溶解硫酸盐硫同位素测试预处理方法优化

为分析水体中硫酸盐硫同位素组成，常将SO^2-₄转化为固体BaSO₄，进行预处理，但是该方法对体积少且低浓度硫酸盐样品（0～20 mg·L^-1）存在耗时长、回收率低等问题，对硫同位素测试结果的影响仍不明确。本文通过对不同Ba²⁺添加量、反应温度、pH和沉淀时间等条件对SO^2-₄回收率及δ³⁴S BaSO₄测试影响进行研究，优化由SO^2-₄转化BaSO₄的方法，并选取四种不同类型水体（雨水、湖水、污水、海水）进行验证。结果表明：1） SO^2-₄的回收率受沉淀生成速率和反应时间控制，在相同反应时间内，沉淀生成速率与过饱和度比、温度成正比，与H⁺浓度成反比，并且搅拌可以加速BaSO₄晶体的生长；2） 改进的预处理流程为：在水样中添加适量盐酸和氯化钡（pH=2.6，过饱和度比≥55），用玻璃棒匀速搅拌1 min，90 ℃水浴加热1 h，冷却至室温后用滤膜收集BaSO₄；3） 优化后的预处理方法测试天然样品时，SO^2-₄可以在1 h内完全转化为BaSO₄固体，并且不发生硫同位素分馏，低SO^2-₄浓度（3.24 mg·L^-1）的雨水样品，其转化率从0提高至98.7%，δ³⁴S约为15.7‰±0.1‰；中等SO^2-₄浓度（30.34 mg·L^-1）的湖水样品，其转化率从26%提高至100%，δ³⁴S为5.8‰±0.2‰；高SO^2-₄浓度（140.4～2 516.4 mg·L^-1）的污水和海水样品，硫酸盐的回收率和δ³⁴S与传统方法得到的结果相近。改进的预处理方法有效提高了低浓度水溶硫酸盐样品的回收率，避免转化过程中的同位素分馏。该方法对于中高浓度的样品同样适用。

Improvement of Pretreating Method for Sulfur Isotope Analysis of Dissolved Sulfate

The prevalent pretreatment method for sulfur isotope determination in water samples is to convert dissolved SO^2-₄ into solid BaSO₄. However, when this method is applied to sulfate samples with small volume and low-concentration (0～20 mg·L^-1), there are some problems such as time-consuming and low recovery rate, which would cause uncertain effects on the results of sulfur isotope determination. In this paper, considering the effects of different conditions(Ba²⁺ addition, reaction temperature, pH, settling time) on recovery rate and determination of δ³⁴S BaSO₄, the method of converting dissolved SO^2-₄ into solid BaSO₄ was optimized. In addition, the optimized method has been tested in four different types of water samples(rain water, lake water, sewage and seawater) . The results show: 1) The recovery rate of SO2-4 was controlled by the precipitation generating rate and reaction time. Under the condition of the the same reaction time, the precipitation generating rate was positively correlated with the super-saturation ratio and temperature, while negatively with the concentration of H+, and stirring can accelerate the formation of BaSO₄ crystals. 2) The optimized pretreatment process is as follows: add appropriate amount of hydrochloric acid and barium chloride to the water sample（pH=2.6, super-saturation ratio≥55）, and stir with a glass rod at a constant speed for 1 minute, heat in a water bath at 90 ℃ for 1 hour, then cool to normal temperature and use filter membrane to collect solid BaSO4. 3) The optimized pretreatment method has been tested in natural samples, the test result shows that dissolved SO^2-₄can be completely converted into solid BaSO₄ within 1 hour without any sulfur isotope fractionization. In rainwater samples with low SO^2-₄ concentration (3.24 mg·L^-1), the conversion rate increased from 0 to 98.7%, and the δ³⁴S value was determined to be about 15.7‰±0.1‰; in lake water samples with medium SO^2-₄ concentration (30.34 mg·L^-1) , the conversion rate increased from 26% to 100%, and the δ³⁴S value was determined to be 5.8‰±0.2‰. While, the recovery rate or δ³⁴S value in sewage and seawater samples with high SO^2-₄ concentration (140.4～2 516.4 mg·L^-1) are close to that got by prevalent method. In conclusion, the optimized pretreatment method effectively increases the recovery rate of low-concentration water-soluble sulfate samples and avoids sulfur isotope fractionization during the conversion process. Evidently, this optimized method is also applicable to samples with medium and high SO^2-₄concentrations.