Seawater Electrolysis Enables Mg(oh)2 Production and Co2 Mineralization

This application claims the benefit of priority to U.S. Provisional Application No. 63/256,888, filed on Oct. 18, 2021, the contents of which are hereby incorporated by reference in their entirety.

This invention was made with Government support under Contract No. DE-FE0031705 awarded by the United States Department of Energy. The Government has certain rights in the invention.

Ocean carbon storage is a pathway for reducing atmospheric carbon concentrations. The oceans represent a vast reservoir of about 38,000 gigatonnes of carbon, stored in a dissolved form as HCO, HCO, and COspecies. Carbon capture from oceans via the formation of divalent metal carbonate solids from ocean water has the potential to decrease its carbon storage capacity due to a pH reduction from this process. But an increase in the pH of ocean water may increase this storage capacity in accordance with Henry's law (see, e.g.,, showing a plot of seawater COuptake with respect to pH). Accordingly, the addition of alkaline materials, such as metal hydroxides, to ocean water has the potential to increase its pH, thereby restoring its carbon storage capacity.

Brucite (Mg(OH)) for industrial uses can be obtained either naturally, through the hydration of MgO produced from calcining magnesium carbonate, or by precipitation from seawater by the provision of alkalinity. Ocean water contains a high amount of Mgions, particularly in the form of chloride and sulfate salts. Thus, ocean water can be a source for brucite production. There is a need for efficient methods of forming brucite from ocean water. There is further a need for methods of increasing pH of ocean water, particularly as a part of carbon capture methods.

The present disclosure relates to methods for producing hydroxide solids, particularly Mg(OH)solids. In some embodiments, the present disclosure provides a method for producing one or more hydroxide solids, the method comprising:

In some embodiments, the electrolyte solution comprises divalent metal cations. In certain embodiments, the electrolyte solution comprises Mg2+, Ca2+, or both Mg2+ and Ca2+ ions. In particularly preferred embodiments, the divalent cations comprise Mg2+ ions.

In certain embodiments, wherein the electrolyte solution comprises a brine or sea water. Preferably, the electrolyte solution comprises sea water.

In certain embodiments, the brine or sea water comprises NaCl in the brine or sea water in a concentration about 1,000 ppm or more, about 2,000 ppm or more, about 3,000 ppm or more, about 4,000 ppm or more, about 5,000 ppm or more, about 6,000 ppm or more, about 7,000 ppm or more, about 8,000 ppm or more, about 9,000 ppm or more, about 10,000 ppm or more, about 15,000 ppm or more, about 20,000 ppm or more, about 25,000 ppm or more, or about 30,000 ppm or more, about 35,000 ppm or more, about 40,000 ppm or more, about 45,000 ppm or more, about 50,000 ppm or more, about 55,000 ppm or more, or about 60,000 ppm or more. Preferably, the NaCl concentration is about 35,000 or more.

In certain embodiments, the electrolyte solution has a Ca-equivalent or Mg-equivalent concentration of about 2 ppm or more, about 10 ppm or more, about 50 ppm or more, about 100 ppm or more, about 200 ppm or more, about 300 ppm or more, about 400 ppm or more, about 500 ppm or more, about 600 ppm or more, about 700 ppm or more, about 800 ppm or more, about 900 ppm or more, about 1000 ppm or more, about 11 ppm or more, about 1200 ppm or more, about 1300 ppm or more, about 1400 ppm or more, or about 1500 ppm or more. Preferably, the electrolyte solution has an Mg-equivalent concentration of about 1000 ppm or more.

In some embodiments, the one or more hydroxide solids comprises Mg(OH)2, Ca(OH), or both Mg(OH)and Ca(OH). Preferably, the one or more hydroxide solids comprise Mg(OH).

In some embodiments, the electroactive mesh cathode comprises a rotating disc cathode. In particular embodiments, the rotating disc cathode has an electroactive mesh disposed thereon.

In some embodiments, the method further comprises removing the one or more hydroxide solids from the surface of the mesh. In particular embodiments, the removing the one or more hydroxide solids from the surface of the mesh comprises scraping the surface of the mesh.

In some embodiments where the cathode is a rotating disc cathode, removing the one or more hydroxide solids from the surface of the mesh comprises rotating the rotating disc cathode past a scraper.

In certain embodiments, the electroactive mesh cathode comprises a metallic composition, non-metallic composition, or hybrid metallic and non-metallic composition.

