Study On Supercritical CO₂ Chelating Extraction Of Copper, Lead, Cadmium And Effect On Catechin And Caffeine In Green Tea

The content of heavy metals in green tea have increasingly increased as the environment polluted, which significantly affect the quality of the green tea and the development of the whole tea industry, restrict the export of tea product. Supercritical carbon dioxide chelating extraction (SCCE), as a new separation and extraction method, compared with traditional extraction methods, has a lot of advantages including higher extraction speed, less comsumption of organic and toxic solvent, less pollution to environment. It's a new technology coming up with the development direction of green chemistry.This paper firstly summarized the extraction mechanism, modifier and chelating agent in the process of SCCE, the application fields of different chelating agent and the applicaition examples in environmental protection, nuclear industry and food industry. We also analyzed the difficults and reviewed the futural development of the SCCE technology. Secondly, our experiment systematically investigated the effect of the threeheavy metals (copper, lead and cadmium) in green tea by SCCE, and analyzed the content variety of the major valid compounds (catechin and caffeine) in green tea after SCCE. The major works of our experiment were listed as follows:(1) extraction and determination of the three heavy metals (copper, lead and cadmium) in green tea. (2)in order to find out the optimum extraction condition of the three metals in green tea by SCCE, we carried out single-factor experiment, four-factor and three-level orthogonal design experiment and central composite design experiment. (3) weinvestegated the varieties on the content of catechin and caffeine in green tea before and after SCCE of the three heavy metals to find out the change law of the major valid compounds (catechin and caffeine) in green tea in the process of SCCE. The experiment results were listed as follows:(1) Flame furnace atomic absorption spectrometry method conbined with microwave digestion was established to precisely determine the content of copper, lead and cadmium in green tea. The content of copper, lead and cadmium in green tea sample determined in the optimum condition was 19.92μg/g,1.63μg/g and 0.42μg/g, respectively.(2) The major factors affecting the the extraction efficiency of the three heavy metals in green tea sample was the extraction pressure and extraction temperature. Central composite design experiment was carried out to optimize the extraction condition of copper, lead and cadmium in green tea sample, and the total extraction yield of the three heavy metals could reached 73.9%, which is the maximum in our experiment, and the extraction yield of copper, lead and cadmium could reached 75.1%,62.3% and 61.9%at the optimum condition, respectively.(3) Ultrasonic-solvent extraction and ultraviolet spectrophotometer were used to determine the content of catechin in green tea. The result showed that the content of catechin was 282.2mg/g. After SCCE of heavy metals, the content of catechin in green tea changed few in the range of 4.7%-7.7%. Solvent extraction and high pressure liquid chromatography were used to determine the content of caffeine in green tea. The result showed that the content of catechin in green tea was 41.15mg/g. After SCCE of heavy metals, the content of caffeine in green tea changed significantly in the range of 52.1%-66.1%.