A Study on the Classification of Geological Background Source Cadmium Migration Phases in Zhejiang Provence, China

Abstract

Abstract The release of cadmium during the natural weathering process of cadmium-containing strata constitutes a significant source of cadmium pollution in both water and soil. The Hetang Formation, located in western Zhejiang Province in China, is a typical example of a cadmium-rich black shale stratum. The bottom coal seam of this formation has been found to contain high levels of cadmium, with concentrations reaching up to 11.2 mg/kg. The continuous weathering and erosion of cadmium-bearing bedrock by fluvial processes results in the release of cadmium, leading to geological background cadmium contamination of downstream soils. This study relies on the strong adsorption characteristics of iron-sediment for cadmium during migration and uses FE-SEM, EDS, optical microscopes and other means to observe the distribution of iron elements in rock fragments of different weathering degrees in the Hetang Formation in river channels. The aim is to explore the state of cadmium from geological background sources during migration. By comparing detritus from upstream and downstream sections of the river channel, it was determined that under weathering conditions, the occurrence state of geological background source cadmium could be divided into three distinct stages: unweathered stage; intra-detritus migration stage; and completely weathered stage. Furthermore, after leaving the Hetang Formation shale outcrop area, there was a sharp decrease in content for Hetang Formation detritus. At this point geological background source cadmium had largely escaped from constraints imposed by rock structure and had come into full contact with water. The findings from this research deepen our understanding regarding migration patterns for geological background source cadmium. They also reveal formation processes for geological background source cadmium pollution and provide a theoretical basis for identification and treatment for geological background source cadmium pollution.