赣南桃江河表层沉积物钨赋存特征及风险分析

陈 明,李凤果,师艳丽,陶美霞,胡兰文



赣南桃江河表层沉积物钨赋存特征及风险分析

陈 明*,李凤果,师艳丽,陶美霞,胡兰文

(江西理工大学资源与环境工程学院,江西省矿冶环境污染控制重点实验室,江西 赣州 341000)

选择赣南桃江河表层沉积物为研究对象,采用改进的BCR提取法分析桃江上、中、下游及支流表层沉积物中钨的含量及赋存形态,并利用富集系数法(EF)和风险指数编码法(RAC)对桃江河表层沉积物中钨的富集程度与环境风险进行评价.结果表明,桃江河表层沉积物钨的总量范围为1.21~39.73mg/kg,均值为18.21mg/kg.研究区域58%的采样点沉积物中钨总量高于江西省土壤重金属背景值;桃江河沉积物中钨的主要赋存形态是残渣态(B4态),沉积物中各形态钨占总钨的比例大小顺序为残渣态>可氧化态>可还原态>弱酸提取态,空间上有效态钨占总钨的比例大小为支流>下游>上游>中游,平均比例分别为22.44%、21.03%、14.45%及10.91%.相关性分析显示,pH值和阳离子交换与钨的各形态及总量呈正相关.EF法分析表明桃江河上游与支流沉积物中钨富集严重;RAC法分析结果显示采样点沉积物中钨含量呈低、中、高生态风险占比分别为33.33%、46.67%、17.78%.上述结果表明,桃江河表层沉积物钨富集程度及环境风险较严重,应引起重视并开展深入研究.

桃江河；沉积物；钨；赋存形态；风险评估

钨(W)是一种水不溶性金属且呈化学惰性[1-2],相当长的时期内被认为无显著的生态毒性及环境效应,其生态环境安全性基本被忽视[3-5].与其他金属物质相比,确定钨及钨化合物可能对人类健康和毒性作用的动物研究相当有限.美国环保署(USEPA)于 2008 年正式将钨列为新兴环境污染物[6-7],俄罗斯也已经将钨列为一种水体污染物[8].虽然到目前为止尚未证实儿童白血病群发与该群体对钨的摄入之间存在确切联系,但实验室动物实验研究确已表明钨有毒性、可致癌[9-13],且在中性-偏碱性水环境中具有较强的迁移能力[14-15].

我国赣南分布着众多钨矿,钨矿开采已有百年历史,早先的钨矿开采、冶炼、工业生产等,将大量钨尾砂和废弃渣质等含大量重金属的污染物直接排入河道,沉积物是是重金属污染的源和汇[16],沉积物中重金属含量能有效反映河流重金属富集情况.重金属污染的生态风险和生物毒性不仅与重金属的总量有关,与重金属赋存形态也密切相关[17].不同形态的重金属生物效应不同,重金属赋存形态可以有效判断重金属的来源、生物毒性、迁移转化及环境行为[18-19].桃江河是赣江重要的支流之一,由于多年的工业生产、钨矿开采、生产养殖等,造成河流污染.目前已有学者对桃江河沉积物中重金属污染进行调查分析[20],但缺少对河流积物中重金属钨的探究.本文对桃江河表层沉积物中钨的含量及赋存形态进行研究,期望为钨污染的预防治理提供科学参考.

1 材料与方法

1.1 研究区域与样品采集

桃江是赣江的二级支流,发源于赣州市全南县饭池嶂,流经龙南县、信丰县、赣县,在赣县大埠、大田乡后汇入贡江.桃江河属于亚热带丘陵山区湿润季风气候,河流以雨水补给为主,径流量随降水变化而变化.近年来,由于养殖业、城镇建设以及早期无序的矿山开采等大量的污染物排入水体,造成河流水体污染.本文充分考虑了桃江河段内生产和生活取水口、废水排放口及污染物排放的位置,采用宏观控制重点布设法布设采样点[21].在桃江干流和支流共选取了45个采样点(图1).

图1 研究区域位置及采样点分布

样品于2017年6月采用GPS定位系统定位,并采用抓斗式采样器采集1.5~2kg的表层沉积物(0~10cm)装入自封袋中,放于4℃保温冰箱中运回实验室.

