Light Electroshock: Coastal Erosion

Global warming and rising sea levels are causing a ton of problems in many parts of the world. One of the results of these processes is coastal erosion, which not only threatens nearby infrastructure, but also the marine ecosystem. Classic methods of combating coastal erosion include building reinforcement structures and introducing special substances into the substrate, but these methods do not provide long-term results. Scientists from Northwestern University (Evanston, USA) have developed a new method of combating coastal erosion, which is based, roughly speaking, on electric shock. What principles underlie this method, how exactly does it work, and how effective is it? We will find answers to these questions in the scientists' report.

Research basis

Coastal areas support the world’s most developed regions and are home to about 40% of the world’s population. However, coastal areas face significant challenges from extreme weather conditions and rising sea levels, particularly erosion. Traditional methods of mitigating coastal erosion include seawalls and beach restoration. However, these approaches are only effective for a few years, representing expensive and temporary solutions that require recursive retrofitting. In contrast, natural systems such as coral reefs and ecological barriers not only increase coastal resilience, but also provide greater structural support and can even promote beach accretion.

Inspired by how marine organisms use metabolic energy to grow their skeletons and shells through mineral deposits in seawater that can withstand even the most extreme disturbances, this paper explores a new approach to mitigating marine soil erosion. This approach involves using electrical energy to deposit mineral deposits comparable to those that build the skeletons and shells of marine organisms into the pores of marine soils to cement and maximize the erosion resistance of such materials.

The ability to precipitate solid mineral binders in seawater by applying weak electrical stimulations results from the large buffering capacity and wide availability of ions that characterize such an electrolyte and the process of electrodeposition: the electrically mediated precipitation of minerals dissolved in solutions. In particular, when an electric current is applied to seawater, a variety of reduction and oxidation (redox) reactions occur, as well as solid precipitation. These reactions begin with the release of hydroxide ions (OH^–) around the cathode interfaces, resulting in an increase in local pH. Under these conditions, the resulting hydroxide ions react with naturally dissolved divalent cations in seawater (Mg²⁺ and Ca²⁺) and bicarbonate anions (HCO₃^–), which results in the otherwise non-spontaneous precipitation of two minerals: magnesium hydroxide (Mg(OH)₂) and calcium carbonate (CaCO₃).

Currently, electrodeposition of minerals in seawater is widely used to protect offshore structures from corrosion, promote marine life and heal cracks in coastal infrastructure. Recent experimental data show that electrodeposition can also cement marine substrates in contact with metallic structures and the minerals formed in this way appear to be durable. However, the understanding of the effect of electrodeposition on the structure and properties of marine soils remains limited. In this context, there is a knowledge gap due to the apparent lack of studies investigating the effect of applied voltage on mineral selectivity, composition, spatial distribution, incorporation mechanisms and effects on soil properties, despite voltage being the driving force behind electrodeposition.

Although a mechanistic understanding of the reactions and products of electrodeposition has been achieved for seawater, this knowledge is not available for soils, where the electrical and kinetic phenomena governing electrodeposition are inherently more complex due to the influence of geometric, physical and chemical constraints imposed by the pore network of such materials. Thus, the influence of the different reaction regimes and mineral formations that can be achieved at different electrochemical potentials on the structure and properties of marine soils remains unexplored.

The main hypothesis of this study is that electrodeposition in marine soils depends mainly on the characteristics of the electrical treatment, soil structure and electrolyte solution. To test this overarching hypothesis, the scientists conducted two different types of experiments in specially designed electrochemical cells on quartz sand under strictly controlled conditions. The first involved short-term experiments without water recirculation to identify the selectivity of electrodeposition in soils. The second involved long-term experiments with periodic water recirculation to identify the permeation mechanisms and effects of electrodeposition in soils under conditions comparable to open seawater.

Overall, these experiments investigated the influence of the following central variables on the characteristics of the applied electrical treatments, the treated porous materials and the electrolyte solutions:

• the magnitude and duration of the applied voltage;
• relative soil density;
• concentration of electrolyte ions, respectively.

Numerous physical, chemical and mechanical characterization methods have been used to investigate the morphology, structure, composition, spatial distribution and effects of electrodeposited minerals in sand.

