N-propylbenzene (n-PBZ) and isopropylbenzene (i-PBZ) are considered hazardous and noxious substances, and once leaked into the sea, they will cause serious harm to the marine ecosystems. Our previous study has found that the removal rate of n-PBZ and i-PBZ at a concentration of 5 mg/L by Rhodomonas reticulata S6A within 7 days were 34.6% and 38.8%, respectively. To further clarify the suitable environmental conditions for the remediation of propylbenzene-contaminated seawaters, the effects of salinity, temperature and light intensity on the removal of n-PBZ and i-PBZ from seawater by R. reticulata S6A were investigated in this study using response surface methodology (RSM) based on the Box-Behnken design. The results indicated that light intensity and temperature were the key factors affecting the removal of n-PBZ and i-PBZ, respectively. Additionally, the interactions between temperature and salinity, as well as temperature and light intensity, were found to be significant. The optimal environmental condition for the microalga to remove n-PBZ was as follows, salinity 27.2, temperature 25.1 ℃ and light intensity 237.9 μmol/(m²·s) with a measured removal rate of 51.2%. The most suitable environmental condition for removing i-PBZ was as follows, salinity 28.4, temperature 25.5 ℃ and light intensity 580.8 μmol/(m²·s) with a measured removal rate of 54.6%. The removal rate prediction model constructed based on the experimental results could be used to estimate the bioremediation effect in propylbenzene-contaminated seawaters, providing a decision-making basis for determining the appropriate occasions and times for using the microalga.

The attenuation characteristics of hazardous chemicals in seawater are crucial for taking targeted emergency measures after their leakage accident occurs. In order to investigate the attenuation trend of n-phenylbenzene (n-PBZ) and isopropylbenzene (i-PBZ) spilled into the sea with changes in environmental conditions, a microcosm system was used to adjust temperature, salinity, pH, light intensity, and wind speed, respectively. Based on the concentration changes of n-PBZ and i-PBZ in seawater, attenuation kinetics analysis was conducted to determine the environmental factors that had a significant impact and the adverse effects of low wind speed on their attenuation. The results showed that temperature was the factor that had the greatest impact on the attenuation of each propylbenzene with a significantly positive correlation with the attenuation rate constant (k). The k values of n-PBZ and i-PBZ at 30 ℃ were 2.11 and 1.90 times higher than those at 5 ℃, respectively. Illumination was also beneficial for the attenuation of n-PBZ and i-PBZ. The k value at light intensity of 800 μmol/(m²·s) were 1.63 and 1.57 times higher than those under dark conditions, respectively. At lower salinity, the k value increased with the increase of salinity, but remained stable at salinity ≥10. The change in seawater pH value (6~9) had no significant effect on the attenuation of propylbenzene. The low wind speed (1 m/s) commonly seen in foggy weather was very unfavorable for the attenuation of n-PBZ and i-PBZ with the k value less than 60% of those at a wind speed of 5 m/s, resulting in the accident area at medium to high ecological risks for a long period (over 20 days). Our findings demonstrated that temperature, light intensity, and wind speed are the main environmental factors affecting the natural attenuation of n-PBZ and i-PBZ in seawater.