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1.
微生物参与铁氧化物矿物的还原性溶解是高砷地下水形成的关键过程,其中具有砷还原功能的微生物如何参与含水层砷释放的生物地球化学过程亟待研究.利用从江汉平原典型高砷含水层中厌氧条件下分离出的四株细菌(Citrobacter sp.JH-1、Clostridium sp.JH-6、Exiguobacterium sp.JH-13、Paenibacillus sp.JH-33),通过室内厌氧模拟培养实验,查明其砷、铁还原能力,并通过分别与铁氧化物矿物及原位沉积物共同培养,探究原位含水层微生物参与的砷释放机理.结果表明:四株细菌均具有厌氧条件下砷、铁还原功能,Citrobacter sp.JH-1砷还原能力最强,96 h内还原的As(Ⅴ)浓度为2.22 μmol/L.其中Citrobacter sp.JH-1不仅可在厌氧和有氧条件下还原溶液中的As(Ⅴ),还可在厌氧条件下还原溶液中的Fe(Ⅲ)和无定型的水铁矿,在与含水层沉积物共培养12 d后,沉积物中铁与砷的释放量分别为510 mg/kg及1 150 μg/kg.江汉平原含水层中的原位微生物兼具砷/铁还原功能,在厌氧条件下可还原沉积物中的铁氧化物矿物并促进砷的释放,为深入揭示高砷地下水成因机理与地下水砷污染的防控提供重要科学依据.   相似文献   

2.
王晶  谢作明  王佳  杨洋  刘恩杨 《地球科学》2021,46(2):642-651
硫在铁和砷的生物地球化学循环中发挥着重要作用,但地下水系统中硫循环的中间产物S(0)对细菌转化铁和砷的影响尚不清楚.采用室内模拟实验,研究硫参与下细菌D2201对液相和载砷针铁矿中Fe(III)和As(V)的还原作用.结果表明:细菌D2201具有很强的铁还原能力,可以将液相中74%的Fe(III)还原;加入硫后,细菌还原S(0)产生的S(-II)使铁还原率提高到94%.但是,硫没有明显影响细菌对砷的还原.在实验初期,细菌明显加速了载砷针铁矿的还原,最终还原释放到液相中的Fe(II)浓度为32.12 μmol/L;硫的加入增强了细菌对载砷针铁矿的还原,还原溶解的Fe(II)增加至284.13 μmol/L,同时,砷的释放量也增加了1.6倍.这些结果表明硫显著促进了细菌对针铁矿的还原溶解并加速砷的释放.XRD和SEM-EDS结果显示,细菌还原针铁矿但不改变其矿相,而硫的加入也仅使矿物发生一定程度的团聚,并没有使其转变为其他矿物,也未导致砷的再吸附.   相似文献   

3.
高砷含水层沉积物矿物学特征及砷的活化   总被引：2，自引：0，他引：2  
利用X射线衍射和X射线荧光分析、沉积物序列提取试验及矿物饱和度的计算,对采自江汉平原中部沙湖地区典型高砷含水层钻孔沉积物样品矿物学进行了分析,并讨论了控制含水层中砷释放和迁移的地球化学机制.对矿物在沉积物与土壤中的分布及组成的对比分析,在一定程度上指示了矿物赋存环境和/或高砷水形成的环境背景:土壤与沉积物中高岭石以低于其他3种粘土矿物的含量普遍存在,指示了含水层沉积物形成过程江汉平原存在一定的湿热古气候环境;沉积物绿泥石含量低于土壤中绿泥石,恰恰反映了土壤比沉积物略强的碱性环境;沉积物中黄铁矿的存在,显示了含水层局部的强还原性环境,指示地下水中广泛存在的Fe2+容易与二价硫发生沉淀并结合砷.砷主要以无定形铁锰氧化物结合态(平均在31％以上)形式存在,其次以碳酸盐和有机质结合态存在.无定形铁锰氧化物的还原溶解可能是控制砷迁移到地下水中主要的地球化学机制.相对高含量的绿泥石容易在含水层中发生风化,其溶解过程可以将铁释放到地下水中,从而成为影响地下水中砷活化的潜在因素.  相似文献   

