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Volume 43 Issue 9
Sep.  2019
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Assessment of the speciation and vertical distribution characteristics of heavy metals in sediments in artificial reef waters of Ma’an Archipelago

  • Corresponding author: Hong HUANG, hhuang@shou.edu.cn
  • Received Date: 2019-05-31
    Accepted Date: 2019-07-31
  • In order to understand the characteristics of speciation and vertical distribution of heavy metals in the sediments, columnar sediments were collected in surrounding waters of Ma’an Archipelago in June 2018. Seven heavy metals (Cd, Cu, Zn, Pb, Cr, As, Ni) were analyzed, with the improved European Community Bureau of Reference (BCR) method, by measuring the grain size composition of columnar sediments, and the vertical distribution characteristics and sources of heavy metals in the sediment of Ma’an Archipelago, by combining the analyzing method, Igeo, correlation and potential ecological risk index, were explored. The results showed that fine silt (D [3,2]) was dominant in the columnar samples; the content of heavy metals in the columnar samples fluctuated serratedly with depth. The total amount of seven heavy metals reached the peak at 0.5 m and 2.0 m; all of them reached the class I standard of marine sediment quality in China; Cd was dominated by secondary phase, which was composed of the exchangeable and carbonate fractions (F1)、iron-manganese oxide states (F2) and organic matter and sulfide fractions (F3), The proportion of three fractions was similar; the mode of occurrence of Cu, Zn, Pb, Cr, As, Ni was residual state, and the trend of effective state of seven heavy metals was periodic and consistent. Take Ni as a reference, with three analytical methods, results showed that Cu, Zn, Pb, Cr, As and Ni are mainly natural deposition, and whether Cd is a threat to the environment still remains for further study. Through three analytical methods, we can observe the content and morphology changing by years of heavy metals by means of isotope dating, aiming to determine the historical records and influencing factors of heavy metal pollution. The pollution degree of heavy metals in surrounding waters near Ma’an Archipelago is much lower, and the ecological environment is totally well protected, and the environment is less affected by human factors, reflecting the results of local sediment background values.
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Assessment of the speciation and vertical distribution characteristics of heavy metals in sediments in artificial reef waters of Ma’an Archipelago

    Corresponding author: Hong HUANG, hhuang@shou.edu.cn
  • 1. College of Ecology and Environment, Shanghai Ocean University, Shanghai    201306, China
  • 2. Shanghai Zhenhua Heavy Industries Co.Ltd, Shanghai    201100, China

Abstract: In order to understand the characteristics of speciation and vertical distribution of heavy metals in the sediments, columnar sediments were collected in surrounding waters of Ma’an Archipelago in June 2018. Seven heavy metals (Cd, Cu, Zn, Pb, Cr, As, Ni) were analyzed, with the improved European Community Bureau of Reference (BCR) method, by measuring the grain size composition of columnar sediments, and the vertical distribution characteristics and sources of heavy metals in the sediment of Ma’an Archipelago, by combining the analyzing method, Igeo, correlation and potential ecological risk index, were explored. The results showed that fine silt (D [3,2]) was dominant in the columnar samples; the content of heavy metals in the columnar samples fluctuated serratedly with depth. The total amount of seven heavy metals reached the peak at 0.5 m and 2.0 m; all of them reached the class I standard of marine sediment quality in China; Cd was dominated by secondary phase, which was composed of the exchangeable and carbonate fractions (F1)、iron-manganese oxide states (F2) and organic matter and sulfide fractions (F3), The proportion of three fractions was similar; the mode of occurrence of Cu, Zn, Pb, Cr, As, Ni was residual state, and the trend of effective state of seven heavy metals was periodic and consistent. Take Ni as a reference, with three analytical methods, results showed that Cu, Zn, Pb, Cr, As and Ni are mainly natural deposition, and whether Cd is a threat to the environment still remains for further study. Through three analytical methods, we can observe the content and morphology changing by years of heavy metals by means of isotope dating, aiming to determine the historical records and influencing factors of heavy metal pollution. The pollution degree of heavy metals in surrounding waters near Ma’an Archipelago is much lower, and the ecological environment is totally well protected, and the environment is less affected by human factors, reflecting the results of local sediment background values.

