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