Arsenic Immobilization by Calcium Arsenate Formation Paper by James V. Bothe, JR and Paul W.Brown Present by Bo-Chou, Lin Professor : J.F.Gaillard February.
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Arsenic Immobilization by Calcium Arsenate Formation
Paper by James V. Bothe , JR and Paul W.Brown
Present 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 >Arsenic
* 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 those 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 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 powder with arsenic acid and deionized water at a liquid to solids 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(OH)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(AsO4)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 ultrapure 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 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
* Arsenate apatite remain stable at PH from 9.5~12.65
* Arsenate apatite, Ca5(AsO4)3OH, was not observed to form when 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(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)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 wt % magnesium.
Formation of the solid solution :
Ca10(AsO4)yo(PO4)6-yo(OH)2
yo = 0.5 ~ 5.0
Equilibrium Concentrations of Ca and As
Stability of Arsenate Apatite
* Ca5(AsO4)3OH becomes more soluble at lower pH and less soluble at high pH
* The equilibrium arsenate concentrations were lowest in those suspensions 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)3OH crystallites may also play a role in immobilizing dissolved arsenate ion , especially within the range, 0 < yo < 2.5
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