Photocatalytic treatment of volatile organic compounds emitted frommosquito repel incense burning

Use your smartphone to scan this QR code and download this article ABSTRACT Mosquito-repellent incense (MRI) is a widely used product in the household. Although the quality of MRI has been strictly managed, it still has certain health effects in practical use. This study investigated the level of volatile organic compounds (VOCs) in MRI smoke and its removal ability by photocatalysis. An online survey was conducted using Google form to select three typical types of MRI which are themost popular use in Vietnam (i.e., MOSFLY, RAID, and JUMBOVAPE) aswell as their use level and condition. These types of MRIs were then burnt in a normal room for 8 h to determine the level of air pollution emission. Results from room test show that the tVOCs concentration emitted from one of the MRIs was as high as 1621 ppb, which was the highest pollution level among the three MRIs tested. This type of MRI was then employed as VOCs pollutant source for testing the treatment ability of a photocatalytic equipment in a closed chamber. The experimental treatment of emitted VOCs by the photocatalytic treatment equipment showed that titania nanotubes (TNTs) modified with metal salt and heat treatment achieved high removal efficiency. It has reached indoor air standards of ≤ 490 ppb after 180 min of treatment with an input concentration of about 1,600 ppb. By changing the conditions of TNTs modification and operation conditions, the highest treatment efficiency was achieved with 2 g of zinc doped TNTs at a Zn/Ti molar ratio of 0.5%, calcined at 500 oC, in which the treatment to meet the standard reached the shortest treatment time. The results in this study indicate that burning MRI could cause indoor air pollution that may affect human health and photocatalysis is a potential technology for treating VOCs from indoorMRI burning.


INTRODUCTION
One of the most popular methods of repelling 2 mosquitoes is to use incense. Mosquito-repellent 3 incense (MRI) or mosquito coil with spiral-shaped 4 contains compounds that repel mosquitoes derived 5 from dried daisies. According to Strickman et al. 6 (2009), components of MRI may include pyrethrum, 7 pyrethrins, allethrin, esbiothrin, butylated hydroxy-8 toluene, piperonyl butoxide, and n-octyl bicyclohep-9 tene dicarboximide 1 . With high mosquito repellent 10 effect at low cost, MRI is widely used, especially in 11 Asia, Africa, South America, and Australia 2 . In Viet-12 nam, the use of mosquito-repellent incense at home is 13 very popular, especially in rural areas.
14 Although it is considered as a safe product for health, 15 MRI smoke contains some air pollutants such as fine 16 dust, formaldehyde 2 , and polycyclic aromatic hydro-17 carbons (PAHs) 3 . This smoke can irritate some parts 18 of the body 2 and affect the lungs in adolescents and 19 infants significantly 4 . The research of Salvi et al. 20 (2016) indicated that the concentrations of PM2.5 and 21 CO by MRI burning were much higher than those 22 by indoor cooking activities using biomass fuels 5,6 . 23 However, there is a lack of information on air pollu-24 tion from MRI burning in Vietnam as well as an effec-25 tive solution for its smoke control. 26 In recent years, the photolysis under the presence 27 of a catalytic material has been extensively stud-28 ied because of its superior properties. In the field 29 of environment, photocatalysis is particularly effec-30 tive in the treatment of pollutants in low concentra-31 tions such as nitrogen oxides 7 , volatile organic com-32 pounds (VOCs) 8,9 , sulfur dioxide 10 , odor 11 , and air-33 borne microorganisms 12 . Titanium dioxide (TiO 2 ) 34 is usually the photocatalytic material of choice be-35 cause of its appropriate oxidation and reduction po-36 tentials, high stability, and suitable band gap energy. 37 Moreover, TiO 2 is chemically stable, environmental-38 friendly, relatively cheap, and easily found in the mar-39 ket. Among different types of TiO 2 , titania nanotubes 40 (TNTs) is recently becoming an efficient and popu-41 larly used type because of its high activity and good 42 properties [13][14][15] .   to ensure that the air is circulated evenly and simu-79 late the actual room condition. The experiments were 80 conducted with two closed chambers, one of which 81 contained treatment equipment (Chamber 2, treat-82 ment chamber) and the other as a control chamber 83 without any treatment (Chamber 1). Pollutant con-84 centrations (tVOCs) were measured in both cham-85 bers for comparison. In treatment tests, the MRI coils 86 were burnt in both chambers until the tVOCs concen-87 tration reached around 1,600 ppb. The coils were then 88 extinguished and the treatment equipment in Cham-89 ber 2 was turned on. The performance of the treat-90 ment equipment was calculated using the following 91 equation: Where: C 1 and C 2 are tVOCs concentrations in 93 Chamber 1 and 2, respectively.

