Removal Of Pb And Hg In Wastewater By Zero-Valent Iron

With the rapid development of industry, agriculture and transportation, and urbanization process, more and more heavy metals enter into the environment. Wastewater discharge is an important pollution source of heavy metals in the environment, lead mainly comes from electroplating, smelting, casting, pesticide, mining, dyes, oil, batteries, machine printed industries, while mercury mainly comes from the mining, chlor-alkali, mercury fulminate, electrical and electronic, pharmacy, dyes industries. Based on the statistical data in2007, the total emission of lead and mercury in industrial wastewater in China were191t and1t; and their contents were2.25～6.78mg/L and0.047～129.8μg/mL, respectively, in recently years. Lead and mercury in the environment can not be degradated by microorganisms, and can pollute atmospheric, water and soil environment for a long time. On the other hand, animal and plant can enrich and accumulate Pb and Hg in the environment, and can enter into human bodies through food chain, which may do harm to central nervous system, endocrine system, immune system and bones of human.At present, the removal methods of Pb2+, Hg2+in wastewater mainly include chemical precipitation, ion exchange, adsorption, electrolysis and membrane separation. However, these methods have disadvantages of high operation cost, complicated operation and easy to cause secondary pollution. It is urgent to develop new technology to remove lead and mercury in wastewater. Zero-valent iron (ZVI) not only possess advantages of strong reduction ability, low cost, and friendly to the environment, but also can produce colloids of iron oxide and iron hydroxides after reduction to strongly adsorb pollutants in wastewater. So far, ZVI oxidation and reduction technology is mainly used to remove some heavy metals, such as Cr, Co, Cu, Zn, and As, and high removal efficiencies can be obtained. To be our best knowledge, few reports can be found to remove Pb and Hg in wastewater.In this thesis, the removal efficiencies of Pb2+, Hg2+in simulated wastewater were studied by ZVI with the static flask experiments, and the removal conditions of the two heavy metal ions were optimized. Moreover, the effect of Cl-, Ca2+, and humic acid (HA) on the removal rates were discussed, and the kinetic characteristics of Pb2+and Hg2+removal by ZVI were obtained. In addition, and the surface morphology and composition changes of ZVI before and after treatment were characterized by scan electron microscope and X-ray diffraction, and the action mechanisms of ZVI and Pb2+, Hg2+were analyzed preliminarily. The research objectives is to provide scientific basises for the application of ZVI oxidation and reduction technology in wastewater treatment, and the improvement of the removal rates for heavy metals. The main results are summarized as follows.1. The optimum removal conditions of ZVI to Pb2+and Hg2+in the wastewater were obtained. When the initial concentrations of Pb2+and Hg2+were8mg/L and0.1mg/L, respectively, the dosage of ZVI was0.0017g/L, the initial pH was5, the temperature was25℃, the removal time was480min, the removal rates of Pb2+and Hg2+can be obtained as98.35%and94.5%, respectively. The effluent concentrations of Pb2+and Hg2+were below the national integrated wastewater discharge standard (GB8978-1996). The removal processes of Pb2+and Hg2+could be described by the first-order reaction kinetic equation, and the removal rate constant of ZVI to Hg2+(0.010min21) was higher than that of Pb2+(0.0081min-1).2. The concentration variation of Cl-, HA in wastewater had no significantly effects on the removal rate of Pb2+, which obviously decreased with the concentration of Ca2+increasing. The concentration variations of Cl-, Ca2+significantly influenced on the removal rate of Hg2+. The removal rate of Hg2+decreased in the presence of Cl" and Ca2+with the concentration of0.01mol/L and0.25mmol/L, respectively. However, the removal rate of Hg2+increased in the presence of Cl+and Ca2+with the concentration of0.03mol/L and0.5～0.8mmol/L, respectively. The removal rate of Hg2+significantly increased in the presence of HA, and that at low concentration (1.5mg/L) was a little higher than that at high concentration (3-5mg/L).The removal processes of Pb2+, Hg2+could be described by pseudo-first order reaction kinetic equation when Cl", Ca+, HA existed alone, and the removal rate constants of Pb2+in wastewater increased2.07,1.09and2.95times higher than that of Pb2+existed only, while that of Hg2+increased1.65,1.69and1.51times than that of Hg2+existed only, respectively. The removal processes of Pb2+, Hg2+could also be described by pseudo-first order reaction kinetic equation when Cl2+Ca2+, Cl2+HA, Cl2+Ca2++HA existed in the wastewater, and the removal rate constant of Pb2+increased3.62,2.46and1.95times, and that of Hg2+increased1.97,2.22and1.44times respectively. However, the removal rates of Pb2+and Hg2+increased little. The reason is that Cl-can enter the interspace of iron oxides and hydroxides, neutralize the partial positive electrical charge of ZVI corrosion products of Fe2+and Fe3+, produce pitting, and enhance the adsorption of iron oxides and hydroxides to Pb2+and Hg2+in wastewater. Ca2+can promote the gather of iron oxides and hydroxides, and remove Pb2+and Hg2+by coprecipitation, but the aggregation of iron oxides and hydroxides may accumulate in the surface of ZVI to form the passivation layer, and reduce the removal rate of Pb2+. Fe2+, Pb2+or Hg2+may form multielement complexes with HA, and coprecipitate Pb2+, and increase the solubility of Hg2+, relieve passivation, and help to remove Hg2+. However, Ca2+, Fe2+, Pb2+or Hg2+may form multielement complexes with HA, improve the removal rate constants of Pb2+, Hg2+through coprecipitation.3. The analytical results of scan electron microscope and X-ray diffraction showed that the passivation layer formed on the surface of ZVI after treatment, its main compositions were FeOOH and Fe3O4-Fe2O3. The main action mechanisms of ZVI to Pb2+and Hg2+in wastewater were redox reaction, adsorption and coprecipitation of iron oxides and hydroxides formed after reduction of ZVI.