  • 重金属作为非生物降解型污染物,经过污水直排、大气沉降及地表径流等多种途径进入海洋,并与悬浮颗粒通过吸附、沉降,随水流搬运,最终进入沉积物[1]。由于沉积物相对稳定的特性,重金属含量的垂向分布可作为研究海域污染历史的重要依据[2-3]。通过研究柱状沉积物重金属的含量及分布,不仅可以判断该海域的污染程度,还可以追溯人类工业文明中各种污染物质的来源、分布、迁移和转化的历史,以此了解不同历史阶段人类活动对所研究区域重金属输送量的变化[4-5]

    马鞍列岛海域位于长江口外的东南部,泗礁岛的东北部,隶属国家级海洋特别保护区,共有136个岛屿,岛礁面积达19 km2。该海域具有丰富的海洋生物资源,独特的岛礁地貌和潮间带湿地生态系统。前人对该海域研究较多集中于马鞍列岛岛礁及附近海域表层沉积物及生物体内的重金属含量[6],而针对重金属垂向分布规律研究相对较少。因此,本实验通过采集马鞍列岛附近海域的柱状沉积物样品,采用改进后的BCR连续提取方法分析沉积物重金属形态,利用地累积指数法(Igeo)、生态危害指数、归一法等分析沉积物中重金属污染状况及来源,探讨重金属污染的历史记录和影响因素,对该海域生态环境修复以及海洋特别保护区的管理提供科学依据。

1.   材料与方法
  • 2018年06月在花鸟山以东-求子山-黄礁人工鱼礁增殖放流区[7]附近海域 (30°46′58″N,122°44′02″E)(图1 站点A),采用大型重力式采泥器采集柱状沉积物。柱长约3.55 m,柱样外径110 mm,内径105 mm。其中0~1.80 m之间柱样每隔0.30 m分割,从上至下分别记为A1、A2、A3、A4、A5、A6,1.80~3.55 m之间柱样每隔0.35 m分割,从上至下分别记为B1、B2、B3、B4、B5。将分割好的柱状沉积物样品装入聚乙烯袋,充氮气后密封,于–4°C冷藏带回实验室。利用真空冷冻干燥机(Christ Delta 1-24 LSC,德国)将沉积物样品冷冻干燥后,剔除贝壳、砾石等杂质,用玛瑙研钵研磨并过100目筛,采用四分缩分法取10~20 g制备好的样品,保存于聚乙烯袋中待用。所有样品的采集、贮存、运输、预处理均按《海洋监测规范》(GB 17378-2007)中的相关规定进行[8]

    Figure 1.  The sampling site in the Ma’an archipelago

  • 利用Nemati等[9]提出修正后的 BCR法对沉积物重金属的不同形态进行提取,采用不同提取剂,将重金属分为可交换态及碳酸盐结合态(可交换态,F1)、Fe/Mn氧化物结合态(可还原态,F2)、有机物及硫化物结合态(可氧化态,F3)以及残存在矿物晶格中的残渣态(F4),其中前3种形态统称为有效态。具体步骤:向1 g沉积物样品中分别加入0.11 mol/L乙酸、0.5 mol/L羟基氯化铵、8.8 mol/L过氧化氢以及20 mL王水分步提取,其中消解液均需通过20 min,3 000 r/min离心,并用2%HNO3溶液定容100 mL。各形态提取液、总量消解液中的重金属含量均采用 ICP-MS (7500a,Agilent Technologies,美国)进行测定。以海底沉积物成分分析标准物质(GBW07314,中国) 作为质量控制样品。

  • 1969 年法国科学家 Müller提出地累积指数(Igeo)法,可直观反映外源重金属污染在沉积物中的富集程度,目前被广泛使用[10-11]。该方法利用重金属的含量与其对应地球化学背景值的关系,评价重金属的污染程度,计算公式如下:

    中,Cn为沉积物中重金属实测含量;Bn为黏质沉积岩(即普通页岩)中该元素的地球化学背景值[12]Igeo法将重金属污染划分为 7个等级:Igeo≤0,无污染;Igeo为 0~1,轻度污染;Igeo为 1~2和2~3分别为偏中度和中度污染;Igeo为 3~4、4~5、5~6分别为为偏重、严重和极重污染。

