Accumulation and distribution of trace tin in soil-paddy system

Sun Hanwen, Qiao Fengxia^#, Liang Shuxuan, Li Lixin, Suo Ran
(College of Chemistry and Environmental Science, Hebei University, Key Laboratory of Analytical Science and Technology of Hebei Province, Baoding, 071002;  ^#Department of Chemistry, Baoding Teacher's College, Baoding , 071051, China)

Received Dec.3, 2003; Supported by the Natural Science Foundation of Hebei Province for the project No.203110.

Abstract A new method has been developed for the determinations of tin in soil and paddy by hydride generation atomic fluorescence spectrometry. With the presence of sodium dodecanesulfonate, boracic acid was used as the reaction medium. The accumulation and distribution of trace tin in soil-paddy system was investigated. It is shown that tin quantum accumulated in the middle section is much lower, while tin quantum contained both end sections are higher. The regularity of tin in different depth of the soil is varied with various structures in different depth of soil-paddy system. The concentration of every organ in paddy is shown as follows: the root > the lowest-node leaves > the lowest node> the lower-node leaves > the lower node > rice > the sub-top node leaves > the top node leaves > the sub-top node > the top node. Contrast to the concentration of tin in soil, the root of the paddy has a concentrated effect on the tin, the other organs of paddy have a diluted effect on the tin. The efficiencies of the soil provided tin for the root and the lowest-node leaves of the paddy are higher than that for other organs of paddy. The proposed method has a detection limit (3s ) for tin was 0.016mg·L^-1 and for a 0.5000g paddy or soil sample, the detection limit for tin was 0.04 mg·g^-1. The linearity can reach up to 80mg·L^-1.
Keywords  Tin; Soil; Paddy; Distribution; Accumulation

1 INTRODUCTION
In the recent years, with the development of the industry and agriculture, the concentration of toxic elements in soil was increased, which not only have serious affection on the plants and crops, but also threaten the people's health by food cycle. The element of tin is indispensable to our body, which speeded the reaction of protein and nucleic acid and accelerated the growth of the creature. However, if the assimilative quantum of tin were in overabundant, it would lead to pathological changes. Therefore tin is a potentially important indicator of anthropogenic contamination. The measurement of low Sn concentrations in soil-paddy system was an important part of a large quantity study to determine the source of Sn contamination found in residential soil and paddy.
    However, the determination of tin in soil at trace level is problematic. Some analytical methods had already been developed for trace tin analysis, such as hydride generation inductively coupled plasma mass spectrometry (HGICPMS)^[1]. Hydride generation electrothermal atomic absorption spectrometry (HGETAAS)^[2], atomic absorption spectrometry^[3], hydride generation atomic fluorescence spectrometry (HGAFS)^[4,5], hydride generation atomic absorption spectrometry(HGAAS)^[6], graphite furnace atomic absorption spectrometry (GFAAS)^[7] and inductively coupled plasma atomic emission spectrometry (ICPAES)^{[8, 9]} and so on, of which the most popular and preferred one in terms of simplicity, sensitivity, precision, speed, and cost is HGAFS, since the analyte is separated from the matrix elements in solution. And to solve the narrow range of acidity in the determination of tin by hydride generation, organic acid was recommended such as tartaric acid^[4] and acetic acid- sodium acetate buffer solution^[5]. But the sensitivity of tin was not high enough, to getting the sensitivity of tin more higher, with the presence of sodium dodecanesulfonate, boracic acid was used as the reaction medium in this work, because both sodium dodecanesulfonate and boracic acid can improve the sensitivity of tin. And boracic acid was also used as covering reagent. All of this making the proposed method has more higher sensitivity and selectivity.
    Furthermore, a literature survey revealed that most of such work concentrated on the determination of total tin content with little work on the distribution in soil. In the research of trace element's concentration and distribution and movement in plant, most of work concentrated on the several necessary metal and fairly toxic metal ^{[10, 11]}, while some metal element such as Sn, Bi, Te and Ge, which both have some biological function and little gentle toxic efficiency to some organs, were neglected, especially their regularity of distribution and movement in plant. However, the regularity of trace elements movement and distribution was not only dependent on the soil’s chemical and physical characteristic, but also dependent on the structure and function of plant's different organs. So the research of tin in soil-crop system was very meaningful.
    Our aim of this work described here was to develop a new sensitive method for determining trace tin in soil and paddy samples and studying accumulation and distribution of trace tin in soil-paddy system.

