Multi-Component Mass Transfer and Chemical Oxidation

The remediation of soil and groundwater contaminated with multi-component non-aqueous phase liquids (NAPLs) such as coal tars from former manufactured gas plants (MGPs) is associated with a number of challenges. Due to thermodynamic considerations, the presence of more than one compound within multi-component NAPLs (especially when they are structurally dissimilar) can restrict intra-NAPL diffusion. Since diffusion is the dominant process in the dissolution of organic compounds, any diffusion limitations can restrict mass transfer between the NAPL and the aqueous phase. Consequently, the efficiency of conventional water-based remediation methods can be restricted. In situ chemical oxidation (ISCO) has been a possible remediation technology touted for the treatment of multi-component NAPLs. However, chemical oxidation occurs only in the aqueous phase and consequently the mass transfer between NAPLs and the aqueous phase indirectly controls the overall treatment efficiency. The primary objective of this research effort was to theoretically and experimentally investigate mass transfer processes from complex multi-component NAPLs subjected to water and chemical oxidants. For the purpose of this evaluation, the feasibility of chemical oxidants to degrade MGP residuals needs to be quantified. A series of physical model trials supported by a host of aqueous and slurry batch experiments were conducted to assess the performance of two chemical oxidants (persulfate and permanganate) using impacted sediments collected from a former MGP site. The results indicated that dissolved components were readily degraded with persulfate or permanganate (except for benzene) in the aqueous batch systems. In addition, in the well-mixed slurry systems when contact with the oxidant was achieved, permanganate, unactivated persulfate, and alkaline activated persulfate were able to degrade >95%, 45% and 30% of the initial mass quantified, respectively. However, insignificant quantifiable mass was lost in all physical models under dynamic conditions which are more representative of in situ conditions. A simple single-cell numerical model was constrained by the experimental results and used to investigate treatment expectations and the potential long-term behaviour of dissolved phase concentrations as a result of treatment using 6 pore volumes of oxidant. A specified inlet oxidant concentration and NAPL composition (22 compounds (34 %), and bulk mass (66 %) composed of unidentified material) were prescribed, and the effluent concentrations of the known soluble constituents were estimated from mass balance considerations. A variety of long-term simulation scenarios were performed. In general, for a NAPL saturation of 6 %, the results indicated that the effluent profiles over a 10-year period were reduced temporality as a result of the oxidant injection and then rebounded to a profile that was coincident with a no-treatment scenario. Based on a sensitivity analyses, neither water velocity or oxidant concentration affected the long-term behavior of dissolved phase concentrations; however, increasing the mass transfer rate coefficient had a dramatic impact, and chemical oxidant injections were only effective for low NAPL saturations (<1 %). Intra-NAPL diffusion is one of the most critical processes which can influence NAPL-water mass transfer processes. A comprehensive experimental and computational study was performed to investigate the role of intra-NAPL diffusion on the mass transfer between multi-component NAPLs and water, and to identify some of the controlling situations where this process should be considered. A diffusion-based numerical model was developed, and two different physical systems were simulated; a spherical single NAPL blob with total surface area available for mass transfer, and an isolated rectangular NAPL with only one side available for mass transfer. A series of batch and physical model experiments were conducted using coal tars collected from a former MGP site to capture multi-component diffusion-limited mass transfer behavior under static and dynamic conditions, respectively. This series of experiments was intended to focus on the direct interaction of multi-component NAPLs with water and a persulfate solution without the presence of sediment. The results from the static experiments indicated that under the diffusion-controlled mass transfer conditions, the estimated mass transfer rate coefficients were lower than typical mass transfer rate coefficients determined under continuous mixed conditions. Although, no overall trend was observed between the mass transfer rate coefficients for the various organic compounds identified, an inverse dependency between the mass transfer rate coefficient and molecular weight was clear but different for BTEX and some PAHs compounds suggesting that the intra-NAPL diffusion behavior of these two organic compound classes are different. The results indicated that molecular weight and concentration of each component are the most important parameters affecting intra-NAPL diffusion coefficients. A combination of NAPL composition, NAPL geometry, and interphase mass transfer rate may result in the depletion of more soluble compounds at the interface which can restrict NAPL-water mass transfer. When the main intra-NAPL diffusion coefficients are in the range of the self-diffusion coefficients, dissolution is not limited by internal diffusion except for high interphase mass transfer rates or long diffusional distances. In the case of complex and highly viscous NAPLs, smaller intra-NAPL diffusion coefficients are expected and even the low range of mass transfer rates can result in the depletion of more soluble compounds at the NAPL-water interface and diffusion-limited dissolution. Depending on the NAPL properties (i.e., constituent components, viscosity, temperature), interfacial depletion of the more soluble compounds can vary and influence mass transfer and dissolved phase concentrations. The comparison of experimental and simulated results indicated that rate-limited intra-NAPL diffusion within complex multi-component NAPLs as well as persulfate-NAPL interactions can restrict mass loss and chemical oxidation efficiency compared to the no-treatment scenario. It was determined that during 64 days of persulfate injection the multi-component mass transfer rate coefficients were ~70 % smaller than those estimated during an equivalent water injection period. The experimental and computational effort described in this study is the first effort to provide comprehensive information about the role of intra-NAPL diffusion on dissolution of multi-component NAPLs and the direct interaction of persulfate with MGP residuals. The diffusion-based model developed in this study provides a realistic platform to capture the temporal and spatial mass fluxes and compositional changes within complex NAPLs. While chemical oxidants (persulfate or permanganate) are able to degrade MGP residuals in well-mixed conditions, rate-limited NAPL-water mass transfer restricts treatment in systems more representative of in situ conditions. Therefore, methods to overcome the mass transfer limitations and intra-NAPL resistances are required for the remediation of complex multi-component NAPLs

