The Adsorption Principle of Activated Carbon

Active charcoal—also referred to as active carbon, activated charcoal, or activated carbon—is a carbonaceous material that appears in the form of a black powder or granules. Structurally, active charcoal consists of irregularly arranged microcrystalline carbon, featuring fine pores within its cross-linked network; the activation process itself introduces structural defects into the carbon matrix. Consequently, it is classified as a porous carbon material characterized by a low bulk density and a large specific surface area, making it a primary material used in the manufacture of filters.

Production of Activated Carbon

The primary raw materials for activated carbon can be virtually any carbon-rich organic substance—such as coal, wood, fruit pits, coconut shells, walnut shells, apricot shells, and date pits. Within an activation furnace, these carbonaceous materials undergo pyrolysis under conditions of high temperature and specific pressure to be transformed into activated carbon. During this activation process, an immense surface area and a complex pore structure gradually develop; the process of adsorption—the very mechanism by which activated carbon functions—takes place precisely within these pores and upon these surfaces. The size of the pores within the activated carbon exerts a selective effect on the adsorbate; this selectivity arises from the fact that large molecules are unable to penetrate pores with diameters smaller than their own molecular dimensions. Activated carbon is a hydrophobic adsorbent produced from carbon-based raw materials through high-temperature carbonization and activation processes. Characterized by a vast network of micropores and an extraordinarily large surface area, activated carbon is highly effective at removing color and odors; it is capable of eliminating the majority of organic pollutants—as well as certain inorganic substances, including various toxic heavy metals—from secondary effluent.

Principles of Activated Carbon

1. Filtration Principle

Activated carbon filtration is a process in which suspended contaminants in water are intercepted. The intercepted suspended solids become lodged within the interstitial spaces between the activated carbon granules. The pore size and porosity of the filter bed increase as the particle size of the activated carbon material increases. In other words, the coarser the activated carbon particles, the greater the available space for accommodating suspended solids. This manifests as enhanced filtration efficiency, increased contaminant-holding capacity, and a higher total contaminant capture volume. Furthermore, the larger the pores in the activated carbon filter bed, the deeper suspended solids can penetrate into the lower layers of the filter medium; provided there is a sufficient protective thickness, a greater quantity of suspended solids can be intercepted, allowing the middle and lower filter layers to more effectively perform their interception function and thereby increasing the overall contaminant capture capacity of the filtration unit.

From a strictly theoretical standpoint, the capacity of activated carbon to intercept suspended solids stems from the extensive surface area provided by the carbon material. At low flow rates, the filtration efficiency of the unit is primarily derived from the sieving action of the activated carbon; conversely, at high flow rates, the filtration efficiency is primarily derived from the adsorption action occurring on the surfaces of the activated carbon particles. During the filtration process, the greater the particle surface area provided by the activated carbon, the stronger the adhesion force exerted upon the suspended solids present in the water.

2. Adsorption Principle

Based on the nature of the forces acting between activated carbon molecules and contaminant molecules during the adsorption process, adsorption can be broadly classified into two categories: physical adsorption and chemical adsorption (also known as activated adsorption). During the adsorption process, when the forces acting between the activated carbon molecules and the contaminant molecules are Van der Waals forces (or electrostatic attraction), the process is termed physical adsorption; conversely, when the forces acting between the activated carbon molecules and the contaminant molecules involve chemical bonds, the process is termed chemical adsorption. The intensity of physical adsorption is primarily determined by the physical properties of the activated carbon and is largely independent of its chemical properties. Since Van der Waals forces are relatively weak, they exert minimal influence on the molecular structure of the contaminants; as these forces are analogous to the cohesive forces existing between molecules, physical adsorption can be likened to a phenomenon of condensation or aggregation. Consequently, during physical adsorption, the chemical properties of the contaminants remain essentially unchanged.

Conversely, because chemical bonds are strong, they exert a significant influence on the molecular structure of the contaminants; therefore, chemical adsorption can be regarded as a chemical reaction—specifically, the result of a chemical interaction occurring between the contaminants and the activated carbon. Chemisorption generally involves the sharing of electron pairs or electron transfer—rather than mere weak perturbations or polarization effects—and constitutes an irreversible chemical reaction process. The fundamental distinction between physisorption and chemisorption lies in the nature of the forces responsible for generating the adsorption bond.

Adsorption is the process by which pollutant molecules become attached to a solid surface; as this occurs, the free energy of the molecules decreases. Consequently, adsorption is an exothermic process, and the heat released during this process is termed the "heat of adsorption" for that specific pollutant on that particular solid surface. Due to the differing forces involved in physisorption and chemisorption, these two processes exhibit distinct differences across various parameters, including heat of adsorption, adsorption rate, activation energy, optimal temperature range, selectivity, the number of adsorbed layers, and spectroscopic characteristics.

In China, activated carbon adsorption technology has a long history of application—spanning many years—in the purification and decolorization processes of industries such as pharmaceuticals, chemicals, and food manufacturing. Its application in the treatment of industrial wastewater began in the 1970s. Practical industrial experience has demonstrated that activated carbon possesses exceptional adsorption capabilities for trace organic pollutants present in water; it yields excellent adsorption results when applied to wastewater generated by industries such as textile printing and dyeing, dye manufacturing, food processing, and organic chemical production. Generally, activated carbon exhibits a unique capacity for removing organic substances found in wastewater—typically quantified via aggregate parameters such as BOD and COD—including synthetic dyes, surfactants, phenols, benzene derivatives, organochlorine compounds, pesticides, and petrochemical products. Consequently, activated carbon adsorption has gradually emerged as one of the primary methods employed in the secondary or tertiary treatment of industrial wastewater.

Adsorption is a gradual process in which one substance adheres to the surface of another. It is an interfacial phenomenon intrinsically linked to changes in surface tension and surface energy. The driving forces that induce adsorption are primarily of two types: the repulsive force exerted by the solvent (water) upon hydrophobic substances, and the attractive affinity exerted by the solid adsorbent upon the solute. In the context of wastewater treatment, adsorption typically results from the combined interplay of these two forces. The specific surface area and pore structure of activated carbon directly determine its adsorption capacity; therefore, when selecting an activated carbon product, the optimal choice should be determined empirically through laboratory testing based on the specific water quality characteristics of the wastewater in question. For the treatment of printing and dyeing wastewater, it is generally advisable to select a type of activated carbon characterized by a well-developed mesoporous (transition pore) structure. Furthermore, ash content also plays a role: the lower the ash content, the better the adsorption performance. Adsorption occurs more readily when the size of the adsorbate molecules closely matches the diameter of the carbon pores. The concentration of the adsorbate also influences the adsorption capacity of the activated carbon; within a certain concentration range, the adsorption capacity increases as the adsorbate concentration rises. Additionally, water temperature and pH value also have an impact, with the adsorption capacity decreasing as the water temperature increases.