Behavior of Typical Pollutants and Their Ecological Consequences
The speciation and behavior of pollutants determine their geochemical processes and bioavailability. Investigation of these characteristics of pollutants, which are not as yet fully understood, is one of the most important foci of research in environmental science. This limits our understanding of the transport and transformation of pollutants in the environment and associated ecological risks.
The above concerns represent frontier issues in environmental sciences, geochemistry and ecology. There have been numerous studies on these questions. Studies on the speciation of pollutants in environments have tended to be more at a micro- than macro-scale. More robust evidence can be obtained via probing the speciation of the pollutants at the molecular level. Synchrotron-based facilities, X-ray diffraction and Fourier-transformed infrared techniques are widely used methods. Solid-liquid partitioning, transformation and bioaccumulation of contaminants are largely controlled by their speciation in the environment. Through investigating the interactive behavior and speciation in simple systems, the processes and mechanisms of their transport and transformation can be understood. Stable and radioactive isotopes and non-destructive techniques are convenient tools. Based on the international status of research and our focal areas for research in this field, we have prioritized our research interests as: (1) Speciation and behavior of contaminants at a micro-scale; and (2) Bioavailability of contaminants in the food web.
Micro-scale speciation and environmental behavior of pollutants (As, Cd, PAHs). Arsenate is believed to be present primarily as ferric arsenate surface precipitate on the surface of ferrihydrite in acidic media (pH<5), rather than solely as bidentate-binuclear surface complexes. In iron-arsenate co-precipitation systems, ferric arsenate may also form and constitutes the major arsenic phase under acidic pH conditions; while at neutral to alkalinic pH conditions, yukonite and ferrihydrite-adsorbed arsenate are the most prevalent arsenic species. Under the impact of microbial activities, the arsenic in sediments may be reduced and re-precipitated as arsenic sulfide. We have also constructed a model of the adsorption and oxidation of arsenite on manganese dioxide. The transport processes and underlying mechanisms in the accumulation of Cd in soil columns were further clarified. Through ex-situ and in-situ leaching experiments, we found that the leaching effect of mixing organic acid with surfactant was greater than the application of organic acid alone under the optimal conditions, resulting in both a significant reduction in the concentration of Cd in topsoil and its accumulation in the lower soil profile. The effects of molecular mass and initial concentrations of PAHs on desorption processes from aged and contaminated pasture soils were investigated. Results show that pore distribution plays an important role in the adsorption and desorption of PAHs. We also found that the concentration of trichlorobenzene in rice was strongly correlated with root uptake. Results of our research have enhanced our understanding of speciation processes of contaminants in the environment, have further contributed to adsorption theory of heavy metals on minerals, and can be used to predict the speciation, behavior and fate of contaminants in the environment.
Bioavailability of organic and inorganic contaminants. Our team found that plant growth inhibitors significantly reduced the accumulation of cadmium in marigold plant tissues, and that after addition of glutathione to the soil, inhibition by maleic hydrazide was effectively alleviated, while actidione had no inhibitive effect. Sulfur deficiency resulted in the reduction of phytochelatins that are synthesized in marigold roots, thereby reducing the absorption of sulfur by plants. Foliar application of sulfate and cystine promoted Cd accumulation in marigold plant tissue, but increasing temperature reduced the accumulation of cadmium, suggesting that high temperature may have interfered with sulfur absorption by the plant. We also found that the thiol gene played an important role in the process of cadmium absorption in marigold plant tissues. We have screened a species of willow that is strongly resistant to cadmium toxicity and able to transport Cd through the phloem to the apex, as evidenced by the extremely high concentration of Cd in the phloem relative to the basal part of the plant.
We studied accumulation of Cd in common woody plants, and found that these plants displayed taxonomic variations in the adsorption of soil cadmium ¾ i.e., poplar > willow > apricot and plum > locust and arborvitae. We concluded that poplar and willow trees are ideal plant species for remediation of Cd-contaminated soils. We have also used 14C-labeling techniques to examine aging, metabolism and absorption processes of phenanthrene in wheat. Results showed that phenanthrene was generally enriched in wheat roots rather than wheat stems, suggesting that it cannot be transferred to the upper part of the plant. These studies have revealed the effects of chemical speciation of heavy metals on biological enrichment processes and are important for predicting environmental and ecological risks of heavy metals and the remediation of soils contaminated with heavy metals.