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

Abstract

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 R oxalate > R citrate > R malate . 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, L 2− oxalic and HL − oxalic jointly enhanced the dissolution of kaolinite, but for malic and citric acids, HL − malic and H 2L − citric made a higher contribution to the total dissolution rate of kaolinite than L 2− malic and L 3− citric, respectively. For oxalic acid, the proposed model was R Si = 1.89 × 10 −12 × [ ( 25 x ) / ( 1 + 25 x ) ] + 1.93 × 10 −12 × [ ( 1990 x 1 ) / ( 1 + 1990 x 1 ) ] ( R 2 = 0.9763 ), where x and x 1 denote the concentrations of HL oxalic and L oxalic, respectively, and x 1 = 10 −3.81 × x / [ H + ] . For malic acid, the model was R Si = 4.79 × 10 −12 × [ ( 328 x ) / ( 1 + 328 x ) ] + 1.67 × 10 −13 × [ ( 1149 x 1 ) / ( 1 + 1149 x 1 ) ] ( R 2 = 0.9452 ), where x and x 1 denote the concentrations of HL malic and L malic, respectively, and x 1 = 10 −5.11 × x / [ H + ] , and for citric acid, the model was R Si = 4.73 × 10 −12 × [ ( 845 x ) / ( 1 + 845 x ) ] + 4.68 × 10 −12 × [ ( 2855 x 1 ) / ( 1 + 2855 x 1 ) ] ( R 2 = 0.9682 ), where x and x 1 denote the concentrations of H 2L citric and L citric, respectively, and x 1 = 10 −11.16 × x / [ H + ] 2 .