  • 根据Hakanson[13]1980年建立的潜在生态危害指数法,综合考虑了重金属的毒性、重金属在沉积物中普遍的迁移转化规律、评价区域对重金属污染的敏感性以及重金属区域背景值的差异,并消除区域差异及异源污染影响,系统反映沉积物中重金属对生态环境的潜在影响力,成为国内外沉积物质量评价中应用最为广泛的方法之一[14-15],其公式如下:

    式中,$C_f^i$为重金属i的污染系数;Ci为重金属i的实测浓度;$C_n^i$为重金属i的评价参比值(表1),$C_n^i$通常采用工业化前沉积物重金属最高背景值;Cd为重金属总体污染程度;$E_r^i$为金属i的潜在生态危害系数;$T_r^i$为重金属毒性响应系数(表1),该值反映重金属的毒性水平及生物对重金属污染的敏感程度;ERI为多种重金属潜在生态危害指数。本实验危害指数计算中的沉积物中重金属的参照值$C_n^i$和毒性系数$T_r^i$取自文献[16]。

    元素 element
    CdCuPbZnCrAs
    $C_n^{i\left( {{{10}^{ - 6}}} \right)}$1 50701759015
    $T_r^i$305 5 1 2 10

    Table 1.  Background reference value (Ci n) and toxicity coefficient (Ti r) of heavy metals

  • 因子分析方法是一种降维映射方法,代表各类信息的综合指标又称之为因子,通过各变量间存在一定的相关关系,综合指标之间彼此不重合的信息,同时根据专业知识和指标所反映的独特含义给指标予以命名[17-18]。同时利用相关性分析重金属元素来源,通过污染物中某种受人为因素影响较小的元素或组份对重金属含量进行校正[19-20]。通常情况下Ni受人为因素影响较小,主要来自于自然沉积,因此本实验选择Ni作为参比元素进行分析。

  • 本实验采用SPSS 19.0软件进行数据相关性分析。相关性分析采用双尾检验,Pearson系数差异显著和极显著水平分别为P=0.05和0.01,处理结果使用Origin 2018软件进行绘图。

2.   结果
  • 沉积物粒度变化与沉积物形成环境的关系极为密切,分析单个粒度参数及其组合特征是研究物质来源、水动力和沉积环境的重要方法之一[21]。根据Folk分类法[22],沉积物颗粒按粒径大小可分为砂砾(2~0.063 mm)、粉粒(0.063~0.004 mm)、粘粒(<0.004 mm) 3个粒级组分。柱状沉积物中粉粒占比55.74%~65.20%,平均含量为61.83%;沙砾含量占比最低,且含量标准差较小,平均含量为2.81%;沉积物粒径范围为5.34~7.73 μm,平均值为6.29 μm。由于沉积物粒度较细且比表面积相对较大,有利于重金属的吸附作用(表2)。

    深度
    depth
    粘粒%
    clay
    粉粒%
    silt
    砂砾%
    gravel
    表面积平均粒径/μm
    D[3,2]
    A134.9564.350.706.19
    A241.4258.450.135.38
    A341.6758.240.095.34
    A434.7965.200.016.14
    A537.7562.090.165.52
    A635.2663.271.476.44
    B136.6562.081.276.23
    B232.8263.743.446.63
    B333.2564.122.636.88
    B426.3255.7417.94 7.73
    B534.1162.823.076.67

    Table 2.  Grain size characteristics of sediments

  • 运用BCR连续提取法提取柱状沉积物重金属,其含量见表3。可以看出,除Cd在2.15~2.5 m(B1~B2)时含量为国家海洋沉积物质量重金属Ⅱ类标准(MSQ-Ⅱ),其他6种重金属及其他深度Cd的含量均满足国家海洋沉积物质量重金属Ⅰ类标准(MSQ-1)[23]。Cd含量变化范围:0.30~0.80 mg/kg,平均含量为0.67 mg/kg,Cu含量变化范围:8.25 ~28.35 mg/kg,平均含量为13.41 mg/kg;Zn含量变化范围:26.70~107.80 mg/kg,平均含量为56.36 mg/kg;Pb含量变化范围;7.05~20.35 mg/kg,平均含量为11.11 mg/kg;Cr含量变化范围:25.15~57.80 mg/kg,平均含量为33.74 mg/kg;Ni含量变化范围:10.75~27.50 mg/kg,平均含量为15.45 mg/kg;As含量变化范围:6.80~19.95 mg/kg,平均含量为12.17 mg/kg,Zn在B2处含量达到最大值,其他6种重金属含量均在B1处达到最大值,总体上该海域底质环境较好。

