01 Properties of activated carbon
Activated carbon is a kind of carbon that has been specially treated. Its surface has countless small pores, and the diameter of the pores is generally between 2 and 50nm. Therefore, activated carbon has a large surface area. The surface area of 1 gram of activated carbon can reach 500m2, and some activated carbon can even reach 1500m2. At present, almost all the applications of activated carbon are based on this feature. The main way for activated carbon to adsorb is physical adsorption, and the smaller the particles of activated carbon, the faster the diffusion rate of its pores, and the stronger the adsorption capacity of the activated carbon.
Activated carbon is dark black in appearance, has good adsorption performance, stable chemical properties, can withstand strong acid and strong alkali, can withstand water immersion and high temperature, and has a lower density than water. It is a porous hydrophobic adsorbent.
02 Mechanism of activated carbon
The main reason for the adsorption of activated carbon is that the atomic force field on the solid surface is unsaturated and has surface energy, so it can adsorb certain molecules to reduce the surface energy. After a solid adsorbs solute molecules from a solution, the concentration of the solution will decrease and the adsorbed molecules will concentrate on the solid surface.
The adsorption characteristics of activated carbon are not only related to the pore structure and distribution, but also related to the surface chemical properties of activated carbon. Activated carbon itself is non-polar, and its content and charge vary with raw material composition and activation conditions. Carbon activated at low temperature (< 500°C) can generate surface acidic oxides, and release H+ after hydrolysis. Due to the weak polarity on the surface of activated carbon, other polar solutes compete for the active position on the surface of activated carbon, resulting in a decrease in the adsorption capacity of non-polar solutes, and the exchange adsorption or complexation reaction of some metal ions in water improves the adsorption effect of activated carbon on metal ions. .
In short, in the adsorption process, it is the microporous structure that really determines the adsorption capacity. The entire specific surface is almost composed of micropores. Coarse pores and transition pores act as coarse and fine adsorption channels respectively, and their existence and distribution affect the adsorption and desorption rates to a considerable extent. In addition, the adsorption properties of activated carbon are also affected by the chemical properties of the activated carbon surface.
03 The role of activated carbon in water treatment
1. Reduce COD
Activated carbon has a good adsorption effect for the removal of organic substances that are difficult to biodegrade and remove in sewage, such as aromatic hydrocarbons, chlorine-containing/toxic phenols, etc.
COD is a key indicator of sewage discharge. Using activated carbon adsorption to remove residual organic matter in sewage has become one of the effective treatment technologies and has been widely used in urban sewage and industrial wastewater treatment processes.
2. Reduce the chroma
Such as printing and dyeing wastewater, coking wastewater, pharmaceutical factory wastewater, etc., usually have the problem of high coloration. Chromaticity is difficult to remove in the primary and secondary treatment of conventional sewage,
With its rich pore structure, activated carbon has great advantages in removing color, and has a significant adsorption effect on color substances in sewage.
3. Water purification (Shangshui)
Urban tap water is treated with activated carbon, which can effectively remove organic impurities, disinfection by-products THM, and PFAS. It also has a good adsorption effect on residual chlorine in tap water.
When activated carbon is treated, it will neither bring in additional impurities, but also retain a certain amount of trace elements. It is an effective means of treating water sources.
Activated carbon is also commonly used in industrial water purification. When the purity of process water is high, activated carbon is usually used in combination with permeable membranes and microporous membranes to prepare purified water.
4. Others
Activated carbon can have a certain chemical adsorption effect on heavy metals, and the presence of oxygen-containing functional groups can also change the chemical properties of some heavy metals. For example, when activated carbon treats electroplating chromium-containing wastewater, it can be reduced to less toxic trivalent chromium ions. Activated carbon also has a certain adsorption effect on low concentrations of mercury in sewage.