3.1. Adsorbent characterization
3.1.2. Thermogravimetric Analysis
In order to observe the thermal stability of the substances, thermogravimetric analyzes (TGA) of Fe3O4, Fe3O4 @ SiO2 and Fe3O4 @ SiO2 @ OPhh2 were used. The TGA curves of Fe3O4 and Fe3O4 @ SiO2 nanoparticles slightly changed and the TGA curves(Fig. 2a) of Fe3O4 @ SiO2 @ OPhh2 nanoparticles change very suddenly with the increase in temperature. In the case of Fe3O4 nanoparticles, the weight loss was Approx. 4% which could be related to the adsorbed water and (/ or) structural water in the course of a temperature section of50°C to 800°C . The TGA curve of Fe3O4 @ SiO2 showed a weight loss about 2% before the temperature of 200°C due to the desorption of water from the surface layers of the silica, and the weight loss was increase 3% more at a temperature range of 200°C to 800°C % which were associated with structural water . For the TGA curves of Fe3O4 @ SiO2 @ OPhh2, organic polymers decompose rapidly between the temperatures of 200°C and 330°C and the water loss in the breakdown of the built-in magnet (OPPh2) are probably the reasons of a weight loss of about 5.80%.
3.1.3. XRD pattern
Figure 2b shows the XRD types of Fe3O4, Fe3O4 @ SiO2 and Fe3O4 @ SiO2 @ OPhh2 nanoparticles. Eight diffraction peaks (111, 220, 311, 400, 422,511,440 and 533) were examined, which suggests a spinel cube made exclusively from magnetite. The similar typical peaks were noted for Fe3O4 @ SiO2 and Fe3O4 @ SiO2 @ OPhh2 nanoparticles, suggesting that a crystalline structure of Fe3O4 was stable during silica coating and ultimately led to surface modification.
3.1.4. FT-IR spectroscopy
The peaks at 588 and 1635 cm−1 became the Fe-O vibration of Fe3O4. Attributed . The Fe-O peak in Fe3O4 @ SiO2 was thus fully confirmed. The presence of the peak at 3431 cm−1 is due to the stretching vibrations of OH band. The peak at 1081 cm−1 can be related to the asymmetric and symmetric stretching vibration of SiO2 bands and the Si-OH band is produced another peak at 798 cm−1 (Fig. 2c).
3.2. Adsorption investigations
3.2.1. Determination of the optimal pH value
In aqueous solution, the pH value is an essential factor parameter for the adsorption performance . In order to obtain the optimum pH, 0.01 g of adsorbent with 25 ml of palladium solution (2 mg L−1) were added to 15 containers. Each of the containers was then adjusted to the desired pH in the range of 1 to 9. The adsorption time were 3.5 hours and the shaking speed was 150 rpm. After the adsorption process the resulting solution was separated using a magnet and analyzed with ICP method. The result of the analysis showed (Fig. 3a) that the optimum pH for maximum adsorption of Pd ions (94.35%) was pH = 6. Therefore, for the next steps this pH value was used as the optimum pH.
3.2.2. Effect of the adsorbent dosage
The optimal amount of the adsorbent has an effect on the process costs of the adsorption. The influence of the amount of adsorbent on the Pd (II) removal ranged from 0.01 to 0.1 g. examined. The adsorbent was added to each container with 25 ml of palladium ions at a concentration of 3 mg L−1 and with the optimal pH (PH = 6) at room temperature. The adsorption has been proceeded in 3.5 hours with 150 rpm shaking speed. The test results are shown in Fig. 3b. The percentage of adsorbent dosage increased from 0.01 to 0.04 g, and a further increase in the adsorbent dosage from 0.04 to 1 g had no effect on the percentage of Pd (II) adsorption. The reason is an overdose of the adsorbent, while the number of active centers of the adsorbent remains constant [22, 23]. The result of the analysis shows that the best amount of adsorbent is 0.04 g at an adsorption percentage of 86.7. The recovery percentage (R %) of Pd(II) ions by Fe3O4/SiO2/OPPh2 was calculated using the concentration balance equation (3).
3.2.3. Determination of optimum metal concentration:
A range of Pd(II) concentrations including 0.5, 1, 1.5, 2, 2.5 and 3 mg L−1 have been tested to reach the maximum removal percent of it from a metal aqueous solution with a volume of 25 ml, PH = 6 and the optimal adsorbent content of 0.04 g. The adsorption time was3.5 hours and the shaker speed was 150 rpm. The obtained results from analysis of separated solutions are shown in Fig. 3c. As the Fig. 3c shows, the highest percentage of metal removal has reached for 2 mg L−1 of Pd(II) which is 89.85.
