3.2.1 Effect of particle size
The removal efficiency and effluent concentration of naphthalene under different particle sizes were shown in Fig. 5. As shown in Fig. 5. (a), the addition of PM with different particle sizes can improve the removal efficiency of naphthalene in bioretention facility. With the particle size of 0, 0 ~ 45, 45 ~ 75 and 75 ~ 150 µm at 50 cm depth, the average removal efficiencies for naphthalene were 81%, 92%, 84% and 87%, respectively. On the one hand, most of PM were trapped in bioretention facility, the adsorbed naphthalene onto PM was removed at the same time. On the other hand, the trapped PM can act as media for naphthalene adsorption and improving the removal efficiency further. Besides, when the particle size was 0 ~ 45 and 75 ~ 150 µm, the average removal efficiency decreased from 74%, 81%, 90% and 92–66%, 74%, 82% and 85% at 10 cm, 20 cm, 40 cm and 50 cm depth in bioretention facility. The similar decreasing phenomenon on removal efficiency with the increase of particle size was also found in Shen's study (Shen et al. 2021). In addition, the removal efficiency of naphthalene in the top 10cm reached a satisfactory level under the influences of 0 ~ 45µm particles. Small particle size with high specific surface area could carry more naphthalene and promote the adsorption of naphthalene by the media at the same time, showing a high removal efficiency (Li et al. 2006, Yi et al. 2012).
Besides, as indicated in the Fig. 5. (a), the average removal efficiency of naphthalene gradually increased with the increase on depth. The removal efficiency of naphthalene at 10 cm depth accounted for 73%, 81%, 78%, and 77% of the overall removal efficiency when the particle sizes was 0, 0 ~ 45, 45 ~ 75, and 75 ~ 150 µm. Therefore, the main removal layer of naphthalene in bioretention facilities was the top layer. Xiong et al. also obtained that the removal of heavy metals by bioretention facility mainly occurred at upper layers (Xiong et al. 2021).Compared with zeolite and gravel at bottom, the sandy layer at top would offer lower porosity and resulted in a large amount of PM and naphthalene retained (Hatt et al. 2009). In addition, due to the trapped PM at top, the removal efficiency of naphthalene at 10 cm depth further improved which may be attributed to the decreased porosity and increased residence time.
As shown in Fig. 5. (b), the difference of effluent concentration with time at different particle sizes was obvious. When particle sizes of PM were 0, 0 ~ 45, 45 ~ 75, and 75 ~ 150 µm in runoff, with the contact time increased from 5 min to 120 min, the effluent concentration of naphthalene increased from 0.28 mg/L, 0.08 mg/L, 0.16 mg/L and 0.20 mg/L to 0.56 mg/L, 0.29 mg/L, 0.38 mg/L and 0.45 mg/L. With the increase of time, the adsorption sites of naphthalene by the media gradually decreased, so the migration gradually increased. The final effluent concentration of naphthalene was low under the influences of PM, indicating that PM was beneficial to reduce the migration of naphthalene in bioretention facilities. Besides, the effluent concentration of naphthalene increased by 0.28 mg/L, 0.21 mg/L, 0.22 mg/L and 0.25 mg/L when particle sizes of PM were 0, 0 ~ 45, 45 ~ 75, and 75 ~ 150 µm in runoff, respectively. Due to the stronger carrying capacity of PM with small particle size, they could greater share the adsorption pressure of naphthalene by the media, so the migration was worse.
3.2.2. Effect of PM concentration
Figure 6 showed the removal efficiency and migration of naphthalene under the influences of different PM concentrations of each layer in bioretention facilities. As can be seen in Fig. 6. (a), it is possible to note that naphthalene removal performance of each layer under the influences of different PM concentrations was better than without PM in bioretention facility. When PM with concentrations of 0, 100, 300, and 500 mg/L, the average effluent concentrations of naphthalene were 0.37 mg/L, 0.32mg/L, 0.28 mg/L, 0.21 mg/L, and the corresponding removal efficiency was 82%, 84%, 86%, and 90%, respectively. The higher the concentration of PM, the higher the efficiency for naphthalene removal and the worse the mobility in bioretention facility, which is similar to Li's research results (Li et al. 2013). Besides, under four different concentrations of PM, each layer had a satisfactory removal effect on naphthalene, more than 60% removal efficiency could be achieved, especially in the influences of 500 mg/L PM could stably keep above 70% removal efficiency. Due to the strong retention of PM at the top, the higher the concentration of PM in runoff, the greater amount of PM and naphthalene was retained at the top. The porosity decreased and the adsorption of naphthalene onto the media increased, so the removal efficiency increased further.
Besides, when there was no PM in runoff, the removal efficiency of naphthalene was 59%, 67%, 77% and 81% at 10 cm, 20 cm, 40 cm and 50 cm depth, respectively. However, when the PM concentration was 500 mg/L, the removal efficiency increased by 11%, 10%, 7% and 6% at 10 cm, 20 cm, 40 cm and 50 cm depth, respectively. The addition of PM had a decreasing effect on the removal efficiency of each media layer from the top to bottom. Compared to the zeolite and gravel at the bottom, the top media was sandy soil, which had a smaller particle size. Therefore, the adsorption and interception of naphthalene by the media in the biretention facility decreased from top to bottom. Besides, a large number of PM were trapped at the top, leading to a further decrease in porosity, so the removal efficiency of naphthalene from the top media was obviously improved.
As can be seen in Fig. 6. (b), unlike the effluent concentration without PM, lower effluent concentration of naphthalene was observed when the PM concentration was 100, 300, and 500 mg/L, respectively. When there was no PM in runoff, the contact time increased from 5 min to 120 min, and effluent concentration of naphthalene increased from 0.28 to 0.56 mg/L. Similarly, when the concentration of PM in runoff was 500mg/L, the effluent concentration of naphthalene increased from 0.07 mg/L to 0.40 mg/L. As can be seen from the final effluent concentration, the migration of naphthalene could be effectively inhibited under the influences of 500 mg/L PM. Besides, the first 30 min of contact increased the effluent concentration by 0.15 mg/L, 0.15 mg/L, 0.14 mg/L and 0.12 mg/L when PM with concentrations of 0, 100, 300 and 500 mg/L, respectively. However, the latter 90 min of contact increased by 0.13 mg/L, 0.12 mg/L, 0.12 mg/L and 0.10 mg/L. The effluent concentration changed relatively faster in the first 30 min, which indicated that the migration of naphthalene was enhanced faster and the migration rate was larger. Some studies have shown that naphthalene with strong hydrophobicity had a higher adsorption affinity with hydrophobic groups on the media surface (Degenkolb et al. 2018). Consequently, the faster the adsorption removal process on the internal media over a shorter period of time, thus the faster migration rate. With the contact time increased, the internal media tend to saturate at a slower rate. In addition, the greater the concentration of PM in runoff, with the lower the migration rate. This is because under the influence of high concentration of PM, the absorption of media enhanced for naphthalene, with a low mobility.