4.
含水层沉积物是江汉平原地下水中砷的主要来源,沉积物地球化学特征对地下水的水化学具有重要控制作用。为查明江汉平原第四系沉积物中砷的垂向分布及赋存环境,在典型高砷地下水分布区内选取2个深钻(JH002孔及YLW01孔,深度分别为230m和201m)采集沉积物样品进行了地球化学分析。结果表明全新统和上更新统含水层沉积物以黏土、粉土、淤泥质黏土、粉砂、细砂为主,指示着弱水动力的沉积环境;2个钻孔沉积物地球化学特征相似,w(As)=2.0~22.6mg/kg(平均9.0mg/kg),w(Fe)=11.8~55.0mg/g(平均37.8mg/g),w(S)=0.1~2.1mg/g(平均0.4mg/g)。中、下更新统沉积物岩相变化较大,以砂和砾石居多,局部含有黏土夹层,指示着沉积时较强的水动力沉积环境;其中JH002孔沉积物w(As)=2.7~160.5mg/kg(平均40.9mg/kg),w(Fe)=20.1~179.5mg/g(平均50.5mg/g),w(S)=0.1~17.7mg/g(平均4.9mg/g);YLW01孔沉积物砷、铁、硫质量分数均低于JH002孔,w(As)=5.2~56.1mg/kg(平均16.2mg/kg),w(Fe)=10.9~117.5 mg/g(平均36.4 mg/g),w(S)=0.3~7.8mg/g(平均1.8mg/g)。YLW01孔中、下更新统沉积物颗粒较JH002孔更细,所处的水动力条件更弱,砷、铁、硫质量分数均低于JH002孔,说明沉积历史环境影响着砷、铁、硫等元素的分布。沉积物地球化学数据聚类分析结果表明全新统和上更新统砷与铁具有显著的相关性,而中、下更新统沉积物砷与硫化物矿物紧密相关。结合不同深度含水层水化学特征差异指示上更新统含水层中含砷铁氧化物的还原性溶解导致浅层地下水中砷的富集,富硫的中、下更新统深层含水层中强还原环境下砷受到硫化物矿物的固定作用难以释放进入地下水中。  相似文献   

5.
硫酸盐还原菌是厌氧环境中参与砷形态转化的重要微生物种群,其介导的生物地球化学循环过程对铁氧化物表面吸附态砷迁移转化的影响亟待深入研究.选取江汉平原典型高砷含水层原位沉积物分离纯化出一株严格厌氧硫酸盐还原菌Desulfovibrio JH-S1,对其进行砷和铁还原能力鉴定,并通过模拟培养实验探究硫酸盐还原菌参与下的铁矿物相转化对吸附态砷迁移的影响.Desulfovibrio JH-S1具有Fe(III)还原能力,无硫和有硫体系中Fe(III)均能被还原,但在硫酸盐充足条件下铁还原量显著增加;该菌株不具备As(V)还原能力,但添加硫酸盐的培养体系中As(V)去除率可达96%以上.Desulfovibrio JH-S1能够还原硫酸盐从而促进载砷的水铁矿还原转化为纤铁矿,并导致吸附的砷释放.江汉平原高砷含水层土著硫酸盐还原菌兼具硫酸盐/铁还原功能,参与了高砷含水层系统中砷-铁-硫耦合循环,对高砷地下水的形成具有重要作用.   相似文献   

