A new approach to neutralize the toxic effect of polycyclic aromatic hydrocarbons on the example of 9,10-dimethylanthracene: a kinetic aspect

. The rate constants of the Diels-Alder reaction of 9,10-dimethylanthracene with tetracyanoethylene, 4-phenyl-1,2,4-triazoline-3,5-dione, maleic anhydride, and N-phenylmaleimide in solution have been determined. It was found that tetracyanoethylene and 4-phenyl-1,2,4-triazoline-3,5-dione can react instantly with 9,10-dimethylanthracene, which is a representative of toxic polycyclic aromatic hydrocarbons. The kinetics of the reactions of 9,10-dimethylanthracene with tetracyanoethylene and N-phenylmaleimide in the solid phase without solvent has been studied. The rate of these reactions in the solid phase appeared to be significantly lower than that of the same reactions in the solution, since the rate of reactions in the solid phase strongly depends on the rate of renewal of contacts between the reagent molecules. The data obtained can be useful for the purification of water bodies and soil from polycyclic aromatic hydrocarbons.


Introduction
It is known that polycyclic aromatic hydrocarbons (PAHs) have pronounced carcinogenic, mutagenic, and teratogenic properties [1].The main sources of PAHs are industrial emissions from coke and chemicals, metallurgical, oil refining industries, as well as heating systems, thermal power plants, aviation, automobile and water transport.When PAHs together with industrial effluents enter a water body, they quickly spread over its surface, forming an emulsion that changes the physicochemical parameters of water.Even the thinnest PAH film isolates water from oxygen in the air, which leads to a deterioration in gas exchange and an increase in the temperature of the surface layer of water.Many species of animals and plants living in the aquatic environment are able to accumulate PAHs.When PAHs enter the soil, the whole range of properties characterizing its fertility is changed: the water-air regime is deteriorated, the content of mobile nitrogen and phosphorus compounds sharply is decreased, and the solonetz process is developed.PAHs reaching the soil disrupt the existing geochemical balance in the ecosystem.Many soil microorganisms turned out to be highly sensitive to the action of PAHs, which changes the existing microbiocenoses and affects the biological productivity of the soil.PAHs strongly inhibit growth of saprophytic microorganisms, but stimulate the reproduction of pathogenic microorganisms.Due to the low accessibility of PAHs to soil bacteria, the process of their destruction is very slow.PAHs from the soil enter the underground parts of plants, which can then be used as feed for farm animals.Taking into account the ability of PAHs to be accumulated and their presence in various parts of the trophic chain, this type of toxicant is one of the most priority environmentally hazardous factors.In this regard, the concentration of PAHs should be greatly reduced in the environment.
A relationship between the structure of PAHs and their carcinogenic properties was previously found.It turned out that PAHs with plane structure, conjugated π-system, and nonlinear arrangement of benzene rings possess carcinogenic properties [1].According to Pullman's concept, the carcinogenic activity of PAHs is associated with the electronic properties of the so-called K-and L-regions of the molecule.Therefore, it is necessary to destroy π-system of PAHs to neutralize their carcinogenic effect.PAHs with a destroyed πsystem do not show carcinogenic properties [2].The π-system of PAHs can be destroyed by hydrogenation, halogenation under ultraviolet radiation, oxidation.However, these methods are characterized either by the complexity of the reaction conditions, or the formation of toxic products.There are biological methods and methods of oxidative destruction of PAHs.Biological methods are based on the use of microorganisms, which have a huge variety of enzyme systems that carry out the biodegradation process, and a large metabolism lability.Microorganisms-destructors are able to use PAHs as the sole source of carbon and energy.The half-life of PAHs under the action of microorganisms reaches 30 days or more [3][4].The evaluation of the effectiveness of decancerogenization by the method of oxidative destruction with caution must be done, since the results of decontamination can be difficult to predict.During radiation oxidation in air of benz(a)pyrene, adsorbed on silica gel, more than 20 products are formed.These products include epoxides and diolepoxides exhibiting mutagenic properties [5].The structure of PAHs can be destroyed by chemical oxidation [6].Ozone, hydrogen peroxide, permanganates and persulfates are mainly used as oxidants.Electrochemical [7,8] and photochemical [9][10] methods for the destruction of PAHs are known.Very often, such methods for the destruction of PAHs lead to the formation of a mixture of substances, among which there may be substances even more toxic than the initial PAHs.Mechanochemical methods for the destruction of PAHs are known [11][12].The reactions are usually carried out in ball mills in the presence of various minerals (magnetite, birnessite, Na-and Cu-montmorillonite).It was found that the dominant pathway of mechanochemical destruction of PAHs is oxidation.
We propose to deactivate PAHs using the Diels-Alder reaction.The proposed approach is cost-effective, since it does not require the use of expensive equipment, is characterized by the simplicity of the reaction conditions, has 100% conversion, is characterized by a high conversion rate, and leads to the formation of a single safe product.In this work we have considered the kinetic aspect of PAH neutralization on the example of 9,10dimethylanthracene.

