Comparison between Ag (I) and Ni (II) removal from synthetic nuclear power plant coolant water by iron oxide nanoparticles
© Salmani et al.; licensee BioMed Central Ltd. 2013
Received: 29 July 2012
Accepted: 3 June 2013
Published: 25 July 2013
The impact of effective parameters such as iron oxide nanoparticles dosage, contact time and solution pH was optimized for removal of Ag(I) and Ni(II) in the nuclear cooling system and the best conditions were compared. Nearly complete removal (97%) of Ni(II) and Ag(I) were obtained at adsorbent dosage of 40 and 20 g/L, respectively. Experiments showed that 4 hours was a good choice as optimum contact time for two ions removal. The effective parameter was pH, so that maximum removal efficiency was obtained for Ag(I) in acidic pH=3 and for Ni(II) in basic pH=10. It seems that removal of Ag(I) was controlled by adsorption-reduction mechanism, but Ni(II) could place only adsorption. Langmuir and Freundlich model was more suitable for nickel and silver removal by this adsorbent, respectively. Ag(I) and Ni(II) removal efficiency trend by this adsorbent is similar at periods but different in the concentrations, pHs and equilibrium model. The obtained results were very promising, as both Ag(I) and Ni(II) were effectively removed from synthetic wastewater and there was a possibility to remove Ag(I) very fast. Hence, the idea of using nanoparticles for application of metal ions removal from wastewaters seems to be very efficient and quite promising.
The primary coolant is an essential cooling medium used to control heat in a nuclear power plant. The most commonly used primary coolant is high-purity water. During the operation of a nuclear power plant, corrosion products are released from the surfaces of the primary circuits into the cooling system. The corrosion products originate from the internal surfaces of piping and steam generator under the condition of high pressure and temperature. Some corrosion species are activated by the neutron flux in the reactor core and various forms of radionuclides are produced . The important radionuclides 60Co and 58Co are present in liquid waters that released from pressurized water nuclear power reactors. The 58Co can be formed by fast neutron capture of 58Ni. Various forms of radionuclides cause activity build-up, contamination of the primary coolant system and occupational radiation exposure. Therefore, the system decontamination is an important task in a nuclear power plant by reducing the radiation and metal concentration level. To achieve this goal, an alternative process is required to remove corrosion products .
Various physicochemical methods have been employed for removal of heavy metals from waste discharges, which include chemical precipitation, flotation, filtration, extraction, ion-exchange, electrochemical operation and adsorption process [3–6]. Ni(II) removal from synthetic nuclear power plant coolant water has been studied by such different methods as ion exchange resins (IRN77 and SKN1), stack configuration of continuous Electro-deionization, coir pith, magnetic filter – electro-deionization hybrid separation system [7–9]. The used conventional techniques for removal of chemical contaminants from wastewaters are adsorption process. These are mainly preferred when the enrichment of trace metal amounts or a high selectivity for a certain metal are required. Adsorption of heavy metals on mineral adsorbents such as Iron oxides and zero-valant iron has been widely studied by researchers. In the recent works, efforts are being made to use nanomaterials because nonomaterials have higher surface area and greater active sites for interaction with metal species [4, 10–12]. As a result, the use of nanoparticles for contaminant removal is more effective than the mass.
Besides lots of advantages of zero-valent Iron nanoparticles (nZVI), one of its major drawbacks is its sensitivity to oxidation in aqueous solution. In the recent years, the iron oxide nanoparticles have been investigated for the removal of organic and inorganic contaminants. In this study, iron oxide nanoparticle, a new and extensively usable material, was investigated for removal of Ag(I) and Ni(II) from synthetic nuclear power plant coolant water. The aim of this study was to compare Ag(I) and Ni(II) removal based on adsorption by iron oxide nanoparticles that was developed a shell layer iron oxide and a core zero-valent iron.