3.2.4. Desorption solutions and their concentration
3.2.5. Influence of the adsorption time:
After adsorption, three different desorption solution have examined to separate the Pd(II)ions from the adsorbent. The desorption solutions were 1M chloric acid, 1M ammonium nitrate, 1M nitric acid and 1M HNO3 + HCl mixture. Four sample solutin of Pd (II) ions (2mg/l) have been provided and added 0.1 g of adsorbent to them and placed in a shaker for 3.5 hours at room temperature. After the adsorption process the samples were filtered and the adsorbent of the Pd (II) ions remained on filter paper. A volume of 10 ml of each desorption solutions were used to wash the adsorbent and separate the Pd(II) ions. Each desorbent solution analyzed to determine the percentage of desorption. The obtained results are shown in Fig. 3d. 1 M HCl + HNO3 was the best desorption solution for Pd metal. A range of various concentrations of this mixture have examined (from 0.5 to 5 M) to improve the desorption percentage. 10 ml of different concentrations of mixed HCl + HNO3 were used to wash the same adsorbents which were prepared by the same way as before (0.04 g of adsorbent with 2 mg / l Pd ions in a volume of 25 ml and a pH of 6 stored in the shaker for 3.5 hours). The obtained desorption percentages are shown inFig.3e. The mixed HCl + HNO3 with a concentration of 0.5 M with 97.95% recovery were the best desorption solution for Pd (II) ions. The contact time of metal ions with adsorbent has an important effect on removal percentage of Pd(II) from the solution. 25 ml of pd(II) solution with a concentration of 2 mg L−1 mixed with 0.04 g adsorbent at pH = 6 and placed in a shaker at three different temperatures of 20, 30 and 40°C. In each cases the samples from the shaker were taken at different times (5, 8, 10, 15, 30, 45, 60, 75, 90, 120min) and analyzed to investigate the influence of contact time. The obtained results are shown in Fig. 3F.
3.2.6. Adsorption isotherms modeling and Thermodynamic properties
The adsorption of Pd (II) ions was investigated at different temperatures of 20, 30 and 40°C. The containers with a volume of 25 ml were used and the 0.04 g of adsorbent, a certain amount of metal solution (from 0.5 mg L−1 to 10 mg L−1) with pH = 6 were added to them. The mixtures were shaking in water bath with 300 rpm for 3.5 h and everything were the same at different temperatures. After removing the samples from the shaker, they were filtered with an external magnetic field and the filtered solutions were separated for analysis. The obtained data were examined with the adsorption isotherm models Jessons, Fritz Schlunder, Weber-Van Viet, generalized and the non-linear isotherm fit curves (Fig. 4). The results of the model parameters obtained are shown in Tables 1 and 2. Thermodynamic parameters indicate the endothermic, irreversible and non-spontaneous nature of the adsorption (Table 1 and 4). The concentration of the Pd (II) ions rose from 0.5 to 10 mg L−1 with an increase in temperature and the influence of the sorption capacities was shown by Pd (II) in equilibrium is shown in Fig. 3F and illustrates the importance of adsorption kinetics as an important factor between contact time and adsorption capacity. To display the experimental kinetic data, the diffusion types pseudo-first and pseudo-second order were analyzed at different temperatures and the associated parameters were calculated; the results are shown in Tables 1 and 3 and in Fig. 4. The influence of temperature on the rate constant of adsorption was investigated using the Arrhenius equation. (Table 1). The k0 of activation energy (Ea) is 8.06× 10 8 g. mg. min−1 with a C0 of 2.0 mg L−1. The magnitude of Ea indicates the adsorption mechanism, for the chemical adsorption Ea should be in the range of 40-800 kJ mol−1 and for physical adsorption it should be between 5 to 40 kJ mol−1 . The evaluated activation energy in this work was 59.96 kJ mol−1, which verifies the chemisorption as the dominant adsorption mechanism.
3.2.7. Effect of co-existing ions
The presence of foreign ions even in low concentrations through the matrix affect the spectrometric analysis of the ions in the trace range. Therefore, under optimal conditions for the adsorption of Pd (II) ions (5 mg L−1), the presence of interfering ions (5 mg L−1) such as Cd2 +, Ni2 +, Ca2 +, Co2 +, Fe3 +, Ba2 + and Pt4 + were determined by means of the adsorbent used (Fe3O4 / SiO2 / OPPh2). The interfering ions have very little influence on the Pd (II) uptake, as shown in Fig. 3g.
3.2.8. Prepare real samples
The sensible application of the presented method was investigated on the basis of the adsorption of palladium from the two different water samples. 25 ml of the tap water and spring water samples at optimized conditions were exposed to the addition of 0.2, 0.4 and 0.6 mg / L Pd (II) ions. The obtained results for the two samples (with and without addition of the Pd (II) ions) are shown in Table 5.