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物探与化探, 2023, 47(3): 826-834 doi: 10.11720/wtyht.2023.1321

生态地质调查

海南省琼中县土壤—茶树中重金属的迁移特征及饮茶健康风险

弓秋丽,1,2, 杨剑洲1,2, 王振亮1,2, 严慧3

1.自然资源部 地球化学探测重点实验室,河北 廊坊 065000

2.中国地质科学院 地球物理地球化学勘查研究所,河北 廊坊 065000

3.自然资源部 长沙矿产资源监督检测中心,湖南 长沙 410007

Migration of heavy metals in the soil-tea plant system and health risks of drinking tea: A case study of Qiongzhong County, Hainan Province

GONG Qiu-Li,1,2, YANG Jian-Zhou1,2, WANG Zhen-Liang1,2, YAN Hui3

1. Key Laboratory of Geochemical Exploration of Ministry of Natural Resources, Langfang 065000, China

2. Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences (CAGS), Langfang 065000, China

3. Changsha Supervision and Inspection Center of Mineral Resources, Ministry of Natural Resources, Changsha 410007, China

第一作者: 弓秋丽(1983-),女,硕士,高级工程师,2009年毕业于中国地质科学院,主要从事勘查地球化学研究工作。Email:gqiuli@mail.cgs.gov.cn

责任编辑: 蒋实

收稿日期: 2022-06-20   修回日期: 2022-11-26  

基金资助: 中国地质调查局地质调查项目(DD20190305)
中央及科研院所基本科研业务费(AS2022P02)

Received: 2022-06-20   Revised: 2022-11-26  

摘要

以海南琼中3个生态茶园为研究区,采集土壤以及对应茶树的根、茎、叶(包含老叶、新叶和嫩芽),对重金属在土壤—茶树系统中的迁移特征进行研究,分析重金属的迁移规律和饮茶所致健康风险。结果表明,土壤中Pb、Cr、Cd、As和Hg含量略高于海南岛土壤背景值,没有明显的累积。重金属在茶树不同器官的富集程度差异明显,其中Cr、Zn、Pb、Hg和Cd在根部富集,而Cu和Ni在叶片富集;Pb、Cd和Hg在老叶中的含量比在嫩叶和芽中的含量高,表现为随着茶叶生长而累积的特征;Cu、Ni和Zn在芽中的含量高于老叶和新叶,表现出在茶叶生长部位富集的特征。生物富集系数(BCF)表明,土壤理化组成、重金属种类和叶龄能够影响茶叶对重金属的吸收程度。风险评价结果显示,所有样品的目标危害系数(HQ)和危害指数(HI)均低于1,表明饮茶所致重金属健康风险处于可接受水平。本研究可为茶园重金属防控提供科学依据,对茶园管理和确保茶叶消费者健康具有积极指导意义。

关键词： 土壤; 茶叶; 重金属; 植物器官; 健康风险评价

Abstract

This study sampled the soil and the corresponding roots, stems, and leaves (including large leaves, new leaves, and sprouts) of tea plants from three ecological tea plantations in Qiongzhong County, Hainan Province. Based on these samples, this study investigated the migration of heavy metals in the soil-tea plant system and analyzed the migration patterns of heavy metals and the health risks caused by heavy metals in tea. As indicated by the results, the Pb, Cr, Cd, As, and Hg concentrations in the soil are slightly higher than the background values of corresponding soil elements in Hainan, showing non-significant accumulation. The enrichment of heavy metals varies significantly in different organs of tea plants. Specifically, Cr, Zn, Pb, Hg, and Cd are enriched in roots, while Cu and Ni are enriched in leaves; Pb, Cd, and Hg have higher concentrations in large leaves than in new leaves and sprouts, indicating that these elements are enriched with the growth of leaves; Cu, Ni, and Zn have higher concentrations in sprouts than in leaves, showing that these elements are enriched in the growing parts of leaves. Bio-concentration factors (BCF) indicate that soil physicochemical composition, heavy metal species, and leaf age have effects on the absorption of heavy metals by tea leaves. The results of the risk assessment show that the target hazard quotients (HQ) and hazard indices (HI) of all samples are less than 1, indicating acceptable health risks caused by heavy metals in tea. This study can provide a scientific basis for the prevention and control of heavy metals in tea plantations and has a positive guiding significance for managing tea plantations and ensuring the health of tea consumers.

