Removal of heavy metals from dredging marine sediments via electrokinetic hexagonal system: A pilot study in Italy

Among the several treatment options, electrokinetic (EK) remediation is recognized as an effective technique for the removal of heavy metals from low-permeability porous matrices. However, most of the EK decontamination research reported was performed on linear configuration systems at a laboratory scale. In this study, a series of experiments were performed on a pilot-scale system where the electrodes were arranged in a hexagonal configuration, to assess the improvement of the EK process in the removal of inorganic contaminants from sediments dredged in the harbor of Piombino, Italy. HNO₃ was used as acid conditioning and both pH effect and treatment duration time were investigated. Sediment characterization and metal fractionation were also presented, in order to understand how the bioavailability of metals affects the process efficiency. The increase in pH due to the buffering capacity of the sediment in the sections close to the cathode favored the precipitation and accumulation of metals. However, the results highlighted that longer treatment times, combined with an efficient pH reduction, can improve treatment performance, resulting in high removal efficiencies for all the target metals considered (a percentage removal greater than 50% was reached for Cd, Ni, Pb, Cu and Zn). Compared to different EK configuration systems, the hexagonal configuration arrangement applied in our study provides better results for the remediation of dredged marine sediment.

1. Introduction

Sediment dredging for waterway maintenance poses an environmental issue in sediment management [1,2]. The main concern is related to the high pollution load introduced during ship transport and industrial activities. In particular, the contamination of harbor sediments by heavy metals and their complexation with organic matter represent crucial bottlenecks, as their high stability hinders their removal [3].

Traditional soil management strategies include environmentally unsustainable alternatives such as landfills or near-shore disposal facilities [4]. Nowadays, process sustainability [5,6] and resource recovery [7] represents a new paradigm for industrial activities [8], but the current technological readiness level (TRL) of some processes is still low [9].

In view of that, the development of new technologies for soil remediation in a circular economy perspective is one of the main environmental challenges. The efficiency of process is closely related to soil characteristics, such as pollutant concentration, organic matter content, particle size distribution, and alkalinity; therefore, based on matrix complexity, specific treatments may be required [10].

Focusing on marine sediments, the high salinity, low hydraulic permeability and high content of carbonates and organic matter often make common treatment technologies (e.g pump-and-treat or soil washing) inadequate or inefficient [11]. Among the available treatment options, electrokinetic (EK) decontamination is recognized as an effective technique for the removal of heavy metals from low-permeability porous matrices [12]. In addition, its feasible implementation both in situ and ex-situ and coupling with other remediation technologies, such as phytoremediation and Fenton processes, represent further advantages [13,14].

EK technology exploits the application of an electric field to promote the mobilization of metal ions towards the electrodes through three main transport mechanisms: electroosmosis, electromigration and electrophoresis [15,16].

Furthermore, the applied electric field promotes the water electrolysis with the production of H⁺ and OH⁻ ions, which generate a charge gradient along the solid matrix due to the transport phenomena described above, making it essential to adjust the pH to avoid undesirable effects that can inhibit the transport mechanisms (e.g precipitation of hydroxides and carbonates) [17].

Nevertheless, the electric field active area is the main driving force of the process, and the efficiency of the system is closely related to the configuration of the electrodes [18]. Regarding inorganic contaminants, over the years, various electrode configurations have been studied to increase the removal efficiency [19]. Compared to a 1-D configuration, a 2-D configuration achieves greater decontamination by reducing specific electricity consumption [20]. In this context, Kim et al. [21] proved that a pilot-scale 2-D hexagonal configuration applied to agricultural saline lands shows high salt removal efficiencies; however, most of the studies focused on heavy metal remediation from soils [22,23]. Therefore, the different behaviour of the sediments due to the high salinity and carbonate content must be taken into account.

In a recent study, the performance of a one-dimensional configuration was compared with that of a two-dimensional configuration in terms of pollutant removal efficiency and electric energy consumption. The 2-D hexagonal configuration showed better performance and allowed an increase in the removal efficiency of Cu and Pb by more than 80% [24]. Consistently, another in situ EK study demonstrated that the hexagonal configuration can be a valid option for the remediation of paddy rice fields polluted with As, Pb, and Cu [25].

At the same time, many authors have addressed this topic, focusing on sediment remediation.

Ammami et al. [26] performed bench-scale EK tests to evaluate the effects of various operation settings for a single-stage treatment aimed at removing heavy metals and PAHs from dredged silt. Citric acid, a chelating agent, and various surfactants have been combined. The tests showed satisfying results (50%, 30%, and 35% of Zn, Pb, and Cd were removed, respectively) using a dosage of 0.1 M of citric acid.

A similar study on the extraction of heavy metals from marine sediments tested various processing fluids such as EDTA, citric acid, HNO₃ and HCl, where HCl has been shown to be the best agent for the extraction of Ni, Cu, Zn, and Pb [3].

To the best of our knowledge, a 2-D hexagonal configuration plant for harbor sediment remediation has not yet been optimized. In this study, a particular two-dimensional hexagonal EK configuration at pilot-scale was applied for the first time for marine sediment remediation, in order to evaluate the heavy metal removal efficiency of EK technology.

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