The Structure Evolution Of Heat-treated Pyrite And Its Performance And Mechanism For Arsenic Removal From Groundwater

Pyrite is the most common sulfide in nature and likely occur surface oxidation when exposed to air, result in acid mine drainage (AMD). At the same time, pyrite releases some of the heavy metal elements, such as Cu, As, Zn, to pollute the environment. However, pyrite can be used to treat polluted water due to its surface activity, crystal structure and physical properties. Arsenic is toxic and it is long term exposure can do great harm to human health. At present, the main sulfide adsorbents for arsenic are natural pyrite and synthetic nanometer FeS. But pyrite has smaller specific surface area and lower surface activity in the removal of arsenic in the water; and synthetic FeS has poor stability, high cost of preparation and difficult problem for separation. Some people studied the thermal decomposition of pyrite and found that pyrite can transform to pyrrhotite which has more specific surface aera and stronger reactivity and shows more excellent performance in the removal of metal ion in water.Therefore, in this thesis, the thermal structure evolution of pyrite under nitrogen atmosphere has been investigated. The study was performed using Differential Thermal Analysis and Thermol-gravimetric Analyzer, X-ray powder diffractometer, Field Emission Scanning Electron Microscopy and Magnetic Susceptibility meter. Then, the heat-treated pyrite was used as adsorbent for trivalent arsenic and pentavalent arsenic, to expound the adsorption mechanism and reaction mechanism. The following are the main achievements of this thesis.1. The thermal decomposition process of pyrite under nitrogen atmosphere is:pyrite transforms to monoclinic pyrrhotite, monoclinic pyrrhotite transforms to hexagonal pyrrhotite and hexagonal pyrrhotite transforms to troilite. The different transformation temperatures for monoclinic pyrrhotite and hexagonal pyrrhotite have important indicating significance for the formation and transformation of iron sulfide minerals in the natural environment.2. The conversion rates of pyrite to monoclinic pyrrhotite and monoclinic pyrrhotite to hexagonal pyrrhotite increase with the temperature rises. At the lower temperature,500-600 ℃, the rate of pyrite transforms to monoclinic pyrrhotite is higher than the rate of monoclinic pyrrhotite transforms to hexagonal pyrrhotite. At the higher temperature,700-900 ℃, the rate of pyrite transforms to monoclinic pyrrhotite is lower than the rate of monoclinic pyrrhotite transforms to hexagonal pyrrhotite.3. Whether in the aerobic or anaerobic environment, the optimum pH for As(Ⅲ) removal (weak acid to weak alkaline) is wider than for As(Ⅴ) removal (weak alkaline) by pyrite and pyrrhotite. Under aerobic environment, the uptake efficiency for As(Ⅲ) and As(V) is stronger than under anaerobic environment, and the removal for As(Ⅴ) is superior to As(Ⅲ).4. The optimum calcinations condition for As(Ⅲ) and As(Ⅴ) removal by heat-treated pyrite is 600℃ temperature and 1 hour time. It means that monoclinic pyrrhotite has best sorptive ability for arsenic. Kinetics adsorption shows inclined to quasi-secondary rate equation; meanwhile, isothermal adsorption can be described accurately by the Langmuir adsorption model both under aerobic and anaerobic environments.5. The adsorption of As(Ⅲ) and As(Ⅴ) on heat-treated pyrite including physical adsorption and chemical adsorption. Physical adsorption is mainly adsorbs on mineral structure itself, therefore, monoclinic pyrrhotite which there is more adsorption sites has better adsorption effect. Chemical adsorption shows the redox process:As(Ⅲ) adsorption, a large number of Fe(Ⅱ) oxides to Fe(Ⅲ), and then generates FeOOH and Fe(OH)3, take places strong complexation and flocculation with As(Ⅲ) respectively, in aerobic condition; By contrast, there is only occurs flocculation of Fe(OH)3/Fe(OH)2 with As(Ⅲ) in anaerobic condition. As(Ⅴ) adsorption is similar to As(V), and As(V) can be reduced to As(Ⅲ), prompting the oxidation of Fe(Ⅱ) in aerobic condition; However, the reduction of As(Ⅴ) can make Fe(Ⅱ) oxides to Fe(Ⅲ) and forms abundant Fe(OH)3 flocculates with As(Ⅲ)/ As(Ⅴ). The XPS analysis revealed that arsenic shows trivalent state in the product adsorbed As(Ⅲ), and pentavalent state coexists with trivalent state in the product adsorbed As(Ⅴ) under aerobic environment. However, under anaerobic environment, arsenic shows trivalent state both in the products afsorbed As(Ⅲ) and As(Ⅴ).