Research results

Physical characterization

Image #1

Physical characterization obtained using scanning electron microscopy (SEM or SEM from scanning electron microscope), Raman spectroscopy and image analysis of electrodeposited minerals highlights key microscopic features of their deposition loci, morphology and polymorphism, as well as obvious macroscopic effects on quartz sand (Figure 1). The first pattern found is that the deposition loci and the type of electrodepositions depend on the applied voltage, which controls the reaction modes and hence the electrodeposition process (1a). At relatively low voltage (2.0 V), sparse electrodepositions surround the sand particles without significantly binding them to each other (row 1, column 1 in 1a). At medium voltage (3.0 V) and high voltage (4.0 V), denser electrodepositions surround and bind sand particles through mineral bridges (row 1, columns 2 and 3 in 1a). SEM results consistently indicate two dominant mineral formations: CaCO₃ in the form of calcite (rhombohedral morphology) and aragonite (acicular morphology), as well as Mg(OH)₂ in the form of brucite (fibrous compounds consisting of plate-like structures). Formations of CaCO₃ appear especially at lower voltages (rows 2 and 3, columns 1 and 2 on 1a), while the formation of Mg(OH)₂ are mainly electrodeposited at higher voltages (rows 2 and 3, column 3 on 1a). When and CaCO₃and Mg(OH)₂ electrodeposited, minerals Mg(OH)₂ coat silica soil particles and form a substrate for the precipitation of CaCO minerals₃ (lines 2 and 3 on 1a).

Analysis of SEM images of selected samples of tested quartz sand shows that Mg(OH)₂ is formed both in the form of lamellar and block brucite, and CaCO₃ is found in increasingly stable forms of vaterite, aragonite and calcite (with aragonite predominating in quantity relative to calcite and vaterite) (1b). Notably, quantitative Raman spectral analysis of precipitated minerals on quartz sands consistently indicates precipitation of CaCO₃ in the form of calcite and aragonite, as well as Mg(OH)₂ in the form of brucite; in addition, Raman spectra also reveal precipitation of hydromagnesite, Mg₅(CO₃)₄(OH)₂ ∙ 4H₂O (1c). X-ray diffraction analyses presented below (2e), also quantitatively confirm these observations.

A second discernible pattern in the electrodeposition of minerals in marine sands is that the size and deposition loci of the electrodeposition depend on the relative density (1d), which is an indicator of porosity. Loose sands exhibit finer minerals and more uniform and widespread sediments (columns 1 and 2 in 1d). In contrast, dense sands show larger minerals and less uniform and widespread sediments, as well as more aragonite compared to calcite, with almost no brucite (columns 3 and 4 in 1d).

Analysis of the volume and mass of 'cemented' (or affected) sand sheds light on a third distinguishable feature of electrodeposition in marine soils (1st). The volume of soil affected by electrodeposition depends on the duration and magnitude of the electrical stimulation applied. After 7 days of treatment, a limited volume of sand has been cemented by the electrodeposition. The largest cemented volume is achieved at 3.0 V, while smaller volumes are achieved (in order) at 4.0 and 2.0 V. After 28 days, a larger volume of sand has been cemented by the electrodeposition. The largest cemented volume is achieved at 4.0 V, while smaller volumes are achieved (in order) at 3.0 and 2.0 V.

For a given voltage, the affected volume of electrodeposition decreases with increasing relative density of the soil, whereas it increases with longer electrical stimulation. Increasing the duration of electrical stimulation from 7 to 28 days does not significantly change the degree of cemented sand volume at 2.0 and 3.0 V, whereas it significantly increases this degree at 4.0 V. After 7 days, the area of ​​soil affected by electrodeposition reaches a radial distance of approximately 2–3 times the electrode diameter at 3.0 V. In contrast, this area exceeds a radial distance of 20 times the electrode diameter after 28 days of treatment at 4.0 V.

Consideration of the macroscopic effects of electrodeposition for different voltage values ​​sheds light on the final feature of mineral electrodeposition in marine soils (1f). There are different mechanisms of penetration and sizes of volumes affected by electrodeposition, depending mainly on changes in the electrochemical reactions that control the electrodeposition. Application of a low voltage (2.0 V) for a long time results in electrodeposition near the cathode, which penetrates into the soil further from the electrode at a medium voltage (3.0 V) and affects a significant proportion of the soil volume at a high voltage (4.0 V). Comparable results are obtained for shorter periods of time, with the difference that the total volume affected by electrodeposition is successively less significant. As the electrodepositions grow in sands, they form a cemented material. The electric current and resistance vary with such mineral formations, the former generally decreasing and the latter increasing due to the constant applied potential over time.

Chemical characterization

Image #2

Chemical characterization of environmental conditions in sand and electrosediments (graphs above) obtained using pH measurements, energy dispersive X-ray spectroscopy (EDS from energy dispersive X-ray spectroscopy), X-ray diffraction (XRD from X-ray diffraction) and thermogravimetric analysis (TGA from thermogravimetric analysis), supports advanced analysis of mineral properties and their formation in closed and open electrochemical systems.

