The adsorption process and action principle involved in activated carbon water treatment are complicated, so there are many influencing factors. The factors are 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.
1.The nature of activated carbon
Since the adsorption phenomenon occurs on the surface of the adsorbent, the specific surface area is one of the important factors affecting the adsorption. The larger the specific surface area, the better the adsorption performance.
The adsorption process can be seen as three stages, the internal diffusion has a greater impact on the adsorption rate, so the microporous distribution of activated carbon is another important factor affecting adsorption.
In addition, the surface chemistry, polarity and charging of activated carbon also affect the adsorption effect.
Activated carbon for water treatment has three requirements: large adsorption capacity, fast adsorption speed, and good mechanical strength. The adsorption capacity of activated carbon is attached to other external conditions, mainly related to the specific surface area, the larger specific surface area, the larger number of micropores, more the adsorbent matter can be adsorbed on the fine pore wall. The adsorption rate is mainly related to the particle size and pore distribution, activated carbon for water treatment, requiring more developed transition pores (radius 20 ~ 1000A), which is conducive to the diffusion of adsorbent matter into micropores. The smaller the particle size, the faster the adsorption rate, but the head loss will increase, generally in the range of 8 to 30 mesh is appropriate. The mechanical wear strength of activated carbon will directly affect the service life.
2. Adsorbent (solute or pollutant) properties
The adsorption capacity of the same activated carbon for different pollutants is differ a lot.
(1). Solubility
The solubility of the same group of substances decreases with the increase of chain length, while the adsorption capacity increases with the increase of series or molecular weight of the homologue. The smaller the solubility, the more easily adsorbed.
Such as the order of activated carbon adsorption organic acid from water increases according to formic acid - acetic acid - propionic acid - butyric.
(2). The molecular structure
The size and chemical structure of adsorbent molecule also have great influence on adsorption. Because the adsorption speed is affected by the internal diffusion rate, the size of the adsorbent (solute) molecule is proportional to the size of the pore size of activated carbon, which is the most conducive to adsorption. In homologues, it’s easier to be adsorbed for larger molecules than smaller ones. Organic compounds with unsaturated bonds are more easily adsorbed than those with saturated bonds. Aromatic organic matter is easier to be adsorbed than aliphatic organic matter.
(3). Polarity
Activated carbon can be regarded as a non-polar adsorbent, the adsorption capacity of non-polar substances in water is greater than that of polar substances.
(4). Adsorbent (solute)
When the concentration of adsorbent is in a certain range, the adsorption capacity increases with the increase of concentration. Therefore, the concentration of adsorbent (solute) changes, activated carbon to the adsorbent (solute) adsorption capacity also changes.
3.The influence of solution pH
The influence of solution pH value on adsorption should be considered together with the influence of activated carbon and adsorbent (solute). The solution pH value controls the degree of dissociation of acidic or basic compounds. When the pH value reaches a certain range, these compounds will dissociate, affecting the adsorption of these compounds. The pH value of the solution also affects the solubility of the adsorbent (solute) and the charge of the adsorbent (solute) of the colloid. Because activated carbon can absorb hydrogen and oxygen ions in water, it affects the adsorption of other ions.
The effect of activated carbon adsorption of organic pollutants from water, generally decreased with the increase of the pH value of the solution, pH value is higher than 9.0, it is not easy to adsorb, the lower the pH value, the better the effect. In practical application, the optimum pH value range is determined by experiment.
4.The influence of solution temperature
Because the heat of adsorption is small during liquid phase adsorption, the effect of solution temperature is small. Adsorption is an exothermic reaction. Adsorption heat is the total heat released by activated carbon when adsorption unit weight of adsorbent (solute). The greater the adsorption heat, the greater the influence of temperature on adsorption. On the other hand, temperature has an effect on the solubility of substances and therefore on adsorption. When treating water with activated carbon, temperature has no significant effect on adsorption.
5.Influence of coexistence of multi-component adsorbents
When water is treated by adsorption, it is usually not a single contaminant, but a mixture of pollutants. During adsorption, they can co-adsorb, promote each other or interfere with each other. In general, the adsorption capacity of multiple components is lower than that of single component.
6.Conditions of adsorption operating
When activated carbon adsorption liquid, diffusion (liquid film diffusion) speed has an impact on adsorption, so the type of adsorption device, contact time (water speed) and so on have an impact on the adsorption effect.
To sum up, there are many factors affecting adsorption, which should be comprehensively analyzed and selected according to the specific situation to achieve the best adsorption effect.
