Surface functionalized magnetic α-Fe₂O₃ nanoparticles: Synthesis, characterization and Hg²⁺ ion removal in water

Abstract

Mercury is a specific concern worldwide because of its high toxicity. Exposure to mercury, even at very low concentrations, induces digestive, brain, kidney, neurological and endocrine system diseases. In this work, Fe₂O₃ nanoparticles were synthesized via one step solution combustion synthesis using glycine as a fuel. The structural features of the as formed Fe₂O₃ nanoparticles were characterized by XRD, SEM, EDX and FTIR. Further, the surface of the prepared Fe₂O₃ nanoparticles were coated with silica groups followed by thiol(-SH)functionalization using l-cysteine by ultra-sonication technique. The surface functionalized magnetic nanoparticles were analyzed by FT-IR technique, to confirm the –SH functionalization. The functionalized magnetic nanoparticles of various amounts (0.005 g, 0.010 g. 0.015 g, 0.020 g) were used for removal of Hg²⁺ ion in 50 ppm aqueous solution. After the adsorption process is over, the Fe₂O₃@SiO₂@SH nanoparticles extracted using strong magnet and the resultant Hg²⁺ ion in water is studied by complexing with dithizone ligand using UV visible spectroscopic technique. The disappearance of absorbance peak corresponds to [Hg(dithizone)₂]²⁺ at 490 nm, for 50 ppm Hg²⁺ ion solution when 10 mg of the Fe₂O₃@SiO₂@SH nanoparticles used, confirms that about 98% Hg²⁺ ion in water is removed.

Introduction

Mercury (Hg) is one of the most toxic metals found in the environment, especially in water. Most of the mercury contamination occurred as a result of human activities including the burning of fossils, compost incinerators, mining and extraction of Hg from the cinnabar, the chlor-alkali industries [1].The pulp and paper industries, plastic industry, paints, fungicides, electronic industry, nonferrous metal smelting, cement production, dye chemicals industry, battery industries medical devices etc. were also considered as sources for mercury ion [2,3]. Exposure to mercury, even at very low concentrations, induces toxicity to the central nervous system, kidney, lung tissues, and reproductive system [4]. This widespread problem of mercury in water is jeopardizing human health. Therefore, it is very much essential to address this issue immediately.

Nanoparticles (NPs) are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures. Nanomaterials such as iron oxide nanoparticles (IONPs) are attractive for removal of heavy metals from the water due to their important features like small size, high surface area, and magnetic property [5,6].One of the major issue in the case of iron oxide particles is rapid agglomeration which decreases the surface area adsorption capacity [7] and magnetism [8] . Due to their large surface-to-volume ratio, magnetic iron oxide nanoparticles possess high surface energies and therefore tend to adhere to each other in order to reduce their surface energy and form clusters thus resulting in increased particle size [9]. As the material size decreases, a major portion of the atoms are found at the surface compared to those inside. The molecules at the surface do not have full allocation of covalent bonds and are in an energetically unstable state. Also nanomaterials are more reactive than the bulk material. With the high reactivity, almost all types of nanomaterials tend to agglomerate into bigger particles. Particle-Particle interactions are the major driving forces for aggregation and deposition of nanoparticles. These interactions can be described by the Derjaguin-Landau-Verwey- Overbeek (DLVO) theory of colloidal stability. The DLVO theory states that the stability of nanoparticles can be explained by the sum (total interaction energy) of vander Waals and electric double layer interactions [10]. When nanoparticle approaches another particle, it experiences total interaction energy and this energy determines whether the net interaction between particles are repulsive or attractive [11]. Magnetic nanomaterials exhibit magnetic dipole moment and the contribution of the magnetic force may dominate the total particle-particle interaction energy, thereby leading to aggregation [10].

Surface modification of IONPs through coating method prevents the oxidation and agglomeration of IONPs and also provides the possibility for further functionalization [12]. Also the presence of adsorbed hydroxyl groups on the surface of magnetic nanoparticles allows the attachment of different functionalities [13]. Among the various coating materials, silica is the best studied and most commonly used substance. The silica layer increases the chemical stability and biocompatibility of magnetic nanoparticles and also serves as a platform for surface functionalization of magnetic nanoparticles [14,15]. Functionalization by thiol groups will enhance the adsorption capacity due to its great affinity towards metal ions. L-cysteine is a sulfur-containing amino acid having three functional groups (-SH, -NH₂, -COOH), which has strong tendency to bind with heavy metal ions [16], [17], [18] . L-cysteine functionalized magnetite synthesized by co-precipitation was effective for the adsorption of lead and chromium ions via binding with the amine group (-NH₂) of L-cysteine [19]. L-cysteine functionalized magnetite which was fabricated continuously used as an adsorbent for lead and cadmium ions via the interaction with sulfhydryl and amine groups in L-cysteine [20]. The thiol group (-SH) in L-cysteine have high affinity towards mercury ions.

Many type of magnetic iron oxide nano-adsorbents are reported for mercury removal like magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) nanoparticles via chemical precipitation using Aloe Vera as stabilizing agent [21], silica coated CoFe₂O₄ functionalized with EDTA [22], polyethylene glycol (PEG)-coated Fe₃O₄ nanoparticles [23], silica coated Fe₃O₄ nanoparticles functionalized with 3-mercaptopropyltrimethoxysilane (3-MPTS) [24], silica coated Fe₃O₄ nanoparticles functionalized with Trimethoxysilyl-propanethiol (TMMPS) [25]. Among various preparation methods of iron oxide nanoparticles, solution combustion synthesis (SCS) having advantages including short reaction time, no need of inert gas and vacuum, low cost, easy preparation and higher yield [26]. In this work, firstly, we synthesized Fe₂O₃ nanoparticles (INOPs) via SCS method using glycine fuel. Secondly, the as formed IONPs are silica coated followed by thiol via ultra sonication method. The prepared magnetic adsorbent was characterized by using suitable techniques and further used them effectively to remove about 98% of mercury from the water.