What Is Activated Carbon Adsorption? A Water Treatment Method for Removing Pollutants
Activated carbon adsorption is one of the most widely used physicochemical methods for removing dissolved pollutants from water. It relies on the enormous internal pore surface of carbonaceous material to capture organic compounds, colour, odour and trace contaminants that sedimentation or filtration alone cannot remove.
How activated carbon is made
Activated carbon is produced from carbon-containing raw materials such as sawdust, nutshells and lignite by carbonisation followed by activation. It is supplied in two main forms: powdered activated carbon (PAC), with a particle size of roughly 10–50 µm, and granular activated carbon (GAC), with a particle size of 0.4–2.4 mm. Both grades are highly porous and have a very large specific surface area, typically 500–1,000 m² per gram.
The two key performance parameters are adsorption capacity and adsorption rate. Adsorption capacity is the mass of solute that one unit weight of carbon holds when it reaches saturation, and depends on the raw material, the manufacturing process and the regeneration method: the higher the capacity, the less carbon is consumed. Adsorption rate is the mass of solute taken up per unit weight of carbon per unit time. Because adsorption is selective, both parameters must be determined experimentally for each application. Granular grades must also have adequate mechanical strength and a defined particle size range.
Adsorption equilibrium and isotherms
Carbon continues to adsorb solute until equilibrium is reached, that is, until the solute concentration in the water stops changing. At a given temperature, the relationship between the mass adsorbed per unit weight of carbon at equilibrium and the concentration remaining in the water is called the adsorption isotherm. It is commonly expressed by the Freundlich equation:
X/M = k · C1/n
where X is the mass of solute adsorbed, M the mass of carbon dosed, C the solute concentration at equilibrium, and k and n are constants obtained from tests.
At the same temperature and pressure, different adsorbents do not have the same affinity for a given molecule, which is why carbon selection is normally based on isotherm or jar tests carried out with the actual wastewater.
Powdered and granular grades
Powdered activated carbon is made from high-grade charcoal by a special process, either physical or chemical, followed by steam activation, refining and grinding. It is a black powder, insoluble in normal solutions, odourless and tasteless, with a large surface area, high purity, fast filtration and stable quality; it also shows a flocculating and filter-aid effect. It is widely used for decolourisation and refining in food, pharmaceutical and fine chemical production, and for water purification.
Granular activated carbon is used where the carbon has to be retained in a bed and reused, for example in pressure filters and tertiary treatment columns. Coconut-shell carbon, produced by high-temperature activation and carbonisation and sometimes loaded with photocatalyst or carbon fibre, is a newer grade whose adsorption capacity for organic vapours is reported to be more than five times that of ordinary carbon.
Activated carbon is also used for gas-phase adsorption, for example in solvent recovery, odour control in air-conditioned and enclosed spaces, and off-gas polishing in chemical plants. This article concentrates on its use in water treatment.
Regeneration
Once the adsorption capacity of granular carbon is exhausted, it is normally regenerated by thermal treatment: the spent carbon is dried and then roasted in a regeneration furnace at about 850 °C. Each regeneration cycle loses roughly 5–10% of the carbon and reduces its capacity, so regeneration efficiency has a strong influence on the operating cost of a carbon filter and therefore on the overall cost of water treatment.
How adsorption works at the interface
Adsorption is a surface phenomenon in which material carried in one phase accumulates at the interface with another phase. In wastewater treatment the relevant case is liquid–solid adsorption: the pollutant is transferred from the water to the surface of the solid adsorbent, its concentration in the water falls, and the water leaves the unit purified. Solid adsorbents used in practice include activated carbon, diatomite, zeolite and ion exchange resins.
Because adsorption is a surface effect, the performance of an adsorbent is governed mainly by three surface properties:
1. Specific surface area. The surface area per unit weight of adsorbent (m²/g) increases with porosity, and a larger specific surface area means a higher adsorption capacity. Porous activated carbon reaches more than 1,000 m²/g, which is why it is such an effective adsorbent in water treatment.
2. Surface energy. Molecules inside a liquid or a solid are pulled evenly in all directions, whereas molecules at the surface have a surplus of energy, known as surface energy, and that surplus is the driving force behind surface adsorption.
3. Surface chemistry. Adsorption also depends on the chemical bonds in the crystal structure of the solid; the selective uptake of electrolyte ions, for example, is controlled by surface chemistry. In short, specific surface area only provides the opportunity for contact, surface energy explains why adsorption occurs spontaneously, and the chemical state of the surface dominates selective adsorption.
Physical, chemical and ion-exchange adsorption
Chemical adsorption involves a chemical reaction between adsorbent and adsorbate through chemical bonding, which binds the two firmly together. Because the reaction requires a large activation energy it normally takes place at higher temperature and releases more heat. Chemical adsorption is selective, since one adsorbent acts only on one or a few specific substances, and because it relies on chemical bonds it forms a monolayer, is relatively stable and is not easily desorbed.
Physical adsorption is caused by molecular forces and is the most common form. The adsorbed molecules are not fixed at specific points on the surface but can move freely along the interface. The heat of adsorption is small, no activation energy is needed and the process can take place at low temperature. It is reversible: adsorbed molecules can leave the surface again through thermal motion, which is called desorption. Physical adsorption can form mono- or multi-molecular layers, and because intermolecular forces are universal, one adsorbent can take up many substances, although the amount adsorbed differs with the properties of each adsorbate and depends closely on the surface area and pore distribution of the adsorbent.
Ion-exchange adsorption takes place when ions held on the adsorbent surface are exchanged for ions of the same charge in solution.
Where it fits in industrial wastewater treatment
In industrial plants, activated carbon is usually the polishing step after oil removal, flotation, biological treatment or oxidation. Removing emulsified oil, suspended solids and most of the dissolved organics upstream protects the carbon bed, extends its service life and cuts regeneration cost, which is why mechanical and physicochemical pretreatment is specified ahead of carbon adsorption in most refinery and petrochemical schemes.
