以设计精准修复方案为目标的土壤重金属形态分布研究

Distribution of Heavy Metals by Form for Precise Remediation of Polluted Farmland Soil

农田土壤原位钝化/稳定化修复是通过向土壤中添加化学药剂以降低重金属活性、抑制土壤重金属向农作物迁移进而降低重金属风险的一种修复技术。针对河南省某重金属污染农田土壤,为制定精准的原位稳定化修复方案,采集32个表层土壤样品测定其Cd、Pb、As、Cu、Zn、Ni、Cr 7种元素全量,并利用BCR连续提取法测定弱酸提取态、可还原态、可氧化态和残渣态4种形态含量。结果表明:土壤Cd、Pb、As、Cu、Zn、Ni和Cr含量的平均值分别为1.90、144.91、7.33、32.24、91.40、28.80和19.76 mg·kg-1,Cd和Pb含量分别超过HJ/T 332—2006《食用农产品产地环境质量评价标准》的3.17和1.81倍。BCR形态分析表明,弱酸提取态Cd含量占全量比例较高,平均值为24.06%;土壤80.35%的Pb以可还原态存在,其余5种元素主要以残渣态存在。基于不同元素的弱酸提取态含量与全量建立回归分析模型,结果显示各元素的弱酸提取态含量随全量呈线性变化且两者的正相关关系显著;根据Cd、Pb 2种元素全量与弱酸提取态含量的空间分布差异对研究区域进行分区,基于土壤全量Cd和弱酸提取态Cd含量的分区结果有一定差异,而基于弱酸提取态Cd与弱酸提取态Pb含量的分区结果大致相同,能够为设计精准的修复方案提供依据。最后,分别以重金属全量与有效态含量为出发点对研究区域重金属的污染程度与存在风险给予评价,Hakanson风险评价结果表明土壤Cd存在极强的潜在生态风险,风险评价编码法评价结果则显示Cd为中等风险,两者之间的差异提示土壤重金属修复需综合考虑不同评价方法的评价结果,这为重金属的修复评价奠定基础。

In-situ immobilization/stabilization of heavy metals in farmland soil is a soil remediation technology through amending the soil with certain chemical agents to reduce activity of the pollutants,inhibit their transport from soil to crops and hence mitigate their risks to crops. A total of 32 soil samples were collected from heavy metal polluted croplands in Henan Province for analysis of Cd,Pb,As,Cu,Zn,Ni and Cr for their totals and various fractions,i. e.,weak-acid extractable,reducible,oxidizable and residue,using the BCR sequential extraction method,and then a precise soil remediation program formed. Results show that the concentration of Cd,Pb,As,Cu,Zn,Ni and Cr in the soil was on average1. 90,144. 91,7. 33,32. 24,91. 40,28. 80 and 19. 76 mg·kg-1,respectively. Among them,Cd and Pb concentration in the soil was far beyond the criteria of the"Standard for Environmental Quality Evaluation Standards of Farmlands Produ-cing Edible Agricultural Produce(HJ/T 332—2006) "reaching 3. 17 and 1. 81 times as high,respectively. The analysis using the BCR sequential extraction method shows that weak acid extractable Cd was relatively high in percentage of the total,reaching up to 24. 06%; Pb existed in the soil mainly in reducible form,which accounted for 80. 35% of the total in the soil,while the other 5 elements existed mainly in residual form. Regression analysis models were established based on contents of the weak acid extractables and their respective total of the heavy metal elements in the soil,indicating that the former varied with the latter exhibiting positive linear relationships between the two. According to the differences weak acid extractable Cd and Pb and their respective total in spatial distribution,the study area was partitioned. The partitioning by total Cd differed somewhat from that by weak acid extractable Cd,while the partitioning by weak acid extractable Cd was quite similar to that by weak acid extractable Pb,which may serve as a basis for formulation of a precise soil remediation program. In the end,levels and risks of the heavy metal pollution of the soil in view of the total and the content of available form of heavy metals,separately. Hakanson Potential Ecological Risk Assessment shows that Cd in the soil posed a strong potential ecological risk,but Cd in the soil was rated as moderate in risk according to the Risk Assessment Code(RAC).The difference between the two evaluations suggests that it is essential to take into account results of all various evaluation methods to program remediation of heavy metals polluted soils. This study may have laid down a foundation for evaluation and remediation of heavy metal polluted soils.