BG4434U1 - Bioelectrochemical reactor for (collective) recovery of noble metals from industrial wastewater - Google Patents

Abstract

The present utility model relates to a bioelectrochemical reactor for microbe-assisted electrochemical recovery of metals using the cathode reduction method. Said reactor does not use any energy input due to the fact that the anode and cathode are operated in short-circuit mode (electrochemical microbial snorkel). The energy is produced by exoelectrogenic microorganisms which, during intracellular oxidation of organic matter, carry out extracellular transfer of electrons to the anode. The present utility model proposes a new way of collective recovery of noble and other valuable metals with positive potential present in mixtures present in modeled and real industrial wastewater which would be applied in areas such as hydrometallurgy, electrochemistry, ecology and environmental protection.

Description

The present utility model relates to a bioelectrochemical reactor for microbiologically assisted electrochemical (co)regeneration of precious metals from industrial wastewater operating in short-circuit mode, which finds application in hydrometallurgy and electrochemical productions, having regard to the purification of metals from solutions and environmental Protection.

Prior art

As a result of rapid industrialization and ever-expanding production activities, huge amounts of waste containing various metals are generated. In recent years, there has been a huge growth in the use of many information and communication technologies, such as mobile phones, computers, tablets, smart TVs, etc. Rapid technological development is constantly leading to the development of new products and shortening the service life of old ones. End-of-life electrical and electronic equipment contains circuit boards containing precious and rare metals, including gold, silver, palladium and platinum, as well as potentially environmentally hazardous metals such as mercury, lead, cadmium and beryllium. Extracting metals for their recycling will not only lead to a reduction in environmental pollution (e.g. metal-contaminated waste water), but they will also be reusable, which in the long run, given the limited natural resources of metals, will require their yield to be reasonably controlled.

For the purification and processing of metals, a number of technologies based on physical, chemical and biological processes have been developed so far, with the greatest efforts directed at the effective regeneration of precious metals. The main problem for the application of existing technologies is that in most wastewaters the concentration of metals is relatively low (of the order of pg/l to mg/l), which necessitates the investment of additional energy and raw materials for their pre-concentration from large water volumes. The traditional technology for extracting gold and other precious metals from ores and solid waste is based on the cyanide method, in which the metals are extracted in the form of cyanide complexes.

The use of potassium cyanide, which is highly toxic to the respiratory chains of various organisms, causes serious environmental problems, and for this reason the cyanide method is banned in many countries. Thiosulfate extraction of gold and silver is a promising alternative to the cyanide method, but to achieve a comparable speed and efficiency of the process, the use of catalysts is required, which further increases the cost of the method and can cause secondary environmental pollution. In general, a problem in the extraction or recovery of precious metals is the high energy intensity of the processes to obtain them in their pure form. For example, the classical electrolytic method of silver extraction requires electrical energy equal to 3.81 kWh per 1 kg of silver.

