Advanced remediation using nanosized zero-valent iron and electrical current in situ − A comparison with conventional remediation using nanosized zero-valent iron alone

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Highlights

  • Significant increase of NZVI longevity.

  • The efficiency of the reductive dechlorination increased 3.7 times with application of DC.

  • Application of DC provide higher degradation rate of problematic cis-1,2-DCE.

  • Method using DC proves even 5 times higher efficiency in the laboratory scale experiment.

Abstract

This study presents an advance on the standard in situ reductive remediation technique using nanosized zero valent iron (NZVI). The initial process used a conventional form of NZVI, NANOFER 25S (NANO IRON, s.r.o.), at a site contaminated with chlorinated hydrocarbons (CHC) in 2014. In 2018, levels of contamination at the site were still too high; hence, a second remediation intervention was performed using a cheaper form of NZVI, NANOFER STAR DC (NANO IRON, s.r.o.), accompanied with application of an electrical current (DC). The physical-chemical parameters at the site showed moderate reductive conditions for both techniques. While remediation with NANOFER 25S showed a continuous decrease in NZVI activity over time, application of NANOFER STAR DC + DC resulted in a long-term reductive effect, observable as a persistent decrease in CHC concentration and evolution of ethene and ethane dechlorination products. NZVI in the STAR DC + DC was still active after around 200 days. Final efficiency of the STAR DC + DC method was calculated at 3.7 times higher than the conventional technique using NANOFER 25S only. Laboratory-scale experiments confirmed the field observations, indicating the significantly elevated effectivity of STAR DC + DC.

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.

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