Sn(IV) anions adsorption onto ferric hydroxide: A speciation-based model

• Ferric hydroxide can be used for deep removal of Sn.
• Ferric hydroxide surface consists of three types of surface hydroxyl groups.
• The adsorption property of Sn can be quantified in alkaline solution.
• A speciation-based model can quantify Sn adsorption.
• pH and equilibrium Sn concentration codetermine Sn adsorption property.

Sn is a detrimental impurity in tungsten hydrometallurgy which should be strictly controlled. Using the freshly precipitated ferric hydroxide to remove Sn impurity from tungstate solution exhibits satisfactory effects. The goal of the present research is to provide further understanding of the adsorption mechanism. The work indicates that the surface of ferric hydroxide consists of three types of surface acid sites denoted as S1, S2, and S3, each of which has a surface density (Гi) of 1.0 × 10− 3, 3.1 × 10− 3, and 9.6 × 10− 3 mol·g− 1 and acid constant (pKH) of 3.6, 5.7, and 10.8 respectively. The pHzpc of 7.9 for the ferric hydroxide is obtained. Since Sn exists in alkaline solution as Sn(OH)5− and Sn(OH)62 −, a speciation-based model is developed and data fitting to the model gives six adsorption equilibrium constants which are 26.2, 1.2 × 10− 11 and 1.5 for Sn(OH)5−, and 4.3, 1.6 × 10− 15 and 2.2 × 10− 3 for Sn(OH)62 − onto S1, S2, and S3 respectively. The average absolute error of the predicting results and experimental data is 5.3%. The speciation-based model can successfully describe the Sn adsorption ratio at different pH values and predict the Sn adsorption ratio with different initial Sn concentrations.