In some embodiments, the electroactive mesh cathode comprises stainless steel, titanium oxide, carbon nanotubes, one or more polymers, graphite, or combinations thereof. Preferably, the mesh cathode comprises stainless steel.

In some embodiments, the electroactive mesh comprises pores having a diameter in the range of about 0.1 μm to about 10000 μm.

In some embodiments, the method comprises forming alkalized effluents having a pH greater than 9, or in other embodiments, greater than 10.

In some embodiments, the anolyte comprises an acid. In certain embodiments, the acid has a pH of less than about 6.

In some embodiments, the method further comprises providing a barrier to separate the catholyte and the anolyte. In some embodiments, the barrier comprises a polymer, such as cellulose, polyvinyl chloride, organic rubber, polyolefin, polyethylene, polypropylene, or any combination thereof.

In other embodiments, the method further comprises cycling the anolyte to a neutralization pool. The neutralization pool may comprise mafic materials, ultramafic materials, calcium-rich fly ash, slag, or any combination thereof.

In some embodiments, the electrolytically generating of hydroxide ions is conducted at a current density of greater than 50 μA/cm.

The process according to the present disclosure is based on electrochemically enhanced electrolysis reactions to form brucite (Mg(OH)) precipitates in order to increase ocean alkalinity and promote atmospheric carbon dioxide dissolution. Such processes include, but are not limited to, those disclosed in International Application No. PCT/US22/35289 filed on Jun. 28, 2022, International Application PCT/US20/37629 filed on Jun. 12, 2020, and U.S. application Ser. No. 17/722,036 filed on Apr. 15, 2022, the entireties of which are hereby incorporated by reference herein.

As shown in, increasing the pH of seawater increases its carbon storage capacity according to the equilibrium constants describing the speciation of HCO, HCO, and COions and Henry's Law. In particular, the dissolution of alkaline (e.g., calcium- and magnesium-rich) solids in the ocean surface could advantageously increase its pH, allowing additional COuptake. The COuptake (quantified as a mass of COincorporated into solid products or as dissolved ions per mass of initial solid or liquid material) describes the material's efficiency in sequestering gaseous COin stable solids or dissolved ions. Enhancing COuptake allows impactful removal of gaseous COresulting from anthropogenic sources.

As shown in, brucite (Mg(OH)) may be added in seawater to equilibrium which results in a pH of 9.1, equivalent to about a threefold increase in the total dissolved CO, compared to that at pH of 8.2, which is the typical pH of seawater. Brucite for industrial uses can be obtained either naturally, e.g., through the hydration of MgO produced from calcining magnesium carbonate, or by precipitation from seawater by the provision of alkalinity.illustrates COuptake capacity enhanced by brucite dissolution. Every mole of dissolved brucite can promote about 1.6 moles of atmospheric COabsorption.

In the present disclosure, metal hydroxide solids, such as brucite, may be produced by an electrochemical process that uses seawater, which contains ˜55 mmol Mg/L, or using other Mg-rich brines as feeds. In some embodiments, a membrane-less reactor may be used to produce brucite precipitates. Advantages of such a membrane-less reactor may include lower energy requirements, reduced maintenance and operating costs, and reduced manufacturing expense at increasing scales.

In some embodiments, a method according to the present disclosure comprises: providing a catholyte comprising an electrolyte solution; contacting the catholyte with an electroactive mesh cathode to electrolytically generate hydroxide ions, thereby precipitating the one or more hydroxide solids.

In some embodiments, the method further comprises removing the one or more hydroxide solids from the surface of the mesh where they may deposit.

A COmineralization process can be achieved by alkalizing a circumneutral Ca- and Mg-containing solution (e.g., seawater, alkaline metal-rich groundwater, industrial wastewater, or desalination brine). In some embodiments, the method uses a single-compartment continuous stirred-tank reactor (CSTR). Operational parameters such as voltage, current density, and hydraulic retention time (“HRT”)) are chosen to minimize the hydroxylation energy intensity of the design.

Turning to, a membrane-less reactor useful for practicing certain embodiments of the present invention is shown. A membrane-less electrolysis reactorwas conceptualized to electrochemically precipitate hydroxide solids from a catholyte. In some embodiments, a hydroxide-forming process can advantageously be achieved by alkalizing a circumneutral Ca- and Mg-containing solution, such as seawater, alkaline metal-rich groundwater, industrial wastewater, or desalination brine. We evaluated the feasibility of the conceptualized multi-compartments reactor, by using a single-compartment continuous stirred-tank reactor (CSTR). Operational parameters (e.g., voltage, current density, and hydraulic retention time (“HRT”)) may also be selected to demonstrate the carbonation energy intensity of the design.

