What Are The Factors Affecting The Adsorption Performance Of VOCs？

There are various adsorption materials for VOCs, such as activated carbon, zeolite and so on. The adsorption and desorption performance of adsorbent materials on VOCs is affected by a variety of factors, the main influencing factors include the structural parameters of the adsorbent, the nature of the VOCs gas molecules and external conditions, etc., which are explained in detail as follows:

• Specific Surface Area And Pore Structure

Surface area provides a place for the adsorption process, increasing the probability of the adsorbent interacting with the VOCs, and a large specific surface area implies superior adsorption performance, and it is possible to increase the specific surface area of the adsorbent by opening the closed pores or forming new pores to increase the specific surface area of the adsorbent. Appropriate acid and alkali treatments can effectively expand the surface area of the material and improve its adsorption capacity, but excessive acid and alkali may also lead to pore destruction or collapse thus reducing the surface area. Activated carbon was used as the adsorbent to investigate the effects of specific surface area and pore structure on the adsorption capacity, and the results showed that the adsorption performance of activated carbon on 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 adsorbent materials, especially the pore size distribution, determines their adsorption capacity for VOCs, and it was found that the preparation conditions affect the specific surface area and pore characteristics. Different activation temperatures have certain influence laws on the physical structure of porous carbon materials, and the pore structure of carbon materials will go through the process from low to high and back to low as the activation temperature increases.

Generally speaking, micropores are the main site of adsorption, but increased diffusion resistance in narrow pores also leads to lower adsorption rate; mesopores enhance intra-particle diffusion and shorten the adsorption time. Therefore, the pore size of the adsorbent material determines the size of the VOCs molecules that can be adsorbed, and according to the size exclusion theory, VOCs molecules can enter the pores of the adsorbent material only when the pore diameter is larger than the molecular diameter of VOCs. Therefore, optimal adsorption occurs where the pore diameter matches the size of the adsorbent molecules, with micropores favouring the adsorption of small-volume VOCs, and large pores, such as mesopores, being more suitable for the adsorption of large-molecule VOCs. For the same type of VOCs, the larger the diameter of the molecules, the stronger the superposition of the pore walls between the adsorbents, the stronger the adsorption bonding energy, and the greater the adsorption capacity for VOCs.

The adsorption and desorption behaviours of different target substances such as n-hexane, toluene and ethyl acetate on adsorbents such as activated carbon, 5A, NaY, 13X, etc., were investigated by chromatography and thermogravimetry, and it was found that the magnitude of the force of physical adsorption was related to the pore size distribution of the adsorbent and the diameter of the molecules, and that when adsorbent molecules were close to the surface of the adsorbent, the Interaction between the solid surface and the molecules occurs; when the molecules are on two surfaces there is a superposition of potentials (e.g., slit pores), and the potentials are greater for cylindrical or spherical pores.

• Surface chemistry

In addition to the morphological structure, the type and number of chemical functional groups on the surface of the adsorbent material may also have an effect on the adsorption 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 related to both the nature of the raw materials and the activation or modification methods such as heating, chemical and electrochemical treatments, and different modification methods yield different surface chemical functional groups.

The heteroatoms of the surface functional groups determine the surface chemistry of the adsorbent, and the heteroatoms mainly include oxygen, nitrogen, halogens, hydrogen, etc., of which 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 oxygens, acidic, basic and neutral, and the acidic functional groups are -COOH, -OH, -C=O, -CO and -COO-, which are associated with oxidative phase related to the oxidative phase, with the carboxyl and hydroxyl groups showing strong electron uptake.

In general, liquid-phase oxidation contributes to the formation of carboxylic acids, while gas-phase oxidation promotes the formation of hydroxyl and carbonyl groups. Most of the oxygen groups are a source of surface acidity, which contributes to the adhesion of hydrophilic VOCs to carbon surfaces, and oxidation by acids and ozone is the most efficient method of introducing surface oxygen groups on carbon materials. The presence of oxygen groups may inhibit specific interactions between hydrophobic VOCs and π-electron rich regions on carbonaceous adsorbents. Therefore, hydrophobic VOCs prefer to adsorb on activated carbons without surface oxygen groups. Nitrogen-containing groups are usually introduced by the treatment of ammonia, nitric acid and nitrogen-containing compounds, which are predominantly alkaline.

• External detection environment

In addition to the influence brought by the adsorbent and adsorbent mass, 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. The HJ644-2013 standard for the adsorption of volatile organic compounds in ambient air tube sampling stipulates that the sampling temperature of adsorption tubes should not exceed 40 ℃; HJ734-2014 standard for fixed source pollution exhaust VOCs using adsorbent material is Tenax GR, Carbopack B, Carbopack C and Carboxen 1000 composite adsorbent material, adsorption sampling temperature of 0 ~ 5 ℃. To achieve the adsorption enrichment of some low-boiling VOCs components, the required temperature is even lower, such as C2 components of the sampling enrichment usually need to be -100 ℃ below the low-temperature, only in recent years to the development of the use of -76 ℃ low-temperature cold traps, to achieve the enrichment of C2 and C2 and above the enrichment of low-boiling hydrocarbons.

When the actual sampling scenario is a high humidity environment and there is a competition for adsorption between the target adsorbent and water on the adsorbent surface, the adsorption of water will have a great impact on the adsorption of VOCs. Therefore, water removal step is usually added before capture to reduce the impact of water vapour.