Enhancement of Naphthalene Biodegradation by Sulfate Application in Brackish Subsurface Systems

Anaerobic biodegradation is the most dominant mechanism in the petroleum hydrocarbon contaminated subsurface systems. Due to depletion of terminal electron acceptors such as sulfate, anaerobic degradation of organic contaminants can be restricted. Hence, engineered sulfate application has been proposed as an effective remediation strategy to enhance the activities of sulfate reducer bacteria (SRB) in the contaminated subsurface systems. However, biodegradation process is significantly affected by environmental conditions and sulfate application in the contaminated saline and brackish coastal regions is unknown. A series of flow-through reactors (FTRs) representative of dynamic anaerobic subsurface conditions were conducted using undisturbed soil samples collected from brackish (semi)-arid coastal environments in Qatar. Dissolved naphthalene as one of the most dominant petroleum hydrocarbons that can be found in most of the contaminated sites was injected into FTRs under different salinity conditions. The relevant geochemical indicators as well as soil adsorption and dissolved phase concentrations were measured. The results confirmed development of reducing conditions as well as SRB activity under experimental conditions. Salinity did not restrict bioremediation and dissolved naphthalene degradation was more stable and enhanced under brackish water conditions because microbial cultures within the undisturbed soil were adapted to the brackish water conditions at the field sampling environment. This paper will provide an overview of the flow-through experiments and key findings.This publication was made possible by funding from NPRP grant # NPRP9-093-1- 021 from the Qatar national research fund (a member of Qatar Foundation)

Influence of Water Table Fluctuation on Natural Source Zone Depletion in Hydrocarbon Contaminated Subsurface Environments

Most of the prediction theories regarding dissolution of organic contaminants in the subsurface systems have been proposed based on the static water conditions and the influence of water fluctuations on mass removal requires further investigations. In this study, it was intended to investigate the effects of water table fluctuations on biogeochemical properties of the contaminated soil at the smear zone between the vadose zone and the groundwater table. An automated 60 cm soil column system was developed and connected to a hydrostatic equilibrium reservoir to impose the water regime by using a multi-channel pump. Four homogenized hydrocarbon contaminated soil columns were constructed and two of them were fully saturated and remained under static water conditions while another two columns were operated under water table fluctuations between the soil surface and 40 cm below it. The experiments were run for 150 days and relevant geochemical indicators as well as dissolved phase concentrations were analyzed at 30 and 50 cm below the soil surface in all columns. The results indicated significant difference in terms of biodegradation effectiveness between the smear zones exposed to static and water table fluctuation conditions. This presentation will provide an overview of the experimental approach, mass removal efficiency, and key findings.This publication was made possible by funding from NPRP grant # NPRP9-093-1-021 from the Qatar national research fund (a member of Qatar Foundation). We acknowledge that all the Gas Chromatography analyses were accomplished in the Central Laboratories unit, Qatar University

