Geochemical Behavior of Cadimum during Early Diagenesis in Tidal Sediment from the Shuangtaizi Estuary, China

English Abstract:

A sediment core and pore-water samples were taken in the Shuangtaizi Estuary for a study on early diagenesis in sediment and its influence on geochemical behavior of cadimum. Sulfate, Fe and Mn ions in the pore-water were analysed to better understand the depth ranges and reactions related to the early diagenesis. In addition, three different types of sulfur, acid volatile sulfur (AVS), pyrite sulfur (PS) and elemental sulfur (ES), and their transformations during early diagenesis were also studied.

<span"> Results showed that low concentrations of both sulfate and organic carbon have occurred between 0-30cm depth of the sediment core, suggesting a reaction between sulfate and organic carbon. The concentration of ES is lower than its detection limitation between 0-25cm, as well as the low concentration of AVS, which is consistent to high PS content in this depth interval, suggesting the production of pyrite will lead to the reduction of ES and AVS. With the reaction between sulfate ion and organic matter, hydrogen ion content in the pore-water also increased accordingly, resulting in a decrease of pH value in the pore-water, and suggesting an influence of early diagenesis. A low concentration of dissolved cadimum ion occurred in the pore-water at a depth interval from 0 to 30cm, while total cadimum concentration in the sediment reached its maximum in this depth interval, indicating that cadimum in the pore-water has been absorbed on the surface of mineral particle and/or combined with the Fe/Mn oxides in the sediment. High percentage of exchangeable cadimum speciation reinforced this presumption. Exchangeable cadimum was the dominant phase in the sediment (57.6%), followed by Fe/Mn oxides phase (25.9%), residual phase (12.5%) and organic/sulfide phase (4.0%).

<span"> Correlation, cluster and factor analysis showed that cadimum is mainly originated from weathering of rock on land. The absorption of clay mineral, Fe/Mn oxides in the sediment and pH values variaton in the pore-water are main influencing factors for the mobilization,migration and transformation of cadimum in sediment. Total diffuse flux of cadimum in the sediment is 0.042 mgm-2a-1. Almost 50% of cadimum in the sediment is redissolved into pore-water and another 50% cadimum is deposited or adsorbed on the surface of solid sediment.