EFFECTS OF ACTIVE SILICA ON STABILIZATION REACTION OF COPPER AND NICKEL IONS BY MAGNESIA

Abstract

Magnesia (MgO) is gaining an interest as an agent for stabilization of some heavy metals in the contaminated soils. MgO also reacts with silica (SiO 2 ) and water at room temperatures and atmospheric pressures to form layer silicates. The presence of SiO 2 may affect the stabilization of heavy metals by MgO. To investigate the effects of SiO 2 , we mixed MgO, SiO 2 , and solutions of Cu(NO 3 ) 2 or Ni(NO 3 ) 2 at a solid : solution ratio of 1 kg : 2 L. The reaction products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and chemical analyses. Suspensions immediately turned alkaline in both systems due to hydration of MgO. XRD and FT-IR results showed that copper nitrate hydroxide (CU 2 NO 3 (OH) 3 ) or nickel hydroxide (Ni(OH) 2 ) as well as brucite (Mg(OH) 2 ) precipitated just after mixing and they gradually disappeared to form poorly ordered layer silicate minerals. The Cu and Ni concentrations were kept low throughout the experiments. We interpreted the experimental results as indicating that a part of the Cu and Ni were incorporated in octahedral sites of the newly formed layer silicate minerals. The sequential extraction with water and MgCl 2 solution indicated that the amounts of extractable Cu and Ni were larger for the products from heavy metal-MgO-SiO 2 systems than for heavy metal-MgO systems. The results show that the presence of active SiO 2 is a little detrimental to the stabilization of Cu and Ni by MgO in a short term but may be advantageous in a long run.