珠江三角洲城郊菜地耕层土壤中Cd、Pb赋存形态及其相互关系

Fractionations of Topsoil Cd and Pb and Interrelationships among Their Own Fractions in Some Rural/Peri-urban Vegetable Fields in Pearl River Delta

选取珠江三角洲典型区域城郊菜地41个耕层土壤样品,采用化学形态分级-连续提取法研究了土壤中Cd、Pb赋存形态,通过统计分析,揭示了菜地耕层土壤Cd、Pb形态分布特征及其相互关系。Cd赋存形态平均含量顺序为:铁锰氧化物结合态(32.4%)≥碳酸盐结合态(27.2%)≥交换态(21.0%)>残渣态(10.4%)>有机态(9.0%);Pb则为:残渣态(54.1%)>铁锰氧化物结合态(27.5%)>有机态(11.6%)>碳酸盐结合态(4.6%)>交换态(2.1%)。非残渣态Cd占各形态Cd总和的平均比例(89.6%)明显高于Pb(45.9%)。碳酸盐结合态Cd分别与铁锰氧化物结合态Cd、有机结合态Cd呈极显著正相关和显著正相关(前者r=0.8482,P<0.01;后者r=0.4090,P<0.05),但与残渣态Cd呈极显著负相关(r=-0.5343,P<0.01),铁锰氧化物结合态Cd和有机结合态Cd分别与残渣态Cd呈极显著正相关(前者r=0.6965,p<0.01;后者r=0.5379,P<0.01);铁锰氧化物结合态Pb与残渣态Pb呈极显著正相关(r=0.5217,P<0.01),但有机结合态Pb却与残渣态Pb呈显著负相关(r=-0.3252,P<0.05)。在散点图矩阵的基础上建立了曲线估计的回归方程,用之可预测土壤中交换态Cd、Pb的大致数量及植物有效性。

This study was conducted to investigate the fractionations of topsoil Cd and Pb, and the interrelationships among their own fractions in some rural/peri-urban vegetable fields in Pearl River Delta. According to a five-stage sequential extraction procedure （a modified Tessier method）, 41 soil samples from typical regions in Guangzhou and Foshan areas were analyzed for Cd and Pb fractions using graphite furnace-atomic absorption spectrophotometry （GF-AAS）. With statistical analyses, the results showed as follows： 1） On average, the order of Cd in each fraction was Fe-Mn oxides-bound （32.4%） ≥ carbonate-bound （27.2%） ≥exchangeable （21.0%） 〉residual （10.4%） 〉organic-bound （9.0%）, respectively, Pb followed the order： residual （54.1%） 〉Fe-Mn oxides-bound （27.5%） 〉 organic-bound （11.6%） 〉 carbonate-bound （4.6%）〉exchangeable （2.1%） （All paired comparisons, P〈 0.01）. The percentage of non-residual fractions of Cd （89.6%） was conspicuously higher than that of Pb （45.9%）. 2） Carbonate-bound Cd was strongly positively correlated with Fe-Mn oxides-bound Cd （r = 0.8482, P〈 0.01） but strongly negatively correlated with residual Cd （r = -0.5343, P〈 0.01）, respectively. The significant positive correlation was also found between carbonate-bound Cd and organic-bound Cd （r = 0.4090, P〈 0.05）. At the same time, Fe-Mn oxides-bound Cd and organic-bound Cd were all strongly positively correlated with residual Cd （r = 0.6965, P〈 0.01 and r = 0.5379, P〈 0.01, respectively）. Fe-Mn oxides-bound Pb was strongly positively correlated with residual Pb （r = 0.5217, P〈 0.01）. However, organic Pb was significantly negatively correlated with residual Pb （r = -0.3252, P〈 0.05）. 3） Exchangeable Cd was subjected to residual Cd and organic-bound Cd. Similarly, exchangeable Pb was subjected to Fe-Mn oxides-bound Pb, residual Pb and organic-bound Pb, only the former two fractions of which were the main factors controlling the change of exchangeable Pb. 4） On the basis of scatterplot matrices for Cd and Pb fractions, curve estimation models were established to predict the amount of exchangeable fractions and heavy metal phytoavailability in the soils. The results from this research could serve as the basis for establishing the environmental quality standards for heavy metals in soils.