1.2 样品预处理与测定

采集的样品自然风干后,剔除贝壳、树枝、碎石等杂质,用玛瑙钵碾碎过200目筛子,保存于聚乙烯袋中待测.钨总量的测定:称取0.50g土样在聚四氟乙烯坩埚中,然后加5mL体积比为 1:1硝酸和1mL浓缩磷酸在95℃下加热30min后加浓缩硝酸2.5mL,再加热30min,重复4次后95℃加热2h,然后加3mL 30%过氧化氢95℃加热15min,再加2mL 30%过氧化氢95℃加热2h,钨形态(弱酸提取态B1、可还原态B2、可氧化态B3、残渣态B4)采用改进BCR三步法提取[22],提取步骤见图2.所有预处理后的钨总量和各形态均用1%硝酸稀释至50mL.

本实验所有提取液均送工程研究院采用电感耦合等离子体质谱仪(ICP–MS,Agilent 8800)测定.实验过程均设置空白对照和3个平行样以减小误差,桃江表层沉积物钨总量的回收率控制在93%~107%之间;钨的形态分析标准物质的回收率控制在89%~ 110%之间;Fe的回收率在91%~105%之间,测量误差小于±10%.沉积物理化性质包括pH值、土壤有机质、阳离子交换、全氮等,具体测定方法见表1.

表1 沉积物理化性质测定方法

1.3 评价方法

1.3.1 富集系数法(EF) 计算公式见文献[23],其钨背景值参照江西省土壤重金属背景值为5.1mg/ kg[24].根据富集系数将污染程度分为5个等级,当富集系数介于0.5~1.5时,表明沉积物中重金属主要源自土壤和岩石圈的自然风化的过程;如果大于1.5,表明沉积物中重金属来源受人为影响.

1.3.2 风险指数编码法(RAC) 沉积物中重金属总量不能反映重金属的潜在生态风险[25].风险评估编码法是基于重金属不同形态对环境有不同程度的威胁而提出的.B1态化学性质不稳定,易在上覆水-沉积物界面交换从而污染水体;B2态在沉积物中氧化电位较低的情况下会被释放出来;B3态在强氧化条件下性质活跃易污染水体环境.因此将B2、B3态视为潜在生物有效态.B4态性质比较稳定生物可利用性低.利用这4种形态在沉积物重金属总量中所占的比例来评价钨的生态环境风险[26].RAC通过河流沉积物中钨的B1、B2、B3态之和占重金属总量的比例来评价钨的生态风险[27-28].具体风险评价等级标准见表2.

2 结果分析

2.1 桃江河沉积物中钨含量及形态分布

由表3可知,桃江河沉积物中钨的总量在0.51~ 39.73mg/kg之间,均值为10.86mg/kg.从空间分布上,桃江上游沉积物中钨总量在1.21~39.73mg/kg之间,均值为18.21mg/kg;桃江中游沉积物中钨的总量在1.61~15.03mg/kg之间,均值为8.07mg/kg;下游沉积物中钨含量在2.58~10.36mg/kg之间,均值为6.38mg/kg;支流表层沉积物中钨总量均值为8.19mg/kg.结果显示,桃江河表层沉积物中钨总量大小顺序为上游>支流>中游>下游.沉积物中钨总量均值均超过了江西省土壤背景值,其中,上游沉积物中钨平均含量高出背景值3.57倍.总体上来看,桃江河表层沉积物钨含量变异系数均高,表明桃江钨为点源污染.

由图3、4可知,桃江河沉积物中钨的4种形态占比大小为B4态>B3态>B2态>B1态,表明桃江河上流、中游、下游及支流沉积物中钨主要以B4态形式赋存,其B4态均值占钨总量比分别为89.72%、88.05%、82.66%和83.50%,平均百分比为85.98%;B3态均值所占总量比例分别为15.55%、10.55%、9.67%和16.11%.平均百分比 12.97%;B2态、B1态均值占总量比相对较低,平均百分比分别为0.07%、0.98%,表明桃江河沉积物中钨的生物可利用性低.但是B3态、B2态、B1态在外界环境条件发生变化时,随时有可能被释放出来,从而对水体造成污染.从潜在生物有效态考虑,钨的生物有效态在桃江上游、中游、下游和支流的占比范围分别为5.42%~39.67%、4.88%~20.36%、9.87%~45.09%和2.5%~52.94%.桃江河中上游沉积物中钨的生物有效态占比较低,平均比例分别为14.45%、10.91%;但下游与支流沉积物中钨的生物有效态相对较高,平均比例分别为21.03% 和22.44%;支流和下游B3态钨含量较高,可能与竹制品厂、养殖场、矿产冶炼等向河流排入废水有关.