In recent years, a new alternative for the regeneration of metals from wastewater without the need for their reconcentration is offered by bioelectrochemical systems that combine the biocatalyzed anodic oxidation of organic products with the reduction of various electron acceptors. Available metal ions can be directly reduced on the cathode of a microbial fuel cell (MFE), with the simultaneous generation of an electric current. Despite the thermodynamically obvious possibility of realizing such a process, the concept of so-called metallurgical microbial fuel cells was proposed only a few years ago and its technology readiness level (TRL) is between 1 and 2, meaning that the idea is still still in the stage of laboratory tests. Among the challenges to be solved are the small generated current, increasing the reduction rate, respectively productivity of the metal regeneration process. In this direction, we recently proposed an innovative idea and for the first time in the world we proved the possibility of regenerating copper from an aqueous solution in an MGE operating in short-circuit mode [1]. With the design used, complete removal of copper ions from the catholyte was achieved within 10 days, with regeneration of copper on the cathode (copper foil) being 10% more efficient when operating the system in short circuit mode compared to on operation load resistance (MGE). By analogy with the cable bacteria found in soils and sediments and connecting the anaerobic part of a biotope with the aerobic layers like a snorkel, the bioelectrochemical system operating in short-circuit mode is called a microbial electrochemical snorkel (MES) for short. We further continued and deepened our research on the regeneration of silver and gold using the MESH method. Model wastewaters containing free silver, gold or their complex ions were used [2, 3]. The possibility of using cathodes of a material other than the metal subject to regeneration was also verified. Within a day, a high degree of removal (>95%) of metal ions from the solutions used as catholytes and an efficiency of regeneration of metals on the cathodes of more than 94% in short-circuit mode (SEC) exceeding by 5 to 7 was achieved % relevant indicators compared to operation through included load resistance (MGE). By X-ray photoelectron spectroscopic (XPR) analysis, it was demonstrated that unlike copper, which is regenerated predominantly in the form of SiO, silver and gold are regenerated on all tested cathodes in their elemental state, i.e. as Ag ^o and Ai° . The same design was used for all three metals, varying the cathode materials, the concentration of metal ions and complexes in the starting solution, the temperature, and the presence of a proton exchange membrane. The accumulated experience from the in-depth analyzes led to the establishment of the mechanisms of the main ongoing processes and showed which components and parameters should be changed in order to increase the productivity and efficiency of the regeneration process. One of the main drawbacks of the used design was the small volume of the cathode compartment, limiting the cathode area as well as the amount of catholyte, which is overcome by the proposed optimized design.

Technical nature of the utility model

In the current useful model, a new construction of a bioelectrochemical reactor is proposed with completely changed parameters, both of the housing and of the individual structural elements and their arrangement, compared to those of the bioelectrochemical reactor used in the cited publications [1-3]. The new bioelectrochemical reactor is specially designed for operation in the short-circuit mode (SEC) with increased dimensions of the anode and cathode compartments, allowing the regeneration of metals to be carried out from larger volumes of solutions, as well as the use of cathodes and anodes with a larger surface, which leads to an increase in the productivity of the regeneration process, shortening the time for complete removal of the available metal ions from the catholyte, as well as to a greater efficiency of regeneration of the noble metals on the cathode. The optimized cathode compartment allows easy assembly and disassembly of the cathodes, enabling the new bioelectrochemical reactor to be used in continuous and (semi)flow mode for (co)regeneration of one or more contained noble metal ions, e.g. Ag and Al from waste leads.

Bioelectrochemical reactor

The developed bioelectrochemical reactor is two-chambered (Figure 1), consisting of a casing performing the function of a cathode compartment; a cathode compartment cover with a centrally located circular internally threaded hole mounted on the upper, outwardly projecting periphery of the housing by mounting bolts; anode compartment, which is movable and is installed through the circular opening of the cathode compartment cover in the central part of the housing; round anode compartment cover with external thread that rotates tightly onto the cathode compartment cover. Electrodes performing the function of anode and cathode respectively, separated from each other by a separator (ion exchange membrane or diaphragm), are installed in the anode and cathode compartments. To study the kinetics of electrode processes, reference electrodes are mounted and fixed.

Corps

The body of the bioelectrochemical reactor can be in the form of a cube with dimensions ranging from 80 mm x 80 mm x 80 mm to 250 mm x 250 mm x 250 mm, a parallelepiped with a length ranging from 100 mm to 250 mm, a width ranging from 50 mm to 120 mm and a height ranging from 80 mm to 120 mm, or a cylinder with a diameter ranging from 80 mm to 250 mm and a height ranging from 80 mm to 250 mm. In the upper part of the case, regardless of its shape and dimensions, there is a peripheral board protruding outwards 20 mm long, with holes for mounting a cover by means of bolts. The body can be made of different materials, mostly engineering plastics, using different production technologies - gluing of individual boards, matrix casting, 3P-printing, etc.

