单一及复合外源镉(Ⅱ)铬(Ⅲ)污染在红壤中的老化过程

Aging Process of Single and Combined Exogenous Cadmium(Ⅱ) and Chromium(Ⅲ) Contamination in Red Soil

为研究复合外源镉(Ⅱ)铬(Ⅲ)污染在红壤中的老化过程以及与单一外源镉铬污染的差异,在未受污染的红壤中添加不同浓度的镉(0、1、5、10、20和40mg/kg)以及铬(0、100、200和400mg/kg),在老化的第1、3、7、15、30、60、90d取样,采用BCR分级提取法分析镉、铬在红壤中的赋存形态,并采用四种动力学方程拟合老化过程中的形态变化。结果表明,外源镉、铬进入土壤后发生形态再分配作用。随着老化时间延长,重金属与土壤的结合强度不断增加,总再分配系数不断降低。老化过程可分为快反应阶段和慢反应阶段。Elovich方程拟合镉铬的弱酸可提取态和残渣态的变化效果最好。90天老化后,土壤中镉的主要赋存形态为弱酸可提取态,铬的主要赋存形态为可还原态和可氧化态。外源重金属污染水平越低,总再分配系数越小,结合强度系数越高,表明低外源污染水平的镉铬形态再分配作用效率更高,与土壤组分的结合更紧密。镉-铬复合污染组的总再分配系数比对应的单一污染处理组低4.2%~25.0%,结合强度系数比对应的单一污染处理组高9.3%~47.0%,这说明复合外源镉铬污染较单一外源镉铬污染的形态再分配效率更高,与土壤组分结合更紧密。外源镉的加入可促进外源铬的老化过程,外源铬的加入亦可促进外源镉的老化过程。

In order to study the aging process of combined exogenous cadmium(Ⅱ)and chromium(Ⅲ)contamination in red soil and the difference with single cadmium and chromium contamination,concentrations of 0,1,5,10,20,and 40 mg·kg-1 of cadmium(Ⅱ)and 0,100,200,300 and 400 mg·kg-1 of chromium(Ⅲ)were spiked in uncontaminated red soils.After 1,3,7,15,30,60 and 90 d of incubation,samples were collected to analyze the speciation of metal by the modified community bureau of reference(BCR)sequential extraction procedure.Four kinetic models were used to fit the change of fractions.The result shows that the redistribution occur after exogenous cadmium and chromium enter in the soil.The bingding intensity of soil and heavy metal increases,and whole-soil redistribution index decreases gradually with time.The aging process of metals can be divided into a fast process and a slow process.The Elovich equation is the best fitted for the change of acid-extractable and residue fractions during the incubation.After 90 d of incubation,cadmium in soil exists mainly in acid-extractable fractions,while chromium exists mainly in the reducible and oxidizable fractions.The lower loading level of metals has a lower value of whole-soil redistribution index and a higher value of reduced partition index after 90 d of incubation,indicating that the efficiency of redistribution is greater and the metals bind to the solid-phase component more strongly when metal is spiked at low concentrations.The whole-soil redistribution index of the Cd-Cr combined contamination group is 4.2%~25.0%lower than that of the corresponding single contamination group,and the reduced partition index was 9.3%~47.0%higher than the corresponding single contamination group.This indicates that the efficiency of redistribution in combined exogenous cadmium and chromium contamination soils is greater and the metals bind to the solid-phase component more strongly compared with the singled exogenous cadmium and chromium contamination soils The existence of chromium accelerates the aging process of cadmium in the soils,and the existence of cadmium also accelerates the aging process of chromium in the soils.