Application Analysis of Microwave Radiation Technology in Environmental Monitoring

The use of microwave radiation technology to promote chemical reactions began in 1986 with the esterification, hydrolysis and oxidation reactions of Gedye R in microwave ovens. The potential of microwave radiation technology in environmental engineering has only been noticed in recent years. Up to now, microwave radiation technology has been successfully used in various environmental engineering research fields such as environmental monitoring, waste gas treatment, sewage treatment and solid waste treatment. The application research in environmental monitoring mainly focuses on microwave extraction and microwave digestion. Sample pretreatment.

2.1 Microwave extraction principle

The basic principle of microwave extraction is to use the difference in the degree of microwave radiant energy absorbed by the medium. By selecting different solvents and adjusting the microwave heating parameters, the target components in the material are selectively extracted, so that the target in the sample (such as organic pollutants) Effectively separated from the matrix material. Microwave extraction has been widely used in the extraction and separation of targets from soils, sediments and various organisms.

2.2 Microwave extraction characteristics

1), fast and efficient

The dipole molecules in the sample and solvent change their positive and negative polarities at a speed of 109/s under the action of high-frequency microwaves, generating dipole eddy current, ion conduction and high frequency friction, thus generating a large amount of heat in a short time. . The weak hydrogen bond rupture caused by the rotation of the dipole molecule, the ion shifting, etc. accelerate the penetration of the solvent molecules into the sample matrix, and the components to be analyzed are quickly solvated, so that the microwave extraction time is significantly shortened.

2), heating evenly

Microwave heating is the energy that penetrates into the material and is absorbed by the material into heat energy to heat the material, thus forming a unique way of heating the material. The whole material is heated without temperature gradient, that is, the microwave heating has the advantage of uniformity.

3), microwave heating is selective

Microwaves exhibit selective heating characteristics for materials with different dielectric properties. Materials with low dielectric constant and dielectric loss can be said to be "transparent" to the incidence of microwaves. The greater the polarity of the solute and solvent, the greater the absorption of microwave energy and the faster the temperature rise, which promotes the extraction rate. For non-polar solvents that do not absorb microwaves, microwaves have little effect. Therefore, the polarity of the solvent must be considered when selecting the extractant to achieve the best results.

4), biological effects (non-thermal effects)

Since most organisms contain polar molecules, they cause strong polar oscillations under the action of microwave field, which leads to relaxation of hydrogen bonds between cells, and electrical breakdown of cell membrane structure, which accelerates the penetration of solvent molecules into the matrix and is to be extracted. Solvation of the ingredients. Therefore, when the components to be analyzed are extracted from the biological substrate by microwave extraction, the extraction efficiency can be improved.

2.3 Comparison of microwave extraction technology with other technologies

Any extraction technique is to quickly and efficiently separate the components to be analyzed from the matrix. However, due to the complexity of the matrix and the different characteristics of the extraction technique, the purpose of the analysis and the analytical method must always be taken into consideration when selecting the method. Costs, simplification of operations, and the amount of time. The characteristics of other methods such as ultrasonic extraction, supercritical fluid extraction and accelerated solvent extraction are shown in Table 1. Soxhlet extraction is a classic extraction method with a long history. It is commonly used in the extraction of active substances, but it is time-consuming, work-intensive and consumes a large amount of solvent. It is easy to cause environmental pollution during concentration. insufficient.

Table 1 Comparison of different extraction methods

Soxhlet extraction ultrasonic extraction microwave extraction super-supercritical fluid extraction accelerated solvent extraction

Time 24～48h 30～60min 4～20min 30～60min 15min

Pre-separation, no filtration, solvent evaporation, no filtration, no filtration

The amount of solvent is large and small

Low cost, high and high

Work intensity is much lower and lower

The degree of pollution is small and small

3.1 Principle of microwave digestion

The basic principle of microwave digestion is to use the microscopic particles of the sample to generate electron polarization (rearrangement of electrons around the nucleus), atomic polarization (rearrangement of atoms in the molecule), orientation polarization (molecular permanent coupling) in the microwave field. Polar reorientation) and surface polarization (rearrangement of free charge). Among the four polarizations, the first two polarizations are much faster than the vibration period of the microwave electromagnetic field (10-9 to 10-12 s) (the relaxation time is 10-15 to 10-16 s and 10, respectively). -12 ~ 10-13s), so there will be no dielectric heating, and the latter two polarizations are equivalent, which can produce dielectric heating, that is, through the polarization process of microscopic particles, the microwave energy is converted into thermal energy.

3.2 Characteristics of microwave digestion

1), the sample is completely decomposed

Since the decomposition reaction is carried out in a closed vessel at a high temperature and a high pressure, the sample is completely decomposed without pollution and loss under the condition of controlling the optimum pressure and the microwave heating time by using a reasonable acid and a solvent.

2), the dissolution rate is fast

Since the reaction between the sample and the solvent is generated after instantaneous absorption of the microwave radiation energy, the heat transfer process is not required, and the instantaneous temperature can be reached, thereby eliminating the loss of energy during the heat conduction process, and thus the time required for sample decomposition is longer than conventional. The method is greatly shortened, and the time required for general dissolution is no more than 20 minutes.

3), economy

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