Impact Of Co-existing Cations On Phytoavailability Of Cadmium In Soils And Its Mechanisms

Cadmium, as a hazardous element in environment, has caused serious pollution all over the world. Currently, there are more than 20,000 ha of Cd-contaminated soil in China.Compared with other heavy metals and organic pollutant, Cadmium could exert a negative influence on humans, animals and plants even at a lower concentration. Moreover, it was difficult to highlight the pollution because of its concealment and delitescence in toxicity. It has been taken as the prior controlling pollutant by U.S.A., Japan and European Union. The acceptable content of Cd in fertilizer and biosolid has also been stnctly limited. The mobility, transformation, and subsequent ecological effect of cadmium in environment have been the focus of attention in the fields of soil and environmental science.Sorption, as the most important reaction in soil chemistry, controls the availability of Cd in soil. Methods mainly based on the theory, therefore, were applied to reduce the availability of Cd in soil. Among which, calcite, clay material, or phosphate were applied to enhance its sorption and decrease its availability in soil. Co-existing cations are important components in soil solution, which could exert significant influence on Cd sorption in soil. The effects of single Ca, Zn, and K on the adsorption and phytoavailability of Cd, as well as the effects of binary Ca, K, and Zn on adsorption and phytoavailabihty of Cd were also studied. The results would make clear the mechanisms of the mutual reactions occurred among ions as well as ions and soils, and might be important in fertilization management that aimed to control Cd-pollution in soil.The single and complex effects of Ca, K, and Zn on the sorption and phytoavailability of Cd were studied in Cd-polluted yellow brown earth and lateritic red soil, using Batch , Low- Pressure Centrifugal Displacement as well as standard methods. The results showed that with the same ionic strength and different concentrations, single or binary Ca, K, and Zn with Cd co-existing could reduce Cd adsorption in lateritic red soil, and decrease the distribution coefficient (K_d^Cd) of Cd significantly. Ca²⁺, K⁺ and Zn²⁺ apparently decreased the adsorption of Cd with the order as follows: Zn⁽2+_<K⁺?Ca⁺, and co-existing binary cations of Ca²⁺, K⁺ and Zn²⁺ reduced adsorption of Cd with the order of K⁺-Zn²⁺?Ca²⁺-Zn²⁺> Ca²⁺-K⁺.The sorption amount of Cd decreased by 68.6-98.7% compared with single Na⁺ system. Zinc could reduce the most of Cd adsorption among three cations. When co-existing with Ca or K, Zn²⁺ played a crucial role in reducing the adsorption of Cd. With the same ionic strength and different concentrations, binary Ca, K, and Zn with Cd co-existing and the rate of Zn showed the significantly negative relation with K_d^Cd. It showed significantly linear (n=7) or polynomials (n=16) regression relation between Zn and K_d^Cd, and Zn accounted for the most significant effect on the adsorption of Cd.Cd content in rape plant was significantly improved with the addition of K fertilizer in two seriously Cd-polluted soils, and the effect was enhanced with the increasing K fertilizer. Cd content of rape plant was increased by 6.20-29.7% in high rate of K than that's no applied K. It showed phytoavailability of Cd was under the impact of the competition between Cd and K. While the effect ofCa on the phytoavailability of Cd was complicated, Ca could decrease the availability of Cd by 4.47-15.1% only at a medium amount comparing with no applied Ca. The domination of Cd sorption in soils became decreased in controlling uptake of Cd by rape with the increasing addition of Zinc fertilizer. Cd content of rape plant decreased by 28.18-31.8% in high rate of Zn than that's no applied Zn. Ca/Cd and Zn/Cd in soil solution showed the significant relation with Cd content in rape plant in the contaminated lateritic red soils both with high and low content of Cd. Thereinto, the influence of Ca/Cd was positive, while the influence of Zn/Cd was negative. The less uptake of Cd was, the larger Ca/Cd, and Zn/Cd in rape plant was. Ca/Cd in soil solution showed significantly negative relation with Ca/Cd in rape plant in contaminated lateritic red soil with low content of Cd. Zn/Cd in soil solution showed significantly positive relation with Zn/Cd in rape plant in Cd-contaminated lateritic red soil.There was a positive relation between the content of Cd in soil solution and the application of Ca, K and Zn both in the high and low contaminated red soils (p<0.05). The concentration of Cd was mainly controlled by the application of Zn. Cd concentration was obviously decreased in rape plant with the decreasing addition of Ca and K and the increasing addition of Zn under Zn co-existed with Ca or K. The application of Ca, K, and Zn could significantly decrease the availability of Cd (p<0.05). The content of Cd in rape plant was mainly controlled by application Zn, and enhanced rate of Zn could obviously reduced phytoavailability of Cd. Cd content in plant was obviously enhanced with the decreasing addition Ca and the increasing a addition of K under Ca co-existed with K in low Cd-contaminated soil. The uptake of Cd was promoted by continuously application of Ca and K, indicating the increase in the phytoavailability of Cd.The results of this research showed that zinc was the key factor to reduce distribution coefficient (/CdCd) and phytoavailability of Cd with Zn co-existed with Ca or K in soils .Ca/Cd and Zn/Cd in soil solution could measure phytoavailability of Cd and the values of Ca/Cd and Zn/Cd in soil solution could reflect the high or low phytoavailability of Cd. The values of Ca/Cd and Zn/Cd in rape plant could reflect the high or low phytoavailability of Cd. These results renewed the former ideas which considered the ratio of total Zn and total Cd in soil as the index of judging availability of Cd in soils, providing a new way of judging the phytoavailability of Cd in soil and evaluating interaction between other cations with Cd in soils.