Electrokinetic Remediation in Marine Sediment: A Review and a Bibliometric Analysis

Abstract

Daily industrial activities pose a significant risk of environmental contamination through the release of toxic chemicals, including heavy metals, radionuclides and organic pollutants. Coastal marine areas, estuaries and harbors serve as primary hotspots for such pollution, with marine sediments acting as the ultimate sink for industrial and urban discharges, posing a serious environmental problem. Addressing this pressing issue requires the adoption of environmentally friendly technologies for the remediation and recovery of contaminated marine sediments. This paper provides a comprehensive review of different approaches for the remediation of contaminated sediments, focusing on the principle of electrokinetic remediation, with special emphasis on the use of microorganisms. A bibliometric analysis of key articles in the field is presented to elucidate the most important findings, particularly in the marine environment. The current state-of-the-art is reported for soil and sediment remediation approaches, with the first large-scale experiments and a preliminary cost estimate reported. However, the limited information available on the applicability of these techniques in the marine environment is highlighted. The limitations and risks associated with an inadequate implementation of this technique are discussed while acknowledging the advantages it offers for in situ remediation in marine environments.

1. Introduction

Every day, industrial activities release many toxic chemicals (heavy metals, radionuclides and organic pollutants) into the environment as a result of accidental spills or inappropriate management [1]. The result is a high number of polluted areas, especially in the marine coastal zone, such as estuaries and harbors, where the majority of industrial and civil discharges are concentrated [2]. The benthic compartment of these environments is the most affected by the accumulation of different types of pollution and is also the most difficult to remediate [3,4,5]. Awareness of the far-reaching effects of these conditions has highlighted, for the first time at a national level, the need to apply appropriate remediation strategies using new environmentally friendly approaches to the recovery and cleansing of contaminated marine sediments [6]. One of the latest technologies that has shown great potential for the remediation of various sites is the electrochemical approach, as reported in recent studies [7,8,9,10,11,12].

The present review focuses on the literature related to electrokinetic remediation on saline sediments and the impact on a microbial community in order to assess the advantages and gaps in its application. Vocciante and co-authors [13] have defined electrochemical remediation as a method for the migration, separation and elimination of contaminants in soil or sediment under an electric field, which is cost-effective and has a low environmental impact.

This technology can be applied both in situ and ex situ; the in situ methods allow the contaminants to be treated directly at the polluted site, whereas the ex situ methods require the contaminated sediment to be dredged from the original site [14,15], with additional costs and the risk of causing dispersion in the environment and the probable risk to human health that can occur during polluted sediment mobilization [16].

The efficiency of treating polluted soil and sediments depends on various factors. These include the grain size distribution, permeability, organic matter content, moisture and salt content [17].

1.1. Electrokinetic Remediation: Principles

Electrokinetic remediation is a technique that applies a low-intensity current directly to the contaminated site. This approach is widely accepted for both in situ and ex situ remediation of low permeable clay-rich soil or fine-grained sediment [18].

Recently, it has been applied in various contexts, such as soils, sediments and groundwater [19]. This electrochemical methodology uses a current to remove toxic chemical ionic species from the soil, and some of them, like solid compounds, precipitate at the electrodes [20,21,22]. This method is commonly used to remove heavy metals, pentachlorophenol, trichloroethylene, petroleum, diesel and dissolved organic matter from contaminated soil [23,24]. An electric field is applied to the soil, which moves interstitial fluid and creates an electro-osmotic flow toward the cathode, allowing for the removal of soluble contaminants [25]. In 1936, Puri and Anand utilized electrokinetic remediation in situ to extract sodium hydroxide from a contaminated area [26].

The electric field caused ions, ion complexes and particles to move via electromigration towards electrodes of opposite charge in the direction of the electric field. Consequently, charged particles migrate toward the oppositely charged electrode and vice versa [27].

