污灌区土壤-棉花系统中铊的分布特征

Distributions of Thallium in soil-cotton system in sewage irrigation area

通过对位于华北平原北部,河北省与天津市接壤处的污灌区土壤-棉花(Gossypium spp.)系统进行实地调查,分样区采集耕作层土壤样品、土壤剖面样品和棉花全植株样品,运用Element2型高分辨电感耦合等离子体质谱仪测定土壤样品中Tl的质量分数和形态,以及棉花不同组织中Tl的质量分数,结果显示:在土壤剖面中,Tl质量分数依次为表土层(0～20cm)>心土层(20～40cm)>底土层(40～60cm),表明污水灌溉使Tl在表土层有所累积;在表土层中,Tl的质量分数在0.257～0.420mg·kg-1之间,与研究区Tl背景值(0.330mg·kg-1)相比,个别样点有明显增高的趋势,Tl的形态特征为残余态>有机结合态>碳酸盐结合态>交换态≥水溶态>铁锰结合态,较高的有机结合态表明长期污灌导致表土层中有机质含量增加,使得Tl易于在表土层积累;在棉花植株各组织中,Tl的质量分数在3.5～78.6μg·kg-1之间,其分布规律为根系>果壳>茎杆>籽粒>纤维。研究表明,Tl主要分布于污灌区土壤-棉花系统中的表土层、棉花根系、茎杆和果壳等生态分室中;棉花植株对Tl的平均吸收系数为2.477,其中棉花根系对土壤中Tl的平均吸收系数达6.186,因此,棉花对重金属的输出作用减缓了土壤中Tl的积累速度,可以有效降低Tl对土壤污染的风险。

By means of investigating the soil-cotton system of sewage irrigation area in contiguous area of Hebei and Tianjin, located in north part of North China Plain, the plow layer soil sample, the soil profile sample and the whole plant of cotton sample in difference type area are collected, and the contents and chemical speciation of Thallium （T1） in soil sample and the contents of T1 in all cotton tissues are tested using Element2 High Resolution Inductively Coupled Plasma Mass Spectrometer. The research results show： the tissues of T1 order is epipedon （0-20 cm） 〉 the middle level （20-40 cm） 〉 the subsoil level （40--60 cm） in the soil profile sample of type area, it is indicated that T1 has accumulated in the surface soil level by sewage irrigation; in epipedon, the contents of Tl is between 0.257 and 0.420 mg·kg^-1, compared with the T1 background value of research area, the contents of Tl of individual sampling has the obvious increasing tendency, the chemical speciation characteristic of T1 is residual fraction 〉 bound organic matter〉 bond to carbonates 〉 exchangeable speciation ≥ water soluble 〉 bond to Fe-Mn oxides, more bound organic matter indicates that the organic content has increased in the epipedon due to the long-term sewage irrigation, which resulted in the T1 accumulating in the epipedon; in each tissues of cotton plant, the contents of T1 is between 3.5 and 78.6 μg·kg^-1, the distribution rule is root 〉 hull of cotton 〉 caudexes 〉 seed 〉 cotton fiber. The research indicated： T1 is mostly distributing in different ecological compartment of soil-cotton system in sewage irrigation area, such as epipedon, roots, candexes and hulls of cotton; the average absorption coefficient of Thallium of cotton plant and roots of cotton is 2.447 and 6.186 respectively, therefore, the function of heavy metal output of cotton slowed down accumulation speed of T1 in soil, cotton planting is effectual to prevent soil contaminated of T1, and decreased the risk of heavy metal pollution.