Microbial Electrochemical Cell and direct salt recovery

Abstract

A method and apparatus for reducing brine effluent from desalination plants by modifying microbial electrochemical cells are disclosed. To reduce brine generation, the contaminant salts from saline water, such as seawater or wastewater, which are to be desalinated, are accumulated in two organic solvent solutions. The generated organic solvent solutions are then mixed in a container wherein the acids and alkalis combine to produce salts which precipitate out of the organic solvent solution due to low solubility. The method may employ bipolar membranes or cation exchange membranes and anion exchanges membranes. Some embodiments of this invention can be used to desalinate brine from any available source and reduce the operational cost of brine treatment.

Claims

1. A microbial electrochemical cell for recovering contaminant salts from saline water comprising: an anode chamber, a desalination chamber comprising saline water, and a cathode chamber in which OH.sup. ions are generated; an anode electrode at least partially contained in the anode chamber; a cathode electrode at least partially contained in the cathode chamber with an electron acceptor at least partially in contact with the cathode electrode; and an electric circuit connecting the cathode electrode and the anode electrode; the anode chamber comprising a plurality of anode-respiring microorganisms to oxidize substrates, transfer electrons through respiratory chains of the microorganisms, release electrons and H.sup.+ ions in the anode chamber, and generate an electric current between the cathode electrode and the anode electrode via the electric circuit; an acid chamber between the anode chamber and the desalination chamber; an anion exchange membrane between the acid chamber and the desalination chamber; such that anions are transferred from the desalination chamber to the acid chamber due to an electric field established by the electric current; an alkaline chamber between the cathode chamber and the desalination chamber; a cation exchange membrane between the alkaline chamber and the desalination chamber; such that cations are transferred from the desalination chamber to the acid chamber due to an electric field established by the electric current; an organic solvent disposed in the acid chamber; the organic solvent having a lower solubility for one or more contaminant salts than their respective acids; an organic solvent disposed in the alkaline chamber; the organic solvent having a lower solubility for one or more contaminant salts than their respective alkalis.

2. The system of claim 1; wherein a cation exchange membrane or a bipolar membrane is sandwiched between the anode chamber and the acid chamber; an anion exchange membrane or a bipolar membrane is sandwiched between the cathode chamber and the alkaline chamber.

3. The system of claim 2; wherein the desalination chamber comprises seawater, brine, wastewater, produced water, or other solutions containing contaminant ions, or combinations thereof.

4. The system of claim 2; wherein the anode chamber or cathode chamber or both comprise one or more species of microorganisms.

5. The system of claim 4; further comprising microorganisms that are prokaryotic or eukaryotic, immobilized or non-immobilized or any combination of these forms.

6. The system of claim 5; wherein at least one species of microorganisms present in the anode chamber are anode-respiring microorganisms.

7. The system of claim 5; wherein the substrate comprises one or more organic compounds that can be oxidized or decomposed by the microorganisms disposed inside the anode chamber.

8. The system of claim 2; wherein the electron acceptor is immobilized or non-immobilized, is present in an aqueous state, or a vapor state, comprises one or more species of a plurality of microorganisms, or one or more electrochemically-active chemicals, or any combination of these forms.

9. The system of claim 2; wherein the electric current is generated from a combination of anode-respiring microorganisms and external power sources including batteries, electrochemical cells, solar power, wind-generated power, or other energy forms.

10. The system of claim 2; further comprising a plurality of redox mediators, pH buffer solutions, or combinations thereof in the anode and/or cathode chambers.

11. The system of claim 2; further comprising one or more electrically conductive chemicals in the acid chamber and/or the alkaline chamber.

12. The system of claim 2; further comprising one or more electrically conductive chemicals in the desalination chamber.

13. The system of claim 2; wherein a precipitate collection system is used to generate precipitated contaminant salts from the effluent solutions of the acid and alkaline chambers; the precipitate collection system comprising a tank, a conveyor belt, an auger, a pump, or a fluid tank or combinations thereof.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0022] The invention is more fully appreciated in connection with the following detailed description taken in conjunction with FIG. 1. FIG. 1 is a diagrammatic vertical sectional view illustrating a five-chamber MEC configured in accordance with an embodiment of the invention. The structural elements identified by the reference numbers in FIG. 1 are as follows: [0023] 1. Anode chamber [0024] 2. Acid chamber [0025] 3. Desalination chamber [0026] 4. Alkaline chamber [0027] 5. Cathode chamber [0028] 6. Cation exchange membrane (CEM) or Bipolar membrane [0029] 7. Anion exchange membrane (AEM) [0030] 8. Cation exchange membrane (CEM) [0031] 9. Anion exchange membrane (AEM) or Bipolar membrane [0032] 10. Anode electrode [0033] 11. Biofilm of an anode-respiring microorganism [0034] 12. Cathode electrode [0035] 13. Anolyte solution [0036] 14. Solution in acid chamber [0037] 15. Saline water [0038] 16. Solution in alkaline chamber [0039] 17. Catholyte solution [0040] 18. Tank [0041] 19. Precipitated contaminant salt [0042] 20. Pump [0043] 21. Dry solid salt [0044] 22. Power supply

