Advanced remediation using nanosized zero-valent iron and electrical current in situ − A comparison with conventional remediation using nanosized zero-valent iron alone
Graphical Abstract
Introduction
Application of nanosized zero valent iron (NZVI) has proved to be very efficient in the reduction of contaminating chlorinated hydrocarbons (CHCs), one of the most common and ubiquitous pollutant groups currently detected in drinking water. While NZVI efficiency has been proven in a number of laboratory experiments (e.g. [19], [20], [22], [24], [25], [31]), the true potential of the method lies in its successful scaling-up for real-world applications. In situ application of ZVI was firstly performed in Trenton, NJ, USA in 1999/2000 [5] and, since then, there have been a number of in situ ZVI remediation applications at contaminated sites around the world [27], [40]. Following 20 years of development in ZVI remediation methodology, in situ remediation techniques are now generally more effective economically than ex-situ methods and a number of companies now offer commercial ZVI remediation services.
The groundwater environment is a dynamic system defined by such factors as the character of the subsoil, its hydraulic permeability and the direction and velocity of groundwater flow. Furthermore, the natural occurrence of indigenous bacterial strains can be a significant factor involved in the natural attenuation of contaminants and, as such, can significantly contribute to the remediation process. One of the key factors affecting remediation efficiency, however, is time. Remediation is usually a long process demanding long-term activity of the agent used, along with continuous monitoring and evaluation of the progress. However, rapid implementing of remediation intervention, and use of reagents with an immediate effect, will substantially reduce the spread of contamination and may reduce the overall costs for remediation of a site.
Use of NZVI for in situ remediation applications first became widespread in the USA [3], [41], with other countries following suite as verified results became available [8], [9], [26], [31]. However, use of NZVI in situ can be limited by a lack of stability - zero valent iron is a highly reductive agent and can react non-selectively with surrounding oxidants [6]; which results in a relatively short lifespan [39]. As such, there have been increasing efforts to prolong its longevity and thus increase overall efficiency (for a summary, see the recent review by [11]).
One approach for prolonging the reactive lifetime of NZVI is to partially protect it from oxidation by applying a DC electrical field (herein DC) at the application site. Moreover, application of DC can enhance the transportation properties of both NZVI [15], [38] and contaminants [34]. For in situ application, however, other aspects affecting mobility have also to be considered, such as soil permeability, homogeneity and heterogeneity. To date, there have been a number of laboratory-scale studies testing variations to methods employing DC [1], [13], [26], [35] and recently a full-scale pilot test was applied at an actual contaminated site [2]. The laboratory studies showed that application of DC has great potential to not only enhance the mobility of NZVI particles but also to prolong NZVI reactivity [1].
Generally speaking, NZVI efficiency during the remediation process is characterised by the rate and degree to which contaminants are reduced. In the case of chlorinated ethenes, the products of abiotic dechlorination are less chlorinated CHCs, i.e. Trichloroethylene (TCE), cis-1,2-DCE, vinyl chloride (VC) and ethene and ethane [18]. The presumed mechanism of abiotic dechlorination is either sequential hydrogenolysis, whereby C-X bonds are sequentially replaced by C-H bonds while X- is released, or through the β-elimination reduction pathway, which provides an acetylene molecule that is then transformed to ethene and ethane without production of the intermediates cis-1,2-DCE or VC [3], [14]. While laboratory-scale studies have confirmed that a decrease in contaminant concentration is a reliable indicator of the ongoing reduction process, field applications will require a different approach as contaminant concentrations can change in real time due to a number of natural factors, including dilution as water flowing through the system increases, natural attenuation, changes in inflow direction or the influx of upgradient contaminated groundwater (at variable concentrations) into the treatment zone. Further, as NZVI reduction progresses, less chlorinated by-products (i.e. cis-1,2-DCE, VC) are produced that are harmful to the environment, and may even be more toxic than the original compounds (i.e. perchloroethylene (PCE) and TCE). In comparison, application of NZVI + DC is expected to increase contaminant degradation (as indicated by an increase in ethene/ethane production) and should also enhance degradation without producing intermediate products, such as cis-1,2-DCE and VC.
Basically, application of NZVI + DC relies on a combination of oxidative-reductive reactions. While reductive dechlorination of CHCs by DC alone is possible, the process demands specialised electrodes and electrolytes [23], and while NZVI alone is able to reduce CHC, it has a relatively short lifespan (due to reactions with oxygen and oxidants; [6]) lasting from two weeks [12] to one year [16], [31]. In an aqueous environment with an excess of electrons, NZVI slowly oxidises to Fe2+, or oxidised Fe3+ can be reduced back to Fe2+ in a reversible reaction (see Eq. (1)). Hydration of Fe2+ then provides a proton that acts on the hydrogenation of chlorinated compounds (see Eq. (2); [30]. In addition, however, the complex composition of the water sample also has to be considered as a large proportion of NZVI capacity can be consumed in reactions with dissolved O2, NH4+ or NO2-, followed by reactions with Mn2+ [11].Fe2+ + 3H2O ↔ Fe(OH)3 + 3H+ + e-R – X + H+ + 2Fe2+ → R - H + X- + 2Fe3+
In a previous study performed within the grounds of an industrial company in which the soil had been exposed to high doses of contamination, we described the effect of NZVI + DC on the occurrence of indigenous bacteria [4]. Building on this research, and previously published research describing the method’s use at a laboratory-scale [1], this paper describes the implementation of NZVI + DC as an in situ remediation method at the same site. The efficiency and costs of the novel method are then evaluated based on a comparison with previous remediation at the same site using NZVI only.
