Controlling CH4 Emissions from Integrated Vertical-Flow Constructed Wetlands by Using Potassium Peroxymonosulfate (PMS) as Oxidant


 It is very important to control methane emissions to reduce global warming. In this study, an attempt was made to adjust the oxidation-reduction potential (Eh) by adding different mass of potassium peroxymonosulfate (PMS) (0 g, 31.25 g, 62.5 g, 125 g, 250 g and 500 g) to reduce methane from integrated vertical-flow constructed wetlands (IVCW). Results show that the reduced CH4 emission from IVCW was the highest with decreased by 43.5% compared to blank group (PMS=0), when adding 125g PMS. Importantly, the reduced CH4 from the root-water system of IVCW was higher than that of the stem-leaf system of IVCW, when adding PMS. It’s found that Eh not only has a significant correlation with CH4 flux, but also has a significant relationship between PMS quality, DO, water temperature and sampling time (yEh= -0.44XPMS + 6.82XDO + 0.38t - 264.1, R2 = 0.99). It concludes that PMS, as an oxidant, is a very feasible method for controlling methane emissions from IVCW. Further research may combine other methods such as microbiology, physical control and hydrology control for mitigating the CH4 emissions from constructed wetlands.


Introduction
shown that 70% of the CH4 produced by CWs is mainly transported by vascular plants, and oxygen 40 will be transported to plant roots under photosynthesis, providing an oxygen environment for root 41 microorganisms and a growth environment for methanotrophic bacteria (Chowdhury and Dick, 42 2013a). At the same time, CH4 produced by methanogens in the wetland transmits through the 43 stems and leaves of plants into the atmosphere. Generally, controlling of CH4 emission from CWs 44 includes three processes of methane production, methane oxidation, and methane release, and 45 ultimately affects the climate change (mainly the greenhouse effect). Therefore, finding feasible 46 ways to control CH4 emissions during wastewater treatment has received increasing attention 47 (IPCC, 2013). 48 The control of methane emissions in CWs can generally be divided into macro-control and 49 potassium peroxymonosulfate (PMS=0 g), and the IVCW 1 and IVCW 2 is used to control methane 136 emission by addding PMS.  (3) 156 Hydrolysis of PMS produces [O], hydroxyl radical (•OH) and sulfate radicals (•SO4 -) with 157 certain oxidative properties. In the first experiment try, mass of 125g PMS was added to the 158 IVCW for pre-experiment of methane control. It was found that the addition of 125g PMS can 159 maintain the lowest Redox potential (Eh) of IVCW for 7 days, resulting in Eh values ranged 160 from -159 mV to 188 mV. It indicates from pre-treatment experiment that the PMS pose of 125 g 161 can significantly stimulate and improve Eh values in IVCW. Therefore, based on the 162 experimental results of 125g PMS, five PMS mass gradients of 31.25g, 62.5g, 125g, 250g and 163 500g were set in the form of a geometric sequence and added to the IVCW to monitor the quality 164 of each layer of the IVCW. Each designed mass of PMS is averagely added to each injected hole 165 of PVC pipes (Fig. 1b). 166 Based on optimal hydrology parameter of water depth of 105 cm from our previous research 167 results (Liu et al., 2018), the designed experiment time of PMS to control methane in IVCW is 168 July to December, and the monthly PMS addition amount from July to October is 31.25g, 250g, 169 61.25g, 125g,500g PMS is added from November to December, while the blank experiment is 170 monitored simultaneously from July to December. The experimental operation processes included 171 six steps are the same to our previous report (Liu et al., 2018). The experiment time of each group 172 was carried out by adding PMS to IVCW 2, and the blank (control) experimental system of IVCW 173 0 without PMS was simultaneously compared. When the Eh value of the third layer of IVCW is 174 lower than -100 mV and no longer increases, the experiment stops. 175

176
The monitoring of CH4 samples is same to previous research (Liu et al., 2018), including CH4 177 monitoring of root-water subsystem in the influent tank, rootwater subsystem in the effluent tank, 178 stem-leaf subsystem in the influent tank, and stem-leaf subsystem in the effluent tank. Water 179 samples are collected at the sampling spots (see Table 1  The data on methane fluxes are processed by using Flux Revision professional software, 188 which is provided by a portable soil gas flux measurement system (WS-LI820, WEST Systems, 189 Italy). Data statistical analysis and graphing are done by software R (R Foundation for Statistics 190   that the addition of PMS has a certain impact on methane emissions. In addition, in the blank group, 203 the methane emission from the stem-leaf system of IVCWs was 0.88 times than that of the root-204 water system, while the methane emissions from the stem-leaf system of IVCWs was 1.178 times 205 than that of the root-water system by adding PMS. It can be seen that the reduced methane 206 emissions by adding of PMS from root-water system of IVCWs were significantly higher than that 207 of stem-leaf system. Methane flux from July to December.

