Rainwater runoff and pore water migrating through soils contain large amounts of fine colloids ranging from a few nanometers to several micrometers in size, including clay minerals as well as organic matter and microorganisms, and metal oxides (Fig. 1). It has been pointed out that the surfaces of these colloidal particles are electrically charged, enabling them to adsorb contaminants and nutrients and thereby act as carriers for their transport.

The mobility of colloids strongly depends on aggregation processes, in which colloids collide and attach to one another (coagulate) to form aggregates, as well as on deposition processes, in which colloids are captured by the soil matrix.

Through research on aggregation and deposition, we aim to contribute to a better understanding of fate and transport of substances/contaminants in water and soil environments.

In particular, by leveraging approaches from colloid and interfacial chemistry, we have been continuously conducting fundamental research that supports the prediction of the environmental fate and transport of nano- and microplastic particles, whose impacts have raised increasing concern in recent years.

Although flow is generally present in environmental systems, the influence of electrostatic interaction forces on aggregation rates under flow had not been sufficiently validated from either experimental or theoretical perspectives (Fig. 2).

To address this gap, we measured aggregation rates in well-defined flow fields using colloidal particles whose surface charge could be accurately quantified, and performed corresponding theoretical analyses.

As a result, we successfully demonstrated qualitative agreement between the theoretical predictions and the experimental measurements.

・T. Sugimoto, M. Kobayashi, and Y. Adachi, "Aggregation rate of charged colloidal particles in a shear flow: Trajectory analysis using non-linear Poisson-Boltzmann solution", J. Applied Mechanics, JSCE, 70, I_475-I_482, 2014.

・T. Sugimoto, Y. Watanabe, and M. Kobayashi, "Kinetics of turbulent hetero-coagulation of oppositely charged colloidal particles", Theoretical and Applied Mechanics Japan, 63, 133-145, 2015.

Phosphate, an essential nutrient, and highly toxic arsenic are present in dissolved form as anions, and are known to adsorb onto positively charged clay minerals and metal oxides in soils.

When such specifically adsorbing ions accumulate excessively at particle surfaces, they can neutralize the original colloidal charge and induce charge reversal, in which the sign of the net surface charge becomes opposite. Once a positively charged colloid undergoes charge reversal, its newly acquired negative charge generates electrostatic repulsion against coexisting negatively charged clay colloids, leading to repulsion-dominated heteroaggregation.

To investigate this problem, we conducted measurements under conditions where collisions occur via Brownian motion, and performed theoretical analyses. As a result, we successfully explained—using a standard theoretical model—the experimental observation that heteroaggregation becomes repulsive within the salt-concentration range where charge reversal occurs.

・T. Cao, T. Sugimoto, I. Szilagyi, G. Trefalt, and M. Borkovec, "Heteroaggregation of oppositely charged particles in the presence of multivalent ions", Phys. Chem. Chem. Phys., 19, 15160-15171, 2017.

・T. Sugimoto, T. Cao, I. Szilagyi, M. Borkovec, and G. Trefalt, "Aggregation and charging of sulfate and amidine latex particles in the presence of oxyanions", Journal of Colloid and Interface Science, 524, 456-464, 2018.

In recent years, nano-bubbles—fine bubbles with diameters of a few micrometers or less—have attracted attention for their potential to remediate contaminated soils.

However, when injecting nanobubbles into soils, it has remained unclear how coexisting soil colloids affect nanobubble transport (Fig. 4).

To address this issue, we conducted column transport experiments using a model soil system in which colloids and nanobubbles coexist. We employed a resonant mass measurement technique that distinguishes and counts solid colloids (nanoplastic particles) and nanobubbles based on differences in particle–fluid density.

We found that when colloids were introduced first, the deposited colloids acted as new adsorption sites for nanobubbles via physicochemical interactions. As a result, nanobubble transport was more strongly hindered than in the absence of colloid deposition.

・T. Sugimoto, S. Hamamoto, T. Nishimura, "Inhibited nanobubble transport in a saturated porous medium: Effects of deposited colloidal particles", Journal of Contaminant Hydrology, 242, 103854, 2021.