Materials and methods
Physical and chemical property of zero-valent iron nanoparticles
Average particle size
< 30 nm
Specific surface area
> 20 m2/g
0.04 ~ 5 g/cm3
<0.3(International standard 0.4)
So, iron oxide nanoparticles were synthesized by exposing nZVI to air and as a result ignition and heat were produced. In order to homogenize the particles, they were grinded following cooling down. It is documented in the literature that the concentrations of heavy metals in nuclear power plant coolant water vary by several orders of magnitude [1, 2, 14]. Attempt was made to use realistic concentration ranges so, the composition of this solution was Sb(V), Co(II), Fe(III), Ni(II), Ag(I), B(III), Cr(III), Li(I), Cs(I) with concentration of 5, 1, 30, 15, 5, 20, 4, 0.5, 0.5 mg/L, respectively. The salts of Sb2O5, Co(NO3)2.6H2O, Fe(NO3)3.9H2O, Ni(NO3)2.6H2O, AgNO3, H3BO3, Cr(NO3)3.9H2O, Li(OH).H2O, CsNO3 of analytical grade (Merck) were used. Stock and diluted solution were prepared by dissolving salts in distillated water. pH adjustment was performed with 0.1 N HCl and NaOH solution. All experiments were performed in environmental chemistry laboratory.
In order to determine the size of nanoparticles in solution, a Scanning Tunneling Microscopy (STM), SS2 model (Iran) was used. A Varian model 20AA atomic absorption spectrometer was applied for measurement of ion concentrations in solution. Chemical substances were weighed at precision of ±0.0001 g with Mettler model digital laboratory scale. Solution pH was measured by a Sension 3 model digital pH-meter.
Batch adsorption experiments
Experimental conditions investigated
Contact time, h
0.17, 0.5, 1, 4, 8 and 24
Iron oxide nanoparticle dosage g/ml
0.5, 1, 10, 15, 25, 30 and 40
Initial concentration of Ni, mg/L
Initial concentration of Ag, mg/L
pH of the aqueous solution
2, 3, 4, 5, 6, 7, 8, 9 and 10
Iron oxide nanoparticle size, nm
The mean equilibrium concentration of metal ions from output atomic absorption (Ce), time (t), pH and amount of adsorbed ions on the adsorbent at any time (q) was saved in data file of SPSS. Removal efficiency (R), t/q, qe and Ce/qe for all cases were calculated by formulate in SPSS and then saved. In order to examine the control of adsorption mechanism and kinetic model, the experimental data were used to test. For obtaining the best adsorption equilibrium, some different isotherm equations were tested and the (Ce/qe) versus Ce was plotted finally. The value of Q0 and b was determined from slope and interception of the obtained straight line. Three different models were tested for obtaining the best kinetic model. Finally, the t/q versus t gave a straight line and then qe and k2,ads were determined from the slope and interception of the plotting, respectively. It is important to notice that the experimental estimation of qe is not necessary for the application of this model.
Results and discussion
Characterization of iron oxide nanoparticle
A literature survey suggests that the precise composition of the oxide shell depends on the fabrication processes and also on environmental conditions. For example, if we heated the obtained Fe3O4 at 250°C in air for 5 h, the color changed from black to red-brown . The X-ray diffraction (XRD) pattern of the red-brown product was very close to that of the Fe2O3 and agreed well with the standard XRD pattern of γ-Fe2O3 . With respect to production method and color (red-brown) of nanoparticles, the prepared nanoparticles were γ-Fe2O3.
nZVI have some disadvantages including: a) Its mechanism in removing some metals like Ni(II) is both reduction and adsorption in which iron ions enter to environment due to reduction process while its permitted limit is 3 mg/L. b) different forms of nZVI (i.e. fresh, aged, and surface modified) are differentially toxic to rodent nerve cells . So, in this study nZVI was oxidized, in which oxide layer thickness, and as a result absorption process is increased and its operation and maintenance are more comfortable.
