稳定同位素^(202)Hg稀释技术测定土壤汞有效性——与化学提取方法比较

COMPARISON BETWEEN STALE ISOTOPE ^(202)Hg DILUTION TECHNIQUE AND CHEMICAL EXTRACTION IN DETERMINING Hg AVAILABILITY IN TWO TYPICAL SOILS OF CHINA

准确评价汞(Hg)在土壤中的有效性对预测污染土壤中Hg的潜在生态风险及其环境质量标准的修订具有十分重要的意义。本研究通过稳定同位素202Hg稀释技术及同位素交换动力学方法(IEK)分析红壤和潮土中同位素可交换Hg含量(E值)及同位素可利用态Hg含量(Ea),并与4种单一提取法和1种连续提取法(改进的BCR法)获得的土壤有效态Hg含量进行比较。结果表明,外源稳定同位素202Hg加入土壤后,红壤和潮土的悬浮液中同位素交换均在24 h后达到稳定状态。同位素交换动力学方程对悬浮液中同位素比值及土壤同位素交换态含量Et值的变化有比较理想的拟合效果,红壤三个间段的E值(E1 min、E1 min-24 h和E>24 h)及所占全量的比例均高于潮土。两种土壤Ea(为E1 min与E1 min-24 h之和)所占全量的比例为38%~60%,显著高于单一提取法中提取率最高的0.03%TGA-1/15 mol L-1Na2HPO4(10%~15%)及连续提取的∑BCR(20%~27%,为酸可提取态、还原态和氧化态占全量之和),这表明与化学方法相比较,用土壤同位素可利用态Hg含量(Ea)作为土壤中有效态Hg含量的表征可能偏高,原因可能与外源同位素被土壤固持及土壤悬浮液的性质有关。

Information on distribution of heavy metals in various chemical forms in soils may have greater chemical and biological significance than that of just total concentration. The amount of an isotopically exchangeable metal in soil solution, also called E-value, may conceptually best represent the fraction of the metal that is in dynamic equilibrium in the solution phase, and is considered to be a better indicator of availability of the chemically reactive metal to plants. Mer- cury （Hg） is of great concern in soil because it is phytotoxic to plants through accumulation in plant tissues and detrimen- tal to human health through the food chain. In order to assess its phytotoxicity to vegetables and potential risk to human health, it is very important to have a better knowledge about labile pools of Hg in soils. The aim of this study is to deter- mine the pools of isotopically exchangeable Hg （E-value） in different soils. E-values of Hg in two typical soils （red soil and fluvo-aquic soil） was measured using the stable isotope ^202 Hg dilution technique and E1 min, El min -24 h and E 〉 24 h values of Hg in the soils were calculated based on the model of isotopic exchange kinetics （IEK） with an iterative nonlinear re- gression algorithm. In addition, pools of labile Hg in the soils were also determined with four different single chemical ex- traction methods and a sequential extraction procedure （modified BCR approach） for comparison. Results show that 24 h after the addition of 202Hg, equilibrium of ^202Hg/^200Hg between soil and solution was established in both soils. The IEK equation was found fitting quite ideally the dynamics of isotope ratios and E, values of Hg in soils. All the three compart- ments of E1 min, E1 min-24 h and E 〉 24 h in the red soil were markedly higher than those in the fluvo-aquic soil. E values, sum of E1 min, and E1 min-24h and around 38%-60% of total Hg, obtained using the isotope method, were significantly higher than that （10%-15% of total Hg） acquired through extraction with 0.03% TGA -1/15 mol L-1 Na2HPO4 being the highest among the 4 single extraction methods, and than ∑ BCR （20%-27% , sum of acid-extractable, reducible and oxi- dizable fractions） of the sequential extraction method. It was also found that soil pH had marked influence on E-values of Hg in the two studied soils. Compared with the chemical extraction procedures, the isotope method may yield E which tends to be a bit too higher when used as indictor of content of soil available Hg. It is, probably, related to fixation of the spiked extraneous 202Hg by the soils and properties of the soil suspensions for E measurement.