6.
江汉平原高砷含水层沉积物地球化学特征   总被引：2，自引：0，他引：2  
选取江汉平原典型地下水砷中毒区仙桃市沙湖原种场为研究区,对3个长50m的钻孔沉积物砷含量与赋存形态及其他化学组成进行了分析。结果表明区内沉积物砷质量分数为1.35~107.5mg/kg(平均值为12.8mg/kg)。黏土或亚黏土层中砷含量较高,这与细粒沉积物中铁锰氧化物、黏土矿物对砷的吸附有关。地下20m左右深度内含水层沉积物中砷含量最高,相应地下水中砷质量浓度高达到1 000μg/L。草酸-草酸铵选择性提取结果指示沉积物中10%~77%(平均38%)的As与无定形铁氢氧化物结合,表明无定形铁氢氧化物还原性溶解可能是控制砷释放与还原的主要地球化学过程,并且有机质生物氧化机制极大地促进了该过程。然而,沉积物中仅1.2%~23%的铁被草酸-草酸铵提取,含水层中砷浓度主要受铁的氢氧化物还原性溶解影响,但其他形式的铁、有机物的吸附作用亦控制着砷的含量。  相似文献   

7.
大同盆地高砷地下水系统沉积物环境磁学特征   总被引：1，自引：0，他引：1  
为了研究高砷含水层沉积物中砷与磁性矿物的关系, 对大同盆地高砷地区含水层钻孔样品进行了环境磁学和地球化学分析, 结果表明亚铁磁性矿物是决定含水层沉积物磁性特征的主要磁性矿物.通过对比砷和磁性参数在垂向上的变化趋势发现, 高砷含量往往对应着低的饱和等温剩磁(SIRM) 值, 揭示出砷和顺磁性矿物之间也存在联系.沉积物砷含量与磁性参数的相关系数均小于0.5 (α=0.05), 表明砷和亚铁磁性矿物及不完整反铁磁性矿物之间不存在明显的联系.在所有磁性参数中磁化率(χ)和砷含量之间相关性最显著, 相关系数均在0.4 (α=0.05) 左右, 低的相关系数与亚铁磁性矿物和不完整反铁磁性矿物对顺铁磁性矿物的稀释有关.该地区高砷地下水的形成可能与水铁矿及纤铁矿等砷的赋存矿物在还原条件下的还原溶解有关.   相似文献   

8.
砷是土壤中重要的(类)重金属污染物,其毒性主要取决于在环境中的形态及氧化还原状态。游离态Fe(Ⅱ)(Fe(Ⅱ)_(aq))驱动铁(氢)氧化物晶相重组过程是土壤铁循环的重要组成,对土壤中重金属的吸附、固定、钝化等环境行为有重要影响。本研究采用~(57)Fe稳定同位素示踪方法研究厌氧条件下Fe(Ⅱ)_(aq)驱动针铁矿晶相重组过程中砷的氧化还原及形态变化过程。结果显示,在只有针铁矿存在的对照处理中,针铁矿本身对As(Ⅲ)没有氧化作用,但83%的As(Ⅲ)被吸附到针铁矿表面。在Fe(Ⅱ)_(aq)和针铁矿共存体系中,Fe(Ⅱ)_(aq)可与针铁矿中结构态Fe(Ⅲ)发生铁原子交换,As(Ⅲ)的存在降低了铁原子交换速率。同时,在Fe(Ⅱ)_(aq)驱动针铁矿晶相转化过程中,77%的As(Ⅲ)被氧化成As(Ⅴ),As活性降低。另外,部分吸附在针铁矿表面的As(Ⅲ)和氧化转化后的As(Ⅴ)通过针铁矿的晶格单元包裹或取代Fe结构位的形式被针铁矿结构化固定,从而进一步降低了As的活性。  相似文献   

