Dissolution of kaolinite induced by citric, oxalic, and malic acids

Kaolinite is a dominant clay mineral in the soils in tropical and subtropical regions, and its dissolution has an influence on a variety of soil properties. In this work, kaolinite dissolution induced by three kinds of low-molecular-weight organic acid, i.e., citric, oxalic, and malic acids, was evaluated under far-from-equilibrium conditions. The rates of kaolinite dissolution depended on the kind and concentration of organic acids, with the sequence Roxalate>Rcitrate>Rmalate. Chemical calculation showed the change in concentration of organic ligand relative to change in concentration of organic acid in suspensions of kaolinite and organic acid. The effect of organic acid on kaolinite dissolution was modeled by species of organic anionic ligand. For oxalic acid, L2−oxalic and HL−oxalic jointly enhanced the dissolution of kaolinite, but for malic and citric acids, HL−malic and H2L−citric made a higher contribution to the total dissolution rate of kaolinite than L2−malic and L3−citric, respectively. For oxalic acid, the proposed model was RSi=1.89×10−12×[(25x)/(1+25x)]+1.93×10−12×[(1990x1)/(1+1990x1)] (R2=0.9763), where x and x1 denote the concentrations of HLoxalic and Loxalic, respectively, and x1=10−3.81×x/[H+]. For malic acid, the model was RSi=4.79×10−12×[(328x)/(1+328x)]+1.67×10−13×[(1149x1)/(1+1149x1)] (R2=0.9452), where x and x1 denote the concentrations of HLmalic and Lmalic, respectively, and x1=10−5.11×x/[H+], and for citric acid, the model was RSi=4.73×10−12×[(845x)/(1+845x)]+4.68×10−12×[(2855x1)/(1+2855x1)] (R2=0.9682), where x and x1 denote the concentrations of H2Lcitric and Lcitric, respectively, and x1=10−11.16×x/[H+]2.