| Thallium is one of the most typical toxic heavy metal pollutants for animals and plants and humans in environment. The toxicity of thallium even higher than the carcinogenic heavy metals such as cadmium, lead for mammal. But due to the scattered characteristic, thallium pollution failed to cause enough attention for a long time. Soil is the destination of heavy metal pollutants. The damage to environment not only depends on the amount, but also depend on the existence speciation of heavy metals in soil. The existence speciation can affect the ability of migration and bioavailability in environment, and have an effection on human health by food chains in the end. The conclusions of studies on the bioavailability of heavy metals in soil-plant system can directly indicate pollution degree of heavy metals in soil and the harmfulness to ecosystem and human health, and thus provide the theoretical bases for remediation of soil heavy metal pollution and pollution prevention and control of agricultural,and heavy metals exposure risk assessment in food.This paper study soil, tea tree leaves, product tea samples of Fenghuangdancong tea tree from Chaozhou Guangdong Procince. By established method,We detacted the different speciation thallium content in sample, analyzed heavy metal thallium distribution and enrichment characteristics in soil-plant system, respectively by Bioconcentration Factor(CF),soil Mobility Factor(MF), Mobility Concentration Factor(MCF) to characterize thallium bioavailability. Thus, obtained thallium bioavailability characteristics by coontrasting with the other heavy metals(copper, lead, zinc, cadmium, chromium, nickel), at the same time verified confidence degree of the results by Correlation analysis; Based on referencing the Health Risk Assessment Model of US EPA,we assessed the health risk of thallium exposed from consumption Fenghuangdancong tea infusion,. Combining previous research, there are five conclusions following:1. Biological samples treated by microwave digestion, soil samples treated by wet mixed acid digestion, with improved BCR sequential extraction procedure to extract various speciation of heavy metals in soil, under the optimized condition of ICP-OES&ICP-MS analysis methods for measuring thallium and other heavy metal content in sample, the method it is considered to be: detection limit satisfy the requirements of trace analysis, high precision,high accuracy, the result can meet the bioavailability research.2.Fenghuangdancong tea soil thallium content range is 506~1098 μg·kg-1, mean is 768μg·kg-1, it belons to the normal range; Thallium is found in all of tea leaves, tender leavesthallium content is range of 30.1~177 μg·kg-1, mean is 88.6 μg·kg-1; elder leaves thallium content is range of 33.7~547 μg·kg-1, mean is 180 μg·kg-1, tea tree leaves thallium content concentration level is high, it is comparable to supergene plant thallium content from thallium mining area that reported by some document in print. The speciation analysis results of soil thallium: Acid extractable state(F1) average is 10.4 μg·kg-1, accounted for 1.4%; Reducible state(F2) average is 40.2 μg·kg-1, accounted for 5.3%; Oxidizable state(F3) average is 2.79μg·kg-1, accounted for 0.4%; Residual state(F4) average is 711 μg·kg-1, accounted for 93.0%.Speciation analysis results show that the soil of tea tree hadn’t polluted by exogenous thallium pollution, for the most part thallium(> 87.2%) exists in Residual state that plant can’t use.3.The total content of soil thallium were not significantly associated with content of plants, the confidence degree is low when characterized bioavailability by CF;Fenghuangdancong tea soil thallium MF in the range of 0.038 ~ 0.124, Fenghuangdancong tea soil thallium MF was negatively related with tea tree thallium content of elder leaves(R=0.667*,P < 0.05), but as a result of MF failed to reflect the internal factor that influencing the bioavailability-biological effect, soil thallium MF cannot characterize the bioavailability comperhensive; Speciation thallium state(F1+ F2) was negatively related with elder leaves thallium content of tea tree(R=0.650*,P < 0.05), esprcially F1 was negatively related with elder leaves thallium content(R=0.712**,P < 0.01). It illustrated the amount of thallium in the elder leaves and state F1&F2 thallium content in soil is closely related,especially F1. Based on the results of Correlations, uesd with Thallium contents both in state F1&F2 and Fenghuangdancong tea tree leaves, calculating the activity of Thallium in soil MCF range of 0.83~7.29, the mean value is 2.65. Though there is difference between kinds of Fenghuangdancong tea MCF, but the resul show by comparing the 11 kinds of Fenghuangdancong tea tree MCFs characteristics with the MCFs of other heavy metal, that Thallium although is a non-essential elements in plant growth, but the thallium MCF(2.65) is just lower than copper(5.09), nickel(3.60), is much higher than zinc(1.16), cadmium(1.05),chromium(0.328), lead(0.025). It is showed that thallium in tea tree leaves is enriched from the active thallium(F1&F2) in soil, the bioavailability of thallium degree is high when characterized by MCF.4.The active content of heavy metal soil and the content in tea tree, at a high confidence level has significant correlation, showed that the content of heavy metals in plants are closely related to the activity content in soil. MCF had a higher confidence degree than CF when described bioavailability of the soil-Fenghuangdancong tea system. MCF more is moreaccurater than CF and MF when tell the differences of bioavailabilities in various heavy metals in this system. MCF can be characterized more adaptive, meanwhile it can obviously distinguish enrichment characteristics of tea tree for different heavy metals.5.Fenghuangdancong product tea content of thallium is in range of 20.5~197 μg·kg-1, the mean value is 78.5 μg·kg-1; thallium dissolution rate is range of 24.5 ~ 38.5%, mean is 33.1%,the dissolution rate is high, rank only second to nickel(51.9%) If according to the daily directly intake water 1.125L·Day-1,that it is all intake by tea infusions, consume 10 g product tea: the concentration range of thallium in tea infusions is 0.045~0.404 μg·L-1, mean is 0.196μg·L-1, exceeded the thallium limits(0.1μg·L-1) both in "Standards for drinking water quality"(GB/T5749-2006), and “Environmental quality standards for surface water "(GB3838-2002);Assessed the health risk of thallium exposed from consumption Fenghuangdancong tea infusion, by referencing the Health Risk Assessment Model of US EPA. Compare with two Reference Dose(RfD) candidate values of appearing different kinds of thallium toxicity effect,the non-cancer risk assessment results for drinking thallium exposure Fenghuangdancong tea infusion:Based on appearing animal Hair follicle atrophy toxicity effect:HQ = 0.364 < 1, there is no risk.Based on appearing human clinical observation in Alopecia toxicity effect:HQ = 1.21 > 1, there is a low risk.Nonetheless, in consideration of the Reference Dose(RfD) is candidate, there are some certain indeterminacy in assessment results. We should have to keep it in reserve.Fenghuangdancong tea leaves have a high enrichment ability of activity thallium from soil, and thallium is non-essential elements for tea tree growth, thallium has high bioavailability, higher than some heavy metal that were viewed as a potential toxic risk in soil pollution, such as cadmium, lead. In view of soil is a distination of environmental pollutants,it is suggested that we are necessary to monitor the thallium content in soil belongs to tea-producing area. At the same time, thallium has a big proportion in the dissolution of in tea infusion. Because of the high toxicity of thallium to human body, it is suggested that we should establish some regulation for thallium content in product tea. |
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