Keywords： soil; tea; heavy metal; plant organ; health risk assessment

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本文引用格式

弓秋丽, 杨剑洲, 王振亮, 严慧. 海南省琼中县土壤—茶树中重金属的迁移特征及饮茶健康风险[J]. 物探与化探, 2023, 47(3): 826-834 doi:10.11720/wtyht.2023.1321

GONG Qiu-Li, YANG Jian-Zhou, WANG Zhen-Liang, YAN Hui. Migration of heavy metals in the soil-tea plant system and health risks of drinking tea: A case study of Qiongzhong County, Hainan Province[J]. Geophysical and Geochemical Exploration, 2023, 47(3): 826-834 doi:10.11720/wtyht.2023.1321

0 引言

土壤重金属污染不仅影响农作物的品质和产量,还会危及环境和人体健康。食品和饮料的食用是人类摄入重金属的重要途径,由于重金属具有高毒性、难降解性和生物可利用性,过度摄入重金属会损害人体器官,对健康造成不可逆的影响[1]。茶叶具有清香止渴、抗氧化、利水消肿等功能,是世界3大饮料之一[2]。中国种植茶叶面积占全球比例为54%,是重要的茶叶出口国[3],其中海南省茶园面积约1 200 hm2[4],产量在全国而言占比较小,但受到环境影响,海南春茶比岛外地区早上市约3个月,加上所属的大叶种茶的品质优势而受到广泛欢迎。

前人报导了大量茶园土壤和茶叶中重金属的分布特征和风险评价。周国华等[5]研究了土壤—铁观音茶叶地球化学特征,认为地质背景和成土母质是影响土壤—茶叶微量元素迁移系数的主要因素。孙镜蔚等[6]通过分析福建茶园土壤—茶树体系中重金属的有效性,认为土壤酸性越大,有机质含量越低,重金属的生物活性越强。此外,茶园施肥状况、茶叶生长期和茶树品种也在一定程度上影响重金属从土壤向茶叶的转运[7-8]。张清海等[9]通过研究贵州云雾茶园茶叶—茶汤系统中重金属含量,发现Hg和Cu具有较高的浸出率,需要引起注意。王峰等[7]参考了茶叶日摄入量,计算出饮用闽中矿区茶叶所致的重金属健康风险主要来自Cr,长期摄入过高重金属含量的茶叶会导致重金属在人体内的累积。前人研究主要集中在茶叶重金属的富集特征或饮茶所致的健康风险,缺少两者相结合的研究,此外,对茶树中其他器官的重金属含量也研究较少,这对重金属在土壤—茶树中运移特征的研究不够系统。为此,本次工作以琼中县生态茶园为对象,综合研究了土壤—根—茎—叶(老叶、新叶、嫩芽)系统中重金属分布特征,同时计算饮茶所致重金属暴露风险,以期为茶园管理和重金属风险防控提供支持。

1 研究区概况

研究区位于海南省琼中县茶叶种植区,该区域属热带季风气候,年平均气温22 ℃,年降水量2 200~2 444 mm,地貌类型以丘陵为主,成土母质主要为酸性侵入岩,是海南省茶叶最适宜种植区之一。

