What uses a graphite rod as a cathode? 4 key points to understand

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Graphite rods are used as cathodes in the Hall-Heroult process for extracting aluminum metal from aluminum oxide.

In this process, both the anode and cathode are made of graphite.

4 key points to understand

1. Hall-Heroult Process

This is a major industrial process for the extraction of aluminum.

Aluminum oxide (Al2O3) is dissolved in molten cryolite (Na3AlF6) and electrolyzed in a cell.

The process requires a high temperature of about 950 to 980 degrees Celsius.

2. Role of Graphite in the Process

In the Hall-Heroult process, graphite serves a dual role as both the anode and the cathode.

The graphite anode is consumed during the process as it reacts with oxygen ions, releasing carbon dioxide.

The graphite cathode, on the other hand, remains relatively stable and provides a surface for the reduction of aluminum ions.

3. Why Graphite is Used

Graphite is chosen for its electrical conductivity, its resistance to high temperatures, and its stability in the electrolytic environment.

It is also relatively inexpensive and easy to fabricate, which is crucial for large-scale industrial processes.

4. Cathode Properties in the Hall-Heroult Process

As per the desirable properties for cathode materials mentioned in the reference, graphite meets the criteria of being a stable material when in contact with the electrolyte, having a useful working voltage, being easy to fabricate, and having a low cost.

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1. Introduction

2,

Numerous industrial effluents produce large quantities of wastewater with varying compositions and vast hydraulic loads compositions [ 1 3 ]. Industrial effluents contain a variety of substances, including organic waste, inorganic chemicals, suspended or dissolved particles, and heavy metal effluents [ 4 ]. The likelihood of such effluent being uncontrolledly dumped into water bodies will have serious negative effects on the social and natural systems. Because of the great complexity and variety of the contaminants in this wastewater, it must be treated in order to fulfil the necessary criteria for water quality, requiring the use of highly effective and affordable methods [ 5 ].

3 of the wastewater is generated per ton of crushed sugarcane [3 of wastewater is produced per ton of cane crushed. Moreover, it was also reported, for around 1.5'2 m3 fresh consumed to process 1 ton of cane, 1 m3 of wastewater is generated [5 in the generated wastewater is in the range of 50,000'60,000 mg/L while the COD can reach from 110,000 to 190,000 mg/L [

The sugar industry is the most prevalent agro-based sector in more than 130 countries, especially in developing ones. This sector today contributes significantly to economic growth and is a key driver of employment creation in many developing countries in Asia, Africa, and South America. The industry is involved in sugarcane processing action to produce raw sugar from more than 70% of the sugarcane produced in the worldwide [ 6 ]. The chemicals, feed stocks, and end products utilized in the sugarcane industry affect the quantity and composition of the produced wastewater. While secondary wastewater sources are mostly obtained from barometric condensers, dust removal in the chimney and scrubbers, and cooling of turbines, including washing water, the majority of the wastewater is produced during the cane processing operations, e.g., evaporation, crystallization, and refining [ 7 ]. In addition, sugarcane that enters the factory usually contains approximately 70'80% moisture, so 0.7 mof the wastewater is generated per ton of crushed sugarcane [ 6 8 ]. In literature, many studies conducted in different countries reported that about 1 mof wastewater is produced per ton of cane crushed. Moreover, it was also reported, for around 1.5'2 mfresh consumed to process 1 ton of cane, 1 mof wastewater is generated [ 9 ]. The discharged wastewater is characterized by high organic loads and intense color. The typical level of BODin the generated wastewater is in the range of 50,000'60,000 mg/L while the COD can reach from 110,000 to 190,000 mg/L [ 10 ].

In general, for the industrial wastewaters, numerous authors have discussed various wastewater treatment systems in the literature. The kind of pollutants present in wastewater affects the treatment procedures [ 5 ]. Some of them are based on advanced oxidation processes [ 11 12 ], chemical coagulation [ 13 ], bio-coagulation [ 14 ], filtration [ 15 ], ion exchange [ 16 ], aerobic and anaerobic treatment [ 17 ], and others [ 18 ]. The majority of them need significant financial investments, and their application is constrained mostly due to economic concerns taking precedence over the significance of pollution management.

2 even by using special substrates [

Paradoxically, it is possible to use the organic pollutants existing in industrial wastewaters as a sustainable and renewable energy source. A wonderful tool that may be used to accomplish this goal is the microbial fuel cell (MFC), which simultaneously treats wastewater and produces electrical energy from it. The MFC is a cutting-edge environmental and energy technology that produces electrical energy from organic contaminants found in wastewater [ 19 20 ]. Unfortunately, this promised device has not shifted to the industrial scale yet due to the low performance. Although, many trials have been reported to enhance the generated power from the microbial fuel cells, most of these reports introduced MFC power less than 100 mW/meven by using special substrates [ 21 ]. Consequently, there are numerous trials underway to enhance the performance of the MFC through different ways, such as cathode or anode development [ 22 23 ], operating conditions optimization [ 24 ], membrane modification [ 25 ], and electrolyte adjustment [ 26 ]. Moreover, other researchers applied various types of microorganisms [ 27 28 ], media (fuel) [ 27 29 ], electrode materials/sub materials [ 30 31 ], and cell configurations [ 32 33 ]. However, anode attracts the most attention among the aforementioned factors affecting the MFC performance due to its strong and direct impact on the generated power and current densities. Consequently, numerous reports have been published about anode modification compared to the other factors [ 34 35 ].

Shewanella oneidensis

or a bacterial consortia found in wastewater, colonized freshly electrospun carbon nanofiber electrodes before being introduced into a lab-scaled system [

Nanomaterials can provide marvelous solutions for many hopeless problems. For anode modification, electrospun nanofibers have been exploited to improve the performance. For instance, double layer and Co-doped carbon nanofibers distinctly enhanced the industrial wastewater-based MFC [ 20 36 ]. In addition, exo-electrogenic microorganisms, such as the model bacteriumor a bacterial consortia found in wastewater, colonized freshly electrospun carbon nanofiber electrodes before being introduced into a lab-scaled system [ 37 ].

2 (rutile) and carbon nanofibers was studied in an MFC for the same purpose [

In addition, the carbon nanofiber mats' nanopores are uncomfortable for the micro-scale biocatalyst (microorganisms). In other words, it is difficult for microorganisms to penetrate the carbon nanofiber anode, which negates the benefit of the nanostructure and abolishes the feature of a high surface area. Other methods have thus been used to provide the electrospun carbon nanofiber-based anodes more characteristics. For instance, a hybrid anode made of carboxylated multiwalled carbon nanotubes and carbon nanofibers was electrospun and utilized to increase the performance of MFCs by increasing cell attachment and lowering anode potential [ 38 ]. A composite anode made of TiO(rutile) and carbon nanofibers was studied in an MFC for the same purpose [ 18 ]. However, the generated power from aforementioned anodes-based MFCs was not sufficient.

Actually, the packing density of the carbon nanofiber layer is a key factor. In more detail, at low packing density, the microorganisms can penetrate through the nanofiber layer due to presence of large pores. Microorganism insertion through the carbon nanofiber layer creates an excellent conductive network to transfer the electrons. However, after a specific packing density threshold, the pores will be too small to pass the microorganisms, which results in a strong decrease in the generated power. In this study, the aforementioned hypothesis was experimentally proven by the fabrication of carbon nanofibers-attached graphite to be exploited as anode in a sugarcane industry wastewater-based MFC. The results were very satisfactory as the performance was strongly enhanced at the optimized amount of the attached carbon nanofibers.

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