Moreover, during this process, the electrolysis of water that generates is at the anode oxygen (${\mathrm{O}}_2\uparrow$) and hydrogen ions (${\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}$) due to oxidation, and at the cathode hydrogen (${\mathrm{H}}_2\uparrow$) and hydroxyl ions (${\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}$) due to reduction, according to the following Equations (1) and (2).

$$2{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}^{+}+4{\mathrm{e}}^{-}+{\mathrm{O}}_2\uparrow \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathrm{E}0=-1.229 \mathrm{V}$$

(1)

$$4{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{O}\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}^{-}+2{\mathrm{H}}_2\uparrow \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathrm{E}0=-0.828 \mathrm{V}$$

(2)

Acidic solutions are generated at the anode, and alkaline solutions are generated at the cathode. As a result, the pH at the cathode increases while the pH at the anode decreases. The protons (H⁺) and hydroxyl (OH⁻) ions move towards the oppositely charged electrode [27], creating a pH gradient that modifies the site state. The pH variations at the electrodes and the movement of the acid front are influenced by the buffering capacity of the soil or groundwater system. In some cases, it is necessary to control the pH variations to prevent the formation of specific species that could hinder the recovery of contaminants. According to DeFlaun and Condee [28], extreme pH levels can inhibit bacteria that play a crucial role in the biodegradation processes, as values outside the range of 5.5–8.5 are detrimental to most bacterial strains [29]. During electrolysis in composite media, such as groundwater or soil and sediment, not only hydrogen and oxygen formation occur. Depending on the composition and the electric field applied, side reactions (as shown in Equations (3)–(9)) may significantly affect the process efficiency.

$${\mathrm{M}\mathrm{e}}^0\to {\mathrm{M}\mathrm{e}}^{\mathrm{n}+}+\mathrm{n}{\mathrm{e}}^{-}$$

(3)

$${\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(4)

$${\mathrm{N}\mathrm{O}}_3^{-}+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{O}}_2^{-}+{\mathrm{H}}_2\mathrm{O}$$

(5)

$${\mathrm{C}\mathrm{a}}^{2+}+{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$$

(6)

$${\mathrm{M}\mathrm{g}}^{2+}+2{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{M}\mathrm{g}\left(\mathrm{O}\mathrm{H}\right)}_2$$

(7)

$${\mathrm{S}\mathrm{O}}_4^{2-}\to {\mathrm{S}\mathrm{O}}_2\uparrow +{\mathrm{O}}_2\uparrow +2{\mathrm{e}}^{-}$$

(8)

$${\mathrm{C}\mathrm{a}}^{2+}+{\mathrm{S}\mathrm{O}}_4^{2-}\to {\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{O}}_4$$

(9)

When considering the marine environment, it is important to note that the anion that is highly present is Cl⁻. Therefore, it is necessary to take into account the side reactions caused by Cl⁻ oxidation at each pH. Bennett et al. [30] discovered that the electric field produced at the anode results in a large volume of chlorine being converted to a hypochlorite solution, which can significantly affect the formation of oxygen. According to several authors [31,32,33,34], the electrokinetic process produces chlorine gas when the pH value of the anode is below 3. On the other hand, if the pH is over 7.5, hypochlorite is produced. When the pH is between 3 and 7.5, hypochloric acid is the main product (see Equations (10)–(12)).

$$2{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{C}\mathrm{l}}_2+2{\mathrm{e}}^{-}\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{E}}^0=+1.36 {\mathrm{V}}_{\mathrm{SHE}}$$

(10)

$${\mathrm{C}\mathrm{l}}^{-}+2{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{C}\mathrm{l}\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{E}}^0=+0.89 {\mathrm{V}}_{\mathrm{SHE}}$$

(11)

$${2\mathrm{C}\mathrm{l}}^{-}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{H}\mathrm{C}\mathrm{l}\mathrm{O}+2{\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\hspace{1em}\hspace{1em}0.89\le {\mathrm{E}}^0\left({\mathrm{V}}_{\mathrm{SHE}}\right)\le 1.36$$

(12)

The oxygen evolution reaction is thermodynamically favored over the chlorine evolution reaction, which requires a greater potential to take place at each pH. However, due to the very fast kinetics, the chlorine evolution reaction overlaps with the oxygen evolution reaction [35]. Chlorine or hypochlorite production can induce electrode corrosion. Additionally, Cl⁻ can slowly reduce or directly react with metal, leading to electrode poisoning and a decrease in its lifespan [31]. Electrode corrosion may also be caused by Mg²⁺, Ca²⁺ and Na⁺. During the reaction, these combine with and supply cations to the electrode, altering its activity and stability [35]. Furthermore, at specific pH levels, insoluble precipitates form on the electrode surface, rendering the electrode inactive and leading to catalyst poisoning or accelerated degradation [35,36]. The main effects of an applied electric field on soil are electro-osmosis, which is the movement of the naturally present solution in the soil, electromigration of charged species, and electrophoresis, which is the motion of ions in a stationary fluid.

