Removal of Cadmium from Polluted Water Environments using Zero Iron Nanoparticles

As a result of the development and expansion of cities, growth and development of industries and technology, various pollutants have entered the environment. The pollutions resulting from the discharge of industrial effluents, consumption of fuel, discharge of municipal sewage and the use of sludge from sewage treatment as a soil fertilizer have caused harmful effects affecting humans and living creatures and changing ecosystems. Meanwhile, as a heavy metal, cadmium is very important due to its high mobility and toxicity in low concentrations.

Various methods have been developed to remove heavy metals from contaminated water and soil, and surface adsorption is one of the most efficient methods. Various surface adsorbents such as clays, zeolites, dry plants, agricultural waste materials, biopolymers, metal oxides, microorganisms, and volcanic ash have been used to remove heavy metals. Iron nanoparticles with zero charge (nZVI) is a new technology that has successfully been used to remove various metal ions. Nanoparticles are special adsorbents that are used for remedial purposes due to the presence of significant specific surface area that leads to high density of exchange sites and metal removal capacity. Basically, iron nanoparticles have been introduced as effective reductants and catalysts for a wide range of common pollutants such as organochlorine compounds and metal ions.

Cleaning water environments of heavy metals can happen as a result of surface absorption by absorbent materials such as nanoparticles. The aim of the present study is to investigate the effect of iron nanoparticles in removing cadmium from water environments.



Materials and methods

Nanoscale iron particles with zero charge can be prepared in aqueous environments by reduction of ferric iron or ferrous iron by sodium borohydride or by decomposition of pentacarbonyl iron in organic solvents or argon. In this study, the synthesis of iron nanoparticles by sodium borohydride has been used. For the synthesis of nanoparticles, 7.823 grams of iron sulfate II (FeSo4.7H2O) was dissolved in 200 ml of distilled water in the presence of ethanol. After setting the pH of the suspension at 6.8, 0.8 grams of starch and 1.8 grams of sodium borohydride were added to the solution. The nanoparticles were separated from the solution using a centrifuge and washed with ethanol. All steps were performed in the vicinity of nitrogen gas and finally the synthesized nanoparticles were dried under nitrogen gas. Finally, by using microscopic methods, images with very high magnification of the material are obtained. In this study, the shape and morphology of produced nanoparticles were investigated using SEM.

To investigate the effect of nanoparticle and pollutant concentration on removing efficiency, factorial statistical design with cadmium concentration treatment at 9 levels (10, 25, 50, 75, 100, 200, 300, 400 and 500 mg/liter), and treatment with zero amount of nano iron in three levels (0.025, 0.05 and 0.1 g) was applied. The effect of contact time on removal efficiency was investigated at 10, 30 minutes and 1, 4, 8, and 24 hours, and the effect of alkalinity was investigated at pHs of 4, 6, 8, and 10. In all stages, the concentration of cadmium in the purified solution was measured using an atomic absorption device.



Results and discussion

With the passage of time, the amount of absorption or removal efficiency increases, but after 4 hours, its changes are not statistically significant. The increase in removal efficiency with the passage of time is due to the fact that with the passage of time the formation of holes and corrosion on the surface of iron increases, as a result of which the cross-sectional area of absorption and removal efficiency also increases. In addition, the active sites for cadmium absorption change and the number of products resulting from the reaction of iron in the water environment increases, which also causes an increase in the removal efficiency with increasing retention time.

The results showed that the absorption efficiency decreases with the increase of the initial concentration of cadmium, which means that iron nanoparticles have a limited capacity to absorb cadmium. Examining the effect of the initial concentration of the ions of the adsorbed material showed that firstly, the more concentrated the solution is in terms of the number of ions, the better the absorption is, and secondly, the number of active sites for absorption gradually increases with the increase in the process time and the increase in the number of ions absorbed on the adsorbent decreases, so that the rate of absorption decreases significantly, leading to the formation of balance in absorption.

The greater the amount of iron nanoparticles, the greater the number of active surfaces participating in metal absorption, and as a result, it holds the absorbed cadmium with greater force. The results of placing cadmium-contaminated nanoparticles in distilled water (Figure 5) showed that in high doses of iron nanoparticles, a smaller amount of cadmium was absorbed and re-entered the environment. With the increase in the initial concentration of cadmium, its release has also increased, because, as mentioned above, the number of occupied sites has increased and the amount of holding energy per ion has decreased, causing release.



Conclusion

As the concentration of the pollutant increases, due to the limited capacity of the adsorbent, the efficiency of absorption decreases, which means that with the saturation of the absorption sites, it is not possible to absorb more of the pollutant. On the other hand, with the increase in the amount of adsorbent, the absorption efficiency increases due to the increase in the number of absorption sites. Also, by increasing the amount of absorbent and pollutant, the possibility of collision between cadmium and iron nanoparticles and the occurrence of absorption reactions increases. The high ratio of absorbent to pollutant causes a stronger bond and as a result less pollutant is released. When the adsorption surfaces of nanoparticles are occupied by the pollutant, it is no longer possible to release the pollutant and reuse these absorption surfaces, and in general, nanoparticles of iron have zero and cannot be used more than once to clean cadmium from the water environment.