17:15 〜 18:45
[MZZ45-P10] 3価の鉄を含むスメクタイトによる水田からのメタン発生抑制
キーワード：粘土鉱物、メタン、水田土壌、スメクタイト
Methane has 25 times the greenhouse effect of carbon dioxide (CO2) and is the greenhouse gas with the second largest total impact on global warming after CO2. It is estimated that paddy fields account for 16% of the global methane production, and various measures are being taken to combat this. In particular, the application of iron (Fe) materials such as ferrihydrite is a promising technology for reducing methane emissions, as it keeps paddy fields in an oxidative environment. On the other hand, ferrihydrite has the problem that the Fe2+ eluted during reduction leaches out of the system and therefore has no lasting effect.
Smectite, which contains Fe in its octahedral structure, affects the redox environment in the soil and basically undergoes a redox reaction while remaining within the structure and not leaching out Fe upon reduction, so it is considered to be oxidised during the period when the paddy field is flooded, reduced by acting as an oxidant during the subsequent flooding period, and then re-oxidised during the non-flooding period. The cycle of oxidation is considered to be repeated. Therefore, if Fe(III)-smectite has methane suppression properties, it could be very effective as a sustainable methane suppression material.
Against this background, this study investigated the methane suppression effect of smectite (montmorillonite and nontronite) added to paddy soil in a laboratory microcosm system. First, a rough time-varying experiment was conducted on methanogenesis and the carbon isotopic composition of the methane produced. The results showed that methane was actively produced after 20 days in the experimental system in this study. The carbon isotope ratios also showed that methane from the CO2 reduction of hydrogen contributed to the small amount of methane production up to around 15 days. On the other hand, the rapid increase in methanogenesis after 20 days was found to be mainly due to methanogenesis from acetate fermentation.
Considering that methane production experiments in such water-soil systems are relatively variable, methane production at 9, 27, and 36 days elapsed was examined in detail, taking into account the timescales described above. The results showed that for the 27- and 36-day-old samples, methane production was suppressed by about 60% when smectite was added, beyond the margin of error; for the 9-day-old samples, there was no difference in the amount of methane produced with and without smectite added, suggesting that the mechanism of methane production may be different. Further clarification of the reasons for this is expected to help elucidate the mechanism of the suppression of methane production by smectite. However, from the XAFS experiments, a decrease in Fe(III) content of smectite in the soil was observed during this period, and it is highly likely that the Eh buffering effect of smectite due to this may contribute to the suppression of methane production. Thus, we think that Fe(III)-bearing smectite can be used to mitigate the emission of methane from paddy field, and its effect can last longer than using ferrihydrite to keep the paddy field not to be highly reducing, since the Fe(II)-bearing smectite reduced during the flooded period can be re-oxidized during the non-flooded period. This is the first study which clearly shows that Fe-bearing smectite has a potential to reduce the emission of methane.
Smectite, which contains Fe in its octahedral structure, affects the redox environment in the soil and basically undergoes a redox reaction while remaining within the structure and not leaching out Fe upon reduction, so it is considered to be oxidised during the period when the paddy field is flooded, reduced by acting as an oxidant during the subsequent flooding period, and then re-oxidised during the non-flooding period. The cycle of oxidation is considered to be repeated. Therefore, if Fe(III)-smectite has methane suppression properties, it could be very effective as a sustainable methane suppression material.
Against this background, this study investigated the methane suppression effect of smectite (montmorillonite and nontronite) added to paddy soil in a laboratory microcosm system. First, a rough time-varying experiment was conducted on methanogenesis and the carbon isotopic composition of the methane produced. The results showed that methane was actively produced after 20 days in the experimental system in this study. The carbon isotope ratios also showed that methane from the CO2 reduction of hydrogen contributed to the small amount of methane production up to around 15 days. On the other hand, the rapid increase in methanogenesis after 20 days was found to be mainly due to methanogenesis from acetate fermentation.
Considering that methane production experiments in such water-soil systems are relatively variable, methane production at 9, 27, and 36 days elapsed was examined in detail, taking into account the timescales described above. The results showed that for the 27- and 36-day-old samples, methane production was suppressed by about 60% when smectite was added, beyond the margin of error; for the 9-day-old samples, there was no difference in the amount of methane produced with and without smectite added, suggesting that the mechanism of methane production may be different. Further clarification of the reasons for this is expected to help elucidate the mechanism of the suppression of methane production by smectite. However, from the XAFS experiments, a decrease in Fe(III) content of smectite in the soil was observed during this period, and it is highly likely that the Eh buffering effect of smectite due to this may contribute to the suppression of methane production. Thus, we think that Fe(III)-bearing smectite can be used to mitigate the emission of methane from paddy field, and its effect can last longer than using ferrihydrite to keep the paddy field not to be highly reducing, since the Fe(II)-bearing smectite reduced during the flooded period can be re-oxidized during the non-flooded period. This is the first study which clearly shows that Fe-bearing smectite has a potential to reduce the emission of methane.