1.2. Electrokinetic Base Methodology Currently Applied in Different Environments

To reduce costs and treatment time, electrochemical remediation can be combined with other methods. Notable integrated or coupled technologies include the following:

• Electrokinetic biobarriers, also known as electrokinetic biofences, are a static in situ approach used to enclose, contain and purify contaminated groundwater. This technology involves positioning a row of electrodes at the border of a contaminated area, perpendicularly to the predominant groundwater flow direction, and placing them at the lowest depth where contaminants are observed. Different layouts can be used depending on the type of contamination, whether it is inorganic or organic.
• Electrolytic reactive barriers, also known as e-barriers, consist of a panel of permeable electrodes closely laid in the channel of polluted groundwater. For more information on this methodology, refer to Sale, Petersen and Gilbert [37].
• Electrokinetic-permeable reactive barriers (PRBs) are an in situ method used to enclose a reactive zone in groundwater for the elimination of halogenated organic compounds and inorganic salts. If the barrier is loaded with microorganisms, it works as a biological reactor to break down the target contaminants. On the other hand, if the barrier is loaded with limestone, hydroxyapatite, activated carbon, or zeolite, it works as a chemical precipitator or adsorbent.
• Electrokinetic–chemical oxidation/reduction is used for wastewater treatment. It is based on the concepts of chemical oxidation and reduction reactions, which reduce organic contaminants by adding oxidants such as hydrogen peroxide, permanganate, persulfate or reductants in the form of nanoscale iron particles.
• Electrokinetic bioremediation, also known as electro-bioremediation or electro-bioreclamation, is a group of cleanup techniques that use microbes for transformation and electrokinetics for the transport of subsurface pollutants, as well as any substances present or produced during the process. This technique is a combination of electrokinetics and bioremediation.
• Electrokinetic phytoremediation is a plant-based technology that involves the use of plants to remove elemental contaminants or reduce their bioavailability in contaminated areas. This is due to plants’ ability to absorb ionic materials in the soil through their root system, even in small amounts.
• Electrokinetic stabilization is mainly used to remove heavy metals. It acts by precipitating contaminants through the injection of alkaline solutions or reducing agents based on the specific contaminant conditions.
• Electrokinetic-thermal treatment is a process that involves the application of electrical energy to a contaminated site, resulting in resistive heating. This heating accelerates many chemical and biological reactions, modifying physical properties, and can be used to remove volatile organic contaminants.

More comprehensive details of these topics are presented in the book edited by Reddy and Cameselle [25].

Electrokinetic bioremediation and electrokinetic phytoremediation are cost-effective and eco-friendly technologies. Electrokinetic bioremediation uses microbes to break down organic compounds, while phytoremediation employs plants and associated microorganisms to remove, degrade or sequester both inorganic and organic pollutants from contaminated sites. According to Ali et al. [38], the latest technique is time-consuming and inefficient due to the moderate growth and low biomass production, particularly when higher plants are used due to moderate growth and low biomass production. Certain types of algae and cyanobacteria, such as Chlorella, Cyclotella, Lyngbya, Scenedesmus, Synechocystis, Spirulina, Oscillatoria and Anabaena, are capable of binding metals through processes such as absorption, physical adsorption and strong binding with various chemical groups (-COOH, -NH2, =NH, -SH, -OH) on their cell walls [39,40,41,42]. The use of microalgae has been shown to significantly increase biomass growth and accumulation [43].

However, similar to the phytoremediation approaches based on higher plants, the application of microalgae is limited by the availability of light, which significantly reduces its potential applications. Additionally, in the marine environment, physically delimiting the treated area and maintaining sufficient light availability pose significant challenges, making an in situ application almost impossible.