2 样品采集与分析测定

2.1 样品采集与前处理

2020年4~5月期间,在海南省琼中县选择种植面积较大的3处生态茶园(图1),分别为新伟农场茶园2处和白马骏红茶园1处。这3处茶园具有5年以上的种植历史,占地面积42 hm2,约为琼中茶园面积的35%,在茶树种植过程中均不使用农药和化肥,只通过诱虫黄板进行虫害防护,最能够代表琼中生态茶园管理模式。经实地调查,3处茶园土壤均为砖红壤,土壤砂黏适中,土层中含大量花岗岩风化产物,周边无明显的水、气污染源。根据茶园面积大小,尽可能均匀布设采样点,共采集5套土壤—根—茎—老叶和15套土壤—根—茎—新叶—嫩芽样品用于重金属测试。

图1

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图1   研究区土壤及对应植物采样位置示意

Fig.1   The position map of soil and corresponding plant samples in the study area


每件土壤样品质量约1.5 kg,由3个子样(0~20 cm深)等量混合而成,子样点相距约15 m并呈三角形分布。土壤样品在去除落叶、根系、石块等杂物后装入干净布制样品袋中。土壤在通风仓库自然晾干,经混匀、过10目尼龙筛后,采用四分法去除多余土壤,盛取部分样品装满200 mL聚乙烯瓶中,运回实验室待测。

在土壤采样点附近,采集多套同株茶树根茎叶样品,每套植物样品尽可能靠近土壤采样点,并保证鲜重大于500 g。其中采集须根作为茶树根系样品,较完整的主茎作为茶树茎样品,茶叶样品分为一年生长期的老叶样品(相对靠近根部的绿色老叶片)、数周生长期的新叶样品(靠近顶端的青色大叶片)和一周生长期的嫩芽样品(一芽一叶,均未展开)。由于同株茶树嫩芽数量有限,会采集临近茶树嫩芽作为样品以确保达到分析重量。新鲜植物样品经自来水去除杂质,再用去离子水冲刷3遍后自然晾干。将植物样品于60 ℃条件下烘干,再经玛瑙研磨仪制成粉末后封装待测。

2.2 样品分析测试与质量保证

土壤样品于中国地质科学院地球物理地球化学勘查研究所完成测定。样品经过玛瑙研磨仪碎至200目后,称取足量样品,经HCl+HNO3+HF+HClO4消解后上机测试。不同重金属对应的分析方法和检出限见表1,同时采用标准物质样品(GSS33、GpH-2)和重复样对土壤重金属、Corg(有机碳)、pH进行质量监控,详细的分析过程参照文献[10]。

表1   分析方法及检出限

Table 1  Analytical methods and corresponding detection limit of analyzed indicators

样品类型分析项目分析方法检出限单位
土壤Cd等离子体质谱法3010-9
Pb等离子体质谱法210-6
As原子荧光光谱法110-6
Hg原子荧光光谱法0.510-9
Cr等离子体光学发射光谱法510-6
Corg高频燃烧—红外碳硫仪0.1%
pH电位法0.1
植物Cd等离子体质谱法310-9
Cu等离子体质谱法0.0510-6
Pb等离子体质谱法0.0210-6
Ni等离子体质谱法0.0510-6
As等离子体质谱法1010-6
Hg等离子体质谱法110-9
Zn等离子体质谱法0.510-6
Cr等离子体质谱法0.0510-6

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植物样品于自然资源部长沙矿产资源监督检测中心完成测定。称取(0.5±0.001)g粉末样品置于微波消解罐中,加入3~5 mL硝酸,加盖放置1 h后进行消解。待消解罐冷却后,打开罐盖排气并清洗内盖,再将消解罐置于100 ℃电热板上加热30 min后提取样品,加水定容至25 mL容量瓶中备用。配制与样品基体匹配的Cd、Pb、Cr、Ni、Cu、Zn、As、Hg混合标准溶液,采用Sc、Rh、Re混合内标,使用等离子体质谱法对重金属进行测试。采用标准物质(GBW10011)和空白样进行质量监控,所有样品100%做重复样分析。

