The adsorption of solute molecules from water by powdered activated carbon is a complex process, resulting from the combined action of several forces. According to the dual-rate diffusion theory of adsorption, the process consists of two stages: rapid diffusion and slow diffusion. Rapid diffusion is completed within a few hours, utilizing 60%-80% of the activated carbon's adsorption capacity. Rapid diffusion is the process by which solute molecules diffuse through the large pores within the carbon particles, where the resistance is low and the molecules are evenly distributed radially. These large pores generate radial diffusion resistance. When molecules further diffuse from the large pores into the interconnected micropores, they encounter significant resistance due to the narrow pore size, resulting in extremely slow diffusion. Micropores are also evenly distributed within the carbon particles but do not constitute radial diffusion resistance. Factors affecting the adsorption of powdered activated carbon include the polarity, molecular weight, and spatial structure of the solute molecules, which depend on the characteristics of the water source. Activated carbon exhibits selective adsorption for different types of molecules.
(I) Selection of Powdered Activated Carbon Application Process
Foreign experts have analyzed and studied the application of powdered activated carbon, concluding that the adsorption and removal of synthetic chemicals by powdered activated carbon mainly depends on the type of compound. When selecting the application point, the degree of mixing and treatment contact time must be considered, and interference from water treatment chemicals should be minimized. Based on the practical experience of a domestic water plant using powdered activated carbon, it is believed that for water sources polluted by domestic and industrial wastewater, leading to eutrophication and rapid proliferation of algae and other microorganisms, the pollution is relatively severe and complex. During the dry season, the water often emits complex odors and tastes. Furthermore, the water intake section is a tidal river, resulting in a long wastewater retention time and significant pollution damage. When selecting the powdered activated carbon application process, the following factors should be considered:
(1) The application point should have sufficient mixing conditions to ensure rapid and thorough mixing of the powdered activated carbon with the treated water.
(2) Extend the contact and adsorption time between the powdered activated carbon and the water as much as possible to fully utilize the adsorption capacity of the powdered activated carbon and improve the adsorption rate.
(3) Select powdered activated carbon with smaller particle size to maximize the adsorption surface area for the same weight of activated carbon; select wood-based activated carbon with well-developed mesopores to improve the adsorption efficiency of activated carbon for organic matter.
(4) Minimize interference from chemical reagents in the water treatment process, such as chlorine, potassium permanganate, and coagulants.
(5) Select dry or wet application methods based on the amount of activated carbon to be added and site conditions.
(6) Based on the water quality... The dosage of powdered activated carbon is determined based on the water pollution status, with dosages ranging from 5-30 mg/L. The process of adding powdered activated carbon at a certain water treatment plant is as follows:
(II) Adding powdered activated carbon significantly improves effluent water quality
(1) Adding powdered activated carbon has a significant effect on removing color. Color removal has been reported to reach 70%. Low color indicates high efficiency in removing organic matter and good removal of iron and manganese. However, the effect of color removal is not directly proportional to the amount of activated carbon added. The complex mechanism requires further research.
(2) Adding powdered activated carbon has a significant effect on removing odor and taste. Eutrophic water bodies in some areas face not only odors produced by algae reproduction and killing processes, but also complex industrial pollution. The abnormal concentration of phenolic substances in the water body over a long period causes odors. Due to the dynamic and uncertain nature of odor-causing substances, quantitative analysis of odor becomes a very difficult task. It is envisioned that only after years of investigation and research on specific water bodies, establishing relevant mathematical models and corresponding analytical methods, can the detection of odor be gradually solved. Odor is generally identified by human senses, which introduces significant human error. Odor removal is an important comprehensive evaluation index for the removal of pollutants by powdered activated carbon, and it is also an extremely important and challenging sensory indicator for the water supply industry to ensure the safety of drinking water.
(3) Adding activated carbon helps remove anionic detergents. The chemical industry at home and abroad has long utilized powdered activated carbon to purify and remove detergents from industrial wastewater. This is an evaluation index for the removal of large-molecule synthetic organic compounds by powdered activated carbon.
