Activated Carbon: From Production to Practical Use.

Activated carbon , also known as activated carbon, is a form of carbon that has undergone activation treatments with the aim of enhancing its characteristic adsorbent capacities , increasing its internal surface area and porosity.

It is produced by the carbonization of organic materials such as wood , coconut or directly from coal , followed by an activation process that increases its porosity and its ability to absorb chemicals and impurities.
Activated carbon owes these properties to its highly porous structure.
This porous structure is not uniform but rather, is composed of a complex network of pores of varying sizes.

Usage

In marine reef aquariums, activated carbon is used as part of the filtration system to improve water quality .
Its ability to adsorb organic matter , harmful chemicals and other impurities makes it an invaluable tool in maintaining a healthy and stable aquatic environment. In a marine reef aquarium, water quality is crucial to the health and well-being of living organisms, and the overall system.

Water must have stable and well-defined parameters to ensure the survival and prosperity of the aquarium inhabitants.
Key water parameters, such as temperature, pH, triad, salinity and nutrient concentration , must be maintained within certain ranges to replicate natural conditions and provide an adequate environment for our guests.
The presence of an excess of decaying organic matter, harmful chemicals or other contaminants can compromise water quality, causing health problems for aquarium inhabitants and negatively affecting the stability of the entire ecosystem.

The main goal of using activated carbon in the marine reef aquarium is to improve water quality by removing unwanted substances and contaminants .
Activated carbon acts as a highly effective selective adsorbent, adsorbing dissolved harmful organic substances such as drug residues, metabolites and toxins produced by corals and other organisms.
It also adsorbs and retains various harmful chemical compounds, including dyes , pesticides and heavy metals , which can be accidentally introduced into the aquarium, as well as actively contributes to improving the clarity of the water.

Production process

Activated carbon is known for its highly porous structure, which gives it a large surface area per unit volume and to which we owe its characteristics.
This porous structure is obtained through the process of carbonization and activation of the basic carbon material.
During carbonization, carbonaceous organic material is heated to high temperatures in the absence of oxygen, producing charcoal. The charcoal is then activated by chemical or physical treatments that open and increase its porosity.

Choice of raw materials

The most commonly used materials for the production of activated carbon include lignin, coconut shells, coal and peat . Each type of material produces an activated carbon with specific characteristics of porosity and adsorption capacity.

Wood: Activated carbons derived from wood generally tend to have a relatively high porosity and a good amount of macro-pores, making them suitable for the adsorption of large molecules. However, this reduces their efficiency in adsorbing small molecules, making them less interesting for our purposes, if not specifically treated.

Peat: Peat, being less dense and more heterogeneous in composition than other carbon materials, leads to the production of activated carbon with a variable mix of pores, but of lower quality. Although they tend to have a good balance between micro and meso-pores, their low surface area makes them less performing and therefore less suitable for advanced applications.

Coal: Activated carbons produced from coal, also known as “mineral carbons” (bituminous coal, lignite, anthracite, etc.) have a highly developed porous structure with a good distribution of micro and meso-pores.
They are particularly appreciated for their ability to adsorb a wide range of molecules, making them versatile for various applications.

Coconut: These activated carbons are known for their high hardness and for having a large amount of micro-pores, which makes them particularly effective in adsorbing small organic molecules.
They are often preferred for applications requiring water purification from small molecular contaminants and are among the most commonly used in our industry.

Activation of carbon

The activation of carbon plays a crucial role in determining its effectiveness as an adsorbent material.
The carbon activation process has a significant impact on the final porous structure of the material, for this reason the quality and final characteristics of the product also depend on the activation technique used.

Chemical Activation: In chemical activation, the carbonaceous material is subjected to the action of chemical activating agents prior to carbonization. These chemical agents (e.g. phosphoric acid, sodium or potassium hydroxide, zinc chloride , etc.) degrade and swell the structure of the starting material, increasing its internal porosity.
The treatment takes place at temperatures between 450°C and 900°C, significantly lower than those used in physical activation.

Phosphoric Acid (H3PO4): Commonly used to activate lignin-based materials. The process results in the formation of a highly porous structure, ideal for adsorption of dyes and small organics from the liquid.

Potassium Hydroxide (KOH): The use of KOH is known to produce activated carbon with a large amount of micro-pores and highly active surfaces, useful for example for the adsorption of gases and vapors, such as ammonia or sulfur dioxide.
This method is particularly advantageous for applications requiring a high density of micro-pores, such as the purification of drinking water, the removal of contaminants from chemical solutions, or the adsorption of harmful gases from the air.