The role of intra-NAPL diffusion on mass transfer from MGP residuals

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.jconhyd.2018.04.002 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/An experimental and computational study was performed to investigate the role of multi-component intra-NAPL diffusion on NAPL-water mass transfer. Molecular weight and the NAPL component concentrations were determined to be the most important parameters affecting intra-NAPL diffusion coefficients. Four NAPLs with different viscosities but the same quantified mass were simulated. For a spherical NAPL body, a combination of NAPL properties and interphase mass transfer rate can result in internal diffusion limitations. When the main intra-NAPL diffusion coefficients are in the range of self-diffusion coefficients (10−5 to 10−6 cm2/s), dissolution is not limited by internal diffusion except for high mass transfer rate coefficients (>180 cm/day). For a complex and relatively high viscous NAPL (>50 g/(cm s)), smaller intra-NAPL diffusion coefficients (<10−8) are expected and even low mass transfer rate coefficients (~6 cm/day) can result in diffusion-limited dissolution.Natural Sciences and Engineering Research Council of Canada Collaborative Research Development Grant (NSERC) (CRDPJ 429357 - 2011

Biodegradation Kinetics of Benzene and Naphthalene in the Vadose and Saturated Zones of a (Semi)-Arid Saline Coastal Soil Environment

Biodegradation is a key process for the remediation of sites contaminated by petroleum hydrocarbons (PHCs), but this process is not well known for the (semi)-Arid coastal environments where saline conditions and continuous water level fluctuations are common. This study differs from the limited previous studies on the biodegradation of PHCs in Qatari coastal soils mainly by its findings on the biodegradation kinetics of the selected PHCs of benzene and naphthalene by indigenous bacteria. Soil samples were collected above, across, and below the groundwater table at the eastern coast of Qatar within a depth of 0 to-40 cm. Environmental conditions combining low oxygen and high sulfate concentrations were considered in this study which could favor either or both aerobic and anaerobic bacteria including sulfate-reducing bacteria (SRB). The consideration of SRB was motivated by previously reported high sulfate concentrations in Qatari soil and groundwater. Low-and high-salinity conditions were applied in the experiments, and the results showed the sorption of the two PHCs on the soil samples. Sorption was dominant for naphthalene whereas the biodegradation process contributed the most for the removal of benzene from water. Losses of nitrate observed in the biodegradation experiments were attributed to the activity of nitrate-reducing bacteria (NRB). The results suggested that aerobic, NRB, and most likely SRB biodegraded the two PHCs, where the combined contribution of sorption and biodegradation in biotic microcosms led to considerable concentration losses of the two PHCs in the aqueous phase (31 to 58% after 21 to 35 days). Although benzene was degraded faster than naphthalene, the biodegradation of these two PHCs was in general very slow with rate coefficients in the order of 10-3 to 10-2 day-1 and the applied kinetic models fitted the experimental results very well. It is relevant to mention that these rate coefficients are the contribution from all the microbial groups in the soil and not from just one.Scopu

Realistic expectations for the treatment of FMGP residuals by chemical oxidants

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.jconhyd.2018.08.007 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/Methods to remediate soil and groundwater contamination at former manufactured gas plant (FMGP) sites are scarce. The objective of this study was to investigate the ability of two chemical oxidants (persulfate and permanganate) to degrade FMGP residuals in a dynamic system representative of in situ conditions. A series of physical model trials supported by aqueous and slurry batch experiments using impacted sediments collected from a FMGP site were conducted. To explore treatment expectations a screening model constrained by the experimental data was employed. The results from the aqueous experiments showed that dissolved components (except for benzene) were readily degraded by persulfate or permanganate. In the well-mixed slurry systems, when contact with the oxidant was achieved, 95%, 45% and 30% of the initial mass quantified was degraded by permanganate, unactivated persulfate, and alkaline activated persulfate, respectively. In stark contrast, the total mass removed in the physical model trials was negligible for both permanganate and persulfate irrespective of the bleb or lense architecture used. Hence the net benefit of flushing 6 pore volumes of permanganate or persulfate at a concentration of 30 g/L under the physical model operating conditions was minimal. To achieve a substantial degradation of mass within the treatment system (>40%), results from the screening model indicated that the hydraulic resident time would need to be >10 days and the average lumped mass transfer coefficient increased by two orders-of-magnitude. Results from long-term (5 years) simulations showed that the dissolved concentrations of organic compounds are reduced temporarily as a result of the presence of permanganate but then rebound to a profile that is essentially coincident with a no-treatment scenario following exposure to permanganate. Neither a lower velocity nor higher permanganate dosing affected the long-term behavior of the dissolved phase concentrations; however, increasing the mass transfer rate coefficient had an impact. The findings from this investigation indicate that the efficiency of permanganate or persulfate to treat for FMGP residuals is mass transfer limited.TECO Peoples Gas, Tampa FLNatural Sciences and Engineering Research Council (NSERC) of Canada Collaborative Research and Development Gran

Effects of dissolved organic phase composition and salinity on the engineered sulfate application in a flow-through system

