Arsenic Immobilization by Calcium Arsenate Formation.ppt

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1、Arsenic Immobilization by Calcium Arsenate Formation,Paper by James V. Bothe , JR and Paul W.BrownPresent by Bo-Chou, Lin Professor : J.F.Gaillard February 24 , 2000,Introduction,* Arsenic is of environmental concern due to its toxic properties ,toxicity : Arsine Arsenite Arsenoxide Arsenates Arseni

2、c* A common method to remove dissolved arsenic in the aresenic- containing watses is though precipitation of low solubility cacium arsenates* The role of calcium arsenate formation in reducing the concentrations of dissolved arsenic has not been well established,Objective of the paper,* Identify tho

3、se calcium arsenates, including the apatite Ca5(AsO4)3OH* Establish the conditions under which they are stable* Establish the processes responsible for the immobilization of arsenic in the presence of lime,Apatites,* Apatites are a class of minerals that are compositionally varied but share the same

4、 crystal structure and have been investigated as host materials for long-term immobilization of a number of environmentally hazardous elements* Apatites are attractive hosts because they tend to be stable over broad ranges of pH.,Experimental Procedure,* Prepare suspensions made by mixing Ca(OH)2 po

5、wder with arsenic acid and deionized water at a liquid tosolids weight ratio of approximately 10 to attain molar Ca/As ratios varying from 0.80 to 4.0.* Analyze the PH and concentration of Ca and As of the solution .* Analyze the solid-phase by x-ray diffractometer , SEM and TGA,Analyze Result,Ca4(O

6、H)2(AsO4)2*4H2O,X-ray diffraction pattern,SEM micrograph,X-ray diffraction pattern for Ca3(AsO4)2*2/3H2O and Ca3(AsO4)2*1/4H2O,(a) Ca3(AsO4)2*2/3H2O made with the calcium source containing 0.5 wt % magnesium,(b) Ca3(AsO4)2*1/4H2O made with the calcium source containing 0.5 wt % magnesium,(c) Ca3(AsO

7、4)2*1/4H2O made with the ultrapure calcium source.,TGA 2050 Thermogravimetric Analyzer,TGA profiles,(a) Ca3(AsO4)2*2/3H2O made with the calcium source containing 0.5 wt % magnesium(b) Ca3(AsO4)2*1/4H2O made with the calcium source containing 0.5 wt % magnesium(c) Ca3(AsO4)2*1/4H2O made with the ultr

8、apure calcium source.,SEM micrographs of Ca3(AsO4)2*1/4H2O and Ca3(AsO4)2*2/3H2O.,(a) the large platy crystals of the hydrate Ca3(AsO4)2*1/4H2O (b) the smaller leafy crystals of the hydrate Ca3(AsO4)2*2/3H2O.,Arsenate apatite,X-ray diffraction pattern of Ca5(AsO4)3OH made with the ultrapure calcium

9、source.,SEM showing its characteristically needlelike morphology,Calcium Arsenate Stability,* Arsenic concentrations equilibrated with Ca4(OH)2(AsO4)2*4H2O remain low (0.5 mg/L) throughout its range of stability* A structural dependence on the pH of the solution from which the hydrate precipitates*

10、Arsenate apatite remain stable at PH from 9.512.65* Arsenate apatite, Ca5(AsO4)3OH, was not observed to formwhen the calcium source containing 0.5 wt % magnesium oxide was used.,Immobilization of Arsenic,Considering that the regulatory limit for soluble arsenic is 5.0 mg/L, the precipitation of Ca4(

11、OH)2(AsO4)2*H2O and/or the apatite Ca5(AsO4)3OH at pH levels above 12.00 , and the hydrate Ca3(AsO4)2*2/3H2O provide adequate means for the immobilization of arsenic.,Solid-Solid Miscibility,A series of experiments were performed to investigate the degree of solid-solid miscibility between Ca5(AsO4)

12、3OH and Ca5(PO4)3OH.,Why ?,* Solid solution formation by apatites is well-recognized * phosphate is frequently present in natural waters !,X-ray diffraction patterns of the apatitic reaction products,Suspensions had varying ratios of phosphate to arsenate and used the calcium source containing 0.5 w

13、t % magnesium. Formation of the solid solution : Ca10(AsO4)yo(PO4)6-yo(OH)2yo = 0.5 5.0,Equilibrium Concentrations of Ca and As,Stability of Arsenate Apatite,* Ca5(AsO4)3OH becomes more soluble at lower pH andless soluble at high pH* The equilibrium arsenate concentrations were lowest in those suspe

14、nsions having compositions with yo less than 2.5. * Small amounts of phosphate facilitate the formation of Ca5(AsO4)3OH as a stable phase, even in the presence of magnesium,Conclusion,* Different molar Ca/As ratios in the suspentions will form different precipitated calcium arsenate hydrates* As the pH increases , the equilibrium arsenate concentrations decrease .* The adsorption of arsenate ion onto the fine Ca5(PO4)3OHcrystallites may also play a role in immobilizing dissolved arsenate ion , especially within the range, 0 yo 2.5,