Electrochemical dechlorination of trichloroethylene in the presence of natural organic matter, metal ions and nitrates in a simulated karst media
Introduction
Chlorinated solvents, hexavalent chromium, selenates, and nitrates are priority groundwater pollutants that pose public health risks [1]. Electrochemical methods are of interest for treating such contaminants in groundwater because of the capability to adjust redox conditions to changes in the influent composition and flow rate [2], [3], [4]. Electrochemical processes are effective for removal of chlorinated solvents such as trichloroethylene (TCE), regulated metals, and nitrates [5] via electrochemically induced oxidation or reduction mechanisms [6], [7], [8], [9], [10]. Indirect electrochemical reduction of TCE via hydrochlorination involves the reaction of chlorinated compounds with atomic hydrogen produced at the cathode due to water electrolysis [11]. For materials with higher hydrogen evolution overpotential, such as silver, copper and lead, the kinetics of hydrogen evolution may be relatively slow, which implies that any other reducible species such as TCE molecules can acquire the atomic hydrogen from the cathode [6], [28]. Another process for trichloroethylene removal is dechlorination in the presence of iron, however, presence of co-oxidants, such as chromate, selenite, or nitrate limit this process [6], [12], [13], [14].
Groundwater from karst aquifers is an important source of drinking water, accounting for 25% of the world and 40% of the US groundwater resources [15]. Karst aquifer, formed from the dissolution of soluble rocks such as limestone and dolomite, are significant routes of contaminant exposure for human and wildlife [16]. They are complex systems not only because the dissolution process creates complex networks of preferential flow pathways but also because it is hard to evaluate the manner in which groundwater flow is transmitted through the system [17]. Surface runoff that often contains hydrocarbons, metal species, nitrate, and other organic contaminants may enter karst groundwater systems through infiltration. Thus, studying the fate and transport process of groundwater in a simulated limestone block is of significant scientific and engineering importance.
Natural organic matter (NOM) is present in the groundwater due to hydrological connectivity and limited filtration between the surface and subsurface in karst aquifers [18]. The presence of NOM can influence the remediation processes due to possible competition for transformation mechanisms of target contaminants. NOM is ubiquitous in karst aquifers with a high tendency to be adsorbed on mineral surfaces such as Fe oxides [19]. Presence of NOM in groundwater negatively impacts the removal of contaminants by Fe but there is still a limited knowledge on the effect of coexisting NOM and other contaminants on remediation, especially in karst aquifers [20]. NOM may suppress the remediation of target contaminants not only by direct electron transfer at the surface of the cathode or reaction with atomic hydrogen produced at the cathode surface [11], but also by complex production and aggregation in solution. Humic acids aggregate with iron species and form precipitates, especially, with calcium and magnesium naturally presented in groundwater [20]. In systems using the iron anode, the inevitable limitation of the iron reduction is driven by the corrosion of zero-valent iron and deposition of minerals, which decrease the reductive capacity and clog the system [21]. The deposition of aggregates may impact the long-term activity of the remedial system due to precipitation on the iron surface which would causes clogging the pores, decreasing the permeability, and developing preferential flow path [20].
Among heavy metals, chromium (Cr) and selenium (Se) are common contaminants in surface water and in numerous industrial activities such as preservation of wood, electroplating and metal finishing [22]. Zero-valent iron is capable of treating dissolved metal ions such as selenium and chromium [23]. The removal of the reduced chromium species Cr(III) occurs through precipitation of the sparingly soluble Cr(OH3) or precipitation of mixed iron(III)-chromium(III) oxyhydroxide solids [24], [25], [26]. Nitrate is one of the main co-contaminants found at sites contaminated with chlorinated solvents and metals and is easily combined with various organic and inorganic substances [27]. Nitrate is also found in groundwater naturally at a very low concentration level. Since nitrate and TCE reduce at the cathode via reaction with atomic hydrogen, a process leading to a competition between target contaminant (TCE) and nitrate, it is important to evaluate its influence on the remediation process even in concentration lower than maximum contaminant level (MCL). Although presence of ferrous due to iron anode electrolysis leads to formation of a reducing environment, it also results in precipitates, which may act as an insulator that gradually cover the surface of electrodes and decrease the removal rate of contaminants by terminating the further redox reactions [28].
