Evaluation of solid polymeric organic materials for use in bioreactive sediment capping to stimulate the degradation of chlorinated aliphatic hydrocarbons

A review of the in-situ capping amendments and modeling approaches for the remediation of contaminated marine sediments

Contaminated sediments can pose long-term risks to human beings and ecosystems as they accumulate inorganic and organic contaminants becoming a sink and source of pollution. Compared to ex-situ technologies (i.e., dredging activities and off site treatments), in-situ capping (ISC) intends to minimize contaminated sediment mobilization and impact into the water column whilst treating contamination. Literature shows that numerous types of ISC amendments in presence of both organic and inorganic pollutants are investigated, although a few are contributions whose experiments have been designed and conducted with a view to future engineering. Against this background of shortcomings, this review paper intends to investigate ISC reliability, applicability and its long-term effectiveness, by also comparing reactive and physical ISCs. Additionally, an examination of the main numerical simulations applied to ISC technology was carried out. We found that activated carbon and organoclay resulted the most studied amendments for organically contaminated sediment, whereas biochar, clay minerals, and industrial-by products were more employed in presence of sediment contaminated by metal(loids). There is no better ISC system in absolute terms, since technological performance depends on many factors and only a few experimental investigations included a long-term modeling phase to predict ISC long-term efficiency. Most of numerical models included simplified transport equations based on diffusion and adsorption, and the goodness of fitting between experimental and modeled data was not always computed. The review finally discusses new research directions such as the need for long-term applications on field-scale and cap effectiveness in presence of site-specific tidal forces and currents.

Pilot-Scale Evaluation of a Permeable Reactive Barrier with Compost and Brown Coal to Treat Groundwater Contaminated with Trichloroethylene

This study evaluates, under field conditions, the efficiency of a permeable reactive barrier (PRB) with compost and brown coal to remove trichloroethylene (TCE) (109 µg/L) from contaminated groundwater. Three stainless steel boxes (1.2 × 0.5 × 0.5 m) with the brown coal-compost mixture at three different mixing ratios of 1:1, 1:3, and 1:5 (by volume) were installed to simulate the PRB. Groundwater from the TCE-contaminated aquifer was pumped into the system at a flow rate of 3.6 L/h. Residence times in the boxes were of: 25, 20, 10 h, respectively. Effluent samples were analyzed for TCE and its daughter products: dichloroethylene (DCE), vinyl chloride (VC) and ethane. During the 198-day experimental period TCE concentrations in groundwater decreased below ≤1.1 µg/L, i.e., much lower than groundwater and drinking water standards in Poland. After 16 days cis-1,2-DCE was monitored indicating possible reductive dechlorination of TCE. However, complete transformation of TCE into non-toxic byproducts was not evidenced during the time of experiments, indicating that reductive dechlorination slowed down or stopped at DCE, and that the designed residence times were not long enough to allow the complete dechlorination process.

Natural attenuation of chlorinated ethenes in hyporheic zones: A review of key biogeochemical processes and in-situ transformation potential

Chlorinated ethenes (CEs) are legacy contaminants whose chemical footprint is expected to persist in aquifers around the world for many decades to come. These organohalides have been reported in river systems with concerning prevalence and are thought to be significant chemical stressors in urban water ecosystems. The aquifer-river interface (known as the hyporheic zone) is a critical pathway for CE discharge to surface water bodies in groundwater baseflow. This pore water system may represent a natural bioreactor where anoxic and oxic biotransformation process act in synergy to reduce or even eliminate contaminant fluxes to surface water. Here, we critically review current process understanding of anaerobic CE respiration in the competitive framework of hyporheic zone biogeochemical cycling fuelled by in-situ fermentation of natural organic matter. We conceptualise anoxic-oxic interface development for metabolic and co-metabolic mineralisation by a range of aerobic bacteria with a focus on vinyl chloride degradation pathways. The superimposition of microbial metabolic processes occurring in sediment biofilms and bulk solute transport delivering reactants produces a scale dependence in contaminant transformation rates. Process interpretation is often confounded by the natural geological heterogeneity typical of most riverbed environments. We discuss insights from recent field experience of CE plumes discharging to surface water and present a range of practical monitoring technologies which address this inherent complexity at different spatial scales. Future research must address key dynamics which link supply of limiting reactants, residence times and microbial ecophysiology to better understand the natural attenuation capacity of hyporheic systems.