Cathode compartment cover

The cover of the cathode compartment is adapted to the size of the body board and is fixed on it by means of bolts. The lid is made of the same material as the body. Centrally on the lid is a circular opening with a diameter varying from 70 mm to 230 mm and a height varying from 20 mm to 40 mm, through which the anode compartment is mounted. The round hole in the housing cover has an internal thread to which the anode compartment cover is screwed.

Anode compartment

The dimensions and shape of the anode compartment are matched to those of the housing. The height of the anode compartment is equal to that of the housing. To provide sufficient volume for the catholyte and at the same time minimize ohmic losses, the width and length (for cubic and parallelepiped body shape) or diameter (for cylindrical body shape) should be 10 mm to 30 mm less than those of the hull. The anode compartment is made of the same material as the housing.

Anode compartment cover

The anode compartment cover is circular with a diameter ranging from 70 mm to 230 mm with an external thread that rotates tightly on the cathode compartment cover. It is made of the same material as the housing, cathode compartment cover and anode compartment.

Anode

Materials with high electrical conductivity, corrosion resistance and biocompatibility must be selected for the manufacture of anodes. The most suitable anode materials are those based on carbon (carbon cloth, cachet, graphite, graphitized paper, graphite granules, granular activated carbon and biocarbon, etc.), the choice being based on the specific construction of the anode compartment, the used microorganisms as anodic biocatalysts and the method used for their immobilization. The shape of the anode can be rectangular or circular, and when using granular materials the shape is determined by that of the anode compartment. The geometric surface of the anode depends on the size of the anode compartment and can vary from 10 to 500 cm ² . It is necessary that an electrochemically active biofilm (EAB) has been formed on the electrode performing the function of a bioanode. EAB formation can be accomplished by continuously applying an appropriate electrode potential in a three-electrode cell inoculated under anaerobic conditions with electrogenic microorganisms; in a microbial fuel cell with a constantly connected load resistance until reaching a sufficiently negative operating potential below -300 mV (relative to a standard hydrogen electrode), or by immobilizing electrogenic microorganisms in a suitable polymer matrix. Microorganisms can be a pure single culture, or a mixed culture consisting of species distributed by electrogenicity, by the ability to live in anaerobic conditions, by a gradient of hydrogen sulfide and substrate.

Cathode

The cathode material is either the same metal in foil, mesh or wire form, or another conductive material (eg graphite, graphitized paper, carbon cloth) providing deposition of the target metal. To ensure better adhesion of metal deposits, electrodes used as cathodes must be pre-cleaned to remove surface oxides and/or grease. Metal electrodes are cleaned by etching in dilute hydrochloric acid, and carbon-based ones - in an ultrasonic bath, in a mixture of acetone and ethanol (1:1).

Depending on the shape and dimensions of the MESH, one or more cathodes can be used. In the proposed design of a bioelectrochemical reactor, up to 4 cathodes are provided. When using two or more cathodes, they are located symmetrically with respect to the bioanode. It is preferable to use an even number of cathodes. The shape of the cathodes can be flat - rectangular or round, and their geometric area is from 18 to 400 cm ² , as well as volumetric, in the case of carbon granules, and the size of both types of electrodes is determined depending on the size and shape of the cathode compartment. The cathode space is filled with an aqueous solution containing ions of the target metal for regeneration or waste water.

Conductors

Each electrode is provided with a current lead which is appropriately led from the anode or cathode compartment and provides the possibility of electrical contact with the other electrodes. A suitable material for the current conductor of flat electrodes is titanium wire, and for granular ones - titanium mesh.

Separator

The presence of a separator (ion exchange membrane or diaphragm) between the anode and cathode compartments prevents partial loss of the metal ions in the catholyte due to passage into the anode compartment and adsorption on the anode. It is preferable to use anion exchange membranes or diaphragms preventing mechanical mixing of anolyte and catholyte.

