INVESTIGATION OF HYDROXYAPATITE AS A MEANS OF REMOVING DISSOLVED ARSENIC FROM POTABLE WATER

Experiments on the adsorption and ion exchange of arsenic by hydroxyapatite, Ca₅(PO₄) ₃OH, were conducted to investigate the use of hydroxyapatite as both an efficient and inexpensive means for reducing dissolved arsenic in potable water sources.  Experiments used synthetic medical-grade hydroxyapatite, raw ground domestic cow (Bos taurus) bone, and collagen-extracted bone.  Batch tests were conducted using a solid to liquid ratio of 1:10 and initial arsenic concentrations of 10, 1, and 0.1ppm over times ranging from 1 hour to 28 days.  Most experiments were run at 30^oC, between pH 5-8, although supplemental experiments investigated changes in temperature and pH.

Under all experimental conditions, raw bone was more effective than both the collagen-extracted bone and the synthetic hydroxyapatite in adsorbing aqueous arsenic.  For example, raw bone lowered arsenic concentrations from an initial 0.1ppm to less than 0.047ppm within one hour.  Similarly, within 48 hours the raw hydroxyapatite lowered an initial concentration of 1ppm to under 0.047 ppm.  In a 48 hour test, starting with the same amount of initial arsenic (10ppm), synthetic hydroxyapatite lowered arsenic concentrations to 6.63ppm (36% removal), extracted bone lowered concentrations to 4.53ppm (57% removal), while raw bone lowered dissolved arsenic to only 2.42ppm (77% removal).  Values of the distribution coefficient, K_D (mL/g), derived from experiments at 48 hours range from 6 for synthetic hydroxyapatite, to 60 for collagen-extracted bone, and to 200 for raw bone.

These experimental results indicate that in addition to being adsorbed by the mineral phase, the arsenic is apparently binding to the collagen in the bone.  Further, results suggest that the initial uptake of arsenic is by surface adsorption and that the exchange of AsO₄^3- for PO₄^3- in the hydroxyapatite occurs over extended periods of time.  Hence, the use of bone to reduce arsenic in drinking water to the World Health Organization’s regulatory limit of 0.050 ppm is shown to be feasible.