大气PM2.5中重金属的化学形态分布

运用多级连续提取法,对广州市不同季节不同采样高度大气PM2.5中重金属的化学形态和生物有效性进行分析。研究表明,PM2.5中各重金属元素之间的化学形态分布差异较大,Zn、Cd、As、Mn主要分布在F1(可溶态与可交换态)和F2(碳酸盐态、可氧化态与可还原态),绝大部分的Pb以F2存在,Ni和Mo主要分布在F1和F3(有机质、氧化物与硫化物结合态),Cu主要以F2和F3存在,Cr主要分布在F3和F4(残渣态),Co则4种形态平均分布。采样高度对重金属的化学形态分布影响不大,同一采样期内楼顶与地面样品中同一元素的化学形态分布结果比较一致。两个采样季节重金属的形态百分比存在不同程度的变化,2007年春重金属的不稳定态(F1)比例比2006年秋普遍增加,次稳定态(F2、F3)比例减少。在10种重金属中,Cd、Zn、Pb和As的生物有效性系数高(>0.7),属于生物可利用性元素,在环境中的活动性要明显高于其它元素;Mn、Cu、Mo、Co、Ni和Cr元素的生物有效性系数值在0.2～0.6之间,属于潜在生物有效性元素,在环境中比较稳定。

Chemical speciation distribution of PM2.5-bound heavy metals in the air

Multi-stage sequential extraction procedures(SEP) were applied to study the chemical speciation of PM2.5-bound heavy metals from different elevations and different seasons in Guangzhou,China.The PM-bound heavy metals were divided into four fractions,including soluble and exchangeable(F1),carbonate-bound,oxides-bound and reducible-bound(F2),organic-bound,oxidisable-bound and sulphidic-bound(F3),and residual such as silicates(F4).Results showed that the relative distribution pattern of the four fractions differed dramatically among the 10 heavy metals tested.The relative abundance of the different forms of metals present in the PM-2.5 was in the order: most of Zn,Cd,As and Mn were distributed in F1 and F2;most of Ni and Mo were in F1 and F3;most of Cu was in F2 and F3,whereas the majority of Pb was in F2.Meanwhile,most of Cr was associated with F3 and F4 and Co distributed evenly in the four fractions.Sampling elevations had little effect on the chemical speciation distribution of heavy metals.However,the distribution pattern of these metals was quite different during different sampling seasons.The percentage of unstable fraction(F1) in spring was generally higher than that in autumn.Among all the 10 heavy metals,Cd,Zn,Pb and As were bioavailable elements with bioavailability indexes(BI) higher than 0.7.Mn,Cu,Mo,Co,Ni and Cr were potentially bioavailable elements with BI values around 0.2 to 0.6.