Dechlorination data from an anaerobic biotrickling filter containing Dehalococcoides spp. fed with single compounds (TCE, cis-DCE, or VC) were fitted to the model to determine biokinetic constants. Experiments with multiple compounds were used to determine inhibition between the compounds. It was found that the Michaelis-Menten half-saturation constants for all compounds were higher than for cells grown in suspended cultures, indicating a lower enzyme affinity in biofilm cells. It was also observed that TCE competitively inhibited the dechlorination of cis-DCE and had a mild detrimental effect on the dechlorination of VC. Thus, careful selection of biotreatment conditions, possibly with the help of a model such as the one presented herein, is required to minimize the production of partially dechlorinated intermediates.

Biogeochemical changes and mercury methylation beneath an in-situ sediment cap

In-situ capping has shown promise as a management strategy for contaminated aquatic sediments, however, little is known about how mercury methylation in underlying sediments will be affected. Changes to the location and extent of sulfate reduction and other biological processes were studied in estuarine sediment using laboratory microcosms. Observations in a model sediment showed increases of in situ total methylmercury concomitant with an upward extension of anaerobic bacterial activity beneath a sediment cap and under anoxic conditions. Increased methylmercury (up to 50%) was observed beneath a sediment cap in a region 2-3 cm higher than in an uncapped sediment. A 1-dimensional, unsteady, reaction transport model was used to simulate the transient response to mercury-related biogeochemical processes. The location, magnitude, and expected duration of the increased methylmercury was such that a significant impact on near cap-water interface methylmercury was not expected for the sediments studied. Explicit consideration of the biogeochemical effects of capping on mercury contaminated sediment, however, may be necessary for very thin or unstable capping layers where the physical sequestration provided by a cap may be compromised.

Effects of natural organic matter on PCB-activated carbon sorption kinetics: implications for sediment capping applications

In situ capping of polychlorinated biphenyl (PCB)-contaminated sediments with a layer of activated carbon has been proposed, but several questions remain regarding the long-term effectiveness of this remediation strategy. Here, we assess the degree to which kinetic limitations, size exclusion effects, and electrostatic repulsions impaired PCB sorption to activated carbon. Sorption of 11 PCB congeners with activated carbon was studied in fixed bed reactors with organic-free water (OFW) and Suwannee River natural organic matter (SR-NOM), made by reconstituting freeze-dried SR-NOM at a concentration of 10 mg L(-1) as carbon. In the OFW test, no PCBs were detected in the column effluent over the 390-d study, indicating that PCB-activated carbon equilibrium sorption capacities may be achieved before breakthrough even at the relatively high hydraulic loading rate (HLR) of 3.1 m h(-1). However, in the SR-NOM fixed-bed test, partial PCB breakthrough occurred over the entire 320-d test (HLRs of 3.1-, 1.5-, and 0.8 m h(-1)). Simulations from a modified pore and surface diffusion model indicated that external (film diffusion) mass transfer was the dominant rate-limiting step but that internal (pore diffusion) mass transfer limitations were also present. The external mass transfer limitation was likely caused by formation of PCB-NOM complexes that reduced PCB sorption through a combination of (i) increased film diffusion resistance; (ii) size exclusion effects; and (iii) electrostatic repulsive forces between the PCBs and the NOM-coated activated carbon. However, the seepage velocities in the SR-NOM fixed bed test were about 1000 times higher than would be expected in a sediment cap. Therefore, additional studies are needed to assess whether the mass transfer limitations described here would be likely to manifest themselves at the lower seepage velocities observed in practice.

Review: In situ and bioremediation of organic pollutants in aquatic sediments

Organic pollutants in sediments are a worldwide problem because sediments act as sinks for hydrophobic, recalcitrant and hazardous compounds. Depending on biogeochemical processes these hydrocarbons are involved in adsorption, desorption and transformation processes and can be made available to benthic organisms as well as organisms in the water column through the sediment-water interface. Most of these recalcitrant hydrocarbons are toxic and carcinogenic, they may enter the food-chain and accumulate in biological tissue. Several approaches are being investigated or have been already used to remove organic hydrocarbons from sediments. This paper provides a review on types and sources of organic pollutants as well as their behavior in sediments. It presents the advantages and disadvantages of traditional sediment remediation techniques in use, such as dredging, capping and monitored natural attenuation. Furthermore, it describes new approaches with emphasis on bioremediation, like biostimulation, bioaugmentation and phytoremediation applied to sediments. These new techniques promise to be of lower impact and more cost efficient than traditional management strategies.

