Simultaneous removal of multiple indoor-air pollutants using a combined process of electrostatic precipitation and catalytic decomposition

Highlights

• ESP and catalytic decomposition combined for indoor-air purification.

• Real indoor VOCs with complex composition were effectively removed.

• Ozone in polluted gas flow enhanced the catalytic decomposition of VOCs.

• Particulate matter, VOCs and ozone were simultaneously removed.

• 100% removal achieved for multiple pollutants.

Abstract

A synergistically combined process was studied for the removal of multi-pollutants in indoor air. Technologies including electrostatic precipitation and catalytic decomposition were integrated and investigated in a 3 m³ environmental chamber. Various sources of pollutants including cigarette smoke and building material were used to generate contaminants with complex composition. Simultaneous and effective removal of particulate matter, VOCs and ozone were achieved. The effective combination of the technologies synergistically improved the decontamination performance. Electrostatic precipitation removed particulate matter (94%-100% removal), while UV-catalysts oxidized VOCs (100% removal) within 20 min. In addition, during this process, the precipitator and VUV lights produced ozone (up to 360 ppb), initiated ozone-catalytic oxidation of gaseous pollutants, consequently enhanced the removal of VOCs. Moreover, an ozone depleting module was installed downstream to decompose the residual ozone, ensuring no ozone was released into ambient air. Notably, comparing with other reported works, the gas hourly space velocity in this work was high as 594,000 h⁻¹, while the contact time of pollutants on the catalyst was short as 0.02 s. However, simultaneous and effective removal of real and complex pollutants was achieved under such circumstance. The experimental results also suggest that the combined process can effectively improve the indoor air quality.

Introduction

Indoor air pollution has been attracting growing attentions. The indoor air pollutants cause serious health risks for residents, especially in the developing countries [1], [2]. In China, half million of premature deaths are caused by indoor air pollution every year [3]. It is urgent to develop novel and effective control method for indoor air pollutions.

Generally, the composition of indoor-air pollutants can be categorized as aerosol and gaseous pollutants. The former mainly includes particulate matter (PM), microorganisms and small droplets in the air [4], [5]; while the latter mainly includes ozone, SO₂, CO, volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs)[6], [7], [8], [9]. In China, PMs, ozone and VOCs are among the commonly detected and concerning pollutants in indoor air [7], [10]. PMs with small diameters can be breathed deep into human lung. It is worse that PMs often carry contaminants such as organic matters, carbon black, heavy metals and pathogens, bringing heavy concerns with human health risks (for instance exacerbating asthma) [11], [12]. Ozone irritates and damages body tissue, causing allergic or chronicle symptoms [7], [10], [13]. It is also responsible for the formation of indoor secondary organic aerosols. Most VOCs are toxic and the exposure of human in VOCs brings about lots of troubles including poisoning, hormone disruption and cancer [14], [15]. The interaction between ozone and VOCs (especially cigarette smoke) could aggravate the injury on respiratory system [16], [17].

Various technologies have been studied to mitigate the indoor air pollution, including ventilation [18], filtration [5], [11], electrostatic precipitation (ESP) [12], [19], adsorption [20], [21], [22], [23], catalytic oxidation [15], [24], [25], [26], [27] and photo-catalytic oxidation (PCO) [20], [28], [29], [30], [31]. However, more studies are still required to tackle the challenges that have been revealed during researches and applications. On one hand, the mentioned technologies have their own advantages and disadvantages. For instance, filtration and ESP effectively remove aerosols, especially PMs, however, they have limited effect on gaseous pollutants; adsorption and catalytic oxidation eliminate gaseous contaminants, such as VOCs, but they demand periodically replacement of adsorbents or catalysts [32]; PCO could work at room temperature with higher removal efficiency, but there is a risk of secondary pollution formation. Besides, ozone emission has been a major concern for the application of ESP and PCO in indoor environment [33], [34]. Few studies have reported the control of ozone in indoor air. It worth emphasizing that, ESP is promising for aerosol/PM removal without the need of replacing filtering materials; while catalytic oxidation is promising for VOCs removal due to its high efficiency [12], [15], [35]. Shimizu et al. [36] developed a corona reactor and simultaneous removed PMs and HCHO in indoor air. The PMs collection rate was >95%, while the concentration of HCHO was controlled to a level below 0.08 ppm. Vargas’s group [37] synthesized the amorphous TiO₂ nanoparticles (NP) using a sol-gel method. The formation mechanism, toxicity and antibacterial properties of the amorphous NPs were investigated. The surface defects were found critical for the toxicity and a “memory effect” to light was also observed [37]. Fang and co-researchers [38] developed a MnOx/SiO₂@AC catalyst that can efficiently decompose and utilize ozone, thus removing benzene via ozone-catalytic oxidation at ambient temperatures. Weon et al. [39] modified TiO₂ catalyst with Pt and F, enhanced the formation of mobile OH radicals, hindered the deposition of carbonaceous intermediates, improved the stability of catalyst and increased the VOCs removal efficiency. On the other hand, so far, the mentioned technologies mainly focus on the removal of one or two pollutants. Very few studies were on the control of multi-pollutants (especially the complex pollutants in real environment) [40], [41]. For instance, few research works reported the simultaneous control of ozone and cigarette smoke in indoor environment. By contrast, the transformation, interference or competition among different pollutants can be important for indoor air pollutions [36], [42], [43]. It has become a bottle-neck for the further study and potential application of indoor-air purification technologies. Therefore, the simultaneous control of multiple pollutants in indoor air is a key problem to be solved. More researches are still required for the effective combination of several technologies to completely clean the indoor air; while further investigations are also needed to understand the mechanism of simultaneous removal of multi-pollutants.

In this work, a novel process combined with ESP and catalytic decomposition was studied for the simultaneous removal of multiple indoor-air pollutants. The pollutants included PM, real VOCs and ozone, which were either released from a building-material sample or using gaseous mixture. The novel gas-cleaning process can be summarized as: ESP removed PMs, VUV lights generated ozone and initiated UV-catalytic oxidation on the catalysts, while an ozone depleting module (ODM) decomposed the residue ozone. The gas cleaning process using combined technologies was investigated and discussed, while the cleaning performance was evaluated and compared with reported works. The results demonstrated a novel and effective process for improving the indoor air quality (IAQ), while it could also provide a new idea for the decontamination of multi-pollutants in indoor air.