Mechanisms And Processes For Plant Uptake Of PAHs And Prediction Of Uptake With A Partition-limited Model

Full understanding of how plants uptake and accumulate organic contaminants like polycyclic aromatic hydrocarbons (PAHs) have considerable benefits for risk assessment of crop contamination and subsequent human exposure and the development of phytoremediation. In this dissertation, the uptake, accumulation and translocation behavior of PAHs in ryegrass was investigated using a hydroponic system. Through comparing the sorption of PAHs by biomass and the uptake of PAHs by ryegrass, the mechanisms for plant uptake of PAHs were discussed. The kinetic uptake of PAHs by ryegrass either from water or air was studied. In addition, a partition-limited model was used to predict and quantify the accumulation and translocation behavior of PAHs in ryegrass, and major influential factors on the model performance were examined, with great emphasis on the impact of foliar uptake. The main valuable conclusions are shown as follows:(1) Root uptake of PAHs from water was very fast. Root concentration factors (RCF) for acenaphthene, fluorene, phenanthrene and pyrene were 5.7,10.3, 19.4, and 63.6 ml/g, respectively, when the steady state was attained. These values were far below the corresponding figures of partition coefficients (127.2 ml/g for acenaphthene, 200.5 ml/g for fluorene, 571.4 ml/g for phenanthrene, and 3935.4 ml/g for pyrene), indicating that the uptake equilibrium was not attained. The velocity and extent of PAH uptake and accumulation by ryegrass shoot was much lower than that by root, and was largely determined by the chemical properties of the PAHs. Although plant shoot was able to uptake greater amount of PAHs with high K_ow values, the transpiration stream concentration factors (TSCF) for these chemicals were generally small. As a result, the uptake rate was greatly confined for PAHs with high K_ow values, and thus more time was needed to reach uptake equilibrium.(2) The partition-limited model had a good performance on predicting PAH uptake by ryegrass. Model predictions of PAH concentrations in roots and shoots of ryegrass were found to be all within an order of magnitude of the observed values. However, the model focused on root translocation only, while excluded foliar uptake, which resulted in a poor performance on predicting PAH concentrations in shoots. The differences between estimated and measured concentrations of PAHs in rootsand shoots were less than 42.1% and 78.4%, respectively. If the impact of foliar uptake on model performance was taken into account, the differences for all the four PAHs would be greatly reduced. Since the impact of foliar uptake increased with the increasing lipophilicity of the PAHs, the differences decreased in an order of pyrene > phenanthrene > fluorene > acenaphthene, with the maximum difference reduced from 78.4% down to 47.1% for pyrene.