Effects of dielectric barrier discharge on diatomite contaminated with oil products

Abstract

Recently, the attention of many researchers has been attracted by the methods of high-energy chemistry, in particular, the dielectric barrier discharge (DBD). The plasma-adsorption of water treatment with subsequent (or simultaneous) regeneration of sorbents using DBR is a new and poorly studied area, therefore, the study of the processes of sorbent treatment in a barrier discharge is an urgent and important task.

The aim of this study was the identification of the regularities of the processes of regeneration of diatomite in the dielectric barrier discharge plasma, and the features of the effect of plasma on the surface properties of the sorbent (diatomite of SMD-Sorb brand) in planar and coaxial DBR reactors.

We used two DBR reactors with coaxial and planar electrodes. Oxygen was used as a plasma-forming gas. The concentration of oil products (OP) and aldehydes was determined by the fluorimetric method. The control of monobasic carboxylic acids was carried out photometrically using a spectrophotometer. The determination of CO and CO₂ contents in the gas phase at the outlet of the reactor, was carried out by gas chromatography using a flame ionization detector. The concentration of ozone formed in the discharge zone of the reactor was determined by the absorption spectroscopy based on light absorption at λ = 254 nm.

For the estimation of the number of "sorption-desorption" cycles of the studied adsorbent, experiments were carried out with multiple treatments in DBR reactors with different arrangements of electrodes. It was found that with repeated regeneration of the contaminated adsorbent, its sorption properties decreased. However, even with the maximum number of regeneration cycles, when using each of the reactors, the sorption capacity only after 5 cycles reached the value of the sorption capacity of the original diatomite (2.63 mg/g). Thus, the optimal number of sorption /desorption cycles was 5, and DBR can be used for multiple regenerations of the same sorbent.

The coaxial system is more efficient than the planar one, both in terms of the destruction efficiency and the sorption capacity of the sorbent after the initial treatment of diatomite by DBR (1.4 times), which is probably due to the fact that in the coaxial system the sorbent distribution in the discharge combustion zone allows a more effective interaction of active plasma particles with the sorbent. The comparison of the results of sorbent processing in various DBR reactors showed that the coaxial system is more energy efficient, the specific energy consumption for sorbent regeneration was 3 times lower than in the planar system. The only advantage of the planar system was the shorter processing time of the adsorbent.

An essential advantage of the processing of sorbents contaminated with OP is not only the possibility of their regeneration, but also a high degree of mineralization of the original toxicants (96-98%). It was found that, during short processing times (up to 300 s), the products of OP destruction (such as carboxylic acids and aldehydes) did not completely decompose and gradually accumulated in the volume of the sorbent, causing a decrease in the sorption capacity upon repeated use. However, an increase in the exposure time of the discharge on the contaminated sorbent led to the oxidation of sorbed aldehydes and acids into carbon dioxide. Full regeneration of the sorbent required ~ 40 minutes of treatment in the discharge, and the contribution of CO₂ in the “carbon” balance was 94% of the carbon contained in the OP before processing in the DBR. Experiments have shown that ozone concentration in the system was insufficient for the complete oxidation of the OP. Thus, other active particles formed in the discharge zone, for example, O, OH^·, and H^· should be involved in the oxidation process.