Influence of removal of organic matter and iron and manganese oxides on cadmium adsorption by red paddy soil aggregates

The ability of soil aggregates to adsorb heavy metals is controlled by their different component contents. However, the influence of specific components on the adsorption behavior is uncertain. In this study, organic matter and iron and manganese oxides in aggregates of different particle sizes from red paddy soil were selectively removed. The influence of these components on cadmium adsorption was also examined. Results showed that Langmuir equation could accurately describe the adsorption characteristics of Cd in bulk soil and aggregates. The <0.002 mm fraction had the largest adsorption capacity for Cd because of its higher content of organic matter and Fe and Mn oxides. Removal of organic matter, amorphous Fe oxides or Mn oxides could cause obvious decrease in Cd adsorption, whereas an opposite result was observed for free Fe oxides removal. Removing unit mass of Fe or Mn oxides showed stronger effect on the adsorption capacity compared with that of organic matter. Desorption characteristics of Cd in various aggregates appeared to be unaffected by organic matter but more determined by free and amorphous Fe oxides.