9.
原生高砷地下水的类型、化学特征及成因   总被引：4，自引：0，他引：4  
由于分布广、危害大,原生高砷地下水严重威胁全球内数亿居民的身体健康。研究原生高砷地下水的分布、化学特征及成因有助于进一步理解地下水中砷的迁移转化规律,并确保高砷区地下水的可持续利用。在查阅大量文献资料的基础上,结合近10年的高砷地下水研究经验,把原生高砷地下水分为还原性中性高砷地下水（Ⅰ 1型）和还原性弱碱性高砷地下水（Ⅰ 2型）、氧化性弱碱性高砷地下水（Ⅱ型）和氧化性弱酸性高砷地下水（Ⅲ型）。Ⅰ 1型高砷地下水主要分布于河流三角洲地区,Ⅰ 2型分布于干旱半干旱封闭内陆盆地,Ⅱ型主要分布于干旱半干旱平原盆地,Ⅲ型主要分布于富含黄铁矿或硫化物矿物的基岩地区。Ⅰ 1型高砷地下水处于还原环境,pH呈中性,Fe/Mn氧化物矿物的还原性溶解是造成As富集的主要原因。Ⅰ 2型高砷地下水处于还原环境,pH呈弱碱性,除了Fe/Mn氧化物矿物的还原性溶解外,As的解吸附是含水层中砷释放的重要原因。Ⅱ型高砷地下水处于氧化弱氧化环境,pH呈弱碱性,As的解吸附是含水层中砷释放的主要原因。Ⅲ型高砷地下水处于氧化环境,pH呈弱酸性,黄铁矿及其他硫化物矿物的氧化溶解导致了含水层中砷的释放。对于Ⅰ 2型高砷地下水,需要深入研究Fe/Mn氧化物矿物的还原性溶解以及As的解吸附对地下水砷富集的相对贡献量。  相似文献   

10.
饮水型砷中毒分布在中国台湾、新疆、内蒙、山西、吉林等地。笔者采用GIS的空间数据叠加技术、化验测试与环境模拟技术,进行了地下水砷的反向地球化学模拟研究。研究表明,受构造运动控制,低洼地带堆积了巨厚的粉砂淤泥质沉积物和富含有机质的湖积物,为砷的赋存提供了空间。地下水砷的富集受水中Fe、Mn、pH、Cl-、PO34-、HCO3-、SO24-、Se的影响,其中,重碳酸钙型水中砷含量最低,氯化物重碳酸钠型水砷含量最高。臭葱石(FeAsO4:2H2O)等含铁、含锰矿物在进入地下水的溶解过程中,形成铁(锰)氧化物和砷化合物(砷酸盐或亚砷酸盐)。随着Eh降低,氧化物被还原形成更为活泼的离子组分,吸附在氧化物表面的砷化合物随之解吸,还原环境有利于砷从沉积物中向水中溶解、迁移。研究结果为实施安全供水提供了重要依据。  相似文献   

11.
蒲阳河流域地下水水化学及同位素特征   总被引：3，自引：2，他引：1       下载免费PDF全文
保定西部山前地区位于太行山及华北平原交界带,为缓解极端气候灾害对生产生活的影响,维持地下水资源的可持续开发利用,开展相关的地下水水化学及同位素特征研究。研究区地下水化学类型以HCO3—Ca·Mg、HCO3·SO4—Ca·Mg及SO4·HCO3—Ca·Mg为主,区内地下水主要来源于大气降水,流域内地表水补给地下水;地下水中化学成分为Ca2＋、Mg2＋、HCO-3、SO2-4,主要来源于岩石风化作用,同时受到人类活动的影响,地下水中硝酸盐含量明显升高;由于受到褶皱构造的控制,流域的上游及平原区均出现年龄大于60年的地下水,多数岩溶水年龄较复杂,从现代水到大于60年的水均有分布。研究成果为流域内水资源的合理开发利用提供建议,区内岩溶地下水的开发将在一定程度上缓解极端天气的影响。  相似文献   

