1. Introduction
The release of heavy metals in aqueous systems is of severe concern due to their hazardous effects on human and the environment. Heavy metal pollution exists in the aqueous waste stream from several industries such as metal plating, battery manufacture, pharmaceutical, mining, tanneries, and painting, as well as farming sources where fertilizers and fungicidal spray are intensively applied [
1]. Several conventional techniques have been reported to remove metal ions from aqueous solutions, such as oxidation, reduction, precipitation, membrane filtration, ion exchange, and adsorption. Among these methods, adsorption is the most favorable process, economically and technically, for removing heavy metals from aqueous solutions [
2,
3]. Recently, Fe
3O
4 nanoparticles were shown to be highly efficient materials for heavy metal ion removal by adsorption; metal ion adsorption by magnetite was demonstrated through a combination of electrostatic attraction and ligand exchange [
4,
5,
6]. Fe
3O
4 nanoparticles can be rapidly and easily separated from aqueous solutions using an external magnetic field due to their magnetic property and have some advantages of sensitivity and high efficiency. Therefore, adding Fe
3O
4 nanoparticles to the adsorbent is an excellent way to resolve separation problems [
7], but there are some challenges that Fe
3O
4 nanoparticles present. The first one is that Fe
3O
4 nanoparticles oxidize and dissolve easily. Also, the recycling process is difficult due to the small size of nanoparticles. Finally, the nanoparticles tend to co-aggregate and thus the effective surface area decreases, reducing their reaction activity. In order to protect Fe
3O
4 nanoparticles, metal, polymer and a silica shell were used [
8]. Several techniques have been developed to minimize the co-aggregation of the Fe
3O
4 nanoparticles and improve their manipulation, for example using polymers and clays [
9]. Moreover, clay soils are widely used as adsorbents that isolate hazardous and other waste materials from surrounding environments.
There are few studies on the adsorption characteristics of heavy metal ions in talc. Talc is known by the chemical formula Mg
3Si
4O
10(OH)
2. It consists of a magnesium hydroxide layer (MgO∙H
2O) sandwiched between two silicate layers (SiO
2), forming a three-layer structure [
10]. Adjacent layers are connected by weak van der Waals forces, providing talc a platy structure. The low energy silicate layers of talc planes, [001] crystal domains, have hydrophobic properties, while the edges displaying the hydroxyl groups (–SiOH) and (–MgOH) are more hydrophilic [
11,
12]. Talc is commonly used as a filler, coating and dusting agent in paints, lubricants, plastics, cosmetics, pharmaceuticals, and ceramics manufacture [
13]. In this work, Fe
3O
4/talc nanocomposite was used for removal of Cu(II), Ni(II) and Pb(II) ions from aqueous solutions and response surface methodology (RSM) was applied for optimization study and screening variables effects on ion removal.
In traditional methods, one variable at a time is employed for monitoring the effect of functional variables. In this optimization method, the analyzed parameter is changed; others are kept at a fixed level. This technique cannot evaluate interactive effects between the variables and uses a large number of experiments, which is time consuming and costly [
14]. Multivariate statistical methods have been preferred to identify the perfect combination of factors and interactions among elements, which are not possible to recognize using the one variety method [
15] . Additionally, these techniques are very beneficial tools to save the time and reduce the cost of research. The actual design consists of estimation of the coefficients in a mathematical model, predicting the response, and checking the adequacy of the model. Essentially the widely used designs to find out response surfaces are factorial designs and the more complex response surface methodologies [
16,
17,
18].
In the present work, the ability of a hybrid material consisting of the talc sheets as support of magnetite particles, for removal of Cu(II), Ni(II), and Pb(II) from aqueous solutions was studied. The metal adsorption capacity of the talc can be manipulated by adding Fe
3O
4 nanoparticles, which increase the amount of heavy metal uptake. The adsorption experiments were performed and the influence of heavy metals ion concentration, removal time and adsorbent amount were analyzed by the response surface methodology. To the best of our knowledge, there are not examples in the literature dealing with the removal of Cu(II), Ni(II) and Pb(II) by Fe
3O
4/talc nanocomposite as an adsorbent. Furthermore, the synthesized of magnetite/talc nanocomposite has not been reported except in our previous study [
19]. The objective of this study was therefore to examine the ability of Fe
3O
4/talc nanocomposite to remove heavy metal ions from aqueous solution according to response surface methodology design.