    深度
    depth
    重金属/(mg/kg) heavy metal
    CdCuZnPbCrNiAs
    A1 0.30 11.25 37.20 9.05 27.85 13.60 8.60
    A2 0.45 22.45 98.70 15.80 45.20 20.55 19.90
    A3 0.45 13.75 47.50 8.70 28.65 11.40 12.65
    A4 0.30 11.40 30.75 8.70 26.60 12.75 8.35
    A5 0.35 9.30 44.95 9.90 28.85 13.90 7.50
    A6 0.35 10.60 42.55 10.20 31.75 14.60 6.80
    B1 0.80 28.35 99.20 20.35 57.80 27.50 19.95
    B2 0.65 15.00 107.80 15.30 47.65 21.90 18.25
    B3 0.30 8.25 56.20 8.10 26.30 12.10 14.60
    B4 0.30 8.55 28.40 7.05 25.15 10.75 8.75
    B5 0.45 8.60 26.70 9.05 25.30 10.95 8.50

    Table 3.  Heavy metal vertical content of columnar sediment

  • 重金属各形态随深度的变化基本呈现相同的趋势(图2)。从图2的形态分布结果发现,Cd在所有深度(A1~B5)有效态百分组成占比均超过70%,有效态中的3种形态(F1、F2、F3)所占百分比相当,分别为22%、26%、21%,Cu、Pb有效态含量相对较高,分别是21.4%和17.5%,比As、Cr、Zn、Ni约高1倍,与表层沉积物重金属形态分布存在相似性,其中Cu 的有机物及硫化物结合态(F3)所占比例相较F1、F2态要高,且Cu在A5(1.50 m)、A6(1.80 m)、B3(2.85 m)、B5(3.55 m)时有效态含量均高于25%,其中B3(2.85 m)深度有效态含量高达40%,因此柱状沉积物中 Cu 的迁移性也较强,Pb随深度变化在A6(1.80 m)、B3(2.85 m)、B4(3.20 m)时有效态含量均高于22%,Pb的铁锰化物结合态(F2)含量相较F1和F3高,因此在缺氧或氧化还原电位下降时较易重新释放;而As、Cr、Zn、Ni4种元素残渣态(F4)均高于85%。总体上,除Cd、Zn外,在B1(2.15 m)深度处,Cu(10.3%)、Pb(10.5%)、Cr(3.6%)、As(4.5%)、Ni(2.1%)有效态含量最低,而在A5(1.50 m)处,各重金属有效态含量均相对较高,在A1(0.30 m)、A5(1.50 m)、B3(2.85 m)、B5(3.55)处有效态含量达到峰值。

    Figure 2.  The percentage of heavy metals speciation of columnar sediment

  • 为定量了解柱状沉积物中重金属不同来源对其含量的贡献率,对重金属含量进行因子分析(表4)。结果表明,各重金属元素之间均存在显著正相关关系。综合因子分析和相关性分析结果表明,沉积物中重金属含量主要受沉积环境的影响。

    NiCdZnPbCrAs
    Ni 1
    Cd 0.859** 1
    Cu 0.898** 0.796**
    Zn 0.900** 0.775** 1
    Pb 0.987** 0.886** 0.892** 1
    Cr 0.990** 0.892** 0.923** 0.989** 1
    As 0.795** 0.735** 0.929** 0.809** 0.837** 1

