VOCs adsorption technology is widely used at present, and adsorption materials are also diverse, such as activated carbon, resin, silica gel, alumina balls, zeolite and so on.The adsorption performance of the adsorbent on VOCs is affected by many factors, including the structural parameters of the adsorbent, the properties of the VOCs gas molecules and external conditions.Today, let’s find out the details~
First、Specific surface area and pore structure
The surface area of the adsorbent provides a place for the adsorption process, increasing the probability of interaction between the adsorbent and VOCs. The large specific surface area means that the adsorption performance is superior, and the specific surface area of the adsorbent is generally increased by opening a closed hole or forming a new hole.
Proper acid and alkali treatment can effectively expand the surface area of the material and improve its adsorption capacity, but excessive acid and alkali may also lead to hole damage or collapse and thus reduce the surface area.The study found that activated carbon with the highest specific surface area did not always exhibit the best adsorption capacity for organic compounds, which proves that the adsorption of VOCs by materials is affected by multiple factors.Using activated carbon as adsorbent, the effects of specific surface area and pore structure on the adsorption capacity were investigated. The results showed that the adsorption of VOCs was mainly controlled by pore diffusion, and the smaller the filling density of the adsorbent, the easier it was to penetrate.
The microstructure of carbon-based adsorption materials, especially the pore size distribution, determines its adsorption ability to VOCs. It is found that the preparation materials and conditions will affect its specific surface area and pore characteristics. The effect of activation temperature on the physical structure of porous carbon materials is found to be significant. With the increase of activation temperature, the pore structure of carbon materials goes from low to high and then to low.
Generally speaking, micropores are the main part of adsorption, but the increase of diffusion resistance in narrow pores will also lead to a low adsorption rate. The mesopore enhanced the diffusion in the particle and shortened the adsorption time. Therefore, the pore size of the adsorbed material determines the size of the VOCs molecules that can be adsorbed. According to the size exclusion theory, only when the pore diameter is greater than the molecular diameter of the VOCs, the VOCs molecules can enter the pores of the adsorbed material. Therefore, the optimal adsorption occurs in the place where the pore size matches the molecular size of the adsorbent, micropores are conducive to the adsorption of small volume VOCs, and large pores such as mesopore are more suitable for the adsorption of large molecules VOCs. For the same type of VOCs, the larger the diameter of molecules, the stronger the superposition of pore walls between adsorbents; The stronger the adsorption bond energy, the greater the adsorption capacity of VOCs, and the larger VOCs molecules showed lower adsorption capacity.
The adsorption and desorption behaviors of n-hexane, toluene, ethyl acetate and other target substances on activated carbon, 5A, NaY, 13X and other adsorbents were investigated by chromatography and thermogravimetric method. It was found that the physical adsorption force was related to the pore size distribution and molecular diameter of adsorbents. When the adsorbent molecule is close to the adsorbent surface, the solid surface and the molecule interact, and when the molecule is at the two surfaces, the potential energy superposition (such as a slit hole) will be generated, and the potential energy of the cylindrical or spherical hole will be larger. The pore shape of activated carbon is mainly crack pore, molecular sieve has multi-dimensional pore system, and the pore shape is more complex.
Second、Surface chemical properties
In addition to the morphology and structure, the chemical functional groups on the surface of the adsorbent also play a role in the adsorption of VOCs, and its type and quantity have a great impact on the adsorption capacity of VOCs, and the surface chemical modification of the adsorbent can change its adsorption capacity and selectivity for VOCs. For example, the surface functional groups of carbonaceous adsorbents are not only related to the properties of raw materials, but also related to activation or modification methods such as heating, chemical and electrochemical treatment, and the surface chemical functional groups obtained by different modification methods are different.
The surface functional group of heteroatoms determines the surface chemical properties of adsorbent. Heteroatoms mainly include oxygen, nitrogen, halogens, hydrogen, etc. Among them, the oxygen and nitrogen groups on porous carbon are considered to be the most important species in the adsorption process. There are three different types of oxygen groups, which are acidic, basic and neutral. The acidic functional groups are – COOH, – OH, – C=O, – CO and – COO -, which are related to the oxidation phase, in which the carboxyl group and the hydroxyl group show strong electron absorption capacity.
In general, liquid phase oxidation contributes to the formation of carboxylic acids, while gaseous phase oxidation promotes the formation of hydroxyl and carbonyl groups.Most oxygen groups are a source of surface acidity that helps hydrophilic VOCs adhere to carbon surfaces, and oxidation by acids and ozone is the most effective way to introduce surface oxygen groups on carbon materials.
The chemical state of transition metals loaded on activated carbon also plays an important role in improving the adsorption capacity of VOCs at low temperatures. At present, the metal modified activated carbon technology is mainly used to treat small relative molecular mass pollutants such as formaldehyde and toluene, and the application of some large relative molecular mass VOCs needs to be further studied.
Third、External detection environment
In addition to the influence of adsorbents and adsorbents, external conditions such as temperature and humidity also have a certain impact on the adsorption performance of VOCs, and the influence of temperature on the adsorption of VOCs is more obvious. According to HJ 644-2013 standard, the sampling temperature of adsorption tube for volatile organic compounds in ambient air should not exceed 40 ℃. The adsorption materials used in HJ 734-2014 standard for VOCs from fixed source pollution are composite adsorption materials composed of Tenax GR, Carbopack B, Carbopack C and Carboxen 1000, and the adsorption sampling temperature is 0 ~ 5 ℃. In order to achieve adsorption and enrichment of some low-boiling VOCs components, the temperature required is lower, such as the sampling and enrichment of C2 components usually requires a low temperature below -100 ℃, and in recent years, it has been developed to use a low temperature cold trap of -76 ℃ to achieve the enrichment of C2 and low boiling point hydrocarbons above C2.
When the actual sampling scene is a high humidity environment and there is competitive adsorption between the target adsorbent and water on the adsorbent surface, the adsorption of water will have a huge impact on the adsorption of VOCs. Therefore, a water removal step is often added before capture to reduce the effect of water vapor. In addition, many researchers improve the hydrophobicity of the adsorption material by modifying it to make it more suitable for the adsorption of VOCs in a high humidity environment.