212
Methane emissions of IVCWs from July to November are shown in Fig. 2b. CH4 flux in 213 the blank group was 2.28, 0.75, 1.67, 1.59, 1.2625 times than that of the PMS group (31.25 g, 62.5 214 g, 125 g, 250 g and 500 g), respectively. Obviously, CH4 flux from IVCWs during the summer 215 (July-August), was at a relatively low level compared with the autumn and winter (September-216 December). The average methane emissions in autumn and winter were 1.59 times than that in 217 summer. 218

219
The relationship of CH4 flux from IVCWs among Eh and mass of PMS is illustrated in Fig.  220 3.   Figure 4b shows the relationship between the total CH4 emission of the IVCW stem-leaf 248 system, the total CH4 emission of the root-water system and the total CH4 emission flux with time 249 when 31.25g PMS is added. The average CH4 emission flux of the stem-leaf system is 0.894 250 mol/m 2 /day higher than that of the root-water system. PMS has a significant impact at 18h, 72h 251 and 120h. Under these conditions, methane emissions are reduced by an average of 274%.  Table 1. It can be seen that the methane emission flux fluctuates greatly with 255 time. In addition, the methane emissions from the effluent system to which PMS is added are only 256 0.02 mol/m 2 /day different from the inlet system, indicating that the effect is more uniform when 257 62.5g of PMS is added.  Figure 5b shows the relationship between the total CH4 emissions of the IVCW stem-leaf 263 system, the total CH4 emissions of the root-water system, and the total CH4 emissions flux over 264 time when 62.5g PMS is added. The average CH4 emission flux of the stem-leaf system is 265 0.127mol/m 2 /day smaller than that of the root-water system. Adding 62.5g of PMS had a 266 significant effect at 18h, 40h and 72h, under these conditions, methane emissions were reduced by 267 an average of 60.5%. 268

CH 4 flux changes with time when 125g PMS is added 269
It can be seen that the methane emissions from the influent tank of IVCW with PMS are about 270 twice than that of the effluent tank of IVCW, which indicates that PMS has a greater impact on the 271 influent tank of IVCW in Fig. 6a. The variations of CH4 are shown in Table 1. 272 Figure 6b shows the relationship between the total CH4 emission of IVCW, the total CH4 273 emission of the root-water system and the total CH4 emission flux when 125g PMS is added. The 274 average CH4 emission flux of the stem-leaf system is 0.585mol/m 2 /day smaller than that of the 275 root water system. In addition, the result of the blank group is exactly the opposite. Moreover, 276 PMS has significant effects at 48h， 60h， 72h and 96h. Under these conditions, methane emissions 277 were reduced by an average of 43.5%. The specific changes are shown in Table 1. It can be seen that the methane emissions from the inlet system with PMS are about 1.27 284 times that of the effluent tank of IVCW, which indicates that PMS has a greater impact on the 285 influent tank of IVCW in Fig. 7a. The specific changes are shown in Table 1. 286 Figure 7b shows the relationship between the total CH4 emissions of IVCW, the total CH4 287 emission of the root-water system and the total CH4 emission flux when 250 g of PMS is added. 288 The average CH4 emission flux of the stem-leaf system is 0.198mol/m 2 /day higher than the average 289 CH4 emission flux of the root-water system. The result of the blank group is just the opposite. In When 500g of PMS was added, the methane emission flux of the stem-leaf system and root-298 water system, and the stem-leaf system and root-water system of IVCW 2 changed with the 299 sampling time as shown in Fig. 8a. It can be seen that the methane emission flux fluctuates greatly 300 with time. In addition, the methane emissions from the effluent system to which PMS is added are 301 only 0.048 mol/m 2 /day different from the inlet system, indicating that the effect is more uniform 302 when 500g of PMS is added. 303 Figure 8b shows the relationship between the total CH4 emissions of IVCW, the total CH4 304 emission of the root water system and the total CH4 emission flux when 500g PMS is added. The 305 average CH4 emission flux of the stem-leaf system is 0.0087mol/m 2 /day larger than that of the root 306 water system. The result of the blank group is just the opposite. In addition, PMS has significant 307 effects at 18h and 264h, under these conditions, methane emissions were reduced by an average 308 of 48%. 309 In summary, the dosages of PMS that have better effects in this experiment are 62.5g, 125g, 314 and 250g, respectively, and the action time is basically within 2 to 4 days. After 62.5g, 125g, and 315 250g PMS were added in 2-4 days, the methane emission flux was reduced by 42.4%, 49.2%, and 316 95.1% compared to the blank experiment. After deducting the effect of temperature, the reduction 317 was 23.6%, 29.5%, and 37.3% respectively.  Table 1. Through regression analysis, it is found that the 321 regression equations numbered 1, 4, 5, 6, 13, 16, 24, 25, 27, 31, 34, 35 conform to equation (6) 322 (A1-A4 are constants). The difference in constants may point to different conditions. The specific 323 formulas are shown in Table 1, and other regression equations that do not meet the formula (6), 324 the main reason will be further discussed in the follow-up experiments of the research group. 325 Also, it can be concluded from Table 1 The relationship between the methane emission flux and the mass of PMS, water 342 temperature and sampling time is following regressive equation (9).  are basically lower than that in blank group, but higher than that of other CWs (Table 3) From the results of correlation analysis in Table 2, temperature is the main controlling factor of 362 CH4 flux. This is because the PMS added in this experiment is greatly affected by temperature, including diffusion, transpiration, and the migration of rooting plants (Søvik and Kløve, 2007). 376 The results of this experiment show that in the blank group, the methane emissions from the 377 stem-leaf system were 0.88 times than that of the root-water system. This is consistent with the 378 findings of previous investigation from us (Liu, 2019). Because plant root exudates and 379 abscission (sugars, organic acids, etc.) will become substrates and affect methane emissions 380  which have shown that the activity of methanogens in substrates was affected by Eh values, and 405 methane production depends on lower Eh values (Inglett et al., 2012a). It's has been reported for 406 methane production that the optimal range of Eh was -315~-500 mV (Jee and Nishio, 1987). It 407 also has indicated for methane production that the optimal Eh value is -150 mV (Masscheleyn et 408 al., 1993), and the methane production amount is larger when Eh is lower than this value. 409 Relevant research shows that methane is produced only when the soil Eh is -140 mV. When the 410 soil Eh ranged from -200 mV to -300 mV, methane production increases by 10 times and 411 emissions are increased by 17 times (Liu et al., 2014).There are also reports that methane is 412 produced when the soil Eh is below +240 mV, and the critical Eh value of methane production is 413 higher than previously reported (Szafranek-Nakonieczna and Stępniewska, 2015). These studies 414 indicate that the critical condition of Eh produced by methane is not necessarily -150 mV. 415 Therefore, Eh with an average value of -46.52 mV in this study was evidently higher the 416 threshold value of Eh (-150 mV), the methane emission can be efficiently controlled by adding 417 PMS. 418

419
The correlation analysis results of the effects of temperature, Eh and DO on methane 420 emissions is presented in Table 4. It can be seen that the correlation between methane and Eh is 421  were reduced by 43.5% compared with blank group. Compared with other reported CWs, the 461 CH4 emission (0.956 mol/m 2 /day) of IVCW in this study is relatively high. In the summer of 462 blank group, the methane emission of the root-water system has reached 63.5%, almost 463 accounting for the main emission. However, the methane emission reduction of the root-water 464 system under the action of PMS is 12.76 times than that of the stem-leaf system, and the 465 reduction of methane emission in summer is higher than that of other seasons. Therefore, the 466 effect of using PMS in summer is mainly to reduce methane emissions in the root-water system. 467 The conclusion of this study is that PMS, as an oxidant, is a very feasible engineering method for 468 mitigating methane emissions from IVCWs.                                        700  701  702  703  704  705  706  707  708  709  710  711  712  713  714  715  716  717  718  719  720  721  722  723  724  725 726 727    Figure 1