    Table 4.  Correlation analysis matrix among heavy metal elements

    运用地累积指数法对该海域柱状沉积物重金属现状进行评价(图3),柱状沉积物中Cd在B1、B2时地累积指数为0~1(0<Igeo<1),显示为轻度污染,其他深度地累积指数均≤0 (Igeo≤0),而Cu、Pb、Zn、Cr、As不同深度地累积指数均≤0(Igeo≤0),呈现零污染,这表明除Cd在B1、B2深度外,Cd、Cu、Pb、Zn、Cr、As都表现为自然沉积的作用。此外,本实验利用文献[16]所推荐的参比值、金属毒性系数、沉积物重金属污染生态危害系数以及生态危害指数与污染程度的划分标准,对柱状沉积物进行了污染评价(表5)。柱状沉积物中Cd、Cu、Pb、Zn、Cr、As潜在生态风险危害系数(${\rm{C}}_f^i$)均小于1,说明单个重金属污染处于低污染程度,柱状沉积物中重金属总体污染程度(Cd)低于8,说明总体污染程度均处于较低水平(背景水平)。柱状沉积物重金属总的潜在生态风险危害指数平均值为24.27,潜在生态风险危害指数较低。

    Figure 3.  Heavy metal Igeo values in columnar sediments

    CdCuZnPbCrAsCdRI
    $C_f^i$ 取值范围 value range 0.3~0.65 0.17~0.57 0.21~0.86 0.10~0.29 0.28~0.64 0.5~1.33 1.56~3.0
    区域平均值 Regional average 0.43 0.27 0.45 0.16 0.38 0.81 2.49
    $E_f^i$ 取值范围 value range 9~19.5 0.83~2.84 0.21~0.86 0.50~1.45 0.56~1.28 5~13.27 16.1~39.21
    区域平均值 Regional average 12.82 1.34 0.45 0.79 0.75 8.11 24.27

    Table 5.  Columnar sediment heavy metal ecological hazard index evaluation system

3.   讨论
  • 沉积物重金属随深度的增加含量呈锯齿状波动,表明周围环境输入量不稳定(图4)。但从柱状沉积物中重金属含量总趋势来看,变化基本呈现一致性,其中在0.5 m和2.0 m重金属浓度出现极值。张志忠[24]等研究发现,浙江近岸海域沉积物的沉积速率为0.91~1.19 cm/a。本实验选取其平均沉降速度1 cm/a估算沉积物的形成年代,人为将柱状沉积物划分为4个阶段:

    Figure 4.  Heavy metal vertical content map of columnar sediment

  • 按照1 cm/a沉积速率估算,2.50 m深度以下沉积物相当于18世纪50年代之前形成的。从图4看,除Cd元素先降低后升高,其他重金属含量基本保持不变,表明该段沉积物样品受人为活动影响较小。

  • 按照1 cm/a沉积速率估算,1.50~2.50 m深度沉积物相当于18世纪50年代—19世纪50年代形成,各元素含量呈锯齿状波动变化,在距今200年(2.00 m)时各元素出现极值,由于变化具有统一性,推测可能与当时水动力条件、沉积物性质、细颗粒物质絮凝作用有关[25]

  • 按照1 cm/a沉积速率估算0.90~1.50 m深度之间,相当于19世纪50年代—20世纪50年代形成的,各元素垂向分布较为平稳,变化趋势统一,可大致分析是与当时气候、潮流特征、地形相关 [26]。白爱娟等[27]在研究中国近百年气候变化时表明,20世纪80年代以前中国气温变化并不明显,升温幅度不大,这与重金属垂向分布较为平稳具有相关性,相较第二阶段,第三阶段各重金属含量有所下降,这可能是由于19世纪中期全球海平面上升速率增加[28],进而引起上升流、沉积流和涡旋流等变化,增加水体交换速率[29]。导致重金属含量有所下降。

  • 按照1 cm/a沉积速率估算0~0.90 m深度之间,相当于20世纪50年代至今,该阶段在20世纪70年代前后再次出现峰值,东南亚季风在20世纪60年代末和70年代末经历了减弱过程[30~31],龚道溢等[32]研究长江中下游以及中国东部地区气候时发现20世纪80年代后夏季雨量出现明显转折,导致亚洲大范围季风雨区明显南移,部分地区持续干旱少雨,加之重工业及生活污水污染持续加重,长期积累的环境污染问题相继爆发,致使在20世纪70年代前后沉积物中重金属达到峰值。近40年中国环境污染调查专项调查与防范,重金属含量呈下降趋势[33]。总体上,综合沉积物性质、流体动力学、人为因素、气候等各方面影响,柱状沉积物中重金属垂向呈锯齿状波动且变化趋势相似。