3. Experimental Section
3.1. Materials and Methods
All reagents in this work were of analytical grade and used as received without further purification. Ferric chloride hexahydrate (FeCl3·6H2O) and ferrous chloride tetrahydrate (FeCl2·4H2O) of 96% were used as the iron precursor and also, talc powder (<10 μg, 3MgO·4SiO2·H2O) were obtained from Sigma-Aldrich (St Louis, MO, USA). NaOH of 99% was obtained from Merck KGaA (Darmstadt, Germany). Copper chloride, lead nitrate (II), and nickel chloride were supplied by Hamburg Chemical (Hamburg, Germany). All aqueous solutions were prepared with deionized water.
3.2. Fe3O4/Talc Nanocomposites Preparation
The chemical co-precipitation technique has been used in preparation of nano particles. For the synthesis of Fe
3O
4/talc nanocomposites, 2.0 g of talc was suspended in 120 mL deionized water, and then a solution of Fe
3+ and Fe
2+ with (2:1) molar ratio was added into the mixture. The ion solution suspended with talc composites were stirred for 24 h for impregnation by the external surface of talc layers to prepare talc/Fe
3+–Fe
2+ composites. Then 25 mL of freshly prepared NaOH (2.0 M) was added to talc/Fe
3+–Fe
2+ composites suspension under continuous stirring. The suspensions were finally centrifuged, washed twice with ethanol and distilled water, and kept in a vacuum stove at 100 °C. All the experiments were conducted at an ambient temperature and under a non-oxidizing oxygen free environment through the flow of nitrogen gas [
33].
3.3. Adsorbate
Three stock solutions (1000 mg/L) of Cu(II), Ni(II) and Pb(II) ions were prepared by dissolving appropriate amounts of copper chloride, nickel chloride, and lead nitrate in deionized water and then transferred to 1-liter volumetric flasks and diluted with deionized water. The stock solutions were then diluted with deionized water to obtain the desired concentration range of Cu(II), Ni(II) and Pb(II) standard solutions.
3.4. Experimental Procedures
Different amount of nano-adsorbent according to RSM design was mixed with 25 mL solution of single metallic ions of Cu(II), Ni(II) and Pb(II) using a shaker at the temperature of 25 °C. After that, the suspension was magnetically separated from the aqueous solution and the residual concentration of metal ions in the solution was analyzed. The heavy metal ion concentration in the filtrate was determined using AAS (Atomic Absorption Spectrophotometry, S Series; Thermo Scientific, Waltham, MA, USA).
The response removal efficiency of heavy metal ions was calculated as Equation (8):
where
Y is the percentage of adsorption, C
0 is the initial concentration of heavy metal ions (mg/L) and
Ct is the concentration of heavy metal ions at time
t.
All experiments were carried out in triplicate and the mean values are reported. The maximum deviation was found to be ±2%.
4. Conclusions
In this work, a new nanoadsorbent was successfully used for heavy metal removal. Such synthesized adsorbent has not only a unique structure with a large surface area, but also a superparamagnetic character. These features make it an effective and convenient adsorbent for heavy metals removal. Response surface methodology and central composite rotary design were appreciable in determining the optimal conditions for adsorption. In addition, the amount of sorption of metal ions on nanoadsorbent increased with increasing adsorbent dosage. The adsorption kinetics abides by pseudo second order kinetic equation, and the Langmuir isotherm fitted well with the adsorption process. The prepared adsorbent performed in neutral actual pH condition to prevent precipitation. In addition, this adsorbent could be adsorbed by an external magnetic field after heavy metal ion adsorbing. A rapid sorption of Cu(II), Ni(II) and Pb(II) was found on the Fe3O4/talc nanocomposite during less than 2 min. Moreover, In aqueous solutions, the high concentrations of heavy metal ions (100, 92 and 270 mg/L) for Cu(II), Ni(II) and Pb(II), respectively were adsorbed in the very low level amount of adsorbent (around 0.12 g).
The actual results were in good agreement with the predicted data by models. Experimental results show that the use of Fe3O4 nanoparticles for the heavy metal ion removal is technically achievable, environmentally friendly, and economically attractive for the treatment of water. Compared to conventional separation, the advantages of adsorption followed by magnetic separation are attributed to its rapidness, effectiveness, and simplicity.