【作者】 谢先军；

【导师】 王焰新； Andre D. Ellis；

【作者基本信息】 中国地质大学 ， 环境工程， 2008， 博士

【摘要】 水中砷含量超过10μg/L就会对人体健康产生威胁。世界范围内,由于饮用高砷地下水而引起的砷中毒事件已有大量的报导。特别是在印度和孟加拉国发现广泛分布的高砷地下水以来,对这类年轻松散沉积物含水层砷污染的研究已成为当前地下水环境领域研究的热点。有关高砷地下水的成因机制,不同的学者提出了不同的假说模型。尽管对于高砷地下水的成因机制还存在争论,但普遍认为,高砷地下水的形成与含水层中Fe氧化物-氢氧化物有密切的联系,而且含水层中原生微生物的活动对砷的迁移、转化具有重要的影响。含水层沉积物中Fe氧化物-氢氧化物主要以赤铁矿（αFe₂O₃）、针铁矿（αFeOOH）、纤铁矿（γFeOOH）、磁赤铁矿（γFe₂O₃）、磁铁矿（Fe₃O₄）和水铁矿（5Fe₂O₃·9H₂O）等不同的类型存在。不同类型的Fe氧化物-氢氧化物其地球化学性质不一样,对砷的吸附能力也存在差别。从环境磁学角度来看,上述Fe氧化物-氢氧化物具有不同的磁学特征,室温下磁赤铁矿属亚铁磁性矿物,赤铁矿和针铁矿具不完整反铁磁性特征,而纤铁矿和水铁矿属于顺磁性矿物,因此,根据磁性参数测量,可提取有关不同Fe氧化物-氢氧化物的信息。同时,含水层中砷的迁移、转化还受到硫的地球化学及生物地球化学过程控制。高砷含水层中硫同位素地球化学的研究,能对影响含水层中砷迁移、转化的地球化学及生物地球化学过程进行指示。此外,对沉积物中生物标志物的分析,能对含水层系统中微生物的活动进行表征。因此,对高砷含水层中Fe氧化物-氢氧化物、硫同位素、沉积物生物标志物等的研究,对理解高砷地下水的成因机制具有重要意义。本研究以大同盆地为研究区,该地区高砷地下水具有高HCO₃、低SO₄^2-、NO₃^-、U、Mo、Cr等及强烈的H₂S气味特征,地下水以Na-HCO₃-Cl为主要水化学类型。本次研究以高砷含水层为研究对象,对含水层沉积物Fe氧化物-氢氧化物、生物标志物及硫同位素开展了研究工作,并取得了以下主要成果:1、通过磁学研究,对高砷含水层沉积物中磁性矿物与砷的关系进行了表征。本研究通过对含水层沉积物采用KLY-3S型磁化率仪、TDS-1型脉冲磁化仪及SMD-88型旋转磁力仪等进行了质量磁化率（χ）、饱和等温剩磁（SIRM）、等温剩磁（IRM300mT）、硬剩磁（HIRM）、F比值（F-ratio）等的测定和计算。并采用氢发生原子荧光光谱仪（AFS-810）对全岩砷含量进行了分析。此外,采用化学提取的方法对沉积物中不同相态的铁进行了提取,并用电感耦合等离子光谱仪（POEMSⅢ）对Fe、As含量进行了分析。结果表明:高砷含水层沉积物中磁性矿物以亚铁磁性矿物（磁赤铁矿等）为主,并含有少量反铁磁性矿物（赤铁矿等）;沉积物全岩砷含量与饱和等温剩磁（SIRM）、等温剩磁（IRM300mT）等磁性参数具有显著的相关性（α=0.05,R=0.686）,说明含水层沉积物中砷主要和亚铁磁性矿物（如,磁赤铁矿等）共存;与磁分析结果相似,化学提取结果表明,沉积物中Fe主要以还原态铁形式（赤铁矿、磁赤铁矿等）存在,砷主要出现在还原态铁相中。因此,有利的还原条件,可以使沉积物中还原态铁还原溶解并释放砷。2、通过硫同位素地球化学研究,对含水层系统中控制砷地球化学循环的地球化学及生物地球化学过程进行了研究。环境中的硫对砷的地球化学循环具有重要的影响。本研究采用配备有元素分析仪（Costech）的同位素比值质谱分析仪（Delta PlusXL）对高砷地下水中硫同位素进行了分析。此外,采用离子色谱仪（Metrohm 761 Compact IC）对主要阴离子进行了分析;采用电感耦合质谱仪（Thermo ExCell）对部分微量元素及不同形态砷进行了分析。结果显示:高砷地下水中δ³⁴S_[SO4]介于-2.5到+36.1‰之间,变化范围较大,表明微生物活动参与了硫的地球化学循环过程。地下水中δ³⁴S_[SO4]背景值为8-15‰,且高砷地下水δ³⁴S同时发生了强烈亏损和富集作用（±20‰）。此外,地下水补给区δ³⁴S_[SO4]值介于8-15‰之间,略高于中国北方大气硫同位素平均值（5‰）,但低于典型蒸发岩硫同位素值（16‰）,因此,大气来源硫（干、湿沉降）及侧向裂隙水溶解蒸发岩来源硫为本地区地下水中硫的主要来源。除两件硫含量很低的样品外,δ³⁴S_[SO4]与地下水中砷含量具有显著的正相关性,即地下水中砷含量随δ³⁴S_[SO4]增高而增加,这表明砷的释放与微生物厌氧还原有关。负的δ³⁴S值为典型微生物还原产物的硫同位素特征,两件低硫含量、低δ³⁴S_[S04]值、高砷含量的样品表明,微生物在铁、锰的氧化物参与下的厌氧氧化低δ³⁴S值的硫化物,对高砷地下水的产生也具有重要作用。其过程可表达如下:3S⁰+2FeOOH-As→SO₄^2-+2FeS+2As_s+2H⁺S⁰+3MnO₂-As+4H⁺→SO₄^2-+3Mn²⁺+3As_s+2H₂O其中-As为与Fe、Mn氧化物共存的砷;Ass为水溶态砷。3、通过对沉积物中生物标志物的分析,对含水层中微生物活动进行指示。本研究通过索氏提取方法对高砷沉积物样品中有机物进行了提取分离,并采用气相色谱质谱仪对提取的饱和烷烃类物质进行了分析。本研究以沉积物中饱和烃类为研究对象,系列化合物采用标准谱库检索和质谱解析相结合的方法进行了鉴定。结果表明:所有沉积物样品中饱和烷烃的碳数分布范围为C₁₄到C₃₅,主要可分为陆源、双峰和石油来源等;部分样品中高碳数正构烷烃含量较高,表现出一定的奇偶优势,表明有机质来源以陆源高等植物的输入为主;部分样品低碳数正构烷烃含量较高,奇偶优势不明显,表明其来源以浮游生物和细菌为主;CPI值、C₂₉甾烷及藿烷的分布特征表明,有机物经历了微生物的降解作用。此外,色谱图中不能分辨包状组分的存在说明,有机物具有典型的石油来源特征,并经历强烈的生物降解过程。因此,沉积物中天然生物可降解有机质的存在,为微生物活动提供了条件。结合硫同位素研究结果认为,微生物活动对砷的地球化学循环具有重要意义。综上,本研究通过对高砷含水层沉积物中环境磁学、硫同位素地球化学及生物标志物研究表明:沉积物中砷主要与可还原态的Fe氧化物-氢氧化物（如磁赤铁矿等）共存,为高砷地下水中砷的主要来源;还原条件是高砷地下水形成的重要因素,微生物厌氧条件下对铁的氧化物的还原是砷发生迁移的重要地球化学过程,但低δ³⁴S_[SO4]及低SO₄^2-也表明微生物在Fe的氧化物参与下对硫化物的厌氧氧化对高砷地下水的形成也具有一定的意义;沉积物生物标志物分析表明有机物表现为多来源,且有强烈微生物降解特征,结合硫同位素研究认为微生物活动对砷的迁移具有重要作用。本论文创新点:1、首次对高砷含水层沉积物进行了较为详细的环境磁学研究,并采用化学全分析及扫描电镜等方法对磁性组分的砷含量进行了分析;2、运用硫同位素地球化学的方法,对影响砷迁移、转化的地球化学及生物地球化学过程进行了研究;3、通过对高砷含水层沉积物中生物标志物的研究,结合硫同位素研究结果,认为生物作用对含水层中砷的循环有重要作用。总之,论文在高砷沉积物环境磁学,生物标志物,硫同位素地球化学等方面取得了创新性进展。更多还原

