Cadmium Removal From The Aquatic Environment By Zero-Valent Iron Nanoparticles

Introduction
    Cadmium as one of the toxic heavy metal, because of harmful effects on health and the Environment, has attracted a lot of attention. The main sources of cadmium emissions are the environment, the waste of electroplating, melting, alloyed manufacturing, pigments, plastics, batteries, extraction and refining processes. This metal easily accumulates in different machines of people and has negative effects such as renal disorders, lung insufficiency, bone lesions, cancer, high blood pressure, on the healt.
The adsorption process as an effective method for removing heavy metals from soil and water has spread. Various adsorbents such as clays, zeolites, the dry plants, waste pile of agricultural, biopolymers, metallic oxides, microorganisms, sewage sludge, ash and activated carbon for the removal of cadmium are used. Absorbed is known As an economic and efficient way with significant potential for removal, recovery and recycling metals of waste.
In study of transmission and absorbed of minerals and compounds on the adsorbent, must exist Equations and relations between concentration and remaining of absorbed matter in constant temperatures, these equations are isotherms, Langmuir and Freundlich models are represent these equations.
Methods
In this study, iron nanoparticles in size 8-18 nm and the effective level of 59-79 m2 / g was used, to prepare cadmium soluble was used of salt of cadmium chloride H2O(2/5) CdCl2.and for changes in PH was used NaOH hydroxide and HCl.
Testing method
The experiments done as batch reactor and changing pH, change in the amount of nanoparticles, change in the initial concentration of soluble,  change in the test time and Sedimentation time are investigated. The initial concentration of soluble is equal to 60, 30, 15, 5 mg / L, which is the value of 60 mg / L is included as the initial concentration of the soluble in other tests. in order to prepare 250 mm of  60 mg / L soluble, amount of 0.0035 gr of cadmium chloride salt is needed.
At first 250 ml of the Concentration of cadmium is prepared and certain amounts of particles with 0.75, 0.5, 0.25 and 0.13 g weight, was poured into 100 ml test tube containing cadmium soluble, Then in order to uniform distribution of nanoparticles in soluble to increase the contact between pollutants and nanoparticles and to enhance the efficiency of adsorption, The Solubles were placed in an ultrasonic device.
After washing with distilled water, magnet was poured into a test tube. And was adjusted onto a magnetic stirrer at high speed in a specified period of 60, 45, 30, 20 and 10 minute, And then was given Sedimentation time to the desired soluble. Also, in order to adjust the pH of concentrated nitric acid and were used normalized profits of 0.1.
Assessing the effects of changes in pH on the removal efficiency of cadmium by iron nanoparticles
The results of  the cadmium absorption in different pH at room temperature Was performed with an initial concentration of  60 milligrams per liter, 0.25 g of nanoparticles in 100 mg of soluble testing time 45 minutes.
with increasing pH, Removal rates increased, And from pH= 5.8 to pH= 7.2  the removal efficiency increases about 60%, However, cadmium soluble in 8.4 = pH was tested under identical conditions without the presence of nanoparticles, became clear that without the presence of nanoparticles at this pH, there is also a 80% removal efficiency. but in pH=7.2 without the presence of nanoparticles, the removal efficiency was obtained 34%, indicating that In PH= 7.2 the presence of nanoparticles increases removal efficiency.
Assessing changes in the amount of iron nanoparticles and its performance in the removal of cadmium
The results of the cadmium absorption was performed in varying amounts of iron nanoparticles at room temperature, with initial concentration of 60 milligrams per liter, a pH value of 7.2 with different values ​​of 0.13, 0.25, 0.5 and 0.75 g in 100 ml of nanoparticles and time-tested 45 minutes.
Removal efficiency increases with increasing amount of nanoparticle and  removal efficiency is  increased with increasing amount of nanoparticle from 13/0 to 25 / 0 g by about 30%, While the the removal efficiency of cadmium with increasing amount of nano-particles from 5/0 to 75 / 0 g has increased only 3%. 0.75 g of nano has the best performance, but given the proximity the result with 0.5 g of nano, this value has been selected as optimal nano. because of the lack of affordable higher amount of nanoparticles.
