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Study on the Accumulation and Remove of Cadmium and Lead in Oysters

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Tutor: ZhangChaoZuo
School: Guangdong Ocean University
Course: Aquatic Products Processing and Storage Engineering
Keywords: chitosan,N,O-carboxymethyl chitosan,cadmium,lead,metallothionein
CLC: X174
Type: Master's thesis
Year:  2011
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Abstract:
With the rapid development of modern industry and agriculture, large amount of industrial wastewater and domestic sewage dumped into water, and the content of heavy metals was continuously increasing in coastal marine environment. Shellfish, which belong to filter feeding organisms, can accumulate heavy metals in seawater, especially cadmium£¨Cd£© and lead£¨Pb£© which are widely existed and have formed serious potential dangers to aquatic product safety. Meanwhile, heavy metals have the characteristics of hard degradation and biological magnification, they will be biomagnified and transferred along with food chains, which would threaten the health of human beings. At present, the shellfish purification technique is mainly against pathogenic microorganism, and there are no available methods to purify the shellfish polluted by heavy metals, so study on how to decrease or remove the heavy metal residues in shellfish is of great significance.Chitosan£¨CTS£© has a series of virtues such as abundance in resources, nontoxicity, biodegradability and efficient metal chelater. In this paper, using the methods of single factor experiment, adsorption dynamics and thermodynamics, the adsorption property of CTS and its derivate N,O-carboxymethyl chitosan£¨N,O-CMC£© for heavy metals Cd2+ and Pb2+ in single and mixed solution were studied. Through establishing Cd2+¡¢Pb2+ contaminative models of oysters, and determining the content of heavy metals and inductive metallothionein£¨MT£© during accumulation process, exsit style of heavey metals in oysters was revealed. Then the drainoff experiment of heavy metals in living oysters were discussed, which can provide theoretical basis for effectively decreasing and removing the heavy metal residues in fresh shellfish, and provide guarantee for the aquatic product safety and human health.The main conclusions were shown as follows:£¨1£© CTS was used to investigate the adoption of two metals ions Cd2+ and Pb2+ in different pH, temperature, reaction time, adsorbent dose and initial metal ions concentration. The results showed that pH, temperature, reaction time, adsorbent dose and initial metal ions concentration had great influence on adsorption of CTS for Cd2+ and Pb2+ in single solution, but the effect of temperature was of no significant on adsorption of CTS for Cd2+ and Pb2+ in mixed solution. By studying CTS adsorption kinetics of Cd2+ and Pb2+ in single and mixed solution, it can be found that they were both according with Lagergren equation adsorption model 2. The adsorption thermodynamics of CTS for Pb2+ in single and mixed solution was well followed as the Langmuir isotherm equation; for Cd2+, the adsorption of CTS in single solution was according with Langmuir adsorption equation, but in mixed solution was not.£¨2£© The N,O-CMC was used to investigate the adoption of two metals ions Cd2+ and Pb2+ in different pH, temperature, reaction time, adsorbent dose and initial metal ions concentration. The results indicated that pH, reaction time, adsorbent dose and initial metal ions concentration had great effect on adsorption of N,O-CMC for Cd2+ and Pb2+ in single and mixed solution. The adsorption kinetics of N,O-CMC for Cd2+ and Pb2+ in single and mixed solution were both according with Lagergren equation adsorption model 2; but in single and mixed solution, the adsorption thermodynamics of N,O-CMC for Cd2+ were agreed with the Langmuir isotherm equation under certain concentration, for Pb2+, it was followed as the Langmuir isotherm equation within the scope of experiment.£¨3£© The biological enrichment capacity of oyster to Cd2+ and Pb2+ was high with certain time and dose effect, and the bioconcentration ability of oysters for Cd2+ was significant higher than Pb2+. The heavy metal content in the oysters in each exposure group were mainly higher than standard limit in 2 d; simultaneously, amounts of MT were synthetized when induced by Cd2+ and Pb2+ in oysters, and the two had correlativity. After Cd2+ and Pb2+ exposure, the oysters were treated with CTS and N,O-CMC, it was found that they can remove Cd2+ and Pb2+ in oysters. It was same to the in vitro experiment that N,O-CMC had better removal ability. N,O-CMC played a great role in the promotion of Pb2+ discharge for the light pollution oyster with Pb2+, it can reach standard limit in 6 d and its content was 0.73 mg/kg.£¨4£© Studies on isolation and depuration of MT which was induced in oysters by Cd and Pb. Through acid treatment experiment, the determination of metal ions content in column liquid and capillary electrophoresis detection, it confirmed that Cd2+ and Pb2+ which enrichment in oysters were binding with MT.
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