Effects of magnesium hydroxide morphology on Pb(ii) removal from aqueous solutions

Abstract

In this study, magnesium hydroxide (MH) particles with distinct morphologies were obtained through direct precipitation and subsequent hydrothermal treatment with various magnesium salts. The synthesized products were systematically characterized and utilized for the removal of Pb(II) ions from aqueous solutions. The adsorption process of Pb(II) by two different MH structures, namely flower globular magnesium hydroxide (FGMH) and hexagonal plate magnesium hydroxide (HPMH), adhered to the Langmuir isotherm and pseudo-second-order model. FGMH exhibited higher Pb(II) removal capacity (2612 mg g⁻¹) than HPMH (1431 mg g⁻¹), attributable to the unique three-dimensional layered structures of FGMH that provide a larger surface area and abundant active sites. Additionally, metallic Pb was obtained by recycling the adsorbed Pb(II) through acid dissolution-electrolysis. Furthermore, Pb(II) removal mechanisms were investigated by analyzing adsorption kinetics and isotherms, and the adsorbed products were characterized. Based on the findings, the removal process occurs in two key stages. First, Pb(II) ions bind with OH⁻ ions on the surface upon diffusing to the MH surface, resulting in Pb(OH)₂ deposits in situ. Concurrently, Mg(II) ions diffuse into the solution, substituting Pb(II) ions in the MH lattice. Second, the resultant Pb(OH)₂, which is unstable, reacts with CO₂ dissolved in water to yield Pb₃(CO₃)₂(OH)₂. Therefore, owing to its outstanding Pb(II) adsorption performance and simple preparation method, FGMH is a promising solution for Pb(II) pollution.