【Abstract】 The bulk geochemistry of sediment samples from three boreholes （each around 50 m deep） drilled at the arsenic contaminated （As content >1060μg/L） aquifers of Datong Basin in Shanxi Province, northern China, indicates that the average bulk concentrations of major and trace elements of the sediments are similar to those of the average upper continental crust. The average content of arsenic （18.7 mg/kg） is higher than that of modern unconsolidated sediments （5-10 mg/kg）. However, the abundance of elements varies with grain size, with higher concentrations in finer fractions of the sediments （clays and silts）. The NH2OH-HCl extracted iron is closely correlated with extracted arsenic, suggesting that iron oxyhydroxides may be the major source of arsenic. Geochemical and environmental magnetic studies were carried out to identify the relationship of iron hydroxides/oxides with the arsenic in aquifer sediment and how it relates to the occurrences of high arsenic groundwater. Arsenic and iron contents showed strong association, however, weak correlation was observed between contents of phosphorus and iron and arsenic. This may be the result of competitive adsorption of phosphorus and arsenic on Fe-hydroxides/oxides. The strong correlations between arsenic contents and magnetic parameters suggest that the ferrimagnetic minerals are the dominated carrier of arsenic though there are some antiferromagnetic minerals present in the aquifer sediment sample. The results of chemical extraction experiments also suggest the arsenic co-exists with the reducible iron oxides such as maghemite and hematite in aquifer. And results of microcosm experiments show that the mobilization of arsenic from sediments at Datong is probably mainly related to changes in redox conditions. Moderately reducing conditions are favorable for release of arsenic into groundwater at Datong.To better understand the sources and mobilization processes responsible for arsenic enrichment in groundwater in the central part of Datong Basin where serious arsenic poisoning cases have been reported, hydrochemical characteristics of the groundwater and the geochemical and mineralogical features of the aquifer sediments were studied. The aqueous arsenic levels are strongly depth-dependent