Method and device for targeted removal of metals from organic sediments

Abstract

Method and device for targeted removal of metals from organic sediment, comprising adding an aqueous medium and a biosurfactant to the organic sediment to obtain a reaction mixture and thereafter subjecting the reaction mixture to a plurality of serially arranged steps of electrolysis in electrolytic cells in which predetermined electric voltages are applied between anodes and cathodes thereof, each electrolytic cell being arranged to remove a specific metal by adapting the voltage potential in each electrolytic cell to the reduction potential of the specific metal to be removed in the specific electrolytic cell.

Description

[0001] Method and device for targeted removal of metals from organic sediments

[0002] The present invention concerns a method for targeted removal of metals from organic sediment as indicated by the preable of claim 1. According to another aspect the present invention concerns a device for targeted removal of metals from organic sediment as indicted by the preamble of claim 12.

[0003] Background

[0004] Organic sediments from marine origin may contain valuable resources, such as energy, sulphur, nitrates, and phosphor, that could be re-utilized in other applications. However, these sediments also contain heavy metals which are known for their toxic effects once they enter into the food chain. The presence of these metals in the sediment is preventing the full utilisation of its potential resources.

[0005] Existing methods used for the removal of heavy metals from sediments often involve a complex, high-cost processes. Several methods have been proven successful, but these are often costly and involve the use of chemicals or adsorbents, which then end up as a waste product.

[0006] Conventional methods may comprise chemical precipitation involving adding chemicals to the sediment to convert the heavy metals into insoluble forms that can be easily separated. Commonly used precipitation agents include lime (calcium hydroxide), iron salts (e.g., ferric chloride), and aluminium salts (e.g., aluminium sulphate). The added chemicals react with the heavy metals to form precipitates, which can then be removed by sedimentation or filtration.

[0007] Other known methods are based om stabilization and solidification, involving addition of certain components to the sediment to chemically bind the heavy metals, reducing their mobility and bioavailability. Common stabilizing agents include cement, fly ash, and lime. The components added react with the heavy metals, forming stable compounds or minerals, thereby reducing their leaching potential.

[0008] A third category of known methods makes use of soil washing, a technique that involves washing the sediment with water or chemical solutions to selectively remove the heavy metals. It can be done using physical agitation or by employing surfactants or chelating agents. The contaminants are extracted into the washing solution, which is then treated to separate and recover the heavy metals.

[0009] Electrolytic processes are commonly known and have been around for centuries. Below are some examples of electrolytic processes used for soil remediation. Electro-kineticSoil Remediation (EK) is a widely used electro-remediation method that relies on the application of a low-intensity direct current through the soil. The process involves the movement of ions in the soil due to electro-osmosis, electromigration, and electrophoresis. These phenomena help to transport contaminants towards electrodes, where they can be removed or transformed. EK is effective for removing heavy metals, radionuclides, and certain organic compounds.

[0010] Electromigration involves the migration of charged contaminants in the soil under the influence of an electric field. It is particularly effective for the removal of heavy metals and ions. Contaminants move towards the electrode of opposite charge due to the electric field, facilitating their extraction or immobilization.

[0011] ElectrokineticBioremediation (EK-Bio) combines electro-kinetic with bioremediation techniques. It utilizes the electric field to enhance the transport of nutrients, electron acceptors, and bacteria into the contaminated soil, promoting microbial degradation of organic pollutants. EK-Bio is often used for the treatment of petroleum hydrocarbons and other organic compounds.

[0012] US 2014 126965 Al concerns a process for washing contaminated soil using a washing solution and chelating agents. The chelating agents form water-soluble complexes with metals and thereby facilitates their removal from soils and sediment into the washing solution. Also organic contaminations may be removed using water solutions amended with surfactants, detergents or organic solvents in addition to the chelating agents.

[0013] US 8 926 814 B2 is a publication generally related to environment protection, describing an apparatus and a method for electrokinetic in-situ leaching remediation of contaminated soil. The apparatus includes a DC electrical source, electrode chambers, electrodes, permeable reactive barriers, electrode solution pH testers and extraction tubes, a heating pipe network, e heat exchanger, and more.

[0014] CN 1048 76409 B concerns a method and a device for removing heavy metals and polycyclic aromatic hydrocarbons in bottom sediment of river channels and belongs to the field of riverchannel pollution treatment in environmental protection. The device comprises an electrolysis device, a biological fuel cell and a galvanometer, wherein the electrolysis device comprises a cathode area, a bottom-sediment area and an anode area. The negative electrode of the biological fuel cell is arranged at the outer side of a round drum, the positive electrode of the biological fuel cell is arranged at the inner side of the round drum, and a compost layer is arranged between the negative electrode and the positive electrode; CN 1066 23401 A concerns a method for remediating contaminated soil based on osmotic adsorption.

