Fine soil particle aggregation in ultra-fine bubble irrigated paddy fields Open Access

Submicron-scale bubbles suspended in water are referred to as ultra-fine bubbles (UFB) by ISO 20298-1 (fine bubble technology). Research on the use of UFB is ongoing, particularly for environmental and agricultural applications. Additionally, interest in UFB–water is growing as it can be prepared using only water and air, making it an inexpensive alternative that can be prepared anywhere. Several stability hypotheses exist for the properties of UFB in water. Regarding the relationship between UFB and impurities, stability theory posits that impurities stabilize UFB (Yasui et al. 2016). We have also shown that UFB can stay in pure water for a long time, depending on the conditions (Ueda et al. 2013). Stability hypotheses suggest that UFB stability results from microscopic particles attached to the bubbles. An increase in the number of UFB may also decrease the electrical conductivity of the water as water molecules and ions may collect around the bubbles, thereby altering the properties of the water.

Regarding the agricultural use of water with UFB, we have examined the environmental response of food crop species, such as soybean (Iijima et al. 2020), cereal, and leguminous species (Iijima et al. 2022a), using deionized water. In experiments, UFB promoted high growth under oligotrophic conditions in hydroponic soybean cultivation. UFB also exhibited the same effect on growth when polyethylene glycol was added as desiccation stress (Iijima et al. 2022b). Thus, we infer that UFB promote plant growth under environmental stress. However, it is currently unknown whether UFB affect crop growth in soil cultivation. We are currently conducting field demonstrations in rice crop trials and are monitoring the parameter characteristics.

UFB have been effectively used in radical generation (Liu et al. 2021), flocculation, sedimentation (Xiao et al. 2018), and flotation beneficiation. Recent research has focused on improving the flotation effect of UFB (Chang et al. 2020; Li et al. 2022; Wang et al. 2022). The current application of UFB in rice cultivation suggests the possibility of genetic changes in the cells as well as the contribution of chemical effects such as radical activity. In addition, limited studies have discussed direct soil and water changes and provided accurate data. Therefore, we focused on the direct effects of UFB on soil and water. UFB are also being used for cleaning oil sands and are expected to be used as an environmentally friendly technology (Thuy Bui et al. 2022). By clarifying the relationship between soil particulates and UFB through field trials, the significance of these applied studies would become more apparent.

In this study, we examined the presence of UFB and their changes over time in several paddy fields. By simulating the soil in the laboratory, we also examined changes in the UFB and particulate mixture using Japanese Industrial Standard (JIS) test powder 1 (Type 11 Kanto loam). We believe that this is a novel approach in that we are attempting to elucidate the process from an engineering-related perspective, in addition to the growth of bot rice plants and soil microorganisms.

In Field 1, rice was cultivated using irrigation water from Lake Biwa. Water samples were collected on July 23, 2021. The water samples were directly collected from the field supply outlet (control) from the normal paddy field and UFB paddy field (see Figure 1). The paddy field water samples were collected far from the supply outlet (paddy field (normal and UFB)). In Field 2, rice was cultivated by pumping groundwater (Figure 2). Water samples were collected again on September 23, 2021, and sample points were collected at similar locations as in Field 1.

The parent materials of the soils in Field 1 and Field 2 were granite and volcanic ash, respectively. The soil type was Gleyic Fluvisol in Field 1 and Haplic Fluvisol in Field 2 (WRB 2015). According to soil type, the dominant soil clays were smectite, vermiculite, fine mica, and chlorite in Field 1 and allophane and imogolite in Field 2. For granite, there were micro-colloids of 450 nm or less, as well as larger soil particles, which might interact with UFB (Vilks & Bachinski 1996). Volcanic ash particles exist as particle aggregates and settle in the soil (Brown et al. 2012). Therefore, the fine soil particles were thought to be fewer than those of granite.

The collected water samples were individually sealed and refrigerated in 80-mL polyethylene terephthalate (PET) bottles, which were opened and measured at the moment each sample was obtained. The sample bottles were kept for up to 7 days (Field 1) and 10 days (Field 2). The number density of air bubbles and fine soil particles was measured using a nanoparticle tracking analyzer (Nanosight LM10, Malvern Panalytical Ltd), and the average values of three measurements were obtained. Turb^® 550 (WTW, Xylem Japan) was used to measure turbidity. Electrical conductivity, dissolved oxygen concentration, and pH were also measured to determine whether changes occurred during sample storage. The supernatant of the PET bottle was sampled during all measurements, so that precipitated impurities did not affect the measurements.