Microorganisms are effectively used in bioremediation because of their ability to transform contaminants into less hazardous compounds or because they can sequester metals and bioconcentrate them into a small volume under aerobic or anaerobic conditions. Indeed, as reported by several authors, Cr(IV) is reduced to Cr(III) by Pseudomonas spp. or by some species of cyanobacteria [44,45,46]. Furthermore, as reported by Zhang and Majidi [47], Stichococcus bacillaris Nägeli can store metals in polyphosphate bodies or bind metals to intracellular proteins. Wang et al. [48] have systematically summarized the progress of this technology, where several studies dealing with the remediation of heavy metal-contaminated sites are reported [49,50,51,52,53,54,55,56,57,58,59]. Wang et al. [60] investigated copper speciation in polluted soil before and after electrokinetic treatment and the correlations of soil microbial with enzyme activities. This study is very important because nowadays, most of the studies on electrokinetic remediation are concerned with the distribution of residual contaminants in the soil and their overall removal efficiency after electrokinetic treatment, but it is well known that the total metal concentration in the soil does not give any indication about metal toxicity [61]. In fact, the ecotoxicological properties of metals depend on the chemical forms, and their speciation determines their reactivity and hazard. It should be mentioned that electrokinetic bioremediation has some limitations when the sediment contains a higher amount of calcium and magnesium. Under such conditions, as reported by Annamalai and Sundaram [62], the mobility of bacteria may indeed be hindered due to the adsorption of calcium and magnesium on the bacterial cell wall. In addition, the formation of white insoluble precipitates, such as Mg(OH)₂ and Ca(OH)₂ on the surface of the cathode/anode, could potentially occur during electrolysis [63], reducing efficiency in long-term experiments due to the interruption of current flow. Typically, cathodes are constructed from chemically inert, electrically conductive or alkali-resistant materials such as graphite, coated titanium or platinum [64], which are widely used due to their low cost and accessibility. As seen above, the formation of oxide films on surfaces causes a reduction in efficiency due to the reduction and modification of the electrode surface. To avoid this, metallic oxides such as SnO₂-SnO₃, IrO₂, Ta₂O₅ and RuO₂ could be used to cover the surface of the base electrode [65]. In addition, synthetic materials such as polypyrrole and polyaniline can be used as auxiliary electrodes to improve performance [66]. These materials, which are widely used in environmental applications, have high electrical conductivity and are superior to graphene oxide and carbon nanotubes [67].

1.3. Electrokinetic Bioremediation in Marine Environments

Although industrial activities are often located close to the coast, and it is not uncommon to find heavily polluted marine sediments, the bibliographic search showed that few articles deal with the remediation of sediments by electrokinetic bioremediation in marine environments.

Pedersen et al. [68] deal with electrodialytic remediation, i.e., a method based on electrokinetic remediation of polluted harbor sediments contaminated with heavy metals (Cd, Cu, Pb and Zn) in a lab-scale reactor (500 mL total volume; distance between the electrodes is nearly 8 cm; current density applied is between 0.2 and 0.5 (mA/cm²)).

Bellagamba et al. [69] described the possibility of electrochemically modifying the redox potential of a crude oil-contaminated marine sediment to fix it in situ, thereby enhancing the biodegradation of pollutants by indigenous microbial communities. The authors used a lab-scale reactor (1 L total volume, DSA (dimensionally stable anode) 44 cm² geometric area; Ti mesh covered with mixed metal oxides, primarily consisting of Ir and Ru (Magneto Special Anodes, Schiedam, The Netherlands)), with the distance between electrodes not reported, and a 2 V potential difference applied in a continuous and in intermittent manner. This study showed that the intermittent application of electrolysis was advantageous in keeping the energy requirements of the process low.

Cappello et al. [70] describe the effects of a long-term study that coupled electrokinetic transport and bioremediation for the remediation of marine sediments contaminated by crude oil. The authors used a ~100 L mesocosm filled with marine sediment and a DSA anode similar to that used by Bellagamba et al. [66], as well as a stainless-steel mesh cathode of approximately 0.045 m² in geometric area that was disposed at ~30 cm of distance to drive seawater electrolysis at a fixed current density of 11 Am⁻². The application of low-voltage direct current increased the redox potential of the sediment, creating conditions potentially conducive to aerobic biodegradation of petroleum hydrocarbons.