Engineered sulfate application has been proposed as an effective remedy to enhance the rate-limited biodegradation of petroleum-hydrocarbon-contaminated subsurface environments, but the effects of dissolved organic phase composition and salinity on the efficiency of this method are unknown. A series of flow-through experiments were conducted for 150 days and dissolved benzene, toluene, naphthalene, and 1-methylnaphthalene were injected under sulfate-reducing and three different salinity conditions for 80 pore volumes. Then, polycyclic aromatic hydrocarbons (PAHs) were omitted from the influent solution and just dissolved benzene and toluene were injected to investigate the influence of dissolved phase composition on treatment efficiency. A stronger sorption capacity for PAHs was observed and the retardation of the injected organic compounds followed the order of benzene 60 PVs influent solution, and benzene slightly degraded following the removal of PAH compounds. The results showed substrate interactions and composition can result in rate-limited and insufficient biodegradation. Similar reducing conditions and organic utilization were observed for different salinity conditions in the presence of the multi-component dissolved organic phase. This was attributed to the dominant microbial community involved in toluene degradation that exerted catabolic repression on the simultaneous utilization of other organic compounds and were not susceptible to changes in salinity. 2020, The Author(s).This study was funded by the NPRP grant no. NPRP9-93-1-021 from the Qatar National Research Fund (a member of Qatar Foundation). AcknowledgementsScopu

Influence of Water Table Fluctuation on Natural Source Zone Depletion in Hydrocarbon Contaminated Subsurface Environments

Most of the prediction theories regarding dissolution of organic contaminants in the subsurface systems have been proposed based on the static water conditions; and the influence of water fluctuations on mass removal requires further investigations. In this study, it was intended to investigate the effects of water table fluctuations on biogeochemical properties of the contaminated soil at the smear zone between the vadose zone and the groundwater table. An automated 60 cm soil column system was developed and connected to a hydrostatic equilibrium reservoir to impose the water regime by using a multi-channel pump. Four homogenized hydrocarbon contaminated soil columns were constructed and two of them were fully saturated and remained under static water conditions while another two columns were operated under water table fluctuations between the soil surface and 40 cm below it. The experiments were run for 150 days and relevant geochemical indicators as well as dissolved phase concentrations were analyzed at 30 and 50 cm below the soil surface in all columns. The results indicated significant difference in terms of biodegradation effectiveness between the smear zones exposed to static and water table fluctuation conditions. This presentation will provide an overview of the experimental approach, mass removal efficiency, and key findings

Enhancement of Naphthalene Degradation by a Sequential Sulfate Injection Scenario in a (Semi)-Arid Coastal Soil: a Flow-Through Reactor Experiment

Engineered sulfate injection has been introduced as an effective technology to enhance the remediation of soil and groundwater contaminated by petroleum hydrocarbons. While some studies indicate that sulfate injection is a promising method for the treatment of hydrocarbon-contaminated subsurface systems, its application in the brackish soil environments is unknown. In this study, we explored related geochemical indicators along with soil adsorption and dissolved phase concentrations to provide an improved understanding of the hydrocarbon-contaminated subsurface responses to the sulfate injection in brackish environments. A series of flow-through experiments representing in situ groundwater anaerobic bioremediation were conducted and two sulfate injection episodes were applied to examine the degradation of dissolved naphthalene under low salinity and brackish conditions. As opposed to the substantial body of previous studies that salinity restricts biodegradation, the results from this study showed that naphthalene anaerobic degradation was more stable once the salinity was as high as that at the sampling location in the coastal brackish environment. While increasing naphthalene concentration from 4 to 12 mg L-1 did not limit biodegradation efficiency under brackish condition similar to the sampling location, it adversely restricted the developed reducing conditions and biodegradation process under low salinity conditions. This highlights the adaption of the microbial communities within the soil to the brackish environment at the sampling location suggesting that changing the salinity during engineered sulfate application can make the remediation process more susceptible against the environmental stresses and substrate toxicity. The results of this study provide insight into the engineered sulfate application as a remediation strategy for potential removal of dissolved naphthalene from the contaminated brackish groundwater. 2020, The Author(s).Open Access funding provided by the Qatar National Library. This study was funded by NPRP grant # NPRP9-93-1-021 from the Qatar national research fund (a member of Qatar Foundation). Acknowledgments The findings achieved herein are solely the responsibility of the authors. We also would like to acknowledge the funding provided by the Canada Excellence Research Chair program in Ecohydrology. We are thankful to Marianne Vandergriendt, Shirley Chatten, Lindsay Norwood, and Jamal Hannun for their valuable assistance during the experiments and sample analyses.Scopu