Mode of operation

The useful model also addresses the specific short-circuit mode of operation by applying the microbial electrochemical snorkel principle to (co)regenerate noble metals from solution. The method for (co)regeneration of noble metals by microbial electrochemical snorkeling is carried out by connecting the terminals of the cathode and bioanode in short, where the cathode and anode potentials overlap almost instantaneously and the process proceeds at the corresponding mixed potential. For cathodic reduction of noble metals to occur, the value of the resulting mixed potential must be more negative than the initially measured open circuit cathodic potential. During the process, the change in the short-circuit current and/or the concentration of the metal ions in the catholyte is monitored. When the concentration drops below the analytically detectable minimum for the respective metals, the process is terminated, the cathode is dismantled, dried and weighed to determine the yield obtained. At low concentrations of the metals in the used solution, the process can be carried out by repeatedly replacing the catholyte without dismantling the cathode or in flow mode. When the process is stationary, the catholyte can be replaced manually or automatically. When operating in flow mode, the catholyte is stored in a separate tank and fed to the cathode compartment by a peristaltic pump. The flow rate at which the catholyte is fed is controlled depending on the capacity of the cathode compartment and the performance of the process. When using a microbial electrochemical snorkel with two or more mounted cathodes in the cathode compartment, the process can be conducted by shorting all cathodes, part of them, or just one cathode to the bioanode. An embodiment is possible in which the cathode compartment is divided into several sub-compartments by an impermeable partition, and a cathode is installed in each sub-compartment. In such an embodiment, a cathode of a different electrode material can be installed in each subdivision and a catholyte of a different composition than the other subdivisions can be used. This allows the simultaneous regeneration of several different metals on different cathodes, short-circuited to a single bioanode, deposited separately on a cathode and yet the deposition proceeds simultaneously. When conducting the process using mixed solutions, two or more metals can be co-deposited on the same cathode.

Microbial electrochemical snorkeling can be used as an effective method without input of additional energy and reagents for (co)regeneration of precious metals from various industrial waste streams, such as spent electrolytes from electroplating baths, eluents from so-called electronic scrap (electronic boards from mobile devices, smart TVs) etc. The electrodeposits of mono-, bi- or poly-metallic nanoparticles obtained on the cathodes can be directly used as (electro) catalysts for various target processes. Additionally, they can be recovered as pure metals or alloys by mechanical scraping from the surface of used cathodes. Another parallel application of the obtained cathodes modified with metal catalysts is that they can be directly used in electrochemical fuel cells and electrolyzers.

Summary of the new device's distinguishing features and how it works:

□ New construction - bioelectrochemical reactor in the form of a cube with dimensions ranging from 80 mm x 80 mm x 80 mm to 250 mm x 250 mm x 250 mm, a parallelepiped with a length varying from 100 mm to 250 mm, width varying from 50 mm to 120 mm and a height ranging from 80 mm to 120 mm, or cylindrical with a diameter ranging from 80 mm to 250 mm and a height ranging from 80 mm to 250 mm.

□ New internal layout, vertical on the anode - with the possibility of increasing the unfolded area.

□ New cathode/cathodes - use of carbon cloth as electrode material.

□ Possibility of positioning one or several cathode cameras.

□ Operation in short-circuit mode.

□ Simultaneous cathodic deposition of more than one metal (bimetallic composites).

□ Simultaneous deposition of several metals on separate cathodes.

□ Obtaining modified electrodes with metal nano-particles with zero consumption of input energy.

Explanation of the attached figures

Figure 1 is a schematic diagram of the constructed bioelectrochemical reactor for (co)regeneration of noble metals from solutions containing their free or complex ions

The bioelectrochemical reactor is built from:

- Cathode compartment (housing);

- Cathodes;

- Catholyte;

- Cathode compartment cover with internal thread, bolted to the housing;

- Separator separating the cathode from the anode compartment;

- Anode compartment;

- Cover of the anode compartment with external thread;

- Reference electrode to anode;

- Reference electrode to the cathode;

- Fixing bolt.

Examples of implementation of the utility model

The examples do not exhaust the scope of the utility model and it can be used for other precious metals as well.