DETAILED DESCRIPTION OF THE INVENTION

[0045] This disclosure provides MECs with direct salt recovery (MECDSR). An exemplary MECDSR enclosed herein includes an anode chamber 1, a cathode chamber 5, an anode electrode 10, a cathode electrode 12, a saline solution chamber 3, and two additional chambers for organic solvent: an acid chamber 2 and an alkaline chamber 4. The anode electrode 10 is disposed inside the anode chamber 1 which contains anode-respiring microorganisms 11. The microorganism forms a biofilm on the anode electrode. Alternatively, it may be immobilized on the electrode or suspended in the anolyte solution 13. The anode chamber 1 may be separated from the acid chamber 2 by a cation exchange membrane or a bipolar membrane 6. The acid 2 and alkaline 4 chambers consist of an organic solvent 14, 16 that has a high solubility for acids and alkalis but low solubility for the contaminant salts. 99% pure Ethanol is one such example of a suitable organic solvent. The acid 2 and desalination 3 chambers are separated by an anion-exchange membrane 7. The desalination chamber 3 comprises the saline water 15 which is a solution comprising contaminant ions that are to be desalinated. An exemplary saline water includes, but is not limited to, seawater, brine, wastewater, produced water, and/or combinations thereof. The desalination 3 and alkaline 4 chambers are separated by a cation-exchange membrane 8. The alkaline 4 and cathode 5 chambers are separated by an anion-exchange membrane or another bipolar membrane 9. The cathode chamber 5 contains the cathode electrode 12 and the catholyte solution 17.

[0046] The acid chamber 2 consists of an organic solvent 14. In some embodiments, the acid chamber 2 and the alkaline 4 chamber may additionally also consist of electrically conductive chemicals or materials. Concentrated HCl is one example of an electrically conductive chemical that can be added to the acid chamber to enhance the electrical conductivity of the organic solvent 14. Concentrated NaOH is one example of an electrically conductive chemical that can be added to the alkaline chamber to enhance the electrical conductivity of the organic solvent 15. Additionally, in some embodiments, the desalination chamber also comprises electrically conductive chemicals such that the electrical conductivity of the saline water is enhanced and MEC desalination efficiency is improved. Exemplary electrically conductive chemicals that can be added to the saline water include, but are not limited to, inorganic salts comprising sodium chloride and/or potassium chloride.

[0047] The anode chamber 1 consists of the anode-respiring microorganisms 11 and the substrates. During operation, some substrates provided in the anode chamber are oxidized by the microorganisms 11. These microorganisms 11 release electrons through their metabolic processes. Optionally, a redox mediator may be supplied in the anode chamber for shuttling the electrons provided by the microorganism to the anode electrode 10. The anode 10 and cathode electrodes 12 are connected via an external circuit 22. Concurrently, the cathode electrode 12 accepts the electrons from the external circuit 22. A suitable electron acceptor 17 in the cathode chamber can be used to accept electrons from the cathode electrode. However, the electron acceptor must be chosen such that the electrochemical reaction in the cathode chamber results in an accumulation of OH-anions in the cathode chamber.

[0048] As the electrochemical reactions in the anode 1 and cathode chambers 5 proceed, H.sup.+ ions accumulate inside the anode chamber 1 and OH.sup. ions accumulate inside the cathode chamber 5. If a bipolar membrane 6 is used between the anode and acid chamber. Then with sufficient potential, water at the bipolar membrane splits into OH.sup. and H.sup.+ ions where the OH.sup. ions travel towards the anode electrode, thus neutralizing the pH of the anolyte solution. The H.sup.+ ions travel from the bipolar membrane 6 towards the acid chamber 2 containing the organic solvent 14. Alternatively, if a cation-exchange membrane 6 is used between the anode chamber 1 and the acid chamber 2, then the H.sup.+ ions are accumulated inside the anode chamber 1 and travel through the cation-exchange membrane 6 towards the acid chamber 2 containing the organic solvent 14. Concurrently, contaminant anions in the desalination chamber 3, which contains a saline solution 15, travel towards the anode electrode 10 but get stopped in the acid chamber 2 by the cation-exchange membrane 6. Thus, the acid chamber 2 containing organic solvent accumulates H.sup.+ ions from the anode chamber 1 and contaminant anions from the desalination chamber 3, forming acids 14.

[0049] In a similar manner, contaminant cations in the desalination chamber 3 are attracted towards the cathode electrode 12 to maintain electrical neutrality, but they get stopped inside the alkaline chamber 4 by the anion-exchange membrane 9. The cathode chamber 9 accumulates OH.sup. anions which get attracted towards the anode electrode 10 but get stopped in the alkaline chamber 4 by the cation-exchange membrane 8. Thus, the alkaline chamber 4 accumulates OH.sup. anions from the cathode chamber 5 and the contaminant cations from the desalination chamber 3, forming alkalis 16.

