Study On Hydrothermal Synthesis Of Arsenic Sodium And Its Toxicity Leaching Characteristics

Arsenic and its compounds are toxic to human beings, animals and plants, and have serious pollution on ecological environment. Nowadadys, there are many ways, such as chemical precipitation, physical and chemical, biological method and solidification/stabilization technology, have been developed for remove and solid arsenic. However, the methods of remove arsenic have its shortage. According to the research, As5+, Si4+ and Al3+ ion can bond oxygen formed as tetrahedral structure, then AsO43-, SiO44-, AlO45- can occur isomorphic replacement under certain conditions to form the relatively stable arsenate solid solution, which will become one of the more promising technique for treat arsenic.In this paper, Arsenic sodalite was synthesized by hydrothermal method. The condition of synthesis arsenic sodalite optimization, the stability of solid solution, the impact of Si and Al on forming arsenic sodalite and synthesis mechanism were mainly researched. Some important results were acquired as follows:(1) The factors influenced on the synthesis arsenic sodalities were systematically studied, including kaolin and metakaolin, the initial solution pH, TEA used, the reaction temperature and reaction time. It was showed that all above list factors had remarkable impacts on the synthesis of arsenic sodalities. The optimum synthesis conditions were as follows:using metakaolin as the raw, initial solution pH=12, using TEA, reaction temperature is 200 ℃ and recation time is 24 h.(2) Arsenic sodalites were synthesized under different molar ratio of (As/(As+Al+Si))aq of initial solution, and the products were characterized by means of XRD, SEM-EDS and FT-IR to research the replacement law. Arsenic contented in the product is about 7.8%. Under different molar ratio of (As/(A1+Si+As))aq of initial solution condition, it has the same law, such as, n(Si/Al)bulk< 1, n(Al/Si)bulk> 1> n((Si+As)/Al)bulk≈1 and n((Al+As)/Si)bulk> 1. The n(Al/Si)bulk=1 in an ideal sodalite crystals. Therefore, As is mainly substitution Si in progress of synthesis, then arsenic sodalities phase can be formed.(3) In shore-term leaching test, the As leaching concentration of precipitate is about 4 mg/L under pH=12 condition. At same leaching condition, the As leaching concentration of calcium arsenate waste is about 150 mg/L, which is far higher than hydrothermal precipitate. Farther more, Arsenic leaching rate of hydrothermal precipitate is lower than 0.7%. But, Arsenic leaching rate of calcium arsenate waste is several times than hydrothermal precipitate, it is indicated that the hydrothermal precipitate is stability in short-term leaching. In long-term leaching test, the lowest arsenic leaching concentration is occur at (As/(Al+Si+As))aq=0.33, and arsenic leaching rate is about 0.45%, it is show that arsenic sodalite has good stability.(4) Improving n(Si/Al) in initial reaction system, we can found that the morphology and phase composition of hydrothermal precipitates have changed significantly. Farther more, with the the improving n(Si/Al), As leaching concentration of hydrothermal precipitates were increased.(5) Under Reducing n(Si/Al) reaction system, the result show that the product is mainly compose with arsenic sodalite. However, with the reducing n(Si/Al), As leaching concentration of hydrothermal,precipitates were increased, it is indicated that the stable of hydrothermal precipitates are reduced. In addition, Al, Si and As cannot synthesized under n(Si+As/Al)aq=1.0 in initial solution enter into crystals.(6) At the same proportion and same reaction condition, precipitates were synthesized by different aluminum source and silicon source were comparative analyzed. The phase composition is only arsenic sodalite. It shows the same law, such as, n(Si/Al)bulk<1, n(Al/Si)bulk> 1, n((Si+As)/Al)bulk≈1 and n((Al+As)/Si)bulk> 1. It is indicated that As is mainly substitution Si in progress of synthesis, then arsenic can enter into crystals.