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Electro-oxidation
Electro-oxidation (EO or EOx), also known as anodic oxidation or electrochemical oxidation (EC), is a technique used for wastewater treatment, mainly for industrial effluents, and is a type of advanced oxidation process (AOP). The most general layout comprises two electrodes, operating as anode and cathode, connected to a power source. When an energy input and sufficient supporting electrolyte are provided to the system, strong oxidizing species are formed, which interact with the contaminants and degrade them. The refractory compounds are thus converted into reaction intermediates and, ultimately, into water and CO2 by complete mineralization.
Electro-oxidation has recently grown in popularity thanks to its ease of set-up and effectiveness in treating harmful and recalcitrant organic pollutants, which are typically difficult to degrade with conventional wastewater remediation processes. Also, it does not require any external addition of chemicals (contrary to other processes), as the required reactive species are generated at the anode surface.
Electro-oxidation has been applied to treat a wide variety of harmful and non-biodegradable contaminants, including aromatics, pesticides, drugs and dyes. Due to its relatively high operating costs, it is often combined with other technologies, such as biological remediation. Electro-oxidation can additionally be paired with other electrochemical technologies such as electrocoagulation, consecutively or simultaneously, to further reduce operational costs while achieving high degradation standards.
The set-up for performing an electro-oxidation treatment consists of an electrochemical cell. An external electric potential difference (aka voltage) is applied to the electrodes, resulting in the formation of reactive species, namely hydroxyl radicals, in the proximity of the electrode surface. To assure a reasonable rate of generation of radicals, voltage is adjusted to provide current density of 10-100 mA/cm2. While the cathodes materials are mostly the same in all cases, the anodes can vary greatly according to the application (see § Electrode materials), as the reaction mechanism is strongly influenced by the material selection. Cathodes are mostly made up by stainless steel plates, Platinum mesh or carbon felt electrodes.
Depending on the effluent nature, an increase of the conductivity of the solution may be required: the value of 1000 mS/cm is commonly taken as a threshold. Salts like sodium chloride or sodium sulfate can be added to the solution, acting as electrolytes, thus raising the conductivity. Typical values of salts concentration are in the range of few grams per liter, but the addition has a significant impact on power consumption and can reduce it by up to 30%.
As the main cost associated to electro-oxidation process is the consumption of electricity, its performance are typically assessed through two main parameters, namely current efficiency and specific energy consumption. Current efficiency is generally defined as the charge required for the oxidation of the considered species over the total charged passed during electrolysis. Although some expressions have been proposed to evaluate the instantaneous current efficiency, they have several limitations due to the presence of volatile intermediates or the need for specialized equipment. Thus, it is much easier to define a general current efficiency (GCE), defined as an average of the value of current efficiency along the entire process and formulated as follows:
Where COD0 and CODt are the chemical oxygen demand (g/dm3) at time 0 and after the treatment time t, F is the Faraday's constant (96'485 C/mol), V is the electrolyte volume (dm3), I is the current (A), t is the treatment time (h) and 8 is the oxygen equivalent mass. Current efficiency is a time dependent parameter and it decreases monotonically with treatment time. Instead, the specific energy consumption measures the energy required to remove a unit of COD from the solution and is typically expressed in kWh/kgCOD. It can be calculated according to:
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Electro-oxidation
Electro-oxidation (EO or EOx), also known as anodic oxidation or electrochemical oxidation (EC), is a technique used for wastewater treatment, mainly for industrial effluents, and is a type of advanced oxidation process (AOP). The most general layout comprises two electrodes, operating as anode and cathode, connected to a power source. When an energy input and sufficient supporting electrolyte are provided to the system, strong oxidizing species are formed, which interact with the contaminants and degrade them. The refractory compounds are thus converted into reaction intermediates and, ultimately, into water and CO2 by complete mineralization.
Electro-oxidation has recently grown in popularity thanks to its ease of set-up and effectiveness in treating harmful and recalcitrant organic pollutants, which are typically difficult to degrade with conventional wastewater remediation processes. Also, it does not require any external addition of chemicals (contrary to other processes), as the required reactive species are generated at the anode surface.
Electro-oxidation has been applied to treat a wide variety of harmful and non-biodegradable contaminants, including aromatics, pesticides, drugs and dyes. Due to its relatively high operating costs, it is often combined with other technologies, such as biological remediation. Electro-oxidation can additionally be paired with other electrochemical technologies such as electrocoagulation, consecutively or simultaneously, to further reduce operational costs while achieving high degradation standards.
The set-up for performing an electro-oxidation treatment consists of an electrochemical cell. An external electric potential difference (aka voltage) is applied to the electrodes, resulting in the formation of reactive species, namely hydroxyl radicals, in the proximity of the electrode surface. To assure a reasonable rate of generation of radicals, voltage is adjusted to provide current density of 10-100 mA/cm2. While the cathodes materials are mostly the same in all cases, the anodes can vary greatly according to the application (see § Electrode materials), as the reaction mechanism is strongly influenced by the material selection. Cathodes are mostly made up by stainless steel plates, Platinum mesh or carbon felt electrodes.
Depending on the effluent nature, an increase of the conductivity of the solution may be required: the value of 1000 mS/cm is commonly taken as a threshold. Salts like sodium chloride or sodium sulfate can be added to the solution, acting as electrolytes, thus raising the conductivity. Typical values of salts concentration are in the range of few grams per liter, but the addition has a significant impact on power consumption and can reduce it by up to 30%.
As the main cost associated to electro-oxidation process is the consumption of electricity, its performance are typically assessed through two main parameters, namely current efficiency and specific energy consumption. Current efficiency is generally defined as the charge required for the oxidation of the considered species over the total charged passed during electrolysis. Although some expressions have been proposed to evaluate the instantaneous current efficiency, they have several limitations due to the presence of volatile intermediates or the need for specialized equipment. Thus, it is much easier to define a general current efficiency (GCE), defined as an average of the value of current efficiency along the entire process and formulated as follows:
Where COD0 and CODt are the chemical oxygen demand (g/dm3) at time 0 and after the treatment time t, F is the Faraday's constant (96'485 C/mol), V is the electrolyte volume (dm3), I is the current (A), t is the treatment time (h) and 8 is the oxygen equivalent mass. Current efficiency is a time dependent parameter and it decreases monotonically with treatment time. Instead, the specific energy consumption measures the energy required to remove a unit of COD from the solution and is typically expressed in kWh/kgCOD. It can be calculated according to: