The influence of the different MCs on the NO2 sorption ability
Figure 2 shows the variation over time in NO2 sorption volume with the four different MCs. The NO2 sorption volume was high soon after the aeration test started and then gradually decreased. After around 12 h, the reduction was small in all conditions. The behavior was confirmed to be almost the same even under different MC conditions. The specimen at 0% MC had the lowest NO2 sorption volume, as a whole. Soon after the aeration test started, the NO2 sorption volume at 8.6% MC was about twice as greater as that at 0% MC. The NO2 sorption volume at 12.7% MC showed almost the same value as that at 8.6% MC, and the NO2 sorption volume at 16.5% MC was the greatest. However, the differences between the NO2 sorption volumes at 8.6% MC, 12.7% MC, and 16.5% MC were very small. The reductions of the NO2 sorption volume at 0% MC and above 8.6% MC were small after around 6 and 12 h, respectively. The behavior of NO2 sorption volume was different between before and after the reduction of the NO2 sorption volume was small. Because the reduction of the NO2 sorption volume of all conditions was small at around 12 h, it was evaluated by dividing the aeration time into two periods, namely, the periods from the start of aeration to 12 h (0–12 h), and from 12 to 24 h (12–24 h). Figure 3 shows the NO2 sorption volume per unit weight and time of the specimens with different MCs for each aeration period. At 0–12 h, the NO2 sorption volume at 0% MC was the lowest (0.48 µmol), and that at 8.6% MC (1.28 µmol) was 2.7 times greater than that at 0% MC. This shows that the NO2 sorption volume increased drastically with the presence of water. The NO2 sorption volume at 12.7% MC was 1.37 µmol, and that at 16.5% MC was 1.50 µmol, which showed that the NO2 sorption volume increased slightly as the MC increased. However, the increase was less than the increase between the NO2 sorption volume at 0% MC and that at 8.6% MC. To evaluate the influence of the MC on NO2 sorption volume, one-way ANOVA was conducted. The results showed significant differences in the NO2 sorption volume between MC conditions (p < 0.01). Multiple comparisons showed that the NO2 sorption volume under the conditions with moisture was significantly greater than that at 0% MC. At 12–24 h, the NO2 sorption volume at 0% MC and 8.6% MC showed almost the same value (0.15 µmol), and those at 12.7% MC and 16.5% MC were 0.20 µmol and 0.28 µmol, respectively. Under the conditions with moisture, the NO2 sorption volume increased slightly as the MC increased. The result of one-way ANOVA showed significant differences in the NO2 sorption volume between MC conditions (p < 0.01). Multiple comparisons showed that the NO2 sorption volume at 16.5% MC was significantly greater than that at 0% MC and 8.6% MC. From the results above, it was suggested that the NO2 sorption volume at 0–12 h was greater than that at 12–24 h under each MC condition. Furthermore, at 0–12 h, it was shown that the presence of water contributed greatly to the NO2 sorption ability.
The following assumptions were made regarding the conditions of water in the specimens at each MC [14]. At 0% MC, it was assumed that water was not present. At 8.6% MC, it was assumed that water was present as monolayer and multilayer adsorbed water. In monolayer adsorbed water, hydrogen bonding between the water molecules and the hydroxyl of amorphous cellulose, hemicellulose, and lignin, which have hydrophilic property was formed on the surface of the tissue. In multilayer adsorbed water, bonding by the van der Waals force and electrostatic attraction was formed onto the surface of the monolayer absorbed water. At 12.7% MC, the amount of multilayer adsorbed water increased to more than that was assumed at 8.6% MC. At 16.5% MC, the water was present as multilayer adsorbed water in a layer thicker than that was assumed at 12.7% MC. Based on the above, under the conditions in which water was present, it seems that the water molecules at 8.6% MC did not react easily with the NO2 gas because some water molecules were present as a monolayer, which was adsorbed to the amorphous cellulose, hemicellulose, and lignin by a strong hydrogen bond. On the other hand, it seems that the water molecules at 12.7% MC and 16.5% MC reacted more easily than at 8.6% MC because the water molecules were adsorbed by the van der Waals force, which is weaker than the hydrogen bond. Therefore, it was considered that the NO2 sorption volume increased as the MC increased, although the increase was slight. The behavior seemed to be different under the conditions with much higher MC due to capillary condensation. However, the experiment in this study was limited to conditions in which the bound water was present. Thus, a condition above 16.5% MC is needed to be examined in the future. In Japan, the equilibrium MC is about 15% [23], and it decreases to 10–12% in an air-conditioned environment. Therefore, it was elucidated that a high NO2 sorption ability is observed in the range of humidity in real-life situations.
The reaction between NO2 and moisture at different MCs
NO generation volume
To evaluate the reaction between NO2 and water (Eq. 5), the NO generation volumes at the four different MCs were compared. The results of the variation over time in NO generation volume are shown in Fig. 4. Under the condition at 0% MC, the NO generation volume was high soon after the aeration started and then decreased drastically. This behavior seemed to be caused by the NO that was generated due to the reaction between NO2 and the surface of the tissue, and the polyphenol [17, 24]. After decreasing drastically, NO generation volume then decreased gradually. In this period, it was possible that water could not be removed from the specimen by the freeze-drying then remained slightly in the specimen. Therefore, NO seems to have been generated by the reaction between NO2 and the water. In addition, it was inferred that NO2 reacted with the amorphous hydroxyl group on the surface of the tissue [17]. Under the conditions in which moisture was present, the NO generation volume was at its highest at about 3 h after the start of the aeration. With regard to the quantitative relationship, the NO generation increased as the MC increased. The greatest value at 16.5% MC was 1.4 times and 1.6 times greater than those at 12.7% MC and 8.6% MC, respectively. Compared with the behavior of the NO2 sorption volume shown in Fig. 2, it took longer until the NO generation volume reached its greatest value. This suggested that NO2 reacts with water on the surface of the tissue immediately after the start of aeration, and then takes time to diffuse. Also, it was presumed that the reaction between NO2 and water continues for a long time because the NO generation volume did not reach equilibrium in 24 h under the conditions with moisture. The NO generation volume was also evaluated by the dividing aeration time into two periods as with the evaluation of the NO2 sorption volume. Figure 5 shows the NO generation volume per unit weight and time of the specimens with the different MCs in each aeration period. At both 0–12 h and 12–24 h, the NO generation volume increased as the MC increased. To evaluate the influence of MC on NO generation volume, one-way ANOVA was conducted. The result showed significant differences in the NO generation volume at different MC conditions (p < 0.01). At 0–12 h, multiple comparisons showed that the NO generation volume of the specimen at 16.5% MC was significantly greater than the others (p < 0.01), and the NO generation volume of the specimen at 9.5% MC was significantly greater than that at 0% MC (p < 0.05). At 12–24 h, multiple comparisons showed that the NO generation volume of the specimen at 16.5% MC was significantly greater than those at 0% MC and 3.8% MC (p < 0.01 and p < 0.05, respectively). Considering that the MC in monolayer adsorbed water is in the range below 5–6% [16], the NO generation volume increased greatly, namely the reactivity between NO2 and water would be enhanced by the presence of the multilayer adsorbed water. As a result, it was strongly indicated that the water in the specimen reacted with NO2, and the NO generation volume increased as the content of multilayer adsorbed water increased.
Concentration of nitrate ion
In the reaction between NO2 and water, HNO3 is generated in addition to NO (Eq. 5). To evaluate the generated HNO3, the concentration of NO3− was measured using the specimen after the aeration test. The results are shown in Fig. 6. The concentration of NO3− of the specimen at 0% MC was 4.9 ppm. This seemed to be formed by the reaction between NO2 and the water that could not be removed from the specimen by freeze-drying and then remained in the specimen in slight amounts and that was present on the surface of the amorphous hydroxyl group [17]. The concentration of NO3− in the specimen at 8.6% MC was 8.78 ppm, which was about twice as high as that at 0% MC. It was suggested that HNO3 was formed by the reaction between NO2 and water under the 8.6% MC condition. Furthermore, the concentration increased linearly in the range from 0% MC to 12.7% MC, and then it decreased slightly in the specimen at 16.5% MC. To evaluate the influence of MC on the HNO3 generation, one-way ANOVA was conducted. The result did not show significant differences in the concentration of NO3− between different MC conditions. It was expected that the concentration of NO3− would increase as the MC increased as in the case of NO generation. However, the concentration of the specimen at 16.5% MC was almost the same as that at 12.7% MC. Based on the discussion of the NO2 sorption volume and NO generation volume, it was shown that the reaction between NO2 and water occurred mainly in the 12–24 h. Because the results shown in Fig. 6 included the values in the whole aeration time of 0–24 h, it seemed that the relationship between the MC and NO3− was not shown clearly. In addition, it was reported that nitrous acid (HNO2) is also formed in the reaction between NO2 and water (Eq. 3). Therefore, it is possible that HNO2 was held in the specimen in addition to HNO3 and the condition was different depending on each moisture content. From the above results, we need to examine the reaction between NO2 and water in more detail. However, it was indicated strongly that HNO3 was formed by the reaction between NO2 and water, and then held in the specimen.