In addition, the treatment of marine sediments and highly saline soils by electrokinetic bioremediation is further complicated by the presence of high concentrations of chloride ions. As mentioned above, these ions can be converted to chlorine or hypochlorite, which affects bacterial mobility and sometimes inhibits bacterial growth [71,72]. Furthermore, as seawater has a high content of aggressive chloride anions, seawater electrolysis requires robust, efficient and usually quite expensive electrocatalysts to help resist chloride corrosion, especially for the anode, as the metallic oxides, such as the SnO₂-SnO₃, IrO₂, Ta₂O₅ and RuO₂ described above, are used to cover the surface of the electrode [63,65,73].

1.4. The Impact of Electrokinetic on Microbial Communities

Electrokinetic technology is used to increase the bioavailability of the contaminant by promoting transfer between the organic contaminant and bacteria capable of degrading it. In addition, electrokinetics can induce current thermal effects and electrode reactions to establish suitable temperature, pH and redox conditions for the biotransformation processes [74]. Electrokinetics has interesting implications for microbial communities. Initially, this technique was investigated as a possible alternative to thermal sterilization methods for the preservation of heat-sensitive ingredients in food processing. In this context, several authors have investigated the induction of cell death in bacteria under an electric field [75,76,77,78,79,80,81,82]. Several laboratory-scale studies have attempted to assess the role of the electric field on the microbial community, with sometimes conflicting results [81,83]. A common conclusion was that changes in microbial community structure and function were due to the side effects of electrokinetics, such as pH changes or contaminant speciation, rather than the electric current itself [81,83,84,85,86,87,88]. For example, Wick and co-authors [81] observed that a current of 1 mA cm⁻² applied for more than 30 days did not significantly affect the composition and physiology of the microbial community in the electrokinetic reactor core. Instead, a pH variation in the vicinity of the electrodes appeared to induce minor changes in the microbial community. The same result was described in a second study [83], where a lower current, 0.314 mA cm⁻², was applied for 27 days, resulting in a decrease in the pH near the anode to 4. Kim et al. and Cang et al. investigated the influence of pH-induced changes in the toxicity or mobility of contaminants on the soil microbial community at sites treated with electrokinetic remediation [84,85,86]. In particular, Kim et al. [86] observed that ethylenediaminetetraacetic acid (EDTA), used as an additive, enhanced the electrokinetic movement of diesel in contaminated soils and reduced microbial activity, particularly at the cathode. In addition, Cang et al. [84] observed that treatment with sodium hypochlorite (NaClO) and a high pH buffer solution to convert Cr(III) to highly mobile Cr(VI) resulted in a significant reduction in the functional diversity of the microbial community. Lear et al. [83] were the first to study in detail the effect of soil electrokinetics on native soil microbial communities and obtained results that contradict the above results due to the significantly low intensity of the applied electric field but are in agreement with Jackman et al. [89], who in their study, showed the stimulation of the activity of sulfur-oxidizing bacteria when applying 20 mA cm⁻² to soils due to the pH changes induced by electrokinetics. Furthermore, Lear et al. [83] confirmed that the important effect on the soil microbial community and activity occurs exclusively in the vicinity of the anode due to the low pH generated by the electrokinetic treatment. Years later, Lear et al. [85] evaluated the effect on soil microbial communities during electrokinetic treatment of ex situ soil that was contaminated with pentachlorophenol, confirming that the electrokinetic treatment reduced the number of cultivable bacteria and fungi near the anode by 17% and 30%, respectively. Although the effect of pentachlorophenol on soil microbial communities has been demonstrated [90], the authors state that microbial communities could be affected by both pentachlorophenol and electrokinetics, which exacerbate the toxic influence of contaminants and compromise the capacity of microbial communities. Indeed, electrokinetics causes a change in the soil pH due to the electrolysis of water, which generates hydrogen ions at the anode, and it is known that pentachlorophenol, which migrates to the anode by electromigration [85,91], becomes even more dangerous because it is in its phenolic, lipophilic form under acidic conditions. The authors emphasized that the direct effect of the applied current on soil bacteria could not be assessed because of the electrokinetically induced changes in the soil, as shown above, which alter the form of the contaminants. Kim et al. [86], in their research, reported on the consequences of electrokinetic remediation on native microbial activity and communities in diesel-contaminated soil, emphasizing that the main removal mechanism of diesel was due to electro-osmosis and most of the bacteria were moved by electro-osmosis. As pointed out by the authors [86], little was known about the effect of electrokinetics on the soil microbial community and activity. Only a few studies have investigated these events [60,83,85] and have not assessed the changes in the microbial community during electrokinetic treatment.

