Study On The Causes Of Pb Contamination In Westlake Longjing Tea And Their Control Techniques

Lead (Pb) is a physiological and neurological toxin affecting almost every important organ and system in the human body. Food and water are the main sources of Pb intoken by the general adult population. Tea is a recognized healthy beverage, which has the properties such as being anti-microbial, anti-inflammatory and anti-caries. Drinking tea continually can prevent cardiovascular disease and some kinds of cancer. Westlake Longjing tea, a very famous one and regarded as the best green tea in China, is a favourite product for a lot of tea drinkers. However, it has been reported that Pb concentration in Wastlake Longjing tea was continuously increased and there were an increasing number of cases reporting that the product had Pb concentration over 2 mg/kg dry weight, the former maximum permissible concentration (MPC) for Pb in China in the past years. It brought great concern by the tea producers and consumers. The objective of this study was to comprehensively investigate the current status of Pb concentration in Westlake Longjing tea and its pollution sources so as to supply information for proposing strategies for combating tea Pb contamination problem.The results showed that the Pb concentration in the shoots of one bud and two leaves, the raw material of Westlake Longjing tea, ranged from 0.3 to 5.6 mg/kg with an average of 2.2 mg/kg. 53.5% of the samples exceeded 2 mg/kg, the maximum Pb limitation of the former national standard. However, only 1.4% of the samples exceeded of 5 mg/kg of Pb, the current MPC. The Pb concentration in tea shoots was varied with the maturity of tea leaves and the age of tea plants. Older tea leaves and plants tended to contain higher concentrations of Pb than young leaves and plants of tea. The Pb concentration in tea shoots had a distinct seasonal variation with the order of spring > autumn > summer, and first flush > second flush in spring tea. The Pb concentration in mature leaves was significantly and positively correlated to that in shoots at P<0.05. Among different parts of a tea plant, the concentrations of Pb in feeding roots and producing branches were the highest, followed by the lateral branches, old and mature leaves, trunk, lateral roots, tap root, tender shoots and seeds. Therefore, the concentration of Pb in Westlake Longjing teacould be obviously increased if bud scale, fish leaf, mature leaves and branches were harvested together with the shoots.The Pb concentration in the air of a tea garden was significantly and positively correlated with that of tea leaves. The Pb in the air came from both the automobile exhaust and the dust. Therefore, the concentration of Pb in tea leaves produced nearby a road was higher than that far from a road. The air Pb concentrations were significantly reduced when it was more than 500 m away from a main road and more than 50 cm high on the ground. Washing leaves using water could decrease the Pb concentration, especially for relative older shoots and mature leaves, by which Pb concentration could be reduced by 23% at the most. However, there was no difference in Pb concentrations of tender shoots before and after washing.The total Pb concentration in soils from tea gardens in Hangzhou Westlake district ranged from 22.8 to 87.2 mg/kg, with a mean of 41.3 mg/kg. The Pb concentration extracted in 0.1 MHC1 ranged from 3.3 to 18.8 mg/kg, with a mean of 6.6 mg/kg. The Pb concentration in tea shoots was positively correlated with the extractable Pb content, and negatively with the soil pH. The Pb concentration in fertilizers including rape seed cake, compound fertilizers, urea, single super phosphate, ammonium phosphate, ammonium sulphate and potassium chloride sold in the market ranged from undetected value to 18.0 mg/kg, which was met with the national standard for Pb in fertilizers. However, some commercial organic fertilizers had higher Pb concentration than the chemical fertilizers, some of them had a level as high as 212.6 mg/kg.The concentration of Pb in tea shoots was significantly reduced by liming in highly acidic soils (pH 3.6). It could be reduced by 18.9-52.5% in the treatment with CaCO3 application rate of 7.2 t/hm2. After CaCC>3 addition into soil, pH was increased, and the proportions of Pb in the exchangeable and carbonate-bound fractions were decreased, the fractions bound to Fe/Mn oxides and residues were concurrently increased with no change observed in the organic fraction.The Pb concentrations in made teas were significantly increased when the fresh leaves were spread on an unclean ground. The Pb concentration increased by 1.3~2.2 fold in tea leaves spread on the cement ground, compared to that on a clean bamboo sheet.However, the processing of Westlake Longjing tea during the shape forming period in an electric heating pan resulted in no change in Pb concentration in the tea leaves. The oils for tea manufacture and packaging bags or cans had also no eflFects on the Pb concentration of Westlake Longjing tea.The current status of Pb concentration and its changing trend, the sources of Pb contamination in tea leaves and the effect of Pb intake from drinking tea on human health were also discussed in this paper. Five techniques or stratagies were proposed for combating the Pb comtamination problem: (1) planting protection trees between the road and tea gardens;(2) liming highly acidic soils;(3) no bud scale, fish leaf and mature leaves was plucked with the shoots;(4) improving the conditions of fresh leaf speading out and processing factory, especially keeping dust away from the workshop;and (5) refining the Westlake Longjing tea, removing the mature leaves and tea dust. These techniques were extended on a large scale by holding workshops, setting up demonstration farms and the application of soil Pb reducing products.