Introduction:
Today what has attracted more than each other category of human thoughts is the problem of environmental pollution by heavy metals, which affect the health of humans and humans due to their lack of absorption and the effects of physiologic effects at low concentrations. Cadmium has been identified as a carcinogen due to severe toxic effects on human organs. The sources of cadmium inputs to aqueous solutions include wastewaters from chemical fertilizers, pesticides, mines, melting, building batteries, pigments, stabilizers, and alloys, plating, sewage sludge, plastics and synthetic rubber.The daily intake of this substance is 10-35 μg / liter. Smoking is another source of cadmium exposur. According to Standard 1053 of Iran, the maximum allowable cadmium concentration in drinking water, based on the average daily consumption of drinking water equal to 2.5 liters, for a human with 70 kg is 0.005 mg/l. According to the World Health Organization's 1996 standard, cadmium concentrations in drinking water were limited to 0.003 mg /l. Newadays, There are several removal processes for heavy metals from wastewater such as; chemical deposition, membrane processes, ion exchange, coagulation, flocculation and absorption. However, absorption method is one of the most effective and efficient methods. In fact, it can be argued that in many cases protective shells not only prevent the oxidation of iron, but can also be used for more function. Magnetic oxide nanoparticles are most often used in aqueous solutions due to their many advantages such as high surface to volume ratio, very small size, high reactivity, and better absorption of toxins and heavy metals. But one of the main challenges of pure iron oxide nanoparticles is their high chemical activity, which makes it easy to oxidize against air (especially magnetite) or in acidic aqueous media. This will result in the loss of magnetism and the dispersion of iron oxide nanoparticles. Therefore, in order to prevent this occurrence, providing the appropriate coverage level and developing some effective protection strategies to maintain the stability of magnetic iron oxides is very necessary.Protective shields (especially silica), in addition to stabilizing iron oxide nanoparticles, have the benefits of preventing the accumulation of iron nanoparticles, increasing the compatibility of magnetic nanoparticles with solid matrices in the environment, creating graphene and bonding more nanoparticles with organic ligands, reducing the release rate of the oxygen molecule To the magnetic nuclei and to stabilize the crystalline structure of nanoparticles at high temperatures.
Materials and methods:
A two-step method was used to prepare magnetic nanoparticles coated with silica. In this study, cadmium adsorption was performed on adsorbent Fe3O4 @ SiO2 iron oxide mesopes in a closed reactor within 100 ml echinoderms. In the first step, the iron oxide nanoparticles were synthesized via a simple co-precipitation and then, in the second step, the tetraethyl ortho-silicate was used to coating it. For this purpose, 22 g of Fe2Cl3.6H2O and 8 g of FeCL2.4H2O were mixed to 400 mL of deionized water and the resulting mixture was stirred for 1 h in the presence of nitrogen gas at 80 ° C. Then, 15 mL NH4OH (25 % w/w) was added drop wise to reach pH 9. The produced nanoparticles were collected from the solution using an external magnetic field (1.2 Tesla) and washed several times with ethanol and deionized water and then, dried in an oven at 60 ° C for 24 hours. In the second step, a tetraethyl ethoxylan (TEOS) material was used to mixter of Fe3O4. therefore, the nanoparticle was synthesized in the previous stage with 15 ml of TEOS, 6 ml of ammonia (25%), 80 ml of ethanol (65%) and some of the deionised water injected into the reactor. At the same time, a litttle amount of ammonia used, because it reduces the size of the nanoparticles and increases the number of free groups of silanol (Si-OH) on the nanoparticle surface and activates them. Finally, the supernatant was isolated by magnet and washed five times with distilled water and ethanol and then, dried in an oven at 60 ° C for 24 hours. X-ray diffraction (XRD), infrared spectrometer (FT-IR), transient electron microscopy (TEM) and scanning electron microscope (SEM) were used to study absorbent properties.
Discussion of the Results:
The calculation of the adsorbent isoelectric point (pHZPC) is an essential part of the adsorption process. The isoelectric point is called the pH where the adsorbant has a neutral load. At this point, due to the low electrostatic interactions between adsorbent and adsorbent, the amount of absorption of pollutants is reduced. However, at pH <pHZPC, adsorption load is negatively charged due to the presence of H + ions and at pH> pHzpc the adsorption load is negatively affected by the presence of active hydroxide ions (OH). The pHZPC of Fe3O4 @ SiO2 nanoparticles is calculated to be about 3.26. Therefore, according to the above, it can be stated that the adsorbent has a negative charge in pH>3/26. The maximum removal percentage and its adsorption capacity were 72.86% and 33 mg/g at pH 5, respectively. In this regard, pH 5 was used to continue the experiments. The cadmium adsorption was investigated at interval of 10–60 min. The results indicated that cadmium adsorption increased with increasing contact time. In the present study, iron oxide magnetite nanoparticles coated with silica as adsorbent in the cadmium adsorption process were used. Then, the effect of important pH and contact time factors on the removal of cadmium from aqueous solutions and kinetic, isotherm and thermodynamic models were investigated. According to studies, It was found that the adsorption reaction rate increased by the adsorbent at initial times. The main reason is the availability of more active cadmium sites on the adsorbent surface, which over time will increase the amount of cadmium removal from the solution.The values of Langmuir and Freundlich isotherms parameters of the cadmium adsorption process by Fe3O4 @ SiO2 are shown in Table 2. The results showed that regression coefficients of Langmuir and Freundlich isotherm models were determined to be 0.984 and 0.957 ,respectively.The values obtained from the kinetic parameters of the first-order, second-order kinetics model are shown in Table 3. This suggests that the adsorption of cadmium on the adsorbent surface is more consistent with and consistent with the Langmuir isotherm model. In order to analyze the results of the cadmium uptake process at the synthesized absorbent level, first-order and pseudo-second order kinetics models were used. According to Table 3, the regression coefficient in a pseudo-second-order kinetic model is higher than that of the pseudo-first kinetics (R2 = 0.991). In addition, in this study, by increasing the solution temperature from 25 to 45 degrees Celsius, the percentage of cadmium removal increased 72.3 to 78.12 percent. Increasing the absorption of pollutants at high temperatures can be due to the combined desire of adsorption sites for metal ions. In this way, the increase in temperature leads to an increase in kinetic energy and an increase in the number of collisions between the adsorbent and the absorbent and decreases the viscosity of the soluble, thus facilitating the addition of absorbed ions to absorption positions . Based on experimental results and regression coefficient, cadmium adsorption on Fe3O4@SiO2 nanoparticles has been followed by a pseudo second-order kinetic and Langmuir isotherm. Finally, the thermodynamic studies showed that the adsorption process is of endothermic and spontaneous nature.
conclusion:
Morphology analysis showed that Fe3O4@SiO2 nanoparticles had a size of 15 nm approximately. The results of adsorption kinetics indicate that cadmium adsorption on adsorbent follows a quadratic kinetic model. The obtained optimal conditions for the cadmium adsorption were included, pH 5 and reaction time 30 min. Based on experimental results and regression coefficient, cadmium adsorption on Fe3O4@SiO2 nanoparticles has been followed by a pseudo second-order kinetic and Langmuir isotherm. Examining isothermic models suggests that this process follows the Langmuir isotherm model, which indicates homogeneity and the single-layer absorption process.
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