Temporary changes in pH at the anode (2a) and cathode (2b), measured in the short term in a closed system, show that the pH in the sand can change significantly, resulting in an acidic environment near the anode and a more alkaline environment near the cathode due to the influence of oxidation and reduction reactions, respectively. For both loose and dense sands, the low voltage level (2.0 V) results in minimal or negligible changes in pH. However, for the highest voltage level (4.0 V), the impact is significant. In all cases, the pH changes are delayed in dense sands due to their lower porosity and greater tortuosity compared to loose sands. After the electrical conditioning is stopped, the pH begins to return to its original value. In contrast, the temporary pH changes at the anode and cathode (2c), measured in long-term tests in an open system, reveal minimal or negligible changes in pH due to periodic recirculation of seawater. By filling the system with Ca ions²⁺ and Mg²⁺these conditions maintain the concentration of ions of the main electrolyte approximately constant.

Analysis of elemental mapping obtained by EDS for loose and dense sands (2d), shows a decrease in the Ca/Mg ratio with increasing stress, which is consistent with all the qualitative results obtained using SEM (1a). The same trend for the Ca/Mg ratio as a function of applied voltage is calculated from the Rietveld refinement of the XRD spectra (2e) and TGA results (2f). The quantitative differences between the obtained trends for the Ca/Mg ratio are related to the different methods used to analyze this variable, while remaining qualitatively consistent. Notably, both XRD and TGA results again confirm the dominant precipitation of calcium-based minerals at limited stresses, in contrast to the dominant precipitation of magnesium-based minerals at significant stresses, as previously shown by SEM analyses (1a). X-ray diffraction analyses confirm the presence of vaterite, calcite, aragonite, brucite and hydromagnesite, as previously noted by Raman spectroscopy (1c).

Hydromechanical characterization

Image #3

The nucleation and growth of mineral electrodepositions alter the structure of marine sands. Consequently, these mineral deposits alter the properties of such materials. The results of this work quantitatively reveal changes in hydraulic conductivity (k) and unlimited compressive strength (UCS from unconfined compressive strength) of marine sands subjected to electrodeposition (graphs above).

Hydraulic characteristics of sands subjected to long-term electrical conditioning (3a), indicates a tendency for hydraulic conductivity to decrease for materials subjected to increasing stresses. Interestingly, the decrease in hydraulic conductivity of cemented sand compared to clean (i.e. untreated) sand can reach one order of magnitude for the highest stress (4.0 V). In all cases, the results show higher hydraulic conductivity of loose sands compared to dense ones due to the more porous packing of the former compared to the latter.

Analysis of the correlation between unconfined compressive strength and cement content in the tested sands (3b) shows that both loose and dense sands benefit from a marked increase in strength with mass fraction of cement. Electroprecipitation can produce cemented sands with UCS reaching several MPa. In other words, electrodeposition can transform initially cohesionless sands into rocks.

For a more detailed look at the nuances of the study, I recommend taking a look at scientists' report And additional materials to him.

Epilogue

Motivated by the lack of sustainable and long-term approaches to mitigate coastal erosion worldwide, this study presented an experimental laboratory investigation of electrically mediated precipitation of mineral binders in marine sands through the electrodeposition process.

In particular, using specially designed electrochemical cells, this study systematically and mechanistically investigated the electrodeposition of calcium and magnesium based minerals in seawater saturated quartz sand. For the first time, this work analyzed the effects of the magnitude and duration of the applied voltage, the relative density of the soil and the concentration of electrolyte ions, which are central variables for any applied electrical treatment, the treated porous material and the electrolyte solution, respectively.

The results of this work shed light on the various reaction mechanisms governing the electrodeposition of CaCO₃Mg(OH)₂ and Mg₅(CO₃)₄(OH)₂ · 4H₂O in seawater-saturated sands. The transition from oxygen reduction to water reduction occurs particularly between 2.0 and 3.0 V, with significant hydrogen evolution at 4.0 V. During oxygen reduction, hydroxide formation is limited by mass transfer and results in predominantly CaCO deposits.₃mainly in the form of calcite, with associated formations of Mg₅(CO₃)₄(OH)₂ · 4H₂O.

During water reduction, hydroxide formation is rate limited and favors the formation of Mg(OH)₂mainly in the form of brucite. By identifying these two different potential regimes, this work highlights that control of the applied electrode potential, taking into account the relative soil density and the electrolyte properties, provides selectivity for the type of mineral and polymorph precipitated, the location of mineral precipitation, and the mechanism of mineral incorporation. Thus, electrodeposition allows for different effects on the structure and properties of electrodeposited sands.

This work extends the knowledge of mineral electrodeposition in seawater-saturated soils, highlighting the potential for using this process to protect coastal areas from erosional processes. In addition, the results of this work improve the analysis of problems related to sedimentation, biomineralization and geological carbon sequestration, which are controlled by mineral nucleation and growth in porous materials.

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