Table 1 Instrument parameters and operating conditions

2 EXPERIMENTAL
2.1 Apparatus
A model AFS-230 double-channel nondispersive atomic fluorescence spectrometer (Beijing Haiguang Analytical Instrument Co., Beijing, China) equipped with a tin hollow cathode lamp (general Research Institute of Non-Ferrous Metals, Beijing, China) was used for all the determinations. The operating parameters used for the AFS instrument are given in Table 1.
2.2 Reagents
The tin stock solution, 1000mg·L^-1, was prepared by dissolving 1.000g metallic tin (high-purity grade) in 100mL of concentrated hydrochloric acid and diluting to 1000ml with sub-boiling distilled water. The working standard solutions were prepared daily by successive dilution of this solution with 2% (V/V) HCl. The KBH₄ solutions were prepared by dissolving the reagent in 0.5%(M/V) sodium hydroxide solution.
    Boracic acid solutions were prepared by dissolving the reagent in 0.005% sodium dodecanesulfonate (SDS) solution.
    All chemicals were of analytical-reagent grade unless specified otherwise, and sub-boiling distilled water was used throughout the experiment. High-pure argon was used as carrier and shield gas.
2.3 Samples pretreatment
2.3.1 Soil samples pretreatment
Approximately 0.5000g of dried soil was accurately weighed and placed in 5mL concentrated HNO₃ solution, after over a night, 15 mL concentrated HCl was added with little H₂O₂, followed by gentle heating until a clear solution was obtained. The solution was transferred to a 50.0 mL calibrated flask with 2%(V/V) hydrochloric acid.
2.3.2 Paddy samples pretreatment
From the rice to the root, the paddy was divided into ten sections: rice, top node leaves, top node, sub-top node leaves, sub-top node, lower-node leaves, lower node, lowest node leaves, lowest node, root, every section of paddy was digested as procedure2.3.1.
2.4 Recommended operating procedure
An aliquot of the sample solution (1.00mL) was put in a 25.0mL volumetric flask, 22.2 mL 4.6% (w/v) boracic acid was added in. then it was diluted to 25.0ml with water and directly used for the determinations of tin by hydride generation atomic fluorescence spectrometry.

3 RESULTS AND DISCUSSIONS
3.1 Effect of lamp current
Studies on the influences of lamp current show that an increase in the lamp current significantly improve the signal intensities of Sn, however, higher lamp currents would produce higher signals noise, and reduce the lifetime of the lamps. A current of 60 mA was, therefore, used as a compromise.
3.2 Effect of carrier and shield gas flow rate
Pure argon was used as both the carrier gas and the shield gas. The argon flow was used to transfer generated hydride from the hydride generator to the atomizer quarts cell. The influences of various flow rates on the sensitivity and stability of the measurement were investigated in this study. The fluorescence signal intensity was increased with increasing argon flow rate from 200 to 400 mL·min^-1. The carrier gas at too low a flow rate could not quickly sweep the vapor of analyte into the inner tube of a quartz furnace, and at too high a flow rate would dilute the introduced analyte in the furnace tube and reduce the residence time of analyte in atomizer quarty cell by the carrier gas. The optimum flow rate of the carrier gas was found to be 400 mL· min^-1.
    The shield gas was used to prevent extraneous air from entering the flame. The fluorescence signal intensity was increased with increasing the shield gas flow rate from 600 to 800 mL·min^-1, while decreased when the shield gas flow rate was higher than 800 mL·min^-1. A flow rate of 800 mL·min^-1 for tin sample gave the highest signal intensity. A carrier gas of 400 mL·min^-1 and a shield gas of 800 mL·min^-1 were employed for the determination of tin.
3.3 Atomizer temperature and observation height
The atomizer used was a quartz furnace in a shielding mode, which consisted of an inner tube and an outer shielding tube, heated by means of an externally wrapped resistance wire. An influence of the atomizer temperature on the signal was studied. The signal intensity of tin increased with increase of the furnace temperature in the range of 30-300ºC, while the signal intensity decreased and the noise levels increased when the furnace temperature was at higher than 300ºC because of the vapor expansion and the furnace radiation increasing at a higher temperature. A temperature of 300ºC was selected.
    The observation height is the distance from the quartz furnace outlet to the point where the atomic fluorescence signal is measured. As the observation height increased, the signal intensity increased, while the background caused by radiation from the furnace decreased. However, a too high observation height would reduce the signals because of oxidation of analyte by the oxygen in air from entering the flame. In this study an observation height of 7.0 mm was used.
3.4 Effect of reacting acidity and KBH₄ concentration
The generation of nascent hydrogen, which is the actual reducing agent from borohydride requires an acidic reaction medium. Comparing to hydrochloric acid, acetic acid, tartaric acid and phosphoric acid, the hydride generation efficiency in boracic acid medium was found to be higher, so boracic acid was used in this work. The influence of the boracic acid concentration in the reaction medium on fluorescence signals of Sn was investigated using 2.0% (w/v) KBH₄ as a reducer. The result shows that fluorescence signal was more sensitive and stable in the range of 30-45g·L^-1, boracic acid solution of 40 g·L^-1 was selected as optimum acid medium in this work. And sodium dodecanesulfonate was used as the sensitizing reagent.
    The influence of acid concentration in carrier liquid on fluorescence signals was compared with hydrochloric acid and boracic acid, the test data shown in the presence of hydrochloric acid carrier liquid, the sensitivity of the signal was higher with the HCl concentration in the range of 1.0-1.2 mol L^-1. Hydrochloric acid solution of 1.1 mol·L^-1 was employed in this work.
    KBH₄ was used as both a reducer and a hydrogen supplier, which was necessary to sustain the argon-hydrogen flame. A lower KBH₄ concentration could give highest signal intensity for tin. In this work, a 2.5%(w/v) of KBH₄ concentration, which provided a good signal-to-noise ratio, was employed for the determination of tin.
3.5 Linearity, detection limits and interference
On the optimized conditions, the linearity can reach up to 80mg·L^-1, the detection limit (3s ) for tin was 0.016 mg·L^-1(r>0.9997), and for a 0.5000g paddy or soil samples, the detection limit for tin was 0.04 mg·g^-1. RSD(relative standard deviation) was 0.18% (n=5) for 20mg·L^-1 tin. 1000 fold Cu²⁺, Zn²⁺, As³⁺, Hg²⁺, Fe³⁺, Co²⁺, Ni²⁺ and 500 fold Cd²⁺, Sb³⁺ and Pb²⁺ have no obvious influence for the determinations of 10 mg·L^-1 tin.
3.6 The content and distribution of tin in soil
The contents of Sn in the soil were determined by HGAFS. The results were listed in Table2. The content of the soil tin in the depth of 20-40 cm was lower than that in the depth of 0-20cm and 40-60cm. This indicated that in different depth, the structure of the soil have difference, in the depth of 20-40 cm, the quantum of soil form clump, the forming clumps make the soil have more pore, which benefits the movement of tin and produce a filter effect, so in the depth of 40-60cm tin concentration was more higher. In the depth of 0-20 cm, for the surface of the soil was directly influenced by the outer factor, so the tin concentration was the highest.

