Rhizosphere environmental factors regulated the cadmium adsorption by vermicompost: Influence of pH and low-molecular-weight organic acids

Abstract

Vermicompost is a promising amendment for immobilization of cadmium (Cd) in soils; however, its effectiveness can be influenced by rhizosphere environment conditions, such as pH and the presence of low-molecular-weight organic acids (LMWOAs). In this study, a batch experiment was conducted to examine the characteristics of Cd adsorption by vermicompost at different pH (pH = 3, 5, and 7) and after the addition of different LMWOAs (oxalic acid; citric acid; malic acid). Furthermore, a series of morphology and structural analyses were conducted to elucidate the mechanisms of observed effects. The results showed that the adsorption capacity of vermicompost for Cd increased as pH increased, and chemisorption dominated the adsorption process. Changes in pH altered adsorption performance by affecting the -OH groups of alcohol/phenol and the -CH2 groups of aliphatics. Further, the addition of oxalic acid promoted Cd adsorption, and the effect was concentration dependent. Modifying the verimicompost surface with more adsorption sites might be the main reason. Conversely, citric acid and malic acid showed the ability to inhibit Cd adsorption by vermicompost. Citric acid caused a blocking effect by covering flocculent substances on the vermicompost surface while reducing surface adsorption sites by dissolving mineral components such as iron oxides. However, the action of malic acid did not appear to be related to changes in morphology or the structure of vermicompost. Overall, the results of this study partially explain the limited effectiveness of Cd immobilization within the rhizosphere by vermicompost, and provide theoretical support for regulating rhizosphere environments to improve the effectiveness of vermicompost immobilization of Cd.