Section snippets
Site description
This research took place in the grounds of a large industrial company (Spolchemie a.s.; Ústí nad Labem, Czech Republic) whose grounds have been exposed to high doses of contamination. Owing to the large-scale and long-term character of remediation processes at the site, it has been thoroughly monitored and a description has been provided by Czinnerová et al. [4]. Here, we provide a general characterisation with some additional information. The site has a Tertiary and Cretaceous subsoil, the
NANOFER 25S
Geochemical redox processes are typically described by measurement of pH-ORP. After application of 25S, an increase in pH was observed as a result of reduction of hydrogen protons, while a concurrent decrease in ORP corresponded to an increase in molecular hydrogen and a simultaneous decrease in molecular oxygen, which was consumed in the reaction with Fe0 (Fig. 2). The greatest change in pH-ORP was observed in well AW-32, the effect possibly being multiplied due to an overall decrease in
Conclusion
This study compared two methods of in situ CHC remediation (application of 25S NZVI and STAR DC NZVI with an additional DC electrical current) at the same contaminated site four years apart. While each test was potentially impacted by slight differences in local conditions, every effort was made to ensure comparable sampling and characterisation of site parameters. Compared with 25S, application of STAR DC + DC resulted in improved reductive conditions that persisted over a longer time,
CRediT authorship contribution statement
Alena Pavelková: Writing − original draft, Conceptualization. Vojtěch Stejskal: Investigation, Methodology. Tomáš Pluhař: Resources. Jaroslav Nosek: Conceptualization, Methodology, Writing − review & editing,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LM2018124 and The Technology Agency of the Czech Republic, project No. FW03010071.
References (41)
- et al.
Electric-field enhanced reactivity and migration of iron nanoparticles with implications for groundwater treatment technologies: proof of concept
Water Res.
(2019) - et al.
Combination of NZVI and DC for the in-situ remediation of chlorinated ethenes: an environmental and economic case study
Chemosphere
(2020) - et al.
Combining nanoscale zero-valent iron with electrokinetic treatment for remediation of chlorinated ethenes and promoting biodegradation: a long-term field study
Water Res.
(2020) - et al.
Kinetics of trichloroethene dechlorination with iron powder
Water Res.
(2005) - et al.
Field assessment of carboxymethyl cellulose stabilized iron nanoparticles for in situ destruction of chlorinated solvents in source zones
Water Res.
(2010) - et al.
Electrokinetic-enhanced nanoscale iron reactive barrier of trichloroethylene solubilized by triton X-100 from groundwater
Electrochim. Acta
(2012) - et al.
Kinetic models for volatile chlorinated hydrocarbons removal by zero-valent iron
Chemosphere
(2004) - et al.
Impact of NZVI stability on mobility in porous media
J. Contam. Hydrol.
(2013) - et al.
Abiotic reductive dechlorination of chlorinated ethylenes by soil
Chemosphere
(2004) - et al.
Effects of ferrous ions on the reductive dechlorination of trichloroethylene by zero-valent iron
J. Hazard. Mater.
(2006)
Optimization of electrochemical dechlorination of trichloroethylene in reducing electrolytes
Water Res.
Nanoscale zero-valent iron application for in situ reduction of hexavalent chromium and its effects on indigenous microorganism populations
Sci. Total Environ.
Sulfidated nano zerovalent iron (S-NZVI) for in situ treatment of chlorinated solvents: a field study
Water Res.
Reduction of chlorinated hydrocarbons using nano zero-valent iron supported with an electric field. characterization of electrochemical processes and thermodynamic stability
Chemosphere
Field demonstration of enhanced removal of chlorinated solvents in groundwater using biochar-supported nanoscale zero-valent iron
Sci. Total Environ.
Electrokinetic soil remediation − critical overview
Sci. Total Environ.
Field assessment of nanoscale bimetallic particles for groundwater treatment
Environ. Sci. Technol.
Selectivity of nano zerovalent iron in in situ chemical reduction: challenges and improvements
Remediat. J.
Biodegradability of chlorinated solvents and related chlorinated aliphatic compounds. 2004
Rev. Environ. Sci. Bio/Technol.
Cited by (4)
Transport of nZVI/copper synthesized by green tea extract in Cr(IV)-contaminated soil: modeling study and reduced toxicity
2024, Environmental Science and Pollution Research