  • 沉积物重金属可交换态及碳酸盐结合态(F1)对沉积物环境中的pH变化敏感,在酸性条件下易被重新释放,本文研究海域呈中性至弱碱性,因此F1态重金属不易释放;Fe/Mn氧化物结合态(F2)被铁锰氧化物包裹或本身已成为氢氧化物沉淀的部分,在氧化还原电位下降或缺氧时易被还原;有机质及硫化物结合态(F3)是指重金属元素与有机活性基团或硫离子结合的部分,不同元素与有机化合物结合的差异较大,通常情况只在强氧化条件才可释放;残渣态(F4)主要赋存在原生矿物和次生硅酸盐矿物晶格中,性质非常稳定,对重金属的迁移影响小,生物毒性不大,所以认为对环境相对安全[34]。一般来说,当沉积物重金属有效态/总量小于50%,认为重金属主要来源于矿物自然沉积的结果,当有效态/总量大于50%,则认为受到人为活动影响的可能性较大。从图3看,重金属Cd有效态的质量分数最高 (71.6%),表明可能受到人为活动影响,也说明Cd 的活性较强更易迁移。其次重金属Cu、Pb有效态百分比较高,Cu有效态平均占比23.9%,Cd有效态平均占比33.3%,表明两种元素的迁移性也相对较强,且二者形态组成变化具有相似趋势,这可能与它们类似的地球化学性质有关。而 As、Cr、Zn、Ni以残渣态为主,主要来源于自然沉积作用,释放迁移能力也较弱。在B1深度处各重金属含量均为较低水平,综合B1所处的历史时期 (第三阶段),该阶段沉积环境良好且人为因素影响小,因此B1深度处各重金属稳定性较强且含量较低。而A5深度处重金属含量较高,此深度所处阶段(第二阶段),存在少量人为活动的影响,重金属分布更可能受当时沉积环境的影响,如悬浮物质、有机质含量以及水体盐度、溶解态和颗粒态之间的转换机制等。整体上,柱状沉积物重金属有效态变化具有周期性和一致性。

    沉积物中重金属的含量主要受环境背景值和人类活动输入的影响,而人类活动造成的污染又通过河流径流(包括近海直接排污)和大气沉降两种方式输入近海[35],通过因子分析方法得出的结果有且只有一个主成分,所占比例在89.2%~99.1%,同时为鉴别沉积物中重金属自然和人为来源的相对贡献率[36],通常利用污染物中某种受人为污染影响较小的元素或组份对重金属含量进行校正。Ni、Fe受人为因素影响较小,且Ni与Fe在含量上有较密切的正相关关系,主要来自于自然沉积[37-38],因此本研究选择化学性质稳定的惰性元素Ni作为参比元素,通过与目标元素之间的相关性判别该海域柱状沉积物重金属含量主要受海域沉积环境的影响。综合Igeo、生态风险危害系数,柱状沉积物中6种重金属(Cd、Cu、Pb、Zn、Cr、As)受人为影响较小,可较准确反映当地沉积物本底值。但重金属Cd在个别深度处(B1、B2)0<Igeo<1,存在轻度污染且Cd形态组成上以迁移性较强的次生相(F1、F2和F3)为主,因此对该海域生态环境存在风险仍需持续关注。

4.   结论
  • (1)柱状沉积物粒度自下而上呈粗-细的趋势,且以细粉砂为主。

    (2)柱状沉积物中重金属含量随深度大致呈锯齿状波动,垂向变化趋势基本一致且呈现周期性,除个别深度Cd为国家海洋沉积物重金属II类标准,其余均为I类标准,底质环境较好。

    (3)柱状沉积物中Cd以有效态(>60%)为主,F1、F2、F3的形态含量差别不大,其余6种重金属均以残渣态(F4)为主要赋存形态。

    (4) Igeo、相关性及潜在生态危害指数分析揭示,马鞍列岛人工鱼礁修复海域重金属受人为因素影响较小,污染程度较低。

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