1. Introduction
A large number of volatile organic compounds (VOCs) are emitted from various industrial processes and with their toxicity that threatens human health, they contribute to severe environmental problems [
1,
2], thus they are tightly controlled in many countries. Styrene and
o-xylene, as typical VOCs, widely coexist in a wide range of industrial production processes, such as pesticide, surface coating and paint manufacturing processes, especially in the coatings industry [
3,
4]. China is regarded as the second largest country for the manufacture of coatings, therefore, large amounts of styrene/
o-xylene mixture have been frequently released into the atmosphere accompanying the production and usage of coatings. Indiscriminate exposure to styrene/
o-xylene mixtures may cause dizziness, neurasthenia and an increased risk of cancer [
3,
4]. Moreover, they could be precursors for the formation of secondary aerosols and photochemical smog. Therefore, styrene/
o-xylene mixture emissions need to be strictly controlled by developing suitable abatement methods.
Some well-established purification methods, such as adsorption, incineration, combustion and photocatalysis have been widely used to control the gas pollution. However, these methods exhibit some drawbacks including low efficiencies and high energy consumption [
5,
6]. Biological methods, developed in the late 80s, have received great attention due to their convenient maintenance, low operating costs and lower likelihood of secondary pollution. Many researchers have reported biofiltration processing as an effective technology for the removal of soluble and biodegradable volatile organic compounds (VOCs) [
7,
8,
9,
10]. However, its application is strongly limited in the case of biopoisonous and less biodegradable VOCs, such as
o-xylene, which as the most recalcitrant BTEX that has demonstrated the lowest biodegradability in biofilters [
11,
12]. A new attempt to solve these problems is to combine biological purification with chemical methods, such as ozonation or photooxidation, and the role of chemical method in the combined processes is as a pretreatment. Recently, several studies have indicated that ultraviolet (UV) irradiation could be effectively utilized as a pretreatment, since it converts some recalcitrant VOCs into several simple intermediates. However, the application of UV is restricted by the relative long residence time required, and the removal rate falls sharply with increasing initial concentrations [
13,
14]. In our group, an integrated biotrickling filter (BTF) system with a non-thermal plasma (NTP) as the main pretreatment step was used, and the performance of the non-thermal plasma oxidation pretreatment step was examined. NTP, an emerging technology, makes the most of the supplied energy for creating energetic electrons, strong oxidizing agents and highly reactive species such as ozone, atomic oxygen O (O·), hydroxyl radical (HO·) [
15,
16]. Due to the presence of the above substances, it can achieve complete VOC oxidation to CO
2 and H
2O or convert them into non bio-poisonous and easily biodegradable byproducts, facilitating the subsequent biodegradation.
In real life, most industrial emissions contain VOC mixtures. However, the studied pollutants have often been single component VOCs [
17,
18,
19,
20]. Mok
et al. studied the abatement of trichloromethane using nonthermal plasma reactors [
21]. Zhang
et al. compared styrene removal in air by positive and negative DC corona discharges, and reported that positive corona processing was more effective [
20]. In the present study, a recalcitrant VOC mixture, styrene and
o-xylene, was employed as the target contamination with a cylindrical DBD plasma configuration at room temperature. In order to identify the influence of key process parameters on the removal efficiency of styrene/
o-xylene mixture to further achieve cost-effective process conditions, the influence of various parameters like initial concentration of the mixture, residence time and relative humidity in the plasma reactor were investigated, and the results are discussed. In addition, biodegradability and biotoxicity of byproduct(s) were also evaluated with the aim of providing fundamental data to evaluate the possibility of success of the subsequent biodegradation.
4. Conclusions
The abatement of the mixture of styrene and o-xylene by non-thermal plasma generated in a DBD reactor was experimentally investigated under ambient temperature and pressure conditions. The experiment results of various technical parameters for mixed VOCs decomposition indicated that the removal efficiency was enhanced significantly with the rising reaction time, but overly high residence time is not favorable because of the low energy efficiency. Styrene was more easily degraded than o-xylene no matter whether as a single-component VOC or in mixed VOCs. The optimum relative humidity was 40%–60%, and the increase of initial concentration of VOCs had a negative effect on the removal efficiency of the plasma system. The removals of styrene and o-xylene decreased significantly when they were mixed together compared to those of the single-component VOCs, and o-xylene decreased more rapidly, indicating that the addition of styrene had an adverse on o-xylene removal. The selectivity of CO and CO2 was relatively low and much carbon deposition was observed on the surface of the reactor. Additionally, aromatic, nitrogenous by-products and O3 were detected in the reactor. It has been observed that the biodegradability of the mixture of styrene and o-xylene was strongly improved in the presence of NTP, which benefitted the subsequent degradation in a bio-trickling filter (BTF). During the research on the biotoxicity of VOCs, it was showed that the mixed VOCs in the absence of NTP has been transformed into water-soluble and biodegradable by-products that had a positive effect on biomass. Furthermore the positive influence of discharge voltage was remarkable. However, it is also necessary to avoid high voltages, which could inhibit the growth of Chlorella in terms of biomass due to the role of high voltage to produce high concentrations of ozone and NOx.