There is a limited literature on the treatability of mixtures of organic and inorganic groundwater pollutants, especially in limestone karst aquifers. In this study, a detailed investigation of the TCE removal efficacy in the presence of other groundwater contaminants (e.g. Chromate, Selenate, Nitrates) was performed and information is provided for assessing the application of designed system in karst environment rich in NOM. Besides the fact that these contaminants are often found in mixtures and are most commonly found in groundwater, we chose them to represent the combination of chemical species with different characteristics that can interfere with the transformation mechanisms. This is an important aspect of the optimization of remediation technologies for the treatment of real groundwater with complex geochemistry. A limestone column was specially designed to simulate the channel flow in the karst aquifer. The application of an electrochemical reactor in a limestone system with an iron anode to remediate TCE in the presence of dichromate, selenate, nitrate, and humic acids is evaluated in this study.
Section snippets
Chemicals and materials
The chemicals used in this study include trichloroethylene (99.5%, Sigma Aldrich), potassium dichromate (reagent grade, JT Baker), sodium selenate (99.8%, Alfa Aesar), sodium nitrate (reagent grade, JT Baker), sodium bicarbonate (reagent grade, Fisher Chemical), humic acid (Alfa Aesar), and calcium sulfate (99.9%, JT Baker). Excess amount of TCE was dissolved in 18.2 MΩ cm deionized water. This saturated solution was used to prepare aqueous TCE solution during experiments. Electrodes were cast
Effect of humic acids on TCE reduction
The effluent TCE concentration decay versus influent by iron anode and copper cathode in the presence of various concentration of humic acids is presented in Fig. 2 (1, 2, and 5 mgTOC L−1).
Several studies have shown the negative effect of NOM on Fe activity for chlorinated hydrocarbon degradation, which is attributed to strong competition for reactive sites [29], [30] and changing reduction potential of surface sites [20]. As observed, in the absence of humic acids, more than 90% of the TCE was
Conclusion
Limestone block column experiments showed that the efficiency of iron anode and copper cathode system for TCE removal is influenced by the presence of humic acid and strong oxidants such as chromate, selenate, and nitrate. The influence on TCE removal is in the following order: humic acid, chromate, selenate, and nitrate. Dichromate and selenate are reduced to the insoluble ions, which presumably forms precipitates result in covering iron anode surface. The layer of sediments on iron anode
Acknowledgment
This work was supported by Award Number P42ES017198 from the National Institute of Environmental Health Sciences (NIEHS). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIEHS or the National Institutes of Health.
References (38)
- et al.
Degradation of TCE, Cr(VI), sulfate, and nitrate mixtures by granular iron in flow-through columns under different microbial conditions
Water Res.
(2002) - et al.
Removal of organic contaminants from secondary effluent by anodic oxidation with a boron-doped diamond anode as tertiary treatment
J. Hazard. Mater.
(2015) - et al.
The influence of cathode material on electrochemical degradation of trichloroethylene in aqueous solution
Chemosphere
(2016) - et al.
Electrochemical degradation of trichloroethylene in aqueous solution by bipolar graphite electrodes
J. Environ. Chem. Eng.
(2016) - et al.
Influence of humic substances on electrochemical degradation of trichloroethylene in limestone aquifers
Electrochim. Acta
(2015) - et al.
An in situ study of the effect of nitrate on the reductuion of trichloroethylene by granular iron
J. Contam. Hydrol.
(2003) - et al.
Spatiotemporal changes of CVOC concentrations in karts aquifers: analysis of three decades of data from Puerto Rico
J. Sci. Total Environ.
(2015) - et al.
Treatment of inorganic contaminants using permeable reactive barriers
J. Contam. Hydrol.
(2000) - et al.
Role of iron anode oxidation on transformation of chromium by electrolysis
Electrochim. Acta J.
(2012) - et al.
