| Water resource is very important for our human survival and development. Yet, serious water quality problems happen in a majority of cities, with the rapid development of urbanization and industrialization. Among the problems, water pollution caused by heavy metal is relatively serious. Heavy metal has a high mobility and low toxic concentration, which cannot be degraded easily in the water and possesses bioaccumulation property. Once the water is polluted by it, the polluted water is not only difficult to be cleaned but will seriously harm aquatic life and human health.Recently, Nanoscale zero-valent iron (NZVI), as a new technology controlling pollution used in the field of environmental pollution control, has attracted widespread attention. Many scholars carried out researches on NZVI’s effect on heavy metal polluted water and it turned out a better result. But NZVI has some shortages, like poor air stability, easy oxidation, spontaneous combustion and easy agglomeration. Due to that, preparation and practical application of NZVI is challengeable. Therefore, the modification of NZVI becomes a hotspot in further researches.In this thesis, based on data compilation and exploration, a pumice stone was chosen as the load material to prepare loaded-type Pumice-NZVI (P-NZVI) and it would be used for processing Cr(Ⅵ), Hg(Ⅱ) solution. The thesis studied the characterization and properties of NZVI, the mechanism that P-NZVI removes Cr(Ⅵ) and Hg(Ⅱ) in the water, the impact of environmental factors on the removal, as well as the kinetic studies. The main content of this paper is divided into the following four parts:1. The first part focuses on best preparation technic of P-NZVI through multiple sets of experiments. Liquid-phase reduction was selected to produce NZVI at room temperature, protected by nitrogen during the entire process. The mixture of alcohol-water, with volume ratio being8:2, was used to dissolve FeCl3·6H2O and the volume was set to100mL. Then, together with3.36g pumice particles, the mixture was put in a three-necked flask, at this moment, N2with high purity was sent in the flask. The mixture should be maintained stirring continuously for1h. After that, by using a peristaltic pump,100mL sodium borohydride solution made up by deionized water (the molar ratio of Fe3+and BH4-being1:5) was added into three-necked flask slowly, at the rate of1drop per second. During the dropping period, the three-necked flask was put into an ultrasonic cleaning machine for2min every10min (frequency:40KHz). After the addition was completed, the mixture still needed to be stirred for1h. Then, cleaned the P-NZVI with deoxygenated deionized water three times, using100mL each time. The P-NZVI was dried by an electric jacket at65℃and was collected after cooling.2. The analysis of characterization showed that NZVI particles were uniform in size, particle size between10～20nm. The pumice looked rough with porous loose shape. With good dispersion character, NZVI particles distributed evenly in the surface and pores of the pumice. The specific surface area of P-NZVI was32.2m2/g (NZVI content being0.28g, mass ratio being7.7%) based on the detection of BET N2method. This was higher than other loaded-type nanometer-ferrous materials. The analysis of P-NZVI by Thermal Analyzer demonstrated that P-NZVI had good thermal stability below550℃. Pumice has hard texture and P-NZVI has good mechanical properties.3. Removing Cr(Ⅵ) and Hg(Ⅱ) in the water by means of P-NZVI includes physical adsorption and chemical reduction. At first, Cr(Ⅵ) and Hg(Ⅱ) were absorbed on the surface and internal of the P-NZVI with the help of the pumice’s excellent adsorption property. Then, Cr(VI) and Hg(Ⅱ) could be reduced to Cr(Ⅲ) and Hg(0) respectively by the NZVI’s strong reduction property so that the Cr(VI) and Hg(Ⅱ) can be removed.4. With growing concentration of heavy metals in water, the P-NZVI’s effect on removing Cr(Ⅵ) and Hg(Ⅱ) was gradually decreasing. With increasing pH of the solution, the P-NZVI’s removal rate for Cr(Ⅵ) was decreasing, while the P-NZVI’s removal rate for Hg(Ⅱ) was gradually increasing. The increasing reaction temperature and the increasing NZVI dosage could raise the removal rate of Cr(Ⅵ) and Hg(Ⅱ). Besides, since the iron hydroxide can absorb and flocculate the Hg(Ⅱ), the increasing temperature will raise the solubility of iron hydroxide. As a result, an excessive temperature and pH can cause fluctuation of removal rate for Hg(Ⅱ). An excessive NZVI dosing can make excess iron be corroded thereby affecting the removal for Cr (Ⅵ) and Hg(Ⅱ). Kinetic studies have also proved the above rules. As it shows that P-NZVI has a very good removal effect for Cr(Ⅵ) and Hg(Ⅱ). Overall, environmental factors have a relatively small impact on the removal.5. Acidic deionized water was applied to immerse the used P-NZVI, which can dissolve NZVI and P-NZVI’s surface recombination sediment and "regenerate" the NZVI. With better reusability, the P-NZVI has a very important practical value in remedying heavy metal polluted water. |
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