It is important to highlight that the effect of aeration on bioremediation treatment is crucial for the optimization of these methods [92], but unfortunately, as reported by Morales Pontet et al. [93], few papers have been found evaluating the inclusion of air in microbial metal removal treatments. As reported by Velizarov [94], when living cells are exposed to direct current, the effects vary depending on the amperage, treatment time, cell type and medium properties. Not only an antimicrobial effect has been observed due to the adverse electrochemical reactions, especially at close electrodes, but also stimulation of cell proliferation has been observed [95,96,97,98,99,100]. Chang et al. [97] described a 140% increase in immobilized cell growth for Escherichia coli cultivation at a current density of 180 Am⁻². Nakanishi et al. [98] reported that a change in the composition of the culture medium due to a direct electric field could stimulate the growth of Saccharomyces cerevisiae.

There is little evidence of the effect of electrokinetics in marine microbial communities. Cappello et al. [70] observed the effects of electrokinetics on marine sediment microbial communities, which seemed to generate a significant increase in the amount of Alcanivorax borkumensis cells and that the five most abundant phyla in the electrolytic mesocosms were Gammaproteobacteria, Alphaproteobacteria, Bacteroidetes, Firmicutes and Deltaproteobacteria. They also concluded that aromatic hydrocarbons were mainly removed from the sediment by electro-osmosis, while low-molecular-weight alkanes were reduced by biodegradation.

1.5. Chemical Changes Generated by the Electric Field in Sediments

Highly contaminated sites usually consist of blackish water or sediments characterized by a foul odor. Most contaminants entering a contaminated site cause a rapid decrease in dissolved oxygen (DO) and a concomitant decrease in oxidation-reduction potential (ORP) [101,102]. As reported by Chen et al. [101], when the ORP in sediments falls below −50 mV, anaerobic microorganisms such as sulfate-reducing bacteria, methanogens, fungi and actinomycetes dominate the microbial community composition [101]. Sulfate-reducing bacteria can reduce sulfate to S₂ and H₂S under anaerobic conditions, which are responsible for the blackening processes [102,103,104]. The unpleasant odors are due to NH₃, H₂S and volatile organic sulfur compounds formed during the anaerobic transformation of sulfur-containing organic matter [103], but also to the production of odor compounds such as the sesquiterpenoid geosmin and 2-methylisoborneo-l, formed during the metabolism of fungi and actinomycetes [105]. Dimethyltrisulphide and β-ionone were also emitted during algal decomposition [104]. Other gases produced are CH₄ under anaerobic conditions and CO₂ under aerobic conditions [106].

DC voltage (2.5, 5 and 10 V) is useful to increase the ORP of the surface sediments to improve the in situ remediation treatment by acting on different fronts as follows: (i) directly generating oxidation processes (O₂ production by H₂O hydrolysis, contributing to the neutralization of part of the toxic substances mentioned above [69]) and (ii) allowing the development of aerobic microorganisms capable of degrading most of the remaining contaminants to H₂O and CO₂ [69,92].

The monitoring of physicochemical parameters (such as the pH, electrical conductivity (EC), DO, ORP, ammonia nitrogen amount (${\mathrm{N}\mathrm{H}}_4^{+}-\mathrm{N}),$ turbidity, sulfate amount (${\mathrm{S}\mathrm{O}}_4^{2-}$) and nitrate amount (${\mathrm{N}\mathrm{O}}_3^{-}$)) during electrokinetic remediation is useful to evaluate the efficiency and trend of remediation.

Stelmach et al. [107] observed that petroleum contamination limited the transformation of humic acid, fulvic acid and humin, commonly referred to as soil organic carbon, which are nutrient sources for soil microbial growth [108,109]. As reported by several authors [110,111], exogenous carbon inputs affect the mineralization of soil organic carbon, which is correlated with variations in the microbial community composition. Although the effects of soil organic carbon on petroleum transformation in petroleum-contaminated soils are unknown, many experiments have shown that the addition of exogenous carbon increases petroleum degradation [112,113,114], and according to Wang and Guo [115], an enhanced soil organic carbon metabolism led to an increased abundance of functional bacteria and a higher alpha diversity of microorganisms over a period of time. In addition, similarity to the original microbial community was maintained.