Evaluation of peat and sawdust as permeable reactive barrier materials for stimulating in situ biodegradation of trichloroethene

Two low cost solid organic materials, sawdust and peat, were tested in laboratory batch microcosm and flow-through column experiments to determine their suitability for application in permeable reactive barriers (PRBs) supporting biodegradation of trichloroethene (TCE). In microcosms with peat, TCE (∼30μM) was sequentially and completely degraded to cis-dichloroethene (cDCE), vinyl chloride, and ethene through reductive dechlorination. In microcosms with sawdust, reductive dechlorination of TCE stopped at cDCE and high methane production (up to 3000μM) was observed. 16S rRNA gene copy numbers of Dehalobacter and Archaea were higher (1000 and 10 times, respectively) in sawdust microcosms than those in peat microcosms. Dehalococcoides and vcrA gene copy numbers were 10 times higher in peat microcosms than in sawdust microcosms. These gene copy number differences are consistent with the extent of TCE degradation and production of methane in the microcosms. Flow-through column experiments showed that hydraulic conductivity reduction with time was consistently greater in the sawdust column compared to the peat column. The greater conductivity reduction was likely due to biofouling and methane gas bubble formation. The experimental observations indicate that peat has potential to be a better solid organic material than sawdust to support reductive dechlorination of TCE in PRB applications.

Impact of a wastewater treatment plant on microbial community composition and function in a hyporheic zone of a eutrophic river

The impact of the installation of a technologically advanced wastewater treatment plant (WWTP) on the benthic microbial community of a vinyl chloride (VC) impacted eutrophic river was examined two years before, and three and four years after installation of the WWTP. Reduced dissolved organic carbon and increased dissolved oxygen concentrations in surface water and reduced total organic carbon and total nitrogen content in the sediment were recorded in the post-WWTP samples. Pyrosequencing of bacterial 16S rRNA gene fragments in sediment cores showed reduced relative abundance of heterotrophs and fermenters such as Chloroflexi and Firmicutes in more oxic and nutrient poor post-WWTP sediments. Similarly, quantitative PCR analysis showed 1–3 orders of magnitude reduction in phylogenetic and functional genes of sulphate reducers, denitrifiers, ammonium oxidizers, methanogens and VC-respiring Dehalococcoides mccartyi. In contrast, members of Proteobacteria adapted to nutrient-poor conditions were enriched in post-WWTP samples. This transition in the trophic state of the hyporheic sediments reduced but did not abolish the VC respiration potential in the post-WWTP sediments as an important hyporheic sediment function. Our results highlight effective nutrient load reduction and parallel microbial ecological state restoration of a human-stressed urban river as a result of installation of a WWTP.

Bioremediation of 1,2-dichloroethane contaminated groundwater: Microcosm and microbial diversity studies

In this study, the effectiveness of bioremediating 1,2-dichloroethane (DCA)-contaminated groundwater under different oxidation-reduction processes was evaluated. Microcosms were constructed using indigenous bacteria and activated sludge as the inocula and cane molasses and a slow polycolloid-releasing substrate (SPRS) as the primary substrates. Complete DCA removal was obtained within 30 days under aerobic and reductive dechlorinating conditions. In anaerobic microcosms with sludge and substrate addition, chloroethane, vinyl chloride, and ethene were produced. The microbial communities and DCA-degrading bacteria in microcosms were characterized by 16S rRNA-based denatured-gradient-gel electrophoresis profiling and nucleotide sequence analyses. Real-time polymerase chain reaction was applied to evaluate the variations in Dehalococcoides spp. and Desulfitobacterium spp. Increase in Desulfitobacterium spp. indicates that the growth of Desulfitobacterium might be induced by DCA. Results indicate that DCA could be used as the primary substrate under aerobic conditions. The increased ethene concentrations imply that dihaloelimination was the dominate mechanism for DCA biodegradation.