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As one of important contaminants of water pollution, the toxic heavy metals have harmful effects on the life of human being and environments. Heavy metal pollution can arise from many sources but most commonly arises from the purification of metals, e.g., the smelting of ores and the preparation of nuclear fuels. Wastewater from many other industries, such as metallurgical, tannery, chemical, mining, and battery manufacturing industries, etc.. contains also one or more of these toxic heavy metals. The concentration of metals is sometimes higher than permissible discharge levels in effluents. The discharge of such solutions not only causes environmental pollution but also adversely affects aquatic and human life, if the concentration of these ions is above the pollution standard. For instance, chromium (Cr), copper (Cu), cadmium (Cd) and nickel (Ni) are listed in the 11 hazardous priority substances of pollutants. Contact with these ions can result in severe health problems ranging from simple skin irritation to carcinoma. Through precipitation of their compounds or by ion exchange into soils and muds, heavy metal pollutants can localize and lay dormant. Unlike organic pollutants, heavy metals do not decay and, thus, pose a different kind of challenge for remediation. Hence, it is utmost important to purify water before its use as it is one of the basic needs for the existence of mankind. And the effective disposal of heavy metals has been arousing worldwide concern in the last few decades.Nanoparticles, often characterized by an extremely high surface-to-volume ratio, have been attracting much interest because of their unique physical and chemical properties distinct from their coarse-sized counterparts. They have been used in many fields, such as the biosensors, drug delivery, lubricants, and solar cells. These special properties are closely related to the disordered crystallographic structures near the surface of nanoparticles. Due to the well-known biocompatibility and excellent ferrimagnetic properties exhibited by the Fe3O4 nanoparticles, their applications have been explored in several areas, such as magnetic recording technology, pigments, catalysis, and photo-catalysis, as well as the medical uses.In this study, we purified the contaminated water using the magnetite Fe3O4 nanoparticles with different particle size, where different particle sizes were successfully controlled by the different fabrication processes, i.e.. the co-precipitation and the polyol method, respectively. For comparison, the surfaces of synthesized Fe3O4 nanoparticles were modified with a layer of hydrophilic and biocompatible polymer [polyethylene glycol (PEG-4000)] and also used to extract poisonous ions from the waste water. Due to the magnetic properties of Fe3O4 nanoparticles, the adsorbents were conveniently separated from the resultant via an external magnetic field. Subsequently, a series of characterization methods, including X-ray diffraction (XRD). scanning electron microscopy (SEM), transmission electron microscopy, Fourier transform infrared (FT-IR) spectra, thermogravimetry analyzer (TGA), Brunauer, Emmett, and Teller (BET) surface area analyzer and Zeta potential analyzer, were used to characterize the structures of nanoparticles obtained before and after the purification process. The main results and conclusions in this work are summarized as follow:1. Fe3O4 magnetic nanoparticles with different average sizes were synthesized and structural characterizations showed that the three kinds of nanoparticles had different sizes, i.e., an average particle size of 8 nm,12 nm and 35 nm was observed for the nanoparticles prepared with the co-precipitation method, the co-precipitation combining a surface decoration process, and the polyol process, respectively. It is indicated from FI-IR analysis that Fe3O4 magnetic polymer microspheres can be effectively synthesized. The corresponding surface areas of three kinds of nanoparticles were 1.9×102m2/g, 1.1×102m2/g and 4.6×101m2/g.2. The aforementioned nanoparticles were contacted with the wastewater containing the toxic metal ions, such as Ni(Ⅱ), Cu(Ⅱ), Cd(Ⅱ) and Cr(Ⅵ). For comparison, coarse-grained (CG) Fe3O4 particles were also contacted with the wastewater. It is found that the adsorption capacity of Fe3O4 particles increased with decreasing the particle size or increasing the surface area. For instance, the adsorption capacity of Fe3O4 nanoparticles with a mean size of 8 nm was as high as 35.46 mg/g, which is almost 7 times higher than that of the CG particles (5.10 mg/g). Whereas the adsorption capability of the surface decorated nanoparticles was 23.04 mg/g, obviously less than that of the co-precipitation process prepared nanoparticles.3. Under room temperature, the co-precipitation process synthesized nanoparticles (7 g, d=8 nm) were used to treat the wastewater (pH=4.0,1000 ml) containing 41.87 Ni(Ⅱ),47.44 Cu(Ⅱ),45.87 Cd(Ⅱ) and 43.61 Cr(Ⅵ) (in mg/L) for 24h. The ion concentration in the effluent was below 0.03mg/L for the each kind of toxic ion (Ni2+<0.01, Cu2+<0.01, Cr6+<0.01 and Cd2+=0.02, in mg/L), which can meet the discharge requirement in China.4. To optimize the conditions such as pH, contacting time, temperature and charge ratio, a series of adsorption experiments were conducted by mixing the wastewater containing 41.87 Ni(Ⅱ),47.44 Cu(Ⅱ),45.87 Cd(Ⅱ) and 43.61 Cr(Ⅵ) (in mg/L) with Fe3O4 nanoparticles with different particle sizes. Present results suggest that the adsorption capacity of Fe3O4 nanoparticles was strongly dependent of the surface area of adsorbent, the pH and temperature in wastewater. An increase in surface area of particles and in the temperature of wastewater is beneficial for the elimination of toxic ions. The optimum operation conditions can be summarized as:pH=4.0,S/L=1:4, t=24h, T=50℃。5. A series of adsorption experiments were conducted by mixing the wastewater with Fe3O4 nanoparticles with an average size of 8 nm. The effect of pH in the range of 1-11 indicated that the pH in solution obviously affected the removal efficiency of metal ions at room temperature (20℃), although the metal extraction rates approach a plateau for Ni2+ and Cd2+ with pH higher than 4.0. However, as far as Cu2+ is concerned, the effect of pH on the removal efficiency can be ignored for whose removal efficiency is higher than 97% at the whole range of pH investigated. At the same time, it is very interesting to note that the Cr6+ concentration in filtration unfavorably increased while the pH was changed from 4.0 to 11.0. The present results also hint that the separation of metal ions can be effectively achieved by the control of the pH in wastewater. The adsorption mechanisms of Cr(VI) have been suggested that the adsorption of Cr(VI) occurred due to the physical adsorption at lower pH caused by electrostatic attraction.
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