Study on the Corrosion of Coral Sand by Oxalic Acid.

A vegetation ecosystem is essential to maintain a livable environment in a tropical reclamation land. In this paper, kinetics of coral corrosion in the presence of the different concentration of oxalic acid was studied. According to the kinetic model established, main factors affecting corrosion of coral sand were elucidated. The results show that Ca²⁺ and Mg²⁺ in the coral sand were rapidly dissolved during the beginning stage of the reaction, followed by the fixation of a large amount of Ca²⁺ by oxalate to form calcium oxalate precipitate. As the concentration of oxalic acid increased, the concentration of Ca²⁺ in the solution gradually decreased and the concentration of Mg²⁺ gradually increased. The maximum concentrations of Ca²⁺ and Mg²⁺ were 0.65 and 2.87 mmol·L^-1, respectively. The corrosion kinetics of Mg²⁺ was fitting with Stumm model, indicating this process was mainly governed by diffusion. In addition, dissolution and loss of Ca²⁺ in coral sand was inhibited at high concentration of oxalic acid. The results of XRD and SEM show that the calcium oxalate precipitates formed by the reaction of high concentration oxalic acid are attached onto the surface of coral sands, and magnesian calcite is preferentially dissolved, followed by aragonite and calcite.