河流沉积物中铊的形态与同位素组成特征研究

Thallium (Tl) is a toxic heavy metal and exists in two speciations of Tl(I) and Tl(III) in nature. The transformation between speciations of Tl(I) and Tl(III) may result in significant fractionation of Tl isotope in the environment, for which intensive studies have been focused on seawater-marine sediment system, using adsorption experiments of Tl(I) to the synthetic birnessite. However, such works have failed to afford the firsthand evidences of compositions of Tl speciations from natural marine sediments, and little is also known on compositions of Tl speciations and isotopes in fresh water sediments. This proposed research aims to study the Tl speciations (Tl(I), Tl(III) and dimethylthallium Me2Tl+) and isotopic compositions in river sediments from a Tl sulfide mineralization area in southwestern Guizhou Province, using chemical separation of Tl speciations and Tl isotope determination. This fundamental research will provide interesting results to reveal Tl speciation and isotopic compositions in river sediments and associated constraint factors, and elucidate the influencing effects of Tl speciation variation on Tl isotopic fractionation in the environment. The findings of this research are significant for providing an in-depth understanding for the compositions of Tl speciation and isotope in river sediment, and also further the understaning for geochemical cycling and isotope fractionation of Tl in the earth surface system.

铊是典型毒害重金属元素，在自然界中存在Tl(I)和Tl(III)两种价态，二者在环境体系中的相互均衡过程会发生较大的同位素分馏。国际上针对水环境中铊形态转化影响铊同位素分馏的研究集中在海水和海洋沉积物上，所采用的证据仅来自合成水钠锰矿吸附Tl(I)的实验结果，缺少天然海洋沉积物中铊形态组成影响铊同位素分馏的直接证据，对表生淡水沉积物中铊形态与同位素组成研究也未见报道。本项目以黔西南铊矿化区河流沉积物中铊形态（Tl(I)、Tl(III)、Me2Tl+）与同位素(205Tl/203Tl)组成特征为研究目标，应用铊形态分离和铊同位素测试技术，研究河流沉积物中铊的形态与同位素组成特征及其关键控制因素，阐明沉积物中铊形态转化对铊同位素分馏的影响机制。研究成果有望在国际上率先揭示河流沉积物中铊形态与同位素组成特征，丰富和深化对铊表生地球化学循环与铊同位素分馏规律的认识。

本项目开展了河流沉积物中铊赋存形态与同位素组成特征研究，取得的主要研究成果包括：（1）建立了基于二乙烯三胺五乙酸（DTPA）吸附Tl(III)的氧化铝(Al2O3)-十二烷基硫酸钠（SDS）吸附剂的水环境介质中铊形态（Tl+和Tl3+）分析新技术方法；（2）建立了锰氧化物(δ-MnO2)凝胶装载新型DGT原位测定水环境中生物有效态Tl的新技术方法，实用于较宽范围的pH（4-9）和离子强度（0.1-200 mM），持续工作性强，可长达117天；（3）黔西南滥木厂清水河沉积物中水溶态和可交换态铊T1的总比例为3.7%-25.9%，具有较强的生有效性；沉积物孔隙水中，Tl含量远远超出河水中铊含量，表明沉积物中的铊存在二次释放污染地表水的风险。河流沉积物孔隙水中主要以Tl(I)形式存在，局部沉积物孔隙水中Tl(I)含量高达245.5 μg/L，Tl(III)含量达20.9 μg/L，较高的Tl(III)含量，显示更高的水生生物毒性风险；（4）沉积物中ε205Tl为0.44±0.35～1.32±0.42（平均1.16），与红铊矿和围岩的ε205Tl（0.84±0.11）相近，偏重的ε205Tl与沉积物中Tl3+含量密切相关；（4）河流沉积物中，Cyanobacteria, Spirochaete, Hydrogenophaga, and Acinetobacter等微生物群落与铊含量呈显著正相关，表明这些微生物耐受铊或促进铊的生物地球化学循环，从而影响铊的赋存形态及同位素分馏；（5）在兰坪铅锌矿区河流沉积物中，DTPA水溶液提取液中主要为Tl+，但Tl3+含量占比也高达7.3%-23.7%。；较高含量Tl3+，可能与紫外照射含Pb溶液促使Tl+氧化为Tl3+。本项目研究初步揭示了河流沉积物中铊的赋存形态与同位素组成特征，阐明了沉积物中铊形态转化对铊同位素分馏的作用机制，丰富和深化了对铊的表生生物地球化学循环和同位素分馏规律的认识。