Wick et al. [81] demonstrated that low densities of both sulfur-oxidizing bacteria and heterotrophs in liquid culture were inhibited by 200 Am⁻² (3 V) DC, again highlighting the risks associated with the incorrect and careless application of electrokinetic agents.

2. Materials and Methods

Network analyses and visualizations for the bibliometric analysis were carried out using VOSviewer (version 1.6.19), an open-source bibliometric software [116]. In the VOSviewer images generated by the software, each node in a network represents the entity selected for analysis, such as the article, author, country, institution or keyword. The size of the node, represented by either a circle or a frame, indicates the frequency of occurrence of the entity, reflecting the number of times the entity appears. Links between nodes indicate co-occurrences between the analyzed entities, with the thickness of the link emphasizing the frequency of co-occurrences. Larger nodes indicate higher occurrence rates, while thicker links between nodes indicate more frequent co-occurrences between entities. Each color corresponds to a thematic cluster, where the nodes and links within this cluster illustrate the coverage of topics (nodes) and the relationships (links) between these topics (nodes) within this particular theme (cluster).

3. Results

3.1. Bibliometric Analysis

This bibliographic review was based on a list of references obtained from Scopus on 7 November 2023. Data were obtained from a systematic search of documents matching the search terms in “Article title, Abstract, Keywords”. The search keywords were “electrokinetic” AND “bioremediation”. The search yielded 465 documents from 1996 to 2023, consisting of 350 articles, 60 reviews, 35 conference papers, 15 book chapters, three books, one editorial and one conference review. Of the total, only 10% referred to sediment, while 90% referred to soil. To obtain comprehensive data suitable for analysis, the information downloaded from the Scopus database includes citation details, bibliographic references, abstracts, keywords and funding details.

3.2. Publication Trends

Although the first electrokinetic remediation process was described in 1936 [26], this search yielded documents from 1996. This is because the nomenclature in the first articles and trials was different. In fact, the title of the first article was ‘Reclamation of alkali soils by electrodialysis’. A robust positive correlation was found between the number of articles and the years of publication, both in the broader field of research and specifically within studies related to seawater (Figure 1).

3.3. Country of Authorship and Affiliation

Considering the 465 documents retrieved by the search, the authors were based in different countries, and the top ten countries of authorship belong to China (152), the United States (56), Spain (52), South Korea (33), India (31), Australia (27), the United Kingdom (25), Canada (21), Denmark (17) and Iran (16), as shown in Figure 2.

According to Figure 3, there are strong collaboration networks between countries. The network analyzed by VOSviewer according to bibliographic coupling country consisted of 23 items divided into five clusters. The colors are defined by dividing by the mean, and the visualization score is based on the average normalized citation.

3.4. Authorship

The top ten authors on this topic are Rodrigo, M.A. (27), Cañizares, P. (24), Guo, S. (19), Li, F. (13), Cang, L. (12), Villaseñor, J. (12), Reddy, K.R. (11), Ribeiro, A.B. (11), Navarro, V. (9), and in fifth place with eight papers are Mateus, E.P., Ottosen, L.M. and Wang, S. The first eight authors, all with two papers, obtained by refined searches within the keyword seawater, are Cañizares, P., Couto, N., Gidudu, B., Guedes, P., Mateus, E.P., Ribeiro, A.B., Rodrigo, M.A. and Saez, C. To evaluate the distribution of highly cited authors, data obtained from the Scopus databases were analyzed using Author Co-Citation Analysis (ACA). The ACA network contained 558 articles grouped into six co-citation clusters (Figure 4).

3.5. Keywords Co-Occurrence Analysis

In order to evaluate the correlation between topics based on the content of the publications, a keyword co-occurrence analysis (KCA) was performed. In this context, the analysis shows the most frequently used author keywords among the retrieved documents cited at least five times. The KCA network contained 580 items divided into seven clusters (Figure 5). According to the map shown in Figure 5, the most frequently used author keywords are electrokinetic remediation, bioremediation, electrokinetic, phytoremediation, remediation, heavy metals, soil, soil remediation and contaminated soil.