表3 桃江表层沉积物中钨各形态含量

续表3

图3 桃江支流沉积物中形态分布比例

图4 桃江干流沉积物中形态分布比例

2.2 桃江河沉积物中钨形态相关性分析

为了研究桃江河沉积物中理化指标对钨形态的影响,将钨的各形态含量与有机质、全磷、全氮、pH值和阳离子交换量进行Pearson相关性分析结果见表4.在桃江表层沉积物中,钨的B2态与pH值呈显著正相关,B3态与阳离子交换呈显著正相关(£0.05),其他形态与pH值和阳离子交换量均有一定的相关性.自然界中钨常以钨酸盐形式存在,但随着pH值的酸化在偏酸性-中性条件时水中大量形成硫代钨酸盐.在偏碱性条件下,钨仍以钨酸盐的形式存在,水中可检出的硫代钨酸盐含量极低.离子强度的增加在酸性条件下会抑制硫代钨酸盐形成[29].钨的B3态与B4态呈显著正相关(£0.01),B3态、B4态与B1态、B2态有一定的相关性但不明显,表明B3态、B4态在外界条件改变时可能相互转化[30]. 4种形态与阳离子交换具有一定相关性,说明水体中离子强度不同会影响水体酸碱度从而对水中硫化物、微生物等产生影响,进一步影响钨的迁移转化. B1态、B3态和B4态与有机质呈正相关,可能随着有机质含量提高,微生物的活跃度相应提高,对钨形态之间的转化也将产生一定的影响.全氮与pH值、B1态、B2态呈正相关,表明水中氨氮会影响水体pH值的变化,从而进一步改变钨的存在形态.B1态是对生态环境和水体生物最具危险性的形态,总磷与其呈负相关,可能说明总磷对钨造成的生态风险影响较小.

表4 桃江沉积物中理化性质与各种形态相关分析

注:**.在0.01水平(双尾)相关性显著;*. 在0.05水平(双尾)相关性显著.

2.3 桃江河沉积物中钨的富集与评价

2.3.1 EF法富集程度评价 本研究参考江西省土壤重金属背景值,以Fe作为参比元素.对桃江河各采样点沉积物中钨含量与Fe的含量进行标准化比值计算[31],得到45个采样点钨的富集系数.钨的富集系数可以有效反映钨的富集程度[32].由图5可知,桃江河上游与支流沉积物中钨出现较强富集,按空间分布来看,桃江上游沉积物中钨富集最明显,平均富集系数为8.42,可能与上游存有赣南两大钨矿区的钨矿开采有关.与中上游相比下游钨的富集量最少,平均富集系数为3.07,表明桃江沉积物中钨含量较高与人为影响有关.

2.3.2 RAC风险评价 与富集系数法相比,RAC风险评价着重考虑了易被生物利用的钨对环境的污染风险[33].根据风险指数编码法,以桃江河表层沉积物中B1、B2、B3态钨之和占总钨的百分比为基础,评价桃江河沉积物中钨的风险程度,结果如图6所示.从空间分布上,桃江河沉积物有效态钨占总钨比例基本在30%以下;其中桃江上游沉积物中有效态钨占总钨的比例均值为14.45%,呈中度风险;桃江中游沉积物中有效态钨占总钨的比例均值为10.91%,呈中度生态风险;桃江下游沉积物中有效态钨占总钨的比例均值为21.03%,呈中度生态风险;支流沉积物中有效态钨占总钨的比例均值为22.44%,呈中度生态风险.数据表明,桃江河支流表层沉积物中钨的风险程度高于干流,结合实地考察,表明支流受人为影响较大,同时桃江支流富营养化比较严重,对有效态钨的释放风险有一定影响[34].根据图6数据分析发现有82.22%的采样点属于低中等风险,只有17.78%的采样点呈高风险.