in the study area and the high arsenic concentrations are found at depths between 15 m to 60 m, with a maximum up to 1820μg/L The hydrochemical characteristics of high arsenic groundwater from the Datong Basin indicate that the mobilization of arsenic is related to reductive dissolution of Fe oxide/hydroxides and/or desorption from the Fe oxide/hydroxides at high pH （above 8.0）. The bulk chemical results of sediments show that the arsenic and iron contents are moderately correlated, suggesting the association of arsenic with iron-bearing minerals. Sequential-extraction experiment results show that solid-phase arsenic is similarly distributed among the different pools of reservoir in the aquifer sediments. Strongly adsorbed arsenic and arsenic co-precipitated in solids are the dominant species of arsenic in the solid-phase. Geochemical studies using chemical analysis, X-ray diffraction and scanning electron microscopy on the magnetically separated fractions demonstrate that iron oxides/hydroxides with residual magnetite and chlorite, illite, iron oxide/hydroxides-coated quartz and feldspar, and ankerite are the dominant carriers of arsenic in the sediments. The major processes of arsenic mobilization should be desorption and reductive dissolution targeting Fe-rich phases in the aquifer sediments under reducing and alkaline conditions.Hydrochemical and sulfur isotope investigation helps understand the release mechanism of arsenic from aquifer into groundwater. The correlation between arsenic contents and values of proxies for redox conditions (Eh, U, and SO₄^2-) all indicate that the occurrences of high arsenic groundwaters are related to the reducing conditions of the aquifer system. The dissolved arsenic is dominated by inorganic species, including arsenite （As（III）） and arsenate （As（V）） in the waters. Distribution of As（III） and As（V） was primarily regulated by redox potential （Eh）. Theδ³⁴S_[SO4] values of groundwater samples from the study area have a wide range （from -2.5 to +36.1‰）, suggesting that the microbiological reducing of sulfate could occur within the aquifer. The springs which were regarded as the major recharge sources of groundwater in this area haveδ³⁴S_[SO4] values of +8.7 and +9.7‰, respectively. The δ³⁴S_[SO4] values fit within the typicalδ³⁴S values of atmospheric SO₄^2-, suggesting that atmospheric SO₄^2- is a major source of groundwater sulfate. Besides, the correlation betweenδ³⁴S_[SO4] and Eh and U indicates the reduction of sulfate in groundwater. The positive correlation betweenδ³⁴S_[SO4] and total arsenic supports that reducing processes are responsible for arsenic mobilization from