Example 1:

A bioelectrochemical reactor (Fig. 1) was constructed, consisting of a cube-shaped housing with dimensions of 80 mm x 80 mm x 80 mm, performing the function of cathode compartment 1, cover 4 of cathode compartment 1 with dimensions of 100 mm x 100 mm with a centrally located 70 mm diameter circular hole with internal thread, mounted on the upper outwardly projecting periphery of the housing by fastening bolts 10, an anode compartment 6 of dimensions 50 mm x 50 mm x 80 mm, which is removable and is mounted through the circular hole in the cover 4 in the central part of the kopnyca of the cathode compartment, a round cover 7 of the anode compartment 6 with a diameter of 70 mm with an external thread, which turns tightly on the cover 4 of the cathode compartment 1. The described components are made of plastic by 3 D-printing. On the four walls of the housing of the cathode compartment 1 at a distance of 20 mm from the bottom, 30 mm from the top opening and 10 mm from the side walls, four rectangular cathodes 2 of graphitized paper with dimensions of 60 mm x 30 mm and a thickness of 1 mm are mounted, on to which titanium wire current conductors are attached. On the side walls of the anode compartment 6, rectangular openings with dimensions of 30 mm x 60 mm are left, and on the inside of the walls a separator membrane 5 ZIRFON PERL UTP 500 is installed, preventing mixing of catholyte and anolyte. The internal free volume of the anode compartment 6 is filled with graphite granules representing a bioanode with a total mass of 200 g placed on a titanium mesh serving as a current conductor, on which an electrochemically active biofilm of anaerobically activated sludge is immobilized. Domestic wastewater with a volume of 100 ml and a COD of 510 mg/l was used as anolyte. A mixed aqueous solution of Na ₃ [Ag(S ₂ O ₃ ) ₂ ] and Na ₃ [Au(S ₂ O ₃ ) ₂ ] with a volume of 200 ml and a concentration of both metals of 100 mg Ag/l and 100 mg Au/l.

Operation of the bioelectrochemical reactor:

In the short-circuit mode, realized by directly connecting the current lines of the cathodes 2 with that of the anode, the bioelectrochemical reactor operates for 6 hours. During operation, with the help of a digital multimeter, the variation of the cathode and anode potential was monitored relative to reference electrodes 8 and 9 mounted in the anode 6 and cathode 1 compartments. After 6 hours of operation of the bioelectrochemical reactor in short-circuit mode, it was determined by spectrophotometric analysis that the gold and silver content of the catholyte was below the detectable limit of the method, indicating a removal efficiency of both metals from the catholyte of >99%. After dismantling the cathodes 2 from the bioelectrochemical reactor and drying to a constant mass, by weighing on an analytical balance, an increase in the mass of each cathode compared to the original was found by 9.2 mg, 9.5 mg, 9.6 mg and 9.8 mg, respectively . The total mass increase of the four cathodes 2 is 38.1 mg, corresponding to 95.25% of the initial amount of the two metals in the catholyte. X-ray structural analysis revealed the presence of both metals in the deposits on the cathodes, and X-ray photoelectron spectroscopy proved that the two metals are in an elemental state in the ratio Ag:Au 0.8:1.0.

For the first time, (co)regeneration of gold and silver has been carried out in a bioelectrochemical reactor operating in short-circuit mode, with high efficiency.

Example 2:

In a bioelectrochemical reactor with a parallelepiped body size of 250 mm x 250 mm x 80 mm and an anode compartment 6 with dimensions of 220 mm x 220 mm x 80 mm, gold regeneration was carried out from an aqueous solution of HAuCU with a volume of 2000 ml containing 500 mg Au/l.