Effect of time on Ni(II) and Ag(I) removal
According to Figure 2, there is a break point at after 4 h contact time. So, 4 h can be chosen as optimum time for both ions. Efecan Nazlı concluded that equilibrium time for Ni(II) removal is 4 h by using nZVI . The importance of contact time comes from the need of identify the possibility of binding and obtaining the optimal time for occupied sites by metal ions and completing removal of the target metal ions. When adsorbent dosage becomes constant in a batch system, number of sites remains constant. After this time, the balance has happened between adsorption and desorption. If the time increases, the removal efficiency does not increase. Then adsorption of Ni(II) and Ag(I) on the iron oxide nanoparticle adsorbent was completed at the 4 hours and it found to be very rapid as evaluated on the basis of time effect.
Effect of adsorbent dosage on Ni(II) and Ag(I) removal
It can be seen when the iron oxide nanoparticle dosage increases, the removal efficiency of Ni(II) and Ag(I) increases. There are nearly complete removal (97%) of Ni(II) and Ag(I) at iron oxide nanoparticle dosage of 40 and 20 g/L respectively. Clearly, it`s due to initial contaminant concentration that Ni(II) is 15 but Ag(I) is 5 mg/L. Hu et al. employed magnetic Fe2O3 nanoparticles as adsorbent for the removal of Cr(VI) from wastewater and the adsorption capacity was found to be very high .
Comparison of two graphs (Figure 3) of removal of Ni(II) and Ag(I) in terms of nanoparticle dosage showed a significant difference in removal. The removal of Ni(II) has a nonlinear relationship with the amount of adsorbent, but for Ag(I) the relationship is nearly linear and the high removal was happened in primary time. This difference indicates that the removal mechanism is different for these ions. So that Ni ions adsorb on the surface of the nanoparticle but the Ag ions adsorb on the surface of particles and then reduced by particles core, so removal occurs faster.
Effect of pH on Ni(II) and Ag(I) removal
According to Figure 5, Ag(I) removal efficiency in acidic condition is more than alkaline but Ni(II) removal efficiency is more in alkaline condition. It is probably a sign that there is still nZVI besides iron oxide nanoparticle. Because nZVI mechanism for Ag(I) removal is only reduction and removal efficiency by reduction is more in acidic pH. Ni(II) removal efficiency in alkaline pH is more and, it`s common for iron oxide nanoparticle. if there is nZVI, its reduction mechanism for Ni(II) removal is trivial compared to adsorption.
Adsorption isotherm study
Freundlich and Langmuir parameters for adsorption of nickel and silver on iron oxide nanoparticles
Adsorption kinetic study
Adsorption rate constants, q e estimated and correlation coefficients associated to the pseudo-second-order kinetic model
The correlation coefficients for the second-order kinetic model are nearly equal to 1 and the theoretical values of qe also agree very well with the experimental values. This suggests that the experiment data of Ni(II) and Ag(I) adsorption on the iron oxide nanoparticles fitted properly with the pseudo-second-order kinetic model. Similar adsorption has been reported such as Pb2+ removal from wastewater by water soluble Fe3O4 nanoparticles as adsorbent in which Langmuir isotherm and pseudo-second-order kinetic model were fitted to experimental Pb2+ adsorption and kinetic data .
The behavior of two ions is similar at different adsorbent dosages, time periods, and kinetic model. They are also similar at pH=7and but the removal efficiency trends of two ions at the tested pH range and equilibrium modeling are different. Removal efficiency in alkaline condition is more than acidic pH for Ni(II) and vice versa for Ag(I). Iron oxide nanoparticles have a medium efficiency for removing Ni(II) and Ag(I) from synthetic nuclear power plant coolant water because it`s more efficient at alkaline pH. By evaluating other metals especially Cr(III) and Co(II) removal efficiency, iron oxide nanoparticles can be used to purify synthetic nuclear power plant coolant water.
The authors express their appreciation to the Advanced Analytical Laboratory of Faculty of Public Health and Iranian Research Organization for Science and Technology for cooperation in this work.
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