湘江下游河床沉积物铅污染特征及铅来源的Pb同位素示踪分析

Lead(Pb)contamination and tracing for its sources using Pb isotopes in bed sediments from lower reaches of the Xiangjiang River,Hunan province of China

利用BCR及1M HCl分离提取技术对湘江下游河床沉积物不同化学形态铅的铅含量及Pb同位素组成进行分析。结果表明,河床沉积物重金属铅含量变化大(Pb含量为(54.4~4379)mg/kg),且富集明显(EF>3.5)。不同河段沉积物铅富集程度明显不同,湘潭、长沙及湘阴河段沉积物铅富集程度相近(EF平均为3.5~4.1),而株洲段沉积物铅富集明显较高(EF平均为82.0)。BCR分离提取结果显示河床沉积物铅主要以赋存于铁锰氧化物中的可还原态铅(43.76%)和赋存于硅酸盐矿物的残渣态铅(41.53%)的形式存在。沉积物全岩铅同位素组成显示株洲段铅为非放射成因铅(^(206)Pb/^(207)Pb平均为1.1744),而湘潭至湘阴段沉积物明显富放射成因铅(^(206)Pb/^(207)Pb平均为1.1827)。由BCR提取的可提取态与残渣态铅的^(206)Pb/^(207)Pb比值平均分别为1.1798±0.002及1.1844±0.008,即可提取态与残渣态铅之间Pb同位素组成变化不明显。而1M HCl提取的可提取态(^(206)Pb/^(207)Pb平均为1.1746±0.004)与残渣态铅(^(206)Pb/^(207)Pb平均为1.1859±0.007)的Pb同位素组成明显不同。这可能是BCR分离提取实验中,株洲段沉积物非放射成因的人为源铅未完全提取所致,故BCR提取法不适合用于矿业活动影响河流沉积物重金属污染的生态风险评价。铅同位素示踪分析表明,株洲段沉积物中铅为来自铅锌矿矿石(62.4%)及燃煤排放(36.8%)人为源铅组成的二元混合铅,而湘潭至湘阴段沉积物铅为来自自然源花岗岩(54.4%)和来自人为源铅锌矿矿石(29.2%)及燃煤排放(15%)组成的混合铅。

Concentrations of Pb and its isotopic ratios in different fractions of bed sediments from lower reaches of the Xiangjiang River(China)were analysed following the three step BCR and 1 M HCl extractions.Results showed that concentrations of Pb in river bed sediments varied from 54.4 to 4379(mg/kg),and sediments were highly enriched with Pb with enrichment factors(EF)>3.5.The enrichment degree of Pb in river bed sediments of different reaches varies greatly.Among them,the enrichment factors of Pb in sediments of Xiangtan,Changsha and Xiangyin reaches were similar,with EF values ranging from 3.5 to 4.1,while the enrichment degree of Pb in Zhuzhou sediments was much higher than that of above sediments,which had average EF value of 82.Results of three step BCR extractions showed that Pb in sediments was dominated by reducible Fe-oxide(43.76%)and residual silicate(41.53%)fractions.Pb isotopes in bulk sediments at Zhuzhou section were of less radiogenic Pb with^(206)Pb/^(207)Pb ratios around 1.1744,while those of the other three sections(Xiangtan,Changsha and Xiangyin)were more radiogenic,with^(206)Pb/^(207)Pb ratios around 1.1827.The leachate and residue fractions from the BCR extraction generally had similar Pb isotope compositions,for which^(206)Pb/^(207)Pb ratios were 1.1798±0.002 and 1.1844±0.008,respectively.This suggested that differentiation of Pb isotopes between leachate and residue fractions of BCR extraction was insignificant.However,the differentiation between leachate and residue fractions of 1 M HCl extraction was distinct,as shown by average^(206)Pb/^(207)Pb ratios of 1.1746±0.004 and 1.1859±0.007 for leachates and residues,respectively.This may be attrubted to the incomplete extraction of leachated(mobile)Pb in sediments at Zhuzhou section.Thus,the present study suggests that the BCR extraction scheme was not appropriate for ecological risk assessment of heavy metal contamination in mining-impacted(ore-Pb dominated)river sediments.It was then concluded that Pb in sediments at Zhuzhou section was mixed from anthropogenic Pb from mining and smelting for Pb-Zn ores(62.4%)and from coal combustion(36.8%),while Pb in sediments from the other three sections was mixed from Pb-Zn ore mining and smelting activities(54.4%)and coal combustion(15%),and from lithologically inherited Pb from granite(29.2%)through weathering.