The synergistic mechanisms of citric acid and oxalic acid on the rapid dissolution of kaolinite

Low molecular weight organic acids (LMWOAs) play an important role in dissolution and structural alteration of the minerals, but the mechanisms of the rapid dissolution of minerals in the presence of mixed LMWOAs have been less investigated. Here, the dissolution kinetics of a kaolinite sample in aqueous citric acid, oxalic acid and citric/oxalic mixture at the same pH were examined. Structural changes of kaolinite were studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The data for Si and Al dissolution from kaolinite fit well with the first-order equation (Ct = a(1-exp(−kt)) whether the kaolinite was treated with a single acid or acid mixture. Note that the Si dissolution rate in aqueous citric acid was higher than in oxalic acid. In contrast, the Al dissolution rate in oxalic acid was greater than in citric acid. These are explained by the characterization results of the kaolintie. The synergetic effects of oxalate and citrate effectively and rapidly dissolves Si and Al from kaolinite, which leads to a kaolinite with a decreased degrees of crystallinity, a samller median diameter and an increased specific surface areas.