场地土壤重金属镉和铅复合污染毒性阈值的推导及其应用

Derivation of Toxicity Threshold for Combined Pollution of Cadmium and Lead in Site Soil and Its Application

镉(Cd)和铅(Pb)是生物体非必要元素,具有毒性大、迁移性强等特点,二者在场地土壤中通常存在复合污染的现象。土壤污染物预测无效应浓度(predicted no effect concentration,PNEC)是污染物自身毒性与不确定性系数共同决定的参数,其中土壤污染物的生物有效性是PNEC值不确定性的重要来源。本研究选择湖南常宁某铜矿冶炼厂和江苏靖江某电镀厂2个不同类型的污染场地及周边土壤,基于土壤理化性质及重金属的有效性浓度,采用经验模型估算出Cd和Pb单一污染对植物根系生长的半效应浓度(median effect concentration,EC_(50))及其土壤中的PNEC,通过浓度加和(concentration addition,CA)模型,结合外推因子法,估算获得场地土壤实际Cd和Pb复合污染的预测无效应浓度(PNEC_(mix))。结果表明,2个场地土壤中Cd的平均含量分别为背景值的102倍和37.6倍,Pb平均含量分别为背景值的15.1倍和3.86倍,Cd和Pb复合污染现象明显。由于2个场地土壤性质差异较大,Cd和Pb的EC_(50)存在较大差异,EC_(50)(Cd)分别为19.1~36.2 mg·kg^(-1)和20.1~35.4 mg·kg^(-1),而EC_(50)(Pb)分别为366~1891 mg·kg^(-1)和682~1575 mg·kg^(-1)。同时由于场地实际污染土壤的理化性质及Pb/Cd含量比不同,所推导的PNEC_(mix)也存在明显差异,Cd含量及其在Cd和Pb总量中的占比较高的土壤样点的PNEC_(mix)较低,2个场地的PNEC_(mix)分别为0.933~37.9 mg·kg^(-1)和32.9~744 mg·kg^(-1)。2个场地调查样点中Cd和Pb含量的实测值均在一定程度上高于相应的PNEC_(mix),对植物生长存在一定的生态风险。因此,进行复合污染生态风险基准值制定和生态风险评价时,需要考虑影响污染物的生物有效性和关键场地特异性(site-specific)因素,如土壤理化性质及不同污染物浓度比等。

Both cadmium(Cd)and lead(Pb)are non-essential elements for organisms and they have the characteristics of high toxicity and strong migration.These two elements usually co-exist in soil of the sites.The predicted no effect concentration(PNEC)of soil pollutants is a parameter determined depending on both the toxic effect and the uncertainty coefficient.The bioavailability of pollutants in soil is a significant source of uncertainty in calculating PNECs.In this study,two contaminated sites were studied,one is a copper smelter in Changning,Hunan Province,and the other is an electroplating plant in Jingjiang,Jiangsu Province.Based on the physical-chemical properties and the bioavailable concentrations of heavy metals of the soil,the median effect concentrations(EC_(50))and PNECs of Cd and Pb in the studied soil were calculated using empirical models based on plant root growth.Through concentration addition(CA)model and extrapolation factor method,the predicted no effect concentration of mixture(PNEC_(mix))of Cd and Pb in the studied soil were also calculated.The results showed the EC_(50) of Cd and Pb are quite different between the two studied sites due to the large difference in soil properties.The EC_(50)(Cd)values are 19.1~36.2 mg·kg^(-1) and 20.1~35.4 mg·kg^(-1),while the EC_(50)(Pb)values are 366~1891 mg·kg^(-1) and 682~1575 mg·kg^(-1),respectively.Meanwhile,great difference in the estimated PNEC_(mix) between the two sites were also observed due to the different physical-chemical properties and Pb/Cd ratios.The soil with higher concentration or proportion of Cd had lower PNEC_(mix).The PNEC_(mix) values of the two sites are 0.933~37.9 mg·kg^(-1) and 32.9~744 mg·kg^(-1),respectively.The measured concentrations of Cd and Pb in the survey points of the two sites were generally higher than the corresponding estimated PNEC_(mix),which posed potential ecological risk.Therefore,for the establishment of ecological risk benchmark value and ecological risk assessment of compound pollution,it is necessary to consider key factors impacting the bioavailability of pollutants in soil and site specificity,e.g.,the physical-chemical properties of the soil and the concentration ratio of different pollutants co-existing.