Efficient Preparation Of Tea-leaf Saponins And The Dual Role Of Tea Saponins In Phytoremediation Of Cadmium-contaminated Soil

Tea(Camellia sinensis(L.)O.Ktze)is one of the most important economic crops in China.Made from tender shoots and fresh leaves picked in spring,the beverage tea has multiple functions,such as anti-oxidation,and is loved by the people all over the world.However,due to the insufficient domestic tea market consumption capacity,tea is facing a relative overcapacity phenomenon in China.At the same time,heavy metal contamination in soils is a severe challenge faced by modern agriculture,and cadmium is the most severe inorganic contaminant in China.The phytoremediation of cadmiumcontaminated soil faces the problem of low remediation efficiency.The application of surfactants has been confirmed to effectively enhancing the bioavailability of heavy metals in soil and improving the efficiency of phytoremediation.As a kind of secondary metabolites of tea,tea saponins are also a kind of natural surfactants.Aiming at the problems of relatively overcapacity of tea leaves and low phytoremediation efficiency of cadmium-contaminated soils,this thesis takes tea-leaf saponins as the research object,confirms the feasibility of extracting tea saponins from the leaves of tea trees,establishes a fast and efficient preparation method for tea-leaf saponins,reveals the distribution and dynamic variations of tea-leaf saponins in tea trees,and discovers that tea-leaf saponins and tea-seed saponins have the same properties as surfactants;through independent soil experiments and hydroponic experiments,this thesis reveals the dual roles of tea saponins in phytoremediation of cadmium-contaminated soil,confirms the feasibility of applying tea saponins in phytoremediation of cadmium-containing wastewater,realizes the in-depth development and comprehensive utilization of tea-leaf resources,and improves the bioavailability of cadmium through the application of tea saponins,thereby achieves the purpose of improving the phytoremediation efficiency of cadmium-contaminated soils.The main research contents and results of this thesis are as follows.(1)Combined ultrasonic-assisted water extraction with acetone precipitation,a rapid and efficient method for the industrial preparation of tea-leaf saponins was established,and the feasibility of extracting tea saponins from tea leaves was confirmed.Through the optimizations of response surface methodology and single-variable experiments,optimized variables of ultrasonic-assisted water extraction were acquired,and they were 75 m L/g of liquid-solid ratio,78 w of ultrasonic power,60 ℃ of extraction temperature,and 20 min of extraction time(ultrasonic treatment time),respectively;optimum acetone-extraction solution ratio was 0.1 for acetone precipitation;after optimizations,the concentration of tea tree variety Jiukengzao in the extraction solution was 3.83 ± 0.0553 mg/m L,and the purity of tea-leaf saponins was76.5% ± 1.13%;the 23.0% yield of tea-leaf saponins for tea tree variety Jiukengzao calculated from the concentration got under optimized extraction conditions was significantly higher than the yield of tea-seed saponins with the same method,ultrasonic-assisted water extraction based on optimizations,whose value was 21.3%.(2)The distribution and dynamic variations of tea-leaf saponins in tea trees were revealed.By measuring the contents of tea-leaf saponins of different tea tree cultivars,different tea-leaf maturity,and different picking seasons,it was discovered that the content of tea-leaf saponins was related to tea tree cultivars,tea-leaf maturity,and tealeaf picking seasons;tea tree cultivars with later germination stage and larger leaf,and tea leaves with higher maturity on the same branch would have higher contents of tealeaf saponins;for the third leaf on the new shoot,summer is the preferred picking season,while for the matured leaf(the first leaf on the matured branch),autumn is the best choice;there was no conflict between picking matured tea leaves as a new source for extracting tea-leaf saponins and picking tender shoots of tea trees in spring.(3)Based on soil experiments,the first role of tea saponins on phytoremediation of cadmium-contaminated soils was confirmed: the addition of tea saponins significantly increased the concentration and proportion of cadmium available fraction in cadmium-contaminated soils.Through acquiring the Fourier transform infrared spectra and ultraviolet spectra,and measuring the critical micelle concentration of tealeaf saponins and tea-seed saponins,it was discovered that there was no significant difference between tea-leaf saponins and tea-seed saponins when using them as surfactants.Soil experiments confirmed the effects of tea saponins on the increase of cadmium available fraction concentration,and the quantitative effects of tea saponins were discovered: the significant interaction existed between cadmium concentration in the soil and tea saponins concentration,and the interaction was negative;the effect of tea saponins treatment time was not significant,but its trend was to rise first and then fall;when the soil was aged for 30 days and the concentration of cadmium in the soil was between 25-75 mg/kg,the optimal treatment time for tea saponins was within 3-9days;soil aged time had no significant effect on the concentration of cadmium available fraction in cadmium-contaminated soils.It had been revealed for the first time that the presence of heavy metal lead interfered with the effect of tea saponins on the increase of cadmium available fraction proportion.The presence of lead significantly shortened the effective treatment time of tea saponins.(4)Based on hydroponic experiments,the second role of tea saponins on phytoremediation of cadmium-contaminated soils was revealed: tea saponins promoted the accumulation of Portulaca oleracea on cadmium,without changing the chemical form of cadmium in the soil.By conducting independent hydroponic experiments,it was discovered that after the addition of 0.015% tea saponins,the absorption capacity of Portulaca oleracea on cadmium had been further significantly improved,indicating tea saponins had the potential to be utilized in the phytoremediation of cadmiumcontaining wastewater.