Referring still to, reactorincludes a reservoircontaining a catholyte, such as seawater, alkaline metal-rich groundwater, industrial wastewater, desalination brine. The reactor further includes an anolyte inletand outlet. Electrode assemblyis in fluid contact with the aqueous sequestration solution reservoirand comprises rotating disk cathodesand anodesseparated by a barrier layer. The rotating disc cathodes(e.g. 316L stainless steel mesh) may be rotated around shaftto pass a scraperfor product removal and collection. The reactor may further comprise a neutralization pool. Omay be produced at the anode, and may be released at an Ooutlet. Hmay be produced at the rotating disk cathode, and may be released at an Houtlet.

In embodiments comprising rotating disc cathodes, inducing the precipitation of the carbonate solid includes rotating a cylinder consisting of the electroactive mesh in the solution, while applying suction to draw the solution onto the outer surface of the mesh.

The electrolytes may be separated with a porous barrier for the following reasons: (1) minimized neutralization reactions between anolytes and catholytes allows stable cathode pH for effective mineralization; (2) separated electrolytes promote higher energy efficiency of the reactor; and (3) the gas streams (Hand O) may need to be divided and collected separately.

Referring still to, an online pH-monitoring system may be used, for example, to control the applied electric current to attain a constant catholyte pH or greater than 9. The anolyte can in some embodiments provide

In some embodiments, the reactor includes a catholyte and an anolyte. The catholyte may be an electrolyte solution configured to flow around or through a cathode. The anolyte may be an electrolyte configured to flow around or through an anode. The catholyte may comprise an electrolyte solution.

In some embodiments, the electrolyte solution comprises divalent metal cations, such as Mg, Ca, or both Mgand Caions. In particularly preferred embodiments, the electrolyte solution comprises Mgions.

In some embodiments, the electrolyte solution comprises seawater or a brine. Preferably, the electrolyte is seawater. In some embodiments, the electrolyte solution has a concentration of NaCl of about 1,000 ppm or more, about 2,000 ppm or more, about 3,000 ppm or more, about 4,000 ppm or more, about 5,000 ppm or more, about 6,000 ppm or more, about 7,000 ppm or more, about 8,000 ppm or more, about 9,000 ppm or more, about 10,000 ppm or more, about 15,000 ppm or more, about 20,000 ppm or more, about 25,000 ppm or more, or about 30,000 ppm or more, about 35,000 ppm or more, about 40,000 ppm or more, about 45,000 ppm or more, about 50,000 ppm or more, about 55,000 ppm or more, or about 60,000 ppm or more, or greater, or any range or value there between. In preferred embodiments, the electrolyte solution has a NaCl concentration of about 35,000 ppm or more.

In some embodiments, the catholyte has a concentration of Ca-equivalent or Mg-equivalent of about 2 ppm or more, about 10 ppm or more, about 50 ppm or more, about 100 ppm or more, about 200 ppm or more, about 300 ppm or more, about 400 ppm or more, about 500 ppm or more, about 600 ppm or more, about 700 ppm or more, about 800 ppm or more, about 900 ppm or more, about 1000 ppm or more, about 11 ppm or more, about 1200 ppm or more, about 1300 ppm or more, about 1400 ppm or more, or about 1500 ppm or more. Preferably, the catholyte solution has an Mg-equivalent concentration of about 1000 ppm or more. Ca-equivalent and Mg-equivalent refer to salts of Ca and Mg in the electrolyte solution. Preferably, the salts are chloride salts or sulfate salts.

In some embodiments, the anolyte comprises an acid. In some embodiments, the anolyte has a pH of less than about 7, less than about 6, less than about 4, less than about 3, less than about 2, down to less than about 1. In particular embodiments, the anolyte has a pH of about 1 to about 6, about 1 to about 5, about 1 to about 4, about 1 to about 3, or about 1 to about 2.

In some embodiments, the one or more hydroxide solids comprise Mg(OH), Ca(OH), or both Mg(OH)and Ca(OH). In particularly preferred embodiments, the one or more hydroxide solids comprise Mg(OH)(also referred to herein as brucite).

