Synthesis and use of magnetic nanoparticles for the adsorption of mercury from water

Abstract

This study used magnetite (Fe3O4) nanoparticles (NPs), mesoporous silica coated magnetite NPs (SCMNPs) and thiol functionalised silica-coated magnetite nanoparticles (SH-SCMNPs) for Hg(II) removal and recovery from water. The Fe3O4 NPs were prepared via conventional co-precipitation methods. Mesoporous silica coating was created on dense liquid-silica coated magnetite NPs (DLSC-Fe3O4 NPs) using cetyltri-methyl-ammonium chloride (CTAC) as molecular templates and followed by a sol-gel reaction. SCMNPs were functionalised with 3-MPTMS using the co-condensation method. Functionalisation of SCMNPs with this specific organic group was performed to enhance the selectivity of the magnetic NPs towards Hg(II). The characteristics of these particles were assessed at different stages in the production process. The hydrodynamic particle size distribution increased from an average diameter of ~75 nm for Fe3O4 NPs to ~105 nm after silica coating, and was found to be ~111 nm after functionalisation with thiol. The particles were found to be almost spherical with a uniform mesoporous structure with a pore size of ~2.1 nm. The particles were strongly responsive to an external magnetic field making separation from solution possible in less than 1 minute using a permanent magnet.Batch tests were used to evaluate the feasibility of the prepared NPs for the adsorption and desorption of Hg (II) from synthetic wastewater. SH-SCMNPs displayed a high removal efficiency for Hg(II) uptake, with 90% of Hg(II) removed during the first 5 minutes and equilibrium in less than 15 minutes. The adsorption efficiency was highly pH dependant. Adsorption was not affected by the majority of coexisting cations and anions under the conditions tested. 3 M HCl and thiourea in a 3 M HCl solution was an effective eluent for the desorption of adsorbed-Hg on SCMNPs and SH-SCMNPs respectively. This did not result in the destruction of the nanoparticles and they could subsequently be reused, without loss of their activity, in further adsorption tests. The adsorption characteristics of the particles were quantified in a series of isotherm experiments using Hg(II) solution concentrations of between 40 and 1000 ?g L?1 at adsorbent concentrations of 4 and 8 mg L-1. The adsorption capacity was higher than for other commonly used adsorbents. Both the Langmuir and Freundlich isotherm models were applied to the isotherm data and the maximum adsorption capacity was achieved when the ratio of adsorbent to adsorbate was low.A semi-continuous method for using the process at a lab scale was developed and was found to be successful in the removal and recovery of Hg(II) and confirmed the results of the batch experiments