aquifer into the groundwater. However, a continuous enrichment ofδ³⁴S_[SO4] was not observed with the increase of the total arsenic. Moreover, there are two high arsenic groundwater samples that have lowδ³⁴S_[SO4] values （-2.5 and +8.5‰, respectively）. This can not be explained by microbial reducing processes which should result in a continuous enrichment ofδ³⁴S_[SO4] values of residual sulfate. Instead, it may be attributed to microbial oxidation under anaerobic conditions. Therefore, it can be concluded that apart from reducing processes, microbial anaerobic oxidation process in the presence of Fe-Mn oxides/hydroxides is also an important factor for the mobilization of arsenic in shallow groundwater in Datong Basin.Biomarker and hydrochemical characteristics of geogenic arsenic-contaminated aquifers at the Datong basin were analyzed to understand the impact of natural organic matter （NOM） biodegradation on arsenic enrichment in groundwater. The hydrochemical characteristics of high arsenic groundwater from the Datong Basin indicate that the mobilization of arsenic is related to reductive dissolution of Fe and Mn oxyhydroxides under the impact of NOM biodegradation in the aquifer. Meanwhile, the elevated concentration of bicarbonate alkalinity is another important factor to mobilization of arsenic into groundwater. Biomarker analysis reveals that all the sediments contain petroleum-sourced hydrocarbons, which may have undergone biodegradation, as suggest by the dominance of C25-C31 n-alkanes, C29 sterane and the distribution pattern of hopanes. The presence of unresolved complex mixtures （UCMs） in all samples also indicates the effect of biodegradation. At some depths （5.4-11.8 m; 31-33.2 m and 40-48.4 m below the land surface）, the samples have low n-alkanes content and no odd-over-even predominance, suggesting that indigenous microbes within the aquifer can preferentially remove the petroleum-sourced n-alkanes. The bioavailable organic carbon is very important to promote the microbial activity and subsequent arsenic release from the aquifer to groundwater.In generally, clay minerals and iron oxides/hydroxides are the dominant sources of arsenic for groundwater. High pH and low Eh condition is favorable for the mobilization of arsenic from aquifer sediments into groundwater. Arsenic can be mobilized into groundwater either via desorption from the surfaces of clay minerals and iron oxides/hydroxides in alkaline environment, or via dissolution driven by the reduction of As-bearing Fe（III） oxides/hydroxides at low Eh conditions. Moreover, microbial activities are also important in controlling the mobilization of arsenic.更多还原