Physical Activation

Physical activation begins with the carbonization of the carbonaceous material at temperatures ranging from 600°C to 900°C in a low-oxygen environment. The carbonized material is then exposed to oxidizing gases (e.g., water vapor, carbon dioxide) at even higher temperatures, typically between 800°C and 1000°C. This process opens the porous structure of the coal, removing volatilized organic matter and creating an extensive network of pores.

Activation with water vapor

The use of steam is prevalent when it is desired to produce activated carbon with a good distribution of pores of all sizes. The resulting material is effective in adsorbing a wide range of compounds, from volatile organic compounds in the gas phase to contaminants in aqueous solutions.

CO2 activation

This method tends to produce an activated carbon with a higher concentration of meso-pores, useful for the adsorption of medium to large sized molecules.

Choice of activation technology

Physical activation is often preferred for the production of activated carbon for a wide range of applications, including air purification, industrial wastewater treatment, and adsorption of contaminants from complex gas streams.
The choice between chemical and physical activation depends on several factors, including cost, the desired properties of the final activated carbon, and the specifics of the application. Chemical activation is often more expensive due to the use of chemical reagents, but can produce activated carbons with superior adsorptive properties for specific contaminants. On the other hand, physical activation is typically more versatile and less expensive, making the resulting activated carbon suitable for a wider range of applications.

Environmental impact

In addition to technical considerations, environmental impact and production costs are also crucial when choosing the activation method.
Chemical activation , although offering specific advantages in terms of control of porosity and adsorptive properties, involves the use and disposal of potentially hazardous chemicals , which can have environmental implications .
As a result, research and development is moving towards more environmentally friendly and sustainable processes, such as the use of safer activating agents or the recovery and reuse of chemical agents .
In contrast, physical activation, while generally more environmentally sustainable , requires significant amounts of energy , especially in the high-temperature carbonization and oxidant gas activation steps. Energy efficiency of these processes is therefore an area of ​​interest to reduce operating costs and the carbon footprint associated with activated carbon production.

Fortunately, technological advances are aimed at developing more efficient, cost-effective and environmentally sustainable activation methods.
For example, research explores the use of microwaves for charcoal activation , which can provide faster, more uniform heating, reducing process times and energy consumption.

Chemical and physical properties

Activated carbon is insoluble in water and does not react with adsorbed substances , making it an inert and relatively safe material to use in aquariums.

The most important physical properties of activated carbon in our field concern its porous structure, which provides a large surface area available for the adsorption of impurities .

The pore size can vary and is calibrated by choosing different materials and activation techniques, allowing the activated carbon to adsorb a diverse range of compounds.

The chemical properties of activated carbon depend on the activation process and the nature of the base material .

Adsorption mechanism

The mechanism of adsorption of impurities in water by activated carbon is based on the physical and chemical attraction between the molecules of the substances to be adsorbed and the surface of the carbon.

Unwanted molecules in the water are captured and retained on the surface of the activated carbon through a series of processes , including physical adsorption , chemical adsorption, and electrostatic attraction (Van der Waals forces).

Activated carbon may contain surface functional groups that can chemically interact with the substances to be adsorbed, further enhancing their effectiveness.
The adsorbed impurities remain trapped in the pores, allowing for their effective removal.

This adsorption process is highly selective, allowing the activated carbon to remove specific classes of substances based on their size and chemical and physical properties.

Porosity and adsorbed molecules

As previously mentioned, activated carbon owes these adsorbent properties to its highly porous structure. This structure is composed of a complex network of pores of variable size that can be classified into micro-pores (diameter less than 2 nm), meso-pores (diameter between 2 nm and 50 nm) and macro-pores (diameter greater than 50 nm).

The distribution and size of pores significantly influence the adsorption capabilities of activated carbon, determining its effectiveness in adsorbing molecules of different sizes.

Micropores (diameter less than 2 nm)

Phenolic compounds and small volatile organic molecules: These organic compounds can arise from various sources, both biotic and abiotic.
They include various types of alcohols, aldehydes, organic acids and other catabolites that may be present in aquarium water as a result of the metabolism of living organisms or the decomposition of organic matter.

They are toxic to many aquatic organisms and can negatively affect the health of the aquarium ecosystem.

Chloramines: Chemicals used in drinking water for disinfection, but harmful to living organisms in aquariums.
Chlorine and chloramine molecules are small enough to be effectively adsorbed by the micro-pores.