12.
Sediments from the Red River and from an adjacent floodplain aquifer were investigated with respect to the speciation of Fe and As in the solid phase, to trace the diagenetic changes in the river sediment upon burial into young aquifers, and the related mechanisms of arsenic release to the groundwater. Goethite with subordinate amounts of hematite were, using Mössbauer spectroscopy, identified as the iron oxide minerals present in both types of sediment. The release kinetics of Fe, As, Mn and PO4 from the sediment were investigated in leaching experiments with HCl and 10 mM ascorbic acid, both at pH 3. From the river sediments, most of the Fe and As was mobilized by reductive dissolution with ascorbic acid while HCl released very little Fe and As. This suggests As to be associated with an Fe-oxide phase. For oxidized aquifer sediment most Fe was mobilized by ascorbic acid but here not much As was released. However, the reduced aquifer sediments contained a large pool of Fe(II) and As that is readily leached by HCl, probably derived from an unidentified authigenic Fe(II)-containing mineral which incorporates As as well. Extraction with ascorbic acid indicates that the river sediments contain both As(V) and As(III), while the reduced aquifer sediment almost exclusively releases As(III). The difference in the amount of Fe(II) leached from river and oxidized aquifer sediments by ascorbic acid and HCl, was attributed to reductive dissolution of Fe(III). The reactivity of this pool of Fe(III) was quantified by a rate law and compared to that of synthetic iron oxides. In the river mud, Fe(III) had a reactivity close to that of ferrihydrite, while the river sand and oxidized aquifer sediment exhibited a reactivity ranging from lepidocrocite or poorly crystalline goethite to hematite. Mineralogy by itself appears to be a poor predictor of the iron oxide reactivity in natural samples using the reactivity of synthetic Fe-oxides as a reference. Sediments were incubated, both unamended and with acetate added, and monitored for up to 2 months. The river mud showed the fastest release of both Fe and As, while the effect of acetate addition was minor. This suggests that the presence of reactive organic carbon is not rate limiting. In the case of the river and aquifer sediments, the release of Fe and As was always stimulated by acetate addition and here reactive organic carbon was clearly the rate limiting factor. The reduced aquifer sediment apparently can sustain slower but prolonged microbially-driven release of As. The highly reactive pools of Fe(III) and As in the river mud could be due to reoxidation of As and Fe contained in the reducing groundwater from the floodplain aquifers that are discharging into the river. Deposition of the suspended mud on the floodplain during high river stages is proposed to be a major flux of As onto the floodplain and into the underlying aquifers.  相似文献   

13.
A stratigraphic profile of solid phase As was measured to investigate the diagenetic cycling of arsenic and related elements in fluvial sediments of the Meghna River delta plain. The distributions of Fe, Mn, and Al are typically characterized by surficial solid phase enrichment, and As is distributed down to 36.6 m showing similar alternate layers of maxima and minima with Fe, Mn and TOC, which reflects the diagenetic remobilization and periodical differences in source materials of As. Lithological characteristics and geochemical data suggest that elevated levels of As are found in organic-matter-rich clay and silty sand rather than sand samples, with occasionally enriched As content in iron-oxyhydroxide-coated sand grains. Arsenic demonstrates a positive and significant co-variation with total organic carbon in sediments, which suggests the important role of particulate and colloidal organic matter and biological activity in controlling the distribution of arsenic in the Bengal delta. However, the concentrations of Fe and Mn weakly correlate with As contents, whereas Al contents show no relationship with As. The results of this study suggest that reactive oxides or hydroxides of Fe and Mn, rather than Fe and Mn with other minerals, might control arsenic distribution.  相似文献   

14.
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 and 60 m, with a maximum up to 1820 μg/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 oxides/oxyhydroxides and/or desorption from the Fe oxides/oxyhydroxides at high pH (above 8.0). The bulk chemical results of sediments show the arsenic and iron are moderately correlated, suggesting that arsenic is associated with iron-bearing minerals. Results of sequential-extraction experiment show that solid-phase arsenic is similarly distributed among the different pools of reservoir in the aquifer sediments. Strongly adsorbed arsenic and co-precipitated arsenic are its dominant species in the solid-phase. Geochemical studies using chemical analysis, X-ray diffraction and scanning electron microscopy on magnetically separated fractions demonstrate that iron oxides/oxyhydroxides with residual magnetite and chlorite, illite, iron oxides/oxyhydroxides-coated quartz and feldspar, and ankerite are the dominant carriers of arsenic in the sediments. The major processes of arsenic mobilization are probably linked to desorption of As from Fe oxides/oxyhydroxides and reductive dissolution of Fe-rich phases in the aquifer sediments under reducing and alkaline conditions.  相似文献   