We investigated the time variation of the total UFB, fine soil particle concentration, and water turbidity data. We also compared the concentration distribution in each field. For the samples of Field 2, only the distribution was considered to determine the similarity of the trend to that of Field 1, as the soil type of Field 2 was volcanic ash, and almost no change occurred in the turbidity conditions. Type 11 Kanto loam, which was used for the experiments, was also examined for changes in the distribution map.

The dissolved oxygen concentration, electrical conductivity, and pH of the refrigerated samples were simultaneously measured over 7 days, but no changes were observed under all conditions. Therefore, we believe that the samples were properly preserved.

In the paddy field case, the first day of generation had a peak at approximately 200–300 nm in the paddy field (UFB). However, the peak almost disappeared after 3 days. In contrast, the paddy field (normal) had a peak at approximately 350 nm, which remained even after 7 days. The same trend was observed at approximately 300–400 nm for the paddy field (UFB). Because the nanoparticle tracking analyzer could not distinguish between UFB and fine soil particles, it was challenging to confirm which of the two peaks was represented. Although coal and glass adsorption has been reported as possible collateral evidence for the adsorption of fine particles and bubbles (Chang et al. 2020) (Rosa & Rubio 2018), this study is the first to confirm the actual temporal variation.

Figure 6 shows that impurities were almost undetectable in control, which remained unchanged after 10 days. However, a peak concentration at approximately 300 nm was noted for the paddy field (normal, day 1), and it remained for 3 days. In contrast, the paddy field (UFB) sample exhibited a high peak at 350 nm after 1 day. Nevertheless, after 3 days, the peak was reduced, and the water became clearer overall. This result indicates that when UFB water is injected into the paddy field, the impurities in the water and the UFB rapidly condense and are either suspended or precipitated and removed from the water. A similar phenomenon was observed in Field 1.

Although only judged by the distribution of the particle size, it can be seen that the average particle size of water in the paddy field is approximately 400–500 nm due to UFB contamination, which is about 100 nm larger than normal (Figure 9). However, the mode value is distributed in the same way as the normal (Figure 10). In addition, since the bubble concentration is lower for UFB (Figure 8), we found that the number of bubbles decreased overall, and the average bubble diameter increased as a result of sedimentation or levitation when UFB and fine soil particles were combined. This is also consistent with a decrease in turbidity. We also calculated the fitting value between the number density and turbidity. The coefficient of determination (COD) values were above 0.93.

In control, the UFB shows some degree of consistency for the mode value, while the mean value varies widely. Since the normal samples (control and paddy field) tend to have a constant value for both mode and mean values, the interaction between particles contained in the raw water and UFB can be seen. Although slight, the UFB concentration is high, which may indicate an increase in turbidity due to UFB turbidity.

Regarding the difference between the mode and mean values in the control sample, we suspect that the bubble size may have been slightly larger because of the fine soil particles adhering to the UFB; in the paddy field, it is clear that the adhered UFB tended to be crushed and reduced, and the turbidity also decreased. Hence, we believe that our hypothesis is plausible. The results of this study are insufficient to suggest any influence from weather conditions. However, we will consider the influence of weather using similar tests in an ongoing study at the University of Shiga Prefecture. We have included a supplementary figure for further information.

In this study, we examined the coagulation effect of fine soil particles when UFB water was used in rice cultivation. The concentration of bubbles and fine soil particles, turbidity, and variation in density distribution over time were the variables considered. Our observations and experiments established a time-dependence relationship between UFB and fine soil particles in rice-cultivated soil, and we found that a relationship existed between the properties of UFB and water turbidity. In particular, the data in the paddy field showed that turbidity reduction (2 NTU or less) occurred over a short period (3 days). In the distribution of bubbles and fine soil particles, the average UFB size increased because of the aggregation of fine soil particles (control), and the decrease in turbidity in the paddy field indicated that UFB attract fine soil particles and thereby improve the water quality.

We thank Eisei Sakata of Eatech Co. Ltd. for their provision of an ultra-fine bubble generator. We also thank Masaharu Kitajima of Wing Amagi Co. Ltd. for conducting water quality surveys in paddy fields. We sampled the paddy field water with the consent of the farmers, and they agreed to the publishing of the results as an original paper.

This work was supported by the Fund for the Promotion of Joint International Research (grant number 19KK0158). The funding agency had no role in study design, in the collection, analysis, and interpretation of data, in the writing of the report, and in the decision to submit the article for publication.

Y. Ueda and M. Iijima conceived the presented idea. Y. Ueda developed the theory and verified the analytical methods. M. Iijima supervised the findings of this work. Y. Izumi, Y. Hirooka, and Y. Watanabe carried out the field experiments. Y. Ueda wrote the manuscript with support from M. Iijima, Y. Hirooka, Y. Watanabe, and Y. Izumi. All authors discussed the results and contributed to the final manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.