土壤和植物样品重金属的分析检出限、准确度和精密度均优于DZ/T 0258—2014和DD2005-03技术标准,数据质量可靠。

3 健康风险评价模型

重金属日摄入量(EDI)、目标危害系数(HQ)和复合危害系数(HI)是评价重金属摄入风险的重要指标[2,11],茶叶饮用过程中重金属的摄入与其他食品不同,还需要考虑泡茶过程中重金属的浸出率。这3种指标的具体计算方式为:

EDI =(Ci × FIR × TR)/(WAB × 1000),
HQ = EDI/RfD,
HI = ∑HQ,

式中:Ci为重金属i的含量;FIR为茶叶日摄入量(由于饮茶群体主要为成人,因此只计算成人饮茶所致健康风险,本文取11.4 g·d-1[12]);TR为重金属i的浸出率(本文取Cd=14.18%、Pb=33.1%、Hg=45.2%、As=23.83%、Cr=11.45%、Ni=67.71%、Cu=28.7%、Zn=19.3%[2,12]);WAB为成年人平均体重(本文取60 kg); HQ和HI分别用来表征单种重金属和多种重金属的风险值;RfD为此重金属日摄入量的风险阈值 (本文取Cd=0.001、Pb=0.003 5、Hg=0.000 7、As=0.05、Cr=0.003、Ni=0.02、Cu=0.04、Zn=0.3 mg·(kg·d)-1[12-13])。当HQ值大于1时,表明此重金属可引起人体健康风险,当HI大于1时,表明多种重金属摄入的人体健康风险为不可接受水平,反之则健康风险为可接受水平[12]。

4 结果与分析

4.1 土壤重金属、Corg和pH特征

研究区土壤中Pb、Cr、Cd、As和Hg的平均值依次为30.5×10-6、36.5×10-6、0.043×10-6、1.99×10-6和0.041×10-6(表2),略高于海南省土壤背景值[14]。所有土壤样品中这5种重金属含量均低于《土壤环境质量农用地土壤污染风险管控标准(试行)》(GB 15618—2018)规定的筛选值[15],表明茶园土壤未受到重金属污染。土壤Corg值为0.826%~2.35%,平均值为1.27%,80%的点位Corg值大于1%,处于尚可和优良级别[6]。土壤pH值为4.20~5.04,平均值为4.65,显著低于海南省土壤平均水平[14],属于酸性—强酸性土壤。茶园土壤较低的pH主要有3种原因: 首先,茶树属于典型的喜酸作物,由于根系和共生菌根菌的酸性生长存活条件导致茶树无法在碱性土壤中生长。相关研究表明,较适宜种植茶树的土壤pH为4.0~6.5,最适宜的pH为4.5~5.5[16],因此,茶园选址会优先考虑土壤pH较低的区域。其次,茶树根系发达,生长过程中分泌大量草酸、苹果酸等有机酸,每年因为种植茶树导致的土壤酸化率可达到0.071 [2,16]。此外,茶叶的生长需要大量的氮肥,氮肥中铵的硝化作用会产生H+[17],进一步加速茶园的土壤酸化。然而,茶树种植导致的土壤酸化会带来一系列不利结果。首先,茶树是深根性植物,种植茶树导致的土壤酸化可以影响到0~200 cm深度。这一过程会导致营养型离子的浸出(K+、Na+、Ca2+和Mg2+),同时加速氮和磷等土壤养分流失[18]。此外,在酸性条件下,土壤重金属具有较高的有效态含量,这会增加土壤重金属迁移能力而增加环境健康风险[2,19 -20]。因此,茶树种植过程中应注意防止土壤过度酸化以确保茶叶质量安全。

表2   研究区土壤重金属基本统计参数

Table 2  Basic statistics of heavy metals in soils of the study area

参数w(Pb)/10-6w(Cr)/10-6w(Cd)/10-6w(As)/10-6w(Hg)/10-6w(Corg)/%pH
最小值19.56.180.0280.8390.0260.8264.2
最大值41.3130.900.0835.110.1312.355.04
平均值30.536.500.0431.990.0411.274.65
中位值30.623.700.0401.750.0341.194.65
变异系数/%18.7100.534.148.858.028.04.2
海南土壤背景[14]24.427.50.041.340.02
农用地土壤环
境筛选值[15]		701500.3401.3