15.
《Applied Geochemistry》1999,14(5):655-667
Sediments sampled at a hydrocarbon-contaminated, glacial-outwash, sandy aquifer near Bemidji, Minnesota, were analyzed for sediment-associated Fe with several techniques. Extraction with 0.5 M HCl dissolved poorly crystalline Fe oxides and small amounts of Fe in crystalline Fe oxides, and extracted Fe from phyllosilicates. Use of Ti-citrate-EDTA-bicarbonate results in more complete removal of crystalline Fe oxides. The average HCl-extractable Fe(III) concentration in the sediments closest to the crude-oil contamination (16.2 μmol/g) has been reduced by up to 30% from background values (23.8 μmol/g) as a result of Fe(III) reduction in contaminated anoxic groundwater. Iron(II) concentrations are elevated in sediments within an anoxic plume in the aquifer. Iron(II) values under the oil body (19.2 μmol/g) are as much as 4 times those in the background sediments (4.6 μmol/g), indicating incorporation of reduced Fe in the contaminated sediments. A 70% increase in total extractable Fe at the anoxic/oxic transition zone indicates reoxidation and precipitation of Fe mobilized from sediment in the anoxic plume. Scanning electron microscopy detected authigenic ferroan calcite in the anoxic sediments and confirmed abundant Fe(III) oxyhydroxides at the anoxic/oxic boundary. The redox biogeochemistry of Fe in this system is coupled to contaminant degradation and is important in predicting processes of hydrocarbon degradation.  相似文献   

16.
The chemical and microbiological characteristics of groundwater from two provinces of central Spain were studied. In some zones of this area, the concentrations of As in groundwater exceed the guideline concentrations, set internationally between 10 g/l and 50 g/l, reaching levels over 100 g/l. A narrow correlation between the contents of arsenic and HCO3– was observed. These data suggest a possible mechanism of the As mobilization from aquifer sediments to groundwater: the bicarbonate ions could displace HAsO42– adsorbed on aquifer oxyhydroxides. Sediments containing relatively high contents of adsorbed arsenic are deposited in surface water environments with low carbonate concentrations. Subsequently, the sediments become exposed to groundwater with highly dissolved carbonate content, and arsenic can be mobilized by displacement from mineral surfaces. In addition, the presence of Pseudomonas genera bacteria, which secrete siderophores (Fe chelating agents) could mobilize As adsorbed on Fe oxides through their dissolution. These combined microbiological and chemical processes might have increased the natural mobility of As.  相似文献   

17.
The importance of accessing safe aquifers in areas with high As is being increasingly recognized. The present study aims to investigate the sorption and mobility of As at the sediment-groundwater interface to identify a likely safe aquifer in the Holocene deposit in southwestern Bangladesh. The upper, shallow aquifer at around 18 m depth, which is composed mainly of very fine, grey, reduced sand and contains 24.3 μg/g As, was found to produce highly enriched groundwater (190 μg/L As). In contrast, deeper sediments are composed of partly oxidized, brownish, medium sand with natural adsorbents like Fe- and Al-oxides; they contain 0.76 μg/g As and impart low As concentrations to the water (4 μg/L). These observations were supported by spectroscopic studies with SEM, TEM, XRD and XRF, and by adsorption, leaching, column tests and sequential extraction. A relatively high in-situ dissolution rate (Rr) of 1.42 × 10−16 mol/m2/s was derived for the shallower aquifer from the inverse mass-balance model. The high Rr may enhance As release processes in the upper sediment. The field-based reaction rate (Kr) was extrapolated to be roughly 1.23 × 10−13 s−1 and 6.24 × 10−14 s−1 for the shallower and deeper aquifer, respectively, from the laboratory-obtained adsorption/desorption data. This implies that As is more reactive in the shallower aquifer. The partition coefficient for the distribution of As at the sediment-water interface (Kd-As) was found to range from 5 to 235 L/kg based on in-situ, batch adsorption, and flow-through column techniques. Additionally, a parametric equation for Kd-As (R2 = 0.67) was obtained from the groundwater pH and the logarithm of the leachable Fe and Al concentrations in sediment. A one-dimensional finite-difference numerical model incorporating Kd and Kr showed that the shallow, leached As can be immobilized and prevented from reaching the deeper aquifer (∼150 m) after 100 year by a natural filter of oxidizing sand and adsorbent minerals like Fe and Al oxides; in this scenario, 99% of the As in groundwater is reduced. The deeper aquifer appears to be an adequate source of sustainable, safe water.  相似文献   

