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Title: Removal of heavy metals from synthetic wastewater by mangrove microcosms and their impacts
Other Titles: Wei guan hong shu lin xi tong dui wu shui zhong chong jin shu de jing hua neng li ji qi ying xiang
Authors: Yim, Ming-wai (嚴明威)
Department: Dept. of Biology and Chemistry
Degree: Master of Philosophy
Issue Date: 1999
Publisher: Dept. of Biology and Chemistry, City University of Hong Kong
Subjects: Mangrove ecology
Sewage -- Purification -- Heavy metals removal
Notes: CityU Call Number: TD758.5.H43 Y55 1999
Includes bibliographical references (leaves 172-180)
Thesis (M.Phil.)--City University of Hong Kong, 1999
xix, 180 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: Mangrove ecosystems are shown to be effective in treating different types of wastewater but the ecological functioning of the system might be damaged by the pollutants contained in wastewater. The present research project aims to study the feasibility of employing a simulated mangrove ecosystem to treat wastewater enriched with heavy metals. The treatment efficiency of different mangrove plant species and wastewater composition will be examined. The impact of wastewater-borne heavy metals on plant growth and soil microbial communities of mangrove ecosystem will also be examined. Three separated experiments were carried out to fulfill these objectives, results and discussions are presented in Chapters 4, 5 and 6, respectively. Chapter 3 describes the materials and methods of this study while Chapter 2 gives a literature review. Mangrove microcosms, consisting of fresh mangrove soil and young mangrove plants of either Kandelia candel or Bruguiera gymnorrhiza, were used to treat synthetic industrial wastewater containing various concentrations of heavy metals. The wastewater loading period of each experiment was around 6 months. The normal strength wastewater had heavy metal concentrations similar to the standard discharged to government treatment plants, i.e., Cu, Zn, Cd, Cr and Ni concentrations were 3, 5, 0.2, 2, and 3 mg L-', respectively. Removal efficiencies of heavy metals from normal strength industrial wastewater were very high in mangrove microcosms planted with Kandelia candel (>90%) but the removal efficiencies of heavy metals from strong wastewater (10 times the normal strength) were reduced, except for Cu. Mangrove microcosms planted with Bruguiera gymnorrhiza had higher removal efficiencies in treating strong wastewater than Kandelia candel, implying that mangrove ecosystems consisting of different plant species would have different heavy metal removal efficiencies. Nevertheless, large quantities of heavy metals in wastewater were retained in the soil component of the mangrove microcosms, and the removal of heavy metals by plant uptake was relatively small, often accounting for less than 2% of the total retention. For the impact of wastewater on mangrove microcosms, no harmful effects on mangrove plant growth parameters (e.g. stem height, leaf number, maximum leaf length and stem basal diameter) and total biomass production were observed even when the synthetic industrial wastewater applied to the microcosms had heavy metal concentration 8 times the normal strength of wastewater. However, very strong wastewater (10 times the normal strength) did exert serious adverse effects on plant growth as well as biomass production. Addition of nutrients to industrial wastewater did not have any significant effect on improving plant tolerance to heavy metals nor enhancing the heavy metal removal efficiency. On the other hand, the mangrove plant growth and vigour was enhanced if the microcosm was pretreated with synthetic municipal wastewater having COD, organic nitrogen, ammonium nitrogen, nitrate nitrogen and phosphate concentrations of 250, 3, 40, 1, and 10 mg L-', respectively, which in turn slightly reduced the harmful effects caused by heavy metals. ATP contents and alkaline phosphatase activities of the mangrove soils treated with different strengths of industrial wastewater were also reduced at the end of the wastewater loading period. The toxicity of soil elutriates increased with the strengths of wastewater. The results of soil microbial parameters show that microbial measurements were generally more sensitive to the toxicity of heavy metals than plant growth. Pretreatment of municipal wastewater had no beneficial effects on microbial communities. The present study suggests that although the mangrove ecosystem was capable of retaining heavy metals from wastewater, the metal toxicity of strong wastewater on mangrove plants and microbial communities must be carefully evaluated.
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