Mesopores (diameter between 2 nm and 50 nm)

Harmful Proteins, Amino Acids, and Peptides: Although they vary in size, many harmful proteins and peptides are able to adhere and become trapped in the mesopores. Many of the toxic compounds used in the ongoing chemical battle between corals and algae are protein, peptide, or amino acid-based toxins.

Eliminating these substances helps to attenuate the inhibitory effects and reduce the manifestations of allelopathy.

Colorants and tannins: These are substances that can give aquarium water an unpleasant color, reducing transparency and negatively affecting the metabolism of the system.
Mesopores are effective in adsorbing these molecules.

Drugs, antibiotics and other synthetic contaminants: After a pharmacological treatment, it is common to find residues of the drugs used and their catabolites in the system.
These can remain in the water for a long time, causing harmful effects on other organisms and on the whole system in general, especially when dealing with algaecide treatments.
The mesopores in activated carbon can adsorb these substances, helping to reduce the harmful side effects of treatments.

Macro-pores (diameter greater than 50 nm)

Large Organic Molecules: Some large organic molecules, such as certain natural or synthetic polymers found in water , can be retained in macropores. Their removal helps prevent the accumulation of substances that might otherwise promote the growth of harmful algae or bacteria.

It is important to note, however, that while micro- and meso-pores play a direct role in the chemical adsorption of dissolved substances, macro-pores are more important in facilitating the access of molecules to the smaller pores and in the removal of larger particles through a more mechanical action.

Practical use of activated carbon

When using activated carbon in granular, flake or pellet form, it is important to thoroughly wash the material to maximize its adsorption capacity and remove dust, impurities and production residues .

Most manufacturers recommend rinsing the activated carbon in osmosis for a few minutes.
While a reverse osmosis rinse is generally more than sufficient and further processes have only marginal benefits, some hobbyists also prefer to soak the carbon in hot water for a period of time before use to ensure it is fully activated and the pores are well opened and free of dust.

Positioning

Activated carbon can be placed in different points of the aquarium filtering system, depending on the needs and use we will make of it.
The choice between a static use (with the bag simply hung in an area of ​​strong flow) or dynamic use (in dedicated reactors or in a section of the sump with forced circulation) depends on the system and the results we want to obtain .

One of the most common methods is to place the carbon inside mesh bags or special containers placed in one of the sections of the sump, after the pre-filtering-mechanical filtering section.

Some enthusiasts prefer to use dedicated reactors for activated carbon, which allow for greater exposure of the water to the filter material, increasing its effectiveness and speed of adsorption.

In some systems, particularly in the case of continuous use , a more moderate absorption speed is preferred and one is prepared to lose a little efficiency in order to have a more delicate reduction of organic compounds .

If, however, it is used to remove a toxic contaminant, it may be more interesting to use it actively with a forced circulation of water that passes through the material in order to increase its efficiency and speed of removal.

Replacement frequency

The frequency with which you should replace activated carbon in your marine reef aquarium depends on several factors , including the biological load of the aquarium, the amount and quality of carbon used, the population of the system , and the presence of contaminants .

Generally speaking, it is recommended to replace the activated carbon every 2-4 weeks , however depending on the specific conditions of different systems, the frequency of replacement can be modified to suit the needs of the tank.

It is essential to learn how to monitor and interpret system conditions to determine the optimal time for carbon replacement.

The duration of the effectiveness of activated carbon can vary significantly depending on the type and quality of the carbon itself, as well as the intensity of use. To maintain optimal effectiveness, it is crucial to periodically evaluate the water conditions and replace the carbon before its adsorption capacity is exhausted.

Limitations and Precautions

Improper use of activated carbon can lead to various negative side effects on the aquarium and its inhabitants.
For example, too much activated carbon can remove beneficial substances from the water , such as trace elements , which are necessary for coral growth and health , or lead to too rapid and aggressive removal of organic compounds , which can destabilize the ecosystem and disturb animals .

It has been observed several times that an excessive speed in the absorption of some organic molecules can be harmful for many corals .

If large quantities are used in systems with forced circulation, several corals may be disturbed and become closed up, or begin to spin.

Additionally, low-quality carbon can release large amounts of phosphate, barium, zinc, aluminum, or other contaminants, further compromising water quality.

We hope that this little insight has been useful to you, helping to clarify some doubts and providing you with valuable information on the use of activated carbon in your aquariums.