Silica coated magnetite particles for magnetic removal of Hg²⁺ from water

Abstract

The magnetic removal of Hg²⁺ from water has been assessed using silica coated magnetite particles. The magnetite particles were first prepared by hydrolysis of FeSO₄ and their surfaces were modified with amorphous silica shells that were then functionalized with organic moieties containing terminal dithiocarbamate groups. Under the experimental conditions used, the materials reported here displayed high efficiency for Hg²⁺ uptake (74%) even at contaminant levels as low as 50 μg l⁻¹. Therefore these eco-nanomagnets show great potential for the removal of heavy metal ions of polluted water, via magnetic separation.

Introduction

Water pollution by trace heavy metals is well-known as a serious environmental and public problem. Among trace metals, there is a great concern about mercury pollution, which is due to its toxicity, persistent character in the environment and biota as well as bioamplification and bioaccumulation along the food chain. In the European Union mercury is considered as a priority and hazardous pollutant and the cessation or phasing out of mercury discharges, emissions and losses will be required by 2020 under Water Framework Directive (2000/60/CE).

In order to provide long term high quality water or to enable water recycling, there has been research in alternative remediation processes involving modified nanometer sized adsorbents, ion exchangers or systems that can also be of interest for bio-applications [1], [2], [3], [4], [5]. In this context several materials have been investigated such as carbon nanotubes, inorganic nanoparticles, zeolites, biopolymers and dendrimers [6], [7]. However, most of the published work deals with relatively high and environmentally unrealistic mercury concentrations (⩾20 mg l⁻¹) even knowing that mercury is very toxic to aquatic organisms in trace levels [8], [9], [10]. This is mainly because the determination of low mercury concentrations (<100 μg l⁻¹) is not a trivial task [11]. Analytical problems arise due to the fact that many chemicals and pharmaceutical preparations contain trace amounts of mercury and/or its compounds, and the presence of other ions in solution may interfere with the determination of Hg²⁺ [12]. Consequently, the development of efficient new materials and clean-up technologies for removing mercury from effluents is extremely urgent [13], [14], [15], [16], [17], [18].

Among the magnetic nanosized materials, iron oxides play a major role in many areas of chemistry, physics and materials science. In particular, magnetic iron oxides such as magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) have been investigated intensively for environmental and bio-applications [19], [20], [21], [22], [23], [24], [25], [26]. In addition to convenient magnetic properties and low toxicity and price, iron oxide (e.g. Fe₃O₄) nanoparticles exhibit high surface to volume ratios, depending on the particle size, which associated to their ability for surface chemical modification can show enhanced capacity for heavy metal uptake in water treatment procedures. Surface modification achieved by the attachment of inorganic shells or/and organic molecules not only stabilizes the nanoparticles, eventually preventing their oxidation, but also provides specific functionalities that can be selective for ion uptake. For example, high selective hollow mesoporous aluminosilicate spheres with nano-sized Fe₃O₄ cores are suitable for adsorption of Hg²⁺ [27]. Iron oxide nanoparticles dispersed in chelating resins or coated with adequate chelating agents have been used for the removal of a wide range of metal ions from wastewater [28], [29], [30], [31], [32], [33], [34], [35], [36], overall displaying higher adsorption capacity than traditional materials such as activated carbon [37], [38], [39]. More recently, silica coated Fe₃O₄ functionalized with γ-mercaptopropyltrimethoxysilane have been successfully applied for extraction of Cd²⁺, Cu²⁺, Hg²⁺ and Pb²⁺ in a wide pH range and even in the presence of foreign ions acting as interferents such as Al³⁺, Fe³⁺ and Cl⁻ [40]. Fe₃O₄ particles encapsulated in thiol containing polymers have also been reported and their efficiency to remove Ag⁺, Hg²⁺ and Pb²⁺ ions has been evaluated [41].

It is well-known that the dithiocarbamate (DTC) group has a strong affinity towards heavy metal cations and their chelates are remarkably stable in aqueous solutions [42], [43], [44], [45], [46], [47], [48], [49], [50], [51]. Thus silica gel grafted with DTC groups have been used to extract Co²⁺ [48], [52], Hg²⁺ [52], [53], [54], [55], Cu²⁺ [48], [54] and Cd²⁺ [48], [54] from aqueous solutions. Here we wish to report a new type of eco-nanomagnets that are composed of SiO₂ coated magnetite functionalized with DTC groups. In this context, we emphasize the exceptional efficiency of these systems for the Hg²⁺ uptake, even at trace levels found in natural waters, as compared to other materials reported in the literature.