[0015] US 10 835 938 Bl relates to in-situ remediation systems useful for reclaiming contaminated soils containing salts, heavy metals, and radionuclides by provision of multiple methods and devices for contaminated soil remediation including electrokinetic methods and devices, leaching solution supply and removal methods and devices, soil negative pressure moisture control method and devices, pressurized leaching / extraction method and devices and sequential leaching / extraction methods and chemicals for the improvement of the remediation efficiency, enhancement of contaminants migration rates, shortening of remediation period, end prevention of secondary subsurface contamination.

[0016] CN 114804585 A concerns a method for treating polluted bottom mud through combination of electroosmosis and incineration, and belongs to the technical field of polluted bottom mud treatment. In this method, pretreated sludge is subjected to electroosmosis treatment, then incineration treatment is conducted. Solid matter is thereby obtained while gas and heat are generated. The method can be used for removing most heavy metals and all harmful organic matters in the sediment as well as harmful organic matters.

[0017] While several devices and methods have been suggested and attempted for removing metals from the sea floor in near-land regions, there is still a significant need for improvements in this technical / environmental field.

[0018] Objective

[0019] It is an objective of the present invention to provide a method and / or device to allow for the purification of sediment contaminated with heavy metals allowing separate extraction and isolation of each of the individual metals to be removed.

[0020] The present invention

[0021] The objective is fulfilled by the present invention which in one aspect concerns a method as defined by claim 1. According to another aspect, the present invention concerns a device as defined by claim 12.

[0022] Preferred embodiments are disclosed by the dependent claims.

[0023] The present invention provides a solution to remove these metals, ex-situ, and in a targeted manner. The metals can be recovered individually and once removed, the sediment can be further processed to extract the remaining resources contained within. The method according to the present invention is conducted in two stages or steps, the first step being to extract the metals which are bound to the sediment by use of biosurfactants and the second step to recover the metals from the solution by means of an electrolytic process. Multiple electric cells are required to target the metals, individually, in the electrolyte solution. The method is preferably conducted as a continuous / semi-continuous process in which the material to be treated is added as a continuous flow into a first reactor inlet accompanied by a continuous addition of bio-surfactant at a rate adapted to the flow rate of the material to be treated. The electrolytic process is typically conducted in a portion-wise (semi-continuous) manner with a certain residence time of the material within each step or cell, i.e. in a semi-continuous manner. The entire method may also be conducted as a batch-wise process.

[0024] The existence of biosurfactants and their ability to extract heavy metals is well studied, but their application has been mostly confined to the laboratory. Recently, the biosurfactant "Rhamnolipid" was introduced as a replacement of synthetic surfactants in soap products.

[0025] Biosurfactants are not only suitable as an environmentally friendly alternative in cleaning products but also for the extraction of heavy metals from organic sediments. Unlike synthetic surfactants, bio-surfactants are produced by bacteria or fungi, and they are biodegradable.

[0026] When dissolved in water, biosurfactants reduce the surface tension and form micelle structures, which attract heavy metals which are bonded to the sediment. Studies have shown that the use of biosurfactants in combination with chelating or complexing agents such as tetrasodium glutamate diacetate is an effective method for the removal of heavy metals from organic sediments. A chelating agent is stimulating the bonding of metal ions in a solution and research shows that its use in combination with biosurfactants is significantly increasing the removal rates.

[0027] While use of chelating agents may be used as an optional addition within the scope of the present invention, it is not a required element or component thereof.

[0028] In addition to their ability to remove heavy metals biosurfactants are also resistant to pH changes and salinity. Last but not least, they have proven to exhibit excellent electrolytic properties which make them suitable for use in an electrolytic process.

[0029] The present invention is a unique combination of technologies in which biosurfactants and electrolysis are combined in an ex-situ process to obtain a new and improved purification of contaminated sediment, allowing each metal element to be separately removed with a purity of at least 90 %) Each metal has its unique reduction potential, which is expressed in Volt and measured against a Standard Hydrogen Electrode (SHE). The reduction potential is the electric potential difference (V) required to initiate the migration of metal ions in a solution (electrolyte) towards the cathode. Below is an overview of the reduction potential of the metals found in organic sediment originating from an aquaculture location.

