1. Introduction
Heavy metals exist naturally in the environment, but anthropogenic activities elevate the concentration of these elements [
1]. These elements cause detrimental effects on human health as well as on the biotic and abiotic environment causing several diseases in living organisms and lowering the quality of water. Pollution caused by heavy metals in the water bodies is a serious environmental threat, as they are highly toxic even at low concentrations, tend to bioaccumulate in tissues, and are non-biodegradable [
2]. Effluents from industries, sewage systems, and tanneries contaminate water bodies. Among the commonly found heavy metals in water bodies, cadmium toxicity is one of the most prevalent problems globally, and its intake causes chronic pulmonary diseases, renal failure, and prostate diseases. Cadmium toxicity also causes several syndromes, liver impairment, and mutation in genes that causes problems in the development of the fetus [
3,
4]. Cadmium poisoning is a global health-related problem and is considered harmful to multiple organs. Cadmium exposure may occur through water, food, air, and even soil [
5]. Some drugs and nutritional supplements may also lead to Cd contamination [
6]. Its chronic exposure can directly affect the central nervous, cardiovascular, respiratory, reproductive, and excretory system and may cause cancer [
5]. Cadmium exposure occurs through consuming contaminated food and water, leading to longstanding health problems.
Many classical methods have been used to reduce the contamination caused by heavy metals, including membrane separation, filtration, electrochemical treatment, ultrafiltration, ion exchange as well as adsorption [
7]. Solid-phase extraction is widely employed for heavy metal remediation for several samples, including aqueous media [
8]. Sorbents for the removal of heavy metals include a variety of materials, clays, bio-composites, and activated carbons [
9]. These materials are not generally used for the adsorption process due to low efficiency, smaller adsorption capacity, and unavailability in the bulk form [
10]. To cope with these limitations and ensure safe methods for conditioning wastewater, nanoparticles, and nanocomposites are widely used, such as composite made up of iron oxide chitosan zero-valent iron nanocomposites and activated carbon, silica-coated iron oxide nanocomposites, which are effective due to their high adsorption capacity [
11,
12].
Magnetic solid-phase dispersion is widely used for the remediation of heavy metals from waste waters using nanosorbents as these particles provide high surface area, offers higher adsorption capacity, and are cost-effective materials that efficiently extract heavy metal ions even if present in trace amount [
7,
13]. Iron oxide-based nanoparticles have a great potential for the removal of contaminants because they are cost-effective, easily synthesized, and modified [
14]. Properties of iron oxide nanoparticles include a higher surface area-to-volume ratio and less toxicity in nature; they are chemically inert, biocompatible, and super-paramagnetic in nature [
15]. Iron oxide nanoparticles attained prime importance due to easy separation of particles from sample solution when an external magnetic field is employed for regeneration [
16]. Several methodologies have been used to synthesize nanoparticles, including co-precipitation, emulsion, sol-gel process, hydrothermal, and chemical vapor deposition, to obtain desired properties, tunable size, structure, and shape [
17].
The co-precipitation process for the synthesis of iron oxide nanoparticles is the easiest and quite proficient chemical route. Synthesis of magnetite typically takes place by mixing a stoichiometric mixture of ferric and ferrous salts in aqueous media [
18]. By regulating pH, temperature, nature of salt used, and ionic strength, the size and structure of nanoparticles are easily tuned. The co-precipitation process involves the synchronized procedures of growth coarsening, agglomeration, and nucleation process [
19]. However, naked iron oxide nanoparticles are prone to oxidation and form aggregates [
20]. Surface modification of magnetic nanoparticles with a particular ligand enhances the selectivity and makes it a suitable sorbent. Iron oxide nanoparticles are modified by different methods, for example, the addition of functionalities such as carboxylic group, aldehydes, doping of metal ions, and coating of organic polymers, surfactant, and silica [
21]. Inert coating of materials such as SiO
2 for surface modification of magnetic iron oxide nanoparticles is employed, which inhibits the formation of aggregates in liquid media and enhances chemical stability. Metal iron oxide nanoparticles have adsorptive properties, which can be improved by adding functional groups to their surface. Iron oxide coated with silica results in less agglomeration, higher stability, and minimum cytotoxic effect [
22]. Different silanes are employed for introducing silica on iron oxide nanoparticles, such as tetraethyl orthosilicate (TEOS), aminopropyl silane (APTES), sodium silicate, etc. [
23,
24].