Kinetic studies of "slow" reactions (9,10-dimethylanthracene with maleic anhydride and N-phenylmaleimide) by spectrophotometric method
The rate constants of the reactions of 9,10-dimethylanthracene with maleic anhydride and N-phenylmaleimide have been determined on the spectrophotometer Hitachi U-2900 (Japan).The rate constants of the reaction of 9,10-dimethylanthracene and maleic anhydride in cyclohexane and carbon tetrachloride have been determined under conditions of equal concentrations.The concentrations of 9,10-dimethylanthracene and maleic anhydride were 5•10 -4 -10 -3 mol•l -1 .In all other solvents, the concentration of maleic anhydride was 20-50 times higher than the concentration of 9,10-dimethylanthracene. Monitoring the progress of the reaction have been carried out by changing the absorption of 9,10-dimethylanthracene in the wavelength range λ 390-410 nm.The rate constant of the reaction of 9,10-dimethylanthracene with N-phenylmaleimide has been determined in 1,4dioxane in the pseudo first order mode at a wavelength λ 400 nm.The concentrations of 9,10-dimethylanthracene and N-phenylmaleimide were 1•10 -4 and 2•10 -3 mol•l -1 , respectively.

Kinetic studies of solid-phase reactions
The Diels-Alder reactions of 9,10-dimethylanthracene with N-phenylmaleimide and tetracyanoethylene have been carried out by grinding equimolar amounts (0.005 mol) of solid reagents in an agate mortar with an agate pestle equipped with a "scraper" made of curved thin copper sheet with a shape similar to that of a mortar.This allowed the powder to be removed from the walls to the bottom of the mortar during the mixing.Grinding has been carried out with the constant uniform force at a speed of 2 revolutions per second.The slow reaction between 9,10-dimethylanthracene and N-phenylmaleimide has been carried out in a ball mill GM9458 (Germany) with two stainless steel balls with a diameter of 10 mm and an oscillation frequency of 25 Hz at room temperature.Due to the enormous absorption coefficient of 9,10-dimethylanthracene the UV monitoring of 9,10dimethylanthracene absorption in solution is possible at the concentration range 1•10 -5 -1•10 -4 mol•l -1 .During the preparation of the solution and the time of analysis (3-5 min, spectrophotometer Hitachi U-2900, Japan), the error due to the additional reaction between 9,10-dimethylanthracene and N-phenylmaleimide is less than 1 %, and reaction between 9,10-dimethylanthracene and tetracyanoethylene proceeds completely.Therefore, the rate of reaction between 9,10-dimethylanthracene and tetracyanoethylene has been monitored by IR spectroscopy (Vertex 70 FTIR spectrometer, Bruker, Germany) by the changing of the band intensities of reagents and adduct.

Results and Discussions
Reactions of 9,10-dimethylanthracene with tetracyanoethylene, 4-phenyl-1,2,4-triazoline-3,5-dione, maleic anhydride and N-phenylmaleimide proceed according to Scheme 1.The cycloaddition of dienophiles occurs at the most active 9,10-reactive center of 9,10dimethylanthracene.In this case, the destruction of a uniform aromatic system occurs, which is the cause of the manifestation of toxic properties by 9,10-dimethylanthracene. Table 1 shows the rate constants for the reactions of 9,10-dimethylanthracene with tetracyanoethylene, 4-phenyl-1,2,4-triazoline-3,5-dione, maleic anhydride, and Nphenylmaleimide in various solvents at 25°C.From Table 1 it can be seen that in the reaction with 9,10-dimethylanthracene the most active dienophile is tetracyanoethylene.The reactivity of 4-phenyl-1,2,4-triazoline-3,5-dione is slightly lower than tetracyanoethylene.Maleic anhydride and N-phenylmaleimide are 6 orders of magnitude inferior to tetracyanoethylene.The observed rate of the 1+5 reaction with continuous uniform grinding of the equimolar mixture of solid reagents has been determined by monitoring the content of diene 1 in the selected samples (Table 2, Figure 2).The 1+5 reaction has been carried out additionally in the ball mill (Figure 3).As can be seen from a comparison of Figures 2 and 3, the 1+5 reaction proceeds in a ball mill about 4 times faster compared to the grinding in an agate mortar.In both cases, an induction period is observed, whose nature is unknown.
Since the 1+2 reaction in the solution proceeds instantly, its rate in the solid phase has been determined from the change in the intensity of the absorption bands in the selected samples (Table 3).

Conclusion
Using 9,10-dimethylanthracene as an example, it has been shown that the Diels-Alder reaction can be used to destroy the uniform aromatic π-system of PAHs, which is responsible for the manifestation of the toxic properties of PAHs.The rate constants of Diels-Alder reactions of 9,10-dimethylanthracene with tetracyanoethylene, 4-phenyl-1,2,4triazoline-3,5-dione, maleic anhydride and N-phenylmaleimide in solution have been determined.The most active dienophiles were tetracyanoethylene and 4-phenyl-1,2,4triazoline-3,5-dione.The neutralization of 9,10-dimethylanthracene by tetracyanoethylene and 4-phenyl-1,2,4-triazoline-3,5-dione (identical diene and dienophile concentrations and equal to 1 M) would require 1 and 8 milliseconds, respectively.The reaction rates of 9,10dimethylanthracene with N-phenylmaleimide and tetracyanoethylene in the solid phase without solvent have been determined.The neutralization of 9,10-dimethylanthracene in the solid phase with tetracyanoethylene and N-phenylmaleenimide requires 19 and 100 minutes, respectively, when the reagents are stirred manually.With automation of stirring, the rate of the solid-phase reaction can be significantly increased.The data obtained can be useful for the purification of water bodies and soil from PAHs.

Fig. 3 .
Fig. 3. Change in the content of diene 1 during the grinding of equimolar mixtures 1 and 5 in a ball mill (2 runs).

Table 2 .
The content (%) of diene 1 in the 1+5 reaction during grinding of an equimolar mixture of solid reagents in an agate mortar at 25 o C.