The Temporal And Spacial Distribution Mechanism Of Pd In Tropical Urban Rainfall-Runoff

Platinum Group Elements(PGEs)have good catalytic performance on VECs(Vehicle Exhaust Catalysts).However,due to the rapid increase of motor vehicles,environmental problems causing by PGEs are becoming more and more serious.With the acceleration of the urbanization process,the impervious layer area in the city increases rapidly.The non-point source pollution is difficult to control.In this study,Haikou city was selected as the research area,and the traffic area,cultural district and commercial district were also selected as the monitoring points to collect runoff samples.Concentrations of dissolved and suspended Pd(palladium)were measured by inductively coupled plasma-mass spectrometry(ICP-MS),while the determination of pH,Eh,Cl(chloride ion)and TSS(Total Suspended Solid)was also measured,respectively.Then the flush process of Pd,the pollution characteristics,the space and time variation,and solid-liquid process were analysed,and the influencing factors are discussed.The results show that the concentration of Pd in runoff decreased gradually in a rainfall event.Both dissolved Pd and suspended Pd present high concentration and obvious nonlinear scouring process in the traffic area.The differences between cultural district and commercial district are not obvious.The runoff process and flow conditions of each area are different.During the flush process,Pd concentration fluctuates a lot,especially in the traffic area.However,the concentration fluctuation of cultural district and commercial district are quite different from those of traffic areas.Meanwhile,there are some spatial variations in the flush effect of Pd,but they are all dominated by the first-flush effect.Moreover,the first-flush effect often occurs in the field with high rainfall intensity.The spatial distribution of EMC of two phases of Pd in rainfall runoff is consistent,showing a high level in the traffic area,but the EMC of dissolved Pd also occupies a large proportion in the commercial area,while the EMC of two phases of Pd in the cultural district is the lowest.The EMC value of dissolved Pd in dry season is higher than that in wet season.Although the EMC of suspended Pd has similar temporal distribution,the seasonal variation is not obvious.The accumulation of Pd increased in the dry period before rain.Because of the long dry period before rain,the concentration of Pd increased.High rainfall intensity lead to excessive runoff dilution and low Pd concentration.Although the decrease of rainfall intensity rose the Pd concentration,low rainfall intensity may prevent particle Pd from being washed away,resulting in low suspended Pd concentration.Water parameters have little effect on suspended Pd.The dissolved Pd concentration could be strongly affected by the water parameters of rainfall runoff.When pH decreases,the concentration of dissolved Pd increased,and the increase of oxidation will also cause the increase of its concentration.Cl and dissolved Pd showed a very significant positive correlation.Under the combined action of acidity and oxidation,the concentration of Pd was greatly affected.The solid-liquid process of Pd in rainfall runoff was also affected by TSS concentration.The concentration of suspended Pd is positively correlated with TSS concentration.Besides,TSS is also an important factor affecting the Pd partition coefficient.The changes of the Pd partition coefficient is consistent with the suspended Pd process.Rainfall characteristics,hydrochemical conditions and other factors have an impact on the distribution.