通过富集系数法和风险指数编码法对桃江河标称沉积物中钨的富集及风险等级的评价可知桃江支流沉积物中钨富集程度和风险等级均高于干流.对比2种评价方法, EF法能够有效反映河沉积物中钨的富集程度,但是忽略了钨的生物可利用性,容易造成污染评价的夸大或低估.相反,RAC法根据不同形态钨在沉积物中的风险等级进行评价,能够更精确反映钨在沉积物中的赋存形态及潜在风险.富集系数法得出研究区域40%采样点沉积物中钨含量呈较强富集,37.78%的采样点呈中度富集,26.67%的采样点呈轻富集或无富集;RAC法可知桃江河33.33%的采样点沉积物中钨含量呈低风险,46.67%的采样点呈中等风险,17.78%的采样点属于高风险.桃江表层沉积物中70%以上的区域存在钨的富集,全河段有效态钨含量存在不同等级的风险,因此吸取21世纪已发生钨污染事件的教训,我国也应将河流沉积物中钨存在的风险考虑到沉积物重金属污染修复中.

图5 桃江河沉积物中钨的富集系数

图6 桃江河沉积物中钨RAC风险指数

3 结论

3.1 桃江河沉积物中钨的形态分析表明,钨的主要赋存形态为残渣态,且钨各形态含量占比为残渣态>可氧化态>可还原态>弱酸提取态,桃江支流有效态含量普遍高于干流,中游与下游个别采样点可氧化态钨含量较高.

3.2 相关性分析显示,弱酸提取态、可还原态、可氧化态、残渣态与pH和阳离子交换呈正相关,表明pH和阳离子交换是钨富集的影响因子;可氧化态与残渣态显著相关.

3.3 富集系数法分析结果显示,桃江河40%的采样点沉积物中钨含量呈较强富集,风险指数编码法分析表明,46.67%的采样点沉积物中钨含量呈中等风险,17.78%的采样点表层沉积物中钨含量呈高生态风险,表明桃江表层沉积物中钨的富集与生态风险相对较高.

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Occurrence characteristics and risk assessment of tungsten in surface sediments of Taojiang River in Southern Jiangxi Province.

CHEN Ming*, LI Feng-guo, SHI Yan-li, TAO Mei-xia, HU Lan-wen

(Jiangxi Key Laboratory of Mining & Metallurgy Enviromental Pollution Control, Jiangxi University of Science and Technology, Ganzhou 341000, China)., 2019,39(4)：1715~1723

Total concentrations and forms of tungsten in surface sediments from Taojiang river were determined and the methods of enrichment coefficient (EF) and risk index coding (RAC) were applied to assess the accumulative degree and environmental risks. The results showed that the tungsten concentrations ranged from 1.21 to 39.73mg/kg, with an average of 18.21mg/kg, while 58% of sampling sites were greater than the background values of the soil in Jiangxi Province. The species of tungsten were presented dominantly in the residual fraction, followed by the oxidizable, reducible and weak acid extraction fraction. Spatially, the effective tungsten in tributary is maximal with a mean value of 22.44%, followed by downstream (21.03%), upstream (14.45%) and midstream (10.91%). Correlation analysisshowed that total concentrations and species of tungsten were positively correlated with the pH and cation exchange, respectively. The EF analysissuggested that the tungsten enrichment was seriously accumulated in the upper reaches and tributaries of the Taojiang River. The RAC analysis demonstrated the ecological risk in different sampling sites was low, medium and high, with a proportion of 33.33%, 46.67% and 17.78%, respectively. Altogether, this study indicated that the tungsten were accumulated in sediments of Taojiang River with seriously environmental risks, which deserve to be additional more extensive researches.

Taojiang River；sediment；tumgsten；occurrence form；risk assessment

X142

A

1000-6923(2019)04-1715-09

2018-09-10

国家自然科学基金资助项目(51664025)

*责任作者, 教授, jxlgdx@qq.com

陈 明(1976-),男,江西赣州人,教授,主要研究方向为重金属污染控制.发表论文30余篇.