Synthesis of mesoporous adsorbents from mine tailings and application for selective metal ions separation

Abstract

Massive amounts of mine tailings have been released to approximately 88 million tons in the surroundings of abandoned mine sites in Korea, posing a potential risk to humans and the ecosystems. In this research, the mesoporous silica SBA-15 was synthesized from mine tailings as metal adsorbent for separation of metal ions in aqueous solutions. The mesoporous silica SBA-15 was prepared by self-assembling pathway using surfactant as the template in silicate solution extracted from mine tailings. The porosity can only be obtained after the surfactant removal process such as calcination or extraction. The SBA-15 synthesized from mine tailings by extraction has the highest hydroxyl surface group density (22.4 nm-2) among other samples synthesized in this research (0.4-7.9 nm-2). The mesoporous materials were dispersed in toluene with organic functional group agents including thiol (-SH) and amine (-NH) groups to graft the silica surface. The functional group of the adsorbent surface increases the adsorption capacity, selectivity of the metal ions and the stability of the adsorbent. Chemically extracted mesoporous materials have higher amounts of functional groups than calcined samples due to their high surface hydroxyl group amounts as anchor points between the mesoporous silica and the functional groups. The adsorption studies were conducted by means of batch tests using metal solutions for separation of targeted metal ions. Adsorption capacity (Qe) is defined as the amount of adsorbate per unit mass of the adsorbent. The selectivity coefficient (Ks) represents adsorption selectivity to the targeted metal ions in multi components solution. These two values are the most important properties of the adsorbent. Selective adsorption characteristics for separation of metal ions such as Co(II) and Mn(II) from lithium ion batteries and Au(III) from CPU of laptops were conducted by the batch test. In this system, the adsorption capacities of functionalized SBA-15 for Co(II), Mn(II) and Au(III) ions were 9.7 mg/g, 6.8 and 216.5 mg/g and the selectivity coefficients of the functionalized SBA-15 were 46.3 (Co-Mn), 43.4 (Mn-Co) and 144.3 (Au-Cu), respectively. The adsorption capacities of functionalized adsorbents are approximately 3-10 times higher than that of as-synthesized SBA-15. For the regeneration of the functionalized mesoporous silica and the recovery of adsorbed metal ions, desorption tests of metal ions from adsorbents were performed using 2M HNO3 and regeneration efficiency of 90-97 % was achieved after 5 times of adsorption-desorption cycles.