Transformation of Chemical Forms and Migration Characteristics of Cadmium and Selenium During Soil Formation in Black Rock Series

Abstract

[Objective] Black rock series contain high amounts of cadmium (Cd) and selenium (Se). During weathering and soil formation on the rocks, the chemical form transformation and release of Cd and Se from the rocks can have an impact on the surrounding ecological environment. [Methods] In this study, a black rock series distribution area of Hetang formation of the Lower Cambrian was selected in northwest Zhejiang. By collecting horizonal soil samples from soil profiles along different parts of the terrain (hill upslope, hill middle slope, hill downslope, inter hill valley bottom and valley terrace), the quantity and chemical form transformation patterns of Cd and Se during the process of weathering soil formation and weathering material migration of the black rock series were discussed. [Results] The results showed that total Cd and total Se in the soil decreased from parent rock to soil. The loss of Cd and Se in the weathered matter of black rock series increased with the increasing of transportation distance along the hilly slope, and the loss of Cd and Se in the process of evolution from slope deposit to diluvium was greater than that from residual to slope deposit. The Cd and Se lost from the weathered materials could affect the contents of Cd and Se in the topsoil derived from the alluvial of the downstream river in the way of water diffusion, but the degree of influence decreased with the increasing of distance. With the increasing of the transportation distance of weathered materials and the evolution of soil, the transformation of carbonate bound Cd and residual Cd to water-soluble Cd, exchangeable Cd, organic matter bound Cd and oxide bound Cd occurred. The residual Se gradually activated and transformed into exchangeable Se, organic matter bound Se and oxide bound Se. The results also showed that soil water-soluble Cd, exchangeable Cd (Se) and organic matter bound Cd (Se) showed an increasing trend toward the surface. The organic enrichment and topsoil enrichment of Se were more obvious than those of Cd. [Conclusion] The results indicated that the weathering of black rock series could not only directly affect the contents of soil Cd and Se in their distribution area through residue, but also affect the contents of soil Cd and Se in the surrounding areas through water diffusion.

Keywords

• black rock series
• weathering
• soil development
• cd
• se
• chemical form
• growth and decline
• environmental effect