Four rectangular cathodes 2 of graphitized paper 2 with dimensions of 220 mm x 180 mm and a thickness of 1 mm were used, on which current conductors of titanium wire were attached. A graphite disc with a diameter of 200 mm and a thickness of 10 mm was used as an anode, buried in a layer of 70 mm high water sediment placed in the anode compartment 6. The graphite anode is mounted 30 mm above the bottom edge of the sediment layer and an insulated copper wire serving as a current conductor is attached to it. The sediment layer was covered by a 10 mm water column with water from the same water source from which the sediment was collected, to which 1% NaC l solution was added to increase conductivity. The rest of the components and operating parameters of the bioelectrochemical reactor are the same as in Example 1. After 6 hours of operation in short-circuit mode, over 98% removal of the gold contained in the catholyte was analytically determined. By X-ray photoelectron spectroscopy, it was found that elemental gold was deposited on the cathodes. The amount of gold deposited on the four cathodes was 235.9 mg, 238.3 mg, 241.4 mg, and 243.7 mg, respectively. The total amount of gold regenerated on the cathodes is 958.3 mg, which corresponds to a 95.8% regeneration efficiency, exceeding by 0.8% the efficiency reported in [3] achieved within 24 hours.

Claims (1)

A bioelectrochemical reactor for the (co)regeneration of precious metals from industrial waste water, consisting of a housing, a cathode compartment, an anode compartment, a cathode, a bioanode, characterized in that the housing has the shape of a cube with dimensions ranging from 80 mm x 80 mm x 80 mm to 250 mm x 250 mm x 250 mm or a parallelepiped shape with a length varying from 100 mm to 250 mm, a width varying from 50 mm to 120 mm and a height varying from 80 mm to 120 mm, or a cylindrical shape with a diameter ranging from 80 mm to 250 mm and a height ranging from 80 mm to 250 mm, is the cathode compartment (1) provided with a cover (4) of the cathode compartment (1) with a centrally located circular opening measuring from 70 mm to 230 mm and height varying from 20 mm to 40 mm and internally threaded, mounted on the upper outwardly projecting periphery of the housing by means of fastening bolts (10), wherein the anode compartment (6) is cylindrical in shape with a diameter varying from 40 mm to 200 mm and a height ranging from 40 mm to 200 mm, it is movable and mounted through the circular hole of the cover (4) in the central part of the housing of the cathode compartment (1), the anode compartment (6) has a circular cover (7) with a diameter ranging from 40 mm up to 200 mm and with an external thread that corresponds to the thread of the cover (4) of the cathode compartment (l); the housing, the cover (4) of the cathode compartment (1) and the cover (7) of the anode compartment (6), as well as the cathode compartment (1) and the anode compartment (6), are made of engineering plastic, and on the four walls of the housing at a distance of 10 mm to 20 mm from the bottom, from 10 mm to 20 mm from the top hole and from 10 mm to 20 mm from the side walls, one to four cathodes (2) of the target metal or gold or silver are mounted, under the form of a foil or mesh or wire, or of another conductive material selected from graphite or graphitized paper or carbon cloth, on which current conductors of titanium wire or titanium mesh are attached, such that, in the case of two or more cathodes (2), they are arranged symmetrically relative to the bioanode, the shape of the cathodes (2) is flat or rectangular or round or volumetric, their geometric area being from 18 cm2 to 400 cm2 , and the volume from 50 cm3 to 500 cm3 in the case of carbon granules, as on both types of electrodes the size is dependent on the dimensions and shape of the cathode compartment (1), wherein the space of the cathode compartment (1) is filled with an aqueous solution containing ions of the target metal or gold and/or silver or other noble metal for regeneration or waste water, and on the side walls of the anode compartment (6) there are rectangular openings with dimensions of 30 mm x 60 mm, and on the inside of the walls a separator partition (5) representing an ion exchange membrane or diaphragm is installed, and in the internal free volume of the anode compartment (6) the bioanode is arranged in the form of carbon cloth or cachet or graphite or graphitized paper or graphite granules or granular activated carbon or biocarbon, placed on a titanium grid representing a current conductor, and the bioanode has an immobilized electrochemically active biofilm of anaerobic activated sludge, the shape of the bioanode is rectangular or round, the geometric surface of the bioanode varies from 10 cm2 to 500 cm2, and the current leads of the cathode (2) and the bioanode are short-circuited