Synthesis And Pre-Treated Of Schwertmannite And Its Efficiency Of Arsenic Removal From Simulated Groundwater

Arsenic is a kind of protoplasm poison and confirmed to be a potential carcinogens by the American Disease Control Centre and International Agency for Research on Cancer. The method of removal arsenic through adorption was considered as one of the most effective ways in the removal of arsenic from groundwater due to its high efficiency, easy manipulation, low cost and stable operation. Therefore, developing arsenic-removing agent of high efficiency and cost-effective have received the increasing attention for the arsenic removal from underground water. Among many arsenic-removing agents, schwertmannite exibites an extensive application prospect in removing arsenic in water and wastewater by virtue of its large reactive surfaces and hydroxy groups, sulfate affiliated to arsenic and other trace elements. Schwertmannite naturally formed could hardly be utilized in the advanced purification and treatment because of the mixture of other mineral phases and a large amount of poisonous elements. Therefore, the purpose of this study are:(1) to synthesize schwertmannite through bio-oxidation of FeSO4 by Acidithiobacillus ferrooxidans or chemical oxidation by H2O2; (2) to investigate the removal of As(Ⅲ) in simulated groundwater via adsorption of schwertmannite formed by biologic or chemical procedures; (3) to explore the pre-treated methods of biogenic schwertmannite to improve the adsorption capacity for As(Ⅲ); and (4) to perform a column experiment for evaluating the granular schwertmannite capacity of arsenic removal in synthetic underground water.It was found that schwertmannite could be synthesized by two different procedures: chemical oxidation of ferrous iron by H2O2 and bacterial oxidation of ferrous iron by resting A.ferrooxidans cells in ferrous sulfate solution. The synthesis rate of schwertmannite by chemical method was faster than that by biological procedure. In the chemosynthesis system, about 43.1% of the ferrous iron was incorported into the yellow-brown precipitates after 24 h for reaction, while in the biosynthesis system, it took 48～60 h to transform 36.7% of the ferrous ions into the red-brown precipitates. But physiochemical properties of the biogenic schwertmannite were more excellent than that formed by chemosynthesis for As(Ⅲ) adsorption. For example, the biosynthesis schwertmannite had 45.63 m2/g of the specific surface area, while chemosynthesis one only 3.17 m2/g. In addition, pHpzc of the former (pHpzc=7.6) was higher than the latter(pHpzc=5.7). Moreover, biogenic schwertmannite displayed a typical appearance of "hedge-hog" and small spheroids with a diameter of 400～600 nm.Static adsorption experiments were carried out in the study to investigate the removal of As(Ⅲ) in simulated groundwater via adsorption of schwertmannite of biologic or chemical origin. Obviouly, biogenic schwertmannite had a bigger adsorption capacity than chemical orgin one. The optimum pH for As(Ⅲ) adsorption by biogenic schwertmannite was wider than others. Moreover, biosynthesized schwertmannite exhibited stronger specific adsorption capacity of As(Ⅲ) than that of chemosynthesized one. In addition, the adsorption isotherm indicated that biosynthesized schwertmannite had a maximum sorption capacity of 102 mg/g for As(Ⅲ), which was 24.1% higher than chemosynthesized schwertmannite (82 mg/g).For further enhancing adsorption capacity of As(Ⅲ) by biogenic schwermannite, the pre-treatment or modification of the biologic schwertmannite was performed in the study including using NaOH, NaCl,thermal activation at 2O0℃, and ethanol-ultrasound. Results showed that the highest sorption capacity of 143.3 mg/g for As(Ⅲ) was obtained when the schwertmannite was pre-treated by NaOH with 0.01 mol/L at room temperature, which was much higher than that in untreated mineral. Besides, the surface area enhanced from 45.63 m2/g to 325.18 m2/g, and the molar ratio of Fe/S also increased, the phenomenon of aggregate weakened remarkably.A column experiment was conducted to explore arsenite removal in stimulated underground water through granular schwertmannite sorption. When the initial arsenic concentration and linear velocity were 0.2 mg/L and 1.2 ml/min, respectively, the adsorbent worked wonderfully in removing arsenic, the 10μg/L breakthrough volumes was 800BV, saturated adsorptive capacity for As(Ⅲ) was 6.6 mg/g. Meanwhile, the As forms in groundwater had no significant impact on the adsorption capacity.It was concluded that biogenic schwertmannite had a good performance in physicochemical properties and adsorption capacity than the chemosynthesis one, the mineral adsorption performance could enhanced effectively through 0.01 mol/L NaOH pre-treatment. What's more, the granular schwertmannite still had good adsorption performance in removal arsenic. Consequently, schwertmannite as a novel material had manesfested an extensive prospect in the engineering application.