Does pH affect the adsorption effect of activated carbon?

This article mainly analyzes and summarizes the effect of pH value on the adsorption capacity of activated carbon. It also explains that in addition to pH value, the characteristics of the adsorbent, coexisting substances, temperature, and adsorption time will also have an impact on the adsorption performance of activated carbon.

 

Activated carbon is an adsorption material widely used in water treatment, waste gas treatment and other fields. Its high adsorption capacity comes from its unique porous structure and surface chemical properties. However, the adsorption effect of activated carbon is affected by many factors, among which pH value is particularly critical. The removal effect of activated carbon on organic matter and impurities in the solution is different under different pH conditions. In acidic solutions, the adsorption rate of activated carbon is higher, while in alkaline solutions, the adsorption rate decreases.

Adsorption principle of activated carbon

To deeply understand the effect of pH on the adsorption of activated carbon, we must first understand the adsorption principle of activated carbon. There are two main types of adsorption of activated carbon: physical adsorption and chemical adsorption.

Physical adsorption: Activated carbon has a highly developed pore structure, which provides a large surface area for adsorption. When impurity molecules in liquid or gas come into contact with the surface of activated carbon, these impurity molecules are adsorbed in the pores of activated carbon due to the van der Waals force between molecules. Physical adsorption is the main mechanism for removing impurities in liquid and gas phases.

Chemical adsorption: In addition to physical adsorption, the surface of activated carbon also has some chemically bonded oxygen and hydrogen groups, such as carboxyl, hydroxyl, phenols, etc. These functional groups can react chemically with the adsorbate, thereby enhancing the adsorption effect. Chemical adsorption is usually used in special application scenarios, such as removing heavy metal ions or toxic compounds in water.

The specific effect of pH on the adsorption effect of activated carbon

The effect of pH on the adsorption effect of activated carbon is mainly reflected in the solubility, existence state and charge characteristics of the adsorbate.  

1.Adsorption effect under acidic conditions Under acidic conditions

The adsorption effect of activated carbon is usually better, especially when the pH value is between 3 and 5. This is mainly because:

Changes in solubility and ionization state: In a low pH environment, the proportion of organic acids in organic matter is smaller, which means that they are more easily adsorbed by activated carbon. This low ionization state can increase the affinity of molecules with the surface of activated carbon, thereby improving adsorption efficiency.

Changes in surface charge: Under acidic conditions, activated carbon has less surface charge and lower polarity, which is more conducive to adsorbing non-polar or weakly polar molecules.

Application examples: In the field of water treatment, adjusting the pH value of wastewater to 3-5 and then adsorbing activated carbon can significantly improve the removal rate of organic matter. This is widely used in the treatment of some highly polluted water sources.

2. Compared with acidic conditions, the adsorption effect under alkaline conditions is poorer, especially when the pH value is greater than 9.

This is mainly because:

Charge repulsion: Under alkaline conditions, the surface of activated carbon is negatively charged, which creates a repulsive force with negatively charged organic matter or other negatively charged adsorbates, resulting in a reduction in adsorption efficiency.

Peptization effect: In some cases, the alkaline environment will cause the organic matter adsorbed on the surface of several activated carbons to "peptize", that is, the adsorbed molecules will be redissolved into the solution, or even "deadsorption" will occur, thus increasing the impurities in the solution.

Application examples: In water treatment, the pH value of alkaline solutions often needs to be adjusted to neutral or acidic in order to effectively utilize the adsorption function of activated carbon. Otherwise, the adsorption effect may be unsatisfactory or even counterproductive.  

Comprehensive factors affecting the adsorption effect of activated carbon

In addition to acidic and alkaline conditions, the effect of pH on the adsorption effect of activated carbon is also affected by other factors such as the properties of the adsorbate, coexisting substances and temperature.

1. Properties of the adsorbate The polarity, solubility, molecular size and structure of the adsorbate will affect the adsorption efficiency. For example, activated carbon is more likely to adsorb non-polar or low-polar organic molecules. The lower the solubility of the adsorbate, the easier it is to be adsorbed by activated carbon. The larger the adsorbate molecule, the larger the contact area with the surface of the activated carbon, thereby improving the adsorption efficiency.

2. The influence of coexisting substances In practical applications, aqueous solutions often contain multiple adsorbates. When multiple adsorbates exist at the same time, the adsorption effect of activated carbon on a certain adsorbate may be inhibited. For example, when a large number of anions and cations exist in water at the same time, these ions will compete with the target adsorbate for adsorption sites, resulting in a decrease in the removal efficiency of the target. Therefore, in the case of the coexistence of multiple pollutants, it is recommended to separate and pretreat the wastewater to reduce interference with the adsorption effect of activated carbon.

3. The effect of temperature The effect of temperature on the adsorption effect of activated carbon is relatively small, but it still needs to be considered in some special cases. For example, at high temperatures, the pore structure of activated carbon may change slightly, thus affecting its adsorption performance. Generally, moderate temperatures (15-30°C) are more favorable for most adsorption processes.

4. The effect of contact time The adsorption process requires sufficient contact time to reach adsorption equilibrium. If the contact time is too short, the adsorption is not completed, resulting in poor results. In practical applications, it should be ensured that the activated carbon has sufficient contact time with the adsorbate to give full play to its adsorption capacity. According to experience, the usual contact time is 30 minutes to 1 hour.

PH adjustment method to improve adsorption effect

In order to improve the adsorption effect of activated carbon, especially in the field of water treatment, it is a common method to appropriately adjust the pH value of water. Acid treatment method: When treating some organic matter that is more difficult to remove, the pH value of water can be adjusted to 3-5. The acidic environment reduces the ionization of the adsorbate, making it easier to be adsorbed by activated carbon. This method is usually used in the purification process of highly polluted water sources such as pharmaceutical wastewater and industrial wastewater.

Pretreatment method: Before the wastewater enters the activated carbon adsorption device, the acid-base adjustment method is used to adjust the pH value to neutral or slightly acidic to create a favorable adsorption environment and improve the use efficiency of activated carbon.