94
The treatment equipment is illustrated in Figure 3, 95 which has many components. The fan (2) mounted at 96 the top of the equipment has an effect of circulating 97 the polluted airflow in the closed chamber through 98 the equipment with an appropriate flow. The UVA 99 lamp system (5) and electronic ballast (9) has the ef-100 fect of generating UV light irradiating on the catalyst 101 coated glass fiber (4) to perform the photocatalytic 102 treatment. The air after being processed by the photo-103 catalytic will pass through the coarse fabric filter (7) to 104 collect dust in the air. All the above components are 105 fixed to a cylindrical stainless steel tube (8) by racks 106 (1, 3, and 6). The equipment length and diameter are 107 400 mm and 49 mm, respectively.

108
The catalytic materials used in this study were 109 TNTs made from TiO 2 by the hydrother-110 mal method 16     to be higher than that with 1 slice, in which the high-185 est tVOCs concentration when burning 2 slices were 186 1.27 times higher than that from 1 slice.

187
The experiment was then conducted to investigate the 188 pollution of MRI smoke in a closed chamber with a 189 volume of 408 L. Due to the very high concentration 190 of air pollutants that exceeds the upper limit of the 191 measuring device when burning 1 slice of MRI in the 192 closed chambers, the experiments were consequently 193 conducted with 1 cm of MRI B and data were recorded 194 every 10 min for 3 h. As seen in Figure 5, the concen-195 tration of tVOCs varied from 1200 to 3000 ppb.   ter that throughout the test period. The experiment was first conducted to investigate the 217 influence of metal doping (e.g, Al, Co, Cu, Zn, Fe, 218 Cd, and Sr) on the activity of TNTs and to find out 219 the suitable photocatalyst for VOCs removal. The re-220 sults are displayed in Figure 6 for the metal/Ti mo-221 lar ratio of 0.5% and the catalyst amount of 1 g. Al-222 though the concentration of tVOCs in both cham-223 bers decreased during 300 min of the experiment, 224 the decrease rate was much faster in the chamber 225 with the treatment equipment than in the blank one, longer times to reach the standard. Thus, the catalyst 262 annealed at a temperature of 500 o C had outstanding 263 treatment ability and was selected for further experi-264 ments.

265
The ratio of metal and titanium may also have an ef-266 fect on the activity of the photocatalyst. In this exper-267 iment, different Zn/Ti molar ratios of 0.5%, 1%, and 268 1.5% were tested for catalyst calcined at 500 o C and 269 catalyst amount of 1 g and the results are exhibited 270 in Figure 8. Among the tested materials, the catalyst 271 with a Zn/Ti ratio of 0.5% gave the shortest treatment 272 time of 220 min. At higher ratios of 1% and 1.5%, the 273 results were not much different, in which the times 274 to reach the standard were 300 and 320 min, respec-275 tively. The suitable amount of catalyst used is also an impor-277 tant consideration for photocatalytic treatment appli-278 cation, where too much catalyst is costly but too less 279 catalyst affects the photocatalytic activity. In this ex-280 periment, the amount of catalyst was varied in the 281 range of 0.5 -2.0 g, and the results are displayed 282 in Figure 9. In general, the more catalyst used, the 283 shorter times to reach the standard were. It took only 284 180 min for tVOCs concentration to decrease to be-285 low the standard when using 2.0 g of catalyst while 286 the use of 0.5 -1.5 g of catalyst needed around 220 -287 240 min. After 180 min, the experiment using 2 g of 288 catalyst achieved the highest efficiency of 70.6% while 289 the lowest efficiency of 65.9% was in the case of 1.5 g. 290 The efficiency when using 0.5 g and 1 g was not much 291 different. Thus, under the condition of this study, the 292 amount of catalyst suitable for the treatment equip-293 ment is 2 g.