Removal mechanisms of copper using steel-making slag: adsorption and precipitation

Slag, which consists of calcium oxide, aluminium oxide, and other metal oxides, is an abundant by-product in steel-making process. It has been used as adsorbents to remove various heavy metals and the major removal mechanisms are precipitation and adsorption on the surface of metal oxide. However, the relative contribution of two removal mechanisms has not reported. In this study, the removal characteristics of copper were investigated in terms of sorption kinetics and sorption isotherms. As initial pH of solution increased, the sorption capacity of slags increased dramatically. We evaluated the relative contribution of two mechanisms in the copper removal by steel-making slag. At above pH 3.0, contribution of precipitation to overall removal of copper was less than 12%. As a result, most of copper was removed by slag as a form or copper hydroxide.

The major removal mechanisms of copper ions are adsorption and precipitation. Regardless of kinds of slag, pseudo second-order kinetic model and Langmuir isotherm was fitted well. The adsorptive amount of copper on slag was 17.4 mg/g, 7.2 mg/g and 6.2 mg/g for Y slag, J slag, and K slag, respectively. The precipitated amount of copper was highly dependent on initial pH of solution and slag. The relative contribution of adsorption on removal of copper using slag decreased dramatically and that of precipitation increased sharply as initial pH increased. As a result, the principle mechanism of copper removal using slag is not adsorption on slag surface but precipitation with hydroxide dissolved from slag.