Table 2 The analysis result of the soil tin in different depth (n=5)

Table 3 The analysis result of tin in different organs of paddy and their corresponding absorptive coefficients (n=5)

3.7 The distribution of tin in paddy
The absorption and transferred function of different paddy organ is different. The result was shown in Table 3. The concentrated ability of different paddy organs was: the root > the lowest-node leaves > the lowest node> the lower-node leaves > the lower node > rice > the sub-top node leaves > the top node leaves > the sub-top node > the top node. In a general, because of the direct contact with soil, the concentration of tin in the root was the highest. There is some quantum tin in rice, which would affect the people's health by biological cycle.
    From top node leaves to the lowest node leaves, or from the top node to the lowest node, the concentration of tin is increasing. It is indicated that through the water the nutrient can be transferred from root to the top, the organ which nearer to the root has more content of tin. The concentration of tin in the top node leaves, the sub-top node leaves, the lower-node leaves and the lowest-node leaves were higher than that in the top node, the sub-top node, the lower node and the lowest node, respectively. This supported the point that the metabolism in the leaves where photosynthesis and water rising being taken places, is more active. So the transferred procedures and the quantum of necessary water were more, which lead to a higher concentration of tin in the leaves.
3.8 The absorptive characteristic of tin in paddy organs
In order to describe the absorption and distributing ability of plant for the absorbed element, an absorptive coefficient (C) was defined as: the ratios of tin content in paddy to the content of tin in the growth soil (0-20cm, the growing depth). The coefficient of different organs in paddy was represented as the effect of successive concentration or dilution in biological organs to the life-chemical substance. It is a size for measure the absorptive ability of different organs in paddy to soil's element. If C>1, it means the paddy has higher concentrated efficiency than the soil, If C <1, it means the paddy’s concentrated ability is lower, in contrast to the soil, it has a diluted effect on the tin.
    From Table 2 it is shown that the concentration of tin in the root was higher than that in the soil, it can be concluded that the organ has the highest absorptive ability. The absorptive coefficient of the lowest node leaves was nearly getting to 1, this indicated that the ability of concentration and dilution of tin in the lowest node leaves was equal. For other organs in paddy (C <1) have some different diluted effect on tin.
    Only the element in soil have some ability of movement, Can the element be absorbed by the paddy, the efficiency of the soil provided tin for the paddy (E_s/p) was employed in this work. The efficiency of the soil provide element for plant or crops was a bridge for soil and crops.^[12] Es/p was consistent with the absorptive coefficient of the paddy organs. With increasing the absorptive coefficients, Es/p was increased. The ability of every organ in paddy contributed to Es/p was as follows: the root > the lowest-node leaves > the lowest node> the lower-node leaves > the lower node > rice > the sub-top node leaves > the top node leaves > the sub-top node > the top node. In the successive transferred procedure of soil-paddy- body cycles though the rice has some diluted affection to tin, but tin still has a great hidden danger to people's health, for tin can be concentrated in our body.
    On the other hand, the unit of paddy leaves has higher concentrated efficiency, which were the major role to protect the rice from tin pollution in soil. We can fertilized paddy in equilibrium, which will keep the paddy not only grow better but also have higher ability of sustaining the toxic element's pollution.

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