PCB dechlorination enhancement in Anacostia River sediment microcosms

In situ treatment of PCB contaminated sediments via microbial dechlorination is a promising alternative to dredging, which may be reserved for only the most contaminated areas. Reductive dechlorination of low levels of weathered PCB mixtures typical of urban environments may occur at slow rates. Here, we report that biostimulation and bioaugmentation enhanced dechlorination of low concentration (2.1 mg PCBs/kg dry weight) historical PCBs in microcosms prepared with Anacostia River, Washington, DC, sediment. Treatments included electron donors butyrate, lactate, propionate and acetate (1 mM each); alternate halogenated electron acceptors (haloprimers) tetrachlorobenzene (TeCB, 25 microM), pentachloronitrobenzene (PCNB, 25 microM), or 2,3,4,5,6-PCB (PCB116, 2.0 microM); and/or bioaugmentation with a culture containing Dehalococcoides ethenogenes strain 195 (3 x 10(6)cells/mL). Dechlorination rates were enhanced in microcosms receiving bioaugmentation, PCNB and PCNB plus bioaugmentation, compared to other treatments. Microcosm subcultures generated after 415 days and spiked with PCB116 showed sustained capacity for dechlorination of PCB116 in PCNB, PCNB plus bioaugmentation, and TeCB treatments, relative to other treatments. Analysis of Chloroflexi 16S rRNA genes showed that TeCB and PCNB increased native Dehalococcoides spp. from the Pinellas subgroup; however this increase was correlated to enhanced dechlorination of low concentration weathered PCBs only in PCNB-amended microcosms. D. ethenogenes strain 195 was detected only in bioaugmented microcosms and decreased over 281 days. Bioaugmentation with D. ethenogenes strain 195 increased PCB dechlorination rates initially, but enhanced capacity for dechlorination of a model congener, PCB116, after 415 days occurred only in microcosms with enhanced native Dehalococcoides spp.

Microbial colonization of an in situ sediment cap and correlation to stratified redox zones

In situ capping is a management technique for contaminated sediments involving the placement of clean material at the sediment-water interface. This work combined porewater geochemical profiling with quantitative microbial data to investigate the intrinsic microbial colonization of a sand cap. Geochemical characterization using voltammetric microelectrodes indicated vertical stratification of biogeochemical processes within a capped sediment column. Following dissection of the column, quantitative real-time PCR (qPCR) enumerated microbial populations within each discrete redoxzone and was accompanied by terminal-restriction fragment length polymorphism (T-RFLP) to elucidate general community shifts. Bacteria and Archaea were present within the cap according to qPCR, with higher concentrations generally observed in the underlying sediment. Iron-reducing populations were detected and quantified using newly designed qPCR primer pairs for Anaeromyxobacter spp. and Shewanella spp. and published primer sets for delta-Proteobacteria and Geobacteracea. Results confirmed geochemical measurements indicating that microbial Fe(III) reduction was a major process in the overlying cap. Genes encoding microbial sulfate reduction (dsrA) and methanogenesis (mcrA) were also present within the cap but were more prevalent in the sediment. Canonical correspondence analysis of terminal-restriction fragment length polymorphism (T-RFLP) patterns verified that spatial changes in bacterial community composition were significantly correlated to depth and Fe2+ and Mn2+ concentration gradients. Cumulatively, results demonstrate that microorganisms indigenous to aquatic sediments colonized the overlying cap to form complex communities mirroring redox stratification. Implications of capping for biogeochemical cycling, contaminant fate and transport, and remedial design are discussed.

Spatial and temporal evolution of biogeochemical processes following in situ capping of contaminated sediments

In situ capping has recently emerged as a remedial method for contaminated sediments and involves placing a layer of clean material at the sediment-water interface. The biogeochemical response of native sediment following capping, as well as the redoxenvironmentsthatdevelopwithinthe cap, are currently unknown. Column experiments were performed using voltammetric microelectrodes to characterize spatial and temporal distributions of biogeochemical processes in capped sediments under stagnant and upflow conditions. Oxygen penetration into sand caps extended only a few centimeters, thus maintaining underlying sediment anaerobic. Chemical species indicative of heterotrophic organic matter degradation (Mn2+, Fe2+, organic--FeIII(aq), FexSy(aq), sigmaH2S) were observed in stratified zones below the oxic layer. The majority of the overlying cap was subject to iron-reducing conditions under stagnant flow, while upflow conditions led to a compression of the redox zones toward the cap-water interface. Controls confirmed that sediment capping induced an upward, vertical shift of biogeochemical processes into the overlying cap, with redox stratification conserved. The redox conditions within the cap, specifically the predominance of iron reduction, should allow for reductive contaminant attenuation processes to extend into the overlying cap. These findings improve our understanding of the dynamics of biogeochemical processes following capping of contaminated sediments.