Analytical data are usually accompanied by several known and unknown errors called uncertainties. The presence of such errors leads to the dispersion of results; thus, their exact estimation is of prime importance. So far, the estimation of uncertainty has been applied to the analytical results only [
25]. In the case of the determination of trace analytes, validation through the estimation of uncertainty is considered a unique tool, which can be performed either by bottom-up or top-bottom approach. The former approach measures all possible uncertainty sources individually, while the latter combines all sources together [
26,
27,
28].
In this work, amine functionalized Fe
3O
4@SiO
2 nanoparticles were utilized as sorbents for fast and efficient preconcentration of Cd
2+ ions. Synthesized nanosorbent was characterized by Fourier transformed infrared (FT-IR) spectroscopy, SEM analysis, and XRD spectroscopy. Magnetic properties, zeta potential, and hydrodynamic size of the sorbent were also studied. Functionalization of the sorbent enhanced its stability and adsorption capacity. The Plackett–Burman design was utilized to find out the optimum conditions for the adsorption of Cd
2+ ions via response surface methodology [
29]. Finally, synthesized nanosorbent was applied for preconcentration of Cd
2+ ions in different water and food samples.
2. Experimental Procedure
2.1. Reagents and Solutions
All reagents utilized were of analytical grade. Fe(NO3)3.9H2O, FeSO4.7H2O, NaOH, tetraethyl orthosilicate (TEOS), 3-aminopropyl silane (APTES), ethanol, ammonium hydroxide, potassium hydroxide, and hydrochloric acid were purchased from Merck (Darmstadt, Germany) and used without further purification. A standard solution of Cd2+ of 1000 mgL−1 was purchased from Fluka Kamica (Buchs, Switzerland).
2.2. Instrumentation
For FT-IR analysis, PerkinElmer UATR Spectrum Two™ was used. The crystalline structure and phase of IONPs were analyzed by using PANalytical X-ray diffractometer model 3040/60 X’Pert PRO operated at 45 kV and 40 mA source having CuKα (λ = 1.54 Å) radiation at step width of 0.02° over angle range of 10–80°. SEM analysis of iron oxide and functionalized iron oxide nanoparticles was carried out by JSM5910 manufactured by JEOL, Japan, with the energy of 30 kV, whose maximum magnification is 300,000× with the maximum resolving power of 2.3 nm.
Dynamic light scattering and zeta-potential measurements were performed via Zetasizer Nano ZSP from Malvern Instruments (Malvern Panalytical Ltd, Malvern, Worcestershire, UK) with the aid of Malvern Zetasizer software (v7.13). The specific surface area of Fe3O4@SiO2@APTES was assessed through BET (Autosorb-iQ-MP/XR by Quantachrome Instruments, Anton Paar QuantaTec Inc. Florida, Boynton Beach, FL, USA) along with N2 as adsorbate at 77 K. Magnetic parameters were investigated through vibrating sample magnetometer (Model-EV9) with ±15 KOe applied field at 25 °C. For the determination of preconcentrated Cd2+ ions flame atomic absorption spectrometer (PerkinElmer AAnalystTM 700, (PerkinElmer Inc., Waltham, MA, USA) was used.
2.3. Synthesis of Surface Modified Iron Oxide Nanoparticles (Fe3O4@SiO2@APTES)
2.3.1. Synthesis of Iron Oxide Nanoparticles
Synthesis of iron oxide nanoparticles was carried out by using the co-precipitation method. The aqueous solution with 0.02 M of Fe(NO
3)
3.9H
2O and 0.01 M was prepared. The solutions of 0.02 M of Fe(NO
3)
3.9H
2O and 0.01 M FeSO
4.7H
2O were mixed together in a ratio of 2:1. The solution was continuously stirred for better homogeneity at 80 °C for 30 min. Then, the precipitating agent NaOH (3 M) was added dropwise to the solution. The stirring is continued for 2 h at 80 °C at pH 11. Synthesized precipitates were filtered and washed several times with distilled water. The nanoparticles were dried overnight at 80 °C in the electric oven. The nanoparticles were obtained in bulk form and were ground with the help of a pestle and mortar [
2,
15]. A percent yield of 95.7% was obtained for Fe
3O
4 through the co-precipitation method.
