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Activated Carbon Adsorption

Adsorption is a phenomenon that when the fluid is in contact with the porous medium, a certain component or multiple components in the fluid will accumulate on the surface of the medium. It is also that the surface of a substance (mainly solid substance) adsorbs molecules or ions in the surrounding liquid, gas.

It can be divided into physical adsorption and chemical adsorption.

• Physical adsorption: The adsorption between the adsorbent and the adsorbate through intermolecular forces.
• Chemical adsorption: The chemical interaction between the adsorbent and the adsorbate is caused by the chemical bond force.

Commonly used adsorbents in wastewater treatment are activated carbon, sulfonated coal, active enzyme, zeolite, activated clay, diatomaceous earth, coke, wood charcoal, sawdust, etc.

The properties of activated carbon

Manufacture of activated carbon

Activated carbon is a hydrophobic adsorbent made from carbon-based materials (such as wood and coal) after high-temperature carbonization and activation. Carbonization is the pyrolysis of raw materials into carbon residue to produce polycyclic aromatic substances similar to graphite. Activation is to turn the pyrolyzed carbon residue into a porous structure.

The pore structure and distribution of activated carbon

Activated carbon consists of fine pores. According to the diameter of the pores, it can be divided into micropores ( < 2nm), mesopores (2-50 nm), macropores (>50 nm).

Specific surface area: the total surface area per unit of mass of adsorbent. The specific surface area of commonly-used activated carbon is 500-1700m2/g, of which 95% are micropores.

It is the micropores that make the activated carbon have the adsorption function, and the function of the macropores is to introduce the solution into the adsorption function zone of the micropores.

Chemical properties

Activated carbon is composed of flat graphite-type microcrystals. The carbon atoms at the edge of the microcrystals, due to the unsaturation of covalent bonds, are easy to combine with other elements such as oxygen and hydrogen to form various oxygen-containing functional groups, which make the activated carbon have some activity. The ones that have been confirmed include hydroxy groups (-OH), carboxyl groups (-COOH).

Adsorption

Equilibrium concentration

When the adsorption rate and the desorption rate are equal, that is, when the adsorbed quantity per unit time is equal to the desorbed quantity, the concentration of the adsorbate in the solution and that of the adsorbent surface will not change and reach equilibrium. At this time, the concentration of the adsorbate in the solution becomes the equilibrium concentration.

Adsorption isotherm

Under certain temperature conditions, the amount of adsorption increases with the increase of the equilibrium concentration of adsorbate. The curve in which the adsorption amount changes with the equilibrium concentration is called the adsorption isotherm. Due to the complexity of the liquid phase, there is no unified adsorption theory, so the gas phase adsorption isotherm is used.

Adsorption models

• Type I adsorption isotherm: Langmuir equation, Freundlich equation;
• Type II adsorption isotherm: Brunauer–Emmett–Teller (BET) theory.

Adsorption rate

Adsorption rate is the amount of material adsorbed by a unit weight of adsorbent in a unit time. The adsorption process can be divided into three stages: the membrane diffusion stage, the internal diffusion stage, and the adsorption reaction stage. Because the adsorption reaction stage is very fast, the adsorption rate is mainly controlled by the first two stages.

Influencing factors

(1) The property of the adsorbent: the larger the specific surface area of the adsorbent, the stronger the adsorption capacity.

(2) The properties of adsorbate

• Solubility: The lower the solubility, the easier it is to be adsorbed.
• Surface free energy: The more the surface free energy decreases, the easier it is to be adsorbed.
• Polarity.
• The size and degree of unsaturation of adsorbed molecules.
• Concentration of adsorbate.

(3) pH value of wastewater: Activated carbon generally has a higher adsorption rate in acidic solutions than in alkaline solutions. In addition, pH has an effect on the existence and solubility of the adsorbate, thereby affecting the adsorption effect.

(4) Coexisting substances

(5) Temperature: The adsorption process is an exothermic process, the amount of adsorption decreases when the temperature rises, and vice versa. The temperature has a greater impact on the gas phase, less on the liquid phase.

(6) Contact time: It should be ensured that the adsorbate and the adsorbent have a certain contact time to make the adsorption close to equilibrium.

Adsorption operation

Static: Under the condition that the wastewater does not flow, the adsorption operation is carried out.

Dynamic: The adsorption operation under the condition of flowing wastewater.

Dynamic equipment commonly used in wastewater treatment includes fixed bed, moving bed and fluidized bed.

Fixed bed

When the wastewater continuously passes through the adsorption equipment filled with adsorbent, the adsorbate in the wastewater is adsorbed by the adsorbent. If the amount of adsorbent is sufficient, the concentration of adsorbate in the wastewater flowing from the adsorption equipment can be reduced to zero. After the adsorbent has been used for a period of time, the concentration of adsorbate in the effluent gradually increases. When it reaches a certain value, the water should be stopped and the adsorbent should be regenerated. The adsorption and regeneration can be performed alternately in the same equipment, or the saturated adsorbent can be discharged and sent to the regeneration equipment.

Moving bed

The raw water flows from the bottom of the adsorption tower into countercurrent contact with the adsorbent, the treated water flows out from the top of the tower, the regenerated adsorbent is added from the top of the tower, and the adsorbent close to adsorption saturation is intermittently removed from the bottom of the tower.

Activated carbon adsorption used in wastewater treatment

The adsorption of organic matter by activated carbon

The active adsorption method is mostly used to remove small amounts of organic matter in a dissolved state that cannot be removed by biological, physical, or chemical methods. However, some organic matter is easy to adsorb, and some organic matter is difficult to adsorb. Whether the activated carbon adsorption method can be used should be determined by adsorption test.

Factors that influence activated carbon adsorption

• Molecular structure — Aromatic compounds are easier to adsorb than general aliphatics.
• Interfacial tension — The more the liquid interfacial tension is reduced, the easier it is to be adsorbed.
• Solubility — Activated carbon is a hydrophobic substance, so the stronger the hydrophobicity of the adsorbed substance, the easier it is to be adsorbed.
• Ionicity and polarity – Some of the organic acids and amines are weakly acidic or weakly alkaline after being dissolved in water. This kind of organic matter has a larger adsorption capacity when it is in a non-dissociated molecular state than when it is ionized.
• Molecular weight — the larger the molecular weight, the stronger the adsorption.
• pH — Reducing the pH value of the wastewater to 2-3, and then performing adsorption, usually can increase the removal rate of organic matter, because the proportion of organic acids in the wastewater to form ions is small, so the adsorption capacity is large.
• Concentration — Generally, as the concentration of organic matter increases, the amount of adsorption increases exponentially.
• Coexisting substances — some metal ions such as mercury, chromic acid, iron, etc. will undergo redox reactions on the surface of activated carbon, and the products will precipitate in the particles and hinder adsorption.