3.6. Publications with the Highest Citation

Some of the most relevant papers in terms of the number of citations related to electrokinetic and bioremediation published are listed in

Table 1. The top 10 cited papers in electrokinetic bioremediation on 7 November 2023.

Title	Journal	Year	N° Citations	References
Remediation technologies for metal-contaminated soils and groundwater: An evaluation	Engineering Geology	2001	1242	[117]
Remediation techniques for heavy metal-contaminated soils: Principles and applicability	Science of the Total Environment	2018	972	[118]
Electrokinetic soil remediation—Critical overview	Science of the Total Environment	2002	761	[64]
Remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs)	Journal of Hazardous Materials	2009	678	[119]
Remediation approaches for polycyclic aromatic hydrocarbons (PAHs) contaminated soils: Technological constraints, emerging trends and future directions	Chemosphere	2017	516	[120]
Characterization and remediation of soils contaminated with uranium	Journal of Hazardous Materials	2009	471	[121]
The use of chelating agents in the remediation of metal-contaminated soils: A review	Environmental Pollution	2008	461	[122]
Polycyclic Aromatic Hydrocarbons: Sources, Toxicity and Remediation Approaches	Frontiers in Microbiology	2020	395	[123]
A comprehensive guide of remediation technologies for oil-contaminated soil—Present works and future directions	Marine Pollution Bulletin	2016	311	[124]
An overview of field-scale studies on remediation of soil contaminated with heavy metals and metalloids: Technical progress over the last decade	Water Research	2018	294	[125]

.

4. Conclusions

Electrokinetic remediation is a well-established approach for soil remediation, and numerous studies have demonstrated its success against various types of contaminants in both industrial and environmental settings. At least two studies demonstrated the scale-up applicability of this technique for the ex situ application applied on dredged sediments and soils with different contaminants: Barba and colleagues [126] used 32 m³ of soil contaminated by a mixture of two herbicides, 2,4-D and oxyfluorfen, and Benamar et al. [18] with 0.4 kg and 40 kg of dredged multi-contaminated harbor sediment. Alshawabkeh et al. [127] introduced the calculation of the total price per volume of soil treated (USD/m³), taking into account various factors such as the soil type, medium pH, electrolyte type, remediation duration and the electric potential that affect the efficiency of electrokinetic remediation [128]. The authors declared that the total cost of repairing each volume of soil in 1999 ranged from USD 39.52 to USD 48 [127]. This cost estimate must be updated with the current costs of labor, raw materials and energy.

Nevertheless, the applicability of electrokinetic remediation in marine environments remains limited, as revealed by the bibliometric analysis, with less than 10% of the articles focusing on the remediation of marine sediments. The corrosiveness of the marine environment, exacerbated by the generation of aggressive chloride anions and other side reactions, significantly complicates and challenges the design of technologies specifically for in situ applications and increases the cost of materials needed to produce durable electrodes.

Combined techniques, such as electrokinetic bioremediation, have the potential to mitigate the negative effects of electrokinetic remediation. For example, the use of lower voltages and the application of pulsed currents can help achieve this goal while stimulating the in situ growth of the microbial communities involved in the bioremediation treatments. Furthermore, unlike electrokinetic phytoremediation, electrokinetic bioremediation offers the possibility of enclosing the site to be treated with appropriate coatings capable of preventing the phenomena of resuspension and diffusion of contaminants into the surrounding environment. This greatly expands the application of this technique.

However, some unclear aspects need to be better investigated, such as the production of toxic metabolites during the biodegradation process. In addition, a more thorough understanding of the main effects of the current (intensity and duration) on microbial communities is required, both to optimize the conditions for their growth and to minimize the side effects due to incomplete metabolism during microbial activities.

We still lack data from large-scale experiments, particularly regarding in situ electrokinetic bioremediation in marine environments. Such data are crucial for modeling the process and providing a potential estimate of the economic costs associated with this approach. For these reasons, a preliminary study of the site-specific conditions for electrokinetic bioremediation should be carried out prior to the actual treatment. This assessment will help determine whether the metabolites produced are less toxic than the initial contaminants and thus guide the process in the most effective way, including from an economic point of view.