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.