Hydrochlorination of TCE in a circulated electric column at high flow rate
Chemosphere
(2016)
Arsenic removal from drinking water by electrocoagulation using iron electrodes- an understanding of the process parameters
J. Environ. Chem. Eng.
Treatment of trichloroethylene and hexavalent chromium by granular iron in the presence of dissolved CaCO3
J. Contam. Hydrol.
Prevention of iron passivation and enhancement of nitrate reduction by electron supplementation
Chem. Eng. J.
Removal of nitrates from groundwater by electrocoagulation
Chem. Eng. J.
Effect of current density on enhanced transformation of naphthalene
Environ. Sci. Technol. J.
Electrokinetic soil remediation: challenges and opportunities
Sep. Sci. Technol. J.
Multicomponent reactive transport in an in-situ zero-valent iron cell
J. Environ. Sci. Technol.
Electrochemically induced dual reactive barriers for transformation of TCE and mixture of contaminants in groundwater
Environ. Sci. Technol.
Electrolytic manipulation of persulfate reactivity by iron electrodes for tricholoroethylene degradation in groundwater
Environ. Sci. Technol. J.
Cited by (18)
Insights into the flow-through electrochemical system in water and wastewater treatment: Influential factors, advantages, challenges, and perspective
2024, Separation and Purification TechnologyUnveiling the role of biochar in simultaneous removal of hexavalent chromium and trichloroethylene by biochar supported nanoscale zero-valent iron
2023, Science of the Total EnvironmentInfluence of nitrate concentration on trichloroethylene reductive dechlorination in weak electric stimulation system
2022, ChemosphereCitation Excerpt :However, both reductive dechlorination and denitrification processes require external electrons for bio-metabolism. They might compete for electrons donors (e.g., hydrogen, acetate) or carbon sources (Fallahpour et al., 2017). Therefore, it is interesting to explore the interaction between two biochemical processes in weak electrical stimulation systems.
Highly dispersed Pd nanoparticles for electrocatalytic dechlorination: Positive and negative effects of carbon supports
2021, Journal of Environmental Chemical EngineeringCitation Excerpt :Electrocatalytic dechlorination has emerged as a promising technology that enables detoxification of chlorinated organic compounds (COCs). It has gained much attention in the field of treatment of COC-contaminated groundwater and wastewater, owing to the high reaction rate, mild and safe operation conditions, and elimination of secondary pollutants [1–6]. The key to dechlorination is the adsorbed hydrogen (Hads*) on the cathode surface, a strong reducible species generated in situ from water electrolysis, which is responsible for the cleavage of the carbon-chlorine (C–Cl) bond.
Removal of Persistent Organic Pollutants (POPs) from water and wastewater by adsorption and electrocoagulation process
2021, Groundwater for Sustainable DevelopmentTransformation of tetrachloroethylene in a flow-through electrochemical reactor
2020, Science of the Total EnvironmentCitation Excerpt :Such knowledge is required for development of a more robust test design for enhanced applicability and feasibility testing at conditions simulating that of potential in-situ applications. Previous work has studied electrochemical removal of mainly TCE in static reactors (Yuan et al., 2012; Yuan et al., 2013b), stirred reactors (Mao et al., 2011; Sáez et al., 2009), recirculated reactors (Mao et al., 2012a; Lakshmipathiraj et al., 2012; Fallahpour et al., 2016), vertical flow-through reactors (Mao et al., 2012b; Rajic et al., 2016a; Rajic et al., 2015a; Rajic et al., 2016b; Rajic et al., 2015b; Rajic et al., 2014; Yuan et al., 2013b; Fallahpour et al., 2017), applied constant voltage (Petersen et al., 2007; Scialdone et al., 2010; Li and Farrell, 2000) or studied electrochemistry in combination with other remediation strategies, e.g. bioremediation (Verdini et al., 2015; Tiehm et al., 2009; Aulenta et al., 2009; Aulenta et al., 2010; Lai et al., 2015). Assessment of the potential of flow-through electrochemical transformation of PCE is important to cover the full range of chlorinated ethene species detected at contaminated sites.