Contribution of water
According to Eq. (5), the NO2 sorption volume can be calculated theoretically using the result of the NO generation volume shown in Fig. 4. The calculated value was compared with the actual measured value of the NO2 sorption volume shown in Fig. 2. In addition, the NO2 sorption volume results for the extracted specimens were compared. An evaluation of the contributions of water and extractives to NO2 sorption was attempted by comparing these values. The results are shown in Fig. 7. In 0–12 h aeration, the calculated value was much lower than the measured value in every MC condition and showed different behavior from the measured value (extracted specimen). It was expected that the factors other than water would make large contributions to the NO2 sorption volume. After 12 h aeration, the calculated value was almost the same as both measured values at 8.6% MC. As the MC increased, it took longer duration of aeration for the calculated value to match the measured values. The calculated values seemed to show the NO2 sorption volume consumed by the reaction between NO2 and water. Thus, it was implied strongly that the NO2 sorption after 12 h aeration was caused by the reaction between NO2 and water. To compare them in detail, the NO2 sorption volume per unit weight and time of the specimen for each MC at 0–12 h and 12–24 h aeration are shown in Fig. 8. To evaluate the reaction between NO2 and H2O on NO2 sorption volume at each aeration time, one-way ANOVA was conducted. The results at 0–12 h, showed significant differences between the measured values and the calculated value at every MC condition (p < 0.01). Multiple comparisons showed that the NO2 sorption volume of the measured value was significantly greater, five times greater, than that of the measured value (extracted specimen) at every MC condition (p < 0.01). This seems to show the contribution of the extractives [12]. Also, it was considered that water contributed mostly to the NO2 sorption of the extracted specimen, because there was no significant difference between the calculated value and the measured value (extracted specimen) of the NO2 sorption volume at 8.6% MC and 12.7% MC. However, there was a significant difference between them at 16.5% MC (p < 0.05). In the case of 16.5% MC, it is possible that the NO2 sorption volume that was calculated from the NO generation volume was found to be greater than it actually was. To elucidate the reason for this, the NO2 sorption ability under the higher MC conditions needs to be evaluated. At 12–24 h, because the result of one-way ANOVA did not show significant differences between the measured values and the calculated value, it was strongly implied that the NO2 sorption was caused mainly by the reaction with water at every MC condition as discussed above.
We tried to compare the contribution ratios, namely, those of extractives only, water only, both extractives and water, and the others, to the NO2 sorption under each MC condition based on the measured values at 0–12 h. The NO2 sorption value in the non-extracted specimen at 16.5% MC was taken to be the reference value because it had the largest MC and the greatest NO2 sorption volume under all conditions. The contribution ratio of extractives only was calculated using the NO2 sorption volume in the non-extracted and extracted specimens at 0% MC using the following equation.
$$Contribution\;ratio\;of\;extractives\;only\;(\% ) = \frac{{\left( {Q_{{W{\text{NO}}_{2} }} \;{\text{of}}\;{\text{non-extracted}}\;{\text{specimen}}} \right) - \left( {Q_{{W{\text{NO}}_{2} }} \;{\text{of}}\;{\text{extracted}}\;{\text{specimen}}} \right)}}{{\left( {Q_{{W{\text{NO}}_{2} }} \;{\text{of}}\;{\text{non-extracted}}\;{\text{specimen}}\;{\text{at}}\;16.5\% \;{\text{MC}}} \right)}} \times 100.$$
The contribution ratio of water only was calculated using the NO2 sorption volume of the extracted specimen at each MC using the following equation.
$${\text{Contribution}}\;{\text{ratio}}\;{\text{of}}\;{\text{water}}\;{\text{only}}\;\left( \% \right) = \frac{{\left( {Q_{{W{\text{NO}}_{2} }} \;{\text{at}}\;{\text{each}}\;{\text{MC}}\;{\text{condition}}} \right) - \left( {Q_{{W{\text{NO}}_{2} }} \;{\text{at}}\;0\% \;{\text{MC}}} \right)}}{{\left( {Q_{{W{\text{NO}}_{2} }} \;{\text{of}}\;{\text{non-extracted}}\;{\text{specimen}}\;{\text{at}}\;16.5\% \;{\text{MC}}} \right)}} \times 100.$$
The contribution ratio of both water and extractives was calculated by subtracting the difference between the NO2 sorption in the extracted specimen at each MC and that at 0% MC, namely the influence of water only (A), from the difference between the NO2 sorption of the non-extracted specimen at each MC and that at 0% MC, namely the influences of water only and of both water and extractives.
$${\text{Contribution}}\;{\text{ratio}}\;{\text{of}}\;{\text{both}}\;{\text{water + }}\;{\text{extractives}}\;\left( \% \right) = \frac{{\left( {Q_{{W{\text{NO}}_{2} }} \;{\text{at}}\;{\text{each}}\;{\text{MC}}\;{\text{condition}}} \right) - \left( {Q_{{W{\text{NO}}_{2} }} \;{\text{at}}\;0\% \;{\text{MC}}} \right) - A}}{{\left( {Q_{{W{\text{NO}}_{2} }} \;{\text{of}}\;{\text{non-extracted}}\;{\text{specimen}}\;{\text{at}}\;16.5\% \;{\text{MC}}} \right)}} \times 100.$$
The left ratio was taken to be the contribution ratio of factors other than water and extractives.
The results of the calculation are shown in Fig. 9. The contribution ratio of extractives only was 23.4%. Under the conditions with moisture, the contribution ratio of water only ranged from 4.1 to 6.7%, which was close to the value found in the previous study [12]. The contribution ratio of both water + extractives was the highest, and it increased slightly as the MC increased. The above results implied that the presence of water and extractives together contributed greatly to the NO2 sorption ability. This might be because the extractives changed due to the presence of water, and the extractives could then make contact with NO2 gas more easily [25]. The details need to be examined in the future.