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4.2 茶树重金属含量特征

茶树不同器官中重金属平均含量见图2,由此可知,重金属在茶树不同器官中具有明显的差异,其中Zn、Cr、Pb、Cd和Hg在根系的含量显著高于其他器官,具有较强的富集作用,而As在根系中基本低于检出限。其次,不同叶龄的茶叶也表现出重金属富集的差异性,其中Cr、Pb、Cd和Hg含量表现为老叶>新叶>嫩芽,显示这4种元素随着植物生长不断累积的特征,而Cu、Ni和Zn含量表现为嫩芽>新叶>老叶,表明这3种元素作为植物生长所必需的营养元素在茶叶生长部位富集的特征,这与前人研究结果类似[5]。

图2

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图2   研究区茶树不同器官中重金属含量

Fig.2   Heavy metals content in different organs of tea trees in the study area


中华人民共和国农业行业标准《茶叶中铬、镉、汞、砷及氟化物限量》(NY659—2003)[21]规定,茶叶中Cr、As、Cd和Hg的安全限量分别为5×10-6、2×10-6、1×10-6和0.3×10-6,中华人民共和国国家标准《食品安全国家标准食品中污染物限量》(GB 2762—2017 )[22]和中华人民共和国农业行业标准《无公害食品茶叶》(NY5244—2004)规定,茶叶中Pb的安全限量为5×10-6,中华人民共和国农业行业标准《绿色食品茶叶》(NY/T 288—2018)规定,茶叶中Cu的限量为30×10-6。我国食品安全国家标准针对茶叶规定了Pb的限量指标,而欧盟、美国等颁布的茶叶食品安全标准主要针对农药残留限量,未提及茶叶重金属残留限量指标。2003年,农业农村部农产品质量安全监管局提出了茶叶中Cr、As、Cd和Hg的限量指标,2018年又提出了绿色食品产业产品中铜的限量指标,目前尚无茶叶中Ni、Zn相关的最大限量标准。研究区茶叶(新叶和嫩芽)中重金属含量分布见表3,重金属含量均远低于对应的标准限制。

表3   茶树新叶与嫩芽重金属含量分布

Table 3  Distribution of heavy metals in young leaves and sprouts of tea

项目新叶嫩芽安全限量/
10-6		依据标准
含量范围
/10-6		平均值
/10-6		含量范围
/10-6		平均值
/10-6
Cr0.22~1.880.770.16~1.640.595.0中华人民共和国农业行业标准《茶叶中铬、镉、汞、砷及氟化物限量》(NY659—2003)
As0.02~0.190.060.03~0.140.052.0中华人民共和国农业行业标准《茶叶中铬、镉、汞、砷及氟化物限量》(NY659—2003)
Cd0.02~0.040.030.01~0.040.021.0中华人民共和国农业行业标准《茶叶中铬、镉、汞、砷及氟化物限量》(NY659—2003)
Hg0.0018~0.0030.00250.0018~0.00350.00240.3中华人民共和国农业行业标准《茶叶中铬、镉、汞、砷及氟化物限量》(NY659—2003)
Pb0.02~0.410.120.01~0.260.085.0中华人民共和国国家标准《食品安全国家标准食品中污染物限量》(GB 2762—2017 )、中华人民共和国农业行业标准《无公害食品茶叶》(NY5244—2004)
Cu15.20~22.3018.9016.10~24.2020.2330.0中华人民共和国农业行业标准《绿色食品茶叶》(NY/T 288—2018)

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4.3 土壤—茶叶元素迁移系数

重金属从土壤迁移到植物,是进入食物链的主要途径[23]。生物富集因子(BCF)用于表征植物对重金属的富集能力[5]:

BCF =植物中重金属含量(干重)/土壤中重金属含量(干重)× 100,

表4为茶树中不同植物器官的重金属生物富集因子。由此可见:①茶树对不同重金属的转运能力具有较大差异,其中对Cd的生物富集因子要远高于Pb、Cr、As和Hg。大量研究表明,农作物对重金属的富集能力与农作物种类[23]、土壤重金属含量和形态特征[24-25]、土壤的理化性质[26]等多种因素有关,其中土壤重金属有效态含量是影响农作物对土壤元素吸收的重要因素[26]。调查表明,我国农用地土壤中Cd的有效态比例要远高于其他重金属[27⇓-29],导致农作物对Cd具有较高的迁移系数。此外, Hg在叶片中也具有较高的生物富集因子,研究表明,植物叶片的Hg不仅来自根系土,大气沉降也是Hg的重要来源[30]。②不同器官重金属的BCF值也具有较大差异,其中Pb、Cr、Cd和Hg的BCF值大致表现为根>茎>叶,表现出远离土壤端逐渐降低的特征。这些元素不是植物生长的必需元素,通过蒸腾作用等伴随着水分转运至植物体内,向上运移过程中受到不同器官阻碍,转移量逐渐降低[31-32]。③不同叶龄对重金属的转运程度也不同。老叶因具有较高的Pb、Cr、Cd、Hg含量从而具有较高的生物富集因子。这些元素与N、P、K、Zn等高移动性营养元素不同,它们较难被重新分配,从而伴随着叶片生长不断累积[2]。

表4   茶树不同植物器官5种重金属的生物富集因子

Table 4  Bioaccumulation factors of the five heavy metals in different organs of tea

植物器
官类型		BCF (Pb)BCF (Cr)BCF (Cd)BCF (As)BCF (Hg)
范围平均值变异
系数		范围平均值变异
系数		范围平均值变异
系数		范围平均值变异
系数		范围平均值变异
系数
根8.91~64.533.442.66.76~79.928.480.5224~205166063.16.11~13846.068.4
茎1.64~23.410.164.70.87~35.79.38106211~593346310.00~5.020.821973.82~21.0413.231.3
老叶1.42~4.853.0346.84.47~16.912.638.143.4~85.757.631.50.00~0.570.1122414.9~19.716.611.9
新叶0.05~1.490.411080.37~11.22.7099.025.7~13575.243.20.45~9.643.4062.02.29~9.106.6927.0
嫩芽0.03~0.930.261030.43~5.121.8970.727.0~10855.244.50.74~8.943.2560.91.91~10.46.6234.5

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4.4 土壤重金属、Corg和pH对茶叶重金属含量的影响

新叶和嫩芽为茶树中可食用部分,其重金属含量同土壤重金属、Corg和pH的皮尔逊相关系数见图3。结果显示,Cr和Hg各自在土壤和新叶中均呈显著正相关(p<0.05),相关系数分别为0.62和0.51,表明茶叶中Cr、Hg与土壤重金属全量密切相关。As和Corg呈显著正相关(p<0.05),相关系数为0.63。研究表明,茶树根部吸收的As主要来自土壤有效As,而加大土壤Corg含量会增加土壤有效As的比例,从而增加茶叶对As的吸收和累积[33]。新叶中 Hg和土壤pH呈极显著负相关(p<0.01),相关系数为-0.86,因此,防止土壤过度酸化是有效降低茶叶Hg含量的有效途径之一。尽管茶叶中其他重金属同土壤理化性质存在一定相关性(图3),但显著性水平较差,并不具有普遍性。因此,茶叶重金属与土壤元素和理化条件的关系非常复杂,并不单一受到某一因素的控制。

图3

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图3   茶叶的新叶和嫩芽中重金属含量与土壤重金属、pH、Corg的相关关系

注:“*”表示 P<0.05,“**”表示 P<0.01

Fig.3   Correlation of heavy metal content in new leaves and buds of tea with soil heavy metal, pH and Corg

note: correlations are significant at p < 0.05 (*) or p < 0.01 (**).