[0030] Zinc (Zn2++ 2e -> Zn): -0.76 V

[0031] Chromium (Cr3++ 3e" -> Cr): -0.74 V

[0032] Cadmium (Cd2++ 2e_-> Cd): -0.40 V

[0033] Nickel (Ni2++ 2e -> Ni): -0.25 V

[0034] Lead (Pb2++ 2e -> Pb): -0.13 V

[0035] Copper (Cu2++ 2e_-> Cu): +0.34 V

[0036] The values mentioned above are values at ideal or standard conditions. The actual reduction potential will be somewhat affected by pH, composition of the electrolyte mixture and the temperature at the cathode.

[0037] Below, the invention is described in further details in the form of non-limiting embodiments illustrated by the enclosed drawings, where

[0038] Fig. 1 is an overall flow scheme of an embodiment of the device and method according to the present invention,

[0039] Fig. 2 is a schematic representation of some elements of the flow scheme of Fig. 1

[0040] Fig. 3 is a schematic representation of an electrolytical cell useful in the method according to the present invention.

[0041] Figure 1 shows a device 10 comprising a sediment feed tank 11 for contaminated sediment, a sediment storage tank 12 for purified sediment, a biosurfactant stock tank 13 and a mixing unit 14 for thorough mixing of sediment from feed tank 11 with biosurfactant from stock tank 13. A flow 15 of combined sediment and biosurfactant leaves the mixing unit and enters a battery 16 of electrolytic cells in which the desired heavy metals are selectively separated from the mixture.

[0042] The stock tank 13 typically holds an aqueous solution of a biosurfactant useful for the task.

[0043] Each cell of the battery of electrolytic cell is arranged to apply a specific voltage targeted to conduct the removal of a specific metal, in correspondence with the table of reduction potential indicated above. In this manner, one cell is configured to separate copper from the mixture, another one lead, a third one nickel etc. The exact voltage may be adjusted in dependency of local factors that may be determined experimentally or otherwise. The sequence of the cells, i.e. which metal is separated first, second and so on, may vary. In a preferred embodiment, the cells are arranged in a sequence with which the first cell has the lowest voltage, the next cell has the second lowest voltage, and so on. What is more important is that the cells are electrochemically separated (isolated) from one another. In each of the electrolytic cells, a specific targeted metal is deposited on the cathode thereof.

[0044] Purified sediment leaves the battery of electrolytic cells in flow 17 and is subjected to dewatering in a dewatering unit 18 and thereafter directed to the storage tank 12 for purified sediment.

[0045] The water separated from the purified sediment is typically treated in a water treatment unit 19 before being released. Remains of sediment following the separated water from the dewatering unit to the water treatment unit 19, may be recycled as flow 20 to the mixing unit 14.

[0046] Auxiliary equipment such as valves, sensors and other control units typically employed in a real life embodiment are omitted from the drawing. It is considered within the craftsmanship in this technical area to include relevant auxiliary equipment in order to control and achieve the desired flow rates, residence times, pH levels etc. in order to reach and maintain desired parameters for the overall process.

[0047] Figure 2 is a schematic representation of an embodiment of the mixing unit 14 and part of the electrolytic cell battery 16, illustrating a possible configuration of the latter in which the electrolytic cells are separated from one another in terms of vertical level, allowing gravity to assist in the transfer of electrolyte from cell to cell.

[0048] Figure 3 is a schematic representation of an electrolytic cell in general. It is provided with an anode 31 which is positively charged and a cathode 32 which is negatively charged by a direct current supply 33 which at least in the present case typically is one of a kind where the output voltage may be quite finely adjusted. The liquid medium, electrolyte 34, inside the electrolytic cells is typically made up of the biosurfactant and water from the environment at which the sediment was collected. Additional components could be added to adjust the salinity and thereby the conductivity of the liquid medium but that is usually not required.

[0049] As illustrated, the positively charged metal cation travels to the cathode where it receives one or more electrons to thereby be reduced to solid metal which accumulates on the cathode. The negatively charged counterpart anion travels to the anode where one or more electrons are released and serves to maintain electric current through the cell. Depending on the nature of the anion, different types of gas may be emitted at the anode, some of which should be collected for their commercial value and / or for environmental reasons.

[0050] After a certain period of time, significant amounts of metals have accumulated on the cathodes, and the cathodes are then replaced to maintain efficiency of the process and to recover and store the metal accumulated thereon. The used cathodes may be discarded or reused after recovery of metal therefrom. Each electrolytic cell may include more than one cathode and more than one anode to thereby increase the cells' inherent electrode surface area while also allowing cathodes and anodes to be replaced one by one, without pausing operation of the device.

