恩施渔塘坝富硒碳质岩石中硒的形态分析

SELENIUM SPECIATION IN SE-RICH ROCKS AT YUTANGBA

湖北恩施渔塘坝是中国人群硒中毒爆发性流行的地区,也是恩施较为典型的高硒地区之一。其北部出露的富硒碳质岩石,通常认为是渔塘坝硒的主要来源。采用7步连续化学浸提技术,将富硒碳质岩石中硒分为水溶态、可交换态、有机态、元素态、酸性提取态、硫化物/硒化物态和残渣态硒,使用氢化物-原子荧光法测定了岩石总硒和硒形态。结果表明,碳质页岩和碳质硅质岩中硒主要以有机态、硫化物/硒化物态和可交换态硒为主,碳质泥岩中硒以有机态、元素态和硫化物/硒化物态硒为主。碳质泥岩中的水溶态硒显著低于碳质页岩和碳质硅质岩中的水溶态硒,但元素态硒的比例显著高于后两者,说明碳质泥岩是富硒碳质岩层中硒易于富集的场所。富硒碳质岩中的可利用态硒主要以Se4+为主,而非是Se6+和有机硒。可交换态和有机结合态硒中Se4+与岩石总硒的显著正相关关系表明,Se4+离子的分布影响着岩石总硒,有机质与粘土在硒富集的地球化学过程中承担了重要作用。

Yutangba, where a sudden incidence of human Se poisoning occurred in 1963,was a typical high-Se area in China. The exposed Se-rich rocks of the Maokou and Wujiaping formations were generally considered to be the main source of Se that was enriched in soils of Yutangba and other places in Enshi, but little is known about the Se speciation trends in these rocks. Proeedurally defined Se fractions in Se-rich rocks were determined mainly based on the modified sequential extraction protocol as follows： soluble （MQ water）, adsorbed [0.1 mol/L K2HPO4 ＋ KH2PO4 （pH=7.0）], base soluble （organic matter： 0.1 mol/L NaOH）, elemental Se （1 mol/L Na2SO3）, carbonate and amorphous oxides （15% CH3CO2 H）, sulfide/selenide （1 mol/L CrCl2 ＋ HC1） and residual Se （HNO3 ＋ HF＋ H2O2） . Selenium in bulk rock, fraction and its speciation were measured by HG-AFS. The results showed that base soluble, sulfide/selenide and absorbed Se were of predominant form in carbonaccons shale and carbonaccons chert, while base soluble, elemental and sulfide/selenide Se were of dominant form in mudstone. The proportion of soluble Se in mudstone is significantly lower than that in carbonaccons shale and chert, as well as sulfide/selenide Se, while the amount of elemental Se is significantly greater than that in both rocks, indicating the current geochemical weathering condition of Yutartgba is favorable for Se mobilization and re-deposition as secondary native Se in suitable positions such as interbedded mudstone layer. Se （Ⅳ） is significantly greater in proportion in soluble, absorbed and base soluble Se fraction, rather than organic-Se and Se（Ⅵ）. This revealed Se（Ⅳ） is the predominant speciation for Se transportation and bioavailability, and also affected the total concentration of Se in whole rocks. The results support that Se was highly associated with organic matter. Adsorption of Se（Ⅳ） by organic matter and clay in carbonaceous Se-rich rocks plays an important role in the weathering process of Se-rich rocks in Yutangba.