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