伴随阴离子对土壤Cd形态转化的影响

Effect of Accompanying Anions on the Cd2+ Fraction Transformation in Soils

化学性质各异的伴随阴离子通过影响进入土壤的外源Cd2＋后的形态转化,而影响其迁移特性和生物毒性.对此进行了解将有助于采取相应的防治镉污染的技术措施.通过室内连续培养试验（0-70 d）,研究了伴随阴离子（NO3^-、C1^-、SO4^2-）对Cd^2＋在土壤中的吸附特性、形态分配与转化特性的影响.结果表明,伴随阴离子对添加外源Cd2＋溶液后的土壤Cd 吸附作用能力表现为：SO4^2-〉NO3^-〉C1^-;SO4^2-处理在20 d 时吸附率达最大值（93.61%）;NO3^-与C1-处理在30 d 时吸附率达最大值,分别为82.25%、67.97%.初始状态土壤中Cd 主要以残渣态存在,可交换态Cd 含量与比例最低,占总Cd 含量不到10%.添加1.0 mmol·L^-1 外源Cd2＋处理后,在培养期内（0-70 d）,表现为可交换态Cd 含量〉铁锰氧化物结合态Cd 含量〉碳酸盐结合态Cd 含量〉残渣态Cd 含量〉有机结合态Cd 含量.土壤可交换态Cd 在0-70 d 分配系数达到38%-61%,其中SO4^2-处理培养10 d 达到最大值（61.09%）,NO3^-与C1^-处理30 d 时达到最大值（分别为43.74%和41.80%）.土壤碳酸盐结合态Cd 含量在30 d 时趋于饱和,其含量（0-70 d）表现为SO4^2-〈C1^-〈NO3^-.3 种伴随阴离子处理的土壤铁锰氧化物结合态Cd 含量在0-50d 无显著性差异.有机结合态Cd 和残渣态Cd 含量随着培养周期延长而逐渐增加,但其吸收转化分配比例相对可交换态、碳酸盐结合态和铁锰氧化物结合态低.从生物利用度系数来评价镉形态的危害程度,初始状态土壤中其危害程度表现为生物潜在可利用态〉生物难利用态〉生物易利用态.添加1.0 mmol·L^-1 的外源Cd2＋溶液后,SO4^2-处理的表现为：生物易利用态〉生物潜在可利用态〉生物难利用态;C1^-和NO3^-处理的表现为生物潜在可利用态〉生物易利用态〉生物难利用态.这表明,镉形态在SO4^2-处理伴随下对土壤造成的危害程度相对C1^-和NO3^-处理严重.外源Cd^2＋溶液进入土壤主要转化为可交换态Cd（SO4^2-〉NO3^-〉C1^-）,SO4^2-处理培养10 d 达到最大值（转化系数为61.09%）.可交换态Cd 在土壤中具有较强的迁移性,容易被生物吸收利用,对环境影响最大.

When the heavy metal ions were entered into soil, it would inevitably carry anion. Due to the various anions with different chemical feature, it will affect heavy metals transformation and result in differences on the mobility and toxicity of heavy metals in soil. Researches on effect of inorganic ligands to heavy metals will advance the understanding of the mechanism of heavy metal’s forms changes and the controls in heavy metals contamination. The plain paddy soil in Chengdu was performed in this study by adding the exogenous Cd2+ to investigate the anionic (NO3-, C1- and SO42-)effects on the uptake, the form distribution and transformation of cadmium. The results showed that the Cd absorption capacity in the soil was expressed the tendency of SO42->NO3->C1-, when added exogenous Cd2+solution. After 20 days, the absorption capacity of Cd reached at 93.61%under the SO42-treatment. After 30 days, the absorption capacity of Cd reached at 82.25% and 67.97% under the NO3- and C1- treatments, respectively. The initial state of cadmium in soil is mainly the residual form and the exchangeable cadmium content is very low, about less than 10% of total Cd content. To add 1 mmol·L-1 exogenous Cd2+ during the treatment of 0~70 day, the content of cadmium form was showed the trend of the exchangeable Cd>Fe-Mn oxide bound Cd>carbonate bound Cd>residual Cd>organic bound Cd. Cadmium in soil is mainly the form of exchangeable bound and its distribution coefficient reached at 38%~61%. The distribution coefficient reached at 61.09%under SO42-treatment of 10 days, 43.74%under NO3-treatment of 30 days, and 41.80%under C1-treatment of 30 days. The content of carbonate bound Cd expressed saturation at the end of 30 days, and its content was showed the tendency of SO42-<C1-<NO3-during the treatment. The content of Fe-Mn oxide bound Cd was no significant during the treatment of 0~50 days. The contents of organic bound Cd and residual Cd were gradually increased with the prolonged treatment, but the ratio of absorption and transformation was less than that of exchangeable Cd, carbonate bound Cd and Fe-Mn oxide bound Cd. The injury extend of cadmium forms were evaluated by bioavailability factor and was showed the trend of potential bioavailability Cd>bioavailability Cd>inert bioavailability Cd in the initial state of the soil. To add 1 mmol·L-1 exogenous Cd2+, the SO42-treatment led to the expression of bioavailability Cd>potential bioavailability Cd>inert bioavailability Cd, whereas the NO3-and C1-treatment resulted in the expression of potential bioavailability Cd>bioavailability Cd>inert bioavailability Cd. In conclusion, the SO42-treatment caused more serious pollution that did the NO3-and C1- treatments. Exogenous Cd2+ was mainly transformed into exchangeable bound and showed the tendency of SO42->NO3->C1-. At the end of 10 days, the transformation coefficient reached at 61.09%under the SO42-treatment. As the exchangeable bound Cd was likely to transfer and be absorbed by organisms, it will possess great impact on the environment.