Brief Analysis of Liquid Phase Adsorption Characteristics of Activated Carbon
Since the adsorption process and the principle of action involved in the treatment of activated carbon water are complicated, there are many influencing factors. It is mainly related to the nature of activated carbon, the nature of pollutants in water, the process principle of activated carbon treatment, and the selected operating parameters and operating conditions.
First, the nature of activated carbon
Since the adsorption phenomenon occurs on the surface of the adsorbent, the specific surface area of ​​the adsorbent is one of the important factors affecting the adsorption. The larger the specific surface area, the better the adsorption performance. Because the adsorption process can be regarded as three stages, the internal diffusion has a great influence on the adsorption speed, so the micropore distribution of activated carbon is another important factor affecting the adsorption.
Activated carbon for water treatment should have three requirements: large adsorption capacity, fast adsorption speed, and good mechanical strength. The adsorption capacity of activated carbon is related to the specific surface area of ​​activated carbon, and the specific surface area is large, the number of micropores is large, and the adsorbate adsorbed on the pore wall is more. The adsorption rate is mainly related to the particle size and pore distribution. The activated carbon for water treatment requires a transition hole (radius 20~1000A) to be developed, which is beneficial to the diffusion of the adsorption mass into the micropores. The smaller the particle size of activated carbon, the faster the adsorption speed, but the head loss should be increased, generally in the range of 8~30 mesh. The mechanical wear resistance of activated carbon directly affects the service life of activated carbon.
Second, the nature of the adsorbate (solute or pollutant)
The same activated carbon has great differences in the adsorption capacity of different pollutants.
(1) Solubility
The solubility of a substance of the same group decreases as the chain lengthens, and the adsorption capacity increases as the series of homologues rises or the molecular weight increases. The smaller the solubility, the easier it is to adsorb. For example, the order in which activated carbon adsorbs organic acids from water is increased by formic acid-acetic acid-propionic acid-butyric acid.
(II) Molecular Structure The size and chemical structure of the adsorbate molecules also have a large impact on adsorption. Because the adsorption rate is affected by the internal diffusion rate, the size of the adsorbate (solute) molecule is proportional to the pore size of the activated carbon, which is most favorable for adsorption. In homologues, molecules larger than molecules are easily adsorbed. The organic matter of the unsaturated bond is more soluble than the saturated one. Aromatic organics are easier to adsorb than aliphatic organics. (3) Polar activated carbon can be regarded as a non-polar adsorbent, and its adsorption capacity for non-polar substances in water is greater than that of polar substances. (4) When the concentration of the adsorption (solute) adsorbate is within a certain range, the adsorption capacity increases as the concentration increases. Therefore, the concentration of the adsorbate (solute) changes, and the adsorption capacity of the activated carbon for the adsorbate (solute) also changes.
Third, the effect of solution pH
The effect of pH on the adsorption should be considered in combination with the effects of activated carbon and adsorbate (solute). The pH of the solution controls the degree of dissociation of acidic or basic compounds. When the pH reaches a certain range, these compounds dissociate and affect the adsorption of these compounds.
The pH of the solution also affects the solubility of the adsorbate (solute) and the charge of the adsorbate (solute) of the colloid. Since activated carbon can adsorb hydrogen and oxygen ions in water, it affects the adsorption of other ions. The effect of activated carbon adsorbing organic pollutants from water generally decreases with the increase of pH value of the solution. When the pH value is higher than 9.0, it is not easy to adsorb, and the lower the pH value, the better the effect. In practical applications, the optimum pH range is determined experimentally.
The adsorption of activated carbon is divided into two types: liquid phase adsorption and gas phase adsorption. The liquid phase adsorption capacity is often evaluated by adsorption isotherm, and the gas phase adsorption capacity is evaluated by the solvent vapor adsorption amount. The following experts from Jiangsu Activated Carbon Factory will give you a detailed introduction:
The adsorption isotherm indicates the relationship between the partial pressure or concentration of the adsorbed material in the adsorption system and the adsorption amount at a certain temperature, that is, when the temperature is kept constant, the relationship between the equilibrium adsorption amount and the partial pressure or concentration can be measured. Taking the remaining concentration as the horizontal axis and the adsorption amount of the single mass of activated carbon as the vertical axis, the relationship curve can be drawn.
When the partial pressure or concentration is kept constant, the relationship between the equilibrium adsorption amount and the temperature can be measured, and the relationship curve, that is, the adsorption isobar line, is drawn. Since adsorption of a small amount of components in an industrial device is carried out substantially in an isothermal state, adsorption isotherms are most important and commonly used.
The solvent vapor adsorption amount indicates the gas phase adsorption performance, and the measurement of the carbon tetrachloride adsorption rate of the granular activated carbon can be exemplified, under the specified test conditions, that is, the specified carbon layer height, the gas flow rate, the adsorption temperature, the measured tube cross-sectional area, Under the condition of carbon tetrachloride vapor concentration, the mixed air stream containing a certain concentration of carbon tetrachloride vapor continuously passes through the activated carbon. When the adsorption saturation is reached, the mass of carbon tetrachloride adsorbed by the activated carbon sample and the sample quality The percentage is taken as the adsorption rate of carbon tetrachloride.
For the adsorption capacity in activated carbon applications, it is best to use the actual activated carbon, operating conditions, and specific treatments for evaluation.
Fourth, the effect of solution temperature
Since the adsorption heat is small when the liquid phase is adsorbed, the influence of the solution temperature is small.
Adsorption is an exothermic reaction. The heat of adsorption, that is, the total heat released by the activated carbon adsorption unit weight of the adsorbate (solute), in KJ/mol. The greater the heat of adsorption, the greater the effect of temperature on adsorption. On the other hand, temperature has an effect on the solubility of the substance and therefore also on the adsorption. When water is treated with activated carbon, the effect of temperature on adsorption is not significant.
5. The influence of coexistence of multi-component adsorbate
When water is applied by adsorption, usually water is not a single pollutant, but a mixture of multi-component pollutants. When adsorbed, they can co-adsorb, promote each other or interfere with each other. In general, the adsorption capacity of the multi-component adsorption is lower than that of the single-component adsorption.
Sixth, adsorption operating conditions
Since the diffusion rate of the external diffusion (liquid film diffusion) affects the adsorption during the liquid phase adsorption of the activated carbon, the type of the adsorption device and the contact time (water passing rate) have an effect on the adsorption effect.
In summary, there are many factors affecting adsorption. Comprehensive analysis should be carried out. According to the specific conditions, the best adsorption conditions should be selected to achieve the best adsorption effect.
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