4.5 健康风险评价

茶叶重金属日摄入量结果见表5,相对于新叶,嫩芽中Zn、Cu、Ni的日摄入较高,Cr、Pb、As、Cd和Hg较低,这与茶叶重金属含量变化一致。无论是新叶还是嫩芽,日摄入量均表现为Zn>Cu>Ni>Cr>Pb>As>Cd>Hg,其中Zn、Cu、Ni的占比都达到99%以上,表明这3种元素是饮茶主要摄入的重金属。健康风险评价结果表明(图4),食用嫩叶和新叶所导致的重金属健康风险值较为接近,Ni和Cu是风险指数的主要贡献者,因此,茶叶种植应该注意Ni和Cu的防控。8种重金属日摄入量均低于重金属日摄入量阈值,且所有茶叶样品的复合危害系数均远低于1,表明食用琼中生态茶园茶叶不会产生重金属危害。

表5   琼中县茶园茶树新叶与嫩芽重金属日摄入量(EDI)计算结果统计

Table 5  Estimated daily intake (EDI) of heavy metal for adults associated with the consumption of infusions of young tea leaves and sprouts from the tea plantations in Qiongzhong Countymg·(kg·d)-1

茶叶类型参数CuPbZnCrNiCdAsHg
嫩芽最小值8.78×10-46.29×10-71.25×10-33.48×10-63.64×10-42.16×10-71.54×10-61.55×10-7
最大值1.32×10-31.64×10-51.96×10-33.57×10-51.26×10-31.19×10-66.34×10-63.01×10-7
平均值1.10×10-35.00×10-61.61×10-31.28×10-55.85×10-45.89×10-72.45×10-62.06×10-7
新叶最小值8.29×10-41.19×10-61.15×10-34.79×10-63.01×10-44.31×10-71.04×10-61.55×10-7
最大值1.22×10-32.58×10-51.75×10-34.09×10-51.28×10-31.13×10-68.60×10-62.58×10-7
平均值1.03×10-37.76×10-61.37×10-31.68×10-55.55×10-47.56×10-72.66×10-62.13×10-7

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图4

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图4   研究区饮茶所致目标危害系数(HQ)及其占比

Fig.4   The calculated target hazard quotients (HQ) of heavy metals and its proportion caused by drinking tea in the study area


然而,本次健康风险研究结果具有较大的不确定性。首先,本次结果是基于鲜样干测而来,但茶叶种类丰富,不同的加工过程也可能增加茶叶重金属含量[2]。其次,泡茶水质、水温、冲泡时间、单次冲泡还是多次冲泡以及“洗茶”等不同条件和饮茶方式均会影响茶叶重金属实际浸出率[34⇓-36],从而影响重金属的摄入。例如海南居民招待客人饮茶时,会将第一次浸出液舍去,多人分饮数次冲泡的茶汤;而居民工作饮茶则多以大壶泡茶形式饮用,这些都会影响重金属的摄入评价。此外,饮茶群体的茶叶消费量具有较大差异,且饮茶也不是人们摄入重金属的主要来源,谷物、薯类、生活环境介质(土壤、水、空气等)中重金属的摄入以及人体对重金属吸收程度均会影响人体重金属健康风险[37]。因此,加强不同环境介质的重金属监测,是保护人类健康的有效途径。

5 结论

1)琼中生态茶园土壤中Cr、Cd、As、Pb和Hg含量略高于海南岛土壤背景值,均低于土壤污染筛选值,表明茶树种植没有导致土壤中这5种重金属的污染。但茶园土壤pH远低于海南岛土壤平均水平,需要引起注意。

