Experimental Method and Application of Rapid and Continuous Extraction of Reduced Inorganic Sulfur from Sediments
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摘要:
还原性无机硫是沉积物硫中最活跃的部分，其含量变化控制沉积物中铁、磷及重金属等元素的地球化学行为，在地质过程和环境污染方面都具有至关重要的影响。化学连续提取法是目前沉积物中硫形态提取基本方法，但常用的冷扩散法处理单个样品耗时长，难以实现对大批量样品的快速连续提取。为实现快速、准确地测定沉积物样品各形态还原性无机硫的含量，本文采用热蒸馏法，改进基于前人的三步提取过程，通过优化实验装置预先制备实验所需的二氯化铬溶液，实现了样品还原性无机硫形态的快速连续提取；以过氧化氢为氧化剂，将提取的各形态硫氧化为$\mathrm{SO}_4^{2-}$后采用离子色谱进行检测。选取三峡库区沉积物样品进行重复实验检验，得到提取酸挥发性硫、黄铁矿硫、元素硫的标准偏差(RSD，n=3)分别为5.26%、1.22%和3.09%，重复性较好。进一步对酸挥发性硫、黄铁矿硫、元素硫的加标回收率进行测定，得到这三种硫形态的回收率分别为92.8%、93.6%、94.1%。本实验方法采用的热蒸馏法对单个硫形态提取时间为1.5h，用时较短，玻璃装置连接便捷、操作简单，分析检测准确度好，实现了一套装置对沉积物还原性无机硫形态的连续提取，可适用于大批量样品的硫形态快速提取与检测。
Abstract:BACKGROUND Sulfur is an active element with multiple chemical forms, which plays a vital role in the regulation of redox chemistry. Reduced inorganic sulfur (RIS) including acid volatile sulfur (AVS), elemental sulfur (ES) and pyrite sulfur (CRS) is the most active part of sulfur species in sediments and plays an important role in controlling the geochemical behavior of iron, phosphorus and heavy metals in sediments. Separation and determination of reduced inorganic sulfur in anoxic sediments are critical to ecological and geological studies of sulfur cycles.
Both distillation and diffusion methods can be effectively used to separate AVS, ES, and CRS in sediments. However, the current methods for extracting sulfur species are difficult to adapt to the large number of samples in geological and environmental research. Due to the reaction time, requirements of 24h for single sulfur species in the diffusion method limits the number of samples that can be processed on a timely basis. This limitation presents a problem for analyzing fresh anoxic sediment samples which have to be processed immediately to minimize the risk of sulfide re-oxidation. The detection method has the disadvantages of having a cumbersome testing process, long analysis time, and easy loss of sulfur components.
OBJECTIVES To achieve efficient and continuous determination of reduced inorganic sulfur forms in bulk sediment samples.
METHODS The method used in this experiment was improved based on the thermal distillation method, which was used to continuously extract the reduced inorganic sulfur from sediments. The reaction flask used in the experiment was a three-head round-bottom flask. A nitrogen flushing pipe, a condenser tube and two injection tubes were connected to each of the small necks. For the AVS procedure, 1.00g sediment sample reacted with 10mL of 6mol/L HCl under nitrogen gas at an elevated temperature (90℃) to convert reduced sulfur species into hydrogen sulfide which was subsequently carried by a nitrogen gas stream into a trap. For the CRS procedure, 20mL of CrCl2 solution was added to the sediment in the distillation flask after the AVS procedure, and 10mL of 6mol/L HCl was immediately placed in the flask at an elevated temperature (90℃), flushed with nitrogen. For the ES procedure, 20mL DMF were poured into the sample flask which contained acid and CrCl2 solutions from the previous procedure. 20mL of CrCl2 solution and 10mL of 6mol/L HCl were injected into the flask, and the reaction was allowed to take place at 90℃ purging with nitrogen. During the entire distillation process, H2S gas was absorbed by NaOH solution, and then oxidized by H2O2 to $\mathrm{SO}_4^{2-}$. The concentrations of $\mathrm{SO}_4^{2-}$ were obtained by ion chromatography.
RESULTS Repeatability experiments (n=3) were conducted on sediment samples from the Three Gorges Reservoir area and the mean values of acid volatile sulfur, elemental sulfur and pyrite sulfur were obtained as 0.19, 0.37 and 3.10μmol/g, respectively. The relative standard deviations (RSD) of the experimental results were 5.26%, 1.22% and 3.09%, respectively. In order to test the effectiveness of the distillation procedures, Na2S·9H2O, pyrite and S were added in the sediment to reveal the corresponding standard recoveries. An average of 92.8% of the added Na2S·9H2O was recovered by the AVS diffusion method. An average of 93.6% of the added pyrite was recovered by the CRS diffusion method. An average of 94.1% of the added elemental sulfur was recovered by the ES diffusion method. In the literature, recovery of AVS by the improved diffusion method ranged from 82.01% to 108.71%, and recovery of ES ranged from 92.25% to 98.08% (n=3), respectively.
The modified apparatus presented in this paper was an economic version which uses rubber and glass parts. The method provided the advantages of lower sample weighing, and simple operation. Compared with the diffusion method (24h), the extraction time for individual sulfur forms by our distillation method was just 1.5h. The recoveries achieved by the method are comparable to those reported for earlier methods. In addition, the results are more like the data on reduced inorganic sulfur content obtained by Hongbin Yin after measuring sediment samples from Taihu Lake using the modified cold diffusion method, indicating that the method designed in this study has a high degree of confidence.
Geochemical processes of sulfur in river aquatic systems play a crucial role in environmental evolution. In this study, the distributions and seasonal variation of reduced inorganic sulfur (RIS) in the Three Gorges Reservoir area surface sediments were investigated. Surface sediment samples were collected from 8 points in the section from Yunyang to Zigui in the Three Gorges Reservoir area in August and December 2017. The result showed that the AVS and ES contents were higher in summer than in winter, and the trend of RIS in the section from Yunyang to Zigui was roughly decreasing, with obvious seasonal and spatial changes. Low sulfur pollution in the Three Gorges Reservoir area was observed.
CONCLUSIONS The improved thermal distillation method and apparatus in this study have significant advantages in the extraction efficiency of reduced inorganic sulfur from sediments. The extraction time of this study for individual sulfur form is 1.5h, less than the diffusion method. The established analytical method has good precision and accuracy, which is suitable for investigation studies with large numbers of samples such as environmental research and geological surveys.
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表 1 重复实验与加标实验结果
Table 1. Results of repeated experiment and spike recovery in AVS, CRS, and ES procedures.
硫形态 重复实验 加标实验(回收率) 第一次实验(μmol/g) 第二次实验(μmol/g) 第三次实验(μmol/g) 平均值(μmol/g) RSD(%) 第一次实验(%) 第二次实验(%) 第三次实验(%) 平均值(%) RSD(%) 酸挥发性硫(AVS) 0.20 0.19 0.18 0.19 5.26 91.6 93.2 93.6 92.8 1.14 黄铁矿硫(CRS) 3.13 3.06 3.12 3.10 1.22 93.4 95.7 91.7 93.6 2.14 元素硫(ES) 0.38 0.38 0.36 0.37 3.09 94.6 93.8 93.9 94.1 0.46 
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