Transfer of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans from Ambient Air to Banyan Leaves

Long-Full Lin1, Wen-Jhy Lee1, Chia-Yang Hung1, and Guo-Ping Chang-Chien2

1Department of Environmental Engineering, National Cheng Kung University, Tainan, Taiwan2Department of Chemical Engineering, Cheng-Shiu University, Kaohsiung, Taiwan

Introduction

Due to the toxicity of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), their transfer and fate in the environment is an issue of great concern. In general, the content of PCDD/Fs in plant leaves is considered the result of short term exposure, and is chosen as an intermediate acceptor of PCDD/Fs in the environment to discuss the possible pathway of PCDD/Fs entering plant leaves. Among the various species of plant, banyan (Fig. 1) is a good indicatory species owing to its wide spread nature in Taiwan.

This study investigated the characteristics of PCDD/Fs in banyan leaves located in urban areas of Taiwan, with the results used to evaluate the transfer of PCDD/Fs from ambient air to plant leaves. In addition, the total degradation coefficient and half-life time of PCDD/Fs in banyan leaves were estimated as well.

Experimental Section

The primary pathway for PCDD/Fs entering the banyan leaves was by particle phase adsorption (86%) and the mean half-life time of PCDDs and PCDFs is 426 hrs and 276 hrs in banyan leaf, respectively.

Results and Discussion

Conclusions

Fig. 2. The stoma of banyan leaf.

The PCDD/F concentrations in the ambient air ranged between 0.038 and 0.136 pg I-TEQ/Nm3 and averaged 0.090 pg I-TEQ/Nm3 (RSD:29.7%; n=28). The percentage of particle-bound PCDD/Fs in the ambient air ranged between 49% (2,3,7,8-TCDD) and 97% (OCDD). The maximum pore diameter that allowed suspended particles to enter the banyan leaves is 3.2μm (Fig. 2), and the particle-bound PCDD/Fs with aerodynamic diameter (dae) less than 3.2μm is 92%.

The composition of PCDD/Fs in the banyan leaves ranged from 1.06 to 8.53 ng I-TEQ/kg dry weight with an average of 4.00 ng I-TEQ/kg dry weight (RSD:44.0%; n=28).

A total of seven ambient air and banyan leaf sites located in urban areas were sampled and analyzed for PCDD/Fs. The particle size distribution of PCDD/Fs in ambient air was measured using a micro-orifice uniform-deposit impactor (MOUDI) and Noll rotary impactor (NRI). The pore size of banyan stoma was measured using a scanning electron microscope (SEM).

The content of PCDD/Fs adsorbed on the surface of banyan leaves ranged from 0.0021 to 0.0084 ng I-TEQ/kg with an average of 0.0062 ng I-TEQ/kg, which is minor compared with that adsorbed in the banyan leaves.

The primary pathway for PCDD/Fs entering the banyan leaves was by particle phase adsorption (86%), which is a different conclusion to a previous study that found gas phase absorption was the main pathway for PCDD/Fs entering the leaves (Lorber, 1995; McLachlan, 1997).

In addition, the degradation coefficient ktot (including the photolysis and the biodegradation) of PCDDs, PCDFs and total PCDD/Fs was 0.0390 d-1 , 0.0603 d-1 and 0.0499 d-1, respectively, using pseudo first order kinetics. The half-life time of PCDDs in banyan leaves was from 327 hrs (1,2,3,4,7,8-HxCDD) to 458 hrs (OCDD) and averaged 426 hrs. For PCDFs, the half-life time was from 240 hrs (1,2,3,7,8,9-HxCDF) to 297 hrs (1,2,3,7,8-PeCDF) and the mean half-life was 276 hrs.

Fig. 1. Banyan

Lober, M. (1995). Development of an air-to-leaf vapour phase transfer factor for dioxins and furans, Organohalogen Compd., 24, 179-186.McLachlan, M. S. (1997). A simple model to predict accumulation of PCDD/Fs in an agricultural food chain, Chemosphere, 34, 1263-1276.

References

Keywords

Dioxins, Gas/Particle Partition, Particle Size Distribution, Banyan Leaf, Half-Life Time