In some embodiments, the cathodecomprises an electroactive mesh. In some embodiments, the electroactive mesh comprises a metallic or a non-metallic composition, or a combination of metallic and non-metallic compositions. In some embodiments, the electroactive mesh comprises, consists essentially of, or consists of a metallic mesh or carbon-based mesh. In some embodiments, the electroactive mesh comprises stainless steel, titanium oxide, carbon nanotubes, polymers, and/or graphite, or other hybrid compositions of these materials. Preferably, the electroactive mesh comprises stainless.

In some embodiments, the electroactive mesh comprises pores having a diameter in the range of about 0.01 μm to about 10000 μm (e.g., about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 1000, 1500, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, or 10000 μm, or any range there between).

In some embodiments, the cathodesare (for example, 316L stainless steel mesh) coupled with OER- (oxygen evolution reactions) selective anodes (e.g., MnO-coated Pt) to produce alkalinity and acidity.

In some embodiments, the method further comprises removing the one or more hydroxide solids from the surface of the mesh. In preferred embodiments, the one or more hydroxide solids are removed by a scraping process. The scraping process may use a metallic brush, blade, or high-pressure nozzles. In particular embodiments where the cathodes are rotating disc cathodes, the one or more hydroxide solids from the surface of the mesh may be removed from the surface of the mesh by rotating the rotating disc cathode past a scraper,

In some embodiments, the reactor further comprises a barrierto separate the anolyte from the catholyte. In some embodiments, the barrier comprises cellulose, polyvinyl chloride, organic rubber, polyolefin, polyethylene, polypropylene, any other suitable material, or combinations thereof. The barrier separates the catholyte and anolyte in order to: (1) minimize neutralization reactions between the anolyte and the catholyte, resulting in a stable cathode pH necessary for brucite production; (2) promote higher energy efficiency of the reactor; and (3) facilitate collection of gas streams (Hand O).

A pH-monitoring system may be used, for example, to control the applied electric current to attain a constant catholyte pH. For example, in some embodiments, the catholyte pH is maintained above 9, such as at about 9.5-9.6. The stainless steel cathodes may be covered by a hydrophobic mesh (e.g., polypropylene (PP) meshes) as hydroxide catalysts, thereby electrolytically generating hydroxide ions at the cathode. The catholyte may be seawater-flushed such that the Mgions react with the electrolytically produced OH ions to produce Mg(OH). Operational parameters including current density and hydraulic retention time, and HRT may be optimized. Within a reasonable HRT (e.g., in seconds to minutes), the production of Mg(OH)is promoted at high current densities. In some embodiments, the current density is greater than 50 μA/cm, greater than 100 μA/cm, greater than 200 μA/cm, greater than 300 μA/cm, greater than 400 μA/cm, or greater than 5000 μA/cm, or at any range therebetween. In addition, high current densities may also yield alkalinized effluents (e.g., pH greater than about 9, or greater than about 10), can advantageously be used to improve COcapture capabilities of an anolyte source, such as seawater.

In some embodiments, PP-covered stainless steel cathodes may be rotated to pass a scraper (e.g., a metallic brush, blade, or high-pressure nozzles) to remove the hydroxides, thereby regenerating the cathode for subsequent hydroxide production as the discs rotate back into the liquid. In some embodiments, a nozzle sprayer may be used to force the detachment of the precipitated hydroxides.

In some embodiments, the anolyte is cycled to a neutralization poolcomprising calcium-rich fly ash, slag, or any combination thereof, and the produced acidity can thus be consumed to restore alkalinity. Ca-rich fly ashes and minerals advantageously may also be used to enrich the Cain anolyte.

As shown in, Mg(OH)according to certain embodiments of the present methods forms a scale at the cathode surface, permitting easy removal via a simple scraping process.shows a plot of the brucite production and removal rates per L seawater as functions of the current densities. A higher current density yields a lower concentration of brucite formed and a higher removal rate.shows a scanning electron microscopy (SEM) image of the brucite precipitates formed on the cathode mesh. The brucite formed is thick, brittle, and with defined cracks, which help promote easy removal.shows an X-ray diffraction (XRD) plot of the precipitates formed. The XRD plot shows that brucite is formed as the same peaks are seen between the precipitates and brucite.

As used herein, the singular terms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.

As used herein, the term “set” refers to a collection of one or more objects. Thus, for example, a set of objects can include a single object or multiple objects.