Study On Bioremediation Of Cadmium-contaminated Water By Chlamydomonas Reinhardtii

Heavy metal pollution in aqueous system is a significant world-wide problem. Heavy metal ions which are present as ions in wastewater are toxic and can be readily absorbed into the human body through the food chain in aquatic ecosystem. The prevention of heavy metal contamination in aquatic environments is often performed by conventional methods. However, these methods have many disadvantages, such as incomplete metal removal, toxic sludge generation and cost inefficiency. Metal uptake by microorganisms has been studied for some years. Researches indicate that algae have the abilities to accumulate trace metal. Based on which, technologies of bioremediation of heavy metal contaminated water by algae in living and non-living form have been developed and got more and more attention around world-wide for its cost-effective and environmentally friendly characteristics. However, there still exist some deficiencies in mechanism and application of bioremediation. So, further investigation is still needed to elucidate the process of bioremediation and optimize the maximum efficiency of removal. In this thesis, we established an adsorption system of Chlamydomonas reinhardtii and cadmium to study the tolerance of C. reinhardtii to cadmium through testing the toxic effects of cadmium and evaluate the practical prospect of the microalgae, use the adsorption isotherms models and biosorption kinetic models to fit the experiment data, study the influence from the surroundings factors such as pH, anions and cations, block groups of the microalgae chemically and identify the physical adsorption ability, study the method of immobilization of C. reinhardtii and the factors which affect the adsorption of the immobilized microalgae. Through our researches, several conclusions can be safely reached:C. reinhardtii was able to survive even at a very high cadmium concentration, especially the higher density the microalgae gathered together in, the more tolerant individuals of the microalgae survived, which will help to utilize the microalgae to remove heavy metals from polluted water.From the study of the microalgae adsorption isotherms and biosorption kinetics, Langmuir equation and Freundlich equation fitted very well the adsorption isotherms, Lagergren equation known as a first order kinetic model and the second kinetic model which based on a combination of Gauss-Newton and Levenberg-Marquardi methods both described the biosorption process andsimulated the experimental transient profiles, but the latter was better.The optimal pH, which was the major factor influencing the adsorption of cadmium, was around 6.0; and the higher concentration of cations such as K⁺, Na⁺, Ca²⁺, Mg²⁺, the lower the sorbing capacity of the microalgae; This effect, on the other hand, also illustrated that the ion-exchange was the main adsorption mechanism.Chemical modification and physical adsorption experiments demonstrated that the dominant functional groups which present on the cell surface are the carboxyl, amide and hydroxyl. They play major roles in the complexation of heavy metal; the maximum contribution of physical adsorption in the overall biosorption process is about 5.5%.C. reinhardtii can be reused several times through desorption, the living microalgae should be eluded with mild chemical reagents such as NaCl. 0.1mol/L HC1 was one of optimal selection for non-living microalgae.