Assessing changes in testing time and the performance impact of nanoparticles in removal of   cadmium
the results of  Cadmium absorption was performed in varying amounts of  testing time at room temperature, with an initial concentration of 60 milligrams per liter, a pH value of 7, 0.5 g nano-particles in 100 ml soluble at various times experiments, 60, 45, the 30, 20 and 10 minutes .
of the Testing time 10 min to 20 min, the removal efficiency increased to 10% and from 20 minutes to 30 minutes, the removal efficiency increased only 3%.
Assessing various amounts of  Initial soluble concentration and the performance of nanoparticles in the removal of cadmium
The results of the cadmium was performed absorption at different values ​​of the concentration of the initial soluble at room temperature, with initial concentrations of 60, 30, 15 and 5 mg l, pH value equal to 7.1, 0.5 grams of nanoparticles per 100 ml of soluble and time-tested 30 minutes.
Whatever the soluble concentrations increases, the removal efficiency is reduced so that the maximum absorption of the soluble has happened at a concentration of 5 and 15 mg / L, which is equal to 98%.
And in fact the removal efficiency for both samples was the same. As can be seen, the speed of absorption process for all four cases are very close.
Assessing  Change in settling time and the performance of particles in the removal of cadmium
The results of the cadmium absorption  was performed at different times of sedimentation at room temperature, with an initial concentration of 60 milligrams per liter, pH value equal to 7, 0.5 grams of nanoparticles per 100 ml of soluble, test time of 30 minutes and the time of sedimentation 90, 60, 30 and 15 minutes.
Eliminated by increasing the retention time increases if the maximum of percent absorption occurs at the retention time of 90 min, With increasing retention time from 15 to 30 minutes increases the percent absorption, but rate of increase is negligible, But in the retention time of 60 to 90 min, the removal efficiency increases to 20%.
 Results and discussion
In this research, it was found that nano-zero valent iron particles are as an appropriate adsorbent for Cadmium reduction. Increasing amount of nano-materials, increases the absorption. Optimal obtained 0.5 gr per 100 ml of solution volume. In the study by Bahrami and his colleagues about removing of Cadmium from aquatic solution by nano-magnetic materials, it has been found that the removal efficiency increases with increasing amount of adsorbent. They concluded that by increasing the amount of adsorbent, the available adsorption sites for better adsorption of cadmium increases.
By increasing of pH from 3 to 9, the removal efficiency increases but as it is clear on the diagram, the promotion rate of this efficiency in pH from 7 to 9 is insignificant and only 10% increased. Deposition has an important role in the removal of cadmium ions in the alkaline range. The likelihood of precipitation of metal hydroxides in the pore spaces around the particles is very low, because the adsorption process is faster than the precipitation process.
Cadmium removal at pH lower than 7 is mainly controlled by adsorption process and this value at pH greater than 7 is significantly increased by cadmium hydroxide precipitation. In the study by L.V and his colleagues about Chrome removal by nano zero valent iron, it has been found that at pH greater than 7 removal efficiency decreases.
At the time of testing, respectively, 20, 30, 45 and 60 min, the removal efficiency increases, but the rate of absorption reduces and the absorption rate is fixed at 30 and 45 minutes. So 30 minutes was considered as the optimal time. In the research by Alqudami and his colleagues about removal of Cadmium and Lead by nano-zero valent iron, it has been found that the best adsorption occurred in 20 minutes. This may be due to the rapid accumulation of nanoparticles which reduce the active surface and thus reduce the absorption capacity.
Maximum adsorption by considering the all optimal situation occurred in a 5 mg/L solution. In the research by Bahrami and his colleagues about removing of Cadmium by nano-magnetic materials, it has been found that by increasing the concentration of the solution, removal efficiency decreases.
At low concentrations of cadmium, specific surface areas and adsorption sites are more And cadmium ions can interaction by Surface of the adsorbent particles.So, The absorption efficiency  increases.