2)茶树各器官中,根部相对富集Pb、Cr、Cd和Hg,叶片相对富集Cu和Ni;在茶叶中,老叶相对富集Cr、Pb、Cd和Hg,嫩芽相对富集Cu、Ni和Zn,反映出重金属在茶树不同生长部位含量的差异。

3)Zn、Cu、Ni是饮茶摄入的主要重金属,其中Ni和Cu是健康风险值的主要贡献者,需要引起关注。成人饮用琼中生态茶园新叶和嫩芽所摄入的重金属量低于参考值,产生的健康风险处于可接受水平。

致谢

本次样品采集过程中得到琼中新伟农场胡治中同志的大力协助,在此深表感谢。

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A field survey was conducted to investigate the concentrations of chromium (Cr), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb) in vegetables, corresponding cultivated soils and irrigation waters from 36 open sites in high natural background area of Wuzhou, South China. Redundancy analysis, Spearman's rho correlation analysis and multiple regression analysis were adopted to evaluate the contributions of impacting factors on metal contents in the edible parts of vegetables. This study concluded that leafy and root vegetables had relatively higher metal concentrations and adjusted transfer factor values compared to fruiting vegetables according to nonparametric tests. Plant species, total soil metal content and soil pH value were affirmed as three critical factors with the highest contribution rate among all the influencing factors. The bivariate curve equation models for heavy metals in the edible vegetable tissues were well fitted to predict the metal concentrations in vegetables. The results from this case study also suggested that it could be one of efficient strategies for clean agricultural production and food safety in high natural background area to breed vegetable varieties with low heavy metal accumulation and to enlarge planting scale of these varieties.Copyright © 2017 Elsevier Ltd. All rights reserved.

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Crop straw biochar incorporation may be a sustainable method of amending soil, but feedstock-related Cd and Pb content is a major concern. We investigated the effects of heavy metal-rich (RC) and -free biochar (FC) on the phytoavailability of Cd and Pb in two acidic metalliferous soils. Biochar significantly increased soil pH and improved plant growth. Pb in soil and plant tissues significantly decreased after biochar application, and a similar pattern was observed for Cd after FC application. RC significantly increased NH4NO3-extractable Cd in both lightly contaminated (YBS) and heavily contaminated soils (RS). The Cd content of plants grown on YBS increased, whereas it decreased on RS. The Cd and Pb input-output balance suggested that RC application to YBS might induce a soil Cd accumulation risk. Therefore, identifying heavy metal contamination in biochar is crucial before it is used as a soil amendment. Copyright © 2015 Elsevier Ltd. All rights reserved.

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The distribution pattern and fractionation of arsenic (As) in three soil profiles from tea (Camellia sinensis L.) gardens located in Karbi-Anglong (KA), Cachar (CA) and Karimganj (KG) districts in the state of Assam, India, were investigated depth-wise (0-10, 10-30, 30-60 and 60-100 cm). DTPA-extractable As was primarily restricted to surface horizons. Arsenic speciation study showed the presence of higher As(V) concentrations in the upper horizon and its gradual decrease with the increase in soil depths, following a decrease of Eh. As fractionation by sequential extraction in all the soil profiles showed that arsenic concentrations in the three most labile fractions (i.e., water-soluble, exchangeable and carbonate-bound fractions) were generally low. Most arsenic in soils was nominally associated with the organic and Fe-Mn oxide fractions, being extractable in oxidizing or reducing conditions. DTPA-extractable As (assumed to represent plant-available As) was found to be strongly correlated to the labile pool of As (i.e. the sum of the first three fractions). The statistical comparison of means (two-sample t-test) showed the presence of significant differences between the concentrations of As(III) and As(V) for different soil locations, depths and fractions. The risk assessment code (RAC) was found to be below the pollution level for all soils. The measurement of arsenic uptake by different parts of tea plants corroborated the hypothesis that roots act as a buffer and hold back contamination from the aerial parts.Copyright © 2011 Elsevier Ltd. All rights reserved.

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