Two immobilization methods (i.e., ice water-soaked using a digital temperature controller vs. freeze-dried using liquid nitrogen) were applied for mass production techniques of TiO2-embedded expanded polystyrene (TiEPS) balls with nanoscale TiO2 particles embedded on EPS balls. No significant changes in crystalline structure of TiO2 nanoparticles embedded on the TiEPS balls were observed during the mass production of TiEPS balls. Greater residuals of freeze-dried TiEPS balls suggested the improved immobilization methods for mass production procedures of TiEPS balls. Although similar growth inhibition between TiEPS balls using two immobilization methods was observed within 10 hrs, both growth and reproduction of M. aeruginosa can be more significantly inhibited by applying the freeze-dried TiEPS balls after 10 hrs. These results were mainly attributed to the difference in exposed surface area of embedded TiO2 nanoparticles which generated various reactive oxygen species peroxidizing and leading to the inactivation and degradation of M. aeruginosa. Relatively greater k value (0.207 day-1) was estimated from freeze-dried TiEPS balls than that (0.089 day-1) from ice water-soaked TiEPS balls, suggesting that both growth and reproduction of M. aeruginosa were effectively inhibited with greater amounts of reactive oxygen species generated from freeze-dried TiEPS balls. Consequently, self-floating freeze-dried TiEPS balls can be readily applied to inhibit the excessive growth of harmful algae in the stagnant water body without the recovery process for long time.
Mass production technique of nanoscale TiO2 particle-embedded expanded polystyrene (EPS) balls with temperature-controlled melting method was developed, and the photocatalytic activity of TiO2-embedded EPS (TiEPS) balls to suppress the excessive growth of Microcystis aeruginosa (M. aeruginosa) cultured from both indoor and outdoor experiments was verified under ultraviolet and solar light irradiation, respectively. According to the experimental results, the growth inhibition of M. aeruginosa was evidently observed by applying TiEPS balls, and increased proportionally with the surface area coverage of TiEPS balls. Based on the comparison of both specific growth rate (μ) and first-order degradation rate (k), the experimental cases using TiEPS balls with surface area coverage of 100% suppressed more significantly the growth of M. aeruginosa cultured from both indoor and outdoor experiments during the initial period of the experiment. However, through the whole experiment, both μ and k values between experimental cases using TiEPS balls and EPS balls with surface area coverage of 100% were not statistically different (p > 0.05). These results indicated that the photocatalytic degradation effect was dominant during the initial period of application, and the irradiation shading became dominant with the elapsed time. Therefore, the growth of M. aeruginosa can be suppressed due to both synergistic and combinational effects of photocatalytic degradation and irradiation shading under natural solar light. Based on the aforementioned results, self-floating TiEPS balls produced from this simple and cost-effective mass production technique can be readily applied to inhibit the excessive growth of harmful algae in the stagnant water body.
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