2.3.2. Surface Modification of Iron Oxide Nanoparticles
Tetraethyl orthosilicate (TEOS) and ethanol were dissolved in a proportion of 1:1. 1 g of Fe
3O
4 was added to a 15 mL suspension of TEOS and ethanol. Ammonium hydroxide was added to this dispersion to adjust pH 11. The mixture was continuously stirred magnetically for 24 h. The product obtained was silica-coated iron oxide nanoparticles (Fe
3O
4@SiO
2) which were washed numerous times with ethanol and water. The product was dried at 60 °C in an electric oven [
30]. The yield for Fe
3O
4@SiO
2 at the end of this step was 91.8%.
2.3.3. Functionalization of Fe3O4@SiO2 with APTES
Briefly, 1 g of silica-coated iron oxide nanoparticles was dispersed in a mixture of ethanol, water, and APTES taken in a ratio of 1:1:1. The mixture was stirred for 5 h at 50 °C. pH of the mixture was adjusted to 11 by adding 0.2 M solution of KOH dropwise. The product obtained was washed with a mixture of ethanol and distilled water several times. Amine functionalized silica-coated iron oxide nanoparticles obtained were dried at 60 °C in an electric oven. The overall percent yield observed was 89.7% for Fe3O4@SiO2 @APTES.
2.4. Microextraction Procedure for Cadmium Ions
The extraction of Cd2+ ions from aqueous solutions was studied in batch mode adsorption. During the sorption step, 50 mgL−1 of Cd2+ solution was used. pH of the Cd2+ solution was adjusted using a 0.1 M solution of sodium hydroxide and hydrochloric acid which was added dropwise. The synthesized sorbent was added to the analyte solution, which was sonicated for a few minutes to ensure complete adsorption of Cd2+ from the solution. Finally, the beaker containing the mixture was left undisturbed until the sorbent aggregated and settled at the bottom of the beaker. Clear solution was disposed of. Then 0.1 M HCl was added to the sorbent for elution of Cd2+ ions, and the mixture was shaken and kept under the influence of a magnetic field to aggregate the sorbent. The clear solution obtained was taken out and analyzed by FAAS to determine the amount of Cd2+.
2.5. Experimental Design Methodology
To study the effect of variables that influence the process of adsorption, each factor must be studied in correspondence with non-linear effects and the interaction among these factors. Multivariate experimental design, in comparison with traditional approaches, permit the optimization of more than one variable simultaneously. This procedure is quick as the number of experiments is reduced, making it cost-effective as well.
2.5.1. Plackett–Burman Design
Plackett–Burman design is an optimization tool for screening significant factors which have an impact on the efficacy and potency of the proposed technique. Minitab software v17.1 (Minitab Inc., State College, PA, USA) was used for designing the experiment. This design yields a proficient, prompt, and potent optimization approach contrary to the univariate process. The two levels of Plackett–Burman design with a set of twenty-four experiments were used to identify the optimum factors. The empirical data were evaluated using Minitab 17.1.
Lower (−) and higher (+) levels of factors and results obtained during their optimization through the Plackett–Burman matrix are shown in
Table 1a and b, respectively. A standardized pareto chart is devised to illustrate the results of the Plackett–Burman design (
Figure 1). Parameters that have an impact on adsorption efficacy on the devised procedure are evident and were identified where the absolute magnitude of given parameters are represented by the horizontal bars. The fitted quadratic response model is shown below as Equation (1):
Here “y” is the predicted response, while xi and xj are coded values of independent factors. Regression coefficients for intercept, linear, quadratic, and interaction terms are shown as β0, βi, βij, and βii, respectively, while “ε” is the random error.
2.5.2. Central Composite Design
The CCD method involves the combination of a two-level factorial design and some extra points, i.e., star points with at least one central point of the experiment. Many different points, such as rotatability and orthogonality, are obtained from this reference central point to adjust quadratic equations. The central composite design is the most recognized experimental design for second-order models, as it allows one to draw a conclusion with a small set of experiments and provides good results. In this study, Minitab 17.1 version and STATISTICA were used for analyzing data obtained from experimental design and response obtained.