[0051] Preferred embodiments

[0052] While a wide variety of biosurfactants are useful within the scope of the present invention, it is preferred that the biosurfactant comprises at least one glycolipid. Rhamnolipids have been found to be very useful but other glycolipids may be equally useful.

[0053] Depending on the sediment concentration it may be beneficial to add a chelating agent, such as tetrasodium glutamate diacetate (GLDA).

[0054] Cadmium, chrome, lead, nickel, zinc, and copper are among the metals important to be able to remove. Preferably, the method is conducted in a manner with which at least two, more preferred at least three and most preferred all of these metals are removed by the process.

[0055] With regard to the sequence of the electrolytic cell, it is convenient to start with one provided with a voltage corresponding to the lowermost reduction potential of the metals to be removed and to increase the voltage successively from cell to cell in accordance with relevant reduction potentials.

[0056] With regard to concentration of the biosurfactant in the mixture entering the battery of electrolytic cells, it is preferred that it is within the range 0.2 - 8.0%, more preferred within the range 0.5 - 5.0%, and most preferred in the range 1.0 - 3.0 % by weight of the reaction mixture.

[0057] With regard to concentration of sediment in the mixture entering the battery of electrolytic cells, it is preferred that it is at least 10 % by weight, more preferred in a concentration of at least 15 % by weight.

[0058] During operation, it is preferred that the pH is maintained at a stable level of 7 or higher. Addition of NaOH or similar alkali may be used for the purpose. It is also preferred that the temperature is maintained at a stable level.

Claims

Claims1. Method for targeted removal of metals from organic sediment, characterised in adding an aqueous medium and a biosurfactant to the organic sediment to obtain a reaction mixture and thereafter subjecting the reaction mixture to a plurality of serially arranged steps of electrolysis in electrolytic cells in which predetermined electric voltages are applied between anodes and cathodes thereof, each electrolytic cell being arranged to remove a specific metal by adapting the voltage potential in each electrolytic cell to the reduction potential of the specific metal to be removed in the specific electrolytic cell.

2. Method as claimed in claim 1, wherein the biosurfactant comprises at least one glycolipid.

3. Method as claimed in claim 1 or claim 2, wherein each step of electrolysis is electrically isolated from any other step of electrolysis.

4. Method as claimed in claim 3, wherein electrical isolation is obtained by arranging each step of electrolysis at a different vertical level than the preceding one.

5. Method as claimed in any one of the preceding claims, wherein at least one, preferably at least two, and more preferred all of the following metals is / are removed by the process: cadmium, chrome, lead, nickel, zinc, copper.

6. Method as claimed in any one of the preceding claims, wherein the cathode in each electrolytic cell is periodically replaced to allow the metal accumulated on said cathode to be recovered.

7. Method as claimed in any one of the preceding claims, wherein the electric potential is successively increased from each electrolytic cell to the next.

8. Method as claimed in any one of the preceding claims, wherein the biosurfactant is present in the reaction mixture in a concentration of 0.2 - 8.0%, more preferred within the range 0.5 - 5.0%, and most preferred in the range 1.0 - 3.0% by weight of the reaction mixture.

9. Method as claimed in any one of the preceding claims, wherein the sediment is present in the reaction mixture in a concentration of at least 10 % by weight and more preferred in a concentration of at least 15 % by weight.

10. Method as claimed in any one of the preceding claims, wherein the temperature and pH of the reaction mixture is maintained at a stable level in each electrolytic cell.

11. Method as claimed in any one of the preceding claims, the method being carried out as a continuous or semi-continuous process.

12. Device (10) for targeted removal of metals from organic sediment, characterised in comprising:- a sediment feed tank (11) arranged to hold an aqueous mixture of sediment and a biosurfactant stock tank (13), said tanks (11, 13) being arranged to charge sediment, water and biosurfactant to a mixing unit,- a mixing unit (14) arranged for thorough mixing of the sediment and the biosurfactant,- electrolytic cells (16) serially arranged downstream of the mixing unit (14), each cell being arranged to be applied with a targeted electric voltage, said electric voltage corresponding to the reduction potential of the metal to be removed in said cell.

13. Device as claimed in claim 12, arranged to operate in a continuous / semi-continuous manner.

14. Device as claimed in claim 12 or 13, wherein each electrolytic cell is electrically isolated from any other electrolytic cell.

15. Device as claimed in any one of claims 12-14, further comprising at least one of a dewatering unit (18) downstream of the electrolytic cells, a sediment storage tank (12) for purified sediment, and a water treatment unit (19) for purification of water

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