18.
Aquifer geochemistry was characterized at a field site in the Munshiganj district of Bangladesh where the groundwater is severely contaminated by As. Vertical profiles of aqueous and solid phase parameters were measured in a sandy deep aquifer (depth >150 m) below a thick confining clay (119 to 150 m), a sandy upper aquifer (3.5 to 119 m) above this confining layer, and a surficial clay layer (<3.5 m). In the deep aquifer and near the top of the upper aquifer, aqueous As levels are low (<10 μg/L), but aqueous As approaches a maximum of 640 μg/L at a depth of 30 to 40 m and falls to 58 μg/L near the base (107 m) of the upper aquifer. In contrast, solid phase As concentrations are uniformly low, rarely exceeding 2 μg/g in the two sandy aquifers and never exceeding 10 μg/g in the clay layers. Solid phase As is also similarly distributed among a variety of reservoirs in the deep and upper aquifer, including adsorbed As, As coprecipitated in solids leachable by mild acids and reductants, and As incorporated in silicates and other more recalcitrant phases. One notable difference among depths is that sorbed As loads, considered with respect to solid phase Fe extractable with 1 N HCl, 0.2 M oxalic acid, and a 0.5 M Ti(III)-citrate-EDTA solution, appear to be at capacity at depths where aqueous As is highest; this suggests that sorption limitations may, in part, explain the aqueous As depth profile at this site. Competition for sorption sites by silicate, phosphate, and carbonate oxyanions appear to sustain elevated aqueous As levels in the upper aquifer. Furthermore, geochemical profiles are consistent with the hypothesis that past or ongoing reductive dissolution of Fe(III) oxyhydroxides acts synergistically with competitive sorption to maintain elevated dissolved As levels in the upper aquifer. Microprobe data indicate substantial spatial comapping between As and Fe in both the upper and deep aquifer sediments, and microscopic observations reveal ubiquitous Fe coatings on most solid phases, including quartz, feldspars, and aluminosilicates. Extraction results and XRD analysis of density/magnetic separates suggest that these coatings may comprise predominantly Fe(II) and mixed valence Fe solids, although the presence of Fe(III) oxyhydroxides can not be ruled out. These data suggest As release may continue to be linked to dissolution processes targeting Fe, or Fe-rich, phases in these aquifers.  相似文献   

19.
Higher levels of arsenic in the aquifers of Chia-Nan Plain in southwestern Taiwan were attributed to the cause of Blackfoot disease in the area half a century ago. Although extensive studies were conducted on the occurrence, speciation, mobility, and transport of arsenic in the region, the relationship between arsenic adsorption by different aquifer materials and the concentration and speciation of arsenic in these aquifers was poorly delineated. This study focused on characterization of sediments in the region, as well as determination of relations between arsenic adsorption and other geochemical and clay mineralogical properties of the sediments. The arsenic adsorption capacity was positively related to the clay minerals, Fe, and Mn contents in the sediments. The higher arsenic adsorption capacity of the sediments served as a sink for the arsenic during its transport from the central mountains to the coast and as a source for its release under reduced environment and in the presence of humic substances.  相似文献   

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