[0050] In some embodiments, if the organic solvent used in the acid and alkaline chambers is soluble in water, then the organic solvent used must be of the highest purity that is feasible in order to eliminate the dissolution of acids and alkalis in the small proportion of water that is present along with the organic solvent. In certain embodiments, if the solubility of acids and alkalis in the preferred organic solvent is lower than that in water. Then, in order to avoid the precipitation of acids and alkalis in the acid and alkaline chambers, a higher quantity of the organic solvent can be used in the acid and alkaline chambers. However, in some such embodiments, if the width of the acid 2 and alkaline chambers 4 is too wide, internal resistance may increase which may impede the transfer of contaminant ions towards the acid and alkaline chambers. In such a scenario, the width of the acid and alkaline chambers should be kept at a thickness corresponding with the lowest feasible internal resistance of the cell. In some such embodiments, a larger quantity of organic solvent can be supplied in through acid and alkaline chambers of small thickness using continuous flow inlets and outlets.

[0051] Therefore, the effluent of the acid chamber contains organic solvent along with acids. It may also contain a slightly higher percentage of water due to the transport of some water molecules into the acid chamber through the ion-exchange membranes. Similarly, the effluent of the alkaline chamber contains organic solvent along with alkalis. It may also contain a slightly higher percentage of water due to the transport of some water molecules into the alkaline chamber through the ion-exchange membranes

[0052] In some embodiments, a precipitate collection system is provided to generate precipitated contaminant salts from the effluents of the acid and alkaline chambers. In some such embodiments, the effluents to the acid and alkaline chambers are mixed in a separate tank 18 wherein the acids and alkalis react to produce the contaminant salts. In certain embodiments, if this mixed solution is allowed to settle, the contaminant salts 19 precipitate out of the solution due to low solubility in the preferred organic solvent. Alternatively, precipitation can be achieved by distillation of the organic solvent if the preferred organic solvent has a low boiling point. In certain other embodiments, the contaminant salt can be separated from the organic solvent which is present in the tank 18 by use of conveyor belts. In some such embodiments, the outlet to the tank is connected to a conveyor belt through an auger. Alternatively, other mechanisms such as pumps 20, can be used to transport the outlet fluid from the tank to the conveyor belt.

[0053] The conveyor belt may receive a certain combination of the precipitated contaminant salt and the organic solvent. In certain embodiments, the conveyor belt may be used such that the precipitated contaminant salts are trapped by the belt and transported to the edge of the belt. In some such embodiments, the organic solvent may pass through the belt and get collected in a fluid tank and recycled back to the inlets of acid and alkaline chambers.

[0054] In some embodiments where bipolar membranes are employed, if the cell does not generate enough potential for water splitting at the bipolar membranes, then the reactions may, additionally, be driven by an external power source 22. Examples of external power sources include, but are not limited to, batteries, electrochemical cells, solar power, wind-generated power, other energy forms, and combinations thereof. In some embodiments of the invention, internal resistance may be reduced by ensuring the least feasible distance between the anode electrode and the cathode electrode.

[0055] Desalination processes according to embodiments of this disclosure include a batch operation of the anolyte, the catholyte, and the saline solutions. Alternatively, a continuous flow design can also be used for one or more of these solutions.

EXAMPLE

[0056] In an exemplary embodiment, batch operation of an MECDSR is performed and operated as per FIG. 1. The anode chamber consisted of activated carbon cloth as the anode electrode. The anode electrode had a biofilm of microorganism, Saccharomyces cerevisiae. The anolyte solution consisted of 10 g/l of glucose as substrate. The cathode chamber consisted of activated carbon cloth as the cathode electrode. The catholyte solution consisted of 2 mg/l of KMnO.sub.4 which is used as the electron acceptor. The desalination chamber consisted of seawater obtained from the Arabian Gulf at the coast of Sharjah, United Arab Emirates. The initial TDS of seawater was 40,000 ppm. The apparatus was operated for 24 hours during which the pH of the anolyte solution 13 changed negligibly from 3.75 to 3.7. The pH increase in the catholyte solution 17 was from 6.61 to 8. This increase in pH is very low compared to the increase in pH of catholyte solutions containing KMnO.sub.4 and employed in conventional microbial desalination cells, where the pH increases drastically from 6.6 to 11.73. The final pH in the solution contained in the acid chamber was 2.1 due to the accumulation of acid from the neighboring chambers. The final pH in the solution contained in the alkaline chamber was 15.77 due to the accumulation of alkali from the neighboring chambers. The final TDS of the seawater that was to be desalinated was 37,000 ppm. This TDS reduction is moderate in comparison to desalination efficiencies of conventional microbial electrochemical systems in 24 hours. Thereafter, the solutions from the acid chamber 14 and the alkaline chamber 16 were mixed in a beaker. Within a few hours, salt had precipitated out of the organic solvent solution and weighed 500 mg.