Removal of PAHs in Tea Seed Oils by Food Grade Activated Carbon
Polycyclic aromatic hydrocarbons (PAHs) refer to a class of hydrocarbons containing two or more benzene rings. They are carcinogenic, and during the processing of tea seed oils, represented by benzo[a]pyrene (BaP), may be produced and become a food safety issue. The Priority Pollutant List of EPA includes 129 pollutants, of which 16 are PAHs.
Other tests showed that there are at least the following seven PAHs: acenaphthene, pyrene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene, exist to varying degrees in the tea seed oils prepared by expeller pressing and solvent extraction.
Methods currently used to remove PAHs in tea seed oils can be categorized into chemical methods, physical methods, and biochemical methods. Considering the safety and cost efficiency of oil processing, physical methods are still the priority solution. Activated carbon is a hydrophobic adsorbent made of carbon-based raw materials. It contains a large number of pores with different sizes, a large specific surface area, and high activation energy, and can effectively remove organic pollutants. In addition, the form activated carbon used in the production of tree seed oils is a food processing aid, so any non-food-grade activated carbon is prohibited from being used in the production. Based on the other tests on the effects of different processes on the residual form and content of PAHs in tea seed oils, this test takes food-grade activated carbon samples with different structures as the PAHs adsorbent in cold-pressed tea seed oils.
Adsorption isotherm and structure of activated carbon
IUPAC has classified the pores into 1. microporous (<2 nm); 2. mesoporous (2-50 nm); and 3. macroporous (>50 nm).
The characteristics of the four activated carbon samples are as follows:
- A: Among the four samples, it has the largest specific surface area, a relatively large pore volume, relatively few meso- and macro-pores, the proportion of micropores is high.
- B: It has the smallest specific surface area, a pore volume slightly larger than that of A, more meso- and macro-pores, fewer micropores. So its adsorption capacity is weaker than that of A.
- C: A specific surface area between A and B, a small pore volume, more meso- and macro-pores, fewer micropores.
- D: Specific surface area and total pore volume slightly higher than those of C, more meso- and macro-pores, fewer micropores.
The ranking of adsorption capacities for PAHs in tea seed oils from high to low is sample A, B, D, C.
Among the four tested activated carbon samples, sample A has a relatively strong ability to adsorb PAHs in tea seed oil, indicating that the PAHs adsorption mainly depends on the micropores of activated carbon. And the adsorption also requires a certain amount and proportion of mesopores and macropores, which provides channels for solutes to be better adsorbed.
According to the four isotherm adsorption and desorption curves, samples A and B with higher N2 adsorption and desorption have higher adsorption rates of PAHs; while samples C and D with lower N2 adsorption and desorption have relatively lower adsorption rates. This indicates that the adsorption of PAHs in tea seed oils by activated carbon conforms to the pattern of BET adsorption isotherm.
PAHs adsorption stability
From the test, almost no PAHs are eluted, indicating that activated carbon has excellent adsorption stability to PAHs. And it also shows that this adsorption is an irreversible process.